Electron-Transfer Dynamics for a Donor–Bridge–Acceptor Complex in Ionic Liquids

Optical and electrochemical effects of triarylamine inclusion to alkoxy BODIPY-based derivatives

Three new triphenylamine-BODIPY dyads BDPT1–3 have been designed and synthesized.

Solvation heterogeneity in ionic liquids as demonstrated by photo-chemical reactions

It has been recognised that ionic liquids (ILs) with long alkyl-chains have a segregated structure due to the inhomogeneous distribution of polar parts and non-polar parts. This inhomogeneity of ILs brings about unique solvation phenomena of solute molecules dissolved in ILs. We have investigated various solvation-state selective phenomena by using laser spectroscopic techniques such as solvation state selective vibrational spectroscopy, translational and rotational dynamics of small molecules in ILs, and solvation state selective fundamental chemical reactions. In this paper, we have reviewed an intramolecular electron transfer (ET) reaction in the Marcus inverted region of N,N-dimethyl-p-nitroaniline and an intramolecular proton transfer (IPT) reaction in 4′-N,N-diethylamino-3-hydroxyflavone as examples of chemical reactions affected by unique solvation in ILs.

Characterization of a New Electron Donor-Acceptor Dyad in Conventional Solvents and Ionic Liquids

Ionic liquids are being tested as potential replacements for current electrolytes in energy-related applications. Electron transfer (ET) plays a central role in these applications, making it essential to understand how ET in ionic liquids differs from ET in conventional organic solvents and how these differences affect reaction kinetics. A new intramolecular electron donor-acceptor probe was synthesized by covalently linking the popular photoacceptor coumarin 152 with the donor dimethylaniline to create the dyad "C152-DMA" for potential use in probing dynamical solvent effects in ionic liquids. Molecular dynamics simulations of this dyad show the considerable conformational flexibility of the linker group but over a range of geometries in which the ET rate parameters vary little and should have minimal effect on reaction times >100 ps. Steady-state and time-resolved fluorescence methods show the spectra of C152-DMA to be highly responsive to solvent polarity, with ET rates varying over the range of 10⁸ to 10¹² s^-1 between nonpolar and high-polarity conventional solvents. The sensitivity to hydrolysis in the presence of acidic impurities limits the dyad's use to ionic liquids of high purity. The results in the few ionic liquids examined here suggest that in addition to solvent polarity, electron transfer in C152-DMA also depends on solvent fluidity or solvation times.

Salt Effect in Ion-Pair Dynamics after Bimolecular Photoinduced Electron Transfer in a Room-Temperature Ionic Liquid

Bimolecular photoinduced electron transfer between perylene and two quenchers was investigated in an imidazolium room-temperature ionic liquid (RTIL) and in a dipolar solvent mixture of the same viscosity using transient absorption on the subpicosecond to submicrosecond time scales. Whereas charge separation dynamics were similar in both solvents, significant differences were observed in the temporal evolution of the ensuing radical ions: although small, the free-ion yield is significantly larger in the RTIL, and recombination of the ion pair to the triplet state of perylene is more efficient in the dipolar solvent. The temporal evolution of reactant, ion, and triplet state populations could be well reproduced using unified encounter theory. This analysis reveals that the observed differences can be explained by the strong screening of the Coulomb potential in the ion pair by the ionic solvent. In essence, RTILs favor free ions compared to highly dipolar solvents of the same viscosity.

Optical and Electronic Properties of Molecular Systems Derived from Rhodanine

Push-pull functional compounds consisting of dicyanorhodanine derivatives have attracted a lot of interest because their optical, electronic, and charge transport properties make them useful as building blocks for organic photovoltaic implementations. The analysis of the frontier molecular orbitals shows that the vertical transitions of electronic absorption are characterized as intramolecular charge transfer; furthermore, we show that the analyzed compounds exhibit bathochromic displacements when comparing the presence (or absence) of solvent as an interacting medium. In comparison with materials defined by their energy of reorganization of electrons (holes) as electron (hole) transporters, we find a transport hierarchy whereby the molecule ( Z)-2-(1,1-dicyanomethylene)-5-[(4-dimethylamino)benzylidene]-1,3-thiazol-4 is better at transporting holes than molecule ( Z)-2-(1,1-dicyanomethylene)-5-(tetrathiafulvalene-2-ylidene)-1,3-thiazol-4.

Photoinduced Bimolecular Electron Transfer in Ionic Liquids: Cationic Electron Donors

Recently, we have reported a systematic study of photoinduced electron-transfer reactions in ionic liquid solvents using neutral and anionic electron donors and a series of cyano-substituted anthracene acceptors [ Wu , B. ; Maroncelli , M. ; Castner , E. W. Jr Photoinduced Bimolecular Electron Transfer in Ionic Liquids . J. Am. Chem. Soc. 139 , 2017 , 14568 ]. Herein, we report complementary results for a cationic class of 1-alkyl-4-dimethylaminopyridinium electron donors. Reductive quenching of cyano-substituted anthracene fluorophores by these cationic quenchers is studied in solutions of acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Varying the length of the alkyl chain permits tuning of the quencher diffusivities in solution. The observed quenching kinetics are interpreted using a diffusion-reaction analysis. Together with results from the prior study, these results show that the intrinsic electron-transfer rate constant does not depend on the quencher charge in this family of reactions.

Photoexcitation dynamics of p-nitroaniline and N,N-dimethyl-p-nitroaniline in 1-alkyl-3-methylimidazolium-cation based ionic liquids with different alkyl-chain lengths

Back electron transfer and vibrational energy relaxation of p-nitroaniline and N,N-dimethyl-p-nitroaniline in ionic liquids were investigated using transient absorption spectroscopy.