Rational Design and Synthesis of Efficient Sunscreens To Boost the Solar Protection Factor.

Skin cancer incidence has been increasing in the last decades, but most of the commercial formulations used as sunscreens are designed to protect only against solar erythema. Many of the active components present in sunscreens show critical weaknesses, such as low stability and toxicity. Thus, the development of more efficient components is an urgent health necessity and an attractive industrial target. We have rationally designed core moieties with increased photoprotective capacities and a new energy dissipation mechanism. Using these scaffolds, we have synthesized a series of compounds with tunable properties suitable for their use in sunscreens, and enhanced properties in terms of stability, light energy dissipation, and toxicity. Moreover, some representative compounds were included in final sunscreen formulations and a relevant solar protection factor boost was measured.

A biomimetic molecular switch at work: coupling photoisomerization dynamics to peptide structural rearrangement.

In spite of considerable interest in the design of molecular switches towards photo-controllable (bio)materials, few studies focused on the major influence of the surrounding environment on the switch photoreactivities. We present a combined experimental and computational study of a retinal-like molecular switch linked to a peptide, elucidating the effects on the photoreactivity and on the α-helix secondary structure. Temperature-dependent, femtosecond UV-vis transient absorption spectroscopy and high-level hybrid quantum mechanics/molecular mechanics methods were applied to describe the photoisomerization process and the subsequent peptide rearrangement. It was found that the conformational heterogeneity of the ground state peptide controls the excited state potential energy surface and the thermally activated population decay. Still, a reversible α-helix to α-hairpin conformational change is predicted, paving the way for a fine photocontrol of different secondary structure elements, hence (bio)molecular functions, using retinal-inspired molecular switches.

Hydantoin-based molecular photoswitches.

A new family of molecular photoswitches based on arylidenehydantoins is described together with their synthesis and photochemical and photophysical studies. A series of hydantoin derivatives have been prepared as single isomers using simple and versatile chemistry in good yields. Our studies show that the photostationary states of these compounds can be easily controlled by means of external factors, such as the light source or filters. Moreover, the detailed investigations proved that these switches are efficient (i.e., they make efficient use of the light energy, are high fatigue resistant, and are very photostable). In some cases, the switches can be completely turned on/off, a desirable feature for specific applications. A series of theoretical calculations have also been carried out to understand the photoisomerization mechanism at the molecular level.

Reversible photocontrol of peptide conformation with a rhodopsin-like photoswitch.

Reversible photocontrol of biomolecules requires chromophores that can efficiently undergo large conformational changes upon exposure to wavelengths of light that are compatible with living systems. We designed a benzylidene-pyrroline chromophore that mimics the Schiff base of rhodopsin and can be used to introduce light-switchable intramolecular cross-links in peptides and proteins. This new class of photoswitch undergoes an ~10 Å change in end-to-end distance upon isomerization and can be used to control the conformation of a target peptide efficiently and reversibly using, alternately, violet (400 nm) and blue (446 nm) light.

Fluorenylidene-pyrroline biomimetic light-driven molecular switches.

A new family of biomimetic photoactivated molecular switches based in the retinal chromophore is described. Expedient synthesis allows a library of compounds with a different substitution pattern, including chiral substituents, to be obtained. The effect of substitution, solvent, and light source on the photoisomerization step has been assessed. The absorption maximum has been red-shifted ca. 50 nm with respect to related systems and rotation is now easily achieved by using visible light.

Regio- and stereochemistry of [2 + 2] photocycloadditions of imines to alkenes: a computational and experimental study.

The [2 + 2] photocycloaddition of isoxazolines to alkenes has been studied by means of CASPT2/6-31G*//CASSCF/6-31G*. The reaction outcome is influenced by the relative ratio of imine deactivation and photocycloaddition. Analysis of the conical intersection points involved in the photoreaction shows that fast deactivation is prevented when an electron-withdrawing group is placed in any position that can affect the imine moiety. Computational data predict that the photoreaction will be regiospecific but without stereoselectivity. Furthermore, the favored regioisomer will be different for alkenes with electron-withdrawing or electron-releasing substituents. The results of a complementary experimental study correlate well with the computational data. Several conclusions included in the present work could prove useful for the generalization of the photocycloaddition of imines.

New light-induced iminyl radical cyclization reactions of acyloximes to isoquinolines.

