Evaluation of near-infrared two-photon absorption response and excited state dynamics of asymmetric aza-BODIPY derivatives.

The study focused on asymmetric aza-BODIPY derivatives, which were modified with 4-bromophenyl (B1), 4-bromo and 4-bromophenyl (B2), 4-N,N-diphenylaminobiphenyl (B3), 4-methoxybiphenyl (B4), 4-methoxyphenyl and 4-bromophenyl (B5), and 2,4-dimethoxybiphenyl (B6) groups at 2,6 and β positions of aza-BODIPY skeleton. The compounds were thoroughly investigated to understand their linear optical properties, near-infrared nonlinear absorption responses, and dynamics of excited states. The linear absorption and fluorescence behaviors of the compounds were studied depending on the substituents and solvents (THF, CHCl). Compounds B1 and B4 showed higher absorption in THF, while compound B5 exhibited higher absorption in CHCl. No significant change was observed in compounds B2, B3, and B6. The fluorescence spectra revealed that compounds B1, B4, and B6 exhibit comparatively higher fluorescence quantum yields in CHCl medium than in THF. The fluorescence of B2, B3, and B5 was significantly quenched due to the strong intramolecular charge transfer (ICT) effects influenced by the electron-donating diphenylamine unit and methoxy groups, as well as the heavy atom effect of bromine atoms. Excited state dynamics of all compounds were revealed with pump-probe spectroscopy. Near-infrared two-photon absorption (TPA) response at 1000 nm was observed for all the studied compounds. Compound B3 indicated the highest TPA cross-section (TPCS) (210 GM) due to high ICT capability of diphenylaminobiphenyl unit. Compounds B3 and B6 stand out with their strong photophysical properties; B3 exhibited high ICT activity and the highest TPCS, while B6 had high fluorescence quantum yield.