Benzophenone excited-state ketyl radicals

The nanosecond-picosecond two-color two-laser flash photolysis method is also useful to study the excited state of radicals, that is, the D, state. We applied nanosecond-picosecond two-color two-laser flash photolysis to detect the absorption and fluorescence spectra of the ketyl radical of benzophenone and its derivatives (BPH and BPDH ) in the excited state in the UV-visible region [111], since BPH and BPDH are well investigated radicals in various fields. Since BPH and BPDH are generated from irreversible ways, such as photoionization, we employed a streak camera to realize the single-shot detection of these intermediates. 

Luminescence is seldom observed from free radicals and radical ions because of the low energy of the lowest excited states of open-shell species, the benzophenone ketyl radical being however a noteworthy exception. There are few reports of actual photochemical reactions of free radicals, but the situation is different with biradicals such as carbenes. These have two unpaired electrons and can exist in singlet or triplet states and they take part in addition and insertion reactions (Figure 4.90). 

Hammond, Wamser, Chang, and Baylor [54] studied the photochemistry of the BP/MK pair formally using flash photolysis techniques and showed that electron transfer occurs via triplet exciplexes that can be formed by excitation of either partner, though under normal circumstances MK absorbs the vast majority of the light. Benzophenone ketyl radical was observed as a transient, and a second species present was identified as either the cation radical of MK or its corresponding deprotonated radical. The coupling product of the latter two radicals (16) was observed as a product of steady-state irradiation, and it was formed with a quantum... 

Radicals and radical ions provide fruitful subjects of research. Room temperature fluorescence from the arylmethyl radicals Ph3C, Ph2CH- and PhCH2 and theoretical studies of rotational barriers in the benzyl cation, radical and anion as well as the singlet and triplet states of diphenylcarbene are typical examples of such contemporary studies. A very detailed paper considers the problems of the state assignment and reactivity of excited states of p-substituted benzyl radicals.Ketyl radicals containing the enthrone moiety and the 4-(methyl sulphonate) benzophenone ketyl radical anion are related studies in this field. 

The results clearly show that the photoinitiating activity of the water soluble benzophenone in the presence of an amine is mainly associated with the ability of the lowest excited triplet - state to abstract an electron from the amine co-synergist via an intermediate exciplex shown in Scheme I. The radical anion will then induce hydrogen atom abstraction to give a ketyl radical and an alkylamino radical. The latter is mainly responsible for inducing polymerisation of the acrylic monomer and supports earlier work on the benzophenone-triethylamine tetramine-induced photopolymerisation of methylmethacrylate during which terminal amine groups were detected (2 ) ... 

The abstraction of a hydrogen molecnle from the co-initiator or the transfer of an electron between the two initiating molecnles takes place once the photoinitiator is in the excited state. In both cases, free radicals result, one or more of which may actnally begin the photopolymerization. Initiation by hydrogen transfer is common in diaryl ketones. The co-initiator is usually an ether or an alcohol with an abstractable or-hydrogen, such as 2-propanol. Equation 4 demonstrates the H-abstraction reaction between the photoinitiator benzophenone and the hydrogen donor tetrahydrofiiran. In this case, the ether radical initiates polymerization, and the ketyl radical only participates in termination. 

In photoinitiation by electron transfer, the photoinitiator, after absorption of the initiating light, forms an excited-state complex (exciplex or charge-transfer complex) with the co-initiator, typically an amine. Electron transfer from the amine to the photoinitiator occurs in this exciplex, immediately followed by proton transfer of an a-hydrogen from the amine to the photoinitiator. This results in two radicals an amine radical that will initiate polymerization and a ketyl-type radical that will most likely terminate by coupling with another free radical species. Figure 2 demonstrates this type of photoinitiation process between benzophenone and an amine co-initiator (3,15). 

Benzophenone (BP) is a very useful photosensitizer in photografting polymerization. BP was transferred to an exciting state as it absorbed in the UV range spectrum, relaxed, and abstracted the tertiary hydrogen from the polymer main chains on the PHBHV film snrface by inelastic collision. Ketyl radical (BP ) dimerized, while polymer radical P is generated at the film surface [72]. 

The photoreduction of benzophenone in benzene has been investigated by many workers [Refs. 63,183, 481, 483]. Benzophenone in the lowest excited triplet state 3(n,n ) abstracts hydrogen from benzene, producing a ketyl and a phenyl radical ... 

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