Synthesis and Photophysical Properties of Glass-Forming Bay-Substituted Perylenediimide Derivatives

A series of perylenediimide-based small molecules (PDII-PD15) containing electron-deficient groups in the bay region were synthesized and characterized. The PDI derivatives were found to be capable of forming molecular glasses with glass transition temperatures ranging from 50 to 102 degrees C. Detailed investigations of the optical properties of the synthesized derivatives were performed and compared with those obtained from quantum chemical calculations. Optimized molecular structures of the PDI derivatives exhibited core-twisting by 16 degrees and torsional angle between the bay substituent and the perylene core in the range of 60-72 degrees. The PDI derivatives exhibited absorption maxima in the range of 2.27-2.36 eV and emission maxima in the range of 2.10-2.28 eV. The impact of the bay substituents on the emission, fluorescence quantum yield, and lifetimes in solutions and thin films was established. The red shift of emission maxima (from 2.282 to 2.095 eV) observed for various PDIs in solutions was accompanied by significant reduction in the emission quantum yield (from 0.73 to 0.44) and corresponding increase of the fluorescence lifetime (from 4.5 to 6.8 ns). This was in agreement with quantum chemical calculations indicating decrease of the radiative relaxation rate due to reduction of the oscillator strength and remarkable decrease of the torsional activation barrier. The spectral properties of the wet-casted perylenediimide films featuring different bay substituents were also studied. The variation in the emission peak (of 0.25 eV) and the considerable increase of the Stokes shift (of 0.4 eV) are explained in terms of the formation of the amorphous state. The influence of the bay substituents on the thermal and spectral properties of the films are discussed.