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J. Phys. Chem. 1987, 91, 4436-4438
Picosecond Transient Absorption Measurements of Geminate Electron-Cation Recombination C. L. Braun* Department of Chemistry, Dartmouth College, Hanover. New Hampshire 03755
and T. W. Scott Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale. New Jersey 08801 (Received: March 24, 1987; In Final Form: June 8, 1987)
We report the decay kinetics of electron-ation pairs based on observation of transient optical absorption by geminate electrons. The pairs were produced by two-photon ionization of dilute solutions of azulene or benzene in liquid hexane. We compare results at 223 K with those obtained from infrared-stimulated conductivity and find that bias in the conductivity technique is not severe. Both methods indicate that when benzene in hexane is photoionized by sequential two-photon excitation at 266 nm, geminate pairs of initial radius near 50 8, are formed.
Introduction When a molecule is photoionized in a hydrocarbon liquid or solid, the photoelectron is expected to thermalize in less than 1 ps.' The resultant cation-electron charge pair is separated by 2-15 nm?4 distances at which the Coulombic binding energy is greater than the average thermal energy of the electron. The majority of such geminate charge pairs thus recombine. Geminate charge pairs are the major initial species produced when ionizing radiation is absorbed by matter. Steady-state experiments have revealed a great deal about the properties of such pairs.',5 Recently, time-resolved studies of geminate cationanion recombination have added additional detail.68 In those studies, geminate electrons produced by pulsed electron-beam excitation are intercepted by added scavenger molecules which act to slow the subsequent ion recombination. It has also been shown that the first half-life of electron-cation pairs in scavenger-free liquid hexane is less than 100 ps at room t e m p e r a t ~ r e . ~ We have studied the time dependence of geminate charge pair recombination for dilute anthracene photoionized in liquid hexane.4*'0v11We used a highly sensitive conductivity technique in which an intense infrared pulse, absorbed by the geminate electrons, increases the probability that the electrons escape recombination. Because the population of geminate pairs decays with time, the infrared-stimulated conductivity decreases as the delay time between ionizing pulse and infrared probe is increased. With the conductivity method, we have demonstrated that (a) there are no long-lived (>5 ps) precursors of the geminate charge pairs,4 (b) the diffusion constant of the electron as measured in time-of-flight mobility experiments governs the recombination kinetics,I0 (c) the distribution g(r) of initial cation-electron separations is fairly broad,I0J1 and (d) the distribution g(r) is not strongly temperature dependent.I0 Despite the high sensitivity of the conductivity technique, it suffers from probable biaslo in detecting geminate pairs of varying ( 1 ) Warman, J. M. NATO Adv. Study Inst. Ser., Ser. C. 1982, 86,433. (2) Choi, H. T.; Sethi, D. S.; Braun, C. L. J . Chem. Phys. 1982, 77, 6027. (3) Holroyd, R. A.; Russell, R. L. J . Phys. Chem. 1974, 78, 2128. (4) Braun, C. L.; Scott, T. W. J. Phys. Chem. 1983,87, 4776. ( 5 ) Yakovlev, B. S.;Lukin, L. V. Adv. Chem. Phys. 1985, 60, 99. (6) Sauer, Jr., M. C.; Jonah, C. D. J . Phys. Chem. 1980,84,2539. Jonah, C. D.; Sauer, M. C.; Cooper, R.; Trifunac, A. D. Chem. Phys. Lett. 1979.63, 535. Smith, J. P.; Trifunac, A. D. J . Phys. Chem. 1981, 85, 1645. (7) Van den Ende, C. A. M.; Luthjens, L. H.; Warman, J . M.; Hummel, A. Radiat. Phys. Chem. 1982,19,455. Van den Ende, C. A. M.; Nyikos, L.; Warman, J. M.; Hummel, A. Radiat. Phys. Chem. 1980, 15, 273. (8) Tagawa, S.; Tabata, Y.; Kobayashi, H.; Washio, M. Radiat. Phys. Chem. 1982,19, 193. Katsumura, Y.; Tabata, Y.; Tagawa, S.Radiat. Phys. Chem. 1982, 29, 267. Tagawa, S.; Washio, M.; Tabata, Y.; Kobayashi, H. Radiat. Phys. Chem. 1982, 19, 217. Tagawa, S.; Katsumura, Y.; Tabata, Y . Radiat. Phys. Chem. 1982, 19, 125. (9) Jonah, C. D. Radiat. Phys. Chem. 1983, 21, 53. (10) Scott, T. W.; Braun, C. L. Can. J . Chem. 1985, 63, 228. (11) Scott, T. W.; Braun, C. L. Chem. Phys. Lett. 1986, 127, 501.
