Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study

The electronic circular dichroism (CD) spectrum of the sesquiterpenoid 1β,10β-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-π* enone transition) and of a new moderately intense band in the π-π* 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 molecules in the crystals. The vibrational structure of the CD spectrum was also reproduced for the isolated molecule by modelling the potential energy surfaces at the harmonic level and taking into account Duschinsky and Herzberg-Teller effects