Effects of benzene-fused position on photophysical properties of benzocoumarin-based two-photon fluorescent probes for HClO detection

Four benzocoumarin-based two-photon fluorescent probes (BH1–BH4) are proposed for ratiometric detection of hypochlorous acid (HClO) and their two-photon sensing performance are evaluated by means of time-dependent density functional theory and quadratic response theory. The effects of benzene-fused position on Stokes shift, fluorescence quantum yield and two-photon absorption are discussed comprehensively. The results show that fusing a benzene ring in coumarin can enhance Stokes shift efficiently. The benzene-fused position has important effects on these photophysical properties. The benzo[g]coumarins (BH1) and benzo[f]coumarins (BH2) derivatives have larger Stokes shifts in comparison with benzo[h]coumarins (BH3) and dihydrophenazine (BH4) derivatives. The two-photon absorption of benzo[f]coumarins (BH2) derivative is much smaller than those of other benzo-coumarins derivatives. The large Stokes shift and increased two-photon action cross section can be achieved simultaneously in the dihydrophenazine (BH4) derivative. Therefore, the designed BH4 is expected to have superior performance for the ratiometric detection of HClO. To explore the reasons behind these effects, the intramolecular charge transfer degrees are illustrated quantitatively by plotting the hole-electron isosurface map, and the relation between charge transfer and Stokes shift is revealed. A two-state model analysis is employed to understand two-photon absorption ability. Moreover, the fluorescence near-quenching mechanism of the product molecules B1 and B3 is explained by analyzing reorganization energy and Huang-Rhys factor, as well as related normal mode. Our research could contribute to the efficient design of ratiometric two-photon fluorescent probes with large Stokes shift and significant two-photon action cross section.