The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
Electron-Transfer Fluorescence Quenching
(19) H. Greenspan and E. Fischer, J . Phys. Chem., 69, 2466 (1965). (20) G. Fischer and E. Fischer, Mol. fhofochem., 8, 279 (1977). (21) J. Saltiel, J. T. D'Agostino, W.G. Herkstroeter, G. Saint-Ruf, and N. P. Buu-Hoi, J . Am. Chem. Soc., 95, 2543 (1973).
2659
(22)0.G. Malan, H. Gusten, and D. Schulte-Frohlinde, J. Phys. Chem., 72, 1457 (1968). (23) D. J. S.Birch and J. 6. Birks, Chem. Phys. Lett., 38, 432 (1976). (24)J. L. Charlton and J. Saltiel, J . Phys. Chem., 81, 1940 (1977).
Electron-Transfer Fluorescence Quenching and Exciplexes of Cyano-Substituted Anthracenes Jens Eriksen and Christopher S. Foote" Department of Chemistry, University of California, Los Angeles, California 90024 (Received July 13, 1978) Publication costs assisted by the Petroleum Research Fund and the National Science Foundation
The fluorescence of 9,lO-dicyanoanthracene(DCA) and 9-cyanoanthracene (CNA) is quenched by a wide variety of electron-rich substrates. The quenching rate constants closely resemble values calculated for an electron-transfer process. Exciplex emission has been observed for the DCA/ 1,3-cyclohexadiene and DCA/ 1,2dimethoxybenzene system. Support for the electron-transfer mechanism is found in the solvent dependence of the exciplex emission.
Introduction Electron-deficient sensitizers, such as 9,lO-dicyanoanthracene (DCA), sensitize the photooxygenation of certain substrates in oxygen-saturated polar solvents.' The reactions do not involve singlet oxygen, and we have proposed the following mechanism involving a donor (D) radical cation and sensitizer radical anion which subsequently reduces oxygen to superoxide. The ultimate products are similar to those of electron-rich substrates with singlet 0xygen.l DCA -!% lDCA*
-D
[DCA-e + D+.] DCA 0 2 - s
+
+ 302
D+.
-+
DO2
Ertciplex emission has been observed in several laboratories in the fluorescence quenching of cyano-substituted hydrocarbons by olefins and dienes. Thus, Taylor2 observed exciplex emission from a-cyanonaphthalene quenched by olefins and dienes and proposed a chargetransfer quenching mechanism based on solvent dependent exciplex emissions. Similar results were reported by Ware and c o - ~ o r k e r s .Ware, ~ Holmes, and Arnold4 observed electron-transfer fluorescence quenching of DCA by substituted 1,l-diphenylethylenes and exciplex emission was detected in the DCA/4,4'-dimethyl-l,l-diphenylethylene system in benzene using time-correlating single photon counting. Caldwell and co-workers5 reported exciplex emission from 9-cyanophenanthrenelp-methylstyrene systems and Yang et a1.6 observed exciplex emission in the quenching of DCA and 9-cyanoanthracene (CNA) fluorescence by conjugated dienes. In connection with our photooxidation studies,l we wished to study fluorescence quenching by a variety of potentially reactive donors. We report that the fluorescence of DCA and CNA is quenched by a wide variety of substrates in acetonitrile a t room temperature. The quenching rate constants are close to those calculated for an electron-transfer quenching process and emission from highly polar exciplexes has been observed in a few cases in nonpolar but not in the highly polar solvents in which photooxidation occurs. 0022-3654/78/2082-2659$0 1 .OO/O
