No abstract

A near-infrared (NIR) polymethine dye (1), consisting of a cyclohepta[1,2-b;4,3-b']dithiophene and two phenol moieties, was synthesized. This dye exhibited pH-responsive changes in its photophysical properties due to a two-step acid-base equilibrium that produced a protonated cation (1H ) and an anion (1 ). While 1H showed an intense fluorescence in the red region of the visible spectrum, 1 exhibited a strong absorption in the NIR region. The tropylium ion character in 1H induces high pK and pK values for 1. Moreover, a stable radical (1 ) was prepared, which showed a NIR absorption band with a maximum at circa 1600 nm. The cyclic voltammogram of 1 revealed a two-step reversible redox process that produced 1 and the cation 1 , which is different from 1H . These redox processes accompany drastic electrochromic changes in the vis-NIR region. Overall, 1 is susceptible to multiple interconversions between five forms, due to the multifaceted character of the cycloheptadithiophene skeleton.

A series of thiophene-fused tropylium ions, containing various electron-donating amino groups at the terminal positions, was synthesized. The fusion of the thiophene rings, as well as the presence of the terminal amino groups endows the cationic tropylium ion with excellent stability and high pK values. X-ray crystallographic analysis of these compounds revealed a pronounced quinoidal character for the amino-substituted dithienotropylium skeletons. These compounds exhibit attractive photophysical properties such as strong absorption in the visible region combined with red fluorescence. Theoretical calculations suggested that the 3,3'-bithiophene substructure should be crucial for attaining these photophysical properties.

The synthesis of optically active poly(fluorene-alt-chiral iminofluorene)s using 1-phenylethylamine as a chiral auxiliary is described. Optically active polyfluorenes were synthesized by the palladiumcatalyzed Suzuki-Miyaura coupling polymerization of 9,9 0 -dioctylfluorene-2,7-bis(trimethyleneborate) with a chiral Schiff base monomer (imino group modified-fluorene). The obtained polymers showed intense green emission with high quantum efficiency. Analysis of the circular dichroism data of the optically active polymers indicates that the polymers exhibit various highly-ordered regular structures in solution.

A cyclic octithiophene containing two β,β'-linkages was synthesized. Due to the large structural change in the excited state, this compound exhibited bathochromically shifted fluorescence. It also showed a small difference between the first and second oxidation potentials, indicative of spin delocalization through the β,β'-linkage in the one electron-oxidized state.