The magnetic circular dichroism (MCD) and absorption studies of 1,8-naphthalimide. The theoretical analysis in terms of density functional (DF) and coupled cluster (CC) theories Tomasz Seidler, Marcin Andrzejak, Marek T. Pawlikowski ⇑ Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Ingardena 3, Poland article info Article history: Received 2 July 2012 In final form 17 October 2012 Available online 24 October 2012 abstract The vibronic coupling theory based on the Franck–Condon (FC) approach was applied to study the absorption and magnetic circular dichroism (MCD) spectra of 1,8-naphthalimide in the excitation region corresponding to the low-energy 1A 1 ? 2A 1 and 1A 1 ? 1B 1 transitions. The computations carried out in terms of PBE0 and CC2 methods have shown that the vertical excitation energies of 2A 1 and 1B 1 states and Franck–Condon parameters from CC2/aug-cc-pVDZ computation lead to a very good agreement with the experiment. MCD analysis suggests that the vibronic coupling effects between 2A 1 and 1B 1 states may be responsible for observed minor discrepancies between the theoretical and empirical MCD spectra. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction In the recent years imides and diimides, like the phthalimide (PI), 1,8-naphthalimide (NDI) and 1,4,5,8-naphthalene-tetracarb- oxydiimide (NTD) have attracted much attention in the biological and technical sciences [1–4]. The interest has emerged since the imides and diimides have found their applications as antitumour drugs [5], DNA-cleaving agents [6], fluorescent tags [7], semicon- ductor materials [8], just to give a few examples. For these reasons the imides and diimides derivatives have been intensively studied by different experimental techniques including the electronic absorption and fluorescence [9,10], the circular dichroism (CD) [11–14] and the magnetic circular dichroism (MCD) [12,15]. Of these methods, the MCD seems to be especially attractive as a tool to study the excited electronic states of variety of the low- symmetry molecules [16]. A particular advantage of MCD is that it allows studying even very weak dipole-allowed transition(s) embedded or hidden in the vibrational manifold of more intense dipole-allowed ones [16]. Specifically, the theory of MCD [17–20] teaches us that a closeness of molecular states strongly favors the magnetic mixing effects (B-term) resulting in a substantial in- crease of magnetic rotatory strength of transitions associated with modest oscillatory strength in absorption. Since that magnetic mixing involves the electronic states of different nuclear geome- tries, the MCD spectra will always reveal an immanent contribu- tion of the vibrational origin. In other words, the Franck–Condon (FC) and/or vibronic coupling effects have to be inevitably taken into account [21,22] when the problem addresses the rotatory strength distributions in the MCD spectrum. For this reason, an analysis of the MCD spectrum may provide a very good test of quality of the quantum chemical methods applied to interpret a gi- ven experimental data [23]. In this short report we wish to examine in some details the MCD and the absorption spectra of 1,8-naphthalimide (NDI) molecule. The geometrical structure of that compound is schematically shown in Figure 1. The absorption spectrum of 1,8-naphthalimide molecule shows two intense absorption bands in the visible and UV excitation regions. The lowest energy band at ca 30 000 cm 1 consists of two 1A 1 ? 2A 1 and 1A 1 ? 1B 1 close energy transitions [12,23] and shows the vibrational structure, mainly due to the activity of totally symmetric oscillations. The absorption and the corresponding MCD spectra are challenged in terms of the vibronic model whose parameters are obtainable from quantum chemical computations based on two different methodologies. Namely, the excited states characteristics like the vertical excitation energies, the transition dipole moments and the FC parameters reported hereafter were ascertained from the computations based on the coupled clusters (CC2) and the time dependent (TD) density func- tional PBE0 methods. That latter has been already examined by us earlier [23] for a different purpose. Both PBE0 and CC2 were ap- plied at the aug-cc-pVDZ basis set level. 2. Computational details The geometrical parameters of the NDI molecule are defined in Figure 1. The NDI molecule is planar in the ground electronic state belonging to C 2v point group. For that group the three kinds of transitions from the totally symmetric ground state are 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.050 ⇑ Corresponding author. E-mail addresses: andrzeja@chemia.uj.edu.pl (M. Andrzejak), pawlikow@ chemia.uj.edu.pl (M.T. Pawlikowski). Chemical Physics Letters 555 (2013) 87–91 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett