Photoinduced electron-transfer processes of tetrathiafulvalene-(spacer)-(naphthalenediimide)-(spacer)-tertrathiafulvalene triads in solution

Intermolecular photoinduced charge-separation and charge-recombination processes of tetrathiafulvalene-(spacer)-naphthalenediimide-(spacer)-tertrathiafulvalenetriad s (TTF-(sp)-NIm-(sp)-TTF), which have been designed to change the length and kind of the spacers, have been investigated in various solvents by time-resolved absorption and fluorescence techniques. The fluorescence lifetimes of the NIm moiety considerably decreased compared with that of the NIm molecule, suggesting the photoinduced intramolecular charge-separation via the singlet excited state of the NIm moiety. From the observed short fluorescence lifetimes, the charge-separation rate constants (k_CS) and quantum yields (Φ_CS) of TTF-(sp)-NIm-(sp)-TTF have been evaluated; the k_CS values for the triads with the rigid cyclohexyl spacers are larger than that with long flexible alkyl chain spacers. In the nanosecond transient spectra in PhCN, the absorption bands were observed at 480 and 760 nm, which were attributed to the radical anion of NIm (NIm^̇-) suggesting the formation of TTF^̇+- (sp)-NIm^̇--(sp)-TTF. From the decays of the radical ion-pair, the charge-recombination rate-constants (k_CR) were evaluated in the range of 9 × 10⁵ - 3 × 10⁷ s^-1. Longer lifetimes of the radical ion-pair were observed for the triads with the cyclohexyl than that of the long flexible spacer. From the temperature dependence of the k_CR values, the reorganization energies and coupling constants were experimentally evaluated in PhCN; small coupling constants of the triads with the cyclohexyl spacers than that of the long flexible spacers support the relatively long lifetimes of the charge separated states of the chyclohexyl spacer.