Image transfer efficiency

Figure 17. Schematic representation of image transfer efficiency for a 1 1 projection printer. (Reproduced with permission from Ref. 1)...

Schematic representation of image transfer efficiency for a 1 1 projection printer.

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

Porphyrin is one of the most widely studied macrocyclic systems suitable for complexation with lanthanide(III) ions. Porphyrins can absorb strongly in the UV-vis region so as to serve as efficient photo-sensitizers, making lanthanide(III)porphyrinate complexes ideal candidates for luminescence imaging agents. Indirect excitation of porphyrin antenna chromopheres in close proximity to lanthanide ions can make the energy in the triplet state of the porphyrin ligand transfer efficiently to the excited state of the lanthanide ion so as to sensitize the lanthanide luminescence, particularly NIR emission. 

A second approach also considers three populations free (unquenched) donors No, free acceptors NA, and a population engaged in FRET pairs Ns that transfer energy with characteristic efficiency E (between 0 and 1). However, in this case, the Ns population emits both donor fluorescence (quenched by a fraction (1 - E)) and sensitized emission (proportional to ENS). To keep in line with the treatment and terminology in other chapters in this volume, this latter approach will be followed here. Note, however, that in other chapters the population of FRET pairs is indicated by the subscript DA whereas we stick to the notation Ns to indicate that this quantity is based on photons emitted from sensitized emission (S image) and to keep the close synonymy with the former approach. Thus, our Io-s equals Ida and Is +1 a equals IAo- Both ways yield essentially identical results. 

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