Switching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and Formate

Abstract

Bifunctional or amphoteric photoacids simultaneously present donor (acidic) and acceptor (basic) properties making them useful tools to analyze proton transfer reactions. In protic solvents, the proton exchange between the acid and the base is controlled by the acidity or basicity strength and typically occurs on two different pathways known as protolysis and hydrolysis. We report here how the addition of a formate base will alter the relative importance of the possible reaction pathways of the bifunctional photoacid 7-hydroxyquinoline (7HQ), which has been recently understood to predominantly involve a hydroxide/methoxide transport mechanism between the basic proton-accepting quinoline nitrogen site toward the proton-donating OH group with a time constant of 360 ps in deuterated methanol (CD 3 OD). We follow the reaction dynamics by probing the IR-active marker modes of the different charged forms of photoexcited 7HQ, and of formic acid (HCOOD) in CD 3 OD solution. A comparison of the transient IR spectra as a function of formate concentration, and classical molecular dynamics simulations enables us to identify distinct contributions of “tight” (meaning “contact”) and “loose” (i.e., “solvent-separated”) 7HQ–formate reaction pairs in our data. Our results suggest that depending on the orientation of the OH group with respect to the quinoline aromatic ring system, the presence of the formate molecule in a proton relay pathway facilitates a net proton transfer from the proton-donating OH group of 7HQ-N* via the methanol/formate bridge toward the quinoline N site.

Fiber Optic Plasmonic Sensors Based on Nanodome Arrays with Nanogaps

“…Complicated equations can accurately predict the functional relationship between concentrations and outputs based on antibody−antigen interactions. 45 As a result, the developed sensors diagnosed Tg in serum samples with an average recovery rate (C SPR /C RIA × 100) of 103%. C SPR and C RIA are the concentrations obtained using the FO-SPR and RIA Kit, respectively.…”

“…The graph for DANG in Figure was refitted with the log3P1 model of Origin software instead of a linear equation to derive accurate values. Complicated equations can accurately predict the functional relationship between concentrations and outputs based on antibody–antigen interactions . As a result, the developed sensors diagnosed Tg in serum samples with an average recovery rate ( C SPR / C RIA × 100) of 103%.…”

Solvent-Triggered Long-Range Proton Transport in 7-Hydroxyquinoline Using a Sulfonamide Transporter Group

“…Intra- and intermolecular proton transfer is a driving force in many processes such as oxidation/reduction, − acid–base neutralization, protolysis, − enol/keto tautomerism, etc., and implements a specific role for chemical energy conversion ranging from light-harvesting and photonic devices − to controlling proton transport in ATP synthesis and hydrogen-bonding interactions of a nucleoside base in DNA and RNA. − The process finds multiple applications in solar cells and OLED fabrication, , sensors, , molecular logic gates, drug delivery, protein labeling, , and many others . Artificial molecular switches, exploiting short-range proton transport via intramolecular hydrogen bonding, are still a challenging field of research because the large π-electron rearrangement in the molecule results in strong changes in their chemical and optical properties. ,,,− The ability to transport a tautomeric proton from one to another site of the molecule opens a perspective horizon for designing new types of molecular switches.…”

“…Proton transport in 7-hydroxyquinoline (7-OHQ) represents a particular interest because the proton from the acidic OH group can be transferred to the basic quinoline nitrogen (N quin ) under suitable conditions. − Direct intramolecular proton transfer from the OH group to the N quin is unlikely to proceed spontaneously in the absence of a proton carrier due to the large distance between proton donor and proton acceptor sites (Scheme A). Indirect proton transport in polar protic solvents such as alcohols or water can be facilitated by an intermolecular solute–solvent hydrogen bonding bridge, ,,, as depicted in Scheme B. Upon photoexcitation, the solvent bridge triggers the proton transfer to the excited-state zwitterionic Z* form, as detailed by Sebastiani and Nibbering. − The overall process is based on the photolysis of 7-OHQ behaving as a bifunctional photoacid.…”

“…Indirect proton transport in polar protic solvents such as alcohols or water can be facilitated by an intermolecular solute–solvent hydrogen bonding bridge, ,,, as depicted in Scheme B. Upon photoexcitation, the solvent bridge triggers the proton transfer to the excited-state zwitterionic Z* form, as detailed by Sebastiani and Nibbering. − The overall process is based on the photolysis of 7-OHQ behaving as a bifunctional photoacid. The only possibility for intramolecular long-range proton transport has been achieved by incorporation of a crane-arm at the eighth position in 7-OHQ. ,− This reveals new perspectives for designing new molecular architectures in long-range proton transport by modifying the rotor (crane-arm), which allows it pick up the tautomeric proton under the appropriate external stimuli through a six-membered intramolecular hydrogen-bonding interaction (IMHBI) (Scheme C).…”