Solvent and Temperature Effects on Photoinduced Proton-Coupled Electron Transfer in the Marcus Inverted Region

Abstract

Concerted proton-coupled electron transfer (PCET) in the Marcus inverted region was recently demonstrated ( Science 2019 , 364 , 471–475). Understanding the requirements for such reactivity is fundamentally important and holds promise as a design principle for solar energy conversion systems. Herein, we investigate the solvent polarity and temperature dependence of photoinduced proton-coupled charge separation (CS) and charge recombination (CR) in anthracene–phenol–pyridine triads: 1 (10-(4-hydroxy-3-(4-methylpyridin-2-yl)benzyl)anthracene-9-carbonitrile) and 2 (10-(4-hydroxy-3-(4-methoxypyridin-2-yl)benzyl)anthracene-9-carbonitrile). Both the CS and CR rate constants increased with increasing polarity in acetonitrile: n -butyronitrile mixtures. The kinetics were semi-quantitatively analyzed where changes in dielectric and refractive index, and thus consequently changes in driving force (−Δ G °) and reorganization energy (λ), were accounted for. The results were further validated by fitting the temperature dependence, from 180 to 298 K, in n -butyronitrile. The analyses support previous computational work where transitions to proton vibrational excited states dominate the CR reaction with a distinct activation free energy (Δ G * CR ∼ 140 meV). However, the solvent continuum model fails to accurately describe the changes in Δ G ° and λ with temperature via changes in dielectric constant and refractive index. Satisfactory modeling was obtained using the results of a molecular solvent model [ J. Phys. Chem. B 1999 , 103 , 9130–9140], which predicts that λ decreases with temperature, opposite to that of the continuum model. To further assess the solvent polarity control in the inverted region, the reactions were studied in toluene. Nonpolar solvents decrease both Δ G ° CR and λ, slowing CR into the nanosecond time regime for 2 in toluene at 298 K. This demonstrates how PCET in the inverted region may be controlled to potentially use proton-coupled CS states for efficient solar fuel production and photoredox catalysis.

Proton-coupled energy transfer in molecular triads

“…( A ) Schematic Jablonski diagram of the possible transitions at 77 K for the triads: radiative decay (downward solid line) and nonradiative decay. The CSS energy depends strongly on the chemical environment ( 16 , 19 ) and is higher than the LES at 77 K. ( B ) Absorption (solid line) and fluorescence spectra normalized to the first An emission peak (dashed line, λ exc : 400 nm) of 1 (black) and the reference compounds of similar concentrations (4 to 7 μM): 2,4-di- tert -butyl-6-(pyridin-2-yl)phenol (PhOH-py, blue) and 9-cyano-10-methylanthracene (CN-MeAn, green). ( C ) Fluorescence spectra of triads 1 , 2 , 4 , 5 , 6 , and 7 with excitation at 400 nm ( 1 and 2 ), 365 nm ( 6 ), and 375 nm ( 5 and 7 ).…”

“…Nonetheless, photoexcitation of the An subunit of the triads results in the formation of a local excited state (LES) on the anthracene (Eq. 1) ( 16 , 19 ). In tenths of picoseconds, the LES forms the charge-separated state (CSS) by electron transfer from the PhOH group to the 1 *An concerted with proton transfer to the pyridine (Eq.…”

A Symmetry‐Broken Charge‐Separated State in the Marcus Inverted Region

“…[22] One of the most insightful and extensively investigated features of the classical Marcus equation is the MIR dependence of CT reaction kinetics, where the rate constant of CT dynamics slows down as the Gibbs free energy change of reaction (À ΔG) becomes more exergonic than the total reorganization energy (λ). [23][24][25][26][27][28][29][30][31] The manifestation of the SB-CS process in photosynthetic RCs and the application of SB-CS materials to organic photovoltaics (OPVs) have motivated researchers to design and develop efficient RC mimics, especially artificial special pairs. [11,20,32,33] Furthermore, the most important and attractive feature of SB-CS processes is that CS proceeds faster at the expense of low energy loss, which is in contrast to conventional OPVs, which require large energy offset between the optical band gap and CT state energy for the efficient exciton splitting.…”

“…The charge recombination (CR) kinetics of RCs is suppressed due to the Marcus inverted‐region (MIR) effect, leading to a long‐lived charge‐separated state in RCs [22] . One of the most insightful and extensively investigated features of the classical Marcus equation is the MIR dependence of CT reaction kinetics, where the rate constant of CT dynamics slows down as the Gibbs free energy change of reaction (−Δ G ) becomes more exergonic than the total reorganization energy ( λ ) [23–31] …”

Contribution Factors of the First Kind Calculated for the Marcus Electron-Transfer Rate and Their Applications

“…For instance, ET rates are estimated for ET from PSII to PSI through chains comprising plastoquinone (PQ), cytochrome b 6 f (Cyt b 6 f ), and plastocyanin (PC). − Additionally, Moser et al rigorously investigated tunneling dynamics in PSII . Recently, the Marcus theory has been widely used, e.g., in molecular design for organic semiconductors, proton-coupled ET, − and extracellular ET. − …”