J . Phys. Chem. 1993, 97, 3662-3667
3662
Mechanism of Fluorescence Quenching of Pyrene with Purines in Polar Media. Formation of the Pyrene Triplet State via Exciplex Formation Shuichi Hashimoto Chemistry Department, Gunma College of Technology, 580 Toriba-machi, Maebashi, Gunma 371, Japan Received: November 10, 1992; In Final Form: January 15, 1993
The kinetics and the mechanism of fluorescence quenching of pyrene by a number of purines, including their deprotonated anionic forms, were investigated in aqueous and acetonitrile solutions. In aqueous solution, the kinetics of the quenching was both dynamic and static, where there is appreciable formation of a ground-state complex between pyrene and purine. Only dynamic quenching was observed in acetonitrile. The quenching mechanism was shown to occur via exciplex formation due to a weak electron-transfer interaction from purines to pyrene. This leads to enhanced formation of the pyrene triplet state, as the pyrene triplet state was observed as the only transient product, and there was little formation of ionic species.
Introduction It has long been recognized’ that purines, via complex formation, can solubilize polycyclic aromatic hydrocarbons (PAH’s) in aqueous solution. Research in this area has been motivated by the fact that PAH’s are carcinogenic and that some purinesconstitute important componentsof DNA. NMRstudies2 have focused on the stability and structure of the complex between PAH’s and purines such as caffeine, while the crystal structure of the 1:1 molecular complex between 1,3,7,9-tetramethyluric acid (TEMU) and pyrene has been elucidated.3 Fluorescence studieslc showed that xanthine-based caffeine does not quench pyrene fluorescence, pyrene-caffeine complexes luminesce, and uric acid based TEMU quenches pyrene fluorescence in aqueous solution. The question arises with regard to the remarkable difference in fluorescence quenching ability between two molecules of similar structure; also, the quenching mechanism by TEMU has not been unraveled. Therefore, we have undertaken a study to clarify the mechanism of fluorescence quenching of PAH, namely, pyrene by purines in solution. Early researchersIb found that the acidity or basicity of solution significantly affects the quenching efficiencyof purines for excited PAH’s. Recently, Nosaka and Kira4 have reported that the protonated forms of caffeine and theophylline act as strong quenchers via energy transfer from the excited states of PAH’s. Hence, attention was also directed to the effect of protonated and deprotonated forms of purines on quenching efficiency. The present results show that dynamic quenching and static quenching are operative in the fluorescence quenching of pyrene by TEMU and other purines in aqueous solution. The anionic forms of theophylline (THEO-), 8-chlorotheophylline (ClTHEO-), theobromine (THEOB-) and 1,3,&trimethyIuric acid (TRMU-) are identified as quenchers for excited pyrene. The quenching ability of purines correlates with the oxidation potentials of purines, and a weak exciplex emission is detected in the pyrene/ TEMU and other systems, even in such polar media as water or acetonitrile. This suggests that the quenching is basically due to electron transfer. The product of the quenching reaction is identified as enhanced formation of the pyrene triplet state observed for quenching by TEMU, TRMU, and CI-THEO-. Consequently, the quenching mechanism is ascribed to the triplet formation from an emissive exciplex state.
Experimental Section Pyrene (P) (Eastman) was purified by silica gel column chromatography. Caffeine (1,3,7hmethylxanthine) (CAF), theophylline (1,3-dimethylxanthine) (THEO), 8-chlorotheoph0022-365419312097-3662$04.00/0
dH3
CH3
TEMU
CH3
C H3
THEO-
CH3
C 1-THEO-
THEOB-
TRMU
TRMU-
Figure 1. Structural formula of purines.
