Fluorescein, photosensitizers

Comparison of the behavior of free (mono) radicals with that of electronically excited sensitizers has led to the assumption that the excited sensitizer, Sensrad, is best described as a phototropic-isomeric diradical. Thus, photodimerization and photodehydrogenation, exhibited by certain sensitizers in the absence of oxygen, reflect radical-combination and hydrogen-abstraction reactions. Furthermore, fluorescein (a photosensitizer) becomes paramagnetic when excited... 

There are two main ways of generating the singlet oxygen molecules (1) photo-chemically by irradiation in presence of a sensitizer and (2) chemically. The photochemical route proceeds as follows, where sens represents the photosensitizer (typically a fluorescein derivative, methylene blue, certain porphyrins, or certain polycyclic aromatic hydrocarbons) ... 

In this case, the activated compound (C) cannot be produced by direct irradiation with light of frequency v. Instead, the activated photosensitizer (Z) transfers energy to tiie monomer or initiator at an appropriate frequency that can be absorbed by C. Typical photosensitizers used in photochemically-initiated polymerizations are benzophenone, fluorescein, and eosin. 

The xanthene dyes can be divided into two main groups viz. the biphenylmethane derivatives (pyronins) and the triphenylmethane derivatives (mainly the phthaleins). Among the phthaleins are fluorescein (hydroxy group) and rhodamin (amino group) and the dual compounds (hydroxy and amino). Both fluorescein and rhodamin, currently used as biological dyes, are also photosensitizers. 

Phenyl-CHj-Rose Bengal, eosin. Photosensitized reactions fluorescein... 

Other mononuclear nickel complexes containing thiolate ligands, such as pyri-dine-2-thiolate [127], 2-mercaptoethanol [128], 2-mercaptobenzimidazole [129], or 2-mercaptobenthiazole [129] have proved active in light-driven systems based on fluorescein, erythrosin B, eosin Y, and rose Bengal as photosensitizers. These very simple systems also proved extremely stable upon turnover with 5500 TON after 40 h for the pyridine-2-thiolate complex [127]. Again, bleaching of the photosensitizer has been recognized to be responsible for the decrease of the activity after a few tens of hours. This issue could be solved with the replacement of the molecular photosensitizer by CdSe semiconductor nanocrystals, capped with... 

The photosensitizers supply molecular oxygen in a photochemical reaction. Examples of usual photosensitizers are Rose-Bengal, eosin-y, fluorescein. The photosensitizers are bound to Merifield type resin via ester bond (Scheme 17). ... 

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