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

Area of Science:

• * Organic Chemistry
• * Computational Chemistry
• * Materials Science

Background:

• * Diradical character in conjugated π-systems is often qualitatively assessed via quinoidal distortion.
• * Quantitative structural descriptors linking molecular geometry to electronic ground states are limited.
• * Understanding these structure-property relationships is crucial for designing novel organic materials.

Purpose of the Study:

• * To establish bond-length alternation (BLA) as a quantitative descriptor for electronic regimes in donor-acceptor indophenines.
• * To correlate BLA with the transition from closed-shell to diradicaloid electronic structures.
• * To investigate the impact of BLA on frontier orbital degeneracy and spin polarization.

Main Methods:

• * Utilized broken-symmetry density functional calculations.
• * Analyzed bond-length alternation (BLA) as a key geometric parameter.
• * Computed singlet-triplet energy separation (ΔE_ST), fractional frontier natural occupations, and spin polarization (⟨S²⟩).

Main Results:

• * Progressive reduction in BLA correlates with a shift from closed-shell to diradicaloid character.
• * A narrow BLA window marks a rapid decrease in ΔE_ST, increased natural occupations, and enhanced spin polarization.
• * Frontier natural orbitals show a loss of bonding-antibonding complementarity and emergence of quasi-degenerate pairs.

Conclusions:

• * Bond-length alternation (BLA) is a practical geometric descriptor for classifying electronic regimes in indophenines.
• * The study reveals a clear structure-electronic correlation driven by BLA.
• * Findings suggest similar correlations may exist in other quinoidal π-systems, aiding material design.