Two chromophores

When two or more chromophores are present in the same molecule, their absorption is usually additive as long as they are separated by two or more single bonds. Two chromophores in conjugation (i.e., separated by only one single bond) give rise to a new tjrpe of absorption with increased Some examples are given in Table IX. 

Rhodacyanines possess two chromophoric systems. They are at the same time neutrocyanine derivatives, which involves position 5 of the ketomethylene, and methine cyanine, which involves position 2. Following lUPAC s standard nomenclature rules, structure 7 is named 3-ethyl-4-phenyl-2- 4-oxo-3-ethyl-5-[2-(3-ethy]-2,3-dihydro-benzo-l,3-thiazo-lylidene)ethylidene]-tetrahydro-l,3-thiazolylidene-methyl -1.3-thiazolium iodide (Scheme 5). It implies that the 4-phenyl thiazole ring having the... 

Each tetracycline possesses a characteristic uv-absorption spectmm and this property is used extensively in stmcture elucidation (12,13). This spectmm results from the contribution of two chromophores the BCD ring system gives a X at approximately 350 nm and the A-ring a X at... 

Metal Introduced Into the Dyestuff Molecule in Manufacture. These dyes fall into two categories according to the ratio of metal cation to dye molecule anion 1 1 metal dye complexes comprise one metal cation and one chromophoric ligand and 1 2 metal dye complexes comprise one metal cation and two chromophoric ligands. 

Clearly the reaction follows first-order kinetics, with a rate constant of k2 (or k f.2). This special case may not be so uncommon as these coincidences imply. If A contains two equivalent substituents and I one, then the relation k2 = k /2 will hold if they react independently. And if P contains two chromophores and I one, the molar absorptivities may well be approximated as eP = 2ei. Cases like this, and others closely related to it, are described by several authors.2-5... 

Figures 3a-f show the emission and excitation spectra for all six humic fractions. The excitation and emission maxima are listed in Table III along with the maxima of the phase-resolved emission spectra. In each case the emission spectrum was scanned with the excitation maximum wavelength held constant, and the excitation spectrum was scanned with the emission maximum wavelength held constant. Several interesting features are noted. The two humic samples ( Figures 3a,b) each have two excitation maxima and it appears that a double peak has been merged into the emission scan as evidenced by the shoulder on the high energy side of the emission peak. Similarly it seems evident that the exaggerated shoulders in the emission spectra of all the fractions point to the inclusion of two emission peaks in each spectrum. This evidence suggests the presence of two chromophores in each humic fraction.

Humic materials fractionated on the basis of hydrophobicity and proton affinity continue to exhibit two fluorophores as discussed in the section "Exciation-Emission Spectra. Strong evidence to establish the existence of at least two chromophores is seen in the phase-resolved spectra. These spectra are shown in Figures 4 a-f. They consist of the phase-resolved emission spectrum of each of the two fluorophores plotted separately and the normal emission spectrum of the humic fraction. If the nulling out of one fluorophore is exact then the sum of the two separate phase resolved spectra should be additive to equal the normal spectrum. In these figures the normal emission spectrum was measured separately from the two phase resolved emision spectra. The phase resolved spectra were then superimposed onto the scan of the normal emission spectrum. 

Because of our obvious prejudices we will describe this study in some detail. The energy levels of the acetophenone and -methylstyrene moieties are ideally situated for intramolecular triplet energy transfer (see Figure 6.12). If these two chromophores are not interacting, then the absorption spectra for these compounds should be the composite of the acetophenone-/ -methylstyrene spectra. Figure 6.13 indicates that this is true for n = 4 (and also 2 and 3) however, it is not correct for n = 1 (Figure 6.14). In the case of n = 1 the increased intensity of the vibronic structure of the n - n ... 

A FRET pair is constituted by two chromophores, one acting as energy donor and the other as acceptor. The efficiency of the energy transfer process is described as ... 

The experiments are based on the fact that the probability of energy transfer between the two chromophores depends strongly on their separation distance [314]. For the Np-Py pair the characteristic distance of the process is 28.9 A [315]. Thus, the occurrence of NRET between Np and Py in dilute mixed solutions of polymers carrying either Np or Py signals the existence of interpolymeric association. By assessing the extent of energy transfer under various circumstances it becomes possible to monitor closely the interactions between polymer chains. 

CTC-HC1 is the HC1 salt of amphoteric CTC it is multifunctional with two chromophores. It is a para-chlorophenol with an ot,3-unsaturated ketone in conjugation. 

For a system containing two chromophores i and j, the exciton chirality (positive or negative) governing the sign and amplitude of the split Cotton effect can be theoretically defined as below59 ... 

In the case of a molecule having two identical chromophores connected by er-bonds in some orientation, it is probable that the state of one excited chro-mophore is the same as that of the other, as the excited state (exciton) delocalizes between the two chromophores. 

A molecule containing two chromophores oriented in chiral positions can be defined to have either negative or positive chirality as depicted in Figure... 

One may easily visualise ET taking place between two chromophores that lie within van der Waals contact (Figure 2a). In this so-called through-space... 

The distance used for calculating fi may be measured either in A or by the number of bonds which connect the two chromophores. For the norbomylogous bridge, one bond is equivalent to 12 A... 

Oxy- and deoxy-hemoglobins are mainly of interest because they are related to the regional cerebral blood flow (rCBF). The focal change in rCBF determines the activation state. The term activation usually refers to the focal increase in rCBF whereas a decrease is called deactivation [78]. With the dual wavelength approach, one can derive two simultaneous equations to be solved for each of the two chromophore concentration changes. To this end, Equation (4) is split into two parts, separating the contributions from HbO and Hb. Equation (4) is then, rewritten as... 

Both the binuclear iron center and the tyrosyl radical contribute to the visible spectrum of active RRB2. By reducing the radical with hydroxyurea, the contributions of the two chromophores can be separated (Figure 4) (60). The radical has sharp absorption peaks at 390 and 410 nm and a broad absorbance at 600 nm which are also found in 2,4,6-tii(rcrt-butyl)phenoxy radical (61). The iron chromophore has a spectrum very similar to that of metHr(OH). 

Interaction of two chromophores exhibiting allowed (strong) n-n absorption bands splits the excited state into two energy levels with the energy gap 2VSj (Davydov splitting), Figure 12. 

Figure 12. Splitting the excited slate into two energy levels by exciton interactions between the two chromophores i and j. Reprinted with permission from N. Harada, K. Nakanishi, Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books, Mill Valley, California, 1983.

For non-coplanar electric transition dipole moments pi0a and pj0a of the two chromophores at Rjj interchromophoric distance the exciton chirality is nonzero and defined by ... 

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