Sequential Formation of Yellow, Red, and Orange 1‐Phenyl‐3,3‐Biphenylene‐Allene Dimers Prior to Blue Tetracene Formation: Helicity Reversal in trans‐3,4‐Diphenyl‐1,2‐bis(fluorenylidene)cyclobutane

1-Phenyl-3,3-biphenyleneallene (2), the base-catalyzed rearrangement product of 9-phenylethynylfluorene (1) yields a yellow, head-to-tail dimer 6 that, upon gentle warming, is converted to the red tail-to-tail isomer trans-3,4-diphenyl-1,2-bis(fluorenylidene)cyclobutane (7), in which the two fluorenylidene moieties severely overlap. The helical sense of the fluorenylidene moieties in 7 matches that of the phenyl substituents, and the …