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Center for Nonlinear Studies |
Large differences in conformation and topology between linear and cyclic conjugated molecules have a large impact on optical properties, thus presenting a fascinating opportunity to study structure-property relationships. The change in optical properties results primarily from a change in symmetry selection rules which cause the lowest singlet electronic transition to be strongly allowed in linear oligomers but forbidden in rings. This effect can be understood using an excitonic model in which individual transition dipole moments of the lowest transition sum in linear oligomers but cancel in rings. For such forbidden transitions, Herzberg-Teller intensity borrowing can contribute significantly to the optical properties. Here we present a detailed quantum-chemical investigation of the electronic excited states of a fully conjugated butadiynelinked zinc-porphyrin nanoring at the DFT and semiempirical levels.1 Our modelled Herzberg-Teller spectrum is in excellent agreement with the low temperature experimental fluorescence spectrum and allows full interpretation its complex structure. Our calculations show how the most strongly coupled HT vibration lifts the symmetry by bisecting the ring, creating two regions characterised by decreased and increased bond length alternation, and thus increased and decreased conjugation respectively. In order to render HT vibronic calculations for such a large system (360 atoms) tractable, we prescreened modes with fast INDO/SCI calculations and recomputed transition dipole derivatives with TD-DFT only for strongly coupled modes. Our approach might thus be well suited to investigating HT vibronic coupling effects in other large systems featuring forbidden transitions (e.g. solid state H-aggregates).
[1] J.K. Sprafke, D.V. Kondratuk, M. Wykes, A.L. Thompson, M. Hoffmann, R. Drevinskas, W.-H. Chen, C. K. Yong, J. Kärnbratt, J.E. Bullock, M. Malfois, M.R. Wasielewski, B. Albinsson, L.M. Herz, D. Zigmantas, D. Beljonne, H.L. Anderson. J. Am. Chem. Soc., 2011, 133, 17262-1727
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