Molecular spectra and molecular organization in thin solid films of bis(neopentylimido) perylene

The synthesis, molecular spectra, and thin solid film properties of a perylene dye, bis(neopentylimido) perylene (BNPTCD), are reported. Absorption and emission molecular spectra are used to probe the structure of vacuum-evaporated thin films and Langmuir-Blodgett monolayers of BNPTCD. The experimental assignment of electronic, infrared, and Raman spectra are aided with calculated molecular structure and spectra using density functional theory (DFT) and Hartree-Fock (HF) computational methods at 6-31G level of theory. Characteristic vibrational modes and local symmetry of the planar perylene tetracarboxylic chromophore (PTCD) are used to extract molecular organization and film packing properties from spectral data collected using transmission and reflection-absorption (RAIRS) infrared spectroscopy. From the infrared spectra, using surface selection rules and polarization properties, it was extracted that the PTCD plane in BNPTCD is organized in the evaporated film with the plane head-on tilted over the substrate. After the molecular organization was identified, the effect of thermal annealing and solvent vapors on the film structure was examined. It is found that the PTCD plane turns to a flat-on orientation when the film is thermal annealed or solvent treated. The aggregation and molecular stacking in films was also inspected using the electronic absorption and fluorescence of monomers and aggregates.