Intramolecular Charge-Transfer Mechanism in Quinolidines: The Role of the Amino Twist Angle

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.