J. Am. Chem. SOC.1981, 103, 7235-1239
Quenching of Singlet Excited States of Electron-Deficient Anthracenes by Allyl- and Benzyltrialkylstannanes+ David F. Eaton Contribution No. 2898 from the E. I . du Pont de Nemours & Co., Inc.. Central Research and Development Department, Experimental Station, Wilmington. Delaware 19898. Received April 30, 1981. Revised Manuscript Received July 16, 1981
Abstract: Allyl- and benzyltrialkylstannanes quench the fluorescent singlet states of several meso-substituted anthracenes. Intermediacy of a charge-stabilized exciplex is supported by correlation of fluorescence quenching rate constants with anthracene reduction potential for the constant quencher benzyltrimethylstannane and with the ionization potential of the quencher stannane for the constant fluorescor 9,lO-dicyanoanthracene (DCA). Quenching of DCA by a series of para-substituted (X = C1, F, H, CH3, OCH3) benzyltri-n-butylstannanes can be analyzed by a Hammett treatment ( p = -1.4 vs. up). Quantum yield measurements of DCA disappearance as a function of both the nature of X and the quencher concentration provide rate constants for product-forming channels during exciplex decay. Limiting quantum yields for product formation are shown to increase with increasing electron-withdrawing nature of X while the rate constant for product formation is nearly invariant with X. Rate constants for radiationless return to unchanged ground states increase dramatically as X becomes electron releasing. A rationale for all observed data in terms of reversible exciplex formation and subsequent decay is presented.
Bimolecular photophysical interactions between donor-acceptor (D-A) pairs, mediated by excited state complex (exciplex) formation, have proved to be pervasive. Charge-transfer (CT) stabilization has been shown to be important during deactivation of electron-deficient excited states (A*) with such diverse groundstate donors as amines,' phosphines,2 thiol^,^ and sulfides4 and electron-rich aromatics,s dienes: olefins,' and hydrocarbons.* In many cases, clear correlations between donor oxidation or ionization potential and rate constants for overall deactivation of A* have been obtained and cited as evidence in favor of intervention of CT-stabilized exciplexes. In other instances, ion radical intermediates have been directly observed spectroscopically. Few systems, however, have provided direct chemical (product) evidence in favor of full electron transfer during exciplex decay. Group 4A organometallics, particularly those containing M-C u bonds 0 to a neighboring ?r system, have been shown to be exceedingly electron rich. Photoelectron spectroscopic studiesg indicate that ionization potentials can be in the range typical of conventional donors effective in CT exciplex schemes, and many such organometallics readily form ground-state C T complexes with electron-deficient acceptors such as TCNE.I0 In ground-state chemistry, the facile reactions of organosilicon, -tin, -germanium, and -lead species with electrophiles have been elegantly shown to proceed by way of charge-transfer intermediates.'' This paper presents details of a study of the deactivation of electron-deficient excited singlet states, particularly 9,lO-dicyanoanthracene (DCA), by allylic and benzylic organostannanes. Steady-state analysis of fluorescence quenching and photochemical product formation as a function of quencher concentration allows nearly complete dissection of the exciplex-mediated decay kinetics of these systems and reveals several unusual facets of the processes which occur.1z
Results Fluorescence Quenching. Rate constants (k,f) for overall fluorescence quenching of singlet DCA* in benzene by the lifetime method are listed in Table I for a variety of organotins. Also listed in Table I are gas phase vertical ionization potential data for the various quenchers; adiabatic values are uniformly about 0.4 eV lower.I0 Generally, ks' is seen to depend inversely on ionization potential. Thus the following trend (compound, k,f, IP): PhCH2SnMe3, 10.1 X lo9 M-' s-I, 8.1 eV > PhCH2SnPh3,4.3 X lo9 M-l s-', 8.5 eV allylSnMe3, 5.1 X lo9 M-' , 8.5 eV > Me&, I X lo7 M-I S-', 9.8 eV. Steric effects counter the effect of increased ease of
'This paper is dedicated to George S. Hammond on the occasion of his 60th birthday.
Table 1. Rate Constants for Quenching of 9,lO-Dicyanoanthracene Fluorescence by Organotin Compounds k,
(M-'s-', ~ 1 0 . ~I,' ) ~(eV)b 0.07 9.8 4.3 8.4 1.9 1.9 10.1 8.1 5.1 8.5
PhCH,SnPh, PhCH,Sn(n-Bu), PhCH2SnMe, allylSnMe, allylSn(n-Bu), a In benzene by lifetime method. tial.
Vertical ionization poten-
Table 11. Rate Constants for Quenching of 9,lO-Dicyanoanthracene Fluorescence by Para-Substituted Benzyltri-n-butylstannanes, p-XC, H,CH, Sn(n-C,H9),
c1 F H a
In benzene by lifetime method.
ionization, however, in both the benzyl and allyl series: PhCH2SnMe3, 10.1 X lo9 M-I SKI,8.1 eV > PhCH2Sn(n-Bu)3, (1) (a) Kuzmin, M. G.; Guseva, L. N. Chem. Phys. Lett. 1969,3, 71. (b) Van, S.-P.; Hammond, G. S. J . Am. Chem. SOC.1978, 100, 3895. (2) Marcondes, M. E. R.; Toscano, V. G.; Weiss, R. G. J . Am. Chem. Soc.
1975. 97. 4485. ( 3 ) Murakami, Y.; Aoyama, Y.; Tokunaga, Commun. 1979, 1018.
