Two-Chromophore Systems

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... 

If 4>no and (A = 1,2) are localized ground- and excited-state wave functions of the chromophores k, the ground state of the two-chromophore system may be described by Pq = 4>,o4>2o. whereas the excited states 4 = N (4>i,4>2o 4> 4>2f) are degenerate in zero-order approximation. The exci-ton-chirality model only takes into account the interaction between the transition dipole moments A/, and localized in the chromophores. Thus, the interaction gives rise to a Davydov splitting by 2Vj2 of the energies of combinations and of locally excited states. From the dipole-dipole approximation one obtains... 

The absorption bands in the visible region of the spectra are attributed to resonance between the quinonic ring and the substituents. In 1,4 disubstituted anthraquinones, an interaction between the two chromophoric systems occurs, giving a resonance effect between two extreme structures. This difference in behaviour has been pointed out 1,4 dihydroxy and diamino compounds show a double-headed peak that is not shown by 1,5 compounds. The latter ones behave as twice the corresponding monosubstituted compound (the molecule would than be divided diagonally) (Fig. 12) [8],... 

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... 

Sassoon and Rabani [79] also prepared a two polymer system in which a chromophore was covalently bound to one polyelectrolyte and a donor or acceptor was electrostatically held by the other polyelectrolyte, and showed that its back ET underwent a similar retardation effect. They employed 26 as a photosensitizer, MV2+ as a mediator, and ferricyanide as an acceptor electrostatically bound to the added polycation (polybrene). 

The chemistry of these phthalides has much in common with that of the compounds reviewed in the next chapter, i.e., the feMco-triarylmethanes, and in fact, the parent dyes of the two classes share the same basic chromophoric system. However, the latter are true redox systems, rather than pH indicators, and consequently have a different range of technical applications. The situation is complicated further in that the triarylmethane cationic dyes can also bleach at high pH, giving a hydroxide addition... 

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 ... 

Figure 22. (a) Two tetra-chromophoric systems 24. (b) Profiles of the (U)HF/ 3-21G gas phase structures for the "collapsed" ground state and giant CS state of syn,syn-24. The "normal" ground state structure was located using the AM1 semi-empirical method. 

In the strong coupling case, the transfer of excitation energy is faster than the nuclear vibrations and the vibrational relaxation ( 10 12 s). The excitation energy is not localized on one of the molecules but is truly delocalized over the two components (or more in multi-chromophoric systems). The transfer of excitation is a coherent process9 the excitation oscillates back and forth between D and A and is never more than instantaneously localized on either molecule. Such a delocalization is described in the frame of the exciton theory10 . 

A colorimetric estimation of inserted PEG-phospholipids was developed based on the two-phase system used to quantify phospholipids (49). The formation of a complex between the phospholipids and Fe(SCN)3 transfers the chromophore Fe(SCN)3 to the organic phase, allowing quantification of the phospholipids present in the solution. This system was applied to PEG-phospholipids (50). It is quite sensitive but obviously limited to PEG-phospholipids. The PEG-lipids, which bear no phosphate group, cannot be quantified by this method. 

Fourfold Heck coupling (twofold on each dibromobenzene moiety) and Jeffery conditions has been applied on the two easily accessible tetrabromo[2.2]paracyclophane isomers 21 and 23, respectively, to prepare new double-layered 1,2- and 1,4-distyrylbenzene chromophoric systems 22-R and 24-R respectively, to enable studies of intramolecular charge-transfer phenomena in such systems (Scheme... 

For this three-state model the values of 52pa can be determined by the Eqs. 10 and 11. In this simple model the chromophore system of the molecule can be modeled by two arbitrarily oriented linear oscillators, (igg and Afigg (for excitation into the first excited electronic state Si), or by the figg and ligg/ (for excitation into the final electronic state Sy), which simultaneously absorb two photons and transfer their energy to the emission oscillator, fi. It has been shown that the limiting value of fluorescence anisotropy T2pa can be written as [23] ... 

Phytochrome is found not only in higher plants but also in algae, where it controls the movement of chloroplasts,611 and also in cyanobacteria.623 54 Cyano-bacterial phytochromes contain histidine kinase domains, which may function in a two-component system with a response regulator similar to protein CheY of the chemotaxis system in E. coli (Fig. 19-5).624/625 Some nonphotosynthetic bacteria also use bacteriophytochromes for light sensing. In some cases biliverdin (Fig. 24-24) is the chromophore.6253... 

When the anthracene chromophore is substituted at the 9-position by a re-system, coplanarity of the two re-systems will be impaired for steric reasons, and so will be electronic conjugation. For example, in ground state 9-phenylanthracene the angle between the plane of the anthracene and that of the phenyl group is about 60° [17,18]. Likewise, in 9-acetylanthracene, the... 

Thus, we can assume that two types of stilbene structures contribute to the chromophoric systems of kraft lignin. If further double bonds or carbonyl groups are conjugated with the stilbene as in XIII, the resulting structure is likely to have absorption within the visible region of the spectrum. 

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