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Elementoorganic dendrimer characterization by Raman spectroscopy
Polymer 45 (2004) 5889–5895 www.elsevier.com/locate/polymer
Elementoorganic dendrimer characterization by Raman spectroscopy V.L. Furera,*, J.P. Majoralb, A.M. Caminadeb, V.I. Kovalenkoc,* a
Department of Physics, Kazan State Architect and Civil Engineering Academy, 1 Zelenaya, 420043 Kazan, Russia Federation b Laboratorie de Chimie de Coordination, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 4, France c A.E. Arbuzov Institute of Organic and Physical Chemistry, RAS, 8 Arbuzov Str., 420088 Kazan, Russia Federation Received 26 January 2004; received in revised form 18 May 2004; accepted 16 June 2004
Abstract The FT-Raman (10–3500 cmK1) spectra of ten generations of the elementoorganic dendrimers have been recorded and analyzed for the first time. Their spectral pattern is determined by the ratio Nt/Nr (Nt—number of terminal groups, Nr—number of repeating units). This ratio trends to mrK1 (mr—branching functionality of repeating unit), and becomes constant, when the generation number is higher than 3–5. Experimental Raman spectra of generations higher than 4 are very close similar, according theoretical approach. The dependence of line full width at half height in Raman spectra on generation number is established. The low frequency Raman spectra in R(n) representation are used to investigate the librations of terminal groups. The possibility appears to separate the lines assigned to the core, repeated units and terminal groups of dendrimers by difference spectroscopy method. From the difference Raman spectra of dendrimers it follows that for the generations higher than six steric congestion disturb the conformations of terminal groups. FT-Raman spectroscopy provides unique detailed information about the structure of the technologically relevant materials, which could not be obtained before with any other technique. q 2004 Elsevier Ltd. All rights reserved. Keywords: Phosphorus-containing dendrimers; Raman spectra; Normal vibrations
1. Introduction Dendrimers are a new type of compounds and the perspective materials studied by supramolecular chemistry [1,2]. A dendrimer molecule has mainly a three-dimensional tree-like structure proceeded from one center—the core (initiator core),—with branches (dendrons). The size of such a molecule is determined by a generation number, i.e. by the length of the dendrons, which consist of equal number of repeating units, and terminate in end groups [1]. According to their molecular topology (repeating units, terminal groups, high molecular masses), dendrimers are representatives of polymer compounds. 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.
* Corresponding authors. Tel.: C7-8432-1047-37; fax: C7-8432387972. E-mail addresses: [email protected] (V.L. Furer), [email protected] (V.I. Kovalenko). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.038
The molecular mass of dendrimers increases exponentially step by step, according to their generation number. So dendrimers come smoothly from typical organic molecule (generation 0 or 1) through oligomers (generation 2–5) to high polymers (higher generations). Heteroatom-containing dendrimers, and particularly phosphorus-containing dendrimers, benefit from the global interest in dendrimers, but they have also their own specificity and properties [3]. 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. 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. The extraordinary versatile chemistry of phosphorus should still lead to more efficient methods of synthesis. The most promising fields of research certainly will concern the applications of these compounds in various aspects of chemistry, biology, physics and material science. One can anticipate in particular that
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compounds having a hydrophobic interior and a hydrophilic surface should have valuable properties [3]. However there are still only few structural studies of dendrimers. The difficulties of applying the traditional arsenal of physical chemistry and structural methods for linear polymers are well known. Such a situation exists in the field of the vibrational spectroscopy of the dendrimers as well, in spite of the obvious success of this method in the structural investigations of polymers. IR spectra of dendrimers up to now were used only for the analytical purposes. Only the Raman spectra of the poly(amidoamine) dendrimers were published [4]. Recently we reported about preparation, IR and Raman spectra of the phosphorus-containing dendrimers built up to 12th generation with terminal aldehyde groups and PCl bonds [5–9]. Twelve generations of one series were used to study connection between peculiarities of structure of these compounds and their vibrational spectra [7–9]. The IR and Raman spectra of the phosphorus-containing dendrimers were interpreted on basis of normal vibrations calculation [7–9]. In this work we try to show the value of FT-Raman spectroscopy for characterization of the phosphorus-containing dendrimers. Our aim was to study the connection between the peculiarities of the structure of these compounds and their FT-Raman spectra. We will elaborate on the spectroscopic data on ten generations of the phosphoruscontaining dendrimers. Due to the precise molecular composition and the large generation number we were able to observe the transformation of the spectral pattern from the molecule-core, through the row of oligomeric molecules to the monodispersed macromolecules, with mass equal to several million of atomic units. The FT-Raman spectra of dendrimers were interpreted on basis of normal vibrations calculation. Due to the predictable, controlled and reproducible structure of dendrimers they may be used as new standards for molecular spectroscopy and thus the information usually inaccessible may be obtained. The control growth of dendrons with increase of generation number enables one to distinguish the lines in the Raman spectra assigned to the specific molecular fragments. The possibility appears to separate lines assigned to the core, repeated units and terminal groups of dendrimers by difference spectroscopy method. The concepts, which emerge from such analysis, are essential for the synthesis, modification and application of dendrimers.
