On the nature of the two low-lying singlet states of anthracene: Vibronic activity in the two-photon excitation spectrum

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Abstract
The two‐photon excitation spectrum of anthracene in a n‐heptane matrix at 10 K has been measured in the energy region 26 000–32 000 cm−1. Experimental evidence of two‐photon band assignment to vibronically induced B1u ×b1u and B2u ×b2u transitions is given. In particular, the two‐photon spectrum above ≂28 000 cm−1 shows several vibronic origins built on b2u vibrations and progressions of ag modes on them. The lowest ππ* absorption region (−1) has, on the contrary, negligible intensity and very weak B1u ×b1u bands are observed. These data can be rationalized in terms of vibronic coupling between electronic states induced by normal coordinates of b1u and b2u symmetry. By means of the Herzberg–Teller theory and displacing the molecule away from equilibrium along the normal coordinate we have calculated the vibronic interaction between electronic states in the orbital following approach. The results based on CNDO/S‐CI wave functions show that B2u ×b2u transitions have a larger vibronic activity than the B1u ×b1u transitions. The 1132, 1384, and 1393 cm−1 b2u modes are particularly strong in inducing two‐photon intensity through a vibronic coupling mechanism involving essentially the ground and the final 1 1B2u state. The B2u ×b2u two‐photon amplitude tensors are not sensitive to the method of calculation. B1u ×b1u tensors have instead a more pronounced dependence on the method used. This is due to the fact that in the two‐photon sums most of the intermediate states play an equivalent role in determining the amplitude tensor, in contrast with the B2u ×b2u case. It is important to use a reasonably correct representation of the excited state wave functions. It is shown that more accurate calculations (INDO/S and CNDO/S with increased CI) lead to better agreement of the total vibronic intensity of the 1 1B1u state with experiment and predict the largest activity for the 648 cm−1 b1u mode, as observed in the spectrum.