Unlocking the potential of Indolo-Carbazole Derivatives: first-principles insights into charge injection and optical switching applications

Abstract

This study systematically investigates eight newly designed donor-π-acceptor (D-π-A) chromophores derived from a 3,8-dimethyl-11H-benzo[4,5]thieno[3,2-b]benzo[4,5]thieno[2,3-d]pyrrole core coupled with a 2-cyanoacrylic acid acceptor, where seven structural variants were engineered to modulate optoelectronic properties. Comprehensive theoretical analyses, including frontier molecular orbital (FMO), natural bond orbital (NBO), density of states (DOS), electrostatic potential surface (ESP), nonlinear optical (NLO) response, light harvesting efficiency (LHE), and charge injection barrier (ϕ_h/ϕ_e) calculations, reveal distinct functional behaviors: four variants (SD1−SD4) exhibit donor-dominant characteristics while three (SD5−SD7) demonstrate ambipolar/acceptor properties. Detailed charge injection analysis identifies SD2 as an exceptional hole transporter (ϕ_h = 0.09 eV with ITO) and SD5 as an efficient electron injector (ϕ_e = 1.28 eV with Al), demonstrating how molecular engineering controls charge transport mechanisms. Donor-dominant variants (SD1−SD4) feature narrowed bandgaps (2.12–2.43 eV) and high-lying HOMOs (−4.79 to −5.13 eV), enabling efficient hole injection, while ambipolar/acceptor groups (SD5−SD7) exhibit enhanced electron affinity via lowered LUMOs (−3.02 to −3.14 eV). SD2 is optimal, with the highest hyperpolarizability (34,680 a.u.), efficient charge transfer (λ_h = 0.226 eV), and long radiative lifetime (12.54 ns). Key findings include: (1) reduced HOMO-LUMO gaps correlate with red-shifted absorption (up to 395 nm) and improved light harvesting (η up to 0.91); (2) D-π-A charge separation enhances nonlinear responses (β_x > 10⁹ a.u.); and (3) ambipolar variants achieve low reorganization energies (<0.3 eV) for balanced charge transport. The two-level model (TLM) (β_CT ∝ (f₀·Δμ)/ΔE³, R² > 0.95) confirms that molecular tuning selectively enhances hole/electron injection while preserving nonlinear performance. This work provides a design framework for multifunctional materials, where donor/acceptor spatial and energetic control enables tailored optoelectronic functions. SD2/SD4 suit hole-transport layers, SD6 serves as ambipolar channels, and SD5/SD7 act as electron acceptors in photovoltaics, optical switches, or frequency doublers. These insights advance molecular engineering for targeted device applications, balancing charge transport and optical properties.