J. Phys. Chem. 1982, 86, 2420-2427
2420
except that the rate for hydrogen transfer (k+)is approximately an order of magnitude faster than kf + k N R . We have therefore studied the quenching of the fluorescence of I in cyclohexane by ethanol. The quenching plot, expressed as the reciprocal of the fluorescence intensity vs. ethanol concentration (Figure l), is curved because of the dependence of the reverse hydrogen transfer in eq 5 on ethanol concentration. The quenching mechanism predicts that
where rpof is the fluorescence intensity in the absence of ethanol and 4fis the intensity in the presence of a concentration [&]. The equation for the analysis of the decay-time quenching datalOJ1has to be similarly modified: [B*] = Cle-”lt C2e-x2t (7)
+
Xi,z = 1 / [ k f+ ~ N + R (k+ + k-)[Q1 + k d 7 ((k-[Q1 + kd - kf - ~ N -R k+[Q1)2 + 4~+~-[Q121”21 (8) (10)C. Lewis and W. R. Ware, Mol. Photochem., 5, 261 (1973). (11)D.V. O’Connor and W . R. Ware, Natl. Bur. Stand. Spec. Publ., No. 256, 193 (1978).
The rate constants k+, k-, and kd may be evaluated1’ by plotting (A, + A,) and AIAz vs. [Q]. Both plots are curved at higher ethanol concentrations which we attribute to the resolution of the instrument, but from the measurements at lower concentrations (Table 111), the forward hydrogen transfer is diffusion controlled (k+ = 6.8 X lo9 dm3 mol-’ s-’) in excellent agreement with the value given by Calvert and Pitts.12 k- has a value of approximately 1.7 X 108dm3 mol-’ s-l and kd a value of 1.0 X 10’ s-’. The calculated curve (eq 6) using these values is superimposed on the experimental points in Figure 1 and can be seen to reproduce the observed data extremely well. If these results are then extrapolated to pure ethanol, values of approximately 10 ps and 2-3 ns are predicted, suggesting that the lower of the two values observed here are very much determined by the limitations of the instrument. However, the overall picture given by our results amply confirm the mechanism put forward by Kondo.
Acknowledgment. Financial support from the SERC in the form of a CASE Research Studentship (for K.W.H.), the Department of Industry, and Lancashire County Council is gratefully acknowledged. ~~~~~
(12)J. G.Calvert and J. N. Pitta Jr., ‘Photochemistry”, Wiley, 1966.
Photochemical Reactions of Triplet Acetone with Indole, Purlne, and Pyrimidine Derivatives Kunlhlko Kasama, Aklko Takematsu, and Shlgeyoshl Aral The Insthte of Physical and Chemical Research, Wako-shi, Sairama 35 1, Japan (Received: December 4, 198 1; I n Final Form: February 9, 1982)
The photochemical reactions of triplet acetone with indole, indole derivatives (1-methyl-,2-methyl-, 3-methyl-, 5-methyl-,and 7-methylindole,and tryptophan),purine derivatives (caffeine,7-methylguanine,adenine, adenosine, and guanosine),and a pyrimidine derivative (thymine) have been studied in aqueous solutions by using a KrF or ArF laser. The quenching processes of triplet acetone by indoles, being diffusion controlled, occur via the following paths: triplet-triplet energy transfer, electron transfer, photoaddition of triplet acetone to the 2-carbon atom of the indole ring, and deactivation without a chemical change. The yields of energy transfer, electron transfer, and photoaddition were determined from absorbance measurements. The transient absorptions due to triplet states were observed for caffeine, 7-methylguanine, and thymine, while weak transient absorptions which showed apparent second-order decays were observed for adenine, adenosine, and guanosine. Triplet acetone is quenched mainly via T-T energy transfer in caffeine, 7-methylguanine, and thymine. The weak absorptions may be attributed to neutral radicals in adenine and adenosine and to a cation radical in guanosine.
