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 Plenary Lecture Abstract 

Stephen F. Nelsen

   65th Birthday Symposium
Madison, Wisconsin
June 3-4, 2005

Jeffrey Zink

Department of Chemistry and Biochemistry, University of California, Los Angeles , California 90095 USA

Spectroscopic Consequences of Excited State Coupling:  Excited State Mixed Valence

Excited state mixed valence exists when a system possesses two or more interchangeably equivalent sites that have different oxidation states in an excited electronic state but a symmetrical charge distribution in the ground electronic state. Some of the most commonly encountered molecular systems having these characteristics contain  

two identical charge bearing units M separated by a bridge B and are represented by the generic M-B-M symbol. The mixed valence excited state occurs when the charge is transferred from the center to one M or the other which produces two localized configurations: M^—-B⁺-M and M-B⁺-M^—. There are two excited state surfaces separated by twice the excited state coupling element H_ab^ex.  Because the two M groups are coupled to each other through the bridge, the excited state can be modeled using the language of Marcus-Hush theory, usually used to describe ground state intervalence behavior.  The spectroscopy, however, is significantly different because the electronic transitions occur from the  M-B-M ground state to the coupled M^—-B⁺-M and     M-B⁺-M^— excited states, whereas for ground state intervalence the transitions occur within the coupled M^—-B⁺-M/M-B⁺-M^— system.  As a result of the fruitful collaboration between the UW and UCLA groups, new spectroscopic features that arise from the ground to coupled excited state transition have been interpreted.  These features will be illustrated by the spectra of the diphenyl hydrazine radical cation 1⁺ (22/Ph₂⁺) and its monophenyl analogue 2⁺ (22/tBuPh⁺).  The diphenyl compound has  

two phenyl to hydrazine charge transfer absorptions while the monophenyl compound has one band.  The absorption band consists of two components separated in energy by twice the effective coupling in the excited electronic state, and the selection rules are governed by the directions and signs of the transition dipole moments.  Emission from the coupled excited state to the ground state is observed at low temperature. The electronic transitions to the mixed valence excited state are calculated using the time-dependent theory of spectroscopy and wavepacket propagation.  Resonance Raman spectra are used to determine the frequencies and displacements of the totally symmetric normal modes that undergo changes between the ground and excited electronic state.  The intensity ratio of the two peaks is dictated by four factors:  the magnitude of the coupling, the sign of the coupling, the spatial orientation between the transition dipoles in the non-linear molecule, and the relative signs of the transition dipoles.  An orbital symmetry analysis is developed to explain the sign of the coupling.  The absorption, emission and Raman spectra are fit simultaneously with one parameter set.  This analysis provides detailed information about the coupling and also about the individual vibrational modes involved in the electron transfer.

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Biographical Sketch

Jeffrey I. Zink

Professor of Chemistry

MAILING ADDRESS

Department of Chemistry and Biochemistry  
University of California
405 Hilgard Ave.    
Los Angeles , CA 90095-1569

e-mail: zink@chem.ucla.edu
Tel: 310/825-1001
Fax: 310/206-4038

a.  PROFESSIONAL PREPARATION

University of Wisconsin , Milwaukee , Chemistry, 1962-1964    University of Wisconsin , Madison , Chemistry, BS 1996 University of Illinois ,  Chemistry, Ph.D. 1970

b.  APPOINTMENTS

1982-Present                 Professor, Department of Chemistry                                         and Biochemistry, UCLA

1976-82                           Associate Professor, Department of Chemistry and Biochemistry, UCLA

1970-76                           Assistant Professor, Department of Chemistry and Biochemistry, UCLA

c.  HONORS AND PROFESSIONAL ACTIVITIES (since 1980)

Member, American Chemical Society, Interamerican Photochemical Society, Phi Kappa Phi, Phi Lambda Epsilon, Alpha Chi Sigma; Glenn T. Seaborg Award, 1983; Herbert Newby McCoy Award, 1985; John Simon Guggenheim Fellow, 1987; Dow-Hansen Distinguished Teaching Award, 1994; DOE Sustained Outstanding Research Award, 1995; DOE Outstanding Scientific Accomplishment in Metals and Ceramic Sciences Award, 1998, Invited Guest Professor, University of Paris VI, 1998; Invited Guest Professor, University of Amsterdam , 1999.

d.  COLLABORATIVE PUBLICATIONS WITH STEVE NELSEN

1.     J. V. Lockard, J. I. Zink, A. E. Konradsson, M. N. Weaver, S. F. Nelsen, “Spectroscopic Consequences of a Mixed Valance Excite State: Quantitative Treatment of Dihydrazine Diradical Dication,” J. Am. Chem. Soc., 125, 13471-13480 (2003).

2.     S. E. Bailey, J. I. Zink, S. F. Nelsen, “Contributions of Symmetric and Asymmetric Normal Coordinates to the Intervalence Electronic Absorption and Resonance Raman Spectra of a Strongly Coupled r-Phenylenediamine Radical Cation,” J. Am. Chem. Soc., 125, 5939-5947 (2003).

3.  J. V. Lockard, J. I. Zink, D. A. Trieber, A. E. Konradsson, M. N. Weaver, S. F. Nelsen, “Excited State Mixed Valence in a Diphenyl Hydrazine Cation”, J. Phys. Chem., ASAP on the web, (2005).

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