29 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
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
Controlling Proton Conductivity with Light: A Scheme Based on Photoacid Doping of Materials
Transducing light energy to changes
in material properties is central
to a large range of functional materials, including those used in
light harvesting. In conventional semiconductors, photoconductivity
arises due to generation of mobile electrons or holes with light.
Here we demonstrate, to our knowledge for the first time, an analogue
of this effect for protons in an organic polymer solution and in water.
We show that when a material is doped with photoacids, light excitation
generates extra mobile protons that change the low-frequency conductivity
of the material. We measure such change both in poly(ethylene glycol)
(PEG) and in water sandwiched between two transparent electrodes and
doped with a well-known photoacid 8-hydroxypyrene-1,3,6-trisulfonic
acid (HPTS). The complex impedance of the material is measured over
a range of 0.1 Hz–1 MHz in both the presence and absence of
light, and it is found that shining light changes the low frequency
impedance significantly. We model the impedance spectra of the material
with a minimal circuit composed of a diffusive impedance (Warburg
element), a parallel capacitance, and a resistance. Fitting the light
and dark impedance spectra to the model reveals that light reduces
the low-frequency diffusive impedance of the material, which is consistent
with generation of extra free carriers by light. We further suggest
that the light-induced conductivity change arises mainly due to those
photoreleased protons that manage to escape the zone of influence
of the parent ion and avoid recapture. Such escape is more likely
in materials with larger diffusion coefficient for protons and shorter
electrostatic screening lengths for the parent ion. This explanation
is consistent with our observed differences in the photoconductivity
of solution of HPTS in water and in PEG. We anticipate that this scheme
can be employed in protonic circuits where direct transduction of
energy from light to protonic gradients or protonic currents is necessary.
This work will also serve as a basis for using photoacids as optical
handles for characterizing the molecular mechanisms of conductivity
in proton conducting materials
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Two-dimensional fluorescence-detected coherent spectroscopy with absolute phasing by confocal imaging of a dynamic grating and 27-step phase-cycling
Ultrafast Intramolecular Electron and Proton Transfer in Bis(imino)isoindole Derivatives
Concerted
motion of electrons and protons in the excited state
is pertinent to a wide range of chemical phenomena, including those
relevant for solar-to-fuel light harvesting. The excited state dynamics
of small proton-bearing molecules are expected to serve as models
for better understanding such phenomena. In particular, for designing
the next generation of multielectron and multiproton redox catalysts,
understanding the dynamics of more than one proton in the excited
state is important. Toward this goal, we have measured the ultrafast
dynamics of intramolecular excited state proton transfer in a recently
synthesized dye with two equivalent transferable protons. We have
used a visible ultrafast pump to initiate the proton transfer in the
excited state, and have probed the transient absorption of the molecule
over a wide bandwidth in the visible range. The measurement shows
that the signal which is characteristic of proton transfer emerges
within ∼710 fs. To identify whether both protons were transferred
in the excited state, we have measured the ultrafast dynamics of a
related derivative, where only a single proton was available for transfer.
The measured proton transfer time in that molecule was ∼427
fs. The observed dynamics in both cases were reasonably fit with single
exponentials. Supported by the ultrafast observations, steady-state
fluorescence, and preliminary computations of the relaxed excited
states, we argue that the doubly protonated derivative most likely
transfers only one of its two protons in the excited state. We have
performed calculations of the frontier molecular orbitals in the Franck–Condon
region. The calculations show that in both derivatives, the excitation
is primarily from the HOMO to LUMO causing a large rearrangement of
the electronic charge density immediately after photoexcitation. In
particular, charge density is shifted away from the phenolic protons
and toward the proton acceptor nitrogens. The proton transfer is hypothesized
to occur both due to enhanced acidity of the phenolic proton and enhanced
basicity of the nitrogen in the excited state. We hope this study
can provide insight for better understanding of the general class
of excited state concerted electron–proton dynamics
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