Low temperature spectra of the ortho-POPOP molecule: Additional arguments of its flattening in the excited state

Low temperature absorption and fluorescence spectra were measured for 1,2-b is-(5-phenyl-oxazolyl-2)-benzene (ortho-POPOP), a sterically hindered molec ule, substantially non-planar in its ground state. Quantum-chemical calcula tions with the optimization of molecular geometry were made using the semie mpirical AMI method. The observed spectral changes, together with the obtai ned theoretical results, evidenced in favor of our previously made assumpti on about the considerable flattening of ortho-POPOP in the lowest singlet e xcited state. The data of spin-orbit coupling between singlet and triplet t erms, which has been estimated in CNDO/S scheme, were used to calculate the efficiency of intersystem crossing in the studied molecule.