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radii. Here we report measurements of the decay kinetics based on optical absorption by geminate electrons. We then compare the new transient absorption measurements with corresponding conductivity-detected decay kinetics and conclude that the bias in those earlier measurements is not severe. Related experiments which rely on isolating absorption by geminate cations in the presence of interfering absorption by neutral excited states have recently been reported.I2 Experimental Section The laser" and the apparatus used for pulseprobe conductivity experiments4J0have been described earlier. Transient absorption experiments employed 35-ps pulses of 1-4 mJ at 355 or 266 nm. The excitation pulse was focused by a 1-m lens to a spot size of about 0.5 mm. The IR probe beam at 1.06 or 1.91 pm was attenuated to a small fraction of a millijoule. Stirred hexane solutions in 1-cm quartz cuvettes were temperature-controlled with flowing N2gas. The solutions were usually air-equilibrated, but occasional deoxygenation by N2 bubbling was found to produce no effect on signal levels or decay kinetics.
Results and Discussion Anthracene at high dilution in liquid hexane was the source of the geminate charge pairs whose decay was observed in our IR-stimulated conductivity experiment^.^*'^-" In the optical absorption experiments reported here, 355-nm excitation of anthracene in hexane at room temperature gave a transient signal at 1.06 pm, but it displayed only the slow decay characteristic of the first-excited singlet state of anthracene. Similarly, slow decays were exhibited by solutions of perylene and rubrene in hexane when excited by a 266-nm pulse and observed at 1.06 pm. Pyrene solutions excited at either 355 or 266 nm exhibited the fast 1.06-pm absorption decay expected for geminate electrons in hexane. Other solutes which allow observation of similarly fast initial decays include azulene, benzene, and triphenylamine. Here we report results for benzene and azulene. The upper curve in Figure 1 is the 1.06-pm transient absorption M solution of azulene in n-hexane excited at from a 8.7 X 355 nm. On the basis of the preliminary work described above, we suspected that a substantial portion of the slowly decaying tail resulted from absorption by neutral azulene excited states. This view is confirmed by the lower curve in Figure 1 which shows the effect of the presence of 0.042 M perfluorohexane (PFH) in the azulene solution. This well-known electron scavenger does a good job of quenching the absorption peak near time zero, thus confirming its geminate electron origins. Following the attenuated maximum in the lower curve, the residual signal rises slowly with ~~
~
(12) Hirata, Y.; Mataga, N.;Sakata, Y.; Misumi, S . J. Phys. Chem. 1986, 90, 6065.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4437
Letters
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120
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240 360 Probe Delay (PSEC)
1
480
I
1.0
600
Figure 1. Upper curve: transient absorption at 1.06 pm of 8.7 X lo4
M azulene in hexane excited at 355 nm; T = 296 K. Lower curve: same solution but with 0.042 M perfluorohexane added as electron scavenger.
time indicating buildup of an unidentified species which may be produced by electron scavenging. Indeed, addition of either CC14 or PFH as electron scavengers causes slow photodegradation of azulene under our experimental conditions. The slow buildup in the lower curve could also result from S, absorption following conversion of S2 (populated by 355-nm excitation) to S,; the S2 lifetime is 1.4 ns.13 Given these several complications, the decay kinetics for the geminate electrons seen in Figure 1 are not quantitative. However, if the transmission observed at 240 ps in the lower curve is taken to be characteristic of excited singlet absorption and is subtracted from the upper curve, the difference signal can be fit’o$”with a mean geminate pair radius of 55 A. While that radius is reasonable, it is highly approximate especially considering the poor time resolution available for examining the short-lived decay kinetics of geminate electrons at room temperature. Transient absorption by azulene solutions was also probed at 1.91 pm in an attempt to minimize complications from excitedstate absorption. The longer wavelength IR pulse was obtained by a one-quantum Stokes’ shift of the 1.06-pm fundamental in high-pressure hydrogen gas. The long-lived tail was a bit less prominent than that in the top curve of Figure 1, but absorption by neutral excited state(s) was not eliminated. The top curve in Figure 2 depicts the transient absorption at 1.06 pm resulting from 266-nm irradiation of 0.07 M benzene in hexane at 223 K. The lower solid curve is for the same solution but after 0.05 M PFH has been added. Again as in the case with azulene as solute, PFH appears to quickly scavenge the geminate electrons. If it is assumed that the plateau absorbance in the lower curve is the result of absorption by benzene SI states, then that absorbance should be subtracted from the upper curve in order to obtain the true geminate pair decay curve. The dashed curve gives the risetime behavior expected for a population of benzene Si's. The dashed plus the horizontal portion of the lower solid curve is subtracted to yield Figure 3. The subtraction procedure is legitimate provided that the rapid decay in the lower solid curve reflects only the rapid scavenging of geminate electrons by PFH. This ignores the possibility that multiphoton absorption to populate hexane excited states is responsible for some of the absorbance near time zero (pulse overlap regime). The data of Figure 3 can be fitl0>”,I4with either the Gauss = 4 9 / ( ~ ’ / ~ Gexp(-r2/G2) ~) or r2EXP = 9/(2L3) exp(-r/L) distribution functions where r is the initial geminate pair radius and G and L are characteristic separation parameters. The latter fit is shown and yields a mean thermalization radius (3L) of 45 %.; a value of G = 45 A in Gauss gives a very similar fit. Figure 3 should be compared with the conductance-mode data of Figure 4. Note that the two curves are very similar with the transient absorbance decaying just a bit more rapidly than the IR-stimulated (13)Berlman, I. B. Handbook of Fluorescence Spictra of Aromatic Molecules; Academic: New York, 1965;p 103. (14) Hong, K. M.; Noolandi, J. J . Chem. Phys. 1978,68,5163; 1978,69, 5026.
