Conformation‐Variable [7]Annulenyl Radicals

A series of benzo‐fused [7]annulenyl radical derivatives are synthesized and studied. By comparing their spin distribution differences revealed by EPR, variable molecular structures adaptive to steric features around the [7]annulenyl moiety are unveiled. Specifically, for radicals having no substituent on the vinylene carbons (C10 & C11), a relatively planar dibenzo[ a , d ][7]annulenyl skeleton containing a delocalized spin is suggested by both experimental data and computation results. Depending on the substituting group on C5, decent to optimal persistency is observed, as represented by a half lifetime of 23 h displayed by a triisopropylphenyl‐substituted radical in air‐saturated solution. When a substituent is added to C10, the radical stability can be further improved, although the [7]annulenyl framework becomes slightly bended. However, the tribenzo[ a , c , e ][7]annulenyl radicals exhibit noticeably inferior stability attributed to the nonplanar conformation assumed by the tetracyclic scaffold, which not only curbs the spin delocalization by disrupting the π‐conjugation, but also makes the molecule more susceptible to various spin‐annihilation reactions. More interestingly, when an anthryl group is attached to C9 of tribenzo[ a , c , e ][7]annulenyl, a pronounced conformational transition to folded state is observed, with the spin concurrently relocated from the tribenzo[7]annulenyl moiety to 9,10‐anthrylene unit, presumably driven by the superior spin delocalization property.