Theoretical study on tertiary amine-fluorophore photoinduced electron transfer (PET) systems Ioannis D. Petsalakis, Nektarios N. Lathiotakis, Giannoula Theodorakopoulos * Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116 35, Greece article info Article history: Received 9 June 2008 Accepted 22 July 2008 Available online 31 July 2008 Keywords: Fluorescent sensors Photoinduced electron transfer Density functional theory Time-dependent density functional theory abstract A series of tertiary amine-fluorophore systems, have been investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. These systems have been proposed as fluorescent sensors for organophosphorus (OP) nerve agent mimics [J. Am. Chem. Soc. 2006, 128, 4500]. The different fluorophores considered are pyrene, coumarin, peryline and coronene. Excitation energies have been determined at the ground state DFT-optimum geometry, with and without the presence of a solvent, yielding information on the absorption spectra of these systems as well as the type of excitation contribut- ing to the relevant excited states. In all these systems, in the initial form, excitations are found characterized by a transfer of electron from the amine to the fluorophore, consistent with the observed quenching of fluo- rescence. This type of excitations are absent in the corresponding quaternary ammonium systems, resulting from reaction with an OP nerve agent mimic, which is also consistent with the observed fluorescence. A sim- ple rule is proposed for the prediction of the strength of the PET process, based on the degeneracy of the HOMO orbitals calculated for the separate constituent molecules of a sensor system. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Chemical warfare agents and in particular nerve agents are some of the most dangerous weapons of mass destruction [1]. It is thus of vital interest to develop visual detection methods, based on molecular sensors that indicate the presence of such agents through changes in their fluorescence properties. Recently, a series of systems which might serve as PET sensors for organophosphorus (OP) nerve agent mimics have been proposed and investigated [2]. These systems combine a primary hydroxyl group in close proxim- ity to a tertiary amine, appended with a fluorophore. Fluorescence is generally quenched in the initial system presumably through a photoinduced electron transfer from the amine to the fluorophore. It was found that upon reaction of these compounds with diethyl chlorophosphate (DCP), the corresponding quaternary ammonium salt was produced, with an accompanying increase in fluorescence intensity, by different factors, depending on the particular system [2 and supporting information]. Molecular PET sensors find use in many applications with new systems being reported constantly [3], and it would be of great interest if theoretical information could be obtained on their elec- tronic structure as well as on the photoinduced charge-transfer process. The systems considered here are too large to study with ab initio configuration interaction methods, which would be appro- priate for calculations on excited electronic states and in particular electron transfer (or charge-transfer) states. However, some useful information on the ground state properties, as well as vertical exci- tation energies and type of excitation involved can be obtained by DFT [4] and TDDFT [5] calculations, respectively. In a previous pub- lication by the present authors DFT and TDDFT calculations on a PET sensor for dicarboxylic acids were reported, where changes in the intensity of the absorption spectra as well as proton NMR shifts were related to the selective recognition of the different car- boxylic acids [6]. In this work, the fluorescent sensor systems of Dale and Rebek [2] mentioned above, have been investigated by DFT and TDDFT calculations, in an effort to obtain information on the electronic structure of these systems and in particular to examine to what ex- tent theoretical information may be used to rationalize the exper- imental findings and offer predictions regarding the construction of possible PET systems. The calculation of excited states of charge-transfer character is not an easy task, and TDDFT has been often considered as having particular inherent difficulties with such states [7]. However, as already mentioned such methods are realistic alternative methods for systems of this size, at least as a starting point for a theoretical description of photoinduced charge-transfer. 2. Calculations and results DFT and TDDFT calculations have been carried out on the fluo- rescent sensor systems presented by Dale and Rebek [2]. In struc- tures I below, compounds 1 and 1+ of [2] are indicated as Ia and Ib, 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.07.025 * Corresponding author. Tel.: +30 210 7273 800; fax: +30 210 7273 794. E-mail address: ithe@eie.gr (G. Theodorakopoulos). Journal of Molecular Structure: THEOCHEM 867 (2008) 64–70 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem