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

It is shown that molecular electronic circular dichroism (CD) can systemati cally be investigated by means of adiabatic time-dependent density function al theory (TDDFT). We briefly summarize the theory and outline its extensio n for the calculation of rotatory strengths. A new, efficient algorithm has been implemented in the TURBOMOLE program package for the present work, ma king large-scale applications feasible. The study of circular dichroism in helicenes has played a crucial role in the understanding of molecular optic al activity. We present the first ab initio simulation of electronic CD spe ctra of [n]helicenes, n = 4-7, 12. Substituent effects are considered for t he 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 spect ra depending on the number of rings and substitution patterns are worked ou t and rationalized. Merits and limitations of TDDFT in chemical application s are discussed.