Vibronic analyses of the lowest singlet–singlet and singlet–triplet band systems of pyridazine Original Research Article

The first singlet–singlet and singlet–triplet band systems of the absorption spectrum of pyridazine vapour are analysed using ab initio and vibronic coupling calculations. The lowest singlet–triplet absorption involves a comparatively unperturbed (π∗,n) 3B1 state, and contrasts with the highly perturbed singlet–singlet spectrum. The major source of vibronic perturbation in the singlet–singlet absorption is attributed to coupling between near-resonant (π∗,n) 1A2 and (π∗,n) 1B1 states, with the former being slightly lower in energy. Many features of this complex and unusual spectrum, and its associated single vibronic level fluorescence spectrum, can be explained using a simple vibronic model. This provides experimental support for recent relaxed CASPT2 and EOM-CCSD calculations, but contrasts with earlier assignments of the spectrum. Theory and experiment suggest that the spacing between the lowest A2 and B1 states is larger in the triplet manifold, leading to a simpler spectrum.