Ultrafast excited-state dynamics of a series of zwitterionic pyridinium phenoxides with increasing sterical hindering

Abstract

A series of pyridinium phenoxides that differ by the dihedral angle between the pyridinium and the phenoxide rings because of substituents with increasing steric encumbrance has been investigated by ultrafast spectroscopy. Like the related betaine-30, these molecules are characterised by a zwitterionic electronic ground state and a weakly polar S(1) state. Their fluorescence lifetime was found to lie between 200 to 750 fs, decreasing with increasing dihedral angle, and increasing with solvent viscosity. This was assigned to a non-radiative deactivation of the emissive state coupled to a large amplitude motion involving the dihedral angle. The transient absorption spectra suggested that emission occurs from the Franck-Condon S(1) state, which decays to a dark excited state, that itself most probably corresponds to the relaxed S(1) state. Finally, this relaxed state decays to the vibrationally hot ground state through an intramolecular charge separation process with a time constant ranging between 0.4 and 3 ps, increasing with the dihedral angle and with the solvent relaxation time. These variations were discussed in terms of the Jortner-Bixon model of electron transfer, where the charge separation dynamics depends on both electronic coupling and solvent relaxation. The results suggested that charge separation slows down with increasing dihedral angle.

Picosecond Switchable Azo Dyes

“…The time constants τ 3 and τ 4 are characterized by sinusoidal spectral amplitudes and ascribed to vibrational cooling in the hot ground state (see the Supporting Information, Figures S4 and S5). The assignment of τ 3 and τ 4 to vibrational cooling is in accordance with the nonexponential character of this process . Such a fast cooling can be explained by the fact that the dye molecules leave the excited state very rapidly (<1 ps) and carry a large amount of vibrational energy.…”

To what extent can charge localization influence electron injection efficiency at graphene–porphyrin interfaces?

“…[1][2][3][4] Understanding and controlling this process on the molecular level is crucial, not only for optimizing the performance of many energy-challenged relevant devices, [5][6][7] but also for enhancing many photocatalytic processes. [10][11][12][13][14] More specifically, Ricks et al prepared a series of molecules to test the effect of cross-conjugated bridges on the charge transfer rate in D-A systems. [10][11][12][13][14] More specifically, Ricks et al prepared a series of molecules to test the effect of cross-conjugated bridges on the charge transfer rate in D-A systems.…”

Concerning restricted rotations in pyridinyl anisoles and pyridinyl phenols bearing saturated handles, and their solid state structures

“…[1][2][3][4][5][6][7][8][9] Unfortunately, until now two difficulties remain. First of all, the formation of aggregates greatly contributes to reduce the solubilities of pyridinium phenolates.…”

“…11 These drawbacks may be avoided by introducing bulky groups at the ortho position of the phenolate. [4][5][6][7][8][9][10][11][12] In this context, we have recently synthesised pyridinium phenolates bearing saturated handles of various lengths (compounds 1a-d, Scheme 1). 13 A moderate twist angle was introduced by anchoring two methyl groups at the meta position of the pyridine ring to retain reasonable NLO responses.…”