Elsevier

Vibrational Spectroscopy

Volume 70, January 2014, Pages 78-88
Vibrational Spectroscopy

Comparative DFT study of Raman spectra of phosphorus-containing dendrimers built from thiophosphoryl, cyclotriphosphazene and phthalocyanine cores

https://doi.org/10.1016/j.vibspec.2013.11.009Get rights and content

Highlights

  • The FT Raman spectra of dendrimers with different cores were studied.

  • The structural optimization and normal mode analysis were performed for dendrimers.

  • The lines of the cores, repeating units and benzaldehyde terminal groups were defined.

Abstract

The FT Raman spectra of the zero and first generations of phosphorus-containing dendrimers built from thiophosphoryl, cyclotriphosphazene and phthalocyanine core with terminal oxybenzaldehyde groups have been recorded and analyzed. The structural optimization and normal mode analysis were performed for dendrimers on the basis of the density functional theory (DFT). The calculated geometrical parameters, harmonic vibrational frequencies and Raman scattering activities are predicted in a good agreement with the experimental data. The experimental Raman spectra of dendrimers were interpreted by means of potential energy distribution. Relying on DFT calculations the lines of the cores, repeating units and terminal groups of dendrimers were assigned.

The influence of the encirclement on the line frequencies and intensities was studied and due to the predictable, controlled and reproducible structure of dendrimers the information, usually inaccessible is obtained. The strong line at 1600 cm−1 show marked changes of intensity in dependence of aldehyde (single bondCHdouble bondO) or azomethyne (single bondCHdouble bondN) substituents in the aromatic ring. The polarizabilities and lipophilicity of dendrimers were estimated.

Introduction

Dendrimers constitute an important class of macromolecular compounds, with more than 16,000 papers published to date, and they continue to attract an increasing interest, due to the numerous potential applications, for instance in the fields of catalysis, materials or even biology and medicine [1], [2], [3], [4], [5], [6], [7]. A dendrimer molecule has mainly a three-dimensional tree-like structure emanating from one center – the core (initiator core), – with branches (dendrons). The size of such a molecule is determined by the generation number, i.e. by the length of the dendrons, which consist of an equal number of repeating units terminated by the end groups. Dendrimers can be considered as polymer compounds, since they are constituted of repeating units, and have often high molecular masses, but in contrast to linear polymers, dendrimers have a tree-like three-dimensional molecular structure with exact numbers of repeating units and terminal groups, i.e. they are monodisperse compounds, thanks to their step-by-step synthesis. The techniques used for characterizing such macromolecular compounds should afford not only their chemical composition but also their morphology, shape and homogeneity. NMR spectrometry, mass spectrometry, size exclusion chromatography, dynamic light scattering and various microscopies (TEM, AFM) have provided important information about the structure of these compounds [8], but FT Raman spectroscopy should afford additional information [9], [10], [11], [12], [13], [14].

Phosphorus-containing dendrimers share several properties with other types of dendrimers, for instance concerning catalysis, creation of new materials, and modification of surfaces of materials [15], [16]. However, they have also some properties never reported up to now for other dendrimers, such as a high dipole moment value and the ability to form hydrogels even at low concentrations in water [17], [18]. One can anticipate in particular that compounds having a hydrophobic interior and a hydrophilic surface should have valuable properties [15], [16]. The most promising fields of research certainly will concern the applications of these compounds in various aspects of chemistry, biology, physics, material science and medicine [15], [16]. A few years ago we reported about preparation, IR and Raman spectra of the phosphorus-containing dendrimers built from thiophosphoryl core up to 12th generation with terminal aldehyde groups and PCl bonds [19], [20], [21].

In this work, we try to show the value of FT Raman spectroscopy combined with DFT calculations for characterizing phosphorus-containing dendrimers built from different cores. The number of terminal groups and the global shape influencing many properties of dendrimers are closely related to the nature of the core. A core offering more functional groups gives a higher number of terminal groups for equal generations. Spherical macromolecular architectures are easily obtained starting from a highly functional core [6]. Our aim was to study the connection between the peculiarities of the structure of these compounds and their FT Raman spectra. We will analyze the spectroscopic data of phosphorus-containing dendrimers built from thiophosphoryl, cyclophosphazene and phthalocyanine cores. Due to the predictable, controlled and reproducible structure of dendrimers they may be used as new standards for molecular spectroscopy and thus information usually inaccessible may be obtained. The control growth of dendrons with generation number enables one to distinguish the lines in the Raman spectra assigned to the specific molecular fragments. The possibility appears to separate bands assigned to the core, repeating units and terminal groups of dendrimers. The concepts, which emerge from such analysis, are essential for the synthesis, modification and application of dendrimers.

Analysis of Raman spectra of dendrons combined with DFT calculations is important for investigation of supramolecular properties of dendrimers as containers for different guest molecules. The calculation of electronic density spatial distributions reveals the existence of regions where appropriate environments would attract either ion or a metal atom. The interpretation of Raman spectra of non-crystalline dendrimers is important for characterization of their structure. The results that emerge from such an analysis contribute to the understanding of the structure, dynamics and properties of dendrimers.

Section snippets

Experimental

The synthesis and main characteristics of the phosphorus-containing dendrimers were described in detail earlier [15], [16], [17], [18]. The molecules built from the trifunctional thiophosphoryl Sdouble bondP(single bondOsingle bond)3, the hexafunctional cyclotriphosphazene (NP)3 and the octafunctional phthalocyanine cores, the bifunctional repeating unit single bondOsingle bondC6H4single bondCHdouble bondNsingle bondN(CH3)single bondP(S)〈 and the 4-oxybenzaldehyde fragments single bondOsingle bondC6H4single bondCHO as the terminal groups were studied (Fig. 1). The molecule Gc1′ contains a thiophosphoryl core, three

Computational method

Calculations of Raman spectra of dendrimers G1′, Gc1′ and P0′ were carried out using the gradient–correlated density functional theory with Perdew–Burke–Ernzerhof exchange–correlation functional (DFT/PBE) [22]. This functional is very satisfactory from the theoretical point of view, because it verifies a lot of the exact conditions for the exchange–correlation hole and it does not contain any fitting parameters [23]. The comparison of the computed with PBE functional binding energies,

Results and discussion

Optimized geometrical structures of G1′, Gc1′ and P0′ molecules were obtained using DFT/PBE TZ2P quantum chemical method. The distances between atoms in dendrimer molecules and the angles between bonds were calculated and presented in Table 1, Table 2, Table 3. The atoms of molecules are numbered as in Fig. 2.

The X-ray diffraction experimental data of G1′ are absent. But we can use the geometric parameters of G0′ dendrimer of the zero generation with terminal aldehyde groups and G1 dendrimer of

Summary

The Raman spectra study provides a proof of the actual structure of the phosphorus-containing dendrimers built from thiophosphoryl, cyclotriphosphazene and phthalocyanine cores. The structural optimization and vibrational analysis were made by DFT method. The intensities of the most prominent lines in the Raman spectra of G1′, Gc1′ and P0′ are reproduced by our calculations. The calculated curves of G1′, Gc1′ and P0′ as a whole correspond to the experimental Raman spectra in the wide frequency

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