Dr Chuanfang F Oohn

Dr ) Zhang
email: zhangjc@tcd.ie

Sang-Hoon Park

Oskar Ronan

Dr Andrew Harvey

Dr Andrés Seral-Ascaso

Dr Zifeng Lin

Dr Niall Mcevoy

Conor S Bo

Prof Nina C G S Berner

Georg S Duesberg

Dr Patrick Rozier

Jonathan N Coleman
email: colemaj@tcd.ie

Valeria Nicolosi

S.-H Park

Prof. J C S N Boland

Prof. J N Duesberg

Prof. V Coleman

Nicolosi

Dr S.-H Park

Enabling Flexible Heterostructures for Li-Ion Battery Anodes Based on Nanotube and Liquid-Phase Exfoliated 2D Gallium Chalcogenide Nanosheet Colloidal Solutions

metal chalcogenide (MC) nanosheets (NS) have displayed high capacities as lithium-ion battery (LiB) anodes. Nevertheless, their complicated synthesis routes coupled with low electronic conductivity greatly limit them as promising LiB electrode material. Here, this work reports a facile single-walled carbon nanotube (SWCNT) percolating strategy for efficiently maximizing the electrochemical performances of gallium chalcogenide (GaX, X= S or Se). Multiscaled flexible GaX NS/SWCNT heterostructures with abundant voids for Li + diffusion are fabricated by embedding the liquid-exfoliated GaX NS matrix within a SWCNT-percolated network; the latter improves the electron transport and ion diffusion kinetics as well as maintains the mechanical flexibility. Consequently, high capacities (i.e., 838 mAh g-1 per gallium (II) sulfide (GaS) NS/SWCNT mass and 1107 mAh g-1 per GaS mass; the latter is close to the theoretical value) and good rate capabilities are achieved, which can be majorly attributed to the alloying processes of disordered Ga formed after the first irreversible GaX conversion reaction, as monitored by in situ X-ray diffraction. The presented approach, colloidal solution processing of SWCNT and liquid-exfoliated MC NS to produce flexible paper-based electrode, could be generalized for wearable energy storage devices with promising performances.

Introduction

The incredible popularity of portable consumer electronics bas greatly stimulated the development of safe, high-energy/ power density, long cycle-life energy storage devices, ( 1 -4 1 espe cially lithium-ion batteries (LiBs).l 5 l Commercial graphite anodes suffer from relatively low theoretical capacity (=372 mAh g-1 ) and sluggish ion kinetics (10-11-10 -10 cm 2 s-1 ) ( 6 1 upon Li + intercalation/de-intercalationVl Engineering the electrode by nanostructuring (S-lO ] or designing porous low dimensional materials[11-14 1 may result in faster Li + diffu sion and thus improve their rate capability. [ 15 • 1 7 - 20 1 Recently, 2D nanomaterials have attracted huge research attention for their use in LiBs due to their exotic properties.l 21 -2 31 As a typical example, the layered metal chalcogenides (MCs) including III-VI/IV-VI layered semiconductors (e.g., GaS, GaSe, InSe, and SnS 2 ) and transition metal dichalcogenides (e.g., MoS 2 , WS 2 , and ReS 2 ) possess higher specific capacities (>550 mAh g-1 ) ( 2 4, 2 51 and larger interlayer spacing compared to graphite, ( 26 -3 °1 allowing a rapid Li + insertion/extraction process without inducing significant volume changes. More over, the weak van der Waals forces among the MC sheets facilitate the production of nanosheet (NS) dispersions via a facile exfoliation strategy.l 31 -33 1 To date, molybdenum disulfide (MoS i ) bas been the most widely investigated lay ered MC for use as anodes in both LiB[ 34 . 35 l and sodium-ion batteries.l 36 1 Others, such as tungsten disulfide (WS 2 )[ 5 l and rhenium disulfide (ReS 2 ), [ 26 • 37 1 have recently been reported as having good electrochemical performances. Undoubtedly, the layered MC family is thus far relatively unexplored but shows remarkable potential for LiBs. Nevertheless, their poor electrical conductivity severely limits the electron transport and leads to a series of issues, such as low measured capaci ties, rapid capacity decay, and thus poor rate capability. ( 38 , 39 1 Incorporating the MC NS into a conductive scaffold such as graphene, ( 401 carbon nanotubes, ( 41 1 or conducting poly mers,( 4 21 for instance, could effectively main tain the structural integrity of the electrode while exposing more active sites to the electrolyte, resulting in much higher capacities as well as better rate capability.

As a representative member of layered MC family, gallium (II) sulfide (GaS) possesses an indirect bandgap of 2.5 eV and is of interest for many applications ranging from photoelectronics[ 43 1 to gas sensing.l 44 1 However, a few studies have been conducted on GaS as an LiB anode mate rial, apart from that of Meng et al., who deposited GaS x thin films of quite low mass onto single-walled carbon nanotubes (SWCNTs) via atomic layer deposition, yielding a prom ising discharge capacity of 575 mAh g-1 (per total composite mass) and 770 mAh g-1 (per mass of GaSx) at 120 mA g-1 .( 24 1 However, this synthesis process is not scalable and can potentially give rise to the hazardous H 2 S leakage in the temperature range used (150-200 °C). We believe for such materials to be competitive in LiB, a simple, scalable and environmentally friendly production of MC NS would be necessary.l 30 1 To this end, liquid-phase exfoliation (LPE) bas been widely recognized as an efficient, scalable and low cost approach to obtain the 2D NS dispersions in suitable solvents.l 33

• 45 • 46 1
Herein, a facile SWCNT percolating strategy for effi ciently maximizing the electrochemical performances of gal lium chalcogenide (GaX, X= So r Se) is reported. Through environmentally friendly solution processing of SWCNT and liquid-phase exfoliated GaX NS, we obtained freestanding, flexible GaX NS/SWCNT heterostructures with NS matrix well embedded in the SWCNT-percolated network. Such a hierarchical nanostructure enables high capacities and fast rate capabilities in the GaX NS/SWCNT paper com posite, holding great promise for practical LiB applications. The charge storage mechanism was studied by in situ X-ray diffraction. The superior electrochemical Li + -storage per formances could be ascribed to the advantages associated with the multiscaled hierarchical nanostructure at the same time: (i) the open channels might result in faster Li + diffu sion. (ii) The entangled SWCNT network forms developed, percolated electron transport paths, lowering the junction resistance. (iii) SWCNT could function as a "conductive binder," maintaining the structural integrity of the thick elec trode without losing electrical contact with the current col lector.l 4 7 • 4 81 Moreover, the absence of an insulating binder leads to dead-volume free electrode, allowing the composite to perform to its full potential. We believe the approach for the preparation of MC NS flexible conductive composites can be generalized, with the porous scaffold ranging from gra phene to SWCNT, etc., while the NS could range from GaTe, InS, InSe, InTe, to other, yet untested, MC NS. The resultant composite films could find applications in wearable elec tronics and optoelectronics, energy-storage systems, or sen sors, to name but a few.

