Coupling between electronic excitation and proton transfer is relevant to the kinetics of redox reactions, in particular those involved in solar-tofuel 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 = {NH 2 , CH 3 O, H, Br, Cl, CN}) were selected and their changes in pK a upon electronic excitation in aqueous solutions were determined. The Hammett parameters σ p of these substituents, indicative of their electron withdrawing capability, span a range of −0.7 to +0.7. Using Forster cycle analysis, the acid dissociation equilibria in the ground and first excited state were determined. The ground state pK a obeys an expected linear relationship with respect to the Hammet parameter σ p . An important finding of our work is that the excited state pK a * also obeys a linear relationship with respect to σ p . Interestingly, the excited state pK a * is ∼5 times more sensitive to the electron-withdrawing power of the substituent than the ground state pK a . We attribute this difference to the larger polarizability of the excited state charge density. Increase in pK a due to optical excitation ranging between 2.2 (R = CN) and 10.6 (R = NH 2 ) units were observed within the set. This substantial range of ΔpK a 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.

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.

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 = {NH, OCH, 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.

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.

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.