In order to investigate the joint influence of the conformation flexibility and the matching of the energies of the charge-transfer (CT) and the localized triplet excited (3LE) states on the thermally-activated delayed fluorescence (TADF) in electron donor-acceptor molecules, a series of compact electron donor-acceptor dyads and triad were prepared, with naphthalimide (NI) as electron acceptor and phenothiazine (PTZ) as electron donor. The NI and PTZ moieties are either directly connected at 3-position of NI and N-position of PTZ moiety via a C-N single bond, or are linked through a phenyl group. The tuning of the energy order of the CT and LE states is achieved by oxidation of the PTZ unit into the corresponding sulfoxide, whereas conformation restriction is imposed by introducing ortho-methyl substituents on the phenyl linker, thus the coupling magnitude between the CT and the 3LE states are controlled. Singlet oxygen quantum yields (FD) of NI-PTZ is moderate in n-hexane (HEX, ΦΔ = 19%). TADF was observed for the dyads, the biexponential luminescence lifetime are 16.0 ns (99.9%)/14.4 μs (0.1%) for the dyad and 7.2 ns (99.6%)/2.0 μs (0.4%) for the triad. The triplet state was observed in the nanosecond transient absorption spectra with lifetimes: in the 4-48 μs range. Computational investigations show that the orthogonal electron donor-acceptor molecular structure is beneficial for TADF. These calculations also help interpreting the ns-TA spectra and the origins of TADF in NI-PTZ, which is ultimately due to the small energetic difference between the 3LE and 3CT states. Conversely, NI-PTZ-O, which has a higher CT state and bear a much more stabilized 3LE state, does not display TADF.
Keywords: Charge-transfer; Electron donor; Intersystem crossing; TADF; Triplet state
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Ye, K.; Cao, L.; Raamsdonk, D. M. E. V.; Wang, Z.; Zhao, J.; Escudero, D.; Jacquemin, D. Beilstein Arch. 2022, 202224. doi:10.3762/bxiv.2022.24.v1
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