Monomeric Dyes

From this summary it is apparent that water solutions of Pseudocyanine above ca. 5 X 10"5M contain a multicomponent mixture of differently absorbing species. Hence, the resulting solution spectra are neither expected nor found to exhibit isosbestic points (12). However, as will be shown, in the presence of appropriate adsorbents the spectra of Pseudocyanine can be drastically modified by imposition of an equilibrium between monomeric dye in solution and its /-aggregate at the substrate surface. 

O Brien, Kelly, and Costa (241) confirmed the earlier conclusion by West that intersystem crossing is of little importance in typical sensitizing dyes and that the major competing processes are fluorescence and internal conversion within the singlet manifold. They concluded that ksens for monomeric dyes 9 -1... 

The present chapter concentrates on the photoelectrochemistry of aggregates of cyanine dyes adsorbed at the semiconductor electrodes with special emphasis given to the difference in the photosensitization action of polymer aggregate species and monomeric dye. 

The CD induced in the visible absorption bands of proflavine cation intercalated to DMA was characterized in terms of the dye-DNA base pair exciton interactions [84]. In this work, the data were obtained at very low amounts of intercalated solute relative to the amount of oligonucleotide, with the goal of observing the CD spectra of isolated and monomeric dye systems. Substantial ionic strength effects were noted, indicating the electrostatic nature of the interaction. Comparison of the data with the results obtained after theoretical modeling indicated that the proflavine molecules insert in the DNA helix with the molecular plane parallel to the stacking plane of the DNA base pairs. 

According to exciton theory [82,83], the excited state energy level of the monomeric dye splits into two upon aggregation, one level being lower and the other higher in energy than the monomer excited state. The transition to the higher state is forbidden for head-to-tail (J-type) dimers, whereas the lower... 

VI), respectively, show the 248-nm and 193-nm polymeric BARC design antireflection coating architectures. Typical examples of monomeric dyes are shown in... 

Fig. 24. Spectral changes during spin-coating of solutions of pseudoisocyanine derivatives PIC 2-18 solution [the time from the moment of placing the solution on the centrifuge is shown in the inset, optical density at the absorption maximum of the monomeric dye in solution (530 nm) as a function of spin-coating time] (a) PIC 2-2 solution with added K2B10H10 during formation of J-aggregate (spectra recorded every 0.05 sec) (b).

Another aspect of the change the local field factor at the /-aggregate formation is the dramatic increase of the J-peak at the last moment of formation of the solid film without a decrease of the optical density at the absorption maximum of the monomeric dye, i.e., the significant growth of the optical density of the J-peak is not compensated by a decrease of the monomer optical density. An additional increase of the J-peak, for example, for PIC 2-2 (+K2B10H10) by 2.15 times can be seen from the measured absorption spectra. 

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