The dominant molecular orbitals contributing into absorption peaks of an organic light-emitting molecule

The dominant molecular orbitals (MO) making roles in electronic transitions responsible for optical processes in an OLED molecule are investigated. Whole molecule transition during absorption of stimulus light in such devices are commonly described first-principally as a summation of electronic transitions between several MOs. Time dependent density functional theory (TDDFT) can determine the percent of each electronic transition contributing into a specific molecular transition between ground and excited states. This scheme, however, ignores electronic correlation during the transitions, which of crucial importance for correcting the difference energies between MO levels. This electron-hole interaction energy can be recovered considering every peak of absorption spectrum of the molecule to one single electronic transition between two specific MOs. The corresponding single-excited molecular orbitals (SEMO) are employed to calculate the intensities of absorption peaks of a well-known OLED molecule Alq3. First, TDDFT is used to simulate the molecule and to obtain its MOs and it is shown that TDDFT is capable to track the dominant electronic transitions and their specific MOs responsible for the optical process, and so, it provides a reliable basis for calculations of SEMO. The SEMO is then applied on MOs contributing dominantly in each absorption peak of the molecule. The calculations, performed within this scheme, move the computational curve excellently closer to experimental measurement of the absorption peaks. •Design process of OLED devices are mostly akin to predicting the dominant electronic transition between molecular orbitals.•An ignored ability of TDDFT is to predict the dominant MOs' transitions contributing to absorption peaks of the molecule.•Single excited molecular orbitals (SEMO) is capable of calculating the absorption spectrum of molecule.•TDDFT+SEMO applied on OLED molecule Alq3 shows that its primary absorption peak is due to HOMO to L+1 transition.