Madhu Chennabasappa

Alain Pautrat

Olivier Toulemonde
email: olivier.toulemonde@icmcb.cnrs.fr

From magnetic cluster glass state to giant vertical magnetization shift induced by ferromagnetic cluster growth in SrFe 0.25 Co 0.75 O 2.63

A charge ordering phenomenon within the crystallographic sites as following Sr 4 (Fe 0.14 3+ Co 0.36 3+ ) 4 8h (Fe 0.11 4+ Co 0.14 4+ Co 0.25 3+ ) 4 8f

O 10.52 was previously reported thanks to neutron diffraction coupled with Mossbauer spectroscopy studies . Such distribution supports a natural magnetic layered structure combining both in-plane ferromagnetic super-exchange interactions mainly on the octahedron "8f" sub-layer alternating and in-plane antiferromagnetic super-exchange interactions on the "8h" sub-layer containing tetrahedral sites and five-fold symmetry polyhedra (i.e. squared based pyramid and/or trigonal bipyramid). Because of the interfacial magnetic interactions between the two types of layers, we report a detailed study of the intriguing magnetic properties. AC magnetic susceptibility shows a frequency dependent peak suggesting a cooperative character due to inter-cluster interactions resulting in a magnetic cluster glass state. When temperature decreases and/or applied magnetic field increase, the ferromagnetic clusters growth is promoted and results in a large vertical hysteretic shift on the Field Cooled magnetization isotherm in relation with minor loop.

The occurrence of horizontal and/or vertical hysteretic shift of the magnetic loop [1] attracted attention in solid-state physics and materials chemistry communities. Leading to horizontal shift of the magnetic hysteresis loop, the so-called exchange bias (EB) is ascribed to interactions between ferromagnetic (FM) and antiferromagnetic (AFM) layers. Positive or negative exchange bias occurs when a positive or negative horizontal hysteretic shift is seen depending on the sign of the cooling field. Historically, EB effect was evidenced by Meiklejohn and Bean [2] in nanostructured Co/CoO system with Co as ferromagnetic (FM) phase and CoO as antiferromagnetic (AFM) phase before being extended to binary alloys [3] -Nogués and Schuller review the phenomenology of exchange bias and related effects [4].

Interestingly, it is more recently that vertical hysteretic shift was pointed out either in artificial bilayered material [5] or in natural materials. In the latter case, the vertical hysteretic shift is often structurally related to either oxygen vacancies as seen in layered ferrite Sr 4 Fe 4 O 11 [6] or cobaltite Sr 3 YCo 4 O 10.5 [7] or in the intrinsic layered Ruddlesden-Popper (RP) Sr 1.5 Pr 0.5 CoO 4 [8] phases. However, some controversies about its origin were pointed out [9] due to the large number of factors like interfacial interactions between different magnetic phases, macroscopic separated phases and/or magnetic training effect.

Few years ago, an exchange bias and memory effect was reported in the perovskite SrFe 0.5 Co 0.5 O 3- 10. The given crystalline framework Sr 2 FeCoO 6 based on "rock-salt"-type ordering [11] is likely not valid because the stabilisation of such very high Fe 4+ and Co 4+ oxidation states require drastic synthesis conditions [12]. Indeed SrFe 0.5 Co 0.5 O 3 is simple cubic perovskite with ferromagnetic transition close to room temperature [13]. However, the reported magnetic properties are likely intrinsic. Furthermore, an oxygen vacancies ordering model in SrFe 1-x Co x O 3- oxygen deficient mixed iron/cobalt perovskite has been reported [14]. And, when Fe/Co ratio is changed, the coexistence of nearly compensated and ferrimagnetic regions in the oxygen vacancies layered structure leads to magnetoresistance and more likely vertical shift than exchange bias [15]. [18]. But it has also been observed in cation ordered perovskite being the signature of long-range magnetic frustration due to competing nearest and next nearest neighbor superexchange interaction [ 19 ]. AC magnetization measurements at different frequency were then carried out to better characterize that behavior.

Peak position on the real part of the AC magnetization around the main peak shows a frequency dependent shift (see inset of figure 1) that rules out the possibility of any long-range magnetic ordering at the peak temperature defined as T f for freezing temperature. To better understand the nature of the frequency dependence of T f , our data has been analyzed using the empirical Vogel-Fulcher (VF) law expected for magnetically interacting clusters [20] because a Néel-Arrhenius law expected for superparamagnetic systems [ 21 ] results in very unphysical parameters as illustrated in the supplementary material (figure S3). The empirical Vogel-Fulcher law given by with three fitting parameters i.e.  * the characteristic attempt time, E a the average thermal activation energy and the Vogel-Fulcher T 0 having value between 0 and T f that is often interpreted as a measure of inter-cluster interaction strength, The frequency dependence of the freezing temperature T f using the VF approach shown figure 2a gives reasonable parameters.

Figure 2a :

The frequency dependence of freezing temperature plotted as a log(τ) vs 1/(T f -T 0 ).

Fit parameters obtained from the Vogel-Fulcher law are sum up in table1.

-

1,40 -1,35 -1,30 -1,25 -1,20 -1,15 -3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 102,05 K <T g <102,35K
collected data linear fit to the power law divergence of the critical slowing down log ()

log (T f -T g /T g ) Equation y = a + b*x Weight No W

Figure 2b :

The frequency dependence of freezing temperature plotted as a log(τ) vs log(

T f -T G )/T G ).
Fit parameters obtained from the power law divergence are sum up in table 1.

Figure 2c :

The frequency dependence of freezing temperature for a fixed characteristic attempt time to 5.08 10 -10 s plotted as T f vs 1/log(/ 0 ).

Fit parameters obtained from the Vogel-Fulcher law are sum up in table1.

The T 0 value ranges between 96.7 K < T 0 < 97.0 K was first estimated minimizing the fitting error for a fixed T 0 value. Interestingly, T 0 nicely matches with the ZFC peak obtained under 100 Oe in the DC magnetic susceptibility. Then E a /k B and  * were obtained from the slope and the intercept of log versus 1/(T f -T 0 ) fixing T 0 value at 96.85K. It results in parameters ranging between 142.5 K < E a /k B < 155.0 K and 2.45 10 -7 s <  * < 4.8 10 -7 s respectively. First, E a /k B temperature range well matches with the one at which an upturn is seen in the difference between ZFC and FCC magnetic DC susceptibility as seen in supplementary material figure S1.

Surprisingly, it also matches with the one obtained using the same empirical law for rich iron perovkite SrFe 0.90 Co 0.10 O 3- 22  for which Co/Fe ratio is reversed with respect to our sample.

The average strength of the super exchange magnetic interactions involved in SrFe 0.90 Co 0.10 O 3- and our SrFe 0.25 Co 0.75 O 2.63 compounds seems on the same range. Secondly,  * range value is typical for spin cluster glass (10 -9 -10 -6 s) supporting a cooperative character due to inter-cluster interactions and not a single atomic spin dynamic (10 -13 s). This is further supported by the Tholence's criterion [23] T* = (T f -T 0 )/T 0 ranging from 0.09 to 0.11 and by the ratio Ea/k B T f ranging from 1.30 to 1.45. Both Vogel Fulcher parameters amplitudes are comparable to those of frustrated spin glass on a concentrated system with ferromagnetic and antiferromagnetic interactions [24] or for short-range interactions magnetic glass as observed in Eu 0.5 Gd 0.5 S material [23]. Then, to quantify the spin glass transition temperature, the frequency dependence of T f has been studied by the dynamic scaling theory [25] (see figure 2b)

where  is the dynamical fluctuation time scale corresponding to measurement frequency at the peak temperature,  0 is here the microscopic flipping time for fluctuating entities, T g is the spin cluster glass transition temperature in the limit of zero frequency, and z and  are the critical exponents. First, the best scaling relation is obtained for the following set of parameters with 102.05K < T g < 102.35 K, 15.3 < z < 16.6 and 2.8 10 -10 s <  0 < 9.3010 -10 s. The value of exponent z is higher to that observed in case of Fe doped cobaltites perovskite structure [11,26] but in the order of magnitude of others mixed cobaltite insulating oxides [27] in which a large distribution of ferromagnetic and antiferromagnetic super exchange interactions is reported. Finally, the characteristic attempt time  * for the empirical Vogel-Fulcher law was fixed to the value of the microscopic flipping time for fluctuating entities to 5.08 10 [14].

Furthermore the ratio Ea/k B T f ranging from 2.81 to 2.97 always nicely matches that obtained for Eu 0.5-x Gd 0.5+x S materials characterized as frustrated spin glass [23].

As Figure 3 shows four isothermal magnetization curves versus applied magnetic field carried out at 15K. Already a small deviation of the hysteresis loop is highlighted after cooling the sample from RT under 0.01T regarding the standard measured after a ZFC process. Both a squared broadening of the hysteresis curve and a slight vertical shift of the magnetization i.e., in the direction of the applied field are observed and unambiguously improved when applied field amplitude increases suggesting the combination of ferromagnetism and spin cluster glass state [28]. Furthermore, an application of +3T shifts the magnetization towards positive axis while reversing the field to -3T results in a shift towards negative magnetization. The amplitude of the phenomenon in our material is not of the same order of magnitude as those published till date. Indeed, almost the entire loop changes its sign. Let us recall that a giant vertical hysteretic shift of 35% at 2 K under ±7T was considered in La 0.3 Sr 0.7 FeO 3 (110nm)/SrRuO3 (10nm) bilayer on STO substrate [5] while our effect is in the order of magnitude of 85% at 15K under ±3 T. To characterize such a high asymmetry, isothermal magnetization loop starting from 14T was made in purpose as previously pointed by Geshev [29] (see supplementary material S4 figure). The magnetization saturation that is never reached under such experimental limit, could roughly be estimated around 2.5 B /Sr 4 FeCo 4 O 10.5 . This relatively low magnetic saturation is typical of a magnetic frustrated system. The frustration occurs at a long-range scale due to competing nearest and next nearest neighbor superexchange interactions. And, the vertical hysteretic behavior of magnetization when the material is cooled under 3 T is thus likely due to minor loop in relation with ferromagnetic clusters aligned by an external magnetic field. It is not a conventional exchange bias phenomenon [28]. FCC magnetic susceptibility (Figure 1) and with the spin cluster glass transition state proposed in Sr 4 Fe 4 O 11 perovskite [30]. The real part isothermal AC magnetization curves under low applied field were then carried out. As shown in the lower insets of figure 4, the magnetic field dependence is first kept symmetric through the paramagnetic state (115K) to the spin cluster glass state (90K) transition while a clear breaking of the symmetric response implying the existence of a spontaneous magnetic moment as seen at lower temperature (65K (black circle with line) and 70K (green line) -upper inset of figure 4) [31]. Such a breaking of the symmetric response is even enhanced when temperature is further decreased. It unambiguously supports long range magnetic ordering at the used frequency probe.

