Singlet state photochemical initiation

In most photochemical systems, the triplet state is populated via a radiationless transition from the lowest excited singlet state following initial excitation. This transition is also known as "intersystem crossing" in photochemistry. Very little is known experimentally about the exact nature of this process, but the rate or... 

In many cases, the proper description of the rates and mechanisms of photochemical reactions also requires knowledge of processes such as fluorescence and phosphorescence that can deactivate an excited state before the reaction has a chance to occur. Electronic absorption takes place in about 10 -10 s, and because fluorescence lifetimes are typically 10" -10 s, an excited singlet state can initiate very fast photochemical reactions in the range from femtoseconds (10 s, the time it takes to excite a molecule) to picoseconds (10 s, the lifetime of the excited state). Examples of such ultrafast reactions are the initial events... 

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. 

The photochemical dissociation of Me2Ge from 7,7-dimethyl-l,4,5,6-tetraphenyl-2,3-benzo-7-germanorbomadiene (14) has been studied by flash photolysis, low-temperature matrix isolation and CIDNP 3H NMR techniques30. The results suggest that a biradical (15) is formed as an intermediate species in the photoreaction. The biradical is initially formed in the singlet state, which undergoes conversion to the triplet state before irreversible decomposition to form Me2Ge and tetraphenylnaphthalene (TPN) (reaction 19). 

Direct conversion systems are threshold devices, that is, there is a minimum photon energy which can initiate the photochemical reaction. This is called the band-gap ei rgy Eg with a corresponding band-gap wavelength Xg. Eg usually corresponds to the 0-0 transition to the lowest excited singlet state of the absorber (see Fig. 1). Hence, it is important to know what fraction of the incident solar power is available at the band-gap energy. 

These photophysical processes often decide the photochemical behaviour of a molecule and reduce the quantum yield of a photochemical reaction to much less than unity. A molecule in the singlet state is a different chemical species from that in the triplet state and may initiate different chemistry. Therefore, for a complete understanding of a photochemical reaction, a clear knowledge of various photophysical processes, that isj how the absorbed quantum is partitioned into different pathways is essential. This account keeping of the absorbed quanta, so to say, may help modify a given chemical reaction if it is so desired. We shall discuss each of these processes one by one. 

The quantum yield of photochemical processes can vary from a low fractional value to over a million (Section 1.2). High quantum yields are due to secondary processes. An initially excited molecule may start a chain reaction and give rise to a great number of product molecules before the chain is finally terminated. For nonchain reactions, the quantum yields for various competitive photophysical and photochemical processes must add up to unity for a monophotonic process if the reaction occurs from the singlet state only ... 

In the case of dianthrylethanone 34, the course of the photochemical isomerization was found to be concentration-dependent (cf. Table 6). Triplet state reaction products are the 47t + 2n cyclomers 35 and 36, which are formed mainly (80%) by irradiation of 34 at low concentration [51], The predominant (98%) formation of the singlet state product, i.e., 47t + 47t cycloadduct 37, is observed when dianthrylethanone 34 at high initial concentrations is... 

The details of the photochemical primary process in cyclopentanone are fairly well understood (33). Since the spectrum in the first absorption region appears to be similar to that of the simple aliphatic ketones, it has been assumed that initial excitation is to the upper singlet state and that the zero-zero band is near 3600 A. 

When two radicals are in close association as a pair surrounded by a cage of solvent molecules, the two odd electrons will interact with one another just as two electrons do within a molecule. The interaction will yield either a singlet state, if the two electrons have spins paired, or a triplet, if the spins are unpaired. If, for example, the caged pair arose by thermal dissociation of an ordinary ground state molecule, in which all electrons would have been paired, the state would initially be a singlet, S, whereas if the pair arose in a photochemical reaction from dissociation of an excited molecule in a triplet state, it would be initially a triplet, T. 

The photochemical cycloreversion of SCBs is known to be initiated from the lowest (a, a ) excited singlet state by cleavage of one of the ring Si—C(2) bonds to form a biradicaloid intermediate. This excited-state intermediate cleaves to silene and alkene, recloses to starting material, or undergoes an intramolecular disproportionation if an alkyl substituent is present at C-2 <1999CJC1136, 19890M1112>. 

This equation can be true only if p is also negative, indicating that the radical pair was initially formed in the singlet state. This is eminently reasonable when you consider that precursor 11-6 had all its electrons paired. This result is consistent with the notion that thermal fragmentation of a chemical bond preserves the spin state of the electrons, normally the singlet. Many photochemical fragmentations, on the other hand, proceed by way of electronically excited triplet states and produce triplet radical pairs. 

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