J. Phys. Chem. B 2008, 112, 5363-5367
5363
Raman Study of the Rigidity of Penta-p-phenylene Derivatives Used as Legs in Molecular Tripods Isabel Lo´ pez-Toco´ n,† Daniel Pela´ ez,† Juan Soto,† Rodrigo Rico,‡ Chengzhi Cai,§ Juan M. Lo´ pez-Romero,‡ and Juan C. Otero*,† Departamento de Quı´mica Fı´sica and Departamento de Quı´mica Orga´ nica, Facultad de Ciencias, UniVersidad de Ma´ laga, 29071 Ma´ laga, Spain, and Department of Chemistry and Center for Materials Chemistry, UniVersity of Houston, Houston, Texas 77204-5003 ReceiVed: February 13, 2008; In Final Form: February 25, 2008
Molecular planarity of penta-p-phenylene (P5P) and several substituted derivatives with four side chains of various lengths, including deca(ethylene glycol) groups, is discussed by considering the changes in the intensity ratio between the Raman bands recorded at 1280 and 1220 cm-1. The intensity ratio between both bands I1280/I1220 shows a small increase with the size of the substituent, indicating a high rigidity for all these compounds, even those with long oligo(ethylene glycol) side chains. This result is important given that these phenylene derivatives are versatile building blocks for the construction of nanometric tripod-shaped adsorbates for biological applications since the side chains should prevent the nonspecific interaction with proteins.
I. Introduction The conformational structure in the oligo(p-phenylene) (OPP) series has been previously addressed due to their interesting optoelectronic properties.1-3 It has been shown that changes in the relative intensity between the Raman bands recorded at about 1280 and 1220 cm-1 is related to the planarity of these molecules. The intensity ratio I1280/I1220 decreases as the number of the conjugated rings increases indicating a larger delocalization and, thus, an increasing molecular rigidity.4-6 In this work, the dependence of the molecular planarity upon substitution in penta-p-phenylene (P5P) has been studied. Moreover, the Raman spectrum of P5P has been analyzed in order to complete the previously published results corresponding to the p-phenylene series.5 We are interested in developing giant, shape-persistent molecules for controlling the location and orientation of individual biomolecules on surfaces. These well-defined model systems will allow us to study the cellular response to extracellular signaling molecules at a single-molecule level. To this end, we decided to use the relatively rigid OPPs as the framework of the giant, shape-persistent adsorbate molecules. To prevent the nonspecific interactions with protein molecules in biological systems, the OPP frameworks need to be modified with long oligo(ethylene glycol) chains. It becomes critical that the OPP frames remain highly rigid after this modification. The results of this work demonstrate that modification of OPP derivatives with deca(ethylene glycol) (OEG, Figure 1) side chains does not reduce the rigidity of the molecule. II. Experimental Methods All samples P5P, P5P-Br2, and P5P-Br2-R4 with R ) CH3, OCH3, CH2Br, CH2OCH3,CH2O(CH2CH2O)3Me, and CH2O(CH2CH2O)10Et (Figure 1) have been synthesized in our laboratory * To whom correspondence should be addressed. E-mail:
[email protected]. † Departamento de Quı´mica Fı´sica, Universidad de Ma ´ laga. ‡ Departamento de Quı´mica Orga ´ nica, Universidad de Ma´laga. § University of Houston.