ylline (CI-THEO), and theobromine (3,7-dimethylxanthine) (THEOB) were purchased from Tokyo Kasei. 1,3,7,9-Tetramethyluric acid (TEMU) and 1,3,7-trimethyIuric acid (TRMU) were from Fluka and used as received. The structural formulas of purines are shown in Figure 1. The pH of the solutions was adjusted with appropriate buffers: 0.1 M acetate (pH 3.2), 0.1 M phosphate (pH 8.3), 0.1 M ammonium (pH 9.2), 0.1 M phosphate-NaOH (pH 113,and 0.1 M glycine-NaCI-NaOH (pH 12.9). Laser photolysis was carried out with a setup described previous1y.s Briefly, a N2 laser (Usho Optical Systems KN1.2M; excitation wavelength 337.1 nm; pulse width 5 ns; energy 5 mJ/pulse) was used for excitation. A 150-W Xe short arc lamp (Ushio UXL-l50DS), whose intensity was increased 1020 times with a pulse generator (Yamashita Denso YXP-15OW) when necessary, was used as a monitor light source. Beam intensities were monitored with a photomultiplier, PMT 0 1993 American Chemical Society
Fluorescence Quenching of Pyrene with Purines
The Journal of Physical Chemistry, Vol. 97,No. IS, 1993 3663
A D B
310.0
370,O
Figure 2. Changes in the absorption spectrum of M pyrene on addition of T R M U in aqueous solution (pH 3.2) at 298 K. [TRMU]: ( I ) 0 M; M;(3) 8.0 X IO-’ M; (4) 2.0 X lo4 M; (5) 3.4 X M;(6) 4.2 X lo4 M. (2) 4.8 X
(Hamamatsu R-928, six stages), through a monochromator (Acton SpactraPro-275,1200 grooves/mm). A signal from PMT was fed into a digital storage oscilloscope, DSO (Tektronix 2440, 300 MHz, 500 megasamples/s), through a back off circuit.6 All data were analyzed by using a personal computer (NEC PC9801DS) interfaced to DSO. The time constant of the detection system was less than 10 ns when a 504 terminator was used. The number of absorbed photons was calibrated by using a standard solution of anthracene in cyclohexane, with cPT(anthracene) = 0.71’ and &anthracene at 422.5 nm) = 64 700 M-I cm-i.8 Fluorescence decay was measured with a PMT (Hamamatsu 1P28, nine stages) and analyzed by a nonlinear least-squares method.9 A Shimazu UV-3 101PC double-monochromator spectrophotometer and a Hitachi F-3010 spectrofluorimeter interfaced to an NEC personal computer were used for stationary measurements. The spectral response of the fluorescence instrument was corrected with fluorescence standards.I0 Fluorescence quantum yields were determined in reference to a quinine bisulfate solution in 0.1 N H2S04(0.54”). Cyclic voltammetry was conducted with a Hokuto Denko HA301 potentiostat and a HB-104 function generator. Acetonitrile (Wako) purified by a literature proCedurel2was used as a solvent and 0.1 M tetrabutylammonium perchlorate (Nakarai) was used as a supporting electrolyte. The half-wave oxidation potential (E1pox)of purines was measured in reference to SCE at 298 K. E l / f x of TEMU was estimated to be +1.31 V in acetonitrile in which completely reversible one-electron oxidation-reduction was observed. The oxidation process of TRMU and Cl-THE0 was irreversible; E 1 / 2 mof TRMU and Cl-THE0 was evaluated to +1.26 and +1.82 V, respectively, by assuming the relation El/? = EFat-0.03 V. Oxidationof CAFand T H E 0 was not observed at potentials below +2.0 V. The E l p o xof Cl-THEO- was estimated to +1.26 in the presence of M triethylamine in acetonitrile, and estimations of E1 poxfor other purines were not possible. The temperature was controlled by circulating thermostated (f0.2 K) water. Doubly distilled water was used throughout the experiments. Sample solutions were deaerated by freeze-pump thaw cycles.
