J. Phys. Chem. 1981, 85, 670-677
670
retical fit of the k dependence of the nonradiative rates is included. The numerical comparison between theory and experiment was carried out by a trial and error procedure. Cia: and p were varied until the best fit was obtained. The solid curve was obtained by using CiA: = 0.09 and /3 = 0.97, the dashed curve by using CiA? = 0.04 and p = 0.96. The difference in the slopes of the curves in Figure 6 suggests (see Figure 5 ) that the effective displacement between ground and excited states along the maximum frequency modes is greater in the case of the distorted complexes than for the complexes with octahedral skeletons. This effect is consistent with the symmetry arguments concerning the number of effective acceptors. The latter is greater in the case of lower symmetry. In Figure 6 one also sees that the larger effective displacement of the maximum frequency modes leads to larger absolute nonradiative rates as it should be expected for the case of weakly coupled states. Conclusions The large structural variety of chromium(II1) complexes permitted us to choose a series, in which one parameter
is gradually changed without affecting the others seriously. As the number of active hydrogen atoms is increased an approximately proportional rise of the nonradiative decay is observed. This is in contrast to the predictions of Robbins and Thornson: who have expected an nth power dependence. They wrongly assumed that the effective displacement is proportional to the number of degenerate oscillators, or strictly that the number of maximum frequency stretching modes has an effect on the nonradiative rate equivalent to increasing the displacement of a single maximum frequency mode. The more comprehensive theory described here demonstrates that the nonradiative decay rate depends on the sum of the squares of the individual displacements, only a few of which are nonzero on account of symmetry. The other influence the decay rate by their frequency change and by the subset of the total number of high-frequency stretching modes. Acknowledgment. We thank Professor Dr. G. Hohlneicher, University of Cologne, for the opportunity to measure the phosphorescence decay times, Miss Laura Hughes, La Jolla, for reading the manuscript, and the Deutsche Forschungsgemeinschaft,for financial assistance.
Photophysics of Indole. Comparative Study of Quenching, Solvent, and Temperature Effects by Laser Flash Photolysis and Fluorescence R. Klein,” I. Tatlscheff,
‘
Instltut Curb, Section de Physique et Chimle, 7523 1 Paris Cedex 05, France
M. Bazin, and R. Santus Laboratoire de Physico-chimie de /‘Adaptation biologique, Museum National d’ Histoire Naturelle, 7523 1 Paris Cedex 05, France (Received: July 30, 1980; In Final Form: October 31, 1980)
A study of both singlet and triplet energy transfer from indole to anthracene has allowed a determination of the triplet extinction coefficient of indole in cyclohexane solution, tT = 47 ( 4 5 ) X lo2 M-’ cm-l. This value was then used to determine the triplet quantum yield of indole in cyclohexane, Q T = 0.43 f 0.05, which is close to the fluorescence quantum yield, QF = 0.49 f 0.05. Dynamic quenching by N20 is observed in cyclohexane solution, QF, QT, and the fluorescence lifetime of indole being 23% lower in the presence of 1 atm of N 2 0than in degassed solution. In aqueous solution, QF and QT increase with decreasing temperature (60 to 1.5 “C)while the quantum yields of solvated electrons and radicals decrease. At 1.5 OC,in water, QF = 0.46 f 0.05 and QT = 0.43 f 0.1. The variation with temperature of the fluorescence quantum yield of aqueous indole has been interpreted in terms of two radiationless processes, a temperature-independent one and a temperature-dependent one. The present study identifies the temperature-independent pathway as intersystem crossing to the triplet state. The photoionization mechanism is discussed with respect to these new data.
Introduction In spite of the great deal of attention devoted to the photophysics of indole and its derivatives (in particular tryptophan), the deactivation scheme following UV excitation in fluid solution is not ~ l e a r . ~Walker ,~ et ala4analyzed fluorescence lifetimes and yields of indole in terms (1)LA 198, associd au C.N.R.S. e t B l’Universit6 P. e t M. Curie. (2) R. Santus, M. Bazin, and M. Aubailly, Reu. Chem. Int., 3, 231 (1980). (3) R. Lumry and M. Hershberger, Photochem. Photobiol., 27, 819 (1978). (4) M. S. Walker, T. W. Bednar, and R. Lumry in “Molecular Luminescence”, E. C. Lim, Ed., W. A. Benjamin, New York, 1969, p 132.
of a radiative rate constant k’ and a nonradiative rate constant K ( T ) that is strongly temperature dependent in water solution. Gally and Edelman5 suggested for the temperature quenching of tryptophan in aqueous solution a model in which two radiationless processes compete with emission for the deactivation of the fluorescent state: one, temperature independent, the other, temperature dependent. This model also fits indole.6-8 The tempera( 5 ) J. A. Gally and G . M. Edelman, Biochim. Biophys. Acta, 60, 499 (1962). (6) E. P. Kirby and R. F. Steiner, J. Phys. Chem., 74, 4480 (1970). (7) E. P. Busel, T. L. Bushueva, and E. A. Burshtein, Opt. Spectrosc., 29, 268 (1970).
