Michaela Fecková

Pascal Le Poul

Filip Bureš

Françoise Robin-Le Guen

Nonlinear optical properties of pyrimidine chromophores

Keywords: Pyrimidine, Two-photon absorption, Second harmonic generation, Nonlinear optic

The large library of nonlinear optical (NLO) chromophores based on pyrimidine scaffold is discussed in this review. Since the first examples described thirteen years ago, the interest in pyrimidine push-pull NLO chromophores has increased dramatically, especially for second harmonic generation and two photon absorption. More than 130 properly selected chromophores are presented to demonstrate fundamental structure-property relationships and to highlight key applications of pyrimidine-derived materials.

Introduction

Nonlinear optics (NLO) was initially studied theoretically by Maria Göppert-Mayer when she was a PhD student at the University of Göttingen. 1 Experimentally, the NLO began to be studied in the 1960s shortly after the development of lasers: a sufficiently powerful source of coherent light. In 1961, Franken et al. reported for the first time the phenomenon of second harmonic generation (SGH) with a quartz crystal irradiated at λ 0 = 694,3 nm with ruby laser leading to emission of the UV light with doubled frequency (λ 2 = 347,2 nm). [START_REF] Franken | Generation of optical harmonics[END_REF] In the same A c c e p t e d m a n u s c r i p t year, two-photon absorption (TPA), a third-order NLO process, was discovered by Kaiser and Garret. [START_REF] Kaiser | Two-photon excitation in CaF 2 :Eu 2+[END_REF] Third harmonic generation (THG), another third order NLO phenomena, was observed at λ 3 = 231,4 nm with the same ruby laser previously used for SHG, later by Terhune et al. [START_REF] Terhune | Optical generation in calcite[END_REF] On a microscopic scale, when an electromagnetic field ⃗ interacts with a molecule, a polarization resulting in induced dipolar moment ⃗ is given by the following equation:

⃗ = ⃗ + ⃗ + ⃗ + ⃗ + ⋯ Eq. 1
Where :

• ⃗ : the electrical dipolar moment of the molecule in ground state

• : the linear polarizability tensor

• : the second order (quadratic) hyperpolarizability tensor

• : the third order (cubic) hyperpolarizability tensor On a macroscopic scale, the polarization ⃗ of the material can be written: ⃗ = ⃗ + ∑ ( ) ⃗ = ⃗ + ( ) ⃗ + ( ) ⃗ + ( ) ⃗ + ⋯ + ( ) ⃗ Eq.2

Where :

• ⃗ : intrinsic polarization of the material • ( ) : linear (1st order) susceptibility tensor • ( ) , ( ) and ( ) : 2nd order (quadratic), 3rd order (cubic), and nth order non linear susceptibility tensors.

The β and γ terms are related to 2 nd and 3 rd order NLO respectively. NLO effects are only observed for high intensity electric field ⃗ , otherwise n order term are not significant and the polarization can be considered as linearly dependent on ⃗ . It should be noted that ( ) and terms are null in case of centrosymmetric materials or molecules. [START_REF] Boyd | Nonlinear Optics[END_REF] On the other hand, ( ) and terms are always present even if not always observable. Even if numerous inorganic NLO materials such as lithium niobate have been initially used, organic and organometallic derivatives emerged during the past four decades, mainly due to their higher and faster NLO response as well as better tunability. 6 Concerning the 2 nd order NLO, the main applications are based on the sum frequency generation, [START_REF] Morita | Theory of sum frequency generation spectroscopy[END_REF] mainly SHG, Pockels effect and optical rectification. [START_REF] Bass | [END_REF] The most investigated application involves conversion of red laser into a blue/green coherent source of light used for example in the Blu-Ray© technology. [START_REF]White Paper Blu-Ray Disc Format-Physical Format Specifications for BD-ROM[END_REF] Other key applications are related to optical signal processing and integrated optics 10 as well as SHG microscopy. 11 The β tensor is generally expressed in esu (1 esu ≈ 3.34 10 -11 C). Three main methods are used to measure the molecular 2 nd order NLO response. The first one is the electric field induced second harmonic generation (EFISH), the second one is the hyper Rayleigh scattering (HRS), [START_REF] Zyss | Nonlinear optics in multipolar media -theory and experiments[END_REF] while harmonic light scattering (HLS) can also be used. [START_REF] Terhune | Measurements of nonlinear light scattering[END_REF] The HRS and HLS methods provide directly the β tensor in the three directions of space with Cartesian coordinates. This method permits to measure NLO response even if the molecule does not present any permanent dipolar moment such as octupolar compounds. The EFISH method can be applied to non-charged polar molecules and is based on the application of a high external electric field ⃗ on the analytical cell to align the chromophores in the same direction. This method provides the NLO response as the scalar product between the permanent dipolar moment of the molecule ⃗ in fundamental state and the vector component of β described as β // . Dispersion of β, in particular for linear push-pull molecules, can be described by a two-state model according to the following equation: 14 = 1 -1 -Eq. 3

The D-π-A push-pull arrangement is typical of 2 nd order NLO chromophores and the value of the β tensor is closely related to the intramolecular charge transfer (ICT) into the molecule, which can be tuned by varying the donor (D)/acceptor (A) pair interconnected by a πconjugated linker. On a macroscopic scale, 2 nd order NLO/electro-optic materials formed by chromophores dissolved in a polymer matrix, can be obtained by electro-poling. The chromophores are oriented via an external electric field at the temperature elevated above the glass transition temperature of the used polymer matrix and cooled with persistent electric field. [START_REF] Dalton | Electric field poled organic electro-optic materials: state of the art and future prospects[END_REF] As far as the 3 rd order NLO response is concerned, the imaginary part of the γ term is related to multiphoton absorption including TPA: two photons can be absorbed simultaneously by a molecule. [START_REF] He | Multiphoton absorbing materials: molecular design, characterizations and applications[END_REF] The real part of γ will induce the Kerr effect leading to change in the refractive index as a response to the applied electric field. The main applications of TPA span biological imaging, 17 microfabrication, 18 optical limiting, 19 3D optical data storage [START_REF] Cumpston | Two-photon polymerization initiators for three-dimensional optical storage and microfabrication[END_REF] and TPA photodynamic therapy. 21 On the other hand, the Kerr effect is used for all optical data processing. 22 TPA cross-section, expressed in Göppert-Mayer unit (GM = 10 -50 cm 4 .s.photons -1 ) can be measured by the Z-Scan or the two-photon excitation fluorescence (TPEF) methods. The TPEF technique is based on upconverted fluorescence emission. [START_REF] Xu | Measurement of two-photon excitation cross-sections of molecular fluorophores with data from 690 to 1050 nm[END_REF] The Z-scan technique is suitable for non-fluorescent materials as it relies on transformation of phase distortion to amplitude distortion during beam propagation through a nonlinear medium. [START_REF] Ajami | Two-photon absorption cross-section measurements of various two-photon initiators for ultrashort laser radiation applying the Zscan technique[END_REF] Both open-and closed-aperture Z-scan methods can be used for TPA characterization. Closed-aperture Z-scan can also be employed for measuring the real part of the γ tensor. [START_REF] Yin | Determination of nonlinear absoption and refraction by single Z-scan method[END_REF] In case of D-π-A push pull derivatives, the TPA maximum is observed exactly at the double of one-photon absorption maxima, but in case of quadrupolar or octupolar structures, the TPA maxima are generally observed at a lower wavelength than the double of one-photon absorption maxima. In these cases, the lowest energy excitation is theoretically two-photon forbidden. [START_REF] Katan | Effects of (multi)branching of dipolar chromophores on photophysical properties and two-photon absorption[END_REF] In general, the branching effect, with quadrupolar and octupolar structures, significantly enhances the TPA cross-section. [START_REF] Terenziani | Enhanced twophoton absorption of organic chromophores: theoretical and experimental assessments[END_REF] TPA properties are highly solvent dependent: the values of TPA cross section plotted versus the solvent polarity generally show a non-monotonic dependence and can be hardly modeled. On the other hand, theoretical calculations can be very useful to predict and rationalized the NLO properties of organic chromophores.

