Kinetic Stabilization of [ <i>n</i> ]Cumulenes by Endgroup Design

The quest to model carbyne has been largely tackled through increasingly longer polyynes. [ n ]Cumulenes are an alternative class of molecules to model carbyne. The major difference between polyynes and cumulenes is that polyynes maintain strong bond length alternation (BLA) while cumulenes do not. The development of longer [ n ]cumulenes is challenged by decreased stability relative to oligoynes. This study compares the persistence of tetraaryl[ n ]cumulenes based on ortho ‐substituent(s) of aryl endgroups. A trend emerges in which bulkier ortho ‐substituents offer more persistent [ n ]cumulenes. The enhanced stability is consistent with a combination of steric shielding and disrupted electronic communication between the endgroups and cumulenic framework. Kinetically stabilized [ n ]cumulenes [ n ] ot Bu ( n = 5, 7, 9, 11, 13) are successfully synthesized. The longest derivatives, [11] ot Bu and [13] ot Bu , are studied in solution, and the electronic absorption properties are compared experimentally and computationally. The terminal alkylidene groups of [ n ]cumulenes have significant effects on the optical properties and result in an additional optical state, S 1 , relative to oligoynes. The S 1 state gives a lower fundamental optical energy gap ( E g ) in [ n ]cumulenes relative to oligoynes and decreases as the molecules transition from D 2 to D ∞h symmetry. The highest energy state ( S 4 ) is allowed by symmetry at any length and shows remarkable similarity to the analogous absorptions of polyynes.