Transient absorption spectroscopy of a heteroaromaticdonor–acceptor-p-conjugated 2,20-bipyridine dye

We report on a study of the excited state dynamics of a heteroaromatic 4,40-p-conjugated 2,20-bipyridine dye, bearing conjugated p-excessive and p-deficient heteroaromatic rings as donor and acceptor substituents, by means of steady state and transient absorption spectroscopy in EtOH solutions and a room temperature. The ground state absorption shows an intense intramolecular charge transfer band in the visible region with a maximum at 515 nm. Excited state dynamics is measured by pumping at 480 nm at shorter wavelengths from the absorption maximum and probing at specific wavelengths corresponding to bleaching, stimulated emission and at the maximum of excited state absorption. The spectral evolution is also evaluated in the frequency domain as a function of the delay between pump and probe pulses. By means of SVD analysis two components with time constants around 2 ps and 150 ps are identified. In addition a relevant dynamical Stokes shift of the stimulated emission occurs in the first 60 ps which is attributed to solvent dynamics. Ab initio calculations allows to assign the ground spectral features and the excited state conformation. The geometry of the ground and of the first excited state are optimized both in the cisoid and in the transoid forms. The latter results more stable of about 0.4 eV.