ИСТИНА

Diarylethenes attract substantial interest from both experimentalists and theorists because of their applicability as molecular machines and switches [1]. Thus, trans/cis photoisomerization of stilbene and its derivatives has been explored in detail [2], and its photochemical dynamics can be considered as a basic model of operation of some molecular devices. After initial photoexcitation of the trans-isomer, twisting around the central ethylenic bond eventually brings about sudden polarization to give zwitterionic perpendicular excited state. Subsequent relaxation to the ground state, the cis-to-trans ratio depending on the exact compound, concludes the photochemical process. To tune the parameters of the photoinduced evolution, one can reduce aromaticity of the molecule by, e.g., replacement of the phenyl rings with thiophene. Accordingly, the effect of substitution on the trans/cis photoisomerization in dithienylethenes is usually investigated by means of stationary rather than ultrafast spectroscopy. As a result, the details of evolution in the excited state remain largely inaccessible. In the present work, we consider photochemical isomerization of the least aromatic of the diarylethenic systems, 3,3’-difurylethene, as well as its symmetrically fluorinated derivatives. The electronic structure of the S1 state across the trans/cis isomerization domain is studied with the use of the extended multi-configuration quasi-degenerate perturbation theory XMCQDPT2. The nature of the first singlet excited state turns out to be largely similar to that in unsubstituted stilbene. In some cases, however, the twisting in the excited state turns out to be barrierless, in contrast to many stilbenes. Halfway between the cis and trans ends, we observe sudden polarization that can be regarded as a manifestation of the pseudo Jahn-Teller effect due to interaction of a quasi-degenerate pair of states of different symmetry. Some of the ring-fluorinated 3,3’-difurylethens exhibit an alternative relaxation pathway that consists in an expressed (up to 90 deg.) out-of-plane bending of one of the fluorine atoms. Importantly, the said bending of the ring fluorine and bending of subsituents at the central ethylenic bond upon twisting do not occur simultaneously. They rather characterize two different domains of the potential energy surface. It is worth noting that the bending of ring fluorines is similar to that observed in highly fluorinated benzenes [3]. Attributable to the interaction with σ* and/or Rydberg states, it is, possibly, a common feature of highly fluorinated aromatic rings and larger systems based thereon. The reported study was funded by RFBR, project number 19-33-90252\19 [1] Irie M. Chem. Rev. 2000, 100, 1685−1716 [2] Ioffe I.N. et al. J. Am. Chem. Soc. 2017, 139, 15265–15274 [3] Hüter O. et al. J. Chem. Phys. 2016, 145, 014302