Can Brønsted Photobases Act as Lewis Photobases?

Abstract

Dative bonding or Lewis acid–base chemistry underpins a large number of chemical phenomena in a variety of fields, such as catalysis, metal–ligand interactions, and surface chemistry. Developing light-controlled Lewis acid–base interactions could offer a new way of controlling and understanding such phenomena. Photoinduced proton transfer, that is, excited-state Brønsted acidity and basicity, has been extensively studied and applied. Here, in direct analogy to excited-state Brønsted basicity, we show that exciting a photobasic molecule with light generates a thermodynamic drive for the transfer of a Lewis acid from a donor to a photobasic molecule. We have used the archetypal BF3 as our Lewis acid and our photoactive Lewis bases are a family of quinolines, which are known Brønsted photobases as well. We have constructed the experimental Förster cycle for this system and have verified it computationally to demonstrate that a significant drive (0.2–0.7 eV) exists for the transfer of BF3 to a photoexcited quinoline. The magnitude of this drive is similar to those reported for Brønsted photobasicity in quinolines. Computational results from TDDFT and energy decomposition analysis show that the origin of such an effect is similar to the Brønsted photoactivity of these molecules, in that they follow the Hammett parameter of substituent groups. These results suggest that photobases may be capable of controlling the chemical phenomena beyond proton transfer and may open opportunities for a new handle in photocatalysis.

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

“…Voegtle and Dawlaty investigated the strength of Lewis adducts created by combining BF 3 to a series of quinoline derivatives, acting as photoactive Lewis bases . The focus of this work was not primarily on the photochemistry of the Lewis adducts but on how different quinoline-based Lewis bases could affect the strength of the B–N bond.…”

“…Voegtle and Dawlaty investigated the strength of Lewis adducts created by combining BF 3 to a series of quinoline derivatives, acting as photoactive Lewis bases. 77 The focus of this work was not primarily on the photochemistry of the Lewis adducts but on how different quinoline-based Lewis bases could affect the strength of the B–N bond. A first observation from this work is that all quinolines showed an increased bonding affinity for BF 3 when the adduct was in its first excited electronic state.…”

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

“…Recent work has shown that the strength of classic Lewis pairs is modified upon light excitation. 12 Studying the interfacial Lewis interactions is important in electrochemistry. However, Lewis bonding is often convoluted with a range of complex phenomena at the surface, including poor characterization of surface sites and geometries, involvement of the band structure of the solids, difficulty in measurement of partial electrosorption valency of the adsorbates, and finally competing reactions.…”

“…For example, Carino et al have demonstrated formation of BF 3 –quinoxiline adducts from a LiBF 4 electrolyte and subsequent modification of redox properties of the Lewis adduct. Recent work has shown that the strength of classic Lewis pairs is modified upon light excitation …”

Synthesis and characterization of new N-Heterocyclic-BF3 adducts and their reactivity vs water

“…Redox-active organic molecules are used in a wide variety of applications, ranging from energy storage, use as photochemical sensitizers, as homogeneous (electro)catalysts, and in biomedical applications. [1][2][3][4][5][6] The generation of charged organic molecules facilitates their solubility in aqueous and nonaqueous solvents. We are particularly interested in boron compounds.…”