Chromophores transfer

Until now, we have not considered the possibility of excitation transfer. However, it is quite common for the following sequence of events to occur (a) light absorption by chromophore /, proportional to Af = Sf Cf (b) given light absorption by chromophore/, transfer of excitation to and emission from chromophore/ with probability in situation k ... 

Most of the work on photoresponsive azobenzene-containing materials is based on polymer matrices. Numerous chemistries have been utilized to graft azobenzene ligands to various polymer chains. The azobenzene chromophores transfer light energy into conformational changes upon photoirradiation, which can be used to control chemical and physical properties of the materials, such as viscosity, conductivity, pH, solubility, wettability, permeability, transport properties, mechanical properties, and structural properties. 

Shichida, Y., Kato, T., Sasayama, S., Fukada, Y., and Yoshizawa, T, Effects of chloride on chicken iodopsin and the chromophore transfer reactions from iodopsin to scotopsin and B-photopsin, Biochemistry, 29, 5843, 1990. 

The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

The deep violet color of pentaphenylbismuth and certain other pentaarylbismuth compounds has been the subject of considerable speculation. It has been shown by x-ray diffraction (173) that the bismuth atom in pentaphenylbismuth is square—pyramidal. WeU-formed crystals are dichromic, appearing violet when viewed in one plane but colorless in another plane. The nature of the chromophore has been suggested to be a charge-transfer transition by excitation of the four long equatorial bonds ... 

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

Ward, W. W., Cody, C. W., Hart, R. C., and Cormier, M. J. (1980). Spec-trophotometric identity of the energy transfer chromophores in Renilla and Aequorea green fluorescent proteins. Photochem. Photobiol. 31 611-615. 

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Since the electrostatic potential sharply decreases with increasing distance from the polyelectrolyte cylinder, the degree of reactivity modification by functional groups fixed to the polyion is strongly dependent on the distance from the cylinder surface. Considerable electrostatic potential effects on the photoinduced forward and thermal back electron transfer reactions, which will be discussed in the following chapters, can be attributed to the functional chromophore groups directly attached to the polyelectrolyte back-bone through covalent bonds. 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

PCSs are systems of chromophores bound into a single macromolecule. Therefore, the study of processes of electronic excitation and energy transfer, as well as the investigation of the ways of deactivation of excited states, should lay a foundation for the understanding of such properties of PCSs as reactivity in photochemical transformations, photosensitizing and photoelectric activity, photoinitiated paramagnetism, etc. 

Barium and strontium salts of polystyrene with two active end-groups per chain were prepared by Francois et al.82). Direct electron transfer from tiny metal particles deposited on a filter through which a THF solution of the monomer was percolated yields the required polymers 82). The A.max of the resulting solution depends on the DPn of the formed oligomers, being identical with that of the salt of polymers with one active end-group per chain for DPn > 10, but is red-shifted at lower DPn. Moreover, for low DPn, (<5), the absorption peak splits due to chromophor-chromophor interaction caused by the vicinity of the reactive benzyl type anions. 

In 159 and 163-166 the tertiary amine function is coordinated to the boron atom and transmits the electronic change due to the ester formation to the chromophore. In 160-162 the boron atom is directly connected to the chromophore. After the complexation of the saccharide, the change of the charge transfer, e.g., for 159 [249-251], or the fluorescence bands, e.g., for 160-166 [252-255], can be measured and interpreted. The most selective binding of n-glucose has been achieved with host 164 that forms a 1 1 complex with a macrocyclic structure (Scheme 1). 

Another class of photochemically relevant polyphosphazenes is formed by macromolecules having chromophores able to absorb light in a selective way and to transfer it to external species, thus inducing different reactions by energy transfer processes. In some cases electron transfer processes are also involved. These situations are described by Formula below and the corresponding polymers and external reagents are reported in Table 26. 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

An extension of this kind of antennae is a first-generation heterometallic den-drimer with appended organic chromophores like pyrenyl units [25,26]. In the tetranuclear species consisting of an Os(II)-based core surrounded by three Ru(II)-based moieties and six pyrenyl units in the periphery, 100% efficient energy transfer is observed to the Os(II) core regardless of the light-absorbing unit. 

Dendrimers with a polyphenyl core around a central biphenyl unit decorated at the rim with peryleneimide chromophores have been investigated both in bulk and at the single-molecule level in order to understand their time and space-resolved behavior [28]. The results obtained have shown that the conformational distribution plays an important role in the dynamics of the photophysical processes. Energy transfer in a series of shape-persistent polyphenylene dendrimers substituted with peryleneimide and terryleneimide chro-mophoric units (4-7) has been investigated in toluene solution [29]. 

Energy hopping among the peryleneimide chromophores, revealed by anisotropy decay times [30], occurs with a rate constant of 4.6x10 s E When three peryleneimide and one terryleneimide chromophores are attached to the dendrimer rim, energy transfer from the former to the latter units takes place with... 

In dichloromethane solutions, excitation of the chromophoric groups of the dendrons causes singlet-singlet energy transfer processes that lead to the excitation of the porphyrin core. It was found that the dendrimer 17, which has a spherical morphology, exhibits a much higher energy transfer quantum yield (0.8) than the partially substituted species 13-16 (quantum yield <0.32). Fluo-... 

---