In addition, from isothermal minor loop magnetization curves collected at 15K after cooling the sample from RT under different magnetic excitation, both the squared loop size and the vertical shift increase with applied magnetic field as seen figure S6. All our magnetization measurements definitively suggest the stabilization of ferromagnetic clusters whose (i) magnetization directions are aligned along the applied field direction when cooling and (ii) sizes are promoted either by the increase of the applied field amplitude when cooling or by lowering the temperature below 50K.

0 2 4 6 8 10 0,2 0,3 0,4 0,5 H bias (T)
number of cycles (n) 0,2 0,4

1/ sqrt(n) The training effect is an important characteristic related to the gradual decrease of the anisotropy interaction commonly found on the exchange bias systems and interpreted by a spin configurational relaxation mode [32]. It is described by a quantitative power law along n consecutive cycles [33] H bias (n) -H E = sqrt(n) where  is both system and experiment dependent constant and H E is the bias field in the limit of infinite loops and has been measured at 15K for the present sample after cooling the sample from RT under 3T. Up to 10 consecutive minor MH loops have been collected and plotted in figure Supplementary material S7. Figure 5 is focused in the dependence of bias field (H bias ) as defined previously along the 10 consecutive cycles. The quantitative power law is satisfied giving in a H bias contraction. Interestingly, two different regimes are indeed depicted. Considering the last cycles, H E  = 0.091  0.013 T is expected from the intercept of the linear fit (see supplementary material S8 figure) . That is why, as already reported in Mg 0.5 Mn 2.5 O 4 [34] spinel oxide the question is how much our loop displacements are associated with the magnetic viscosity defined as S = dM/dln(t) where M is the magnetization and t the elapsed time. The magnetization (see figure 6a) and the bias field (see figure 6b) time dependence along the consecutive number of cycles collected at 15K were then plotted in a time log scale. The magnetization relaxation phenomenon is here investigated as follows, the magnetization as function of time under a given applied magnetic field lower than the applied during the cooling process. Two extremes M(1T, 15K, t) linear behavior are underlined figure 6a and they obeyed to the magnetic viscosity's log scale time law resulting in 5.4 10 -3 < S(1T,15K) < 5.8 10 -3 (a.u) and 7.3 10 -3 <S(-1T,15K) <7.9 10 -3 (a.u). The observed logarithmic dependence of the magnetization in spin glass systems means that the relaxation of the system is determined by a broad distribution of activation energies. This is indeed expected for our materials exhibiting a large distribution in magnetic exchange constant sign and/or amplitude considering all the nearest and the next nearest magnetic interactions plus the chemical distribution. Then the two previously reported regimes for the bias field are also underlined in figure 6b showing two different logarithmic dependence of the bias field. A slightly modified magnetic viscosity law with the equation H bias (or M(1T, 15K, t)) = const + S1n(t/o), where t is the elapsed time since changing the field and o the characteristic time of the relaxation has been checked.

However unsuccessful tests were carried out aiming to extract from our data any characteristic time of the relaxation arising either from the thermal fluctuation of the bias field or from the magnetization. Even if a o of 100s was inputted as observed in the case of phase separation manganites perovskites [35], a large range of fixed o from 100s to 10 [15] , magnetic moments at the 8h crystallographic site i.e. in the oxygen deficient layers is in average higher than those at the 8f crystallographic site i.e. in the oxygen replete layers. The relatively higher content of ferromagnetic interactions in the octahedral observed in our SrFe 0.25 Co 0.75 O 2.63 further supports the higher frustration at the in-plane interface between the magnetic stacking layers. Thus at the sub-layers scale, the iron and cobalt distribution gives rise to a natural multilayers stacking with dominant nearest neighbor magnetic parallel interactions within the oxygen replete layers and dominant nearest neighbor magnetic anti-parallel interactions within the subsequent oxygen deficient sub-layers. From macroscopic measurements, a ferromagnetic ordered state is found to coexist at low temperatures with a G-type AFM short range ordering.

It results in a glassy magnetic state at longer range scale at T g = 102.2  0.15 K that can be reduced in size when the application of magnetic fields induces a percolation of the FM domains. Such macroscopic phase's separation was previously proposed in Bi 0.67 Ca 0.33 MnO 3 perovskite manganites [30].

In conclusion, supported by the structural and chemical framework showing subsequent layers along the c axis and following the Sr

  an intermediate conclusion, from a dynamic scaling theory analysis, the low temperature magnetic behavior of our SrFe 0.25 Co 0.75 O 2.63 material is significant of magnetic clusters stabilization that are frozen at "macroscopic" time scales below T g = 102.20  0.15 K and fluctuate above it. Clusters growth occurs at different scale of temperature, likely depends on the local oxidation state distribution in the layered structure, and then, on the distribution of magnetic super-exchange interactions. Two preferential growth temperatures are then suggested. A first one's occurring around 310 K in relation with the Vogel-Fulcher T 0 = 92.05  0.25 K and the characteristic attempt time  * = 5.08 10 -10 s as supported by the thermal activation temperature ranging 306 K < E a /k B < 316 K related to the loss of the Curie -Weiss behavior for the DC magnetic susceptibility data. A second one's would occur around 150K in relation with the Vogel-Fulcher T 0 = 96.85  0.15 K and the characteristic attempt time  *' = 3.45 10 -7 s as supported by the average thermal activation temperature range between 142.5 K < E a /k B < 155.0 K which matches with the asymptotic temperature behavior of the DC susceptibility difference between the ZFC and FCC process. That is why, regarding (i) the layered character of our SrFe 0.25 Co 0.75 O 2.63 material, (ii) its frozen spin cluster glass state below 102.2 K and (iii) the long-range magnetic ordering observed around room temperature in related SrFe 0.15 Co 0.85 O 2.62 oxide [14], the possibility of any vertical and/or horizontal hysteretic shift phenomenon has been probed.

Figure 3 Figure 4 :

 34 Figure 3: Isothermal magnetization curves at 15K after ZFC or FCC under different applied field.

Figure 5 :

 5 Figure 5 : Decrease of the bias field (H bias ) with the consecutive number (n) of cycles of the hysteresis loop (bottom & left axis) or with 1/sqrt(n) (top & right axis) exhibiting a so-called training effect.

  Figure 6 a.

  Figure 6 b.

  Previously, detailed investigations of SrFe 0.25 Co 0.75 O 2.63 material by means of Mössbauer spectroscopy, Mohr salt titration and neutron diffraction using the Fe and Co neutron cross sections contrast, have been carried out. Such an approach allows us to propose a Following the structural and chemical investigations of our SrFe 0.25 Co 0.75 O 2.63 material, the magnetic properties were carried out using commercial MPMS SQUID (Quantum Design, USA) instrument. Our results suggest that our natural framework shows similarly behavior than the one observed in ferromagnetic / antiferromagnetic heterostructure resulting in a biased coercive field leading to giant vertical shift magnetization. Its magnitude can then be tuned either by the temperature or by the applied magnetic field.

		0.10			1.2 emu/g)					Figure 1:
	M (emu/g)	0.04 0.06 0.08			1.1 M' -3 (10 1.0	96	0.3Hz 100 104 108 112 H ac =2Oe 3Hz 33Hz) H dc =5Oe 66Hz	Typical Zero Field Cooled (ZFC) and Field Cooled Cooling (FCC) magnetization curves
								T (K)
		0.02					H=100 Oe	ZFC	Inset displays Real part of
									FCC	the AC susceptibility at
		0 0.00	50	100	150	200	250	300	350	different frequency
					T (K)
	As shown in figure 1, magnetization measurements under 0.01T on SrFe 0.25 Co 0.75 O 2.63
	material reveal a weak magnetic peak around 95  5K. Further, a deviation between data
	collected in a field-cooled mode (FCC) and a zero field cooled mode (ZFC) is observed. It
	becomes larger and larger when cooling with a clear asymptotic divergence around 150K
	whereas the Curie-Weiss law divergence already occurs around 310K (see Supplementary
	material S1 & S2 figures). Such a thermal hysteretic behaviour and the magnetic peak
	temperature were expected in view of similar measurements carried out on layered
	SrFeO 3- 17  and SrFe 0.5 Co 0.5 O 3- 11]. Below 50K, similar constant branch on the FCC curve
	has already been observed either in ferromagnetic clusters embedded in a weakly magnetic
	matrix in a concentrated nanoparticles system
	crystallographic formula as the following Sr 4 (Fe 0.14	3+ Co 0.36 3+ ) 4	8h (Fe 0.11 4+ Co 0.14	4+ Co 0.25 3+ ) 4	8f O 10.52

[16]

. It supports a natural layered structure with high Fe and Co oxidation states in the oxygen replete layers.

