Supramolecular assembly and spectroscopic characterization of indolenine–barbituric acid zwitterions

Abstract

A series of indolenine and barbituric acid zwitterion scaffolds were synthesized with a maximum yield of 98% via the formation of C–C single bonds. The structures were unambiguously elucidated by various spectroscopic techniques such as ¹H, ¹³C NMR (1D and 2D), FT-IR and high-resolution mass spectrometry (HRMS). Single crystal X-ray crystallography analysis on 22, as the 22.DMF 1 : 1 solvate, confirms the presence of well-separated iminium and enolate centres, and also confirms that the BA ring is highly twisted with respect to the indolenine ring due to steric hindrance. The presence of N–H⋯O⁻ and N–H⋯O⁻ groups favours 1D-supramolecular assembly in the solid-state. The orange or yellow solutions of the zwitterion exhibit an intense molar absorption coefficient, ε, ranging between 0.21 × 10⁴ and 2.93 × 10⁴ M⁻¹ cm⁻¹ in the UV-vis region. Furthermore, the Intramolecular Charge Transfer (ICT) peak of the zwitterion displays a hypsochromic shift in the absorption behavior when the polarity of the solvent increases. Moreover, treatment with a small amount of trifluoroacetic acid (TFA) with the DMF solution of 19 resulted in the protonation of the enolate of the BA ring. This fundamental work provides valuable structural design and information for the construction of supramolecular chemistry and synthetic dyes based on indolenine substituted BA derivatives.