Carbonyl triplets, decay

Thus we see that in molecules possessing ->- 77 excited states inter-combinational transitions (intersystem crossing, phosphorescence, and non-radiative triplet decay) should be efficient compared to the same processes in aromatic hydrocarbons. This conclusion is consistent with the high phosphorescence efficiencies and low fluorescence efficiencies exhibited by most carbonyl and heterocyclic compounds. 

The results presented in this report correspond to systems where reactions 2 and 3 account for the decay of carbonyl triplets A series of copolymers of phenyl vinyl ketone and o-tolyl vinyl... 

It is found that carotenoids with carbonyl functional groups increase the triplet decay rate constants of the porphyrins TPP or TPPS in frozen toluene/ethanol glasses possibly by triplet-triplet transfer. The triplet-triplet transfer could be facilitated by the presence of hydrogen bonding between carotenoids containing carbonyl groups and the prophyrins. Carotenoids with no polar functional groups like 3 Totene do not interact with porphyrin so as to affect the triplet life time of the latter. 

Role of Hydrogen Bonding. Qualitatively, these results can be understood as effects of hydrogen bonding on the triplet-state decay, as explained below. Hydrogen bonding is known to occur at carbonyl triplet states (12), and probably will also occur at the three methoxyl oxygens of I. 

The Paterno-Biichi reaction between excited carbonyl groups and olefins leads to the formation of four-membered cyclic ethers (oxetanes). This reaction has been reviewed by Arnold [94], Jones [95], Wilson [96]. The scope of the Paterno-Buchi reaction is impressive. It might be initiated by either singlet or triplet, n, re or, less frequently re, n excited carbonyls, and both electron-rich as well as extremely electron-poor olefins participate. It is now widely accepted that the initial intermediate in the oxetane formation is an exciplex between the excited carbonyl triplet and the olefin. The exciplex is then thought to decay to the triplet 1,4-diradical which, upon ISC to the singlet diradical, may either fragment back to the starting components or cyclize to oxetanes. 

This mechanism recognizes the fact that 02(JAg) is an electronically excited state and will be a good electron acceptor. The analogy has been drawn [110] between the behavior of the latter species and of ketone triplets. Replacement of 02( Ag) by a carbonyl triplet in Eq. (60) would give a scheme with which no organic photochemist would argue. Of particular importance is that decay of each species, and of each species complexed to an organic substrate, requires... 

Nanosecond Flash Photolysis Measurements.—A computer-controlled ns flash photolysis spectrometer has been described. " The system was employed in a study of the photochemistry of xanthene dyes in solution. A nitrogen laser was used to provide 2—3 mJ excitation pulses at 337.1 nm for a ns flash photolysis study of electron-transfer reactions of phenolate ions with aromatic carbonyl triplets. " A PDP II computer was used to control the transient digitizer employed for detection, and to subsequently process the data. A nanosecond transient absorption spectrophotometer has been constructed using a tunable dye laser in a pulse-probe conflguration with up to 100 ns probe delayA method for reconstructing the time-resolved transient absorption was discussed and results presented for anthracene in acetonitrile solution. The time-resolution of ns flash photolysis may be greatly increased by consideration of the integral under the transient absorption spectrum. Decay times comparable to or shorter than the excitation flash may be determined by this method. 

So far, the solid state type I reaction has been reliable only when followed by the irreversible loss of CO to yield alkyl-alkyl radical species (RP-B or BR-B) in a net de-carbonylation process. The type 11 reaction relies on the presence of a y-hydrogen that can be transferred to the carbonyl oxygen to generate the 1,4-hydroxy-biradical (BR C). The type-1 and type-11 reactions are generally favored in the excited triplet state and they often compete with each other and with other excited state decay pathways. While the radical species generated in these reactions generate complex product mixtures in solution, they tend to be highly selective in the crystalline state. 

Mechanistic evidence indicates 450,451> that the triplet enone first approaches the olefinic partner to form an exciplex. The next step consists in the formation of one of the new C—C bonds to give a 1,4-diradical, which is now the immediate precursor of the cyclobutane. Both exciplex and 1,4-diradical can decay resp. disproportionate to afford ground state enone and alkene. Eventually oxetane formation, i.e. addition of the carbonyl group of the enone to an olefin is also observed452. Although at first view the photocycloaddition of an enone to an alkene would be expected to afford a variety of structurally related products, the knowledge of the influence of substituents on the stereochemical outcome of the reaction allows the selective synthesis of the desired annelation product in inter-molecular reactions 453,454a b). As for intramolecular reactions, the substituent effects are made up by structural limitations 449). 

