3644
J. Phys. Chem. 1987, 91, 3644-3649
In Figure 6 we plot the experimental values of 1/R vs. D. The predicted linear variation is well obeyed; from the intercept we obtain a value of R = 7.6 f 0.7 A, and from the slope, the value k, = (1.2 f 0.5) X lo-" cm3 s-I. The value of R = 7.6 A is very close to the sum of the van der Waals radii of two pyrene molecules (R = 7.1 A). The value of R obtained compares very well with the published value by Heuman,17 7.0 8, < R < 9.0 A, for the pyrene excimer formation reaction in different solvents at room temperature. Also our value for the diffusion coefficient of the pyrene derivative (DM= D / 2 ) at room temperature agrees very with the Heuman's values for the pyrene diffusion coefficient. The value of R obtained does not support the assumption made by Donner et a1.I8 of a critical radius of 10 8, for excimer formation in membranes. The value (17) Heuman, E. Z. Naiurforscfi. A 1981, 36A, 1323. (18) Donner, M.; Andre, J. C.; Bouchy, M. Biocfiem. Biopfiys. Res. Commun. 1980, 97, 1183.
of k, is usually compared9 with the predicted value from the kinetic theory of gases. The calculated value k, = 1.2 X lo-" cm3 SKI is lower than kgas 1.0 X lo-'' cm3 s-l. The difference can be attributed to the steric effect of the side chain of our pyrene derivative molecule. An Arrhenius plot of D M / Tis shown in Figure 7. A reasonable straight line can be drawn through the experimental points and an activation energy of 2.5 kcal mol-] could be obtained. In the same figure are also plotted the values of the reciprocal of viscosity (l/v). The same slope should be expected from the two plots. A small difference (2.2 vs. 2.5 kcal mol-I) can be attributed to the failure of the Stokes e q ~ a t i o n ' ~ relating the diffusion coefficient with the macroscopic viscosity of the medium.
Acknowledgment. The authors thank NSERC Canada and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for their financial support. Registry No. 1, 70570-29-5. (19) Alwattar, A. H.; Lumb, M. D.; and Birks, J. B. In Organic Molecular Pfiotophysics, Vol. 1, Birks, J. B., Ed.; Wiley-Interscience: New York, 1973.
Fluorescence Quenching of 9,lO-Dlcyanoanthracene by Dlenes and Alkenes Susan L. P. Changt and David I. Schuster* Department of Chemistry, New York University, New York, New York 10003 (Received: September 2, 1986; In Final Form: January 9, 1987)
The fluorescence of 9,lO-dicyanoanthracene (DCNA) is quenched by a variety of acyclic and cyclic dienes and alkenes. The observed quenching rate constants are consistent with values calculated for an electron-transfer process. Solvent-dependent exciplex emission was observed for DCNA with 1,3-~ycloheptadiene,2,3-dimethyl-2-butene, and 1,3-~yclohexadiene,giving calculated exciplex dipole moments of 10.8, 13.5, and 11.2 D, respectively.
Introduction Quenching by dienes of the fluorescence of arenes was originally rationalized by the formation of arene/diene exciplexes,' which was later confirmed by the observation of exciplex fluorescence in several systemsS2 Compelling evidence for the intermediacy of such exciplexes in photocycloaddition reactions was first obtained for phenanthrene/olefin systems by C a l d ~ e l land , ~ indirect evidence for the intermediacy of reversibly formed exciplexes in anthracene/diene systems has been reported by Yang4-5and SaltieL6 Woodward-Hoffmann rules7 predict that concerted suprafacial [4 + 41 and [2 + 21 cycloadditions are photochemically allowed, while similar [4 + 21 cycloadditions are photochemically forbidden. The simultaneous occurrence of both forbidden and allowed reaction pathways was interpreted by Yang in terms of competing concerted and stepwise reactions of the intermediate e x c i p l e x e ~ .Yang ~ ~ ~ ~et al.4c,dfound a correlation between the polarity of the arene/diene exciplexes and the mode of cycloaddition: for exciplexes with relatively little polarity, the presumably concerted [4 + 41 cycloaddition process was predominant, while for relatively polar exciplexes, the stepwise [4 + 21 pathway was observed. Triplet pathways for cycloadditions are possible, as demonstrated by Saltiel for additions of benz[a]anthracene to 1,3-~entadienes,~ but the singlet pathway has been shown to operate in the anthracene/diene system^.^" The polarity of the diene/anthracene exciplex depends in large part on the donor-acceptor characteristics of the diene and anthracene, respectively. Thus, when an electron-rich diene such 'Chemistry Department, Universidad Simon Bolivar, Apt. 80659, Caracas, Venezuela 1081 A.
