Theoretical investigations of the UV spectra of coumarin derivatives

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Abstract

The UV absorption spectra of substitued coumarins have been investigated with a wide panel of theoretical methods. In the density functional theory framework (DFT), various basis sets, as well as several functionals have been tested. It turns out that the Becke–Lee–Yang–Parr functional (B3LYP), combined with the 6-311+G(2d,2p) atomic basis set, provides reliable λmax when the solvent effects are included in the model.

Introduction

Coumarins (Fig. 1) are dyes owing their success to their efficient light emission properties, high stability, and easy synthesis [1]. They are of considerable biological and medical interest, showing important physiological effects (anticoagulant activity, weak toxicity, etc.,) [2]. Compounds derived from the basic coumarin structural unit are also widely used as laser dyes. As most of the coumarins provide blue–green colours, there is a huge interest in the design of new derivatives able to emit in the yellow-red region of the visible spectrum. To achieve this purpose, it is essential to isolate and correctly describe the excited state(s) of the molecular structures involved in the fluorescence process. Nowadays, the molecular modelling techniques offer a competitive alternative for the interpretation of experimental data arising from industrial interest and applications. This work aims at defining an optimal theoretical scheme for computing the absorption energies associated to these excited states. Time-dependent density functional theory (TD-DFT) is often found to be a robust and accurate method for low lying excited states, and has frequently been applied to solve numerous chemical and physical problems [3], [4], [5], [6], [7], [8], [9].

Section snippets

Methodology

The calculations have been performed with Gaussian 03 [10], following a two stages procedure. (i) The optimisation of the ground-state geometry, for which a large panel of quantum chemical methods has been used, from Hartree–Fock approach to correlated techniques like density-functional theory (DFT) or second-order Møller-Plesset (MP2). After the minimisation process, the vibrational spectrum is evaluated to check that no imaginary frequency is present. (ii) The determination of the vertical

Results

For the methodological part, we have used a training set of five molecular systems directly deriving from coumarin, for which we use the standard substitution position labelling (5-Me; 6-Cl; 7,8-diOH; 6,7-diOH; 6-NH2). The experimental λmax in ethanol are known for each of these molecules: 315, 321, 335, 354 and 370 nm, respectively [26]. Previous coumarin’s excited states evaluation by Cave et al. [27], [28] were carried out for specific derivatives and did not explicitly take into account

Conclusions and outlooks

An adaptable theoretical scheme that allows to evaluate the maximum absorption wavelength of coumarin dyes has been designed. It turns out that:

  • (i)

    6-311G(2d,2p) is a suitable basis set for ground-state geometry optimisation;

  • (ii)

    for UV spectra calculation, only one set of diffuse functions is needed;

  • (iii)

    B3LYP provides valid geometries. For a better accuracy, geometries have to be optimised in solvent-phase;

  • (iv)

    B3LYP produces λmax in good agreement with experimental data;

  • (v)

    solvent effects on the UV spectra are

Acknowledgements

J.P. acknowledges the FRIA (Belgian ‘Fonds pour la formation à la Recherche dans L’Industrie et dans l’Agriculture’) for his PhD Grant. D.J. and E.A.P. thank the Belgian National Fund for their respective Research Associate positions. The calculations have been performed on the Interuniversity Scientific Computing Facility (ISCF), installed at the Facultés Universitaires Notre-Dame de la Paix (Namur, Belgium), for which the authors gratefully acknowledge the financial support of the FNRS-FRFC

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