[1] It is unstable and isomerizes with a half-life of 50 minutes at room temperature to 3a,7a-dihydro-1H-indene via a thermal 6π disrotatory electrocyclic ring closing.

[2] Upon exposure to ultraviolet light, it undergoes a photochemical 8π electrocyclic ring closing to give bicyclo[6.1.0]nona-2,4,6-triene.

[4] The all-cis isomer of C9H9− can be prepared by treatment of 9-chlorobicyclo[6.1.0]nona-2,4,6-triene (1) with lithium or potassium metal.

[5] Despite the ring strain resulting from having C–C–C bond angles of 140° instead of the ideal 120° for sp2 carbon, this species is believed to be planar and to possess D9h symmetry.

Computation and experimental evidence suggest that C9H9+ is a rare example of a ground state species that exhibits Möbius aromaticity.