0022-3654/81/2085-0670$01.25/00 1981 American Chemical Society
Photophysics of Indole
ture-independent process was assumed to be intersystem crossing to the triplet On the basis of flash photolysis results obtained by Grossweiner and J o ~ c h e k , ~ Walker et al.1° proposed that one decay path for excited indole is electron ejection; the temperature-dependent process might be solvated electron formation."JZ A controversy exists, however, about the origin of the solvated electron, which may be the fluorescent statelZJ3or a prefluorescent state such as a vibrationally excited state.'"'* Hopkins and Lumry" suggest at least two pathways for electron formation, outright electron ejection and collisional scavenging. This latter process probably occurs in cyclohexane from the lowest excited state of indole on collision with NzO. However, Grand et al.la find that N20does not quench the indole fluorescence but only scavenges electrons ejected from a prefluorescent state. Photophysical and photochemical parameters such as the fluorescence quantum yield (QF),the fluorescence lifetime ( T F ) , and the solvated electrons quantum yield (8,) of the indoles also have been extensively studied but are still controversial.2~3~19 The triplet quantum yield (QT) has been determined in an aqueous solution of N-acetyltryptophanamide (NATA) and t r ~ p t 0 p h a n . lFor ~ indole, the reported QT values were not directly determined. Onem was derived from the Q F value by assuming Q F + QT = 1 (in cyclohexane). Another was deduced from the phosphorescence quantum yield in ethylene glycol-water glasses at low temperature.2' Q T in aqueous solution was approximately determined by subtracting the temperaturedependent yield plus the fluorescence yield from unity.22 Reliable photoionization data also are lacking. For aqueous indole excited in its first absorption band, 240 < X < 300 nm, the electron quantum yield at room temperature, respectively determined by transient absorption and scavenging by NzO, H+, or Fez+has been reported as 0.26,16 O.ll,I1 0.11,l2 and 0.009.23 In this work, we attempted to obtain an energy balance by measuring not only the quantum yield of fluorescence but also the quantum yields of triplet, solvated electron, and radical formation. In order to obtain the QT value from T-T absorption, we needed to know the triplet extinction coefficient (ET)at the wavelength of the maximum T-T absorption. We shall first describe in this paper the determination of ET by triplet energy transfer from indole to anthracene and the subsequent determination of QT in degassed cyclohexane. Some results of a study of NzO quenching effect are then described, although full details (8)R. Klein and I. Tatischeff, Chem. Phys. Lett., 51, 333 (1977). (9)L. I. Grossweiner and H. I.Joscheck, Adu. Chem. Ser., No. 50,279 (1965). (10)M.S. Walker, T. W. Bednar, and R. Lumry, J. Chem. Phys., 45, 3455 (1966). (11)T. R. Hopkins and R. Lumry, Photochem. Photobiol., 15, 555 (1972). (12)J. Feitelson, Photochem. Photobiol., 13,87 (1971). (13)R. F. Evans, R. R. Kuntz, W. A. Volkert, and C. A. Ghiron, Photochem. Photobiol., 27, 511 (1978). (14)W.A. Volkert, R. R. Kuntz, C. A. Ghiron, R. Evans, R. Santus, and M. Bazin, Photochem. Photobiol., 26, 3 (1977). (15)D. V. Bent and E. Hayon, J. Am. Chem. SOC.,97,2612 (1975). (16)J. F. Baugher and L. I. Grossweiner, J. Phys. Chem., 81, 1349 (1977). (17)E. Amouyai, A. Bernas, and D. Grand, J. Photochem., 9, 236 (1978). (18)D. Grand, A. Bernas, and E. Amouyal, Chem. Phys., 44,73 (1979). (19)I. Tatischeff and R. Klein, Photochem. Photobiol., 22,221(1975). (20)C. Pernot and L. Lindqvist, J. Photochem., 6, 215 (1976/77). (21)D. Muller, M. Ewald, and G. Durocher, Can. J. Chem., 52,409 (1979). (22)G.R. Fleming, J. M. Morris, R. J. Robbins, G . J. Woolfe, P. J. Thistlethwaite, and G. W. Robinson, Proc. Natl. Acad. Sei. U.S.A., 75, 4652 (1978). (23)H.B. Steen, M. K. Bowman, and L. Kevan, J. Phys. Chem., 80, 482 (1976).
The Journal of Physical Chern;stv, Vol. 85, No. 6, 7981 671
250
300
350
400
A (nm) Figure 1. Absorption spectra in degassed cyclohexane solution: (--) 2 X lo4 M indole; (0) 2 X lo4 M anthracene; (-) 2 X lo4 M Indole 4- 2 X lo-' M anthracene.
will be published elsewhere.24c Then the determination of quantum yields in aqueous solution of indole at three temperature are described, and the photoionization mechanism and the photophysics of indole are discussed in view of these new data.
Experimental Section The solutions, kept in the dark, were made with triply distilled water (on permanganate and baryte) or purified Merck cyclohexane (passed through a column of activated Woelm silica). Anthracene (Hopkins and William), naphthalene (Eastman), indole (Calbiochem), argon, and NzO (Air Liquide) were used as received. Degassing was obtained by five successive freeze-thaw cycles. The absorption spectra were measured with a Cary 118 CX spectrophotometer. Observed optical densities in indole and anthracene mixed solution were the sum of the respective absorbance of indole and anthracene in pure solutions of identical concentrations (Figure 1). Corrected emission spectra and fluorescence quantum yields were obtained with the apparatus described in earlier modified first by use of photocounting and by data treatment with Hewlett-Packard 9835 microcomputer. Two perpendicular micrometers positioned the edge corner of a 10 X 10 mm quartz cell in the exciting and analyzing light beams to reduce screening and reabsorption phen0mena.2~~ The fluorescence quantum yields in a mixture of two solutes were calculated by taking into account the fractional absorption defined by B1 = [Al/ ( A , + A,)] [ 110-(A1 A z ) ] . AI and Az are respectively the absorbances of solutes 1 and 2 for the optical pathway in the fluorescence cell. The slit widths of the emission monochromator were set to a half-maximum bandwidth of 1.6 nm for cyclohexane solutions and 6.4 nm for aqueous solutions. The 265-nm laser flash photolysis setup has been already described.*,% Solutions were excited by using 20-11s fwhm laser pulses. The maximal photon density irradiating the cell was 6 X 10l6photons cm-z pulse-'. I(t),the transmitted analyzing light as a function of time elapsed after the laser +
~
(24)(a) I. Tatischeff and R. Klein in "Excited States of Biological Molecules", J. B. Birks, Ed., Wiley, New York, 1976,p 375. (b)P. Vigny and M. Duquesne, Photochem. Photobiol., 20, 15 (1974). (c) R. Klein, J. P. Ballini and I. Tatischeff, to be published. (25)R. Bensasson, C. Chachaty, E. J. Land, and C. Salet, Photochem. Photobiol., 16,27 (1972). (26)C.Salet and R. Bensasson,Photochem. Photobiol., 22,131 (1975).
672
The Journal of Physical Chemlstty, Vol. 85, No. 6, 1981
Klein et al.