Introduction
The photochemistry of nucleic acids and their components has been investigated extensively. In the UV irradiation of nucleic acid components, the fluorescence is hardly observed at room temperature’2 and photochemical reactions do not occur to an appreciable e ~ t e n t .The ~ total yields of fluorescence and phosphorescence have been reported to be 0.001-0.1 in various rigid solvents at 77 K.23435These results imply that the excited states of nu(1)M.Daniels and W. Hauswirth, Science, 171, 675 (1971). (2)E.P. Gibson and J. H. Turnbull, J.Photochem., 11, 313 (1979). (3)D.A.Angelov, P. G. Kryukov, V. S. Letokhov, D. N. Nikogosyan, and A. A. Oraevsky, Appl. Phys., 21, 391 (1980).
cleic acid components are relaxed through some nonreactive and nonradiative processes. Wilson and Callis have examined the fluorescence of adenine and 7methyladenine in the mixed solvent of ethylene glycol and water at 140 K.6 The fluorescence spectrum of 7methyladenine showed the mirror-image relation with respect to the absorption and fluorescence-excitation spectra. In adenine, however, the mirror-image of the fluorescence spectrum was different from the absorption (4) V . Kleinwachter, Collect. Czech. Chem. Commun., 37,1622(1972).
(5)R.S.Becker and G. Kogan, Photochem. Photobiol., 31,5 (1980). (6)R. W.Wilson and P. R. Callis, Photochem. Photobiol., 31, 323 (1980).
QQ22-3654/82/2Q86-242Q$Ql.25/ Q 0 1982 American Chemical Society
Photochemical Reactions of Trlplet Acetone
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982 2421
Scheme I
spectrum, although it accorded with the fluorescence-excitation spectrum. Furthermore, the fluorescence intensity of 9-methyladenine was 200 times weaker than that of 7-methyladenine. They concluded that the fluorescence of adenine arises from N(7)-H tautomer molecules instead of abundant N(9)-H molecules.
N(9)- H
N(7) - H
Similar results have been obtained with guanine and 7methylguanine.6 The photochemical reactions of nucleic acid bases, nucleosides, and nucleotides with alcohols and amines in the presence of di-tert-butyl peroxide or acetone as a sensitizer have been studied by using a high-pressure mercury lamp.'s8 The irradiation of purine derivatives resulted in the substitution of the hydrogen atom at the C(8) position by moieties of alcohols or amines. For example, the reaction of adenine with isopropyl alcohol is shown in Scheme I. Acetone photosensitization of pyrimidine derivatives was found to give pyrimidine dimer, 5,6-dihydropyrimidine, and a small amount of pyrimidine-alcoho1 adduct as products. Although the reaction of pyrimidine was suppressed in the solution of equimolar pyrimidine and purine, that of purine was unchanged. This fact indicates that purine has a higher reactivity than pyrimidine. The photochemical reactions of aromatic ketones with purines and pyrimidines in aqueous solutions have been investigated by using a flash photolysis techniqueSg Triplet benzophenone was quenched at a diffusion-controlled rate by pyrimidine derivatives, but ketyl radical was not formed significantly. On the contrary, the reactions of triplet benzophenone with purine derivatives produced ketyl radical due to H-atom abstraction from purines. The photochemical processes of indole derivatives have been studied more widely than those of nucleic acid components because fluorescence and T-T absorption are easily observable at room temperature.'&l2 Wilkinson and Garner have demonstrated in laser flash photolysis that the quenching processes of the triplet states of aromatic ketones such as acetophenone and benzophenone by indole derivatives in benzene result in the H-atom abstraction from the N-H bond with various efficiencies. In addition to these processes, charge-transfer interaction leading to ground-state ketones and indoles has been suggested for residual quenching proces~es.'~ (7)J. Salomon and D. Elad, Photochem. Photobiol., 19,21 (1974). (8)A. A. Frimer, A. Havron, D. Leonov, J. Sperling, and D. Elad, J. Am. Chem. SOC.,98,6026 (1976). (9)M.Charlier and C. Helene, Photochem. Photobiol., 16,71 (1972). (10)D.V. Bent and E. Hayon, J. Am. Chem. SOC.,97,2612 (1975). (11)F. D.Bryant, R. Santus, and L. I. Grossweiner, J.Phys. Chem., 79. . -,2711 - . - - (19751. -. .. ,. (12)R.Klein, I. Tatkcheff, M. Bazin, and R. Santus, J.Phys. Chem., 85,670 (1981). (13)F. Wilkinson and A. Garner, Photochem. Photobiol., 27, 659 (1978).
