Novel D-π-A-π-D structural 2, 2′-bipyridine derivatives with different electron donors: Structure, one- and two-photon excited fluorescence and bioimaging

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

“…It is noteworthy that the TPA maximum cross section of bisbtz-VI is ca. 6 times larger than that of bipyridine-cored congener bpy , measured in toluene under identical TPEF conditions (literature data for bpy are 219 GM at 690 nm and 425 GM at 780 nm in chloroform and DMF, respectively). , Although the TPA performance of bisbtz-VI in chloroform is somewhat lowered (δ TPA = 1018 GM at 840 nm), it is still about 4.6 times higher than that of bpy , yet with an emission maximum shifted to the orange spectral region (Figures S4 and S12 in SI). The TPA superiority of the bisbtz scaffold can be ascribed according to the three-state model to a notably larger transition-dipole moment between the ground and the first excited state (μ 01 = 17.8 D), which is also reflected in the higher molar extinction coefficient (cf.…”

“…[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.…”

“…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.…”

“…In this regard, employment of suitable heteroaromatic scaffolds in π-conjugated systems with intramolecular charge-transfer (ICT) is a useful design strategy in the construction of TPA active dyes allowing researchers to meet specific application requirements and to achieve large δ TPA values within a confined chromophore space by proper arrangement of heteroatoms or entire heteroaromatic blocks within the π-system. − Of particular popularity are low-cost and readily available N -heteroaromatics, such as pyridines (py’s), pyrazines (pz’s), 2,2′-bipyridines (bpy’s), or benzothiazoles (btz’s), owing to their electron-accepting properties, photochemical resistance, and ease of their further functionalization through protonation, N -alkylation, or N -coordination to metal fragments allowing to tune solubility and NLO properties of π-systems thereof. ,− …”

“…While dipolar C-6 substituted 2- H -benzothiazoles without an additional EWG substituent display small TPA action cross sections comparable with other commonly used OPA fluorophores, 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 ). 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, ,− but with higher TPA efficiency.…”