The present study provides mechanistic insights into the role of positional isomerism in dictating the kinetics for charge recombination as well as aggregated state emission behavior of three 9-Styrylanthracene based isostructural donor-π-acceptor systems (E)-2-(2-(anthracen-9-yl)vinyl)benzonitrile [ortho], {(E)-3-(2-(anthracen-9-yl)vinyl)benzonitrile [meta] and (E)-4-(2-anthracen-9-yl) vinyl benzonitrile [para], the only difference between them being the substitution position of the cyano group on the styryl moiety. It is observed that the meta isomer exhibits aggregation-caused quenching (ACQ) behavior, whereas the para and the ortho isomers exhibit enhanced emission (AIE) upon aggregation. Moreover, the rate constants of back electron transfer (kBET) in a non-polar solvent medium between an electron donor {N, N-diethylaniline [DEA]} and ortho and para isomers are 7.1x104 s-1 and 6.6x104 s-1 respectively, which are two orders of magnitude lesser compared to DEA-meta isomer system [~1x106 s-1]. The results suggest that ortho and para substitution of electron-withdrawing cyano group results in a more substantial inherent “push-pull” effect in the donor-π-acceptor systems than meta substituted. As a result, in the excited state, upon electron transfer, the ortho and para-substituted isomers generate a longer-lived charge-separated state via better charge delocalization than the meta isomer. Furthermore, in the aggregated states, the ACQ-to AIE transformation can be attributed to the presence of interplay between their dark intramolecular charge transfer state (ICT)-to- locally excited state (LE), which is achieved via tweaking the inherent “push-pull” effect of the isomers. The details of the work will be presented.

Regulation of aggregated state emission and back electron transfer in anthracene based push-pull dyes via positional isomerism

ACS Spring 2022

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