6533b7d6fe1ef96bd1266e6b

RESEARCH PRODUCT

Structure, energy and charge transport in two-dimensional crystals of cyanine dyes

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Abstract In order to develop systems with controlled energy or charge transfer across molecular dimensions it is highly desirable to build two-dimensional crystals of functional dye molecules: they can be addressed from the third dimension and the lateral intermolecular interactions can be well characterized due to a fixed and defined geometry. In an effort to study these possibilities we prepared crystals of different, negatively charged cyanine dyes, formed after adsorption from a water subphase to a positively charged monolayer. These crystals are one monolayer thick, of uniform dimensions between 10 and 100 μm (depending on nucleation conditions) and of rectangular shape. Single crystals are studied by transmission electron diffraction and by polarized absorption and emission spectroscopy. We show that the crystals consist of two rows of densely stacked molecules with two different orientations of the long molecular axes. This leads to two perpendicular polarized absorption bands. The measured splitting is in accordance with results of extended dipole calculations. The latter were performed for crystals of three slightly different molecules where the angles between the long axes varied between 70° and 100°. The aliphatic tails form a lattice which is epitaxially related to the centred rectangular one of the dyes. It is incommensurate and the tails are tilted with tilt azimuth either parallel to the a or to the b axis (depending on the type of dye) of the dye lattice. The procedure allows formation of mixed crystals with structural parameters, depending on composition, between those of the pure compounds. Hence also the optical spectra can be tuned via composition maintaining sharp band edges and emission bands. Replacing the positively charged lipid molecule with saturated bonds by one with a diacetylene group in the aliphatic region, one can sensitize the photopolymerization of diacetylenes. In accordance with the established mechanism for photopolymerization this indicates efficient charge transfer from the dye crystal to the linear polymer.