Tris-Azo Triangular Paraphenylenes: Synthesis and Reversible Interconversion into Radial π-Conjugated Macrocycles

We report the synthesis of cycloparaphenylene derivatives featuring tris-azo groups. The smaller derivative, [3]cycloazobenzene ([3]CAB-0), adopts a triangular all-cis form and exhibits thermally and photochemically stable characteristics due to significant ring strain, as well as symmetric Kagome-patterned crystal packing. In contrast, the as-synthesized [3]cycloazobenzene with three biphenylene bridges ([3]CAB-1) adopts a similar triangular all-cis form; however, it undergoes photoinduced isomerization, leading to a mixture of cis and trans forms at a photostationary state. Interestingly, the addition of an excess of acid selectively leads to the formation of the all-trans form. DFT calculations reveal that the interconversion from a triangular to a circular shape correlates with an increase in HOMO and a decrease in LUMO, characteristics intrinsic to radial π-conjugated systems.