Singlet or triplet excitation

The Jablohski diagram plays an important role in molecular spectroscopy (Fig. 8.19). It shows three energy levels the ground state (G), the first excited singlet state (S), and the metastable in-between state. Later on researchers identified this metastable state as the lowest triplet (T).  

Let us compute the energy difference between the singlet and triplet states  

Aleksander Jabtohski (1898-1980), Polish theoretical physicist, professor at the John Casimirus University in Vilnius, then at the Nicolaus Copernicus University in Toruh, studied photoluminescence problems. 

Electronic Motion in the Mean Field Atoms and Molecules 

The difference between the energies of the ground and triplet states is  

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The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

S = Sensitizer S = singlet or triplet excited S (SA), (S02) = electronically excited complexes. 

The discovery that azo compounds undergo singlet sensitized decomposition is particularly relevant to the problem of spin correlation effects in free radical reactions. Any radical pair precursor that gives a difference in products depending upon whether it is produced as a singlet or triplet excited state is said to show a spin correlation effect. 

Consistently, the PIA spectra of toluene solutions containing MP-Ceo and OPVn (n = 2, 3 or 4) in a 1 1 molar ratio, recorded using selective photoexcitation of MP C60 at 528 nm (Fig. 1.28b), invariably exhibit an absorption at 1.78 eV with an associated shoulder at 1.54 eV, characteristic of MP-C6o(7i) [103]. The monomolecular decay (—AT oc Ip, p = 0.89-0.96) with lifetime 150-260 ps associated with these PIA bands supports this assignment. Furthermore, weak fullerene fluorescence at 1.73 eV (715 nm) is observed under these conditions for all three mixtures. No characteristic PIA bands of OP Vw+ radical cations or MP-Cg0 radical anions are discernible under these conditions. From these observations we conclude that electron transfer from the ground state of the OPVn molecules to the singlet or triplet excited state of MP-Cgo does not occur in toluene solution. 

Use of photoexcited fullerenes (i.e., the singlet or triplet excited state) widens the scope of electron-transfer reactions. This assumption is because excitation of fullerenes enhances both the electron-acceptor and -donor behavior of the photoexcited fullerenes. For example, the triplet excited state of C o, which is formed by efficient intersystem crossing (i.e. with a quantum yield close to unity) [18, 19] has a reduction potential of E°red = 1.14 V relative to the SCE [18, 19]. This potential is clearly more positive than the reduction potential of the ground state (—0.43 V) [20]. Thus, the triplet excited state of Ceo can be reduced with a variety of organic compounds yielding the Cgo radical anion and the oxidized donor [18]. 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

Various types of photochemical reactions such as addition or substitution reactions, atom abstractions or rearrangements can arise from either singlet or triplet excited states of molecules. They can be described as secondary photochemical processes. Many examples of these are to be found in works concerned with the mechanistic aspects of photochemistry [4, 5]. 

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