Dye absorption band

The physical and optical properties of the NPs used in this investigation are described in Table 6.1. The optical absorption properties of the ruthenium dye complex are also detailed in the table. It can be seen that there is good overlap between 7 of the pure silver and alloy NPs and the absorption band of the complex, while the gold NPs lie outside the absorption peak and are used as a negative control. The dependence of the excitation spectra of the dye complex on NP-dye distance is shown in Figure 6.14 for the case of the pure silver NPs. Also included in the figure is the excitation spectrum for the complex coated on the PEL layer in the absence of NPs. From Table 6.1, it can be seen that there is very good overlap between 7 of the silver NPs and the dye absorption band which constitutes the optimum plasmonic enhancement condition for the case of excitation enhancement. 

Kuhn H 1958 Oscillator strength of absorption bands in dye molecules J. Chem. Phys. 29 958-9... 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

Figure 9.33 shows examples of SVLF spectra obtained by tuning a frequency-doubled dye laser to the Og absorption band of the system of pyrazine (1,4-diazabenzene)... 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

Unlike PMDs having a single chromophore, the absorption band of a bis-dye spHts into two components, so that one maximum is shifted bathochromicaHy and the other hypsochromicaHy with respect to the absorption maximum of the parent dye. The distance between bis-dye maxima depends on the magnitude of the chromophore interaction. Interaction of this kind has been discovered to be universal (45,46). 

There are two main kinds of dye aggregates, characterized by their typical spectral properties J-aggregates and H-aggregates. The absorption band maximum (f-band) of the J-aggregates is shifted bathochromicaHy with respect to that of an isolated molecule (M-band) the absorption maximum of the H-aggregates is shifted hypsochromicaHy (H-band). The dyes can also form dimers with a shorter absorption wavelength (D-band). 

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

The interaetion of PVP for various moleeular masses (8T0 - 360T0 ) with 3 groups of organie reagents sueh as triphenylmethane, trioxyfluorone and azodyes is established with the methods of UV-, VIS- and IR- speetroseopy. The ehanges of speetroseopie properties of dyes at addition of polymer ar e shown in shift of maximum of absorption bands, displaeement of reagent dissoeiation. The influenee of moleeular masses of polymer on properties of azodye-PVP adduets is studied. 

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

The merocyanine dye mentioned above shows solvatochromism, which means that the absorption band maximum of the quinoid form (D form) is sensitive to solvent polarity [40,41]. In Fig. 3, the absorption maximum of the solvatochromic band for M-Mc (a low molecular weight merocyanine analog) is plotted against the dielectric constant of 1,4-dioxane/water mixtures [42]. With the relationship... 

It has become recognized during recent years that the color of dyes is associated with the resonance of electric charge from atom to atom of the dye molecule.2,3> 4 6 6 Because of the complexity of the problem, however, it has not been easy to expand this idea into a theory of color permitting the rough quantitative calculation of the frequencies and intensities of the absorption bands of dyes. I have now developed a theory of this nature the theory and some of the results of its application are described briefly in the following paragraphs. 

The long-wave-length absorption band of a dye such as benzaurin or Dobner s violet has been associated with resonance of the type... 

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

Compound 20 is noteworthy in that it was the first azo dye reported whose absorption band is shifted beyond the visible region into the... 

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