Results and Discussion

In general, successful exfoliation of 2D materials is achieved when the energy cost of mixing with the solvent is mini mized. This tends to occur for solvents with similar surface energy to the 2D material to be exfoliated. ( 33 , 45 ] In a pre vious report, we found that GaS could be well exfoliated in isopropanol (IPA); ( 33 1 therefore, in this work we similarly bath-sonicated the commercial GaS and GaSe powders, ( 49 1 utilizing the sonic energy to overcome the weak van der Waals coupling of the layers and forming an NS-enriched dis persion. This is schematically illustrated in ,:::i;/j:_�\:

. . ,•�. :� . To investigate if defects were introduced to the GaS NS during sonication, both X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were performed. The decon voluted XPS of the Ga 3d core level (Figure lg) shows that the GaS NS exhibits only a small amount of degradation from a GaxOy component which can be found at a slightly higher binding energy than the dominant GaS species. The characteristic lattice vibrations at 195, 301, and 370 cm-1 corresponding to the GaS A\ 53 1 Therefore, the effect of SWCNT mass fraction on both prop erties of the GaS NS/SWCNT composites is explored. In the case of the MC NS only film, the electrical conductivity was too low to measure. The 5% SWCNT composite exhibits 0.2 S m-1 but dramatically increases up to 1491 S m-1 for 50% SWCNT, as shown in Figure 2b. Such an increase in electrical conductivity is usually interpreted via percolation theory.1 54 The measured conductivity was further analyzed using the following equation:

�-1.2 (f) E --Extinction --Absorbance (g) !\ Gas JI \ Ga 3d 1.0 è 0.8 ïii ( h ) --Scattering 1' � 0.

{1)

where a is the conductivity, Mr is the SWCNT mass loading Although we were not able to measure the conductivity for samples below 5% SWCNT, the data are consistent with a percolation threshold of =3.5%, somewhat larger than the value of ssl % reported by Liu et al.

In addtion to the electrical percolation, the SWCNT loading fraction also determines the mechanical proper ties of such composites.l 41 1 As illustrated in Figure 2c, upon increasing SWCNT content from 2% to 50%, both the strain and stress performances are dramatically improved. Further more, the toughness ( energy density absorbed at fracture) as a function of SWCNT mass fraction (Figure 2d) was plotted, showing a drastic improvement in the toughness as the SWCNT fraction is initially increased up to 10%. We would expect such a toughness increase to help the composite survive the expansion/contraction cycles associated with repeated charging/discharging.[ 41 1 Raman spectra of the GaS NS/SWCNT composite films show the characteristic peaks of GaS (Figure 2e) as well as peaks from SWCNT ("'1340 cm-1, "'1580 cm-1, and "'2675 cm-1 , indexed as the characteristic D, G and 2D bands, respectively).l 55 l A plot of relative GaS Raman intensity versus SWCNT loading fraction is shown in Figure S2 (Sup porting Information). At high SWCNT fraction, the contribu tion from radial breathing modes of SWCNTl 56 l is seen in the low-frequency regime, partially overlapping with the GaS A 1 8 mode (Figure S2a, Supporting Information). A color-coded map, showing contributions from both SWCNTs and GaS, is presented in Figure S2b (Supporting Information). On the basis of the normalized Raman intensity, the Ioasf l sw cr-rr as a function of the electrode composition (1/M L -1) was plotted, revealing a good linear fit (Figure S2c, Supporting Informa tion). is worth mentioning that, since the 30% SWCNT composite possesses good conductivity (Figure 2b ), mechan ical toughness (Figure 2c,d) and uniformity, as seen from the homogeneous distribution of SWCNT and GaS signals over the entire mapped area in its scanning Raman spectroscopy (Figure 2f,g), we selected 30% as the optimized SWCNT loading fraction in the GaS/GaSe NS/SWCNT composite films.

Figure 2h is an SEM image of the GaS NS/30% SWCNT, showing that the GaS NS are uniformly embedded in the SWCNT network. The cross-sectional SEM image of the GaS NS/30% SWCNT composite shown in Figure 2i displays a thickness of "'4-5 µm, consistent with the thickness obtained with a digital micrometer. The enlarged SEM image shows that the GaS NS is uniformly spread within the SWCNT net work (Figure 2j), which creates sufficient void space for rapid Li + diffusion as well as volume expansion and also prevents the GaS NS from aggregating. The homogeneous distribution of both nanotubes and nanosheets within the composite was confirmed by elemental mapping of Ga, S, and C performed by energy-dispersive X-ray spectroscopy (EDX) in the SEM (Figure 2k).

As for the GaSe NS/30% SWCNT composite, it exhibits a slightly higher electrical conductivity of 954 S m-1 (Figure 2b ). The exfoliated GaSe NS are well embedded in the SWCNT percolated network (Figure 21-n), forming a homogeneous composite. The EDX mapping of Ga/Se/C elements in Figure 2o also indicates a uniform composition across the electrode. For the sake of simplicity, we term the MC NS/30% (5 µm) SWCNT composites as GaS NS/SWCNT and GaSe NS/SWCNT for the further studies unless specifically noted.

The electrochemical performance of the GaS NS/ SWCNT composite was investigated through cyclic voltam metry (CV) and galvanostatic charge/discharge (GCD), and compared to that of GaS NS. Figure 3a shows CV curves at a scan rate of 0.2 mV s-1 . The sharp cathodic peak starts at 1.1 V in the first cycle, then gradually weakens in the fol lowing cycles, and eventually disappears in the tenth cycle, suggesting that the composite film possibly undergoes struc tural change, which could be expressed by the following conversion reactions:1 25

1 GaS+2Li + +2e-➔ Ga+Li 2 S ( 2
)
as well as the formation of solid electrolyte interphase (SEI).l 57 l This process is similar to the conversion reaction of MoS i fSWCNT anode.l 41 l The obvious plateau at 1.�1.1 V in the first discharge profile (Figure S3a, Supporting Informa tion) agrees well with the CV result. The cathodic peaks cen tered at 0.6 and 0.1 V could be attributed to the formation of LiGa and Li 2 Ga alloy phases, respectively, according to the following overall reaction:[ 58 , 59 1

(3)

Upon anodic scanning, peaks centered at 0.5 V and 0.8 V, are found, corresponding to de-alloying of LiiGa and LiGa phases to Ga, respectively.l 58 .5 9 l The absence of sharp peaks in anodic scanning should indicate that the conversion mecha nism of GaS (reaction (2)) is irreversible. Based on the above reactions, a theoretical capacity of around 1053 mAh g-1 for GaS (assuming four electrons transfer during the lithiation) was obtained. Although the initial discharge capacities of the two materials are similar (1730 mAh g-1 , Figure S3a, Sup porting Information), the composite retains a much higher discharge capacity (838 mAh g-1 ) than that of pure GaS NS (576 mAh g- 1 ) after five cycles at 100 mA g-1 (Figure 3b). Close examination reveals that broad CV curves (Figure 3a) as well as sloping GCD profiles (Figure S4a, Supporting Information) are found in the composite film, in contrast to the sharp CV peaks and discharge plateau in the GaS NS (Figure 4b). This implies that the charge-storage mechanism bas changed from traditional battery type to pseudocapaci tive type upon the introduction of SWCNT.[ 60 1 Such a crit ical adjustment, we believe, is important, as charge could be injected/ejected much faster in the case of pseudoca pacitive behavior.1 15 , 61 , 62 1 As indicated in Figure 2a, the ran domly distributed SWCNT could simultaneously serve both as the conductive "spacer" and "binder" between the GaS NS, forming a multiscaled hierarchical nanostructure with many surface Li + storage sites and shortened ion diffusion paths, which could be responsible for the broad CV and sloping GCD curves in the composite films. In the absence of SWCNT, the GaS NS easily restack to form the multi layered GaS bulk phase (as shown in Figure lb) and thus decrease the surface Li + storage sites, leading to apparent lower capacity values and fast capacity decay. Actually, the fact that the MC-based conductive composite exhibits a pseu docapacitive charge storage mechanism is not unexpected. Very recently, Dunn and co-workers studied a TiS 2 composite electrode, which demonstrated pseudocapacitive intercalated behavior in its 2D nanoscaled morphology that is not exhib ited in the corresponding bulk TiS 2 .l 63 1 Another study by Park and co-workers also shows that the electrochemical behavior of MoS 2 could change from the dominated sluggish diffusion control to pseudocapacitive provided the layered MoS 2 was scaled down into nanometric sheets and percolated with a reduced graphene oxide network.[ 64 1 To further understand the charge storage mechanisms, in situ X-ray diffraction (XRD) of the GaS NS/SWCNT composite electrode was performed, and the patterns were