8 , O M
0
0
with 0.05 M pemuorohexane
,
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200
400
600
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800
I
loo0
PROBE DELAY PSEC)
Figure 2. Upper curve: transient absorption at 1.06 pm of 0.07 M benzene in hexane excited at 266 nm;T = 223 K. Lower curve: same
solution but with 0.05 M perfluorohexane added as electron scavenger. The solid curves are meant only to guide the eye. The dashed curve is the estimated early time contribution by SIstates of benzene.
2.0
i
AT% 1
.o
PROBE DELAY (PSEC)
Figure 3. Data of Figure 2 corrected for benzene excited-state absorption as described in text. The smooth curve is for an PEXP distribution with a mean radius of 45 A. At 223 K, the Onsager radius r, = e2/(4rcc0kT) = 375.6 A and the electron diffusion constant D = 1.92 X lo4 cmz s-l. The smooth curve is based on the assumption that both the UV and IR pulses are Gaussian with a full width at half-maximum (fwhm) of 35 ps.
conductivity. The solid curve in Figure 4 is the decay expected for an GEXP distribution of mean radius 57 A; a value of G = 57 A in Gauss gives a very similar fit. Apparently, the true geminate pair population as measured by transient absorption decays at almost the same rate as that indicated by the time dependence of the IR-stimulated conductivity. This might seem odd, because the conductance experiment is biased in favor of detecting large (100 A) pairs rather than small (20 A) pairs,I0 and larger pairs recombine at a slower rate. Moreover, as was first shown by Hummel,I5 the mean radius of the surviving geminate pairs increases monotonically with time. There are thus two reasons why IR-stimulated conductivity should yield decays that are slower than the true geminate recombination kinetics: the technique preferentially observes larger, slower decaying pairs and also signals the presence of surviving pairs with (1 5 ) Hummel, A. In Aduances in Radiation Chemistry; Burton, M., Magee, J. L., Eds.; Wiley: New York, 1974;Vol. 4,p 28.
J. Phys. Chem. 1987,91,4438-4440
4438
4-
3Ai
2-
have rate constants of 1-2 X 10l2 L mol-' s-l at 295 K. The electron diffusion constant is roughly 10 times smaller at 223 K, and the scavenging rate constant is expected to fall by a factor of 10 as well. Indeed, A G H observed the expected change with temperature for trichloroethylene as scavenger.16 Using 1 X 10" L mol-' s-l as the rate constant appropriate to 223 K, one estimates a first half-life of 200 ps for electron scavenging by 0.05 M PFH. The scavenging seen in Figure 3 is clearly faster; most of the geminate electrons disappear on a time scale shorter than the pulse-overlap time of about 60 ps. While more work is needed, the result suggests that PFH may act to scavenge nonthermalized electrons. Further work could reveal the time scale on which electrons fall into the low-mobility state characteristic of the liquid at 223 K. We note that Lee and Lipsky have seen evidence of epithermal electron scavenging by perfluorinated hydrocarbons in various hydrocarbon solvent^.'^ The present transient absor tion results indicate that, at least for initial pair radii near 50 , the IR-stimulated conductivity technique provides a reasonably unbiased view of geminate pair decay kinetics. Further work will be required to assess the detailed shape and possible temperature dependence of the distribution function of initial thermalization lengths.