.
Figure 1. Absorption spectra of triplet acetone (-), neutral isopropyl alcohol radical (- - -), and acetone anion radical (- -) in water.
-
The knowledge of excited states and primary photochemical reactions of nucleic acid components is necessary for a better understanding of photobiological response. In the present work we investigate the quenching processes of triplet acetone by indole, purine, and pyrimidine derivatives. Since the energy level of triplet acetone, Le., 28000 cm-', is higher than those of the quenchers, energy transfer is expected to occur efficiently in the initial step.14 Although triplet acetone and its reduced form, isopropyl alcohol radical, absorb the light above 25 000 cm-', their extinction coefficients are relatively small. Therefore, it is possible to observe the transient absorptions produced from solutes in a wide wavenumber region. Experimental Section Indole, indole derivatives (1-methyl-, 2-methyl-, 3methyl-, &methyl-, and 7-methylindole, and tryptophan), purine derivatives (caffeine, 7-methylguanine, adenine, adenosine, and guanosine), and thymine were of the highest grade commercially available (Sigma Chemical Co.) and used without further purification. Disodium 1,5naphthalenedisulfonate (extra pure grade, Tokyo Kasei Kogyo Co.) was recrystallized twice from water-ethanol solutions. Benzophenone (guaranteed reagent grade, Wako Junyaku Co.) was recrystallized twice from ethanol solutions. Water was distilled 4 times. Laser photolysis was carried out by using a Tachisto Tac I1 excimer laser, which provided the 248-nm (KrF) or 193-nm (ArF) light pulse with a duration of 20 ns. The maximum energy was 250 mJ in KrF and 150 mJ in ArF. The source of analyzing light was a 500-W Ushio xenon lamp, whose intensity was increased about 15 times during the detection of transient absorptions by using an additional dc power supply. The laser and analyzing light beams passed collinearly through a Suprasil quartz cell with an optical path length of 10 mm. The spot of the laser beam was confined to 6 = 10 mm on the cell surface by means of an iris. The pear-shape glass bulb was connected to the cell for evacuating solutions. The transmitted light entered a Narumi R23 spectrograph monochromator equipped with a Hamamatsu R374 photomultiplier. The signals were displayed as a function of time on a Tektronix 7904-7A13 oscilloscope. Transient spectra were determined from a series of oscilloscope traces obtained with the same solution in a point-by-point manner with respect to wavenumber. Solutions were degassed by freeze-pump-thaw cycles. The pH was stabilized by using phosphates (pH 4.6-9.1) or carbonates (pH 9.4-10.7). In the solutions of acetone and indole derivatives, their concentrations were chosen to give an absorbance of 2.0 for acetone and 0.6 for the derivatives at 248 nm. The absorption spectra of sample solutions were measured with a Hitachi 330 spectropho(14)R.F.Borkman and D. R. Kearns, J.Chem. Phys., 44,945(1966).
2422
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
Kasama et ai.
h
i
, $&A.
u
+
h.
