Dianisylamino donor (D) and dimesitylboryl acceptor (A) substituents were introduced at the 1,6- and 2,7-positions of pyrene to demonstrate that the substitution patterns influence the photophysical properties. The different pictures in orbital interactions between the pyrene core and the D-A substituents led to the outcome that 1,6-substituted pyrene derivative 1 had stronger electron-donating and electron-accepting properties in conjunction with a small HOMO-LUMO gap, as compared to the 2,7-substituted derivative. For these pyrene derivatives, modest (Φ = 0.2) to strong (Φ = 1.0) fluorescence was detected in degassed organic solvents; 1 exhibited a typical intramolecular charge transfer (ICT) emission obeying energy-gap law, while 2 displayed a moderate inverse energy-gap law, originating from the different substitution patterns. Although theoretical calculations predicted that both 1 and 2 adopt highly twisted ICT excited states (TICT excited states) even in the gas phase, but practically, it was suggested that the observed photophysical properties could be determined by the extent of twist angle of the TICT-like excited state in accordance with the solvent polarity. Moreover, the bulky D-A substituents inhibit the intermolecular direct π-π interactions, thereby resulting in the bright and moderate solid-state emissions for 1 (Φ = 0.76) and 2 (Φ = 0.21), respectively.

Orbital interaction between 2,7-pyrenylene and two nitrogen redox-active centers effectively reduces the energy difference between HOMO and HOMO-1, both of which were distributed over the two nitrogen centers. In fact, one- and two-electron oxidation of 2,7-bis(dianisylamino)pyrene 3 generated a persistent radical cation and a persistent dication, respectively, and we succeeded in the isolation and single crystal X-ray structural analyses of all three oxidation states. The radical cation was considered as a spin and charge delocalized mixed-valence compound with a semiquinoidal structure. The dication was in an open-shell singlet state with a small singlet-triplet energy gap. The molecular and electronic structures for all three oxidation states of 3 were studied in comparison with the data reported for each oxidation state of closely related bis(triarylamine)s, of which structures were determined by X-ray crystallography.

Three kinds of para-phenylenediamine (PDA) derivatives bearing nitronyl nitroxide (NN) groups were prepared and characterized on the basis of the electrochemical, electron spin resonance (ESR) spectroscopic, absorption spectroscopic, and magnetic susceptibility measurements. It was clarified that the oxidation potential of the central PDA unit is strongly influenced by the numbers of substituted electron-withdrawing NN groups. In addition, the intervalence charge transfer in the central PDA unit was detected in the monocationic states of the PDAs with two NN groups, indicating the coexistence of the localized spins and the delocalized spin on theses molecules. Moreover, pulsed ESR measurements confirmed that the delocalized spin on the central PDA unit and the localized two spins on the NN groups were ferromagnetically coupled in the monocationic states.

Radical cations of bis(triarylamine)s, 3 and 4, in which the triarylamine redox centers are bridged by an ortho-phenylene and ortho-carborane cluster, respectively, have been prepared to elucidate the difference in intramolecular charge/spin-transfer (ICT/IST) pathway owing to the two different bridging units affording similar geometrical arrangements between the redox centers. Electrochemistry, absorption spectroscopy, VT-ESR spectroscopy, and DFT calculations reveal that 3 and 4 are classified into class II and class I mixed-valence systems, respectively, and therefore, through-bond and through-space mechanisms are dominant for the ICT/IST phenomena in 3 and 4 , respectively. Moreover, SQUID measurements for dicationic species provide the fact that virtually no spin-exchange interaction is observed for spins in 4 , while the antiferromagnetic interaction for spins in 3 , in accordance with the existence of a conjugation pathway for the ortho-phenylene bridge.

A new derivative of hexaaza[1]paracyclophane in which p-phenylenes are alternately replaced by 9,10-anthrylenes was prepared to investigate the impact on overall π-conjugation as well as conformational change of the macrocycle. The charge and spin distribution for one-electron and three-electron oxidation of the macrocycle was elucidated by means of electrochemical, spectroelectrochemical, EPR spectroscopic, and SQUID magnetometric methods. In particular, the triradical trication was successfully isolated as an air-stable salt, and moreover, its structure was disclosed by X-ray analysis. The triradical trication was characterized as a spin-frustrated three-spin system with the antiferromagnetic exchange interaction (J/k ≃ - 74 K).