The pyrimidine ring, also called 1,3-diazine, is a six membered heterocycle with two nitrogen atoms in positions 1 and 3. This heterocycle is present within the structure of many natural and biologically active products 28 and pyrimidine scaffold can also be found in cytosine, thymine and uracil nucleobases. Pyrimidine is a π-deficient heterocycle and the pyrimidin-4-yl and the pyrimidin-2-yl fragments are therefore electron-withdrawing groups as

shown by their Hammett constants (σ P = 0.63 and 0.53), close to the cyano group (σ P = 0.66). [START_REF] Hansch | A survey of Hammett sustituent constants and resonance and field parameters[END_REF] Due to the position of the nitrogen atoms, these fragments exhibit a stronger electron accepting character than other diazinyl moieties. Many push-pull pyrimidine derivatives exhibit interesting optical properties. 30 The electron-withdrawing character of the pyrimidinyl fragment induces a strong emission solvatochromism observed for π-conjugated pyrimidine derivatives substituted by electron-donating groups. 31 Engaging the electron lone pairs of the nitrogen atoms of pyrimidine by protonation, [START_REF] Achelle | Tuning the photophysical properties of push-pull Azaheterocyclic chromophores by protonation : a brief overview of a French-Spanish-Czech project[END_REF] complexation, [START_REF] Hadad | Metal cation complexation studies of 4-arylvinyl-2,6-di(pyridin-yl)pyrimidines : effect on the optical properties[END_REF] or N-methylation [START_REF] Aranda | Vinyl-diazine triphenylamine and their N-methylated derivatives : Synthesis photophysical properties and application for stainig DNA[END_REF] is an easy and useful way to further increase the electron-withdrawing character of the pyrimidine core.

The synthesis of π-conjugated pyrimidine chromophores is now well established and is often based on functionalization of commercially available pyrimidine building blocks by palladium catalyzed cross coupling reactions 35 or condensation reactions. [START_REF] Pascal | Synthesis and characterization of novel para-and meta-phenylenevinylene derivatives: fine tuning of the electronic and optical properties of conjugated materials[END_REF] Many halogensubstituted and methyl substituted pyrimidines are indeed readily available. Moreover, pyrimidine functionalization is often more tunable and selective than in case of well-known 1,3,5-triazine core. 37 In this contribution, we will summarize various examples of pyrimidine NLOphores described during the last fifteen years and try to highlight the key structural parameters that affect their NLO properties. The main applications of pyrimidine NLOphores will also be briefly highlighted. The first part of the review will focus on 2 nd order NLO properties, while the 3 rd order, mainly TPA, properties will be discussed subsequently.

nd order NLO properties

This chapter focuses on pyrimidines with second order NLO properties that are generally measured in solution using EFISH, HRS or HLS techniques. The discussed derivatives are sorted into groups according to their structural similarities. The main aim of this chapter is to discuss and confront the following structural features: i) electron-donating group, ii) length and arrangement of the π-conjugated linker, iii) N-complexation or quaternization of the heterocycle, iv) the number of arms attached to the pyrimidine central core.

4-Styrylpyrimidines 1 (Figure 1) were synthetized in our lab as model push-pull molecules capable of further N-complexation with tungsten carbonyl or N-methylation leading to 2 and

3.

Their fundamental nonlinear optical properties such as µβ and µβ 0 are summarized in Table 1. Chromophores in series 1, compounds 1a, 1b and 1e in particular, vary in the electron donor, which significantly affects the measured µβ and µβ 0 products. These values follow the electron releasing strength of the particular donors within the order of diphenylamino < piperidine < dimethylamino and are consistent with the theoretical calculations. 38 The diphenylamino group in 1b and 1d decreases the µβ values even though this group lowers the ICT energy, which is most likely due to a partial delocalization of the amino lone electron pair into two appended phenyls, while the lower donor strength shifts the ground-state to a less favorable bond alternation. 2). [START_REF] Achelle | Donor-linker-acceptor (D-p-A) diazine chromophores with extended p-conjugated cores: synthesis, photophysical and second order nonlinear optical properties[END_REF] Well-evaluated series 5a-c showed a clear trend in increased µβ when going from OMe to NPh 2 and NMe 2 donors. It is well-known that 2,5-thienylene unit allows efficient intramolecular charge-transfer (ICT) due to its lower resonance energy as compared to Measured in CHCl 3 at 1907 nm c Two level corrected β value. 14 NLOphores with push-pull arrangement based on an aromatic scaffold have been extensively studied. 41 However, the aromatic arrangement is preferred in the ground state and the ICT in aromatic D-π-A systems generally causes loss of the resonance energy. This contradiction reduces effective donor-acceptor coupling and potential NLO response. [START_REF] Marder | Approaches for Optimizing the First Electronic Hyperpolarizability of Conjugated Organic[END_REF] Employment of a pro-aromatic unit/donor, which is capable to gain aromaticity upon the ICT, seems to be very smart solution to overcome this issue. Hence, we have prepared a series of pyrimidine/pyrimidinium chromophores 6/7 (Figure 3, Table 3) bearing pyranylidene proaromatic fragment, and investigated their NLO properties. 43 The effect of proaromatic donor can be demonstrated when comparing µβ values of 6a (400 × 10 -48 esu) with structural analogues 1a (330), 1b (190) and1e ( the π-linker (6a vs. 6b) has detrimental effect on the NLO response. As can be seen from the data gathered in Table 3, further π-linker variation/extension by introducing ferrocene (6d), phenyl(ethynyl) n units (6e, n = 1 and 6g, n = 2), and thienyl-ethynyl (6f) units has also negatively affected the µβ values compared to parent 6a. On the contrary, N-methylation of the pyrimidine ring enhanced the molecular nonlinearity of 7b-g dramatically. Compared to parent 6f, quaternized NLOphore 7f bearing thienylethynyl π-linker possesses the NLO response more than ten-times higher. It should be mentioned that 8a, a quadrupolar 4,6distyryl analogue of compound 6a, gave almost two times higher µβ value (770 × 10 -48 esu). 43a were employed as a part of the π-linker. In general, an incorporation of metal σ-acetylide into the chromophore backbone results in planar arrangement, efficient coupling between the d orbitals of the metal and π * system of the σ-acetylide bridge, pronounced ICT, and significantly increased NLO response. [START_REF] Cifuentes | Alkynyl compounds and nonlinear optics[END_REF] Based on the EFISH measurements shown in Table 4, the following conclusions can be drawn: i) replacement of 1,4-phenylene by So far in this chapter and based on its well-known electron deficiency, the pyrimidine has been utilized as electron-withdrawing part of D-π-A systems. Recently, we have focused on incorporation of the pyrimidine ring into a π-linker as in chromophores 13 (Figure 5, Table 5). [START_REF] Klikar | Dipolar NLO Chromophores Bearing Diazine Rings as π-Conjugated Linkers[END_REF] Whereas the N,N-diphenylamino group was used as donor, cyclopenta [c]thiophen-4,6dione (ThDione) acts as electron-acceptor; both were linked to pyrimidine core at positions 2 It should be noted that the µβ of all previous compounds described in this chapter are positive indicating excited states more polarized than the ground state (µ e > µ g ) and both states are polarized in the same direction. These positive values are in accordance with the emission solvatochromism observed for these compounds.

9b (R = Ph) 10b (R = Ph) 9c (R = tBu) 10c (R = tBu) N N N N I 11a (R = Ph) 12a (R = Ph) N N N N I 11b (R = tBu) 12b (R = tBu) O Ru R R A PPh 2
In 2009, He et al. reported on push-pull chromophores 14 based on 2-aminoazapyrimidines linked to 2-nitrothiazole that are easily available via azo-coupling (Figure 6). [START_REF] He | Novel nonlinearity-transparencythermal stability trade-off of thiazolylazopyrimidine chromophores for nonlinear optical application[END_REF] Their molecular hyperpolarizabilities were investigated by HRS technique (Table 6). Considering the amino donor and nitro acceptor, pyrimidine acts as a part of the π-conjugated path similar to 13. The replacement of the thiazole linker of compound 14a by a phenylene one (compound 14c) significantly reduces the NLO response. The amino group may further be N-substituted with hydroxyethyl to provide anchoring group. 8). [START_REF] Doddamani | Enhancement of nonlinear optical and thermal properties of polyurethanes by modifying the chromophores with fused heterocyclic and pyrimidine rings[END_REF] After corona poling, these materials exhibit SHG -strength and nature of electron-donating group: strong mesomeric and proaromatic electron releasing fragments increase the NLO response.

-length and composition of π-conjugated linker: increasing the length enhances moderately the NLO response.

