Triarylpyridinium salts: Synthesis and electrochemical properties

Investigation of the Geometric and Spectroscopic Properties of Four Twisted Triphenylpyridinium Donor-Acceptor Dyes

The geometric and spectroscopic properties of four cationic N-aryl-2,4,6-triphenylpyridinium-based donor-acceptor dyes─1-[4-(9H-carbazol-9-yl)phenyl]-2,4,6-triphenylpyridinium, 1-[4-(N,N-diphenylamino)phenyl]-2,4,6-triphenylpyridinium, 1-(9-phenyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridinium, and 1-(9-ethyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridinium─are reported. The four dyes exhibited a twisted, quasi-perpendicular geometry about the central donor-acceptor bond, shown by X-ray crystallography and supported by Raman spectroscopy and DFT calculations. The electronic absorption spectra show weak charge transfer (CT) transitions at about 400 nm (ε ∼ 3000 L mol^-1 cm^-1). Time dependent (TD) DFT supported the nature of the CT transition, displaying an 89-97% shift in electron density from the donor to the acceptor upon electronic excitation. Excited state geometry calculations revealed significant geometry changes upon electronic excitation. Enhancement of vibrational modes attributable to this transition was also recognized in the resonance Raman spectra. Emission spectroscopies showed two distinct emission bands. The lower energy band, resulting from radiative decay of the CT excited state, exhibited large anomalous Stokes shifts of ∼9000 cm^-1. Much of the Stokes shift was a consequence of geometry changes between the ground and excited states. This was confirmed by variable temperature emission studies, with Stokes shifts reducing by up to 3000 cm^-1 upon cooling from 293 to 80 K. Additionally, a high energy aggregation induced emission band was present for two of the dyes, resulting from the inhibition of excited state geometry reorganization and supported by solid-state emission spectra. These phenomena exemplify the importance of geometry in short range donor-acceptor dyes such as these.

Synergetic Effect during the Electrocatalytic Reaction of Hydrogen Production in the Presence of 2.2'-Bipyridine

Abstract

A study is performed of the electrocatalytic properties of 2,2'-bipyridine in the presence of acids of different natures (HBF4, HClO4, TsOH, CF3COOH) and the nature of the synergistic effect. It is shown that the pKa of the acids greatly affect the mechanism of the reactions that occur. Based on electrochemical and electrocatalytic data obtained via cyclic voltammetry and preparative potentiostatic electrolysis, main mechanisms of the electrocatalytic production of molecular hydrogen are proposed that depend on the nature of the acids. The thermodynamic and kinetic parameters of the considered systems are calculated.