High Pressure Spectroscopic Investigation on Proton Transfer in Squaric Acid and 4,4′-Bipyridine Co-crystal

In attempt to the obtain detailed geometric information of proton transfer compound (subsequently denote as SQBP) formed between squaric acid (SQ) and 4,4'-bipyridine(BP), and to investigate the mechanisms of pressure-induced double proton transfer and related structural phase transition, we carried out in-situ high pressure Raman spectroscopy of SQBP up to 20 GPa. A solid-solid phase transition together with double proton transfer phenomenon was confirmed by Raman spectroscopy at about 1.5 GPa, and the activation of C = O stretching mode in Raman spectra indicates a squarering structure of SQ with four symmetric C = O bond formation. These results are further supported by first-principals calculations and in-situ high pressure infrared absorption spectroscopy. Additionally, Raman intensity analysis suggests that a higher-order phase transition with planar BP molecular structure occurred in the pressure range of 3 similar to 6 GPa. As a result, the pi electron delocalization effect in BP dominated the intensity enhancement of C = O stretching mode in SQ. To the best of our knowledge, this is the first time observation of the intensity enhancement of proton donor's normal modes induced by proton acceptor's pi electron delocalization.