; •• ••••• . g l A g •1 -1 ••••• ••••• 2 Ag •••••
. . . • ..... ::::: • "' 400 Figure 3 . a) Cyclic voltammograms of Gas NS/SWCNT composite (30% SWCNT, s µm) at a scan rate of 0.2 mv s-1 . b) Fifth gal vanos tatic charge/ discharge profiles of SWCNT, Gas NS, and Gas NS/SWCNT composites (30% SWCNT, s µm) at a current density of 100 mA g -1 . c) ln situ XRD patterns of Gas NS/SWCNT composites at different potentials with a fix step of 0.2 V versus Li/Li + . d) Rate performance of SWCNT, Gas, and GaSe NS-based samples at different current densities ranging from 100 mA g-1 to 2 A g-1 ; ail the samples have very similar thickness (5 µm). e) Specific capacities as a function of current density; here, the capacity at the fifth cycle under each current was selected. 1) Coulombic efficiency and g) cycling performance of Gas NS and Gas NS/SWCNT composite at 1 A g -1 .

••• ... •• • •• .....
•• •• •• •• • ••...... Gas N5/30% SWCNT . ••• • ••• •• • • • ••• ••• ••••• •• ••• • • • •••••••••••••••••••••••• • ••
••• ... •••••••••••••••••••••••••••••• •••••••••••• .. ••••• .. •••••••••••••••••••••• GaSNS u 0 10 20
recorded at various potentials with a fix potential step of 0.2 V as seen in Figure 3c. Peaks centered at 28 °, 36 °, and 38 ° corne from the in situ cell setup. Beside the broad signal around 23 °, corresponding to the SWCNT scaffold, three other peaks at, respectively, 11 °, 23 °, and 34 ° are found at open circuit potential (2.0 V), corresponding to (002), (004), (006) planes of GaS. The evidence of these (001) peaks sug gests the periodic stacking of the GaS NS. The (001) peaks are maintained down to 1.2 V, then start to weaken at 1.0 V and eventually disappear below 0.8 V, indicating the disor dering of the structure induced by the lithiation of the GaS during the discharge process, which is irreversible, as no (001) peaks are recovered upon the following charge cycle. Said otherwise, there is no evidenoe for the growing of crystalline GaS in agreement with the irreversible conversion of GaS (reaction (2)) deduced from the CV, the capacity is majorly contributed by the Ga alloy process ac.cording to reaction (3). However, the absence of the Ga and Li x Ga characteristic peaks suggests that either the crystal growth of these two species is suppressed, or they are small and disordered.l 59 l

The rate performances are shown in Figure 3d. Com pared to pure GaS NS, the GaS NS/SWCNT composite dis plays much better capacity and rate capability (Figure 3e). For example, at 2 A g-1 , the specific capacity of the GaS NS/ SWCNT composite is 530 mAh g-1 , almost six times higher than that of pure GaS NS (92 mAh g-1 ). When the current is switched back to 100 mA g-1 after 25 cycles, the capacity returns to 700 mAh g-1 . GCD profiles of two electrodes at various current densities could be seen in Figure S4a,b (Supporting Information). We believe the rapid capacity decay of GaS NS could be attributed to the absence of SWCNT, which provides the efficient conductive network and maintains the structural integrity. Moreover, the compos ite's capacity greatly exceeds that of GaS NS and SWCNT when the current density is beyond 200 mA g- 1 , indicating that the synergistic effect between the two components bas been the reason for the superior Li + storage performances.

The cycling performances of GaS NS and GaS NS/SWCNT are evaluated at a high rate (1 A g-1 ), as shown in Figure 3f,g. GaS NS exhibits a low first cycle Coulombic efficiency (iCE, 19%; Figure 3f) and a poor lifetime performance (only 77 mAh g-1 is retained after 100 cycles, Figure 3g). After embedding the GaS NS in the SWCNT scaffold, both the first cycle CE and the capacity retention have been improved con siderably (iCE = 60% and retains 300 mAh g-1 , Figure 3g). The improved iCE should be ascribed to the enhanced elec trical conductivity and mechanical property brought by the SWCNT-percolated network. Further increasing of iCE could be realized by adding Jess SWCNT with higher conductivity but lower surface area. As such, the irreversible capacity Joss from SEI formation is minimized. Although the iCE of the composite is moderate, it is much higher than the graphene/ SWCNT/nickel composite anode material (iCE = 30% ).( 651

