Photochemical activation exciplexes

PHOTOCHEMICAL ACTIVATION OF ORGANOSILICON COMPOUNDS FORMATION AND REACTIVITIES OF EXCIPLEXES, RADICAL IONS, AND NEUTRAL RADICALS... 

Research on carbon-centered radical cations in solution accelerated dramatically with the development of time-resolved optical absorption and emission techniques. The research group of Th. Forster in Germany pioneered photochemical methods of production of radical cations and anions, as well as exciplexes." While the Forster group focused on structure and lifetimes, the later work of D. R. Arnold in Canada, and of H. D. Roth in the United States," reported the reactivity of photochemically generated radical cations from a mechanistic perspective. These studies of radical ion chemistry evolved into the field we now know as electron donor-acceptor interactions, arich area of science in which carbon-centered radical cations are stiU actively smdied. 

The electrogenerated chemiluminescence (ECl) of five l-amino-3-anthryl-9-propane derivatives has been studied in tetrahydrofuran. Emission from intramolecular exciplexes in ECl spectra and weak emission from the locally excited anthracene moiety were observed. The influence of triplet state interaction in ECl emission is discussed. The chemiluminescent decomposition of three a-peroxy-lactones gives CO2 and the corresponding ketone in high yield. The chemiluminescent species produced has been investigated in some detail by measurements of lifetime, energy-transfer activation parameters, and photochemical reactions. 

In previous papers we reported on the photochemistry and photopolymerisation activity of a series of water- and oil-soluble thioxanthone derivatives (1-6). In that work, all the thioxanthone structures were found to operate essentially by a primary photochemical process of electron abstraction via an exciplex with an amine co-synergist. However, whereas the oil soluble structures were found to operate solely through their lowest excited triplet state (1,2,6) the water-... 

Recent studies on enantiodifferentiating photoisomerization reveal that the photosensitization of C Cg (Z)-cycloalkenes with optically active aromatic esters affords chiral ( )-cycloalkenes in good to excellent ees. The mechanism involves enantiodifferentiating rotational relaxation within the exciplex intermediate formed from the excited singlet state of chiral sensitizer and prochiral substrate. The exciplex structure and hence the stereochemical outcome are very sensitive not only to the internal factors, such as sensitizer energy and structure, but also to the external entropy-related factors such as temperature, pressure, and solvent. This leads to a novel idea of multidimensional control of product chirality by several environmental factors. The asymmetric photosensitization can be applied to the photochemical asymmetric synthesis and also be used as a powerful tool for exploiting the reaction mechanisms in the excited-state chemistry. 

Photosensitized enantiodifferentiating reactions are synthetically attractive and mechanistically interesting photochemical processes. The chiral information of the sensitizer is transferred to the substrate by short-Hved interactions in the excited state (i.e., during the lifetime of an exciplex of a reaction intermediate and the chiral sensitizer that is involved in the reaction mechanism) hence, the chirahty is multiplied, and only catalytic amounts of the optically active sensitizer are required. The stabilization energy of an exciplex compared to the locally excited state and its lifetime are often found to be strongly dependent on both electronic and steric properties of its components. Chiral induction can be achieved by different stabilization energies or lifetimes of the exciplex between the sensitizer and the intermediates that lead to the enantiomeric photoproducts. The absence of other reaction pathways without intimate contact to the sensitizer and of racemization processes in the further course of the reaction mechanism is an additional requirement to ensure effective chirality transfer. 

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