ARTICLE pubs.acs.org/JPCA
Integrated NMR and Computational Study of PushPull NLO Probes: Interplay of Solvent and Structural Effects Alberto Marini, Sara Macchi, Sandro Jurinovich, Donata Catalano,* and Benedetta Mennucci* Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, 56126, Pisa Italy
bS Supporting Information ABSTRACT: In this study we combined QM calculations and NMR measurements to understand at a detailed level the complex interplay of structural/electronic properties with the effects of the solvent in the NLO activity of pushpull systems, quantified in terms of variations of the static hyperpolarizability. Different parameters (bond lengths and bond length alternation, vibrational frequencies, electronic charge distribution) are introduced and tested to rationalize both the solvent sensitivity of three molecular systems (namely, p-nitroaniline, ethyl 4-ammino benzoate, and 5-nitro-1H-indole) and the differences among them. This analysis has finally allowed us to establish a clear correlation between the charge transfer behavior of the systems, their NLO properties, and NMR parameters also validating simplified but effective chemical analyses based on resonance limit forms.
1. INTRODUCTION In recent years much attention has been focused on a specific type of organic molecule as potential efficient nonlinear optics (NLO) chromophores,1 the pushpull molecules characterized by a polarizable π-electron system, and donor and acceptor groups to create an asymmetric polarizability.25 These compounds are promising candidates for applications as NLO materials due to the fact that they typically possess large molecular dipoles and often have extremely high molecular hyperpolarizabilities, a fundamental property to design efficient NLO materials.6 To explain these characteristics, pushpull molecules are generally represented in terms of two resonance limit forms showing a neutral and a zwitterionic character. In Figure 1 we report a pictorial representation of the limit forms for a typical pushpull molecule. By modulating the relative weight of the two forms a very different behavior can be induced in the systems with corresponding amplification (or depletion) of the NLO activity. A common way to control the nonlinear optical response is to vary the electronic asymmetry of the molecule, and this is generally done by changing the relative strengths of the donor and acceptor groups. Alternatively, or in addition, also the molecular shape and dimensionality can be tuned to improve selected linear and nonlinear properties.7 A further factor which can strongly affect the NLO properties is the polarity of the solvent: its influence, combined with a proper structural design, can in fact lead to an extraordinarily wide range of β values.8,9 Solvent effects on the relative contributions of the limit forms reflect significant changes of the πelectron structure as shown by theoretical investigations.1013 The latter are commonly quantified in terms of single-to-double bond changes passing from the neutral to the zwitterionic form. This structural analysis can be effectively performed introducing r 2011 American Chemical Society
a structural index indicated as bond length alternation (BLA, see Figure 1 for typical values). Together with the BLA, another promising parameter to consider is the NMR chemical shift of the carbon nuclei involved in the possible single-to-double bond change passing from the neutral to the zwitterionic form. In particular, one expects that differences in chemical shifts of 13C nuclei linked to the donor and acceptor groups represent a very effective tool to both rationalize and predict a potential NLO activity. 13C chemical shifts are in fact much simpler to measure than the geometrical parameters needed to define BLA parameter; moreover, they can be easily obtained in all solvents where the probe is fairly soluble. Despite these evident advantages, studies which exploit NMR spectra to investigate promising candidates for NLO applications are not common.1416 This might be due to the difficulty in finding a simple and an effective relation between NMR parameters and NLO properties. In the present study we focus on this aspect and show that such relation can be found by integrating a computational study based on accurate quantum-mechanical methods accounting for solvent effects with NMR measurements in different solvents. Three NLO probes will be investigated, namely, 4-nitroaniline (PNA), ethyl 4-ammino benzoate (EPAB), and 5-nitro-1Hindole (5NI). These molecules, shown in Figure 2, comply to the scheme of Figure 1 and present an aromatic ring para substituted by a donor group (NH2 for PNA and EPAB, NH for 5NI) and an acceptor group (NO2 for PNA and 5NI, COO for EPAB). The three systems show relevant similarities from a Received: April 26, 2011 Revised: July 15, 2011 Published: July 28, 2011 10035
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Figure 1. Limit structures of a typical pushpull molecule, and dependence of BLA on the neutral/zwitterionic character. BLA is here defined as the average length difference of adjacent CC bonds involved in the single-to-double bond change, passing from the neutral to the zwitterionic form.
