Structure of merocyanine dyes with a hydrocarbon chain studied by vibrational spectroscopy .2. IR study in cast and Langmuir-Blodgett films

IR spectra have been measured for cast films of 3-carboxymethyl-5-[2-(3-decylbenzothiazolin-2-ylidene)ehtylidene] rhodanine (C(10)MD) and 3-carboxy-methyl-5-[2-(3-octadecylbenzothiazolin-2-ylidene)ethylidene]rhodamine (C(18)MD), and cast and Langmuir-Blodgett (LB) films of C(18)MD treated with BaCl2, CaCl2, and NaCl neutral solutions. The following conclusions can be reached: (1) At room temperature C(18)MD in the cast films assumes a free form as well as an intramolecularly hydrogen-bonded form. However, as the temperature is increased to 120 degrees C, the free form is converted into the intramolecularly hydrogen-bonded form; further increase in temperature leads to formation of the free form again. (2) Treatment of the cast films of C(18)MD with the chloride solutions causes the formation of J-aggregates. The hydrocarbon chain is highly ordered and C(18)MD forms weak intramolecularly hydrogen-bonds via a cation in the J-aggregates. (3) The IR spectra of the LB films of C(18)MD, on formation of J-aggregates, are similar to those of the cast films treated with the solutions. The electronic structure in the conjugated system undergoes a significant change upon the formation of J-aggregates. (4) The C=O group, and thus the chromophore plane, are nearly perpendicular to the substrate surface in both the cast and LB films.