as 2,5-dimethyl-2,4-hexadiene and an electron-deficient anthracene such as 9,lO-dicyanoanthracene (DCNA) were employed, a polar exciplex was formed which led in a stepwise manner to a 4a, + 2n, a d d ~ c t . ~As. ~the solvent polarity increases, the polarity of the exciplex leading to photocycloaddition also increases, resulting in a system in which electron-transfer processes leading to radical (1) Stephenson, L. M.; Whitten, D. G.; Vesley, G. F.; Hammond, G. S . J . Am. Cfiem.Soc. 1966,88, 3665,3893. Stephenson, L. M.; Hammond, G. S . Pure Appl. Cfiem. 1968, 16, 125. Stephenson, L. M.; Hammond, G. S . Angew. Chem., Int. Ed. Engl. 1969,8, 261. Evans, T. R. J. Am. Cfiem.SOC. 1971, 93, 2081. Labianca, D. A.; Taylor, G. N.; Hammond, G. S. Ibid. 1972, 94, 3679. Taylor, G. N.; Hammond, G. S. Ibid. 1972, 94, 3684, 3687. (2) (a) Taylor, G. N. Cfiem. Phys. Lett. 1971, IO, 355. (b) Saltiel, J.; Townsend,D. E. J. Am. Cfiem.Soc. 1973,95, 6140. (c) Saltiel, J.; Townsend, D. E.; Watson, B. D.; Shannon, P. Ibid. 1975, 97, 5688. (d) For a review of the literature, see Caldwell, R. A,; Creed, D. Acc. Chem. Res. 1980, 13, 45. (3) Caldwell, R. A,; Smith, L. J. J . Am. Cfiem. SOC.1974, 96, 2994. Caldwell, R. A.; Creed, D.; DeMarco, D. C.; Melton, L. A,; Ohta, H.; Wine, P. H. Ibid. 1980, 102, 2369. See also Majima, T.; Pac, C.; Sakurai, H. Bull. Cfiem.Soc. Jpn. 1978,51, 1811. Pac, C.; Sakurai, H. Chem. Lett. 1976, 1067. Itoh, M.; Takita, N.; Matsumoto, M. J . Am. Cfiem.SOC.1979, 101, 7363. (4) (a) Yang, N. C.; Libman, J. J . Am. Cfiem.SOC.1972, 94, 1405. (b) Yang, N. C.; Libman, J.; Barrett, L.; Hui, M. H.; Loeschen, R. L. Ibid. 1972, 94, 1406. (c) Yang, N. C.; Srinivasachar, K.; Kim, B.; Libman, J. Ibid. 1975, 97, 5006. (d) Yang, N. C.; Yates, R. L.; Masnovi, J.; Shold, D. M.; Chiang, W. Pure Appl. Cfiem. 1979, 51, 173. (5) Yang, N. C.; Shold, D. M.; McVey, J. K. J . Am. Chem. SOC.1975, 97, 5004. (6) Smothers, W. K.; Meyer, M. C.; Saltiel, J. J. Am. Cfiem.SOC.1983, 105, 545. Saltiel, J.; Dabestani, R.; Schanze, K. S.; Trojan, D.; Townsend, D. E.; Goedken, V. L. Ibid. 1986, 108, 2674; see also ref 2b,c. (7) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry; Academic: New York, 1971; p 70 ff. (8) Saltiel, J.; Townsend, D. E.; Metts, L. L.; Wrighton, M.; Mueller, W.; Rosanske, R. C. J . Cfiem. SOC.,Cfiem. Commun. 1978, 588.
