J. Phys. Chem. 1982, 86, 3173-3177
3173
Picosecond Laser Photolysis of Aqueous Indole and Tryptophan J. C. Mialocq, Department de PhyslcoChlmle, C€N Sacley, 9 1 19 1 Glf/Yveite cedex. France
E. Amouyal, A. Bemas,* and D. Grand ERA 718, Univmit6 Paris-Sud, at.350, 91405 Orsay, France (Recelved: December 16, 1981; In Final Form: March 11, 1982)
Photoionization of aqueous solutions of indole (IH) and tryptophan (Trp) has been investigated in a double-beam picosecond spectrometer device with single pulses of 27-pa duration and 265-nm wavelength. Hydrated-electron (e,;) absorption was monitored at 660 nm, and, in the presence of Cd2+as an electron scavenger, absorption of the indole radical cation IH+.(at 600 nm) and of the neutral radical I. (at 530 nm) was recorded. For both solutes, two main conclusions can be derived. (i) The eaq-absorption appears within the laser pulse; hence, electron ejection and solvation occur in a time which is several orders of magnitude shorter than the fluorescent-state lifetime. It is shown that, under the present experimental conditions, e,; formation results predominantly from a one-photon mechanism, besides a biphotonic contribution. Thus, in the monophotonic ionization process, eaq-would originate not from the fluorescent state but from a nonrelaxed prefluorescent state. A straightforward correlation between fluorescence and e,; quantum yields may then be misleading. (ii) The eaq-optical density remains constant up to 1.7 ns (time limit of our apparatus) and, from nanosecond literature data, up to about 15 ns. Such observations imply that no appreciable ion-pair recombination occurs in times shorter than e 1 5 ns. Introduction The photolysis and photochemistry of aqueous tryptophan (Trp) and of various indole (IH) derivatives have been the subject of extensive studies for more than two decades since UV inactivation of proteins has been shown to proceed from the photodegradation of some amino acid residues, Trp in particular.' Among the various deactivation paths of electronically excited Trp, and IH,, it was also recognized that electron ejection constitutes an important primary photoprocess.2 However, some disagreement still persists on the photoionization mechanism, inter alia, on the nature of the ionized-state precursors for either the monophotonic or sequential biphotonic ionization process. In the monophotonic mechanism, e, - has been considered to derive either from the relaxed krst singlet S13" or from a nonrelaxed prefluorescent state.&l0 In the consecutive biphotonic process, the intermediate states have been identified" with S1-and T1to a lesser degree-or alternatively with an unidentified state X populated in competition with internal conversion to the fluorescent state? To our knowledge, apart from a short communication,'2 the pulsed-excitation studies of Trp and IH have been restricted to microsecond and nanosecond resolution times. The present report deals with picosecond photoionization (1) A. D.Mc Laren and D. Shugar, 'Photochemistry of Proteins and Nucleic Acids", MacMillan, New York, 1964. (2)L. I. Grosaweiner, G. W. Sweneon, and E. F. Zwicker, Science, 141, 805 (1963);G. W.Swenson, E. F. Zwicker, and L. I. Grossweiner, ibid., 141, 1042 (1963). (3)J. Feitelson, Photochem. Photobiol., 13,87 (1971). (4)J. Zechner, G. KBhler, N. Getoff, I. Tatischeff, and R. Klein, Photochem. Photobiol., 34,163 (1981). (5)R. Klein, I. Tatischeff, M. Bazin, and R. Santus, J. Phys. Chem., 86,670 (1981). (6)H.B. Stem, J. Chem. Phys., 61,3997 (1974). (7)D.V. Bent and E. Hayon, J. Am. Chem. SOC., 97,2612 (1975). (8)E.Amouyal, A. Bernas, and D. Grand, Photochem. Photobiol., 29, 1071 (1979). (9)L. I. Grosaweiner, A. M. Brendzel, and A. Blum, Chem. Phys., 57, 147 119811. (10)J. C. Mialocq, E. Amouyal, A. Bernaa, and D. Grand, J. Photochem., 17, 132 (1981). (11) B. FinnstrBm, F. Tfibel, and L. Linqvist, Chem. Phys. Lett., 71, 312 (1980). (12)R. Devonehire, one-day symposium on "Fast Reactions in Chemistry and Biology", Royal Institution, London, March 22, 1979.
