J. Phys. Chem. 1988, 92, 183-185
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Pump-Probe Photoionization Detection of Singlet and Triplet Decay in Anisole and p-Cresol in a Supersonic Free Jet Robert J. Lipert, Steven D. CoIson,* Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 06511
and Abha Sur Department of Chemistry, Mt. Holyoke College, South Hadley, Massachusetts 01 075 (Received: July 6, 1987)
The singlet and triplet decay of anisole and p-cresol in a supersonic free jet were determined by delayed photoionization mass spectrometry. A biexponential decay was observed in p-cresol yielding a triplet lifetime of 400 h 10 ns and a singlet lifetime 1 5 ns. Very different behavior was observed for anisole. Evidence suggests the triplet state is decaying at a rate approximately 10 times greater than in p-cresol while the anisole S1 state decays at least 5 times more slowly than the p-cresol
s,. Introduction The near-ultraviolet spectroscopy of proteins and their chromophores has been an active area of research for many While early studies were mainly performed in solution, the advent of lasers and supersonic expansion techniques4 allowed the photophysics of isolated protein chromophores to be probed in great detail.5-7 Recently, the gap between the isolated and the fully solvated molecule has begun to be bridged through the study of van der Waals complexes between solute and solvent molecules.8-10 Sur and Johnson” have used the pump-probe photoionization technique developed by Duncan et al.’* to study the effects of hydrogen bonding on the gas-phase photophysics of phenol, the chromophore of the amino acid tyrosine. It was found that complexation of a single water molecule with phenol lengthened the singlet-state lifetime and increased the quantum yield for intersystem crossing while decreasing the rate of intersystem crossing. They reported internal conversion, which was found to be the dominant decay channel for isolated phenol, was effectively eliminated. These results suggest that 0-H vibrations may be of great importance in the radiationless decay of phenol since it is the 0-H hydrogen of phenol that hydrogen bonds to the oxygen atom of water in a 1:l complex.I0 Moreover, the frequencies and/or frequency shifts of the 0-H vibrations make them good candidates for accepting modes in radiationless tran~itions.’~The alterations of 0-H vibrations which are a consequence of hydrogen-bond formation may be sufficient to greatly reduce this source of SI decay. It was also noted that the photophysics of anisole (which contains a methyl group instead of a hydroxyl hydrogen) in hy(1) Konev, S. V. Fluorescence and Phosphorescence of Proteins and Nucleic Acids; Plenum: New York, 1967. (2) Steiner, R. F., Weinryb, I., Eds. Excited States of Proteins and Nucleic Acids: Plenum: New York, 1971. (3) Demchenko, A. P. Ultrauiolet Spectroscopy of Proteins: SpringerVerlag: Berlin, 1986. (4) Levy, D. H. Annu. Reu. Phys. Chem. 1980, 31, 197. ( 5 ) Hays, T. R.; Henke, W. E.; Selzle, H. E.; Schlag, E. W. Chem. Phys. Lerr. 1983, 97, 347. (6) Bersohn, R.; Even, U.; Jortner, J. J . Chem. Phys. 1984, 80, 1050. (7) Rizzo, T. R.; Park, Y. d.; Levy, D. H. J. Chem. Phys. 1986,85, 6945. ( 8 ) Hager, J.; Ivanco, M.; Smith, M. A.; Wallace, S. C. Chem. Phys. 1986, 105, 397. (9) Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1982,86, 1768. (IO) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M.J. Phys. Chem. 1983, 87, 5083. (11) Sur,A.; Johnson, P. M. J . Chem. Phys. 1986, 84, 1206. (12) Duncan, M. A.; Dietz, T. G.; Liverman, M. G.; Smalley, R. E. J . Phys. Chem. 1981, 85, 7. (13) Avoures, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. Rev. 1977, 77, 793.
