Structure and photophysics of indigoids for singlet fission: Cibalackrot

We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model. Published under license by AIP Publishing

Characteristics of brown carbon in Western United States wildfires

Brown carbon (BrC) associated with aerosol particles in western United States wildfires was measured between Jul. and Aug. 2019 onboard the NASA DC-8 research aircraft during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) study. Two BrC measurement methods are investigated; highly spectrally-resolved light absorption in solvent (water and methanol) extracts of particles collected on filters and in-situ bulk aerosol particle light absorption measured at three wavelengths (405, 532, 664 nm) with a photo acoustic spectrometer (PAS). A light absorption closure analysis for wavelengths between 300 and 700 nm was performed. The combined light absorption of particle pure black carbon material, including enhancements due to internally mixed materials, plus soluble BrC and a Mie-predicted factor for conversion of soluble BrC to aerosol particle BrC, was compared to absorption spectra from a power law fit to the three PAS wavelengths. For the various parameters used, at a wavelength of roughly 400 nm they agreed, at lower wavelengths the individual component-predicted particle light absorption significantly exceeded the PAS and at higher wavelengths the PAS absorption was consistently higher, but more variable. Limitations with extrapolation of PAS data to wavelengths below 405 nm and missing BrC species of low solubility that more strongly absorb at higher wavelengths may account for the differences. Based on measurements closest to fires, the emission ratio of PAS measured BrC at 405 nm relative to carbon monoxide (CO) was on average 0.13 Mm−1 ppbv−1, emission ratios for soluble BrC are also provided. As the smoke moved away from the burning regions the evolution over time of BrC was observed to be highly complex; BrC enhancement, depletion, or constant levels with age were all observed in the first 8 hours after emission in different plumes. Within 8 hours following emissions, 4-nitrocatechol, a well characterized BrC chromophore commonly found in smoke particles, was largely depleted relative to the bulk BrC. In a descending plume where temperature increased by 15 K, 4-nitrocatechol dropped possibly due to temperature-driven evaporation, but bulk BrC remained largely unchanged. Evidence was found for reactions with ozone, or related species, as a pathway for secondary formation of BrC under both low and high oxides of nitrogen (NOx) conditions, while BrC was also observed to be bleached in regions of higher ozone and low NOx, consistent with complex behaviors of BrC observed in laboratory studies. Although the evolution of smoke in the first hours following emission is highly variable, a limited number of measurements of more aged smoke (15 to 30 hours) indicate a net loss of BrC. It is yet to be determined how the near-field BrC evolution in smoke affects the characteristics of smoke over longer time and spatial scales, where its environmental impacts are likely to be greater

Structural and Photophysical Considerations of Singlet Fission Organic Thin Films for Solar Photochemsitry

Singlet fission (SF) is a multichromophore charge multiplication process in organic systems in which a singlet exciton shares its energy with a neighboring chromophore, thus generating two triplet excitons from one photon. SF chromophores can boost photocurrent in solar cells, raising the maximum theoretical power conversion efficiency of a single-junction solar cell from ~33% to ~45. Thin film (TF) preparation techniques, steady-state and time-resolved spectroscopic methods, and numerous advanced calculations were used to study the three systems presented here, all of which exhibit polymorphism. TFs of 1,3-diphenylisobenzofuran (1), were prepared and two polymorphs, α-1 and β-1, were discovered and characterized. α-1films exhibit ΦT near 200% and low ΦF, whereas the dominant photophysical processes in the β-1 polymorph are prompt and excimer emissions, with ΦT around 10%. Absorption fitting revealed that the S1 state of β-1 is lower than α-1, and therefore SF and the correlated triplet 1(TT) is energetically inaccessible to β-1. The SF mechanism in TFs of each polymorph is outlined in great detail. Polymorphism in tetracene (Tc), a near 200% ΦT SF material, has been previously documented, although morphology considerations have been neglected. While crystallite size has been shown to affect dynamics, the two Tc polymorphs, I and II, have not been analyzed in a thorough comparison of dynamics and photophysics. Tc II films show SF rates that are independent of crystallite size and SF occurs more rapidly than in Tc I. The slower Tc I SF rates are highly dependent on grain size. Coupling calculations suggested that Tc I should be faster, but these calculations are limited,and more sophisticated, multimolecule calculations are needed to support experimental results. Two extremely stable indigo derivatives, Cibalackrot (2) and a tert-butylated derivative(3) were structurally and photophysically characterized in solution and in TFs. Two crystalline polymorphs (2α, 2β) and an amorphous phase (2a), as well as a crystalline (3α) and amorphous (3a) phase of 3 were deposited by thermal evaporation. ΦT values of less than 25% were observed for all morphologies, except in 2β(ΦT= 50%). Excimer formation dominates relaxation pathways in TFs of 2 and 3

Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran

Two isomers of both the lowest excited singlet (S₁) and triplet (T₁) states of the directly para, para′-connected covalent dimer of the singlet-fission chromophore 1,3-diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such “excitation isomerism” is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly

Photocurrent Enhanced by Singlet Fission in a Dye-Sensitized Solar Cell

Investigations of singlet fission have accelerated recently because of its potential utility in solar photoconversion, although only a few reports definitively identify the role of singlet fission in a complete solar cell. Evidence of the influence of singlet fission in a dye-sensitized solar cell using 1,3-diphenylisobenzofuran (DPIBF, <b>1</b>) as the sensitizer is reported here. Self-assembly of the blue-absorbing <b>1</b> with co-adsorbed oxidation products on mesoporous TiO₂ yields a cell with a peak internal quantum efficiency of ∼70% and a power conversion efficiency of ∼1.1%. Introducing a ZrO₂ spacer layer of thickness varying from 2 to 20 Å modulates the short-circuit photocurrent such that it is initially reduced as thickness increases but <b>1</b> with 10–15 Å of added ZrO₂. This rise can be explained as being due to a reduced rate of injection of electrons from the S₁ state of <b>1</b> such that singlet fission, known to occur with a 30 ps time constant in polycrystalline films, has the opportunity to proceed efficiently and produce two T₁ states per absorbed photon that can subsequently inject electrons into TiO₂. Transient spectroscopy and kinetic simulations confirm this novel mode of dye-sensitized solar cell operation and its potential utility for enhanced solar photoconversion

Two Thin Film Polymorphs of the Singlet Fission Compound 1,3-Diphenylisobenzofuran

Polycrystalline thin films of 1,3-diphenylisobenzofuran (<b>1</b>) with a morphology referred to here as α exhibit highly efficient singlet fission, producing two triplet states for every absorbed photon at 77 K, and about 1.4 triplet states per absorbed photon at room temperature. However, the triplet yield depends strongly on the specific crystalline form of <b>1</b>, and for the morphology referred to as β the triplet yields are roughly an order of magnitude smaller. In this study, α, β, and mixed α/β films of <b>1</b> are prepared by thermal evaporation and solution drop-casting, and the structural and photophysical differences that may account for the very different triplet quantum yields are explored. The crystallites of <b>1</b> in thin films have been identified with two bulk crystal polymorphs grown from solution and structurally characterized. Analysis of absorption spectra of the films reveals a 600 cm^–1 blue shift in the onset and a unique spectral profile for the form α crystallites as compared to form β. Intermolecular interactions between columns of slip-stacked molecules are different in the two polymorphs, and this likely gives rise to the much smaller triplet quantum yield for β-<b>1</b>

Black Carbon Emissions from the Bakken Oil and Gas Development Region

Black carbon (BC) emission rates from the Bakken oil-producing region of North Dakota have been quantified with a NOAA airborne single-particle soot photometer (SP2). Flights in May 2014 led to six measurements of the BC emission rate in the region. Oil and gas operations (associated flaring, diesel engines associated with pumping and drilling, and oil production-related transport), limited agricultural burning, and sparse urban/transport sector activity contribute to these emissions. The BC emission rate was 1400 ± 360 t year^–1, implying that Bakken production activities are unlikely to contribute to large-scale biases in estimates of BC emissions. An upper limit on the BC emission factor from flaring based on these observations is 0.57 ± 0.14 g/m^–3. Flaring BC was not associated with optically significant internally mixed non-BC material or with significant emissions of non-BC-containing primary aerosol. BC in the outflow from the region was also generally externally mixed