460
J. Phys. Chem. 1983, 87, 460-466
and that which occurred over the time of observation were much less for 3M0/5% 2MD/N20 than for 3M0/5% SQ/N20. This was indicated by a much smaller reduction in the value of Gem,,(3MO+) by adding 5% 2MD (AGcm,(3MO+) = 0.26 X lo4) than by adding 5% SQ (AGtm,(3MO+) = 0.96 X lo4). In addition, the rate of decay of 3MO+ was not measurably greater for the 2MD glass than for 3MO/N20. The relative inefficiency of charge transfer in 3M0/2MD/N20 is likely due to the small difference in ionization potentials between 3M0 and 2MD, compared to the larger difference between 3M0 and SQ. The rates of transfer for 3MP/2MD/N20 and 3MP/SQ/N20 are likely about the same, as indicated by
the similarity in growth and decay of the solute cations. This suggests that the difference in ionization potentials becomes less important as this difference increases. On the basis of the results described here and in our earlier work,l we expect IP3m> IP3M0> IPSm> IPW The total difference (IPsMp - IPSQ)is expected to be less than 1 eV on the basis of typical gas-phase values of ionization potentiaL2'
Acknowledgment. We gratefully acknowledge fruitful discussions with Dr. H. A. Gillis during this work. Registry No. 3MP, 2216-33-3; SQ, 111-01-3;3MO+., 6415602-1; SQ'., 79054-29-8;NZO, 10024-97-2.
Surface Photochemistry. The Effects of Coadsorbed Molecules on Pyrene Luminescence and Acenaphthylene Dimerization on Silica Gel' Richard K. Bauer,' Paul de Mayo,* Keijl Okada, William R. Ware," and Kam C. Wu3 Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 587 (Received: Ju/y 8, 1982; In Final Form: September 28, 1982)
The effects of coadsorbed molecules on the emission of pyrene and the dimerization of acenaphthylene have been studied on silica gel. Alkanols, acenaphthene, glycerol, malonic acid, and 1-adamantanol were the coadsorbates used. The stronger adsorbates apparently displace pyrene from the preferred adsorption sites, reduce the concentration of ground-state associated pairs, and permit the observation of the "growing-in" of the excimer emission. Coadsorption also appears to increase the acenaphthylene mobility and to change the polarity of the surface, as indicated by the cis/trans (C/T) dimer ratio in the rose bengal sensitized reaction.
Introduction The photochemistry and photophysics of molecules adsorbed on surfaces have received comparatively little attention and are poorly understood. The complex nature of surfaces and their varied interactions with adsorbates, coupled with many experimental difficulties unique to these systems, have made progress (1).Publication 283 from the Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Canada. (2)On sabbatical leave from N. Copernicus University, Torun, Poland. (3)Present address: 3M Co. Ltd., Oxford Street East, London, Canada. (4)C. H. Nicholls and P. A. Leermakers, Adu. Photochem., 8,315 (1971);M. Donati, M. Fiorenza, and P. J. Sarti-Fantoni, Heterocycl. Chem., 16,253(1979);E. F.Kiefer and D. A. Carlson, Tetrahedron Lett., 1617 (1967);J. Griffiths and H. Hart, J . Am. Chem. Soc., 90,5296(1968); D. Fassler, R. Gade, and W. Guenther, J. Photochem., 13,44(1980);Z. Chem., 20,229 (1980),and papers there cited; G. Kortum and K. Braun, Ann., 632,104 (1960). (5)H. Werbin and E. T. Stron, J . Am. Chem. Soc., 90,7296 (1968); C.Balny and P. Douzou, C. R. Hebd. Seances. Acad. Sci., Ser. c, 264,417 (1967);C. Balny, K. Djaparidze, and P. Douzou, ibid., 265,1148 (1967). (6)K. Otsuka, M. Fukaya, and A. Morikawa, Bull. Chem. SOC.Jpn., 51 367 (1978);K. Otsuka and A. Morikawa, ibid., 47, 2335 (1974);M. Anpo, T.Wada, and Y. Kubokawa, ibid., 50,31 (1977),and earlier papers in this series. (7)H. Moesta, Discuss. Faraday Soc., 58,244(1974);C. Eden and Z. Shaked, Isr. J. Chem., 13, 1 (1975);H. G. Hecht and L. R. Crackel, J . Photochem., 15, 263 (1981);H. G. Hecht and J. L. Jensen, ibid., 9,33 (1978);D. Oelkrug, M. Radjaipour, and H. Erbse, Z. Phys. Chem. (Frankfurt am Main), 88, 23 (1974);D. Oelkrug, H. Erbse, and M. Plauschinat, ibid., 96,283 (1975);D.Oelkrug, G. Schrem, and L. Andra, ibid., 106,197(1977);D. Oelkrug, M. Plauschinat and R. W. Kessler, J . Lumin. 18/19,434(1979);D. Oelkrug and M. Radjaipour, Z. Phys. Chem. (Frankfurt am Main), 123,163 (1980);R. W. Kessler and F. Wilkinson, J. Chem. Soc., Faraday Tram. 1,77,309(1981);H.Ishida, H. Takahashi, and H. Tsubomura, Bull. Chem. SOC.Jpn., 43,3130 (1970);I. Barthel, H. Dunken, A. Kohler, S. Schinkothe, and J. Shnieke, Wiss. Z.Friedrich-Schiller-~niu.Jena, Math-Naturwiss. Reihe, 25,867(1976). 0022-365418312087-0460$0 1.5010