Results and Discussion Fluorescence Quenching. The fluorescence of DCA and CNA in nitrogen-saturated acetonitrile was quenched by a wide variety of electron-rich substrates (Table I). In each case, the quenching followed the well-known Stern-Volmer e q ~ a t i o n : ~ (1) I o / I Q = 1 + kq~s[QI where Io and IQ are the relative fluorescence intensities in the absence and presence of quencher ( Q ) ,respectively, k , is the bimolecular quenching rate constant, and 7s is the singlet lifetime of the fluorescer. Plots of Io/IQvs. [Q] gave straight lines with intercepts of 1.0 and correlation coefficients typically >0.999. From the slopes ( h q ~ Sof) these plots and T~ in nitrogen-saturated acetonitrile for DCA (15.3 f 0.1 ns8) and CNA (17.2 f 0.1 ns8), the h values for a large number of quenchers were obtaine2 (Table I). The free energy change (AG) involved in an electrontransfer process is given by9 AG = 23.06(E(D/D+) - E(A-/A) - e,2/at -AE0,,) (2) where E(D/D+) is the oxidation potential of the donor (D), E(A-/A) is the reduction potential of the acceptor (A, DCA, or CNA), eO2/aeis the energy (-0.06 eV in acetonitrileg) gained by bringing the two radical ions to the encounter distance ( a ) in a solvent of dielectric constant t, and AEo,O is the electronic excitation energy of the fluorescer. For DCA, E(A-/A) = -0.98 VIo and aEo,o= 2.89 eV," while for CNA, E(A-/A) = -1.58 VIo and AEo,o= 3.04 eV." From the oxidation potential of each quencher and eq 2, AG could be estimated for an electron transfer from quencher to fluorescer (Table I). A plot of h, vs. AG gave a very good correlation with a theoretical line calculated by Rehm and Wellerg for an electron-transfer process (Figure 1). This correlation strongly supports an electron-transfer mechanism for the quenching of the fluorescence of DCA and CNA by the substrates listed in Table I. Exciplex Emission. Solutions of DCA 1 M in the quenchers 1,3-cyclohexadiene (CHD, k , = 1.8 X 1O'O M-l s-l) or 172-dimethoxybenzene(DMB, hq = 1.7 X 1O1O M-l 0 1978 American
Chemical Society
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The Journal of Physlcal Chemlstry, Vol. 82, No. 25, 1978
J. Erlksen and C. S. Foote
TABLE I : Fluorescence Quenching Rates (kq),Half-Peak Oxidation Potentials ( E ,/2(ox)),and Calculated Energies (AG) for Electron Transfera
k fluorescerb DCA DCA DCA DCA DCA DCA DCA CNA DCA CNA CNA DCA CNA DCA DCA CNA CNA CNA CNA CNA CNA CNA
quencher 1,2,4-trimethoxybenzene 2-methoxy-1,l-diphenylethylene tetraphenylethylene diphenyl sulfide 1,8-dimethoxybenzene trans-stil bene 1,l-diphenylethylene 1,2,4-trimethoxy benzene anisole tetraphenyle thylene 1,4-dimethoxy benzene p-xylene 1,2-dimethoxybenzene mesitylene biphenyl 1,3-dimethoxybenzene trans-stilbene 1,l-diphenylethylene durene naphthalene flu or ene anisole
(lolo
114-1
s-1)
1.83 1.63f 1.78 1.67 1.72 1.88 0.849 1.37 1.22
0.971 1.38 0.0485 1.10 0.0517 0.306 0.237 0.360 0.105 0.0175 0.00721 0.0163 0.00036
E ,/ * (OXY (V vs. SCE) 1.12e 1.19f 1.33R 1.45f 1.45e 1,51h 1.52h 1.12: 1.76' 1.33R 1.34e 1.861 1.45e 1.90h 1.91h 1.49' 1.51h 1.52h 1.62h 1.643 1.65h 1.76i
A G , ~
kcal/mol - 19.6
- 18.0 - 14.8 - 12.0
- 12.0
- 10.6
- 10.4 - 9.2 - 4.8 - 4.4 - 4.2 - 2.5
- 1.6 - 1.6 - 1.4
- 0.7
-0.2 0.0 t2.3 t 2.8 t3.0 t5.5
a In acetonitrile at room temperature. 9,10-Dicyanoanthracene (DCA); 9-cyanoanthracene (CNA). In acetonitrile except as noted. Calculated from eq 2, see text. e A. Zweig, W. G. Hodson, and W. H. Jura, J. Am. Chem. SOC., 86, 4124 (1964). Measured by D. S. Steicheyi,T. L. Parker, and S. Mazur, in this laboratory. J. D. Stuart and W. E. Ohnesorge, J. A m . Chem: SOC., 93, 4531, ( 3 :+'/l). In HOAc-0.5 M NaOAc, L. Eberson and K. Nyberg, J. Am. Chem. SOC.,88, 1686 (1966). Reference 9. vi!, C. Neikam, G. R. Dimeler, and M. M. Desmond, J. Electrochem. SOC., 111, 1190 (1964). J
r
WAVELENGTH, nm
Figure 2. Exciplex emission of the DCAICHD system: (A) DCA fluorescence in methylcyclohexane. Exciplex emission in methylcyclohexane (B), diethyl ether (C), and 1,2-dichIoroethane (D). The relative sensitivity setting of the spectrofluorimeter is shown. The concentration of DCA was 2 X M, and that of CHD was 1 M.