-number and position of arms around the pyrimidine core: V-shaped 4,6-disubstituted pyrimidine tends to have µβ values more than double compared to their 4-substituted analogues.

-pyrimidine quaternization and complexation increase the NLO response very significantly -using the pyrimidine as a π-conjugated fragment has positive effect on the NLO properties but is dependent on the ring orientation.

Two-photon absorption (TPA)

In general, various structural elements can affect the TPA properties. In this chapter, we will try to highlight the effect of electron-donating substituents, π-conjugated linker, substituent on the pyrimidine core, the branching effect and the concentration of chromophores on the TPA properties of pyrimidine derivatives.

Effect of the electron donating groups

Compounds 16 with a 2-thiomethyl-4,6-distyrylpyrimidine scaffold are displayed in Figure 9. 49 Whereas methoxy and anthracenyl derivatives 16a and 16d exhibit low TPA crosssection, the presence of amino electron-donating groups significantly increases the TPA cross-section up to 901 GM for 16b in MeCN due to the enhanced ICT (Table 8). The presence of a diethylamino group (16b) leads to significantly higher TPA cross-section than imparts dimethylamino group (16c, δ = 352 GM in MeCN). This can be related to the stronger electron-donating character of the diethylamino group. It should be noted that the TPA crosssections of these compounds are largely solvent dependent. 10, Table 9), the TPA cross-section increases in the following order: NMe 2 (17a) < NEt 2 (17b) < NPh 2 (17c). [START_REF] Liu | Synthesis and photophysical properties of novel pyrimidine-based two-photon absorption chromophores[END_REF] Similarly to 16, a strong solvent effect is observed; the highest cross cross-section values were measured in toluene. The TPA properties of compounds 18 and 19 displayed in Figure 11 indicates a strong enhancement of the TPA cross-section when the alkyl chains of the amino groups are replaced by methoxyethoxyethyl chains (Table 10). In this respect, compound 18b (δ = 1450 GM) exhibits higher TPA cross-section than 18a (δ = 786 GM) in MeCN. In benzene, the compound 19b exhibits a TPA cross-section 1.5-times higher than that of 19a. A higher electron donating strength of the latter donor group has been proposed to explain this increase. [START_REF] Zhang | synthesis, linear and nonlinear photophysical properties of novel pyrimidine-based imidazole derivatives New[END_REF] Moreover, the methoxydiethylene glycol chains induce water solubility, a property valuable for biological imaging application. Zhang and coworker have designed a series of (diphenylaminostyryl)pyrimidine derivatives 20 and 21 displayed in Figure 12. [START_REF] Zhang | Nonlinear optical response and two-photon biological applications of a new family of imidazole-pyrimidine derivatives[END_REF] Introducing ethoxy fragments as auxiliary electrondonating group in para position of the phenyl rings of NPh 2 group results surprisingly in emission quenching. Nevertheless, open aperture Z-scan measurements in DMF reveal increased TPA cross-sections for compounds 20b and 21b compared to their unsubstituted analogues 20a and 21a (measured with TPEF method) (Table 11). It should however be mentioned that the TPA cross section measurement can be strongly affected by the method but also by the laser characteristics. The TPA properties of styrylpyrimidine derivatives 6a and 8a (Figure 3) substituted by proaromatic pyranylidene electron-donating groups have also been studied by open-aperture Z-scan technique in dichloromethane (Table 12). The TPA cross-section has been revealed in the same order as for diphenylamino analogues. 43a A series of porphyrin substituted 4,6-diethynylpyrmidines 22 and 23 have been designed

(Figure 13). 55 These compounds exhibit moderate TPA cross-sections but 22 possesses TPA cross-section higher than ethynylporphyrin (~20 GM). [START_REF] Drobizhev | Extremely stron near IR Two-photon absorption in conjugated porphyrin dimmers: quantitative description with three-essential state model[END_REF] Compared to other porphyrin dimers with a butadiyne linker (δ more than 5000 GM), TPA activity of 22 and 23 remains lower. [START_REF] Drobizhev | Extremely stron near IR Two-photon absorption in conjugated porphyrin dimmers: quantitative description with three-essential state model[END_REF][START_REF] Hammerer | Influence of carbohydrate biological vector on the two-photon resonance of porphyrin oligomers[END_REF] It should be noted that compound 22a exhibits two emission bands at 648 [Q(0.0) band] 13 correspond to TPEF collected at 702 nm; the highest recorded cross-section is significantly lower and blue-shifted when collected at 648 nm. In case of compounds 22b-e and 23, the TPA was only studied at 880 nm. This wavelength corresponds to double of the Soret Band on one-photon absorption but may not correspond to the maxima of the TPA spectra, which may explain the lower crosssection compared to 22a. No significant difference has been observed in TPA cross-section between Zn-porphyrin derivatives and their free bases.

Effect of π-conjugated linker

Another key parameter that controls the ICT between the donor and acceptor in push-pull structure is the nature of the π-conjugated linker. [START_REF] Bureš | Fundamental aspects of perperty tuning in push-pull molecules[END_REF] In this context, Chen and coworkers have designed 4,6-bis(thien-2-ylvinyl)pyrimidine derivatives 24 (Figure 14). [START_REF] Chen | A new building block, bis(thiophene vinyl)pyrimidine, for constructing excellent two-photon absorption materials: synthesis, crystal structure and properties[END_REF] These compounds exhibit large cross-section values (Table 14) independently of the peripherical donor (weak for 24b, strong for 24a). It is unfortunately not possible to estimate the influence of the 2,5thienylene linkers since the TPA properties of analogues compounds without these fragments are not known. The comparison of 1,4-phenylene and 2,5-thienylene linkers can be done with tripodal compounds 25 bearing central triphenylamine core and pyrimidine peripheral fragments (Figure 15). [START_REF] Cvejn | Tripolal molecules with triphenylamine core, diazine peripheral groups and extended π-conjugated linkers[END_REF] As expected, 40 the 2,5-thienylene linker leads to a red-shifted TPA band as well as a 1.7-increase of the cross-section in CH 2 Cl 2 (Table 15). These compounds exhibit relatively moderate TPA cross-section but their fluorescence quantum yield is rather interesting (Φ > 0.5 in CH 2 Cl 2 ). 16) allow to compare 1,4-phenylene and fluorenylene π-spacer. [START_REF] Savel | Metalinduced dimentionality tuning in a series of bipyrimidine-based ligands: a tool to enhance two-photon absorption[END_REF] As expected due to the increased conjugated path, the fluorene derivative 26 exhibit a 2.4-higher TPA cross-section than analogue 15h (Table 16). Fluorene fragments have also been incorporated in the scaffold of chromophores 27 (Figure 17). [START_REF] Denneval | Synthesis and photophysical properties of push-pull structures A c c e p t e d m a n u s c r i p t incorporating diazines as attracting part with a fluorine core[END_REF] In these examples, the 4,6-dimethylpyrimidin-2-yl group is used as electronwithdrawing part, while the dimethylamino group is used as electron-donating group. The TPA maxima in the red region of the spectra are slightly red-shifted concerning the double of 17). Surprisingly, addition of two ethynyl linkers between the central fluorene core and the peripheral fragments leads to a significant decrease of the cross-section (27a vs. 27b). On the contrary, replacement of only one triple bond by a triazole unit on the pyrimidine side (compound 27c) increases the TPA cross-section up to 148 GM. When two triazole rings are present on each side of the fluorene (compound 27d), the cross-section is dramatically decreased and the TPA maxima is blue shifted. Influence of the C2 substituent on the pyrimidine core

