Evaluation of the role that photoacid excited-state acidity has on photovoltage and photocurrent of dye-sensitized ion-exchange membranes

Detailed-balance limits for sunlight-to-protonic energy conversion from aqueous photoacids and photobases based on reversible mass-action kinetics

“…1a). Because H + and OH − are highly mobile charged species in many phases, in practice they can most easily be collected at selective contacts to perform useful work, such as when they are incorporated into an ionic diode that possesses a built-in space–charge region 2,4–6 or a depletion region that transiently forms due to species transport. 3,45,46 Useful work due to H + and/or OH − can be performed using processes, such as, directly altering the concentration of reactants and/or products of a reversible chemical reaction involving H + and/or OH − ( e.g.…”

“…Even though the model is spatially zero-dimensional, an optical pathlength over which light is absorbed, c = 10 nm, is assumed so that areal rates of photon absorption can be converted into average volumetric reaction rates, via division by c. This value for c is the approximate thickness of space-charge regions in state-of-the-art bipolar ion-exchange membranes 6,32,35 that are responsible for charge separation in our previously reported visible-light-driven proton pumps. 2,4,6 Moreover, the model assumes spatially invariant rates of photon absorption and subsequent reactions. For the base-case model we assume excited-state lifetimes, t 7,PAH* = t 8,PA À * = 5 ns, for all singlyexcited photoacid and photobase species, because this is the approximate excited-state lifetime of several well-known visiblelight-absorbing organic photoacid and photobase dyes (Table S1, ESI †).…”

“…Several common photoacid and photobase motifs exhibit |DpK a | E 8, yet non-optimal values of pK a and pK Ã a . 4,[47][48][49][50][51][52][53] This suggests that half-maximum changes in total protonic chemical potential may be possible in practice, as long as absolute values of pK a and pK Ã a can be altered, which should be possible using the tools of synthetic chemistry. 52 Electrostatically, this requires that changes in free energy due to alterations in electron density from absorption of a photon significantly influence regions of the dye molecule that are involved in localized proton transfer.…”

“…This suggests that models for solar energy conversion based on electronic processes in photovoltaic and solar fuel constructs are also applicable to protonic processes in sunlight-driven proton pumps. While most reports of proton pumps driven by visible light are modeled after biological systems whose mechanisms differ significantly from their electronic counterparts, recent efforts from our group [1][2][3][4][5][6] have taken the new approach of designing and analyzing visible-light-driven proton pumps based on knowledge gained from the solid-state-physics electronic semiconductor and photovoltaic communities since their pioneering efforts in the 1940s-1950s. [7][8][9][10] This approach supports the need to quantify the detailed-balance limit for conversion of sunlight into protonic, or more generally ionic, power.…”

Elementary Reaction Steps That Precede or Follow a Unimolecular Reaction Step Can Obfuscate Interpretation of the Driving-Force Dependence to Its Rate Constant

“…For example, photoacid generators, used in photolithography, generate ground-state Brønsted acids as products of their irreversible photochemistry and metastable photoacids, commonly used as photoswitches, photochemically generate long-lived species that undergo reversible PT reactions on ground-state potential energy surfaces . Excited-state photoacids and photobases are of practical interest for use in artificial light-driven proton pumps, − in the study of proton-coupled reaction mechanisms using pump–probe spectroscopies, − as fluorescence sensors of local pH and pOH, − and as triggers to initiate processes in biological organisms via on-demand (de)­protonation of amino acids . Excited-state photoacids and photobases are analogous to photoinduced ET dyes, except that most excited-state photoacids and photobases persist in an electronic excited state after undergoing PT, whereas ET dyes do not.…”