C-13 NMR AND ELECTRONIC ABSORPTION SPECTROSCOPIC STUDIES ON THE EQUILIBRIUM BETWEEN THE COLORLESS LACTONE AND THE COLORED ZWITTERION FORMS OF A FLUORAN-BASED BLACK COLOR FORMER

The equilibrium between the colorless lactone (L) and the colored zwit terion (Z) forms of the fluoran compound 1, which has been widely used as a typical black color former in data-recording systems, has been s tudied by (CNMR)-C-13 and electronic absorption spectroscopies. Fluora n 1 showed no visible absorption in aprotic solvents, while a black co lor appeared in phenolic solvents. The C-13 NMR and signal of the spir o carbon of 1 in CDCl3 appeared at 84.2 ppm, indicating that 1 exists substantially as L in aprotic solvents. In phenol d(6) at 50 degrees C , the signal of the spiro carbon in 1 shifted to a lower magnetic fiel d and appeared in the sp(2)-hybridization region (delta = 162.7), sugg esting that in phenol-d(6) a cleavage of the C(spiro)-O bond in the la ctone ring occurs and that the ring-opened Z form is produced. The equ ilibrium between L and Z depended strongly on the temperature and solv ents. The high temperature and inhibition of the solute (1)-solvent in teraction by steric hindrance, self-association and intramolecular che lation of the solvent shifted the L-Z equilibrium toward L. The thermo dynamic parameters for the equilibrium reaction in phenolic solvents w ere also estimated.