Figure 2. Molecular structures and atom labeling for the studied systems: PNA, EPAB, and 5NI .
structural and an electronic point of view, but 5NI is notably asymmetric with respect to the para axis of the aromatic ring. The present study will use DFT calculations, combined with a continuum solvation model,17 of geometries and infrared frequencies and NBO population18 to interpret the changes in the hyperpolarizability β for the selected molecules and their tuning by the solvent. The soundness of this investigation will be validated by the comparison between computed and experimental 13C and 1H chemical shifts and 1H1H and 1H13C scalar (J) couplings, which reflect both structural and electronic modifications in the π-electron system. Finally, a direct correlation will be evidenced between the trends of selected chemical shift differences in various solvents and the trends of relevant computed electronic properties, such as charge differences on the corresponding nuclei.
2. MATERIALS AND METHODS 2.1. Materials. The NLO probes, namely, 4-nitroaniline (PNA; chemical purity g 99%), ethyl 4-ammino benzoate (EPAB; chemical purity g 98%), and 5-nitro-1H-indole (5NI; chemical purity g 99%), the solvents, namely, cyclohexane-d12 (C6D12; 99.5 atom % D), carbon tetrachloride (CCl4; chemical purity g 99.8%), chloroform-d1 (CDCl3; 99.8 atom % D), dichloromethaned2 (CD2Cl2; 99.9 atom % D), acetone-d6 (99.5 atom % D), dimethyl sulfoxide-d6 (DMSO-d6; 99.9 atom % D), and deuterated water (D2O; 99 atom % D), and the reference compound tetramethylsilane (TMS; chemical purity > 99.9%) were purchased from Sigma-Aldrich and used for NMR investigations without further purification.
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2.2. NMR Measurements and Spectral Parameters. Solutions of each probe in each solvent mentioned above were prepared or attempted. In effect, the solubility of the probes in C6D12, CCl4, and D2O is low or, particularly in the case of PNA, almost vanishing. Therefore, some samples proved to be unsuitable for yielding detectable 13C or even 1H NMR spectra. The probe concentration in the useful samples was estimated to range from about 0.05 (5NI in DMSO-d6) to about 0.005 M (PNA in D2O). 1 H NMR and 13C NMR spectra (both 1H decoupled and coupled) were recorded at 22 °C on a Varian VXR 300 spectrometer, operating at 299.93 MHz and 75.42 MHz for 1H and 13C, respectively, using standard pulse sequences and parameters. The number of averaged scans was in the range 1600 for the 1H spectra and 400020 000 for the 13C spectra, depending on the sample concentration and, in the case of the 13C spectra, on being or not being the 1H decoupling procedure active. In order to analyze the 1H multiplet structure and assign the experimental 1H1H J couplings, a series of homonuclear decoupling experiments was run on 5NI in DMSO-d6. In order to assign some long-range 13C1H J couplings, HETCOR bidimensional spectra were recorded on EPAB in CDCl3, acetone-d6, and DMSOd6, limited to the interval of the aromatic signals. The atom labeling used for the three solutes is shown in Figure 2. Selected examples of the NMR spectra, tables of the 13C and 1H chemical shifts referred to TMS, and tables of 1H1H and 1H13C scalar couplings are supplied as Supporting Information. 