Molecular orientation in solid state crystals has a profund effect on the electron transport and spectroscopic properties of these materials. There are many electronically active model systems we can use to fine tune the unit cell interactions in a crystalline material by varying the pendent side chains of the molecular species. We report the polarized absorption spectra of high quality, thin crystals of a perylene diimide (PDI) species with branched side chains (B2). The absorption spectrum shows exemplary Davydov splitting (DS) of 1230 cm-1 and peak to peak splitting of 3040 cm-1. The experimental results are compared to theory with remarkable agreement. The spectral shifts show evidence of a high degree of intermolecular charge transfer, which, along with Coulombic coupling, create an unprecedented DS for this family of molecules. The presence of the unique orthogonal spectra obtained in these crystals is attributed to the twisted unit cell structure apparent in B2. In contrast, perylene molecules with extended side chains (B6 for example) exhibit a more simplistic unit cell. The molecules are not twisted with respect to one another, so the Davydov splitting effect in these crystals is minimized. Therefore, the electron transport in these materials will be one dimensional upon unpolarized excitations while B2 can exhibit bidirectional electron transport, which can be useful to compare the electronic properties of the H-like and J-like aggregates formed in the crystals.

Crystal structure effects on the Davydov splitting of electronically active conjugated systems

253rd National Meeting (2017)

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