Investigation of ultrafast optical nonlinearity in a nickel-dithiolene complex: mechanism of pulse-selective response in different solvents

The ultrafast optical nonlinear of a dithiolene complex of nickel (bis(1,2-diphenyl-1,2-ethenedithiolato-S,S’)-nickel) (1) has been systematically investigated. Results of Z-scan with picosecond and nanosecond pulses show that compound 1 in N,N’-dimethylformamide (DMF) has a selective reverse saturable absorption (RSA), which could be observed with picosecond pulses and would be dramatically suppressed under ns pulses. The pulse-selective response vanishes in dichloromethane (DCM) and toluene solvents, indicating a connection with the decrease of solvent polarity. The ultrafast decay processes of excited state in different solvents are further investigated via femtosecond transient absorption (TA) experiment. The results are studied with global & target analysis and show that the excited state lifetime of 1 is remarkably shortened with the increasing polarity of the solvents, which suppresses the accumulation of excitons on excited-state and result in the substantially decrease of RSA in DMF solvent under nanosecond pulses. Our results are expected to shed light on the mechanism of selectively tuning optical nonlinearity in metal dithiolene complexes on an ultrafast time scale.