Optical excited states of archetypical blue light emitter, dimethyl-acridine diphenyl-sulfone derivative (DMAC-DPS) in solid films, studied by electroabsorption spectroscopy

Detailed characterization of the lowest energy electronic excited states for dimethyl-acridine diphenyl-sulfone derivative (DMAC-DPS) in solid neat films has been performed by electroabsorption (EA) spectroscopy. The EA spectra were parametrized in the framework of Liptay theory, providing changes in the dipole moment and electronic polarizability. The extracted values of the corresponding parameters in the lowest energy optical absorption bands were assigned to molecular dipoles upon optical excitation associated with the intramolecular charge transfer (CT) from the electron-donating part (DMAC) to the electron-accepting part (DPS) of the molecule. The standard treatment of EA spectra based on Liptay theory was successfully reinterpreted applying the time-dependent density functional (TDDFT) theory, without additional assumptions regarding the shape of the EA signal, characteristic of the Liptay formalism. To correctly describe the EA spectrum, the TDDFT calculations have to be performed not for a single molecule, but for a dimer structure, established by the Monte Carlo (MC) calculations. Although the main EA features are ascribed to the excited states of intramolecular CT origin, the lowest energy excited states gain oscillator strengths due to the geometry distortion of the ground state or excited state interaction introduced by the dimer structure. The implications of these results for the first singlet excited state (S1) are discussed in the context of the thermally activated delayed fluorescence (TADF) emitters employed currently in organic light emitting diodes (OLEDs).