A quantum chemical study of the ground state ring opening/closing of photochromic 1,3,3-trimethylspiro[indoline-2,3′-naphtho[2,1-b][1,4]oxazine]

Abstract

Ground state ring opening/closing reactions of 1,3,3-trimethylspiro[indoline-2,3′-naphtho[2,1-b][1,4]oxazine] have been studied using DFT calculations. An extensive description of the ground state ring opening/closing and isomerization processes undergone by the eight open forms is proposed. The pathways for ring opening show that the most stable merocyanines, TTC and CTC, can be obtained after C–O bond cleavage of the (R)- and (S)-enantiomeric closed forms, respectively. Two s-cis isomers, CCC and TCC, were localized on the potential energy surface and characterized as key intermediates involved in the two previous pathways. The less stable merocyanines, CTT and TTT, revert to the corresponding closed form through an inversion mechanism at the N1′ nitrogen atom. This mechanism is suggested to explain the fast component of the kinetics of the thermal fading reaction. The ground state reaction of enantiomerization between the closed forms (R) and (S) is also reported and two competing mechanisms were found. Enantiomerization can be achieved by after ring opening by cis–trans isomerization either between two merocyanines (i.e. TTC and CTC) or two s-cis intermediates. The high energy barrier of the bond rotation necessary for the interconversion between TTC and CTC isomers makes this process less favorable than direct interconversion between the s-cis intermediates (CCC and TCC).