Proton-Coupled Electron Transfer in a Hydrogen-Bonded Charge-Transfer Complex

A proton-coupled electron transfer (PCET) reaction in a hydrogen-bonded charge-transfer (CT) complex of 4-([2,2′-bipyridin]-4-yl)­phenol (bpy-phenol) with a F^– ion has been investigated by ultrafast time-resolved transient absorption spectroscopy. The phenolic receptor molecule, bpy-phenol, binds to the F^– ion through a hydrogen bond and senses the F^– ion via the Stokes-shifted CT band. Upon photoexcitation, CT from the phenol residue to the bpy residue promotes proton transfer from the phenol radical cation (ArOH^•+) to the fluoride ion at ultrafast time scales of <150 fs (instrument response function limited) and 3 ps, separately. The fast and slow proton-transfer times are linked to two different types of hydrogen-bonding networks between the phenol residue and fluoride ion. Crystalline water in the fluoride salt hydrates mediates the proton-transfer reaction. This work demonstrates the participation of a hydrogen-bonded water bridge within a PCET reaction in a water-restricted environment

Hydrogen Bond and Ligand Dissociation Dynamics in Fluoride Sensing of Re(I)–Polypyridyl Complex

Hydrogen bonding interaction plays an essential role in the early phases of molecular recognition and colorimetric sensing of various anions in aprotic media. In this work, the host–guest interaction between <i>fac</i>-[Re­(CO)₃Cl­(L)] with L = 4-([2,2′-bipyridin]-4-yl)­phenol and fluoride ions is investigated for the hydrogen bond dynamics and the changing local coordination environment. The stoichiometric studies using ¹H NMR and ESI-MS spectroscopies have shown that proton transfer in the H-bonded phenol–fluoride complex activates the dissociation of the CO ligand in the Re­(I) center. The phenol-to-phenolate conversion during formation of HF₂^– ion induces nucleophilic lability of the CO ligand which is probed by intraligand charge transfer (ILCT) and ligand-to-metal charge transfer (LMCT) transitions in transient absorption spectroscopy. After photoexcitation, phenol–phenoxide conversion rapidly equilibrates in 280 fs time scale and the ensuing excited state [Re^II(bpy•^–phenolate¯) (CO)₃Cl]* undergoes CO dissociation in the ultrafast time scale of ∼3 ps. A concerted mechanism of hydrogen cleavage and coordination change is established in anion sensing studies of the rhenium complex

Counteranion Driven Homochiral Assembly of a Cationic <i>C</i>₃‑Symmetric Gelator through Ion-Pair Assisted Hydrogen Bond

The helical handedness in achiral self-assemblies is mostly complex due to spontaneous symmetry breaking or kinetically controlled random assembly formation. Here an attempt has been made to address this issue through chiral anion exchange. A new class of cationic achiral <i>C</i>₃-symmetric gelator devoid of any conventional gelation assisting functional units is found to form both right- and left-handed helical structures. A chiral counteranion exchange-assisted approach is successfully introduced to control the chirality sign and thereby to obtain preferred homochiral assemblies. Formation of anion-assisted chiral assembly was confirmed by circular dichroism (CD) spectroscopy, microscopic images, and crystal structure. The X-ray crystal structure reveals the construction of helical assemblies with opposite handedness for (+)- and (−)-chiral anion reformed gelators. The appropriate counteranion driven ion-pair-assisted hydrogen-bonding interactions are found responsible for the helical bias control in this <i>C</i>₃-symmetric gelator