In plasmon-enhanced heterogeneous catalysis, illumination accelerates reaction rates by generating hot carriers and hot surfaces in the constituent nanostructured metals. In order to understand how photogenerated carriers enhance the nonthermal reaction rate, the effects of photothermal heating and thermal gradients in the catalyst bed must be confidently and quantitatively characterized. This is a challenging task considering the conflating effects of light absorption, heat transport, and reaction energetics. Here, we introduce a methodology to distinguish the thermal and nonthermal contributions from plasmon-enhanced catalysts, demonstrated by illuminated rhodium nanoparticles on oxide supports to catalyze the CO methanation reaction. By simultaneously measuring the total reaction rate and the temperature gradient of the catalyst bed, the effective thermal reaction rate may be extracted. The residual nonthermal rate of the plasmon-enhanced reaction is found to grow with a superlinear dependence on illumination intensity, and its apparent quantum efficiency reaches ∼46% on a Rh/TiO catalyst at a surface temperature of 350 °C. Heat and light are shown to work synergistically in these reactions: the higher the temperature, the higher the overall nonthermal efficiency in plasmon-enhanced catalysis.

Conventional descriptions of excitons in semiconducting polymers do not account for several important observations in polymer:fullerene photovoltaic blends, including the ultrafast time scale of charge photogeneration in phase separated blends and the intermediate role of delocalized charge transfer states. We investigate the nature of excitons in thin films of polymers and polymer:fullerene blends by using broadband ultrafast photoluminescence spectroscopy. Our technique enables us to resolve energetic relaxation, as well as the volume of excitons and population dynamics on ultrafast time scales. We resolve substantial high-energy emission from hot excitons prior to energetic relaxation, which occurs predominantly on a subpicosecond time scale. Consistent with quantum chemical calculations, ultrafast annihilation measurements show that excitons initially extend along a substantial chain length prior to localization induced by structural relaxation. Moreover, we see that hot excitons are initially highly mobile and the subsequent rapid decay in mobility is correlated with energetic relaxation. The relevance of these measurements to charge photogeneration is confirmed by our measurements in blends. We find that charge photogeneration occurs predominately via these delocalized hot exciton states in competition with relaxation and independently of temperature. As well as accounting for the ultrafast time scale of charge generation across large polymer phases, delocalized hot excitons may also account for the crucial requirement that primary charge pairs are well separated in efficient organic photovoltaic blends.

The structural and electronic properties of three carbazole containing copolymers used in organic photovoltaic applications, poly[N-1-octylnonyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2′,1′,3′-benzothiadiazole)] (PCBT), and poly[N-1-octylnonyl-2,7-carbazole-alt-4,7-(2′,1′,3′-benzoselenadiazole)] (PCBSe) have been studied using resonance Raman (RR) and transient absorption (TA) spectroscopies and density functional theory (DFT) calculations. Enhancement of Raman modes centered on the acceptor unit when a Raman excitation wavelength is coincident with lowest energy electronic excitation suggests that the excitation involves charge transfer from the carbazole donor to the varying benzodiazole acceptors. The pattern of the enhancement when the excitation wavelength is coincident with the higher energy transition indicates that this transition is π to π* in nature; this is consistent with TD-DFT calculations. Nanosecond transient absorption studies show long-lived excited state signals for PCDTBT (126 ± 4 ns and 1.56 ± 0.1 μs) and PCBSe (1.82 ± 0.1 μs), suggesting that population of the triplet state is appreciable. No transient signal could be detected in PCBT. B3LYP TD-DFT calculations of the monomer through to the hexamer indicate a broadly delocalized excited state orbital for PCDTBT as indicated by the linear decrease in excitation energy with an increased number of repeat units, while for PCBSe and PCBT, the reduction in excitation is sublinear. The highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) of PCBSe and PCBT polymers compared to PCDTBT are similarly diffuse, but the population of higher order orbitals is decreased when compared with PCDTBT. CAM-B3LYP calculations reduce the delocalization of the frontier orbitals and show less reduction in excitation energy with additional repeat units for each polymer.

Photoluminescence (PL) spectroscopy was used to infer that oxygen adsorption changes the band bending of the anatase phase of TiO2 within P25 nanopowder in different ways. On the one hand, oxygen can adsorb through irreversible reaction with defects which reduces the intrinsic upward band bending at the TiO2 surface and results in increased PL emission. On the other hand, oxygen exposure also leads to molecular chemisorption that yields an outermost negative charge at the surface which increases the upward band bending of TiO2 and decreases the PL emission. Since band bending plays an active role in directing charge carrier migration to the surface, the finding that oxygen adsorption can have two different, and quite opposite, effects on the band bending of TiO2 provides a new perspective on how oxygen may influence photocatalytic reaction efficiencies.

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The structural and electronic properties of a highly solvatochromic merocyanine dye, 2-(3-cyano-5,5-dimethyl-4-(3-(1-octadecylpyridin-4(1H)-ylidene)prop-1-enyl)furan-2(5H)-ylidene)malononitrile (pyr3pi), have been investigated using UV-vis, NMR, hyper-Rayleigh scattering, and Raman spectroscopies and further interpreted using computational chemistry. Spectroscopic data indicate that pyr3pi exists in its zwitterionic form even in low polarity solvents with electronic absorption spectra showing a hypsochromic shift with an increase in solvent polarity and NMR experiments indicating an increasingly zwitterionic structure in chloroform as the temperature is lowered. Raman spectra in increasingly polar solvents show small variations of the structure that are consistent with a change toward a structure with more zwitterionic character. However, comparison of the calculated and experimental vibrational energies and intensities and comparison of NMR coupling constants with calculated bond order indicate that calculations underestimate the amount of charge separation seen in low polarity solvents. Although for this system density functional theory (DFT) calculations and the two-state model qualitatively reproduce negative solvatochromism, they fail to reproduce the trends in hyperpolarizability seen experimentally. This is attributed to solvent field DFT calculations underestimating the degree of charge separation in reaction fields representing low polarity solvents.

The electronic properties of the donor−acceptor (DA) polymer poly{5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c]-1,2,5-thiadiazole} (PTBT) have been investigated using spectroscopic and computational techniques. Electronic absorption and emission spectra reveal the presence of an ordered and a disordered phase in solution. Franck−Condon modeling of the ordered phase yields Huang−Rhys factors of 0.55 (20 °C) and 0.51 (−180 °C), indicating little structural distortion between ground and excited state. DFT calculations with resonance Raman spectroscopy are consistent with a lowest energy excited state that is electronically delocalized and has little charge-transfer character, unexpected for a copolymer with a low bandgap (∼1.8 eV). Transient absorption spectroscopy of PTBT:fullerene blends reveals near-unity internal charge-transfer yields in both ordered and disordered film morphologies. In the disordered blend, charge transfer is complete within the laser pulse (100 fs), whereas the ordered blend also features a slower phase due to exciton diffusion in the phase separated morphology. In the ordered blend, the spectra and dynamics of charge transfer reveal that excitons and charges promptly occupy delocalized states on extended polymer chains. The pervasive use of donor−acceptor structures in polymer devices makes understanding the interplay of morphology and electronic structure of these polymers essential and here a spectroscopic and computational investigation gives an extensive picture of the electronic properties and their effect on charge dynamics in a DA polymer.

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