Theoretical studies on the mechanisms of proton transfer in Schiff bases

Proton transfer reactions of salicylideneaniline (SA), N-tetrachlorosalicylideneaniline (Cl₄SA) and N-tetrachlorosalicylideneaniline-1-pyrenylamine (Cl₄SPY) have been investigated by using a semiempirical SCF MO method with an energy gradient technique. Thermochromic reactions involve the proton transfer from the hydroxyl oxygen to the imine nitrogen via six-membered ring transition states. From the calculated potential barriers, 81.27 and 87.27 kJ mol^-1 for Cl₄SA and Cl₄SPY, respectively, it can be seen that the thermochromic reactions proceed easily at room temperature. The analyses of molecular orbitals and electronic distributions show that the thermochromism is also due to a change in the π-electron configuration induced by the proton transfer. Intermolecular interaction makes the thermochromic reaction of Cl₄SPY easier to proceed. Photochromic reaction of SA is initiated by excitation of the enol form to its first excited singlet state S₁. The population in the S₁ state is transferred either to the second excited singlet state of the keto form (S′₂) along an adiabatic pathway or to the lowest excited triplet state of the keto form (T′₁) via intersystem crossing, and finally relaxing to the trans-keto form (S′'₀) or cis-keto form (S′₀).