Search

The concept of aromaticity is essential for chemists to understand the nature of many chemical substances. “In-plane” aromaticity is a unique type of aromaticity found in some π-conjugated systems consisting of radially oriented p-orbitals. Although in-plane aromaticity has been theoretically predicted about 40 years ago, its intrinsic properties are much less well understood than those of conventional Hückel aromatic systems with perpendicularly oriented p-orbitals because of lack of available examples of in-plane aromatic molecules. Here we demonstrate, from both theoretical and experimental aspects, that in-plane aromaticity of cycloparaphenylenes is reversed upon excitation to the lowest triplet state. In-plane non-aromatic neutral forms with a 4N-electron system and in-plane aromatic dications with a 4N+2-electron system turn out to be aromatic and anti-aromatic, respectively, indicating that Baird’s rule holds for the in-plane aromatic system. Armchair carbon nanotube fragments based on the cycloparaphenylene were also found to follow Baird’s rule from a magnetic viewpoint.

Novel boron dipyrromethene (BODIPY) derivatives having significant absorption and emission in the longer wavelength region were synthesized. BODIPY showed anomalous S 2 emission from the second excited (S 2) state at shorter wavelength. The photodynamics of the S 2 state was investigated by fluorescence up-conversion and transient absorption spectroscopic measurements. The decay time of the S 2 emission was dependent on the internal conversion and the intersystem crossing process. The assignments of transitions were supported by a theoretical calculation based on the time-dependent density functional theory.

Oligofluorenes attract wide attention due to their excellent fluorescent properties. For the detailed understanding of the excited state properties, ultrafast processes have to be clarified. Here, we have investigated the structural relaxation in the singlet excited state of star-shaped oligofluorenes with a truxene or isotruxene core, to which oligofluorenes (n = 1-4) were attached. The transient absorption peak showed red-shift with time upon excitation. The fluorescence decay profiles in the picosecond domain showed the fast component in addition to the component corresponding to the singlet excited state lifetime. These ultrafast phenomena can be attributed to the structural relaxation, i.e., planarization, in the singlet excited state. The planarization process was supported by the theoretical calculation based on the time-dependent density functional theory. Furthermore, dependence of two-photon absorption cross section on the core of the star-shaped oligofluorene has been elucidated.