Chiroptical properties of axially chiral indolenine merocyanine dimers: role of the Huang–Rhys factor and interchromophore mixing of electric and magnetic transition dipole moments†

One major goal in designing chiral molecules for technical applications is maximising the chiroptical response, especially the anisotropy factor (g). In this study, axially chiral indolenine merocyanine dimers were synthesized in which the electron-withdrawing strength of the acceptor was systematically varied. Based on electro-optical absorption measurements of their respective monomers, the electronic character of these dyes ranges from polyene-like molecules up to cyanine-like derivatives. In the ECD spectra, dyes closer to the cyanine limit exhibit stronger chiroptical responses with higher g factors compared to those nearer the polyene limit. This was attributed to greater exciton coupling for the chromophore near the cyanine limit while in the polyene-like chromophores enhanced mutual cancellation of the opposing exciton couplet signals was observed. Quantum chemical calculations further confirmed that a decrease in the Huang–Rhys factor leads to an enhancement in rotational strength and g factor due to the concentration of oscillator strength in few vibronic transitions. Additionally, large exciton coupling and a small inhomogeneous linewidth were identified as key factors for achieving high g values. We also find that mutual geometries, where the electric transition dipole moment of one chromophore aligns well with the magnetic transition dipole moment of the other leads to an enhancement of the rotational strength of the dimer.