Converting light into chemical energy often occurs through redox reactions that require transfer of several electrons and protons. Using light to control proton transfer has the potential for driving otherwise unfavorable protonation reactions or producing transient pH changes. Photoacids and photobases are fundamental functional elements that could serve this purpose. Previously, we have reported the thermodynamic drive for proton removal in a series of quinoline photobases using Forster cycle analysis of the singlet states. Because the existence of thermodynamic drive does not imply that the molecules can indeed capture protons in the excited state, in this work we report the kinetics of proton removal from water by 5-R-quinolines, R = {NH2, OCH3, H, Cl, Br, CN}, using ultrafast transient absorption spectroscopy. We found that the time constants and mechanisms of proton capture from water are highly sensitive to the substituent. In some cases, proton transfer occurs within the singlet manifold, whereas in some others intersystem crossing competes with this process. We have evidence that the triplet states are also capable of proton capture in two of the compounds. This renders the excited state proton transfer process more complicated than can be captured by the linear free energy relationships inferred from the energetics of the singlet states. We have measured proton capture times in this family to be in the range of several tens of picoseconds with no discernible trend with respect to the Hammett parameter of the substituents. This wide range of mechanisms is attributed to the high density of excited electronic states in the singlet and triplet manifolds. The ordering between these states is expected to change by substituent, solvent, and hydrogen bonding, thus making the rate of intersystem crossing and proton transfer very sensitive to these parameters. These results are necessary fundamental steps to assess the capabilities of photobases in prospective applications such as photomediated proton removal in redox reactions, steady state optical regulation of local pH, and pOH jump kinetics experiments.

Interfacial electric fields and the related molecular polarization are the central quantities that govern charge transfer between an electrode and a molecule. The presence of the interfacial field is often inferred indirectly through transport and capacitance measurements. It is desirable to measure such fields directly via the Stark shift that they induce on molecular vibrations. We report the Stark shift of a well-known vibrational chromophore tethered near an electrochemical interface measured using vibrational sum frequency generation spectroscopy. We have two important findings. First, we observe that the measured local field scales with respect to the ionic concentration in the electrolyte according to a model that combines the Gouy–Chapman theory with the capacitive response of a molecular layer. This behavior holds over 3 orders of magnitude in ionic concentration, therefore lending support to the validity of the model. Our results along with this model allow for estimation of the electric field near the electrode as the potential and ionic concentration are varied. Second, we observe that the mentioned variation of the local field with changing potential only occurs for positive potentials, for which the electrode is polarized but negligible current flows. For negative potentials, a sustained electrochemical current is observed that likely arises due to electron transfer and subsequent reduction of protons in the electrolyte. Interestingly, we observe that, under this condition, the local field does not vary with increasingly negative applied potential, reminiscent of the field within a leaky capacitor. The important consequence of this observation is that an increase in the thermodynamic drive for an electrochemical reaction does not necessarily translate to increased molecular polarization near the surface when a sustained current is passing. This study will serve as a baseline in all areas of chemistry in which understanding the role of local fields near interfaces is important and will provide a new perspective for interfacial charge transfer theories.

Interfacial electric fields are important in several areas of chemistry, materials sciences, and device physics. However, they are poorly understood, partly because they are difficult to measure directly and model accurately. We present both a spectroscopic experimental investigation and a theoretical model for the interfacial field at the junction of a conductor and a dielectric. First, we present vibrational sum frequency generation (VSFG) results of the nitrile (CN) stretch of 4-mercaptobenzonitrile (4-MBN) covalently attached to a gold surface and in contact with a variety of liquid dielectrics. It is found that the CN stretch frequency red-shifts with increasing dielectric constant. Second, we build a model in direct analogy to the well-known Onsager reaction field theory, which has been successful in predicting vibrational frequency shifts in bulk dielectric media. Clearly, due to the asymmetric environment, with metal on one side and a dielectric on the other, the bulk Onsager model is not applicable at the interface. To address this, we apply the Onsager model to the interface accounting for the asymmetry. The model successfully explains the red-shift of the CN stretch as a function of the dielectric constant and is used to estimate the reaction field near the interface. We show the similarities and differences between the conventional bulk Onsager model and the interfacial reaction field model. In particular, the model emphasizes the importance of the metal as part of the solvation environment of the tethered molecules. We anticipate that our work will be of fundamental value to understand the crucial and often elusive electric fields at interfaces.

We report evidence that intermolecular vibrations coherently drive charge transfer between the sites of a material on ultrafast time scales. Following a nonresonant stimulated Raman pump pulse that excites the organic material quinhydrone, we observe the initial appearance of oscillations due to intermolecular lattice vibrations and then the delayed appearance of a higher-frequency oscillation that we assign to a totally symmetric intramolecular vibration. We use the coherent dynamics of the transient reflectivity signal to propose that coherence transfer drives excitation of this intramolecular vibration. Furthermore, we conclude that the dynamical frequency shift of the intramolecular vibration reports the formation of a quasi-stable charge-separated state on ultrafast time scales. We calculate model dynamics using the extended Hubbard Hamiltonian to explain coherence transfer due to vibrationally driven charge transfer. These results demonstrate that the coherent excitation of low-frequency vibrations can drive charge transfer in the solid state and control material properties.

