A novel class of near-infrared absorbing squarylium sensitizers with linearly extended π-conjugated structures, which were obtained by Pd-catalyzed cross-coupling reactions with stannylcyclobutenediones, has been developed for dye-sensitized solar cells. The cells based on these dyes exhibited a significant spectral response in the near-infrared region over 750 nm in addition to the visible region.

Novel π-extended squarylium dyes have been synthesized as sensitizers for use in dye-sensitized solar cells (DSSCs). Four quaternary salts with extended π-systems, quinolinium, benz[c,d]indolium, benzopyrylium, and benzo-1-thiopyrylium, were selected as electron-rich heterocyclic components and condensed with indole-based semisquarylium bearing a carboxyl group as an anchor for their immobilization on TiO2 to obtain four unsymmetrical squarylium dyes (SQ1–4). They exhibited intense absorption in the near-infrared region in solution and on the TiO2 surface (absorption edge: approximately 900 nm). Investigation of their electrochemical properties revealed that electron injection from the excited dyes to the TiO2 conduction band was thermodynamically permitted. A molecular-orbital calculation indicated that the electron distribution moved from the cyclobutene core to the heterocyclic components bearing the carboxyl group by photoexcitation of SQ1–4. The photovoltaic performances of DSSCs based on the squarylium dyes significantly depended on the structure of heterocyclic components. Among the series of squarylium dyes, SQ2–4 achieved comparatively high performance in metal-free NIR sensitizers (η = 1.1–1.6%), in accordance with the calculation results. These results suggest that the structure of heterocyclic components strongly affects not only the absorption properties of the dyes but also the photovoltaic performances of DSSCs based on these dyes.Highlights► Four unsymmetrical squarylium dyes with π-extended heterocycles were synthesized. ► π-Extended heterocyclic components lead to red-shifted absorption of the dyes. ► The squarylium dyes achieved acceptable performance in the NIR region (η < 1.6%). ► The photovoltaic performances depended on the structure of heterocyclic components.

We fabricated molecularly doped, polymer-based light-emitting diodes possessing a single emitting layer containing a hole-transporting host polymer poly(N-vinylcarbazole) and an electron-transporting auxiliary, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, doped with novel phosphorescent cyclometalated Pt(II) complexes bearing arylpyridine and 1,3-diketone ligands. These novel cyclometalated Pt(II) complexes emit pure red color both in steady-state emissions (poly(methyl methacrylate) films)) and electrophosphorescence. They exhibited pure red emissions with the Commission Internationale de l’Eclairage coordinates (X = ∼0.67, Y = ∼0.33), which is almost identical to the coordinates of standard red (0.66, 0.34) demanded by the National Television System Committee. The color coordinates remained unchanged over a range of operating voltages, even at luminances greater than 1 × 104 cd/m2. The maximum external quantum efficiency of these devices exceeded 3.6% and the maximum brightness was greater than 1 × 104 cd/m2.