Cocrystals versus Salts of Fluorescein

Mihai Răducă,Augustin M Mădălan

doi:10.3390/cryst11101217

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.

Highlights

Fluorescein is a xanthene derivative widely used as platform for designing various chromogenic or fluorogenic probes [1]
Fluorescein derivatives can serve as fluorescent chelators for Zn(II) [2,3], Cu(II) [4], Co(II), Ni(II) [5], Hg(II) or Ag(I) [6] metal ions
The chromogenic mechanism of fluorescein is based on protonation–deprotonation reactions

Summary

Introduction

Fluorescein is a xanthene derivative widely used as platform for designing various chromogenic or fluorogenic probes [1]. As biological applications of the fluorescein derivatives we can mention detection of nitric oxide production in some tissues [7], detection of homocysteine and cysteine [8], detection of hypochlorite [9], study of serotonin receptors [10], enzymes [11] and live bacteria detection [12]. Due to the biological applications of the fluorescein dyes, solution studies have attracted particular attention. Depending on pH, in solution can be identified cationic (H3 Fl+ ), neutral (H2 Fl) or anionic species (HFl− and Fl2− ). The neutral form (H2 Fl) presents in solution three tautomers: zwitterion (H2 Flz), quinonoid (H2 Flq) and lactone (H2 Fll). For the monoanionic form (HFl− ), the phenolate tautomer appears in small quantities only in pure solvents such as DMSO, acetonitrile or acetone [13]

Methods

Results

Conclusion