Chromophores in Solution

Aminocyclopentene-l-dithiocarbamic acid (LH) forms a series of complexes, MIjCM = Ni", Co", or Cu"), in which the ligands are S,S-bonded (427). Diethanoldithiocarbamic acid forms Cu[(HOC2H4)2-NCS2]2, which is a monomeric, planar, CUS4 chromophore in solution, but Cu - S interaction probably occurs between neighboring molecules in the solid state (428). 

Kummer AD, Kompa C, Niwa H, Flirano T, Kojima S, Michel-Beyerle ME (2002) Viscosity-dependent fluorescence decay of the GLP chromophore in solution due to fast internal conversion. J Phys Chem B 106 7554-7559... 

Beer s law says the absorbance A of a chromophore in solution increases in direct proportion to its concentration. 

A number of Se(ll) and Te(Il) complexes with dithiocarbamate (and related) ligands apparently form paramagnetic MS2(C2v) chromophores in solution (150). The electronic spectra of these systems can be treated as p-p spectra, and have been successfully interpreted using the AOM. 

The excited-state kinetics of the chromoprotein were found to differ markedly from the one of the isolated chromophore in solution. A strong and fast biexponential decay is observed and seems to sign a specific deactivation channel, still to be properly identified. This process might well be an electron transfer from the chromophore to the protein, as earlier works had suggested [12]. It is additionally possible to suggest that the nonexponential nature of the fast decay could reveal a structural heterogeneity in the oxyblepharismin-protein complex. 

The approach here is based on the work of Mukamel [1], to which the reader is referred for a much more complete description. Consider an optical transition between the ground and excited states of a chromophore in solution. The optical transition frequency of the ith chromophore can be written as... 

The blue-shift results from the chemical environment inside the pores of MCM-41, which is dominated by residual silanol and, especially, non reacted amino groups, and thus more basic compared to that in solution. The interaction of the dye molecules with the host is further confirmed by a broadening of the absorption bands of anchored dyes (e.g. rhodamine B sulfonylchloride, Figure 4) in comparison to the main bands of the free chromophores in solution. 

Next, we should survey the fate of the excited states and the different relaxation mechanisms. Such processes may follow a photoinduced excitation of organic chromophores in solution. An overview of the different relaxation pathways in solution will be presented in the following sections. In addition, we will discern practical ways to identify these processes by conventional spectroscopic methods. 

Because the chromophores are tethered to the surface, rotational motion of the chromophore is confined to a conical volume. In contrast, for chromophores in solution, R(00) = 0, since the chromophores attain random orientations at infinite time. In monolayers, R(oo) > 0, since they are motionally restricted, and R(00) is related to the average tilt angle of the chromophores according to the following relationship ... 

Furthermore, as it is seen from numerous experiments, the far red wing of the absorption profile of chromophores in solutions does not appear to follow a Lorentzian decay, as implemented in the conventional expression for the frequency dependent linear absorption cross section, but rather some kind of fast exponential, Urbach-like, decay [22, 23]. 

The described approach to spectroscopic properties of pp chromophores in solution is semiempirical in nature it is based on a specific model for the electronic structure, molecular vibration, and solute-solvent interactions that allows to calculate a large variety of (low-energy) spectroscopic properties, including steady-state... 

Fig. 12.8. Ground state CASSCF optimized structures for the Rh chromophore in solution, in vacuo and in the protein compared to the crystallographic and NMR structure (adapted from Ref. [36]).

Kim Y R, Share P, Pereira M, Sarisky M and Hochstrasser R M 1989 Direct measurements of energy-transfer between identical chromophores in solution J. Chem. Phys. 91 7557-62... 

The results of intramolecular excimer fluorescence studies indicate that the helical polypeptide is a rigid enough molecular framework to hold chromophores in solution without large fluctuations for about 25 ns at 20°C and 40 ns at — 40°C. 

It should be possible, in principle, to separate the two contributions to by control of the temperature. Extensive experiments with solutions of aromatic chromophores unbound to polymer chains have shown that at sufficiently low temperatures, excimer formation will be diffiision controlled. At sufficiently high temperatures, the Birks dynamic equilibrium regime will be reached, and the binding energy of the system will be the important parameter. This treatment of the photophysics appears to work quite well for the free chromophores in solution and even for the end-labeled chains... 

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