We have recently demonstrated8-" the occurrence of intra- and intergranular motion of aromatic hydrocarbons on dry silica gel, the former on time scales set by singlet or triplet lifetimes or that of singlet and triplet radical pairs.12J3 Evidence to this effect was drawn inter alia from emission and excitation spectroscopy. We have also studied the photophysics of pyrene adsorbed on various substrates and have obtained persuasive evidence for a ground-state association which serves as a precursor to an excited dimer. This paper deals specifically with the influence of coadsorbed species on both the photophysics and photochemistry of adsorbed molecules. Incentive for these studies was provided by two observations: (a) pyrene emission in the excimer region exhibits a growing-in characteristic of dynamic exciplex formation only in the presence of coadsorbed molecules such as long-chain alcohols. (b) The cis/trans ratio for the photodimerization of acenaphthylene (1) shows a strong dependence upon the presence and concentration of coadsorbed species. Since the very short-lived singlet is thought to give rise, almost entirely, to the cis dimer (2), the triplet giving both cis and (8)P. de Mayo, K. Okada, M. Rafalska, A. C. Weedon, and G. S. K. Wong, J . Chem. Soc., Chem. Commun., 820 (1981). (9)R. K. Bauer, R. Borenstein, P. de Mayo, K. Okada, M. Rafalska, W. R. Ware, and K. C. Wu, J . Am. Chem. Soc., 104,4635 (1982). (10)R. K. Bauer, P. de Mayo, W. R. Ware, and K. C. Wu, J . Phys. Chem., 86,3781 (1982). (11)K. Hara, P. de Mayo, W. R. Ware, A. C. Weedon, G. S. K. Wong, and K. C.Wu, Chem. Phys. Lett., 69,105 (1980). (12)D. Avnir, P. de Mayo, I. Ono, J . Chem. Soc., Chem. Commun., 1109 (1978). (13)D. Avnir, L. J. Johnston, P. de Mayo, and S. K. Wong, J . Chem. Soc., Chem. Commun., 958 (1981).
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 3, 1983 461
Surface Photochemistry
trans (3), the cis/trans ratio was considered to provide, potentially, significant insight into the dynamics of motion on the surface, especially in view of the demonstrated possibility of sensitizing the dimerization with coadsorbed dyes. Paramagnetic resonance measurements had previously indicated an effect of an additive on the mobility of a stable radical adsorbed on silica gel and alumina.14J5 I t was found that the presence of coadsorbates increased the mobilities, particularly the rotational mobilities, of the radical. However, in the paramagnetic resonance investigation the silica gel and alumina surfaces were implicitly assumed to be homogeneous and the effect of additives on the homogeneity of the surface was not considered. This paper examines in detail the consequences of these two observations and also reports on the results of several experiments with 1,3-bis(l-pyrenyl)propane(BP).
Experimental Section Materials. Silica gel (35-70 mesh, Merck) was activated by heating at 200 "C under vacuum (0.3 torr) for 5-7 h before use. Pyrene was purified by passing through a silica gel column (mp 156 "C). 1,3-Bis(l-pyrenyl)propanewas obtained from Molecular Probes Inc. and was used directly without any purification. Acenaphthylene was purified by decomposition of the corresponding picrate, followed by sublimation and recrystallization from ethanol (mp 94-95 "C). 1-Heptanol and 1-decanol were distilled before use. 1-Hexadecanol was recrystallized from benzene-ethanol mixture (mp 48-49 "C). Acenaphthene and 1-adamantanol were recrystallized from ethanol (mp 93-94, -290 "C, respectively). Cyclohexane and methylene chloride was spectral grade. For emission spectra and kinetics studies, cyclohexane was further purified by passing through a silica gel column. The silica gel surface area (560 m2/g) was determined by the method of Hoffman et a1.16 Monolayer coverage was -370, -100, and 195 mg/(g of silica gel) for acenaphthylene, pyrene, and 1-decanol,respectively. Photophysical Studies. Samples were prepared by adding silica gel to cyclohexane solutions of the molecules to be adsorbed. The cyclohexane solvent was slowly removed at about 40 "C with a rough vacuum line. About 0.5 g of the sample was degassed at 60 "C torr) for about 40 min. After degassing, the sample was sealed in a 2-mm Suprasil sample cell. Emission and excitation spectra were taken with a Perkin-Elmer Hitachi MPF-4 spectrophotometer equipped with a special cell holder. The emission from the front face of the Suprasil cell was monitored for both steady-state and pulse measurements. Emission decay profiles were measured with a PRA (Photochemical Research Associates, London, Ontario) Model 3000 nanosecond fluorometer. Most of the samples were not shaken. Some were tumbled overnight, but there was no significant difference between the tumbled and undisturbed samples. Photochemical Studies. Direct Irradiation of Acenaphthylene in the Presence of Additives. Acenaphthylene (24.0 mg) and a suitable amount of additive (typically 0.19, 0.38, or 0.58 mmol) in a cyclohexane solution were coadsorbed on 500 mg of silica gel by evaporating the solvent in a special cell described el~ewhere.~ The surface coverage of acenaphthylene corresponds to 13%. The sample was degassed under vacuum torr), filled with
-
(14) V. I. Eweinov, V. G . Golubev, and Z. V. Lunina, Russ. J . Phys. Chem. (Engl. Transl.), 49, 564 (1975). (15) A. K. Selivanowskii,R w s . J. Phys. Chem. (Engl. Traml.),50,990 (1976). (16) R. L. Hoffman, D. G. McConnell, G. R. List, and C. D. Evans, Science, 157, 590 (1967).
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