J. Phys. Chem. 1984, 88, 2798-2803
2798
Photophysical Behavior of Exciplexes of 1,4-Dicyanonaphthalene with Methyl- and Methoxy-Substituted Benzenes' H. F. Davis: S . K. Chattopadhyay, and P. K. Das* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: October 27, 1983)
With methyl- and methoxy-substituted benzenes in n-heptane, 1,4-dicyanonaphthalene (DCN) forms exciplexes that are characterized by fluorescence yields in the range 0.01-0.55 and intersystem crossing yields in the range 0.17-0.6. These data combined with exciplex lifetimes have been used to obtain the rate constants for individual photoprocesses, namely, fluorescence, intersystem crossing, and internal conversion, as functions of oxidation potential of the quenchers. The radiative rate constant decreases monotonically from 2.5 X lo7 to 1 X lo6 s-' as the ionic (charge transfer) character of the exciplexes increases in the region of relatively high values (1.2-1.6 V in acetonitrile vs. Ag/O.l M Ag'). On the other hand, the rate constants for both intersystem crossing and internal conversion show distinct increasing trends in the limit of 100% ionic character of the exciplexes as decreases in the region 0.7-1.2 V. While the former behavior is a reflection of the diminishingimportance of locally excited configurations which determine the emissive transition probability in the exciplexes, the latter result is explainable in terms of progressively smaller energy gap between the states involved in the respective radiationless transitions. N
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Introduction Cyanoaromatics are widely used as excited-state acceptors in organic photoreactions mediated by photoinduced electron transfer. The chemical changes of interest in the course of these reactions occur in the exciplexe~,~"ion pairs,3" or solvated radical ions in which relevant bonds are broken or weakened. Some examples of photoreactions affected by electron transfer sensitization by aromatic nitriles are as follows: photoo~ygenation~~" and phot ~ a d d i t i o nof~alkylbenzenes, ~ oxiranes, aryl-substituted ethanes, etc., isomerization* of small-ring compounds, namely, cyclopropanes and oxiranes, cycloadditiong and cycl~dimerization~ involving alkenes, photorearrangementIO of cyclopropanes/ cyclopropenes, and carbon-carbon bond in ethers, pinacols, pinacol ethers, etc. The work described in this paper represents a detailed study of the exciplexes of 1,4-dicyanonaphthalene (DCN), a commonly used singlet acceptor, with two graded series of donor compounds, namely, methylated and methoxylated benzenes. Data from steady-state emission spectral measurements and time-resolved nanosecond laser flash photolysis in n-heptane have been analyzed to obtain various photophysical parameters related to the exci(1) The work described herein was supported by the Office of Basic Energy Sciences, U S . Department of Energy. This is Document No. NDRL-2518 from the Notre Dame Radiation Laboratory. (2) Undergraduate Research Student (Summer, 1983) from the University of Waterloo, Waterloo, Ontario. (3) Gordon, M. S.; Ware, W. R., Eds. "The Exciplex"; Academic Press: New York, 1975. (4) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312-8. (5) Mataga, N.; Ottolenghi, M. In "Molecular Association"; Foster, R., Ed.; Academic Press: New York, 1979; Vol. 2, pp 1-78. (6) Ottplenghi, M. Acc. Chem. res. 1973, 6 , 153-60. (7) (a) Saito, I.; Tamoto, K.; Matsuura, T. Tetrahedron Lett. 1979, 2889-92. (b) Schaap, A. P.; Lopez, L.; Gagnon, S. D. J . Am. Chem. SOC. 1983, 105, 663-4. (c) Eriksen, J.; Foote, C. S. J . Phys. Chem. 1978, 82, 2659-62. (d) Mattes, S. L.; Farid, S. J . Chem. SOC.,Chem. Commun. 1980, 457-8. ( e ) Reichel, L. W.; Griffin, G. W.; Muller, A. J.; Das, P. K.; Ege, S. Can. J. Chem. 1984, 62, 424-36. (f) Albini, A.; Fasani, E.; Oberti, R. Tetrahedron 1982, 38, 1027-34. (8) (a) Albini, A.; Arnold, D. A. Can. J . Chem. 1978, 26, 2985-93. (b) Hixson, S. S.; Boyer, J.; Galluci, C. J. Chem. SOC.,Chem. Commun. 1974, 540-2. (c) Das, P.K.; Muller, A. J.; Griffin, G. W. J . Org. Chem., in press. (9) (a) Mizuno, K.; Ishii, M.; Otsuji, Y . J . Am. Chem. SOC.1981, 103, 5570-2. (b) Arnold, D. R.; Borg, R. M.; Albini, A. J . Chem. SOC.,Chem. Commun. 1981, 138-9. (c) Maroulis, A. J.; Arnold, D. R. Zbid. 1979, 351-2. (d) Mattes, S. L.: Farid, S Acc. Chem. Res. 1982, 15, 80-6. (10) (a) Padwa, A.; Chou, C. S.; Reiker, W. F. J . Org. Chem. 1980, 45, 4555-64. (b) Arnold, D. R.; Humphreys, R. W. R. J. Am. Chem. SOC.1979, 101, 2743-4. (11) Arnold, D. R.; Maroulis, A. J. J . Am. Chem. SOC.1976, 98, 5931-7.
