The structure and energetics of TPD ground and excited states

We analyse the optimized geometric structure and energies of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) neutral molecule in ground and excited states. Ab initio calculations are performed at the Hartree-Fock (HF) or single-excitation (CIS) level with 6-31G basis set using Gaussian 98 program. We got a good qualitative agreement with the results for the neutral molecule in the ground state obtained by density functional theory methods, which were previously reported in the literature. TPD in the ground state has a pronounced twisted geometry, with both central and peripheral dihedral angles of about 40°. On the other hand, the structure of excited state is more planar, as the central dihedral angle becomes smaller. We estimated the reorganization energies associated with the optical transitions, corresponding transition dipole moments, as well as HOMO and LUMO. On the basis of ab initio calculations, we developed a semi-empirical model for the observed absorption and photoluminescence and interpreted the large Stokes shift of about 0.4 eV.