The rapid capacity decay in the first ten cycles could be attributed to the irreversible GaS conversion, as seen in Figure 3a,c. The graduai capacity decay phenomenon is com monly observed in conversion/alloy-type electrodes and has been attributed to the incomplete conversion, pulveriza tion, and Joss of electrical contact with the SWCNT upon cycling.l 13 , 5 7J Moreover, the dissolution and shuttling of inter mediate Li 2 S could be responsible for the capacity fading, similar to the cycling issues that exist in Li-sulfur batteries. Since this is an early-stage report on the GaS NS/SWCNT composite, there is certainly much scope to improve the cycle life by gaining increased understanding of its electrochemical process, i.e., ensuring intimate contact between the active GaS and the SWCNT scaffold, facilitating more complete conversion and alloying processes, etc. GCD curves of GaSe NS and GaSe NS/SWCNT are shown in Figures S3c andS4c,d (Supporting Information), revealing first discharge capacities of 1090 and 1320 mAh g-1 , respectively. A more sloping curve is found in the composite in sharp contrast to plateaus seen for the GaSe NS at ail cur rent densities, further confirming that the addition of SWCNT is beneficial for increasing the Li + storage sites, shortening ion diffusion paths and resulting in a pseudocapacitive behavior. The rate performances of GaSe NS and GaSe NS/SWCNT are summarized in Figure 3d,e. The composite shows a discharge capacity of 713 mAh g-1 at 100 mA g-1 and 413 mAh g-1 at 2 A g-1 , slightly lower capacity values but similar rate capa bilities compared to the GaS NS/SWCNT counterpart. Therefore, GaS NS was selected for fabricating MC NS/ SWCNT films with different compositions and thickness. The capacities normalized to the total mass of the electrodes are plotted as a function of current density and SWCNT loading fraction as seen in Figure 4a,b. The rate performance of the composites can be found in Figure SS (Supporting Infor mation). Both the capacities and the rate performances are remarkably improved upon increasing the SWCNT loading fractions from 5% to 30%, consistent with the improvement in electrical conductivity as well as mechanical properties demonstrated by Figure 2b-<I. However, excessive SWCNT (50%) addition results in a smaller portion of redox-active GaS NS in the composite and thus decreases the capacity substantially, as seen in Figure 4a,b. While it is of more prac tical importance to investigate the specific capacities nor malized to the total composite mass, it is also fundamentally valuable to understand the utilization of the GaS NS, more precisely how their inherent capacity contributes to the total capacity of the composite electrode. The utilization of GaS NS in each composite was quantitatively determined according to Equation (4): cG.s = ( ccom pos ite -CswcNT X % swCNT )! % Gas (4) \Vhere CGaS> Ccom pos iw and CswcN T represent, respectively, the specific capacity per GaS NS, composite, and SWCNT, while % swcNT and % Gas are the mass loading fractions of the SWCNT and GaS NS, respectively. Here, the CswcNT in each composite is supposed to be the same as that of Fîgure 3d. Fîgure 4c plots the utilization of GaS NS in var ious composites as a function of current density. When increasing the % swCNT from 5% to 50%, the CGas increases dramatically at first and then saturates (Fîgure 4d), sug gesting that 30% SWCNT seems to allow the GaS NS to reach its intrinsic capacity values, as shown by the plateau. If so, then the corresponding capacity at the plateau should match the theoretical value based on Equations ( 2) and (3). Encouragingly, the CGas at 100 mA g-1 agrees well with the theoretical capacity (1107 vs 1053 mAh g-1, respectively). As for the GaSe NS, its utilization reaches 928 mAh g-1 in the composite at 100 mA g-1 . Such a remarkably high capacity per MC NS component greatly exceeds that of the atomic layer-deposited GaSx-SWCNT composite (766 mAh g-1 ).l 24 1 The electrochemical impedance spectroscopy (EIS) in Figure S6 (Supporting Information) also suggests that by gradually adding SWCNT up to 30%, the semicircle at high/ medium-frequency region decreases, implying a smaller charge-transfer resistance (R 01 ) and easier semidiffusion pro cess of Li + into the GaS NS host. The effect of mass loading on the areal capacity of GaS NS/SWCNT composite is ana lyzed in detail in Figures S7-S10 (Supporting Information). After cycling, the GaS NS/SWCNT composite still preserves its 2D layered morphology, demonstrating the percolated network maintains the structural integrity upon repeated charging/discharging (Figure 4e,f).

Figure S compares the electrochemical performances of MC-containing electrode systems in detail (note that Table Sl (Supporting Information) provides details and references). lt should be specially noted that, in order to eliminate the effect of the materials' morphology on the capacities, only materials with a 2D layered structure, such as nanosheets, nanoflakes, or nanoplatelets, were chosen from the large MC family. We emphasize that it is rather challenging to make a direct comparison among various systems due to a large number of variables, such as mass loading/film thickness, potential range, binder/conductive additive fractions, and nanosheet size. Taking this into account as much as possible, firstly the litera ture-specified capacities were normalized based on the mass of active MC and total electrode, and then categorized the values according to the mass loading as well as the potential range. It is worth mentioning that an anode with lower poten tial is more desirable for achieving a higher energy/power density in LiBs. Unlike the half-cell configuration that emphasizes the discharge capacity regardless of the electrode being anode or cathode, a full-cell battery system relies on the charge capacity of the anode. Therefore, for those systems tested in the 0.01-3 V window, the charge capacities that were achieved in the range 0.01-2 V were extracted to facilitate our comparison. The compiled plot shows that our binder free GaS NS/SWCNT and GaSe NS/SWCNT flexible com posites perform much better than any other 2D MC-based electrode systems in terms of capacity per active MC and per electrode in the 0.01-2 V window. For example, the utilization of GaS NS reached remarkably high (1107 mAh g-1 ). In addi tion, our composite film outperforms most other electrode systems in the high-mass loading region (2 mg cm-2 ), except for the SnS/graphene anode by Zhao et al, who tested the at a lower current density.1 16 1 This clearly indicates the advantages of the flexible, binder-free composites and highlights its state-of-the-art performance.

Conclusion

In summary, the facile, environmentally friendly produc tion of multiscaled hierarchical GaX NS/SWCNT hetero structures is demonstrated. Through a colloidal solution processing approach, the NS matrix embeds well within the SWCNT percolated network, enabling excellent rate capa bility and high specific capacity (838 mAh g-1 in GaS NS and 713 mAh g-1 in GaSe NS) as the LiB anode material. More over, due to the efficient Li + diffusion and electron transport kinetics, the composite film exhibits remarkably high utiliza tion of GaX NS (1107 mAh g-1 in GaS NS and 928 mAh g-1 in GaSe NS), demonstrating electrochemical performances competitive with those of other reported 2D MC-based elec trode systems. This work opens up vast opportunities for other families of MC NS to be scalably processed into flex ible conductive composite films with a broad range of appli cations such as wearable electronics and optoelectronics, energy storage systems, sensors, and fuel cells.

Experimental Section

Exfoliation of MC NS and Preparation of SWCNT Dispersion:

Gas NS were obtained through lPE of commercial Gas powder (99.999% GaS-05-P, American Elements) using an ultrasonic bath (P30 H Ultrasonic from Fischer scientific) in IPA.1 50 1 First, the Gas powder was added into 20 ml of IPA at a concentration of 45 mg ml-1 , then the suspension was sonicated for 6 h at an amplitude of 100% and a frequency of 37 kHz. During sonication, the temperature of the bath was cooled to 20-30 °C by contin uous flow of cooling water. Once sonicated, the dispersion was subjected to centrifugation (Hettich Mikro 220R) for 180 min at 1000 rpm. The top 60% supernatant was collected while the sed iment was recycled to produce an adequate quantity of Gas NS. SWCNT (P3-CNT, Carbon Solutions, Inc.) was dispersed in IPA at a concentration of 0.1 mg ml-1 using bath sonication. Ga Se NS was similarly prepared by sonication the commercial GaSe pow ders in IPA.

Flexible MC NS/SWCNT Composite: The composites were pre pared through a facile vacuum filtration approach of blended dispersion though a polyethylene membrane (Celgard K2045, 0.06 µm pore size). Severa! Gas NS/SWCNT composite films were prepared with SWCNT contents being 5%, 10%, 20%, 30%, and 50%, respectively, while keeping the filtrated volume constant (20 ml). Composites with different thickness were also prepared by con trolling the filtrated volume to 20, 40, 80, and 160 ml, respec tively. To avoid degradation over time, the films (including the membrane) were immediately transferred to a glove box (oxygen and water content <1 ppm). The natural dried films were then eut into individual electrodes with 7 mm x 7 mm size (geometric area .. o.s cm 2 ). lt is worth noting that the attached membrane was utilized as a second separator in the coin cells. This was clearly different from many reported freestanding electrodes, which were obtained through complicated peeling off or film transfer procedures.1 39 • 66 1 For the GaSe NS/SWCNT composite, the SWCNT loading fraction was controlled at 30%.

