Cycloreversion quantum yield

The results indicated that the cycloreversion quantum yields enhanced significantly when the electron-donating groups were substituted at 2-position of the benzene ring of these diarylethenes, while the cyclization quantum yields improved remarkably when the electron-withdrawing groups were substituted at the same position. 

Hereafter, a and b indicate the closed- and open-ring form isomers, respectively. The photocyclization and cycloreversion quantum yields were determined to be 0.46 and 0.015, respectively.1121 In the absence of oxygen, the coloration/decoloration cycle could be repeated more than 2000 times.[13] The basic performance of diaryl-ethenes is described below. 

The cycloreversion quantum yield of 17b (0 = 0.00013) was much lower than that of 15b (O = 0.075). The very low cycloreversion quantum yield was found to increase steeply as much as 34-fold when the temperature was raised from 25 to 150°C. The large temperature dependence provides 17 with thermal-gated reactivity. The recorded memory in this system can be read many times with a weak laser, which does not raise the temperature of the medium. The memory can be erased with a high-intensity laser, which can raise the temperature of the medium to as high as 100-150 °C. 

The cyclization and cycloreversion quantum yields of 34 were increased to 0.51 and 0.26, respectively. Both yields of 34 were reasonably high. 

Upon addition of an acid, such as trichloroacetic acid, the absorption maximum showed the hypsochromic shift and the cycloreversion quantum yield increased as much as 10 -fold. Protonation to the dimethylamino group decreased the electron-donating ability of the substituent and resulted in the change in reactivity. This acid-gated reactivity is potentially applicable to nondestructive readout in a polymer memory medium. 

Shibata, K., Muto, K., Kobatake, S., and Irie, M., Photocyclization/cycloreversion quantum yields of diarylethenes in single crystals, /. Phys. Chem. A, 106, 209, 2002. 

These experiments proved that a light-excited, reduced flavin is indeed able to photoreduce cyclobutane pyrimidine dimers and that these dimers undergo a spontaneous cycloreversion. The quantum yield of about 0=5% clarified that the overall dimer splitting process is highly efficient, even in these simple model systems ((]) photolyase 70%). 

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. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

Although isomerizations of singlet-excited dianthrylethanes 7 typically proceed by 4n+4n cycloaddition to give cyclomers 8, intramolecular 4n + 2n cycloaddition can be a competing low-quantum yield process, as has been established for the 10,10 -dimethyl derivative 7f [54], Due to the thermolytic cycloreversion of the 4n+4tt cycloadduct 8f, (its half-life in solution at 25°C is only 33 min), irradiation of 7f at elevated temperature actually leads to the formation of 13 as the main product. It is of interest in this context that spectroscopic evidence for dynamic conformational flexibility, and for the existence of distinct conformers in solution has been obtained for substituted dianthrylethanes 7i and 7k [55,56], Two structured emission spectra, separated by about 10 nm, and associated with two structured excitation... 

As far as the excited state chemistry of cis-dianthrylethylene 38a is concerned, in cyclohexane solution, the quantum yield for its isomerization by 47t+ 27t cycloaddition to give 40 is as low as 0.0007, and the concomitant isomerization by 47t + 4n cycloaddition to give 41 proceeds with a quantum yield of < 0.00007. Both cyclomers 40 and 41 smoothly undergo photolytic cycloreversion in cyclohexane to give cis-dianthrylethylene 38a with quantum efficiencies of 0.61 and 0.20, respectively [76]. 

Evidence for adiabatic photolytic cycloreversions at room temperature has been obtained more frequently in recent years [121,122], The adiabatic generation of singlet oxygen by photochemical cycloreversion of the anthracene and 9,10-dimethylanthracene endoperoxides 105 and 106 proceeds with wavelength-dependent quantum yields of 0.22 and 0.35, respectively, and involves the second excited singlet state of the endoperoxides [123]. Photodissociation of the 1,4-endoperoxide from l,4-dimethyl-9,10-diphenylanthracene was found to yield both fragments, i.e., molecular oxygen and l,4-dimethyl-9,10-diphenylanthracene, in their electronically excited state [124]. 

