J. Phys. Chem. B 2000, 104, 5395-5398
5395
A Theoretical Study of the Excited States of Chlorophyll a and Pheophytin a Andreas B. J. Parusel*,† and Stefan Grimme‡ NASA Ames Research Center, MS 239-4, Moffett Field, California 94035, and Organisch-Chemisches Institut der UniVersita¨ t Mu¨ nster, Corrensstrasse 40, 48149 Mu¨ nster, Germany ReceiVed: January 28, 2000; In Final Form: March 16, 2000
The first six excited states of pheophytin a and chlorophyll a with the phytyl chain replaced by a hydrogen atom have been calculated using a combination of density functional theory and the multireference configuration interaction method (DFT/MRCI). The calculations accurately reproduce experimental absorption energies and intensities. Four and three excited states are calculated in the visible region of pheophytin a and chlorophyll a, respectively. We thus predict the existence of more than the two Qx and Qy electronic states in this energy range. These additional states are of significant double-excitation character and cannot be explained within the four frontier orbital model. Two strongly allowed states are calculated in the Soret region at 2.9 eV.
1. Introduction Pheophytin a and its magnesium-complexed derivative chlorophyll a (see Figure 1) play an essential role in the conversion of sunlight into chemical energy in green plants and various algae.1 Both systems are dihydrogen-reduced porphyrins and are found to be important units for electron transfer in the photosynthetic reaction center. Chlorin is the dihydrogenreduced ring skeleton of porphyrin and, thus, is directly related to pheophytin a and chlorophyll a in its macrocyclic structure. Because of their significance to many biological processes, chlorophyll a, pheophytin a, and related porphyrin-type systems have been studied extensively by both experimental and theoretical methods (see, e.g., refs 2 and 3). Many theoretical studies have been published for the highly symmetric free base porphyrin (D2h point group), for which computationally demanding methodologies such as coupled-cluster,4 MRMP/ CASSCF,5 and CASPT26 calculations are computationally feasible. The computational challenge is significantly higher in the study of pheophytin a or chlorophyll a for which an additional ring with a carbonyl group and various substituents in β and meso positions (see Figure 1) lower the symmetry of the molecules to C1. To the best of our knowledge, the symmetry-adapted cluster configuration interaction method (SAC-CI, using a simplified model system)7 and the timedependent density functional theory (TDDFT, without the phytyl chain8 and for chlorophyll a9) are the only first principles methods including large fractions of the electron-correlation contributions that have, as yet, been applied to these systems. Experimentally, the substituents of pheophytin a have almost no effect on the excitation energies compared to those of its unsubstituted derivative chlorin.10,11 However, four absorption maxima appear in the visible region for chlorin compared to six for pheophytin a. These peaks are commonly assigned to the Qx(0,0), Qy(0,0) transition and its vibrational excitations Q(1,0) and Q(2,0), respectively.10,11 This assignment is corroborated by a SAC-CI study,7 in which two excited states are * Corresponding author. Fax: +43-1-31336-790. E-mail: andreas.parusel@ univie.ac.at. † NASA Ames Research Center. Present address: Institute for Theoretical Chemistry and Structural Biology, Althanstr. 14, A-1090 Vienna, Austria. ‡ Organisch-Chemisches Institut der Universita ¨ t Mu¨nster.
Figure 1. Structure of pheophytin a and chlorophyll a and definition of the molecular axes.
calculated in this region. In contrast, the experimental spectrum by Fragata et al.12 and the TDDFT study by Sundholm8 assign the six absorption bands to six different electronic excited states. Only four absorption maxima are observed in the Q region of chlorophyll a,10,11 nine according to Frataga et al.12 Again, the SAC-CI7 study locates only two, and the TDDFT8 study only four, electronic excited states in the low-energy region of this system. The lower symmetry of chlorophyll a compared to its unsubstituted derivative magnesium porphyrin causes the degenerate absorption band in the visible (Q band) and UV (Soret or B band) regions to split into x- and y-polarized components. In addition, the Q transition gains considerable intensity relative to those of free base chlorin and pheophytin.10-12 It is the main goal of this article to gain some insight into the character of the excited states of pheophytin a and chlorophyll a and the controversial assignment of their absorption peaks in the visible and near-UV regions. Are the additional peaks due to further excited states or vibrational excited states? How does coordination of pheophytin a with magnesium affect the absorption spectrum? What is the effect of the pheophytin a and chlorophyll a substituents on the absorption spectra compared to that of the parent compound of chlorin? We investigate these questions using a recently developed method
10.1021/jp000346w CCC: $19.00 © 2000 American Chemical Society Published on Web 05/12/2000
5396 J. Phys. Chem. B, Vol. 104, No. 22, 2000
Parusel and Grimme
that combines density functional theory with the multireference configuration interaction approach (DFT/MRCI).13 This method has already been applied successfully to a series of porphyrintype systems,14 and we present here its application to the calculation of the electronic absorption spectra of pheophytin a and chlorophyll a. 2. Computational Methods It has been shown recently8,9 that the phytyl group has only a minor effect on the excited-state properties of the chlorophyll a system. We therefore use the molecular structures of Sundholm,8 with the phytyl group replaced by a hydrogen atom, as obtained by DFT optimizations applying the Becke-Perdew parametrization using a split-valence basis set augmented with polarization functions on C, N, O, and Mg. All excitation energies are calculated with the DFT/MRCI method.13 The basic idea behind this method is that information about dynamical electron-correlation effects from Kohn-Sham density functional theory (DFT) is included in conventional multireference CI calculations. The method thus emphasizes states of multiconfigurational character that cannot be represented by conventional DFT. The approach is based on an effective Hamiltonian in a basis of spin- and space-symmetry-adapted configuration state functions (CSFs). The CI matrix elements between CSFs are constructed empirically by using the exact Hamilton operator and corrections from DFT. Both the optimized Kohn-Sham orbitals from the parent determinant and the corresponding Kohn-Sham potential from the parent state density enter the calculation. A double counting of electron correlation is avoided explicitly by construction of the effecive Hamiltonian. The selection of the most important CSFs describing nondynamical correlation effects is based on a simple energy gap criterion. In typical applications, converged results can be obtained by considering less than one promille of the full MRCI space in the variational problem. Together with the use of an approximate resolution of the identity (RI) for the evaluation of the twoelectron integrals, this allows for a very efficient DFT/MRCI treatment for molecules with several hundreds of electrons. In general, it is found that the DFT/MRCI approach gives results of high accuracy (e.g., rms errors for excitation energies of
