Quantum yield of the dimerization

While the photodimerization of bis(l-naphthylmethyl) ether was acknowledged somewhat earlier 39), the photodimers were first characterized and the quantum yield of the dimerization determined by Todesco et al. U2). Both the syn- and anti-photodimers were formed in roughly equal amounts, and the quantum yield for formation of the anti-dimer was independent of solvent. However, the quantum yields for formation of the syn-dimer and for excimer fluorescence were found to vary with solvent such that their sum was independent of solvent. The fact that irradiation of l,3-bis(l-naphthyl)-1-propanol yields only the syn-photodimer 113> indicates that the conformational properties of oxygen are largely responsible for anii-dimerization in the ether compound. The possibility of photodimerization was unfortunately not considered in the fluorescence studies of protonated bis (1-naphthylmethyl) amine 115>, l,3-bis(4-methoxy-l-naphthyl) propane 116>, and meso-bis( 1 -(1 -naphthyl-ethyl) ether 13). 

Owing to the bimolecularity of the initiation reaction the quantum yield of the dimer molecules (M2/Nji ) is proportional to the absorbed light quanta N bs and to the ratio kj/ko, characterizing the competition of the chemical dimer initiation process (kj) with the deactivation processes (ko) of the monomer excitation. A comparison of the dimer A absorption intensities of different diacetylene crystals shows that the ratio kj/ko is about a factor of 10 to 10 larger in the TS-6 crystals than in... 

Finally a few sentences are deserved for the vast area of DNA photochemistry. Thymine dimerization is the most common photochemical reaction with the quantum yield of formation in isolated DNA of all-thymine oligodeoxynucleotides 2-3% [3], Furthermore, a recent study based on femtosecond time-resolved transient absorption spectroscopy showed that thymine dimers are formed in less than 1 ps when the strand has an appropriate conformation [258], The low quantum yield of the reaction in regular DNA is suggested to be due to the infrequency of these appropriate reactive conformations. 

The cycloreversion experiments showed a clean Tf=T-DNA to T/T-DNA transformation. No by-products were detected, which supports the idea that DNA may be more stable towards reduction compared to oxidation. Even heating the irradiated DNA with piperidine furnished no other DNA strands other then the repaired strands, showing that base labile sites - indicative for DNA damage - are not formed in the reductive regime. The quantum yield of the intra-DNA repair reaction was therefore calculated based on the assumption that the irradiation of the flavin-Tf=T-DNA strands induces a clean intramolecular excess electron transfer driven cycloreversion. The quantum yield was found to be around 0=0.005, which is high for a photoreaction in DNA. A first insight into how DNA is able to mediate the excess electron transfer was gained with the double strands 11 and 12 in which an additional A T base pair compared to 7 and 8 separates the dimer and the flavin unit. 

Where MPe represents the phthalocyanine in an excited triplet state. The CBr3 species is expected to dimerize rapidly following the reaction. The quantum yields of the resulting phthalocyanine n cation radical are shown in Table H. The EPR spectra obtained at 79 K for the ZnPc and RuPc n C U ion radicals give isotropic g values (Table II), which are very close to the free electron value of 2.0023 and are characteristic of the n caJtion radic ds of phthalocyanines (10,15). 

Electron photodetachment upon laser excitation of the solvent anion above 1.76 eV was observed (Fig. 2a,c) [18]. The cross section of photodetachment linearly increases between 1.76 and 3 eV (Fig. 2b). Under the same physical conditions, the photodetachment and absorption spectra of the solvent anion are identical (Fig. 2b) [20], suggesting a bound-to-CB transition the quantum yield of the photodetachment is close to unity. The photodetachment spectrum is similar to the photoelectron spectra of (C02) 9 clusters observed by Tsukuda et al. [24] in the gas phase it is distinctly different from the electron photodetachment spectra of CO2 in hydrocarbon liquids [27]. This suggests that a C-C bound, 7)2, symmetric dimer anion constitutes the core of the solvent radical anion [18,19]. 

The type II direct photooxygenation of anthracene and its derivatives has been studied quantitatively by Bowen,2 Livingston,3 and their co-workers. Quantum yields of fluorescence, dimerizations, and... 

