Sanford Lipsky

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.

Internal Conversion from Upper Electronic States to the First Excited Singlet State of Benzene, Toluene, p‐Xylene, and Mesitylene

The variation with excitation wavelength of the fluorescence efficiencies of some alkyl benzenes has been studied from 2700 to 1600 A. Measurements have been made in both liquid (pure and dilute solution) and vapor phases. For pure liquids the internal conversion efficiency βλ from upper electronic states to the first excited singlet state has been found to be significantly less than unity and to increase in the order benzene, toluene, p‐xylene, mesitylene when comparisons of βλ are made at comparable excitation wavelengths. Solvent effects on βλ have been noted. No emission is observed from any alkyl benzene studied in the vapor phase for excitations beginning on the long‐wavelength side of the second absorption band and extending to shorter wavelengths.Dissociation, internal conversion to the ground state, and intersystem crossing to a triplet state are considered as mechanisms for the disappearance of the upper electronic singlet states. Measurements of the benzene‐sensitized phosphorescence of biacety...

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.

Yields of excited singlet states of some saturated hydrocarbon liquids when excited by fast electrons

Abstract G values have been measured for the production of the lowest excited singlet states of cyclohexane, methylcyclohexane, 2, 3-dimethylbutane, dodecane, hexadecane, cis -decalin and bicyclohexyl. The technique is based on comparison of the fluorescence intensities of these liquids with that of neat benzene. Comparison of the results obtained in this manner with those obtained via a solute sensitization technique indicate, at least for cyclohexane, that the solute technique may strongly underestimate the true G value.

Photochemistry of Benzene Vapor at 1849 Å

The benzene‐disappearance quantum yield at 1849 A and 1 torr has been determined to be 0.25±0.02 and independent of the number of absorbed quanta to ∼15% conversion. Nitrogen pressures from 0.1 to 50 atm reduce the disappearance yield. At 1 atm N2, the yield is reduced approximately fourfold. Addition of 200 torr propane reduces the yield more than one order of magnitude. At 2 and 3 torr benzene, the quantum yield is reduced to 0.13 and 0.10, respectively, and extrapolates at zero pressure to ∼1.0. Only two major products are found. One is volatile with structured absorption in the region 2600 to 2100 A. Seventeen peaks are resolved in the spectrum with constant separation of ∼500 cm−1. Assuming that the initial yield of the product is equal to the disappearance yield of benzene, an oscillator strength of 0.2 is calculated for the band with maximum absorptivity of 8300 liters/mole·cm at 2350 A. The compound is tentatively identified as an isomer of benzene. The second product is of high molecular weight a...

The electron impact spectra of some mono‐olefinic hydrocarbonsa)

Electron impact spectra of ethylene, propylene, isobutene, trans‐butene, cis‐butene, trimethylethylene, and tetramethylethylene have been obtained at scattering angles of 0° and 90° and at impact energies from ?20 to 150 eV. The spectra scan an energy‐loss region from 2.5–15 eV. All of the observed Rydberg transitions of the methyl derivatives are correlated to corresponding Rydberg transitions of ethylene. The missing π→3p transitions of ethylene are tentatively located via this correlation. Evidence is also presented for assigning the N→3R′ system of ethylene (at 8.26 eV) to a π→3px transition. Possible assignments of some broad continua above ?8–9eV as σ→π* and σ→σ* transitions are considered. In agreement with other reported large‐angle electron impact spectra, no evidence is obtained for transitions that could be assigned to triplet Rydberg states. However the π→π* triplet transitions are all clearly located with transition energies in good agreement with those obtained by a variety of other techniques.

The fluorescence of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) in nonpolar organic liquids for excitation energies below and above the photoionization threshold

The fluorescence from TMPD has been studied in the solvents n‐hexane, methylcyclohexane, cyclohexane, isooctane, and tetramethylsilane as a function of excitation wavelength from λex =300 nm to λex?170 nm. In no solvent is there observed any apparent decline in the fluorescence yield Φf for excitation energies to ?0.7–0.9 eV above the photoionization threshold. At higher energies Φf declines slightly. However, in the presence of low concentrations of either carbon tetrachloride, chloroform, ethylbromide, or n‐perfluoroheptane there is observed an abrupt increase in the quenching efficiency of the TMPD fluorescence almost precisely at an excitation energy equal to the threshold energy for photoionization et. The quenchable species Se generated at et is found to have the following properties: (a) the probability of its quenching follows the usual Stern–Volmer form characteristic of the quenching of a spatially coherent excited state and not of a geminate ion pair; (b) the quantum yield ηe for the production...

Magnetic field effects on recombination fluorescence in liquid iso-octane

The 123.6 nm photoionization of deuterated isooctane at −10 °C in the presence of hexafluorobenzene has been studied by examining the effect of a magnetic field to alter the quantum yield of recombination fluorescence. This fluorescence results from geminate recombination of hexafluorobenzene anions with isooctane positive ions. The use of a deuterated as contrasted to a protonated alkane makes the intensity of the recombination fluorescence much more sensitive to the magnetic field and permits observation of two maxima in the fluorescence yield at field strengths of 0 and 411 G and a possible third maximum at 822 G. The theory of the hyperfine induced spin evolution predicts these resonances at selected multiples of the C6F−6 hyperfine constant of 137 G. Utilizing the diffusion theory of geminate recombination in a Coulomb field, the experimental magnetic field spectrum is found to be well predicted over most of the range of magnetic field strengths studied (up to 2.5 kG) by a simple, one parameter, expo...