Size‐Selective Formation of C 78 Fullerene from a Three‐Dimensional Polyyne Precursor

Multicyclic cagelike cyclophanes 2 a and 2 b containing cyclobutene rings have been prepared as precursors of three‐dimensional polyynes C 78 H 18 ( 1 a ) and C 78 H 12 Cl 6 ( 1 b ), respectively. Laser irradiation of 2 a and 2 b induced expulsion of the aromatic fragment, indane, to give the three‐dimensional polyyne anions C 78 H 18 − and C 78 H 12 Cl 6 − , respectively. Whereas the former anion lost only four hydrogen atoms to form C 78 H 14 − , complete loss of all hydrogen and chlorine atoms was observed from the latter anion, to yield a C 78 − ion that has a fullerene structure which was proven by its characteristic fragmentation pattern.