Spectroscopic characterization of aggregates of squaric acid and squaric acid/bipyridine incorporated within basic zeolites

Adsorption of squaric acid (SQH2) (pKas 1.4 and 3.5) in basic NaY, CsY and NaMor zeolites results in the coloration of the solids and the appearance of a broad band (λmax 470 nm) that is not observed in acid or basic aqueous solutions of SQH2. This band in the visible does not appear when SQH2 is incorporated in acid HMor and HY zeolites. FT-IR shows that the species responsible for the color is ionized squarate. Analogously, the charge-transfer complex of SQH2 and bipyridine (BPY) has been characterized within zeolites. This complex has only been previously observed in SQH2/BPY single crystals.