A new optical nickel(II) cyclen-naphthalene probe for molecular thermometry using fluorescence and NMR techniques.

A new Ni(II) complex was prepared and used as a luminescent molecular thermometer. Fluorescence intensity and 13C NMR chemical shift results showed excellent linear correlations with temperature from 10 to 60 °C in acetonitrile solutions. [Display omitted] A new luminescent molecular thermometer (LMT) was prepared, characterized, and investigated. The sensor works based on the temperature effect on the spin-crossover equilibrium between two spin states of a nickel-cyclen derivative (cyclen = 1,4,7,10-tetraazacyclododecane) containing a naphthalene fragment as the emitting unit. DFT and TD– DFT calculations revealed that photoelectron transfer (PET) can partially quenches the fluorescence of the naphthalene fragment to varying degrees in both high- and low-spin states of the sensor. UV–visible spectra in coordinating solvents such as acetonitrile revealed that 80% of the predominant species adopt five-coordination in the triplet state ([Ni(cycna)(CH 3 CN)]2+) at 20 °C, while 20 % of the molecules are square planar in the singlet spin state. The fluorescence spectra showed excellent correlation of luminescence intensity with temperature (R2 = 1.00) from 10 to 70 °C, proving that the coordination compound can function as LMT. [ABSTRACT FROM AUTHOR]