0022-3654/87/2091-3644$01.50/00 1987 American Chemical Society
Fluorescence Quenching of 9,lO-Dicyanoanthracene
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3645
anions and radical cations p r e d ~ m i n a t e . ~ The importance of polar exciplexes in which charge-transfer processes occur when cyanoarenes interact with dienes and alkenes has been previously n ~ t e d . ~ JTaylorza ~ ' ~ observed a linear free energy correlation between the logarithm of the rate constant for fluorescence quenching of 1-cyanonaphthalene by diene and alkene quenchers and the ionization potential of the respective quenchers. Furthermore, a solvent dependence of the exciplex emission maximum of the 1-cyanonaphthalene/ 1,2-dimethylcyclopentene exciplex was observed, which could be used to calculate a value of 10.8 D for the exciplex dipole moment. Foote and collaborators'O.'' as well as Zhang and collaborators'* have observed that a plot of the logarithm of the rate constant for fluorescence quenching of DCNA by selected alkenes and diene quenchers in a solvent of high polarity (acetonitrile) vs. the free energy change in an electron-transfer process showed a good correlation with the theoretical line calculated by Rehm and Weller.I6 Foote and collaborators1°also observed a solvent-dependent exciplex emission maximum for the DCNA/ 1,3-~yclohexadieneexciplex, which led to a value of 11 D for the exciplex dipole moment. Ware and c o l l a b ~ r a t o r s observed l~ fluorescence quenching of DCNA by 1,l-diphenylethylenes in benzene and acetonitrile; they also report an exciplex from the DCNA/4,4'-dimethyl-l,l-diphenylethylene system. Hirayama and PhillipsI5 report exciplex formation between DCNA and l ,2-dimethoxybenzene, hexamethylbenzene, and 2,5-dimethyl-2,4-hexadienein the vapor phase, where the ratio of exciplex to monomer usually exceeds that in solution. Schaap and collaborator^'^ report the ESR spectrum of DCNA'- obtained by photolysis of solutions of DCNA in acetonitrile in the presence of selected alkenes. In view of the documented importance of polar exciplexes in the photoprocess of DCNA in the presence of dienes and alkenes, we have studied the fluorescence quenching of DCNA by an extended series of dienes and alkenes. We also report the solvent dependence of the exciplexes formed between DCNA and 2,3dimethyl-2-butene, 1,3-~ycloheptadiene,and 1,3-~yclohexadiene.
Experimental Section Materials. DCNA (Kodak) was recrystallized from pyridine. Quenchers were as follows: 2,3-dimethyl-2-butene, cis&- 1,3cyclooctadiene, 1,3-~ycloheptadiene, 1,3-~yclohexadiene,and cyclohexene (Aldrich); 2,5-dimethyl-2,4-hexadiene,cis,trans2,4-hexadiene, cis,cis-2,4-hexadiene, trans- 1,3-pentadiene, and 2-ethyl-l,3-butadiene (Chemical Samples); 2-methyl-2-butene (MC/B distilled, bp 39-40 "C). These compounds were typically 99.4% pure as analyzed by gas chromatography on HewlettPackard 5840A and 5710A instruments equipped with flame ionization detectors, employing the following analytical columns: 20% XE-60 on Chromosorb P/AW-DMCS (10 ft) and GE-XE-60 on Chromosorb W/AW-DMCS (6 ft). Solvents: cyclohexane, benzene 1,2-dichloroethane, and acetonitrile (Aldrich, Spectrophotometric grade) and diethyl ether (Fisher, Anhydrous Reagent) were used as received. Instrumentation and Procedures. Fluorescence emission and excitation spectra were recorded at room temperature on a Spex Fluorolog instrument equipped with a photon-counting detector. 4X M, and quencher in Solutions of DCNA, 1.5 X long-stemmed 1-cm fluorescence cells were outgassed by bubbling nitrogen through the solution for 15 min. Quenching data were analyzed according to the Stern-Volmer equation. Corrected exciplex emission spectra were obtained by subtracting the re(9) Knibbe, H.; Rolling, K.; Schafer, F. p.; Weller, A. J . Chem. Phys. 1967, 47, 1184. (10) Eriksen, J.; Foote, C. S. J . Phys. Chem. 1978, 82, 2659. (11) Araki, Y.; Dobrowolski, D. C.; Goyne, T. E.; Hanson, D. C.; Jiang, Z. Q.;Lee, K. J.; Foote, C. S. J . Am. Chem. SOC.1984, 106, 4570. (12) Zhang, B.-W.; Ming, Y.-F.; Cao, Y. Photochem. Photobiol. 1984.40, 581. (13) Schaap, A. P.; Zaklika, K. A.; Kaskar, B.; Fung, L. W.-M. J. Am. Chem. SOC.1980, 102, 389. (14) Ware, W. R.; Holmes, J. D.; Arnold, D. R. J . Am. Chem. SOC.1974, 96, 7861. (15) Hirayama, S.; Phillips, D. J. Phys. Chem. 1981, 85, 643. (16) Rehm, D.: Weller, A. Isr. J . Chem. 1970, 8, 259.
