Vibronic Structure of PTCDA Stacks: Monomer-Dimer Equilibrium
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Perylenetetracarboxylic dianhydride (PTCDA, Fig. 1) has been extensively characterized, both structurally and electronically [1]. It forms ordered thin films and super lattices, has high mobility for holes, and its spectra indicate both Frenkel and charge-transfer (CT) excitons [2]. PTCDA also provides a welldefined model system for conjugated polymers whose electronic structure [3,4], including nonlinear optical (NLO) spectra and photophysics, offer applications for optoelectronics and light-emitting diodes. PTCDA's molecular excitation corresponds to the 1 1 Bu exciton polarized along the backbone, while CT between adjacent molecules mimics a polaron pair on adjacent strands. Moreover, the π-π* excitation couples to the effective conjugation coordinate, the same out-of-phase C=C and C-C stretch as in polyacetylene (PA) or polyenes, and the PTCDA frontier orbitals are directly related to those of conjugated chains [2]. We have recently carried out a vibronic analysis of PTCDA spectra using linear coupling to mixed Frenkel-CT states [2]. The mixing depends on the wavevector k: it is strong at k = 0, negligible at k = π. We postulate that the k = 0 state at the top of the band is localized to a dimer, while the emissive state at k = π is delocalized over the stack. The model accounts for absorption and electroabsorption in terms of PTCDA dimers, and for emission in terms of PTCDA stacks. In DMSO solution, the absorption indicates aggregate formation related to CT [5]. In this paper, we discuss solution spectra in terms of an equilibrium between PTCDA monomers and dimers. Perylene derivatives with identical solution spectra crystallize in face-to-face stacks. The deep solid -state colors are related to the displacements along the long (x) and short (y) axes [6]. PTCDA is a red dye whose absorption is compared in Fig. 1a with spectra [7] in CH 2 Cl2 . The changes are due to stacks with interplanar separation R = 3.38 Å. Such contacts generate Frenkel excitons and broader vibronic structure than in solution. The fit in Fig. 1a also has CT for adjacent radical ions in the stack [2]. The point is that dimers approximate the infinite stack. We will take the CH 2 Cl2 spectrum for individual molecules M and the film spectrum for dimers M 2 . As there is no structural information about
dimers in solution, using the solid -state dimer is an oversimplification that turns out surprisingly well. Stacking produces far more extensive changes in emission, which we associate with stacks rather than dimers. The solution fluorescence in Fig. 1b almost mirrors the absorption. The quantum yield is over 50% and the Stokes shift is ~70 meV. The solid -state spectrum in Fig. 1b is strongly shifted to the red, has narrower vibronic structure and ~100-fold lower quantum yield. The molecular π-π* transition is x-polarized, along the long axis and normal to the stack. The Frenkel-exciton band consequently has k = 0 at the top, k = π at the bottom, and 0-0 is dipole forbidden at k = π. The principal (1.71 eV) peak is assigned at 1-0 in the ind
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