Bridge-mediated hopping or superexchange electron-transfer processes in bis(triarylamine) systems

Hopping and superexchange are generally considered to be alternative electron-transfer mechanisms in molecular systems. In this work we used mixed-valence radical cations as model systems for the investigation of electron-transfer pathways. We show that substituents attached to a conjugated bridge connecting two triarylamine redox centres have a marked influence on the near-infrared absorption spectra of the corresponding cations. Spectral analysis, followed by evaluation of the electron-transfer parameters using the Generalized Mulliken–Hush theory and simulation of the potential energy surfaces, indicate that hopping and superexchange are not alternatives, but are both present in the radical cation with a dimethoxybenzene bridge. We found that the type of electron-transfer mechanism depends on the bridge-reorganization energy as well as on the bridge-state energy. Because superexchange and hopping follow different distance laws, our findings have implications for the design of new molecular and polymeric electron-transfer materials.

[1]  B. Albinsson,et al.  Bridge-dependent electron transfer in porphyrin-based donor-bridge-acceptor systems. , 2001, Journal of the American Chemical Society.

[2]  B. Giese.  Long-distance charge transport in DNA: the hopping mechanism. , 2000, Accounts of chemical research.

[3]  S. Lindeman,et al.  Disproportionation and Structural Changes of Tetraarylethylene Donors upon Successive Oxidation to Cation Radicals and to Dications , 1998 .

[4]  R. Ismagilov,et al.  Adiabatic electron transfer: comparison of modified theory with experiment. , 1997, Science.

[5]  R. Ismagilov,et al.  Charge-localized p-phenylenedihydrazine radical cations: ESR and optical studies of intramolecular electron transfer rates , 1997 .

[6]  M. Ratner,et al.  Electron Transfer Rates in Bridged Molecular Systems 2. A Steady-State Analysis of Coherent Tunneling and Thermal Transitions† , 2000 .

[7]  R. Ismagilov,et al.  Solvent effects on charge transfer bands of nitrogen-centered intervalence compounds. , 2001, Journal of the American Chemical Society.

[8]  M. Newton,et al.  Metal—lingad and metal—metal coupling elements , 1994 .

[9]  N. Hush.  Distance Dependence of Electron Transfer Rates , 1985 .

[10]  M. Newton,et al.  Estimation of Electron Transfer Distances from AM1 Calculations , 2000 .

[11]  T. Esch,et al.  Organic Electron Transfer Systems, II Substituted Triarylamine Cation‐Radical Redox Systems – Synthesis, Electrochemical and Spectroscopic Properties, Hammet Behavior, and Suitability as Redox Catalysts , 1991 .

[12]  Christoph Lambert,et al.  The Class II/III Transition in Triarylamine Redox Systems , 1999 .

[13]  I. Bersuker,et al.  Electron delocalization in trinuclear mixed-valence clusters , 1982 .

[14]  M. Ondrechen,et al.  Adiabatic potentials for a bridged three-site electron-transfer system , 1982 .

[15]  Lluís Blancafort,et al.  Intramolecular Electron Transfer: Independent (Ground State) Adiabatic (Chemical) and Nonadiabatic Reaction Pathways in Bis(hydrazine) Radical Cations , 2000 .

[16]  Jean-Pierre Launay,et al.  Long-distance intervalence electron transfer , 2001 .

[17]  M. Ondrechen,et al.  Line shape of the intervalence transfer band in bridged mixed-valence dimers: the delocalized case , 1985 .

[18]  Mark A. Ratner,et al.  Molecular-wire behaviour in p -phenylenevinylene oligomers , 1998, Nature.

[19]  B. Pfennig,et al.  Synthesis, characterization, and photophysical properties of a series of supramolecular mixed-valence compounds. , 2001, Inorganic chemistry.

[20]  V. Balzani.  Electron transfer in chemistry , 2001 .

[21]  F. A. Neugebauer,et al.  Über Mono-, Di- und Triarylamin-Radikalkationen , 1975 .

[22]  E. Steckhan,et al.  Über organische Elektronenüberträgersysteme, I. Elektrochemische und spektroskopische Untersuchung bromsubstituierter Triarylamin-Redoxsysteme , 1980 .

[23]  M. Newton,et al.  Quantum chemical probes of electron-transfer kinetics: the nature of donor-acceptor interactions , 1991 .

[24]  C. Lapinte,et al.  Molecular Wires: Synthesis and Properties of the New Mixed-Valence Complex [Cp*(dppe)Fe−C⋮C−X−C⋮C−Fe(dppe)Cp*][PF6] (X = 2,5 -C4H2S) and Comparison of Its Properties with Those of the Related All-Carbon-Bridged Complex (X = −C4−) , 2000 .

[25]  R. Ismagilov,et al.  Effects of bridge redox state levels on the electron transfer and optical properties of intervalence compounds with hydrazine charge-bearing units , 1998 .

[26]  S. Lindeman,et al.  X-ray structure analysis and the intervalent electron transfer in organic mixed-valence crystals with bridged aromatic cation radicals. , 2002, Journal of the American Chemical Society.

[27]  Robert J. Cave,et al.  Generalization of the Mulliken-Hush treatment for the calculation of electron transfer matrix elements , 1996 .

[28]  Norman Sutin,et al.  Optical transitions of symmetrical mixed-valence systems in the Class II-III transition regime. , 2002, Chemical Society reviews.

[29]  C. Lambert,et al.  One- and Two-Dimensional Electron Transfer Processes in Triarylamines with Multiple Redox Centers. , 1998, Angewandte Chemie.