2. Theoretical background A dendrimer molecule has mainly a three-dimensional tree-like structure of dendrons, proceeding from a center, the so called initiator-core. A dendrimer molecule of nth generation consists of three types of units: initiator-core (c), repeating unit (r), and terminal group (t). The multiple functionality of the original molecule, which becomes the
initiator-core, may vary from 3 up to 6 and higher [1]. It determines the multiplicity of the core juncture (mc), and the number of dendrons. The repeating unit has multiple juncture (mr) at its outer end to join the terminal groups, as a rule mrZ2, but it may be equal to 3, 4, 5 etc. The number of repeating units Nr Z mc !ðmnr K 1=ðmr K 1ÞÞ; and the number of terminal groups Nt Z mc !mnr may be calculated for each generation number n of dendrimer. The ratio of Nr/Nt and the only initiator-core altogether determines the spectral pattern. In fact, when the generation number nZ0 there are NcZ1, NrZ0, and NtZmc; at nZ1 NcZ1, NrZmc, and NtZmc!mr, further at nZ2 NcZ1, NrZmc!(mrC1), and NtZmc!m2r etc. For higher generations the share of initiator-core becomes negligible, and the major role is played by the ratio Nt/Nr, which tends to the constant value, which is mrK1. In the most common case, when mcZ3, and mrZ2, this ratio Nt/Nrz1, when generation number is higher than 4. So, the most drastic changes in spectra of dendrimers and in other properties as well, are obviously to be seen at the beginning of the generation row. The control growth of dendrons with increase of the generation number enables one to assign lines in the Raman spectra to the specific molecular fragments. The possibility to separate lines in the difference spectra is defined by structure of dendrimers. A clear example of the above opportunity is given when in the difference spectra of dendrimers generations GnKGnK2 compensation is made by n(CaO) line, that is by terminal groups. The result of subtraction will be the difference spectra in which by one side of zero line will be situated the lines of the core (GnK2), and on the other side—the lines of the repeated unit (Gn). Such ‘spectral arithmetic’ appeared, for example, from the equations NcZ1, NtZ3 (G0) and NcZ1, NrZ9, NtZ12 (G2). Then the subtraction G0 from G2 look like NcC9NrC 12NtK4(NcC3Nt)Z9NrK3Nc. Thus in the difference spectra on side of G0 the lines of the core, and on side of G2 the lines of the repeated unit must be observed. The same results will be obtained during subtraction of Raman spectra of dendrimers G4KG2: NcC45NrC48NtK 4(NcC9NrC12Nt)Z9NrK3Nc and G6KG4: NcC189NrC 192NtK4(NcC45NrC48Nt)Z9NrK3Nc. In such case it may be expected that the pictures of difference spectra of all pairs G2KG0, G4KG2, G6KG4, G8KG6, G10KG8, will be similar to each other. Any differences in spectra must be connected with the changes of the environment of these fragments, variation of the conformational state, modification of the intra- and intermolecular interactions and so on.
3. Experimental The synthesis and main characteristics of the phosphorus-containing dendrimers were described in detail earlier [3,5] (Scheme). The molecular structure of the series
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generations are the following: the trifunctional core SaP–(– O–) 3, the repeating unit –O–C6H4–CHaN–N (CH3)–P(S)o, and the terminal groups are the 4-oxibenzaldehyde fragments –O–C6H4–CHO (Scheme). The studied dendrimers are amorphous compounds except the generation G0, which have a crystalline structure [6]. The IR spectra in the region 3500–150 cmK1 were recorded with an IFS-113v Bruker FTIR-spectrophotometer (4 cmK1 resolution). The Raman spectra in the region 3500– 10 cmK1 were excited by Nd: YAG laser line 1064 nm with power at sample 300 mW and were recorded with an FRA106/S Bruker FT-Raman module with a 4 cmK1 resolution. The G0 spectra were recorded in the crystalline state at room temperature and in the melt at 120 8C. The low-frequency Raman spectra (at 10–400 cmK1) of non-crystalline states include a strong broad Rayleigh component. It makes the distinguishing of Raman bands from Rayleigh background very difficult. This problem could be partly overcome by R(n) representation [11]. The R(n)-spectra were obtained directly from the Raman data according to RðnÞf n½1 K expðKhvc=kTÞIðnÞ; where n is the Raman shift, I(n) is the intensity in the Raman spectrum at n cmK1 and other symbols have their usual meanings. Fig. 1. Experimental Raman spectra of the generation’s series: G0 (1), G2 (2), G4 (3), G6 (4), G8 (5) and G10 (6).
4. Results and discussion The Raman spectra of studied the dendrimers are represented in Fig. 1. The assignment of lines was fulfilled on the basis of the calculated potential energy distribution (PED) [7–9]. The medium line at 1701 cmK1 is assigned to the CaO stretching vibrations of the aldehyde group. The line at 1598 cmK1 refers to the CCph stretching vibrations of the para-substituted aromatic ring. The medium line at 1576 cmK1 may be assigned to the n(CaN) stretching vibrations of the hydrazone fragment pCaN–No. A weak line at 1468 cmK1 refers to the symmetric deformation vibrations of the CH3 groups. A weak line at 1444 cmK1 is connected with the antisymmetric deformation vibrations of the CH3 groups. The medium line at 1205 cmK1 refers to the C–O stretch mixed with the CCph stretch and CCH bend. The medium line at 1152 cmK1 was assigned to the CCH bending. The weak line at 972 cmK1 may be assigned to the mixed mode of the CCph stretch, the CCC and CCH bend. The medium double bands at 853, 835 cmK1 are the pure CH out-of-plane bend. In the difference spectra of dendrimers generations GnK GnK2 the compensation was made by the n(CaO) band, that is by the terminal groups. From ‘spectral arithmetic’ it is clear that the result of the subtraction will be the difference spectra in which by one side of the zero line will be situated the band of the core (GnK2), and on the other side—the lines
of the repeated unit (Gn). This fact enables one to distinguish the lines of the core, repeated units and terminal groups vibrations. The positive lines in the difference Raman spectra of generations G2KG0 may be assigned to the vibrations of repeated units (Fig. 2). The line at 1601 cmK1 corresponds to the CCph stretch. The line at 1576 cmK1 assigned to the CaN is not seen in the experimental IR spectra but reveal itself in the Raman spectra. The weak line strong band at 1222 cmK1 is attributed to the CCph and P–N stretch. The weak line at 1163 cmK1 is the pure CCH bend. The weak line at 1137 cmK1 is assigned to the C–N stretch mixed with CNH bend. The very weak lines at 963, 914 cmK1 correspond to the P–O stretch and to the mixed vibration of the CNH bend, the P–N and N–N stretch. The location of the PaS stretching vibrations usually is not definitely obtained due to its low intensity in the IR spectra and strong dependence on the electronic properties of the substituents. There are two types of PaS bonds in the studied dendrimers: in the core and in the repeated unit. The negative in the difference Raman spectra G2KG0 line at 664 cmK1 may be connected with the n(PaS) in the core and the positive line at 639 cmK1 with the n(PaS) in the repeated unit. The low-frequency Raman spectra (at 10–400 cmK1) of
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Fig. 2. The difference Raman spectra (G2KG0melt) (1), (G4KG2) (2), (G6K G4) (3), (G8KG6) (4) and (G10KG8) (5).
