Intermolecular Exciton Coupling and Vibronic Effects in Solid-State Circular Dichroism: a Case Study

The electronic circular dichroism spectrum (CD) of the sesquiterpenoid 1?,10?-epoxydesacetoxymatricarin (1) measured in the microcrystalline solid state differs from the solution one in the appearance of a pronounced vibrational fine structure in the long-wavelength region (n-?* enone transition) and of a new moderately intense band in the ?-?* region. TDDFT CD calculations were run on input structures derived from the X-ray geometry of 1 to reproduce the impact of exciton-type couplings between proximate dimers in the crystals. The vibrational structure of the CD spectrum was also reproduced on the isolated molecule modeling the potential energy surfaces at harmonic level and taking into account Duschinsky and Herzberg-Teller effects