A long-lived charge-separated state of spiro compact electron donor-acceptor dyads based on rhodamine and naphthalenediimide chromophores

Spiro rhodamine (Rho)-naphthalenediimide (NDI) electron donor-acceptor orthogonal dyads were prepared to generate a long-lived charge separation (CS) state based on the electron spin control approach, i.e. to form the ³CS state, not the ¹CS state, to prolong the CS state lifetime by the electron spin forbidden feature of the charge recombination process of ³CS → S₀. The electron donor Rho (lactam form) is attached via three σ bonds, including two C-C and one N-N bonds (Rho-NDI), or an intervening phenylene, to the electron acceptor NDI (Rho-Ph-NDI and Rho-PhMe-NDI). Transient absorption (TA) spectra show that fast intersystem crossing (ISC) (<120 fs) occurred to generate an upper triplet state localized on the NDI moiety (³NDI*), and then to form the CS state. For Rho-NDI in both non-polar and polar solvents, a long-lived ³CS state (lifetime τ = 0.13 μs) and charge separation quantum yield (Φ_CS) up to 25% were observed, whereas for Rho-Ph-NDI (τ_T = 1.1 μs) and Rho-PhMe-NDI (τ_T = 2.0 μs), a low-lying ³NDI* state was formed by charge recombination (CR) in n-hexane (HEX). In toluene (TOL), however, CS states were observed for Rho-Ph-NDI (0.37 μs) and Rho-PhMe-NDI (0.63 μs). With electron paramagnetic resonance (EPR) spectra, weak electronic coupling between the Rho and NDI moieties for Rho-NDI was proved. Time-resolved EPR (TREPR) spectra detected two transient species including NDI-localized triplets (formed via SOC-ISC) and a ³CS state. The CS state of Rho-NDI features the largest dipolar interaction (|D| = 184 MHz) compared to Rho-Ph-NDI (|D| = 39 MHz) and Rho-PhMe-NDI (|D| = 41 MHz) due to the smallest distance between Rho and NDI moieties. For Rho-NDI, the time-dependent e,a → a,e phase change of the CS state TREPR spectrum indicates that the long-lived CS state is based on the electron spin control effect.