Regulation of photodynamic interactions in 1,8-naphthalimide–linker–phenothiazine dyads by cyclodextrins

Abstract

This study examines the effects of cyclodextrin (CD) on the photophysical properties of 1,8-naphthalimide (NI) derivatives including NI–linker–phenothiazine (NI–L–PTZ) dyads in a H₂O/CH₃CN solution (v/v = 9 : 1). Peculiar excimer and exciplex emissions were observed with reference NI and NI–L–PTZ, respectively, in an aqueous environment. The NI–CD complexes exhibited monomeric emission spectral features, which resulted from their inclusion into the CD cavity. The complex formation constants of CDs with the NI derivatives were determined. The T₁–T_n absorption band of ³NI* was observed at around 470 nm during nanosecond-laser flash photolysis. Photoinduced electron transfer in NI–L–PTZ led to NI˙⁻ and PTZ˙⁺ from the T₁ state. In protic polar solvents, NI˙⁻ easily formed a ketyl radical (NIH˙) by hydrogen abstraction from H₂O, and showed a transient absorption band at around 400 nm. The charge recombination (CR) time of the transient species at 400 nm showed a strong CD dependency, and the CR time of the dyad in γ-CD was increased markedly compared with in β-CD. This suggests that the PTZ moiety can also be included in the γ-CD cavity.