Chemical reactions of the excited states

Chemical reaction of the excited state (secondary charge separation, isomerisation, disassociation etc). 

Hydrogen abstraction from lipids by triplet states of benzophenone derivatives followed directly by the use of laser flash techniques allows the separation of physical quenching processes from chemical reactions of the excited state. 98 xhe production of 2 by the photochemical decomposition of aromatic endoperoxides has also been studied by ps kinetic procedures. 99 mechanistic study has also been reported on the phototransformation of 3-nitrophenol in aqueous solution. 99 There is a strong wavelength dependence of the low quantum yield for the phototransformation in this system. 

With this one exception of vibrational photochemistry through multiphoton infrared light absorption, photochemistry is restricted to the chemical reactions of electronic excited states of molecules. Radiation chemistry is outside the scope of this book, so a very short section is devoted to it to conclude this introduction. 

In addition to chemical reaction, weak fluorescence was detected from 50 at room temperature (acxc 460 nm, Xem 552 nm, cj)f = 0.04). Temperature effects on reaction and fluorescence from 77-310 K have been studied 68). A steady decrease in quantum yield for reaction (r) and a complementary increase in fluorescence quantum yield (< )f) were observed down to about 150K where a sharp increase in f occurred. Photochemical reaction was negligible at 77 K (436 nm). The fluorescence lifetime at 77 K was a few nanoseconds and the estimated value at room temperature is on the order of 60 ps. Detailed analysis of the data showed that two thermally-activated processes are involved (1) chemical reaction of the singlet state with an Arrhenius activation energy of 1.5 kcal/mol and (2) radiationless decay of the singlet with Eact =1.1 kcal/mol. Both processes would appear to be associated with certain vibrational modes of the excited state which become progressively less populated with decreasing temperature. 

Fully thermalised excited states may be treated as distinct chemical species with their own equilibrium thermodynamic properties, including redox potentials. We may therefore define standard redox potentials U°.. and f/%. for reactions of the excited states D and A ... 

Figure 1. Cartoon outlining the steps involved in converting solar energy (hv) to stored chemical energy or electricity. Electron transfer reactions of the excited state (denoted by an asterisk) with donors (D) or acceptors (A) to yield charge-separated species (D or A-) take place in competition with nonproductive emission (hv ), nonradiative excited-state decay, and hack electron transfer, which...

THE MAIN CHEMICAL REACTIONS OF ORGANOMETALLIC EXCITED STATES. 

All non-radiative electronic processes are isoenergetic transitions to another electronic state, which may or may not be the ground state. This other state may be localised on the same molecule, or on molecules produced by unimolecular chemical reaction from the excited-state or the process may involve interaction with states on another system which acts as a quencher of the excited-state, e.g. by energy transfer or bimolecular reaction. (The resultant electronic state(s) may themselves deactivate non-radiatively or radiatively, e.g. the phosphorescence from a triplet state formed from a higher singlet, emission from excimers and exciplexes formed from ground-state excited state interaction (see Sect 1.13.5.5), or emission from quencher states produced by energy transfer or chemical reaction.)... 

The thermal effects induced by light become more pronounced at flie tissue level and will be discussed later. Photochemical processes involving a chemical reaction in the excited state of a cellular constituent (or a chemical unit such as thymine in DNA) are varied, as exhibited in Table II. Here are some examples (Grossweiner and Smith, 1989 Kochevar, 1995). 

The photolysis of dimethyl sulphoxide (at 253.7 nm) in a wide range of solvents has been studied in detail176. Three primary reactions occur, namely (i) fragmentation into methyl radicals and methanesulphinyl radicals, equation (60), (ii) disproportionation into dimethyl sulphone and dimethyl sulphide, equation (61) and (iii) deactivation of the excited state to ground state dimethyl sulphoxide. All chemical processes occur through the singlet state. Further chemical reactions of the initial photochemical products produce species that have been oxidized relative to dimethyl sulphoxide. 

---