.----,-
of Trp, and IH,, using picosecond absorption spectroscopy to track the e,; formation and decay and to investigate triplet and radical production. The excitation wavelength A,, = 265 nm corresponds to a photon energy greater than the photoionization threshold values determined from continuous, low-intensity excitation of IH and Trp.8J3J4 Thus, a monophotonic electron ejection is energetically feasible, besides the sequential biphotonic mechanism inevitable at the laser intensities used. On the other hand, processes where the triplet is the intermediate state which have to be considered in the nanosecond time scale may be disregarded under the present experimental conditions. The purpose of the present study was twofold: (1)to provide complementary information on the nature of the ionized-state precursor in the monophotonic ionization mechanism; (2) to examine the extent of the ion-pair recombination reaction in the 20 ps-2 ns time range. Experimental Section Double-Beam Picosecond Absorption Spectrometer. The time-resolved absorption spectrometer has been described in detail elsewhere15J6and is represented in Figure 1. It is based on a mode-locked Nd:YAG laser. A single pulse of 27-ps duration (1.06 p ) is converted to 530- and 265-nm pulses, using two KDP crystals. The analyzing light is a continuum produced by focusing the unconverted fraction of the infrared beam at the center of a 4-cm long water-filled cell. Such cell length was chosen to provide the most reproducible continuum while minimizing the optical path so as to avoid time dispersion effects due to group velocity variations. The two analyzing beams defined by two diaphragms of 1-mm diameter at the entrance of the cell C are focused onto the target of a 500-channel optical multichannel analyzer (OMA), after filtering by band-pass and narrow (13)D.Grand, A. Bernas, and E. Amouyal, Chem. Phys., 44,73 (1979). (14)A. Bernas, D.Grand, and E. Amouyal, J. Phys. Chem., 84,1259 (1980). (15)J. C. Mialocq, J. Sutton, and P. Goujon, J.Chem. Phys., 72,6338 (1980). (16)J. C. Mialocq, J. Sutton, and P. Goujon, Springer Ser. Chem. Phys., 14,212 (1980).
0022-3654/82/2086-3 173$0 1.2510 0 1982 American Chemical Society
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The Journal of Physlcal Chemistry, Vol. 86, No. 16, 1982
TABLE I : Hydrated-Electron Optical Density, d , , , , as a Function of the Laser Pulse Energy
I
I
l
I
l
E , mJ
d,,,
0.220 0.350
0.23 0.31
d6601E
d6 6 0
9
mJ-'
1.04 0.89
E , mJ 0.440 0.530
If,
d,,,
mJ'
0.37 0.40
0.84 0.75
Tryptophan.H,o
1 I 4
Figure 1. Doubiabeam picosecond absorption spectrometer: (WC) water cell; (D) diaphragm; (Diff) diffuser; (L)lens; (C) cell; (PD) photodiode; (SIT) siilcon Intensifier target; (OMA) optical multichannel analyzer.
O+ 0.0 0
i 50
Flgue 3. Hydrated-electronoptical density, d,, for [Trp,,] = 2.0 X M.
as a function of time
i
150
TIM ElOOps Figwe 2. Hydrated-electron optical density, d,, for [IH,,] = 1.8 X lo4 M.
200
$2:
as a function of time
band interference filters. The ratio of the intensities of the two signals measured at each delay time with and without excitation gives the induced optical density variation. The recorded data represent the mean of at least five laser shots. The UV pulse is focused to a spot size of about 2-mm diameter onto the cell. Ita energy is checked by using a fast RTC photodiode (Model UVHC 20) with a 200-ps rise time and a Tektronix 7912 oscilloscope. The exciting pulse energy, as measured behind the 1-mm thick cell C by using a Laser Precision Corp. RjP-734 energy meter, is in the range 200-600 d. Chemicals. Indole (Fluka puriss, >99%), tryptophan (Merck, 99%), and cadmium perchlorate (Cd(C10J2.6H20, G. Frederic Smith Chemical Co.) were used as supplied. All aqueous solutions were prepared with singly distilled water, since water purity is not critical at the high solute concentrations employed. Purity and concentration of solutes were monitored by absorption spectrophotometry using a Cary 118 spectrophotometer. The optical density of the mlutions was adjusted to 0.95 f 0.05 at 265 nm. The pH of the solutions was 5.9, and experiments were all performed at room temperature.
