Energy barrier to TTC-TTT isomerisation for the merocyanine of a photochromic spiropyran

The H-1 NMR signal of a proton of a spiropyran photomerocyanine was resolve d at low temperature into two signals attributed to the two isomers, TTT an d TTC. The equilibrium distribution and free energy difference between the TTT and TTC isomers were calculated to be 0.15 and 4.6 +/- 0.7 kJ mol(-1), respectively. By H-1 NMR lineshape analysis at various temperatures, an act ivation barrier for the TTC --> TTT isomerisation of 43.6 +/- 3 kJ mol(-1) was calculated.