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

摘要: Laser-induced fluorescence of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) oligomers embedded in helium nanodroplets has been studied the spectral region where strongest absorptions PTCDA crystals and single molecules dissolved organic solvents have observed before. The cold droplet environment allows a separation excitonic molecular excitations. We find narrow $(l1{\mathrm{cm}}^{\ensuremath{-}1})$ as well broad $(\ensuremath{\approx}500{\mathrm{cm}}^{\ensuremath{-}1})$ which can be attributed to electronic transitions dimer and, respectively, sandwichlike oligomers. maximum absorption lies at $22190{\mathrm{cm}}^{\ensuremath{-}1},$ slightly shifting lower energies upon addition more molecules. results are discussed comparison with existing theories exciton states thin films or crystals.