Metal complexes of the merocyanine form of nitrobenzospyran: Structure, optical spectra, stability

The reversible conversion of the photochromic compound, 1′,3′,3′-trimethyl-6-nitro-1′,3′-dihydrospiro[2H-1-benzopyran-2,2′-2′H  -indole] into its merocyanine form by ring-opening, SP ⇌⇌ MC and the metal ion binding by the MC isomer in a coupled reaction, Me + MC ⇌⇌ Me·MC, with Me = Li+, Mg2+, Ca2+, Ba2+ have been studied in acetone, by optical and NMR spectroscopy and by DFT quantum chemical calculations. The absorption coefficient of the pure MC form has been measured, facilitating the determination of the equilibrium constants. From the four complexes Mg·MC has been found the most stable, with an association constant of 5.9 × 103 M−1.The association of the metal ions is accompanied by a hypsochromic shift of the absorption and fluorescence bands of the MC form, and by an increase of the fluorescence lifetime and quantum yield. The fluorescence decay curves of the complexes are biexponential suggesting excited state dissociation. The spectroscopic experiments as well as the DFT calculations indicate that the lowest energy conformer of the MC ‘monodentate’ ligand is a TTT (full trans) structure in the complexes.

► The merocyanine form of 6-nitro-BIPS associates metal ions, as a monodentate ligand.
► The stability of these metal complexes vary in order Mg > Ca ≫ Ba ∼ Li.
► These complexes are fluorescent with biexponential fluorescence decay.
► The merocyanine ligand in these complexes is a TTT (‘all trans’) isomer.