Chromophores aggregation

C1.5.4.5 CONJUGATED POLYMERS, CHROMOPHORE AGGREGATES, AND MICROHETEROGENEOUS MATERIALS... 

The photochemistry of small molecule LC materials has been an active area of research for many years and has been reviewed recently [9]. The photochemistry of LC polymers, per se, has received much less attention although two brief reviews have appeared [5,10], and there has been a considerable effort to apply some simple photochemical transformations such as trans-cis photoisomerization, to the development of practical devices [1-6]. This section is divided into three parts. In Part A, chromophore aggregation, which seems to be important in almost all the cases in which careful UV-Vis and/or fluorescence studies of films of pure LC polymers have been made, is explicitly discussed. Part B is devoted to a thorough review, organized by chromophore type, of the photochemistry and related photophysics of LC polymers. No attempt has been made to extensively cross-reference the work on LC polymers to the hundreds of papers and reviews on analogous non-LC compounds. However, when it seemed particularly appropriate or interesting, experiments related to optical applications of the photochemistry of LC polymers are briefly described. In Part C, a few experiments are described in which a classical photophysical method, fluorescence spectroscopy, is used to probe the microstructures of some LC polymers. 

The purpose of this experiment is to determine up to which PNP concentration, the Beer-Lambert-Bouguer law is still linear. In fact, this law cannot be applied when the optical saturation of the photomultiplier is reached and/or when the chromophore aggregates. Each spectrophotometer cannot count more than a precise number of photons. When this number is reached, the OD given by the spectrophotometer will be the highest, and the photomultiplier is saturated. Beyond the saturation concentration, the spectrophotometer will always give the same OD that corresponds to that of saturation. 

We discussed how the numbers, size, and nature of the akyl chain(s) can influence the formation of inverted vesicles or micelles which could open new perspective toward the synthesis of nanomaterials and, in particular, of photocatalysts. Multi-chromophoric aggregates can also have an important role as light-harvesting units or for the constitution of multicolor emitting materials. 

Menzel, H., Riither, M., Fischer, T., and Stumpe, J. Discrimination of Structural Order and Chromophore Aggregation as Factors Effecting the Photo-reorientation of Azobenzene in Copolyglutamate LB Films. Supramol. Sci. 5, 49 (1998). 

Very highly excited states can be produced by cavitational collapse caused by ultrasonic irradiation alkanes under argon produce emission from C2H, and CH, whereas under nitrogen, emission from CN predominates. Creed et al. have demonstrated photochemically induced chromophore aggregation in a liquid crystalline poly(arylcinnamate). 

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