V
0
1
[Ql
2 ( M x lo2)
3
Figure 1. Stern-Volmer plots for fluorescence quenching of DCNA in acetonitrile by linear dienes: 0,cis,cis-2,4-hexadiene; 0,cis,trans-2,4hexadiene; 0 , 2,5-dimethyl-2,4-hexadiene;V, trans-l,3-pentadiene.
7
6
5 0
yo 4
-
3
2
1
0
1
[Ql
2 ( M x IO2)
3
Figure 2. Stern-Volmer plots for fluorescence quenching of DCNA in acetonitrile by cyclic dienes: 0, 1,3-~yclohexadiene;0, 1,3-cyclooctadiene; A, I ,3-cycloheptadiene.
maining DCNA fluorescence emission at the given quencher concentration from the observed exciplex emission spectra. UV spectra were recorded on Perkin-Elmer 320 and Cary 14 instruments. IH N M R spectra were recorded on a GE QE-300 instrument.
3646 The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
Chang and Schuster TABLE I: DCNA Fluorescence Quenching Rates, Ionization Potentials, Half-Peak Oxidation Potentials, and Calculated Free Energies for Electron Transfer EIp(ox),L? AG,b kq7,,C k,, 10'" quencher IP, eV V kcal/mol M-' M-' s? 2,5-dimethyl-2,47.67d 1.12 -19.6 286 1.9
'0
1
[Ql
2 3 ( M x lo2)
hexadiene 1,2-dimethylcyclohexene cis,trans-2,4-hexadiene 1,3-cyclohexadiene cis,cis-2,4-hexadiene 2,3-dimethyl-2butene 1,3-~ycloheptadiene a-pinene cholesterol 1-methylcyclohexene @-pinene trans-l,3-pentadiene cis,cis-1,3-cyclooctadiene 2-methyl-2-butene 2-ethyl- 1,3-butadiene cyclohexene
4
.,
Figure 3. Stern-Volmer plots for fluorescence quenching of DCNA by 1,3-~yclohexadienein: 0,cyclohexane: benzene; 0 , acetonitrile.
5
4
5
T0 3
-
2
1
0
2
1
[Ql
.,
3
( M x lo2)
Figure 4. Stern-Volmer plots for fluorescence quenching of\DCNA by 2.3-dimethyl-2-butene in: 0, cyclohexane; benzene; A, 1.2-dichloro-
ethane; 0 , diethyl ether; 0 , acetonitrile.
Results Fluorescence Quenching. The fluorescence of DCNA in nitrogen-saturated acetonitrile was quenched by a variety of linear and cyclic dienes and alkenes (Figures 1-4, Table I). In each case, the quenching followed the Stern-Volmer equation:I7 zo/zq
= kq~JQl+ 1
(1) ~~
(1 7) Turro, N. J. Modern Molecular Photochemistry: Benjamin: Menlo
Park, CA, 1978; p 246.
1.49e
-11.1e
1.54
-9.9
365
2.4
8.29 1.56 8.27d 1.58 8.2781~ 1.58
-9.5 -9.0 -9.0
327 592 309
2.1 3.9
-8.3
220
1.4 1.1' 0.42e 0.88'
8.22d
8.311
1.61 1.63' 1.75'
1.3e
-7.8'
1.82'
-5.1' -3.4'
2.0
1.82'
-3.5'
8.59d 8.68'
1.82 1.89
-3.5 -1.8
227 206
0.67' 1.5 1.3
8.688 8.77d
1.89 1.96
-1.8 -0.2
217 25
1.4 0.16
8.958*h
2.09
+2.8
0.76 0.0049
'Taken from Figure 8, eq 2, unless otherwise noted. bCalculated from eq 3. cThe value of r S for DCNA in nitrogen-saturated acetonitrile is taken as 15.3 & 0.1 ns (ref 10). dReference 22. eReference 11. /Reference 23. 8Reference 24. hReference 25. 'Reference 12. where Zo and I , are the relative fluorescence intensities in the absence and the presence of quencher Q, k, is the bimolecular quenching rate constant, and T~ is the singlet lifetime of the fluorescer. Plots of Zo/Zq vs. [Q] gave straight lines with intercepts of 1.0. Typical plots are shown in Figures 1 and 2. From the slopes (kq7J of these plots and the reported value of 7sfor DCNA (1 5.3 f 0.1 ns)'O in nitrogen-saturated acetonitrile, the k, values of the diene and alkene quenchers given in Table I were obtained. A study of the fluorescence quenching of DCNA by 1,3-cyclohexadiene, 1,3-~ycloheptadiene,cis&- 1,3-~yclooctadiene,and 2,3-dimethyl-2-butene was conducted in a variety of nitrogensaturated solvents of different polarities. Stern-Volmer plots were again linear with intercepts of 1.0 (see Figures 3 and 4 for typical data). The slopes kq7, are given in Table 11. However, the fluorescence quenching of DCNA by 2,5-dimethyl-2,4-hexadiene in nitrogen-saturated cyclohexane gave a nonlinear Stern-Volmer plot, in which values of kqTawere found to increase with 2,5dimethyl-2,4-hexadiene concentration (Figure 5, Table 11). Exciplex Emission. The DCNA fluorescence quenching by 1,3-~yclohexadiene,1,3-~ycloheptadiene,2,3-dimethyl-2-butene, and 2,5-dimethyl-2,4-hexadienewas accompanied by weak structureless exciplex emission in some solvents (Figures 6 and 7, Table 11). In all cases, the exciplex excitation spectra were identical with the DCNA fluorescence excitation spectra. Furthermore, no change was observed in the UV absorption spectra of DCNA in the presence of 1,3-cyclohexadiene, 1,3-cycloheptadiene, and 2,3-dimethyl-2-butene at the concentrations required to observe exciplex emission.