.. I La.i .
i-
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I
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200
400
500
800
J
1000
Time (psec)
Figure 4. IR-stimulatedconductance signal for 0.1 M benzene in hexane excited at 266 nm; T = 225 K. The maximum IR-stimulated signal Ai was approximately twice that of the signal excited by a UV pulse alone. The smooth curve is for an ?EXP distribution with a mean radius of 57 A, D = 2.09 X lo4 cm2 s-', r, = 372.8 A, and a fwhm for both pulses of 40 ps.
higher efficiency as their radii grow larger. It is surprising that the kinetics of Figure 4 are only marginally slower than those of Figure 3. The high efficiency of geminate electron scavenging which is seen in Figure 3 is also surprising. While the rate constant for electron scavenging by PFH in hexane does not seem to have been measured, it is expected to be at or near the diffusion-controlled value of other good electron scavengers. Allen, Gangwer, and Holroyd (AGH) have shown16that a number of such molecules
w
Acknowledgment. Grant DE-FG02-86ER13592 from the Department of Energy provided partial support for this work such support does not constitute an endorsement by DOE of the views expressed in the article. We also thank reviewer 2 for making us aware of Hummel's r e ~ u 1 t . l ~ (16) Allen, A. 0.; Gangwer, T. E.; Holroyd, R. A. J . Phys. Chem. 1975, 79, 25. (17) Lee, K.; Lipsky, S. J. Phys. Chem. 1982, 86, 1985.
Selective Enhancement of Proline Raman Slgnals wlth Ultraviolet Excitation' Leland Mayne and Bruce Hudson* Department of Chemistry and Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403 (Received: November 17, 1986; In Final Form: June 24, 1987)
Preresonant Raman spectra of Gly-Pro, Pro-Gly, and N-acetylproline methylamide with excitations between 240 and 200 nm are presented. It is shown that, with excitation near 230 nm, vibrations of the X-Pro bond are preferentially enhanced by approximately a factor of 30 relative to vibrations of the normal peptide bond.
The far-UV resonance Raman spectroscopy of protein components has been of considerable recent The aromatic amino acids have been studied and analyzed in terms of benzene modes,j and N-methylacetamide has been studied as a model for the peptide bond, leading to further understanding of the nature of its vibrational modes and the complex excited electronic state manifold of this chromophore.2 The resonance Raman spectroscopy of the X-proline bond is of particular interest because the structure around the nitrogen in this group distinguishes it from the normal secondary amide peptide bond in terms of its electronic and vibrational spectra. Proline bonds are also of interest (1) Presented in part at the 30th Annual Meeting of the Biophysical
Society, Feb 9-13, 1986; abstract. Mayne, L.; Harhay, G.; Hudson, B. Biophys. J. 1986, 49, 330a. (2) Mayne, L. C.; Ziegler, L. D.; Hudson, B. J . Phys. Chem. 1985, 89, 3395.
( 3 ) (a) Hudson, B.;Mayne, L. Methods Enzymol. 1986, 130, 331. (b) Hudson, B. Spectroscopy 1986, 1 , 22. (4) (a) Johnson, C. R.; Ludwig, M.; O'Donnell, S.; Asher, S . A. J . Am. Chem. Soc. 1984,106,5008. (b) Dudik, J. M.; Johnson, C. R.; Asher, S. A. J. Phys. Chem. 1985, 89, 3805. ( 5 ) (a) Rava, R. P.; Spiro, T. G. J . Am. Chem. Soc. 1984,106,4062. (b) Copeland, R. A,; Spiro, T. G. Biochemistry 1985, 24, 4960.
because of their implication in studies of protein renaturation kinetics6 In particular, the "slow folding" component observed in renaturation is believed to be due to the presence of incorrect X-proline isomers in the unfolded form. There is, however, no direct evidence for this involvement due, in part, to the lack of good spectroscopic techniques that are sensitive to proline isomerization and applicable to proteins. In this study we present preliminary results which may lead to a technique for the detection of cis-trans isomers a t X-proline bonds. Specifically, it is demonstrated that the use of radiation near 230 nm results in selective excitation of the resonance Raman spectrum of the peptide linkage to proline relative to that of the normal secondary peptide linkage. Preliminary studies reported elsewhere7~* indicate that there are small but significant differences between the ultraviolet resonance-enhanced Raman spectra of cis and trans X-proline bonds. (6) (a) Mui, P. W.; Konishi, Y.; Scheraga, H. A. Biochemistry 1985, 24, 4881. (b) Brandts, J. F.; Halvorson, H. R.; Brennan, M. Biochemistry 1975, 14, 4953. (c) Schpid, F. X.;Baldwin, R. L. J . Mol. Biol. 1979, 135, 199. (7) Hudson, B.; Mayne, L. C. In Biological Applications of Raman
Spectroscopy; Spiro, T. G., Ed.; Wiley: New York, 1987; p 181. (8) Mayne, L.; Ramahi, T.; Oas, T.; Hudson, B. Biophys. J. 1984, 45,
322a.
0022-3654/87/2091-4438$01.50/00 1987 American Chemical Society
