W. Albert Noyes

Photosensitization by Benzene Vapor: Biacetyl. The Triplet State of Benzene

Biacetyl strongly quenches the fluorescence of benzene vapor. There is a resulting dissociation of biacetyl which occurs presumably because the second excited singlet state of biacetyl is produced by energy transfer from the benzene. Emission by biacetyl also occurs, but the ratio of phosphorescence to fluorescence is very large and may be infinity. The triplet state of biacetyl seems to be produced preferentially by energy transfer from a triplet state of benzene. Emission efficiencies are such that nearly every singlet‐state molecule of benzene which does not fluoresce must undergo an intersystem crossover to the triplet state. Since this crossover predominates over fluorescent emission, the life of the singlet state of benzene is presumably determined mainly by the crossover. The effective cross section for self‐quenching of the singlet state of benzene is about 0.036×10−16 cm2, while the effective cross section for quenching by biacetyl is about 2.5×10−16 cm2. Since the life of the triplet state of be...

Some Aspects of the Photochemistry of Benzene

Three methods of measuring absolute gas‐phase‐emission yields from benzene are described. Two of the three proved to have large possible errors due to scattered light. One was used to give a value of Q2537=0.18 at a pressure of 10 torr at 25°C. This is in agreement with one value in the literature and slightly lower than the other although all three agree within the limits of error prescribed by the various authors. There is no significant variation with pressure but there is a marked wavelength effect. Below 2500 A the emission decreases rapidly and it is essentially zero at 2400 A and below in agreement with the findings of Poole. Variation at wavelengths longer than 2537 A may exist but seems to be small. By implication the emission yield of biacetyl vapor as determined by Almy and Gillette is probably accurate within the errors specified by those authors. By use of these data it is shown that there is now excellent agreement on the extent of cross‐over of excited‐singlet benzene molecules to the tripl...

Energy Balance in the Photochemistry of Benzene

Quantum yields of triplet‐state formation in benzene vapor have been determined at several wavelengths by the method of Cundall. From 2520 to 2668 A, values average about 0.63 if one excludes those at a single wavelength of 2535.5 A. At this wavelength a higher value is obtained, and its possible significance is briefly discussed. At 2420 A the triplet‐state yield is small and may be zero. Fluorescent yields have been scanned for ranges of a few Angstrom units from 2400 to 2600 A. Above 2520 A there is no significant change with wavelength. The trends in these results are briefly discussed, and it is pointed out that a detailed discussion of the theory of the photochemistry of benzene has little value because data now available are not sufficient to test any theory.

Photo‐Chemical Studies. XL. The Mechanism of the Photo‐Chemical Decomposition of Acetone

The data now available indicate quite strongly that the photo‐chemical decomposition of acetone proceeds mainly if not entirely by a free radical mechanism. The various steps are reviewed and the evidence for them given. At high pressures where wall effects may be neglected certain definite conclusions are possible. In the region where some reactions are both homogeneous and heterogeneous the proof for all steps is not available and would be difficult to obtain. The analyses for some of the known products are not satisfactory for the small amounts formed. Nevertheless, it is believed that the steps proposed are adequately substantiated, and that the difficulties of obtaining further information are so great as probably not to warrant more effort along certain lines.

Photochemical Primary Process in Biacetyl Vapor at 4358 A

At 4358 A at room temperature the quantum yield for the primary dissociation of biacetyl increases with intensity. It is thus implied that the primary process is second order in some active species. By use of the rotating sector it is shown that the species responsible for this effect has a mean life close to that ascribed to an excited triplet state for biacetyl. At higher temperatures this intensity effect disappears and one of the products strongly inhibits both the phosphorescence and the primary dissociation. At these temperatures the data may best be treated by assuming that the triplet state of biacetyl undergoes a unimolecular dissociation with an activation energy of about 15 kcal.

The Photo‐Chemical Decomposition of Acetone at 3130A

The photo‐chemical decomposition of gaseous acetone in light of 3130A has been investigated at 26°C under conditions of high acetone pressure and low absorbed light intensities. The ratio C2H6/CO has been found to decrease substantially below unity at sufficiently low absorbed intensities, under which condition methane has been found as a major product, exceeding the amount of ethane at very low intensities. A kinetic analysis of these data has shown that only a free radical process for the formation of ethane and methane suffices to explain the experimental observations. The energy of activation for the reaction of a methyl radical with an acetone molecule to form methane has been determined as E = 6.5 kcal./mole.

The Mechanism of Acetone Vapor Fluorescence

The decay rates for the fluorescence of acetone vapor have been determined at several temperatures and pressures, and in the presence and absence of foreign gases, particularly oxygen. The quenching of the fluorescence has also been studied over a similar range of conditions. Since both the presence of oxygen and an increase in temperature tend to eliminate the fluorescence from one of the upper states, it has proved possible to obtain rate constants for the state not so eliminated. Approximate constants for the other state may be obtained by use of the data on total fluorescence in conjunction with data on the temperature and oxygen insensitive state. The data indicate that these two states are formed by independent or nearly independent paths so that molecules in one state are not transformed into molecules in the other under the conditions of these experiments. A consideration of this and previous work permits a further elucidation of the fluorescence phenomena.

The Fluorescence of Biacetyl Vapor at 4358A

The fluorescence efficiency of biacetyl vapor when excited by radiation of wavelength 4358A is independent of the number of quanta absorbed per cubic centimeter per second, almost if not quite independent of biacetyl pressure, and decreases with increase in temperature but there seems to be a transition from one type of variation with temperature to another between 60° and 100°. Oxygen quenches the fluorescence strongly, and isobutylene quenches the fluorescence little or not at all.The marked difference in fluorescence behavior of biacetyl when excited by 4358A radiation from that when excited by 3660A radiation as pointed out by previous authors makes desirable an attempt to correlate fluorescence with photochemical behavior. Some suggestions are made with regard to mechanism.

Fluorescence and Photochemical Kinetics of Polyatomic Molecules in the Gas Phase

Various mechanisms for the excitation and quenching of fluorescence are discussed. It is pointed out that the classical Stern‐Volmer mechanism may apparently be obeyed in cases where the process is not that of simple absorption followed by fluorescence and collisional deactivation. For polyatomic molecules a spectroscopic examination will not always disclose definitely the nature of the emitting radical or molecule. A study of the effects of several variables on the intensity of the fluorescence will frequently give much information about the lifetimes of the emitting molecules and the course of the photochemical reactions taking place. Quantitative studies of the fluorescence in the gas phase have been made for very few polyatomic molecules. Nitrogen dioxide and acetone are cited as examples.

Photochemical Studies. XLVI. Photosensitization by Benzene and Pyridine Vapors

Attempts to sensitize the dissociations of the following molecules by means of benzene vapor have been made: hydrogen, oxygen, hydrogen chloride, nitrous oxide, methyl chloride, methyl bromide, methyl iodide. Positive results were obtained only with methyl iodide. The decompositions of methyl chloride and methyl bromine sensitized by pyridine vapor were also studied with negative results. The reasons for the results are discussed briefly.