Conjugated prototropic and ring opening rearrangements in Schiff base derivatives of formyl functionalized 2-oxaindane series spiropyran: synthesis, NMR, IR, UV/Vis, and DFT study

Abstract

Condensation reaction products of formyl derivative of the 2-oxaindane series spiropyran (7-hydroxy-3′,3′-dimethyl-3’H-spiro [chromene-2,1′-isobenzofuran]-8-carbaldehyde) with several 2-amino-4-R-phenols (R = H, tBu, Сl, NO2) were synthesized and studied with the help of IR, UV/Vis, and NMR spectroscopy. It was shown that in contrast with mother spiropyran existing in closed spiro-form, all the compounds in the solution (DMSO, acetonitrile) and in solid state exist in thermodynamically stable merocyanine keto-enamine tautomeric form due to migration of the proton of 7-hydroxy group to azomethine nitrogen, conjugated with electrocyclic opening of the spirounit in the non-stable Schiff bases. In DMSO solution of merocyanine keto-enamines (R = H, tBu, Сl), dynamic equilibrium of Z,E-isomers in respect to C=C-N exocyclic double bond is observed (of which Z-one is the major component); in case of R = NO2, two additional isomers are registered. Upon protonation, strong absorption bands with maxima at the 580–590-nm spectral region are developed, while second protonation leads to hypsochromic shift of the longest wavelength absorption band to ca. 480 nm. Experimental results are supported with the DFT quantum chemical modeling of possible isomers of the obtained substances and their spectral properties (B3LYP/6-311++G(d,p) level of theory for geometry optimizations and spectra modeling).