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(10 H )‐anthracenone ( 6 ), 10‐(11′ H ‐benzo[ b ]fluoren‐11′‐ylidene)‐9(10 H )‐anthracenone ( 7 ), and 10‐(1′,8′‐diaza‐9′ H ‐fluoren‐9′‐ylidene)‐9(10 H )‐anthracenone ( 8 ) were synthesized by applying Barton's twofold 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 Δ G 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)‐9 H ‐fluorene ( 1 ), [10′‐oxo‐9′(10′ H )‐anthracenylidene]‐9(10 H )‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)