Fig. 3. Reduced Raman spectra of the generation’s series: G0cryst (1), G0melt (2), G2 (3), G4 (4), G6 (5), G8 (6) and G10 (7).
dendrimers include a strong broad Rayleigh component. It makes the distinguishing of Raman bands from Rayleigh background very difficult. This problem could be partly overcome by the R(n) representation Raman spectra of dendrimers (Fig. 3). The frequencies of lines in the reduced spectra are shifted to the higher values relatively the ordinary spectra and for generations G0, G2 correspond to the frequencies in the Far IR spectra (Fig. 4). Structure of studied dendrimers is such that they have selection rules allowing the same bands in both IR and Raman spectra, but with different intensities for any given transition. The comparison of the experimental Raman and Far IR spectra of the G0 and the G2 molecules show their marked difference. Line at 383 cmK1 in the reduced Raman spectra of the G0 in the crystalline state may be compared with band at 378 cmK1 in the Far IR spectra and assigned to the out of plane and torsion vibrations of aromatic units [7–9]. Line at 331 cmK1 of the reduced Raman spectra corresponds to the band at 328 cmK1 in the Far IR spectra of the G0 and also refers to the out of plane and torsion vibrations of aromatic units. Line at 305 cmK1 in the reduced Raman spectra has its counterpart in the Far IR spectra at 298 cmK1 assigned to bending vibrations of the SPO and CCH angles. The frequency at 255 cmK1 in the reduced Raman spectra and the Far IR spectra is connected with bending vibrations of
the OCC and CCC angles. Line at 214 cmK1 in the reduced Raman spectra correspond to the band at 219 cmK1 in the Far IR spectra and refer to the bending vibrations of the CCH and OCC angles. The same conformity may be established between lines at 195, 156 cmK1 in the reduced Raman spectra and bands at 196 and 159 cmK1 in the Far IR spectra. The lines assigned to the core, repeated units and terminal groups of dendrimers may be separated by the difference FT-Raman spectroscopy method. The core SaP– (–O–)3, reveal itself by the medium line at 664 cmK1 of the PaS stretch. The repeated units show line at 1576 cmK1 assigned to n(CaN) vibrations of fragment pCaN–No, mixed with the stretching vibrations of the CCph bonds. The 4-oxibenzaldehyde terminal groups are characterized by the well-defined line at 1701 cmK1 assigned to the CaO stretching vibrations. The different phases of dendrimers can be nicely distinguished and recognized by Raman spectroscopy. There are some changes of lines intensity in the Raman spectra of G0 in the crystalline state and in the melt. In melt the intensity of the lines at 106, 255, 304, 330, 357, 384, 416, 442, 462, 539, 578, 633, 664, 833, 854, 934, 948, 961, 972, 1009, 1095, 1152, 1205, 1245, 1289, 1390, 1596, 1682, 2855, 3061, 3074 cmK1 decrease. The lines at 76, 97, 123,
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Fig. 4. Reduced Raman spectra of the generation’s G0cryst (1), G2 (3), and far IR spectra of the G0cryst (2) and G2 (3).
Fig. 5. Experimental Raman spectra of the generation G0 at room temperature (1) and in the melt at 120 8C (2).
322, 625, 673, 848, 1161, 1197, 1229, 1299, 1583, 1709, 3037, 3052, 3084 cmK1 increase their intensity in the melt. The intense narrow lines in the Raman spectra of G0 in the crystalline state are substituted by the weak wide lines in the melt (Fig. 5). In the Raman spectra of crystal there are two intense lines at 1691 and 1701 cmK1 in the region of the CaO stretching vibrations. At temperature 120 8C the only one band n(CaO) with higher frequency remains in Raman spectra of melt. Thus the low-frequency band is due to the crystalline structure. This shift of the n(CaO) line to the higher frequencies may be explained by the relaxation of dipole–dipole interactions in the melt. The relative decrease of the intensity of the carbonyl line at 1701 cmK1 in the Raman spectra of the melt is less than for the aromatic group line at 1597 cmK1. For line at 1701 cmK1 the full width at half height (FWHH) in the Raman spectra of the melt is much higher than that value in the spectra of the crystalline state. Thus the mutual orientation of these groups in the crystalline state and in the melt is different due to the conformational disorder. The relative decrease of the line intensity at 973 cmK1 in the Raman spectra of the melt is higher than for the most other lines. These changes may be ascribed to the conformational mobility in the P–O–Ph– bridge. The different phases of the dendrimer show sharp changes in the low frequency region of the reduced
Raman spectra of G0 (Fig. 4). One may expect that the specific three-dimensional structure of dendrimers molecules will be appeared if the low frequency region of reduced Raman spectra. Unfortunately the interpretation of the IR and Raman spectra in this region is not sufficiently reliable. But the whole similarity of spectra in this region for all dendrimers generations is obviously seen (Fig. 4). The characteristic feature of the studied Raman spectra of the generations G0KG10 is their similarity to each other (Fig. 1). The line FWHW and intensity show that very little changes occur for the first four generations and then achieve saturation. The Raman spectra of dendrimers depend on the ratio Nt/Nr. This ratio is strictly determined by the generation number and is equal to 1.3333, 1.0666, 1.0158, 1.0039 and 1.0009 for the generations G2, G4, G6, G8 and G10. In the Raman spectra of G2KG10 the line at 1576 cmK1 is observed. This line is absent in the Raman spectra of G0 and its intensity may be used for the characterization of the repeated units content. The intensity of the line 1701 cmK1 is proportional to the number of the terminal aldehyde groups. The ratio I1701/I1576 is equal to 0.38 (G2), 0.25 (G4), 0.23 (G6), 0.23 (G8), and 0.23 (G10). The curves of dependence I1701/I1576 and Nt/Nr versus the generation number are the very similar to each other (Fig. 6). Thus the marked differences in the Raman spectra of the first successive generations aspire to zero for the higher ones.