C 6 H 13 C 6 H 13 N N NMe 2 27a-d X Y 27a N N N N N N N N N X Y _ _ 27b
As seen before, the majority of the pyrimidine chromophores are based on the 4,6distyrylpyrimidine moiety. In these chromophores, the substituent in C2 position of the pyrimidine core can significantly affect the photophysical properties including TPA. Liu and coworkers have designed a series of 4,6-bis(N,N-diethylanimostyryl)pyrimidine derivatives 28a-f with various substituents in C2 position (Figure 18). [START_REF] Liu | Two-photon absorption of a series of V-shape molecules : the influence of acceptor's strength on twophoton absorption in a noncentrosymmetric D-π-A-π-D system[END_REF] The electron-donating substituent in the C2 position such as a p-N,N-dimethylaminophenyl group (compound 28a) tends to decrease the TPA cross-section. On the other hand, electron-withdrawing fragment such as pcyanophenyl (compound 28e) or a pyridin-4-yl (compound 28f) significantly enhances the TPA cross-section up to 720 GM in CHCl 3 (Table 18). The pyrazol-1-yl derivative 28h exhibits cross-section significantly lower than imidazol-1-yl and thiophen-2-yl analogues 19a

and 28g (Figures 11 and 18, Tables 10 and18). [START_REF] Tang | Synthesis, crystal structures, two-photon absorption and biological imaging application of two-novel bentshaped pyrimidine derivatives[END_REF][START_REF] Zhang | synthesis, linear and nonlinear photophysical properties and biological imaging application of a novel Λ-type pyrimiine-based thiophene derivative[END_REF] The TPA cross-section of the latter measured with Z-scan method was significantly higher than with TPEF method. The authors admitted that this value is probably overestimated. This illustrates the difficulties to properly compare data obtained by different methods. Yang and coworkers have designed chromophores 29 (Figure 19) for bioimaging application. [START_REF] Yang | Facile synthesis of a two-photon fluorescent probe based on pyrimidine 2-isocyanate and its application in bioimaging[END_REF] The isocyanate group in compound 29a is intended for coupling with the amino group of a biomolecule. Compound 29b bearing a D-glucosamine pendant in the C2 position of the pyrimidine core, exhibits a TPA cross-section slightly lower (337 GM) than that of the parent isocyanate precursor 29a (512 GM, Table 19).

Branching effect

As presented in the introduction, a cooperative enhancement of TPA is generally observed in multi-branched structures. This is also the case of 4,6-distyrylpyrimidines that exhibit higher TPA cross-section, which is generally more than double of the corresponding 4styrylpyrimidine analogues. For example chromophore 8a showed TPA cross-section of 271 GM, 3.2-times higher than that of 4-styryl analogue 6a (Figure 3, Table 11). 43a Similarly, chromophore 16b and 19b exhibit significantly higher TPA cross-section than their monostyryl analogue 30a and 30b (Figure 21). 49a,51 Distyryl derivatives 21 possess also higher TPA cross-section than compounds 20, despite the cooperative branching effect seems to be lower in this case (Figure 11, Table 10). [START_REF] Zhang | Nonlinear optical response and two-photon biological applications of a new family of imidazole-pyrimidine derivatives[END_REF] A comparison of compounds 31-33 displayed in Figure 21, confirms the significant enhancement of the TPA cross-section when going from mono to distyryl derivatives (Table 20). [START_REF] Kournoutas | Branching effect on the linear and non linear optical properties of styrylpyrimidines[END_REF] Incorporation of a third branch in the C2 position of the pyrimidine core (compounds 33) does not lead to C3 symmetry system and cannot be considered as real octupole. Nevertheless, an enhancement of the TPA response up to 492 GM has been observed for 33b whereas the spectral position of TPA is maintained with regards to distyryl analogues 32. The cooperative branching effect on the TPA cross-section of distyrylpyrimidines can further be demonstrated on compounds 34-35, measured by either TPEF or Z-scan techniques (Figure 22, Table 21). [START_REF] Wang | Synthesis, two-photon absorption properties and bioimaging application of mono-, di-and hexa-branched pyrimidine derivatives[END_REF] Hexabranched molecule 36 exhibits higher TPA cross-section than 35

(870 GM in DMF with TPEF method), however its figure of merit (TPA cross-section per molecular weight) is lower. The TPA properties of 3-dimensional octupolar structures 15b-h (Figure 7) were studied in CH 2 Cl 2 (Tables 16 and22) 47b,60 providing interesting TPA properties. When comparing octupolar bipyrimidine structure 15h with its quadrupolar compound 32b, an enhancement of the TPA cross-section can be noticed, which is, however, less than doubled. The complexation of compounds 15g-h with ZnCl 2 induces a red shift of the TPA band and a decrease of the cross-section by a factor of 1.5 and 1.6, respectively. The complexes maintain a centrosymmetric character but the loss of three-dimensionality caused by their planarization has detrimental effect on the TPA response. In the case of 26 (Figure 16), its Zn complex maintains a 3D octupolar structure, which is attributed to the steric hindrance of the fluorenylvinylene branches as shown by DFT calculation. [START_REF] Latouche | Theoretical evidence of metal-induced structural distortions in a series of bipyrimidine-based ligands[END_REF] This leads to a strong enhancement of the TPA cross-section upon complexation (Table 16 and22 23. [START_REF] Chen | A two-dimentional molecule with large conjugation degree: synthesis, two-photon absorption and charge transport ability[END_REF] The observed TPA cross-section in DMF (1352 GM) is similar to other bipyrimidinebased multibranched structure with strong amino donor such as 15g-h. Another strategy to obtain a media with high TPA cross-section involves enriching the chromophore by more pyrimidine cores or arranging the chromophore either as a nanoparticle or an oligomeric structure. [START_REF] Verlhac | Enanced twophoton brighness in molecular-based organic nanoparticles built from articulated-dipoles[END_REF] In the structure of chromophore 38 (Figure 24), two 4,6distyrylpyrimidine units are linked by a non-conjugated spacer. 49a Surprisingly, this compound exhibits a TPA cross-section in toluene that is 7-times higher than the distyrylpyrimidine parent analogue 16b. Liu and coworkers have designed two phthalocyanine derivatives 39 bearing four pyrimidine fragments (Figure 24). [START_REF] Liu | Synthesis, optical properties and singlet oxygen generation of a phthalocyanine derivative containing strong two-photon absorbing chromophores in the periphery[END_REF] As expected, the chromophore bearing the distyrylpyrimidine fragments (39b) exhibit significantly higher TPA response than the phthalocyanine bearing dimethylpyrimidine moieties (39a) (Table 24).

Nevertheless, the phthalocyanine core does not seem to enhance the cross-section, which is not higher than four independent 4,6-distyrylpyrimidine fragments. The beneficial effect of more pyrimidine cores can also be demonstrated by aforementioned triazine-based chromophore 36 (Figure 22), which possesses TPA cross-section up to 1885 GM (Table 21). 25). [START_REF] Li | Two-photon absorption enhancement induced by aggregation with accurate photophysical data: spontaneous accumulation of dye in silica nanoparticules[END_REF] Surprisingly, the authors observed negligible TPA cross-section for the free dye in the 720-860 nm region and claimed a significant increase of the cross-section for the dye-concentrated nanoparticles (δ = 284 GM) in DMF (Table 25). Malval and coworkers have designed a conjugated copolymer 41 alternating 4,6divinylpyrimidine and triphenylamine part comprising 8 to 10 repeating units. [START_REF] Malval | Two-photon absorption in a conformationally twisted D-π-A oligomer: a synergic photosensitizing approach for multiphoton lithography[END_REF] This compound showed a helical structure leading to a weak interchromophore coupling within the oligomer. A significant cooperative enhancement with exceptional TPA activity (δ = 5093 GM) has been recorded.

