13C 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 zwitterion (Z) forms of the fluoran compound 1, which has been widely used as a typical black color former in data-recording systems, has been studied by 13C NMR and electronic absorption spectroscopies. Fluoran 1 showed no visible absorption in aprotic solvents, while a black color appeared in phenolic solvents. The 13C NMR and signal of the spiro carbon of 1 in CDCl3 appeared at 84.2 ppm, indicating that 1 exists substantially as L in aprotic solvents. In phenol d6 at 50 °C, the signal of the spiro carbon in 1 shifted to a lower magnetic field and appeared in the sp2-hybridization region (δ = 162.7), suggesting that in phenol-d6 a cleavage of the C(spiro)–O bond in the lactone ring occurs and that the ring-opened Z form is produced. The equilibrium between L and Z depended strongly on the temperature and solvents. The high temperature and inhibition of the solute (1)–solvent interaction by steric hindrance, self-association and intramolecular chelation of the solvent shifted the L–Z equilibrium toward L. The thermodynamic parameters for the equilibrium reaction in phenolic solvents were also estimated.