Structure determination of thiacyanine dye J-aggregates in thin films: Comparison between spectroscopy and wide angle X-ray scattering

Abstract

The structural properties of J-aggregates in thin films were studied by wide-angle synchrotron X-ray scattering, UV/VIS absorption and fluorescence spectroscopy and time-correlated single photon counting. The system of investigation was the cyanine dye (C33H32N2O6S4−·HN(CH2CH3)3+; 2-{3-[1-(3-sulfopropyl)-naphtho-[1,2-d]thiazol-2(1H)-yliden]-2-ethyl-1-propen-1-yl}-1-(3-sulfo-propyl)-naphtho-[1,2-d] thiazolium-betain·triethylammonium salt) incorporated into a Na-polystyrene-sulfonate (PSSNa) matrix. It could be shown that, depending on the preparation conditions, it is possible to create either a monomer phase or a J-aggregate phase in PSSNa films. Both could optically be distinguished through their typical bathochromic shift of the J-aggregate band in the UV/VIS spectra. Wide angle X-ray scattering experiments confirm intermolecular distances of 7 Å in the J-aggregate and an angle of inclination of 20°. Out of optical studies, the angle of inclination was determined to be around 30°. A more detailed interpretation of the wide angle X-ray scattering signals suggests that 80% of the formed J-aggregate consists of dimers and 20% of a helical arrangement with a diameter of ca. 16 Å. The used stacking model was built out of mono-cis conformer units which were confirmed to be the most stable conformation by DFT calculations (6-31G* basis set, BeckeLyp functional).