Materials Characterization:

The SEM images were obtained on a Zeiss Ultra Plus (Carl Zeiss, Germany) working at 2 keV of acceler ation voltage. EDX was performed in the same microscope and ana lyzed using INCA program. TEM analysis of the MC NS and SWCNT was performed on an FEI Titan at 300 kV (FEI, USA). The nanosheet dispersion was drop-casted onto holey carbon grids (Agar Scien tific, UK). The flake dimensions were statistically analyzed on TEM by measuring the longest axis of the nanosheets and assigning it as the length. Optical spectroscopy was done on Gas NS disper sion using an UV-vis absorbance spectroscopy (Cary 6000i Spec trometer, Agilent, USA). Both the extinction and absorption spectra of the dispersion were measured using an integrating sphere that subtracted the absorption spectra from the extinction spectra. To show the quality and defects of the as-exfoliated Gas NS, X-ray photoelectron spectroscopy and Raman spectroscopy were con ducted. XPS was performed using monochromated Al Ka X-rays from an Omicron XM1000 Mkll X-ray source with an EA125 energy analyzer, which yielded a maximum energy resolution of .. o.65 eV. Ali the binding energy scale was referenced to the aliphatic carbon 1s core-level at 284.8 eV. Raman spectra of Gas NS were acquired using a WITec Alpha 300 R with a 532 nm excitation laser at a power of .. 150 µW. A 100x objective was used with a spectral grating of 600 lines mm-1 . Characteristic spectra were obtained by averaging 20 discrete point spectra for each sample. For the composite electrodes, Raman maps were obtained by acquiring 80 x 80 spectra over an area of 20 x 20 µm. Maps representing the presence of SWCNT were generated by mapping the intensity of the SWCNT G band whereas the presence of Gas was represented by the intensity of the Gas A\g band. The DC electrical conductivity of the composites was measured using a two-point probe technique. Two parallel electrode contacts were created on the film surface using conductive silver paint (Agar Scientific). The resistance of each sample was measured using a Keithley 2400 source meter. The electrode thickness was determined by a digital micrometer (Mitutoyo). ln order to perform mechanical measurements, the Gas NS/SWCNT was peeled off composites (with different SWCNT loading fractions) after vacuum filtration. The free-standing films were then eut into stripes, 2.25 mm wide. The mechanical meas urements were conducted on the Zwick tensile tester using a gauge length of 5 mm and a strain rate of 1 mm min-1 . Each data point was an average of four measurements.

Electrochemico/ Characterization: The electrochemical per formances of the Gas and GaSe NS/SWCNT flexible composites were investigated using CR-2032 coin cells. Two layers of poly• propylene membrane (K2045 Coated PP, Celgard LLC, Charlotte, NC) were used as a separator. The composite electrode (attached on the PP) was used as the working electrode while a lithium metal dise with a diameter of 16 mm was used as both the counter and reference electrodes. The electrolyte was 1 M lithium perchlorate (LiCIO,.) dissolved in ethylene carbonate and dimethyl carbonate in a volume ratio 1:1. Electrochemical tests were performed through 0/, GCD, and EIS with a voltage range of 0.05-2 V on a potentiostat (.VMP3, Biologie). ln the 0/, electrodes were cycled for ten times at 0.2 mV s-1 . For GCD, tests were conducted with current densities ranging from 100 to 2000 mA g-1 . The capacities reported here were those obtained from the sixth discharge profile at each current density and normalized to either the mass of active material (Gas NS) or the total mass of composite (Gas NS/SWCNl). The cycling performance of the electrodes was evaluated at 1000 mA g-1 for 100 cycles. The EIS was conducted at the open-circuit potential with the AC frequencies ranging from 200 kHz to 10 mHz.

  Figure la. Scan ning and transmission electron microscopy (SEM and TEM, respectively) of the obtained dispersions show large quanti ties of exfoliated NS with well-defined edges and no apparent damage in the basal planes, which confirms the good quality of the exfoliated material (Figure lb-e).l501 Statistical anal ysis of the TEM images determines that the GaS NS have (L) = 413 ± 26 nm (the inset of Figure le), in good agreement with (L) = 410 nm derived from the measured extinction and absorption spectra, as well as the subtracted scattering spectra (Figure lf)_( 5 o, 5 i ] Using previously determined met ries from the extinction spectra the GaS NS concentration is found to be 2 mg mL-1 . ( 50 ,51 1 As for the GaSe, the as-prepared solution exhibits an orange color (the inset of Figure ld). The TEM in Figure ld indicates the curved morphology of the GaSe NS with mean

  ..: ...... : ..

Figure 1

 1 Figure 1. a) Schematic of MC NS production through a liquid-phase exfoliation approach. b) SEM and c) TEM of exfoliated Gas NS. lnset in part (c) is the Gas NS size distribution. d) TEM and e) high-resolution TEM of exfoliated GaSe NS. lnsets in parts (d) and (e) are the size distribution and fast Fourier transform pattern of exfoliated GaSe NS, respectively, showing a highly crystalline structure. O Optical extinction, absorbance, and scattering spectra of a typical Gas NS dispersion. Here, intensity represents Extinction, Absorbance, or Scattering as appropriate. g) Ga 3d XPS and h) Raman spectrum of exfoliated Gas NS. flake size of 338 ± 38 nm (Figure ld and its inset). The high resolution TEM and its corresponding fast Fourier transform pattern are shown in Figure le and its inset, respectively, showing the highly crystalline structure of the exfoliated GaSeNS.To investigate if defects were introduced to the GaS NS during sonication, both X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were performed. The decon voluted XPS of the Ga 3d core level (Figurelg) shows that the GaS NS exhibits only a small amount of degradation from a GaxOy component which can be found at a slightly higher binding energy than the dominant GaS species. The characteristic lattice vibrations at 195, 301, and 370 cm-1 corresponding to the GaS A\

8 , E 1

 81

2 8 ,

 28 and the A 1 2 8 modes, respectively, are found in the Raman spectrum displayed in Figure lh.l 43 .s 2 1 Next, binder-free flexible composite electrodes were pre pared, assisted by a vacuum filtration of the mixture of MC NS/SWCNT dispersions, as shown in Figure 2a. As references, pure MC NS and SWCNT films were similarly produced. A typical TEM image of SWCNT is shown in Figure Sl (Sup porting Information). According to previous reports, the mass fraction of the conductive agent is of critical importance to the electrical conductivity and mechanical properties (flex ibility) of the NS/SWCNT and NS/graphene composites.[ 41 ,