AG, kcal/mole Figure 1. Plot of k, vs. AG,see Table I and text. The solid line was calculated by Rehm and Wellerg for an electron-transfer process.
s-l, Table I) exhibited weak, structureless exciplex emission in some solvents (Figures 2 and 3). The absorption spectrum of DCA was unchanged upon addition of 1 M quencher and the exciplex excitation spectra were identical with the DCA fluorescence excitation spectrum. The DCA/ CHD exciplex emission in methylcyclohexane was very similar to that observed by Yang and co-workers.6 For most of the quenchers listed in Table I, it was not possible to dissolve enough substrate to observe possible exciplex
WAVELENGTH, nm
Figure 3. Exciplex emlssion of the DCAIDMB system: (A) DCA fluorescence in methylcyclohexane. Exciplex emission in methylcyclohexane (B), diethyl ether (C), and 1,2-dichIoroethane (D). The relative sensitivity setting of the spectrofluorimeter Is shown. The concentration of DCA was 2 X M, and that of DMB was 1 M.
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978 2661
Electron-Transfer Fluorescence Quenching
TABLE 11: E x c i p l e x Emission D a t a a exciplex vmS,
l o 3 cm-’
1,2-di1,3-cyclo- m e t h o x y solvent (f- ~ / ~ f ’ ) hexadiene ~ benzene methylcyclohexane (0.106) d i e t h y l ether (0.256) 1,2-dichloroethane (0.324) v,( extrapolated)c 2/.~*/hcu~,~ c m - ’ p(a = 5 D
18.7 17.5
17.5
16.3
-15.9 (18.3) 7.2
(19.9)
lo3
10.5 11
16.5
9
a2 X M DCA a n d 1M quencher a t room temperaFrom e q 3 a n d F i g u r e 4. ture. E q u a t i o n 3, r e f 12. Exciplex dipole moment.
c
f-hf‘ Figure 4. Plot of vmX vs. f - ’/,f’(see ( 0 )and DCAIDMB (X).
eq 3 and Table 11) for DCAXHD
emission on the tail of residual DCA fluorescence. The solvent dependence of the position of the exciplex emission is given by2J2J3
vo - 2p2/hca3 (f - Yd? (3)
where urn, is the wave number of the emission maximum, uo is the hypothetical gas-phase emission frequency, w is the exciplex dipole moment, h is Planck’s constant, c is the velocity of light, a is the charge separation in the exciplex, 6 is the solvent dielectric constant, and n is the solvent refractive index. Table I1 lists the solvent dependence of the DCA/CHD and DCA/DMB exciplex emissions. Plots of ,,v vs. f - I/2f’ (eq 3) gave reasonably straight lines (Figure 4),from which the dipole moment of the exciplexes could be estimated, assuming a charge separation of 5 A (Table 11): DCA/CHD, 11 D; DCA/ DMB, 9 D. These data support the electron-transfer mechanism and are in good agreement with dipole moments reported for related exciplexes,2 e.g., l-cyanonaphthalene/2,5-dimethyl-2,4-hexadiene (9.2 D). The intensity of the DCA/CHD and DCA/DMB exciplex emissions decreased with increasing solvent polarity (Figures 2 and 3), presumably because of rapid dissociation of the exciplex into solvent-separated radical ions in polar solvents. The occurrence of DCA sensitized photooxygenation1J4 only in polar solvents, where no exciplex emission was observed suggests that fully solvated radical ions are intermediate in the reaction. The results of the DCA photosensitized oxygenation of phenyl-substituted ethylenes are described elsewhere.lJ4
Experimental Section Materials. DCA (Eastman) and CNA (Pfaltz & Bauer)
