NIR dyes based on [M(R,R ' timdt)(2)] metal-dithiolenes: Additivity of M, R, and R ' contributions to tune the NIR absorption (M = Ni, Pd, Pt; R,R ' timdt = monoreduced form of disubstituted imidazolidine-2,4,5-trithione)

With the aim of preparing new dyes for Q-switching and/or mode-locking Nd-based lasers, such as Nd:YLF and Nd:YAG (emission wavelengths 1053 and 1064 nm, respectively), the syntheses of about 30 new neutral dithiolenemetal complexes belonging to the class [M(R,R′timdt)2] have been carried out [M = Ni (4a−c; 4g−j), Pd (5a−j), Pt (6a−j); R,R′timdt = monoreduced form of disubstituted imidazolidine-2,4,5-trithione]. The examination of the effects induced on the intense NIR absorption by changing M, R, and R′ has allowed for the recognition of the additive contributions (Δλ) of the central metal ion and of the substituents to the λM,R,R′max position of the NIR absorption maximum, which is in the region of 1000 nm. The high Δλ values due to aromatic substituents have been elucidated by means of Hybrid-DFT calculations performed on the model compound [Ni(H2timdt)(Ph,H-timdt)]. Both the λM,R,R′max values and the molar extinction coefficients vary with the solvent. Complex 5f [M = Pd, R = p-F-C6H4, R′ = m,m,p-(MeO)3-C6H2] shows that it has been possible to exactly match the emission wavelength of the Nd:YLF and Nd:YAG lasers in chloroform and toluene, respectively (by choosing the appropriate combination of metal ion and substituents). The highest extinction coefficient ever observed for a metal-dithiolene (120000 M−1·cm−1 at 1034 nm in toluene) is also reported for complex 6g [M = Pt, R = p-F-C6H4, R′ = naphthyl]. Time-resolved absorption dynamics measurements on 5f have also been carried out by means of pump-probe experiments, with a picosecond Nd:YAG Q-switched and mode-locked laser.