The triplet energy of thianaphthene 1,1-dioxide was determined by two indirect methods. The first involved the use of several sensitizers of decreasing triplet energy. The results summarized in Table 1 indicate that triplet lies between 53 and 49 kcal mol-1. The second method is more precise and involves the use of thianaphthene 1,1-dioxide as a sensitizer to establish a photostationary state of the a—methylstilbenes. The composition of the photostationary state of a-methylstilbene has been determined as a function of the triplet energy level of the sensitizer. The results indicate a triplet energy for thianaphthene 1,1-dioxide of 50 1 kcal mol-1. Quantum yields of the photodimerization of thianaphthene 1,1-dioxide were determined in benzene as a function of concentration. Oisc is 0.18. The product distribution as a function of solvent polarity demonstrates the ratio of the head-to-head to head-to-tail dimer (HH/HT) increases with the polarity of the solvents. This is consistent with preferential solvatation of the head-to-head transitions state. 

The substituent effect of 10,10 -positions of bis[2-(9-anthryl)ethyl] glutarates 353a-e, which have more flexible chains, was investigated by irradiation with 350-nm light in dilute (10-4 M) deoxyganated benzene [349], The quantum yields for the dimerization (dimer) falls with the steric bulk of the substituents. 

Crystalline mixtures of the dimers (150-152) with tetracyanoquinodi-methane (153) undergo photoinduced intermolecular electron transfer when irradiated at >350 nm. Cleavage of the C-C bond and the formation of the monomers then occurs. The photochromic properties of the chromene (154) have been studied.The ring opening kinetics using 357 nm were measured and ring closure was effected by irradiation at 422 nm. The quantum yields of the forward and back processes were also measured. Kaneko and coworkers have studied the photochemical cyclization of the enediyne (155) which forms the aromatic compound (156). The best yields are obtained using hexane as the solvent. 

Acridizinium salts also undergo [4+4] dimerization to yield (287) and aspects of the intramolecular interaction and reaction of the arene chromophores in compounds of type (288) with n=l-6 and 8 have been publishedf The quantum yields of the reversible intramolecular dimerization have been determined in several solvents and are reported to depend upon the value of n. Formation of the dimer arises from the intramolecular excimer and a common pericyclic transition state is assumed for the forward and back reactions. 

High chemical but low quantum yields of the two products (233) and (234) are obtained on irradiation (X > 500 nm) of the tetraketones (235). The primary photochemical step is the conversion of the tetraketones into carbon monoxide and the ketenes (236). The reaction process occurs from the singlet state and is thought to be concerted. The final products are formed by the addition of these ketenes (236) to ground state tetraketone. The ketene (236b) was studied in a little more detail and it was shown that irradiation in benzene or toluene at room temperature results in the formation of the dimer (237). Addition of the same ketene to diketones such as biacetyl was also reported. 

The quantum yields for the dimerization of coumarin with or without and the inclusion complex were measiired. The quantum yield, is defined by the equation =-[d(coumarin)/dt]/I(abs)... 

The initial quantum yields for the dimerization at the lower limit are 0.5, 0.3 and 0.2 without and with 6-cyclodextrin, and in the cavity of the cyclodextrin at the solid state, respectively, at 312 nm. This indicates that these photodimerizations all belong to one and the same type of reaction and they proceed more efficiently than that in solution The quantum efficiencies for formation of cis- and trans-head-to-head dimers were determined as 4. 4 X lO in acetonitrile and 4.4-35 X 10 in various solvents such as acetonitrile, dioxane and carbon tetrachloride, respectively [17]. The reason for the difference between the quantum yields in the solid and the solutions [15,17] could not be ex-... 

Here O is the quantum yield for a solution of concentration [M], Oq is the quantum extrapolated to infinite dilution, and [M]i is the concentration at which the quantum yield has fallen to half its limiting value Oq. (Note that the quantum yield refers to the monomer fluorescence, not to that of the excimer.) The Stern-Volmer relation holds only when the fluorescence of an excited molecule M is quenched by reaction of M with another molecule. If in this case the dimer were formed in the ground state and were undergoing fluorescence of the light it had absorbed as a dimer, the quantum yield of the monomer fluorescence would not follow equation (6.23). 

Such a photoprocess could involve the reaction of the excited state Mo2(02P(OPh)2)4 as a diradical, whereby it abstracts a chlorine atom from 1,2-dichloroethane. Alternately, the excited state Mo2(C>2P(OPh)2)4 could act as a reducing agent to give chloride ion and the alkyl radical. The quantum yield of the reaction is decreased if the photoreaction is carried out in a coordinating solvent such as THE or acetonitrile because such compounds coordinate into the axial coordination position of the excited state dimer, which is the site of reactivity. 

Fig. 2 Effect of the laser intensity on the quantum yield of formation of oxidized bases (j)ox) and pyrimidine dimeric photoproducts (0dim)...

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