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Fig. 6. Ratio of terminal groups and repeated units Nt/Nr (1) and ratio of lines intensity 4.35!I1702/I1576 in the Raman spectra (2) versus generation number.
The saturation of the line intensity ratio is observed because the ratio Nt/Nr aspires to one when the generation numbers increase. The mobility of specific chemical groups may affect the mean lifetime of vibration levels. That’s why an analysis of the line shapes and line FWHH in Raman spectra may be very useful. The characteristic feature of studied Raman spectra of generations G0KG10 is their similarity to each other (Fig. 1). The FWHH values for the lines at 1155, 1226, 1576, 1596, 1702 cmK1 show very little changes in Raman spectra for the first four dendrimers generations and than achieve the saturation (Fig. 7). It is interesting that the studied lines refer to the vibrations of the repeated units and terminal groups. From these data it may be supposed that molecular skeleton contain the definite shape and conformation irrespective to dendrimers generation number. The rather rigid repeated units with little conformational freedom lead to the perfect microstructure of the studied phosphorus-containing dendrimers up to the 10th generation. The value of the FWHH for most of lines is less then 20 cmK1. For linear macromolecules, the lines in the Raman spectra are usually more diffuse and broad due to various types of the structural disorders, including polydispersity. From the composition of dendrimers it follows that the difference spectra of all pairs G2KG0, G4KG2, G6KG4,
Fig. 7. Full width at half height of Raman lines at 1596 cmK1 (1), 1155 cmK1 (2) 1702 cmK1 (3) 1576 cmK1 (4) and 1226 cmK1 (5) versus generation number.
G8KG6, G10KG8, must be similar to each other (Fig. 2). Really it is true for the first three pairs. But in the difference Raman spectra of G8KG6, G10KG8 in the region 1600 cmK 1 the pictures become more complicated. Instead of two positive lines at 1576 and 1605 cmK1 observed in the difference Raman spectra of (G2KG0), G4KG2, G6KG4 some EPR-like lines appeared in the difference spectra of the G8KG6, G10KG8 (Fig. 2). The lines in this region belong to the stretching vibrations of the bonds in the repeated units and terminal groups. The lines frequencies and intensity depend on the electronic properties of substituents and local conformation of the aromatic groups [10]. Thus the n(CCph) frequencies of the bonds in the repeated units and terminal groups are slightly different. One may expect that when generation number of dendrimer is sufficiently high (8–10) the steric interactions of the terminal groups occur and lead to the changes of the torsional angles around C–O and P–O bonds on the molecule periphery. These small changes are not seen in the ordinary Raman spectra but reveal their self in the difference spectra. Poley-type absorption due to the librations (hindered rotation) of the molecule in a ‘cage’ formed by its nearest neighbours is observed for many liquids and amorphous polymers in the region 100–125 cmK1. This band is very broad and its maximum is proportional to (V/2I)1/2, where V
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kind of vibrations, are gradually suppressed as generation number is increased. The FWHW of this line increases from 20 to 35 cmK1 when going from G0 in crystalline state to the melt and up to 40 cmK1 in the reduced Raman spectra for the higher generations. In summary Raman spectra study provide a proof of the actual structure of the phosphorus-containing dendrimers of the ten generations with aldehyde groups. The analysis of the line intensities and FWHW in the Raman spectra of the generations G0KG10 reveals their rather quick saturation and reflects strong homogeneity of the dendrimers molecules.
Acknowledgements Author is sincerely grateful to Carolin Lehner and Bruker IR Department for recording the Raman spectra.
References
Fig. 8. Frequencies of the librations line in reduced Raman spectra versus generation number.
is the height of the potential barrier to librations and I is the moment of inertia of the molecule [11,12]. In reduced Raman spectra of dendrimers the broad line near 100 cmK1 is observed for all generations (Fig. 3). Its frequency increases from 99 cmK1 (G0) to 117 cmK1 (G10) (Fig. 8). The intensity of this line is proportional to the number of the terminal oxybenzaldehyde groups. The line shift to higher frequencies when generation number increases may be explained by growth of the potential barrier for terminal groups librations. It indicates, that such
[1] Newkome GR, Moorefield CN, Vogtle F. Dendritic molecules. Deerfield Beach, FL: VCH; 1996. [2] Reinhoudt DN, Stoddart JF, Ungaro R. Chem Eur J 1998;4:1349. [3] Majoral JP, Caminade AM, Maraval V. Chem Commun 2002;2929. [4] Davis AP, Ma G, Allen HC. Anal Chem Acta 2003;496:117. [5] Lartigue ML, Donnadieu B, Galliot C, Caminade AM, Majoral JP. Macromol 1997;30:7335. [6] Majoral JP, Caminade AM. Chem Rev 1999;99:845. [7] Kovalenko VI, Furer VL, Vandyukov AE, Shagidullin RR, Majoral JP, Caminade AM. J Mol Struct 2002;604:45. [8] Furer VL, Kovalenko VI, Vandyukov AE, Majoral JP, Caminade AM. Spectrochim Acta Part A 2002;58:2905. [9] Furer VL, Kovalenko VI, Vandyukov AE, Majoral JP, Caminade AM. Vibr Spectrosc 2003;31:71. [10] Katrizky AR, Topsom RD. Chem Rev 1977;77:639. [11] Larkin IW. J Chem Soc Faraday Trans II 1973;69:1278. [12] Berstein VA, Ryzhov VA. J Macromol Sci Phys 1984;23:271.
