Intramolecular through-space versus through-bond charge transfer in new donor-carbazole-acceptor type luminophores

Abstract

Five new donor–acceptor luminophores consisting of an acridan or phenoxazine donor and a benzonitrile (1–3) or benzothiadiazole (4, 5) acceptor connected via a carbazole linker are reported. All the synthesized compounds were electrochemically active and could be oxidized in either a two-step (1–4) or three-step (5) process consisting of electron abstraction from the donor substituent and central carbazole unit. The electrochemically determined ionization potential (IP) values depended on the type of the donor used, and the acridan-substituted carbazole derivative (compound 1) exhibited the highest IP value of 5.27 eV. Alternatively, the IP values of phenoxazine-substituted carbazoles (2–5) were lower by about 0.2 eV. Compounds 4 and 5, which were the derivatives containing benzothiadiazole acceptors, were electrochemically reduced in a quasi-reversible one-electron process, yielding the absolute value of electron affinity (|EA|) of 2.98 eV for 4. It was found that the |EA| of 5 increased to 3.12 eV as a result of the extended conjugation of its acceptor unit. The synthesized compounds can also be considered potential TADF luminophores as they exploited the so-called “through space charge transfer” (TSCT) effect. This was owing to their molecular architecture imposed by the rigid carbazole bridge, which enabled appropriate mutual orientation of the donor and acceptor units. They exhibited fluorescence in the blue-orange spectral range (480–653 nm), which was strongly dependent on the solvent polarity. The spectra recorded for molecular dispersions of 1–5 in Zeonex showed a significant increase in PLQY of 43% for 1 and 94% for 5. TDDFT calculations suggested that the observed emissions for compounds 1–4 stemmed from local electron density redistribution upon excitation. The donating nature of the bridge (carbazole) gave rise to an appreciable through-bond charge transfer. However, in case of 5, the emission process could have predominantly resulted from stronger through-space interactions. These calculation results fully corroborated with the obtained crystallographic data, indicating significantly closer vicinity of the donor and acceptor units in 5 as compared to the remaining compounds (1–4).