While the CSS ground state for lower acenes and cumulenes is well recognized in literature, but there is no clear consensus on the higher members of the series, yet. Thus, to establish the size-dependent onset of open-shell nature, ground state electronic properties are first examined for the isolated acenes and cumulenes, shown in insets of Figure . The relative energy difference between the OSS and CSS states, reveals that upto n=6 acene (Figure ) and n=12 cumulene (Figure ), OSS solutions converge to CSS state and yields ∆E OSS-CSS ∼ 0, indicating that CSS state is ground state for the lower members of both the series. For n=7 acene, ∆E OSS-CSS = -65 meV marks the transition in ground state from CSS to OSS state. The increasing ∆E upto -650 meV for n=12, ultimately outstrip the CSS behavior of the system for higher acenes. The emergence of OSS ground state, for n ≥ 7, is in complete agreement with previous DFT calculations on higher acenes. Interestingly, the similar kind of build-up of OSS character is also observed for cumulenes from n=14 onwards (Figure ). Although, ∆E OSS-CSS for cumulenes is significantly lower than acenes, however a maxima of ∆E OSS-CSS = -138 meV for n=24 cumulene clearly reveals an OSS ground state for the higher cumulenes as well.