Torsional potentials and full-dimensional simulation of electronic absorption and fluorescence spectra of para-phenylene oligomers using the semiempirical self-consistent charge density-functional tight binding approach

A systematic study on the structural properties of para-phenylene oligomers based on the self-consistent charge density-functional tight binding approach (SCC-DFTB) and its time-dependent (TD) version is presented. Our goal is to investigate the applicability of DFTB for the present class of compounds and to use its computational efficiency for on-the-fly dynamics calculations and to perform in this way simulations of absorption and fluorescence spectra. For this purpose geometry optimizations have been performed for the ground state and for the electronically lowest excited state of oligomers containing two to seven aromatic rings. The torsional potential curves have been computed for para-biphenyl and para-terphenyl in the ground and lowest excited state. Agreement with previously computed DFT results is quite encouraging and DFTB seems to be well suited for the treatment of the class of conjugated pi systems investigated in this work. The intrachain vibrational broadening of absorption and emission spectra computed from dynamics simulations are presented and compared with experimental spectra.