Application of 2PA properties of pyrimidine chromophores

The main application of pyrimidine TPA chromophores involves fluorescence microscopy imaging, which requires water-solubility. In this context, Liu and coworkers have designed a series of amphiphilic 4,6-distyrylpyrimidine derivatives 42 and 43 (Figure 26A) decorated with peripheral carboxylates. [START_REF] Liu | Novel pyrimidinebased amphiphilic molecules: synthesis, spectroscopic properties and application in twophoton fluorescence microscopic imaging[END_REF] Single and two-photon fluorescence microscopy imaging experiments revealed that these compounds can effectively stain the cytoplasmic region of Hep-G2 cells (Figure 26B). Similarly, the chromophores 19b, 28g, 29b, 35 and the conjugate of 29a with transferring protein can be used as cytoplasmic stainer for two-photon microscopy. [START_REF] Zhang | synthesis, linear and nonlinear photophysical properties of novel pyrimidine-based imidazole derivatives New[END_REF][START_REF] Tang | Synthesis, crystal structures, two-photon absorption and biological imaging application of two-novel bentshaped pyrimidine derivatives[END_REF][START_REF] Zhang | synthesis, linear and nonlinear photophysical properties and biological imaging application of a novel Λ-type pyrimiine-based thiophene derivative[END_REF][START_REF] Yang | Facile synthesis of a two-photon fluorescent probe based on pyrimidine 2-isocyanate and its application in bioimaging[END_REF][START_REF] Wang | Synthesis, two-photon absorption properties and bioimaging application of mono-, di-and hexa-branched pyrimidine derivatives[END_REF] Compounds 44 (Figure 27), with hexafluorophosphate watersolubilizing groups have also been used for two-photon fluorescence microscopy and possess high specificity for mitochondria. [START_REF] Zhang | Small watersoluble pyrimidine hexafluorophosphate derivatives with high two-photon absorption activities in the near-IR region and their biological applications[END_REF] The chromophore 45 (Figure 27), exhibiting TPA crosssection of 39 GM at 820 nm in water/DMSO mixture, effectively targets the endoplasmic reticulum in living cells. [START_REF] Hu | A molecular probe based on pyrimidine imidazole derivatives for stable super-resolution endoplasmic reticulum imaging in living cells[END_REF] Very similar results were observed with chromophores 20a and 21a. Compounds 47 (Figure 28) were designed as DNA fluorescent binders. [START_REF] Doan | Two-photn spectroscopy as new sensitive method for determining the DNA binding mode of fluorescent nuclear dyes[END_REF] In both cases, a TPA cross-section enhancement up to 13.6-fold is observed in the presence of DNA due to a groove binding mechanism. Data recording experiments with compound 16b proved its potential for optical data storage. 49a,80 The oligomer 41 has been used to improve the two-photon induced polymerization efficiency of a bicomponent photoinitiator system implying a hexaarylbiimidazole used as a H-abstractor and an aliphatic amine used as H-donor. [START_REF] Malval | Two-photon absorption in a conformationally twisted D-π-A oligomer: a synergic photosensitizing approach for multiphoton lithography[END_REF] Pyrimidine chromophores 48 and 49 with porphyrin and ferrocene moieties have revealed to exhibit optical power limiting properties (Figure 29). 81 part Im as well as the effective value of the third-order NLO susceptibility χ [START_REF] Kaiser | Two-photon excitation in CaF 2 :Eu 2+[END_REF] can be obtained. 82 In general, tuning the Re/Im ratio is rather complicated and no clear structural guidelines have been established so far. Only few examples of pyrimidine chromophores have been investigated for their third-order NLO susceptibility γ (Table 26) and, therefore, it is rather less-straightforward to comment on structure-property relationships. When comparing ethoxy-substituted triphenylamine derivatives 20b and 21b, the monostyrylpyrimidine 20b exhibits a higher value of the real part of χ [START_REF] Kaiser | Two-photon excitation in CaF 2 :Eu 2+[END_REF] than its 4,6-distyryl analogue 21b. [START_REF] Zhang | Nonlinear optical response and two-photon biological applications of a new family of imidazole-pyrimidine derivatives[END_REF] For the ferrocene derivative 49, the absolute value of the real part of χ [START_REF] Kaiser | Two-photon excitation in CaF 2 :Eu 2+[END_REF] is 8-times higher than its imaginary part. 81b In case of 48, Z-scan experiments revealed appreciable NLO refraction in addition to the NLO absorption, but no quantitative values were provided. Recently, Z-scan measurement of a co-crystal of 2-amino-4,6-dimethlypyrimidine with 4-nitrophenol 50 (Figure 30) was reported revealing an effective value of third order susceptibility of 2.48×10 -8 esu. 83 In summary, the pyrimidine heterocycle appears as a wonderful platform for the construction of NLO chromophores. In particular methylpyrimidinium push-pull derivatives, that can be easily obtained in a gram scale, seem to be promising material for optical data processing.

Organometallic methylpyrimidinium chromophores incorporating ruthenium fragment within the π-conjugated spacer are among the best metal-diyne NLO chromophores. The 4,6distyrylpyrimidine moiety is now a well-established TPA chromophore with advanced applications, especially for two-photon fluorescence microscopy. The pyrimidine C2 position offers a unique possibility to link various biomolecules. The resulting systems are very promising probes for biosensing of cancer cells or neurodegenerative tissues. In this context, the development of pyrimidine chromophores bearing bioorganic fragments specifically targeting these cells and tissues is probably the key points in the future of this field.

Figure 1

 1 Figure 1 Structures of 4-styrylpyrimidines 1-3 Influence of mesomeric donors such as NMe 2 , NPh 2 and MeO along with the π-system composition on NLO activity has been screened in 4-styrylpyrimidines 4 and 5 (Figure 2, Table2).[START_REF] Achelle | Donor-linker-acceptor (D-p-A) diazine chromophores with extended p-conjugated cores: synthesis, photophysical and second order nonlinear optical properties[END_REF] Well-evaluated series 5a-c showed a clear trend in increased µβ when going from

  phenylene one.40 However, its influence in chromophores 4 and 5 is less straightforward and depends on the used donor. Whereas weaker NLO response has been recorded for N,Ndimethylamino-substituted chromophore 5a (compared to 4a), in the N,N-diphenylamino series b is the situation opposite.

Figure 2 2

 2 Figure 2 2,5-Thienylene vs. 1,4-phenylene linkers in styrylpyrimidines

  290) bearing N,N-dimethylamino, N,N-diphenylamino, and piperidin-1-yl donors. Further pyranylidene replacement by ferrocene (6c) or shortening

Figure 3

 3 Figure 3 Pyrimidines 6-8 with proaromatic pyranylidene donor

Figure 4

 4 Figure 4 Organometallic pyrimidine derivatives 9-12

b

  s c r i p t and 5. Besides pyrimidine, benzene, pyridazine, and 2,5-thienylene were also employed as part of the π-backbone for comparison. The latter allowed the highest ICT from the diphenylamino donor to the ThDione acceptor. However, pyrimidine ICT transparency strongly depends on the mutual orientation of the appended donor and acceptor. Whereas linking ThDione/N,N-diphenylamine at positions 2/5 brings significant NLO response (13a and 13c), the opposite arrangement has detrimental effect on the optical nonlinearity (13b and 13d). It can be explained by lower µ values in these later cases. Further planarization and extension of the π-system by olefinic unit as in 13c/13d play significant role especially in 13c with the µβ value enhanced up to 410×10 -48 esu. NLO response for 13b and 13d are rather similar and in the Table 5: 2 nd order NLO response of chromophores 13 measured by EFISH methodCompds λ abs (nm) a µβ, b 10 -48 esu µβ 0 , c 10 Measured in CHCl 3 at 1907 nm c Two level corrected β value. 14

Figure 5

 5 Figure 5 Pyrimidine as a part of π-conjugated linker

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

Figure 6 2 -

 2 Figure 6 2-Aminopyrimidine azo derivatives 14

  octupolar chromophores 15 bearing styryl units with amino or alkoxy electron donors are displayed in Figure7.47 Results of HRS/HLS measurement are given inTable 7. Noncentrosymmetric chiral derivatives 15a are organized into left-and right-handed supramolecular aggregates/flexible thin films and allow translation of the NLO response from the molecular to the bulk level. 47b Hence, property tuning in 15 (Table7) can be achieved by varying the electron-releasing unit (RO vs. R 2 N) as well as by the structure of R-pendants.NLO response of NEt 2 and NPh 2 derivatives 15g and 15h are similar. Bipyrimidine chromophores 15 also proved to be strongly emissive and TPA active (see TPA part).

Figure 7

 7 Figure 7 Bipyrimidine octupolar push-pull derivatives 15

  s c r i p t coefficient d 33 of 95 and 86 pm/V at 532 nm. These polymers do not show any decrease of their NLO response below 100 °C and retained 95 % of their NLO response even up to 500 h.The presence of the peripheral pyrimidine fragment into the chromophores significantly enhances the NLO response.

Figure 8 :

 8 Figure 8: Structure of polyurethane polymers P1 and P2 To generalize, second-order nonlinear optical properties of pyrimidine-based NLOphores are

  pyrimidine derivatives have also been extensively used as 3 rd order NLOphores, mainly TPA-active molecules. These chromophores are generally based on 4-or 4,6-(di)styrylpyrimidine scaffolds. More limited studies of the real part of the χ[START_REF] Kaiser | Two-photon excitation in CaF 2 :Eu 2+[END_REF] of pyrimidine derivatives have been published so far.

