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.