2.3. Computational Details. All calculations were performed using the Gaussian 09 package.19 Geometry optimizations, IR intensities, and frequencies were calculated at the DFT level using combination of the B3LYP functional20 with the 6-311G(d,p) basis set. Static hyperpolarizabilities were calculated with two different functionals, namely, B3LYP and CAM-B3LYP;21 for both the 6-311++G(2d,p) basis set was used. Calculations of chemical shielding tensors (CSTs) and spin spin (J) coupling constants were performed by using different exchange-correlation functionals. PNA, the simplest of the investigated systems, was used for testing five different hybrid GGA functionals, namely, PBE1PBE,22 MPW1PW91,23 and its variants MPW1LYP, MPW1PBE, MPW3PBE, and one meta-GGA functional, M06L.24 For CSTs M06L, MPW1PW91, and MPW3PBE have been shown to be the most accurate as already found in previous studies,25 whereas for scalar coupling constants MPW1PW91 and MPW3PBE have shown the best correlation to experimental data. For these reasons, calculations on EPAB and 5NI were performed with M06L, MPW1PW91, and MPW3PBE for CST and MPW1PW91 and MPW3PBE for J. The 6-311++G(2d,p) basis set was used for all calculations of NMR parameters in combination with the gaugeindependent atomic orbital (GIAO) method.26 In order to relate the calculated chemical shielding values (σ scale) to the corresponding experimental chemical shifts (δ iso 1 13 scale, where δi = σiso TMS σi ), both the H and the C isotropic chemical shielding values were calculated for TMS at the same level of theory as done for the molecular probes. This procedure is clearly affected by possible inaccuracies in determining the reference TMS value; to reduce such effects we finally shifted all the calculated δi by a constant value determined for each level of QM calculation, as to eliminate any systematic shifts with respect to the experimental data. Population analysis was computed using the MPW1PW91 functional and 6-311++G(2d,p) basis set, exploiting the natural bond orbital method (NBO version 3)18 implemented in Gaussian 09. 10036
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Figure 3. Variation of bond lengths with respect to the vacuum values (Δd parameter) versus the solvent dielectric constant for selected bonds of (A) PNA, (B) EPAB, and (C) 5NI . See Figure 2 for atom labeling.
The integral equation formalism (IEF) version of PCM27 was used to describe the effects of the environment on both the structures and the properties of NLO materials. Following this model, the solute molecule, quantum-mechanically treated, is hosted in a cavity inside a dielectric representing the solvent; the solvent polarization is described in terms of surface charge induced on the cavity. The shape of the cavity is obtained as the envelope of a set of spheres centered on selected atoms of the solute and is thus determined by the solute geometry. In the present study, the PCM cavities were constructed by applying the united atom topological model and the atomic radii of the UFF force field28 as implemented in the Gaussian code. According to this model, a sphere is associated to each atom (excluding the hydrogens where not explicitly expressed) with radii defined according to the type of atom and of the bonds, namely, we have R(C) = 1.925, R(CH) = 2.125, R(CH2) = 2.325, R(CH3) = 2.525, R(O) = 1.75, R(N nitro group) = 1.83, R(N amino group) = 1.6, and R(H connected to N atom) = 1.2 (all values are in Å). The solvents computationally investigated were carbontetrachloride (CCl4, ε = 2.23), chloroform (CHCl3, ε = 4.71), dichloromethane (CH2Cl2, ε = 8.93), acetone (Acet, ε = 20.49), dimethyl sulfoxide (DMSO, ε = 46.83), and water (H2O, ε = 78.36).
