Synergistic effect of trimethylsilane for photoinduced electron transfer on 1,8-naphthalimides in polar solvent

Photoinduced electron-transfer (PET) process of 1,8-naphthalimidelinkerTMS (NISOSTMS, where SOS=dithiaoxaundecyl and TMS=trimethylsilane) has been investigated using transient absorption measurements in CH3CN and CH3CN/H2O (v/v=9:1). The femtosecond pulsed laser excitation of NISOSTMS produced the NI radical anion (NI−) with a transient absorption band around 415nm, via the intramolecular PET from heteroatom nearby TMS to NI in the singlet excited (S1) state. However, in case of NISOS, the transient band at around 415nm increased concomitantly with the decay of 1NI* (or 3NI*) at around 470nm. This is implied that NI− is primarily generated via the intermolecular quenching of 1NI* (or 3NI*) by NI. In contrast, in a protic polar solvent mixture of CH3CN/H2O, a proton abstraction process occurred from NI− to generate the NI ketyl radical (NIH), which showed a transient absorption band around 405nm. The decay time constants of NIH were quite long compared to those of NI− in CH3CN. Both the rates of charge transfer and deactivation processes of transient species largely depended on the protic polar solvent.