Composition and thermal stability of bis(dipyrrolylmethenato)zinc(II) crystal solvates with N,N-dimethylformamide

• The zinc(II) bis(dipyrrolylmethenates) [Zn2L2] form the stable [Zn2L2(DMF)2] solvates.
• The crystal solvates of [Zn2L2(DMF)2] are stable in solid phase up to 100 °C.
• Additional coordination of DMF molecules causes the fluorescence quenching of [Zn2L2] dyes.
• Results are of interest for the development of the sensors of the electron donor molecules.

Fluorescence of zinc(II) bis(dipyrrolylmethenates) [Zn2L2] is very sensitive to the medium polarity. Fluorescence quantum yield (φ) of [Zn2L2] reaches 0.99 in nonpolar media and close to zero in the electron donor solvents (amines, alcohols and other). Fluorescence quenching may be caused by the additional coordination of the electron donor molecules on the zinc atoms with formation of stable supramolecular structures [Zn2L2(Solv)n]. For the confirming this assumption, the samples of the crystal solvates of 2,2′-, 2,3′- and 3,3′-bis(dipyrrolylmethenato)zinc(II) with N,N-dimethylformamide were obtained by slow crystallization. Spectroscopic studies showed that the quantum yield of crystal solvates in cyclohexane solutions is noticeably lower than φ for corresponding complexes [Zn2L2]. The thermal dissociation processes of crystal solvates in an argon atmosphere have been investigated. It is shown that helicates of the zinc(II) with decamethylsubstituted 2,2′-, 2,3′- and 3,3′-bis(dipyrrolylmethenes) form with DMF stable supramolecular complexes of the composition [Zn2L2(DMF)2]. Crystal solvates [Zn2L2(DMF)2] are stable up to a temperature below ∼100 °C in an inert argon atmosphere and the values of the enthalpies of their thermal destruction process are 88–143 kJ/mol. The obtained results are of interest for the development on the basis of [Zn2L2] of the fluorescent sensors of the electron donor molecules.