Wavenumber (cm")
Flgurr 3. Transient absorption spectra obtained with the 248-nm excitation (50 ml)of 0.16 M acetone and 2.2 X lo-' M indole in the buffered solution at pH 7.1 (phosphate concentration, 1 X lo-' M).
kv"" 1
25.01 c
3SN* -% SN (5) where k, is a quenching rate constant, and k3 and k5 are triplet decay rate constants of acetone (A) and sodium 1,5-naphthalenedisulfonate(SN), respectively. The time dependence of 3A* and 3SN* concentrations, [3A*],and [3SN*] are expressed by the following equations: [3A*], = [3A*Ioe-(k3+kq[SN1)t (6)
where [3A*]ois the triplet-acetone concentration immediately after laser excitation. The concentration of acetone was set at 0.16 M and that of SN was changed in the range of 0.4 X 10*1.3 X 10" M. The overall decay constant of 3A*, k, = k3 + k,[SN], was evaluated from the growth curves of %N* at 445 nm, and k, was plotted against [SN] in Figure 2a. From the slope and the intercept we obtained k3 = 2.0 X lo5 s-' and k, = 3.6 X lo9 M-' s-l. The decay constant of 3SN*was found be k5 = 1.5 X lo4 s-l. If k3 and k5 are negligible in comparison to k,[SN], the total concentration of triplet acetone should be equal to that of 3SN* and, therefore, the ratio between their absorbances corresponds to the ratio between their extinction coefficients. Because of relatively large values of k3 and k,, we made a correction to the maximum absorbance of 3SN*in a manner similar to that of Salet and Bensasson.'* Figure 2b presents the observed and corrected maximum absorbances of 3SN*vs. [SN]. On the basis of the molar extinction coefficient of 3SN* at 445 nm, 9900 M-' cm-l, qTwas estimated to be 600 f 100 M-' cm-' at 300 nm (33300 cm-'). Indoles. Figure 3 shows the transient absorption spectra on the 248-nm excitation (50 mJ) of 0.16 M acetone and 2.2 X lo4 M indole in the phosphate buffer solution at pH 7.1. Only a weak transient absorption was observed for the aqueous solution of indole alone under the same excitation condition. Previous studies have reported that the fluorescence lifetime of indole is 4.0-4.9 ns and its quantum yield is about 0.25 in a neutral aqueous solution at room The quenching rate constant of singlet excited indole by acetone has been determined to be 1.0 X 10'O M-' s - ' . ~ ~ Therefore, singlet excited indole, if produced, should be quenched almost completely within 1 ns. In fact, only the absorption due to triplet acetone was observed immediately after the pulse. Triplet indole (31H*)has an absorption peak at 23 260 cm-l (=430 nm), and neutral indole radical (I-),where the hydrogen atom of the N-H bond is missing, has two peaks at 19600 cm-' ( 4 1 0 nm) and 30300 cm-' (=330 nm).lo The most recent extinction coefficient of triplet indole in water is 3.64 X lo3 M-' cm-' at 23260 cm-'.12 The extinction coefficient of the radical was determined here to be 1850 M-' cm-' at 19600 cm-I and 3110 M-' cm-' at 30300 cm-' by comparing the initial absorbance of hydrated electron to that of I. in direct photoionization of indole in the air-saturated solution. Such a method has
(15)M.Simic, P.Neta, and E. Hayon, J.Phys. Chem.,73,3794(1969). (16)K.-D. Asumus, A. Henglein, A. Wigger, and G. Beck, Ber. Bunsenges. Phys. Chem., 70,756 (1966). (17)M. Nemoto, H.Kokubun, and M. Koizumi, Bull. Chem. SOC. Jpn., 42,2464 (1969).
(18)C.Salet and R. Bensasson, Photochem. Photobiol., 22,231 (1975). (19)J. Feitelson, Isr. J. Chem., 8, 241 (1970). (20)R. W. Ricci, Photochem. Photobiol., 12,67 (1970). (21)A. G.Szabo and D. M. Rayner, J. Am. Chem. Soc., 102, 554 (1980). (22)E.P.Kirby and R. F. Steiner, J. Phys. Chem., 74,4480 (1970). (23)R. W. Ricci and J. M. Nesta, J . Phys. Chem., 80, 974 (1976). (24)M.L. Posener, G. E. Adams, and P. Wardman, J. Chem. SOC., Faraday Trans. 1 , 72,2231 (1976).
