Solvent-induced polarization phenomena in the excited state of composite systems with identical halves. 2. Effects of solvent polarity upon the fluorescence of [2.2](1,3)pyrenophane

Abstract

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.

One-dimensional aromatic crystals in solution. 4. Ground- and excited-state interactions of poly(L-1-pyrenylalanine) studied by chiroptical spectroscopy including circularly polarized fluorescence and fluorescence-detected circular dichroism

“…A similar discussion has been given for the excimers of [2,2] (l,3)pyrenophane in polar and nonpolar solvents. 19 Brittain et al reported the CPF spectrum of the excimer of optically active l-(l-hydroxyhexyl)pyrene in methanol. 20 They found a decrease of the magnitude of gem below Xem = 460 nm and interpreted the decrease as the presence of more than one emitting species.…”

Photophysics of preassociated pyrenes in aqueous polymer solutions and in other organized media

“…73 A marked solvent-dependence is displayed by the emission of [2.2](l,3)pyrenophane. 69 In nonpolar solvents, such as methylcyclohexane the fluorescence originates entirely from locally excited pyrene at all temperatures. In polar solvents, such as acetonitrile and acetone, an intense excimer emission centered at 475 nm is observed in addition to pyrene monomer emission.…”

Cyclophanes containing large polycyclic aromatic hydrocarbons

“…The absorption spectra of the (1,3)pyrenophanes 7-10 are similar to that of pyrene, although the bands are a little broader and somewhat red-shifted (20-30 nm). 25,26 Since very similar changes also occur upon simple substitution at the 1 and 3 positions, it was concluded that there is little interaction between the two π systems. More interesting is the effect of solvent polarity on the fluorescence spectrum of anti- [2.2](1,3)pyrenophane 10.…”