were recrystallized from toluene. CHD (Aldrich, bp 80 “C) and DMB (Eastman, bp 105-6 OC/16 mmHg) were distilled immediately prior to use. Acetonitrile (MCB) of spectroquality was used. Instrumentation a n d Procedure. Fluorescence quenching experiments were carried out a t room temperature using a spectrofluorimeter constructed from a 200-W mercury-xenon lamp (Schoeffel Instrument Corp.), Jarrell-Ash excitation and emission monochromators, and a Heath Model EU-701-93 photomultiplier. The relative fluorescence intensities a t an appropriate wavelength were measured using a digital voltmeter (Tektronix TM 503) connected to the photomultiplier. Solutions of the M) and quencher in 1-cm cuvettes were fluorescer (2 X degassed by bubbling nitrogen through the solutions for 3 min. The quenching results were analyzed according to eq 1 using least-squares analysis. Exciplex emission and excitation spectra were recorded at room temperature on a Spex Fluorolog instrument equipped with a photoncounting detector. Reduction Potentials. Half-peak reduction potentials of DCA (-0.98 V) and CNA (-1.58 V) were obtained by cyclic voltammetry a t a platinum foil electrode vs. a standard calomel electrode (SCE) in acetonitrile. The supporting electrolyte was tetrabutylammonium hexafluorophosphate (0.2 M); scan speed, 30 V/min; full scale deflection, -2.0 V. Reversible reductions were observed for both DCA and CNA. The measured reduction potentials for DCA and CNA were somewhat different from those reported by Chandross and Ferguson15 (-0.82 and -17 (!) V, respectively) for reasons which are not clear. The instrument was calibrated using ferrocene (Eli,(ox) = +0.379 V vs. SCE).16 Singlet Excited State Lifetimes in acetonitrile of DCA (15.3 f 0.1 ns) and CNA (17.2 f 0.1 ns) were determined by single photon counting17 on an Ortec Model 9200 nanosecond fluorescence spectrometer. Corning glass filters were used to isolate the excitation (filters 7-37 or 7-60) and emission bands (filters 3-71 or 3-72). Because of the relatively long lifetimes compared with the halfwidth of the lamp (2.3 ns) it was unnecessary to use convolution18in the data treatment. Solutions of 2 x M DCA or CNA in acetonitrile were deoxygenated by bubbling nitrogen through the solutions for 10 min. The lifetime of DCA agreed with a reported value of 15.2 ns4 in acetonitrile. Acknowledgment. This work was supported by National Science Foundation Grant No. CHE73-08502 A03 and A04 and the Petroleum Research Fund, administered by the American Chemical Society. The Spex Fluorolog was purchased from funds provided by National Science Foundation Grant No. MPS75-06135. The cyclic voltammetry experiments were carried out on an instrument in Professor M. F. Hawthorne’s group and for single photon counting an instrument in Professor V. N. Schumaker’s group was used; thanks are given to these groups for assistance.
References and Notes (1) J. Eriksen, T. L. Parker, and C. S . Foote, J . Am. Chem. SOC.,99, 6455 (1977). (2) G. N. Taylor, Chem. Phys. Lett., 10,355 (1971). 96, (3) W. R. Ware, D. Watt, and J. D. Holmes, J . Am. Chem. SOC., 7853 (1974). (4) W. R. Ware, J. D.Holmes, and D. R. Arnold, J . Am. Chem. SOC., 96,7861 (1974). (5) R. A. Caldwell and L. Smith, J. Am. Chem. SOC., 96,2995 (1974); R. A. Caldwell, N. I. Ghali, C.-K. Chien, D. DeMarco, and L. Smith, ibid., 100, 2857 (1978).
(6) N. C. Yang, private communication; we thank Professor Yang for sending us exciplex emission spectra; see also N. C. Yang, D. M.
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The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
Shold, and J. K. McVey, J . Am. Chem. SOC.,97, 5004 (1975). (7) N. J. Turro, “Molecular Photochemistry”, W. A. Benjamin, New York, N.Y., 1967, p 94. (8) Measured by single photon counting, see Experimental Section. (9) D. Rehm and A. Weller, Isr. J . Chem., 8, 259 (1970). The authors used a value of e t / a t = 0.06 eV as an average value in acetonitrile for a large variety of quenchers. (10) Haif-peak potentials vs. SCE in acetonitrile, see Experimental Section. (1 1) Determined from the absorption and fluorescence spectra in acetonitrile. (12) H. Beens, H. Knibbe, and A. Weller, J. Chem. Phys., 47, 1183 (1967).
