FCPM is very similar to FCM, with two basic distinctive features: (a) the incident light is polarized and (b) the fluorescent dye molecules are aligned by the "host" material.
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Imagine a nematic liquid crystal cell doped with a fluorescent dye. For simplicity, assume that the nematic is of a calamitic type (elongated molecules) and that the fluorescent molecules are also elongated; one such dye molecule is shown as an ellipsoid. The transition dipoles of both excitation and fluorescence (double arrow at the ellipsoidal dye molecule) are along the long axis of the dye molecule. As it is well known, from the earlier studies of the so-called "guest-host" display modes, the anisometric "guest" molecules are aligned by the nematic "host".
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Requirements to the fluorescent probe for FCPM:
1. Anisometric shape
2. Transition dipole moment τ along the long axis
3. Uniformly distributed and well aligned in the liquid crystal.
Examples of some different types of dye alignment in lamellar lyotropic LCs due to hydrophilic-hydrophobic effects are shown below
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Some of the used fluorescent dyes
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Acridine Orange Base |
Rhodamine 6G, perchlorate |
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5-decyl-4,4-difluoro-4-bora-3a,4a- |
DiOC16 (3) |
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Nile Red |
N,N'-Bis(2,5-di-tert-butylphenyl)- |
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