Nonlinear Optical Pigments. Two-Photon Absorption in Crosslinked Conjugated Polymers and Prospects for Remote Nonlinear Optical Thermometry

Abstract

Nonlinear optical (NLO) pigments are compounds insoluble in solvents that exhibit phenomena related to nonlinear optical susceptibilities (χ(n) where n = 2,3,...), e.g., two-photon absorption (2PA) which is related to the imaginary part of χ(3). Determination of spectrally-resolved 2PA properties for NLO pigments of macromolecular nature, such as coordination polymers or crosslinked polymers, has long been a challenging issue due to their particulate form, precluding characterizations with standard techniques such as Z-scan. In this contribution, we investigate thus far unknown spectrally-resolved 2PA properties of a new subclass of NLO pigments—crosslinked conjugated polymers. The studied compounds are built up from electron-donating (triphenylamine) and electron-withdrawing (2,2’-bipyridine) structural fragments joined by vinylene (Pol1) or vinyl(4-ethynylphenyl) (Pol2) aromatic bridges. 2PA properties of these polymers have been characterized in broad spectral range by specially modified two-photon excited fluorescence (TPEF) techniques: solid state TPEF (SSTPEF) and internal standard TPEF (ISTPEF). The impact of self-aggregation of aromatic backbones on the 2PA properties of the polymers has been evaluated through extended comparisons of NLO parameters, i.e., 2PA cross sections (σ2) and molar-mass normalized 2PA merit factors (σ2/M) with those of small-molecular model compounds: Mod1 and Mod2. By doing this, we found that the 2PA response of Pol1 and Pol2 is improved 2–3 times versus respective model compounds in the solid state form. Further comparisons with 2PA results collected for diluted solutions of Mod1 and Mod2 supports the notion that self-aggregated structure contributes to the observed enhancement of 2PA response. On the other hand, it is clear that Pol1 and Pol2 suffer from aggregation-caused quenching phenomenon, well reflected in time-resolved fluorescence properties as well as in relatively low values of quantum yield of fluorescence. Accordingly, despite improved intrinsic 2PA response, the effective intensity of two-photon excited emission for Pol1 and Pol2 is slightly lower relative to Mod1 and Mod2. Finally, we explore temperature-resolved luminescence properties under one- (377 nm), two- (820 nm), and three-photon excitation (1020 nm) conditions of postsynthetically Eu3+-functionalized material, Pol1-Eu, and discuss its suitability for temperature sensing applications.

Nonlinear Optical Properties of Emerging Nano‐ and Microcrystalline Materials

“…Our group proposed ISTPEF (internal standard two-photon excited fluorescence) for measurements of 2PA cross sections of luminescent dispersions of microcrystalline CPs in solvents, [6a] a technique that was later employed for characterization of other two-photon active nonsoluble compounds (NLO pigments) such as crosslinked conjugated polymers. [7] The Z-scan method has been for the first time employed to characterize optical nonlinearities of CPs in the nanoparticle form, as was demonstrated for strongly threephoton absorbing Prussian Blue (ferric hexacyanoferrate(II)) NPs [24] as well as for ZIF-8 NPs, which turned out to be weak two-photon absorbers. [25] The above and other aspects of nonlinear optics of CPs, although thematically scattered, provided the first signposts on what to look and where to look for in future studies.…”

“…In fact, in recent papers [6] we have argued that such microcrystalline materials can still find various applications of their NLO properties (especially the nonlinear absorption and the effects arising from it), forming a class of "NLO pigments." [7] This actually can be further expanded to define a subclass of materials that luminesce under nonlinear excitation (which can be called "NLO phosphors") as well as materials that perform other functions, not necessarily requiring good optical transparency, e.g., photocatalysis, photochemical transformations, generation of photocurrents, sensing etc.…”

Oxidative C–H Homocoupling of Push–Pull Benzothiazoles: An Atom-Economical Route to Highly Emissive Quadrupolar Arylamine-Functionalized 2,2′-Bibenzothiazoles with Enhanced Two-Photon Absorption

“…[5][6][7][8][9] Of particular popularity are lowcost and readily available N-heteroaromatics, such as pyridines (py), pyrazines (pz), 2,2'-bipyridines (bpy), or benzothiazoles (btz), owing to their electron-accepting properties, photochemical resistance and ease of their further functionalization through protonation, N-alkylation or Ncoordination to metal fragments allowing to tune solubility and NLO properties of π-systems thereof. 7,[9][10][11][12][13][14] In most benzothiazole-derived π-conjugated systems reported so far, electron-donating triarylamine moieties are attached to the heteroarene C-2 position due to its higher reactivity, which however limits their further facile functionalization at the less reactive benzene ring. 12 In addition, we have shown that placing triarylamine branches to the C-6 position of the btz unit is more favourable in terms of larger TPA activity, especially when the auxiliary electron-withdrawing group (EWG) is attached to the C-2 carbon.…”

“…7,[9][10][11][12][13][14] In most benzothiazole-derived π-conjugated systems reported so far, electron-donating triarylamine moieties are attached to the heteroarene C-2 position due to its higher reactivity, which however limits their further facile functionalization at the less reactive benzene ring. 12 In addition, we have shown that placing triarylamine branches to the C-6 position of the btz unit is more favourable in terms of larger TPA activity, especially when the auxiliary electron-withdrawing group (EWG) is attached to the C-2 carbon. 8 While dipolar C-6 substituted 2-H-benzothiazoles without additional EWG substituent display small TPA action cross-sections comparable with other commonly-used OPA fluorophores, 8 our latest DFT computational studies revealed that the TPA activity of these derivatives should be largely enhanced (> 20-fold) by their transformation to hitherto elusive quadrupolar 2,2'-bibenzothiazoles (Scheme 1).…”

“…8 While dipolar C-6 substituted 2-H-benzothiazoles without additional EWG substituent display small TPA action cross-sections comparable with other commonly-used OPA fluorophores, 8 our latest DFT computational studies revealed that the TPA activity of these derivatives should be largely enhanced (> 20-fold) by their transformation to hitherto elusive quadrupolar 2,2'-bibenzothiazoles (Scheme 1). The latter derivatives are also expected to show high emission quantum yields due to the structural similarity of their π-core with the skeleton of firefly luciferin and they could benefit from feasible bidentate N^N coordination, resembling the chemistry of bipyridine-cored π-systems widely used in NLO applications, 7,[11][12][13] but with higher TPA efficiency. Although unsubstituted 2,2'-bibenzothiazole (bisbtz) has recently attracted the attention of material chemists as a bidentate N^N ligand in the construction of efficient red-or NIR-light emitters for OLED devices, 16 its triarylaminefunctionalized derivatives have remained so far synthetically unavailable and their properties unexplored.…”