The reversible Z-E photoswitching properties of the (Z) and (E) isomers of the severely constrained bridged azobenzene derivative 5,6-dihydrodibenzo[c,g][1,2]diazocine (1) were investigated quantitatively by UV/vis absorption spectroscopy in solution in n-hexane. In contrast to normal azobenzene (AB), 1 has well separated S(1)(n pi*) absorption bands, peaking at lambda(Z) = 404 nm and lambda(E) = 490 nm. Using light at lambda = 385 nm, it was found that 1Z can be switched to 1E with very high efficiency, Gamma = 92 +/- 3%. Conversely, 1E can be switched back to 1Z using light at lambda = 520 nm with approximately 100% yield. The measured quantum yields are Phi(Z-->E) = 72 +/- 4% and Phi(E-->Z) = 50 +/- 10%. The thermal lifetime of the (E) isomer is 4.5 +/- 0.1 h at 28.5 degrees C. The observed photochromic and photoswitching properties of 1 are much more favorable than those for normal AB, making our title compound a promising candidate for interesting applications as a molecular photoswitch especially at low temperatures. The severe constraints by the ethylenic bridge apparently do not hinder but favor the Z-E photoisomerization reactions.
The high photostability of DNA is commonly attributed to efficient radiationless electronic relaxation processes. We used femtosecond time-resolved fluorescence spectroscopy to reveal that the ensuing dynamics are strongly dependent on base sequence and are also affected by higher-order structure. Excited electronic state lifetimes in dG-doped d(A)20 single-stranded DNA and dG.dC-doped d(A)20.d(T)20 double-stranded DNA decrease sharply with the substitution of only a few bases. In duplexes containing d(AGA).d(TCT) or d(AG).d(TC) repeats, deactivation of the fluorescing states occurs on the subpicosecond time scale, but the excited-state lifetimes increase again in extended d(G) runs. The results point at more complex and molecule-specific photodynamics in native DNA than may be evident in simpler model systems.
Photofragment velocity map imaging was used to study the H atom elimination mechanism in the first excited state of pyrrole at l ¼ 243.1 nm. Two major channels were observed. The first one (76%) produces very fast H atoms and appears to be due to a rapid direct N-H bond breaking in the excited electronic state. The respective H atom kinetic energy distribution has a strong narrow peak at high energies, showing that %72% of the available energy is transferred into relative fragment translation. The observed angular recoil distribution which is described by an anisotropy parameter of b ¼ À0.37 AE 0.05 indicates that the excited optical transition is preferentially perpendicular with respect to the N-H dissociation coordinate. From the maximal kinetic energy release, the value of the N-H bond dissociation energy was found to be D 0 (N-H) ¼ (32 400 AE 400) cm À1 . The other channel (24%) leads to much slower H atoms with a very broad kinetic energy distribution, consistent with subsequent unimolecular decay reactions of the molecules in the ground electronic state after internal conversion. This conclusion was supported by similar experiments for N-methylpyrrole which showed only H atoms from the second channel and no fast component. The results corroborate the conclusion that the lowest electronic state of pyrrole has ps* anti-bonding character and is repulsive with respect to the stretching of the N-H bond.
The excited electronic state lifetime of the guanosine−cytidine (G···C) Watson−Crick (WC) base pair has been directly measured in comparison to free G and C. Measurements have been carried out in solution in chloroform, where the formation of H-bonded base pairs is strongly favored, using the technique of femtosecond fluorescence up-conversion spectroscopy. The results show that the formation of the H-bonded WC pair leads to steep acceleration of the ultrafast nonradiative electronic deactivation compared to the free nucleosides, especially G, which can be explained by an intermolecular G-to-C electron-induced proton-transfer mechanism in the excited state. The results are of vital interest for bridging the huge gap between the well-known electronic properties of the isolated nucleobases and the strikingly different dynamics of DNA molecules.
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