TABLE I: Singlet Energy Transfer Data in Cyclohexane
: 10-4 2 x 10-4 2 x 10-4
0.94 0.88 0.78
10-4 2 x 10-4 5 x 10-4
7 7.7
0.06 0.12 0.22
6.4
This result can be interpreted as a singlet energy transfer. The relative enhancement of anthracene fluorescence by energy transfer makes it possible to determine that singlet quenching without transfer can be neglected with respect to quenching with transfer. This leads to the following mechanism (rate constants are in parentheses): anthracene absorption
i
U
Figure 2. Fluorescence emission spectra in degassed cyclohexane solution. Excitation 265 nm: (I) 2 X M indole; (A) 2 X IO-' M anthracene; (I A) 2 X lo-' M indole i2 X M anthracene.
+
pulse, and the diode signal measuring the laser intensity are recorded through the use of a double-beam oscilloscope. Analysis of the transient absorbance OD@)= log [ ( I o / I ( t ) ] yields the transient lifetime and, by extrapolation, the initial absorbance at the end of the laser pulse. Initial absorbances obtained from oscilloscope traces were measured at different laser intensities, normalized to take into account laser intensity variations, and extrapolated to zero laser intensity. Photolysis experiments were performed with a Phillips low-pressure mercury Lamp (no. 93109) by using 1M NaI as the filtering solution and 6 X M potassium ferrioxalate as the actinometer (4254 = 1.26).27 The intensity a t 254 nm was 2.8 X lox4photons ~ r n s-l. - ~ The quartz irradiation cell could contain 100 mL of M indole aqueous solution, degassed by freeze-thaws cycles. Solvated electrons were determined with the scavenging system earlier described by Feitelson.12 It consists of lo9 M HC104 + 1M 2-propanol which leads to H2 formation according to eaq- H+ Ho k = 2.3 X 1O1O M-l s-'
-
+ Ho + ROH
-
H2
k =5X
lo7 M-l
s-l
Hydrogen formation was determined by gas chromatography. Methods and Results Singlet Energy Transfer from Indole to Anthracene. Anthracene in degassed cyclohexane emits a fluorescence with a maximum at 400 nm. By taking indole in degassed cyclohexane solution (maximum a t 300 nm) as a standard with QFI = 0.49,19we obtain, from corrected emission spectra, QyA = 0.34 in good agreement with Berlman's value, 0.36.28 A mixture of 2 X M indole and 2 X 10" M anthracene in degassed cyclohexane exhibits the anthracene fluorescence with the same quantum yield as a pure 2 X 10" M anthracene solution when excited at 357 nm. Upon 265-nm excitation the mixture shows two emissions, the indole fluorescencewith a quantum yield lower than in pure indole (QFr)o and the anthracene fluorescence with a quantum yield (QFA)IA higher than in pure anthracene solution (&FA)o (Figure 2). (27) C. G. Hatchard and C. A. Parker, Proc. R. SOC.London, Ser. A , 235, 518 (1956). (28) J. B. Berlman in "Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press, New York, 1971, p 356.
indole absorption indole fluorescence
I
+ - ++ - + - +A
+ hv,
hv,
'I*
'I*
I
(IoB')
(k3
hvf
indole nonradiative decays
'I*
singlet transfer quenching
'I*
anthracene fluorescence
(IoBA)
IA*
'A*
I A
A
I
(ki) lA*
hvi
(k,) (kj)
anthracene nonradiative decays 'A* A (k0 We can calculate the quantum efficiency of singlet transfer quenching of indole fs for an anthracene concentration [A] as
f s = ks[AI/(ks[Al + ki + k 3 = 1 - [(&F')IA/(QFI)OI Then, applying the Stern-Volmer relationship (&FI)O/(QF')IA = 1 ~STF'[AI where TFI = 9 is the fluorescence lifetime of indole in the absence of anthracene, we can calculate the bimolecular rate constant kS for the singlet transfer reaction. The mean value found with three different concentrations of anthracene (Table I) is KS = 7 (f0.7)X 1O1O M-l d, Indole Triplet in Cyclohexane. Triplet indole in degassed cyclohexane was observed by its absorption at 430 nm. However, transient absorption at 430 nm by radicals must also be taken into account when calculating the triplet yield. Thus, transients absorptions at 500 and 430 nm were compared in oxygenated and degassed cyclohexane. The ratio ODm,dwassed/OD500,0 = 3.5 corresponds to the quenching of fluorescence by OP1%Thus the 500-nm transient is not the triplet but chiefly the radical, as it appears on reported spectra.20 Therefore, from OD430,0a/OD500,~2 = 0.33, the radical contribution to the transient a t 430 nm (4%) can be subtracted to obtain the triplet absorbance of indole in cyclohexane. ODT = 0.960D430
This optical density is proportional to the triplet extinction coefficient CTand to the triplet concentration observed a t the end of the laser pulse [TI,,, the optical pathlength of the analysis beam being 1cm: ODT = CT[T]-. [TI,, depends on the triplet quantum yield (QT) and on the number of quanta absorbed by the solution. If the incident is low, the concentration of the molelaser intensity (lo) cules in the ground state [GI does not vary during excitation. The absorption will be B = 1- 10-'c[GlL where
EG
is the extinction coefficient at the laser wave-
Photophysics of Indole
The Journal of Physical Chemistry, Vol. 85, No. 6, 1981 673
OD
fA
T t-----i,
1 psec
Flgure 4. Oscilloscope traces of the transient absorption at 422.5nm In degassed cyclohexane solution of 1 0-4 M indole 1 0-4 M anthracene. a, b, c, d, e, and f corres ond respectlvely to OD, OD,,,,, (ODAhA,(OD1hA~, and (OD 1,. OD,,
+
P
1
2
3 4 5 LASER INTENSITY
Figure 3. Dependence of triplet absorbance on laser Intensity in degassed cyclohexane solution. Laser intenstty in arbitrary units, 1 au = 2.75 X 1015photons cm-'. (I) 2.6 X M indole, 430 nm; (A) 2 X lo-' M anthracene, 422.5nm; (N) 1.7 X M naphthalene, 414 nm.
