Photoinduced charge transfer in bichromophoric molecules in the gas phase

Mapping the intrinsic absorption properties and photodegradation pathways of the protonated and deprotonated forms of the sunscreen oxybenzone

Sunscreens provide vital protection against the photodamaging effects of UV radiation, however, many fundamental questions remain about the detailed mechanisms by which they dissipate UV energy. One such issue is the extent to which the pH environment of an organic sunscreen molecule alters its effectiveness, both in terms of ability to absorb UV radiation, and also its potential to photodegrade. Here, we use gas-phase laser photodissociation spectroscopy for the first time to measure the intrinsic UVA-UVC absorption spectra and associated photodegradation products of protonated and deprotonated oxybenzone, away from the complications of bulk mixtures. Our results reveal that protonation state has a dramatic effect on the absorption and photodissociation properties of this sunscreen. While the UV absorption profile of oxybenzone is only modestly affected by protonation across the range from 400-216 nm, deprotonated oxybenzone displays a significantly modified absorption spectrum, with very low photoabsorption between 370-330 nm. Protonated oxybenzone primarily photofragments by rupture of the bonds on either side of the central carbonyl group, producing cationic fragments with m/z 151 and 105. Additional lower mass photofragments (e.g. m/z 95 and 77) are also observed. The production spectra for the photofragments from protonated oxybenzone fall into two distinct categories, which we discuss in the context of different excited state decay pathways. For deprotonated oxybenzone, the major photofragments observed are m/z 211 and 212, which are associated with the ejection of methane and the methyl free radical from the parent ion, respectively. Implications for the suitability of oxybenzone in its protonated and deprotonated forms as an optimum sunscreen molecule are discussed.

Photoinduced ICT vs. excited rotamer intercoversion in two quadrupolar polyaromatic N-methylpyridinium cations

The excited state deactivation of two quadrupolar polyaromatic N-methylpyridinium cations is ruled by either Rotamer Interconversion (RI) in the molecule bearing two naphthyl side groups or Intramolecular Charge Transfer (ICT) by extending the aromaticity in the pyrenyl derivative.

Photophysics of sunscreen molecules in the gas phase: a stepwise approach towards understanding and developing next-generation sunscreens

The relationship between exposure to ultraviolet (UV) radiation and skin cancer urges the need for extra photoprotection, which is presently provided by widespread commercially available sunscreen lotions. Apart from having a large absorption cross section in the UVA and UVB regions of the electromagnetic spectrum, the chemical absorbers in these photoprotective products should also be able to dissipate the excess energy in a safe way, i.e. without releasing photoproducts or inducing any further, harmful, photochemistry. While sunscreens are tested for both their photoprotective capability and dermatological compatibility, phenomena occurring at the molecular level upon absorption of UV radiation are largely overlooked. To date, there is only a limited amount of information regarding the photochemistry and photophysics of these sunscreen molecules. However, a thorough understanding of the intrinsic mechanisms by which popular sunscreen molecular constituents dissipate excess energy has the potential to aid in the design of more efficient, safer sunscreens. In this review, we explore the potential of using gas-phase frequency- and time-resolved spectroscopies in an effort to better understand the photoinduced excited-state dynamics, or photodynamics, of sunscreen molecules. Complementary computational studies are also briefly discussed. Finally, the future outlook of expanding these gas-phase studies into the solution phase is considered.

Precursor and interstitial Cajal cells in the human embryo liver

Interstitial Cajal Cells (ICCs) were only proven in human adult hepatic tissue. The immune phenotypes of various cell types in the human embryonic liver (HEL) are scarcely described. It was hypothesized that in HEL ICCs are present and distinctive to the precursor/progenitor cells populations. It was aimed and performed a qualitative study of HEL by use of antibodies against CD117/c-kit, CD31, CD34, CD90, CD105, DOG1, Ki67, and adiponectin. Five human embryos of 23-29 mm were used. Blasts and hematopoietic cells were comprising the two major cell populations in late stage embryos. The general population of blasts in the HEL was CD34-/CD105, although scarce CD117/c-kit+ and CD90+ such cells were found. Hematopoietic precursors were Ki67+. Adiponectin-positive plasmalemmas were found mostly in blasts. Endothelia were CD31+/CD34+. Interstitial cells with moniliform prolongations were found; such cells were scarcely CD117/c-kit+ but consistently DOG1+. They were diagnosed as ICCs but based on the morphology of their prolongations they can be equally viewed as being telocytes (TCs). Further studies should better correlate the precursor cell-types and immune phenotypes during human liver organogenesis. Liver ICCs and/or TCs should be also investigated in the human fetal liver.

Unified Interpretation of Exciplex Formation and Marcus Electron Transfer on the Basis of Two-Dimensional Free Energy Surfaces

The mechanism of exciplex formation proposed in a previous paper has been refined to show how exciplex formation and Marcus electron transfer (ET) in fluorescence quenching are related to each other. This was done by making simple calculations of the free energies of the initial (DA*) and final (D+A-) states of ET. First it was shown that the decrease in D-A distance can induce intermolecular ET even in nonpolar solvents where solvent orientational polarization is absent, and that it leads to exciplex formation. This is consistent with experimental results that exciplex is most often observed in nonpolar solvents. The calculation was then extended to ET in polar solvents where the free energies are functions of both D-A distance and solvent orientational polarization. This enabled us to discuss both exciplex formation and Marcus ET in the same D-A pair and solvent on the basis of 2-dimensional free energy surfaces. The surfaces contain more information about the rates of these reactions, the mechanism of fluorescence quenching by ET, etc., than simple reaction schemes. By changing the parameters such as the free energy change of reaction, solvent dielectric constants, etc., one can construct the free energy surfaces for various systems. The effects of free energy change of reaction and of solvent polarity on the mechanism and relative importance of exciplex formation and Marcus ET in fluorescence quenching can be well explained. The free energy surface will also be useful for discussion of other phenomena related to ET reactions.