Can Brønsted Photobases Act as Lewis Photobases?

Excited-State H-Aggregation Enables Proton Abstraction from Water in Weak Photobases

Photobasic molecules can abstract a proton from water in the picosecond range, enabling efficient photochemical reactions, especially photocatalytic water splitting. However, such molecules are rarely reported due to the challenge of achieving an excited-state pKa (pKa*) close to the pKa of water ( 15.7). Here, we demonstrate that excited-state H-aggregation can substantially enhance proton-attracting capability by raising the energy of unprotonated species. According to the Förster cycle, the thermodynamic driving force for proton attraction is determined by the energy difference between the protonated and unprotonated species. For the model weak photobasic molecule citrazinic acid, excited-state H-aggregates are formed simply by adjusting the concentration. The energy of the unprotonated species increases by 0.38 eV upon aggregation, while that of the protonated species remains unchanged. The increased energy difference significantly increases the pKa* from a ground-state value of 2.5 to 15.4. Consequently, the weak photobasic aggregates are enabled to abstract a proton from water, a capability absent in the isolated form. Meanwhile, the lower-energy state in H-aggregates functions as a rate-limiting intermediate state, delaying the proton transfer dynamics, but the delay can be modulated by the excitation wavelength. This work provides fundamental insights into H-aggregation-induced photobasicity, opening new avenues for modulating photochemical reactions simply through concentration and light.

A dual experimental-theoretical perspective on ESPT photoacids and their challenges ahead

Photoacids undergo an increase in acidity upon electronic excitation, enabling excited-state proton transfer (ESPT) reactions. A multitude of compounds that allow ESPT has been identified and integrated in numerous applications, as is outlined by reviewing the rich history of photoacid research reaching back more than 90 years. In particular, achievements together with ambitions and challenges are highlighted from a combined experimental and theoretical perspective. Besides explicating the spectral signatures, transient ion-pair species, and electronic states involved in an ESPT, special emphasis is put on the diversity of methods used for studying photoacids as well as on the effects of the environment on the ESPT, illustrated in detail for 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and the naphthols as examples of prototypical photoacids. The development of exceptionally acidic super-photoacids and magic photoacids is subsequently discussed, which opens the way to applications even in aprotic solvents and provides additional insight into the mechanisms underlying ESPT. In the overview of highlights from theory, a comprehensive picture of the scope of studies on HPTS is presented, along with the general conceptualization of the electronic structure of photoacids and approaches for the quantification of excited-state acidity. We conclude with a juxtaposition of established applications of photoacids together with potential open questions and prospective research directions.

A Theoretical Perspective on the Photochemistry of Boron-Nitrogen Lewis Adducts

Boron-Nitrogen (B-N) Lewis adducts form a versatile family of compounds with numerous applications in functional molecules. Despite the growing interest in this family of compounds for optoelectronic applications, little is currently known about their photophysics and photochemistry. Even the electronic absorption spectrum of ammonia borane, the textbook example of a B-N Lewis adduct, is unavailable. Given the versatility of the light-induced processes exhibited by these molecules, we propose in this work a detailed theoretical study of the photochemistry and photophysics of simple B-N Lewis adducts. We used advanced techniques in computational photochemistry to identify and characterize the possible photochemical pathways followed by ammonia borane and extended this knowledge to the substituted B-N Lewis adducts pyridine-borane and pyridine-boric acid. The photochemistry observed for this series of molecules allows us to extract qualitative rules to rationalize the light-induced behavior of more complex B-N-containing molecules.

Inductive Effect Alone Cannot Explain Lewis Adduct Formation and Dissociation at Electrode Interfaces

Understanding breaking and formation of Lewis bonds at an electrified interface is relevant to a large range of phenomena, including electrocatalysis and electroadsorption. The complexities of interfacial environments and associated reactions often impede a systematic understanding of this type of bond at interfaces. To address this challenge, we report the creation of a main group classic Lewis acid-base adduct on an electrode surface and its behavior under varying electrode potentials. The Lewis base is a self-assembled monolayer of mercaptopyridine and the Lewis acid is BF₃, forming a Lewis bond between nitrogen and boron. The bond is stable at positive potentials but cleaves at potentials more negative of approximately -0.3 V vs Ag/AgCl without an associated current. We also show that if the Lewis acid BF₃ is supplied from a reservoir of Li⁺BF₄^- electrolyte, the cleavage is completely reversible. We propose that the N-B Lewis bond is affected both by the field-induced intramolecular polarization (electroinduction) and by the ionic structures and ionic equilibria near the electrode. Our results indicate that the second effect is responsible for the Lewis bond cleavage at negative potentials. This work is relevant to understanding the fundamentals of electrocatalytic and electroadsorption processes.