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The optical properties of monomolecular layers at an air–water interface and also of Langmuir–Blodgett (LB) films of several photochromic spiropyrans with a single hydrophobic long chain and either a hydroxyl or a hydroxymethyl group have been investigated. Changes in the optical spectra of these films were induced by either UV irradiation or heating. UV-visible, and IR absorption spectra showed that spiropyran SP145 formed J-aggregates and SP147 formed H-aggregates. The half-decay period of the J-aggregate to the spiropyran (SP) form was longer than that of the colored monomeric photomerocyanine (PMC) to the SP form. It was concluded that hydrogen bonding between neighboring colored PMC molecules via hydroxyl, hydroxymethyl, and nitro groups stabilizes the aggregate state.

The formation of J-aggregates in Langmuir monolayers of a neat merocyanine dye (3-carboxymethyl-5-[2-(3-octadecyl-2-benzothiazolinyldene)ethylidene]-2-thioxo-4-thiazolidinone) has been examined by measuring the normal incidence absorption spectra. Five types of J-aggregates were found to be formed, depending on the temperature and the concentration and type of cations present in the subphase. The shift in the absorption peak wavelength on the J-aggregates formation is accounted for in terms of the number and the geometrical arrangement of the chromophores in each J-aggregate. The spectral shifts were analyzed numerically using both a point dipole model and a transition density model. These calculations indicate that the five different J-aggregates are linear ones with aggregation numbers ranging from 2 to 6.

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The aggregate formation was studied for Langmuir–Blodgett films of a spiropyran 5Me-SP1822 (1′-octadecyl-3′,3′-dimethyl-5′-methyl-6-nitro-8-(docosanoyloxymethyl)spiro[2H-1-benzopyran-2,2′-indoline]). Both a J-aggregate (λmax = 605 nm) and an H-aggregate (λmax = 493 nm) were formed, depending on the nature of the matrix molecule and the annealing temperature. The compound, having a substituent at the 5′-position, seems to prefer H-aggregation to J-aggregation by heat treatment at elevated temperatures. The molecular orientation in these aggregates was evaluated via analysis of the shifts in the first two absorption bands appearing after formation of the aggregates. Slip angles of 20—30° and 70—80° between the chromophore’s transition moment and the center-to-center line of the molecules, were found for the J- and H-aggregates, respectively, and a tilt angle of 20—30° between the chromophore’s plane and the film surface were obtained using the transition density model. These are in line with the results of Fourier-transform infrared (FT-IR) analysis.