Tuning the Nonlinear Optical Properties of MoS₂ by Interfacing with Bay- or Imide-Substituted Perylene Diimides

We have investigated the tuning of nonlinear optical (NLO) properties in semiconducting MoS₂ nanosheets through covalent functionalization with bay- or imide-substituted perylene diimide (PDI) derivatives. Five MoS₂-PDI hybrid nanomaterials 1a-5a were synthesized by attaching PDI variants featuring azobenzene or other substituents, characterized via Raman, IR, TGA, STEM-EDS, ultraviolet-visible (UV-vis), photoluminescence, and electrochemical analyses, and evaluated for NLO response under nanosecond (4 ns, 1064/532 nm) and femtosecond (70 fs, 800/400 nm) laser excitations. The hybrids exhibit enhanced and tunable NLO-absorptive (switching from saturable to reverse-saturable absorption with intensity) and NLO-refractive (self-focusing) responses compared to exfoliated MoS₂, with 1a and 2a (azobenzene-substituted PDIs) showing the strongest effects due to efficient electron-transfer and -resonant excitations. Hybrids demonstrate superior NLO susceptibility (χ³) values, particularly under resonant 532 nm excitation, with Im{χ³} values of up to -514.7 ± 57.4 × 10^-13 esu·mL/mg for 2a at low intensities, attributed to Pauli blocking, defect states, and two-photon processes. Hybrids having bay-substituted PDIs grafted on MoS₂ enhance conjugation and performance over imide variants, enabling applications in optical-limiting, mode-locking, and photonic devices. Threshold intensities for NLA switching (150-250 MW/cm²) exceed those of MoS₂, highlighting the stability for high-power uses. The developed hybrid materials advance two-dimensional transition-metal dichalcogenide-based optoelectronics by overcoming light-matter interaction limits through molecular antenna effects from PDIs. The study underscores the substituent position and azobenzene integration as levers for NLO optimization in hybrid nanoarchitectures.