Excited states of porphyrin and porphycene aggregates: Computational insights

Computational studies of the structure and electronic spectroscopy of porphyrin and porphycene aggregates has been carried out using Density Functional Theory. A comparison of different DFT approaches has been undertaken, and we show that the proper treatment must include dispersion, here through the B97-D functional, to correctly find bound species. Accurate prediction of absorption spectra for aggregates is possible if care is taken when choosing an appropriate methodology. Excitonic bands are observed in the aggregates, which highlight the importance of using long-range corrected DFT functionals in order to capture aggregate excitations. Excited state stabilisation models developed in the past, and used for spectra analysis in the case of aggregation, are shown to be qualitatively valid for excitations along an aggregation slipping coordinate. It is shown that natural transition orbitals provide the best framework for analysing aggregate spectra.