Elementoorganic dendrimer characterization by Raman spectroscopy V.L. Furera,*, J.P. Majoralb, A.M. Caminadeb, V.I. Kovalenkoc,* a
Department of Physics, Kazan State Architect and Civil Engineering Academy, 1 Zelenaya, 420043 Kazan, Russia Federation b Laboratorie de Chimie de Coordination, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 4, France c A.E. Arbuzov Institute of Organic and Physical Chemistry, RAS, 8 Arbuzov Str., 420088 Kazan, Russia Federation Received 26 January 2004; received in revised form 18 May 2004; accepted 16 June 2004
Abstract The FT-Raman (10–3500 cmK1) spectra of ten generations of the elementoorganic dendrimers have been recorded and analyzed for the first time. Their spectral pattern is determined by the ratio Nt/Nr (Nt—number of terminal groups, Nr—number of repeating units). This ratio trends to mrK1 (mr—branching functionality of repeating unit), and becomes constant, when the generation number is higher than 3–5. Experimental Raman spectra of generations higher than 4 are very close similar, according theoretical approach. The dependence of line full width at half height in Raman spectra on generation number is established. The low frequency Raman spectra in R(n) representation are used to investigate the librations of terminal groups. The possibility appears to separate the lines assigned to the core, repeated units and terminal groups of dendrimers by difference spectroscopy method. From the difference Raman spectra of dendrimers it follows that for the generations higher than six steric congestion disturb the conformations of terminal groups. FT-Raman spectroscopy provides unique detailed information about the structure of the technologically relevant materials, which could not be obtained before with any other technique. q 2004 Elsevier Ltd. All rights reserved. Keywords: Phosphorus-containing dendrimers; Raman spectra; Normal vibrations
1. Introduction Dendrimers are a new type of compounds and the perspective materials studied by supramolecular chemistry [1,2]. A dendrimer molecule has mainly a three-dimensional tree-like structure proceeded from one center—the core (initiator core),—with branches (dendrons). The size of such a molecule is determined by a generation number, i.e. by the length of the dendrons, which consist of equal number of repeating units, and terminate in end groups [1]. According to their molecular topology (repeating units, terminal groups, high molecular masses), dendrimers are representatives of polymer compounds. 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.
* Corresponding authors. Tel.: C7-8432-1047-37; fax: C7-8432387972. E-mail addresses: [email protected] (V.L. Furer), [email protected] (V.I. Kovalenko). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.038
The molecular mass of dendrimers increases exponentially step by step, according to their generation number. So dendrimers come smoothly from typical organic molecule (generation 0 or 1) through oligomers (generation 2–5) to high polymers (higher generations). Heteroatom-containing dendrimers, and particularly phosphorus-containing dendrimers, benefit from the global interest in dendrimers, but they have also their own specificity and properties [3]. 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. 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. The extraordinary versatile chemistry of phosphorus should still lead to more efficient methods of synthesis. The most promising fields of research certainly will concern the applications of these compounds in various aspects of chemistry, biology, physics and material science. One can anticipate in particular that
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compounds having a hydrophobic interior and a hydrophilic surface should have valuable properties [3]. However there are still only few structural studies of dendrimers. The difficulties of applying the traditional arsenal of physical chemistry and structural methods for linear polymers are well known. Such a situation exists in the field of the vibrational spectroscopy of the dendrimers as well, in spite of the obvious success of this method in the structural investigations of polymers. IR spectra of dendrimers up to now were used only for the analytical purposes. Only the Raman spectra of the poly(amidoamine) dendrimers were published [4]. Recently we reported about preparation, IR and Raman spectra of the phosphorus-containing dendrimers built up to 12th generation with terminal aldehyde groups and PCl bonds [5–9]. Twelve generations of one series were used to study connection between peculiarities of structure of these compounds and their vibrational spectra [7–9]. The IR and Raman spectra of the phosphorus-containing dendrimers were interpreted on basis of normal vibrations calculation [7–9]. In this work we try to show the value of FT-Raman spectroscopy for characterization of the phosphorus-containing dendrimers. Our aim was to study the connection between the peculiarities of the structure of these compounds and their FT-Raman spectra. We will elaborate on the spectroscopic data on ten generations of the phosphoruscontaining dendrimers. Due to the precise molecular composition and the large generation number we were able to observe the transformation of the spectral pattern from the molecule-core, through the row of oligomeric molecules to the monodispersed macromolecules, with mass equal to several million of atomic units. The FT-Raman spectra of dendrimers were interpreted on basis of normal vibrations calculation. Due to the predictable, controlled and reproducible structure of dendrimers they may be used as new standards for molecular spectroscopy and thus the information usually inaccessible may be obtained. The control growth of dendrons with increase of generation number enables one to distinguish the lines in the Raman spectra assigned to the specific molecular fragments. The possibility appears to separate lines assigned to the core, repeated units and terminal groups of dendrimers by difference spectroscopy method. The concepts, which emerge from such analysis, are essential for the synthesis, modification and application of dendrimers.