length (265 nm) of ground state molecules and L the optical pathlength of the laser be- (0.05 cm). It follows that [TI,,, = QT1Jl/0.05 and, therefore ODT = C&TI$/O.OF)
of the transient corresponding to the absorption of indole and anthracene triplets directly populated via intersystem crossing, then a slow rise (in the microsecond range) of anthracene triplet due to triplet energy transfer from indole triplet. Triplet energies of indole and anthracene, respectively taken as ET' = 2.47 X lo4cm-' and ETA= 1.47 X lo4 cm-1,32satisfy Birks' energy condition, ET' - E T A > 1500 cm-', for which triplet-triplet transfer can be assumed to occur a t a diffusion-controlled rate with an efficiency of 100%.33 Thus, we represent the mechanism as follows (rate constants are in parentheses): II* 31* (klisc) 'A*
-+ -+
'A*
31*
R and I representing reference and indole, respectively, one obtains
Actinometry of the laser flash was obtained by using naphthalene and anthracene in degassed cyclohexane as references. The naphthalene triplet was monitored at its 414-nm absorption peak for which C T =~ 2.45 X lo4 M-l cm-l 30 and QTA = 0.71.'' The anthracene triplet was monitored at 422.5 nm for which ET* = 6.47 X lo4 M-l cm-' 3o and QTA= 0.71.31The intensity of the laser pulse was obtained by applying the Bensasson et aL31b equation, Io = -log ( 1 - l ) / Q & G R where 1 = [T],,/[G] is the fraction of molecules converted to the triplet state and eGR the extinction coefficient of ground state molecules at 265 nm. CG was taken as 5 X lo3 and 1.02 X lo3 M-l cm-' respectively for naphthalene and anthracene.31b As shown in Figure 3, the triplets absorbances are proportional to the laser intensity until about 5 X 1015 photons cm-2 pulse.-l Anthracene and naphthalene actinometers lead to an identical value of ET'&$ = 2.03 X lo3 M-l cm-l. Triplet Energy Transfer from Indole to Anthracene. The formation of anthracene triplet by energy transfer from triplet indole appears on the oscilloscope trace in Figure 4 which shows a fast rise (in the nanosecond range) (29)W.B. De Lauder and P. Wahl, Biochim. Biophys. Acta, 243, 153 (1971). (30) R. Bensasson and E. J. Land, Trans. Faraday SOC.,67, 1904 (1971). (31) (a) B. Amand and R. Bensasson, Chem. Phys. Lett., 34,44 (1975). (b) R. Bensasson, C. R. Goldschmidt, E. J. Land, and T. G. Truscott, Photochem. Photobiol., 28, 277 (1978).
31* + 'A
11
(kAk,) ( ~ T J
3A* &,[A]) Optical densities of the transient were determined at delay times from 0.1 to 5 ps (Figure 4 ) . By taking log (OD,, - OD(t) ) vs. time, the risetime of the slow part of transient is determined. This corresponds to the triplet lifetime of indole when the latter is quenched by anthracene. In the absence of anthracene, the indole triplet decays with a lifetime TT' = l / k n , where kTi is the sum of the rate constants of the indole triplet decay paths. In the presence of anthracene concentration [A], the indole triplet decays with a lifetime 7TIA = l/(kTi + kT[Al) The triplet transfer efficiency f T will be given by fT = ~ T [ A I / ( ~ + T~ ~T [ A I=) 1- (7TU/7T1) and the bimolecular rate constant k T will be kT = [ ( T T ' ~ ) --~ (TT')-~]/[A] It should be noted that the efficiency of the triplet transfer, f T , already reaches 0.94 with lo4 M anthracene (TTI = 16 ps in pure M indole solution, 7TU = 0.94 ps in the presence of M anthracene). The indole triplet yield in the presence of anthracene is (QT1)u = (QT')I(1 - f s ) where (&TI)' is the indole triplet yield in the absence of anthracene, so that the maximum concentration of indole triplet will be [31]IA = Io(13')u( Q T ' ) I (-~f s ) where (B')IA is the fractional absorption of the laser beam by indole in the mixture. The correlI
(32)F.Wilkinson and A. Gamer, J. Chem. SOC., Faraday Trans. 2,77, 222 (1977). (33) J. B. Birks in "Photophysics of Aromatic Molecules", Wiley, New York, 1970,p 540.
Klein et al.
The Journal of Physical Chemistty, Vol. 85, No. 6, 198 1
674
TABLE 11: Indole Triplet Data in Cyclohexane [indole], M 10-4 2 x 10-4 2 x 10-4
[anthracene], M 10-4 2 x 10-4 5 x 10-4
mean values
h ~M'', S-' 9.7 x 109
9
(t
1) x 109
ODfast = (0D')IA + (ODA)IA ODht was obtained by extrapolating the appropriate delay curves to 40 ns after the start of the laser pulse which is the time required to reduce the concentrations of singlet states to less than 10% of their initial values. By taking f20 ns of the extrapolation time, we introduce an uncertainty of less than 5 % if the anthracene concentration is lower than 2 X lo4 M and less than 10% if the anthracene concentration is 5 X M. The maximum transient absorption, after a long delay time (Figure 4), is only due to fast rising anthracene triplets, (ODA)IA and those arising from the transfer, (ODA),:OD, = (ODA)IA + (ODA)*(ODA)tis proportional to the concentration of anthracene triplets due to the triplet transfer [3A]t which corresponds to the indole triplets effectively transferred (ODA),= [3A]tE~A= [31]IAf~€~A The slow growth (Figure 4) of the transient absorption - ODfmt,therefore effectively measured is ODdow= OD, = (ODA),- (OD')IA= [31]IAfT€TA - [31]IA($
It follows [ 3 1 1= ~ ODsiow/(fTCTA- ET')
= IO(B')IA(QT~)I(~ - fs)
In the absence of anthracene we have
L3III= Io(BI)I(QTI)I= ODTI/€TI Thus 4/(fT€TA
46 X 10' 46 X l o 2 50 X l o 2 47 (t 5) x lo2
8 . 5 x 109 8.4 x 109
sponding absorbance will be (OD1), = [31]IA6T'. The absorbance of anthracene triplets formed from anthracene and indole singlet states is (ODA),. The first part of the triplet growth (Figure 4) corresponds to formation of the indole and anthracene triplets during the laser pulse and decay of the singlet states, hence
ODslow
eT1at 430 nm, M-' cm-'
- ET') = [ODT1(B1)IA(l- fs)I /[OD810w(B')11
Y = [ODTI(B')IAU- fs)I /[OD,iow(B3~I it follows *I
YfTETA =-
l + Y
and finally QTI
= 2.03 x 103/€~I
The results obtained with different concentrations of indole and anthracene are summarized in Table 11. Increasing the temperature from 10 to 60 OC results in a negligible OD$ decrease, less than 5 % Quenching Effect of N20in Cyclohexane. Solutions of indole in cyclohexane were degassed and then saturated with N20 at atmospheric pressure. From the solubility of N20 in cyclohexane at room t e m ~ e r a t u r ethe ~ ~ concentration of N20 is evaluated to be 0.12 M.
