Fluorescent-state switching of a 10-hydroxybezo[h]quinoline skeleton through the electronic nature of substituents

Abstract

The spectroscopic and photophysical properties of novel 10-hydroxybenzo[h]quinoline (HBq) derivatives with various para-substituted phenyl groups at the 7- and 9-positions (1R; R = –NMe₂, –OMe, –Me, –H, –F, –Cl, –CF₃, –CN, and –NO₂) were evaluated by the electron-donating and electron-withdrawing ability of the substituent R in terms of the Hammett substituent constant. The electronic nature of the substituents controlled the excited-state properties. In particular, 1NO₂ exhibited a larger nonfluorescence rate constant than that predicted from the energy gap dependence, and the fluorescence from 1NO₂ was solvent-dependent, distinct from that of the other derivatives. The TD-DFT calculations revealed that the excited-state derivatives except for 1NO₂ adopted the keto-form structures generated by ESIPT, whereas 1NO₂ exhibited non-ESIPT excited-state geometry. The difference originates from the variation in the charge-transfer characters: in the former, charge transfer occurs within the HBq moiety, whereas in the latter, charge transfer proceeds from the HBq moiety to the 4-nitrophenyl groups. The systematic tuning of the electronic transition demonstrated control of the excited-state geometry of HBq derivatives and provided important insights into proton transfer in the excited state. The introduction of 4-substituted phenyl groups also controlled the basicity of the derivatives in the ground states and, notably, 1NMe₂ exhibited pronounced fluorescence color variations, ranging from the near-infrared to the blue region, as a function of the concentration of trifluoroacetic acid.