Quantum yield photodimerization

By examining any correlation between excimer formation (as evidenced by characteristic excimer fluorescence) and dimerization quantum yield, one could perhaps determine whether dimerization is dependent upon prior excimer formation. Excimer fluorescence from anthracene solutions at room temperature is negligible although it has been observed in the solid state at low temperature.<75) Unfortunately, the data for substituted anthracenes allow no firm conclusions to be drawn. Some derivatives dimerize but do not exhibit excimer fluorescence. Others both dimerize and show excimer fluorescence. Still others show excimer fluorescence but do not dimerize and finally, some neither dimerize nor show excimer fluorescence. Hopefully, further work will determine what role excimer formation plays in this photodimerization. 

Photochemical reactions of the pyrimidine polymers in solution were studied to determine the quantum yields of the intramolecular photodimerization of the pyrimidine units along the polymer chains. Photoreactions of the polymers were carried out in very dilute solutions to avoid an intermolecular(interchain) photodimerization. Quantum yields determined at 280 nm for the polymers (1-6 in Figure 1) are listed in Table I. The quantum yield of the 5-bromouracil polymer [poly(MAOU-5Br)] could not be determined because of side reactions of the base during the irradiation. 

Table III. Quantum Yields and Maximum Photodimerization Conversion of Thymine Bases in Polymethacrylates with Pendant Thymine Bases in the Film State...

QUANTUM YIELD PHOTOCHROMISM PHOTO-CROSS-LINKING PHOTOAFFINITY LABELING PHOTODIMERIZATION PHOTOISOMERIZATION PHOTOLYSIS FLASH PHOTOLYSIS... 

As in the former cases, k2 was calculated from the integrated extinction coefficients,149 k3 + kt was derived from fluorescence quantum yields,149 while k3 and k4 were separately estimated from the maximum quantum yields of photooxygenations at high oxygen concentrations.150 Flash spectroscopy techniques were used in order to determine k5 and k7, while kB was obtained from the Stern-Volmer quenching constant of oxygen.149 The ratio ke/kg was determined from the variation of AOz with the concentration of the anthracene.71 When photodimerization occurred, k13l(kia + k13) was calculated from the maximum yield of... 

Lamola and Eisinger169 have shown that the oriented adjacent pairs of 1,3-dimethyl-thymine monomers formed by photosplitting of the appropriate dimer in frozen solution will photodimerize again with a quantum yield of close to unity, and that the pair of monomers shows no fluorescence, but an exciton splitting in the absorption spectrum. 

Although the quinolizinium ion (1), like naphthalene, does not undergo photodimerization, its linear benzo derivative, the acridizinium ion, like anthracene, does so readily (Scheme 27) (57JOC1740). The photodimer dissociates when heated in ethanol. It has been reported that both the dimerization and dissociation in methanol are light-catalyzed and that the quantum yields for the two reactions are 0.23 and 0.49 respectively (78JPR739). 

In 1963, E. J. Bowen published his classic review The Photochemistry of Aromatic Hydrocarbon Solutions, in which he described two major reaction pathways for PAHs irradiated in organic solvents photodimerization and photooxidation mediated by the addition of singlet molecular oxygen, 02 ) (or simply 102), to a PAH (e.g., anthracene). For details on the spectroscopy and photochemistry of this lowest electronically excited singlet state of molecular oxygen, see Chapter 4.A, the monograph by Wayne (1988), and his review article (1994). For compilations of quantum yields of formation and of rate constants for the decay and reactions of 02( A), see Wilkinson et al., 1993 and 1995, respectively. 

In the absence of photodimerization (kg = 0) the ratio of excimer/molec-ular fluorescence quantum yields is given by (Eqs. 1-3)... 

A comparison of the quantum yield of photodimerization given (in accordance with Scheme 1) by... 

Recent attempts to correlate quantum yields of photodimerization of /3-alkoxynaphthalenes with values of [M]y- from spectroscopic data using Eq. (47) have not been entirely successful thus Selinger et al.46 conclude that... 

Irradiation of frozen aqueous thymine solutions produces the cis head-to-head (chh) dimer, the high quantum yield (0.5-1.0) being attributed to the preferred orientation of adjacent molecules in the microcrystalline thymine hydrate. The gradual isolation of substrate molecules in the photodimer matrix is associated135 with the appearance and increase in intensity of molecular fluorescence as photodimerization proceeds identical behavior... 

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). 