.
(34) S. Sato, R. Yugeta, K. Shinsaka, and T. Terao, Bull. Chem. SOC. Jpn., 39, 156 (1966).
0.44 0.44 0.40
0.43t 0.05
The fluorescence quantum yield of 2 X M indole in N20-saturated cyclohexane solution (QfI)Nfi varies with the excitation wavelength as already described for the fluorescence quantum yield of pure indole in degassed cyclohexane (QF1)019, but the ratio (Qd)N,o/(QF')Oremains constant from 200 to 290 nm. We found (QF1)N20/(QF1)0 = 0.76 f 0.03. The triplet quantum yield measured under 265-nm laser flash excitation by looking at the 430-nm transient absorption of 2 X low4M indole was lower in the presence of N20, (QT1)N20, than in degassed solution, (QT')~. We obtain (QT')N,o/(QT~)o = 0.78 f 0.04. The fluorescent lifetime, measured with synchrotron radiation, equally decreases in the presence of N20 by exciting at 265 and 210 nm. ($)N o/(TFI)O = 0.77 f 0.02 (this result was obtained at Lure, drsay, in collaboration with J. P. B a l l i ~ ~ i ) . ~ ~ " Triplet and Radical Quantum Yields in Water. Indole triplet in aqueous solution is also generally detected by ita transient absorption at 430 nm. However, transient absorptions due to solvated electron and neutral radical interfere. The solvated electron contribution was eliminated by performing experiments in the presence of 1atm of N20 and 0.2 M 2-propanol which did not interfere with the formation of the triplet stat@ as hereunder c o n f i i e d (see Table IV). In presence of oxygen, after the fast initial indole triplet decay, the transient absorption at 430 and 530 nm was shown to be due only to the radical. Thus, the ratio OD43o/OD530 was found to be 0.2. This value was also obtained by using the pulse radiolysis technique and the N3*radical attack on ground state indole. In parallel with the results obtained with tryptophan,36 one can assume that indole oxidation by N3*leads to the neutral indole radical for which f430/e63? = 0.21, in good agreement with the laser flash photolysis value. In the presence of N20 + 2-propanol, the measured transient absorbances a t 430 and 530 nm are the sum of triplet (OD,) and radical (OD,) absorbances: OD430 = ODT,430 ODRw and ODm = ODT,~, ODKm It follows that ODT,430 = [1/1- KK'J [OD430- K(OD,,,)], with K = OD,,/OD,, and K'= ODT,,/ODT,m From the above determinations, K was taken as 0.205 (f0.005)and K'was deduced from data at 1.5 "C. At this temperature the radical quantum yield (QR), if it is taken as equal to the solvated electron yield (Qe-), is very low: 0.04 (this work, vide infra). Taking for the indole radical the extinction coefficient obtained with tryptophan radical ER = 1.58 X lo3 at 530 nm,38we can calculate O D R ,from ~~~
+
If we take
QTI
+
where Io is the incident laser intensity, B the absorption, R represents the radical, x the sample, and ref the acti~ ~be ~ nometer. From ODR,bmvalue, ODT,530 and O D T ,can calculated at 1.5 OC. It is found K' = 0.17 and,therefore OD~,430= 1.0340D430 - 0.210D530 (35) E. J. Land and W. A. Prutz, Znt. J. Radiat. Biol., 32, 203 (1977). (36)J. L.Redpath, R. Santus, J. Ovadia, and L. 1. Grossweiner, Int. J.Radiat. B i d , 27, 201 (1975).
