Quenching of n,π*-Excited States in the Gas Phase: Variations in Absolute Reactivity and Selectivity Dieter Klapstein, ² Uwe Pischel, ‡ and Werner M. Nau* ,‡ Contribution from the Department of Chemistry, St. Francis XaVier UniVersity, Antigonish, NoVa Scotia B2G 2W5, Canada, and the Department of Chemistry, UniVersity of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Received March 18, 2002. Revised Manuscript Received June 26, 2002 Abstract: The quenching of the n,π*-excited azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene by 19 heteroatom- containing electron and hydrogen donors, that is, amines, sulfides, ethers, and alcohols, was investigated in the gas phase. Deuterium isotope effects were measured for 9 selectively deuterated derivatives. The data support the involvement of an excited charge-transfer complex, that is, an exciplex, for tertiary amines and sulfides, and a competitive direct hydrogen transfer from the C-H bonds of ethers or from the N-H or O-H bonds of secondary and primary amines or alcohols. The recently observed “inverted” solvent effect for the fluorescence quenching of azoalkanes by amines and sulfides in solution is supported by the observed rate constants in the gas phase, which are substantially larger than those in solution. A more pronounced inverted solvent effect for the weaker electron-donating sulfides and a presumably faster exciplex deactivation result in a switch-over in absolute reactivity relative to tertiary amines in the gas phase. Most importantly, the kinetic data demonstrate that the reactivity of the strongly dipolar O-H and N-H bonds in photoinduced hydrogen abstraction reactions shows a larger decrease upon solvation than that of the less polar C-H bonds. The azoalkane data are compared with previous studies on quenching of n,π*-triplet- excited ketones in the gas phase. Introduction The photoreactions of n,π*-excited ketones and azoalkanes with amines, sulfides, ethers, and alcohols can be understood as a competition between charge transfer (CT) to form exci- plexes or radical ion pairs and hydrogen transfer to form radical pairs. 1-7 Solvent effects have been investigated to elucidate the relative contributions of the various reaction pathways and, in particular, to test for the formation of radical ion pairs, which should be strongly disfavored in nonpolar solvents. We have recently communicated an “inverted” solvent effect for the fluorescence quenching of 1 n,π*-excited 2,3-diazabicyclo[2.2.2]- oct-2-ene (DBO) by amines. 7-9 Although this photoreaction involves the formation of exciplexes with partial CT, 6-10 it is accelerated in nonpolar solvents because the highly dipolar excited chromophore experiences a relative stabilization in polar solvents, thereby retarding the quenching process. 8 The importance of solvent effects for the mechanistic understanding of the quenching of n,π*-excited states encour- ages the study of these basic photoreactions in the absence of solvation, that is, in the gas phase. However, the study of quenching of n,π*-excited states in the gas phase has been restricted to case studies. 11-18 Moreover, the vapor pressure of benzophenone, which is the most extensively studied ketone in solution, is too low to allow gas-phase studies at room temperature. We have presently employed DBO to gain further insight into gas-phase photoreactivity. The strongly fluorescent DBO is photophysically well characterized 19 and sufficiently volatile even at room temperature to permit gas-phase quenching studies, the first of which were reported by Steel. 20 This provides the exceptional opportunity to study quenching in both the gas phase and in solution under identical conditions with standard laser-flash photolysis equipment. Moreover, the fluorescence lifetime of DBO in the gas phase amounts to 1 μs, which opens * To whom correspondence should be addressed. E-mail: Werner.Nau@ unibas.ch. ² St. Francis Xavier University. ‡ University of Basel. (1) Scaiano, J. C. J. Photochem. 1973, 2, 81-118. (2) Wagner, P. J.; Truman, R. J.; Puchalski, A. E.; Wake, R. J. Am. Chem. Soc. 1986, 108, 7727-7738. (3) Hubig, S. M.; Rathore, R.; Kochi, J. K. J. Am. Chem. Soc. 1999, 121, 617-626. (4) Coenjarts, C.; Scaiano, J. C. J. Am. Chem. Soc. 2000, 122, 3635-3641. (5) Pischel, U.; Nau, W. M. J. Am. Chem. Soc. 2001, 123, 9727-9737. (6) Pischel, U.; Nau, W. M. J. Phys. Org. Chem. 2000, 13, 640-647. (7) Pischel, U.; Zhang, X.; Hellrung, B.; Haselbach, E.; Muller, P.-A.; Nau, W. M. J. Am. Chem. Soc. 2000, 122, 2027-2034. (8) Nau, W. M.; Pischel, U. Angew. Chem., Int. Ed. 1999, 38, 2885-2888. (9) Sinicropi, A.; Pischel, U.; Basosi, R.; Nau, W. M.; Olivucci, M. Angew. Chem., Int. Ed. 2000, 39, 4582-4586. (10) Pischel, U.; Allonas, X.; Nau, W. M. J. Inf. Rec. 2000, 25, 311-321. (11) Rebbert, R. E.; Ausloos, P. J. Am. Chem. Soc. 1965, 87, 5569-5572. (12) Abuin, E. B.; Encina, M. V.; Lissi, E. A.; Scaiano, J. C. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1221-1229. (13) Berger, M.; Camp, R. N.; Demetrescu, I.; Giering, L.; Steel, C. Isr. J. Chem. 1977, 16, 311-317. (14) Lendvay, G.; Be ´rces, T. J. Photochem. Photobiol. A 1987, 40, 31-45. (15) Borisevich, N. A.; Karberuk, D. V.; Lysak, N. A.; Tolstorozhev, G. B. Zh. Prik. Spektrosk. 1989, 50, 745-749. (16) Matsushita, Y.; Kajii, Y.; Obi, K. J. Phys. Chem. 1992, 96, 4455-4458. (17) Matsushita, Y.; Yamaguchi, Y.; Hikida, T. Chem. Phys. 1996, 213, 413- 419. (18) Zalesskaya, G. A.; Baranovskii, D. I.; Sambor, E. G. J. Appl. Spectrosc. 1999, 66, 76-80. (19) Nau, W. M.; Greiner, G.; Rau, H.; Wall, J.; Olivucci, M.; Scaiano, J. C. J. Phys. Chem. A 1999, 103, 1579-1584. (20) Solomon, B. S.; Thomas, T. F.; Steel, C. J. Am. Chem. Soc. 1968, 90, 2249-2258. Published on Web 08/30/2002 10.1021/ja020400h CCC: $22.00 © 2002 American Chemical Society J. AM. CHEM. SOC. 2002, 124, 11349-11357 9 11349