Inspired by the synthetic literature, we also test a number of [10]annulenes based on the cyclopenta[cd ]indene core, which is shown at the bottom of Figure . Several substituted analogs of this bridged [10]annulene perimeter exhibit excited-state Hund's rule violations. As in the case of the [14]annulene perimeters, the [10]annulene perimeters are symmetric when possible based on the bridge, and the excited states are always described by a linear combination of two excitations (as in Figure , for example). The class of annulenic perimeters with excited-state Hund's rule violations thus covers a range of both [10]-and [14]annulene motifs. The nature of the bridging atom connecting the rings (and its substituent) can have a strong impact on the energy gap between the S 1 and T 2 state energies, as highlighted in Figure .