Theoretical Study of the Isomerization Mechanism of Azobenzene and Disubstituted Azobenzene Derivatives

Christina R. Crecca and Adrian E. Roitberg*
Department of Chemistry and Quantum Theory Project, University of Florida, Gainesville, Florida 32611
J. Phys. Chem. A, 2006, 110 (26), pp 8188–8203
DOI: 10.1021/jp057413c
Publication Date (Web): June 15, 2006
Copyright © 2006 American Chemical Society
*

 To whom correspondence should be addressed. E-mail:  roitberg@ufl.edu

Abstract

A series of azobenzenes was studied using ab initio methods to determine the substituent effects on the isomerization pathways. Energy barriers were determined from three-dimensional potential energy surfaces of the ground and electronically excited states. In the ground state (S0), the inversion pathway was found to be preferred. Our results show that electron donating substituents increase the isomerization barrier along the inversion pathway, whereas electron withdrawing substituents decrease it. The inversion pathway of the first excited state (S1) showed trans → cis barriers with no curve crossing between S0 and S1. In contrast, a conical intersection was found between the ground and first excited states along the rotation pathway for each of the azobenzenes studied. No barriers were found in this pathway, and we therefore postulate that after n → π* (S1 ← S0) excitation, the rotation mechanism dominates. Upon π → π* (S2 ← S0) excitation, there may be sufficient energy to open an additional pathway (concerted-inversion) as proposed by Diau. Our potential energy surface explains the experimentally observed difference in trans-to-cis quantum yields between S1 and S2 excitations. The concerted inversion channel is not available to the remaining azobenzenes, and so they must employ the rotation pathway for both n → π* and π → π* excitations.

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History

  • Published In Issue July 06, 2006
  • Received December 20, 2005
    Revised May 7, 2006

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