Cocrystals versus Salts of Fluorescein

Abstract

A series of nitrogen-containing organic molecules (4,4’-bipyridyl; trans-1,2-bis(4-pyridyl)ethylene; 1,2-bis(4-pyridyl)ethane; 4-aminopyridine and trans-1,4-diaminocyclohexane) was envisaged for cocrystallization experiments together with fluorescein. These compounds, containing pyridyl or/and amino nitrogen atoms, can act either as hydrogen bond acceptors for the phenol groups of fluorescein-generating cocrystals or as proton acceptors forming organic salts. Five cocrystals were obtained with the partners containing only pyridyl groups: {(H2Fl)2(bipy)} (1); {(H2Fl)2(bipy)(MeOH)2} (2); {(H2Fl)2(bpete)(EtOH)2} (3); {(H2Fl)(bpete)} (4); {(H2Fl)(bpeta)} (5). The compounds bearing amino groups deprotonate fluorescein producing salts: [(HFl)(Hampy)]∙2H2O (6); [(HFl)(Hampy)] (7); [(Fl)(H2diach)]∙3H2O (8); [(HFl)2(H2diach)]∙2H2O∙EtOH (9); and [(HFl)2(Fl)2(H2diach)3]∙4H2O (10). Optical properties of the cocrystals and salts were investigated.