Electronic structure and photostability of benzothiadiazole-based D–A–D small molecules: An integrated theoretical–experimental analysis of frontier orbital alignment, polaron reorganization, and π-conjugation effects

Benzothiadiazole-based donor–acceptor–donor (D–A–D) conjugated small molecules constitute well-defined model systems for elucidating structure–property relationships in organic semiconductors. Here, we report a combined theoretical–experimental analysis of two previously reported molecules, BT-TPA and BT-TP-TPA , synthesized via a palladium nanoparticle (PdNP)-mediated Suzuki–Miyaura protocol, with the aim of establishing quantitative correlations between π-conjugation length, frontier-orbital alignment, charge polaron reorganization, and operational photostability. Density functional theory calculations at the B3LYP-D3(BJ)/6-31G(d,p) level with PCM (chlorobenzene) solvation reveal that π-extension narrows the fundamental adiabatic gap from 2.43 to 1.99 eV, in close agreement with the experimental optical gap (2.23 → 1.93 eV) and the electrochemical gap (2.17 → 1.87 eV). Internal reorganization energies computed via the four-point Nelsen–Marcus scheme exhibit an asymmetric response to π-extension: the electron reorganization energy decreases substantially (λ e = 364 → 219 meV; Δ = −145 meV, −40%), reflecting enhanced LUMO delocalization over the extended π-framework, while the hole reorganization energy shows a modest increase (λ h = 209 → 244 meV; Δ = +35 meV) attributable to additional torsional flexibility introduced by the inter-ring single bonds. The total polaron reorganization energy (λ h + λ e ) decreases from 573 to 463 meV upon π-extension, predicting a global reduction of the energetic cost of bipolar charge injection. Photostability measurements on non-optimized bulk-heterojunction devices reveal that BT-TP-TPA retains approximately 78% of its initial PCE after 300 min under AM 1.5G illumination, compared to approximately 43% for BT-TPA and approximately 37% for the PTB7 reference, in line with the lower total polaron reorganization energy of BT-TP-TPA (λ total = 463 meV vs. 573 meV for BT-TPA). Although device photostability is influenced by multiple factors, including morphology and interfacial effects, this trend suggests a possible correlation between reduced electron reorganization energy and enhanced operational stability. Collectively, these results establish BT-TPA and BT-TP-TPA as theoretically well-characterized model platforms linking molecular architecture, polaronic relaxation, and photostability in organic semiconductors.