Figure 9 2 -

 92 Figure 9 2-Methylthiodistyrylpyrimidines 16 with TPA activity

Figure 10 2 -

 2 Figure 10 2-Aminodistyrylpyrimidines 17

Figure 11

 11 Figure 11 Distyrylpyrimidines 18 and 19 with various pendants X and peripheral donors

Figure 12 2 -

 122 Figure 12 2-Imidazolylpyrimidines 20 and 21 with triphenylphenylamino donors

  [Q(0.1) band]. The data provided in Table

Figure 13

 13 Figure 13 Ethynylpyrimidine-substituted porphyrins 22 and 23

Figure 14

 14 Figure 14 Structure of bis(thienylvinyl)pyrimidines 24

Figure 15

 15 Figure 15 Tripodal chromophores 25 based on triphenylamine core and pyrimidin-4-yl peripheral acceptors

Figure 16

 16 Figure 16 Branched bipyrimidines 26

Figure 17 :

 17 Figure 17: Fluorene-based chromophores 27

Figure 18 :

 18 Figure 18: Structure of 2-substituted 4,6-bis(N,N-diethylaminostyryl)pyrimidines 28

Figure 19 :

 19 Figure 19: Distyrylpirimidines 29 for bioimaging

Figure 20 :Figure 21 :

 2021 Figure 20 : Monostyryl-substituted chromophores 30

Figure 22 :

 22 Figure 22: Multibranched structures of compounds 34-36

  ).

Figure 23 :Table 23 :

 2323 Figure 23 : Dipyrimidine bithiophene multibranched system 37 Table 23 : TPA properties of dipyrimidine bithiophene multibranched system 37

Figure 24 :

 24 Figure 24: Pyrimidine multicore systems 38 and 39.

Figure 25 :Table 25 :

 2525 Figure 25: Structure of system 40 and 41 Table 25: TPA properties of chromophores 40 and 41

54

 54

Figure 26 :Figure 27 :

 2627 Figure 26: (A) Structure of compounds 42 and 43. (B) Differential interference contrast (right) and two-photon microscopy image (left) of Hep-G2 cells stained with 43b.Reproduced with permission from reference[74] (copyright 2007 Royal Chemical Society)

Figure 28 :

 28 Figure 28 : DNA fluorescent binders 47

Figure 29 :

 29 Figure 29: Pyrimidine chromophores 48 and 49 with optical power limiting properties Other 3 rd order NLO properties As mentioned before, the nonlinear refractive index γ can be measured by the close aperture Z-scan technique. Combined with open aperture Z-scan technique, the real Re and imaginary

Figure 30 : 50 Conclusion

 3050 Figure 30: Structure of co-crystal 50

Table 1 :

 1 2 nd order NLO response of chromophores 1

	m a n u s c r i p t
	A c c e p t e d
	38a However, the calculated bond length alternation (BLA) values
	of the vinylic linker are similar and, therefore this phenomenon would needs further
	investigation. 38c A coordination of the pyrimidine ring by tungsten pentacarbonyl afforded
	2a-d with significantly improved NLO responses (Table 1). Moreover, N-methylation as in

-3 measured by EFISH method Compds λ abs (nm) a µβ, b 10 -48 esu µβ 0 , c 10 -30 esu Ref 1a

  Measured in CHCl 3 at 1907 nm c Two level corrected β value.14

		393	330	262	38a
	1b	400	190	150	38a
	1c	386	470	377	38a
	1d	384	200	161	38b
	1e	386	290	233	38a
	2a	462	770	555	38b
	2b	462	700	504	38b
	2c	443	490	363	38b
	2d	435	460	345	38b
	3b	564	2350	1394	38b
	3d	520	1080	702	38b
	a Measured in CH 2 Cl 2	, b

Table 2 :

 2 2 nd order NLO response of chromophores 4

-5 measured by EFISH method Compds λ abs (nm) a µβ, b 10 -48 esu µβ 0 , c 10 -30 esu

					Ref
	4a	491	560	446	39
	4b	494	280	222	39
	5a	419	440	338	39
	5b	416	400	308	39
	5c	484	150	121	39
	a Measured in CH 2 Cl 2	, b

Table 3 :

 3 2 nd order NLO response of chromophores 6

-8 measured by EFISH method Compds λ abs (nm) a µβ, b 10 -48 esu µβ 0 , c 10 -30 esu

  Measured in CHCl 3 (in CH 2 Cl 2 for 6a, 6c and 8a) b Measured in CHCl 3 at 1907 nm c Two level corrected β value. 14 d No stable µβ value was obtained. Aforementioned acetylene derivatives were further used to prepare organometallic derivatives 9-12 whose structures are presented in Figure 4.

	6g	410	160	120	43d
	7b 7c	624 626	2220 < 550 d	1133 < 279 d	43b 43b
	7d	684	1320	558	43b
	7e	540	950	594	43c
	7f	579	2300	1318	43c
	7g	540	770	480	43d
	8a	458	770	480	43a
					Ref
	6a	436	400	300	43a
	6b	452	310	227	43b
	6c	482	160	149	38a
	6d	512	60	40	43b
	6e	408	380	295	43c
	6f	383	220	177	43c

A c c e p t e d m a n u s c r i p t a 43b-d Metal σ-acetylide fragments are generally used as electron-releasing part but Pt(P(Tol) 3 ) 2 in 9-10 and Ru(dppe) 2 in 11-12

Table 4 :

 4 2 nd order NLO response of chromophores 9-12 measured by EFISH method

Compds λ abs (nm) a µβ, b 10 -48 esu µβ 0 , c 10 -30 esu

  Measured in CHCl 3 at 1907 nm c Two level corrected β value.14

					Ref
	9a	407	260	203	43c
	9b	436	170	127	43c
	9c	398	390	308	43c
	10a	527	2000	1283	43c
	10b	584	3600	2039	43c
	10c	521	3600	2338	43c
	11a	458	1700	1200	43d
	11b	460	2200	1600	43d
	12a	683	14000	5900	43d
	12b	678	14000	6000	43d
	a Measured in CHCl 3	b

Table 6 :

 6 2 nd order NLO response of chromophores 14 measured by HRS technique Measured in DMF b Measured in DMF at 1064 nm c Two level corrected β value. 14

	Compds	λ abs (nm) a	β HRS , b 10 -30 esu	β 0 , c 10 -30 esu	Ref
	14a	544	510	316	46
	14b	540	500	312	46
	14c	475	80	56	46

a

Table 7 : 2 nd

 7

order NLO response of chromophores 15 measured by HRS/HLS

technique Compds λ abs (nm) a λ laser (nm) β HRS , a 10 -30 esu β 0 , c 10 -30 esu

  Measured in CH 2 Cl 2 , b HLS technique was used instead of HRS

						Ref
	(R)-15a	-	800	670 ± 190	48 ± 14	47a
	(S)-15a	-	800	700 ± 160	66 ± 15	47a
	15c	373	800	504 ± 80	38 ± 8	47b
	15d 15g 15h	379 437 421	800 1640 1640	485 ± 25 166 ± 33 b 190 ± 38 b	42 ± 7 110 130	47b 47c 47c

a c Three level corrected β value for octupole Doddamani and coworkers have described polyurethanes P1 and P2 incorporating a pyrimidine-based chromophore (Figure

Table 8 :

 8 TPA properties of methylthiodistyrylpyrimidines 16

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

Table 9 :

 9 TPA properties of 2-

mminodistyrylpyrimidines 17 Compds Technique Solvent λ TPA (nm) δ (GM)

						Ref
	17a	TPEF	Toluene	800	475	50
	17a	TPEF	MeCN	840	366	50
	17b	TPEF	Toluene	800	596	50
	17b	TPEF	MeCN	840	461	50
	17c	TPEF	Toluene	800	819	50
	17c	TPEF	MeCN	840	625	50

Table 10 :

 10 TPA properties of distyrylpyrimidines 18 and 19

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	18a	TPEF	MeCN	850	786	52
	18b	TPEF	MeCN	820	1450	52
	19a	TPEF	Benzene	840	1890	53
	19a	TPEF	MeCN	840	620	53
	19b	TPEF	Benzene	840	2844	51
	19b	TPEF	THF	840	3335	51

Table 11 :

 11 TPA properties of 2-Imidazolylpyrimidines 20 and 21

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	20a	TPEF	DMF	890	120	54
	20b	Z Scan	DMF	730	490	54
	21a	TPEF	DMF	890	138	54
	21b	Z Scan	DMF	820	641	54

Table 11 :

 11 TPA properties of pyranylidene derivatives 6a and 8a

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	6a	Z Scan	CH 2 Cl 2	880	86	43a
	8a	Z Scan	CH 2 Cl 2	900	271	43a

Table 13 :