Results I. Fluorescence Quenching. With increasing amounts of purines, theground-state absorption spectrumof pyrene in aqueous solution shows a systematicchange, as shown typically for TRMU in Figure 2 (THEOB- is an exception). In most cases, a bathochromic shift and a reduction in theabsorbanceareobserved at high concentrations of purines insofar as absorption by purines themselves does not interfere with the absorption spectrum of pyrene. These data indicate the formation of ground-state complexes between pyrene and purines. The systems were excited at their isosbestic points for the fluorescence intensity measurements. The systems contained pyrene saturated in water (concentration less than 1 V M). Fluorescence intensities and fluorescence decays were measured as a function of purine concentration (10-4-10-3 M). pH’s were adjusted so as to render more than 99% of the free bases or deprotonated species prcsent in solution. Fluorescencequenching takes place with the purines TEMU, THEO-, Cl-THEO-, THEOB-, TRMU, and TRMU-. Under the present experimental conditions, the fluorescence spectral shape is not changed in the presence of purines. The time-dependent fluorescence intensities were single-exponential over more than 95%of the decay. SternVolmer plots of both steady-state fluorescence intensities, fo/f (monitored at 397 om), versus concentration of purines, [HI, and lifetimes, T O / ? (monitored at 400 nm), versus [HI are shown in Figure 3, for TEMU, THEO-, Cl-THEO-, and THEOB-. With the exception of THEOB-, the plots of fo/f do not tally with those of r o / r ,but deviate upward. These results indicate the presence of static quenching in addition to dynamic quenching,I3 and therefore, the quenching curves are analyzed by the following model: P+HePH
-+ + + -
P* P*
hv
P
H
PH*
--P
PH
H
Hashimoto
3664 The Journal of Physical Chemistry, Vol. 97, No. IS, 1993
0
1.0 [CI-THEO-]
0
1.o [THEO-1 (
3.0
t
3 .O
2.0
2.0 (lO-3M )
M1
Figure 3. I ( 0 ) and ~ ( 0Stern-Volmer ) plots for quenching of pyrene by purines at 298 K: (A) TEMU; (B) THEO-, (C) CI-THEO-, (D) THEOB-.
TABLE I: Quenching Parameters for Purines' (298 K) PK~
K,.,, K";
k,
TEMU
THEO- (PH 11.5)
CI-THEO- (PH 8.3)
10' 1.0 x 103 4.8 x 109
8.7 4.5 x 10' 6.0 X lo2 2.9 X lo9
5.3 3.9 x 102 5.4 x 102 2.5 X lo9
4.9
X
k
TRMU (PH 3.2)
10.0
TRMU- (PH 9.2) 6.1
7.2 X lo2 3.5 x 109
3.8 X 10' 7.1 X lo2 3.5 x 109
1.2 x 10' 5.3 x 102 2.6 x 109
9.2 x 105
2.2 x 106
208 ns 4.8 X IO6
TO
1/Tn
THEOB- (PH 12.9)
9.8 x 105
6.4 x 107
6.4
X
lo6
Errors 45%.
where P and H denote pyrene and purines, respectively. Photoexcited pyrene (P*) either emits or is quenched (with a quenching rate constant of k,) while its excited complex (PH*) is assumed to give no detectable emission a t the monitoring wavelength. The modell* leads to
where Ksv (=kqrO where TO is the lifetime in the absence of a quencher) is thestern-Volmer constant and Keqis the association constant. A least-squares fitting was performed for the experimental quenching curves (best-fitted curves are given in Figure 3), and quenching parameters obtained are listed in Table I. In acetonitrile and benzene, the absorption spectrum of pyrene is hardly affected by TEMU and only dynamic quenching is observed with a k, of (9.6 f 0.2) X lo9 M-I s-I in acetonitrile and (2.2 f 0.2) X 1Olo M-I s-I in benzene. 2. Exciplex Emission. A new broad emission band centered at X = 500 nm (20 000 cm-1) appeared at high concentrations of both pyreneand TEMU in aqueoussolution (this is not apparent in the above quenching studies owing to its low quantum efficiency). The time-dependent rise in the emission at A = 500 nm is rapid and within the laser pulse, and the apparent decay time is significantly short (10.4 ns). We ascribe this emission to the exciplex of pyrene and TEMU. The possibility of phosphorescence emission by pyrene or TEMU can be ruled out, because the phosphorescence peak of pyrene is located at A = 600 nm and the phosphorescence spectrum of TEMU (A = 40&500 nm) is structured and extremely weak. The excitation spectrum of this exciplex emission in aqueous solution shows a bathochromic shift and a broadening of the spectrum compared to that of pyrene monomer emission. This suggests that the exciplex emission originates from excitation of a ground-state complex. This exciplex emission is rather general for pyrene/purine systems,
Wovenumber (
)
Figure 4. Exciplex emission spectrum in various media excited at 343 nm (Absorbance 0.20, deaerated) at 298 K: (A) benzene (intensity X 1); (B) CH2C12 (intensity X 3.6); (C) CH'CN (intensity X 8.3); (D) water (intensity
X
8.3).
