Chi‐Chiu Ko

A versatile bis(2,5-dimethyl-3-thienyl)-1,10-phenanthroline photochromic ligand has been successfully synthesized via a Suzuki cross-coupling reaction. The excitation wavelength for photochromic reaction of the dithienylphenanthroline could be extended from lambda </= 340 nm in the UV region to ca. 480 nm in the visible region, corresponding to the MLCT excitation, through the incorporation into the rhenium(I) tricarbonyl system. Reversible switching of the emissive state by the photochromic reaction has also been demonstrated.

The design of highly efficient and selective photocatalytic systems for CO2 reduction that are based on nonexpensive materials is a great challenge for chemists. The photocatalytic reduction of CO2 by [Co(qpy)(OH2)2](2+) (1) (qpy = 2,2':6',2″:6″,2‴-quaterpyridine) and [Fe(qpy)(OH2)2](2+) (2) have been investigated. With Ru(bpy)3(2+) as the photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant in CH3CN/triethanolamine solution under visible-light excitation (blue light-emitting diode), a turnover number (TON) for CO as high as 2660 with 98% selectivity can be achieved for the cobalt catalyst. In the case of the iron catalyst, the TON was >3000 with up to 95% selectivity. More significantly, when Ru(bpy)3(2+) was replaced by the organic dye sensitizer purpurin, TONs of 790 and 1365 were achieved in N,N-dimethylformamide for the cobalt and iron catalysts, respectively.

High-performance deep-blue emitting phenanthroimidazole derivatives with a structure of donor–linker–acceptor were designed and synthesized. By using different linkers and different linking positions, four deep-blue emitters were obtained and used as emitters or bifunctional hole-transporting emitters in OLEDs. Such devices show low turn-on voltages (as low as 2.8 V), high efficiency (2.63 cd/A, 2.53 lm/W, 3.08%), little efficiency roll-off at high current densities, and stable deep-blue emissions with CIEy < 0.10. Performances are among the best comparing to recently reported deep-blue emitting devices with similar structures. The results suggest that the combination of the phenanthroimidazole and the donor–linker–acceptor structure can be an important approach for developing high performance deep-blue emitters in particular for lighting applications.

Photochromic compounds are well-known for their promising applications in many areas. In this context, many different photochromic families have been developed. As the early study of these photochromic compounds was mainly focused on the organic system, their photochromic reactivity was mainly derived from the singlet excited state. We hypothesized that the incorporation of the photochromic ligand to the transition metal complex and coordination complex systems would not only render the triplet state of the organic photochromic system more readily accessible due to the large spin-orbit coupling of the heavy metal center but also would lead to ready extension of the excitation wavelength to less destructive longer wavelength low-energy excitation. On the other hand, the long-lived triplet excited states of the metal complexes are also suitable for energy or electron transfer processes, which should lead to new photochromic behavior and photoswitchable functional properties. Through the incorporation of the stilbene-, azo-, spirooxazine-, and dithienylethene-containing ligands to transition metal complex systems with heavy metal centers and suitable excited states, triplet state photosensitized photochromism has been achieved. With the triplet state photosensitization, the photochromism of these compounds could be extended from the high energy UV region to the visible region. In the development of dithienylethene-containing ligands, we have adopted an alternative strategy, which involves the incorporation of nitrogen and sulfur heterocycles that directly form part of the dithienylethene framework as ligands to exert a much stronger perturbation and influence on the excited state properties of the photochromic unit by the metal center. On the basis of the new design, wide ranges of dithienylethene-containing ligands, including phenanthrolines, 2-pyridylimidazoles, N-pyridylimidazol-2-ylidenes, cyclometalating thienylpyridines, β-diketonates, and β-ketoiminates have been designed and incorporated into various coordination systems. Apart from the photosensitization, tuning of the closed form absorption and photochromic behavior based on the perturbation of the metal center, coordination-assisted planarization, modification of the ancillary ligands and introduction of various electronic excited states derived from the coordination system have been successfully demonstrated. This strategy can be used for developing NIR photochromic dithienylethenes. With the above effects observed upon the coordination to different transition metal centers and central atoms, this strategy offers a simple and effective way for the modification of the photochromic characteristics. Moreover, the emission and other functional properties of the coordination systems could also be photoswitched by the photochromic reactions.

A new series of diarylethene-containing 1-aryl-substituted 2-(2-pyridyl)imidazole (L) ligands have been prepared and characterized. Subsequent reactions of the 2-(2-pyridyl)imidazole ligands with Re(CO)5Cl under reflux conditions afforded [Re(CO)3(L)Cl]; one of the complexes has also been structurally determined. The open forms of the ligands showed an intense absorption band at ca. 320 nm corresponding to π → π* and n → π* transitions of the 1-aryl-2-(2-pyridyl)imidazole moiety, with mixing of π → π* and n → π* transitions of the thiophene moieties. The corresponding rhenium(I) complexes showed IL absorption bands at ca. 352 nm, with additional MLCT [dπ(Re) → π*(L)] transitions at ca. 425 nm. Upon photocyclization of the diarylethene moieties by UV irradiation, two new absorption bands emerged at ca. 410−425 and 576−586 nm for the ligands, while the rhenium(I) complexes showed three new absorption bands at ca. 288−290, 475−480, and 712−713 nm upon excitation into either the IL or MLCT bands. Such a large shift of the absorption bands of the closed forms of the rhenium(I) complexes to the NIR region could be attributed to the planarization of the four heterocyclic rings relative to the open forms. On the contrary, the organic ligand systems alone did not display such a shift of the absorption maxima to the NIR region upon photocyclization.