Ab Initio Calculation of the Electronic Spectrum of Azobenzene Dyes and Its Impact on the Design of Optical Data Storage Materials Per-Olof Åstrand,* 1 P. S. Ramanujam, ² Søren Hvilsted, ‡ Keld L. Bak, § and Stephan P. A. Sauer ⊥ Contribution from the Condensed Matter Physics and Chemistry Department and Optics and Fluid Dynamics Department, Risø National Laboratory, POB 49, DK-4000 Roskilde, Denmark, UNI-C, Olof Palmes Alle ´ 38, DK-8200 Aarhus N, Denmark, and Chemistry Laboratory IV, Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 Copenhagen Ø, Denmark ReceiVed August 31, 1999. ReVised Manuscript ReceiVed NoVember 29, 1999 Abstract: Electronic excitation energies of 16 azobenzene dyes have been calculated by ab initio methods within the second-order polarization propagator approximation (SOPPA). Good agreement with expriment is found for the lowest singlet and triplet states for both the trans- and cis-azobenzene molecules. The differences are in the range of (0.3 eV, with the exception of the lowest n f π * transition in trans-azobenzene, where a deviation of -0.64 eV is found. The lowest π f π * transition in trans-azobenzene, on the other hand, is particularly well reprensented with a deviation of only -0.15 eV. Furthermore, the experimental singlet π f π * transitions are reproduced for a set of azobenzene dyes with different electron donor and acceptor groups and the correct shifts in excitation energy are obtained for the different substituents. It has also been demonstrated that ab initio methods can be used to determine suitable candidates for azo components used in materials for data storage. I. Introduction The photophysical properties of azo compounds are of large interest in the development of nonlinear optical materials and materials for optical storage of data. In particuler, azobenzene dyes linked to side chains of different polymers and oligomers have been exploited extensively for holographic and digital storage of information in thin films of these materials. 1-11 The principle of the holographic storage process in azobenzene polymer films is that polarized light is used to excite the trans conformation of the azobenzene dyes which then isomerizes to the cis conformation. Since polarized light is used, the dyes with a transition dipole moment orthogonal to the external field are not excited. The cis conformation relaxes back to the trans conformation in a new arbitrary orientation. If the process is carried out repeatedly, the azobenzenes will be aligned. The diffraction is different for the part of the material where the dyes are aligned and this information is read by another laser. Fast optical recording in a permanent manner with high read- out efficiencies has been demonstrated. Total erasure of the inscribed information and multiple reuse of the material have also been achieved. One crucial aspect of this process that can be studied by theoretical models is the required laser wavelength to obtain the trans-to-cis isomerization in an azobenzene dye. The isomerization is induced by an electronic excitation of an electron in either the highest occupied nonbonded orbital (denoted n) or in the highest occupied π orbital to the lowest unoccupied π orbital (denoted π*). In trans-azobenzene (TAB), the lowest excited singlet state, S 1 , is mainly due to the n f π * transition whereas the S 2 state is mainly due to the π f π * transition. 12 If we assume that the geometry of TAB is planar, the oscillator strength of the n f π * transition is zero due to symmetry arguments, which means that it is small also for slightly nonplanar conformations. Thus, to excite the molecule a laser wavelength is often chosen to match the π f π * transi- tion. However, the isomerization process of azobenzene involves also the S 1 state as well as the lowest triplet states, T 1 and T 2 . 12 In some storage devices, it may be desirable to use lasers with long wavelengths. At red and infrared wavelengths, cheap * Address correspondence to this author at the Condensed Matter Physics and Chemistry Department, Risø National Laboratory. E-mail: per- olof.aastrand@risoe.dk. ² Optics and Fluid Dynamics Department, Risø National Laboratory. ‡ Condensed Matter Physics and Chemistry Department, Risø National Laboratory. § UNI-C. ⊥ Department of Chemistry, University of Copenhagen. E-mail: sps@ ithaka.ki.ku.dk. (1) Eich, M.; Wendorff, J. H.; Reck, B.; Ringsdorf, H. Makromol. Chem. Rapid Commun. 1987, 8, 59-63. (2) Wiesner, U.; Antonietti, M.; Boeffel, C.; Spiess, H. W. Makromol. Chem. 1990, 191, 2133-2149. (3) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2268-2273. (4) Hvilsted, S.; Andruzzi, F.; Ramanujam, P. S. 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In Photochromism-Molecules and Systems; Du ¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; Chapter 4, pp 165- 192. 3482 J. Am. Chem. Soc. 2000, 122, 3482-3487 10.1021/ja993154r CCC: $19.00 © 2000 American Chemical Society Published on Web 03/25/2000