Results Hydrated-Electron Generation. In a 1.8 x M aqueous solution of indole excited at 265 nm, the rise time of the optical density at 660 nm due to the hydratedelectron absorption (em = 1.73 X 104 M-' cm-l)17has been (17) E. J. Hart and M. Anbar, 'The Hydrated Electron", Wiley-Interscience, New York, 1970, p 42.
u
o.oo
50
100
150
200
TIME ps Flgue 4. Hydratd4ectron optical density, d,,, as a function of time M in the presence of 0.24 M Cd2+. for [IH,,] = 1.8 X
determined. It is displayed in Figure 2, for an average exciting pulse energy of 220 id. As the absorption of the exciting pulse by the indole ground state is almost total, the calculated estimated maximum decrease of concentration is 0.6 X M, which is notably smaller than the initial concentration. When the exciting pulse energy is increased in the range 220-530 pJ, the ratio of the measured 660-nm optical density to the pulse energy decreases as shown in Table I. Such a decrease could be expected since the concentration of the indole ground state is greatly reduced at the higher pulse excitation energies. A similar experiment has been performed on a 2.0 X M aqueous solution of tryptophan (Figure 3). For these two solutes the rise time of the 660-nm optical density is very similar to that previously observed for phenol.15J8 After this initial rise time, the 660-nm optical density remains constant up to 1.7 ns, the time limit of the apparatus. In order to verify that the 660-nm absorption is indeed due to the hydrated electron, cadmium ion was used as a scavenger. The rise and decay of the 660-nm optical density are shown in Figure 4. The absorption spectrum of the indole molecule was not modified by adding cadmium perchlorate (0.24 M). However, the fluorescence intensity of the same solution excited at 265 nm in a conventional spectrofluorimeter was quenched by 20.8%, (18) J. C.Mialocq, J. Sutton, and P. Goujon,European Conference on the Dynamics of Excited States, Pisa, April 14-16,1980; Nuouo Cimento B , 63, 317 (1981).
Laser Photolysis of Aqueous Indole and Tryptophan
TABLE 11: Optical Density Ratio d 6 T o / d T gupon IH and Trp Photoionization for Two Different Laser Pulse Intensity Values d E 0.18 0.24
d?:? 0.15
0.31
The Journal of Physical Chemistry, Vol. 86, No. 16, 1982 3175
TABLE 111: Recent Literature Values of Hydrated-Electron Quantum Yields from Aqueous IH and Trp under 265-nm-Light Excitation qeaq-
d 2 J Id% 1.2 0.8
indicating somewhat different deactivation processes. Such a quenching has already been mentioned for acety1trypt0phan.l~ It may be asked whether the quenching process is static or dynamic. The cadmium cation could scavenge e, thus inhibiting geminate recombination and the associated delayed fluorescence. On the other hand, it has been shown that Cd2+does not scavenge the hydrated-electron precursor in the photoionization of phenol.l6J8 In the absence of Cd2+,no geminate recombination, leading to a decrease of the e,; red absorption, is observed for phenol, indole, or Trp solutions in the time range investigated. It is suggested then that a dynamic fluorescence quenching occurs by interaction of the indole fluorescent state with cadmium, enhancing the intersystem crossing efficiency. When Cd2+is present, the 660-nm optical density remains constant (dm = 0.03) for times greater than 230 ps and up to 1.6 ns. A decay rate constant can then be calculated, k = (5.3 f 0.6) X 1O'O M-l s-l, a value which is equal to that already obtained from the phenol experiments,16 namely, k = 5.4 X 1O1O M-' s-l. Most of the 660-nm absorption is then due to the hydrated electron, and the residual absorption, upon e,; scavenging, could correspond to the indole cation, characterized by a wide absorption band around 585 nm.7 The ratio of the 660-nm absorbances obtained upon indole and tryptophan photoionization under the same excitation conditions are given in Table I1 for two different laser pulse intensity values. The observed decrease of the ratio d g / d % with the increase of the laser pulse intensity can be interpreted as a higher efficiency of a sequential biphotonic process in the case of Trp. At any rate, we can conclude that e, absorption appears within the 27-ps laser pulse; then it remains constant in the absence of an electron scavenger. Hence, no appreciable geminate recombination takes place in the time range 50 ps-2 ns following IH or