Time-resolved EPR (TREPR), transient absorption, and phosphorescence spectra have been measured in solid
matrixes at 77 K for the covalently linked L-shaped dimers of naphthalene, in which the in-plane long axes
of the two naphthalene moieties are parallel and the in-plane short axes make an angle. The zero-field splitting
(ZFS) parameters obtained by simulation of the TREPR spectra are consistent with the dimer geometries, if
it is assumed that excitation exchange interaction is responsible for the small D values of the dimers relative
to the monomer. The tilt angle in the triplet state is significantly smaller for the dimers exhibiting excimeric
phosphorescence than those exhibiting monomeric emission. Comparison of the spectral position and lifetime
of phosphorescence from the L-shaped dimers with those from the sandwich dimers indicates that the excitation
exchange interaction is substantially greater for the L-shaped dimers as compared to the sandwich dimers.
The proposal that the preferred conformation of the triplet excimer of naphthalene is L-shaped, rather than
sandwich, is supported by the measurements.
Comparisons of the temporal characteristics of the transient absorption with those of the delayed fluorescence for dinaphthylmethanes and dinaphthyl ethers provide compelling kinetic evidence for the formation of intramolecular triplet excimers.
The initial monomeric phosphorescence of Agosta pinacol dimer of naphthalene in hydrocarbon glasses at 77 K changes to an excimer-like phosphorescence of the corresponding diketone dimer upon irradiation. The close spectral resemblance between the diketone emission and the room-temperature excimer phosphorescence of naphthalene and 1 ,/i-di-a-naphthylal kanes ( = 1-4) in fluid solutions provides compelling evidence for the stability of the naphthalene triplet excimer and enables us to address the question of its likely conformation.
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