  -10 s obtained at T g = 102.2K. The parameters 91.8 K < T 0 < 92.3 K and 306 K < E a /k B < 316 K (see figure2c) can then be extracted thanks to the empirical Vogel-Fulcher law from the intercept and the slope respectively. It appears that T 0 parameter is not really impacted by characteristic attempt time adjustment contrary to E a /k B. The E a /k B temperature range is now unambiguously matching with the temperature at which the paramagnetic Curie-Weiss law is lost and with the ferrimagnetic ordering temperature of a similar compounds SrFe 0.15 Co 0.85 O 2.62

  -10 s can been imputed giving similar linear quality factors. Consequently, it is tempting to speculate consecutive loop displacements with the magnetic viscosity moving the state of the sample toward an ideal not displaced loop as the apparent equilibrium at much larger time scale.Fe 4+ start to be active[6] that would pin clusters becomes significant. It results in the upturn around 150K seen in the difference between the DC magnetic susceptibility curves collected in a ZFC and FCC process under 100 Oe. With further temperature decreases, as supported by the frequency dependence of our AC susceptibility, a magnetic phase transition toward a spin cluster glass state takes place around 95 K. The broad distribution in sign and amplitude of the exchange coupling factors is at the origin of the broad size clusters. The spin cluster glass state resulting from the freezing of the different size clusters. Such magnetic frustration at the clusters interface precludes any long range magnetic ordering[14].Interestingly, if one considers the previous study for SrFe 0.15 Co 0.85 O 2.62

	The ferromagnetic clusters are originated from the ferromagnetic Fe 4+ -O(2p)-Co 4+ ,
	Co 4+ -O(2p)-Co 4+ and Co 3+ -O(2p)-Co 4+ superexchange interactions. Their interaction strengths
	are large enough to induce ferromagnetic ordering around room temperature in Sr 1-x La x CoO 3
	(0<x<0.5) [36] and SrFe 1-x Co x O 3 (0.2<x<1) [37]. Interestingly, we already pointed out the
	following iron and cobalt distribution in the crystalline framework with
	Sr 4 (Fe 0.56	3+ Co 1.44 3+ ) 8h (Fe 0.44	4+ Co 0.54 4+ Co 1.02 3+ ) 8f O 10.52 . One can then expect that ferromagnetic
	cluster grow around room temperature resulting from preferential in-plane ferromagnetic super-
	exchange interactions on the octahedron "8f" sub-layer. The slight deviation observed in the
	inverse magnetic susceptibility around room temperature likely result from those clusters
	behaving independently from each other's. When the temperature decreases, the nearest
	antiferromagnetic super exchange interactions as for example Fe 3+ -O(2p)-Co 3+ and Fe 3+ -O(2p)-

  , complex magnetic properties are reported. The ferromagnetic super-exchange interactions that preferentially takes place on the octahedron 8f sub-layer, give raise to ferromagnetic cluster growth when temperature decreases and/or applied magnetic field increases embedded in a spin cluster glass state below 102.2K. It results in a vertical hysteretic shift on the Field Cooled magnetization isotherm at the ferromagnetic clusters / spin cluster glass matrix interface that becomes giant when percolation of the ferromagnetic clusters occurs.AcknowledgementAuthor MC acknowledge Université de Bordeaux1 for his PhD fellowship. This work was supported by the European project "SOPRANO" under Marie Curie actions (Grant No.

	4 (Fe 0.56 3+ Co 1.44 3+ ) 8h (Fe 0.44 4+ Co 0.54 4+ Co 1.02 3+ ) 8f O 10.52
	paramagnetic cation ordering

PITNGA-2008-214040).


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Mechanochemical functionalization of mesoporous carbons for the catalytic transformation of trans-ferulic acid into vanillin

  Noelia Lázaro, Jimena Castro-Gutiérrez, Pamela Ramírez-Vidal, Alain Celzard, Vanessa Fierro, Tahani Saad Algarni, Antonio Pineda, Rafael Luque To cite this version: Noelia Lázaro, Jimena Castro-Gutiérrez, Pamela Ramírez-Vidal, Alain Celzard, Vanessa Fierro, et al.. Mechanochemical functionalization of mesoporous carbons for the catalytic transformation of transferulic acid into vanillin. ACS Sustainable Chemistry & Engineering, 2021, 9 (13), pp.4704-4710. 10.1021/acssuschemeng.0c09028. hal-03406246

Textural properties

GC-MS spectra

*


Annette Leclerc
email: <annette.leclerc@inserm.fr>

Julie Gourmelen

Jean-François Chastang

Sandrine Plouvier

Isabelle Niedhammer

Jean-Louis Lanoë

Plouvier Sandrine

Niedhammer Isabelle

Level of education and back pain in France: the role of demographic, lifestyle and physical work factors

Keywords: Adult, Age Factors, Aged, Demography, Educational Status, Employment, Female, France, epidemiology, Humans, Life Style, Low Back Pain, epidemiology, etiology, psychology, Male, Middle Aged, Occupational Diseases, epidemiology, etiology, psychology, Occupational Exposure, adverse effects, Prevalence, Risk Factors

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A low level of education is associated with various diseases and conditions, including musculoskeletal disorders and more specifically low back pain (LBP) ( ; ; ;

). Associations between LBP andDionne et al. 2001 Aittom ki et al. 2007 ä Hagen et al. 2005[START_REF] Schmidt | Back pain in the German adult population: prevalence, severity, and sociodemographic correlates in a multiregional survey[END_REF] occupation have been much studied, since physical demands at work play a role in incidence and prevalence of LBP ( ;Lund et al. 2006 ) The causal processes linking education and health are not well known (

). This is the situation also for LBP in adulthood. Blane 2003 Access to and quality of care are probably relatively unimportant, since most patients recover quickly from episodes of common LBP (

). Causal mechanisms that may play a role in the consequences of back pain, such as sick leave, probably differ from those Andersson 1999 related to its incidence and recurrence ( ; ;

). Generally speaking, the outcome of LBP Dionne et al. 2001Andersson 1999Hagen et al. 2006 is likely to affect the magnitude of and explanations for the associations between LBP and socioeconomic status ( ;Dionne et al. 2001 ). Similarly, the magnitude of the associations between LBP and most of its risk factors will depend on the specific Hagen et al. 2006 definition and outcome measures chosen ( ; ). Griffith et al. 2007[START_REF] Ozguler | Individual and occupational determinants of low back pain according to various definitions of low back pain[END_REF] Education may affect the incidence and duration of LBP through lifestyle factors such as smoking or obesity ( ;Goldberg et al. 2000 ; ). While height (tallness) is reported to be a risk factor for LBP in some studies ( ; Lake et al. [START_REF] Ozguler | Individual and occupational determinants of low back pain according to various definitions of low back pain[END_REF][START_REF] Power | Predictors of low back pain onset in a prospective British study[END_REF]Kopek et al. 2004 ) it should not explain the higher frequency of LBP among the less educated, since being tall is associated with a Leino-Arjas et al. ). Occupational exposure might be an [START_REF] Skovron | Sociocultural factors and back pain. A population-based study in Belgian adults[END_REF][START_REF] Van Tulder | European guidelines for the management of low back pain[END_REF] intermediate factor. Most studies of the role of occupational risk factors have emphasized occupation more than education, partly because the proximal factor (occupation) is expected to be a stronger determinant than the more distant factor. Another reason is that occupational risk factors are a clear target for prevention (

). [START_REF] Schneider | Workplace stress, lifestyle and social factors as correlates of back pain: a representative study of the German working population[END_REF] This study of more than 15 000 members of the French general population aged 30 to 69 years describes the associations between LBP and education for a specific outcome LBP for more than 30 days during the previous 12 months, a definition similar to long-lasting or -"

frequent LBP ( ). Our objective was to describe the frequency of LBP and associated factors and to help improve our " Griffith et al. 2007 understanding of pathways from education to LBP. A trained interviewer visited each subject at home three times. In addition to providing oral information to the interviewer, participants were asked to complete a self-administered questionnaire. Our study sample is composed of the 18 002 subjects aged 30 to 69 years who were visited three times. Weighting based on this sample enabled us to calculate figures for the entire population of France.

Methods

Subjects

Low back pain

The self-administered questionnaire included a French version of the Nordic Questionnaire for the analysis of musculoskeletal symptoms (

). The main definition used here for LBP was pain in the previous year for at least 30 days (at least 30 days Kuorinka et al. 1987 but not daily, or pain everyday).

The interviewer considered that 210 of the 18 002 subjects in the target population (1 ) were unable to complete a questionnaire.

%

Another subgroup of 114 subjects (0.6 ) had to be excluded because of missing data for several LBP questions. A final group (n 2144, % = 12 ) did not complete the LBP questionnaire at all. Hence the study population consisted of 15 534 subjects: 7292 men and 8242 women.

%

Study variables

Personal factors in this study were those considered to be potential risk factors for LBP: gender, age (in four categories), height (two categories), body mass index (BMI) (three categories), and tobacco consumption (three categories). For height, the cutoff between the categories was 180 cm for men and 170 for women.

Three variables described socioeconomic status: level of education, present or past occupation, and household income. Education had five categories: none or did not complete elementary school; elementary level or other low level general diploma; low level vocational diploma, medium-high level general diploma ( baccalaureate ), higher education (at least two years after baccalaureate).

" "

Occupation was coded according to the French nomenclature, for the present occupation or (for those not in the labor force at the time of the survey) for the longest job held in the past. Only nine men and 321 women had missing data for occupation.

Household income was classified in four categories.

Two variables describing physical working constraints were based on the self-administered questionnaire: tiring work posture and handling heavy loads. The subjects were asked to indicate the duration of exposure (never, less than 10 years, 10 to 20 years, more than 20 years) in the present job, or in the longest job held in the past for those not in the labor force at the time of the survey.

In addition, employment status and income source was used for descriptive purposes only, with six mutually exclusive categories: employed; unemployed looking for a job; retired; housewife; receiving permanent disability benefits; other unemployed. These categories have previously been used for the study of chronic back problems in France (

). Of the three variables describing Leclerc et al. 2006 socioeconomic status, educational level was the only one taken into account in the models, since our objective was to study the respective roles of education and physical constraints at work in the causal pathway leading to LBP. Income level may be a consequence of chronic LBP (

). Occupation may be considered an intermediate variable for it is both a consequence of education and a [START_REF] Stronks | The interrelationship between income, health and employment status[END_REF] determinant of work-related risk factors.

Analyses

In a preliminary stage, we compared the study and target populations. Population employment status and income source (weighted results) were also described according to LBP.

The frequency of LBP in the population was described separately for men and women, according to the variables mentioned above.

Confidence intervals were calculated with the SAS Proc Surveymeans procedure.

The main part of the analyses focused on education and the personal and occupational factors that might explain the associations between education and LBP. For bivariate analyses in the study population, chi-square tests were used. Logistic models were also used, with LBP as the variable of interest. Unweighted models were used, since previous results indicated that weighted and unweighted results were very similar.

Four models were compared, separately for men and women: In models 2A and 2B, the variables in the initial models were those associated with LBP at a p-level of 20 (unadjusted) for men or % women, except occupation and income, which were not included. The variables in final models 2A and 2B were education, age, and those associated with LBP in the model at a p-level of 10 or less. % Model 3 included education, age, and all the variables in final models 2A and 2B.

Since duration of exposure to physical occupational risk factors is closely associated with age, the two occupational risk factors were considered in the models with two categories, never or ever exposed, irrespective of duration.

All the models were based on a sample without any missing values (6811 men and 7013 women).

The third part of the analyses (tables not given) consisted of additional analyses aimed at a better understanding of complementary aspects: relations between education, lifestyle risk factors and height; role of occupation as an intermediate risk factor (associations with education, and physical work exposure), links between education and physical work exposure, and the sensitivity of the results to the LBP " and no LBP definitions used.