Trp photoionization. Triplet and Radical Formation. For aqueous solutions of indole (1.8X M) containing 0.24 M Cd2+,the optical densities a t 1.6 ns have been measured a t various wavelengths: X = 440 nm (triplet 31H), X = 530 nm (neutral radical I-),X = 600 nm (cation radical IH+., X = 660 nm (residual absorption after e, decay). We find dm = -0.02 f 0.02, d5m = 0.105 f 0.025, d , = 0.091 f 0.017, and d m = 0.03 f 0.01. Under the same excitation conditions (Table I), the 660-nm optical density of an indole solution free of cadmium cation was found to be dm = 0.37 f 0.03. As the indole cation radical decays with a rate constant k N 106 s-l,' we can ascertain that its concentration i equal to that of e,; Le., 2.0 X 10" M, a value calculated from the difference of the 660-nm optical densities measured in the absence and the presence of Cd2+and from the ea< molar extinction ~0efficient.l~ The 440-nm absorbance is negligible. This is not surprising, taking into account the molar extinction coefficient of the indole triplet (ew = 3.64 X lo3 M-' cm-') and the triplet quantum yield in water = 0.23 f 0.06 calculated by Klein et al.5 Moreover the buildup of the indole triplet absorption is not complete (19) R. F. Stainer and E. P. Kirby, J. Phys. Chem., 73, 4130 (1969).
exptl technique
ref
0.26 0.15 i 0.04 0.16 f 0.03
Indole 254-nm steady-state irradiation, eaq4 scavenging steady-state irradiation, eaq; scavenging 23 steady-state irradiation, eaq scavenging 24 pH 5.7 nanosecond laser flash experiment 7 nanosecond laser flash experiment 9 nanosecond laser flash experiment 5
0.08 f 0.025 0.004
Tryptophan steady-state irradiation, eaq: scavenging 8 steady-state irradiation, eaq scavenging 24
0.12 0.14 0.01
f
0.021
0.05
f
pH 5.7 0.002 steady-state irradiation (254 nm), eaq-
scavenging 0.08 nanosecond laser flash experiment 0.10 f 0.01 nanosecond laser flash experiment 0.04 nanosecond laser flash experiment (monophotonic component) 0.073 f 0.010 nanosecond laser flash experiment
25 7
20 11 9
at time t = 1.6 ns, since the indole fluorescence lifetime is TF = 4.0-4.9 n ~ . ~ The optical densities measured at 530 and 600 nm at 1.6 ns are in the same ratio as the optical densities recorded at 50 ns by Bent and H a y ~ n .The ~ triplet absorption is negligible in this wavelength region and, in the absence of the hydrated electron, the only possible absorbing species are indole cation radical IH+. (Amm 585 nm) and the longer-lived neutral radical I- (Amm 530 nm).7 According to previous studies,'Pz0 the neutral radical I. would derive from the deprotonation of IH+-. The assignment of the IH+ absorption is well supported by pHeffect observations and the attribution of the 530-nm band to I. radical is also well substantiated.21 However, the nanosecond experiments'~~~ did not provide evidence for the formation of I. from the decay of IH+-. Considering the neutral radical and the electron as coproducts, Bryant et a1.22have calculated the molar extinction coefficient of the tryptophan radical eTV - 1800 f 200 M-' cm-'. Klein et al.5 have obtained $8 1580 M-' cm-l and the quantum yield of 1-formation cpr. = 0.16 f 0.04, assuming the same molar coefficients for the I- and Trp. neutral radicals. Pernot and Lindqvist21have measured by the same method a value &, = 4000 M-l cm-l. The relatively high 530-nm absorbance measured in our experiment at 1.6 ns indicates either a high absorbance of IH+-,very similar to its 600-nm absorbance, or the primary formation of the neutral radical I. by N-H bond rupture. In the latter event the exact contribution of the two mechanisms of I- formation should be reexamined: homolytic rupture of the N-H bond and deprotonation of the cation radical IH+.. The 600-nm absorbance of the neutral radical I. must be quite small at 1.6 ns for two reasons: its formation is incomplete since IH+-is relatively long-lived and, although its spectrum in water seems to extend to the red,' no absorbance due to I- is found at 600 nm in cyclohexane solution.21 From the measured 600-nm optical density and the known IH+. concentration (equal to that of e,;: [e,;] = 2.0 X M) we can calculate the maximum limit of
--
5
(20) J. F. Baugher and L. I. Grossweiner, J. Phys. Chem., 81, 1349 (1977). (21) C. Pernot and L. Lindqvist, J . Photochem., 6, 215 (1976/77). (22) F. D. Bryant, R. Santus, and L. I. Grossweiner, J.Phys. Chem., 79, 2711 (1975).
