40 research outputs found
Molecular Seesaw: How Increased Hydrogen Bonding Can Hinder Excited-State Proton Transfer
A previously unexplained effect in the relative rate of excited-state intramolecular proton transfer (ESIPT) in related indole derivatives is investigated using both theory and experiment. Ultrafast spectroscopy [J. Phys. Chem. A, 2015, 119, 5618–5625] found that although the diol 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole exhibits two equivalent intramolecular hydrogen bonds, the ESIPT rate associated with tautomerization of either hydrogen bond is a factor of 2 slower than that of the single intramolecular hydrogen bond in the ethoxy-ol 1,3-bis(2-pyridylimino)-4-ethoxy-7-hydroxyisoindole. Excited-state electronic structure calculations suggest a resolution to this puzzle by revealing a seesaw effect in which the two hydrogen bonds of the diol are both longer than the single hydrogen bond in the ethoxy-ol. Semiclassical rate theory recovers the previously unexplained trends and leads to clear predictions regarding the relative H/D kinetic isotope effect (KIE) for ESIPT in the two systems. The theoretical KIE predictions are tested using ultrafast spectroscopy, confirming the seesaw effect
Carrier Recombination and Generation Rates for Intravalley and Intervalley Phonon Scattering in Graphene
Electron-hole generation and recombination rates for intravalley and
intervalley phonon scattering in Graphene are presented. The transverse and the
longitudinal optical phonon modes (-modes) near the zone center
(-point) contribute to intravalley interband carrier scattering. At the
zone edge (-point), only the transverse optical phonon mode
(-mode) contributes significantly to intervalley interband scattering
with recombination rates faster than those due to zone center phonons. The
calculated recombination times range from less than a picosecond to more than
hundreds of picoseconds and are strong functions of temperature and electron
and hole densities. The theoretical calculations agree well with experimental
measurements of the recombination rates of photoexcited carriers in graphene.Comment: 6 pages, 9 figure
Molecular Seesaw: How Increased Hydrogen Bonding Can Hinder Excited-State Proton Transfer
A previously unexplained effect in the relative rate of excited-state intramolecular proton transfer (ESIPT) in related indole derivatives is investigated using both theory and experiment. Ultrafast spectroscopy [J. Phys. Chem. A, 2015, 119, 5618–5625] found that although the diol 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole exhibits two equivalent intramolecular hydrogen bonds, the ESIPT rate associated with tautomerization of either hydrogen bond is a factor of 2 slower than that of the single intramolecular hydrogen bond in the ethoxy-ol 1,3-bis(2-pyridylimino)-4-ethoxy-7-hydroxyisoindole. Excited-state electronic structure calculations suggest a resolution to this puzzle by revealing a seesaw effect in which the two hydrogen bonds of the diol are both longer than the single hydrogen bond in the ethoxy-ol. Semiclassical rate theory recovers the previously unexplained trends and leads to clear predictions regarding the relative H/D kinetic isotope effect (KIE) for ESIPT in the two systems. The theoretical KIE predictions are tested using ultrafast spectroscopy, confirming the seesaw effect
Measurement of the Optical Absorption Spectra of Epitaxial Graphene from Terahertz to Visible
We present experimental results on the optical absorption spectra of
epitaxial graphene from the visible to the terahertz (THz) frequency range. In
the THz range, the absorption is dominated by intraband processes with a
frequency dependence similar to the Drude model. In the near IR range, the
absorption is due to interband processes and the measured optical conductivity
is close to the theoretical value of . We extract values for the
carrier densities, the number of carbon atom layers, and the intraband
scattering times from the measurements
Measurement of the Optical Absorption Spectra of Epitaxial Graphene from Terahertz to Visible
We present experimental results on the optical absorption spectra of
epitaxial graphene from the visible to the terahertz (THz) frequency range. In
the THz range, the absorption is dominated by intraband processes with a
frequency dependence similar to the Drude model. In the near IR range, the
absorption is due to interband processes and the measured optical conductivity
is close to the theoretical value of . We extract values for the
carrier densities, the number of carbon atom layers, and the intraband