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drocarbon solvents resembled that of hydrogen-bonded phen01.l~ In order to make a more accurate evaluation of the extent to which hydrogen bonding is comparable to the complete elimination of the 0-H bond as it pertains to radiationless transitions, we have used the pump-probe photoionization technique to study the photophysics of anisole in a supersonic expansion. In addition, since anisole contains a methyl rotor which can be important in enhancing radiationless transition^,'^ p-cresol, which contains both a methyl rotor and an 0-H oscillator, yet is isomeric with anisole, was also studied.
Experimental Section The room temperature vapor of anisole (Aldrich, Gold Label) and p-cresol (MCB) was seeded into helium and expanded C W through a 12.5-hm pinhole at a backing pressure of 20 atm into a vacuum chamber maintained at Torr. One centimeter from the nozzle the free jet was crossed with the unfocused frequency-doubled output of a Nd:YAG pumped dye laser (Quanta-Ray DCR-2,PDL). The electronically excited molecules were then ionized with either 193- or 308-nm radiation from a Questek 2200 excimer laser, counterpropogating to the pump beam and also unfocused. A repelling field of 200 V/cm accelerated the resulting ions into a time-of-flight mass spectrometer. Parent ion signal was monitored as a function of the delay between the firing of the two lasers. The firing of both lasers was triggered by a Stanford DG535 digital delay/pulse generator interfaced to an LSI-11/23 computer. Delays were incremented in steps ranging from 0.5 ns for rapid decays to 2.0 ns for slower decays. Typically 10-30 laser shots were averaged at each delay setting and the entire decay was scanned from 5 to 10 times and averaged. The jitter in both lasers is estimated to be 3 ns. Results and Discussion As has been discussed by Dietz et a1.,16 if one wants to monitor the time evolution of the triplet state, the probe pulse must be of sufficiently short wavelength to produce the ion in approximately the same vibrational state as the highly vibrationally excited triplet state formed through intersystem crossing. This means the energy required is approximately equal to the ionization potential plus the singlet-triplet energy splitting, hEsT. The ionization potential of anisole is about 8.2 eV” while AEsT = 1 eV.18 Therefore (14) Kohler, G.; Kittel, G.; Getoff, N. J . Photochem. 1982, 18, 19. (15) Ito, M. J . Phys. Chem. 1987, 91, 517. (16) Dietz, T. G.; Duncan, M. A,; Smalley, R. E. J. Chem. Phys. 1982, 76, 1227. (17) Franklin, J. L.; Dillard, J. F.; Rosenstcck, H. M.;Herron, J. T.; Draxl, K.; Field, F. H. “Ionization Potentials, Appearance Potentials and Heats of Formation of Gaseous Positive Ions”; NSRDS-NBS 26; National Bureau of Standards: Washington, DC, 1969.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 1, 1988
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Figure 1. Decay curves obtained by pumping S , 0-0 and ionizing with a 193-nm ArF excimer laser. Solid lines are nonlinear least-squaresfits of a convolution of an excitation pulse with the data. (A) p-Cresol, fit to a biexponential decay; T~ = 4.1 ns, T~ = 400 10 ns. (B) Anisole, fit to a single-exponentialdecay; T = 31 ns
Figure 2. Decay curves obtained by pumping S, 0-0 and ionizing with a 308-nm XeCl excimer laser: (A) p-cresol; (B) anisole, fit to a singleexponential decay; 7 = 20 ns.
