Morphology and optical properties of tetracyano-p-xylene single crystals

We report on a class of organic single-crystal semiconductors (tetracyano-p-xylene) with tunable blue-green emission. The crystalline structure, the shape and orientation of the constituent molecules, and the bond length have been determined by atomic force microscopy and compared with the results of x-ray diffraction. The obtained structural parameters were used to evaluate the electronic states and to calculate the optical absorption spectra of the crystal. Due to the nonplanar geometry of the tetracyano-P-xylene molecules and to the closely packed arrangement, the individual character of the molecule is not preserved in the crystalline phase, and a precise description of the optical spectra is obtained by considering molecular clusters consisting of at least three molecules with bond length and symmetry identical to that obtained by the structural experiments. [S0163-1829(98)52316-8].