Excited-State Quenching of Porphyrins by Hydrogen-Bonded Phenol-Pyridine Pair: Evidence of Proton-Coupled Electron Transfer

Abstract

A series of porphyrins containing methoxy-substituted phenols were treated with different pyridine bases. Besides hydrogen bonding (H-bonding), the pyridine bases have imparted oxidation to the phenol rings resulting in coupled electron and proton movement. It has been shown that reduction of an excited substrate/porphyrin macrocycle by phenols with adjacent methoxy groups is facilitated by the movement or transfer of the phenolic proton toward H-bonded bases. Rates of electron transfer are accomplished by associated proton displacements within the redox reaction complex. Demonstrated fluorescence quenching of meso-(4-hydroxyphenyl derivatives)substituted porphyrins in aprotic solvents is attributed to electron transfer from the phenol moiety by added bases (different pyridine derivatives), and rates of quenching are found to be correlated with Bronsted base strength rather than H-bonding equilibria. The rate of quenching is observed to be a function of the extent of hydroxy and methoxy substitutions to the phenyls and the solvent polarities. Replacement of 4hydroxy by 4-methoxy completely eliminated the quenching indicating the disappearance of reduction in the porphyrin macrocycle. The dependence of the extent of fluorescence quenching of studied porphyrins on pyridine concentration led to phenol-pyridine H-bonding equilibrium constants, and these values closely resemble the values obtained directly from the corresponding absorption spectra. The quenching agent is thus revealed to be H-bonded phenol. Further, positive deuterium isotope effects on quenching upon deuteration of the hydroxyl confirm that the electron transfer is coupled to the proton movement.

Local Hydrogen Bonding Determines Branching Pathways in Intermolecular Heptazine Photochemistry

“…Of particular interest are PCET processes in hydrogen-bonded molecular complexes. The importance of preassociative hydrogen bonding for PCET has been explored in many organic − and organometallic − systems where O–H and N–H groups in the substrate can be activated for further chemical reactions. − Also, it is widely understood that hydrogen-bond networks appear to be critical in PCET processes of biological systems such as photosystem II. − These studies indicate that relatively small changes in the hydrogen-bonding environment may substantially influence the preassociation steps of molecular complexes and the outcome of the PCET reaction. − The influence of hydrogen bonding is particularly important for photoinduced PCET of intermolecular complexes, where the hydrogen bonding not only significantly facilitates the intermolecular PCET process through preassociation , but also improves chemoselectivity of certain photoreactions, , which otherwise have to occur through rather slow, randomly oriented diffusional collisions within the solvent.…”

“…22−24 These studies indicate that relatively small changes in the hydrogen-bonding environment may substantially influence the preassociation steps of molecular complexes and the outcome of the PCET reaction. 25−29 The influence of hydrogen bonding is particularly important for photoinduced PCET of intermolecular complexes, where the hydrogen bonding not only significantly facilitates the intermolecular PCET process through preassociation 12,28 of certain photoreactions, 30,31 which otherwise have to occur through rather slow, randomly oriented diffusional collisions within the solvent. While PCET processes are widely recognized to be critical in molecular photochemistry, reports considering the importance of such processes for designing materials for solar photocatalysis are less abundant.…”

A photobasic D–A fluorophore with a high intramolecular charge-transfer as a model strategy for designing organic proton-coupled electron-transfer modulators: an analysis based on steady-state fluorescence, isotopic effect and theoretical study

“…Most examples are based on electron–acceptor polycyclic π-systems like porphyrin rings, anthracene, and conjugated poly-fused rings (Scheme 1B). 26–35 As can be seen, many of the organic photobasic PCET modulators bear a basic moiety in the structure, which favors trapping of the proton for the subsequent occurrence of the PCET mechanism from phenols. In the search for an organic modulator of PCET processes, herein we introduce the inclusion of a weak basic moiety into the D–A chain of the fluorophore and this together with the occurrence of an ICT along this D–A chain upon excitation could be two essential points for constructing a convenient organic photobasic PCET modulator (Scheme 1C).…”

“…Most examples are based on electron-acceptor polycyclic π-systems like porphyrin rings, anthracene, and conjugated poly-fused rings (Scheme 1B). [26][27][28][29][30][31][32][33][34][35] As can be seen, many of the organic photobasic PCET modulators bear a basic moiety in the structure, which favors trapping of the proton for the…”

Organic Based Additives Impact on Thermal Behavior of Ammonium Perchlorate: Superior 4, 4′‐Bipyridine Versus Inferior Biphenyl

“…4,4′‐Bipyridine as one of the most important derivatives of pyridine has been reported as an efficient catalyst for different types of chemical reactions such as the electrochemical and photoelectrochemical reduction of CO 2 to methanol. It is assumed that the catalytic performance of BPy is due to interfacial proton‐coupled electron transfer (PCET) [32]. The impact sensitivity of some tetrazole based energetic materials co‐crystals in presence of different types of symmetric bipyridine based moiety was investigated recently.…”