Dyeing crystals: 19th century phenomenology to 21st century technology
Benedict, Jason Brown
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The linear dichroism of organic light-absorbing molecules (dyes) contained within varied solid matrices are analyzed. Hosts of increasing disorder from single crystals, to organized polycrystalline formations, to polymers are examined. Organic chromophores contained within single crystal hosts serve as optical reporters of the local chemical environment within the crystal and on the growing crystal surfaces. The selectivity for various crystallographic facets combined with knowledge of the structure of the surfaces allows inferences to be drawn regarding the non-covalent interactions that govern crystal growth from solution. Peculiar concepts from the descriptive crystallographic literature of the 19th century are framed in contemporary terms. Attempts to utilize non-centrosymmetric dyed crystals as gain media for up-conversion lasing are described. Acentic media are likewise considered for electrooptic modulation of light. In principle, the growth front of radial non-centrosymmetric polycrystals, known as spherulites, would orient dyes dissolved in the melt in a polar manner. The linear optical properties of dyed spherulites are described. While the strong linear dichroism of the spherulite was indicative of dye alignment, the absence of an enhanced second harmonic generation signal reveal that the dye domains remain effectively centrosymmetric. These materials are nevertheless instructive in illustrating the principles of the orientational dependence of linear dichroism, a subject of long standing confusion. Strong electric fields applied to thin polymer films containing a large proportion of organic chromophores continues to be a popular approach to generating polar ensembles of molecules. Measurements of the linear optical properties of poled polymer films were modeled to assess the magnitude of chromophore alignment and degradation during poling.
- Chemistry