Ultraviolet Photolysis of 1,6-Methano[10]annulene Generates the Singlet Methylene

exist in nonplanar conformation and show non-aromatic characteristics.1 Di-trans-[10]annulene is also expected to be non-planar due to the steric hindrance of two inner hydrogen atoms and has never been synthesized. On the other hand, 1,6-methano, 1,6-epoxy-, and 1,6-imino-[10]annulenes, in which the 10-membered rings are forced to form an approxi-mately planar conformation by replacement of 1,6-position-ed hydrogen atoms by CH2, O or NH groups, respectively, are found to show aromaticity.2-8 Among these three 1,6-bridged[10]annulenes, 1,6-methano[10]annulene stands out because it is stable and synthesizable in quantity.2 Due to its unique characteristics, 1,6-methano- and the similar type of the molecule have been subject to intense investigation on diverse aspects of structural, chemical, and spectroscopic properties. Especially, a series of spectro