1

 1

Figure 2

 2 Figure 2. a) Schematic describing the critical rote of SWCNT in constructing a multiscaled hierarchical nanostructure. lnsets are the Gas NS-based thin films with 0%, 5%, and 30% SWCNT. b) Electrical conductivity of Gas and GaSe NS-based composites with different SWCNT-loading fractions. The fitted line was obtained via percolation theory. c) RepresentatiVe stress-strain curves and d) tensile toughness plotted as a function of SWCNT-loading fraction in Gas NS/SWCNT composite films. e) Raman spectra of Gas NS/SWCNT composites. Scanning Raman spectroscopy of f) GaSA 1 2g and g) SWCNT G band. h) SEM and i,j) cross-sectional SEM images of Gas NS/SWCNT composite (30% SWCNT, s µm) at i) low and j) high magnification. k) EDX mapping of Ga, 5, and C in Gas NS/SWCNT composite. Scale bar is 10 µm. 1) SEM and m,n) cross-sectional SEM images of GaSe NS/SWCNT composite (30% SWCNT, 5 µm) at m) low and n) high magnification. o) EDX mapping of Ga, Se, and C in GaSe NS/SWCNT composite. Scale bar is s µm.

fraction,

  M�c is the threshold of SWCNT fraction to form the first conductive paths, and n is the exponent.( 54 1 While per colation analysis is usually performed on data plotted versus tiller volume fraction, it can easily be shown that, in porous films, the mass and volume fractions are directly propor tional, making the analysis below appropriate. The good fit of this model to the data shown in Figure 2b suggests that the addition of SWCNT follows percolation theory.l 4 7J From the fitted results an n = 2.5 is found, similar to the universal percolation exponent (n = 2.0) and very close to the value of 2.3 obtained by Liu et al. for MoS 2 /SWCNT composites.l 41 1

  ----. ---. G .... a "" s N"' s =-= s w.,.., c"' N "" T,..

  results at each step 1+...-������-�� ........ 90 ( f) <> 0 8 06a 8 8o6 g o8eoe 8 oCe6 80 o 80 o o 8 80 oo 66880688 80 o o 8 oo g 6 o 80o8088 eo e o 800 8 o 800 o oo 6 ooO

  ................ ....... . 200 0 .. ...... ......

2 Figure 4 .

 24 Figure 4. Specific capacities of Gas NS/SWCNT composites as a function of a) current density and b) SWCNT-loading fractions. Here, the capacity at the fifth cycle under each current was selected. Utilization of Gas NS calculated based on Equation (4) as a function of c) current density and d) SWCNT loading fraction. SEM images of cycled Gas NS/SWCNT electrode after 100 cycles at e) low and f) high magnification.

2400 1 -

 1 ----------low or unknown M/A----------➔ 1-M/A: ~ 1 mg cm• 2 -I 1-M/A: ~ 2 mgcm•�-1 1-0ther anodes--I

Figure 5 .

 5 Figure 5. Comparison of the specific capacities of this work to various 20 MC-containing electrode systems when normalized to the mass of active MC as well as the total electrode material. Values are classified according to the mass loading and potential range. Table 51 (Supporting Information) provides details and references. For the purpose of comparison, we also include graphite, CNT/graphene and Si-based anodes.

Acknowledgements C.F.Z. and S.-H.P. contributed equal/yto this work. V.N., C.F.Z., S.-H.P., A.S.-A., O.R. wish to acknow/edge the following funding support: SFI AMBER, SFI PIYRA, ERC StG 20NanoCaps, ERC PoC 2DUSD, ERC PoC 20/nk, FP7 MC ITN MoWSeS, Horizon2020 NMP Co-Pilot. J.N.C. acknowiedges the ERC SEMANT/CS and SFI (11/Pl/1087) for {inancial support. N.M. was supported by the SFI under 15/SIRG/3329, and N.C.B. and G.S.D. acknow/edge SFI P/_10/IN.1/I3030. Z.L. was sup ported by China Scho/arship Council (Grant No. 201304490006). Ali authors wish to thanks the Advanced Microscopy Laboratory ('AML) in CRANN, Trinity College. S. O'Brien and A. Pok/e are a/so acknowi edged and thanked for the he/p provided. S. O'Brien, A. Pok/e, and J. Coe/ho are appreciated for the experimental he/p.

Supporting Information

Supporting Information is available from the Wiley On/ine Library or from the author.

Conflict of lnterest

The authors dec/are no conflict of interest.


Anissa Amar

* Abdou Boucekkine

* Frédéric

Olivier Mongin

DFT study of two-photon absorption of octupolar molecules

Keywords:

The two-photon absorption (TPA) properties of octupolar molecules based on a triphenyl-isocyanurate cyclotrimer, a 1,3,5-triphenyl-benzene or a triphenyl-triazine core were theoretically investigated using DFT and TD-DFT computations. These compounds are very promising regarding their potential application especially for optical limitation. These systems, which exhibit a three-fold axis, contain three arms with a terminal electron donating group linked in 1, 3 and 5 positions to the central C 3 N 3 O 3 isocyanurate, benzene or triazine ring. The SAOP functional and a DZP basis set were selected for the TPA computations. The so-computed TPA energies and cross sections are in good agreement with the observed data. Increasing the strength of the donor terminal group enhances the TPA cross-section values. The compound with triazine core presents the highest two photon cross-section value compared to the values found for the isocyanurate or the 1,3,5-phenyl core as central ring. Furthermore, this study brings to light a cooperative enhancement of the TPA property between the three arms attached to the isocyanurate ring.

Introduction

The process of simultaneous two-photon absorption (TPA) is a nonlinear optical (NLO) phenomenon predicted theoretically by Maria Göppert-Mayer in 1931 [1] and the first experimental evidence was performed by W. Kaiser and C. G. B. Garret [2] 30 years later. The TPA process has attracted growing attention due to its potential applications, among them threedimensional optical storage [3][4][5], two photon-fluorescence imaging [6][7][8], two-photon photodynamic therapy [9] and optical limitation [10][11].

Over the past decades, several theoretical studies have been carried out to understand the relation between chemical/electronic structures with TPA properties and to evaluate the reliability of TPA energies and cross-sections computations [12][13][14][15][16][17][18][19][20][21][22][23].

In this paper, we investigate TPA properties of some octupolar derivatives, namely triphenylisocyanurate, triphenyl-benzene and + (Scheme 1). In previous experimental studies, some of us reported on the linear and NLO properties of substituted triphenyl-isocyanurates, tristyrylisocyanurates and triphenyl-benzene [24][25][26][27][28]. Triazinane-2,4,6-triones, more commonly known as isocyanurates are important cyclotrimeric molecules that attract considerable attention in polymer chemistry in regard to their numerous industrial applications, particularly in the field of foams, elastomers, paints, fibers, and surface coatings. [25] After proper functionalization, they can also give rise to anion receptors, drug-delivery agents or tensioactive building blocks. The recent realization that these molecules were also octupoles draw the attention of some of us toward their (nonlinear) optical properties and led to the discovery that some of them are highly fluorescent when peripherally functionalized by electron-donating arms and present intense nonlinear optical (NLO) responses, in particular fairly large two-photon absorption (2PA) cross-sections, a property of interest for various key developments in bioimaging, microfabrication or fields related to optical information storage and handling. [25] 1,3,5-Triphenylbenzenes and triphenyltriazine are more ubiquitous octupolar derivatives, well studied for their two-photon absorption properties, which constitute interesting reference compounds. [3] The two first derivatives i.e. 1-NMe 2 and 2-NMe 2 [24][25][26] exhibit a good NLO activity/transparency trade-off, and, in terms of third-order activity, they were shown to possess high TPA cross-sections. The TPA properties of the studied compounds will be computed using DFT techniques. The aim of our work is (i) to compare the three considered cores regarding their TPA response; (ii) to investigate the influence of the strength of the terminal donating group on the considered property and (iii) to investigate if any cooperative effect between the three arms born by the isocyanurate core occurs or if only additivity of their own individual responses takes place.

