Observation of intramolecular singlet and triplet excimers of tethered naphthalene moieties under the geometric constraints imposed by the host framework of zeolites

This paper describes the emission spectroscopic investigation of singlet and triplet excimers of tethered bisnaphthalene species at room temperature. 1,1'-Dinaphthylethane (DNE) and 1,1'-dinaphthylpropane (DNP) were incorporated into a 3-D network of a 1.3 nm diameter pseudo-spherical supercage of zeolites Y (with an entry aperture of 0.74 nm), and a 1-D channel of zeolite L with a 0.71 nm aperture. In the straight channel of zeolite L, DNP adopts a parallel-sandwich conformation as suggested by the observation of entirely excimeric fluorescence, while DNE can only enter the channel in an open form. As a result, incorporation of DNP into zeolite L exchanged with Tl(+) ions, which induce enhanced intersystem-crossing, allowed the observation of a broad and featureless phosphorescence spectrum that is ascribable to the triplet excimer of DNP. In contrast, an entirely monomeric phosphorescence is observed for DNE. Furthermore, we observed a monomeric phosphorescence spectrum for DNP in Tl(+)-exchanged zeolite Y, in which DNP was shown to assume its open conformation. Thus the parallel-sandwich structure is responsible for the triplet excimer of the bisnaphthalene species as well as for the singlet counterpart. The parallel-sandwich conformation of the triplet excimer of DNP immobilized by the host framework is significantly different from the previously proposed L-shaped configuration in solution. However, it is very similar to that of the triplet excimer of triple-tethered carbazolophane, as revealed quite recently by Ohkita's group (J. Phys. Chem. B, 2007, 111, 10905). Thus the present study helps tackle the long-term issue of triplet excimer through the use of zeolites.