Study of localized and extended excitons in 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) .1. Spectroscopic properties of thin films and solutions

Excitonic transitions responsible for low energy absorption and fluorescence in dilute solutions and thin films of the organic molecule 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) have been investigated. Combining the results of concentration-dependent solution and thin film absorption and fluorescence with results on thin film exciton diffusion lengths (presented in the following paper, Paper II), we propose a comprehensive model for the low energy excitonic states in this archetype molecular crystal. We unambiguously identify an extended free charge transfer state at E(CT)(F) = 2.23 +/- 0.03 eV which has previously been observed to exhibit effects of quantum confinement in ultrathin layers grown under ultrahigh vacuum. This state self-traps at an energy of E(CT)(ST) = 2.11 +/- 0.04 eV due to the strong exciton-phonon coupling characteristic of this closely packed organic crystal.