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The Journal of phvsical Chemktty, Vol. 86, No. 16, 1982
the molar extinction coefficient of IH+-: e g S 4.5 X lo3 M-' cm-', as compared with E%+' = 2.9 X lo3 M-' cm-1.22
Discussion Hydrated-electron formation is a very efficient process in the monophotonic excitation at 265 nm of indole and tryptophan, in aqueous solution. Table I11 gathers the most recent measurements of pew-for 265-nm irradiation, at room temperature and neutral pH unless otherwise stated. Despite the current uncertainty about the e - quantum yields, one may remark that the values obtain2 for indole are systematically higher than p%-relative to Tq and also that the "high" pe - values for Trp ( N 0.10) compare well with that (0.146)&rived from a thorough model describing the photophysics of aqueous Trp.26 Under the present experimental conditions of high laser intensities, saturation effects are expected when all the molecules in the exposed solution take part in the reaction. A sequential biphotonic process can also intervene involving an intermediate excited state. Its occurrence has been discussed in terms of intensity dependence.27 For the lowest picosecond pulse energy (200 pJ) used in this study, only a fraction of the solute molecules is affected by excitation. However, the buildup of the first singlet excited state S1can give rise to a 265-nm S1absorbance increase, matching the 265-nm Soabsorbance decrease. If the S1absorbance is high enough, a consecutive biphotonic process is p ~ s s i b l e . ' ~ J ~ In the present picosecond laser excitation of indole and tryptophan solutions, the e,- formation efficiency is almost the same, in contradiction with ublished values of pe& (Table 111). Moreover the ratio dm/@ is found to vary with the laser average intensity (Table 11). These two findings support the occurrence of a biphotonic process besides a one-photon ionization, the relative contribution of the latter being higher in the case of indole. For indole excited with a 220-pJ average energy, the concentration of the excited singlet has been calculated to be 6 X lo4 M (see above) and the eap concentration is 1.3 X 10-4M, obtained from the 660-nmabsorbance (Figure 2), dseo= 0.23. This indicates a quantum yield p%- = 0.21, which is about 50% higher than the steady-state irradiation average value (Table 111). In view of the experimental uncertainties, such an estimate may be in error by N 30% but it definitely indicates that the most important fraction ( -2/3) of the eaq-population arises from a monophotonic process. If the monophotonic e, generation were to take place from the relaxed fluorescent state, its rise time would be equal to the fluorescence lifetime, rF = 4.82 f 0.02 ns.% According to this assumption, the eaq-formation at time t = 1.6 ns would represent only 30% of the total ea; expected production but the red absorption increase could be easily observable between 100 ps and 1.6 ns. This is not the case however since the 660-nm absorbance remains constant (Figure 2). This observation has been repeatedly confiied in particular at low UV pulse energy (- 180 CJ), where the contribution of a two-photon process is expected to be the smallest. (23)D. Grand, E.Amouyal, and A. Bemas, unpubliahed results. (24)C. Pigault, D.Ggrard, and G. Laustriat, 10th Intemational Conference on Photochemistry, Iraklion, Greece, Sept 6-12,1981. (25)G. K6hler, G. Grabner, J. Zechner, and N. Getoff, J.Photochem., 17,21 (1981). (26)R.J. Robbins, G. R. Fleming, G. S. Beddard, G. W. Robinson, P. J. Thistlethwaite, and G. J. Woolfe, J. Am. Chem. Soc., 102,6271(1980). (27)U.Lachish, A. Shafferman, and G. Stein, J. Chem. Phys., 64,4205 (1976). (28)A. G. Szabo and D. M. Rayner, J. Am. Chem. Soc., 102, 554 (1980).
Mialocq et ai.
