Calculation of the structure and absorption spectra of phthalocyanines in the gas-phase and in solution

doi:10.1016/S0166-1280(98)00238-3

Journal of Molecular Structure: THEOCHEM

Volume 455, Issue 1, 4 December 1998, Pages 33-50

https://doi.org/10.1016/S0166-1280(98)00238-3 Get rights and content

Abstract

Ab initio calculations have been carried out on both gas-phase and solvated phthalocyanines in order to determine equilibrium structures, vibrational spectra, and electronic spectra. Density functional theory (DFT) optimized geometries have the expected symmetries of D_2h for free-base phthalocyanine, D_4h for copper phthalocyanine, and C_4v for tin phthalocyanine, whereas Hartree–Fock optimized geometries, which also include lead phthalocyanine, are slightly displaced from the expected symmetries. Both sets of optimized geometries agree reasonably well with the available experimental structures. Vibrational spectra, calculated by the Hartree–Fock method, are in partial agreement with measured spectra, whereas visible absorption spectra calculated using molecular geometries obtained at three different levels of theory (DFT, HF, and PM3) showed good agreement with the measured vapor-phase spectra in the Q-band, and using the DFT optimized geometry resulted in very good agreement with measurements in the B-band. The self-consistent reaction field method for including bulk solvent effects in HF calculations had a small, nearly negligible, effect on the molecular structure of SnPc, whereas the DFT/cosmo solvation model yielded the predictable result of increasing the Sn–N bond length by pulling the Sn atom further out of the molecular plane.

Introduction

The excited-state absorption properties of metallo-phthalocyanines [1]make them of interest for optical limiting applications 2, 3, 4. In order to enhance optical limiting properties and tune the absorption spectra, a series of calculations on these derivatives was carried out. However, reasonably “good” geometries for spectra predictions have to be first obtained [5]. Ab initio Hartree–Fock calculations have been carried out on four phthalocyanine compounds: the metal-free phthalocyanine (H₂Pc), copper phthalocyanine (CuPc), tin phthalocyanine (SnPc), and lead phthalocyanine (PbPc). Density functional theory (DFT) calculations have been carried out on H₂Pc, CuPc and SnPc. Note that experimental structures are available for these molecules from X-ray diffraction or neutron diffraction studies. X-ray crystal structures are available for CuPc [6], SnPc [7], and for two structures of PbPc, from both the monoclinic crystal [8]and the triclinic crystal [9]. A geometry for H₂Pc is available from a neutron diffraction study [10].

Ab initio calculations on copper phthalocyanine have been carried out by Szczepaniak and Bragiel [11]. However, a relatively small basis set (3-21G) was used and no geometry optimization was carried out. Instead, the geometry from the X-ray diffraction crystal structure study [6]was employed (modified to an idealized D_4h symmetry). In this study, we report geometry optimizations at both the Hartree–Fock and DFT levels of theory to estimate equilibrium geometries of the isolated H₂Pc, CuPc, SnPc, and PbPc molecules and to evaluate the appropriate approach to be used for organometallic systems of interest to us. The geometries obtained by both theoretical methods agree well with experiment. The DFT method was more successful at obtaining the expected gas-phase symmetry. The structures obtained for the free-base phthalocyanine were used to evaluate the vibrational spectra via calculation and diagonalization of the Hessian matrix, and a fair agreement is indicated with the available experimental values.

Both calculated and experimental geometries were used to calculate electronic absorption spectra using configuration interaction (CI) within a semiempirical formalism, INDO/s. The electronic absorption spectra for phthalocyanines show a strong peak in the blue (300–400 nm) region, referred to as the B-band, and an even stronger Q-band in the red to near-infrared (600–700 nm). This differs from the spectra of porphyrins, where the Q-band tends to be much weaker than the B-band. The spectra for free-base phthalocyanine were calculated using three computationally optimized geometries (HF, DFT, and PM3) and one experimental geometry. For the free-base phthalocyanine, the Q-band is split into two strong peaks, and the calculations from all four geometries correctly predicted this result, whereas only the more symmetrical DFT structure was successful in predicting the single strong peak in the B-band for the gas-phase free-base phthalocyanine.

These structures can also be used as starting points for evaluating the influence of a solvent. The effects of solvation on these molecules have been investigated using continuum reaction field models. The use of such a model with DFT predicted a reasonable deviation from the gas-phase geometry for tin phthalocyanine, whereas no appreciable change in the geometry was found with the self-consistent reaction field (SCRF) included in the Hartree–Fock calculation.

Section snippets

Methods

The gamess [12]program was used to carry out the Hartree–Fock and semiempirical calculations. dmol¹ [13]was used for the DFT calculations. Restricted-spin Hartree–Fock (RHF) calculations were carried out on the metal-free, tin, and lead phthalocyanine molecules, whereas for copper phthalocyanine, restricted-spin open-shell Hartree–Fock (ROHF) calculations were performed. Semiempirical calculations with the PM3 Hamiltonian were also carried out for the

Structure and energetics

For the metal-free phthalocyanine, a stationary point with D_2h symmetry was found in the Hartree–Fock calculations with both the STO-3G and 3-21G basis sets, proving to be a saddle point having one imaginary frequency with a value of 2079i cm⁻¹ using the STO-3G basis and 1290i cm⁻¹ using the 3-21G basis. The structure of this saddle point and the components of the mode corresponding to the imaginary frequency are illustrated in Fig. 1. With the use of the IRC method in gamess, a true minimum was

Conclusions

Geometry optimizations of the phthalocyanines H₂Pc, CuPc, SnPc, and PbPc have been carried out using ab initio calculations. DFT calculations were successful in obtaining the expected symmetries of D_2h for H₂Pc, D_4h for CuPc, and C_4v for SnPc for the equilibrium geometries, whereas the Hartree–Fock calculations yielded geometries slightly displaced from these symmetries, in having only C_2v symmetry for H₂Pc and CuPc, and C_s symmetry for SnPc and PbPc. The lower symmetry obtained in the

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Calculation of the structure and absorption spectra of phthalocyanines in the gas-phase and in solution

Abstract

Introduction

Section snippets

Methods

Structure and energetics

Conclusions

J. Alloys Compd.

Vacuum

Spectrochim. Acta Part A:

Science

Appl. Phys. Lett.

Optics Lett.

Inorg. Chem.

Acta Crystallogr. Sect. B:

Acta Crystallogr. Sect. B:

J. Comput. Chem.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Am. Chem. Soc.

J. Chem. Phys.

J. Chem. Phys.