Triplet species

Phosphinidenes (R-P) differ from other low-coordinate organophosphorus compounds, such as phosphaalkynes (R-C=P), phosphaalkenes (R2C=PR), and phosphaaromatics, in that the phosphorus atom carries only a single a-bonded substituent [7-9]. They relate to carbenes, nitrenes, and silylenes and likewise can exist as singlet and triplet species. The advances that led to stable carbenes [10, 11] and silylenes [12] stimulated an exploration of the chemistry of phosphinidenes. 

UV-vis spectra of matrix-isolated intermediates are not so informative as matrix IR spectra. As a rule, an assignment of the UV spectrum to any intermediate follows after the identification of the latter by IR or esr spectroscopy. However, UV-vis spectra may sometimes be especially useful. It is well known, for example, that the energy of electronic transitions in singlet ground-state carbenes differs from that of the triplet species. In this way UV spectroscopy allows one to identify the ground state of the intermediate stabilized in the matrix in particular cases. This will be exemplified below. 

The agreement between the indirect measurements of Hammond and the direct measurements of Bell and Linschitz is surprisingly good and the fact that a triplet species is involved in the photoreduction is nicely confirmed by these complimentary techniques. There remains, however, one rather... 

The quenching of the trans dimer with oxygen and ferrocene indicates that this product is formed almost entirely from the triplet state. It is possible to calculate the amount of triplet-derived product in benzene by subtracting the amount of product obtained in the presence of oxygen from the amount of product obtained in the absence of oxygen. Such a calculation indicates that acenaphthylene triplets in benzene give both trans and cis dimers in the ratio of 74 26. The triplet state accounts for almost all of the trans product and about 10% of the cis product. The break in the slope of the Stem-Volmer plot for the trans dimer (Figure 10.3) may be attributed to the presence of two excited species which are quenched at different rates. These two species could be (a) two different monomeric acenaphthylene triplet states 7 and T2 or (b) a monomeric acenaphthylene triplet state 7 and a triplet excimer. This second triplet species is of relatively minor importance in the overall reaction since less than 5% of the total product in an unquenched reaction is due to this species. 

When the decay of the triplet species is first order with respect to the triplet concentration, the following equations are applicable ... 

The quantum yield for isomerization in the direct photolysis was found to be d> = 0.94. The reaction could also be sensitized with acetophenone ( = 1.02) and quenched with piperylene, indicating a reactive triplet species with a rate constant kr of 3 x 10l°sec-1. With a 3-(p-methoxyphenyl) derivative two products were obtained<81) ... 

The stereospecificity of addition suggests a singlet(10) carbene although the ground state of cyclopentadienylidene is known to be a triplet. Attempts to produce a triplet species, which would be expected to react nonstereospecific-ally, in a 4-methyl-cw-2-pentene matrix at 77°K or by dilution of mixtures of the azo compound and olefin with hexafluorobenzene or octafluorocyclo-butane (inert diluents) were unsuccessful. It was concluded that the singlet carbene produced upon photolysis reacts more rapidly with the olefinic... 

Fluorenone and sodium sorbate served to quench the unsensitized reaction, indicating a triplet excited state. Similarly, recent investigations of the photosubstitution of 3,5-dinitroanisole indicate a reactive triplet species(143) ... 

Addition of carbethoxynitrenes to olefinic double bonds occurs readily. Addition of both the singlet and the triplet species can take place, the former stereospecifically, the latter not 49>. Additions of sulphonyl nitrenes to double bonds have not been demonstrated except in two instances in which metals were present. The reason is that either addition of the starting sulphonyl azide to the double bond occurs to give a triazoline that loses nitrogen and yields the same aziridine as would have been obtained by the direct addition of the nitrene to the olefin, or the double bond participates in the nitrogen elimination and a free nitrene is never involved 68>. The copper-catalyzed decomposition of benzenesulphonyl azide in cyclohexene did give the aziridine 56 (15%), which was formulated as an attack by the sulphonyl nitrene-copper complex on the double bond 24>. 

Minima in Ti are usually above the So hypersurface, but in some cases, below it (ground state triplet species). In the latter case, the photochemical process proper is over once relaxation into the minimum occurs, although under most conditions further ground-state chemistry is bound to follow, e.g., intermolecular reactions of triplet carbene. On the other hand, if the molecule ends up in a minimum in Ti which lies above So, radiative or non-radiative return to So occurs similarly as from a minimum in Si. However, both of these modes of return are slowed down considerably in the Ti ->-So process, because of its spin-forbidden nature, at least in molecules containing light atoms, and there will usually be time for vibrational motions to reach thermal equilibrium. One can therefore not expect funnels in the Ti surface, at least not in light-atom molecules. 

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

Figure 10.13 Integrated rate-law plot of the decay of quinine triplet species...

Fig. 10 Theoretical curves [equation (12)] representing the temperature dependence of the signal intensities due to a triplet species that has a singlet manifold in equilibrium. The A -values represent the energy gap of the two states.

When two triplet species are allowed to interact weakly, singlet, triplet and quintet states are produced as shown in Fig. 18. Since the exchange integral... 

The arylcarbenes are also bent, the angle being about 150—155° for phenyl-and diphenylcarbenes 63,64) jj g zero-field splitting parameters were shown to be appreciably dependent on the host matrix used >. However diphenylcarbene prepared from different precursors proved to be identical and is believed to be the triplet species of diphenylcarbene. 

If identical addition rates for singlet and triplet diphenylcarbene are assumed, this ratio corresponds to 70% of the triplet species. 

The photosensitized decomposition reduces the hydride shift, as may be expected for a triplet species. However, the stereochemical change in the cycloaddition is relatively small. Similieir results were obtained by Baer and Gutsche for the sensitized photolysis of o-n-butyl-phenyl-diazomethane 63... 

Figure 6. Scheme for photoreaction of a triplet species with a nucleophile (N) in the presence of a quencher (Q). 

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