Shared-reference analysis of pathway-dependent excited-state responses in donor-side and terminal-side substituted...

Distinct substitution pathways within a conjugated chromophore may produce comparable changes in conventional energetic or spectral descriptors without necessarily producing comparable underlying excited-state electronic responses. Here, donor-side and terminal-side substitution pathways in a phenyl-thiophene framework were evaluated independently relative to the same molecular reference using a common gas-phase DFT/TD-DFT protocol. Ground-state structures and frontier orbitals were obtained at the B3LYP/6-311++G(d,p) level, while vertical singlet excitations were evaluated using TD-CAM-B3LYP/6-311++G(d,p). The lowest singlet state, S₁, and the state with the largest oscillator strength within the common S₁-S₁₀ manifold, S_bright, were treated separately and analyzed using canonical orbital contributions, natural transition orbitals, quantitative real-space hole-electron descriptors, and interfragment charge-transfer (IFCT) analysis. Because the two pathways contain chemically different substituent sets, the comparison is interpreted in terms of pathway-dependent responses rather than as an independent separation of substitution-position and substituent-identity effects. The two predefined substitution pathways exhibit distinct electronic and excited-state response patterns within the examined molecular set. In the donor-side series, D0H and D1-OCH₃ exhibit S₁ ≠ S_bright, with the lowest and brightest states showing distinct hole-electron organization and qualitatively different fragment-resolved redistribution. In contrast, D2-NH₂ and the shared reference J retain S₁ = S_bright. All terminal-side derivatives preserve S₁ = S_bright within the calculated S₁-S₁₀ manifold while exhibiting substituent-dependent spatial and fragment-resolved responses. T2-NO₂ is the only terminal-side derivative to show a calculated red shift relative to J, with its S₁ excitation energy lower by 0.310 eV, together with the largest hole-electron centroid distance among the terminal-side derivatives and pronounced redistribution toward its terminal NO₂ region. Targeted TD-ωB97X-D calculations for J, D1-OCH₃, and T2-NO₂ preserve the principal state assignments and the relative energetic directions examined for D1-OCH₃ and T2-NO₂, while showing functional sensitivity in the absolute excitation properties. Together, the NTO, real-space, and IFCT analyses show that transition-density pattern, spatial hole-electron organization, and interfragment redistribution provide complementary information and should not be treated as interchangeable descriptors. The shared-reference, state-resolved comparison therefore identifies pathway-dependent electronic responses within the investigated molecular set and shows that similarity in energetic or spectral descriptors alone does not establish similarity in the underlying excited-state electronic organization.