 13 TPA properties of ethynylpyrimidine-substituted porphyrins

22 and 23 Compds Technique Solvent λ TPA (nm) δ (GM)

						Ref
	22a	TPEF	CH 2 Cl 2	930	120	55a
	22b	TPEF	CHCl 3	880	57	55b
	22c	TPEF	CHCl 3	880	36	55b
	22d	TPEF	CHCl 3	880	76	55b
	22e	TPEF	CHCl 3	880	82	55b
	23a	TPEF	CHCl 3	880	66	55b
	23b	TPEF	CHCl 3	880	80	55b

Table 14 :

 14 TPA

	Ref

properties of bis(thienylvinyl)pyrimidines 24

Compds

Technique Solvent λ TPA (nm) δ (GM)

Table 15 :

 15 TPA properties of tripodal chromophores 25

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	25a	TPEF	CH 2 Cl 2	750	240	60
	25b	TPEF	CH 2 Cl 2	800	410	60
	Bipyrimidine chromophores 15g-h and 26 designed by Savel and coworkers (Figures 7 and

Table 16 :

 16 TPA properties of bipyrimidine chromophores

15h-g and 26 Compds Technique Solvent λ TPA (nm) δ (GM)

						Ref
	15g	TPEF	CHCl 3	775	530	61
	15h	TPEF	CHCl 3	790	460	61
	26	TPEF	CHCl 3	790/860/960	1022/880/41	61

Table 17 :

 17 TPA properties of fluorene-based chromophores 27

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	27a	TPEF	CH 2 Cl 2	740	123	62
	27b	TPEF	CH 2 Cl 2	760	82	62
	27c	TPEF	CH 2 Cl 2	760	148	62
	27d	TPEF	CH 2 Cl 2	700	39	62

Table 18 :

 18 TPA

	Ref

properties of 4,6-bis(N,N-diethylaminostyryl)pyrimidines 28 Compds Technique Solvent λ TPA (nm) δ (GM)

Table 19 :

 19 TPA properties of distyrylpirimidines 29

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	29a	TPEF	CH 2 Cl 2	820	512	65
	29b	TPEF	CH 2 Cl 2	810	337	65

Table 20 :

 20 TPA properties of chromophores 30-33

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	30a	TPEF	Toluene	-	16	49a
	30a	TPEF	MeCN	-	83	49a
	30b	TPEF	Benzene	840	842	51
	30b	TPEF	THF	860	1273	51
	31a	TPEF	CH 2 Cl 2	795	73	66
	31b	TPEF	CH 2 Cl 2	795	109	66
	32a	TPEF	CH 2 Cl 2	790	235	66
	32b	TPEF	CH 2 Cl 2	790	387	66
	33a	TPEF	CH 2 Cl 2	775	324	66
	33b	TPEF	CH 2 Cl 2	775	492	66

Table 21 :

 21 TPA properties of chromophores 34-36

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	34	TPEF	DMF	-	151	67
	34	Z Scan	DMF	830	350	67
	35	TPEF	DMF	840	1319	67
	35	Z Scan	DMF	750	1000	67
	36	TPEF	DMF	870	1885	67
	36	Z Scan	DMF	750	1000	67

Table 22 :

 22 TPA properties of bipyrimidines 15b-f and Zn complexes of bipyrimidines 15g-h and 26.Chen et al. have described another 2D multibranched conjugated system 37 depicted in Figure

	Compds Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	15b	TPEF	CH 2 Cl 2	705	325	47b
	15c	TPEF	CH 2 Cl 2	720	380	47b
	15d	TPEF	CH 2 Cl 2	705	372	47b
	15e	TPEF	CH 2 Cl 2	690	118	47b
	15f	TPEF	CH 2 Cl 2	680	113	47b
	Zn15g	TPEF	CH 2 Cl 2	820	353	61
	Zn15h	TPEF	CH 2 Cl 2	840	280	61
	Zn26	TPEF	CH 2 Cl 2	800/870/980	733/19996/774	61

Table 24 :

 24 TPA properties of pyrimidine multicore systems 38 and 39.

	Compds	Technique Solvent	λ TPA (nm)	δ (GM)	Ref
	38	TPEF	Toluene	840	2280	49a
	38	TPEF	MeCN	-	1890	49a
	39a	Z Scan	DMF	830	377	71
	39b	Z Scan	DMF	870	1153	71
	Li et al. have designed dye-concentrated nanoparticles (DCNs) 40 giving rise to a large
	enhancement in TPA properties (Figure

Table 26 :

 26 Third-order NLO data for compounds 20b, 21b, 28g, 49 and 50 obtained by Z-scan measurements.

	Compds	Solvent	λ ex (

nm) γ (m 2 W -1 ) Re (10 -8 esu) Im (10 -8 esu) χ (3) (10 -8 esu) Ref

	20b 21b 58g 49	DMF DMF DMF DMF	730 820 780 920	8.21 10 -9 2.23 10 -9 6.69 10 -16 -3.35 10 -14	4.26 1.16 3.47 10 -6 -1.74	3.36 4.19 1.45 0.218	5.42 4.35 1.45 1.75	53 53 63 80b
	50	crystal	532				2.48	83

Acknowledgements

M. F. acknowledges the Région Bretagne, France, for her Ph.D. funding (HOLED project)


Yixuan Zhao

Gaël Talbi

Sébastien Clément

Pierre Toulemonde

Thomas Hansen

Martine Cambon

Olivier Cambon

Mickael Beaudhuin

Romain Viennois

Julien Haines
email: julien.haines@umontpellier.fr

Mickaël Beaudhuin

High-pressure, high temperature synthesis of a mesoporous α-quartz/bismuth nanowire composite

Keywords: Nanocomposites, Silica nanotubes, High Pressure, Guest Insertion, Bismuth

Introduction

Nanostructuration has been proposed as a solution to improve the efficiency of thermoelectric materials, which are characterized by their figure of merit ZT=α 2 T/ρλ with α being the Seebeck coefficient, T the temperature, ρ the electrical resistivity and λ the thermal conductivity. [1,2] The decrease of the dimensionality not only can lower the thermal conductivity by increasing the phonon scattering, but also can improve the electronic properties by quantum confinement. [1][2][3][4][5][START_REF] Dresselhaus | Chapter 1 -Quantum Wells and Quantum Wires for Potential Thermoelectric Applications[END_REF] Bismuth and its alloys are materials of choice for thermoelectric applications at low temperature in Peltier devices for cooling of optoelectronic systems, superconductors and for thermal management of electronic devices. [5,[START_REF] Yim | [END_REF][START_REF] Lenoir | Chapter 4 An overview of recent developments for BiSb Alloys[END_REF] Bismuth nanowires have been prepared using different techniques such as physisorption, vapor phase deposition, liquid phase deposition with organic liquids and liquid injection under high pressure [5,[START_REF] Bogomolov | [END_REF][START_REF] Huber | International Conference on Thermoelectrics (IEEE Cat. No. 03TH8726[END_REF][START_REF] Vedernikov | Modules, Systems, and Applications in Thermoelectrics[END_REF][START_REF] Borisov | Proceedings of XIX Session of the Russian Acoustical Society[END_REF][START_REF] Ivanova | [END_REF][14][15].

Although more efficient for obtaining complete pore filling of the mesoporous matrices, the last technique has been investigated in a limited number of studies [START_REF] Bogomolov | [END_REF][START_REF] Huber | International Conference on Thermoelectrics (IEEE Cat. No. 03TH8726[END_REF][START_REF] Vedernikov | Modules, Systems, and Applications in Thermoelectrics[END_REF][START_REF] Borisov | Proceedings of XIX Session of the Russian Acoustical Society[END_REF][START_REF] Ivanova | [END_REF]. It was shown that molten Bi can be inserted in the nanometric channels of porous materials at high pressure and high temperature. [5,[START_REF] Bogomolov | [END_REF][START_REF] Huber | International Conference on Thermoelectrics (IEEE Cat. No. 03TH8726[END_REF][START_REF] Vedernikov | Modules, Systems, and Applications in Thermoelectrics[END_REF][START_REF] Borisov | Proceedings of XIX Session of the Russian Acoustical Society[END_REF][START_REF] Ivanova | [END_REF] Various systems have been studied as host for insertion of liquid Bi including mesoporous silica, chrysotile asbestos, Vycor glass and opal. [START_REF] Bogomolov | [END_REF][START_REF] Huber | International Conference on Thermoelectrics (IEEE Cat. No. 03TH8726[END_REF][START_REF] Vedernikov | Modules, Systems, and Applications in Thermoelectrics[END_REF][START_REF] Borisov | Proceedings of XIX Session of the Russian Acoustical Society[END_REF][START_REF] Ivanova | [END_REF] These materials are ideal for designing a network of Bi nanowires. This nanostructuration resulted in major changes to the electrical and thermal conductivities and the Seebeck coefficient. [5,[START_REF] Bogomolov | [END_REF][START_REF] Vedernikov | Modules, Systems, and Applications in Thermoelectrics[END_REF]16] In the present study, high pressure and high temperature are used to inject liquid bismuth in 4-6 nm diameter pure amorphous silica nanotubes. The critical insertion pressure was estimated to be about 0.1 GPa based on the work of Bogomolov. [START_REF] Bogomolov | [END_REF] The Bi insertion was performed in a diamond anvil cell and millimeter-sized samples were prepared in a conac28 large volume press. The successful synthesis of Bi nanowires was confirmed by transmission electron microscopy (TEM). Neutron powder diffraction (NPD) provided structural information on the phases present in the synthesized nanocomposite and preliminary electronic properties were measured.