and similar emission was observed in aqueous solution for other systems including THEO- and Cl-THEO-. However, the emission is very weak for TRMU and TRMU-. The exciplex emission of pyrene and TEMU was also detected in solvents other than water. Figure 4 compares the exciplex emission spectra in various media. In these spectra, the contribution of pyrene monomer emission, which occurs simultaneously on excitation, was subtracted. In aqueous solution, the contribution of monomer emission is negligible at high concentrationsofTEMU (>5 X 10-3M) andaquantumyieldofexciplex emission can be measured: @(exciplex) = 0.018 a t 298 K. As seen in Figure 4, in less polar media, a shift of the peak position to higher energy occurs concomitantly with the occurrence of higher emission yields. The time-dependent rise and decay of
-
Fluorescence Quenching of Pyrene with Purines
SCHEME I P*
+
TEMU
k3
The JournaI of Physical Chemistry, Vol. 97, No. 15, 1993 3665 TABLE 111: * T ~for TEMU and CI-THEO- Quenching at Different Temperatures' TEMU CI-THE0 Hi0 T(K) HzO CHlCN benzene
(P--TEMU+)*
216 298
323 P + TEMU + hv'
P + TEMU
produd
TABLE II: Kinetic Parameters of the Exciplex in Acetonitrile and Benzene CHCN
+ k:
benzene
3.1 X IO" 3.2 X IO" k , + k,, + k- 3.3 X IO' 3.6 X IO' 1.1 X 10"' 2.7 X IO' k," 1.4 X IO' 1.4 X IO' e IO" 2.0 X IO" kqc 1.1 X IO"' 2.4 X IO'
ki
kJ
Errors f 10%. k, = k j (kc =
CH ICN
benzene
+ k h + k7).
+ k h + k 7 ) / ( k 4 + ks + k h + k 7 ) .
0.42 0.42 0.43
0.14 0.08
0.40
0.1 1 0.08
0.04
Errors *O.O I .
(3P + TEMU)
kl
0.16
k7
The quantum yield of triplet formation was determined by using an extinction coefficient of 30 400 M-1cm-1 in cyclohexane solution. The reliability of the present quantum yield estimation was checked with literature values19 where the same method is employed: the pyrene triplet yield in cyclohexane in the absence and the presence of 1.75 X 10-2 M DABCO (1,Cdiazabicyclo[ 2.2.2loctane) has been reportedZoto be0.30and 0.96, respectively, while our estimation gave 0.38 f 0.01 and 0.79 f 0.02, respectively. Reaction Scheme I1 for the deactivation of excited
@Tm(k
+0.71 >+0.71
CI-THE0 TEMU
+O.S5
+0.02
TRMU CI-THEO-
-0.03 -0.03
AG, = E l p ( p u r i n e ) - Eljzred(pyrene)- Eo.o(pyrene) where El,pd(pyrene) = -2.06 eV and Eo.o(pyrene) = 3.33 eV. in aqueous solution and TRMU in methanol. The pyrene triplet is the only species observed in TRMU/methanol and C1THEO-/water systems with CPT- of 0.09 f 0.01 and 0.08 f 0.01 (298 K), respectively: these two systems also give the enhanced triplet formation with k,lSc of 5.9 f lo8 and 2.0 X lo8 M-I SKI, respectively. Finally, the intersystem crossing rate constant (k7 in Scheme I) of the pyrene-TEMU exciplex is evaluated from the values of and the overall decay constant of the exciplex, ks k6 k7, and is shown also in Table 11. The value of k7 obtained in acetonitrile is the same as that in benzene. This suggests that there is no effect of the medium polarity on k7 in the present pyrene-TEMU exciplex system.
