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The optical and resonance Raman spectra (RR) of bis(phloroglucinol)squaraine (I) and bis(resorcinol)squaraine (II) were investigated, revealing that the equilibria between the neutral and anionic species present in solution explain the experimental observations much better than the previously proposed aggregation equilibria. In particular, the RR spectra were invaluable to assign characteristic modes of (I) and its monoanion, since the tuning of the excitation line to the electronic transition band of the neutral or monoanionic specie leads to the enhancement of the respective modes that are exclusive of each species. In addition, small angle X-ray scattering (SAXS) experiments with (I) did not show any evidence of aggregation.

The thiourea/squarate/tetraethylammonium adduct and the thiourea/croconate adduct were investigated by Raman spectroscopy. The former had been previously investigated by X-ray crystallography which revealed an extended network built from squarate and thiourea, connected by hydrogen bonding. The assignment of the vibrational modes in the thiourea/squarate adduct was facilitated by the preparation of the species containing deuterated thiourea. The vibrational shifts observed in the spectrum of the adducts indicate that the involved hydrogen bonds are only of moderate strength. In addition, the well-known Jahn-Teller pattern present in the Raman spectrum of oxocarbon anions is also observed to a great extent in the spectra of their thiourea adducts.

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