"

" "

Results

The 210 subjects unable to complete a questionnaire were older (28 were 60 years or older), and 76 were in the lowest educational % %

category. The 114 with missing data for LBP were also older and less educated. The 2144 who did not complete the LBP questionnaire did not differ much from the rest of the sample, except that they were less educated.

The population estimates for LBP at least one day in the previous 12 months were 53.4 for men and 57.4 for women. LBP for 30 women the situation was rather similar; 3.2 of women with LBP were receiving permanent disability benefits.

% LBP according to personal, socioeconomic and occupational factors ( ) Table 2 Prevalence of LBP increased with age for men and women. Taller women had a higher risk, whereas the association with height was not significant for men. The risk was higher among ex-smokers of both sexes and was strongly associated with high BMI. LBP was associated with education, occupation, and income; however, the strongest association for both sexes was with education.

A dose-response relation was observed with duration of exposure to tiring postures and handling of heavy loads.

Generally speaking, the weighted and unweighted prevalences were very close.

Contribution of personal factors and physical working constraints to the association between LBP and education

For men (

) both age and education were associated with LBP in model 1. The association with education was not modified Table 3 when tobacco consumption, height and BMI were taken into account (model 2A). These three personal risk factors were significantly associated with LBP at a p-level of 10 . When tiring postures and handling heavy loads were taken into account (model 2B), the % association with education was no longer significant at a p-level of 10 . The strongest association with LBP in this model was for % "

handling heavy loads . The associations between LBP and physical working constraints were similar in model 3, which took into account " both personal and occupational factors.

For women ( ) the association with the lowest educational level in model 1 was smaller than for men. The association with table 4 education was weaker when personal factors were taken into account (model 2A). Overweight and height were more closely associated with LBP in women than men. Model 2B indicated that physical working constraints played an important role; controlling for them, the only educational category with a significantly elevated OR was low vocational . No significant association with education remained in " " model 3.

Additional analyses

Relationship between education and lifestyle risk factors and height

Men with low educational levels were more often smokers, while those with more education were relatively frequently ex-smokers.

Strong associations between education and BMI were observed, especially among women. Among them the frequency of a BMI of 27 or more was 35.1 in the lowest educational category and 11.4 in the highest.

% %

Tallness was more frequent among those most educated. The percentage of men taller than 180 cm ranged from 12.7 in the group % with the least education to 24.1 for those with the most; the corresponding figures for women taller than 170 cm were 4.4 and 11.9 .

% % %

Occupation, education, and physical work exposure

Our first hypothesis was that a part of the pathway from education to LBP was through occupation. The links between education and occupation were not very strong, however. Logistic models conducted separately for men and women (figures not given) indicated that education remained a risk factor of LBP for men when occupation was taken into account and that having the lowest educational level had a stronger effect than being a blue-collar worker. LBP in women was also more strongly associated with education than with occupation.

The level of exposure to physical work factors was associated not only with occupation but also education. This held true especially for men and women classified as farmers and self-employed. Similar results were observed among women in intermediate occupations, a category that includes health care workers. Education (alone) was a strong predictor for physical work exposure. For men, the frequency of tiring postures was highest (63 ) % among those with the least education and 53 for those with a vocational diploma; the lowest frequencies were for the highest (15 ) and % %

next highest (28 ) educational levels. Results were similar for handling of heavy loads: in ascending order of educational levels, % frequency of exposure was 65 , 53 , 56 , 32 and 13 . A strong contrast between educational categories was also observed among

% % % % %
women. The highest percentages were observed for the lowest educational levels: 55 for tiring postures and 41 for handling of heavy % % loads. The lowest frequencies, conversely, were for those with the highest level: 22 and 15 , respectively.

% %

Sensitivity of the results to the definition used for LBP, and to the reference group

With a broader definition for the outcome -LBP at least one day in the previous 12 months -the associations were much weaker. Among women, no significant association was observed with education; among men the relation to education was borderline for significance (p 0.051), and the trends were similar to those for LBP for more than 30 days.

=

If LBO for at least 30 days was compared to a reference group comprising only those without LBP, the link between LBP and level of education was slightly stronger for men, and slightly les strong for women. For men the OR for no diploma in model 1 was 1.79 instead " " of 1.75, for women it was 1.21 instead of 1.27. As in the models presented in and , the excess of LBP in the lowest educational tables 3 4 levels disappeared if physical work exposure variables were included in the models.

Discussion

This study suggest that the main pathways from education to LBP are through occupational exposure and lifestyle factors. Height had a special position, for it is both a risk factor for LBP and associated with a higher educational level. is intended to illustrate the Figure 1 results, rather than provide a general overview of mechanisms leading to LBP. It would not necessarily apply to LBP at all ages or to all dimensions of LBP (

). [START_REF] Mustard | Childhood and early adult predictors of risk of incident back pain: Ontario Child Health Study 2001 follow-up[END_REF] A national population-based survey offers the advantage of yielding estimates for the entire population, taking into account weights derived from the sample design (

). The study includes self employed workers as well as those not in the labor force, Leclerc et al. 2006 which is not the case for other surveys of LBP. Information on past occupational exposure was more accurate than in most comparable population surveys, and LBP was assessed with a rather detailed questionnaire.

Risk indicators and risk factors for LBP, comparisons with other studies

Most but not all cross-sectional and longitudinal studies have observed a relationship between education and LBP prevalence (Dionne ). In this study, associations with LBP were due mainly to two categories those with the lowest educational level and those et al. 2001 with vocational training, which are sometimes grouped with other categories. This might explain results from other studies in which an association with education was not found ( ) or observed only for women ( ). Kopek et al. 2004 Croft and Rigby 1994

The associations with occupational factors were expected. Concerning the role of those factors, a German study concluded that they did not explain the relation between education and severe back pain (

). However, these results were based on a rather Latza et al. 2000 small sample of subjects with past history of back pain. Other authors have suggested that a lifetime perspective is important in understanding the mechanisms leading to musculoskeletal disorders ( ; ). Khatun et al. 2004[START_REF] Sainio | Educational differences in mobility: the contribution of physical workload, obesity, smoking and chronic conditions[END_REF] Tobacco consumption and BMI were associated with LBP in this study. Some previous studies found these associations only for men or only for women ( ; ; ; ; ; Leboeuf-Yde 1999 Goldberg et al. 2000Lake et al. 2000Leino-Arjas et al. 1998Mustard et al. 2005 Power et al. ). 2001

In the logistic models, LBP was associated with height, especially for women. A more detailed analysis of the frequency of LBP according to height also indicated that the association was stronger for women. Some studies have observed such an association only for men (

) or only for women ( ). Kopek et al. 2004 Leino-Arjas et al. 1998

Potential limitations of the study

The principal limitation of this survey was its cross-sectional design. However, most large population surveys comparable to the EDS are also cross-sectional ( ). Subjects with missing data for LBP were less educated, and the weights that we used for et al. 2004 estimating prevalence in the general population did not correct for the completeness of specific questionnaires such as the LBP questionnaire. For that reason the prevalence of LBP 30 days or more in the population could be slightly underestimated. However, consequences for associations between LBP and various factors are expected to be negligible.

Exposure to risk factors was self-assessed. For educational level, height, BMI, or tobacco consumption, self-assessed measures may be imprecise or over-or underestimated. Differential misclassification (distribution of errors depends on presence of LBP) is unlikely however.

Those with LBP may overestimate their past occupational exposure (

). Here differential recall bias for past exposure [START_REF] Stock | Reproducibility and validity of worker s self-reports of physical work demands[END_REF] was probably limited, since the survey did not focus on LBP but on health in general.

Another potential problem with a cross-sectional design is reverse causation or selection. Suffering from LBP might lead to a reduction of occupational exposure. Past exposure, however, includes exposure before LBP onset. There is no such problem for education, since LBP at school age is not expected to have consequences on education. Psychosocial factors at work, which are potential risk factors for LBP, were not taken into account (

). Since Hoogendoorn et al. 2000 these variables were available only for those who held a job at the time of the survey. Additional analyses limited to this subsample indicated that associations between LBP and both education and physical work factors were unchanged when psychosocial factors were also taken into account.

We did not take into account the presence of psychological problems. Associations between LBP and psychological problems would have been difficult to interpret, since the presence of psychological problems is both a potential risk factor, and a consequence of chronic LBP ( ; ; ;

). Andersson 1999 Linton 2000Hurwitz et al. 2003Dersh et al. 2007 The role of two possible pathways from socioeconomic position to LBP could not be studied in this study: the first one is chronic stress in general (

), which could be associated to LBP and also related to report of physical work exposure, especially Kopek et al. 2004 since physical work exposure was self-assessed. The second one is sociocultural factors such as beliefs about back pain ( , Buchbinder, 2001 ; ). How people understand and interpret back pain is expected to be related to education. [START_REF] Skovron | Sociocultural factors and back pain. A population-based study in Belgian adults[END_REF][START_REF] Van Tulder | European guidelines for the management of low back pain[END_REF] However, the effects on LBP in general (rather than chronic or disabling LBP) has not been much studied. It would be interesting to study the role of these factors as an explanation to social differences in prevalence of LBP in the general population.

Mechanisms, role of occupation and education

In studies of LBP, occupation is often taken as a proxy for occupational exposure. An explanation for the limited role of occupation in our study may well be that occupation was not known very precisely, especially for self-employed workers. On the other hand, occupation may be less informative than education about LBP if the objective is to assess socioeconomic status or structural determinants of health, and less informative than occupational factors if the objective is to study the role of proximal factors.

Education is often considered first as a proxy for socioeconomic status in childhood or early adulthood and then as a determinant of occupation. Our results suggest that the role of education remains important in adulthood. A low educational level seems to predict career-long exposure to occupational factors, even within occupations often considered homogeneous for physical exposure. The effects might be slightly different for men and women, both for the level of exposure and for the number of years of exposure. In manual occupations, a low educational level may also limit the possibility of upward mobility or transfer to less physically demanding tasks.

Conclusion

These results from a large population survey suggest that physical work factors and lifestyle factors play an important role in social inequalities in LBP. In addition to reducing physical exposure and promoting healthy lifestyles, improving adults educational level and ' skills may be an important means of prevention.

