Influence of peripheral phenyls to excited state relaxation and aggregation induced fluorescence in nitrile compounds

Fluorescent nanoparticles attract great interest in the field of technological applications. They were found to be suitable for constructing OLEDs, OFETs and other optoelectronic systems. They are also fit for tailoring biological probes for bio-sensing and bio-imaging. Fluorescent organic nanoparticles are usually composed of non-planar geometry molecules experiencing the aggregation induced emission (AIE) phenomenon. [1] Three nitrile-based compounds with different number of peripheral phenyl moieties are studied in this work (Fig. 1). The compounds are designed to show AIE properties. The cyano-vinylene bridge is intentionally introduced into the core to provide a steric twist in the molecules. The pyrazole groups have the ability to deliver high fluorescence efficiency in addition to causing strong intermolecular hydrogen bonds and π-π interactions, which are important for supramolecular constructions. [2] Nanoparticles based on the nitrile compounds have been formed using precipitation method. The compounds are non-fluorescent in diluted solutions, but demonstrate induced emission upon aggregate formation. The nitrile compound containing no peripheral phenyls demonstrated broad absorption and fluorescence spectra in aggregated state. It also exhibited an amorphous aggregate structure in the neat film and a moderate (10-fold) emission enhancement as compared to that in solution. The compound with two peripheral phenyl rings featured 100-fold emission enhancement in the solid phase, which was accompanied by vibronically-resolved spectrum of aggregates indicating their crystalline nature. The nitrile compound bearing four peripheral phenyls showed similar features as the compound with two phenyl moieties. Thus, the peripheral phenyl groups in turn affected the morphology of nitrile-based nanoaggregates formed by precipitation method.