Figure 2. Triplet decay rates (a) and maximum triplet absorbances (b) plotted against the concentration of disodium 1,Bnaphthalenedisulfonate (SN): (0) observed value and (0)corrected value.
tometer. Laser irradiation was always carried out at room temperature.
Results and Discussion Acetone. Figure 1 shows the absorption spectra of triplet acetone, neutral isopropyl alcohol radical, and acetone anion radical in water. The pK, value for the radicals in water has been reported to be 12.2.15J6 (CH3)2C-OH + H 2 0 S(CH&C-O- + H30+ PK.
The molar extinction coefficient of triplet acetone cAT was determined from energy transfer between triplet acetone and sodium 1,5-naphthalenedisulfonate. The singlet acetone produced in direct laser excitation is converted with an efficiency of unity into triplet acetone via rapid intersystem cr~ssing.'~ The triplet energy levels of acetone and sodium 1,5-naphthalenedisulfonate1' are 28 OOO and 20 800 cm-', respectively. Hence, the energy transfer mechanism is as follows: A hv -.+'A* (1)
+
'A*
-+
3A*
3A* + SN
4
k3
k.4
(2)
3A*
A
A
3SN*
(4)
,,
Photochemical Reactions of Triplet Acetone
The Journal of Physical Chemistry, Vol. 86,No. 13, 1982 2423 h
1
o..o..
..............
I
0
J
I
0
2
6 8 1012 [ PO:-] ( x I O - ~M )
4
Figure 5. Plots of the decay rate constant of the transient X vs. phosphate concentration. The pH of the solutions should be always 7.1, because the concentration ratio, [NaH,PO,]/[Na,HPO,], was adjusted to 2/3.
Wavenumber (cm-' )
Figure 4. Transient absorption spectra obtained with the 248-nm excitation (50 MJ) of 0.16 M acetone and 2.2 X lo4 M indole (IH) (a), and 0.16 M acetone and 2.1 X loJ M 5-methyiindoie (5-MI) (b) in the nonbuffered solution at pH 6.0.
been applied successfully to tryptophan and its derivatives." The observed spectra show that the quenching process of 3A* by indole gives rise to 31H* and I.. Furthermore, the process yields a transient species X with the absorption peaks at around 27 500 and 32 000 cm-l. The decay of the species was found to fit the first-order kinetic law with a rate constant of 1.2 X lo5 s-' at room temperature. Figure 4a presents the transient absorption spectra obtained with the same solution as Figure 3 except for the absence of a buffer (pH 6.0). The absorptions due to 31H*, I., and X appear similarly in the spectra. However, the spectrum determined at 1.2 p s after the pulse shows an additional short-lived absorption in the region of 1500018500 cm-', which can be attributed to indole cation radical (IH+.). The formation of the cation radical in the nonbuffered solution was also confirmed for other indoles, for example, 5-methylindole as seen in Figure 4b. Since the absorptions due to 31H*, I., and X grow concurrently with the decay of 3A*, it is clear that these species are produced from triplet acetone in both solutions with and without a buffer. The neutral radical I- may originate from either the direct H-atom abstraction from IH by ,A* or the electron transfer between IH and 3A* followed by proton transfer from IH+. to water. The considerable enhancement of the absorption due to the solute cation radical was observed for 1-methylindole,where the H atom liable for proton transfer is substituted by a CH, group. Therefore, we consider that the electron-transfer process mainly contributes to the formation of I.. However, the two processes together will be referred to as charge transfer in this paper. The pK, of IH+. and I. has been evaluated to be about 3-4.lopz4 IH+-+ H 2 0 .pK,I. + H,O+ The equilibrium is established fairly rapidly in the buffered solution, but slowly in the nonbuffered solution. Such resulta were obtained with all of the indoles studied here. Figure 5 shows a plot of the decay rate constant of X vs. phosphate concentration, where the rate constant decreases significantly below 2 X lo-, M. Since the decay of X was found to depend on the pH, this fact suggests the local
Figure 6. Transient absorption spectra obtained with the 248-nm excitation (50 MJ) of 0.16 M acetone and 2.0 X loJ M 2-methylindole (2-MI) in the buffered solution at pH 7.1 (phosphate concentration, 1 X
lo-,
M).