2. Theoretical background A dendrimer molecule has mainly a three-dimensional tree-like structure of dendrons, proceeding from a center, the so called initiator-core. A dendrimer molecule of nth generation consists of three types of units: initiator-core (c), repeating unit (r), and terminal group (t). The multiple functionality of the original molecule, which becomes the
initiator-core, may vary from 3 up to 6 and higher [1]. It determines the multiplicity of the core juncture (mc), and the number of dendrons. The repeating unit has multiple juncture (mr) at its outer end to join the terminal groups, as a rule mrZ2, but it may be equal to 3, 4, 5 etc. The number of repeating units Nr Z mc !ðmnr K 1=ðmr K 1ÞÞ; and the number of terminal groups Nt Z mc !mnr may be calculated for each generation number n of dendrimer. The ratio of Nr/Nt and the only initiator-core altogether determines the spectral pattern. In fact, when the generation number nZ0 there are NcZ1, NrZ0, and NtZmc; at nZ1 NcZ1, NrZmc, and NtZmc!mr, further at nZ2 NcZ1, NrZmc!(mrC1), and NtZmc!m2r etc. For higher generations the share of initiator-core becomes negligible, and the major role is played by the ratio Nt/Nr, which tends to the constant value, which is mrK1. In the most common case, when mcZ3, and mrZ2, this ratio Nt/Nrz1, when generation number is higher than 4. So, the most drastic changes in spectra of dendrimers and in other properties as well, are obviously to be seen at the beginning of the generation row. The control growth of dendrons with increase of the generation number enables one to assign lines in the Raman spectra to the specific molecular fragments. The possibility to separate lines in the difference spectra is defined by structure of dendrimers. A clear example of the above opportunity is given when in the difference spectra of dendrimers generations GnKGnK2 compensation is made by n(CaO) line, that is by terminal groups. The result of subtraction will be the difference spectra in which by one side of zero line will be situated the lines of the core (GnK2), and on the other side—the lines of the repeated unit (Gn). Such ‘spectral arithmetic’ appeared, for example, from the equations NcZ1, NtZ3 (G0) and NcZ1, NrZ9, NtZ12 (G2). Then the subtraction G0 from G2 look like NcC9NrC 12NtK4(NcC3Nt)Z9NrK3Nc. Thus in the difference spectra on side of G0 the lines of the core, and on side of G2 the lines of the repeated unit must be observed. The same results will be obtained during subtraction of Raman spectra of dendrimers G4KG2: NcC45NrC48NtK 4(NcC9NrC12Nt)Z9NrK3Nc and G6KG4: NcC189NrC 192NtK4(NcC45NrC48Nt)Z9NrK3Nc. In such case it may be expected that the pictures of difference spectra of all pairs G2KG0, G4KG2, G6KG4, G8KG6, G10KG8, will be similar to each other. Any differences in spectra must be connected with the changes of the environment of these fragments, variation of the conformational state, modification of the intra- and intermolecular interactions and so on.
3. Experimental The synthesis and main characteristics of the phosphorus-containing dendrimers were described in detail earlier [3,5] (Scheme). The molecular structure of the series
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generations are the following: the trifunctional core SaP–(– O–) 3, the repeating unit –O–C6H4–CHaN–N (CH3)–P(S)o, and the terminal groups are the 4-oxibenzaldehyde fragments –O–C6H4–CHO (Scheme). The studied dendrimers are amorphous compounds except the generation G0, which have a crystalline structure [6]. The IR spectra in the region 3500–150 cmK1 were recorded with an IFS-113v Bruker FTIR-spectrophotometer (4 cmK1 resolution). The Raman spectra in the region 3500– 10 cmK1 were excited by Nd: YAG laser line 1064 nm with power at sample 300 mW and were recorded with an FRA106/S Bruker FT-Raman module with a 4 cmK1 resolution. The G0 spectra were recorded in the crystalline state at room temperature and in the melt at 120 8C. The low-frequency Raman spectra (at 10–400 cmK1) of non-crystalline states include a strong broad Rayleigh component. It makes the distinguishing of Raman bands from Rayleigh background very difficult. This problem could be partly overcome by R(n) representation [11]. The R(n)-spectra were obtained directly from the Raman data according to RðnÞf n½1 K expðKhvc=kTÞIðnÞ; where n is the Raman shift, I(n) is the intensity in the Raman spectrum at n cmK1 and other symbols have their usual meanings. Fig. 1. Experimental Raman spectra of the generation’s series: G0 (1), G2 (2), G4 (3), G6 (4), G8 (5) and G10 (6).
