PHOTOCHROMIC BEHAVIORS OF LONG ALKYL CHAIN SPIROPYRANS AT THE AIR-WATER-INTERFACE AND IN LB FILMS

Systematic monolayer characteristics and photochromic behaviors of long alkyl chain spiropyran compounds were observed in a monomolecular layer at the air-water interface and in LB films. UV-induced merocyanines of l´-octadecyl- (SP12), 1´- hexadecyl- (SP16), and l´-octadecyl-3´,3´-dimethyl-6-nitrospiro[2H-l-benzopyran-2,2´-indoline] (SP18) gave stable monolayers at the air-water interface. These monolayers changed from spiropyran (SP) to photomerocyanine (PMC) forms with UV irradiation for lower surface pressures and formed J aggregates with increase in surface pressure. Photochromic reactions including J aggregation were reversible at the air-water interface, and the half-decay periods of J aggregates were 103 times longer than those of transferred films. The formation of the aggregates had not been previously observed for the reason that J aggregates in LB films immediately returned to SP via the PMC form. This behavior is quite different from that of SP1822, which is the first J-aggregated photochromic compound observed. From measurement of the absorption with various angles of incidence of the polarized beam at the air-water interface, the change of anisotropy of the absorption in a layer plane was observed with increase in the formation of J aggregates depending on the surface pressure. The transition dipole moments of chromophores were randomly oriented on the subphase for lower surface pressures, corresponding to PMC form, and then took up a nearly flat orientation parallel to the direction of compression at higher surface pressures, corresponding to J aggregates. A model of molecular arrangements of long alkyl chain spiropyrans at the air-water interface in the process of the photochromic reactions, including the formation of J aggregates, is proposed. The molecular arrangement at the air-water interface suggests the formation and stability of J aggregates in LB films, and furthermore it indicates the difference for photochromic behaviors of the spiropyrans. © 1990, American Chemical Society. All rights reserved.