Charge separation and intersystem crossing in compact, orthogonal and sterically encumbered 6,12-diphenyl indolo[3,2-b]carbazole-naphthalimide electron donor–acceptor dyad

Abstract

A compact electron donor–acceptor dyad, NI-ICz, with 6,12-diphenyl-indolo[3,2-b]carbazole (ICz) as the electron donor and naphthalimide (NI) as the acceptor, was prepared to study its electron transfer (ET) and thermally activated delayed fluorescence (TADF) properties. The rigid and bulky electron donor reduced the reorganization energy (λ = 0.96 eV) for ET, which facilitated the formation of a long-lived charge separated (CS) state in NI-ICz by exploiting the Marcus inverted region effect on charge recombination (CR). Transient absorption (TA) spectroscopy revealed the formation of CS triplet (³CS) states (τ = 8.6 μs in n-hexane, 1.4 μs in toluene, and 0.13 μs in acetonitrile). TADF of this dyad was observed in n-hexane but was absent in polar solvents, indicating that ³CS → ¹CS reversed ISC (RISC) was inefficient, which is solid experimental evidence for the spin-vibronic coupling mechanism of TADF. The small zero-field splitting (ZFS) parameter (|D| = 900 MHz) for the triplet state of NI-ICz confirmed the formation of the ³CS state. These results demonstrate the feasibility of achieving a long-lived CS state in compact electron donor–acceptor dyads. The findings also highlight the crucial role of the closely lying ³LE, ¹CS and ³CS states in enabling TADF, where the ³LE state serves as an essential intermediate state to facilitate RISC in TADF systems.