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Abstract: The mechanistic aspects of the photo induced reactions of gaseous amyl nitrite have been investigated using ab initio calculations. We show that the C5H11O–NO bond dissociation mechanism is different on the S1 and S2 surfaces, respectively. The subsequent conformational changes in the ground state 1-pentoxy radical involves several barriers. Interestingly, we show that the intramolecular 1,5-H transfer in the 1-pentoxy radical proceeds in the opposite direction on the S1 state compared to the S0 reaction. Moreover, the results show that the excited state reaction of the alkoxy radical may be a proton transfer that proceeds on a repulsive surface. [Copyright &y& Elsevier]

The front cover artwork is provided by the Hansen and Mikkelsen groups from the University of Copenhagen. The image shows how excited state aromaticity affects the photochemistry of dihydroazulene (DHA). Inducing aromaticity enhances an excited state barrier, so DHA can no longer reach the photoproduct. Read the full text of the Article at 10.1002/cptc.201900088. [ABSTRACT FROM AUTHOR]

Photostability is an essential property of molecular building blocks of nature. Disulfides are central in the structure determination of proteins, which is in striking contradiction to the result that the S–S bond is a photochemically labile structural entity that cleaves to form free radicals upon light exposure. In an earlier contribution we hypothesized that the key to the photostability of some disulfides may be found in a cyclic structural arrangement. Here we provide further evidence to support this hypothesis by showing that straight chain disulfides undergo ultrafast S–S dissociation on a sub 50 fs timescale without further ado. In a cyclic motif resembling the cysteine–disulfide bond in proteins, light can perturb the S–S bond to generate short-lived diradicaloid species, but the sulfur atoms are conformationally restricted by the ring that prevents the sulfur atoms from flying apart. Conversely, in a straight chain conformation, light perturbation results in two separated R S · radicals because there is no restoring force to counteract the repulsive motion of the sulfur atoms. For the cyclic conformation this restoring force is provided by the cyclic framework, and thus the photostability of disulfide-bonds must be ascribed a cyclic structural arrangement. [ABSTRACT FROM AUTHOR]

The photochemistry of nitro-substitutedpolyaromatic compoundsis generally determined by the rapid decay of its S1stateand the rapid population of its triplet manifold. Previous studieshave shown that such an efficient channel is due to a strong couplingof the fluorescent state with specific upper receiver states in thetriplet manifold. Here we examine variations in this mechanism throughthe comparison of the photophysics of 2-nitrofluorene with that of2-diethylamino-7-nitrofluorene. The only difference between thesetwo molecules is the presence of a diethylamino group in a push–pullconfiguration for the latter compound. The femtosecond-resolved experimentspresented herein indicate that 2-nitrofluorene shows ultrafast intersystemcrossing which depopulates the S1emissive state withinless than a picosecond. On the other hand, the amino substituted nitrofluoreneshows a marked shift in its S1energy redounding in theloss of coupling with the receiver triplet state, and therefore amuch longer lifetime of 100 ps in cyclohexane. In polar solvents,the diethylamino substituted compound actually shows double peakedfluorescence due to the formation of charge transfer states. Evaluationof the Stokes shifts in different solvents indicates that both bandscorrespond to intramolecular charge transfer states in equilibriumwhich are formed in an ultrafast time scale from the original locallyexcited (LE) state. The present study addresses the interplay betweenelectron-donating and nitro substituents, showing that the additionof the electron-donating amino group is able to change the couplingwith the triplet states due to a stabilization of the first excitedsinglet state and the rapid formation of charge transfer states inpolar solvents. We include calculations at the TD-DFT level of theorywith the PBE0 and B3LYP functionals which nicely predict the observeddifference between the two compounds, showing how the specific S(π–π*)–T(n−π*)coupling normally prevalent in nitroaromatics is lost in the push–pullcompound. [ABSTRACT FROM AUTHOR]

We have investigated the deep-UV photoinduced, homolytic bond cleavage of amyl nitrite to form NO and pentoxy radicals. One-color multiphoton ionization with ultrashort laser pulses through the S2 state resonance gives rise to photoelectron spectra that reflect ionization from the S1 state. Time-resolved pump--probe photoionization measurements show that upon excitation at 207 nm, the generation of NO in the v = 2 state is delayed, with a rise time of 283 (16) fs. The time-resolved mass spectrum shows the NO to be expelled with a kinetic energy of 1.0 eV, which is consistent with dissociation on the S1 state potential energy surface. Combined, these observations show that the first step of the dissociation reaction involves an internal conversion from the S2 to the S1 state, which is followed by the ejection of the NO radical on the predissociative S1 state potential energy surface. [ABSTRACT FROM AUTHOR]

For a molecule to survive evolution and to become a key building block in nature photochemical stability is essential. The photolytically weak S-S bond does not immediately seem to possess that ability. We mapped the real-time motion of the two sulfur radicals that result from disulfide photolysis on the femtosecond time scale and found the reason for the existence of the S-S bridge as a natural building block in folded structures. The sulfur atoms will indeed move apart on the excited state but only to oscillate around the S-S center of mass. At long S-S distances there is a strong coupling to the ground state and the oscillatory motion enables the molecules to continuously revisit that particular region of the potential energy surface. When a structural feature such as a ring prevents the sulfur radicals from flying apart and thus assures a sufficient residence time in the active region of the potential energy surface the electronic energy is converted into less harmful vibrational energy thereby restoring the S-S bond in the ground state. [ABSTRACT FROM AUTHOR]