Kinetic and thermodynamic investigations of the photochromism and solvatochromism of semipermanent merocyanines

The photochromic and solvatochromic behavior of four phenanthroline-containing spirooxazines bearing various alkyl long-chain substituents has been investigated in different polar and nonpolar solvents at various temperatures. These derivatives always appeared in an equilibrium mixture of spiro forms and open merocyanines. Depending on the substituents and the solvents, the percentage of the spontaneous merocyanine open form ranged from less than 10% to more than 30%. These results have been corroborated by independent NMR and photokinetic analysis. The main spectroscopic, thermodynamic, and photochromic parameters of such photo- and solvatochromic systems have been determined. Special illustrative diagrams showing quantitative substituent and solvent effects on absorption wavelengths, molar absorption coefficients, thermal equilibrium, interconversion rate constant, and quantum yields have been established. The two O-alkyl long-chain-substituted compounds 3 and 4 behave similarly, as do the N-alkyl derivatives 1 and 2. Opposition between polar and nonpolar solvents was also clearly illustrated. A global energy diagram including a short-lived cisoid merocyanine intermediate allows the interpretation of all solvent and temperature effects in terms of the relative solvation of the various short- and long-lived species and transition states.