following the synthetic routes described in ref 7. The key step in the synthetic pathway is based on the Suzuki cross-coupling reaction. µ-Raman spectra of solid samples have been recorded with a Renishaw’s In Via Reflex µ-Raman spectrometer using a Leica microscope with a 50× objective by exciting with the 647.1 nm line from a krypton ion laser. The laser power was 5 mW at the sample position, and the resolution was 2 cm-1. Standard deconvolution programs have been used in order to quantify the intensity of each component in overlapping bands, and DFT force field calculations of P5P and P5P-Br2 have been carried out with the B3LYP nonlocal exchange correlation functional as implemented in the Gaussian98 set of programs8 in conjunction with the 6-31G* basis set. III. Results and Discussion A. Raman Spectra. We are interested in generating patterns of single molecules to study multivalent and multicomponent interactions in biological systems. The development of such molecular-scale devices requires a precise control of the orientation and location of the individual functional moieties on surfaces. For this purpose, we decided to use tripod-shaped adsorbate molecules to attach biomolecules at the focal point of the tripod. The framework of the tripod consists of three oligo(phenylene) heptamers as the tripod legs and one bromophenyl group as the functional arm. The prototype tripodshaped giant molecules have been synthesized and chemisorbed on silicon surfaces.9,10 However, these molecules are highly hydrophobic and will interact nonspecifically with proteins, thus interfering with the specific interaction of target molecules with the ligand on the focal point of the tripod. To overcome this problem, we must develop tripod systems modified with side chains that do not interact with protein molecules. The most common materials for resisting these nonspecific interactions are poly or oligo(ethylene glycol) (PEG or OEG). We chose the deca(ethylene glycol) as the side chains for the penta-pphenylenes, since we anticipated that long OEG groups are needed to mask the P5P backbone to render it resistant to such interactions. Moreover, P5Ps without alkyl substitution are
10.1021/jp8012893 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008
5364 J. Phys. Chem. B, Vol. 112, No. 17, 2008
Lo´pez-Toco´n et al.
Figure 1. Molecular structure of the penta-p-phenylenes (P5P-Br2-R4, R ) H, CH3, CH2Br, CH2OCH3, OCH3, CH2O(CH2CH2O)3Me, and CH2O(CH2CH2O)10Et).
TABLE 1: Raman Intensity Ratios I1280/I1220 from Biphenyl (2P) up to Hexa-p-phenylene (P6P) and the Selected Penta-p-phenylenes Derivatives oligo(p-phenylene)s
I1280/I1220
penta-p-phenylenes
I1280/I1220a
2P P3P P4P P5P P6P
25.01b 5.00b 2.14b 1.31a 1.07b
P5P-Br2 P5P-Br2-[CH3]4 P5P-Br2-[OCH3]4 P5P-Br2-[CH2Br]4 P5P-Br2-[CH2OCH3]4 P5P-Br2-[CH2O(CH2CH2O)3Me]4 P5P-Br2-[CH2O(CH2CH2O)10Et]4
1.48 1.37 1.50 1.60 1.71 1.62 1.64
a
This work. b Ref 5.
barely soluble in common organic solvents such as chloroform, ethyl ether, THF, or toluene, whereas P5Ps modified with four side chains are soluble in these solvents, and the solubility increases with the chain length. The OEG-substituted P5Ps are also soluble in aqueous solvents, which is necessary for biological applications. However, we need to address the potential issue that this modification may reduce the rigidity of the molecule. It has been demonstrated5,6 that the Raman intensity ratio I1280/ I1220 in oligo(p-phenylene)s is related to the number of phenyl rings in the molecule and its planarity. It can be seen that this intensity ratio decreases as the number of conjugated phenyl rings increases, therefore indicating a higher conjugation in the system and an increase of the molecular rigidity. This behavior has been explained on the basis of results obtained from doped oligo(phenylene) in which I1280/I1220 decreases due to the presence of a quinoid structure.11 In addition to this, molecular simulations show a dependence of the oligomer length and the torsional angle between the phenyl rings: a higher number of aromatic rings results in a lower torsional angle12 and, therefore, an increasing planarity. Table 1 and Figure 2 show the intensity ratio I1280/I1220 from biphenyl (2P) up to hexa-p-phenylene (P6P). The result obtained for P5P in this work agrees satisfactorily with the published values for the remaining oligo(p-phenylene)s5 and completes the series. The I1280/I1220 of P5P amounts to 1.31, being positioned between those of P4P, 2.14, and P6P, 1.07, as expected. When using the 514.5 exciting line, the I1280/I1220
Figure 2. Dependence of the Raman intensity ratio I1280/I1220 on the length of the oligo(p-phenylene)s (blue) and on the size of the pentap-phenylene derivatives (red).
