Naphthalimide–Carbazole Compact Electron Donor–Acceptor Dyads: Effect of Molecular Geometry and Electron-Donating Capacity on the Spin-Orbit Charge Transfer Intersystem Crossing
Geliang Tang, Wenbo Yang, Jianzhang Zhao
Abstract
We prepared a series of naphthalimide (NI)–carbazole (Cz) compact electron donor–acceptor dyads showing different substitution positions, C–N/C–C linkers, and conformation restriction magnitudes to study the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The varied conformation restrictions lead to different dihedral angles between the donor and acceptor (37°–81°) and electronic coupling magnitude (matrix elements V: 1290–3070 cm–1). Based on the comparison between the dyads containing C–N and C–C linkers, we found that a large dihedral angle between the donor and acceptor is favorable to efficient SOCT-ISC. For one dyad, the singlet oxygen quantum yield (ΦΔ) is up to 84.4% (in dichloromethane), which is much higher than that of the previously reported NI–phenothiazine (PTZ) analogue dyad (ΦΔ = 16.0% in n-hexane). The intrinsic triplet state lifetime (τT) is 270 μs, longer than that accessed by the heavy atom effect (75.2 μs). As compared with the NI–PTZ analogue dyad, the Cz unit in the current dyads is a weaker electron donor than PTZ. Thus, a higher CT state energy in NI–Cz dyads was observed, which makes the SOCT-ISC efficient in solvents with a wide range of polarities. Meanwhile, the localized triplet state (3LE) becomes the lowest-lying state in the NI–Cz dyads, which is different from the triplet charge transfer (3CT) state observed in the analogue NI–PTZ dyad. Moreover, the large energy gap between the CT and 3LE states inhibits the reverse ISC; as a result, no thermally activated delayed fluorescence was observed for the current NI–Cz dyads.