[reaction: see text] An efficient photochemical approach for the unusual generation of six-membered heterocyclic rings is reported. Iminyl radicals, generated by the irradiation of acyloximes, participate in intramolecular cyclization processes and in intermolecular addition-intramolecular cyclization sequences.

Benzylidene-oxazolones as molecular photoswitches.

The synthesis and photochemical study of a family of molecular switches inspired by the green fluorescent protein (GFP) chromophore is presented. These compounds can be easily synthesized, and their photophysical properties may be tuned. Due to their efficient photoisomerization and high stability, these compounds can be switched on/off by using light and heat or light with different wavelengths.

Photocyclization of iminyl radicals: theoretical study and photochemical aspects.

The irradiation of acyloximes was studied by theoretical methods. CASPT2/6-31G*//CASSCF/6-31G* calculations, using an active space of 14 electrons in 11 orbitals, indicate that S2 should be the spectroscopic state, and its relaxation leads directly to N-O bond breakage due to coupling between the imine pi* and the sigma* N-O orbitals. Subsequent calculations at the B3PW91/6-31+G* level suggest that the resulting iminyl radicals are able to cyclize to the five- or six-membered ring, depending on the presence of a phenyl group as a spacer, a process that has been verified experimentally. The photochemical aspects of the more common five-membered ring formation, such as excited-state quenching, quantum yield, excited-state sensitizers, laser flash photolysis experiments, Stern-Volmer plot, and luminescence measurements, were investigated. These studies indicate that singlet and triplet excited states undergo the same reaction. Emission lifetimes of ca. tau = 10.6 micros for compound 11 are suggestive of triplet parentage, while no fluorescence was detected, in agreement with the computed MEP energy profile.

Irradiation of imine-group VI carbene complexes in the presence of alkynes. 2. Control of product distribution.

The photoreactivity of iminecarbene complexes in the presence of alkynes has been explored. Up to four different reaction paths are available depending on the alkyne and carbene complex substituents, although in each case only one type of product is isolated. 2H-Pyrrole derivatives are formed mainly from aryl alkynes. When alkyl alkynes are used, the method affords a new type of aza-dendralene product in good yields. Isoquinoline derivatives can also be formed in a two-step one-pot photochemical process when the appropriate substituents are present. Finally, indene derivatives are also available through a benzannulation reaction. To explore the underlying mechanism, we carried out computations using DFT methods. Experimental and theoretical results compare well, which allows control over the reaction and product distribution.

The N-cyclopropylimine-1-pyrroline photorearrangement as a synthetic tool: scope and limitations.

The scope and limitations of the photorearrangement of N-cyclopropylimines to 1-pyrrolines are presented. The influence on the reactivity of different substituents throughout the cyclopropane ring and at the iminic position of the N-cyclopropylimine structure is discussed. The observed effects are interpreted from computational studies. The principal findings relate to (1) the enhanced reactivity of 1-substituted compounds toward rearrangement, (2) the lack of reactivity of crowded cyclopropanes, and (3) the high chemoselectivity of the process.

Mechanism of the N-cyclopropylimine-1-pyrroline photorearrangement.

We present here a combined experimental and computational investigation into the photorearrangement of N-cyclopropylimines to yield pyrrolines. We show that the photochemistry, regiochemistry, and stereochemistry of the reaction can be understood in terms of a mechanism involving barrierless evolution in three different (S(2), S(1), S(0)) singlet states and sequential decay through two different (S(2)/S(1), and S(1)/S(0)) conical intersection funnels. We provide evidence that the reaction mechanism involves the generation of a nonequilibrated (i.e., transient) excited state diradical, whose decay can lead not only to pyrrolines but also to cyclopropylimine isomers. It is concluded that the reaction outcome depends on the details of the structure of such transient diradical and on the nature of the dynamics of its decay through the S(1)/S(0) conical intersection.

Irradiation of imine-group VI carbene complexes in the presence of alkynes: a theoretical and experimental study.

The photoreaction between imine-substituted Fischer carbene complexes and alkynes is studied at both experimental and theoretical levels. 2H-Pyrrole derivatives are easily obtained as main products in moderate to good yields, with complete control of the regiochemistry. High-level theoretical calculations are carried out in order to explore and fully understand the reaction pathway. On the basis of the theoretical results, a mechanism that accounts for the experimental findings is proposed.