Methods of calculation

Computational details

A c c e p t e d m a n u s c r i p t

All the TPA and one-photon absorption (OPA) calculations have been carried out using the Amsterdam Density Functional (ADF2017 and ADF2018) suite of programs [START_REF]ADF Amsterdam Density Functional; Scientific Computing & Modelling[END_REF]. These properties have been calculated using the SAOP model potential [START_REF] Schipper | [END_REF]31] (statistical average of orbital model exchange-correlation potential) using the damped cubic response theory module of Lasse Jensen and Coll. [12] implemented in the ADF program package. The latter authors showed that SAOP leads to TPA results in satisfying agreement with experimental measurements. The geometries used in the calculations are those optimized at the MPW1PW91/6-31G* [32] level using the Gaussian09 suite of programs [START_REF] Frisch | c r i p t[END_REF] taking into account the dichloromethane solvent used in the TPA experiments by the means of the PCM model [34].

We wish to remind here to the reader that the SAOP model potential cannot be used for geometry optimizations. It is also worth noting that the geometry considered has a non negligible effect on the computed TPA property. For this reason we decided to use for all systems the optimized geometry computed at the level of theory indicated above. In a previous work, this functional led us to very satisfying results regarding the ground state and OPA properties of these molecules [27]. Moreover, we checked that an extension of the DZP basis set used in our TPA and OPA calculations, i.e. to TZP and TZ2P, had only a small effect on the computed properties. The TPA computations have been carried out considering isolated molecules; it is expected that solvent effects should be the same for the series of homologous molecules considered in the present work.

The lifetime of the electronic excited states is included in the theory using a damping parameter of 0.0034 au (∼0.1 eV ∼ 800 cm -1 ) value which was found suitable for TPA computations [12,35,36].

Finally, it must be recalled that, at our level of theory, the computation of the TPA properties for molecules of large size as ours necessitates a lot of computational time, and that the computation must be repeated for different laser energies in order to simulate the TPA spectra.

Two photon absorption cross section

A c c e p t e d m a n u s c r i p t

The TPA cross-section σ TPA has been obtained from the imaginary part of the third-order hyperpolarizability γ using the following expressions [12,13,37]:

(1)

TPA TPA e N γ ω α π σ 4 2 3 2 3 15 h = (2) ) Im Im Im ( αβαβ αββα ααββ γ γ γ γ αβ + + = ∑ TPA
Where α is the fine structure constant (not to be confused with the α index of the summation in eq 2), e is the elementary charge, ω is the photon energy, ℏ is the reduced Planck's constant whereas the integer value N=4 is used for all simulated TPA spectra [12,13]. The σ TPA value is usually given in Göppert-Mayer units (1 GM = 10 -50 cm 4 s photon -1 ) [1]. We first evaluate σ TPA in atomic units and then multiply it by (0.529177 × 10 -8 cm/a.u.) 4 × (2.418884 × 10 -17 s/a.u.) to obtain its value in the conventional units (cm 4 s photon -1 ).

As said before, to simulate the TPA profiles, we calculated point by point the TPA cross-sections for different laser energies.

Results and discussion

As said before, the geometries of the studied species were optimized at the MPW1PW91/6-31G* level in dichloromethane (DCM) as solvent since this solvent has been used in most experimental measurements discussed here. The geometry plays an important role especially when it concerns possible conjugation paths between different moieties of the considering molecule thus influencing the HOMO-LUMO gap which is an important property considering optical and NLO properties.

As it can be seen from the optimized geometry of the isocyanurate 1-NMe 2 species (Fig. 1), the three peripheral arms adopt a tilted conformation relatively to the central core, the average deviation from planarity being equal to 70° (table 1). In the case of the 1-NMe 2 derivatives with one and two arms, this deviation is higher and respectively equal to 81° and 89°.

3-NMe 2 0

We note also that the mean deviation from planarity for the phenyl core species 2-NMe 2 is less pronounced when compared to isocyanurate 1-NMe 2 , being equal to 37° for the former species vs. 70° for 1-NMe 2 . Finally, the triazine system 3-NMe 2 adopts a planar structure between the three arms and the central core, so differs drastically from the two other systems.

As written before, the TPA properties for the studied systems, as well as the OPA spectra, were computed at the SAOP/DZP level in vacuum. The TPA-simulated profiles are represented in Fig. 2, the experimental and theoretical OPA, TPA maximum energies and cross-sections are summarized in Table 2. The computed σ TPA cross-sections for the cyclotrimers using the SAOP model potential are higher than the measured ones, by a factor of the order of ten. Such deviations between measured and theoretical TPA cross-sections are not unusual and has been discussed by several authors [13,15]. The limitation of the non-linear response TD-DFT theory used for the TPA computations [38] is at least partly responsible for that, in particular the approximation done for the kernel.

Obviously, the neglect of solvent effects is also responsible for a part of this deviation. Finally, we remind that we checked that in our case the extension of the basis set from the DZP to the TZ2P one has only a small effect on the computed TPA cross section.

However, our results reproduce the same trend as the experimental observations, i.e. that the 4

A c c e p t e d m a n u s c r i p t

The enhancement of the maximum TPA cross-sections when increasing the strength of the electron-donating terminal groups i.e. going from 1-OMe to 1-NH 2 and 1-NMe 2 correlates well with the decrease of the energy gap between HOMO and LUMO (Gap H-L ; see Fig. 3).

Fig. 3 SAOP/DZP HOMO-LUMO gaps for 1-X (X = OMe, NH 2 , NMe 2 ), 2-NMe 2 and 3-NMe 2 .

Experimentally, 1-NMe 2 and 2-NMe 2 molecules possess similar two-photon cross-sections; this result was not expected, because the central ring in the 1-X (isocyanurate) derivatives was believed to be more electron-deficient than in the 2-X (phenyl) compounds. The computations give also almost the same σ TPA value, with a small increase of this property when passing from 1-NMe 2 to 2-NMe 2 , as also observed experimentally.

Our results indicate a significant enhancement on the σ TPA with the triazine core compared to the isocyanurate and phenyl centers, in good agreement with experimental observations (work in progress). We note that the calculated value of 3-NMe 2 is two times higher than for 1-NMe 2 and

A c c e p t e d m a n u s c r i p t 2-NMe 2 , whereas, experimentally, this value is three times higher. The enhancement of the σ TPA of 3-NMe 2 , which can be related to the significant decrease of Gap H-L when going from 1-NMe 2 and 2-NMe 2 to 3-NMe 2 originates (at least in part) from the planarity of the entire π system in 3-NMe .

On the other hand, the SAOP calculated TPA energies reproduce well the experimental TPA peak energies for the studied compounds. Our calculations provide the exact value of the TPA energy measured for the 1-NH 2 and deviates by 0.17 eV and 0.12 eV in the case of 1-NMe 2 and 2-NMe 2 .

