Interpretation of resonance Raman spectra of Zn-phthalocyanine and Zn-phthalocyanine-d16 based on the density functional method

Abstract

We have used the density functional method (DFT) to calculate the geometric structure and the frequencies of even (gerade) vibrations of the Zn-phthalocyanine and Zn-phthalocyanine-d 16 molecules, and we have refined the interpretation of the resonance Raman spectra and the fine-structure fluorescence spectra. We observed a dependence of the frequencies of both stretching and bending vibrations of the aza bridges on the nature of the central metal atom.Introduction. Many scientific publications have been devoted to study of the physicochemical properties of phthalocyanine molecules and their analogs because of their broad application in science and technology. An important part of such investigations has been study of the vibrational and vibronic states of compounds in this class to establish correlations between spectra and structure and to develop methods for monitoring and diagnostics of the state of these compounds in technical devices. This is possible after detailed interpretation of the corresponding spectra.The review paper [1] provides discussion of the results of theoretical and experimental work up to 2002 on interpretation of the spectra of phthalocyanine and its metal complexes. In [2], the resonance Raman spectra of Znphthalocyanine (ZnPc) and its isotopically substituted derivative ZnPc-d 16 are obtained. The experimental data are compared with the results of the normal vibration calculation using a valence force field. The calculated data describe satisfactorily the changes in the spectrum upon deuteration. Moreover, for some normal vibrations there are significant discrepancies between the calculated and experimental values of the frequencies. Furthermore, within the D 4h symmetry point group, to which the ZnPc molecule belongs, we cannot use polarization data to separate vibrations of B 1g and B 2g symmetry. Vibrations of these symmetries appear in the resonance Raman spectra as depolarized lines. For ZnPc and ZnPc-d 16 , the calculations showed that a number of normal vibrational frequencies of these symmetries are close. Therefore assignment of the corresponding frequencies requires additional testing.The Calculation. Recently the quantum mechanical density functional method has been successfully used for calculation of the vibrations of macrocyclic molecules of the porphyrin type (see, for example, [3,4]).In order to refine the interpretation, we did the calculations for the equilibrium geometry of the ZnPc and ZnPc-d 16 molecules and their vibrational frequencies by the indicated method, using two exchange-correlation functionals BPE and BLYP with the help of the algorithm proposed in [5] and realized within the quantum chemical program in [6]. The geometry calculation began with an asymmetric structure, but during optimization for ZnPc we obtained an equilibrium configuration belonging to the D 4h symmetry point group with the Zn atom in the plane of the molecule. Its deviation from the plane was 5⋅10 -4 Å, although initially it was equal to 0.4 Å. Then we calculated the force constants and...

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