A comprehensive computational evaluation of substituent effects on the biradical character and photo-physical properties of bicyclic fused heterocyclic cores.

Biradicaloid organic chromophores with open-shell character are promising candidates for near-infrared (NIR) and nonlinear optical (NLO) applications. In this work, bis-substituted quinoxaline (QX), benzo[c]thiadiazole (BT), thieno[3,4-c]thiadiazole (TT), thiadiazolo[3,4-d]azaborinine (ABT), and azaborinino[5,4-e]triazaborinine (ABTB) fused cores with donor/acceptor substitutions of thiophene (Th), N,N-dimethylthiophen-2-amine (NMe2-Th), oxadiazole (Oxad) and 2-(trifluoromethyl)-1,3,4-oxadiazole (CF3-Oxad) were systematically investigated using density functional theory. It was found that HOMO destabilization through donor substitution is more effective than LUMO stabilization through acceptor incorporation, which on further analysis indicates that donor substitution is a major modulator for enhanced photophysical properties. Donor substitution, particularly with NMe2-Th, significantly reduced the HOMO-LUMO gap (HLG) and enhanced the biradical character (BRC), reaching >50% in TT- and ABTB-based systems. The enhanced open-shell character induced by donor substitution was independently verified through reduced singlet-triplet energy gaps (ΔEST) obtained from spin-flip time dependent density functional theory (TDDFT) calculations together with increased odd electron density distributions. Strong bathochromic shifts were observed in the one-photon absorption (OPA) spectra, with NMe2-Th-ABTB exhibiting intense NIR absorption at 973 nm. The static second hyperpolarizability (γ) showed remarkable enhancement at moderate BRC, consistent with established biradicaloid NLO theory. Two-photon absorption cross sections varied over several orders of magnitude, with NMe2-Th-ABTB and NMe2-Th-TT displaying exceptionally high values, identifying them as promising candidates for NIR bioimaging and optical limiting applications. To achieve adequate BRC and enhanced NLO response, this study highlights the need for strong donor substitution, electron-deficient fused cores with biradical resonance forms, and a planar conjugated backbone that facilitates efficient charge transfer.