Triplet states electronic excitations

Energy transfer from a donor that is in an electronically excited triplet state to produce an electronically excited acceptor which is also in its triplet state. 

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. 

Molecules with a singlet ground state may have an electronically excited triplet state accessible by photoexcitation via a radiationless intersystem crossing. Indeed, photochemical study of excited triplet states coupled with ESR spectroscopy is perhaps one of the most exciting chapters in the recent advances in photochemistry. 

Table 7.3 The fine-structure parameters D and from selected molecules in the electronically excited triplet state Ti and from methylene, (CH2), in its electronic triplet ground state. The parameters are weakly dependent both on the temperature and also on the host matrix.

It is known that photochemistry is the study of chemical changes produced by electronically excited species. Most of the photochemical transformations come from triplet excited state, due to its higher life-time. But oxygen molecule is a rare case, its ground state is a triplet while reactive state is electronically excited singlet state. The life time of electronically excited triplet state is relatively low, so significantly populated is singlet excited state. 

Fig. 1. Schematic energy-level diagram for a dye molecule. Electronic states Sq = ground singlet state = first excited singlet state S2 = second excited singlet state Tj = first excited triplet state T2 = second excited triplet state EVS = excited vibrational states. Transitions A = absorption excited states ...

If the cross-coupling is strong enough this may include a transition to a lower electronic level, such as an excited triplet state, a lower energy indirect conduction band, or a localized impurity level. A common occurrence in insulators and semiconductors is the formation of a bound state between an electron and a hole (called... 

In agreement with this expectation Sjogren (16) found that when bombarding C02 with Ne+ ions (RE 21.6 e.v.) of low velocity and at low pressure, vanishing fractions of 0+ ions were obtained. This result indicates that when using electron or photon impact, O + (4S) is formed at 19.1 e.v. after preionizing a highly excited triplet state of neutral C02. 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

Finally, we note the informative work of Garcia-Garibay and co-workers, who have extensively studied QMT in hydrogen transfer reactions in the excited triplet states of ort/zo-alkylarylketones, for example, 6 —> 7, which are electronically similar to radical rearrangements. 

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Figure 9.1. Potential energy diagram for the electronic states of ethylene N, ground state (w)2 r(3Biu) first excited triplet state (mr ) V, first excited singlet state (wir ) Z, two-electron excitation (w )2. For the ion C2H + R and R, Rydberg states, /, / ground and excited states. [From Ref. 2(c).]...

Moreover, carotenoids may quench electronically excited states and scavenge free radicals formed in the retina, and therefore protect biomolecules from oxidative damage. Due to the low energy level of the first excited triplet state ( Car), carotenoids (Car) can act as efficient acceptors of triplet state energy from photosensitizers (S) (Equation 15.1), such as all-tra .s-retinal, the photosensitizers of lipofuscin (Rozanowska et al., 1998), or singlet oxygen C02) (Equation 15.2) (Cantrell et al., 2003) ... 

Using the OPENCORE spectrometer, a research group led by M. Kitagawa in Osaka have developed an experimental setup for dynamic nuclear polarization (DNP) using electron spins in the photo-excited triplet state. This nuclear hyperpolarization technique, called hereafter triplet DNP,... 

The fact that dynamic 13C polarization is only possible through the indirect way via tire 1H spins suggests the mechanism of polarization transfer. Since the polarization transfer between the electrons and nuclei are driven by the dipolar interactions between them, and the fraction of the guest triplet molecules was small, it would be natural to assume that the polarization of the electron spins in the photo-excited triplet state is given to those H spins which happen to be close to the electron spins, and then the 1H polarization would be transported away over the whole volume of the sample by spin diffusion among the 1H spins. 

Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

Cycloadducts have been successively obtained by reaction of MCP with maleic anhydride (116) and a number of related electron-deficient alkenes (137,486,487) under photolytic conditions in the presence of a sensitizer (Table 38, entries 5-8) [132b]. Analogous cycloadditions in mild conditions with high yields have also been performed with electron-donor substituted alkenes, such as vinylene carbonates 483 and 484 and the imidazolinone 485 (entries 2-4) [132], In the case of the unsymmetrical anhydride 137 (entry 6), an almost equimolar mixture of both the possible regioisomers has been obtained [132b]. In all these cases the reaction has also been proposed to occur via diradical intermediates formed from the reaction of 1 with the alkene in its excited triplet state [132]. 

These energy-transfer processes are especially interesting in those chemiluminescence reactions where the primary electronically excited product is formed in its triplet state (autoxidation reactions, radical-ion recombination reactions see Sections III and VIII), although some reactions have been reported to involve direct emission from the excited triplet state 14>. 

If chemiluminescence is to occur, a critical electron-transfer enthalpy value must be exceeded. This value is similar to the energy of the lowest excited triplet state, as was found in the case of fluoranthene 150>, for example. 

Most of the interest in mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer in solution from chlorophyll a to p-benzoquinone (21, 22) which has been shown to occur via the excited triplet state of chlorophyll a. However, these solution studies are not very good models of the in vivo reaction center because the in vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other in the reaction-center protein. 

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