  1998 higher educational level ( ). Cavelaars et al. 2000 Other potential intermediate factors are general stress and occupational psychosocial factors ( ), and sociocultural factors Kopek, 2004 such as beliefs about back pain, which have an important role in disability associated with LBP, and might also play a role for duration of pain and self-assessment of back pain (Buckbinder, 2001, ;

  The data come from the EDS (Enqu te D cennale Sant ), a national survey conducted in 2002 2003 by INSEE, the agency

Figure 1

 1 Figure 1Illustration of the results, pathways between education and low back pain (LBP)

  . A study in Finland found that physical work factors explain most of the occupational class gradient inHoogendoorn et al. 1999

	musculoskeletal disorders (	Aittom ki et al 2007 ä	). A study of injury rates among Massachusetts hospital workers reached similar
	conclusions (	d Errico et al 2007	).

'

Table 1 %

 1 (compared with 75 for those reporting no LBP or shorter duration). Among those unemployed, retirement was the most frequent Among those in the LBP group, 3.1 were classified in permanent disability benefits (1.3 in the rest of the sample). For

	%	%	%

% %

days or more in the previous year was reported by 15.6 of the men and 19 of the women, which corresponds to estimates of 15.8 and % % situation.

11

11

11

11

11

Ackowledgements:

Mission de la Recherche, French ministry of health. Funding:

Footnotes:

: This study has been conducted in accordance with the French and European legislation concerning ethical standards and Ethical standards protection of confidentiality.

References:

Aittom ki (1) Number of subjects in the study population (2) Prevalence and confidence interval in the target population (weighted).

(3) p-value, test comparing prevalences in the study population, Chi-square test.

(4) More than 180 cm for men, more than 170 cm for women.

(5) nine men and 321 women with missing data for occupation.

10 11 (1) Not in the model


V Langlais

K Schneider

H Tang

Light assisted synthesis of poly-para-phenylene on Ag(001)

Keywords: light, on-surface-synthesis, hot electrons, surface plasmon, poly-para-phenylene

A detailed study of poly-para-phenylene obtained by light-assisted on-surface-synthesis on Ag(100) was carried out by Scanning Tunneling Microscopy and Spectroscopy together with Density Functional Theory calculations. The use of light in combination with heat allows to lower by 50 K annealing temperature the each stage of the Ullmann coupling. Debromination of the 4,4'' dibromo-p-terphenyl precursors was thus realized at 300 K, the formation of the first oligomers from the organometallic intermediate by silver bridging atom release at 423 K and poly-para-phenylene by complete elimination of the silver at 473 K. This approach to lower the reaction temperature permits to enhance the Ag(100) surface reactivity to become comparable to that of Cu (111). The underlying mechanism of light effect was proposed to occur via surface mediated excitation, with the creation of photoexcited electrons known as hot electrons correlated with surface plasmon excitation. This original pathway combining both light and heat provides an additional parameter to control on-surface-synthesis by separating the precursor activation stage from the diffusion.

Introduction

Poly-para-phenylene (PPP) has been considered for long as the prototype of organic semiconductor conjugated polymers and could be also seen as the narrowest graphene nanoribbon (GNR). In its neutral form, PPP is a large band gap semiconductor that can be doped to increase its conductivity, moreover, its mechanical properties, low density and high stability regarding temperature, oxygen exposure and moisture make PPP a promising candidate for organic electronics [START_REF] Friedrich | Scratch resistance of high performance polymers[END_REF]. Numerous approaches to synthesize it in wet chemistry were reported [START_REF] Pavlović | Controlled synthesis of unsubstituted high molecular weight poly( para -phenylene) via Suzuki polycondensation-thermal aromatization methodology[END_REF][START_REF] Remmers | Poly( p -phenylene)s. Synthesis, Optical Properties, and Quantitative Analysis with HPLC and MALDI-TOF Mass Spectrometry[END_REF]. Finally, the chemical pathway to access this long-desired polymer in its pristine form was only established in 2016 [START_REF] Abdulkarim | A New Solution to an Old Problem: Synthesis of Unsubstituted Poly( para -phenylene)[END_REF].

On the other hand, PPP can be also regarded as a model system for on-surface-synthesis (OSS). During the past decade, this approach has gained increasing research attention, since new molecular species and nanoarchitectures not achievable by conventional wet chemistry can be successfully formed by chemical reactions directly onto surfaces [START_REF] Wang | Confined on-surface organic synthesis: Strategies and mechanisms Surface[END_REF][START_REF] De Oteyza | Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis[END_REF][START_REF] Shen | Frontiers of on-surface synthesis: From principles to applications[END_REF][START_REF] Sun | On-Surface Synthesis of Carbon Nanostructures[END_REF][START_REF] Méndez | On-surface synthesis of cyclic organic molecules[END_REF][START_REF] Giovannantonio | Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization[END_REF][START_REF] Dong | Surface-Activated Coupling Reactions Confined on a Surface[END_REF][START_REF] Björk | Towards Design Rules for Covalent Nanostructures on Metal Surfaces[END_REF][START_REF] Zuzak | On-Surface Synthesis with Atomic Hydrogen[END_REF]. OSS is based on the use of a substrate not only as physical support but also as active catalyst component to initiate a chemical reaction from precursors that moreover could be carried out in an ultra-clean environment such as ultrahigh vacuum (UHV). This provides the great advantages to give access to a larger range of temperatures than in solution, to provide structural order to achieve atomic scale precision thanks to 2D confinement, to carry out chemical reactions in a solvent free environment and to benefit of additional parameters such as nanostructuration of the surface to further confine the reaction and the reactants [START_REF] Fan | Confined Synthesis of Organometallic Chains and Macrocycles by Cu-O Surface Templating[END_REF]. Up to date, a large number of reactions such as Ullmann coupling [START_REF] Nacci | Covalent Assembly and Characterization of Nonsymmetrical Single-Molecule Nodes[END_REF], Glaser coupling [START_REF] Cirera | Synthesis of Extended Graphdiyne Wires by Vicinal Surface Templating[END_REF], Bergman reaction [START_REF] De Oteyza | Direct Imaging of Covalent Bond Structure in Single-Molecule Chemical Reactions[END_REF], aryl-aryl dehydrogenation [START_REF] Treier | Surface-assisted cyclodehydrogenation provides a synthetic route towards easily processable and chemically tailored nanographenes[END_REF], Diels-Alder reaction [START_REF] Aragonès | Electrostatic catalysis of a Diels-Alder reaction[END_REF], Schiff-base reaction [START_REF] Gong | Structural Variation in Surface-Supported Synthesis by Adjusting the Stoichiometric Ratio of the Reactants[END_REF], boronic acid condensation [START_REF] Domínguez-Celorrio | Symmetry-Driven Formation of Chiral Boroxine-Based Organometallic Oligomers on Ag(001[END_REF], cyclotretramerization [START_REF] Piantek | Manganese Phthalocyanine Derivatives Synthesized by On-Surface Cyclotetramerization[END_REF], etc. have been successfully performed on different metal surfaces in UHV and further investigated with the help of surface science techniques combined with theoretical calculations. The final products obtained by OSS, obviously depend on the initial molecular precursors, but are also strongly influenced by the underlying substrate, its chemical nature, its crystallographic orientation and symmetry which fully determine its reactivity. In addition, the surfaces can even directly participate with their adatoms to the formation of organometallic species (OGM). The syntheses of GNRs [START_REF] Liu | Tuning On-Surface Synthesis of Graphene Nanoribbons by Noncovalent Intermolecular Interactions[END_REF][START_REF] Cai | Atomically precise bottom-up fabrication of graphene nanoribbons[END_REF] and long polyacenes [START_REF] Zuzak | Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene[END_REF] on different metallic surfaces are definitively among the most successful and remarkable examples of OSS. Among the chemical reactions that were successfully tested on metal surfaces, Ullmann coupling is one of the most studied and one of the most popular [START_REF] De Oteyza | Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis[END_REF][START_REF] Giovannantonio | Reversibility and intermediate steps as key tools for the growth of extended ordered polymers via on-surface synthesis[END_REF][START_REF] Lackinger | Surface-assisted Ullmann coupling[END_REF]. Using Ullmann coupling, PPP has been achieved starting from various different precursors such as 1,4-diiodobenzene [START_REF] Ivasenko | Synthesis of Polyphenylene Molecular Wires by Surface-Confined Polymerization[END_REF], diiodo-biphenyl and dibromo-diphenyl [START_REF] Zhou | On-surface synthesis approach to preparing onedimensional organometallic and poly-p-phenylene chains[END_REF], 4,4''-dibromo-p-terphenyl [START_REF] Wang | Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction[END_REF][START_REF] Jang | Tetragonal porous networks made by rod-like molecules on Au(111) with halogen bonds[END_REF][START_REF] Basagni | Molecules-Oligomers-Nanowires-Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order[END_REF][START_REF] Yuan | Resolving Quinoid Structure in Poly( paraphenylene) Chains[END_REF]. In addition, wider nanoribbons were also on-surface synthesized from PPP by dehydrogenation and C-C coupling [START_REF] Song | Wide graphene nanoribbons produced by interchain fusion of poly(pphenylene) via two-zone chemical vapor deposition[END_REF]. Belonging to the same family class, hyperbenzene and zigzag poly-m-phenylene were also successfully achieved from halogen functionalized-m-terphenyl precursors [START_REF] Fan | Surface-Assisted Organic Synthesis of Hyperbenzene Nanotroughs[END_REF][START_REF] Judd | Ullmann Coupling Reactions on Ag(111) and Ag(110); Substrate Influence on the Formation of Covalently Coupled Products and Intermediate Metal-Organic Structures[END_REF]. This process can be described as follows: after halogenated precursors adsorption onto a surface, two activation steps have to be induced: (1) radicals creation by cleavage of the C-X bond(s), with X=Cl, Br, I and (2) diffusion to provoke the chemical reaction between the reactive species. One of the key parameters to achieve covalent coupling is the temperature at which the reaction takes place. Several strategies have been imagined to lower the temperature, such as co-deposition of reactive adatoms such as Ni or Ag together with dibromobiphenyl and diodo-biphenyl molecules on Au(111) has permitted to synthesize stable intermediates [START_REF] Zhou | On-surface synthesis approach to preparing onedimensional organometallic and poly-p-phenylene chains[END_REF].

