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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational ...

Kyushu University

Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird's rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials.

Structural Monitoring of the Onset of Excited-State Aromaticity in a Liquid Crystal Phase.

Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0-1.1) × 10 s as a major component and (3.5-4.3) × 10 s as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5-8.4) × 10 s. Thus, although it has a large driving force (-Δ0CR ∼ 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster ([BNAH (0.2 M)] = (3.5-3.8) × 10 s) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit ( > 2 × 10 s) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of , picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(fluorophenyl)phosphine ligands (), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1-benzo[]imidazole (BIH),  was measured as  = (1.4 ± 0.1) × 10 s. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO, which is one of the main reasons why they work so efficiently.

Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(ii)-Re(i) supramolecular photocatalysts for CO reduction using time-resolved IR measurements.

Ken Onda

Graduate School of Science

Kyushu University

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals