Calculation of electronic spectra of porphin and its derivatives by a modified INDO/S method

Abstract

539.19Quantum-chemical calculations of excited electronic states of porphin (H 2 P) and Mg-porphin (MgP) have been carried out in the framework of the INDO/S method with varying off-diagonal matrix elements of the one-electron Hamiltonian H µν and electron-electron interaction integrals γ µν as functions of the internuclear distance. It has been found that a simultaneous increase in the π-type overlap factor for H µν and decrease in the γ µν integrals, as compared with γ µν NM calculated by the Nishimoto-Mataga formula, make it possible to reproduce the positions of the Q-and B-transitions in the experimental absorption spectrum with a precision of 300 cm -1 . In this case, the N x -transition intensity of the H 2 P molecule is halved, which means that only two B-transitions should be related to the Soret band as in the four-orbital model. Using the proposed parameterization (INDO/Sm), electronic spectra have been calculated for a number of porphyrin compounds including chlorine, bacteriochlorin, tetrabenzoporphin, and tetraazaporphin. The results obtained agree with the experimental data to an accuracy of 300-700 cm -1 whereas the accuracy of the standard INDO/S calculation is no better than 3000 cm -1 .Introduction. We took the following approach for quantum-chemical calculations of a series of porphyrin compounds [1][2][3][4][5]. The geometric structure of the ground state of the molecules was determined by the limited and unlimited Hartree-Fock method (LHF and UHF) with Hamiltonian AM1 [6]. The geometry found from the optimization was used as starting data in calculations of excited states by the CNDO/S method [7,8]. The results of such an approach indicate that the characteristics of the electronic spectra of the examined series (primarily the shifts of the levels) relative to the prototype of the porphyrin class, porphin (H 2 P), reproduce very accurately the experimental data.Moreover, a comparison of the calculated and experimental data for H 2 P itself (Table 1) shows that there are not only several insufficiencies typical of the CNDO/S method but also those noted earlier more than once [19][20][21].1) The energy of levels 1 1 B 3u (Q x ) and 1 1 B 2u (Q y ) is systematically lowered with an uncertainty δE Q ~3000 cm -1 . The energy of levels 2 1 B 3u (B x ) and 2 1 B 2u (B y ), on the other hand, is systematically raised. The oscillator strength of transitions G → B x and G → B y (G is the ground state 1 1 A g ) is raised by four times. It must be noted that use of the random phase approximation [22] instead of overlap of singly excited configurations and the INDO/S Hamiltonian [23,24] instead of the CNDO/S ** Hamiltonian improves the characteristics of the B-states. However, the energy of the Q-states becomes even lower.2) The configurational composition of a pair of Q-states and a pair of B-states is determined mainly by the electron configurations of the four-orbital model [19][20][21]. However, the increased energies of the a u b 3g and b 1u b 2g configurations results in their contribution to the ...

Calculation of the electronic structure and spectra for bacteriochlorophyll analogs

“…It should be noted that the semi-empirical AM1 method with the unbounded Hartree-Fock (UHF) method was used previously [7] to optimize the geometry of H 2 P. The results of the present calculations and the published ones [7] are similar and lead to the same qualitative conclusions about the contribution of the inner 18-membered azacyclopolyene to the electronic structure of H 2 P.…”

“…Its use enabled the accuracy of calculated excited electronic states to be increased substantially. In particular, the energy of the Q-transitions for fundamental porphyrin systems including porphine (H 2 P), chlorin, bacteriochlorin (H 2 BC), tetrabenzoporphine, and tetraazaporphine agreed with experimental results within ~300-700 cm -1 whereas the uncertainty of calculations by the standard INDO/S method, the currently widely used ab initio method, and density-functional theory (DFT and TDDFT) was 2500-3000 cm -1 [7]. Recently Italian and…”

“…Parameterization of the INDO/Sm method, which was specially designed for examination of tetrapyrrole systems, was developed before [7]. Its use enabled the accuracy of calculated excited electronic states to be increased substantially.…”

“…Its application to tetrapyrrole structures was reported before [12]. Excited electronic states of the molecules were calculated using the INDO/Sm method [7] (modified INDO/S method) using our own method to calculate about 400 singly excited configurations. The electronic spectra of the tetrapyrrole compounds that were calculated using the INDO/Sm program were demonstrated to have good accuracy by our group [13][14][15], including the Mg complex of a complicated tetraazaporphine derivative [14], an analog of Mg-phthalocyanine.…”

Spectral-luminescence properties of a norbornene-substituted tetraazachlorin and its metal complexes

“…The calculations predicted weak transitions into N y (361 nm) and N x (347 nm) states, into a state with B 2 symmetry (X, 325 nm), and strong B y (295) and B x (282) transitions. Quantum chemical calculations are known to elevate systematically G → B transition energies ([19] and references therein). This is due partially to the use of solution absorption spectra for comparison with the experiment whereas the calculations are performed for the free molecule.…”