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

Adsorption of squaric acid (SQH(2)) (pKas 1.4 and 3.5) in basic NaY, CsY an d NaMor zeolites results in the coloration of the solids and the appearance of a broad band (lambda(max) 470 nm) that is not observed in acid or basic aqueous solutions of SQH(2). This band in the visible does not appear when SQH(2) is incorporated in acid HMor and HY zeolites. FT-LR shows that the species responsible for the color is ionized squarate. Analogously, the cha rge-transfer complex of SQH(2) and bipyridine (BPY) has been characterized within zeolites. This complex has only been previously observed in SQH(2)/B PY single crystals. (C) 1999 Elsevier Science B.V. All rights reserved.