TD-DFT investigation on the low-lying excited states of spiro-bithiophene

Modifications of the optical properties and charge mobility of spiro-bithiophene (SCPDT) were investigated by time-dependent density functional theory (TD-DFT). Geometry optimizations have been performed for the ground state and excited state of bithiophene (BT), 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) and SCPDT. Vertical excitations and emission transition energies have been calculated. The computed vertical and emission transition energies of BT and CPDT are in good agreement with the experimental data. Compared to CPDT and BT, SCPDT is expected to be an effective EL material by considering that smaller reorganization energy is presented and fluorescence quantum efficiency of non-spiro isolated fragment is preserved in the molecule with spiro-structure.