Diphenylamino Radical Dimer as a Color Center in a Molecular Crystal

Radical pairs of diphenylamino (DPA) can be formed in a single crystal of tetraphyenylhydrazine (TPH) by uv photolysis. The structure of these radical dimers can be elucidated by ESR measurements. These studies show that eight configurationally different dimers occur, which fall into two classes with 3- and 4-Å separation, respectively. Optical measurements reveal long-wavelength charge resonance absorptions arising from the dimers; this interpretation is supported by polarization measurements. Temperature-dependent ESR measurements were used to determine the singlet–triplet splittings of the two spin systems, yielding about 130 and 40 cm−1 for the 3- and 4-Å dimers, respectively. The radical dimer crystal defect can be regarded as a color center in a molecular crystal.