Unraveling the photophysical and semiconducting properties of color converter luminogens with aggregation induced emission characteristics

Abstract

Aggregation-induced emission (AIE) has attracted increasing attention in recent years in the search for luminescent materials with biomedical and optoelectronic applications. Understanding of its underlying mechanisms is fundamental for the rational design of new AIE materials. In the present work, density functional theory (DFT) calculations were carried out for a set of organic fluorophores which have been recently studied as candidates for color converters in visible light communication. Three of them were (Z)-4-benzylidene-2-methyloxazol-5(4H)-one derivatives combined with an electron-donating moiety of trimethylamine, tetraphenylethene or dimethoxy-tetraphenylethene (TPA-BMO, TPE-BMO or DM-TPE-BMO, respectively). The fourth compound was a diaminomaleonitrile-based Schiff base combined with a dimethylamine moiety (DMA-AM). Our calculations suggested that the restriction of the intramolecular vibration (RIV) mechanism is responsible for the AIE effect observed for TPE-BMO and DM-TPE-BMO in the solid state. On the other hand, the uncommon photophysical behavior reported for DMA-AM (including the AIE phenomenon) in the solid state was related to excitonic coupling effects. Fluorescence spectroscopy experiments confirmed the presence of strongly-coupled DMA-AM aggregates in the solid state. Finally, some electronic properties related to the semiconductor behavior were investigated to explore the potential of applying these fluorophores in light-emitting electronic devices. It must be highlighted that the largest electron-transfer rate constant was calculated for TPE-BMO using the semi-classical Marcus theory, while the largest hole-transfer rate constants were obtained for DMA-AM and TPA-BMO.