ionization of the triplet requires about 9.2 eV. Pumping the SI 0-0 at 4.51 eV and probing with 193-nm light or 6.42 eV provides 10.9 eV, clearly sufficient for ionization of both SI and the triplet state formed through intersystem crossing. For p-cresol the IP is about 8.8 eV" while we estimate AESTto be 1 eV so that approximately 9.8 eV are required for triplet ionization. Since the SI 0-0 lies at 4.4 eV, the 10.8 eV available for ionization again is enough energy to ionize SI and T. Shown in Figure 1 are the decay curves obtained by 193-nm ionization after pumping the SI 0-0 transitions in the respective molecules. The general form of the p-cresol decay curve is similar to those exhibited by toluene,I6 benzene,I9 and phenol." There is an initial rapid drop off of excited-state population attributed to singlet-state decay followed by a much slower decline due to triplet relaxation. Also shown is a nonlinear least-squares fit of the p-cresol data to a convolution of a biexponential decay with an excitation pulse modeled by assuming the pump and probe pulses are Gaussians with a fwhm of 8 and 10 ns, respectively. Since this excitation pulse is only an approximation to the true instrument response function which we have yet to measure ac-
curately, the singlet-state lifetime derived from this fit, 4.1 ns, is also only approximate. The hydrocarbon-solution value is 2.3 ns.*O The triplet lifetime of 400 i 10 ns is more reliable and can be compared to 300 ns for phenol." The decay of anisole is strikingly different. There is no obvious evidence for triplet-state ionization. A convolution with a single-exponential decay is shown which gives a lifetime of 3 1 ns, but the fit is clearly not very good. However, attempts to fit the data to a biexponential function were unsuccessful. In Figure 2 are shown the respective decays obtained with 308-nm ionization. In contrast to 193-nm ionization, 308-nm radiation is of too long a wavelength to ionize the triplet state in a one-photon process, again assuming A V = 0. Thus, these curves display the singlet-state decay profiles and are equivalent to fluorescence decays. A fit to a single-exponential decay is shown for the anisole data and gives a lifetime of 20 ns. This shorter lifetime suggests that there is indeed some intersystem crossing occurring in anisole and that the decay obtained with 193-nm ionization is composed of two similar decay functions, one with T = 20 ns, the other with T = 20-40 ns. No fitting of the p-cresol data was attempted since its lifetime is clearly so short that an
*
(18) McClure, D. S.J . Chem Phys. 1949, 17, 905. (19) Otis, C. E., Knee, J. L.; Johnson, P. M. J . Phys. Chem 1983, 87,
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(20) Berlman, I. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic. New York, 1965.
J . Phys. Chem. 1988, 92, 185-189 accurate instrument response function would be needed in order to extract a meaningful lifetime. These results show that the gas-phase excited-state dynamics of anisole are much different from those of p-cresol in that anisole displays a singlet lifetime that is on the order of 5 times longer and no long-lived triplet state is observed. The former finding, which parallels condensed-phase studies, is consistent with the idea that 0-H vibrations are in some way responsible for rapid internal conversion in phenol and p-cresol. Possible explanations for the latter observation are (1) anisole does not undergo intersystem crossing, ( 2 ) the efficiency for ionization of the triplet is very low, (3) ionization from the triplet-state results in an ion with enough vibrational energy to dissociate, (4) the triplet state itself is unstable and falls apart, or (5) the triplet state undergoes a rapid radiationless transition to an unstable state, either So (intersystem crossing to So gives the molecule 47 kcal/mol more vibrational energy than required for dissociation2’) or another triplet state. With regard to the first possibility, anisole is known’4 to undergo intersystem crossing in solution with a quantum yield of 0.63 and preliminary studies in this laboratory using pump-probe photoelectron spectroscopy confirm the presence of a short-lived triplet state in the gas phase.22 This also established that we are indeed able to ionize the triplet. If ionization of the triplet resulted in an unstable ion which fragmented, daughter ions would be seen in the mass spectrum. Under the conditions of the experiment, the mass spectrum consisted of a single peak corresponding to the parent ion. This does not exclude the possibility that the triplet state itself fragments. However, the singlet-triplet gap in anisole is about 8000 cm-I which is insufficient to cleave any bonds. We believe anisole (21) Paul, S.; Back, M. H. Can. J. Chem. 1975, 53, 3330. (22) Miller, P. J.; Lipert, R. J.; Sur, A.; Chupka, W. A., unpublished results.
