Formation of pi-pi complexes between phenanthrene and model pi-acceptor humic subunits

Pi-pi interactions may play a role in association of aromatic compounds with natural organic substances. Complexation in aqueous solvents was studied between the pi donor, phenanthrene (PHEN), and model pi-acceptor species (quinones and N-heteroaromatic cations) that represent certain functional units of humic substances. Charge-transfer bands in the UV and ring-current shifts in the proton nuclear magnetic resonance (NMR) spectrum confirmed the face-to-face, pi-pi donor-acceptor nature of the bond. Complexation constants were obtained by the solubility enhancement method; solubility enhancements up to 2500 were found. Ruled out as predominant causes of solubility enhancement were monomer desolvation (i.e., "hydrophobic" effects), partitioning into micelles, pi-cation interactions, and pi-hydrogen bonding. Acceptor self-stacking and formation of higher-stoichiometry acceptor-donor complexes had to be considered in evaluating donor-acceptor equilibria in some cases. The affinity of acceptor for PHEN followed the order of increasing pi-acceptor strength and varied strongly with the degree of ring overlap with PHEN. Complexation between PHEN and the free solution faces of an acceptor was less favorable than intercalation of PHEN between two acceptor units in a stack. A positive hydrophobic effect on complexation was evident in water mixtures with acetone or methanol and found to correlate with the number of faces of PHEN requiring desolvation to form the complex. When hydrophobic effects are subtracted out, the pi-pi complex actually becomes favored as the solvent water content and polarity decline. The results suggest that phenanthrene, and by implication other donor aromatic compounds, are capable of forming pi-pi interactions with appropriate humic fragments.