Discussion The Rehm-Weller equation,16 which relates the oxidation potential of the donor, the reduction potential of the acceptor, and the electron excitation energy of the fluorescer to the rate of fluorescence quenching by electron transfer, has been applied successfully to fluorescence quenching of DCNA in acetonitrile, employing arenes1°J4 and, in selected cases, dienesL0and alkenes as quenchers.l ','*J~Nevertheless, an extensive quenching study using a series of dienes which in many cases are known to react with anthracenesM has apparently not been previously undertaken. Interpretation of diene and alkene quenching rate is hindered by the lack of available literature on the oxidation potentials of these
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3647
Fluorescence Quenching of 9,lO-Dicyanoanthracene TABLE II: Excidex Emission Data
solvent (fcyclohexane (0.10 1) exciplex exciplex system DCNA/ 1,3-cyclohexadiene DCNA/ 1,3-cycloheptadiene DCNA/ 1,3-~yclooctadiene DCNA/2,3-dimethyl-2-butene
DCNA/2,5-dimethyl-2,4-hexadiene
l/f')"
')maw
kqrs,
Vman
Umpm
M-1
kqrw
Pm 18.7 19.6
wm
M-'
w
133.6 28.7 0.27 6.4 91-430'
17.2 18.2
125 34.6
17.5b 18.3
18.5
67
18.9
21.3 17.7
1,2-dichloroethane (0.328) exciplex
diethyl ether (0.256) exciplex
benzene (0.1 17) exciplex
Vman
kq7v
wn
M-1
16.3b 17.5
139 3 163
18.0
kq7w
M-1 76.9 39 100
"Calculated from eq 4. bReference 10. 'kqr, varies with diene concentration, 91 M-l at 0.0046 M diene to 430 M-I at 0.46 M diene. See text for discussion.
20 I
-
P
\
0
500
400
600
7 0
X (nm)
10
L
-
0 1 2 3 4 5
[Ql
>/ OO
[Ql
Figure 7. Exciplex emission of the DCNA/2,3-dimethyl-2-butene system. (A) DCNA fluorescence in cyclohexane. Corrected exciplex emission in (B) cyclohexane; (C) benzene; (D) diethyl ether; (E) 1,2dichloroethane. The relative sensitivity settings of the spectrofluorimeter are shown. The concentration of DCNA was 2 X M and that of 2,3-dimethyl-2-butene was 0.85 M in cyclohexane and benzene, and 0.55 and 1.7 M in diethyl ether and 1,2-dichloroethane, respectively.
(M x 10')
1 (M x 10')
Figure 5. Stern-Volmer plots for fluorescence quenching of DCNA by 2,5-dimethyl-2,4-hexadienein cyclohexane.
TABLE 111: Ionization Potentials and Half-Peak Oxidation Potentials in Acetonitrile compd IP, eV ethylene 10.51" 2.90" 1-butene 9.58" 2.78" 2-methylpropene 9.23" 2.65" 2-butene 9.13" 2.21" 1,3-butadiene 9.07" 2.03" cyclohexene 8.95" 1.98" 8.40" 1,4-~yclohexadiene 1.60" 1,3-~yclohexadiene 8.25b 1.58b 2,3-dimethyl-2-butene 8.27" 1.84"
" Reference 20.