We are thus led to the conclusion that, in the monophotonic ionization of IH, the ionized-state precursor is definitely not the relaxed first singlet but a prefluorescent state. This conclusion again points out that a straightforward correlation between pew-and c p may ~ be misleading due also to the well-known fact that the experimentally determined pe- does not represent a primary electron ejection yield but contains a recombination probability term.142930 The present conclusion agrees with the statement of Lumry et al.31that electron ejection from IH occurs during Franck-Condon relaxation and should be over in the picosecond time range. It also confirms recent observations9 which rule out the participation of the relaxed S1state in the monophotonic ionization process. Finally, the present conclusion is compatible with the existence of an ionization threshold (A&- = 285 nm for IH, Xthres = 275 nm for Trp) evidenced by N20scavenging experimentd3J4in conditions where N20 has proved not to quench indole fluorescen~e.~,~,'~ The exact nature of such a prefluorescent state remains an open question. Vibrationally excited singlet state has been proposed7* and the partial Rydberg character of the indole 'La excited state has been emphasized by Lami.32 Solvent cage molecules might also be involved: exciplex states formed by singlet excited indole compounds with polar solvent molecules have been invoked3' as well as charge transfer to solvent ~ t a t e s . ~ B ~ In the case of Trp, as indicated above (Table 11), a sequential biphotonic process notably contributes to electron ejection, depending on the laser pulse intensity. If the monophotonic process does take place from the relaxed fluorescent state (main lifetime 3.2 ns26,28,34), 40% of the total monophotonic ea; contribution should have grown in by 1.7 ns. We have never observed a significant increase of the ea; absorbance even under the lowest pulse energy (150 pJ). However, its occurrence cannot be completely rejected in view of the experimental uncertainties particularly at low ea; optical densities. The observed constancy of dS0, hence of e.;, implying no appreciable ion-pair recombination in the time range 50 ps-1.7 ns-and also in the time range 5-15 ns from Bent and Hayon's data'-may seem at first sight surprising. Such observations contradict previous assertions of a rapid ion-electron cage n e ~ t r a l i z a t i o n . ~They ~ explain however why systematically different vea- values are not found when determined either from steady-state irradiations and ea; scavenging or from nanosecond pulsed excitation, provided a one-photon ionization mechanism prevails in the latter case. Present observations might also explain why the pew-values based on scavenging experiments appear not to depend on the scavenger concentration4v8in a concentration range corresponding to electron decay lifetimes in the range 5 ns-0.5 ps. Finally, one might also recall that flash photolysis studies have clearly shown the decay of either Trp+-at 490 nm36or ea; at 715 nm in the submillise~ond~~ or milli~ e c o n d ~time ~ , ~range, ' the main competitive decay reac(29)K. Lee and S. Lipsky, Radiat. Phys. Chem., 15,305 (1980). (30)J. Bullot, P.Cordier, and M. Gauthier, J. Phys. Chem., 84,1253 (1980). (31)R. Lumry and M. Hershberger, Photochem. Photobiol., 27,819 (1978),and references cited therein. (32)H.Lami, J. Chem. Phys., 67,3274 (1977). (33)T.B. Truong, J.Phys. Chem., 84,960(1980). (34)E.Gudgin, R.Lopez-Delgado, and W. R. Ware, to be submitted for publication. (35)L.I. Grcasweiner and J. F. Baugher, J. Phys. Chem.,81,93(1977). (36)L. I. Grossweiner and Y . Usui, Photochem. Photobiol., 13, 195 (1971).
J. Phys. Chem. 1982, 86, 3177-3184
tions and rate constants being the following: e,- + Trp, k = 3.6 X lo8 M-' s-l (ref 38), 1.2 X lo8 M-' s-l (ref 39); e,, - e, 5 X log M-' s-l (ref 38). should also be emphasized that the absence of a notable decay of e, in the picosecond-nanosecond time range does not characterize specifically IH, and Trp, photoionization. A similar situation has been encountered in particular for aqueous phenol and phenolate photoionization.15J8 We have also shown that the 440-nm absorbance is negligible at 1.6 ns. This fact confirms that the triplet state arises from the relaxed fluorescent state and not from a nonrelaxed excited complex state or an excited FranckCondon state. This conclusion agrees with that of Klein et based on quenching experiments. The N-H bond dissociation yield is known to be negligible for IH in water?p4 However, we have noticed a high 530-nm absorbance at 1.6 ns, attributable to the neutral I. radical. This assumption is based on the relatively weak absorption of the radical cation IH+. at 530 nm.' If this is correct, N-H bond dissociation of IH could arise from a consecutive biphotonic process.
