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.
Control of proton transfer is relevant to many areas in chemistry, particularly in catalysis where the kinetics of (de)protonation reactions are often rate limiting. Photobases, which are molecules with enhanced basicity in the excited state, allow for control of proton transfer with light and have the potential to be used as functional units in catalytic systems. Alcohols are the feedstock in many catalytic reactions, where their deprotonation or dehydrogenation is often important. We report that the photobase 5-methoxyquinoline can deprotonate a series of alcohols upon excitation by light. We measure both the thermodynamic limits and the relevant kinetics of this process. A series of alcohols and water spanning the p K range of 12.5-16.5 were used as the proton donors. First, we show evidence from absorption and emission spectroscopy that photoexcited 5-methoxyquinoline deprotonates all donors more acidic than methanol and fails to deprotonate donors that are more basic. Interestingly, in methanol a quasi-equilibrium between the protonated and unprotonated forms of the photobase is established in the excited state, suggesting that the excited state p K of the photobase is near the p K of methanol (15.5). Second, using ultrafast transient absorption spectroscopy, we find that the time constants for excited state proton transfer range from a few picoseconds to tens of picoseconds, with faster speeds for the more acidic donors. Such a correlation between the thermodynamic drive and kinetics suggests that the same mechanism is responsible for proton transfer throughout the series. These results are necessary fundamental steps for applying photobases in potential applications such as deprotonation of alcohols for catalytic and synthetic purposes, optical regulation of pH, and transfer of protons in redox reactions.
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.
Photobases convert light energy to proton removal power. What limits their applicability? Hydrogen bonding, solvation, and photon energy cost.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.