Cis‐Trans Isomerism Inducing Cocrystal Polymorphism with Thermally Activated Delayed Fluorescence and Two‐Photon Absorption

Abstract The cocrystals of 2‐(benzo[d]thiazol‐2‐yl)‐3‐(pyren‐1‐yl)acrylonitrile (Py‐BZTCN) and 1,2,4,5‐tetracyanobenzene (TCNB) are constructed through intermolecular charge‐transfer (CT) interactions between donor and acceptor. Cis‐ and trans‐isomers of Py‐BZTCN induce the formation of two different crystal packing with 1:2 and 1:1 stoichiometric ratios between Py‐BZTCN and TCNB, corresponding to the cocrystals of PCNTC‐O and PCNTC‐R, respectively. PCNTC‐O cocrystal exhibits orange‐yellow normal fluorescence with a CT from pyrene (Py) monomer to TCNB, while PCNTC‐R cocrystal presents red thermally activated delayed fluorescence (TADF) with a CT from Py dimer to TCNB. This photophysical difference originates from that, relative to Py monomer, Py dimer has an elevated energy level of highest occupied molecular orbital (HOMO), which results in a stronger CT in PCNTC‐R cocrystal. Both cocrystals possess unique two‐photon absorption (TPA) properties, and changing monomer into dimer as a donor in the CT cocrystal effectively alters the supramolecular properties. Noteworthily, PCNTC‐R is a rare case in which TADF and TPA are integrated into a cocrystal, opening up a new supramolecular strategy to explore the multi‐functional materials.