Naphthalimide-Carbazole Compact Electron Donor-Acceptor Dyads: Effect of Molecular Geometry and Electron-Donating Capacity on the Spin-Orbit Charge Transfer Intersystem Crossing.

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.