Flexible Organic Charge-Transfer Cocrystals with Near-Infrared Emission

Flexible organic crystals have been recently explored for optical transduction in flexible small-scale optical devices; however, most studies are limited to the visible range (400-600 nm). Due to the photonic well confinement and low optical losses, optical telecommunications require transducive media with tunable optical emission in the 850-1550 nm region. While chemical substitution is a viable option to shift the emissive properties of organic crystals to the telecom regime, depending on the target modification, this could be a resource-intensive approach. Here, with two cocrystals of 1,1'-biphenyl (BP), 4,4'-dimethylbiphenyl (DM), and 7,7,8,8-tetracyanoquinodimethane (TCNQ), we demonstrate that cocrystallization provides an alternative and straightforward access to organic materials with emissions in the NIR region. The crystal between DM and TCNQ is mechanically plastic and emissive in the NIR region and can be used as an NIR optical waveguide. Both experimental and computational analyses point to the structure of the component molecules as determining factors of the CT interaction and, therefore, the emission window. The optical band gap is conducive to narrowing by strong CT interaction, resulting in emission in the NIR region.