KEY INTERMEDIATE PRODUCT OF OXIDATIVE DEGRADATION OF PHOTOCHROMIC SPIROOXAZINES - X-RAY CRYSTAL STRUCTURE AND ELECTRON SPIN RESONANCE ANALYSIS OF ITS 7,7,8,8-TETRACYANOQUINODIMETHANE ION-RADICAL SALT

摘要

The preparation and characterization of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) ion-radical salt of the key oxidation product of the photochromic spirooxazines are described. This salt is prepared through a thermal oxidation reaction of TCNQ on spirooxazines. X-ray diffraction analysis shows it to be a complex salt, consisting of neutral TCNQ, TCNQ(.-) radical anion, and a rearranged oxidized product of spirooxazine (nominal 1:1:1 stoichiometry), which crystallizes in the monoclinic space group P2(1)/c with a = 17.014(3) Angstrom, b = 25.912(5) Angstrom, c = 8.809(1) Angstrom, beta = 90.18(1)degrees, V = 3883.6(11) Angstrom(3), and D-calcd = 1.3064 Mg/m(3) for Z = 4. Least-squares refinement of the model based on 4321 reflections (I > 3 sigma(I)) converged to a final R = 0.0468 and wR = 0.0536. TCNQ/TCNQ(.-) are organized as tetrads and arranged in a helical fashion along the crystal b axis. The cations and TCNQ/TCNQ(.-) form segregated stacks. Variable-temperature (77-303 K) electron spin resonance CESR) data of-single crystals and powders are presented. The powder ESR spectra of the complex salt have a temperature dependence typical of spin-coupled radical-pair systems. The separation energy (J) between the singlet ground state and triplet excited state is calculated to be 0.043 eV. The ESR signal line-width dependence on the single crystal orientation within the applied magnetic field requires a helical arrangement of the TCNQ/TCNQ(.-) couples along the b axis as found in the X-ray study. When the temperature is lowered, the doublet (monoradical) signal experiences line broadening, and at 103 K the triplet transitions appear. The values of the D and E zero-field parameters are calculated to be 2.71 x 10(-3) and 2.15 x 10(-3) cm(-1), respectively. The complex salt has a resistivity typical of a semiconductor. [References: 31]