Europium 2-benzofuranoate: Synthesis and use for bioimaging V.V. Utochnikova a, b, c, * , D.S. Koshelev b, c , A.V. Medvedko b , A.S. Kalyakina b, d , I.S. Bushmarinov e , A.Yu Grishko b , U. Schepers d , S. Br € ase d , S.Z. Vatsadze b a P. N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow, Russian Federation b Department of Chemistry, M. V. Lomonosov Moscow State University, Moscow, Russian Federation c Faculty of Materials Science, M. V. Lomonosov Moscow State University, Moscow, Russian Federation d Karlsruhe Institute of Technology (KIT), Institute of Toxicology and Genetics, Eggenstein-Leopoldshafen, Germany e A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russian Federation article info Article history: Received 20 December 2016 Received in revised form 16 May 2017 Accepted 21 May 2017 Available online xxx Keywords: Bioimaging Europium complex 2-Benzofuranoate Luminescence HeLa Luminescence thermometry abstract Europium 2-benzofuranoate Eu(BFC) 3 (H 2 O) 3 was successfully used for bioimaging in cellulo due to the combination of high solubility and high luminescence intensity in solution. It was possible due to the purposeful variation of the aromatic core of carboxylate anion. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Unique luminescent properties make lanthanide coordination compounds (CCs) perfect materials for bioimaging due to long lifetimes of the excited state and the high values of the Stokes shift [1e3]. Europium complexes are among the mostly used due to they combine high photoluminescence quantum yields (PLQYs) with the emission in the long wavelength range, which can easily penetrate through the tissues [4e6]. Among these materials, aromatic car- boxylates play important role due to the high PLQY values and high chemical stability [7e12]. However, the crucial drawback is that they usually possess low solubility, particularly in water, due to the polymerization, which is a result of the lanthanide coordination sphere saturation [13e18]. To increase the solubility in water, the ligand fluorination approach was recently used with great success [19e21]. This, however, leads to the ligand's optical absorption decrease, which is important for bright photoluminescence, because its intensity is proportional to I ~ ε  PLQY (ε is absorption coefficient) [17e22]. Also complex dissociation in water was observed due to the high acidity of fluorobenzoic acids. The best approach to increase light absorption by ligand is to increase the size of conjugated system, that is the number of con- jugated double bonds (conjugation degree) [22e31]. So, the molar absorption coefficient increases by three orders of magnitude on gosing from benzene to naphthalene, anthracene, and tetracene [28]. At the same time, it results in significant decrease of the excited state energy [28], which in the case of lanthanide com- plexes leads to the absence of sensitization of lanthanide lumi- nescence by organic ligand if triplet state energy becomes lower than the lanthanide excited state one. To restrain triplet state decrease, aromatic ring constriction and heteroatom introduction can be used. For instance, the triplet state of picolinate (25773 cm 1 ) is higher than those of benzoate (21300 cm 1 ), and 2- furanoate triplet state is even higher (29200 cm 1 ) [32]. Lanthanide complexes with this anion were obtained, and europium 2-benzofuranoate was studied in order to reveal if it answers four basic requirements for the biomarker materials [1,2,33,34], namely: 1) high luminescence intensity, 2) high solu- bility, 3) the absence of dissociation in solution, 4) high absorption. * Corresponding author. P. N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow, Russian Federation. E-mail address: valentina.utochnikova@gmail.com (V.V. Utochnikova). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat http://dx.doi.org/10.1016/j.optmat.2017.05.038 0925-3467/© 2017 Elsevier B.V. All rights reserved. Optical Materials xxx (2017) 1e6 Please cite this article in press as: V.V. Utochnikova, et al., Europium 2-benzofuranoate: Synthesis and use for bioimaging, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.05.038