Excitation-energy dependence of the phosphorescence quantum yields of pyridinecarboxaldehyde vapors

Emission and excitation spectra of 3- and 4-pyridinecarboxaldehyde vapors have been measured at different pressures down to 10−2 Torr. The phosphorescence quantum yield measured at low pressure as a function of excitation energy is nearly constant in the range of excitation energy corresponding to the S1(n, π*) state, but it decreases abruptly at the S2(π, π*) threshold. The onset of the abrupt decrease of the yield corresponds to the location of the S2 absorption origin of each molecule, indicating that the nonradiative pathway depends on the type of the excited singlet state to which the molecule is initially excited. The relaxation processes are discussed based on the pressure and excitation-energy dependence of the phosphorescence quantum yield.