Spectrochimica Acta Part A 81 (2011) 560–569 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy jou rn al hom epa ge: www.elsevier.com/locate/saa Experimental and theoretical study of the interaction of 3-carboxy-5,6-benzocoumarin with some 1,2,3,4,5,6,7,8-octahydroacridines and the corresponding N-oxides Maria Marinescu a , Cristina Tablet b , Francisc Potmischil a , Mihaela Hillebrand b,∗ a University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Bd. Panduri 90-92, Bucharest, Romania b University of Bucharest, Faculty of Chemistry, Department of Physical Chemistry, Bd. Regina Elisabeta 4-12, 0330018, Bucharest, Romania a r t i c l e i n f o Article history: Received 10 December 2010 Received in revised form 16 June 2011 Accepted 20 June 2011 Keywords: Coumarin derivatives 1,2,3,4,5,6,7,8-Octahydroacridines Static and dynamic quenching Photoinduced electron transfer DFT calculations a b s t r a c t The interaction of 3-carboxy-5,6-benzocoumarin (BzCum) with 1,2,3,4,5,6,7,8-octahydroacridine (OHA), 9-amino-1,2,3,4,5,6,7,8-octahydroacridine (H 2 N-OHA) and the corresponding N-oxides (OHA-NO and H 2 N-OHA-NO) was studied by fluorescence (steady state, time resolved) and absorption spectroscopy. The analysis of the fluorescence data in terms of Stern–Volmer plots indicated a predominant dynamic quenching for OHA and OHA-NO, and a more complex process for H 2 N-OHA and H 2 N-OHA-NO. The dynamic process was assigned to a photoinduced electron transfer (PET) from the acridine derivatives to the excited state of BzCum. The application of the Rehm–Weller–Marcus theory leads to a good agreement with literature data and allows for the estimation of the solvent organization energy. The presence of the PET mechanism was also supported by DFT results. The absorption spectra evidence the formation of a ground state complex assigned to a hydrogen bond complex involving the carboxylic hydrogen of BzCum. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Fluorescence spectroscopy represents one of the most pow- erful methods for the investigation of molecular interactions in different conditions and between a large series of partners [1]. Sev- eral classes of compounds were used as fluorescence probes and the drug–protein interactions were mostly studied on the same bases. The main problem remains the elucidation of the mechanism for the experimentally observed effects, either a quenching or an enhancing of the emission. Depending on the mechanism, several molecular parameters (kinetic, thermodynamic) of the interaction process can be obtained as well as information on the changes in the local polarity around the fluorophores [2–4]. During the last couple of years we were interested in the fluo- rescence quenching mechanisms in organic solvents and aqueous media of several organic compounds containing a carboxylic group, in order to get more information on their general behavior in sev- eral environments and especially on the interactions with proteins. The choice of the carboxylic acids among other possible compounds was based firstly on the possibility to find them in both neutral and dissociate forms and, secondly, on their satisfactory water sol- ubility. Previous works dealt with the photophysical properties ∗ Corresponding author. E-mail addresses: mihaela.hillebrand@gmail.com, mihh@gw-chimie.math.unibuc.ro (M. Hillebrand). of 2- and 3-carboxy-phenoxathiin derivatives [5], 3-carboxy-5,6- benzocoumarin [6–8] and 3-carboxy-coumarin and their behavior in the presence of cyclodextrins and bovine and human albu- mins (HSA and BSA) [9–13]. We have found that these compounds present a good enough fluorescence quantum yield to ensure a reli- able answer to the changes in the local polarity, the presence of electron donors and the hydrophobic cavity of cyclodextrins. In this paper, we report an analysis of the steady state (SS) and time-resolved (TR) fluorescence quenching of 3-carboxy-5,6- benzocoumarin (BzCum) in acetonitrile (ACN) in the presence of some 1,2,3,4,5,6,7,8-sym-octahydroacridines. Taking into account our previous results on the quenching of BzCum in the pres- ence of aromatic amines [7], we expect that the excited state quenching process would be an electron transfer (ET) from the acridines (the donors) to the excited BzCum (the acceptor). Besides the unsubstituted parent compound in which the reactive cen- ter should be the ring nitrogen atom, we have also chosen their derivatives with 9-substituent amino group, NH 2 , which can also be involved in ET interactions. The corresponding N-oxides were used to evidence the role of the negative charge localized on the NO oxygen in the interaction processes. The synthesis and the experimental and theoretical characterization of their molecular structure were previously reported [14–17]. In order to evidence the possible specific interactions due to the COOH group, additional information on the overall process was obtained using instead of BzCum the 5,6-benzocoumarin-3-carbonyl chloride (Cl-BzCum) (Fig. 1). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.06.051