decreases to 1.16 in agreement with the experimental behavior found in other oligomers.6,13 The Raman spectra of P5P and its substituted derivatives are shown in Figure 3, where the 1100-1400 cm-1 region has been expanded. All spectra are dominated by three characteristic bands recorded at about 1220, 1280, and 1600 cm-1 usually assigned to C-H in-plane bending, C-C inter-ring stretching, and aromatic C-C ring stretching, respectively. These three bands are always present in the Raman spectra of oligo(pphenylene)s and can be used as a test for quality of synthetic routes in these samples.1,14 The behavior of the intensity ratio I1280/I1220 for this series of derivatives is complex given that a correlation with the size of the substituent is not evident. Some
Raman Study of Penta-p-phenylene Derivatives
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5365
Figure 3. Raman spectra of penta-p-phenylene (P5P) and its derivatives (P5P-Br2-R4, R ) H, CH3, CH2Br, CH2OCH3, OCH3, CH2O(CH2CH2O)3Me, and CH2O(CH2CH2O)10Et).
samples only present the two characteristic and expected bands, like P5P or P5P-Br2-[CH2OCH3]4, whereas in the remaining ones the band recorded at 1220 cm-1 appears broad or splits into two components as happens in P5P-Br2, for instance. The splitting of these bands was already detected in the Raman of the OPP series, and it is due to the presence of inequivalent benzene rings.5 In this case one mode is still dominant, but all
of them contribute to the total intensity of the bands at 1220 and 1280 cm-1, and therefore, they have to be taken into account in order to analyze the I1280/I1220 ratio.15 Consequently, the intensity ratio I1280/I1220 has to be estimated taking into account the splitting or the broadening of the low-frequency band at 1220 cm-1. The values of Table 1 have been obtained in this way, and they have been plotted in the inset of Figure 2. The
5366 J. Phys. Chem. B, Vol. 112, No. 17, 2008 excellent correlation found in the whole series of derivatives is noticeable. Moreover, a small increase of the I1280/I1220 ratio with the size of the substituent can be appreciated. All the compounds exhibit similar I1280/I1220 values, between 1.37 and 1.71, whereas in P5P it is 1.31; therefore, the substitution has a minor effect in the I1280/I1220 ratio, even dealing with the largest substituents. B. Normal Modes Calculation. The splitting of the band at 1220 cm-1 is larger for the studied P5P derivatives than in the OPP series5 possibly because they are less symmetric thus allowing a strong mixing between nearby vibrations. For instance, in the case of P5P-Br2-[CH2Br]4 this produces an unresolved pair of bands with similar intensity at ca. 1220 cm-1, each one exhibiting half the intensity of the only recorded band in the spectra of P5P-Br2-[CH2OCH3]4 or P5P-Br2-[OCH3]4 (Figure 3). The largest splitting, which amounts to 20 cm-1, could be a consequence of interactions in the solid. It is a wellknown fact that the presence of several molecules in the unit cell can originate crystal field splitting. In other cases, intermolecular interactions could be responsible for small changes in the force field leading to some mode mixing between vibrations of similar wavenumbers. In order to know the nature of the involved normal modes, force field calculations of P5P, where splitting is absent, and P5P-Br2, which undergoes the clearest one, have been carried out at the B3LYP/6-31G* level of theory under D2h symmetry. Table 2 collects the DFT corrected wavenumbers, by using the standard scaling factor (0.96),16 of the totally symmetric vibrations related to the three characteristic Raman bands at 1220, 1280, and 1600 cm-1 of oligo(p-phenylene)s. The results are very similar for both molecules showing two pairs of calculated vibrations related to each one of the 1280/1220 cm-1 bands. The normal modes calculated at 1262 and 1257/1259 cm-1 should be related to the band at 1280 cm-1 and the wavenumbers at 1217 and 1196/ 1197 cm-1 to that at 1220 cm-1. The latter band shows the largest splitting, amounting to 20 cm-1, in agreement with the observed one in the spectrum of P5P-Br2. This fact could explain why the low-frequency band shows the largest splitting in the Raman spectrum.