Reference 2 1.
compounds. This problem was circumvented by utilizing the linear relationship between the ionization potential and the oxidation potential of organic compounds, which has been established by several research The appropriate data for dienes and alkenes20*21 are presented in Table 111. A linear relationship between the ionization potential and oxidation potential of these dienes and alkenes was observed, as shown in Figure 8. This relationship, given by 400
600
500
I
-.
El12(ox) = 0.76 IP (eV) - 4.71
(2)
h (nm)
Figure 6. Exciplex emission of the DCNA/l,3-cycloheptadiene system. (A) DCNA fluorescence in cyclohexane. Corrected exciplex emission in (B) cyclohexane: (C) benzene; (D) diethyl ether; (E) 1,2-dichloroethane. The relative sensitivity settings of the spectrofluorimeter are M and that of 1.3shown. The concentration of DCNA was 2 X cycloheptadiene was 0.6 M.
(18) Neikam, W.C.; Dimeler, J. R.; Desmond,M. M. J . Electrochemical Soc. 1964, 1 1 1 , 1190. (19) Yang, N. C.; Pysh, E. S.J . Am. Chem. SOC.1963, 85, 2124. (20) Miller, L. L.; Nordblom, G. D.; Mayeda, E. A. J. Org. Chem. 1972, 37, 916. (21) Breslow, R.; Johnson, R. W. Tetrahedron Left. 1975, 40, 3443.
3648
Chang and Schuster
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 10"
[ 0
80 10'0
10
i -z
-z 9
109
Y U
1
108
5f:
107 I
a
7
'0
-20
2
1
3
E,ox(V) Figure 8. Plot of ionization potentials, IP, vs. oxidation potentials, E;)/*. Data taken from ref 16 (0)and 17 (a).
was used in conjunction with reported ionization potential^^^-^^ to calculate the oxidation potentials for the compounds used in this study, shown in Table I. With these oxidation potentials, the corresponding free energy change, AG, for an electron-transfer process was calculated from the Rehm-Weller relationshipi6 AG = 23.06[E(D/D+)
- E(A-/A)
- eo2/ac - AE0,0l (3)
where E(D/D+) is the oxidation potential of the donor, D; E(A-/A)is the reduction potential of the acceptor, A; eo2/atis the energy obtained by bringing two radical ions to encounter distance, a, in a solvent of dielectric constant, t; and A&, is the electronic excitation energy of fluorescer. For DCNA, E(A-/A) = -0.98 V and AEo,o = 2.89 eV.Io The value of eoz/atwas taken to be -0.06 eV in acetonitrile.I0J6 The calculated values of AG are found in Table I. A plot of k,, vs. AG gave a reasonably good correlation with the theoretical line calculated by Rehm and Weller,I6 shown in Figure 9, strongly indicative of an electrontransfer mechanism for the fluorescence quenching of DCNA by these dienes and alkenes in acetonitrile.z6 Yang et al.5 studied the fluorescence quenching of DCNA by 2,5-dimethyl-2,4-hexadiene(DMH) in methylcyclohexane and report an anomalously high quenching constant kqr, of 2000 M-'. This was rationalized by formation of a ground-state complex, supported by UV spectra of this system at high diene concentrations. Yang reports an exciplex Y,,, for this system at 535 nm, while Hirayama and PhillipsIs find v, at 505 nm for the DCNA/DMH exciplex in the vapor phase where the exciplex/ monomer ratio is much greater than in solution. The value observed here for the fluorescence quenching of DCNA by D M H (22) Bocquet, J. F.; Mascalet, P.; Mouvier, G. J . Chim. Phys. 1981, 78, 99. (23) Bischof, P.; Heilbronner, E. Helu. Chim. Acta 1970, 53, 1677. (24) Bieri, G.; Burger, F.; Heilbronner, E.; Maier, J. P. Helu. Chim. Acta 1977, 60, 22 13. (25) Kiser, R. W. Introduction to Mass Spectrometry and Its Applications; Prentice-Hall: Englewood Cliffs, NJ, 1965; pp 308-318. (26) As noted by a referee, some of the quenching constants in Table I seem to be greater than the diffusion-controlled limiting value of k, of ca. 2 X 10" M-' s-' (see ref 10). The greatest discrepancy is for cis,cis-2,4-hexadiene. High values of k, can be explained in terms of contributions of static quenching or ground-state complexation; houcver, such effects should be manifested, respectively, by curved Stern-Volmer plots and changes in UV absorption and fluorescenceexcitation spectra at high quencher concentrations (see ref 2c for a full discussion of such phenomena). Since such effects were not observed, these observations remain anomalous, at the present time.