3. RESULTS AND DISCUSSION The presentation and discussion of the results is organized in two main sections. The first focuses on analysis of solvent effects calculated for ground state geometries, IR frequencies, electronic charge distributions, and static hyperpolarizabilities in the three investigated pushpull molecules. The second section presents a comparison between calculated and experimental NMR data (chemical shifts and scalar couplings). The analyses reported in the two parts are finally combined to shed light on the possible correlations between specific spectroscopic signals and structural-electronic changes induced by solvent in the NLO systems. 3.1. Structure and NLO Property Relationships: A Quantum Chemical View. All molecules (see Figure 2) are consti-
tuted by a benzene ring where a donor group (NH2 for PNA and EPAB, NH for 5NI) and an acceptor group (NO2 for PNA and 5NI, COO for EPAB) are attached in the para position in order to confer a pushpull character to the molecule. We note,
however, that the NH group of 5NI is part of a five-membered ring condensed to the benzene one: this peculiarity has to be taken into account in the following discussion of the results. These systems were chosen since they show relevant similarities from both the structural and the electronic point of view, with the aim of revealing possible analogies in the relation between solvent effects and NLO properties. 3.1.1. Structural Changes, Bond Length Alternation, and Vibrational Frequencies. The geometry optimizations performed on the three investigated systems always provided planar minimum energy conformations, except for the hydrogens of the EPAB ethyl chain. The solvent modifies the structural parameters of the equilibrium geometry in vacuo, here considered as the reference geometry. In the case of pushpull systems, the structural modifications induced by increasing the solvent dielectric constant are commonly rationalized looking at the bond length variations associated to a continuous charge delocalization from the donor to the acceptor group, as indicated in Figure 3. To this purpose, it is useful to look at Δd, the difference between the bond length computed in the solvent and in vacuo. The trends of the Δd values relative to the bonds most affected by the solvent are shown in Figure 3. In all cases, a visible change of the Δd parameters is obtained passing from ε = 1 (vacuum) to ε = 20 (acetone), while the trends become almost flat for ε > 20. By increasing the solvent polarity, CC and CN bond lengths are changed much more effectively than CH ones: the former group of bonds shows maximum length variations of about 1.8% for PNA and about 1% for the other probes, while for CH, the maximum variations are of about 0.3%. For all probes, C1N1 and C4N2 (C4C(O) in EPAB) bond lengths decrease with increasing solvent polarity; the C2C3 bonds are only slightly shortened. In addition, the C1C2 and C3C4 bonds lengthen as does the CO double bond in EPAB . It is worth noting that in 5NI which is asymmetric with respect to the para axis, the CaCb bond length increases and the CbN1 one slightly decreases with solvent polarity. All these data confirm the pushpull character of the three probes according to the scheme of Figure 1 and also suggest that a further resonance structure, shown in Figure 4, could be relatively important for 5NI in solvents with high dielectric constant. This structure involves changes in the bond 10037
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Figure 4. Possible resonance limit forms for 5NI.
Figure 5. ΔBLA = BLA(sol) BLA(vac) (103 Å) as a function of the solvent dielectric constant for PNA, EPAB, and 5NI systems.
lengths of the five-membered ring and shows that in 5NI a further alternative delocalization pattern can be active. As pointed out in the Introduction, the results of the structural analysis of pushpull molecules are commonly and usefully summarized by means of the bond length alternation (BLA) parameter, defined as the average difference between the lengths of adjacent CC single and double bonds in a polyene chain. In this work, the BLA parameter for each molecule was evaluated including the pairs of bonds involved in the single-to-double bond change passing from the neutral to the zwitterionic form, as illustrated in Figure 1, namely, N1C1, C1C2, C2C3, C3C4, C4N2(C). BLA values in vacuo (0.001 Å in PNA, 0.006 Å in EPAB, 0.006 Å in 5NI) reveal a cyanine-like structure for PNA (BLA ≈ 0) and a prevailing neutral character (BLA > 0) for EPAB and 5NI . Moving to solvated systems, BLA values become negative for the three compounds in all solvents, even in the less polar ones, with the exceptions of EPAB and 5NI in CCl4, where BLA almost vanishes. These results are summarized in Figure 5, where we report the trend of the variation of BLA with respect to the vacuum value by increasing the polarity of the