The photodimerization of t-1 via a nonfluorescent singlet excimer is analogous to the behavior of anthracene (41,42). A possible explanation for the absence of excimer fluorescence is provided by the high limiting quantum yield for photodimeriza- tion (0.77 0.12) obtained from the intercept of a plot of versus [t-l] l according to eq. 9 (40). Excimer fluorescence is, in general, a slow process (< 1 x 10" s l), which evidently does not compete with cycloaddition in the exciplexes of t-1 or anthracene (42). 

Mechanistic investigations by Chapman and co-workers (99) indicated that these reactions occurred via a nonfluorescent singlet exciplex intermediate. While the rate constant for quenching of - -t5 by 2,3-dimethy 1-2-butene is slower than the rate of diffusion (Table 8), the limiting quantum yield for cycloaddition is 1.0. Thus, highly efficient exciplex cycloaddition may account for the absence of exciplex fluorescence, as in the case of t-1 photodimerization. Photochemical [2+2] cycloaddition reactions have also been observed to occur upon irradiation of the cyclic c-1 analogues diphenylcyclobutane (7) and diphenyl-vinylene carbonate (10) with 2,3-dimethyl-2-butene (96) however, the mechanistic aspects of these reactions have not been investigated. 

The photodimerization is a reaction competing with the previously discussed photoisomerization (Section III). The quantum yields of photodimerizations are affected by the concentration of the photosubstrate (c > 0.1 molL-1) and the polarity of the... 

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. 

Intramolecular photocycloaddition of naphthalene and anthracene has been studied by Chandross and Schiebel [327], At concentrations above 10 3 M, bi-molecular photodimerization of the anthracene occurs in deaerated methylcyclo-hexane solution. In contrast, irradiation of much more dilute ( 2 X 105 M) solutions resulted in the formation of intramolecular adduct 342 (Scheme 94). Bouas-Laurent et al. showed that the CH2—O—CH2 link is more efficient than the (CH2)3 chain in bridging the two chromophores [328], Irradiation of diethyl ether or methylcyclohexane solution of 343 (5 X 10 5 M) with a high-pressure mercury lamp and liquid filter (X > 335 nm) gave a single photoadduct 344, which was isolated quantitatively. The quantum yield of 344 is 10 times higher than that of 342. 

Table 9 Intramolecular Photodimerization and Photocycloreversion Quantum Yields for 351a-f and 352a-f in Methylcyclohexane at 20°C...

Table 11 Quantum Yields for Intramolecular Photodimerization Compound3 Odlmer...

TABLE 2 Photodimerization Quantum yields of the Hg2+ and Pb2+ Complexes of CSDs (E)-4a-ea... 

It should be noted here that thymine photodimerization may occur by a non-concerted mechanism, involving free radical intermediates. Indeed, photoproducts other than cis-syn dimer, such as the next most abundant thymine dimer, so-called 6 4 adduct, were observed in irradiated DNA. However, the quantum yield of cis-syn photodimer formation (r/j 0.02) is more than an order of magnitude higher than that of the 6 4 adduct ( 0.0013) which in turn is an order of magnitude higher than the quantum yields for other thymine isomers [68]. This specificity can lead to the conclusion that the thymine photodimerization occurs predominantly via concerted 2 + 2 cycloaddition mechanism. A time-resolved study of thymine dimer formation demonstrated that thymine cyclobutane dimers are formed on a timescale of less than 200 nsec, while the 6 4 adduct is formed on a timescale of few milliseconds [69]. The delay in the formation of the latter was attributed to the mechanism of its formation through a reactive intermediate. 

The photostationary Z/E ratio of stilbene, (Z/E)pss, is known to be significantly dependent on the excitation wavelength. Thus, the Z/E ratio is almost unity (48 52) upon irradiation at 254 nm, but is remarkably enhanced up to 93 7 upon irradiation at 313 nm. This apparently surprising change is readily interpreted in terms of the following equation, which relates the (Z/E)pss ratio with the relative extinction coefficient of the two isomers at the excitation wavelength and the relative efficiency (quantum yield) of the forward and reverse reactions (Z/E)pss = ee/ezx z E/E z [201]. Preparative-scale direct irradiation should be done at low stilbene concentrations, since photodimerization of (fi)-stilbene may compete with the photoisomerization as the concentration increases [202-206]. 

According to mechanism (III) even if the concentration of A in one of the two sites (7) is much smaller than the amount in site X, appreciable photoreaction from AY may occur through AxA Y energy transfer as outlined in Sch. 21 or if the quantum yield of reaction from A Y is much larger than that from A x. An example in which situation (III) operates is the photodimerization of 9-cyanoanthracene in the crystalline state presented above (Sch. 23) [138,139]. On a statistical basis, many more molecules within the crystalline bulk phase are expected to be excited than those at defect sites. However, the reaction cavities capable of supporting reaction are specific to defect sites. Efficient photodimerization is believed to occur from exciton migration from the inert bulk sites to the defect sites. 

