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

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Arhangelskis, Mihails  ORCID logo
Eddleston, Mark D 
Reid, David G 
Bučar, Dejan-Krešimir  ORCID logo


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.



density functional calculations, fluorescein, optical spectroscopy, surface analysis, tautomerism

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European Regional Development Fund (ERDF) (via Lille University of Science and Technology) (unknown)
EPSRC (1127438)
Medical Research Council (MR/J007692/1)
Isaac Newton Trust (1238(j))
The Royal Society (nf100747)
M.A. thanks EPSRC for a PhD studentship. M.D.E. acknowledges support from the Interreg V “2 Mers Seas Zeeën” cross-border cooperation program. D.G.R. acknowledges financial support from the MRC. D.K.B. acknowledges University College London for an UCL Excellence Fellowship. A.J.M. acknowledges the support from the Winton Program for the Physics of Sustainability. G.M.D. thanks the Royal Society for funding. This work was performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (, provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council.