Organic aromatic compounds can self-assemble through overlap in π-π orbitals. When the band gap alignment is appropriate, a charge-transfer (CT) complex may be formed. Modifying the side chain on either the donor or acceptor will modulate the intermolecular packing of such complexes, tuning the degree of CT. Since molecular morphology influences the mobility of charges and the diffusion length of excitons, understanding the orientation of the donor-acceptor pairs is key to improving the efficiency and conductivity of charge-transfer systems. In this work, we focused on dipropylaminonaphtalene (DAN) and derivatives of naphthalenetetracarboxylic diimide (NDI) as the donor and acceptor molecules respectively. Specifically, we use the shifts in vibrational modes of NDI-C6H13, -C8H17, -methoxyethoxy, and -3-N-dimethyl to evaluate the influence of packing on the DAN-NDI CT interaction. To do so, we use infrared reflectance-absorbance spectroscopy (IRRAS) to resolve congested spectral bands as a function of the molecular orientation relative to the surface, to gain insight into morphology changes in DAN-NDI films compared to KBr powder spectra. Additional characterization explores the effects of annealing on CT films as well as their optical properties.

Effects of orientation and intermolecular packing on charge-transfer complexes in thin films analyzed by infrared reflectance

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