The cycloaddition of enones to olefins is a reaction of considerable synthetic interest 14°). Oxetane formation and cyclobutane formation are sometimes competitive 141>, but the latter reaction is the more common. The photodimerization of enones 142> is a special case of such cycloaddition. It has been shown that triplets are involved in these cycloadditions, since intersystem crossing quantum yields are unity 143> and cycloaddition is totally quenchable by triplet quenchers. Careful kinetic analysis indicates an intermediate which can partially revert to ground state reactants, since quantum yields are lower than unity even when extrapolated to infinite substrate olefin concentration. That a diradical is... 

CDx and in particular 7-CDx are known to accommodate two aromatic moieties under certain circumstances. Hence if two appropriate prochiral guest molecules are included in the same CDx cavity, regio- and enantioselective bimolecular photoreactions are expected to occur [108]. Indeed, it is known that the presence of CDx not only accelerates the rate but also modifies the product distribution of photocyclodimerizations of anthracene derivatives [109-111], coumarin derivatives [113-115], stilbene derivatives [116,117], stilbazole [118], and tranilast [119]. For instance, Tamaki and coworkers reported significantly enhanced quantum yields of photodimerization of anthracenesulfonates and anthracenecarboxyl-ates in the presence of 3- and 7-CDx [109-111]. These anthracene derivatives form 2 2 and 2 1 guest-host complexes with (3- and 7-CDx, respectively, and... 

That is, the ordered structure of the cholesteric mesophase affects the formation of the traTO-adduct advantageously. Furthermore, the trans/cis product ratio depends significantly on the initial acenaphthylene concentration. In isotropic solutions, the dimerization of singlet-excited acenaphthylene molecules is known to yield exclusively the czv-adduct, whereas a mixture of cis- and traTO-adducts results from triplet-excited solute molecules. The lowering of cu-adduct production in the mesophase has been attributed to the enhanced efficiency of the triplet reaction in comparison with the singlet reaction, as shown by quantum yield measurements [732]. The increase in triplet reaction efficiencies has been ascribed to the increase in the fraction of acenaphthylene-acenaphthylene collisions which have coplanar or parallel-plane orientations with respect to the surrounding solvent molecules, and not to the increase in the total number of collisions per unit time [732]. See references [713, 732, 733] for a more detailed discussion of this photodimerization reaction. 

The photocyclodimerization of JV-vinylcarbazole, which was reported by Ledwith and Shirota, can be accounted for by this mechanism [71-73]. A chain process is involved in this photoreaction, and the quantum yield exceeds unity (the maximum quantum yield is 66). The hole transfer from the cyclobutane radical cation to a neutral JV-vinylcarbazole is a key reaction for the chain process. Similar photodimerizations of electron-rich alkenes such as aryl vinyl ethers [74-76], indenes [29, 77, 78], styrenes [79-80] and enamines [71] have been reported by several groups. The DCA-sensitized photodimerization of phenyl vinyl ether gives cis- and trans-1,2-diphenoxycyclobutanes. This photoreaction also involves a chain process although the chain length is short [75]. 

Ito Y, Matsuura T. A simple method to estimate the approximate solid state quantum yield for photodimerization of trans-cinnamic acid. J Photochem Photobiol 1989 50 141-145. Ito Y, Matsuura T, Fukuyama K. Efficiency for solid-state photocyclization of 2,4,6-triisopropylbenzophenones. Tetrahedron Lett 1988 29 3087-3090. 

Photoreduction of the excited state of phenazine by THF occurs via the (n, TT ) and (rr, rr ) states of phenazine. The photodimerization of deazaflavin has been investigated, in particular its excitation in the presence of oxalate anion, which causes photodimerization with quantum yield Both electrochemical reduction and photochemical reaction of some i-(2-... 

These concepts are in very good agreement with experimental findings. There are relatively few examples of photodimerization of simple noncon-jugated acyclic olefins because these compounds absorb at very short wavelengths. Irradiation of neat but-2-ene, however, yields tetramethylcyclobu-tane with a quantum yield d> = 0.04. For very low conversion, the observed stereochemistry of the adducts is the stereospecific one expected from Scheme 13 for a concerted [ 2s + cycloaddition. However, since the major pathway is cis-trans isomerization with a quantum yield d> = 0.5 (cf. Section 7.1.2), it has been concluded that the molecules that undergo cis-trans isomerization are not involved in photodimerization (Yamazaki et al., 1976). 

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