Proton exchange and isomerisation reactions of photochromic and reverse photochromic spiro-pyrans and their merocyanine forms

Photochromic 1',3',3'-trimethyl-6,8-dinitro-spiro- [2H-1-benzopyran- 2,2-indoline], (6,8-dinitro BIPS) has been studied in different solvents using ¹H-NMR techniques, UV/visible spectroscopy, X-ray diffraction and electrospray ionisation mass spectroscopy (ESI-MS). Molar decadic absorption coefficients for 6,8-dinitro BIPS were found to range between 35 000-45 000 dm³ mol^-1 cm^-1. ¹H-¹H NOESY and ¹H-NMR experiments have established that in solution the merocyanine form of 6,8-dinitro BIPS is in rapid exchange between the trans-trans-cis (TTC) and trans-trans-trans (TTT) isomers. The 3B proton on the central bridging double bond of spiro-pyran merocyanines is labile in organic solvents. The closed form of 6,8-dinitro BIPS enantiomerises about the spiro C(2) carbon at a rate of 18 s^-1 at 300 K. The activation energy for this process was estimated to be 46 kJ mol^-1 using dynamic NMR.