Effects of Multibranching on 3-Hydroxyflavone-Based Chromophores and the Excited-State Intramolecular Proton Transfer Dynamics

A series of one-, two-, and three-branched chromophores based on 3-hydroxyflavones (1-3) have been synthesized as the first example of multibranched chromophores demonstrating excited-state intramolecular proton transfer (ESIPT). Coupling between the 3-hydroxyflavone branches connected by an electron-donating triphenylamine core is manifested in the red-shifted and asymmetric absorption band of 2, whereas the absorption of 3 is governed by the divided donor strength. Their excited-state charge-transfer (ESCT)-coupled ESIPT dynamics is investigated via femtosecond fluorescence upconversion and is proved to be well correlated with the ratio of normal/tautomer emission in the fluorescence spectra. For 1 and 2, with increased donor strength compared with the 4'-N,N-dialkylamino-3-hydroxyflavone analogue, ESIPT appears to cease in the more polar solvent of acetonitrile. Nevertheless, similar dependence of 1-3 on solvent polarity signifies resembling charge-transfer character at the normal excited states (N*), despite their varying structures. As evidenced by the theoretical approach, the frontier orbitals of vibrationally relaxed (geometry-optimized) N*, from which fluorescence and ESIPT should take place, are localized on one specific branch, leading to similar emission patterns and dynamics, whereas the orbitals contributing to Franck-Condon excitation (absorption) spread over the entire molecule. The localization is found to be facilitated by rotation of a specific branch pivoting on the central nitrogen atom, while planarity is maintained within each 3-hydroxyflavone chromophore.