Effect of Positional Substitution on the Optical Response of Symmetrically Disubstituted Azobenzene Derivatives A. A. Blevins and G. J. Blanchard* Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824-1322 ReceiVed: NoVember 11, 2003; In Final Form: February 10, 2004 We have synthesized a series of symmetrically substituted p-diamidoazobenzenes, and we compare their steady-state and time-resolved responses to those of p-diaminoazobenzene and azobenzene. Our experimental and computational data indicate that the identity of the azobenzene ring substituent plays a significant role in determining the ordering of excited electronic states and the oscillator strengths for the various S n r S 0 transitions. We find experimentally that the symmetric disubstitution of azobenzenes also influences the S 0 isomerization surface, with the largest S 0 barrier being found for the unsubstituted compound and the smallest barrier for p-diaminoazobenzene. We understand these data in the context of the relative electron-donating character of the para side groups. Introduction Azobenzenes are a family of compounds that has found wide use because of their facile photoisomerization and relatively high barrier to thermal (ground state) isomerization. These properties are of potential utility in the development of molecular-scale information storage and optical switching strategies. 1-11 Among the key issues under investigation for the azobenzenes is the ability of substitution about their aromatic rings to mediate their optical properties and isomerization behavior. While it is typically assumed that the linear optical response and isomer- ization behavior of these chromophores are linked, this relation- ship has not been established. The purpose of this work is to examine how the presence of substituents on the azobenzene phenyl rings influences the spectroscopic and isomerization behavior of these compounds. We have synthesized a family of symmetrically para-disubstituted azobenzenes and have studied their steady-state optical properties. We have also evaluated the branching ratio for isomerization and the rate of trans isomer recovery for these species. Our findings indicate that the addition of substituents to the azobenzene chromophore can influence the steady-state and time-resolved optical proper- ties of the chromophore significantly, and the isomerization surfaces for these molecules are affected as well. This is not a surprising result because of the relationship between electronic structure and state ordering and the electron density distribution for the bond(s) that dominate the isomerization coordinate. We understand this finding in the context of the NdN bond dominating the isomerization behavior of these molecules, and the effective bond order of this moiety is influenced by the presence of electron-donating or electron-withdrawing substit- uents on the phenyl rings. Our finding that the isomerization barrier is reduced when electron-donating substituents are placed on the rings indicates that it is the π* state that is influenced most strongly, and these findings are supported by the steady- state spectra. Our data also point to the potential importance of asymmetric substitution of the azobenzene rings in structurally mediating isomerization. 12 Experimental Section Steady-State Absorption Spectroscopy. The absorbance spectra of the various azobenzenes reported here were obtained on a Varian/Cary model 300 UV/vis absorption spectrometer. Spectral resolution for all measurements was 1 nm, and the normalized absorption spectra are shown in Figure 1. The molar absorptivities and absorption maxima of the compounds are given in Table 1. Calculated Results. Semiempirical calculations were per- formed on a Windows-based PC using Hyperchem v. 6.0. Dihedral angles were incremented for isomerization barrier calculations using macros written in Excel. 21 For these calcula- tions, initial geometry optimization was performed using PM3 parametrization, and then single point calculations were made for the molecule at each incremented angle without additional geometry optimization. * To whom correspondence should be addressed. E-mail: blanchard@ chemistry.msu.edu. Figure 1. Normalized absorption spectra of azobenzene, p-diaminoa- zobenzene, and the series of p-diamidoazobenzenes 1-5, as indicated. Molar absorptivities of these compounds are given in Table 1. 4962 J. Phys. Chem. B 2004, 108, 4962-4968 10.1021/jp037436w CCC: $27.50 © 2004 American Chemical Society Published on Web 03/25/2004