Circular dichroism of helicenes investigated by time-dependent density functional theory

It is shown that molecular electronic circular dichroism (CD) can systematically be investigated by means of adiabatic time-dependent density functional theory (TDDFT). We briefly summarize the theory and outline its extension for the calculation of rotatory strengths. A new, efficient algorithm has been implemented in the TURBOMOLE program package for the present work, making large-scale applications feasible. The study of circular dichroism in helicenes has played a crucial role in the understanding of molecular optical activity. We present the first ab initio simulation of electronic CD spectra of [n]helicenes, n = 4-7, 12. Substituent effects are considered for the 2,15-dicyano and 2,15-dimethoxy derivates of hexahelicene; experimental CD spectra of these compounds were newly recorded for the present work. The calculations correctly reproduce the most important spectral features and greatly facilitate interpretation. We propose assignments of the low-energy bands in terms of individual excited states. Changes in the observed spectra depending on the number of rings and substitution patterns are worked out and rationalized. Merits and limitations of TDDFT in chemical applications are discussed.