We present the first derivative of the cethrene family that displays features previously unobserved in this class of helical diradicaloids. In contrast to all other reported cethrenes, dimethylnonacethrene is energetically more stable than the product of its 6π electrocyclization, because of the methyl substituents installed in the fjord region. The result is a stable and isolable open form, which could not be detected previously for the unsubstituted parent compound on account of high reactivity. Compared to the shorter homolog dimethylcethrene, π-extension results in lowering of the singlet–triplet energy gap (ΔEST) of dimethylnonacethrene and appearance of an EPR signal at room temperature. In comparison with planar isomer nonazethrene and linear analog nonacene, featuring the same number of benzenoid rings, helical dimethylnonacethrene exhibits significantly lower ΔEST and remarkable stability under ambient conditions. Our results suggest that further adjustment of the steric bulk in the fjord region can enable realization of a hydrocarbon-based magnetic photoswitch.