The first examples of air-stable 20π-electron 5,10,15,20-tetraaryl-5,15-diaza-5,15-dihydroporphyrins, their 18π-electron dications, and the 19π-electron radical cation were prepared through metal-templated annulation of nickel(II) bis(5-arylamino-3-chloro-8-mesityldipyrrin) complexes followed by oxidation. The neutral 20π-electron derivatives are antiaromatic and the cationic 18π-electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20-tetraarylporphyrins.

The first examples of air-stable 20π-electron 5,10,15,20-tetraaryl-5,15-diaza-5,15-dihydroporphyrins, their 18π-electron dications, and the 19π-electron radical cation were prepared through metal-templated annulation of nickel(II) bis(5-arylamino-3-chloro-8-mesityldipyrrin) complexes followed by oxidation. The neutral 20π-electron derivatives are antiaromatic and the cationic 18π-electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20-tetraarylporphyrins.

The first examples of pyrrole- and thiophene-bridged 5,15-diazaporphyrin (DAP) dimers are prepared through Stille coupling reactions of nickel(II) and copper(II) complexes of 3-bromo-10,20-dimesityl-5,15-diazaporphyrin (mesityl=2,4,6-trimethylphenyl) with the respective 2,5-bis(tributylstannyl)heteroles. The effects of the heterole spacers and meso nitrogen atoms on the optical, electrochemical, and magnetic properties of the DAP dimers are investigated by UV/Vis absorption spectroscopy, density functional theory calculations, magnetic circular dichroism spectroscopy, cyclic voltammetry, and EPR spectroscopy. The heterole spacers are found to have a significant impact on the electronic transitions over the entire π-system. In particular, the pyrrole-bridged DAP dimers exhibit high light-harvesting potential in the low-energy visible/near-infrared region owing to the intrinsic charge-transfer character of the lowest excitation.

Phosphole is a chemically tunable heterole, and its π-conjugated derivatives are potential candidates for optoelectronic materials. This account describes recent developments in the synthesis and structure–property relationships of π-conjugated phosphole derivatives made by my research group. Thiophene–phosphole–styrene, phosphole–acetylene–arene, oligophosphole, polyphosphole, areno[c]phosphole, and phosphole–heterole π systems are synthesized using titanacycle-mediated metathesis and palladium-catalyzed cross-coupling reactions. The structural, optical, and electrochemical properties of selected compounds are discussed. Initial results on some applications of thiophene–phosphole copolymers, acenaphtho[c]phospholes, and amine–terthiophene–phosphole donor–π–acceptor dyes in organic solar cells are described. These results give valuable information and guidelines for designing new phosphorus-containing organic materials for molecular electronics.

Two kinds of phosphole- and benzodithiophene-based copolymers bearing P^V=O or P^V=NSO₂C₈H₁₇ functions were prepared by Pd–CuI-promoted Stille coupling reactions, and their optical and electrochemical properties were investigated. Bulk heterojunction organic photovoltaic devices comprised of the new P,S-bridged copolymers and (6,6)-phenyl-C71-butyric acid methyl ester (PC₇₁BM) exhibited power conversion efficiencies of up to 0.65 % under AM1.5 irradiation at 100 mW cm^–2.

New donor–π–acceptor organic (D–π–A) dyes composed of triarylamine, oligothiophene, and phosphole subunits were prepared, and their optical and photovoltaic properties were investigated. The regioselective α-lithiation of the thiophene ring of 2-(thiophen-2-yl)phospholes bearing an ester group, followed by treatment with tributyltin chloride afforded 2-(5-(tributylstannyl)thiophen-2-yl)phosphole derivatives, which underwent Stille coupling with 5′-(p-(diarylamino)phenyl)-5-bromo-2,2′-bithiophene to give triarylamine–terthiophene–phosphole hybrid π systems bearing the terminal ester group. The alkaline hydrolysis of the ester group yielded the target dyes, bearing the carboxylic acid anchoring group. The UV–vis absorption spectra of the new N,S,P-hybrid dyes displayed broad and intense π–π* transitions with two absorption maxima in the visible region. Density functional theory (DFT) calculations of two dye models revealed that each highest occupied molecular orbital (HOMO) resides on the triarylamine–oligothiophene π network, whereas each lowest unoccupied molecular orbital (LUMO) is basically located on the phosphole subunit. In addition, the time-dependent DFT calculations of the models showed that the lowest energy bands of these hybrid dyes are mainly consisted of the HOMO-to-LUMO+1 and HOMO-to-LUMO transitions with the large intramolecular charge-transfer character. The N,S,P-hybrid-dye-sensitized TiO₂ cells exhibited moderate power conversion efficiencies of up to 5.6%. The present findings corroborate the potential utility of the phosphole skeletons as the acceptor components in the D–π–A sensitizers.

The first examples of β–β directly linked, acetylene-bridged, and butadiyne-bridged 5,15-diazaporphyrin dimers have been prepared by palladium-catalyzed coupling reactions of nickel(II) and copper(II) complexes of 3-bromo-10,20-dimesityl-5,15-diazaporphyrin (mesityl=2,4,6-trimethylphenyl). The effects of the linking modes and meso-nitrogen atoms on the structural, optical, electrochemical, and magnetic properties of the distributed π systems were investigated by using X-ray crystallography, UV/Vis absorption spectroscopy, DFT calculations, cyclic voltammetry, and ESR spectroscopy. Both the electronic and steric effects of the meso-nitrogen atoms play an important role in the highly coplanar geometry of the directly linked dimers. The direct β–β linkage produces enhanced π conjugation and electron-spin coupling between the two diazaporphyrin units.