Styrylbenzene Organogels And How The Cyano Groups Tune The Aggregation-Induced Emission

Fluorescence switching of pi-gelators has emerged as a field of great interest because of their promising applications. In this work we present two families of luminogens based on bis(styrylbenzene) and bis(alpha-cyanostyryl) benzene that incorporate acetamido groups and hydrocarbon chains to induce gelation. The presence of cyano groups in the molecular skeleton produces Aggregation Induced Emission (AIE) from solution to the gel while their absence gives the Aggregation Caused Quenching (ACQ) effect. Density Functional Theory calculations were performed in order to shed light on the origin of these opposite phenomena. The bis(styrylbenzene) compound showed a high quantum yield in solution and nearly planar structure was theoretically predicted. A significant drop in the quantum yield was observed after gelation, probably due to the pi-pi intermolecular interactions favoured by the planar scaffold. In contrast, the more twisted structure predicted for the cyano compounds could prevent such pi-pi intermolecular interactions and this would favour fluorescence emission. In addition, the twisted molecular structure predicted for the cyano derivative could induce Z/E photosiomerisation that suppresses the emission in solution. However, the framework of intermolecular contacts that arises during the gelation process could restrict torsional motions to the detriment of photoisomerization, thus resulting in AIE-active compounds. Furthermore, the reorganisation energy and Huang-Rhys factors associated with the nonradiative relaxation were calculated. The highest values were found for some vibrational modes in the low energy region in the case of the cyano compound and these could contribute to the non-radiative relaxation in solution. These intramolecular motions could be blocked in the gel, thus increasing the fluorescence emission. The introduction of cyano groups in the carbon scaffold could therefore provide a strategy for the rational design of new active AIE organogels.