Otto Hahn and coworkers reported clusters up to C15 as early as 1942. They were never prepared in macroscopic amounts as condensed-phase chemical species. It is known that C4, C3, C2 and even C1 have heats of formation per carbon atom bigger than that of C6. The heat of formation is a very useful quantity as it permits evaluation of the heat of reaction of any chemical process. However, it is a variable term which depends on the definitions of reference states and a stable species does not necessarily have a negative heat of formation.
Notorious explosives exhibit positive as well as negative heats of formation.
In fact, energy contents are not good predictors of explosive strength. What really matters is kinetics, reaction rates. Indeed, the time for complete reaction is by five orders of magnitude shorter in explosions than in fuel burning. Hinshelwood and Semenov received the Nobel for such works in 1956. C6 can only be prepared in small amounts in the gas phase or on inert-gas matrices.