The fluorescence spectrum as well as the absorption spectrum of [2.2] (1,3)pyrenophane (metapyrenophane) in nonpolar solvents such as methylcyclohexane are rather similar to those of pyrene. In polar solvents, metapyrenophane showed a broad structureless fluorescence band with maximum at 475 nm along with pyrene monomer-like emission at shorter wavelengths. The experimental results reported here suggest that the shorter wavelength fluorescence is from the locally excited (LE) state of metapyrenophane and the longer wavelength fluorescence is from the intramolecular excimer state. Furthermore, it was found that this excimer state may be formed through the intramolecular charge transfer intermediate state, namely, the "solvent-induced polarization" state.
IntroductionInvestigations on the solvent effects upon the fluorescence and S, +--SI absorption spectra of 9,9'-bianthryl have revealed that a CT (charge transfer) state is produced in polar solvents between separated intramolecular aromatic rings.lt2 In case of 9,9'-bianthryl, the CT state is a fluorescent state, and its fluorescence band is red shifted with increasing solvent polarity. 1,2-Dianthrylethanes9 as described in the preceding show intramolecular excimer fluorescence along with the fluorescence from the LE state. The study of the solvent dependence of the fluorescence yields of 1,2-dianthrylethanes suggested the existence of an intermediate CT state in the excimer formation p r o c e~s .~*~ In view of its molecular structure, 1,Zdianthrylethanes can pass into a partially overlapped sandwich structure which emits excimer fluorescence, whereas 9,9'-bianthryl cannot form such a structure. No other example of such a solvent-induced polarization in the excited state has been found for a molecule or molecular complex containing the same two aromatic groups, except for metapyrenophane (mePy), the results of which will be reported in the following.Intramolecular excimer formation of mePy also depends upon solvent polarity. In contrast to the case of 1,2-dianthrylethanes, intramolecular excimer formation in nonpolar solvents as well as any effective photochemical reaction did not occur in case of mePy, which makes the analysis of experimental results much more explicit.