Bichromophores as a Molecular Platform to Explore Chiroptical Responses: Resonant Coupling and Beyond

Abstract

Circular dichroism (CD) and circularly polarized luminescence (CPL) in molecular assemblies are governed by excited-state coupling spanning from on- to off-resonance regimes. Herein, these effects are investigated in bichromophoric systems where two 1,4-bis(phenylethynyl)benzene chromophores are arranged obliquely on RR- and SS-1,2-cyclohexanediamine scaffolds. The interaction is progressively detuned by varying the transition energy of one chromophore through substitution with diphenylacetylene or phenyl units, while retaining the other chromophore as 1,4-bis(phenylethynyl)benzene. The CD band shape evolves from bisignate to monosignate as exciton coupling changes from degenerate to quasi-degenerate and nondegenerate, since one of the CD peaks undergoes a hypsochromic shift and eventually leaves the spectral window of interest, concomitant with a decrease in gabs. All bichromophoric systems exhibit similar emission spectra, quantum yields, and lifetimes, whereas glum decreases upon detuning, indicating that CPL originates from 1,4-bis(phenylethynyl)benzene due to the chiral electrostatic environment imposed by the neighboring chromophore.