Control of Photophysical Properties of 1,8-Naphthalimides by Electron-Withdrawing Substituents Introduced into<i>N</i>-Alkyl Side Chains
作者:Kazuhiko Matsubayashi、Chisato Kajimura、Yasuo Kubo、Toshitada Yoshihara、Seiji Tobita
DOI:10.1246/bcsj.20100276
日期:2011.5.15
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.
在二氯甲烷中,发现萘醛亚胺1–7的荧光强度因在N-烷基侧链中引入电子吸引取代基(如OCOMe、CO2Me、Cl和CN)而显著增强。通过使用皮秒单光子计数法测量荧光生命周期τf,并通过时间分辨热透镜法测定的跨跃量子产率Φisc,确定了1–7在二氯甲烷和乙腈中的荧光发射速率常数kf、系间交叉速率常数kisc和内部转换速率常数kic。这些结果表明,kisc的值随着取代基电子吸引能力的增加而减小。此外,在二氯甲烷中kisc的对数与取代基常数σI(通过σ键的诱导效应)之间几乎呈线性Hammett关系(ρ = −1.48),在乙腈中为ρ = −1.85。因此,电子吸引取代基所引起的荧光强度增强可被解释为从1(ππ*)到3(nπ*)的系间交叉效率降低,后者的能量因取代基通过σ键的诱导效应而增加。