Distortions of Metalloporphyrin Forms at Low Temperatures

The planar form and two types of distorted molecular conformations in the ground electronic state for a number of metalloporphyrins in solid matrices exist simultaneously. A comparison of experimental data and results from quantum chemical calculations is discussed in order to analyze the structure of distorted porphyrin macrocycles at cryogenic temperatures.


INTRODUCTION
Molecules of porphyrins and metalloporphyrins are, along with their analogs, chromophores in such biologically important compounds as green plant chlorophyll and blood heme (see [1] and references therein). These compounds have high degrees of symmetry and a variety of spectral and photophysical characteristics, and are also good probes for studying many processes of intramolecular and intermolecular interactions. It is known that porphyrins and their metal complexes have planar structure and high degrees of symmetry in neutral solvents at room temperature [1,2]. However, distortions of the planar molecular structure of the porphyrin macrocycle are possible under certain conditions. This leads to spectral manifestations of the effects of nonplanarity for these compounds. Introducing various groups into different positions of the macrocycle (e.g., forming the twice protonated form of porphyrin [3,4] and introducing alkyl groups in various combinations and into different positions of the macrocycle [5-7]) can distort the planar structure of the porphyrin macrocycle. The structure of metalloporphyrin is also distorted upon the extraliganding of the central metal atom [8,9] and the disturbing effect of the crystal field for different types of crystal structure.
The spectral, energetic, physical, and chemical properties of these compounds vary considerably, compared to planar tetrapyrrole macrocycles with the nonplanarity of the porphyrin macrocycle in the ground and excited states [10,11]. Distorted structures of metalloporphyrins are most often recorded and analyzed. Analysis of resonant Raman scattering (RRS) spectra of the metal complexes of porphyrins (with Ni, Pd, Fe, Zn, and other atoms) shows that lines in the 1500-1650 cm −1 region are the ones most shifted [12]. A comparison of the RRS spectra of metalloporphyrins with various substituents (e.g., alkyl and phenyl groups) and metal complexes of porphyrins with various metals makes it difficult to interpret experimental data.
One of the greatest causes of nonplanar distortions for metalloporphyrins [8,9] is the interaction between the metal complexes of porphyrins and extraligands. Raman spectroscopy is not selective enough to allow us to obtain information on the frequencies of normal vibrations of different spectral forms. In addition, complexes with ligands are normally less than stable at room temperature, and adding other organic solvents (e.g., ligands) to a solution makes the recorded spectra much more complicated. It is therefore harder to analyze experimental results.
The aim of this work was to study via low-temperature selective spectroscopy different spectral forms of the simplest metal complexes of porphin under conditions of their stabilization at helium temperatures.

EXPERIMENTAL
Spectral characteristics of metal complexes of porphin with Mg(II), Zn(II), Pd(II), and Pt(II) ions, and of analogous compounds with alkyl substituents on pyrrole rings and the corresponding ions, were recorded when these compounds were placed in a Shpol'skii matrix, a tetrahydrofuran matrix, and hard matrices of inert gases according to the procedure in [13].   Table 1. Conformation with the 0-0 transition position at 552 nm corresponds to a planar molecular structure (F0 form) [9].  It should be emphasized that a planar molecular structure (F0 form) formed mainly when molecules were deposited in inert gas matrices. Concentrations of the distorted ZnP forms in the Ar matrix were relatively small: the concentration of the FI form was less than 10%, and that of the FII form was no more than 5%.
This testifies to the relatively weak disturbing effect the matrix has on the molecular structure of the metal complex.
The dome conformation is usually associated with the presence of extra ligands on the metal atom.
The glow of only the FII form (see Fig. 1) is visible in the fluorescence spectrum when a small amount of pyridine (a strong extra ligand) is added to the n-octane, and there is no glow at all from the F0 and FI forms at a temperature of 4.2 K.
It was established in [13] that the pyridine ligand attached to the metalloporphyrin in the hard matrix of the inert gases of metalloporphyrins breaks off during deposition, and no traces of it are recorded in the absorption spectrum. This confirms there is no initial liganding of the molecules embedded in the inert gas matrices and nitrogen matrices. Only relatively weak van der Waals complexes with gas atoms can form in matrices when preparing samples by means of isolation. This effect is not observed at room temperature, but the distance between the matrix (the inert gas) and a chromophore is less than one nanometer when the molecules are condensed into an inert gas matrix, which can greatly magnify the role of van der Waals complexes [14]. It should be considered that argon does not have its own dipole moment, but it does have fairly high polarizability (1.63 Ǻ [15]). This leads to induction interactions, the magnitude of which for πelectron systems (aromatic compounds and porphyrins) can reach values of several kcal ⋅ mol −1 and will be maximal if the chromophore and matrix are located in parallel. As is described below, the complexing energies of metalloporphyrin with extraligands (the results from quantum chemical calculations) have comparable values and do not exceed 10 kcal mol −1 for ZnP.
Note too that the allowed vibrational structure has been recorded in the absorption spectrum (fluorescence excitation) of ZnP in a hard tetrahydrofuran matrix, in the region of the Soret band.
The spectrally resolved fluorescence excitation spectrum for the FI form of ZnP in solid tetrahydrofuran (4.2 K) in the region of the Soret band is shown in Fig. 4a. The same spectrum for the FII form of ZnP is correspondingly shown in Fig. 4b. We determined a number of normal vibration frequencies for the highlying electronic transitions of the Soret band. These were fully symmetrical vibrations with frequencies of 360, 719, 986, 1359, and 1477 cm −1 (Fig. 4). The fluorescence excitation spectra in the region of the Soret band had a pronounced doublet structure with different splitting values between the components when there were two spectral forms. The splitting value was almost 30% lower for the more distorted FII form of ZnP than for the FI form. From an empirical viewpoint, the reason for this is not entirely understood.

CONCLUSIONS
The existence of different spectral forms in the luminescence spectra of metal complexes of porphyrins at low temperatures is of a fairly general nature.
There are two forms that have 0-0 transition positions with a spectral shift of 360 cm −1 in the fluorescence spectra of MgP in the THF matrix at cryogenic temperatures, while the analogous shift in the THF matrix does not exceed 180 cm −1 for ZnP [9].
The existence of two spectral forms in the ground electronic state was observed in the phosphorescence spectra of the metal complexes of porphin with Pd(II) and Pt(II) when they were placed in the Shpol'skii matrix at a temperature of 4.2 K. The energy gap for two spectral forms in the phosphorescence spectra of the porphin metal complex with the Pd(II) ion in the n-octane matrix at 4.2 K was 78 cm −1 [16], while the analogous energy gap for the Pt(II) complex of porphin had a much lower value: only 38 cm −1 [17]. Our results demonstrate the effectiveness of the fine-structural low-temperature spectroscopy of molecules in combination with modern quantum-mechanical ways of calculating the structure and spectral parameters of impurity molecular centers in low-temperature matrices for the study of structurally distorted macrocyclic compounds.

ACKNOWLEDGMENTS
This work was supported by the Belarus Foundation for Basic Research, project no. Ф16РА-007; and by the European Union as a part of the Horizon 2020 program, grant no. 645628.