Boronic acid sensors for saccharides: A theoretical study Ioannis D. Petsalakis ⇑ , Giannoula Theodorakopoulos Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., Athens 116 35, Greece article info Article history: Received 15 July 2013 In final form 8 September 2013 Available online 19 September 2013 abstract Selective detection of saccharides by fluorescent boronic acid sensors has been the object of active research over the last two decades with numerous experimental reports published. A theoretical study is presented here on pyrene- and anthracene-boronic acid systems and their fluorescent sensing of D-glucose, employing Density Functional Theory and Time-Dependent Density Functional Theory. The difficulties encountered by straight-forward computational approaches are described while it is shown that it is possible to obtain a physically correct description of the photoinduced electron transfer in these systems from diagrams of the molecular orbital energies of the separate donor and acceptor moieties. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Detection of the presence and concentration of biologically important sugars in aqueous solution is necessary for medicinal and industrial purposes [1]. In the past two decades boronic acid-based chemosensors of saccharides have been examined as alternatives to enzymatic detection methods, given that boronic acids react with 1,2 or 1,3 diols of saccharides to form five- or six-membered cyclic esters in basic aqueous media [1,2]. Fluores- cent PET (photoinduced electron transfer) sensors of saccharides based on boronic acids have been designed and in particular since 1994 [3,4] ‘on–off’ fluorescent sensors containing a boronic acid and an amine group have been reported in many variations, in over 220 publications on the subject to date. The basic module of the design of these sensors involves a fluorophore linked by a short spacer to a tertiary amine which is also linked to a phenyl-boronic acid, as for example in structure I with a pyrene fluorophore [5,6]. In (I) there is suppression of fluorescence presumably due to pho- toinduced electron transfer (PET) from nitrogen to pyrene. However, significant enhancement of fluores- cence is observed upon reaction of the boronic acid with 1,2 diols of saccharides to form complex (II), which has been proposed to in- volve donation of electronic charge to boron from nitrogen, [4], thus inhibiting any PET process. Theoretical investigations on model systems have been devoted mainly to the elucidation of the interaction between B and N in these sensors [7–10] and determination of the most stable con- former of the sensors and as shown in (I) structures involving intramolecular B–O–HÁÁÁN interaction are found to be generally lower in energy than those involving a B–N bond [7–9]. Inference on the photoinduced electron transfer capabilities and fluores- cence properties of protonated and neutral sensor systems has been made on the basis of electron-density plots of the highest occupied (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of protonated and neutral sensors molecules [11]. Charge-transfer processes pose a great challenge to theoretical treatments. This is especially true for calculations on actual systems, rather than models, of size for which at best Density 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.09.025 ⇑ Corresponding author. Fax: +30 210 7273 794. E-mail address: idpet@eie.gr (I.D. Petsalakis). Chemical Physics Letters 586 (2013) 111–115 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett