*2.3. Single-Crystal X-ray Diffraction*

The single-crystal X-ray diffraction data for DPA and DPA\_PA salt were collected at 93 K. The measurements were carried out in ω-scan mode with an R-AXIS RAPID II (Rigaku Co., Tokyo, Japan) using the Cu-Kα X-ray obtained from rotating the anode source with a graphite monochromator. The integrated and scaled data were empirically corrected for absorption effects using ABSCOR [50,51]. The initial structure was solved using the direct method with SIR 2004 and refined on *F* <sup>2</sup> with SHELXL 2014 [52,53]. All non-hydrogen atoms were refined anisotropically. The hydrogen atom attached to the nitrogen N2 and N5 atom in pure DPA and the hydrogen atom attached to the nitrogen N2 as well as hydrogen H5A present in between the O3 and O5 oxygen in the DPA\_PA salt were located using the differential Fourier map and refined isotropically. All other hydrogen atom positions were calculated geometrically and included in the calculation using the riding atom model; the calculations were performed for all the hydrogen atoms. Moreover, in the DPA\_PA salt, protonated DPA molecules display disordered structure in which two molecules with opposite configuration share the same site. Their occupancies were fixed to 0.5.

The molecular graphics were produced using Mercury 4.1.0 software [54]. CCDC 2065231 contains the supplementary crystallographic data for the DPA and CCDC 2065230 contains the supplementary crystallographic data for the DPA\_PA salt, and can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data\_request/cif (accessed on 2 April 2021).