Experimental details

The silica nanotubes were prepared from pure chrysotile fibers by using a nitric acid solution [17]. Then, the nanotubes were washed with distilled water and dried in air at 600 °C for 12 hours to remove water and hydroxyl groups from the silica walls of the nanotubes (Fig. 1).

A resistively-heated, membrane diamond anvil cell (DAC) was used for the high pressure, high temperature experiments. Ruby and SrB4O7:Sm 2+ were placed in the 200 μm hole in a 115 μm thick rhenium gasket placed on one diamond anvil. These materials were used to measure the pressure and temperature by optical luminescence. [18][19][20] NaCl was then added and pressed to form a pellet to protect these materials from the molten bismuth. Half of the pellet was then removed and the silica nanotubes and Bi powder were added in a glovebox under argon. A programmable heater controller was used to control the heater power and a thermocouple placed inside the heater near the diamond was used to measure the temperature.

The pressure was increased to 0.5 GPa and the bismuth was melted at close to 500 K, then the liquid Bi was inserted in the nanotubes by increasing the pressure to 2.4 GPa at temperatures around 550 K.

Preparation of millimeter-sized sample Bi/silica nanotubes was carried out using a conac28 high-pressure apparatus. A mixture of bismuth and silica nanotubes with approximately a 1:3 volume ratio was placed in a 5mm inner diameter PTFE capsule in a glovebox and sealed under vacuum in a glass tube.

The tube was then heated in order to melt the bismuth to form a compact sample. The PTFE capsule was inserted in the graphite furnace of the lithographic stone gasket, which was then placed between the two WC anvils of the conac press. Pressure was increased to 1 GPa to connect the graphite furnace. The temperature was increased to 613 K to ensure melting of Bi followed by pressure increase up to 2.1 GPa for insertion. The PTFE capsule was intact and was easily removed after the high P-T treatment. The melting point of PTFE rises steeply with pressure [21] and already reaches 692 K at 0.06 GPa, and will be even higher between 1 and 2 GPa at which the heating was performed. The diameter and thickness of the sample were 4.494 mm and 0.674 mm, respectively. The P-T conditions used ensure both filling of the nanotubes and full densification of the resulting pellet.

The millimeter-sized samples were studied by NPD on the D20 diffractometer at the Institut Laue

Langevin. An incident wavelength of 1.87Å was used. Acquisition times were of the order of 30-60 minutes. Fullprof [START_REF] Rodriguez-Carvajal | Applied Crystallography[END_REF] was used to perform Le Bail fit in order to obtain the unit cell parameters.

The recovered sample from the DAC was placed in IR-white Resin (Sigma Aldrich). This sample and the millimeter-sized samples were cut into 70 nm slices using ultramicrotomy. TEM and electron diffraction measurements were performed with FEG JEOL 2200 FS -200 KV electron microscope equipped a CCD GATAN USC camera with 4092×4092 pixels and a 200 kV electron gun. The probe diameter in STEM mode is about 1nm with a resolution, which can reach 1Å.

The electrical resistivity and the Seebeck coefficient were measured at room temperature by using respectively the Van der Pauw and the hot probe method on the millimeter-sized nanocomposite.

Results and discussion

The initial sample consists of nanotubes with typical outer and inner diameters of 30-35 nm and 4-6 nm [17], respectively (Fig. 1). The TEM results on the Bi containing samples after HP-HT treatment (Fig. 2) show that Bi atoms had been inserted in the initial nanotubes and form nanowires with a diameter of 5-6 nm (Fig. 3) corresponding to the inner diameters of initial nanotube. Some nanowires are found to retain the initial curvature of the chrysotile fibers (Fig. 3). The distance between the atomic planes in the nanotubes are found to be in the order of 6-7 Å, which is similar to the Bi-Bi distance along the c direction in the rhombohedral form. STEM-EDX results confirm that the Bi nanowires are surrounded by SiO2. Importantly, the oxygen distribution is identical to that of Si. Electron diffraction data (Fig. 4) from zones with the same morphology as those corresponding to SiO2 could be indexed based on the -quartz lattice (Table 1) indicating that the nanotubes had crystallized in the form of the -quartz.

Due to the high level of absorption of bismuth in the visible and x-ray regions of the spectrum, NPD appeared to be the technique of choice to obtain structural information on the synthesized nanocomposite. Two phases were observed in the NDP pattern of the mm-sized nanocomposite prepared at 2.1 GPa and 613 K (Fig. 5), highly oriented rhombohedral bismuth (cell parameters: a= 4.5518(5) Å, c= 11.882(1) Å) and -quartz (cell parameters: a= 4.9217(6) Å, c= 5.4169(9) Å) obtained due to the crystallization of the silica nanotubes. The strong preferred orientation present did not allow quantitative analysis to be performed using the Rietveld method.

Preliminary global measurements of the electronic properties were performed on the millimetre-sized nanocomposite. The measured electrical resistivity of 0.3 mΩ.cm is 2.5 times higher than that of bulk Bi. [START_REF] Es-Said | [END_REF] It is known that Bi nanowires present a higher resistivity than the bulk form due to quantum confinement, which induces a semimetal to semiconductor transition. [24][25][26][27] Therefore, this higher measured resistivity may indicate that the confined Bi nanowires increase the electrical resistivity of the nanocomposite. In addition, the insulating quartz also contributes to composite's resistivity. The value obtained for the Seebeck coefficient of -46 ± 2 μV/K (Fig. 6) is comparable to values obtained for Bi nanowires with larger diameters in previous work. [5,[28][29][30][31] This negative value indicate that electrons are the major charge transport carriers. Finally, the thermoelectric power factor for the nanocomposite is about 11 μW/cmK 2 .

Conclusion

These results clearly show that the HP-HT treatment resulted in the insertion of the liquid bismuth to form 5-6 nm wires and in the crystallization of the nanotubes to form -quartz. This nanostructuration can be expected to modify the properties of Bi with respect to its bulk form. In addition, as in previous work [15], selective dissolution may provide a route to obtain isolated 5-6 nm bismuth nanowires. According to theoretical calculations [24], these nanowires may have an exceptional thermoelectric efficiency due to their modified physical properties such as electronic conductivity, Seebeck coefficient and thermal conductivity. Preliminary global electrical measurements on a nanocomposite pellet are consistent with the properties expected due to nanostructuration.

Fig. 1 .Fig 2 .

 12 Fig. 1. TEM images of pure SiO2-nanotubes after acid treatment of chrysotile fibers

Fig. 3 .

 3 Fig. 3. TEM images of Bi nanowires surrounded by SiO2. A typical zone retaining the curvature of the

Fig. 4 .Fig. 2 .Fig. 5 .

 425 Fig. 4. Electron diffraction pattern of zones with the same morphology as the SiO2 containing zones in

Fig. 6 .

 6 Fig.6. Voltage-temperature difference plot for the Bi-filled nanotubes.

Table 1 .

 1 Electron diffraction data for a SiO2-containing zone in Bi-filled nanotubes with indexation based on an -quartz lattice.

	d (Å)

Acknowledgments

We would like to thank Véronique Viguier and Erwan Oliviero from the "Plateforme de Microsocpie Electronique et Analytique" from the Université de Montpellier and Murielle Legendre from Institut Néel.