4. Results and discussion The Raman spectra of studied the dendrimers are represented in Fig. 1. The assignment of lines was fulfilled on the basis of the calculated potential energy distribution (PED) [7–9]. The medium line at 1701 cmK1 is assigned to the CaO stretching vibrations of the aldehyde group. The line at 1598 cmK1 refers to the CCph stretching vibrations of the para-substituted aromatic ring. The medium line at 1576 cmK1 may be assigned to the n(CaN) stretching vibrations of the hydrazone fragment pCaN–No. A weak line at 1468 cmK1 refers to the symmetric deformation vibrations of the CH3 groups. A weak line at 1444 cmK1 is connected with the antisymmetric deformation vibrations of the CH3 groups. The medium line at 1205 cmK1 refers to the C–O stretch mixed with the CCph stretch and CCH bend. The medium line at 1152 cmK1 was assigned to the CCH bending. The weak line at 972 cmK1 may be assigned to the mixed mode of the CCph stretch, the CCC and CCH bend. The medium double bands at 853, 835 cmK1 are the pure CH out-of-plane bend. In the difference spectra of dendrimers generations GnK GnK2 the compensation was made by the n(CaO) band, that is by the terminal groups. From ‘spectral arithmetic’ it is clear that the result of the subtraction will be the difference spectra in which by one side of the zero line will be situated the band of the core (GnK2), and on the other side—the lines
of the repeated unit (Gn). This fact enables one to distinguish the lines of the core, repeated units and terminal groups vibrations. The positive lines in the difference Raman spectra of generations G2KG0 may be assigned to the vibrations of repeated units (Fig. 2). The line at 1601 cmK1 corresponds to the CCph stretch. The line at 1576 cmK1 assigned to the CaN is not seen in the experimental IR spectra but reveal itself in the Raman spectra. The weak line strong band at 1222 cmK1 is attributed to the CCph and P–N stretch. The weak line at 1163 cmK1 is the pure CCH bend. The weak line at 1137 cmK1 is assigned to the C–N stretch mixed with CNH bend. The very weak lines at 963, 914 cmK1 correspond to the P–O stretch and to the mixed vibration of the CNH bend, the P–N and N–N stretch. The location of the PaS stretching vibrations usually is not definitely obtained due to its low intensity in the IR spectra and strong dependence on the electronic properties of the substituents. There are two types of PaS bonds in the studied dendrimers: in the core and in the repeated unit. The negative in the difference Raman spectra G2KG0 line at 664 cmK1 may be connected with the n(PaS) in the core and the positive line at 639 cmK1 with the n(PaS) in the repeated unit. The low-frequency Raman spectra (at 10–400 cmK1) of
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Fig. 2. The difference Raman spectra (G2KG0melt) (1), (G4KG2) (2), (G6K G4) (3), (G8KG6) (4) and (G10KG8) (5).
Fig. 3. Reduced Raman spectra of the generation’s series: G0cryst (1), G0melt (2), G2 (3), G4 (4), G6 (5), G8 (6) and G10 (7).
dendrimers include a strong broad Rayleigh component. It makes the distinguishing of Raman bands from Rayleigh background very difficult. This problem could be partly overcome by the R(n) representation Raman spectra of dendrimers (Fig. 3). The frequencies of lines in the reduced spectra are shifted to the higher values relatively the ordinary spectra and for generations G0, G2 correspond to the frequencies in the Far IR spectra (Fig. 4). Structure of studied dendrimers is such that they have selection rules allowing the same bands in both IR and Raman spectra, but with different intensities for any given transition. The comparison of the experimental Raman and Far IR spectra of the G0 and the G2 molecules show their marked difference. Line at 383 cmK1 in the reduced Raman spectra of the G0 in the crystalline state may be compared with band at 378 cmK1 in the Far IR spectra and assigned to the out of plane and torsion vibrations of aromatic units [7–9]. Line at 331 cmK1 of the reduced Raman spectra corresponds to the band at 328 cmK1 in the Far IR spectra of the G0 and also refers to the out of plane and torsion vibrations of aromatic units. Line at 305 cmK1 in the reduced Raman spectra has its counterpart in the Far IR spectra at 298 cmK1 assigned to bending vibrations of the SPO and CCH angles. The frequency at 255 cmK1 in the reduced Raman spectra and the Far IR spectra is connected with bending vibrations of
the OCC and CCC angles. Line at 214 cmK1 in the reduced Raman spectra correspond to the band at 219 cmK1 in the Far IR spectra and refer to the bending vibrations of the CCH and OCC angles. The same conformity may be established between lines at 195, 156 cmK1 in the reduced Raman spectra and bands at 196 and 159 cmK1 in the Far IR spectra. The lines assigned to the core, repeated units and terminal groups of dendrimers may be separated by the difference FT-Raman spectroscopy method. The core SaP– (–O–)3, reveal itself by the medium line at 664 cmK1 of the PaS stretch. The repeated units show line at 1576 cmK1 assigned to n(CaN) vibrations of fragment pCaN–No, mixed with the stretching vibrations of the CCph bonds. The 4-oxibenzaldehyde terminal groups are characterized by the well-defined line at 1701 cmK1 assigned to the CaO stretching vibrations. The different phases of dendrimers can be nicely distinguished and recognized by Raman spectroscopy. There are some changes of lines intensity in the Raman spectra of G0 in the crystalline state and in the melt. In melt the intensity of the lines at 106, 255, 304, 330, 357, 384, 416, 442, 462, 539, 578, 633, 664, 833, 854, 934, 948, 961, 972, 1009, 1095, 1152, 1205, 1245, 1289, 1390, 1596, 1682, 2855, 3061, 3074 cmK1 decrease. The lines at 76, 97, 123,
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Fig. 4. Reduced Raman spectra of the generation’s G0cryst (1), G2 (3), and far IR spectra of the G0cryst (2) and G2 (3).
Fig. 5. Experimental Raman spectra of the generation G0 at room temperature (1) and in the melt at 120 8C (2).