When monitoring the transient due to triplet carbenes is difficult because of the inherent weak nature of the bands and/or severe overlapping with the absorption bands of the parent diazo compounds, it is more convenient to follow the dynamics of the triplet carbene by measuring the rate of the products formed by reaction of triplet carbenes with quenchers such as radicals (Section 5.3) and carbonyl oxides (Section 6.5). In this case, note that the observed rate constant (feobs) of a triplet carbene reaction is the sum of the decay rate constants of the triplet. These may include decay via an associated but invisible singlet with which the triplet is in rapid equilibrium. Thus in general. 

Carbonyl oxides are also easily detected by LFP with fast TRUV-vis spectroscopy. For example, LFP of a-diazophenylacetate in deaerated Freon-113 generates a transient absorption band at < 270 nm ascribable to triplet methoxycarbonylphenyl-carbene (53), which shows a pseudo-first-order decay with lifetime of 460 ns. When LFP is carried out in aerated solvents, a new transient band appears at 410 nm at the expense of the transient band due to triplet carbene. The decay rate of triplet car-bene increases as the concentration of oxygen increases. This correspondence indicates that the triplet carbene is trapped by oxygen to form the carbonyl oxides (53-O2). This result confirms that the transient absorption quenched by oxygen... 

The behavior of 2-methyl-2-octen-4-one demonstrates the speed with which a,/S-unsaturated ketones relax to unreactive excited states. The compound is essentially photoinert.273 No matter what kind of twisting occurs around the enone system, the free butyl group is readily accessible to the carbonyl oxygen. Nevertheless, no intramolecular hydrogen transfer takes place, indicating that any k,tt states formed decay very rapidly to a relatively unreactive state, presumably the T,r,7t triplet. 

The high value for the quenching of 3,4-dimethoxyacetophenone by phenol suggests that it is probable that within the lignin structure hydroxyl groups are able to quench carbonyls by a static mechanism to yield phenoxy-ketyl radical pairs which decay on a timescales faster than the time resolution of our laser flash photolysis apparatus. Intersystem crossing rate constants for triplet radical pairs in the restricted environments of micelles have been demonstrated to be of the order of 2 -5 x 106 s-1 (25, 24). However, in the lignin matrix where diffusional processes are likely to be... 

An Example The Phosphorescence of Dithiosuccinimide Many thio-carbonyls have photostable excited (n > ji ) and (ti —> ti ) states that tend to relax by photophysical rather than photochemical processes.177,178 Recently, the electronic spectra of dithioimides have been under experimental and theoretical investigation.179-181 The spin-forbidden radiative decay of the lowest-lying triplet state of dithiosuccinimide may serve as an example to illustrate the results of the previous sections. Experimentally a lifetime of 0.10 0.01 ms was determined for the Ti state.179 This value has been corrected for solvent effects, but the transition may include radiative as well as nonradiative depletion mechanisms. 

After the application of the microwave pulse at r = to. Curve a of Fig. 14-10 was found to be changed to Curve b of Fig. 14-10. The b-a difference is illustrated by Curve d in Fig. 14-12(a). This curve shows that a-b value gradually increases immediately after the microwave pulse irradiation and that the value slowly decreases after its maximum at t=/Mi- The initial increase corresponds to the disappearance of [M] with its rate constant (kp) given by Eq. (14-8a). In principle, this rate constant for radical pairs generated by the photoreduction of carbonyl and quinone molecules in micelles can be obtained from their A(t) profiles with ns-laser photolysis measurements. In the actual measurements, however, this rate constant has never been obtained from their A(t) profiles because there are many components due to other species such as triplet precursors and reaction products in the profiles. From the present ODESR measurement, Sakaguchi et al. could purely generate the mixed M state with the microwave pulse and succeeded in direct measurement of the kp value for the first time. From the decay of the a-b value, the kj value can be obtained. They, therefore, could determine the kp value for the first time from their ODESR method. 

High intensity irradiation of dibenzyl ketone in hexane/isopropanol mixtures yields 1,2-diphenyl ethane and toluene as the sole products. A study of the photochemical behaviour of dibenzyl ketone adsorbed on zeolites has been carried out and the results found to be dependent upon the Sl/Al composition of the zeolite. A laser flash examination of the behaviour of dibenzyl ketone in Nafion membranes has demonstrated that a modest yield of benzyl cations is formed. A study of the physical photochemistry of the ketone (1) has shown that the excited state is a carbonyl localized singlet state. This ultimately decays to afford a naphthalene localized triplet state. Norrish Type I cleavage does occur from the singlet state to afford products derived from the radicals (2) and (3). The authors report also that the 1-naphthyl methyl radical is formed from the triplet state. A study of the modification of the photochemical behaviour of the a-alkyldibenzyl ketones (4) by complexation in... 

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