Quantum yield radicals

Figure 18 Schematic representation of the potential energy curves of the ground, MLCT, MLCT, and an states of Re(CO)3R(dmb) with R = Et, Bz, and i-Pr (a) and R = CH3 and CD3 (b). Note that the barrier to dissociation for CH3/CD3 is substantially larger than that of Et, Bz, and i-Pr accounting for the differences in radical quantum yield. (Ref. 293a. Reproduced by permission of Wiley-VCH)...

Figure 6. The reciprocals of the primary methyl radical quantum yields in the 184.9 nm photolysis of 2,3(0) and 3,3-dimethyl-l-butene ( ), cis-3-hexene ( ), and 4-methyl-cis-2-pentene ( ) versus the pressure. From Ref. 67.

In spite of the large number of available photoinitiators [4], the search for new initiators is ongoing. For example, S-(4-benzoyl)phenylthiobenzoate, BpSBz, has been found to be a type I photoinitiator. Upon exposure to light it is cleaved into free radicals (quantum yield 0.45), which initiate the polymerization of methyl methacrylate. In contrast, BpOBz (see Chart 10.1) is not cleaved. It forms a long-lived triplet state rather than free radicals [43]. 

Smith, G.D., Molina, L.T., Molina, M.J. Measurement of radical quantum yields from formaldehyde photolysis between 269 and 339 run. J. Phys. Chem. A 106, 1233-1240 (2002)... 

Flowers, B.A., M.E. Angerhofer, W.R. Simpson, T. Nakayama, and Y. Matsumi (2005), Nitrate radical quantum yield from peroxyacetyl nitrate photolysis, J. Phys. Chem. A, 109, 2552-2558. 

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

The blue luminescence observed during cool flames is said to arise from electronically excited formaldehyde (60,69). The high energy required indicates radical— radical reactions are producing hot molecules. Quantum yields appear to be very low (10 to 10 ) (81). Cool flames never deposit carbon, in contrast to hot flames which emit much more intense, yellowish light and may deposit carbon (82). 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

The more efficient system of genera ting OH radicals in the homogeneous phase is H2O2/uv, where the quantum yield, < >254 0.50 (20,21). There... 

In 2-propanoI, the quantum yield for photolytic conversion of benzophenone to the coupled reduction product is 2.0. The reason is that the radical remaining after abstraction of a hydrogen atom from 2-propanol transfers a hydrogen atom to ground-state benzophenone in a nonphotochemical reaction. Because of this process, two molecules of benzophenone are reduced for each one that is photoexcited ... 

A simple aliphatic ketone such as acetone, when promoted to its n,n excited state, undergoes a single unimolecular photochemical reaction in high quantum yield namely a-cleavage giving a methyl and acetyl radical which react further in secondary dark processes. In general, competition... 

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

Photoinitiation with a high quantum yield of radical production in the visible light is of practical importance for photocuring processes [5,6]. 

The ion-pair complex formed by the interaction of hydroxobis(8-quinolyloxo) vanadium (V) [VOQ2OH] and /i-butyl amine is also effective in photoinitiation of polymerization of MMA in bulk and in solution [40]. The quantum yield of initiation and polymerization determined are equal to 0.166 and 35.0, respectively. Hydroxyl radical ( OH) is reported to be the initiating radical and the following photoreaction is suggested ... 

Delaire et al. [124] have reported that laser photolysis of an acidic solution (pH 2.8) containing PMAvDPA (23) and MV2 + allows the formation of surprisingly long-lived MV + - and DPA cation radicals with a very high charge escape quantum yield. The content of the DPA chromophores in PMAvDPA is as low as less than 1/1000 in the molar ratio DPA/MAA. Figure 20 shows a decay profile of the transient absorption due to MV + monitored at 610 nm [124]. The absorption persists for several milliseconds. As depicted in Fig. 20, the decay obeys second-order kinetics, which yields kb = 3.5 x 10s M 1 s. From the initial optical density measured at 610 nm, the quantum yield for charge escape was estimated to be 0.72 at 0.2 M MV2 +. ... 

The quantum yield < >) is the yield of initiating radicals produced per photon of light absorbed (eq. 5)... 

The acyl phosphonates, acyl phosphine oxides and related compounds (e.g. 81. 82) absorb strongly in the near UV (350-400 nm) and generally decompose by rescission in a manner analogous to the benzoin derivatives.381"285 Quantum yields vary from 0.3 to 1.0 depending on structure. The phosphinyl radicals are highly reactive towards unsaturated substrates and appear to have a high specificity for addition v.v abstraction (see 3.4.3.2). 

They are initiated or accelerated by typical free-radical sources, such as the peroxides referred to, or by light. In the latter case the concept of quantum yield applies (p. 1316). Quantum yields can be quite high, for example, 1000, if each quantum generates a long chain, or low, in the case of nonchain processes. 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

---