Theoretical study on the excited states of rotational isomers of pyridinecarboxaldehyde vapors

The O-trans and O-cis rotamers in the electronic ground states and the low-lying excited states of 2-, 3- and 4-pyridinecarboxaldehydes have been investigated theoretically. The detailed studies of the electronic excited states reveal that the lowest triplet states (T1) and singlet states (S1) correspond to the nπ* excitations and all T1 states mainly come from the electron transition of the lone-pair electrons located on the carbonyl oxygen (n(O)) to π* orbitals and the electron transition of S1 states depends on the geometrical conformations. The mechanism has been further confirmed by the phosphorescence and fluorescence obtained from the transition dipole auto-correlation function approach. In addition, the calculated spectra agree well with the available experimental data.