Synthesis, structure, and electronic properties of syn-[2.2]phenanthrenophanes: First observation of their excimer fluorescence at high temperature

Two syn-[2.2](1,6)- and -(3,6)phenanthrenophanes, 1a,b, were synthesized for the first time by means of intermolecular [2 + 2] photocycloaddition of the corresponding divinylphenanthrenes. Phenanthrenophanes 1a,b were obtained as mixtures of two (exo,exo and exo,endo) and three (exo,exo, exo,endo, and endo,endo) structural isomers, respectively, which were isolated by reversed-phase HPLC and gel permeation chromatography. All of the isomers, whose structures were characterized mainly on the basis of H-1 NMR spectroscopy, were in a syn conformation. X-ray crystallographic analysis of exo,endo-1a was successful, also in agreement with the results of H-1 NMR. Birch reduction of 1b, followed by DDQ oxidation, afforded [4.4](3,6)phenanthrenophane 5b in an anti conformation, due to opening of the cyclobutane rings. The absorption spectra of 1b were relatively similar to that of phenanthrene itself, while those of 1a were rather broadened and red-shifted compared to those of phenanthrene and 1b. In both cases, the spectra were independent of the configuration of the cyclobutane rings. The fluorescence spectra of 1b exhibited sharp vibrational structures, as in phenanthrene, suggesting fluorescence from the locally excited state. On the other hand, 1a afforded a broad and structureless emission due to the excimer fluorescence, even at room temperature. This is the first observation of the excimer emission almost free from the monomer-like emission for phenanthrene derivatives at rather high temperature. Such differences in the absorption and fluorescence spectra between 1a,b can be explained reasonably in terms of differences in the arrangement of the two phenanthrene rings; they are tightly held almost in parallel for 1a, according to the X-ray structural analysis, while tilted by the dihedral angle of ca. 30 degrees for 1b on the basis of MM2 calculations.