Long-Lived Charge-Separated State in Naphthalimide-Phenothiazine Compact Electron Donor-Acceptor Dyads: Effect of Molecular Conformation Restriction and Solvent Polarity

To study the charge separation (CS) and long-lived CSstate, weprepared a series of dyads based on naphthalimide (NI, electron acceptor)and phenothiazine (PTZ, electron donor), with an intervening phenyllinker attached on the N-position of both moieties. The purpose isto exploit the electron spin control effect to prolong the CS-statelifetime by formation of the (CS)-C-3 state, instead of theordinary (CS)-C-1 state, the spin-correlated radical pair (SCRP),or the free ion pairs. The electronic coupling magnitude is tunedby conformational restriction exerted by the methyl groups on thephenyl linker. Differently from the previously reported NI-PTZ analoguescontaining long and flexible linkers, we observed a significant CSemission band centered at ca. 600 nm and thermally activated delayedfluorescence (TADF) with a lifetime of 13.8 ns (population ratio:42%)/321.6 & mu;s (56%). Nanosecond transient absorption spectroscopyindicates that in cyclohexane (CHX), only the (NI)-N-3* statewas observed (lifetime & tau; = 274.7 & mu;s), in acetonitrile(ACN), only the CS state was observed (& tau; = 1.4 & mu;s), whereasin a solvent with intermediate polarity, such as toluene (TOL), boththe (NI)-N-3* (shorter-lived) and the CS states were observed.Observation of the long-lived CS state in ACN, yet lack of TADF, confirmsthe spin-vibronic coupling theoretical model of TADF. Femtosecondtransient absorption spectroscopy indicates that charge separationoccurs in both nonpolar and polar solvents, with time constants rangingfrom less than 1 ps in ACN to ca. 60 ps in CHX. Time-resolved electronparamagnetic resonance (TREPR) spectra indicate the existence of the (NI)-N-3* and CS states for the dyads upon photoexcitation. Theelectron spin-spin dipole interaction magnitude of the radicalanion and cation of the CS state is intermediate between that of atypical SCRP and a (CS)-C-3 state, suggesting that the longCS-state lifetime is partially due to the electron spin control effect.