Do inverse dithienylethenes behave as normal ones? A joint spectroscopic and theoretical investigation

We investigate an inverse (I) dithienylethene, the bis(3,5-dimethyl-2- thienyl) perfluorocyclopentene, using absorption, emission and NMR spectroscopies as well as state-of-art first-principles (TDDFT) calculations. First, we find in addition to the expected antiparallel AP1 and parallel P2 conformers, a new stable antiparallel conformer AP3, but its energy is too high to be significantly populated at working temperature. More importantly, we demonstrate that, instead of an equal proportion of an AP and a P conformer as in normal (N) diarylethenes, the AP conformer is present in large excess. This result is confirmed by both DFT thermodynamical analysis and temperature-dependent NMR experiments modelized with an AP1 ↔ P2 fast interconversion model. With the latter, the relative populations are estimated to be ca. 3/1 for AP1/P2. Furthermore, the 0-0 energies simulated with a model that accounts for both vibrational and state-specific media effects of the ground and the excited states indicate that AP1 and P2 have very similar absorption signatures while only the P2 conformer should give rise to emission. Eventually, within excited state manifold, important topological points along the ring-closure reaction coordinate, and more specifically the unprecedented S1(opt) of the closed isomer, have been identified.