Vibronic line shapes of PTCDA oligomers in helium nanodroplets

Abstract

Oligomers of the organic semiconductor PTCDA are studied by means of helium nanodroplet isolation (HENDI) spectroscopy. In contrast to the monomer absorption spectrum, which exhibits clearly separated, very sharp absorption lines, it is found that the oligomer spectrum consists of three main peaks having an apparent width orders of magnitude larger than the width of the monomer lines. Using a simple theoretical model for the oligomer, in which a Frenkel exciton couples to internal vibrational modes of the monomers, these experimental findings are nicely reproduced. The three peaks present in the oligomer spectrum can already be obtained taking only one effective vibrational mode of the PTCDA molecule into account. The inclusion of more vibrational modes leads to quasi continuous spectra, resembling the broad oligomer spectra.

The dimer-approach to characterize opto-electronic properties of and exciton trapping and diffusion in organic semiconductor aggregates and crystals

“…64 It is possible to generalize the vibronic Hamiltonians to include several intra-monomer vibrations. 65 This was found to be necessary, for example, in the case of macro-cyclic squaraine dyes. 66 Also, harmonic approximations are not essential and may be relaxed.…”

Probing weak dipole-dipole interaction using phase-modulated nonlinear spectroscopy

“…For such molecules, the electronic transition dipoles are typically in the order of several Debye. Dimers with specific interaction strength could be formed for example by linking the organic molecules by spacers [21,22], by selfassembly in helium-nanodroplets [11,23] or by depositing the molecules on a surface with a small coverage [24,25] (using single molecule spectroscopy techniques one then can select close-by molecular pairs). The distances between the molecules can range from a fewÅngström to several nanometer.…”

Spectroscopy of 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) attached to rare gas samples: Clusters vs. bulk matrices. II. Fluorescence emission spectroscopy

“…Moreover, molecules of this type embedded in helium nanodroplets have already been extensively characterized in our group. [21][22][23][24] The present paper is the second part of the spectroscopic study of PTCDA attached to rare gas samples and is devoted to fluorescence emission measurements (absorption and excitation measurements are presented in Ref. 15, later referred to as Paper I).…”