Thermochromic perimidine

Minkin and co-workers35 recently developed a series of novel photo- and thermochromic perimidine spirocyclohexadienones for which the mechanism involved a ring-chain tautomerism. For instance, for 48 (Scheme 6) the absorption maxima for the closed and open forms are, respectively, 390 and 584 nm (log e = 3.8) in octane as solvent. 

Valence and prototropic tautomeric reactions are among the most important mechanisms that govern transformations of a broad variety of photochromic organic systems.1,2 Until recently, no examples of photochromic compounds have been reported whose photochromic behavior was due to a combination of these two tautomeric reactions. Such a combination, which is characteristic of ring-chain tautomerism3 has been implemented in the photochromic and thermochromic rearrangements of a novel type of heterocyclic photochromes, derivatives of 2,3-dihydro-2-spiro-4 -(2, 6 -di-iert-butylcyclohexadien-2, 5 -one)perimidine la and its analogs.4 The occurrence of a proton transfer step is in accord with the fact that the AvV -dimethyl derivative of la exhibits no photochromic properties. 

---