,
I
I
-1 0
I
I
,
,
l
,
0
AG (kcal/mole) Figure 9. Plot of fluorescence quenching rate constant, k,, vs. free energy, AG, for the electron-transfer process involving DCNA interacting with dienes and alkenes in acetonitrile at 20 OC. Data taken from this work and references 12 ( 0 )and 11 ((3). The solid line was calculated according to eq 3 for electron-transfer process.16
in acetonitrile, kqTs = 286 M-I, is in fair agreement with the value of kqT, = 214 M-' predicted by the theoretical Rehm-Welleri6 plot in Figure 9, for a system with AG = 19.6 kcal/mol. We found no evidence for ground-state complex formation in the UV spectra of DCNA in acetonitrile in the presence of up to 0.03 M DMH. However, the quenching constant for this system in cyclohexane was found to vary markedly with the D M H concentration: k q T s = 90 M-' at a diene concentration of 0.0046 M and k q T , = 430 M-' at a diene concentration of 0.46 M (see Figure 5). A study of the DCNA/DMH system in cyclohexane revealed an exciplex vmax at 565 nm in the DMH concentration range 0.023-0.46 M. The exciplex excitation spectra were identical with the DCNA fluorescence excitation spectra over the same diene concentration range, in excellent agreement with earlier finding^.^ The upward curvature in Figure 5 is probably due to the formation of ground-state complexes, as also observed in the 9,l O-dichloroanthracene/DMH system in acetonitrile,zcalthough in the present instance there was no observable change in the UV absorption spectrum of DCNA in cyclohexane as a function of D M H concentration. The possibility of triplex formation in this system cannot be excluded.zc A comparison of the fluorescence quenching constant for DCNA in the presence of selected diene and alkene quenchers in solvents of differing polarity is reported in Table 11. The following trend in the order of quenching efficiency is observed: 1,3-~yclohexadiene> 2,3-dimethyl-2-butene 1,3-cycloheptadiene > 1,3-cyclooctadiene. Exciplex emission was observed for DCNA and 1,3-cyclohexadiene, 1,3-~ycloheptadiene,and 2,3-dimethyl-2-butene; no detectable exciplex emission was observed for the DCNA/ 1,3-~yclooctadienesystem. Considering both the highly twisted nature of the diene unit in cyclooctadiene (the extent of twist was cited to be 42' by electron diffraction, 42-65' by force-field calculations, and 60' by vibrational spectroscopy)*' as well as the difference in the calculated oxidation potentials of the cyclic dienes (Table I), the latter observation is readily rationalized in terms of the reduction in stability of a DCNA/ 1,3-~yclooctadieneexciplex. Both the relative position of the vmax, as well as the intensity of the observed exciplex emission are found to be solvent-dependent in the DCNA/ 1,3-~yclohexadiene,DCNA/ 1,3-~ycloheptadiene, and DCNA/2,3-dimethyl-2-butene systems, as seen in Table I1 and Figures 6 and 7. In all cases, the exciplex excitation spectra were identical with the DCNA fluorescence excitation spectra. The intensity of the exciplex emission in all cases was found to decrease with increasing solvent polarity, presumably due to the tendency of the exciplexes to decompose into radical cations and
-
(27) Giordan, J. C.; McMillian, M. A,; Moore, J. H.; Staley, S. W. J . Am. Chem. So?. 1980, 103, 4870, and references cited therein.
Fluorescence Quenching of 9,lO-Dicyanoanthracene
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
3649
I
22 21
‘i
20
E
0
5
: 19 E
a
18
16 0.0
I
0.1
0
I
0.2 f-’/zf’
0.3
Figure 10. Plot of v- vs.f- ‘/#(see eq 4 and Table 11) for the systems: 0, DCNA/2,3-dimethyl-2-butene; 0 , DCNA/ 1,3-~ycloheptadiene;0 , DCNA/ 1,3-cyclohexadiene; 0 , DCNA/l,3-cyclohexadiene (ref IO).
