Control of Photophysical Properties of 1,8-Naphthalimides by Electron-Withdrawing Substituents Introduced into N-Alkyl Side Chains

Abstract

Abstract The fluorescence intensity of naphthalimides 1–7 in dichloromethane was found to be remarkably enhanced by the introduction of an electron-withdrawing substituent, such as OCOMe, CO2Me, Cl, and CN, into the N-alkyl side chain. The rate constants of the fluorescence emission kf, the intersystem crossing kisc, and the internal conversion kic for 1–7 in dichloromethane and in acetonitrile were determined based on the measurements of the fluorescence lifetimes τf, by using picosecond single photon counting, and the quantum yields of the intersystem crossing Φisc determined by time-resolved thermal lensing. These results indicate that the value of kisc decreases with an increase in the electron-withdrawing ability of the substituent. Furthermore, an almost linear Hammett relationship (ρ = −1.48 in dichloromethane, ρ = −1.85 in acetonitrile) between the logarithm of kisc and the substituent constant σI for inductive effect of the substituent through the σ bond was observed. The enhancement of the fluorescence intensity by the introduction of electron-withdrawing substituents was thus explained by a decrease in the efficiency of the intersystem crossing from 1(ππ*) to 3(nπ*), whose energy was increased by the inductive effect of the substituent through the σ bonds.