Rhodanine-modified thiophene oligomers: Tuning structure for enhanced nonlinear optical responses

Rhodanine-based molecules are known for their strong nonlinear optical (NLO) properties. In this work, two new donor–acceptor thiophene oligomers, RO1 and RO2 , were synthesized using Stille coupling. The design comprises of thiophene and thieno[3,2- b ] thiophene as donor units, and a 3- ethylrhodanine substituted thiophene (3-ERT) as the acceptor, allowing tunable photophysical and nonlinear optical (NLO) properties. Both oligomers showed excellent solubility, thermal stability, and broad UV–Vis absorption (300–600 nm). Positive solvatochromism indicated strong intramolecular charge transfer. SEM analysis revealed distinct morphologies of the synthesized oligomers. The NLO characteristics of RO1 and RO2 integrating 3-ERT acceptor group were examined. Second-order NLO performance was assessed by the help of Hyper-Rayleigh Scattering (HRS ), a single-photon excitation method that produces emission at twice the input frequency. The obtained first hyperpolarizabilities (β) for oligomers RO1 and RO2 246 × 10 −30 esu and 372 × 10 −30 esu. Respectively and both are superior than reference material p-nitroaniline (pNA) in chloroform solvent at 1064 nm, validating robust SHG activity. Third-order NLO characteristics were investigated by means of open-aperture Z-scan method with 532 nm, 5 ns laser pulses , showing substantial three-photon absorption (3PA) and optical limiting properties. The combination of thiophene with 3-ethylrhodanine moiety improves intramolecular charge transfer, promoting superior NLO response of these new oligomers.