322, 625, 673, 848, 1161, 1197, 1229, 1299, 1583, 1709, 3037, 3052, 3084 cmK1 increase their intensity in the melt. The intense narrow lines in the Raman spectra of G0 in the crystalline state are substituted by the weak wide lines in the melt (Fig. 5). In the Raman spectra of crystal there are two intense lines at 1691 and 1701 cmK1 in the region of the CaO stretching vibrations. At temperature 120 8C the only one band n(CaO) with higher frequency remains in Raman spectra of melt. Thus the low-frequency band is due to the crystalline structure. This shift of the n(CaO) line to the higher frequencies may be explained by the relaxation of dipole–dipole interactions in the melt. The relative decrease of the intensity of the carbonyl line at 1701 cmK1 in the Raman spectra of the melt is less than for the aromatic group line at 1597 cmK1. For line at 1701 cmK1 the full width at half height (FWHH) in the Raman spectra of the melt is much higher than that value in the spectra of the crystalline state. Thus the mutual orientation of these groups in the crystalline state and in the melt is different due to the conformational disorder. The relative decrease of the line intensity at 973 cmK1 in the Raman spectra of the melt is higher than for the most other lines. These changes may be ascribed to the conformational mobility in the P–O–Ph– bridge. The different phases of the dendrimer show sharp changes in the low frequency region of the reduced
Raman spectra of G0 (Fig. 4). One may expect that the specific three-dimensional structure of dendrimers molecules will be appeared if the low frequency region of reduced Raman spectra. Unfortunately the interpretation of the IR and Raman spectra in this region is not sufficiently reliable. But the whole similarity of spectra in this region for all dendrimers generations is obviously seen (Fig. 4). The characteristic feature of the studied Raman spectra of the generations G0KG10 is their similarity to each other (Fig. 1). The line FWHW and intensity show that very little changes occur for the first four generations and then achieve saturation. The Raman spectra of dendrimers depend on the ratio Nt/Nr. This ratio is strictly determined by the generation number and is equal to 1.3333, 1.0666, 1.0158, 1.0039 and 1.0009 for the generations G2, G4, G6, G8 and G10. In the Raman spectra of G2KG10 the line at 1576 cmK1 is observed. This line is absent in the Raman spectra of G0 and its intensity may be used for the characterization of the repeated units content. The intensity of the line 1701 cmK1 is proportional to the number of the terminal aldehyde groups. The ratio I1701/I1576 is equal to 0.38 (G2), 0.25 (G4), 0.23 (G6), 0.23 (G8), and 0.23 (G10). The curves of dependence I1701/I1576 and Nt/Nr versus the generation number are the very similar to each other (Fig. 6). Thus the marked differences in the Raman spectra of the first successive generations aspire to zero for the higher ones.
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Fig. 6. Ratio of terminal groups and repeated units Nt/Nr (1) and ratio of lines intensity 4.35!I1702/I1576 in the Raman spectra (2) versus generation number.
The saturation of the line intensity ratio is observed because the ratio Nt/Nr aspires to one when the generation numbers increase. The mobility of specific chemical groups may affect the mean lifetime of vibration levels. That’s why an analysis of the line shapes and line FWHH in Raman spectra may be very useful. The characteristic feature of studied Raman spectra of generations G0KG10 is their similarity to each other (Fig. 1). The FWHH values for the lines at 1155, 1226, 1576, 1596, 1702 cmK1 show very little changes in Raman spectra for the first four dendrimers generations and than achieve the saturation (Fig. 7). It is interesting that the studied lines refer to the vibrations of the repeated units and terminal groups. From these data it may be supposed that molecular skeleton contain the definite shape and conformation irrespective to dendrimers generation number. The rather rigid repeated units with little conformational freedom lead to the perfect microstructure of the studied phosphorus-containing dendrimers up to the 10th generation. The value of the FWHH for most of lines is less then 20 cmK1. For linear macromolecules, the lines in the Raman spectra are usually more diffuse and broad due to various types of the structural disorders, including polydispersity. From the composition of dendrimers it follows that the difference spectra of all pairs G2KG0, G4KG2, G6KG4,
Fig. 7. Full width at half height of Raman lines at 1596 cmK1 (1), 1155 cmK1 (2) 1702 cmK1 (3) 1576 cmK1 (4) and 1226 cmK1 (5) versus generation number.
G8KG6, G10KG8, must be similar to each other (Fig. 2). Really it is true for the first three pairs. But in the difference Raman spectra of G8KG6, G10KG8 in the region 1600 cmK 1 the pictures become more complicated. Instead of two positive lines at 1576 and 1605 cmK1 observed in the difference Raman spectra of (G2KG0), G4KG2, G6KG4 some EPR-like lines appeared in the difference spectra of the G8KG6, G10KG8 (Fig. 2). The lines in this region belong to the stretching vibrations of the bonds in the repeated units and terminal groups. The lines frequencies and intensity depend on the electronic properties of substituents and local conformation of the aromatic groups [10]. Thus the n(CCph) frequencies of the bonds in the repeated units and terminal groups are slightly different. One may expect that when generation number of dendrimer is sufficiently high (8–10) the steric interactions of the terminal groups occur and lead to the changes of the torsional angles around C–O and P–O bonds on the molecule periphery. These small changes are not seen in the ordinary Raman spectra but reveal their self in the difference spectra. Poley-type absorption due to the librations (hindered rotation) of the molecule in a ‘cage’ formed by its nearest neighbours is observed for many liquids and amorphous polymers in the region 100–125 cmK1. This band is very broad and its maximum is proportional to (V/2I)1/2, where V
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kind of vibrations, are gradually suppressed as generation number is increased. The FWHW of this line increases from 20 to 35 cmK1 when going from G0 in crystalline state to the melt and up to 40 cmK1 in the reduced Raman spectra for the higher generations. In summary Raman spectra study provide a proof of the actual structure of the phosphorus-containing dendrimers of the ten generations with aldehyde groups. The analysis of the line intensities and FWHW in the Raman spectra of the generations G0KG10 reveals their rather quick saturation and reflects strong homogeneity of the dendrimers molecules.
Acknowledgements Author is sincerely grateful to Carolin Lehner and Bruker IR Department for recording the Raman spectra.
References
Fig. 8. Frequencies of the librations line in reduced Raman spectra versus generation number.
is the height of the potential barrier to librations and I is the moment of inertia of the molecule [11,12]. In reduced Raman spectra of dendrimers the broad line near 100 cmK1 is observed for all generations (Fig. 3). Its frequency increases from 99 cmK1 (G0) to 117 cmK1 (G10) (Fig. 8). The intensity of this line is proportional to the number of the terminal oxybenzaldehyde groups. The line shift to higher frequencies when generation number increases may be explained by growth of the potential barrier for terminal groups librations. It indicates, that such
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