Vibronic structure of PTCDA stacks: the exciton-phonon-charge-transfer dimer

Perylenetetracarboxylic acid dianhydride (PTCDA) stacks face-to-face in cry stals and multiple quantum wells (MQWs). Excitations of PTCDA stacks are mi xed molecular (Frenkel) and charge-transfer (CT) states coupled to a molecu lar vibration. Eclipsed stacks and molecular conjugation imply strong Frenk el-CT mixing in absorption and electroabsorption, with k=0 at the top of th e exciton band, and negligible mixing at k=pi for emission from the bottom. The exciton-phonon-CT dimer developed for k=0 processes is a nonadiabatic approximation for narrow CT bands. Quantitative dimer spectra are obtained in the vibronic basis of displaced harmonic oscillators for excited PTCDA a nd radical ions. We present a joint analysis of absorption and emission in PTCDA stacks and MQWs using parameters from solution, molecular calculation s, and related conjugated systems. Polarized single-crystal absorption deci sively relates the entire 2-3 eV system to molecular pi-pi* transitions, wh ile electroabsorption with field along the stack implicates adjacent ions i n the stack. The simple structure and extensive PTCDA spectra make possible detailed modelling of mixed Frenkel-CT vibronics that were far less access ible in previous organic molecular crystals. Since the coupled mode is clos ely related to polyenes and conjugated polymers, PTCDA provides a bridge be tween molecular insulators and extended systems capable of charge transport . (C) 1999 Elsevier Science B.V. All rights reserved.