R. D. Small and J. C. Scalano (13) (14) (15) (16) (17)
F. D. Lewis and T.4. Ho, J . Am. Chem. SOC.,99, 7991 (1977). J. Eriksen and C. S. Foote, to be published. E. A. Chandross and J. Ferguson, J . Chem. Phys., 47, 2557 (1967).
According to manufacturer’s specifications. W. R. Ware in “Creation and Detection of the Excited State”, A. A. Lamoia, Ed., Vol. 1, Part A, Marcel Dekker, New York, N.Y., 1971, Chapter 5. (18) H. E. Zimmerman, D. P. Werthemann, and K. S. Kamm, J. Am. Chem. Soc., 96, 439 (1974); L. J. Cline Love and L. A. Shaver, Anal. Chem., 48, 365A (1976); J. Eriksen, Ph.0. Thesis, New York University, New York, N.Y., 1976.
Electron Transfer Reactions of the Biradicals Produced in the Norrish Type I1 Process R. D. Small, Jr., and J. C. Scaiano” Radiation Laboratory,‘ University of Nofre Dame, Notre Dame, Indiana 46556 (Received June 29, 1978) Publication costs assisted by the Department of Energy
The biradicals generated in the Norrish type I1 reaction are efficient electron donors. They are shown to react with a series of bipyridilium cations, pyridine derivatives, and aromatic nitro compounds. The rates of reaction follow the same trends as those observed for monoradicals. For example, the biradical from y-methylvalerophenone reacts with benzyl viologen with k 3 = 3.9 X lo9 M-’ and with 2,5-dinitrobenzoic acid with k 3 = 7.6 X lo8 M-l
Introduction The biradicals produced in the photochemical Norrish type I1 process (reaction 1) are good electron donors, as Ph
Ph
J
O
H
which formally makes the generation of biradicals an “instantaneous” process and largely simplifies the treatment of the dataa8In all cases the experiments were carried out at room temperature in acetonitrile containing 20% water (v/v), which is a convenient solvent for ymethylvalerophenone and all the electron acceptors used. The samples were excited using the pulses (3 mJ, -8 ns, 337.1 nm) from a nitrogen laser and the reaction was followed by monitoring the time profile for either the decay of B (at 415 or 500 nm) or the buildup of A-. produced in reaction 3. Both approaches yield the same pseudoPh
Ph
I
B
R
MlN@@)Le
eM N 2-
1
I
+ A -
2
indicated, for example, by the rate and efficiency with which they transfer an electron to paraquat (1,l’-dimethyL4,4’-bipyridiliurn, 1) dications, reaction 2,2,3where the asterisk in reaction 1 indicates a triplet state. Our earlier experiments were designed in order to examine biradical properties, in particular their lifetime,2,4,5 using reaction 2 as a monitor and applying nanosecond laser flash photolysis techniques. Reaction 2 can be expected to be a member of a larger family of electron transfer processes in biradicals. This paper reports the results of a study aimed at establishing how general electron transfer processes in biradicals are, and how the rates correlate with those of other electron donors and with redox potentials.
Results and Discussion All the experiments described herein were carried out using y-methylvalerophenone as the biradical precursor. The choice of substrate reflects the short triplet lifetime2,69 0022-3654/78/2082-2662$01 .OO/O
’’
Po+
A-.
+
H+
(3)
first-order kinetic behavior, with the experimental firstorder rate constant given by2 kexpt
=
TB-’ -I- b [ A I
(4)
where TB is the biradical lifetime (81 ns in this case) and k3 is the second-order rate constant for the trapping reaction. A number of electron acceptors can be used in reaction 3 in addition to paraquat dications (l),and structures 3-10 illustrate those examined in this paper. In the case of the bipyridilium type of acceptors we have prefered to use hydrochlorides in all cases (some are commercially available as hydrobromides) in order to avoid the possibility of heavy atom effects on the value of T B . ~ Figure 1 shows typical growth traces obtained in the case of benzyl viologen, 3 (Figure lA), and diquat, 6 (Figure 1B). They can be regarded as the two limiting cases; in the former the measurement is straightforward while in the case of diquat we observe a “jump” followed by the expected growth. Control experiments showed that this jump (Figure 1B)is due to absorption resulting from direct
0 1978 American
Chemical Society
