Vibronic couplings and radiationless transitions between the lowest 1Bu and the first excited 2Ag state of linear polyenes

Transoctatetraene is studied as a model compound by means of ab initio (GAMESS) calculations to study the vibronic coupling between the lowest excited Ag state and the dipolar allowed second excited Bu state. It is found that the evaluated couplings to totally symmetric modes in combination to those of Bu-symmetry can describe the high resolution absorption excitation and emission spectra in more detail. The strong vibronic couplings can further explain the fast internal conversion between the 1Bu and the 2Ag state. It is argued that they can also induce a resonance energy transfer which can compete with a quadrupolar coupling at room temperature. The vibronic transfer mechanism is also discussed for photosynthetic systems including the transfer from carotenoids to bacteriochlorophylls.