Electron Spin Exchange in Linked Phenothiazine-Viologen Charge Transfer Complexes Incorporated in "Through-Ring" (Rotaxane) α-Cyclodextrins

Charge Transfer and Intersystem Crossing in Compact Naphthalenediimide-Phenothiazine Triads: Synthesis and Study of the Photophysical Property with Transient Optical and Electron Paramagnetic Resonance Spectroscopic Methods

In order to obtain a long-lived charge separation (CS) state in compact electron donor-acceptor molecular systems, we prepared a series of naphthalenediimide (NDI)-phenothiazine (PTZ) triads, with phenylene as the linker between the donor and acceptor. Conformation restriction is imposed to control the mutual orientation of the NDI and PTZ units by attaching methyl groups on the phenylene linker to tune the electronic coupling between the donor and the acceptor. Moreover, the PTZ moiety was oxidized to sulfoxide to tune the ordering of the CS state and the 3LE state (LE: locally excited state). UV-vis absorption spectra indicate electronic coupling between NDI with the phenylene linker as well as the PTZ units, manifested by the appearance of a charge-transfer (CT) absorption band, whereas this coupling is devoid in the triads with conformation restriction imposed. Fluorescence is strongly quenched in the triads compared to the reference compound, indicating electron transfer upon photoexcitation. Femtosecond transient absorption spectra indicate that the CS takes 0.8 ps, and then the 3LE state is formed by charge recombination in 83 ps. Nanosecond transient absorption (ns-TA) spectra show that the 3NDI state was observed in nonpolar solvents such as cyclohexane (triplet state lifetime: 95.7 μs), whereas the CS state was observed in more polar solvents. The CS state lifetimes are up to 1.2 μs (in toluene). Time-resolved electron paramagnetic resonance spectra of the triads in toluene consist of two types of signals: CS states (narrower signals, ∼10 mT) and 3LE states (broader signals, ∼50 to 200 mT). In the spectra of the triads containing PTZ, the CS state signals dominate, whereas for the triads containing oxidized PTZ, the 3NDI signals (zero-field splitting D ≈ 2000 MHz) prevail, both observations being in agreement with the ns-TA spectral studies. The electron spin polarization phase pattern of the 3NDI states of the triads indicates that the intersystem crossing (ISC) mechanism is spin-orbit charge-transfer ISC. Considering the 3CS state as ion pairs, the electron-exchange energy (J) is determined to be -39 to -59 MHz, and the electron spin dipolar interaction is 83-92 MHz.

A redox-active ionic liquid manifesting charge-transfer interaction between a viologen and carbazole and its effect on the viscosity, ionic conductivity, and redox process of the viologen

Redox-active ionic liquids (RAILs) are gaining attention as a material that can create a wide range of functions. We herein propose a charge-transfer (CT) RAIL by mixing two RAILs, specifically a carbazole-based ionic liquid ([CzC₄ImC₁][TFSI]) as a donor and a viologen-based ionic liquid ([C₄VC₇][TFSI]₂) as an acceptor. We investigated the effect of CT interaction on the physicochemical properties of the CT ionic liquid (CT-IL) using the results of temperature-dependent measurements of UV-vis absorption, viscosity, and ionic conductivity as well as cyclic voltammograms. We employed the Walden analysis and the Grunberg–Nissan model to elucidate the effect of the CT interaction on the viscosity and ionic conductivity. The CT interaction reduces the viscosity by reducing the electrostatic attraction between the dicationic viologen and TFSI anion. It also reduces the ionic conductivity by the CT association of the dicationic viologen and carbazole. The electrochemically reversible responses of the viologens in [C₄VC₇][TFSI]₂ and CT-IL are consistent with the Nernstian and the interacting two-redox site models. Notably, the transport and electrochemical properties are modulated by CT interaction, leading to unique features that are not present in individual component ILs. The inclusion of CT interaction in RAILs thus provides a powerful means to expand the scope of functionalized ionic liquids.

A redox-active ionic liquid (RAIL) consisting of a carbazole and viologen shows charge transfer (CT) interaction. The physicochemical properties are modulated by the CT interaction by comparison with the individual RAILs.