The Journal of Physical Chemistry, Vol. 85,No. 6, 1987 675
Photophysics of Indole
TABLE 111: Triplet and Radical Quantum Yields in Water temp, "C
OD430
1.5 25 60
0.050
OD, 0.010 0.0125 0.021
0.028
0.0125
ODT,ao
QT
QR
0.05 0.026 0.0085
0.43 5 0 . 1 0.23 2 0.06 0.07 t 0.02
0.04 f 0.01 0.16 f 0.04 0.39 t 0.1
TABLE IV : Fluorescence Quantum Yields QF,air
temp. oc-
'
1.5 25 60
water
0.2 M 2-ProH
1 M 2-propanol QF.deeassed
QFXO
0.46 t 0.05 0 . 4 6 t 0.05 0.48 t 0 . 0 5 0.47 5 0.05 0.28 t 0.03 0.28 t 0.03 0.35 t 0.03 0.33 t 0.03 0.07 t 0.01 0.07 t 0.01 0.12 f 0.01 0.10 f 0.01
o'D
I
t
t
f
To obtain the triplet quantum yield, the molecular extinction coefficient of indole triplet in water at 430 nm must be known. The absorption spectrum of the indole 0.1 triplet is broader in water than in cyclohexane. The width at half-height deduced from reported spectra is 4 X lo3 cm-l in cyclohexane.20and 5.2 X lo3cm-' in water.15 With 4 a (a.u the hypothesis that oscillator strengths are independent LASER I N T E N S I T Y of solvent, we may infer that ET could be about 1.3 times Flgure 5. Dependence of solvated electron absorbance on laser lower in water than in cyclohexane, i.e., ET = 3.64 X lo3 intensity in aqueous solution of indole at (0) 1.5 OC;(0)25 "C.and M-l cm-' at 430 nm in water. The extinction coefficient (A)60 OC. Laser intensity in arbitrary units, 1 au = 8.8 X lOI5 photons ern-'. Insert shows (absorbancellaser Intensity) vs. laser intensity at of ground state indole being temperature independent, we (0)1.5 "C, (0)25 "C, and (A) 60 "C. assumed ET temperature independent too. Triplet quantum yields in 2 X lo4 M indole are obtained TABLE V : Solvated Electron and from QT = O D T , ~ ~ ~ B ~ . where ~ ~ BR ~ =~ 9.8 / ~ x D THydrogen ~ B ~Quantum ~ ~ ~ Yields B' = 1.14 X ET = 2.45 X lo4, QTR = 0.75, ODTR tepp, = 0.05, and ETI = 3.64 X lo3 therefore QT = 8.70D~. C ce-,e,oo ODwo ODe-,wo Qe" QH,b The crude approximation used to calculate ET in water and the measure of absorbances at two wavelengths in 1.5 1 4 1 0 0 0 . 0 2 0 0.0194 0.04 t 0.01 0.16 f 0.03 0.13 t 0.03 25 13000 0.070 0.067 order to deduce the radical contribution result in a low 60 10900 0.185 0.178 0.52 t 0.1 0.33 f 0.08 precision, estimated as 25-30% on QT values. From the transient absorption at 530 nm, we can calculate In 0.2 M 2-propanol. In 1 M 2-propanol. ODR,530 = OD530 - ODT,530 = OD530- 0.170DT,430 Extinction coefficients of the solvated electron at 600 nm and QR, taking t R = 1.58 X lo3 at 530 nm.36 The triplet deduced from Figure 1in ref 39 are reported in Table V. and radical quantum yields in water at three temperatures These values make it possible to calculate the solvated are reported in Table 111. electron quantum yield from Fluorescence quantum Yields. The fluorescence Qe- = OD,-BR~~RQTR/ODTRB1~e= 3.2 x 1O4ODe-/~,quantum yield of indole is higher in an alcohol-water mixture than in pure water.6 When the temperature deThe last column of Table V contains values for the hycreases, the solubility of NzO in water3s and the fluoresdrogen yield from electron scavenging by H+,obtained in cence lifetime of indole* increase, so that conditions occur photolysis experiments a t 254 nm. for a possible quenching of indole fluorescence by N20 as Discussion found with cyclohexane (vide supra). As EL check on these above effects, the fluorescence Indole Triplet i n Cyclohexane. The first step of our quantum yield of indole (2 X M) was determined at study on indole triplet in cyclohexane confirms the results three temperatures in both pure aqueous solution and of Pernot and Lindqvist.20 At low laser intensity, the 2-propanol solutions saturated with 1 atm of N 2 0 or detriplet lifetime is 16 ps and our value of CTQT = 2.03 X lo3 gassed. The results are in Table IV. M-' cm-l is nearly identical with their value of 2 X lo3M-l Solvated Electrons. In argon-saturated aqueous solucm-l. The absence of temperature effect on the triplet tions of 2 X lo4 M indole, the transient absorption at 600 yield they observe between 180 K and room temperature nm is due both to the solvated electron and the radical. is also observed between 10 and 60 OC. The shape of curves OD's vs. laser intensity is temperature In a second step, the study of both singlet and triplet dependent as shown in Figure 5. The extrapolation to zero energy transfer from indole to anthracene has allowed a laser intensity of curves OD/laser intensity vs. laser indetermination of the triplet extinction coefficient ( E T )of tensity (insert in Figure 5) leads to OD,, values. indole in cyclohexane solution. At 430 nm, ET = 47 (f5) From Bent and Hayon spectrum,16we deduced cR,6W/ X lo2M-' cm-'. For N-methylindole in benzene at 460 nm, 'R,530 = 0.36 therefore OD,- = ODgw- 0.360DR,530. ET = 5 X lo3 M-l cm-1.32 The bimolecular rate constant The shape of the optical absorption spectrum of the of triplet transfer reaction from indole to anthracene kT solvated electron in water is temperature dependent.39 = 9 (k1)X lo9 M-' s-l may be compared to the rate constant obtained by Wilkinson and Garner32in pulse ra(37) R.Bensasson and E. J. Land, Photochen. Photobiol. Reu., 3,163 diolysis experiment for quenching of the triplet state of (1978). N-methylindole by naphthalene, k, = 6.5 (fl.1)X logM-l (38) H. Stephen and T. Stephen in "Solubilities of Inorganic and s-l. The bimolecular rate constant of singlet transfer reOrganic Compounds", Vol. 1, Pergamon Press, Oxford, 1963, p 330. (39) F. Y. Jou and G . R. Freeman, J. Phys. Chem., 83, 2383 (1979). action from indole to anthracene found here, ks = 7 (&0.7)
676
The Journal of Physical Chemistry, Vol. 85, No. 6, 1981
Klein et al.
