Fumio Hirayama

Fluorescence of saturated hydrocarbons. III. Effect of molecular structure

Fluorescence spectra and fluorescence quantum yields have been determined for a wide variety of alkanes, cycloalkanes, and polycycloalkanes excited at 147 and 165 nm. Correlations between emission characteristics and molecular structure are noted and discussed.

Excimer Fluorescence of Benzene and Its Alkyl Derivatives—Concentration and Temperature Dependence

Fluorescence spectra and quantum yields have been obtained for methylcyclohexane solutions of benzene, toluene, ethylbenzene, cumene, p‐, m‐, o‐xylene, and 1,3,5‐, 1,2,3‐, 1,2,4‐trimethylbenzene as a function of aromatic concentration over the temperature range from 25 to −100°C. At low temperatures distinct excimer emissions were observed for all compounds studied. The intrinsic emission quantum yields of monomer φm and of excimer φe have been determined by a simple technique which requires no assumptions regarding the details of the monomer–excimer kinetics. With decreasing temperature, φe is observed to decrease contrary to the behavior of φm, suggesting that the rate constant for the excimer radiative transition decreases strongly as the temperature is lowered. Such temperature dependence is explained as arising from the existence of a substantial vibronic component in the transition moment that is induced by thermal excitation of upper‐state vibrational motions (e.g., torsional, tilting, etc.) of one monomer with respect to the other. From analysis of the temperature dependence of the fluorescence, lower bounds on the excimer binding energies Eb have been determined. The difference between this lower bound and Eb is approximately equal to the activation energy for radiative decay of the excimer. An estimate of this activation energy indicates that, for the case of benzene, Eb > 0.36 eV. The probability for association of excited monomer to form excimer and the probability for dissociation of excimer to an excited and unexcited monomer have been determined for benzene at 25 and −78°C from an appropriate analysis of the fluorescence quenching effect of CCl4. Additionally it has been demonstrated that the observed increase in CCl4 quenching efficiency at high benzene concentrations is predominantly due to an energy migration process. The probability per encounter for formation of excimer has been determined for benzene to be ≈1.0 and to decrease with alkyl substitution in a manner consistent with the steric requirements of sandwich‐type excimer configurations.Fluorescence spectra and quantum yields have been obtained for methylcyclohexane solutions of benzene, toluene, ethylbenzene, cumene, p‐, m‐, o‐xylene, and 1,3,5‐, 1,2,3‐, 1,2,4‐trimethylbenzene as a function of aromatic concentration over the temperature range from 25 to −100°C. At low temperatures distinct excimer emissions were observed for all compounds studied. The intrinsic emission quantum yields of monomer φm and of excimer φe have been determined by a simple technique which requires no assumptions regarding the details of the monomer–excimer kinetics. With decreasing temperature, φe is observed to decrease contrary to the behavior of φm, suggesting that the rate constant for the excimer radiative transition decreases strongly as the temperature is lowered. Such temperature dependence is explained as arising from the existence of a substantial vibronic component in the transition moment that is induced by thermal excitation of upper‐state vibrational motions (e.g., torsional, tilting, etc.) of one...

Effect of Solvent Perturbation on the S3 → S1 Internal Conversion Efficiency of Benzene, Toluene, and p‐Xylene

The S3 → S1 internal conversion efficiency of benzene is found to depend critically on the effectiveness of environmental perturbation in mixing the S3 and S1 states. Evidence for this derives from the experimental correlation of the conversion efficiency with the intensities of the 0–0 absorptive and emissive transitions in solutions with the solvents benzene, methanol, isopropyl alcohol, tetrahydrofuran, ethyl ether, acetonitrile, hexane, cyclohexane, methylcyclohexane, decalin, isooctane, and perfluorinated hexane. Additionally, the importance of S3 − S1 mixing is shown to explain the marked increase in the efficiency of internal conversion observed upon methyl substitution of benzene.

Fluorescence of saturated hydrocarbons. II. The effect of alkyl substituents

Abstract Fluorescence spectra and quantum yields have been determined for a variety of normal and cyclic alkanes and for some of their alkyl derivatives. In the case of normal alkanes, the quantum yield increases almost linearly with number of carbon atoms from pentane to heptadecane, whereas the emission spectrum remains unchanged. The addition ofa methyl group strongly reduces the quantum yield and shifts the spectrum to the red. For cycloalkanes, a relatively intense emission is observed only for cyclohexane and its alkyl derivatives, whereas cyclopentane, cycloeptane, cyclooctane, and cyclodecane exhibit no fluorescence.

Fluorescence of mono-olefinic hydrocarbons

A weak fluorescence has been observed from propylene, 1−butene, cis−2−butene, trans−2−butene, 1−hexene, cis−2−hexene, trans−2−hexene, 2−methyl−2−pentene, and 2,3−dimethyl−2−butene excited at 184.9 nm in vapor and/or liquid phases. Emission spectra and emission quantum yields are reported. For 2,3−dimethyl−2−butene (tetramethylethylene) the emission intensity was sufficiently strong to permit more detailed study of its properties, and accordingly the effect of longer wavelength excitation, the effectiveness of CCl4 and O2 as quenchers, and the effect of solution in low−temperature hydrocarbon matrices are additionally reported. Possible assignments for the olefin emissions are discussed.

The mechanism of excited state formation in radiolysis of hydrocarbon liquids

Abstract The fluorescence intensity of bicyclohexyl liquid excited both with β particles and with 184.9 nm light has been studied as a function of the concentration of perfluorocarbon electron scavengers. From this it has been possible to estimate (a) the fraction of all excited singlet states which derive from geminate ion pairs and (b) the probability (averaged over the spur size distribution) that a geminate ion pair will generate an excited singlet state on recombination. The latter value is compared with theoretical values and it is concluded that in bicyclohexyl the geminate electron does not spin-relax prior to recombination. Preliminary results for p -xylene are also presented and important differences between the saturated and aromatic hydrocarbon liquids are discussed.

The effect of crystalline phase on the fluorescence characteristics of solid cyclohexane and bicyclohexyl

Abstract Emission from cyclohexane and bicyclohexyl solids has been found to critically depend on their crystalline structure. The emission spectrum of cyclohexane suddenly blue-shifts 2900 cm −1 on cooling below its solid—solid phase transition temperature (−87.1°C). The bicyclohexyl emission intensity sharply decreases on freezing to solid and then again sharply recovers at a heretofore unreported phase transition temperature (≈ 0°C) with the spectrum 2500 cm −1 blue-shifted from that of the liquid. The results are discussed utilizing the partial Rydberg nature of the excited cycloalkane states.

SATURATED HYDROCARBONS AS DONORS IN ELECTRONIC ENERGY TRANSFER PROCESSES

Abstract Electronic energy transfer from saturated hydrocarbons (cyclohexane and heptane) to aromatic solutes (2,5-diphenyloxazole and p−xylene) has been studied using optical excitation at 1470 A. Evidence for the existence of two transferring states is presented. The shorter−lived state is definitely identified with the fluorescent state of the saturated hydrocarbon and is the precursor of a much longer−lived transferring state, tentatively suggested to be the triplet state. The gases O2, SF6, N2O and CO2 have been observed to strongly quench the donor fluorescent state and thereby the energy transfer process. These optical results are compared with those obtained using gamma ray excitation.