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, CHCl₃). Compounds B1 and B4 showed higher absorption in THF, while compound B5 exhibited higher absorption in CHCl₃. 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 CHCl₃ 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.