plexes. A knowledge of these parameters is important for a proper understanding of the mechanisms of electron-transfer-photosensitized organic reactions7-" in which a great deal of interest is noticeable presently.
Experimental Section 1,4-Dicyanonaphthalene (DCN) was synthesized from 1,4dibromonaphthalene (Eastman) by a procedure described for 1-cyanonaphthalene in the literature.I2 It was purified by treatment with charcoal followed by recrystallization (X4) from toluene. The methoxy- and methylbenzenes were of best grades commercially available; they were either fractionally distilled or recrystallized from benzene or ethanol. n-Heptane (Aldrich, spectral grade) was used as received. Chloroform (Aldrich) was distilled (twice) under nitrogen and stored under argon. Quinine bisulfate (Merck) was recrystallized (X2) from hot water. The absorption spectra were recorded in a Cary 219 spectrophotometer (1-nm bandpass). The emission spectral measurements were carried out in an SLM photon-counting spectrofluorimeter equipped with a double monochromator (MC 640) for excitation and two monochromators (MC 320) for monitoring emission at directions normal to that of the excitation. The excitation source was a 450-W xenon lamp (OSRAM XBO) and the detectors were comprised of EM1 9635 QA photomultiplier tubes. Further details regarding the spectrofluorimeter are available e1~ewhere.l~ Measurements of the quenching of DCN fluorescence or the enhancement of exciplex emission at varying quencher concentrations were carried in a ratio mode with respect to the emission output from a glycol solution of rhodamine B (quantum counter) excited by a constant fraction of the exciting light incident on the sample solution. Square quartz cells (1 cm X 1 cm) with optically flat surfaces were used for all emission spectral measurements. The laser flash photolysis apparatus has been described in detail in previous p u b l i ~ a t i o n s ' ~from J ~ this laboratory. A front-face excitation configuration was employed; the laser pulses (337.1 nm, 2-3 mJ/pulse, -8 ns) from a Molectron UV-400 nitrogen laser system intersected the analyzing light beam at -20'. The solutions for photolysis were kept in rectangular quartz cells with pathlengths of 2-3 nm along the monitoring beam. For actinometric measurements necessitating comparisons of transient ab(12) Newman, M. S. Org. Syn. 1941, 21, 89. (13) Chattopadhyay, S. K.; Das, P. K.; Hug, G. L. J . Am. Chem. SOC. 1982, 104, 4507-14. (14) (a) Das, P. K.; Encinas, M. V.; Small, Jr., R. D.; Scaiano, J. C.J . Am. Chem. SOC.1979, 101,6965-70. (b) Das, P. K.; Bhattacharyya, S. N. J . Phys. Chem. 1981, 85, 1391-5. (c) Das, P. K.; Bobrowski, K. J . Chem. SOC.,Faraday Trans. 2 1981, 77, 1009-27.
0022-3654/84/2088-279t?$01.50/00 1984 American Chemical Society
Photophysical Behaviors of Exciplexes
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2799 Scheme I 1 *
A
k 101
t
Q
-
22.