+ +
Discussion Difference in Quenching of Purines. In CAF- and THEOsolubilized aqueous systems, the fluorescence spectrum is reminiscent of that of pyrene, with a slight broadening in thevibronic structure of the spectrum. N o broad emission a t wavelengths longer than ca. 450 nm was observed. The fluorescence decay is single-exponential with a lifetime of 250 ns in an aqueous CAF solution and 175 ns in an aqueous THEO solution; these lifetimes are more or less similar to that of pyrene in water. Neverthelesss, significant quenching by other purines including deprotonated anions of purines is observed. The quenching kinetics is shown to be static as well as dynamic in nature. Exciplex emission is also observed for quenching by some purines, Le., TEMU, Cl-THEO-, and THEO- in aqueous and highly polar media. Several pertinent points can be deduced for the purine quenching. ( 1 ) When the four neutral purines CAF, THEO, TEMU, and TRMU are compared, the extent of ground-state complex formation with pyrene in water is not greatly different (see Table IV for CAF and THEO), yet thequenching behavior is markedly different: both TEMU and TRMU act as strong quenchers while neither CAF nor THEO quenches. Thus, xanthine-based CAF and THEO are strikingly different in behavior from uric acid based TEMU and TRMU. This can be explained from the difference in the oxidation potential of these purines. The free enthalpy change, AGr, of electron transfer (ET) from purines to excited pyrene was estimated and listed in Table V. According to this table, if the quenching is caused by an E T mechanism, then the value of AGf for TEMU and TRMU is close to zero while the reaction is expected to be appreciably endothermic for CAF and THEO. (2) Anionic purines, THEO-, CI-THEO-, THEOB-, and TRMU-, are identified as efficient quenchers for excited pyrene regardless of whether they are based on xanthine or uric acid. Association constants with pyrene in the ground state in water
Hashimoto are smaller than those of their neutral counterpart due probably to the effect of their negative charge. These anions are expected to be solvated more easily than the neutral form in water, and this contributes to the reduction in the association constants. Therefore, the contribution of dynamic quenching is more important for these anionic quenchers. Actually, dynamic quenching is observed uniquely for quenching by THEOB-. The origin of activated quenching by anions of purines is ascribed to the stronger electron-donating ability of the anions. The AGr of ET for Cl-THEO- estimated from its oxidation potential is shown in Table V: AGr is lower for Cl-THEO- than that of Cl-THEO. Although the measurement of the oxidation potential is not possible for THEO-, THEOB-, and TRMU-, the ET mechanism of the fluorescence quenching may also be applicable to these anionic quenchers. (3) The quenching rate constants (k,’s) of purines are close to, but less than, that of a diffusion-controlled reaction, i.e., 7.4 X IO9 M-l s-I in water at 298 K. No great difference is seen for the value of k, among the purines studied. The value of k, for the present purine quenching is that expected from electrontransfer theory,2’ considering the values of AGf estimated for purines (see Table V): these values are close to zero. Unfortunately, an estimation of AGf was possible only for some of the purines and purine anions studied, and a systematic discussion of the relation between k, and AGr cannot be made. Triplet Enhancement via the Exciplex. Previously, Watkins22 studied the quenching of PAH’s with inorganic anions such as I-, SCN-, B r , and NO3- in acetonitrile. H e observed the formation of the triplet state of PAH’s with a quantum yield of nearly unity. His explanation is based on the assumption that triplet formation takes place from an electron-transfer state to a lower lying triplet state and depends on the energetic proximity of the electron-transfer state and the triplet state. Masuhara et alaz3observed the triplet formation on quenching of PAH’s with metal ions, Ag+ and T1+,in aqueous and N,N-dimethylformamide solutions. In these systems, the differences in the electronic structure of the transient complexes from those in Watkin’s studies were emphasized. Thus, the conclusion is that the quenching is due to a nonfluorescent complex leading to rapid intersystem crossing. Delouis et aLi9reported that triplet enhancement of pyrene to a quantum yield of unity takes place on quenching of pyrene fluorescence with DABCO in cyclohexane. In this system, Okada et al.24 observed a charge-transfer state that is a superposition of pyrene anions and DABCO cations: this is the precursor of the triplet pyrene. In the present pyrene/TEMU system, emissive exciplex formation takes place by a weak ET mechanism, where TEMU serves as a donor and pyrene acts as an acceptor, and the triplet state of pyrene is produced from the exciplex state. In the work of Watkins and Masuhara et al., no