Christof Hättig,^†‖ Arnim Hellweg,^{‡‖ Andreas Köhn*§⊥,
Contribution from the Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany, Institut für Physikalische Chemie, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany, and Center for Theoretical Chemistry, Department of Chemistry, Århus University, DK-8000 Århus, Denmark}

J. Am. Chem. Soc., 128, 15672-15682 (2006).
Received June 14, 2006; E-mail: andreas.koehn@uni-mainz.de

Quantum-chemical calculations with the approximate coupled-cluster singles-and-doubles model CC2 have been carried out for 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6). For this molecule dual fluorescence was experimentally observed, raising the discussion about the importance of the amino twist angle for this process. The calculations suggest that both the ground state and the normal fluorescent state are significantly twisted by 30°-40° and that the molecule is flexible enough to move into an even stronger twisted conformation (60°-70°) in its intramolecular charge-transfer (ICT) state which is responsible for the anomalous fluorescence band. Such a conformation both minimizes the total energy in the S₁ state and maximizes the dipole moment. The barrier from the normal fluorescent state to the ICT state region is very small. Comparison to the situation in the 1-methyl-derivative NMC6 suggests that a large alkyl substituent makes the preferably planar normal fluorescent state energetically unfavorable compared to the ICT state and thus promotes the occurrence of dual fluorescence.

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