The photolysis of anthracene-benzene adducts 111 and 112 has been studied in detail [128], Photodissociation of 111 was found to give electronically excited anthracene with a quantum yield of 0.80, but the isomeric 47i + 27i adduct 112 photodissociates mainly diabatically, leading to electronically excited anthracene with a quantum yield of 0.08. The different efficiencies of adiabatic cycloreversions have been rationalized by correlation diagrams involving doubly excited states. Evidence for biradicals as intermediates in the photolyses of 111 and 112 has not been obtained. 

Photoexcitation of lepidopterene 118 (L Y = H in cyclohexane solution results in cycloreversion and gives the electronically excited product E whose deactivation to ground state is characterized by the structureless emission around 600 nm (see Figure 32). The quantum yield of the E emission is 0.58 (0.80), while that of the emission from the locally excited state L is only 0.005 (0.016). (The lower quantum yield data have been reported by... 

TABLE 27 Adiabatic Photolytic Cycloreversion of Lepidoptereue iu Methylcyclohexane/isopentane upon Excitatiou at 274 um. Temperature Dependent Fluorescence Quantum Yields and Lifetimes [134 ... 

The photolytic cycloreversion of lepidopterene to give E occurs at room temperature with a quantum yield of 90%. Upon lowering the temperature,... 

Photoexcitation of lepidopterene in solution also gives rise to a structured emission of low intensity around 400 nm. This emission is attributable to the deactivation of the locally excited state of the E rotamer A, formed mainly by inadvertent direct excitation of the ground state cycloreversion product 114 [131]. The absorption and emission spectra of 114 are typical of the anthracene chromophore (see Figure 33). Selective excitation of 114, experimentally possible because of the suitable ground state [L]/[A] equilibrium ratio, gives rise to locally excited A, which in cyclohexane solution at room temperature has a fluorescence quantum yield of 0.84 [131]. The adiabatic conversion of A into E is difficult to detect because it proceeds at 298 K... 

According to the model for [2+2] cycloaddition shown in Fig. 2, it should be possible to reach the pericyclic intermediate upon irradiation of the cycloadduct. If a common intermediate is attained from the cycloaddition and cycloreversion processes, then the sum of the quantum yields for the two processes should equal unity. This has, in fact, been observed to be the case for several exciplex and anthracene excimer systems (49b,52). Stereospecific cycloreversion of stilbene dimers 11 and 12 to t-1 has been observed to occur upon 254 nm... 

Becker et al. prepared a series of 10-substituted and 10,10 -disubstituted l,2-di(9-anthryl)ethanes 349 and their photochemical properties have been studied by determining the quantum yields for intramolecular (4tt + 4 77) photocyclomerization and the quantum yields of fluorescence in both cyclohexane solutions [347] (Scheme 96). The quantum yields for the intramolecular cyclization of monosubstituted 1,2-dianthrylethanes are higher than those for disubstituted analogs (see Table 8). Bulky substituents increase the fluorescence efficiencies and decrease the quantum yields for cycloaddition. The rate of the cycloreversion is enhanced by the addition of trifluoroacetic acid. 

Miranda and his co-workers have studied the cycloreversion of the cyclobutanes (127) and (128) and of the oxetane (129). This work made use of the pyrylium salt sensitizers (130). The reactions arise from the triplet state of the sensitizers since there is clear evidence that the reactions are quenched by molecular oxygen. The ring opening involves an electron transfer, and the best sensitizer is the thiapyrylium salt (130b). The quantum yields for the three products using the three sensitizers are shown in Table 2. ... 

Notably, photodimers of the cyclobutane type are cleaved by irradiation with far-UV light (240 nm) with a quantum yield of almost unity by way of the so-called [2+2] cycloreversion reaction. In living cells, dimer lesions can be repaired by the nucleotide excision repair pathway, which is based on the excision of a small piece of DNA around the lesion. Lesions not removed from the genome lead to cell death or mutagenesis. 

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