1-Ferrocenylethynylanthraquinone (1-FcAq), which is a donor (D)-acceptor (A) conjugated compound consisting of a ferrocene (Fc) acting as a donor, an anthraquinone (Aq) acting as an acceptor, and an ethynyl linker, undergoes a cyclocondensation reaction with strong organic acid, and forms 2-ferrocenyloxodihydrodibenzochromenylium salts ([1-FcPyl](+)X(-) where X = TFSI, TfO, PF(6), and BF(4)). [1-FcPyl](+) were also characterized as conjugated donor-acceptor compounds, and electrochemical properties, UV-vis absorption spectra, single-crystal X-ray analysis, and TD-DFT calculations have indicated that the LUMO level of [1-FcPyl](+) is lower than that of 1-FcAq because of the much larger pi-conjugated system in [1-FcPyl](+). Variable-temperature Mossbauer spectroscopy (12-300 K) showed that Fe(II) was dominant for the TFSI(-), PF(6)(-), and BF(4)(-) salts of [1-FcPyl](+); although the Fe(III) species was also observed at all temperature ranges, the molar ratio of Fe(III) species increased at higher temperatures in the TFSI(-) and PF(6)(-) salts. This finding indicates that valence tautomerization (VT) between 1-FcPyl(+) and 1-Fc(+)Pyl occurs in the solid state of the TFSI(-) and the PF(6)(-) salts, but not in the BF(4)(-) salt. Variable-temperature (3.5-310 K) IR spectroscopy showed that the frequencies of the skeletal vibration of the ferrocene moiety decreased with increasing temperature in the TFSI(-) and PF(6)(-) salts, indicating the development of a ferrocenium-like character. The precision of the bond lengths of the [1-FcPyl](+) moiety (0.003-0.004 A) determined by single-crystal X-ray analysis (113 and 273 K) is not sufficient to demonstrate the effect of the counterion on VT. The dihedral angle between the ferrocene and the pyrylium moieties in the BF(4)(-) salt (11.25(15) degrees) is larger than that in the TFSI(-) (6.63(12) degrees) and PF(6)(-) (9.55(15) degrees) salts. Furthermore, the planarity of the acceptor moiety (estimated from the dihedral angle between Ph1 and Ph2) is lower in the BF(4)(-) salt compared with that of other salts. These increased dihedral angles might cause a weaker D-A interaction and a destabilization of the acceptor moiety (i.e., raising a LUMO level), leading to lower stability of the Fe(III) (1-Fc(+)Pyl) species. Variable-temperature X-ray powder diffraction (VT XRPD, 100-300 K) revealed that the temperature dependence of the Fe-P distance in the PF(6)(-) salt was smaller than that of the Fe-B distance in the BF(4)(-) salt. Our interpretation of this phenomenon is that the molar ratio of the Fe(III) species is increased in the PF(6)(-) salt, and that the Coulombic force between the ferrocene moiety and PF(6)(-) anion increases, preventing an increase in the Fe-P distance. This indicates that the electrostatic interaction between the [1-FcPyl](+) moiety and the counteranion may affect the occurrence of VT.

The photoisomerization properties of tris(bipyridine)cobalt complexes containing six or three azobenzene moieties, namely, [Co(II)(dmAB)3](BF4)2 [dmAB = 4,4'-bis[3''-(4'''-tolylazo)phenyl]-2,2'-bipyridine], [Co(III)(dmAB)3](BF4)3, [Co(II)(mAB)3](BF4)2 [mAB = 4-[3' '-(4' ''-tolylazo)phenyl]-2,2'-bipyridine], and [Co(III)(dmAB)3](BF4)3, derived from the effect of gathering azobenzenes in one molecule and the effect of the cobalt(II) or cobalt(III) ion were investigated using UV-vis absorption spectroscopy, femtosecond transient spectroscopy, and 1H NMR spectroscopy. In the photostationary state of these four complexes, nearly 50% of the trans-azobenzene moieties of the Co(II) complexes were converted to the cis isomer, and nearly 10% of the trans-azobenzene moieties of the Co(III) complexes isomerized to the cis isomer, implying that the cis isomer ratio in the photostationary state upon irradiation at 365 nm is controlled not by the number of azobenzene moieties in one molecule but rather by the oxidation state of the cobalt ions. The femtosecond transient absorption spectra of the ligands and the complexes suggested that the photoexcited states of the azobenzene moieties in the Co(III) complexes were strongly deactivated by electron transfer from the azobenzene moiety to the cobalt center to form an azobenzene radical cation and a Co(II) center. The cooperation among the photochemical structural changes of six azobenzene moieties in [Co(II)(dmAB)3](BF4)2 was investigated with 1H NMR spectroscopy. The time-course change in the 1H NMR signals of the methyl protons indicated that each azobenzene moiety in [Co(II)(dmAB)3](BF4)2 isomerized to a cis isomer with a random probability of 50% and without interactions among the azobenzene moieties.