Germanium vs Silicon Bridge in Photoinduced Electron Transfer Leading to Blue Emission

Abstract

The germanium-grafted compound with a Donor–Bridge–Acceptor architecture, (E)-3-(dimethyl(4-vinylstyryl)germyl)-9-isopropyl-carbazole (1), was designed and synthesized as a potential blue-light emitter. Its previously reported silicon analogue (2) displays efficient blue-light emission resulting from photoinduced intramolecular electron transfer (ET) from carbazole to the divinylbenzene moiety through the dimethylsilyl bridge. In the current work, comparative photophysical studies combined with theoretical computations (DFT and NBO analysis) were aimed at understanding the role of the bridge atom in the photoinduced electron transfer as the source of blue emission. It was found that similarities of the bridge atoms (Ge and Si) translate into numerous similarities in the investigated organometallic compounds, endowing compounds 1 and 2 with unique properties and making them a good platform for complete electron transfer. By contrast, theoretical calculations of their carbon analogue (3) show that upon excitation only half of the electron charge can be transferred from the donor to the acceptor part. Despite many similarities, replacing silicon with germanium atom leads to a significant (about 10-fold) diminution of the ICT fluorescence quantum yield, resulting from the internal heavy-atom effect of germanium. Enhanced intersystem crossing of compound 1 (15-times larger spin-orbit coupling than that of compound 2) competes with electron transfer, making ET less efficient. Our findings show that the bridge atom controls excited-state dynamics via the modulation of competing deactivation pathways in the investigated systems.