Kinetic Stabilization of [n]Cumulenes by Endgroup Design

Abstract: (299 words). The quest to model carbyne has been largely tackled through the design of increasingly longer polyynes. [n]Cumulenes are an alternative class of molecules to model carbyne. The major difference between polyynes and cumulene as model compounds, however, is that polyynes maintain strong BLA while cumulenes do not. The synthetic development of longer [n]cumulenes has been challenged by decreased stability relative to oligoynes. This study compares the persistence of tetraaryl[n]cumulenes as a function of the ortho-substituent(s) of aryl endgroups. A clear trend emerges in which aryl endgroups with bulkier alkyl ortho-substituents offer more persistent [n]cumulenes. The enhanced stability is consistent with combination of steric shielding and disrupted electronic communication between the endgroups and cumulenic framework. Kinetically stabilized odd [n]cumulenes of the series [n]otBu (n is an odd integer, n = 5, 7, 9, 11, and 13) are successfully synthesized. The longest derivatives, [11]otBu and [13]otBu, are studied in solution, and the electronic absorption properties are compared experimentally and computationally with polyynes. The terminal alkylidene groups of [n]cumulenes have significant effects on the optical properties and result in the observation of an additional optical state, S1, relative to oligoynes. Access to the S1 state in [n]cumulenes gives a lower fundamental optical energy gap (Eg) relative to oligoynes, although the intensity of S1 decreases as a function of length as the molecules transition from D2h to D∞h symmetry. The highest energy state of [n]cumulenes (S4) is fully allowed by symmetry at any length and shows a remarkable similarity to the analogous absorptions of polyynes. At infinite length, this state would be expected to dominate the spectra of both polyynes and cumulenes.