precursor of the triplet was detected directly and an electron-transfer state or a nonfluorescent complex was assumed. The exciplex state as a precursor of triplet formation was unambiguously observed in the pyrene/TEMU system. In the pyrene/DABCOsystem, where thecharge-transfer interaction is much stronger, exclusive formation of the triplet state takes place only in nonpolar media, in which the ions are expected to be destabilized; pyrene anions are produced in acetonitrile. On the other hand, the feature characteristic of the present system is that exciplex formation takes place in highly polar media, resulting in the triplet enhancement without producing separate ions. It should be noted that the free enthalpy change AGf of singlet E T is close to zero for pyrene/TEMU. Kikuchi et al.25 have suggested that fluorescence quenching is not due to a full ET, but predominantly due to partial ET, i.e., exciplex formation when AGrof ET isclose to zero (approximately -0.28 to +0.20 V). The present pyrene/TEMU system fulfills this criterion. This explains the observation of exciplex emission in polar media. Charge-transfer interaction26within the exciplex
Fluorescence Quenching of Pyrene with Purines can be responsible for the enhancement of intersystem crossing, since other factors which affect spin-orbit coupling such as a heavy atom effect or a paramagnetic effect are absent in the pyrene/TEMU. Production of ions from the exciplex seems unfavorable on considering the AGr of ET. Temperature effects on the yields of triplet formation were not observed in acetonitrile, but were observed in water. This can be due to a reduction in the association constant, Keq,at higher temperatures. Therefore, we assume that any temperature effect on the triplet yield is small. This suggests that the activation energy for the formation of the triplet from the exciplex state is low. Triplet enhancement of pyrene occurred also for C1THEO-/water and TRMU/methanol. In thesesystems, thevalue for AGr of E T is estimated to be close to zero, and a mechanism of triplet formation similar to that of TEMU quenching is expected. Although no product of the quenching reaction was detected, we presume that triplet formation is a rather general mechanism for fluorescence quenching of pyrene with purines, because of their similar quenching behavior. Failure to observe triplet formation in systems other than TEMU, TRMU, and Cl-THEO- can be due to a small value of kqlSccompared to k,. Effect of Ground-StateComplexationon the Triplet Yield. The present result indicates that ground-state complexation seems unfavorable for the production of high yields of the pyrene triplet: the yield is higher in acetonitrile and benzene where the formation of the ground-state complex is smaller than in aqueous solution (here, more than 98% of the pyrene is expected to form a ground-state complex with TEMU at concentrations >5 X 10-3 M). The excitation spectrum of exciplex emission in water suggests that the exciplex state consists of an excited groundstate complex while the exciplex in the solvents represents an encounter complex. This leads to a presumption that the excited ground-state complex is held in a plane-parallel geometry2’ in the ground state. This geometrical restriction may enhance the rate of internal conversion, leading to a reduced triplet yield in water.
Summary The recombination of geminate radical pairs is known28to be responsible for triplet formation in fluorescence quenching due toelectron transfer (ET) in highly polar solvents. The production of triplet states from intersystem crossing of exciplex states has been observed in nonpolar media,28 but not in polar solvents. Very recently, Kikuchi et alazshave proposed that E T fluorescence quenching is due to exciplex formation even in polar media when the free enthalpy change AGfof ET is close to zero. They actually observed exciplex emission in a few ET fluorescence quenching systems in acetonitrile. Kikuchi et also observed the formation of the triplet state as well as the production of separate ions in acetonitrile in the fluorescence quenching of 9,lOdicyanoanthracene (DCA) with anisole (AS). Although they interpreted this result in terms of spin-orbit coupling between the singlet exciplex and the locally excited triplet state, they failed
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3667 to find direct evidence of the exciplex state in the DCA/AS system. The present results show the formation of the exciplex by the observation of its emission followed by exclusive intersystem crossing to produce the pyrene triplet state. This is the case for pyrene fluorescence quenching with tetramethyluric acid and some purines in highly polar solvents, i.e., water and acetonitrile. This observation is classified as a unique situation in ET quenching in highly polar media where the exciplex is still formed and decays without producing separate ions but instead undergoes the intersystem crossing.
