title: Asymmetry between absorption and photoluminescence line shapes of TPD: spectroscopic fingerprint of the twisted biphenyl core.
abstract: We analyze absorption, photoluminescence (PL), and resonant Raman spectra of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD), with the aim of providing a microscopic interpretation of a significant Stokes shift of about 0.5 eV that makes this material suitable for stimulated emission. The optical spectra were measured for TPD dissolved in toluene and chloroform, as well as for polystyrene films doped with varying amounts of TPD. In addition, we measured preresonant and resonant Raman spectra, giving direct access to the vibrational modes elongated in the relaxed excited geometry of the molecule. The experimental data are interpreted with calculations of the molecular geometry in the electronic ground state and the optically excited state using density functional theory. Several strongly elongated high-frequency modes within the carbon rings results in a vibronic progression with a calculated spacing of 158 meV, corroborated by the observation of vibrational sidebands in the PL spectra. The peculiarities of the potential energy surfaces related to a twisting around the central bond in the biphenyl core of TPD allow to quantify the asymmetry between the line shapes observed in absorption and emission.
authors.affiliation: Walter Schottky Institut, Technische Universität München, D-85748 Garching, Germany. reinhard.scholz@wsi.tum.de
authors.forename: Reinhard
authors.initials: R
authors.lastname: Scholz
authors.forename: Linus
authors.initials: L
authors.lastname: Gisslén
authors.forename: Cameliu
authors.initials: C
authors.lastname: Himcinschi
authors.forename: Igor
authors.initials: I
authors.lastname: Vragović
authors.forename: Eva M
authors.initials: EM
authors.lastname: Calzado
authors.forename: Enrique
authors.initials: E
authors.lastname: Louis
authors.forename: Emilio
authors.initials: E
authors.lastname: San Fabián Maroto
authors.forename: María A
authors.initials: MA
authors.lastname: Díaz-García