Propertiesof Triplet-Excited [n]Cycloparaphenylenes(n= 8–12): Excitation Energies Lower thanThose of Linear Oligomers and Polymers.

Cycloparaphenylenes (CPPs), a classof hoop-shaped conjugated macrocycles,have attracted the attention of researchers in various fields becauseof their interesting properties. Although their properties in thesinglet-excited state have been reported, there is no systematic informationon the triplet-excited state. In the present study, the propertiesof triplet-excited [n]CPP (n= 8–12,where ndenotes the number of phenyl rings) werecomprehensively investigated. The phosphorescence peak shifted tothe shorter-wavelength side with increasing ring size of the CPPs,indicating smaller triplet energy for smaller CPPs. It was found thatthe triplet energy of a smaller CPP is even smaller than those ofpoly(p-phenylene)s, indicating that small hoop-shapedconjugated macrocycles are effective in realizing low-band gap materials.By applying laser flash photolysis, the Tn–T1absorption spectra of CPPs were obtained, fromwhich the size-dependence of energy levels of higher triplet stateswere also determined. Generation of singlet oxygen by the energy transferfrom a triplet-excited CPP was confirmed. The generation yield becamesmaller with increasing size of the CPP in accordance with the tripletyield. From these observations, the size-dependence of the deactivationpathways is explained. [ABSTRACT FROM AUTHOR]