solvent. As shown in the figure, the overall variation of BLA is maximum for PNA and slightly higher for 5NI than for EPAB . All this clearly supports the growing importance of the zwitterionic limit form, with respect to the neutral one, for representing the three probes in solvents with increasing dielectric constant. To conclude the analysis on structural changes induced by the solvent effects, we present an analysis of calculated infrared (IR) vibrational frequencies for selected modes involving the donor and acceptor groups and the carbon atoms bonded to them. For EPAB, the carbonyl stretching band is also considered: in fact, its sensitivity to the solvent is commonly used in the literature to quantify solvent effects. As done for bond lengths, also here we report the shifts (Δν) induced by the solvent on selected
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frequencies; these data are collected in Figure 6, where they are plotted against the solvent dielectric constant (a complete list of the selected frequencies in the different solvents is reported in the Supporting Information) The Δν trends show the strongest variation within the first 20 units of dielectric constant, as already seen for all the parameters previously discussed. All selected frequencies decrease, except the one relative to the D mode, which slightly increases. Infact, an important component of this mode is the stretching of the CN1 bond, whose double-bond character is enhanced in polar solvents. As far as the C, C0 , and CCO modes are concerned, we recall that, going from the neutral to the zwitterionic form, the double character of the NO bonds (or of the CdO bond in EPAB) is partially lost and the corresponding stretching frequencies decrease. Also, the vibrational frequencies relative to modes involving the NH bonds (A, A0 , and B modes) decrease in polar solvents due to enhanced electron donation, which weakens the NH bond. The largest variations occur for PNA and EPAB, while the shifts are smaller for 5NI . 3.1.2. NBO Charges and Hyperpolarizability. Electronic charge variations due to different solvents were analyzed by the NBO method. As before, also here the results are presented in terms of a difference parameter Δqsol, defined as the difference between the local NBO charge computed for a given atom in solvent and in vacuo. In Figure 7, the Δqsol parameters for selected atoms in each molecule are plotted against ε. For this purpose, the charges located on the atoms of the donor group are considered as all in one and added together; the resulting Δqsol parameter is labeled D. The global charge on the acceptor group is analogously computed, and the pertinent parameter is labeled A. Moreover, the charge formally assigned to each aromatic carbon is the sum of the charges on the carbon itself and on the hydrogen atom bonded to it. As expected, the Δqsol values for the acceptor and donor groups become more positive and more negative, respectively, with increasing solvent polarity (see red and blue lines in Figure 7). The variations shown by the charges on the aromatic carbons are less pronounced. The electron density decreases on C1 and C3 and increases on C2 and C4 for PNA and EPAB, reflecting both the charge transfer process and the mesomeric effects within the benzene unit. It can be noticed that the Δqsol trends of C1 and C4 follow, on a reduced scale, those of the adjacent donor and acceptor groups, respectively. The results for 5NI are somewhat peculiar: in particular, Δqsol on C4 (adjacent to the acceptor group) slightly increases passing from the vacuum to CCl4 solution and its trend is quite flat throughout the investigated range of ε. Moreover, NBO analysis confirms the importance of the other possible zwitterionic resonance structure for 5NI, in which negative donor group charge can be located on the Ca atom as shown in Figure 4. The differences between the Δqsol parameter of the donor and the acceptor groups in apolar (polar) solvent are about 20 (50) 103, 10 (20) 103, and 5 (15) 103 au for PNA, EPAB, and 5NI, respectively. Therefore, the CT efficiency in the three probes follows the order PNA > EPAB > 5NI . The same ordering was pointed out by the analysis of vibrational frequencies (Δν, Figure 6). The CT character of a system is due to the interplay between the electron mobility properties of the π system and the difference of electronic affinity of the donor and the acceptor groups. In PNA, the strong CT character can be ascribed to the large difference between the electron affinities of the amino and nitro groups, which is attenuated in EPAB and in 5NI by the presence of a less efficient acceptor (COO) and donor (enaminic NH) group, respectively. 10038
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Figure 6. Δν (cm1) for selected modes of (A) PNA, (B) EPAB, and (C) 5NI systems as a function of the solvent dielectric constant.
