Triplet mechanism

The Triplet Mechanism (TM) [36, 37], unlike the RPM is much more restrictive with the origin of the polarisation arising from the excited states of the parent molecule. In the TM the polarisation arises from the intersystem crossing from the excited singlet 

The triplet state eigenfunctions at zero field are not the same as the non-zero field Zeeman eigenfunctions, but are rather a linear combination which can be expressed as [38] 

To retain the polarisation generated by the TM mechanism the parent molecule must dissociate to yield radicals (with rate kd), which retains the spin multiplicity of the precursor. If kd can compete with the spin-relaxation time Tis, then the time-dependence of the polarisation generated by the TM can generally be described 

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Figure Bl.16.14. Top, the canonical axes for triplet naphthalene. The z-axis is directed out of the plane of the paper. Bottom, energy levels and relative populations during the CIDEP triplet mechanism process. See text...

Contradictory evidence regarding the reaction to fonn 8 and 9 from 7 led the researchers to use TREPR to investigate the photochemistry of DMPA. Figure B1.16.15A shows the TREPR spectrum ofthis system at 0.7 ps after the laser flash. Radicals 6, 7 and 8 are all present. At 2.54 ps, only 7 can be seen, as shown in figure B1.16.15B. All radicals in this system exliibit an emissive triplet mechanism. After completing a laser flash intensity sPidy, the researchers concluded that production of 8 from 7 occurs upon absorption of a second photon and not tiiemially as some had previously believed. 

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

Figure Bl.16.22. Schematic representations of CIDEP spectra for hypothetical radical pair CH + R. Part A shows the A/E and E/A RPM. Part B shows the absorptive and emissive triplet mechanism. Part C shows the spin-correlated RPM for cases where J and J a.. ...

Koga T, Ohara K, Kuwata K and Mural H 1997 Anomalous triplet mechanism spin polarization... 

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

The lack of any difference in the rate of isomerization between fluoro-sulfonic acid solutions of 34 which had been thoroughly degassed, and those which were saturated with oxygen, suggests that the reaction does not proceed via a triplet mechanism. In fluorosulfonic acid no unproton-ated acid is detected, ruling out the possibility of n,7r excitation. Thus, there is little doubt in this case that it is the Tr,Tr singlet state which is the reactive species. Experiments carried out with a variety of methyl-substituted protonated cydohexadienones have likewise ruled out the... 

Zhang RB, Eriksson LA (2006) A triplet mechanism for the formation of cyclobutane pyrimidine dimers in UV-irradiated DNA. J Phys Chem B 110 7556-7562... 

The value of the pss calculated from Eq. (9.24) is 2.74 and the measured value<7> is 2.64. While this agreement could be taken as evidence in favor of a triplet mechanism, it could also mean that the twist singlet and twist triplet have similar geometries (see Figure 9.2), and therefore similar decay ratios. In this mechanism we have neglected fluorescence from fraws-stilbene and dihydrophenanthrene formation from cw-stilbene. (See Problem 4.)... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. 

The inhibition of product formation by a triplet quencher readily allows a triplet mechanism to be identified where the absence of inhibition by a triplet quencher indicates a singlet mechanism. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

In an attempt to probe further into the complex photochemical system involved, the isomerization of stilbenes adsorbed on silica gel was examined.9 It was observed that the time required for establishment of the photostationary state was significantly increased (by a factor of 3) and that the composition of the photostationary state changed from 93y cis isomer in cyclohexane solution to 60% cis isomer in the silica gel matrix. Though not definitive, this evidence supports Fischer s triplet mechanism, 53 as we have previously reported.9... 

Hutton and Stevens49 have observed that while naphthacene vapor alone produces no delayed fluorescence, pyrene vapor can sensitize the delayed fluorescence of naphthacene vapor. It is true that they explained their results in terms of a long-lived excited dirtier but in a recent private communication Stevens has agreed that delayed fluorescence in the vapor state is probably also produced by triplet-triplet quenching and hks suggested that the results with mixtures of pyrene and naphthacene vapor provide evidence for the occurrence of the mixed triplet mechanism. 

Approximate relative values of 0D (for anthracene) and 6A (for naphthacene) at various concentrations of naphthacene [A] are shown in Table XIV. It will be seen that both efficiencies follow the qualitative predictions calculated above and the results thus provide good confirmation that the mixed triplet mechanism operates. 

Most of the fairly extensive photochemistry of aromatic compounds has not been studied in sufficient detail to permit disentanglement of singlet and triplet mechanisms. Theoretical calculations indicate that the pattern of substituent effects on side-chain reactions of excited disubstituted benzenes should be quite different from that observed in the ground states of the molecules. One problem associated with these predictions is the question of whether or not they are appropriate for triplets as well as for corresponding singlet excited states. Consider the following system ... 

Although this fact alone does not demand that the species be a triplet, presumptive evidence for the triplet mechanism is provided by the demonstration that the reaction can be carried out using benzophenone as a photosensitizer and yields the same product distribution. 

Optically active aryl sulfoxides undergo facile photoracemiza-tion.472 The photo-inversion is sensitized by naphthalene but only partially quenched by piperylene.473 The quenching action was originally interpreted as indicating a triplet mechanism for isomerization. 

Lewis et al. examined the photocycloaddition of 9-methoxycar-bonylphenanthrene with 2,3-dimethyl-2-butene in the absence or presence of a Lewis acid [213], The conformations of the free and complexed molecules in the ground states have been investigated by means of NMR and Gaussian 88 calculations. In the absence of a Lewis acid, a second unidentified adduct and the photodimer are observed as well as the cycloadduct at the 9,10-position (Scheme 41). In the presence of a Lewis acid, neither the second adduct nor the photodimer is detected, and the yield of the photocycloadduct is inferior to that obtained in the absence of Lewis acid. From the fluorescence quenching experiments, it seems likely that the photocycloaddition occurs via a triplet mechanism, whereas the photocycloaddition of their Lewis acid complexes occurs via a singlet mechanism (Table 2). 

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

Deviations from Boltzmann intensities were first noted in 1963 [72] they are known as CIDEP [73,74]. These effects can be caused by two different mechanisms, each giving rise to CIDEP spectra with different characteristic intensity patterns. CIDEP effects can have two sources they may be transferred to the intermediates from their immediate precursor, typically a triplet state (triplet mechanism, TM), or generated early during their existence (radical pair mechanism, RPM) [75]. 

CIDEP originates in two independent processes, the triplet mechanism and the radical pair mechanism. The last one arises in spin correlated pairs [60]. The final spectrum gives a direct insight in the working mechanism. 

The tetramethylene diradical was also proposed as the intermediate for dimerization of olefins and [2 + 2] mixed additions between olefins. Hospool [105] suggested that if a triplet mechanism was operative, the produced diradical intermediate would undergo free rotation before the closure of the final bond and... 

CIDEP (Chemically Induced Dynamic Electron Polarization) Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscope... 

The complete absence of ISC in PRSB implies that the direct-excitation isomerization of this molecule (and also of RSB) proceeds exclusively via the singlet manifold (see the discussion below on the feasibility of a triplet mechanism in rhodopsins). 

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