X 1O'O M-l s-l, is one order of magnitude higher than the cannot be made since the triplet quantum yield of aqueous indole has never been determined experimentally. diffusion-controlled rate in cyclohexane.40" This is the case for other singlet-singlet electronic energy transfers.40b Comparison between QF and QT a t three temperatures shows that, within experimental error, QF/QT = 1 f 0.2. The determination of CTleads to the first experimental This value may be compared to Hook and D r i ~ k a m e r ~ ~ determination of the triplet quantum yield of indole. In results. In a study of the influence of pressure upon the cyclohexane, QT = 0.43 (f0.05) at room temperature. The fluorescence and phosphorescence quantum yield (Qp)of decrease of the triplet yield with temperature is less than indole in poly(methy1 methacrylate), they found, at low 5% from 10 to 60 "C. This may be compared with the pressure and temperature, QF = QP = 0.50 (see Figure 3 fluorescence quantum yield in cyclohexane QF = 0.4919 in ref 45). Furthermore, studies of the temperature dewhich decreases by about 10% between 20 and 60 "C. The sum (QF QT) in cyclohexane rises to 0.92 f 0.1. pendence of the fluorescence quantum yield of aqueous solutions of indole led several authors to consider the raThe residual 0.08 could be accounted for by experimental errors. However, two pathways other than fluorescence diationless rate constant k,, as the sum of a temperaand intersystem crossing have already been postulated. ture-independent term km0 and a temperature dependent The first one was the dissociation of the N-H bond with term W ( T ) having a high activation Where kf is the radiative rate constant, it was found kf/k,,O = 1.06or a quantum yield of 0.05.20This N-H dissociation pathway 1.1: i.e., a value close to the QF/QTratio. From this effectively occurs at higher excitation energy as evidenced agreement and from the fact that the sum QF + QT apby the drop of fluorescence quantum yield41and the formation of H242afor indole in cyclohexane but not for N proaches unity at 1.5 "C, we conclude that the temperature-independent deactivation pathway is the intersystem methylindole. The second one was photoionization. This crossing to the triplet state as earlier postulated.Bp' We can way was postulated to occur in nonaqueous solvents on the basis of photoionization current measurement~l~ and forthus identify kn: to kist, the intersystem crossing rate constant. Now, as kf/Wilc = 1.0-1.1, QF cannot be higher mation of N2 from electron scavenging by N2Ol1J7but this evidence is ambiguous. The simultaneous decrease of QF, than 0.50-0.52 as it has been found.6.8 Solvated Electron Yield in Aqueous Solution. The QT, and T F in the presence of N 2 0 implies a dynamic value of solvated electron quantum yield a t 25 "C deterquenching of the fluorescent state. Recent data42bshow mined from transient absorption by Bent and Hayon,15 Qethat the formation of N2 results from a reaction between = 0.26, is higher than ours, Q; = 0.16. This difference may NzO and indole in its first excited state. In conclusion, be explained by a greater contribution to the photoionithe photophysics of indole in cyclohexane may be simply zation from the biphotonic way of their experimental explained in terms of excitation to the excited singlet state followed by intersystem crossing to the triplet state, conditions (3.6-11s pulse duration) than in ours (20-11s pulse duration at half-maximum height). The biphotonic phofluorescence, and a low N-H dissociation. This latter toionization from the first excited state has been recently mechanism increases by exciting a t X < 240 nm.42a evidenced by double-pulse laser experiments in the case Fluorescence Quantum Yield in Aqueous Solution. The of tryptophan, an indole spread in literature QF data has been extensively discussed As shown by Figure 5, photoionization of aqueous indole is very dee l s e ~ h e r e . ~ We * ~ Jwill ~ retain as a reference value QF = pendent on both laser intensity and temperature. A t 60 0.28 for aqueous indole a t 25 "C.19 The increase in the "C, where the fluorescent state lifetime is short, 1.3 11~:~ fluorescence quantum yield of aqueous indole as the temwith the duration of the laser pulse, the shape perature decreases also is w e l l - k n o ~ n . ~ ~ J ~ * ~ ~ ~compared ~ of the graph OD vs. laser intensity is indicative of a An earlier study8 of the temperature influence on monophotonic process with a saturation effect, increasing fluorescence quantum yields for indole in pure water, in a t high energy. At 1.5 "C, the lifetime of the fluorescent 5 M NaC104, and in cyclohexane, suggests that all fluorescence yields tend to a common value QF = 0.52 at state rises to about 6 ns (extrapolated value from ref 43), and the photoionization becomes both mono- and bipho0 "C (see Figure 1in ref 8). Kirby and Steiner6found the tonic with a saturation effect a t high intensity, At 25 "C, same value QF = 0.50 for the indole fluorescence quantum T~ = 4 f 0.5 ns11*43*44 a linear relation was observed for yield in water and in deuterated water by extrapolating electron absorbance vs. laser intensity. This linearity did to 0 "C. The present values at 1.5 "C (Table IV)approach not imply a pure monophotonic process but, probably, an the value QF = 0.50-0.52 which appears to be the maxiexact compensation of the biphotonic contribution by the mum one found in fluid solution. The increase of QFupon saturation phenomenon, a situation predicted by Lachish addition of alcohol6is confirmed with solutions containing et al.48 This compensation may depend, as shown here, 1 M 2-propanol; the effect increases a t higher temperaon the ratio of the intermediate state lifetime to the pulse tures. However, no QF increase was observed in presence duration. At 25 "C, this ratio is higher than unity in Bent of 0.2 M 2-propanol whatever the temperature. Fluoresand Hayon experiments (4/3.6) but much lower than unity cence quenching of indole by N 2 0 was not significant in in ours (4/20). aqueous solutions. The observation of a very fast buildup of the electron Triplet Quantum Yield in Aqueous Solution. Bent and transient ab~orption'~ could be then explained by a very Hayon's results16 are qualitatively confirmed here; the fast biphotonic photoionization rather than a photoionitemperature effect upon the transient triplet is opposite zation from a vibrationally excited state. The argued to the temperature effect upon the radical and electron. temperature effecP is not a convincing argument for the A direct quantitative comparison with literature data latter mechanism. Indeed, if the photoionization takes place from a prefluorescent vibrationally excited state and (40) J. G. Calvert and J. N. Pitta in "Photochemistry",Wiley, New York, 1966: (a) p 627; (b) p 338. not from the fluorescent state, the temperature should
+
(41) I. Tatischeff, R. Klein, T. Zemb, and M. Duquesne, Chem. Phys. Lett., 54, 394 (1978). (42) (a)J. Zechner, G. Kohler, N. Getoff, I. Tatischeff, and R. Klein, J . Photochem., 9, 307 (1978); (b) to be published. (43) M. S. Walker, T. W. Bednar, R. Lumry, and F. Humphries, Photochem. Photobiol., 14, 147 (1971). (44) G. Laustriat and D. Gerard in "Excited States of Biological Molecules",J. B. Birks, Ed., Wiley, New York, 1976, p 388.
(45) J. W. Hook and H. G. Drickamer, J. Chem. Phys., 69,811 (1978). (46) B.Finnstrom, F.Wibel, and L. Lindqvist, Chem. Phys. Lett., 71, 312 (1980). (47) L. I. Grossweiner, private communication. (48) U. Lachish, A. Shafferman, and G. Stein, J . Chem. Phys., 64,4205 (1976).
