Thermochromism at Room Temperature in Overcrowded Bistricyclic Aromatic Enes:Closely Populated Twisted and Folded Conformations

摘要

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)-an-thracenone (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 eguilibrium between the yellow anti-folded conformations and the thermochromic purple,blue,or red twisted conformations.The NMR experiments demonstrate a fast intercon-version 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 DELTA G_c(t) = 65.5 kJ/mol.B3LYP/6-311++G(d,p) calculations predict anfi-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'-yhdene)-9H-fluorene (1),[10'-oxo-9'(10'H)-anthracenylid-ene]-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 anfi-folded conformations by 14-18 kJ/mol.