Spectral properties and dynamics of the excited state structural relaxation of the ortho analogues of POPOP - Effective abnormally large Stokes shift luminophores

The spectral behaviour of the sterically hindered, substantially non-planar molecules of the ortho analogues of POPOP was studied by steady state absorption and fluorescence methods, time-resolved fluorescence methods and quantum chemical calculations. It was shown that the investigated molecules undergo noticeable flattening in their excited state, which leads to the essential lowering of the energy of the fluorescent term and, in turn, enlargement of the Stokes shift of the fluorescence. The flattening of the ortho analogues of POPOP in the excited state is a very effective process (estimated rate constants are of the order of 10(8)-10(9) s(-1)); it has a low activation energy and mainly intramolecular nature with a minor influence of the environment. The replacement of one of the oxazole rings in the ortho-POPOP molecule by an oxadiazole ring causes a near doubling of the flattening activation energy.