Photophysics of Indole
The Journal of Physical Chemistry, Vol. 85,No. 6, 198 7 677
affect only the fluorescence quantum yield and not the fluorescence lifetime. In fact, both the fluorescence quantum yield and the fluorescence lifetime equally vary with the t e m p e r a t ~ r e . ~ ~ ? ~ ~ Another argument for the solvated electron originating from a prefluorescent state could be the observation of a photoionization threshold at 275 nm18 in aqueous tryptophan. However, in the case of aqueous indole, Hopkins and Lumryl' obtained photoionization by exciting at lower energy than the threshold energy. Furthermore, a similar quenching effect of H+ on fluorescence and solvated electron yields12is observed with aqueous indole. These two latter observations, the temperature effect on fluorescence and the constancy of fluorescence quantum yield in the first absorption band,lg lead to the same conclusion: solvated electrons obtained in steadystate experiments by exciting aqueous indole in the first absorption band ( A > 240 nm) are formed from the fluorescent state. In water, the sum E = Q F + QT + Q , approaches unity at 1.5 "C (E = 0.93) but not a t either 25 "C (E = 0.67) or 60 " C (E = 0.66). The difference (1- C ) might correspond to a fourth deactivation pathway, such as an internal conversion from the fluorescent to the ground state. This pathway, if it occurs, is negligible at 1.5 "C and should remain constant from 25 to 60 "C which is rather unlikely. It may be suggested that only a part of the solvated electrons escapes a fast instantaneous reaction and can be detected by transient absorption in the 0.1-2-ps range. That the radical quantum yield is equal to the solvated electrons yield within experimental error may imply that the above fast reaction is the recombination of the initial ion pair. We can estimate the escape probability by pest = Qe-/l - (QF QT). We found paac= 0.36 a t 1.5 "C, 0.32 a t 25 "C, and 0.60 at 60 "C. With the above hypothesis, the deactivation of the first excited state of aqueous indole may be simply explained by the following mechanism: excitation with A > 240 nm So + hv SI* (1)
+
intersystem crossing photoionization
-- + - +
Si*
fluorescence
So
-
hv
S1* T1*
S1*
ea;
R+
(2) (3) (4)
N-H bond dissociation can be neglected in water.12s42i49 As aforesaid, kf and kisc are temperature independent. The temperature dependence of quantum yields in water originates from the temperature dependence of reaction 4.
Hydrogen quantum yields, obtained in electron-scavenging experiments, are lower than electron yield obtained by transient adsorption which is expected since Q F in 1 M 2-propanol is higher than Q F in 0.2 M 2-propanol. However, at 60 "C the difference between QH1 = 0.33 and 8,- = 0.52 cannot readily be attributed to this effect nor to experimental error; incomplete scavenging of H atom by alcohol is a more likely cause. The H adduct to the indole ring has been detected.2 Our value at 25 "C, QH2 = 0.13 is close to Feitelson's value Q H 2 = 0.11.l2 ~~
~~
(49) R.R.Kuntz, C. A. Ghiron, and W. A. Volkert, J. Photochem., 7, 363 (19'77).
The great number of intermediate radicals (e-, H., HOz., OH.) in the Fricke dosimeter used as the scavenging system by Steen et increases the probability of side reactions and certainly explains the very low electron quantum yield obtained at 220 nm, Qe- = 0.09, and only 10% of this yield in the first excitation band (A > 240 nm).
Conclusion The study of both singlet and triplet energy transfer from indole to anthracene has allowed a determination of the triplet extinction coefficient of indole in cyclohexane solution, + = 47 (A 5) X lo2M-l cm-l. This value was then used to determine the triplet quantum yield of indole in cyclohexane, QT = 0.43 f 0.05 close to the fluorescence quantum yield, QF = 0.49 f 0.05. In aqueous solution, QF and QT increase with decreasing temperature (60 t o 1.5 "C) when solvated electron and radical quantum yields decrease. At 1.5 "C in water, Q F = 0.46 f 0.05 and QT = 0.43 f 0.1. A first conclusion appears by comparing quantum yields in cyclohexane and water, Whatever hypothesis is made to explain the red shift of the emission in water,5w52the resulting excited state in water decays at 1.5 " C by the same pathways as in cyclohexane, Le., mainly by fluorescence and intersystem crossing to the triplet state. A difference appears in the third deactivation pathway, which is N-H bond dissociation in cyclohexane, but photoionization in water. This latter way increases with temperature, probably as a consequence of the water destructuration.e-8J2 The second conclusion concerns the hypothesis of the triplet state formation from nonrelaxed excited complex statess3of from excited Franck-Condon state" rather than via the relaxed fluorescent state. Such a mechanism does not occur in the case of indole; the parallel decrease of QF and QT by N20 quenching in cyclohexane or by temperature quenching in water clearly shows that triplets states are issued from fluorescent states. The present study also identifies the temperature-independent pathway for radiationless deactivation of aqueous indole as the intersystem crossing to the triplet state. The mechanism of photoionization involving either fluorescent state as deduced here and proposed earliersJ2J3 or a prefluorescent state15JB-lewith a photoionization thresholdla remains an open question. Experiments are in progress in order to decide this alternative. Acknowledgment. We thank Dr. Land for his help in performing the pulse radiolysis experiments and J. P. Ballini for the fluorescence lifetime measurements with synchrotron radiation at Lure, Orsay. We thank Dr. J. Belloni for giving us full facilities to use a Perkin-Elmer gas chromatograph at the Laboratoire de Physico-Chimie des rayonnements, Orsay. ~~~
~~
(50) M.S.Walker, T. W. Bednar, and R. Lumry, J . Chem. Phys., 45, 3455 (1966). (51) J. Eisinger and G. Navon, J . Chem. Phys., 50, 2069 (1969). (52) B.Skalski, D.M. Rayner, and A. G. Szabo, Chem. Phys. Lett., 70, 587 (1980). (53) N. Orbach and M. Ottolenghi in "The Exciplex", Gordon-Ware, Ed., Academic Press, New York, 1974, p 79. (54) N. Tsujino, H.Masuhara, and N. Mataga, Chem. Phys. Lett., 15, 357 (1972).