References and Notes ( I ) (a) Weil-Malherbe,H.Biochem.J.1946,40,351. (b) Weil-Malherbe, H. Eiochem. J . 1946,40,363. (c) Boyland, E.; Green, B. Er. J. Cancer 1962. 16, 347. (d) Boyland, E.; Green, B. Er. J . Cancer 1962, 16, 507. (2) (a) Donesi, A.; Paolillo. L.; Temussi, P. A. J . Phys. Chem. 1976,80, 279. (b) Stamm, H.; Stafe, J. 2.Naturforsch. 1981,366, 1618. Jaeckel, H.; Stamm, H. 2.Nafurforsch. 1986, 416. 1416. (3) Dmiani, A.; Santis, P. DE.; Giglio, E.; Liguori, A. M.; Puliti, R.; Ripamonti, A. Acta Crystallogr. 1965, 19, 340. (4) Nosaka, Y.; Kira, A. Photochem. Photobiol. 1991, 54, 15. (5) Hashimoto, S.; Hoshino, E.; Kato, N.; Ota, M.; Kakegawa, H. Macromolecules 1992, 25, 1686. (6) Janata, E. Reo. Sci. Instrum. 1986, 57, 273. (7) Amand, B.; Bensasson, R. Chem. Phys. Left. 1975, 34, 44. (8) Bensasson, R.; Land, E. J. Trans. Faraday SOC.1971.67, 1904. (9) Demas, J. N. Excifed Stare Lifetime Measurements; Academic Press: New York, 1983; p 77. (10) Lippert, E.; Naegel, W.; Seibold-Blankenstein, 1.; Staiger, U.;Voss, W. 2.Z. Anal. Chem. 1959, 170, I . ( I 1) Melhuish, W. H. J . Phys. Chem. 1961, 65, 229. (12) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals; Pergamon Press: Oxford, 1988. (13) Demas, J. N. Excited State Lifetime Measuremenfs; Academic Press: New York, 1983; p 53. (14) Birks, J. B. PhotophysicsofAromaficMolecules; Wiley: New York, 1970; Chapter 7, p 403. ( 1 5 ) Heinzelmann, W.; Labhart, H. Chem. Phys. Lett. 1969, 4, 20. (16) Shida, T. Electronic Absorpfion Spectra of Radical Ions; Elsevier: Amsterdam, 1988. (17) Michael, B. D.; Hart,E. J.;Schmidt. K. H.J.Phys. Chem. 1971,75, 2798. (18) (a) Kasama, K.; Takematsu, A.; Arai, S. J. Phys. Chem. 1982.86, 2420. (b) Quinones, E.; Arce, R. J . Am. Chem. Soc. 1989, 109, 115. (19) Delouis, J . F.; Delaire, J. A.; Ivanoff, N. Chem. Phys. Lett. 1979,61, 343. (20) W1in aqueous solution cannot be obtained directly because of the
low solubility of pyrene; instead, a caffeine-solubilized aqueous solution of pyrene was used for the estimation of W. (21) (a) Rehm. D.; Weller, A. Isr. J . Chem. 1970, 8, 259. (b) Marcus, R. A. Annu. Reo. Phys. Chem. 1964, I S , 155. (22) Watkins, A. R. J . Phys. Chem. 1974, 78, 1885. (23) Masuhara, H.; Shioyama. H.; Saito, T.; Hamada, K.; Yasoshima, S.; Mataga, N. J. Phys. Chem. 1984,88, 5868. (24) Okada, T.; Karaki, 1.; Matsuzawa, E.; Mataga, N.; Sakata, Y.; Misumi, S. J. Phys. Chem. 1981.85, 3957. ( 2 5 ) Kikuchi, K.; Niwa, T.; Takahashi, Y.; Ikeda, H.; Miyashi, T.; Hoshi, M. Chem. Phys. Lett. 1990, 173, 421. (26) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of rhe Triplet Sfafe; Prentice Hall: New York, 1969; p 284. (27) Nosaka, Y.; Kira, A,; Imamura, M. J . Phys. Chem. 1981.85, 1353. (28) Ottolenghi, M. Ace. Chem. Res. 1973, 6, 153. (29) Kikuchi, K.; Hoshi, M.; Niwa, T.; Takahashi, Y.; Miyashi, T. J . Phys. Chem. 1991, 95, 38.