Figure 7. Δqsol (103 au) parameter relative to (A) PNA, (B) EPAB, and (C) 5NI systems as a function of the solvent dielectric constant.
These findings are now compared with the NLO character of the three systems which is here quantified in terms of their static hyperpolarizabilities. By definition, the hyperpolarizability β is a third-rank tensor. In the experiments, however, it is common to define the vector components of β in the direction of the permanent dipole moment which defines the z axis; within this framework, the quantity of interest is the so-called βz29 βz ¼
1 3
∑j ðβzjj þ βjzj þ βjjz Þ
ðj ¼ x, y, zÞ
ð1Þ
All three systems studied are (almost) planar; as a result, the outof plane βy component is practically null. In the case of PNA, which exhibits an exact axial symmetry along the para or z axis, also βx is zero; on the contrary, for EPAB and 5NI, for which the symmetry is lower, βx is not zero. However, it is still 1 order of magnitude lower than βz: this confirms that our analysis in terms of a single component remains valid for all the three probes. The variation of βz with respect to the solvent is shown in Figure 8 for the three compounds. In order to have a better
Figure 8. Δβz = βz(sol) βz(vac) (1030 esu) for PNA, EPAB, and 5NI systems as a function of the solvent dielectric constant. Full and dashed lines refer to B3LYP and CAM-B3LYP functionals, respectively. 10039
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Figure 9. Correlation of calculated and experimental 1H (top) and 13C NMR (bottom) chemical shifts (ppm) for the three compounds in all solvents. Mean absolute deviation values for MPW1PW91/MPW3PBE/ M06L are 0.37/0.35/0.28 (1H) and 1.78/1.64/2.34 (13C). All values are reported in the Supporting Information.
appraisal of the reliability of the chosen QM level of theory in calculating hyperpolarizabilities, two different DFT functionals have been compared, namely, B3LYP (the same used in all analyses reported before) and CAM-B3LYP, which has been successfully used to calculate linear and nonlinear optical properties.30 The values of βz computed in vacuo are 14(12) 1030, 10(9) 1030, and 9(7) 1030 esu for PNA, EPAB, and 5NI, respectively, at the B3LYP(CAM-B3LYP) level. The results obtained with the two functionals are very similar, especially if we focus on the trends moving from one molecule to the other and changing the solvent. These similarities confirm that B3LYP is a reliable level of theory for reproducing optical properties of these probes. Moving now to analysis of the trends, as expected, for all systems, βz significantly increases with increasing solvent polarity. In particular, going from vacuum to water the percent increase is 470/542 for PNA, 280/278 for EPAB, and 476/466 for 5NI at the B3LYP/CAM-B3LYP level. It is noticeable that this much larger solvent sensibility of PNA was already suggested by the previous analysis of the structural changes (Δd, Figure 3),
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Figure 10. Correlation of calculated and experimental 1H1H (top) and 13C1H (bottom) scalar couplings (Hz) for the three compounds in all solvents. Mean absolute deviation values for MPW1PW91/ MPW3PBE are 0.71/0.55 (1H1H) and 0.72/1.97 (13C1H). All values are reported in the Supporting Information.
vibrational frequencies (Δν, Figure 6), and NBO charges (Δqsol, Figure 7). What is partially unexpected is instead the larger change of 5NI with respect to EPAB passing from vacuum to solution, as previously suggested only by the variation of BLA. The larger solvent sensitivity shown by βz of 5NI leads to an inversion in the order of NLO character with respect to EPAB moving from vacuum to solution. This is an interesting result which shows the importance of properly considering solvent effects when designing systems for NLO applications. This larger solvent sensitivity of 5NI can be explained looking at the % variations in the three compounds: namely, PNA and 5NI behave very similarly, while EPAB presents much smaller variations which evidently are related to the weaker acceptor group. The fact that the other investigated parameters did not lead to the same result but instead suggested a less pronounced sensitivity of 5NI with respect to EPAB (and PNA) can be explained in terms of the three resonance forms possible for 5NI (see Figure 4). In fact, the solvent-induced charge transfer which characterizes all the investigated donoracceptor systems is reflected in changes in local properties, such as NBO charges, but these changes will be reduced if the system has different 10040
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Figure 11. Trends of the experimental and computed 1H1H spinspin couplings (Hz) for the three compounds against the solvent dielectric constant. The labels refer to Figure 2. Empty circles refer to experimental data, whereas solid and dashed lines collect MPW3PBE and MPW1PW91 calculations, respectively.
