Molecular versus excitonic transitions in PTCDA dimers and oligomers studied by helium nanodroplet isolation spectroscopy

Laser-induced fluorescence of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) oligomers embedded in helium nanodroplets has been studied in the spectral region where the strongest absorptions of PTCDA crystals and single PTCDA molecules dissolved in organic solvents have been observed before. The cold helium droplet environment allows a separation of excitonic and molecular excitations. We find narrow (<1cm^-1) as well as broad (≈500cm^-1) absorptions which can be attributed to electronic molecular transitions of the PTCDA dimer and, respectively, to excitonic transitions of sandwichlike PTCDA oligomers. The maximum of the excitonic absorption lies at 22190cm^-1, slightly shifting to lower energies upon addition of more PTCDA molecules. The results are discussed in comparison with existing theories of exciton states in thin organic films or crystals.