Lo´pez-Toco´n et al. TABLE 2: B3LYP/6-31G* Scaled Wavenumbers (ν, cm-1, Scale Factor 0.96) and Calculated Relative Raman Intensities (I) of the Totally Symmetric Modes of P5P and P5P-Br2 Related to the Three Characteristic Raman Bands of Oligo(p-phenylene)s characteristic band (cm-1) 1600 1280 1220
P5P
P5P-Br2
ν
I
ν
I
1609 1596 1584 1262 1257 1217 1196
0 2 100 7 4 49 0
1610 1591 1578 1262 1259 1217 1197
0 74 100 12 6 83 0
Figure 4 displays the structures of P5P-Br2 at the turning points of the vibrational period of these four normal modes. As can be seen, all of them contain contributions of C-H in-plane deformation and C-C inter-ring stretching coordinates, respectively. Small changes in the molecular structure, produced by the substituents or by intermolecular interactions in the solid, could originate different mixings between both kinds of coordinates in this set of vibrations. Additionally, it could explain why the Raman intensity of the low-energy vibration is shared by two bands in specific cases when stronger coupling in the solid occurs. The force fields calculations carried out for isolated molecules do not account for these factors given that the results for P5P and P5P-Br2 are almost identical, in wavenumbers and intensities, thus predicting only a strong band at 1217 cm-1, the most symmetric one. On the contrary, the splitting of the band at ca. 1600 cm-1, only observed in the spectrum of P5P-Br2, is predicted by the DFT results, which yield two strong bands at 1578 (I ) 100) and 1591 (I ) 74) cm-1 in this case and just a strong band at 1584 cm-1 in the case of P5P. Although this agreement between theoretical and experimental intensities can be considered as a simple coincidence, it is an illustrative example of the mode mixing occurring under strong interaction in the solid. This effect could be taken as an alternative explanation for the doublet at 1600 cm-1 observed in the Raman of some of these compounds,
Figure 4. B3LYP/6-31G* calculated structures of P5P-Br2 at the turning points of the four normal modes related to the 1220 (left) and 1280 (right) cm-1 Raman bands.
Raman Study of Penta-p-phenylene Derivatives which has previously been discussed on the basis of the existence of a Fermi resonance between the aromatic C-C stretching fundamental and a combination band.17 Concerning the intensities, the failure of the theoretical estimations in reproducing the experimental behavior does not constitute a surprising result. This is due to two facts: (i) Raman intensity is very hard to predict by means of static polarizabilities calculations since it is a third-derivative property; moreover, this problem is further increased when dealing with large molecules such as the OPPs, and (ii) intermolecular interactions owing to the solid are not taken into account in the theoretical model. Therefore, Raman intensities cannot be adequately estimated by means of quantum chemical calculations. As an example of this, the simple ethylene molecule in gas-phase ethylene can be addressed.18 In this case, a factor of 2 agreement can be considered satisfactory if one takes into account the difficulty of both the theoretical prediction of Raman intensities and their experimental measurement. IV. Conclusion The Raman study indicates that the length of the four substituents in the penta-p-phenylenes does not significantly affect the planarity and rigidity of the backbone of the oligomers, probably due to a sufficient separation between the sites which reduces steric hindrances. This is a good result in order to use these compounds as building blocks in the construction of shapepersistent adsorbate molecules for biological applications. Acknowledgment. This research has been supported by the Spanish Ministerio de Educacion y Ciencia (Project Nos. NAN2004-09312-C03, CTQ2006-02330, and CTQ2007-62386) and Junta de Andalucia (Project No. FQM-01895). C.C. acknowledges supports from The Welch Foundation and the NSF CAREER Award (CS-0349228).
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