Photoinduced reactions, molecular

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

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. 

Based on some interesting reactions in certain inorganic crystalline compounds, Kohlschutter [9,10] proposed that the nature and properties of the products obtained take place on the surface or within the solid state. Indeed, he coined the term topochemistry for such reactions in the solid state. However, systematic investigations of photoinduced reactions in crystals began from 1964 onward by Schmidt and Cohen [11], Their studies on the 2tt + 2tt photoreaction of cinnamic acid derivatives in the crystalline state and correlation with the molecular organization in these crystals led to what are now known as Topochemical Principles. The most important conclusions reached by them are as follows (1) The necessary conditions for the reactions to take place are that the reactive double bonds are parallel to one another and the center-to-center distance be within 4.1 A (2) there is one-to-one correspondence between the stereochemistry of the photoproduct and the symmetry relationship between the reactants. The centrosymmet-ric relationship (called the a-form) leads to centrosymmetric cyclobutane (anti-HT), whereas the mirror symmetric arrangements (called the (5-form) produce mirror symmetric dimer (yy -HH). 

The properties of molecular solids are well known to depend not only on the structure of the molecule itself, but also on the three-dimensional architecture adopted during crystallization. Such is the variation in properties with crystal structure that the existence of different polymorphs can, for example, switch on and off photoinduced reactions as well as profoundly affect the pharmaceutical activity of drugs [1-3]. 

In a photochromie system all of the refractive-index change is a result of photoinduced reactions of isolated molecules, and there is no mass transport over distances larger than molecular dimensions. Since each molecule functions independently, the spatial frequency response of photochromic systems extends from zero to the diffraction limit of the recording light. (This is frequently referred to as "molecular resolution.") While our definition of a photochromic system does not require that the process be reversible, many photochromic systems are reversible, optically and/or thermally (31). In fact, it is in general only with photochromic processes that one can obtain, reversible image recording. 

Figure 4.28. Molecular structures and photoinduced reactions of common photoresists. Shown (top) is the positive tone resist containing the active diazonapthoquinone (DNQ) chromophore group. Chemical amplification (CAM) reactions are illustrated in (i)-(iii). Reaction (i) represents photoinduced acid generation step (ii) is an acid-catalyzed deprotection mechanism (positive tone resist) and step (iii) is an acid-catalyzed crosslinking mechanism (negative tone resist).

Porphyrin derivatives have been extensively tested as photosensitizers for the PDT of cancer for two sets of reasons. First, their strong absorption of light in the phototherapeutic window and efficient photoinduced reactions with molecular oxygen offer a photochemical tool to induce localized cytotoxicity in targeted tissues. Second, porphyrin derivatives have an intrinsic affinity for tumors (4-6). Whereas the spectroscopy and photochemistry of porphyrin derivatives are very well understood, the same is not (yet) true for the mechanisms that contribute to their preferential localization and accumulation in tumors. This latter subject is outside the scope of this work, and it will only be briefly mentioned in the context of in vivo studies with porphyrin derivatives. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

The photochemical behavior of these oligomers was first examined with the dimers which were irradiated with a medium-pressure mercury lamp through a Pyrex filter to remove any wavelength below about 300 nm. In dilute methylene chloride solutions, all the dimers (la and Ila-IId) isomerized to the cis conformation until the cis/trans thermodynamic equilibrium mixture was reached. No other photoinduced reaction was detected in these conditions. Irradiation of concentrated solutions, as well as crystalline solid samples, gave rise to molecular dimerization through the alkenyl bridge with all the dimers, through the classical [n2+7t2] cycloaddition process, e.g. for dimer la ... 

Coherent control means the coherent preparation of a molecular wavefunction through the absorption of coherent radiation. The dream of chemists is the controlled selection of wanted reaction channels and the suppression of unwanted channels in a photoinduced reaction. This is illustrated by Fig. 10.9, where the excitation of the triatomic molecule ABC can induce either of the reactions AB -b C or AC -b B, depending on the wavefunction in the excited state of ABC, which is controlled by the form of the excitation pulse. 

The Born-Oppenheimer adiabatic approximation represents one of the cornerstones of molecular physics and chemistry. The concept of adiabatic potential-energy surfaces, defined by the Born-Oppenheimer approximation, is fundamental to our thinking about molecular spectroscopy and chemical reaction djmamics. Many chemical processes can be rationalized in terms of the dynamics of the atomic nuclei on a single Born Oppenheimer potential-energy smface. Nonadiabatic processes, that is, chemical processes which involve nuclear djmamics on at least two coupled potential-energy surfaces and thus cannot be rationalized within the Born-Oppenheimer approximation, are nevertheless ubiquitous in chemistry, most notably in photochemistry and photobiology. Typical phenomena associated with a violation of the Born-Oppenheimer approximation are the radiationless relaxation of excited electronic states, photoinduced uni-molecular decay and isomerization processes of polyatomic molecules. 

Apart from foreseeable applications related to the development of nanotechnology, investigations on photochemical molecular devices and machines are important to increase the basic understanding of photoinduced reactions and other important processes such as self-assembly, as well as to develop reliable theoretical models. This research has also the important merit of stimulating the ingenuity of chemists, thereby instilling new life into chemistry as a scientific discipline. 

Figure 4.42. Molecular structures and photoinduced reactions of common photoresists. Shown are (a) the diazonaphthoquinone (DNQ) positive tone photoresist, and (b) the SU-8 epoxy-based negative tone photoresist.

It is now believed from studies on the natural photosynthetic systems that microenvironments for the photoinduced ET reaction play an important role in the suppression of the back ET [1-3]. As such reaction environments, molecular assembly systems such as micelles [4], liposomes [5], microemulsions [6-8] and colloids [9] have been extensively investigated. In them, the presence of microscopically heterogeneous phases and interfacial electrostatic potential is the key to the ET rate control. 

Functionalized polyelectrolytes are promising candidates for photoinduced ET reaction systems. In recent years, much attention has been focused on modifying the photophysical and photochemical processes by use of polyelectrolyte systems, because dramatic effects are often brought about by the interfacial electrostatic potential and/or the existence of microphase structures in such systems [10, 11], A characteristic feature of polymers as reaction media, in general, lies in the potential that they make a wider variety of molecular designs possible than the conventional organized molecular assemblies such as surfactant micelles and vesicles. From a practical point of view, polymer systems have a potential advantage in that polymers per se can form film and may be assembled into a variety of devices and systems with ease. 

In contrast, if photoactive chromophores are covalently tethered to the polyelectrolyte main chain, the chromophore reactivity in the photoinduced ET is much more greatly changed because the reaction sites are totally constrained to the polyelectrolyte molecular surface. 

Warmuth R The Inner Phase of Molecular Container Compounds As a Novel Reaction Environment J. Inclusion Phenom. Macrocyclic Chem. 2000 37 1-38 Keywords inciusion reaction, photochemistry, photoinduced eiectron transfer, fuiierenes... 

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... 

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