Article pubs.acs.org/JPCA
Fluorescence Spectra of (Dibenzoylmethanato)boron Difluoride Exciplexes with Aromatic Hydrocarbons: A Theoretical Study A. A. Safonov,† A. A. Bagaturyants,*,†,‡ and V. A. Sazhnikov† †
Photochemistry Center, Russian Academy of Sciences, ul. Novatorov 7a, Moscow, 119421 Russia National Research Nuclear University “MIFI”, Kashirskoe sh. 31, Moscow, 115409 Russia
‡
ABSTRACT: An approach is proposed for the quantum-chemical calculation of the structure and fluorescence spectra of exciplexes. The procedure involves the geometry optimization of exciplexes using the CIS method with empirical dispersion correction (CIS-D) and the subsequent single-point calculation of the transition energy using the CIS method with perturbative correction for double excitations (CIS(D)). Calculated fluorescence band positions for exciplexes of (dibenzoylmethanato)boron difluoride with substituted benzenes are in reasonable agreement with the experimental data.
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INTRODUCTION In recent years, difluoroboron β-diketonates have attracted considerable attention due to their interesting and rich photophysical properties. In particular, (dibenzoylmethanato) boron difluoride (DBMBF2) and its derivatives have been used in several studies related to complexation with mono- and polyaromatic hydrocarbons in the ground and excited states,1−8 solvatochromism,9−11 room-temperature phosphorescence,12,13 two-photon absorption,9 white light emission,14 etc. One of the most remarkable features of the DBMBF2 fluorophore is its ability to form emissive exciplexes with benzene and methyl-substituted benzenes (SB) in cyclohexane or acetonitrile solutions,2−6 on the silica-gel surfaces15,16 and in polymer matrices.17,18 Therefore, DBMBF2 can be used in practice as a promising material for fluorescence-based optical chemical sensors selectively detecting the presence of benzene and its derivatives in various media (gas phase, solutions, solid surfaces etc.). Comprehensive data on the fluorescence properties of DBMBF2-SB exciplexes were published by Valat et al.5 They reported exciplex fluorescence spectra for DBMBF2 dissolved in a series of neat aromatic solvents such as benzene, toluene, m-xylene, p-xylenes, mesitylene, and isodurene. The aim of this work is to find a reliable quantum-chemical description of structural and fluorescent properties of DBMBF2 exciplexes with some simple (mononuclear) aromatic hydrocarbons: benzene, toluene, o-, m-, and p-xylenes, mesitylene (1,3,5-trimethylbenzene), and isodurene (1,2,3,5-tetramethylbenzene). Modern quantum chemistry offers a variety of methods for computing structures and electronic properties of molecular systems in the ground and excited electronic states. However, none of these methods is universal, and any quantum-chemical study usually starts from a selection of a © XXXX American Chemical Society
suitable method or methods to solve the problem in hand. In the case of exciplex fluorescence, a method of choice should adequately reproduce (1) van der Waals forces (because these mainly control the structure of molecular complexes) and (2) transition energies and geometries of excited states, especially for intermolecular charge-transfer transitions, which commonly give rise to exciplexes. Reliable data on the structure and energy characteristics of molecular systems in the ground electronic state can be obtained using density functional theory (DFT). The known drawback of DFT in the description of van der Waals interactions, which are particularly important for DBMBF2 complexes with aromatic hydrocarbons considered in this work, can be eliminated by using an empirical dispersion correction (DFT-D method).19−21 The DFT-D method was validated for a large number of systems including van der Waals complexes for which accurate ab initio calculations were performed; it was found that the DFT-D method and the coupled cluster method CCSD(T) gave rather similar results.19−22 Calculations of exciplex fluorescence spectra involve geometry optimization for the first excited singlet state of the system and, possibly, subsequent (single-point) calculations of transition energies using a more sophisticated method. This procedure is rather complicated and computationally demanding. Relatively simple methods like configuration interaction with single excitations (CIS) and time-dependent DFT (TDDFT) do not reproduce dispersion interactions and, hence, are unsuitable for optimizing geometries of complexes and exciplexes. Moreover, TDDFT with standard density Received: April 12, 2015 Revised: June 11, 2015
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DOI: 10.1021/acs.jpca.5b03519 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A functionals poorly describe charge-transfer states23 and grossly underestimate their excitation energies.24−26 However, Huenerbein and Grimme27 successfully applied the TDDFT technique in combination with an empirical dispersion correction (TDDFT-D) for calculations of structures and binding energies of some excimers and exciplexes. Although the dispersion correction was parametrized for the ground rather than excited state, the authors argued that the dispersion correction yields only a lower limit for the van der Waals contribution, because molecules in excited states have a higher polarizability than in the ground state, for which the parameters have been determined. They assume that the state-independent dispersion correction works well for the large number of “inactive electrons” that are not excited and that the more relevant orbital interactions, the involved electron correlations, and the electrostatic contributions are treated adequately by the density functional used. Thus, in the case of excited states, the use of the dispersion correction will provide more accurate results. To overcome another problem of TDDFT related to the incorrect description of charge-transfer transitions, it was suggested to use a hybrid functional with a large amount of the Hartree−Fock (HF) exchange, e.g., BHandHLYP with 50% of (HF) exchange. The use of this approach resulted in the dissociation energies of the benzene and pyrene excimers and the styrene and trimethylamine exciplexes consistent with experimental data.27 A similar approach was used in ref 28 for calculations of the structures and formation energies of DBMBF2 exciplexes with aromatic hydrocarbons. However, the BHandHLYP functional performs rather poorly in calculations of electronic transition energies, and BHandHLYP results are not completely free from the incorrect description of charge-transfer states. Thus, TDDFT-D calculations of exciplexes reported in ref 27 encouraged us to test other methods involving a dispersion correction for calculations of fluorescence spectra of exciplexes. Currently, dispersion correction parameters are available not only for DFT with different functionals but also for the Hartree−Fock (HF) method.29 It seems reasonable to use this correction in calculations by the CIS method, which is free from some disadvantages of TDDFT related to charge-transfer transitions. CIS transition energies are known to be highly overestimated; hence, CIS with dispersion correction (CIS-D) is used only for geometry optimization. Final transition energies at optimized geometries can be calculated by a more sophisticated method, e.g., CIS with perturbative correction for double excitations (CIS(D)).30,31
we used a larger grid for the COSX approximation with the option GridX9, which eliminated the convergence problem.
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RESULTS AND DISCUSSION The calculated structure of the DBMBF2 molecule with the geometry optimized in the first excited singlet state is shown in Figure 1. The energy of the electronic transition between the
Figure 1. Structure of the DBMBF2 molecule in the first excited singlet state.
first excited and ground singlet states corresponding to this structure calculated at the CIS-D level was 3.53 eV (wavelength 351 nm), which is somewhat higher than the experimental value of 3.19 eV.3,6 Geometries of DBMBF2 complexes with aromatic hydrocarbons (benzene, toluene, o-, m-, and p-xylenes, mesitylene, and isodurene) in the first excited singlet state were optimized using the CIS-D method. For each system, several structures corresponding to minima on the potential energy surface were found. All structures exhibit a stacked arrangement of molecules and are characterized by short (
