Abstract.
In the present paper we report the results of a multiconfigurational computational study on potential-energy curves of azobenzene along the NN twisting to clarify the role of this coordinate in the decay of the S2(ππ*) and S1(nπ*) states. We have found that there is a singlet state, S3 at the trans geometry, on the basis of the doubly excited configuration n2 π*2, that has a deep minimum at about 90° of twisting, where it is the lowest excited singlet state. The existence of this state provides an explanation for the short lifetime of S2(ππ*) and for the wavelength-dependence of azobenzene photochemistry. We have characterized the S1(nπ*) state by calculating its vibrational frequencies, which are found to correspond to the recently observed transient Raman spectrum. We have also computed the potential-energy curve for the triplet T1(nπ*) at the density functional theory B3LYP level, which indicates that in this state the isomerization occurs along the twisting coordinate.
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Acknowledgement The financial support from MIUR (project “Modellistica delle proprietà spettroscopiche di sistemi molecolari complessi” funds ex 60% and project “Dinamiche molecolari in sistemi di interesse chimico” funds ex 40%), from the University of Bologna (Funds for Selected Research Topics) is gratefully acknowledged.
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Gagliardi, L., Orlandi, G., Bernardi, F. et al. A theoretical study of the lowest electronic states of azobenzene: the role of torsion coordinate in the cis–trans photoisomerization. Theor Chem Acc 111, 363–372 (2004). https://doi.org/10.1007/s00214-003-0528-1
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DOI: https://doi.org/10.1007/s00214-003-0528-1