Ultrafast Ring-Opening/Closing and Deactivation Channels for a Model Spiropyran–Merocyanine System

Abstract

The photochemistry of a model merocyanine-spiropyran system was analyzed theoretically at the MS-CASPT2//SA-CASSCF(14,12) level. Several excited singlet states were studied in both the closed spiropyran and open merocyanine forms, and the paths to the different S(1)/S(0) conical intersections found were analyzed. After absorption of UV light from the spiropyran form, there are two possible ultrafast routes to efficient conversion to the ground state; one involves the rupture of the C(spiro)-O bond leading to the open form and the other involves the lengthening of the C(spiro)-N bond with no photoreaction. From the merocyanine side the excited state can reach a very broad S(1)/S(0) conical intersection region that leads the system to the closed form after rotation of the central methine bond. Alternatively, rotation of the other methine bonds connects the system through different S(1)/S(0) conical intersections to several merocyanine isomers. The present work provides a theoretical framework for the recent experimental results (Buback , J. J. Am. Chem. Soc. 2010, 132, 1610-1619) and sheds light on the complex photochemistry of these kinds of compounds.

Detailed Molecular Dynamics of the Photochromic Reaction of Spiropyran: A Semiclassical Dynamics Study

“…On the other hand, the C 1 -O bond can be lengthened since it points to the stable MC product, while the dissociation of C 1 -N or C 1 -C 3 would result in the ionization of SP, which is more active than both SP and MC stable structures. Hence, no dissociation of C 1 -N or C 1 -C 3 can be discovered on the dynamic simulations of the real SP, which is different from the previous conclusions that the internal conversion occurs accompanying the dissociation of C 1 -N bond from the model compounds [38][39][40].…”

“…Unlike the possible photodissociation of all the four bonds around the central C 1 atom of the model SP, containing the previously mentioned C 1 -N bond cleavage [38][39][40], the real molecule would usually be ionized with only the C 1 -C 2 dissociation. The ultrafast process of the photoinduced ionization and isomerization of SP plays very important roles in the effective internal conversion and less quantum yields of MC products after the photon excitations, which agrees with the previously experimental observations [24,28].…”

“…using the model molecule [30][31][32][33][34][35][36][37][38][39][40][41]. There are three different pathways for the photochromic reaction proposed by computations.…”

“…There are three different pathways for the photochromic reaction proposed by computations. The first and second kinds of paths are both associated with the C spiro -O stretching and hydrogen-out-ofplane (HOOP) torsion modes (the C atom is connected by Spiro-C) of the excited SP [38][39][40]. The last pathway involves the lengthening C spiro -N bond and the internal conversion from the electronically excited states to the SP ground state through CI S1/S0 (CN) [38][39][40].…”

“…The first and second kinds of paths are both associated with the C spiro -O stretching and hydrogen-out-ofplane (HOOP) torsion modes (the C atom is connected by Spiro-C) of the excited SP [38][39][40]. The last pathway involves the lengthening C spiro -N bond and the internal conversion from the electronically excited states to the SP ground state through CI S1/S0 (CN) [38][39][40]. In order to more directly interpret these mechanisms, the semiclassical dynamics of the photochemical reaction have been carried out by the real SP following the excitation using several laser pulses, which is necessary for the photoinduced chemical reaction [42,43].…”