Photochemical deactivation

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

This Section deals with photochemical deactivations in which the primary photochemical step is a non-redox, heterolytic central atom - ligand bond breaking, and in the final products both the central atom and the tetrapyrrole ligand retain their oxidation state. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

In the first step of the direct stilbene photoisomerization, the stUbene molecule is excited to the singlet or triplet state when it absorbs a quantum of energy. After that, solvent molecules undergo reorganization, thus reducing the energy of the system. Next step is the exdted-state decay by two competitive processes radiative (fluorescence or phosphorescene emission) or nonradiative (photochemical) deactivation. 

The foDowtng possible photochemical deactivation channels have been discussed ... 

This photochemical deactivation reduces the probability of fluorescence. Thus, this process has to be considered in competition using high-intensity fluorescence excitation sources [7]. For this reason one has to take care in any fluorescence measurement, especially examining photochemically instable compounds. Repetitive quantitative examination of the same sample yields a decreasing fluorescence signal. 

Proton transfer processes in the excited state are one of the most important photochemical deactivation routes in biological systems. It has been shown that an intramolecular proton transfer can suppress the double-bond isomerisation that would normally be expected in the excited salicylideneaniline (Fig. lOa). A similar effect was found in an analogue for a recently synthesised green fluorescence protein (GFP) chromophore, 4-(2-hydroxybenzylidene)-lFf-imidazol-5(4Ff)-one (OHBI,... 

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