A workflow for screening and manufacturing co-crystals was used to produce 4:1 co-crystal of cytosine-oxalic acid dihydrate, 2:1 co-crystal of cytosine-malonic acid, and 2:1 co-crystal of cytosine-succinic acid with melting points of 270.2, 219.2, and 249.5 degrees C, and K(sp) values of 8.21 x 10(-12) m(5), 4.90 x 10(-5) m(3), and 3.11 x 10(-6) m(3) in water at 25 degrees C, respectively. 4:1 co-crystal of cytosine-oxalic acid dihydrate crystallized in the P2(1)/c space group. Both 2:1 co-crystal of cytosine-malonic acid and 2:1 co-crystal of cytosine-succinic acid crystallized in the P (1) over bar space group. Co-crystals of cytosine with dicarboxylic acids all possessed it self-assembled cyclic pattern of R(2)(2)(12) forming by a pair of N-H center dot center dot center dot O hydrogen bonds within a dimeric motif of cytosine. The self-recognition pairing of cytosine here resembled the standard hydrogen bonding pattern in the Watson-Crick complementary base pairing of guanine and cytosine but without the middle N-H center dot center dot center dot N interaction. Both photoluminescence (PL) emission intensity and the solubility of co-crystals at a given temperature followed the same descending order of 2:1 co-crystal of cytosine-malonic acid > 2:1 co-crystal of cytosine-succinic acid > 4:1 co-crystal of cytosine-oxalic acid dihydrate. The weakening of the hydrogen bonding strength of 2:1 co-crystal of cytosine-malonic acid due to the angling of the interlayers might explain its lowest melting point of 219.2 degrees C, its highest solubility and its highest PL emission intensity.
