title: Emission spectra and electronic energy levels of the rotational isomers of pyridinecarboxaldehyde vapors.
abstract: Emission and excitation spectra of 2-, 3- and 4-pyridinecarboxaldehyde (2-, 3- and 4-PCA, respectively) vapors have been measured at different temperatures and compared to one another. The emission spectra of these vapors are shown to consist of the T(1)(n, pi) --> S(0) phosphorescence accompanied by the weak thermally activated S(1)(n, pi) --> S(0) delayed fluorescence. Two peaks originating from the two rotamers (syn and anti) have been identified in the fluorescence, phosphorescence and excitation spectra of 3-PCA vapor. Analyses of the temperature dependence and vibrational structure of the spectra of 3-PCA vapor provide the syn-anti energy difference of 190 +/- 30 cm(-1) in the T(1) (n, pi) state, 200 +/- 30 cm(-1) in the S(1)(n, pi) state, and 290 +/- 35 cm(-1) in the ground state. The ground-state energy difference is in agreement with the result of density functional theory (DFT) calculation for 3-PCA vapor. DFT calculation demonstrated also that the syn rotamer exists as a less stable isomer in the ground state for 2- and 3-PCA vapors.
authors.affiliation: Graduate School of Integrated Arts and Science, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima City, 739-8521 Japan. titoh@hiroshima-u.ac.jp
authors.forename: Takao
authors.initials: T
authors.lastname: Itoh