The results of spectral and luminescent studies of polydiphenylenphthalide (PDF) solutions and thin films as well as molecule structure and its energy levels DFT modelling within the Gaussian-03 software package are presented. It is shown that structural unit (diphenylenephthalide) can form four polymer conformations with a similar energy gap. The most probable polymer conformation is the spiral cis-form with phthalide groups turned in opposite directions. At long-wave optical transition the electronic density moves on a chain of a molecule from diphenyl part to somehow distant phthalide one. Fluorescence excitation spectra reveal a transition band on a long-wave tail of the absorption spectrum. Spectral and time measurements of the luminescence allow to assume the presence of conformation transformations of PDF molecule under its optical excitation.http://dx.doi.org/10.5755/j01.ms.17.3.591