Computational challenges in simulating and analyzing experimental linear and nonlinear circular dichroism spectra. R-(+)-1,1'-bis(2-naphthol) as a prototype case.

One- and two-photon circular dichroism spectra of R-(+)-1,1'-bis(2-naphthol), BINOL, a prototype system showing structural chirality, were calculated, in both the gas and solvated phases, in a region of wavelengths extending down to 200 nm, by applying the time-dependent density functional theory response theory. Here we emphasize the computational challenges of such a simulation. The effect of the choice of the exchange-correlation functional is carefully analyzed. We compare results obtained, with correlation-consistent basis sets of double-ζ quality, with both the popular Becke's three-parameter exchange, Lee, Yang, and Parr correlation (B3LYP) functional, and the Coulomb attenuating method based on B3LYP (CAM-B3LYP) functional. For a better analysis of the reliability of the computational model, also one- and two-photon absorption spectra are calculated. Two-photon absorption and dichroism spectra of BINOL have been measured in our group recently. Experimental features are characterized in terms of molecular excitations and the differences in the response of each state in the one- and two-photon processes are highlighted. We analyze and discuss the possible causes for the theory-experiment discrepancy in intensity of the two-photon absorption and circular dichroism response. Conformational effects, which are mostly related to the rotation of the two naphthyl moieties, and/or the coupling of excitonic and charge-transfer excitations, are investigated as possible causes of enhancements of the signal.