Here, an original approach was adopted combining both sun like UV-visible-near IR light and heat at each step of the Ullmann reaction towards PPP synthesis on Ag(100) and Ag(111) single crystal surfaces. The study was carried out by low temperature Scanning Tunneling Microscopy (LT-STM) and Density Functional Theory (DFT) from 4,4''-dibromo-pterphenyl (DBTP) as precursor in order to get a detailed understanding of the mechanism of the on-surface chemical reaction assisted by light. A detailed analysis of light exposition effect at the different stages of the Ullmann reaction i.e. from self-assembly to intermediate OGM phase until complete polymerized PPP was carried out by comparing the reaction temperatures, on both surfaces yielding a lowering of the reaction temperature by 50 K and thus enhancing the surface reactivity of Ag(100) to become comparable to Cu(111). The underlying mechanism was then discussed in terms of substrate mediated electronic excitation and asymmetric energy barriers for bond cleavage and polymer formation.

Results and discussion

Methodology

Ag(100) and Ag(111) single crystal surfaces were prepared by cycles of Ar-sputterimg at 1 keV (ionic current ~10 μAmp) and annealing at 773 K for 30 minutes with a slow cooling down (10K/min) at the last cycle. DBTP and DITP (4,4''diiodo-p-terphenyl) molecules were placed in a home-made mini transferable evaporator and carefully degassed until the UHV base pressure reached again the low 10-10mbar range. The cleanness of the samples and powder was checked directly in the STM. All the STM images were recorded at 5K, unless explicitly mentioned. A Xe arc discharge lamp, Oriel Newport, operating power 150 W, sun-like emission spectrum renders them a popular choice for solar simulation, was used to shine light on the samples (Fig. 1SI). The emission range extends from about 200 nm to 2400 nm, with an irradiance at 0.5 m greater than 100 mWm -2 nm -1 from 800 nm to 1000 nm. The experimental setup consists of an optical fiber located at the exit of the Xe arc discharge lamp together with an optical lens located in front of the UHV window to focus the photon beam on the sample surfaces. The power at the exit of the fiber was about 350 mWm -2 nm -1 at 350 nm.

The density functional theory (DFT) calculations were performed by using the Vienna Ab-initio Simulation Package (VASP) [START_REF] Kresse | Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[END_REF][START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a planewave basis set[END_REF]. The projector augmented wave (PAW) potentials in a plane wave basis were used to describe the valence electron ion core interaction of each element [START_REF] Blöchl | Projector augmented-wave method[END_REF]. The exchange-correlation energy corresponds to that given by the Perdew-Beck-Eneker (PBE) functional within the generalized gradient approximation (GGA) [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF]. The van der Waals (vdW) interactions were considered by a modified version of the semi-empirical DFT-D2 method proposed by Grimme [START_REF] Grimme | Semiempirical GGA-type density functional constructed with a long-range dispersion correction[END_REF]. In this version the correction was not considered for metal-metal interactions within surface slab. The kinetic energy cut-off was fixed to 440 eV, which gives a lattice parameter of 4.162 Å for the optimized fcc bulk silver (with a reciprocal lattice mesh of 15x15x15 to sample the 3D Brillouin zone). The self-consistent electronic convergence criterion was set to 10-6 eV and the geometrical relaxation was considered to be converged when the forces on each unfixed atom was lower than 0.02 eV/Å. The slab which represent the surface of Ag(001) was constructed by a 4 atomic plans with each plan contains of 48 Ag atoms (with a lateral dimension of 16.65 x 24.97 Ų), with x axis along the (100) direction and y axis along the (010) direction. The Brillouin zone was sampled with only the gamma point in the reciprocal space for preliminary calculations and a 3x3x3 k-point sampling for the electronic structure analysis. The vacuum space was set to 18.32 Å (about 12 Å from the Ag surface). The dipole correction was considered in the z direction to compensate the interface dipole effect during the energy minimization.

The activation barriers to detach iodine or bromine atom from the DITP and DBTP precursors, and to detach Ag atoms from the organometallic complexes were estimated with the transition stats obtained by using the Climbing Image Nudged Elastic Band (CI-NEB) method as implemented by the Henkelman group [START_REF] Henkelman | A climbing image nudged elastic band method for finding saddle points and minimum energy paths[END_REF][START_REF] Henkelman | Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points[END_REF]. With this method, the minimum energy reaction path was identified, by optimizing the forces perpendicular to this path to be lower than 0.5 eV/Å on each of equally separated intermediate images.

Halogen bonding driven self-assembly

When deposited onto Ag(100) maintained at 78 K, DBTP formed self-assembly islands stabilized through halogen bonding, indicating that at this temperature, diffusion was high enough to allow molecular rearrangement onto the surface (fig. 1a). Nevertheless, self-assembly did not take place spontaneously but was induced by rising the voltage up to a threshold bias of 1.2 V. In fig 1a, a constant current STM image recorded at 78 K revealed a pseudo square molecular reconstruction extending over the whole terraces with some disordered areas along the step edges and on narrow terraces. Noisy areas corresponding to locations where the molecules were still diffusing during the scan are also visible at the top left corner of the STM image in fig. 1a. The transition between ordered and noisy areas was fuzzy and structural ordering was clearly induced by repeated scans and self-healing defects evolving with time and number of scans on the same area.

Lowering the bias below the threshold value resulted in noisy STM images dominated by molecular diffusion, indicating that the reconstruction is unstable. A closest look at the nodes of the reconstruction, where a windmill shape (WM) is observed, reveals a mixture of left and right handedness giving rise to square, rectangular and trapezoidal cells. For the sake of comparison, the same experiment was conducted with DITP precursors. The stability of the molecular layer arrangement with respect to the tip bias was found to be much stronger for DITP compared to DBTP allowing scanning at lower bias without losing the structural order. These results are in good agreement with previous halogen bonding studies that have proven that the strength of the interaction is related with the increasing polarizability of the halogen (I>Br) and decreasing electronegativity [START_REF] Cavallo | The Halogen Bond[END_REF]. Another difference worth to be mentioned is that the same chirality was found at each node of the WM arrangement yielding a perfect square arrangement (see fig2.SI). In order to elucidate the phenomenon at the origin of this bias induced molecular arrangement, a series of scans were conducted on non-ordered areas (diffusing molecules) at various tip-sample distances by approaching the tip by steps up to ΔZ=-1nm from a setpoint at 10 pA, 100 mV in order to check for electric field effect. No any order was induced by proceeding in such a way, excluding readily electric field effect on the molecular ordering. In contrast, another study on terarylene molecules on Cu(111) reporting on tip-induced formation of a supramolecular network where the 2D assembly stabilized by π-π stacking and van der Waals interactions was observed [START_REF] Calupitan | Scanning Tunneling Microscope Tip-Induced Formation of a Supramolecular Network of Terarylene Molecules on Cu(111)[END_REF]. The main difference between the two systems DIPT/Ag(100) and terarylene/Cu(111) is that the dipole of the latter was found to be aligned with the vertical electric field induced by tip pulses, while in our case, the dipole would be lying rather parallel to the surface. Therefore, for our system, the ordering is energy dependent and related to a resonance of electronic states corresponding to the lowest unoccupied state (LUMO) (see fig3.SI). By increasing the surface coverage of DBTP, a brick-wall ordering was found to become more favorable with higher density packing as shown in fig. 1c. The surface area covered by unit cell in the WM arrangement was found to be about 4.81 nm 2 with 2 molecules per unit cell while the brick wall unit cell occupies about 1.6 nm 2 as well for 2 molecular units confirming the more compact structure for higher coverage [START_REF] Jang | Tetragonal porous networks made by rod-like molecules on Au(111) with halogen bonds[END_REF]. This brick-wall arrangement with interconnected digits is more compact as already reported for both DBTP and DBQP on Au(111) and was also obtained for higher surface coverage. The structure is stabilized by halogen bonding as for the WM arrangement. In opposition to the WM structure, the brick wall selfassembly was not induced by tip scanning but spontaneously formed on the surface after deposition. WM on Ag(100) surface corresponds to a meta-stable configuration with a local minimum of potential energy surface (PES). This "stability" could be reached only when an additional energy was provided by applying a bias voltage imaging. The DBTP precursors were then exposed to light and annealed at different temperatures to complete Ullmann coupling. At 300 K, the C-Br bonds remain intact yielding the same windmills ordering induced by tip bias as observed for the 78 K deposition. Light exposition at 300K or annealing at 340 K without light provokes debromination and generates radicals with unsaturated C-bonds which form an organometallic intermediate (OGM) by picking up adatoms from the substrate as shown in the STM overview presented in fig. 2a. The Ag atoms involved in these OGM wires are either adatoms diffusing from low coordinated step edge sites, but also atoms from the substrate lattice can participate in the chain formation. The chains thus formed by alternating terphenyl groups (Ph)3 and Ag are aligned along the [100] and [010] main crystallographic directions of the substrate but some misalignment along the chains is observed provoking fluctuation along the main direction as could be seen in fig.

From DBTP to PPP

2b. When integrated in the OGM structure, the bridging Ag atoms are positively charged, in contrast with the halogen atoms which are negative due to charge transfer from the substrate. Therefore, along the OGM wires, the Ag atoms are very often surrounded by these negatively charged halogen atoms, which can be nevertheless, found also at other locations along the chains. After light exposition and annealing at 373 K for 3 hours, the same OGM structure was observed on the surface without noticeable change. The next stage of the onsurface Ullmann reaction consists of eliminating the bridging silvers giving rise to short oligomers obtained by light-assisted annealing at 423 K. This stage is illustrated in the high resolution STM image displayed in fig. 2c, where some (Ph)6 oligomers resulting from the elimination of 1 bridging silver atom are clearly identified. In the last stage, the complete Ullmann coupling yielding PPP formation (fig. 2d) was reached for light-assisted annealing at 473 K. The PPP wires are surrounded by halogen rows and are now aligned along the <110> direction. More details on the OGM chains could be extracted from fig. 3a where atomic resolution was achieved on the silver substrate. The OGM is oriented along the <100> direction and along the perpendicular <010> direction. In gas phase, the Ag-(Ph)3-Ag distance was found to be dAg-Ag=1.564 nm. Experimentally, a statistical analysis of the Ag-Ag distance gave a distance dAg-Ag= 1.596 nm ± 0.02 nm, proving an adaptation of the monomer to fit the substrate lattice by stretching the C-Ag bonds. On Cu(111), this distance was reported to be 1.62 nm [START_REF] Wang | Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction[END_REF] which demonstrates the strong substrate influence on the adsorption geometry. The energetically most stable locations of the Ag bridging atoms are the 4-fold hollow sites at the center of the Ag(100) reduced unit cell.

