Abstract
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)