Effect of the Ultraviolet Radiation on the Lens.

The lens is a transparent, biconvex anatomical structure of the eyes responsible for light transmission and fine focusing on the retina. It is fundamentally constituted by water-soluble proteins called crystallins which are responsible for lens transparency due to their stable and highly organized disposition in the lens fiber cells. Some conformational changes and the subsequent aggregation of crystallins lead to loss of transparency in the lens and are the beginning of cataracts, which is the most frequent cause of reversible blindness in the world. Ultraviolet radiation is considered one of the risk factors for cataract development. The lens is exposed to radiation between 295 and 400 nm. This UV radiation may induce several processes that destroy the crystallins; the most significant is the oxidative stress due to increased free radicals formation. The oxidative stress is directly involved in modifications of the crystallin proteins leading to the formation of high molecular weight aggregates and then the subsequent opacification of the lens, known as cataracts. This review aims to summarize current knowledge about the damage of the lens proteins caused by ultraviolet radiation and its role in developing cataracts.

Unravelling the solvent polarity effect on the excited state intramolecular proton transfer mechanism of the 1- and 2-salicylideneanthrylamine. A TD-DFT case study.

Time dependent density functional theory has been used to investigate the photochemical and photophysical processes involved in the excited states relaxation of 1- and 2-salicylideneanthrylamine in different solvent environments. This investigation reveals that the pathways involved in the relaxation of the first excited state depend on the solvent polarity. The emission spectrum in acetonitrile and methanol is dominated by the cis-keto tautomers, while in cyclohexane, the spectrum is dominated by the fluorescence emission of the locally excited trans-enol form. Our results showed that, for each compound, two nearly isoenergetic trans-enol conformers can coexist in equilibrium, which upon photoexcitation, can relax by two competitive processes: rotation about the azomethine N[double bond, length as m-dash]C bond leading to the twisted-enol conformer, and the excited state intramolecular proton transfer leading to the fluorescent cis-keto tautomer, which can undergo a cis-trans isomerization producing the trans-keto photochromic product. The TD-DFT relaxed potential energy profiles for the ESIPT show that the effect of changing the solvent from polar to nonpolar solvents results on an increment of the energy barrier, and therefore, the ESIPT become kinetically less favoured. In constrast, this change favours the relaxation of the excited trans-enol form towards the twisted conformers, in both the enol and keto regions.

Primary photochemistry of nitrated aromatic compounds: excited-state dynamics and NO· dissociation from 9-nitroanthracene.

We report results of femtosecond-resolved ex-periments which elucidate the time scale for the primary photoinduced events in the model nitroaromatic compound 9-nitroanthracene. Through time-resolved fluorescence measurements, we observed the ultrafast decay of the initially excited singlet state, and through transient absorption experiments, we observed the spectral evolution associated with the formation of the relaxed phosphorescent T(1) state. Additionally, we have detected for the first time the accumulation of the anthryloxy radical which results from the nitro-group rearrangement and NO(•) dissociation from photoexcited 9-nitroanthracene, a photochemical channel which occurs in parallel with the formation of the phosphorescent state. The spectral evolution in this molecule is highly complex since both channels take place in similar time ranges of up to a few picoseconds. Despite this complexity, our experiments provide the general time scales in which the primary products are formed. In addition, we include calculations at the time-dependent density functional level of theory which distinguish the molecular orbitals responsible for the n-π* character of the "receiver" vibronic triplet states that couple with the first singlet state and promote the ultrafast transfer of population between the two manifolds. Comparisons with the isoelectronic compounds anthracene-9-carboxylic acid and its conjugated base, which are highly fluorescent, show that in these two compounds the near-isoenergeticity of the S(1) with an appropriate "receiver" triplet state is disrupted, providing support to the idea that a specific energy coincidence is important for the ultrafast population of the triplet manifold, prevalent in polycyclic nitrated aromatic compounds.

Measurements of peroxy radicals using chemical amplification-cavity ringdown spectroscopy.

The peroxy radical chemical amplification (PERCA) method is combined with cavity ringdown spectroscopy(CRDS) to detect peroxy radicals (HO2 and RO2). In PERCA, HO2 and RO2 are first converted to NO2 via reactions with NO, and the OH and RO coproducts are recycled back to HO2 in subsequent reactions with CO and O2; the chain reactions of HO2 are repeated and amplify the level of NO2. The amplified NO2 is then monitored by CRDS, a sensitive absorption technique. The PERCA-CRDS method is calibrated using a HO2 radical source (0.5-3 ppbv), which is generated by thermal decomposition of H2O2 vapor (permeated from 2% H2O2 solution through a porous Teflon tubing) up to 600 degrees C. Using a 2-m long 6.35-mm o.d. Teflon tubing as the flow reactor and 2.5 ppmv NO and 2.5-10% vol/vol CO, the PERCA amplification factor or chain length, Delta[NO2]/([HO2]+[RO2]), is determined to be 150 +/- 50 (90% confidence limit) in this study. The peroxy radical detection sensitivity by PERCA-CRDS is estimated to be approximately 10 pptv/60 s (3sigma). Ambient measurements of the peroxy radicals are carried out at Riverside, California in 2007 to demonstrate the PERCA-CRDS technique.

Singlet excited-state dynamics of nitropolycyclic aromatic hydrocarbons: direct measurements by femtosecond fluorescence up-conversion.

Understanding the dynamics of the electronically excited states of nitrated polycyclic aromatic hydrocarbons (NPAHs) is of great importance since photochemical reactions determine the atmospheric stability of these toxic pollutants. From previous studies, it is known that electronically excited NPAHs evolve through two parallel pathways: The formation of the first triplet state and the dissociation of nitrogen (II) oxide. In this contribution, we present the first time-resolved emission measurements of the singlet excited states which are the precursors in the aforementioned photoprocesses. We analyzed 1-nitronaphthalene, 9-nitroanthracene, 1-nitropyrene, 6-nitrochrysene, and 3-nitrofluoranthene in solution samples. Although these compounds are considered nonfluorescent, with the frequency up-conversion method it was possible to detect the emission from the S1 states despite their femtosecond and picosecond lifetimes. Except for 1-nitronapthalene, where a single exponential is observed, for the rest of the compounds, the emission shows double-exponential decays indicating ultrafast structural changes in the excited states. From anisotropy measurements, we conclude that no significant internal conversion occurs in the singlet manifold after excitation in the first absorption band. In accord with El-Sayed rules and with previous calculations, the highly efficient intersystem crossing implied by the large triplet yields and the ultrafast S1 decays is accounted by the pi-pi* nature of the S1 and T1 states together with the existence of higher triplet configurations which act as receiver states. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule.