Negative Photochromism of 3,1′-Trimethylene-Bridged 6-Nitroindolinospiropyran

A new photochromic nitroindolinospiropyran with a 3,1′-trimethylene bridge was synthesized. Different from the non-bridged spiropyrans it exists mainly as the colored twitter-ionic open form in polar solvents. The following values in CHCl3, DMSO, and CH3OH at 20 °C were determined: (1) molar absorption coefficient of the open form at λmax, (2) Arrhenius’ activation energy, ΔG‡, ΔH‡, and ΔS‡ of thermal coloring and bleaching, (3) ΔG, ΔH, and ΔS of the thermal coloring.