depression of H+ at low phosphate concentration. Figure 6 shows the transient absorption spectra for 0.16 M acetone and 2.0 X lo4 M 2-methylindole (2-MI) in the buffered solution at pH 7.1. The spectra consist of the absorption due to triplet 2-MI at 23000 cm-'and thm due to neutral 2-MI radical at 19500 and 31 200 cm-'. We could not observe the absorption which corresponds to the transient X in indole, although it was evidently produced in the irradiated aqueous acetone solutions of 1-methyl-, 3-methyl-, &methyl-, and 7-methylindole as well as tryptophan. This result suggests that the transient X arises from the reaction which takes place at the 2-carbon atom in the indole ring. The transient X is considered to be either I or 11; I is produced via H-atom abstraction from
.J(....~, ',
m , f, , ( C H 3 ) ~
:
'.I-''
H
'N' H
(n)
(I 1
indoles by triplet acetone and I1 via addition of triplet acetone to indoles. In order to obtain further informations about X, we examined the pH dependence of its decay rate constant in phosphate buffer solutions (see Figure 7). The rate constant decreases with increasing pH, although it tends to level off above pH 9; therefore, the hydrogen ion plays a certain role in the decay of X. We identify the transient species X with the indole-acetone adduct I1 rather than I. If X were radical I, its decay would obey pH-independent second-order kinetics. The observed first-order decay of X can be explained by the following reactions:
+H30*/H20
-
a
+KH~)~~OH+H~O/OH-
H
The first reaction dominates at high pHs and the second
2424
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
Kasama et ai.
ib
Wavenumber(cm-')
1
.oi
Figure 8. Transient absorption spectra obtained with the 248-nm excttation (50 mJ) of 0.16 M acetone and 2.5 X lo4 M 1-methylindole (1-MI) in the buffered solution at pH 7.1 (phosphate concentration, 1 X lo-* M).
9
IH
'
----
0061 3124s 3
PH
~
Try
6 ,us
$0041 ' 3 5 F r
Flgwe 7. Plots of the decay rate constant of the transient X vs. pH in the buffered solution (the phosphate concentration was kept at 1 X lo-* M): (0)indole (IH) and (0)3-methylindole (3-MI).
reaction becomes more important with decreasing pH. Nowada et al. have proposed a similar intermediate (X')
15000
20000
25000
30000
Wavenumber ( cm" )
PhZCO
Figure 0. Transient absorption spectra obtained with the 248-nm exdtatbn (50 mJ) of 0.16 M acetone and 2.8 X lo4 M tryptophan (Try) in the buffered solution at pH 7.1 (phosphate concentration, 1 X lo-* M).
PhO X' in the quenching process of triplet benzophenone by diphenyl ether.25 The quantum yield of diphenoxydiphenylmethane (Ph,C(OPh),) was found to be 0.03, and the ratio k d / k , was estimated to be 30.
+ Ph--O-Ph X' PhzC=O + Ph--O-Ph
3Ph2C=O*
X'
kd +
+ Phi)
-!!+Ph2C--O-Ph Ph2C-0-Ph
+ PhO
-+
PhzC(0Ph)z
I t is generally accepted that electrophilic substitution reactions of indoles take place at the 3-carbon atom,%while radical reactions occur mainly at the 1-or 3-carbon atoman However, several substitution reactions involving the electron-transfer complex as an intermediate have been found to produce 2-substituted indoles.2G30 The calculated charge distribution of an indole cation radical shows the highest positive charge at the 2-carbon atom.29 We suggest that acetone anion radical, indole cation radical, and acetoneindole photoadduct are formed via a common electron-transfer complex 3 x
A
t H I
sz
3 ~ * . . . ~ f ~
encounter complex
.A-.IH+.
electron transfer complex
-
\
A-0
t I H ' .