scattering times from the measurements
Polarizable Anionic Sublattices Can Screen Molecular Dipoles in Noncentrosymmetric Inorganic-Organic Hybrids
We report the growth and photophysical characterization of two polar hybrid lead halide phases, methylenedianiline lead iodide and bromide, (MDA)Pb2I6 and (MDA)Pb2Br6, respectively. The phases crystallize in noncentrosymmetric space group Fdd2, which produces a highly oriented molecular dipole moment that gives rise to second harmonic generation (SHG) upon excitation at 1064 nm. While both compositions are isostructural, the size dependence of the SHG signal suggests that the bromide exhibits a stronger phase-matching response whereas the iodide exhibits a significantly weaker non-phase-matching signal. Similarly, fluorescence from (MDA)Pb2Br6 is observed around 630 nm below 75 K whereas only very weak luminescence from (MDA)Pb2I6 can be seen. We attribute the contrasting optical properties to differences in the character of the halide sublattice and postulate that the increased polarizability of the iodide ions acts to screen the local dipole moment, effectively reducing the local electric field in the crystals
Continuous Representation of the Proton and Electron Kinetic Parameters in the pH–Potential Space for Water Oxidation on Hematite
Understanding the mechanisms of multielectron
and multiproton electrochemical
reactions, particularly in the context of solar-to-fuel water splitting,
is an outstanding challenge. Historically, Pourbaix diagrams are used
to show the influence of potential and pH on the thermodynamic stability
of electrode–electrolyte systems. These diagrams do not carry
kinetic or mechanistic information, which often restricts their use
to cases in which the thermodynamic limit can be assumed. We introduce
and construct from experimental data two new types of diagrams that
demonstrate the kinetic variations of electrochemical reactions as
a function of pH and potential. These diagrams show the variation
of the electron-transfer parameter (α) and the proton reaction
order (ρ) in a wide range of potential and pH. We present α(pH,<i> E</i>) and ρ(pH, <i>E</i>) for water electrolysis
on an iron oxide electrode in the range of pH 7 to 13. In these plots,
regions of acidic and basic mechanisms, relationship to surface protonation
equilibria, and switching between acidic and basic mechanisms due
to electrochemical production of protons can be easily identified.
The proton reaction order is zero in the acidic side, while it is
nonzero in the basic limit. A larger empirical electron-transfer parameter
is observed in the basic compared to the acidic region. These observations
are related to the differences in oxidation mechanism between the
two regions. We propose the use of such diagrams to gain an expanded
and enhanced view of the kinetics of multielectron and multiproton
electrochemical reactions
Modeling and characterization of exciplexes in photoredox CO2 reduction: Insights from quantum chemistry and fluorescence spectroscopy
Interactions between excited state arenes and amines can lead to the formation of structures with distinct emission behavior. These excited state complexes or exciplexes can reduce the ability of the arene to participate in other reactions, such as CO2 reduction, or increase the likelihood of degradation via Birch reduction. Exciplex geometries are necessary to understand photophysical behavior and probe degradation pathways but are challenging to calculate. We establish a detailed computational protocol for calculation, verification, and characterization of exciplexes. Using fluorescence spectroscopy, we first demonstrate the formation of exciplexes between excited state oligo-(p-phenylene) (OPP), shown to successfully carry out CO2 reduction, and triethylamine (TEA). Time-dependent density functional theory (TDDFT) is employed to optimize the geometries of these exciplexes, which are validated by comparing both emission energies and their solvatochromism with experiment. Excited state energy decomposition analysis confirms the predominant role played by charge transfer interactions in the red-shift of emissions relative to the isolated excited state OPP*. We find that although the exciplex emission frequency depends strongly on solvent dielectric, the extent of charge separation in an exciplex does not. Our results also suggest that the formation of solvent-separated ionic radical states upon complete electron transfer competes with exciplex formation in higher dielectric solvents, thereby leading to
reduced exciplex emission intensities in fluorescence experiments