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-ind oline], (6,8-dinitro BIPS) has been studied in different solvents using H-1 -NMR techniques, UV/visible spectroscopy, X-ray diffraction and electrospra y ionisation mass spectroscopy (ESI-MS). Molar decadic absorption coefficie nts for 6,8-dinitro BIPS were found to range between 35000-45000 d mol(-1) cm(-1). H-1-H-1 NOESY and H-1-NMR experiments have established that in solu tion 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 proto n on the central bridging double bond of spiro-pyran merocyanines is labile in organic solvents. The closed form of 6,8-dinitro BIPS enantiomerises ab out the spiro C(2) carbon at a rate of 18 s(-1) at 300 K. The activation en ergy for this process was estimated to be 46 kJ mol(-1) using dynamic NMR.