. .
A-IH
(25) K. Nowada, M. Hisaoka, H. Sakuragi, K. Tokumaru, and M. Yoshida, Tetrahedron Lett., 137 (1978). (26) R. J. Sundberg, "The Chemistry of Indole",Academic Press, New York, 1970, pp 1-92.(27) J. Hutton and W. A. Waters, J. Chem. Soc., 4253 (1965). (281 T. Matsuo. S. Mihara. and I. Ueda. Tetrahedron Lett., 4581 (1976). (29) K. Yoshida, J.Am. Chem. Soc., 101, 2116 (1979). (30) 0.Ito, I. Saito, and T. Matsuura, J. Am. Chem. Soc., 102, 7535 (1980).
The free enthalpy change AG (kcal mol-') for the above electron transfer is given approximately by the equation31 AG 23.06(E0, - Ered)- AET
-
where E,, and E d are the half-wave potentials in volts for the oxidation of an electron donor and for the reduction of an electron acceptor, respectively. AET (kcal mol-') is the energy level of triplet acetone. Erd for acetone has been reported to be -2.305 V against a saturated calomel electrode E , for indole was estimated to be 0.94 V (vs. SCE) from the empirical relationship between ionization potential (IP) and IP of indole is 7.76 eV.% Therefore, AG for the triplet acetone and indole pair was evaluated to be -5.2 kcal mol-'. In a similar manner AG for the triplet acetone and adenine pair was estimated as +10.1 kcal mol-' on the basis of 8.48 eV for IP(adenine).35 Neither electron transfer nor photoaddition occurs in this pair as described later. All indoles gave similar results except 1-methylindole, 2-methylindole, and tryptophan. The absorption with peaks at 17 500 and 29 500 cm-! can be assigned to the solute cation radical in the transient spectra for 1methylindole (1-MI) (see Figure Its yield (0.32) is comparable to those of neutral radicals in other indoles. The triplet state of 1-MI has the absorption at around 21 700 cm-', which disappears completely within a few microseconds after pulse irradiation.% The absorption due to the photoadduct shows the peaks a t 23 300 and 32 000 cm-'; the former peak shifts considerably toward lower wavenumbers as compared with the corresponding peaks (31) D. Rehm and A. Weller, Zsr. J. Chem., 8, 259 (1970). (32) R. 0. Loutfy and R. 0. Loutfy, Can. J. Chem., 50, 4052 (1972). (33) L. L. Miller, G. D. Nordblom, and E. A. Mayeda, J.Org. Chem., 37,916 (1972). (34) H. Giisten, L. Klasinc, and B. Ruscic, Z . Naturforsch.A, 31,1051 (1976). (35) J. Lin, C. Yu, S. Peng, I. Akiyama, K. Li, L. K. Lee, and P. R. LeBreton, J . Am. Chem. Soc., 102, 4627 (1980). (36) F. Willtinson and A. Garner, J. Chem. Soc., Faraday Trans. 2,73, 222 (1977).
Photochemical Reactions of Triplet Acetone
TABLE I: Total Quenching Rate Constants, Triplet Yields, Radical Yields, and Relative Photoadduct Yields of Indole and Its,Derivatives 10-9h M-l s': indole 2-methylindole 3-methylindole 5-methylindole 7-methylindole
triplet yield"
re1 radical photoadduct yield yield
The Journal of Physical Chemisfty, Vol. 86, No. 13, 1982 2425
0.06- (a) O
0 pi
a16
,US
-
0.04
4.0 4-5 4.0 4.1 4.8 4.2
0.30 0.26 1.0 0 0.43 0.46 0.26 0.32 1.2 0.28 0.31 1.5
