Fine-tuning of the charge-separated state energy in compact orthogonal naphthalene–phenoxazine dyads and its effect on the thermally-activated delayed fluorescence

In this study, we synthesized three electron donor–acceptor dyads, NI-PXZ-Ph, NI-PXZ-PhF, and NI-PXZ-PhOCH3, by attaching an electron-donating or withdrawing group to the imide N-position of the naphthalimide (NI) moiety (electron acceptor) to study the effect of the relative energy order of the charge transfer (CT) state and the 3LE (locally excited) state on the photophysical property of the compact electron donor–acceptor dyads, especially their thermally activated delayed fluorescence (TADF) property. The reduction potentials of the NI moieties changed by ca. 0.03–0.05 V by introducing different substituents, while the other features of the dyads remained unchanged. All three dyads showed the TADF property, for example, the prompt and delayed fluorescence lifetimes of NI-PXZ-PhF were determined to be 20.2 ns (99.2%) and 6.4 μs (0.80%), respectively, in deaerated n-hexane. Their lifetimes were found to be much shorter than the previously reported NI-PXZ [16.4 ns (99.2%)/17.0 μs (0.80%)]. Mixed LE and CT states were observed in non-polar solvents (n-hexane) via nanosecond transient absorption spectroscopy. Our experiments showed that in polar solvents (toluene and acetonitrile), although the long-lived 3CT state was populated (0.47 μs), no TADF was observed because of the lower CT state energy (2.10 eV) than the 3LE state energy (2.29 eV). The results suggest that the presence of the 3LE state sharing a similar energy with the CT states is critical for the occurrence of TADF, and the equilibrium between the three states of 3LE and 1CT/3CT favors the efficient TADF process. These experimental results confirm the spin–vibronic coupling mechanism of TADF emitters based on electron donor–acceptor dyad molecular structure motifs.