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 the electron transfer (ET) and the thermally-activated delayed fluorescence (TADF) properties. The rigid and bulky electron donor reduces the reorganization energy (λ = 0.96 eV) of the ET, which facilitates the long-lived charge separated (CS) state formation in NI-ICz, by exploiting the Marcus inverted region effect on charge recombination (CR). Transient absorption (TA) spectroscopy reveals the formation of the CS triplet (³CS) states (τ = 8.6 μs in n-hexane, 1.4 μs in toluene, 0.13 μs in acetonitrile). TADF of this dyad is only present in n-hexane but absent in polar solvents, indicating ³CS → ¹CS reversed ISC (RISC) is inefficient, which is a solid experimental evidence for the spin-vibronic coupling mechanism of TADF. A small zero-field splitting (ZFS) parameter (ΙDΙ = 900 MHz) of the triplet state of NI-ICz confirming the formation of ³CS state. These results demonstrate the feasibility of achieving 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 system.