TABLE I V Exciplex Dipole Moments and Hypothetical Gas-Phase Emission Frequencies exciplex system dipole moment,’.b D vO: wm DCNA/ 1,3-cyclohexadiene 11.2 19.9 DCNA/1,3-~ycloheptadiene 10.8 20.6 DCNA/2,3-dimethyl-2-butene 13.5 22.7 ‘Calculated from eq 4. *Due to the uncertainty in a (eq 4). the uncertainty in the dipole moments is estimated to be &30%. cExtrapolated from Figure 10.
radical anions in more polar solvents?J3 Beens et al?* have derived an expression for the solvent dependence of the relative position of Y,,, for exciplex emission, given by v,
= vo - 2p2/hca3([(c - 1)/(2c + l)] - 1/2[(n2 1)/(2n2 + l ) ] ) = vo - 2p2/hca3(f-
Yd’)(4)
where vmax is the wave number of emission maximum, vo is the hypothetical gas-phase emission frequency, p is the exciplex dipole moment, h is Planck’s constant, c is the velocity of light, u is the interaction distance, taken to be 5 &lo t is the solvent dielectric constant, and n is the solvent index of refraction. In Table I1 values of the appropriate solvent parameters (fI//’) and of vmax are listed. Plots of vmaxvs. (f- ‘/Zf’) are found in Figure 10. If the points corresponding to benzene are disregarded, because of specific solute-solvent interactions in this solvent,28these data give reasonably straight lines, which lead to the dipole moments and values of vo found in Table IV. The larger dipole moment observed for the DCNA/2,3-dimethyl-2-butene exciplex can be ascribed to one or more of the following: (a) a greater amount of charge transferred in the exciplex; (b) a difference in the distribution of the charge in a DCNA exciplex formed with an alkene vs. that formed with a diene; (c) a difference in the charge separation in the exciplex. On the basis of the present data it is not possible to distinguish between these alternatives. (28) Beens, H.; Knibbe, H.; Weller, A. J . Chem. Pkys. 1967, 47, 1183.
16
17
20
18 19 Vmax (kcm-’ 1
Figure 11. Plot of vmpXvs. vmlx for the systems: 0, DCNA/2,3-dimethyl-2-butene vs. DCNA/1,3-~ycloheptadiene; 0 , DCNA/2,3-dimethyl-2-butene vs. DCNA/ 1,3-cyclohexadiene; 0 , DCNA/ 1,3-cycloheptadiene vs. DCNA/ 1,3-~yclohexadiene. For the DCNA/ 1,3-cyclohexadiene system some data was taken from ref 10; these points are indicated with a slash.
TABLE V Ratios of Dipole Moments DCNA/ 1,3-~yclohexadiene:DCNA/ 1,3-cycloheptadiene DCNA/2,3-dimethyl-2-butene:DCNA/ 1,3-cyclohexadiene DCNA/2,3-dimethyl-2-butene:DCNA/ 1,3-cycloheptadiene
D/D‘ 1.0
slopeb 1.1
1.2
1.4
1.3
1.6
“Ratio of dipole moments calculated from eq 4. bSlope of line from Figure 11.
The deviation of the points corresponding to the exciplex v, emission in benzene has been ascribed to the particular solvating characteristics of this solvent.28 Such interactions are expected to be similar for each of the exciplexes reported here. Thus, when for any two exciplexes were plotted the exciplex emission ,v against each other, including the point for benzene, a linear correlation was observed, as seen in Figure 11. The agreement observed in Table V between the slope of the lines in Figure 11 and the ratio of the respective dipole moments indicates that the dipole moments obtained from eq 4 for similar systems can be properly compared. Acknowledgment. This study was supported in part by a grant from the National Science Foundation, CHE-83120154. Susan Chang thanks the Universidad Simon Bolivar in Caracas for grant of a sabbatical leave, which made this work possible. Registry No. DCNA, 1217-45-4; 2,5-dimethyl-2,4-hexadiene,76413-6; 1,2-dimethyIcyclohexene, 1674-10-8; cis,trans-2,4-hexadiene, 5194-50-3; 1,3-cyclohexadiene, 592-57-4; cis,cis-2,4-hexadiene, 610861-8; 2,3-dimethyl-2-butene, 563-79-1; 1,3-~ycloheptadiene,4054-38-0; a-pinene, 80-56-8; cholesterol, 57-88-5; 1-methylcyclohexane, 591-49-1; 0-pinene, 127-9 1-3; trans- 1,3-pentadiene, 2004-70-8; cis,cis-1,3-cyclooctadiene, 3806-59-5; 2-methyl-2-butene, 5 13-35-9; 2-ethyl- 1,3-butadiene, 3404-63-5; cyclohexene, 110-83-8.
