Small, apolar aromatic groups, such as phenyl rings, are commonly included in the structures of fluorophores to impart hindered intramolecular rotations, leading to desirable solid-state luminescence properties. However, they are not normally considered to take part in through-space interactions that influence the fluorescent output. This presentation will detail (1) the photoluminescence properties of a series of phenyl-ring molecular rotors and (2) how their conjugated alkenyl backbones influence their propensities to undergo photocyclization reactions. The fluorescent emissions from some of the rotors are found to originate, not from the localized excited state as one might expect, but from unanticipated through-space aromatic-dimer states.<sup>1</sup> We demonstrate that these relaxed dimer states can form as a result of intra- or intermolecular interactions across a range of environments in solution and solid samples, including conditions that promote aggregation-induced emission. We also show that this emission is 'turned off' upon breaking the conjugation of the alkenyl backbones by epoxide formation.<sup>2</sup> While this modification is deleterious to photoluminescence, it enables formation of extended polycyclic frameworks by Mallory reactions.

Excited-state aromatic interactions and photocyclizations in the aggregation-induced emission of molecular rotors

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ACS Fall 2022

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