Biacetyl sensitization

The phosphorescence of a 5 X lO" M solution of biacetyl in de-aerated 2-propanol at room temperature could be quenched completely by 1 a,d,e (10 8 M) 84). In all three cases, the corresponding photoreduction products 2a,d,e emerge from analogous preparative scale biacetyl sensitized runs. Since 2e is also formed, steric hindrance to hydrogen abstraction from solvent cannot be too effective when a (probably longer-lived) triplet is populated, whereas it might be effective in the direct photolysis ot 1 e 88) where isomerisation competes with reduction probably in the (short-lived) singlet state. 

When irradiated at 2420 A or above, the sum of the fluorescent and triplet-state yields of fluorobenzene is unity or slightly greater than unity. Use of the correct branching ratio is impossible because it has not been determined for fluorobenzene.30,31 This has been substantiated recently, using the Cundall technique, to measure triplet-state yields rather than the biacetyl sensitization technique. The... 

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

The first quantitative photochemical study of a Rh111 amine was reported by Moggi,8 who noted that both 254 nm (LMCT) and 365 nm (ligand field) excitation of [Rh(NH3)5Cl]2+ caused chloride labilization (equation 131). Other early reports include Basolo s study of the photoinduced stereo-retentive halide aquation from [M(en)2X2]+ (M = Rh, Ir X = Cl, Br, I), and Broomhead s observation of chloride aquation from [RhCl2(phen)2]+.726 While halide labilization dominates upon photolysis of [Rh(NH3)5Cl]2+, both bromo and ammine loss occur upon photolysis of the bromo analog (equation 132)685,707 and ammine is labilized from the iodo analog (equation 133).70 Biacetyl sensitization of the bromo complex quenches the biacetyl phosphorescence, but not the fluorescence,707 consistent with a photoreactive triplet state. 

This explanation was shown to be incorrect by investigation 95) of reactions of a number of a-diketones (benzil, biacetyl, 1-phenyl-1,2-propanedione) in the presence of olefins in oxygen saturated solutions. Slow consumption of diketone was observed with relatively rapid consumption of olefin and concomitant formation of epoxides, often in high yield. Many of the olefins which underwent this reaction do not form epoxides at all with singlet oxygen. For example, tetraphenylporphin-sensitized photooxygenation of tetramethylethylene afforded hydroperoxide 111 quantitatively while a biacetyl-sensitized reaction yielded the epoxide 112. Further, it was shown that the... 

Considerable data are available for triplet yields of benzene in dilute solutions of different solvents (see Table 13). In the main, two techniques have been used sensitized phosphorescence of biacetyl, sensitized cis-trans isomerization of butene-2, octene-2, and stllbene. All yield comparable results. In saturated hydrocarbon solvents at room temperature, the triplet yield for CgHg is found to be about 0.24 0.01. There is a solvent dependence of this quantity, the yield dropping to 0.15 in ethanol, 0.13 in methanol, and 0.09 in acetonitrile (91). In determining the effect of environment on the rate constant controlling intersystem crossing, values for emission lifetimes in the various systems are needed. These are, as mentioned previously, often unreliable. Cundall and Pereira (91) have reported... 

The dioxygen adduct of Co(Il) tetraphenylporphyrin is reported to undergo facile photoinduced dissociation of Biacetyl-sensitized photodecomposition of aqueous solutions of [ Co (phen)jplc ] , fransfO -[Co(plc)j,phen] , and trana(N)... 

The dimerization processes are, not surprisingly, influenced by geometrical effects and for example, while the tetrachloro compound (280) behaves analogously to the hydrocarbon (278) in both direct and biacetyl sensitized irradiations, only the 9,10-,9, 10 -dimer is formed from the tetrachloro isomer (281) under both sets of conditions. Bouas-Laurent and co-workers recently reported that with a -Si(Me)2 unit connecting the two anthracenes at the... 

The biacetyl-sensitized vapor-phase grafting technique was... 

Following the biacetyl-sensitized grafting study, we explored the effect of certain physical and chemical variables on unsensitized photografting of monomer vapors on pre-wetted wool (26, 27) and on pre-wetted polyamide and polyester fabrics (28). 

ESR parameters for triplet carbenes15 and nitrenes16 have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices.17 The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the S0 nitrene (Amax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, Amax 415 nm) with a rate constant of about 4.4 x 107 sec. Tt is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (T2) was about 7 nsec. T dimerizes to azopyrene.18... 

In order to increase the quantum yield of cis -> trans isomerization, it is proposed to use aromatic hydrocarbons such as anthracene as a carrier of the quantum chain process. (Z,E)-(cis-18) and (E,E)-l,4-di(3,5-di-tert-butylstyryl) benzene (trans-18) underwent mutual isomerization on biacetyl sensitization with a quantum chain process to afford a 4>c >t value of 1.2-1.3 in a cis isomer concentration of 1 X 10 M [96]. The quantum chain process proceeds through the energy transfer from the trans triplet (H ) to the ground state cis isomer ( c). On addition of anthracene (AN) as a quencher or a carrier of the excited state, the quantum... 

Another example of additives to increase the quantum yield has been reported in biacetyl-sensitized isomerization of 8-styrylfluoranthene in the presence of acridine has been reported [98]. 

For most 1,2-diarylethylenes in liquid solution at ambient temperature triplet state can be generated by nanosecond laser flash photolysis using a high-energy triplet sensitizer. With this method the T-T absorption spectra of a series of trans isomers were recorded, such as 1-StN [122] or 2-StN [421], The same T-T absorption spectra were recorded on biacetyl-sensitized excitation of the three isomers of a substituted 4-styrylstilbene [150] this was also found for all-cis and aU-trans isomers of [26]orthoparacyclophene, a derivative containing 6 phenyl rings and 5 double bonds [445]. For a,ty-diphenylpolyenes, similar T-T absorp-... 

On biacetyl sensitization 22 undergoes adiabatic one-way ZZ ZE and ZZ - EE isomerizations in the triplet manifold (Scheme 8) [98]. The ZE isomer also isomerizes to EE. The quantum yield of ZE - EE isomerization (0ze->ee) nd the photostationary state isomer ratios, ([EE]/[ZE])p , increase with increasing diene concentration. The ze->ee value is about 14 at... 

While the factors resulting here in a given distortion are intramolecular, the conclusions we can draw may be applied to cases where an isolated molecule is distorted by its environment. We would like to examine this approach as starting point for treating the effect of the 11-cw retinal-opsin interaction on the photoisomerization quantum yield of 11-cw to aW-trans retinal. This quantum yield increases from 0.2 for the free chromophore [9] to 0.66 in rhodopsin [10]. This value is close to that of the biacetyl sensitized reaction of the free chromophore, 0.75 [11], and indicates that the triplet state is involved to a large extent in the photoisomerization of ll-cis retinal and that intersystem crossing is inefficient. 

Alcoholic solvents can be an alternative medium for photosubstitution reactions. As an example chlorobenzene yielded anisole (with a quantum yield of 0.049) when irradiated in MeOH [251] and phenetole in aqueous ethanolic solutions, though in the latter case phenol was the main photoproduct [252]. Analogously, monochlorotoluenes underwent photosubstitution in MeOH forming methylanisoles [253,254] and the quantum yield of decomposition ranged from 0.1 to 0.4 with the smallest values applying to the para and ortho isomers. Biacetyl sensitized experiments confirmed... 

---