In the OGM chain, they occupy approximately every 4th position with a distance of 4*aAg = 1.632 nm where aAg is the lattice constant of the Ag(100) face centered cubic single crystal. Every fourth monomer is therefore tilted in order to compensate the remaining mismatch of 0.036 nm between the stretched molecular length and the lattice parameters of the substrate (fig. 3b). In long chains, it might necessarily result in dislocations or chain disruptions to reduce strain and to allow relocation of Ag atoms and (Ph)3. The distance between.the blue lines drawn in fig. 3a equal to aAg/2=0.144 nm corresponds to half unit cell. On various other STM images, multiples of aAg/2 are also found.

Other studies conducted on similar precursors report a bending of the outer phenyl rings toward the Ag surface caused by the C-Ag-C bonds [START_REF] Kawai | Organometallic Bonding in an Ullmann-Type On-Surface Chemical Reaction Studied by High-Resolution[END_REF]. In order to understand which are the parameters governing the reaction and the effect of light exposition on the different stages of PPP formation, a comparison of the temperatures required for each step of the Ullmann coupling was carried out with and without light on both Ag(100) and In the OGM chain, they occupy approximately every 4th position with a distance of 4*aAg = 1.632 nm where aAg is the lattice constant of the Ag(100) face centered cubic single crystal. Every fourth monomer is therefore tilted in order to compensate the remaining mismatch of 0.036 nm between the stretched molecular length and the lattice parameters of the substrate (fig. 3b). In long chains, it might necessarily result in dislocations or chain disruptions to reduce strain and to allow relocation of Ag atoms and (Ph)3. The distance between the blue lines drawn in fig. 3a corresponds to half unit cell equal to aAg/2=0.144 nm. On various other STM images, multiples of aAg/2 are also found. Other studies conducted on similar precursors report a bending of the outer phenyl rings toward the Ag surface caused by the C-Ag-C bonds.

In order to understand which are the parameters governing the reaction and the effect of light exposition on the different stages of PPP formation, a comparison of the temperatures required for each step of the Ullmann coupling was carried out with and without light on both Ag(100) and Ag(110) surfaces. The main results are summarized in table I I: Synthetic view of the results obtained on Ag(100) and Ag(111) at different annealing temperatures with and without light exposition. As could be seen directly from this table, the different steps of the reaction occurred at a temperature lowered by 50 K when a combination of light and annealing temperature was used.

To achieve only PPP islands, the temperature of annealing has to rise up to 473K with light and up to 523 K without light. We can conclude that the use of light in our case allows to lower the reaction temperature by 50 K at all stages of the Ullmann reaction i.e. (i) activation of the precursors by C-Br bond cleavage, (ii) formation of the oligomers by some C-Ag bond cleavages and (iii) complete Ullmann coupling by the remaining C-Ag bond cleavage and reorientation of the PPP along the <110> axis of the Ag(100) surface. The quinoid forms inside PPP islands as shown by Yuan et al. [START_REF] Yuan | Resolving Quinoid Structure in Poly( paraphenylene) Chains[END_REF] on Cu(111) surface were not observed instead these PPP wires adopted a planar conformation as confirmed by DFT calculation. Ag( 111) is known to be more reactive than Ag(100), therefore, as displayed in table I, the reaction temperature stages were found to be lower than the ones on Ag(100), as expected. The interaction of DBTP with the metal surface seems to be rather weak since the same square WM arrangements of the intact molecules were found on this 3-fold symmetry substrate. The WM centres present alternate right and left handedness (SI. Fig4a). This is in striking contrast with respect to the 3-fold symmetry of the self-assembly precursors reconstruction that was found on Cu(111) [START_REF] Wang | Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction[END_REF]. As on Ag(100), the molecular self-ordering was bias energy driven and not stable at low bias. As mentioned previously, the stability of WM structures is ensured by the intermolecular halogen bonds as demonstrated by Chung et al. [START_REF] Chung | Polymorphic porous supramolecular networks mediated by halogen bonds on Ag(111[END_REF] In contrast with the OGM wires obtained on Ag(100), the OGM were not anymore aligned with the main crystallographic axes of the substrate but rather formed loops starting and ending at a step edge (SI fig. 4b). Another difference regards the location of the halogen atoms which are not anymore located around the Ag bridging atoms but besides the (Ph)3 (SI fig. 4c). The PPP were achieved at 373 K using light assistance and at 423 K without light (SI fig. 4d). Without light, thermal annealing duration of 15 min and longer yield the same results. In opposite, a minimum light exposition duration of 5 hours was needed to achieve PPP, which is partly due to our experimental setup, since at the exit of the optical fibre, the intensity was measured to drop at least by a factor 10. This is worth to mention that ordering was largely improved by the use of light with respect to only heat specially on Ag(111) as already reported in Basagni et al. [START_REF] Basagni | On-surface photo-dissociation of C-Br bonds: towards room temperature Ullmann coupling[END_REF] on 5,11-dibromotetracene on Au(111). The comparison between thermal and light dissociation has shown that at 410 K, thermal process yields disordered area with rare zones where OGM dimers were observed while a significant increase of surface order was obtained by using milder conditions (room temperature) together with 405 nm light irradiation. In order to better understand, the energy barriers at play in this process, DFT calculations were carried out. The diffusion barrier calculated by using the Climbed Image-Nudge EB method shows a relatively small value of 0.172 eV and 0.171 eV for DITP and DBTP molecules respectively. These values explain the free diffusion of these molecules even at relatively low temperature of 78 K. Debromination follows a two-step process as depicted in fig. 4a. To detach the first bromine atom from the DBTP molecule adsorbed along the <100> direction, the energy to overcome the barrier was estimated to 0.59 eV. The detachment of the second Br atom requires a higher energy of 0.73 eV (fig. 4a). To provide enough energy to overcome these debromination barriers, an annealing of the substrate at 373 K was therefore necessary. The detached bromine atoms diffuse easily as their diffusion barrier was estimated as low as 0.1 eV. Remaining at this temperature of 373 K, the thus formed para-terphenyl diradical should spontaneously coordinate with diffusive Ag adatoms to form Ag-(Ph)3-Ag chains, thanks to the low coordination barriers of 0.36 eV to attach the first Ag adatom and 0.39 eV to attach the second Ag atom. In addition, the diffusion barrier of Ag adatoms is estimated to be about 0.3 eV, which favours the approach of these atoms to the reactive terphenyl diradicals. The formation of OGM is an exothermic reaction releasing 1.05 eV. The thus formed OGM complex adsorbed along the <100> direction has a length of 1.596 nm, which is 4% shorter than the distance between surface cells (1.665 nm). This length mismatch is at the origin of the misalignment detailed in fig. 2b andfig 3a. Furthermore, the adsorption energy of OGM is -6.311 eV along <100> and -6.083 eV along <110> direction. This difference in adsorption energy could be the reason why only OGM along <100> (or <010>) are observed. Starting from the OGM structure, the C-C coupling also requires the formation of para-terphenyl diradicals. Fig. 4b shows the CI-NEB calculated the activation barrier to cleave the Ag-C coordination bond which is much higher (respectively 0.97 eV and 0.83 eV to detach the first and second Ag atom) than the energy necessary to attach Ag adatoms to this diradical (0.39 and 0.36 eV). This higher activation energy could be the reason why an annealing temperature as high as 473 K was necessary for this last step. However, a question could be raised here, which is why no direct formation of PPP through C-C coupling immediately after the p-terphenyl diradical formation is observed. To answer this, we have calculated the diffusion barrier of the terphenyl diradical to be about 1.98 eV. Thus, the direct C-C coupling when diradicals are lying parallel along the <100> direction is strongly diffusion limited. This conclusion is at first glance in contradiction with the case of benzene radicals coupling on Ag(111) and Au(111) where no energy barrier was found to establish the C-C bonds as calculated by Borjk et al. [START_REF] Björk | Mechanisms of Halogen-Based Covalent Self-Assembly on Metal Surfaces[END_REF].

However, a closer look reveals that the weak activation barrier corresponds in fact to the C-C coupling when two phenyl radicals were positioned face to face at a distance close to the C-C bond. To bring these radicals to such positions, the diffusion barrier was evaluated to be about 0.3 eV on Ag(111). In the present case of diradical, there is one radical at each extremity and the whole diradical is 3 times longer than a single phenyl radical. Such a diradical requires an activation energy to diffuse much higher than the energy required for diffusion of a coordinated species Ag-(Ph)3-Ag and for the (Ph)3 core. Finally, the co-existence of OGM and PPP could be explained by the competition between C-C covalent bond formation and C-Ag bond formation among the remaining diradicals which might form either PPP or OGM complexes. Therefore, to ensure 100% of the diradicals to interact to form pure C-C bonds, a substrate temperature as high as 473 K was necessary. This qualitative and quantitative DFT study fully agrees with the experimental results and provides a clear picture of the Ullmann coupling process taking place at surfaces.

From these results, a number of conclusions can be drawn. Using light in combination with annealing temperature permits the formation of OGM at 300 K. Furthermore, the catalytic activity of the Ag(100) substrate is enhanced and becomes similar to that of Cu(111) Moreover, X-ray Photoemission Spectroscopy (XPS) analysis has revealed coverage decrease due to thermal treatment while the disorder was assigned to molecular fragmentation. Another study carried out on 4-bromo-4' hydroxybiphenyl on Ag(111) has shown that upon radiation using a 266 nm UV laser at 80 K, debromination and dihydroxylation were achieved giving rise to covalently bonded polyphenylene polymer chains after annealing at RT [START_REF] Shen | Self-assembled two-dimensional nanoporous molecular arrays and photoinduced polymerization of 4-bromo-4′-hydroxybiphenyl on Ag(111[END_REF]. Other photoinduced reactions involving C-X (X=Br, Cl) bond breaking have been also used as a direct route to form covalent bonding between molecular bilayers on the Si(111)-(7×7) surface [START_REF] Zhang | Architecturing Covalently Bonded Organic Bilayers on the Si(111)-(7 × 7) Surface via in Situ Photoinduced Reaction[END_REF]. In all these studies, light effect was studied only on the first step of the Ullmann reaction regarding C-X bond cleavage. Here, we have used light combined with temperature at each stage of the reaction to benefit from milder conditions. This strategy allows as well to decouple the diffusion step (activated by temperature) from the bond cleavage.