K. J . Chem. SOC.,Chem.
(4) Brimage, D. R. G.; Davidson, R. S.; Lambeth, P. F. J . Chem. SOC.C
1971 . -, 1741
(5) Leonhardt, H.; Weller, A. Z . Electrochem., 1963, 67, 791. (6) Stephenson, L. M.; Hammond, G. S. Pure Appl. Chem. 1968,16, 125. (7) Taylor, G. N. Chem. Phys. Lett. 1971, 10, 355. (8) (a) Murov, S. L.; Cole, R. S.; Hammond, G. S. J . Am. Chem. SOC. 1968, 90, 2957. (b) Murov, S.; Hammond, G. S. J . Phys. Chem. 1968, 72, 3797. (9) (a) Brown, R. S.; Eaton, D. F.; Hosomi, A,; Traylor, T. G.; Wright, J. M. J . Organomet. Chem. 1974,66, 249. (b) Schweig, A.; Weidner, U. Ibid. 1974, 67, C4. (c) Hosomi, A,; Traylor, T.G. J . Am. Chem. SOC.1975, 97, 3682. (10) (a) Traylor, T. G.; Berwin, H. J.; Jerkunica, J.; Hall, M. L. Pure Appl. Chem. 1972, 30, 599. (b) Fukuzumi, S.; Mochida, K.; Kochi, J. K. J . Am. Chem. SOC.1979, 101, 5961. (c) Reutov, 0. A,; Rozenberg, V. I.; Gavrilova, G. V.; Nikanorov, V. A. J. Organomel. Chem. 1979,177, 101. (d) Eaton, D. F. J . Am. Chem. SOC.1980, 102, 3278.
0 1981 American Chemical Society
1236 J. Am. Chem. SOC.,Vol. 103, No. 24, 1981 +.6r
x u \
x u 1 Y
Table 111. Rate Constants for Quenching of Substituted Anthracene Fluorescence by Benzyltrimethylstannane anthracene 9,lO-dicyano 9-cyano 9,lO-dibromo 9,lO-dichloro anthracene 9-m eth y1 9,lO-dimethyl
E',,*a 2.57 2.18 2.17 1.95 1.78 1.69 1.57
(M-' s-', ~ 1 0 - ~ ) b 7.69 0.146 0.339 0.027 0.006' 0.997. Quenching rate constants were calculated from the slopes by using the measured lifetime of the fluoror. Quantum yields were measured by using uranium glass filters (A >410 nm) and ferrioxalate actino~netry.~~ Solutions in benzene containing 9,lO-dicyanoanthracene (OD at 426 nm -3) and added tin compound ([Q] = 0.001-0.05 M) were photolyzed in a merry-go-round for various times. Aliquots were removed and diluted and the loss of 9,10-DCA determined spectrophotometrically, Do = od initial, D, = od at time t . Plots of log (Do/D,)vs. t were linear to ca. 80% reaction. From the plots, pseudorate constants for DCA loss were obtained and quantum yields calculated from these and the light intensity. Values were reproducible to *lo%. Isolation of Photoproducts. A benzene solution ( 1.O L) of DCA ( 1.14 g, 5 mmol) and PhCH2SnBup(5.10 g, 20 mmol, 0.02 M) was irradiated, with continuous nitrogen sparging, with a Hanovia 450 W mediumpressure lamp through a Pyrex sleeve (A >290 nm). The degree of fluorescence quenching under these conditions is roughly 55%. The mixture was irradiated for 4.75 h. After concentration, the residue was triturated with cold methanol and filtered to give 0.42 g (37%) of recovered DCA. The filtrate was concentrated and chromatographed (650 g activity I neutral alumina, 5.8 X 35 cm), eluting with CHCl,/petroleum ether (5% CHCI3increasing to 50%) to give in order of elution: recovered benzylstannane (3.6 g, 70%), a mixed fraction (mixture A) of hexabutyldistannane and bibenzyl (1.60 g), and a second mixed fraction (mixture B) containing cyanoanthracenes. Mixture A was rechromatographed on silica gel with petroleum ethers to give hexabutyldistannane (1.41 g) and bibenzyl (.008 g). Mixture B was rechromatographed on silica to give 9-benzyl-10-cyanoanthracene (0.242 g, mp 148-151 OC, 163 O C after ethanol recrystallization; lit.%mp 166 "C), and 20 mg of an oily solid whose IR and NMR suggest the structure 9-cyano-IO-(tri-n-buty1)stannylanthracene: IR (KBr) 2230 (CN); NMR (CDC13/Me4Si) 6 8.4-8.2 and 7.8-7.5 (m, 8 H), 2 . M . 8 (m, 27 H). All known compounds exhibited appropriate physical and spectroscopic characteristics.
Acknowledgment. Technical assistance was provided by J. T. Horgan and J. W. Lockhart. I thank George S. Hammond for stimulating and critical comments during early stages of this work. (27) Davies, A. G.; Roberts, B. P.; Smith, J. M. J . Chem. Soc., Perkin Trans. 2 1972, 2221. (28) Tamborski, C.; Ford, F. E.; Lehn, W. L.; Moore, G. J.; Soloski, E. J. J . Org.Chem. 1962, 27, 619. (29) Hatchard, C. G.; Parker, C. A. Proc. R. SOC.London, Ser. A 1956, A235, 518. Cf.: Murov, S. L. "Handbook of Photochemistry"; Marcel Dekker: New York, 1973; pp 119-23. (30) Rio, G.; Sillon, B. Bull. Soc. Chim. Fr. 1961, 831.