From symmetric to asymmetric: tuning photophysical and rectifying properties in [2.2]paracyclophanes

Abstract

[2.2]Paracyclophane (PCP) scaffolds, with their rigid, non-planar geometries and through-space π-conjugation, offer a unique platform for the development of advanced optoelectronic materials. In this study, we synthesize and characterize a series of racemic PCP derivatives bearing electron-donating (e.g., –OCH₃) and electron-withdrawing (e.g., –CN) substituents to explore their photophysical properties and potential as charge-transporting and rectifying materials. Solution-phase spectroscopy reveals that donor–acceptor (D–A) PCPs exhibit pronounced charge-transfer (CT) emission, with large Stokes shifts up to 166 nm, minimal spectral overlap, and solvent-dependent photoluminescence maxima shifting from 396 nm in hexanes to 474 nm in DMF. Photoluminescence quantum yields also increase substantially, from 1% in symmetrically substituted to 31% in D–A compounds. Aggregation studies reveal distinct photoluminescence modulation: D–A compounds form both J- and H-aggregates, while symmetric analogs show J-aggregation. Electrical measurements using a eutectic liquid gallium–indium (EGaIn) top contact electrode reveal that while conductance can be varied over four orders of magnitude depending on substitution pattern, D–A PCPs exhibit pronounced current rectification, achieving rectification ratios up to 200 in amorphous thin films. These findings establish functionalized PCPs as promising dual-mode materials for organic optoelectronics, capable of both environmental photoluminescence modulation and efficient molecular rectification.