1344 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Znorg. zyxwvutsr Chem. 1994, 33, 1344-1347 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK Ligand-Specific Charge Localization in the MLCT Excited State of Ru(bpy)~(dpphen)~+ Monitored by Time-Resolved Resonance Raman Spectroscopy Claudia Turro,t Stefan H. Bossmann,* zyxwvu George E. Leroi,s Jacqueline K. Barton,ll and Nicholas J. Turro'*+ Department of Chemistry, Columbia University, New York, New York 10027, and Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 1125 Received May 7, 1993' Time-resolved resonance Raman spectroscopy has been employed to examine the location of the promoted electron in the metal-to-ligand charge-transfer (MLCT) excited state of R~(bpy)~(dpphen)~+ (bpy zyxw = 2,2'-bipyridine; dpphen = 4,7-diphenyl-1,lO-phenanthroline). Variations in the environment about Ru(bpy)z(dpphen)2+ shift the localization of charge in the MLCT excited state from bpy in neutral micelles (Brij 35) to dpphen in the presence of DNA and anionic surfactants ( CI~H~~OSO~N~, C10H230S03Nar and CsH210SO3Na), whereas in water the electronis localized on both ligands. The shifts in the electronic absorption spectrum and the dependence of the ground-state resonance Raman (rR) signal with excitation wavelengths coincident with the high- and low-energy sides of the MLCT absorption band are consistent with a lowering of the energy of the Ru(II)-dpphen transition with respect to that of bpy in anionic micelles. Introduction Molecular probes whose photophysical properties are sensitive to their immediate environment are of great interest, since their interactions with microheterogeneous systems can provide dy- namic and static informationconcerningthe organized medium.1-4 Heterogeneous microenvironments, such as those in micelles, cyclodextrins, and DNA,' may possess hydrophobic and hydro- philic spaces and may be ionic or neutral. It has been demon- strated that micelles and DNA are able to increase the luminescence quantum yield and lifetimes of many Ru(I1) c~mplexes,~-~ and in the case of DNA stereoselectiveinteraction may be ~perative.~ A potential probe of stereoselective inter- actions with DNA is R~(bpy)z(dpphen)~+ (bpy = 2,2'-bipyridine; dpphen = 4,7-diphenyl-1,lo-phenanthroline), which has been shown by time-resolved resonance Raman (TR3)spectroscopy to exhibit localization of the promoted electron on both bpy and dpphen in water owing to the similar energies of the two ligands.* It was recently established that localization in the metal-to- ~~~ ~ ~ * To whom correspondence should be addressed. t Columbia University. Present address: Engler-BunteInstitut,Universitat Karlsruhe, Karlsruhe, Germany. 48824. I Department of Chemistry, Michigan State University, East Lansing, MI I California Institute of Technology. Abstract published in Advance ACS Abstracts, March 1, 1994. (1) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: Orlando, FL, 1987. (2) Turro, N. J.; Barton, J. K.; Tomalia, D. A. Acc. Chem. Res. 1991, 24, (3) Fox, M. A., Channon, M., Eds. Photoinduced Electron Transfer; Elsevier: Amsterdam, 1988; Vol. 2. (4) Gritzel, M. Heterogeneous Electron Transfer; CRC Press: Boca Raton, FL, 1989. (5) (a) Snyder, S. W.; Buell, S. L.; Demas, J. N.; DeGraff. B. A. J. Phys. Chem. 1989,93,5265-5271. (b) Hauenstein, B. L. J.; Dressick, W. J.; Gilbert, T. B.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1984,88, 1902-1905. (c) Dressick, W. J.; Hauenstein, B. L. J.; Gilbert, T. B.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1984,88,3337-3340. (d) Hauenstein, B. L. J.; Dressick, W. J.; Buel1,S. L.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. SOC. 1983, 105, 4251-4255. (6) Gopidas, K. R.; Leheny, A. R.; Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. SOC. 1991, 113, 7335-7342. (7) (a) Jenkins,Y.;Friedman,A. E.;Turro,N. J.;Barton, J. K.Biochemistry 1992,31,10809-10816. (b) Ottaviani, M. F.;Ghatlia, N. D.; Bossmann, S. H.; Barton, J. K.; Dtirr, H.; Turro, N. J. J. Am. Chem. zyxwvutsr Soc. 1992, 114,8946-8952. (c) Kirsch-DeMesmaeker, A.; Orellana, G.; Barton, J.K.;Turro,N. J. Photochem. Phorobiol. 1990,52,461-472. (d) Barton, J. K. Science 1986,233, 727-734. 332-340. 0020-1669/94/1333-1344$04.50/0 ligand charge-transfer (MLCT) excited state of Ru(bpy)s2+ and other bpy-containing chromophores is intrinsic to the complex and is not a solvent-driven p h e n o m e n ~ n . ~ J ~ The mechanism for localization has been explained by utilizing Raman data." However, we report here the selective localization of the excited electron in Ru(bpy)~(dpphen)~+ on either bpy or dpphen, dictated by distinct solvation environments about each ligand. The perturbations of the MLCT excited state of Ru(bpy)2(dpphen)2+ were observed in the presence of DNA as well as anionic and neutral surfactants and may be attributed to the difference in hydrophobicity between bpy and dpphen. Since the initial determination of the localized nature of the lowest MLCT excited state of R ~ ( b p y ) 3 ~ + , ' ~ J ~ numerous studies have been conducted on this and related systemsO1&l6 Time- resolved resonance Raman (TR') spectroscopy has proven to be a powerful tool in the characterization of electron localization in the MLCT excited state of Ru(I1) diimine comple~es.'~-2~ As (8) Kumar, C. V.; Barton, J. K.; T w o , N. J.; Gould, I. R. Inorg. Chem. (9) DeArmond, M. K.; Myrick, M. L. Inorg. Chem. 1989, 28, 981-982. (10) (a) Myrick, M. L.; DeArmond, M. K.; Blakley, R. L. Inorg. Chem. 1989,28,4077-4084. (b) Myrick, M. L.; Blakley, R. L.; DcArmond, M. K. J. Am. Chem. SOC. 1987, 109,2841-2842. (1 1) Turro, C.; Wagner, P. J.; Leroi, G. E. 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