J. Phys. Chem. 1984,88, 2964-2970
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surface interaction with the hydroxyl groups than in the 4-BZP. The IR spectra showed shifts in the carbonyl vibration consistant with those observed in the phosphorescence spectra. We were unable to resolve shifts in the carbon-nitrogen vibration in 3- and 4-BZP upon adsorption. However, in the difference spectra, in which the alumina absorption spectrum was subtracted from the spectra of the adsorbed molecules, the OH absorption at about 3730 cm-' appeared as a positive going band, which indicates a shift in the intensity of the OH band upon adsorption by the adsorbate. We are doing a more thorough band-to-band analysis of the absorption spectra. The eventual aim of this study is to quantitatively correlate the zero field triplet state parameters to models which describe molecules adsorbed on surfaces. For this purpose, we are currently looking to other spectroscopic techniques including CP/MAS N M R and further work with diffuse reflectance IR to supplement the ODMR results. The wide distribution of orientations of large molecules adsorbed on surfaces makes it difficult to yield precise information regarding molecular geometry and orientation. However, because of the sensitivity of the ODMR technique to detect the small number of spins, the method appears to hold promise in the study of this and other adsorbed species on surfaces.
Conclusion The sensitivity of optically detection methods yields sufficient signal-to-noise ratio to allow even small numbers of molecules such as those encountered on surfaces to be accessible to study. At the least, these results indicate that some orientational information may be postulated as a result of ODMR studies of molecules adsorbed on surfaces. Acknowledgment. Acknowledgment is made to the donors of the Petroleium Research Fund, administered by the American Chemical Society, for the support of this research. K.A.M. was supported by a ACS-PRF Summer Research Fellowship. We also thank Professor Dino S. Tinti for the high-resolution phosphorescence spectra of the samples of adsorbed molecules taken at liquid helium temperatures. In addition, we are grateful to Dr. W. D. Perkins of the Perkin-Elmer Corporation for the diffuse reflectance FTIR spectra. Initial work on the isotherms was done by David Lockhart and Eric Johnson. Registry No. BP, 119-61-9; Cl,BP, 90-98-2; Br2BP, 3988-03-2; Me2BP,61 1-97-2; DPK, 19437-26-4; 2-BZP, 91-02-1; 3-BZP, 5424-19-1; 4-BZP, 14548-46-0; alumina, 1344-28-1.
Intramolecular Exciplex Formation and Fluorescence Quenching as a Function of Chain Length in o-Dimethylaminoalkyl Esters of 2-Anthracenecarboxylic Acid Paul Vanderauwera,t Franz C. DeSchryver,t Albert Weller,$ Mitchell A. Winnik,s and Klaas A. Zachariasse*$ Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F. B-3030 Heverlee, Belgium, Max- Planck- Institut fur biophysikalische Chemie, Abteilung Spektroskopie, 0 - 3 4 0 0 Gottingen. West Germany, and Erindale College and Lash Miller Laboratories, Department of Chemistry, University of Toronto, Canada MSS 1 A1 (Received: October 28, 1983)
The kinetics of intramolecular exciplex formation and electron transfer in the series of w-dimethylaminoalkyl esters of 2-anthracenecarboxylic acid, A(n)D with n = 2, 3, 5, 9, and 11, has been studied by time-correlated single-photon counting. The occurrence of exciplex formation with n = 9 and 11 (in a nonpolar solvent, methylcyclohexane (MCH)) as well as the value of the reaction parameters of intramolecular electron transfer for all n (in a polar solvent, acetonitrile) is determined by the number of the methylene groups ( n ) in the chain. The chain-length dependence can be explained on the basis of the difference in free energy AG between the exciplex (MCH) or the solvent-separated radical ion pair 'A--?DD+(acetonitrile) and the locally excited state 'A*(n)D. This energy difference is a function of n and depends strongly on the polarity of the solvent. In acetonitrile, the singlet excited state of the anthroate moiety of A(n)D is regenerated by thermal back electron transfer from the primarily produced solvent-separated ion pair. The results show that electron transfer can indeed occur over the relatively large distances (of the order of 0.9 nm) between A and D prevailing in A(2)D and A(3)D.
Introduction When two molecules are connected by an oligomeric chain as in the w-dimethylaminoalkyl esters of 2-anthracenecarboxylic acid, 2-ACOO(CH2),N(CH3)2,A(n)D, this chain serves two functions. In the first place, by holding the chromophores together, it can promote rapid intramolecular reactions at low overall concentrations (