Regarding the mechanism of the photochemical effect, shining light on solids to activate surfaces has been used for the last 40 years, in particular in the domain of catalysis to enhance activity or selectivity properties of metals or oxides towards more effective dissociation of small molecules such as oxygen or water [START_REF] Hasselbrink | Mechanisms in photochemistry on metal surfaces[END_REF][START_REF] Franchy | Surface and bulk photochemistry of solids Rep[END_REF]. The mechanisms at play involve different processes, either direct photon absorption by the adsorbates, or photon absorption by the substrate, or a combination of both of them. Then, in case of direct absorption by the adsorbate, electronic excitation might decay into photon emission, vibrational states de-excitation or through other de-excitation channels opened by the presence of the substrate [START_REF] Franchy | Surface and bulk photochemistry of solids Rep[END_REF]. When the photons are absorbed by the substrate, different phenomena might occur such as photoelectrons emission, surface plasmon excitation, surface phonon, electron-hole creation and subsequent electron transfer to the adsorbate giving rise to negative ion resonances (NIRs) [START_REF] Engel | Precision Spectroscopy of Negative-Ion Resonances in Ultralong-Range Rydberg Molecules[END_REF][START_REF] Illenberger | Formation and evolution of negative ion resonances at surfaces Surface[END_REF][START_REF] Marinica | Negative ion resonance of a molecule adsorbed on a metal surface covered with a rare gas monolayer Surface[END_REF]. The main advantage to use light instead of heat is to favor the polymerization process under milder conditions, but also to enhance selectivity promoting only certain chemical reactions.

In this study, due to the interaction between precursors and metal, direct excitation of the vibrational transitions of DBTP or (Ag-Ph)n through light absorption can be readily discarded since it would relax faster into the heat bath provided by the substrate than into energy transfer to the low frequency molecule surface mode required for bond dissociation. In the same way, short-lived electronic excitation of the adsorbate decays into the molecule-surface vibration. However, due to short lifetimes, only a small fraction of the excited molecules, would gain sufficient energy to allow bond cleavage. Therefore, the dissociation of molecular bonds by direct adsorbate excitation occurs very rarely on metal surfaces. The origin of electronic excitation short lifetimes lies in the strong coupling of the adsorbate excited states with the nearly infinite number of electron-hole pair states provided by the substrate. Therefore, on metallic flat surfaces, the direct photoexcitation channel of the adsorbate is quenched [START_REF] Kim | Hot-Electron-Mediated Photochemical Reactions: Principles[END_REF][START_REF] Zhang | Surface-Plasmon-Driven Hot Electron[END_REF]. From these considerations, we can conclude that the underlying mechanism of light effect on the temperatures at which occurs the different bond cleavages could not be explained by a direct photon absorption exciting either vibrational or electronic transitions of DBTP and (Ag-Ph)n groups.

The mechanism with the highest cross section and thus more likely to occur upon light irradiation is plasmon excitation [START_REF] Brongersma | Plasmon-induced hot carrier science and technology[END_REF][START_REF] Christopher | Visible-lightenhanced catalytic oxidation reactions on plasmonic silver nanostructures[END_REF][START_REF] Liu | Plasmonic Hot Electron-Mediated Hydrodehalogenation Kinetics on Nanostructured Ag Electrodes[END_REF]. Resonant optical excitation of the metal surface plasmon produces hot electrons that facilitate chemical reactions. Two processes might concomitantly occur (i) when hot electrons have energy enough to directly populate the LUMO states of the adsorbate, they transfer, in our case, into the antibonding state of the C-Br bond producing the bond cleavage (ii) if hot electrons do not have energy enough to induce a direct electronic excitation, they might result in bond dissociation depending on the cumulated vibrational energy by thermal effect. This latter will induce an increase of the sample temperature which should be reflected in the temperature measurement. This hot electron mediated mechanism is supported by numerous examples where the initial photoexcitation of the substrate produces hot electrons that scatter into the molecular resonance state, leading to the dissociation of the adsorbate [START_REF] Brongersma | Plasmon-induced hot carrier science and technology[END_REF][START_REF] Lee | Observation of an Intermediate State for a Surface Photochemical Reaction Initiated by Hot Electron Transfer[END_REF][START_REF] Palmino | Photochemistry Highlights on On-Surface Synthesis[END_REF].

As pointed out in Barragán et al. [START_REF] Barragán | Power discontinuity and shift of the energy onset of a molecular de-bromination reaction induced by hot-electron tunneling[END_REF] , heating induced by vibrational modes plays a negligible role in bond dissociation with respect to electronic population of the C-Br* antibonding state whenever the electron energy is high enough [START_REF] Erpenbeck | Current-induced dissociation in molecular junctions beyond the paradigm of vibrational heating: The role of antibonding electronic states[END_REF]. Spatially, photon absorption extends over the range of the penetration depth of the incoming light. The electron mean free path, which depend on the electron energy, is of the same order. Therefore, the initial hot electrons, created by photon absorption, will reach the surface with high probability. On metallic surfaces, the penetration depth of photons with an energy of a few eV is in the order of ~ 10nm, corresponding to the electron mean free path at this energy [START_REF] Hasselbrink | Mechanisms in photochemistry on metal surfaces[END_REF]. The hot electron generation phenomenon appears to be correlated with localized surface plasmon resonance enhancing the life time of these hot carriers to convert them into an efficient pathway for photochemistry on metal surfaces or semiconductor metal heterojunctions [START_REF] Hasselbrink | Mechanisms in photochemistry on metal surfaces[END_REF][START_REF] Franchy | Surface and bulk photochemistry of solids Rep[END_REF][START_REF] Zhang | Surface-Plasmon-Driven Hot Electron[END_REF][START_REF] Brongersma | Plasmon-induced hot carrier science and technology[END_REF]. The hot electrons created by metal photon absorption might then tunnel into the LUMO state of the molecules forming thus a transient anion by "Hot Electron attachment" (HEA) that leads to intramolecular bond dissociation, by "Dissociative Electron Attachment" (DEA). In addition, in noble metals as Ag, strong plasmon resonances arising from the Drude-like property of sp-electrons enhance HEA efficiency. An angle-resolved high-resolution electronenergy-loss-spectroscopy (AR-HREELS) electron study reports on a single loss energy observed at 3.69 eV on Ag(100) [START_REF] Rocca | Surface-plasmon spectrum of Ag(001) measured by high-resolution angleresolved electron-energy-loss spectroscopy[END_REF], corresponding to a wavelength of 336 nm. The Xe-arc discharge lamp used in this study covers a wide range of wavelength rendering surface plasmon excitation and hot carrier generation highly probable to occur. To conclude this discussion, the temperature of the different reaction stages is most probably reduced by 50 K with respect to purely thermal processes through photon absorption by the Ag(100) substrate generating hot electrons whose lifetime is enhanced by surface plasmon excitation occurring through the same photon absorption process.

Conclusion

In summary, a detailed study of light assisted Ullmann reaction at Ag(100) surface was carried out starting from DBTP precursors. We found that the temperature at each stage was reduced by 50 K upon light exposition for each stage of the reaction including debromination, C-Ag bond cleavage of the OGM intermediate to achieve oligomers and finally PPP wires. The energy barriers to overcome were calculated by DFT and explained the experimental results. The underlying mechanism of light was explained by hot electron generation supported by surface plasmon excitation through photon absorption by the metallic substrate. A better ordering of the intermediate and final products was found when light was used. This original method to lower the reaction temperature provides an additional parameter to better control on-surfacesynthesis by separating the activation phase from the diffusion occurring at the same time in case of purely thermal activation. This approach could be extended to other systems in particular for fragile precursors to avoid desorption or undesired fragmentation.

Figure 1 :

 1 Figure 1: Constant current STM images of DBTP deposited on Ag(100) maintained at 78 K (a) bias-induced self-assembly monolayer of DBTP on Ag(100) (imaging conditions: 1 pA, 300 mV); (b) Zoom in the same area revealing halogen bonding with windmill of different chirality (imaging conditions: 1pA, 300 mV); (c) Brick-wall arrangement obtained by increasing the surface coverage. (imaging conditions: 10pA, 100 mV).

Figure 2 :

 2 Figure 2: Current constant STM images of DBTP precursors on Ag(100) (a) light activated precursors yielding the formation of OGM intermediate phase obtained at 300K (imaging conditions: 1pA, 300 mV). (b) OGM wires along the [100] direction of the silver substrate presenting misalignment with a jump of half unit cell. (imaging conditions: 20pA, 100 mV). (c) After annealing at 423 K together light, some short oligomers start to be formed along the OGM wires by elimination of one bridging silver atom. (imaging conditions: 5pA, 30 mV). In the first row, Ag bridging atoms are depicted as blue circles and halogen atoms as red circles. (d)

Figure 3 :

 3 Figure 3: (a) Current constant STM images of OGM chain on Ag(100) with atomic resolution on Ag(100) together with OGM chain allowing to locate precisely the Ag bridging atom and the main crystallographic directions of the substrate (imaging conditions: 10pA, 100 mV). (b) Schematic representation of OGM wires along the [100] direction of the silver substrate presenting a tilted molecule explaining the misalignment of the chain with a jump of half unit cell. (c) Current constant STM images of OGM chain on Ag(100) recorded at 1.9V where the tilted monomer appear with a different brightness revealing a change of the LUMO state associated with a different adsorption site and bonding. Imaging conditions:1.9V, 3pA.

Figure 4 :

 4 Figure 4: Energy barriers calculated by CI-NEB to overcome in order to detach (green arrow) and to attach (red arrow) (a) the first and the second bromine atoms from/to the DBTP precursors; (b) the first and the second Ag bridging atoms from/to the OGM.

  .

	Ag	Light 300 K 373K	423K	473K	523K
	100	ON -	OGM OGM WM OGM	Olig,+ PPP OGM	PPP Olig. + PPP	PPP
	111	ON -	OGM OGM	PPP Olig.	PPP
	Table

Table II :

 II . A comparative table of Ullmann coupling reaction of the brominated precursors onto various substrates is shown in table II. Comparative table of on-surface Ullmann reaction temperatures on different substrates. ( a ) refers to this work. For the same DBTP precursors, the use of light enhances the catalytic activity of the Ag(100) to Cu(111) level.

	Precursor	Subst.	OGM	Oligomer PPP
	DBBP [29]	Au111	No	500 K	No
	DBT [49]	Au111	410 K	-
	DBTP a	Ag100	RT+	423 K+	473 K+
			light	light	light
	DBBP [29]	Ag111	RT	373 K	473K
	BHydroxyB	Ag111	80 K	-	RT (λ=266
	P [50]		diradical		nm)
	DBTP [30]	Cu111	RT	393K	473K
	DBBP [29]	Cu111	<RT	-	450K

Acknowledgments

We acknowledge the support of the CALMIP computer center.