Bond-Length Alternation as a Structural Coordinate for Electronic Regime Crossover in Indophenines

The emergence of diradical character in conjugated π-systems is often described qualitatively in terms of quinoidal distortion, yet simple quantitative structural descriptors that link molecular geometry to electronic ground-state regime remain scarce. Here, we show that bond-length alternation (BLA) serves as a compact geometric descriptor that correlates with the electronic regime crossover in donor-acceptor indophenine derivatives. Broken-symmetry density functional calculations show that progressive reduction of BLA is accompanied by a transition from a closed-shell electronic structure to weak diradicaloid character and, for the smallest-BLA members, approach to a near-degenerate frontier orbital manifold. This crossover is consistently reflected by a rapid reduction of the singlet-triplet separation (ΔE_ST), accompanied by increased fractional frontier natural occupations and enhanced spin polarization in the broken-symmetry solutions (⟨S²⟩), all converging within a narrow BLA window. Frontier natural orbitals exhibit a progressive loss of classical bonding-antibonding complementarity and the emergence of a quasi-degenerate symmetry-related frontier pair. These results identify BLA as a practical structural parameter for classifying electronic regimes within this indophenine family and suggest that related quinoidal π-frameworks may exhibit similar structure-electronic correlations.