Effects of solvent and micellar encapsulation on the photostability of avobenzone.

The photodegradation of avobenzone (AV), the only ultraviolet filter molecule approved by the Food and Drug Administration to absorb UVA radiation, is an important problem in sunscreen formulations. In this paper, the photophysics and photostability of AV in various solvent systems and in aqueous micelles are studied. AV in its keto-enol tautomer functions as an effective UVA protection agent. AV is highly susceptible to photoinduced diketonization in both nonpolar solvents and in aqueous aggregates but is considerably more stable in polar, protic solvents like methanol. By studying its stability in different surfactant solutions, we show that incorporation of AV into sodium dodecylsulfate (SDS) micelles can achieve stability levels comparable to neat methanol. Steady-state spectral shifts, fluorescence anisotropy, and time-resolved fluorescence decay measurements are all consistent with AV experiencing a polar environment after micellar encapsulation. It is proposed that AV is encapsulated in the palisade layer of the SDS micelles, which allows access to water molecules that facilitate the re-formation of the enol form after photon absorption and relaxation. Although the detailed mechanism of AV tautomerization remains unclear, this work suggests that tuning the chemical microenvironment of AV may be a useful strategy for improving sunscreen efficacy.

Using sulfur bridge oxidation to control electronic coupling and photochemistry in covalent anthracene dimers.

Covalently tethered bichromophores provide an ideal proving ground to develop strategies for controlling excited state behavior in chromophore assemblies. In this work, optical spectroscopy and electronic structure theory are combined to demonstrate that the oxidation state of a sulfur linker between anthracene chromophores gives control over not only the photophysics but also the photochemistry of the molecules. Altering the oxidation state of the sulfur linker does not change the geometry between chromophores, allowing electronic effects between chromophores to be isolated. Previously, we showed that excitonic states in sulfur-bridged terthiophene dimers were modulated by electronic screening of the sulfur lone pairs, but that the sulfur orbitals were not directly involved in these states. In the bridged anthracene dimers that are the subject of the current paper, the atomic orbitals of the unoxidized S linker can actively mix with the anthracene molecular orbitals to form new electronic states with enhanced charge transfer character, different excitonic coupling, and rapid (sub-nanosecond) intersystem crossing that depends on solvent polarity. However, the fully oxidized SO2 bridge restores purely through-space electronic coupling between anthracene chromophores and inhibits intersystem crossing. Photoexcitation leads to either internal conversion on a sub-20 picosecond timescale, or to the creation of a long-lived emissive state that is the likely precursor of the intramolecular [4 + 4] photodimerization. These results illustrate how chemical modification of a single atom in the covalent bridge can dramatically alter not only the photophysics but also the photochemistry of molecules.

Unusual concentration dependence of the photoisomerization reaction in donor-acceptor Stenhouse adducts.

Donor-acceptor Stenhouse adducts comprise a new class of reversible photochromic molecules that absorb in the visible and near-infrared spectral regions. Unimolecular photoisomerization reactions are usually assumed to be insensitive to photochrome density, at least up to millimolar concentrations. In this paper, the photoisomerization kinetics of a third-generation donor-acceptor Stenhouse adduct molecule (denoted DASA) are examined over a range of concentrations. DASA switches efficiently at micromolar concentrations in both liquid solution and in polymers, but as the photochrome concentration is increased there is a dramatic inhibition of the photoisomerization. A kinetic study of both the reactant and photoproduct decays at varying concentrations and in different hosts indicates that the forward photoisomerization and the thermal backward reaction can change by factors of 20 or more depending on DASA concentration. Femtosecond transient absorption experiments show that the initial cis → trans step of the isomerization is not affected by concentration. It is hypothesized that long-range coulombic interactions interfere with the ground state electrocyclization stage of the isomerization, which is unique to the DASA family of photochromes. The physical origin of the inhibition of photoswitching at high photochrome concentrations must be understood if the DASA class of molecules is to be used for applications that require high photochrome concentrations, including photomechanical actuation.

Exciton dynamics in heterojunction thin-film devices based on exciplex-sensitized triplet-triplet annihilation.

Exciton dynamics in a solid-state exciplex sensitized triplet-triplet annihilation (ESTTA) system are studied using transient photoluminescence (TrPL) measurements. The ESTTA system is a trilayer structure with 4,4',4''-tris(N-3-methyphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) acting as the electron donor, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) as a triplet-diffusion-singlet-blocking (TDSB) layer, and 9,10-bis(2'-naphthyl) anthracene (ADN), acting as the electron acceptor and emitter. The thicknesses of the m-MTDATA and ADN layers are 30 nm, while the thickness of the DMPPP layer is varied to characterize its effect on the singlet quenching of the ADN emission. We find that electron transfer via tunneling through the DMPPP layer is the dominant quenching channel, with a characteristic length of ∼5 nm. Doping the high photoluminescence quantum yield molecule 4,4'-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) into the ADN layer enhanced the overall intensity of the ESTTA signal but did not prevent quenching by exciplex formation. The trilayer configuration (m-MTDATA/DMPPP/ADN) can effectively prevent ADN singlets from being quenched by electron transfer and exciplex formation, and a key property of the DMPPP is its tendency to not undergo electron transfer to the ADN.

Efficient Triplet-Triplet Annihilation Upconversion in an Electroluminescence Device with a Fluorescent Sensitizer and a Triplet-Diffusion Singlet-Blocking Layer.

Solid-state triplet-triplet annihilation upconversion (TTAUC) blue emission in an electroluminescence device (i.e., an organic light-emitting diode (OLED)) is demonstrated. A conventional green fluorophore, tris-(8-hydroxyquinoline)aluminum (Alq3 ), is employed as the sensitizer that generates 75% triplet under electrical pumping for the blue triplet-triplet annihilation emitter, 9,10-bis(2'-naphthyl) anthracene (ADN), with the heterojunction bilayer structure. The operation lifetime is elongated both for ADN blue (4.1x) and Alq3 green (34.8%) emission due to efficient use of excitons and separation of recombination and emission zone. To reduce the singlet quenching (SQ) of blue TTAUC signal by the Alq3 sensitizer with lower bandgap, 1-(2,5-dimethyl-4-(1-pyrenyl)phenyl)pyrene (DMPPP) is inserted between the Alq3 and ADN as a triplet-diffusion-and-singlet-blocking layer. DMPPP exhibits triplet energy close to Alq3 and higher than ADN, as well as higher singlet energy than both Alq3 and ADN. It allows triplet diffusion from Alq3 to ADN, but blocks the SQ of the blue TTAUC signal by Alq3 . 86.1% intrinsic efficiency of TTAUC is demonstrated in this trilayer (Alq3 /DMPPP/ADN) OLED.

Protection of Molecular Microcrystals by Encapsulation under Single-Layer Graphene.

Microcrystals composed of the conjugated organic molecule perylene can be encapsulated beneath single-layer graphene using mild conditions. Scanning electron and atomic force microscopy images show that the graphene exists as a conformal coating on top of the crystal. Raman spectroscopy indicates that the graphene is only slightly perturbed by the underlying crystal, probably due to strain. The graphene layer provides complete protection from a variety of solvents and prevents sublimation of the crystal at elevated temperatures. Time-resolved photoluminescence measurements do not detect any quenching of the perylene emission by the graphene layer, although nonradiative energy transfer within a few nanometers of the crystal-graphene interface cannot be ruled out. The ability to encapsulate samples on a substrate under a graphene monolayer may provide a new way to access and interact with the organic crystal under ambient conditions.