Origin of formation of blue-shifted aggregates including H-aggregates in mixed Langmuir–Blodgett films of merocyanine dye investigated by polarized visible and infrared spectroscopy

The structure of a merocyanine dye (MS) in red- and blue-shifted aggregates involving J- and H-aggregates in mixed LB films of MS–arachidic acid (C₂₀) binary and MS–C₂₀-n-octadecane (AL₁₈) ternary systems (MS]:C₂₀]:AL₁₈] = 1:2:x; x = 0, 1.0, 2.0, 3.0, 4.0 and 5.0) fabricated using an aqueous subphase containing Cd²⁺ ions has been investigated by means of polarized visible and infrared spectroscopy. Not only the AL₁₈ molecules mixed as a third component but also the Cd²⁺ ions added to distilled water for the subphase used turn out to be the significant elements for the formation of the MS aggregates in all the ternary systems. It has been found that the degree of the MS intramolecular charge transfer strongly depends on the aggregation states; the J-aggregate (x = 0), H-aggregate (x = 1.0), oligomer (x = 2.0) and relatively small aggregates (x = 3.0, 4.0 and 5.0), and that the stabilization by the intramolecular charge transfer for the formation of the MS aggregates is induced according to the individual aggregation states. The gauche conformation is clarified to be slightly involved in the MS hydrocarbon chains for all the binary and ternary systems. The long axis of the MS hydrocarbon chains in the J-aggregates, being fairly tilted from the film normal, gradually approaches to the film normal with the increasing AL₁₈ content up to x = 2.0 with the orientation of the long axis of the MS hydrocarbon chains remaining constant in the subsequent ternary systems. It can be concluded that the formation of the red- and blue-shifted aggregates is ascribed to the difference in the MS slippage which can also be regulated by the degree of the orientation of the MS hydrocarbon chains, and that the decrease in the size of the MS aggregates accompanied by the increasing AL₁₈ content is due to the blocking by the AL₁₈ molecules being out of the empty space on the MS chromophore.