Thermochromism at room temperature in overcrowded bistricyclic aromatic enes: Closely populated twisted and folded conformations

The overcrowded thermochromic bistricyclic aromatic enes (BAEs) 10-(9′H-fluoren-9′-ylidene)-9(10H)-anthracenone (6), 10-(11′H-benzo[b]fluoren-11′-ylidene)-9(10H)-anthracenone (7), and 10-(1′,8′-diaza-9′H-fluoren-9′-ylidene)-9(10H) -anthracenone (8) were synthesized by applying Barton's two-fold extrusion diazo-thione coupling method and their crystal and molecular structures were determined. BAEs 6-8 exhibit thermochromic behavior at room temperature due to the equilibrium between the yellow anti-folded conformations and the thermochromic purple, blue, or red twisted conformations. The NMR experiments demonstrate a fast interconversion of the twisted and the anti-folded conformers of 6-8 in solution. BAE 7 readily undergoes E,Z-topomerization at room temperature with the coalescence point at 297 K and the relatively low energy barrier of ΔG_c^‡(t_⊥) = 65.5 kJ/mol. B3LYP/6-311++G(d,p) calculations predict anti-folded a-6 and a-7 to be less stable than twisted t-6 and t-7 by 0.8 and 1.3 kJ/mol, respectively, whereas a-8 is more stable than t-8 by 10.7 kJ/mol. DFT calculations of 6-8, 9-(9′H-fluoren-9′-ylidene)-9H-fluorene (1), [10′-oxo- 9′(10′H)-anthracenylidene]-9(10H)-anthracenone (2) and their 1,8-diaza-substituted derivatives show that substitution in the fluorenylidene unit destabilizes the twisted conformations by 11-22 kJ/mol, while introduction of nitrogen atoms at the 1 and 8 positions of anthracenylidene unit destabilizes the anti-folded conformations by 14-18 kJ/mol.