Chromophores photoreactions

The greater intensity of the band of the metabolite at 220 mis probably due to the presence of a second, superimposed chromophore which could also account for the shift of the minimum. On the other hand, the band near 300 m/u. has the expected intensity. Its broadness and displacement towards longer wavelength are probably due to the presence of a substituent on the double bond or benzenoid ring. That the assignment to a coumaroyl chromophore is essentially correct is evidenced by the fact that both M and the model compound underwent the same type of reaction on irradiation in the near-ultraviolet (Figure 4). The observed isosbestic points imply that the photoreaction is a simple one, such as A -> B or A = B, and is obviously the well-known light-induced trans- to c/r-isomerization (7) of cinnamic acid derivatives. 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Abstract Ultrafast photoreactions in PNS of PYP have been studied by means of fs fluorescence up conversion method. Conclusions obtained are (a) Photoreaction in PNS (chromophore twisting) occurs from vibrationally unrelaxed fluorescence state and coherent oscillations in the fluorescence decay curves have been observed for the first time, (b) Comparative studies on fluorescence dynamics of mutants and w.-t. PYP have proved that the w.-t. PYP is best engineered for the ultrafast reaction, (c) The coherent oscillations in the fluorescence decay completely disappeared and the reaction was much slower in the denatured state, demonstrating the supremely important role of PNS for the photoreaction. 

From such a viewpoint, we are examining primary processes of photoreactions of PYP [1] which functions as a blue light photoreceptor for a negative phototaxis of the purple sulfur bacterium Ectothiorhodospira halophila, some FP s [2] and Rh [3] by means of the fs fluorescence up-conversion measurements. In this article, we will discuss our latest results of fs fluorescence dynamics studies on PYP, because PYP is very stable for repeated irradiation which induces photocycles so that the very accurate experimental results can be obtained rather easily and also the preparation of the site-directed mutants as well as the PYP analogues with modified chromophores are rather easy. However, before that, we will summarize briefly results of our previous investigations. 

The ultrafast photoreactions in PNS of these proteins take place immediately after conversion from the FC state to vibrationally unrelaxed or only partially relaxed FI state [1-3]. For PYP [1] and Rh [3], the primary process is twisting of the chromophore, which causes the ultrafast fluorescence quenching, in the course of the isomerization, while the primary process for FP [2] is the ultrafast electron transfer leading to the fluorescence quenching reaction in PNS. Thus, in spite of the different molecular structures of PYP, Rh and FP chromophores and different kind of photoinduced reactions, these photoresponsive proteins show ultrafast and highly efficient photoreactions from FI state of similar nature (vibrationally unrelaxed or only partially relaxed), suggesting the supremely important role of the PNS controlling the reactions. 

The L-B films offer some advantages over aqueous-hydrocarbon interfaces of micelles and the related assemblies discussed above in terms of the magnitude of their orienting ability and the ease of interpretation of selectivity in photoreactions conducted in them. Molecules in the films have very little freedom of motion (stiff reaction cavities), their interfaces are very well defined, and therefore the alignment of reactant molecules can be readily expressed in the products. Photodimerization of stilbazole derivatives 62, N-octadecyl-l-(4-pyridyl)-4-(phenyl)-l,3-butadiene, (63), surfactant styrene derivatives 64 and 65, and cinnamic acids have been carried out in L-B films [18, 196-200], In all cases, single isomeric head-head dimers are obtained. Geometric isomerization of olefins has not been observed in competition with photodimerization. Independent of the location of the chromophore (i.e.,... 

The contribution of fluorescence to the deactivation of the excited singlet state of the phytochromobilin chromophores in Pr and Pfr (see Section II.D) is negligible in quantitative terms. The total fluorescence quantum yield of the photochromic P and P3 components amounts only to from Tables 1 and 4). This means that deactivation proceeds predominantly through nonradiative channels, i.e., via internal conversion back to the electronic ground state of Pr and via primary photoreaction(s). Nevertheless, the fluorescence efficiency suffices to serve as a sensitive tool to monitor certain aspects of the competing primary reaction(s) of P (see Sections III.A and III.C). 

Evidence from protein fluorescence indicates that the equilibrium conformation of the central regions of the polypeptide backbone, which are amenable to probing by emission from the tryptophan residues, is not changed overall to any major extent in the Pr -< Pfr transformation. The conformational reorganization of the protein, which is induced by the Z - E isomerization (which in turn is presumed to represent the primary photoreaction of the overall Pr -> Pfr transformation), appears to be confined mostly to the domain housing the bilatriene chromophore. 

In recent investigations about the intramolecular meto-addition processes, the effects of heteroatoms in the linking chains are obvious. Blakemore and Gilbert reported the photoreaction of 185 linked by the amide group [240], Pho-toinduced intramolecular electron transfer can be inhibited by /V-acetyl or N-car-bomethoxy groups. Fluorescence is then observed from the arene chromophore and intramolecular meta photocycloaddition occurs through high selectivity. 

In the last 15 years, Wagner and his co-workers have developed the intramolecular ortho photocycloaddition of alkenes to the benzene ring of the acetophenone chromophore. In these photoreactions, the ring opening products of ortho photocycloadducts, 1,3,5-cyclooctatriene derivatives (220), and their secondary photocycloadducts, tricyclic compounds (221), were often produced [271,272] (Scheme 63). The triplet exciplexes generated from ir, ir -triplet states of acetophenone derivatives were postulated as the reactive intermediates [273],... 

Compound 99 was made more hydrophobic by butylation of the 2-OH and 3-OH groups [88], The prepared CD derivative (100) solubilized 9-anthrylmethyl pivalate (AP) much more effectively than 99 or (3-CD. In the presence of AP, the naphthyl emission (Xex = 314 nm) of 100 was efficiently quenched and intense AP emission was observed. Because essentially all of the light is absorbed by the naphthyl chromophores, the strong AP fluorescence demonstrates that energy transfer occurs efficiently. The naphthyl antenna chromophores of 100 were shown to sensitize a selective photoreaction of AP included in the CD cavity (Scheme 2). The photoirradiation of AP in methanol gives 9-neopentylanthracene... 

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