'A*
k-4
A f
Wavelength, nm
Figure 1. (A) Corrected emission spectra observed with DCN (0.2 mM) in n-heptane in the presence of varying concentrationsof anisole. [Anisole] = 0.0, 2.30, 4.60, 6.90, and 18.40 mM for the spectra a, b, c, d, and e, respectively. (B) Triplet-triplet absorption spectra of DCN (0.6 mM) in n-heptane observed at 0.5 ps following the laser flash.
sorbance changes (AOD), photolysis cells with optically flat surfaces were used. For determination of short emission lifetimes, use was made of the time-correlated photon-counting technique. A setup13from Photochemical Research Associates equipped with a thyratronoperated, hydrogen-filled spark lamp was employed to obtain experimental emission profiles; the best fit of the latter with profiles calculated by convolution of the system response function (lamp profile) based on one- or two-exponential decay functions gave the emission lifetimes. The deoxygenation of solutions was effected by bubbling oxygen-free argon (-20 min). In experiments where loss of solvent during degassing was undesirable, argon was presaturated with the solvent in use before passage through the sample solution.
Results Photophysical Parameters of DCN in n-Heptane. In n-heptane, DCN is characterized by moderately strong and fairly resolved = 354 nm) with the 0,O band located at 342 fluorescence (A,, nm (curve a in Figure 1A). The quantum yield of fluorescence (c@),measured with quinine sulfate (in 1 N H2S04) as the standard (& = 0.55)15is 0.41 f 0.06. The observed fluorescence lifetime (7;) is 3.4 f 0.4 ns, giving a value of 1.2 X 10's-I for the radiative rate constant (k,) for the excited-state singlet. Upon 337.1-nm laser flash photolysis, transient absorptions assignable to DCN triplet are observed with maxima at 455 and 275 nm, respectively (Figure 1B). A comparison of end-of-pulse transient absorbances (AOD) at 455 nm, observed with optically matched solutions of DCN in n-heptane and in acetonitrile gave a value of 0.36 & 0.05 for intersystem crossing efficiency (+?) in the former solvent. This was based on the previously measuredSCvalue of 0.19 for &' in acetonitrile and the assumption that the maximum extinction coefficients of triplet-triplet absorption a t 455 nm are equal (that is, (7 f 1) X lo3 M-I cm-1)8cin the two solvents. Thus, in n-heptane the intersystem crossing of DCN is nearly as important as the fluorescence, the rate constant for the former ( k 2 )being 1.1 X 10, s-'. Spectra, Quantum Yields, and Lifetimes of Exciplex Emissions. Upon pulsed (laser) or steady-state (lamp) excitation of an n-heptane solution of DCN in the presence of various methyland methoxy-substituted benzenes (10-3-10-2 M), blue-to-yellow emissions, usually intense, are observed. A typical case is illustrated by the spectra W in Figure 1A with anisole as the quencher (Q). The red-shifted emission increases in intensity as that of the monomer (DCN) fluorescence decreases at increasing quencher concentrations and is ascribed to the singlet exciplex (15) Melhuish, W. H. J . Phys. Chem. 1961, 65, 229-35
hv
3A*
-
A
A t Q t
'A*
hv
Q
k&l
t Q - A t
products
Q
formed via bimolecular interaction between DCN singlet and the quencher. The corrected exciplex emission maxima (v;,,,,) observed under 80-90% quenching of DCN fluorescence range from 399 nm for o-xylene to 565 nm for 1,2,4-trimethoxybenzeneSWith the latter quencber, the observed exciplex emission is very weak and had to be measured in the limit of the sensitivity of the spectrofluorimeter (16-nm band pass for emission). The dynamics of formation and decay of exciplexes ('A*Q) are described by Scheme I, where A represents DCN (acceptor, monomer). Kinetic treatments of the photophysical processes related with ,A* and IA*Q are described in detail in the literat ~ r e . ' ~ J 'Under steady-state conditions one obtains the following relationships:
eo/G' = 1 + ky7F[QI ky = k4(1/7$ -t ks[Q1)/(1/7$
f
k[Q1 +
(1) k-4)
(2)
In eq 1-3, and represent monomer fluorescence intensities in the absence and presence of the quencher (at [Q]), respectively, TP is the observed monomer fluorescence lifetime ( l / x ; = l k , ) in the absence of a quencher, 7; is the exciplex lifetime given by l/C:=,k,, 4: is monomer fluorescence yield, that is, k,~$',and && ds, and +F,obsd represent the integrated areas under the monomer fluorescence and the exciplex emission curves, respectively, in the presence of the quencher at [Q]. For all of the quenchers under study, Stern-Volmer plots of vs. [Q] based on eq 1 were found to be linear for 0-60% quenching, suggesting that the dependence of ky on [Q] implied in eq 2 is negligible. The insensitivity of ky to [Q] apparently arises from one or both of the conditions: k,[q]