The calculated TPA peak has the same energy for 1-NMe 2 and 2-NMe 2 , and that calculated for 3-NMe 2 is lower by 0.4 eV. Experimentally, this trend is verified, but the computed value is 0.36 eV lower this time. We note also that the SAOP calculated OPA energies are underestimated compared to the experimental OPA energies. Finally, the comparison of the TPA and OPA results (Table 2) shows that the TPA peak energies are calculated at approximately (but not exactly) twice the OPA energies for the studied cyclotrimers, due to the method of calculation.

To assess a possible cooperative effect vs. simple additivity regarding the σ TPA property of each individual arm of the isocyanurate core, we considered two models derived from 1-NMe 2 bearing one or two arms attached to the isocyanurate core instead of three (scheme 1). The onearm ( 5) is a D-π-A dipole with the isocyanurate core as acceptor and a dimethylamino substituent as donor group, while the two-arms (4) and the three-arms (1-NMe 2 ) contain respectively two and three of these D-π-A units (by sharing the acceptor group). First of all, it can be noted that these three species exhibit almost the same HOMO-LUMO gap (Fig. S1), in line with a weak electronic coupling between the various arms. [38] The OPA and TPA results obtained are given in Table 3 and Fig. 4. a Experimental values for the TPA cross-sections have been obtained by twophoton excited fluorescence (TPEF) on a similar equipment for all compounds in dichloromethane using fs pulses. See ref. [25] for typical conditions. b Work in progress (to be published). We find that 1-NMe 2 displays a σ TPA value which is 2.5 times larger than that of 4 which exhibits itself a value three times larger than that of 5. This result is in good agreement with experiment which indicates that the measured σ TPA of 1-NMe 2 is 3 times higher than the measured one for compound 4.

We note that the TPA peak energy for 4 deviates by 0.07 eV compared to its congeners 5 and 1-NMe 2 . It appears then that the TPA cross-section increases significantly as the number of arms As a result, simple additivity does not apply in our case for σ TPA suggesting existence of a positive synergy between the arms. Such cooperative enhancement has already been observed experimentally in the case of planar triazine derivatives exhibiting one, two and three arms attached to the triazine core [39]. This issue has also been studied earlier theoretically by Beljonne et al. for related compounds having a C 3 symmetry axis [40]. It can be traced back to the octupolar symmetry of these derivatives.

Conclusions

In this work, we have reported a theoretical DFT study of the TPA properties of promising octupoles, namely triphenyl-isocyanurate, 1,3,5-triphenyl-benzene and triphenyl-triazine derivatives functionalized by extended conjugated arms at their peripheral para-positions. These compounds exhibit a three-fold axis and contain attached arms in 1, 3 and 5 positions, each of them bearing a terminal electron donating group.

The used SAOP functional provided a good qualitative estimate of the TPA cross-sections and energies. To our knowledge, it is the first time that such TPA computations are carried out for octupolar molecules at this level of theory. In agreement with experiment, it is found that an increase of the strength of the donor terminal group (replacing OMe and NH 2 by NMe 2 ) enhances the TPA cross-section. The calculated TPA properties bring to light a small difference for σ TPA when replacing the isocyanurate ring by the phenyl one. In addition, the compound with the triazine core exhibits a much higher TPA cross-section value than the two other compounds with isocyanurate and phenyl cores. In all cases, the HOMO-LUMO gap is a determining factor. This property is for a large part related to the planarity of the considered systems, the triazine core system being perfectly planar contrary to the two other systems. We found also that the twophoton absorption cross-section increases more significantly than three time that of the model compound (5) with only one electron-releasing arm, indicating the existence of a cooperative enhancement of the TPA cross-sections and not a simple additivity of the σ TPA of each arm.

Supplementary data

A c c e p t e d m a n u s c r i p t Scheme 1 .

 1 Scheme 1. Structures of the derivatives under study

Fig. 1

 1 Fig. 1 Optimized geometries in CH 2 Cl 2 of the derivatives under study.

A c c e p t e d m a n u s c r i p t

a

  Experimental values for the TPA cross-sections have been obtained by two-photon excited fluorescence (TPEF) on a similar equipment for all compounds in dichloromethane using fs pulses except for 1-NH 2 for which the TPA cross-section has been derived by Z-scan (open aperture) measurements. See refs.[25] and[28] for typical TPEF and Z-scan conditions, respectively. b Work in progress (to be published).

A c c e p t e d m a n u s c r i p tFig. 2

 2 Fig. 2 Theoretical TPA profiles for 1-OMe (green line), 1-NH 2 (red line), 1-NMe 2 (blue line), 2-NMe 2 (purple line) and 3-NMe 2 (orange line) computed at the SAOP/DZP level.

compounds 1 -NH 2 , 1 -OMe, 1 -NMe 2 and 2 -

 12112 NMe 2 are correctly ranked regarding σ TPA . We must remind that our calculations consider isolated molecules whereas the measured TPA crosssections are made in solution. Experimental data indicate that increasing the strength of the terminal donor group X namely replacing NH 2 by NMe 2 in the 1-X cyclotrimers enhances the TPA cross-section, as experimentally observed; the theoretical computations indicate the same increase of σ TPA when going from 1-NH 2 to 1-NMe 2 .

Fig. 4

 4 Fig. 4 Theoretical TPA profiles of 1-NMe 2 (blue line), 4 (green line) and 5 (red line) at SAOP/DZP level

A c c e p t e d m a n u

  s c r i p t increases, the two-photon absorption of one arm and the core (i.e. a single D-π-A unit) being modelled by 4.

A c c e p t e d m a n u s c r i p t The HOMO-LUMO gaps for 1 -NMe 2 , 4 and 5 ,

 125 Cartesian coordinates of the optimized geometries of the studied systems.

A c c e p t e d m a n u s c r i p t

Table 1

 1 Calculated mean deviation from planarity of the studied systems

	Cpds	Mean deviation (°)
	4	81
	5	89
	1-NMe 2	70
	2-NMe 2

Table 2

 2 Experimental and calculated OPA, TPA energies ω (eV) and cross-sections σ

TPA (GM) of 1-X (X = OMe, NH 2 , NMe 2 ),

2-NMe 2 and 3-NMe 2

			Experimental a			Theoretical
	Cpds	ω (eV)	σ TPA (GM) Refs	ω (eV)	σ TPA (GM)
		OPA	TPA			OPA	TPA
	1-OMe	-	-	-	-	2.93	1.85	860
	1-NH 2	3.80	1.77	130	28	2.76	1.77	1071
	1-NMe 2	3.52	1.72	360	25	2.63	1.55	4246
	2-NMe 2	3.47	1.67	380	26	2.44	1.55	4322
	3-NMe 2	3.06	1.51	1250

Table 3

 3 Calculated OPA, TPA energies ω (eV) and cross-sections σ TPA of the 1-NMe 2 , one arm (compound 5) and two arms (compound 4).

	m a n u s c r i p t
	Experimental a A c c e p t e d Cpds	Theoretical
	ω(eV)	ω(eV)	σ TPA (GM)

Acknowledgements

The authors are grateful to Prof. Lasse Jensen for his help regarding TPA calculations using the AORESPONSE module of ADF2017. The financial support of ANR (Isogate Project) is acknowledged as well as GENCI-IDRIS and GENCI-CINES for an allocation of computing time (Grant No. 2017-2018-080649).