Rationalization of the Color Properties of Fluorescein in the Solid State: A Combined Computational and Experimental Study.

Mihails Arhangelskis,Dejan-Krešimir Bučar,William Jones,Mark D Eddleston,Graeme M Day,David G Reid,Andrew J Morris

doi:10.1002/chem.201601340

Abstract

Fluorescein is known to exist in three tautomeric forms defined as quinoid, zwitterionic, and lactoid. In the solid state, the quinoid and zwitterionic forms give rise to red and yellow materials, respectively. The lactoid form has not been crystallized pure, although its cocrystal and solvate forms exhibit colors ranging from yellow to green. An explanation for the observed colors of the crystals is found using a combination of UV/Vis spectroscopy and plane‐wave DFT calculations. The role of cocrystal coformers in modifying crystal color is also established. Several new crystal structures are determined using a combination of X‐ray and electron diffraction, solid‐state NMR spectroscopy, and crystal structure prediction (CSP). The protocol presented herein may be used to predict color properties of materials prior to their synthesis.

Highlights

Organic pigments are of great importance to society
We describe how recent developments in both experimental and computational methods can be utilized to characterize the optical solid-state properties of the model pigment fluorescein
To elucidate the crystal structure we have studied the crystal energy landscape of possible flsZ structures using the crystal structure prediction (CSP) methods described previously.[36,37,38]

Summary

Introduction

Organic pigments are of great importance to society. They are used in the production of photovoltaic materials,[1] optical data storage devices,[2] and in the coloration of plastics[3] and in the textile industry. The resulting “free” flsL molecules convert into the yellow flsZ form, generating a colored layer on the surface of the crystals. It was considered instructive to establish whether the mechanism of color generation in the cocrystals of flsL is the same as in the solvates (namely, through the formation of the flsZ surface layer), or whether the presence of another molecule (i.e. cocrystal former) in the crystal lattice plays an active role in the optical properties of the resulting multicomponent form.

Results

Conclusion