Intramolecular Charge Transfer of Push–Pull Pyridinium Salts in the Singlet Manifold

Abstract

The solvent effect on the photophysical and photochemical properties of the iodides of three trans (E) isomers of 2-D-vinyl,1-methylpyridinium, where D is a donor group (4-dimethylaminophenyl, 3,4,5-trimethoxyphenyl and 1-pyrenyl), was studied by stationary and transient absorption techniques. The results obtained allowed the negative solvatochromism and relaxation pathways of the excited states in the singlet manifold to be reasonably interpreted. Resorting to ultrafast absorption techniques and DFT calculations allowed information on the excited state dynamics and the role of the solvent-controlled intramolecular charge transfer (ICT) processes to be obtained. The structure-dependent excited state dynamics in nonpolar solvents, where the ICT is slower than solvent rearrangement, and in polar solvents, where an opposite situation is operative, was thus explained. The push-pull character of the three compounds, particularly the anilino-derivative, suggests their potential application in optoelectronics.

Inclusion of Two Push–Pull N-Methylpyridinium Salts in Anionic Surfactant Solutions: A Comprehensive Photophysical Investigation

“…Since these molecules are sensitive to changes in both the polarity and viscosity of the medium in which they are dissolved, the position of the absorption and emission bands together with the fluorescence intensities are a clear sign of the environment experienced by the molecules. 36 In nonmicellar solution, compounds 1 and 2 show nonstructured bell-shaped absorption bands, which undergo a negative solvatochromism; i.e., the band shifts toward lower energies by decreasing the solvent polarity. As for the fluorescence spectra of the two molecules, the emission band shape and position remain almost insensitive to polarity, The Journal of Physical Chemistry B Article although the fluorescence quantum yield decreases by 2 orders of magnitude for compound 1 and almost 1 order of magnitude for compound 2 when going from DCM to the more polar W.…”

“…36 In case a, the Franck− Condon (FC) state, populated by direct absorption of the excitation light, leads to a relaxed locally excited state (LE), defined by a decreased dipole moment with respect to the highly polar ground state. The LE state then evolves into an ICT state, by back-transferring the positive charge to the methylpyridinium unit, thus increasing the dipole moment of The Journal of Physical Chemistry B Article the relaxed excited state.…”

“…Through quantum-mechanical calculations, the latter was found to be twisted (TICT) in the case of compounds 1 and 2 in polar solvents. 36 Hence, in polar environments, the two molecules experience lower energy barriers during the ICT process and lose the net separation between the LE and ICT states. The latter is directly populated along with solvent relaxation (case b).…”

“…36,37 A substantial negative solvatochromism has been found in the absorption spectrum of these molecules, whereas their emission is affected to a minor degree by the solvent polarity. 36 These spectral features have been explained by taking into account the different nature of the electronic states presumably involved in the absorption and emission processes, whose character has also been suggested by quantum-mechanical calculations. 36,38 The ground state is defined by a large dipole moment, owing to the positive charge localized on the strong electron-withdrawing cationic pyridinium ring, while the excited Franck−Condon state shows a much lower dipole moment, because of a charge migration from the D to the A unit.…”

“…In solvents with high polarity and in the presence of suitable D substituents, the formation of this state is thought to come with the twisting of the N-methylpyridinium group around the quasi-single bond with the ethene bridge (TICT). 36 When a TICT state is formed, the back-charge transfer is extremely efficient and the internal conversion becomes the main deactivation channel for this excited state. Depending on the efficiency of the D unit, its hindrance and solvent physical properties, the relative level of stability between ground and excited states and also between the locally excited (LE) and ICT states can therefore be considerably altered; additionally, utterly different spectral photophysical and kinetic properties can be retrieved.…”