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Vapor-phase fluorescence from the S1 (n, π*) states of pyridine-h5 and -d5, α- and β-picolines, and 2,6-lutidine has been studied. The fluorescence quantum yields of these compounds are, respectively, 5.9×10−5, 6.0×10−5, 3.5×10−5, 5.4×10−5, and 2.5×10−5 for excitation to the zero-point level of S1. The quantum yields of the first four compounds decrease sharply as the excitation energy is raised to the value corresponding to the S2 (π, π*) state, whereas the yield of 2,6-lutidine is fairly constant throughout the S1←S0 and S2←S0 absorption regions. The fluorescence quantum yields and their excitation-energy dependence undergo no significant change upon pressure increase from 0.5 to 20 Torr or addition of a foreign gas up to 760 Torr. The rate constants for the intersystem crossing (kISC) and the other nonradiative processes (kFQ) are evaluated as functions of the excitation energy for pyridine-h5 and α-picoline with the aid of the intersystem-crossing yield values which are available from the literature. The results show that kFQ increases rapidly with increasing excitation energy, while kISC is nearly constant. The fast nonradiative decay responsible for the sharp decrease of the fluorescence quantum yield is attributed to the S1→S0 internal conversion.

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The effect of hydrogen bonding on the electronic absorption spectra of phenol, α naphthol, and β naphthol has been investigated with particular attention to the relation between the nature of electronic transitions and their behavior in hydrogen bond formation. The spectra were obtained down to 2000 A in isooctane solution in the presence of varying concentrations of dioxane. From the analysis of the observed spectra, the equilibrium constants for the hydrogen bonds and the spectra of the hydrogen-bonded species were determined. The hydrogen bond energies are given for the ground and excited states of the solute molecules. The experimental results clearly indicate that effects of hydrogen bonding on electronic spectra differ markedly with transitions. Both the frequency shifts and the intensity changes differ in magnitude and even in sign according to the properties of the transitions concerned. It is shown that the transition at 47 000 cm−1 of α naphthol is displaced to higher frequencies upon formation of the hydrogen bond. No appreciable changes are produced by hydrogen bonding in the spectral patterns of the individual transitions, aside from broadening or smoothing of the vibrational structure. The behavior of the transitions in hydrogen bond formation is interpreted on the basis of the electronic structure of the solute molecules. Two factors are shown to be important for accounting for the mechanism of the hydrogen bonding effect: (a) a change in the electron density at the oxygen atom of the O–H group accompanying an electronic transition; (b) a decrease in the electronegativity of the same oxygen atom resulting from hydrogen bond formation.

The phosphorescence quantum yields of benzaldehyde, acetophenone, p-fluorobenzaldehyde, p-chlorobenzal-dehyde, and benzophenone vapors, measured at low pressure down to the order of 10−3 Torr as a function of excitation wavelength, are essentially constant in the region of excitation energy corresponding to the S1(n, π*) state, whereas the yields decrease very sharply as the excitation energy is raised to the value corresponding to the S2(π, π*) state. This indicates that the nonradiative pathway in the carbonyl compound varies according to the type of the singlet state to which the molecule is initially excited. When excited to the S1 and S2 states, the molecule passes to the phosphorescent triplet state and a decomposing state, respectively. Two mechanisms are suggested and examined for understanding the nonradiative properties of the carbonyl compound vapors: (I) The internal conversion S2→S1 is slow compared with the conversion from S2 to the decomposing state; (II) the vibrational energy redistribution in S1 between the vibrational mode populated through S2→S1 and the mode to be populated optically is slow compared with the conversion from the former mode to the decomposing state.