Vibronic perturbations in the electric field spectra of 1,3-diazaazulene and azulene

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Abstract
The electric field spectra of the 3500 and 4500 Å absorption systems of azulene and 1,3‐diaza‐azulene in naphthalene host reveal that the a1 fundamental vibrational levels which are known to be vibronically induced exhibit permanent dipole moments which are significantly different from the dipole moment of the zero‐point level. It is shown theoretically that, by virtue of an a1 level’s interstate vibronic activity, the same level can derive permanent dipole by (i) borrowing permanent dipole from the electronic state, which couples to the state of interest and (ii) borrowing transition dipole, which connects the two coupled states. Calculations indicate that the latter route is of primary importance for both of the above systems. In addition, the theory is easily used to rigorously compute the crude adiabatic permanent dipole of an electronic state from the experimentally observed adiabatic dipole moments. This is done for the two states above.