Synergy i

The Na salt of dye 3 was obtained from Fujifilm. The other dyes were synthesized as their Na salts using the well-known azo-coupling method (Alsantali et al., 2022 ▸ ; Kennedy et al., 2001 ▸ ). Na salts were converted to NH₄ or K salts by reaction with a slight excess of either NH₄Cl or KCl in warm water. Solutions were filtered to give clear aqueous solutions and then allowed to evaporate for 2 to 7 d. This gave yellow or yellow–orange crystals of the desired salt forms that were suitable for single-crystal diffraction analysis. FT–IR spectra were measured as KBr discs using a Nicolet Avatar 380 spectrometer. Raman data were measured from solids using a Reinshaw Ramascope with excitation at 785 nm. Aqueous UV–Vis spectra were measured using a Cary 300 Bio spectrophotometer and solid-state UV–Vis spectra were measured using a Phillips PU8749 spectrophotometer. Measurements on K1 were made at Station 9.8 of the Daresbury SRS. All other measurements were made using standard Rigaku Synergy-i (NH₄4) or Enraf–Nonius KappaCCD (all other structures) laboratory diffractometers equipped with CCD detectors.