Vibronic analysis of circular dichroism spectra of dimeric systems. Chiral molecules consisting of two polyacene chromophores

The CD spectra of (6R,15R)-(+)-6,15-dihydro-6,15-ethanonaphtho [2,3-c]pentaphene (DEP) and (7R,14R)-(+)-7,14-dihydro-7,14-ethanondibenz [a,h] anthracene (DEA) are analyzed as dimers of anthracene and naphthalene, respectively. It is shown that vibronic coupling effects are essential in understanding the vibronic structure of CD spectra in these molecules. The simple molecular dimer model is shown to explain the main vibronic features of the CD spectra, but an extension of that model is needed to achieve good quantitative agreement between experimental and theoretical CD spectra for physically reasonable exciton and vibronic coupling parameters. This extended dimer model allows for a mixing of electronic states of monomers via electronic intermonomer interaction. It is shown that the CD spectrum of DEP locates the hidden 1Lb state of the anthracene molecule about 2150 cm−1 above the origin of the 1La state, and that these two states are electronically coupled in DEP. Only these two assumptions taken together can well explain the CD and absorption spectra of DEP for the same set of parameters. We show that the CD spectra of DEA indicate strong mixing of the 1Lb and 1Bb and of the 1Bb and 1Cb states of naphthalene molecule via intermonomer interaction. The calculations presented here point out that the main reasons of previous failure to explain the vibronic structure of the CD spectra of DEP and DEA is the brakedown of the Born–Oppenheimer approximation in these dimers.