Coupling between electronic excitation and proton transfer is relevant to the kinetics of redox reactions, in particular those involved in solar-to-fuel light harvesting. A prime example of such coupling occurs in photoacids, where electronic excitation leads to proton release in the excited state. Here, we systematically study the inverse of this effect, photobasicity, in which a molecule becomes more basic in the excited state compared to the ground state. This endows the molecule with light induced proton removal capability which is anticipated to be of use in driving reactions where proton transfer is kinetically challenging. To investigate the origins and tunability of photobasicity, a set of 5-R-quinoline derivatives (R = {NH2, CH3O, H, Br, Cl, CN}) were selected and their changes in pKa upon electronic excitation in aqueous solutions were determined. The Hammett parameters σp of these substituents, indicative of their electron withdrawing capability, span a range of −0.7 to +0.7. Using Förster cycle analysis, the acid dissociation equilibria in the ground and first excited state were determined. The ground state pKa obeys an expected linear relationship with respect to the Hammet parameter σp. An important finding of our work is that the excited state pKa* also obeys a linear relationship with respect to σp. Interestingly, the excited state pKa* is ∼5 times more sensitive to the electron-withdrawing power of the substituent than the ground state pKa. We attribute this difference to the larger polarizability of the excited state charge density. Increase in pKa due to optical excitation ranging between 2.2 (R = CN) and 10.6 (R = NH2) units were observed within the set. This substantial range of ΔpKa values may find use in applications such as oxidation catalysis, in which optically induced removal of protons could speed up reaction kinetics. Finally, we comment on the correlation between photobasicity and enhancement of electronic charge density on the heterocyclic nitrogen upon optical excitation.

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.

Iron pyrite (FeS2) is an abundant natural mineral with interesting physical and chemical properties, including its near IR bandgap and extremely high absorption coefficient throughout the visible range. The dynamics of photoinitiated carriers and their interactions with intrinsic and surface defects are still not fully understood, yet clearly are responsible for pyrite’s underwhelming photovoltaic and photocatalytic performance. Here we report, to our knowledge for the first time, broadband ultrafast transient reflection from single-crystal natural iron pyrite with several excitation wavelengths both higher and lower than the accepted nominal bandgap of pyrite. We also demonstrate a method to transform transient reflection to transient absorption, without requiring any assumptions regarding the magnitude of either the absorption coefficient or the refractive index, allowing for a more direct interpretation of our results. An important finding from this work is the observation of a long-lived weak signal when pumping with 0.58 eV, an energy well below the accepted bandgap, which may be evidence for direct optical excitation of either intrinsic trailing edges of the bands or midgap defect states. We identify that after approximately 10 ps the transient spectra due to pumping at 2.59, 1.58, and 0.91 eV all appear qualitatively similar, suggesting relaxation to a common carrier distribution. This common distribution appears to decay on two time scales of about 30 and ≫200 ps. Our results should play a role in understanding charge carrier dynamics within the intricate and complex band structure of pyrite and hopefully provide clarification and direction for future efforts in the development of iron pyrite based technologies.

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.

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.

The coupling of electron and lattice phonon motion plays a fundamental role in the properties of functional organic charge-transfer materials. In this Letter we extend the use of ultrafast vibrational quantum beat spectroscopy to directly elucidate electron–phonon coupling in an organic charge-transfer material. As a case study, we compare the oscillatory components of the transient reflection (TR) of a broadband probe pulse from single crystals of quinhydrone, a 1:1 cocrystal of hydroquinone and p-benzoquinone, after exciting nonresonant impulsive stimulated Raman scattering and resonant electronic transitions using ultrafast pulses. Spontaneous resonance Raman spectra confirm the assignment of these oscillations as coherent lattice phonon excitations. Fourier transforms of the vibrational quantum beats in our broadband TR measurements allow construction of spectra that we show report the ability of these phonons to directly modulate the electronic structure of quinhydrone. These results demonstrate how coherent ultrafast processes can characterize the complex interplay of charge transfer and lattice motion in materials of fundamental relevance to chemistry, materials sciences, and condensed matter physics.

Hematite (Fe2O3) is a promising earth-abundant, visible light absorber, and easily processable photocatalytic material. Understanding the dynamics of photogenerated electrons and holes in hematite and their optical signatures is crucial in designing hematite thin film devices such as photoanodes for water oxidation. We report carrier dynamics in hematite films as measured by ultrafast transient absorption spectroscopy (TA) with a pump pulse centered at 400 nm (3.1 eV) and a probe pulse spanning the visible range. We observe a small negative response for wavelengths shorter than 530 nm (2.34 eV) and a large positive response for longer wavelengths. We interpret the spectrally resolved TA data based on recent electronic band structure calculations, while accounting for excited state absorption, ground state bleach, and stimulated emission within the relevant bands. We propose that the origin of the positive TA response is absorption of the probe by photoexcited electrons within the conduction bands. This interpretation is consistent with features observed in the data, specifically an abrupt boundary near 530 nm, a diffuse edge at lower energy probes with a ∼ 250 fs decay time characteristic of carrier relaxation, and slower decay components of ∼5.7 and >670 ps characteristic of carrier recombination. We propose that the negative TA signal arises at short wavelengths where excited state absorption within the conduction bands is no longer possible and ground state bleach and stimulated emission dominate. This study will assist in understanding the origins of transient optical responses and their interpretation in hematite-based devices such as photoanodes.