ATOMIC SCALE SURFACE STUDIES OF CONDUCTIVE ORGANIC-COMPOUNDS .2. SCANNING TUNNELING MICROSCOPY AND CRYSTAL-STRUCTURE OF THE CHARGE-TRANSFER COMPLEX OF 4-ETHYLPYRIDINE-TCNQ2 (4-EP-TCNQ2)

The scanning tunneling microscopy (STM) studies of the conductive charge transfer complex of 4-ethylpyridine (4EP) with tetracyanoquinodimethane (TCNQ) were combined with X-ray diffraction measurements. The existence of two different crystalline modifications of the complex was revealed. The block-type monocrystals have a triclinic crystallographic structure with the following parameters: a = 7.112(2) angstrom, b = 7.862(7) angstrom, c = 13.954(4) angstrom, alpha = 72.95(4)-degrees, beta = 78.40(2)-degrees and gamma = 69.92(3)-degrees. The existence of a columnar structure of TCNQ molecules with diadic repeat unit is shown. The interplanar distances between neighboring TCNQ molecules in stack is 3.21 angstrom. The STM images of one of the surfaces of such monocrystals correspond well with the distribution of TCNQ atoms in the surface layer constructed from the crystallographic (a, b) plane. The quality of needle-shaped monocrystals does not permit detailed crystallographic measurements. Only the triclinic crystal system and the lattice constants were determined: a = 3.865(2) angstrom, b = 6.658(9) angstrom, c = 26.386(7) angstrom, alpha = 92.63(5)-degrees, beta = 94.12(3)-degrees and gamma = 106.80(8)-degrees. STM images provide additional structural information about this crystal modification. The formation of stacks of TCNQ molecules with the average parameters of 4.1 angstrom (periodicity in stack) and 13.1 angstrom (distance between neighboring stacks) seems to be the characteristic feature of this structure. Observed structures are compared with the known crystallographic data of other charge transfer systems. The pecularities of STM imaging in charge transfer systems are discussed in due course.