A new series of nickel N,N'-dialkylimidazolidine-2,4,5-trithione complexes ([Ni(R(2)timdt)(2)]) carrying linear or branched carbon chains (R = n-C4H9, n-C8H17, n-C12H25, n-C16H33, 2-ethylhexyl, 2-butyloctyl, phytanyl) have been obtained through an original synthetic pathway starting from easily available imidazolium-based ionic liquids. The chemical structures of these nickel complexes were confirmed by NMR and high-resolution mass spectroscopies as well as by X-ray analysis performed on single crystals. The complexes strongly absorb in the near-IR (NIR) region around 1000 nm, with high extinction coefficients reaching 80 000 M-1 cm(-1). They are able to efficiently convert NIR light into heat under laser irradiation with very high photothermal conversion efficiencies (around 30%).