It
So
+ hv
+
Conclusion The picosecond laser-induced photoionization of aqueous indole and tryptophan at 265 nm can be satisfactorily represented by the following reaction scheme: (37)M.T.Pailthorpe, J. P. Bonjour, and C. H. Nicolls, Photochem. Photobiol., 17, 209 (1973). (38)M.Anbar, M.Bambenek, and A. B. Roes, Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.), 43 (1973). (39)H. Templer and P. J. Thietlethwaite,Photochem. Photobiol., 23, 79 (1976).
3177
265 nm
S1*nr
--- + + -
Si*nr (nonrelaxed excited state) Si*nr R+ + e,,;
SI* (relaxed fluorescent state)
(1) (2)
(3)
SI*
So (internal conversion)
(4)
S1*
T (intersystem crossing)
(5)
S1*
So
Si*nr or S1*
hv' (fluorescence) hu
(6)
R+ + e,,;
(7)
We have shown, at least in the case of indole, that the monophotonic e,,; generation takes place from an unrelaxed excited singlet state, and we believe that this conclusion can be extended to tryptophan. The sequential biphotonic process described by reactions 1 7 or 1 3 + 7 depends on the laser intensity and is more efficient in tryptophan. Intersystem crossing to the triplet state involves the fluorescent state, and a possible N-H bond dissociation can be due to a consecutive biphotonic process. It must be emphasized that the extent of the recombination of e,,< and IH+- or Trp+. produced by either a monophotonic or a consecutive biphotonic process is relatively small, or zero, in the picosecond time range.
+
+
Acknowledgment. We thank Professor L. I. Grossweiner (Illinois Institute of Technology, Chicago), Professor G. R. Fleming (The University of Chicago),Professor H. Lami (UniversiW Louis Pasteur, Strasbourg), and Dr. R. Lopez Delgado (UniversiW de Paris-Sud, Orsay) for sending us copies of their work before publication and for several fruitful discussions.
Laser-Photolysls Study of the External Magnetic Field Effect upon the Photochemical Processes of Carbonyl Compounds in Micelles Yoshlo Sakaguchi, Hlsaharu Hayashi, and Saburo Nagakura' The Institute of phvslcel end Chemical Research, Weko, Saitema 351, Japan (Re~slved:December 16, 198 1; I n Final F m : March 24, 1982)
The external magnetic field effects upon the primary photochemical processes of benzophenone and dibenzyl ketone in sodium dodecyl sulfate (SDS) and other micelles have been studied by nanosecond laser photolysis. The decay rate constants of the ketyl and benzyl radicals in the micelles were found to decrease in the presence of weak external magnetic fields and were determined for the ketyl radical in the SDS micelle as (2.82 f 0.05) X loe s-l at zero field and (1.89 f 0.05) X los s-l at 700 G. On the other hand, the amount of the escaping ketyl and benzyl radicals was found to increase in the presence of magnetic fields. Magnetic isotope effects were studied and the decay rate constants in SDS micelles at 700 G were determined as (2.16 f 0.05) X lo6 s-l for (C6H5)z13COH and (1.86 f 0.05) X lo6 s-l for (C6D5),COH.The observed magnetic field and magnetic isotope effects were interpreted in terms of reductions in the triplet-singlet conversion rates of intermediate radical pairs in the presence of magnetic fields.
Introduction The magnetic field effect upon chemical reactions in solutions has been studied extensively and successfully during the past decade by measuring the product yields of photochemica1172or thermochemical3reactions and also *Address correspondence to this author at the Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444 Japan. 0022-3654/82/2088-3177$01.25/0
by detecting directly reaction intermediates by pulse radiolysis4or laser photolysis." Concerning the interme(1)Y.Tanimoto, H. Hayashi, S. Nagakura, N. Sakuragi, and K. Tokumaru, Chem. Phy5. Lett., 41,267(1976). (2)Y.Sakaguchi, H.Hayashi, and S. Nagakura, Bull. Chem. Soc. Jpn., 53, 39 (1980). (3)R.Z.Sagdeev, Yu. N. Molin,K. M.Salikhov, T. V. Leshina, M. A. Kamha, and S. M.Shein, o g . Magn. Reaon., 5 , 603 (1973).
@ 1982 American Chemical Society
