Emission spectra of 3,4- and 3,5-difluorobenzaldehyde vapors: Conformer-specific phosphorescence from the 3,4-derivative

Emission spectra of 3,4- and 3,5-difluorobenzaldehyde (3,4- and 3,5-DFB, respectively) vapors have been measured at different temperatures along with the absorption spectra. Doublet structure was observed in the main phosphorescence bands of 3,4-DFB vapor, which was assigned as originating from the two rotational isomers, whereas such a doublet structure was not observed for 3,5-DFB vapor for which no stable rotamer exists. Temperature dependence of the emission spectrum of 3,4-DFB vapor provides the energy difference of 90 ± 20 cm−1 between the two rotational isomers in the T1(n, π∗) state and 140 ± 20 cm−1 for those in the S0 state. The results of DFT calculation at the B3LYP/6-311++G∗∗ level and vibrational analysis of the emission spectra indicate that the T1 phosphorescence origins of the syn and anti conformers of 3,4-DFB vapor are located at 25280 and 25230 cm−1, respectively. The S1 and T1 origins of 3,5-DFB vapor are determined at 26570 and 24775 cm−1, respectively. The energy levels of 3,4-DFB vapor are compared with those of 3-fluorobenzaldehyde vapor.