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is not crossing directly from the singlet to an unbound triplet state since the triplet lifetime in EPA at 77 K is 3 s.lS It therefore appears that triplet anisole is decaying by way of a radiationless process that is accelerated relative to p-cresol and quenched by rapid relaxation in low-temperature glasses. A possible source of this rate enhancement is more favorable intersystem crossing to So. A lack of information on the triplet manifolds of the respective molecules makes it difficult to assess this possibility. An alternative explanation is that triplet anisole is predissociating via an unbound triplet state. Assuming the 0-H dissociation energy in p-cresol is about the same as in phenol, the energy required to dissociate anisole into phenoxy and methyl radicals is about 30 kcal/mol less than that required to dissociate p-cresol into a phenoxy radical and atomic hydrogen. The unbound triplet state in anisole that correlates with the radical fragments will therefore be lowered in energy, making this decay channel more accessible.
Conclusions It has been found that the substitution of CH3 for H in the para position in phenol has relatively little influence on the excited-state decay of the molecule. Thus the decay dynamics of p-cresol is similar to that of phenol. In contrast, the substitution of CH, for the 0-H hydrogen in phenol results in large changes in excited-state decay. The singlet-state lifetime of anisole is longer than that of phenol, in parallel with the effect of hydrogen bonding to the 0-H hydrogen in phenol. The vibrationally excited triplet state in anisole was found to be much shorter lived than similar states in phenol and p-cresol, possibly because of the lower energy dissociation channel to form phenoxy and methyl radicals. Acknowledgment. This research was supported by the National Institutes of Health. Registry No. Anisole, 100-66-3; p-cresol, 106-44-5
Two Types of Surfactant Phases and Four Coexisting Liquid Phases in a Water/Nonionic Surfactant/Triglyceride/Hydrocarbon System Hironobu Kunieda,* Hiroshi Asaoka, and K6z6 Shinoda Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku. Yokohama 240, Japan (Received: March 27, 1987; I n Final Form: July 29, 1987)
A three-phase region consisting of reversed micellar solution (Om), surfactant (D’), and excess water (W) phases was observed in a wide range of water/oil ratios in a ~ater/R,~EO,/triglyceride(1,2,3-[tris(2-ethylhexanoyloxy)]propane, TEH) system.
The composition of middle phase (D’) remains in the vicinity of a water-surfactant axis, and its phase behavior is different from that in a water/nonionic surfactant/saturated hydrocarbon system, in which the composition of surfactant phase (D) changes from water-rich to oil-rich with increasing lipophilicity of surfactant. The D’ phase is identified with the surfactant phase known as the L3 phase in which an oblate spheroid aggregate is present. In a four-component system of water/ R,,EO,/TEH/hexadecane, a four-phase region consisting of water, two surfactant (D and D’), and oil phases appears due to the overlapping of two three-phase regions. The mechanism for the formation of the four-phase region and the existence of four types of three-phase regions were concluded and actually discovered in a carefully selected system.
Introduction Three coexisting phases (water (W), surfactant (D), and oil (0)phases) appear under certain conditions in a water/nonionic surfactant/oil system or in a brine/ionic surfactant/cosurfactant/oil system.’-3 In the three-phase region, the solubilizing (1) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1954; p 68. (2) Shinod;, K.; Saito, H. J . Colloid Interface Sci. 1968, 26, 70. (3) Kunieda, H.; Friberg, S . E. Bull. Chem. Soc. Jpn. 1981, 54, 1010.
power of surfactant reaches its maximum (formation of microemulsion) and the minimum interfacial tensions are attained due to the critical solution These phenomena are very important for practical applications such as an enhanced oil reco~ery.~.’ (4) Kunieda, H.; Shinoda, K. Bull. Chem. SOC.Jpn. 1982, 55, 1777. (5) Shinoda, K.; Kunieda, H.; Arai, T.; Saijo, H. J. Phys. Chem. 1984,88,
5126. (6) Shah, I>. 0.; Schechter, R. S., Eds. Improved Oil Recovery by Surfactant and Polymer Flooding; Academic: New York, 1977.
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