Figure 12. Trends of the experimental and computed 1H13C spinspin couplings (Hz) for the three compounds against the solvent dielectric constant. The labels refer to Figure 2. Empty circles refer to experimental data, whereas solid and dashed lines collect MPW3PBE and MPW1PW91 calculations, respectively. 10041
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Figure 13. Comparison between trends of (a) Δq[C1C4] (au) and (b) experimental and computed Δδ[C1C4] (ppm) for the three compounds. (b) Empty circles refer to experimental data, whereas solid/dashed/dotted lines collect MPW3PBE/MPW1PW91/M06L calculations, respectively.
delocalization patterns which can distribute the charge on more sites. This is exactly what was found for 5NI . 3.2. NMR Analysis of the PushPull Effect. The analysis performed in the previous sections has shown a clear interplay between solvent effects and different structural and electronic characteristics of the three investigated compounds. Here such a correlation is further analyzed using a direct comparison between calculated and experimental NMR parameters, namely, 1H and 13C NMR chemical shifts and 1H1H and 13C1H spinspin couplings. The prediction of 1H and 13C NMR spectra by DFT methods has become an established tool31 due to the reliability and accuracy reached by recent functionals in this kind of applications.32 In particular, 13C chemical shifts of a wide variety of organic molecules have been calculated by DFT methods with an accuracy of a few ppm both for isolated and for solvated systems. This general good behavior is
confirmed here as shown in the correlation plots reported in Figure 9 in which the chemical shieldings of the three investigated systems in all solvents are collected. The graphs clearly show that the three selected functionals give a very good description for both nuclei. In the case of 1H, some discrepancies are however evident. These data refer to hydrogens bonded to N1 (HR,R0 of PNA and EPAB at 57 ppm for PNA and EPAB and at 12 ppm for 5NI) in H-bonding acceptor solvents (acetone, DMSO, and water). The specific solutesolvent effects that in these cases are clearly active are not explicitly included in our continuum solvation model. As far as the theoretical prediction of spinspin couplings J is concerned, the available literature suggests the use of DFT as a very good compromise between the accuracy of the results and computational cost, provided that the functional and basis set are properly 10042
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The Journal of Physical Chemistry A chosen.33,34 In the present study the complexity is further increased, as solvent effects also have to be included in the calculation of the spinspin couplings. The results obtained for direct (1J) and indirect (nJ with n = 2, 3, 4) 1H1H and 13C1H couplings for the three systems in all solvents are reported in Figure 10; here the analysis on spinspin couplings will be limited to MPW1PW91 and MPW3PBE results due to the worse agreement found at the M06L level of calculation. The plots reported in Figure 10 indicate that the general agreement is very good and that the two functionals behave very similarly but MPW1PW91 better predicts the 1H13C coupling values. Moving to a more detailed analysis of the interplay of solvent and structural effects, in Figures 11 and 12 we report the solvent dependence of selected direct (1 J) and indirect (3 J and 4 J) spinspin couplings for 1 H1 H and 1 H13 C scalar couplings, respectively. What we observe is a good reproduction of both the observed small increase/decrease of the various couplings with solvent and the differences found among corresponding couplings of different probes. As observed before, for J[1H1H ] the two tested functionals show a very similar behavior: for instance, both systematically overestimate the 3J[γ β] of about 10% with respect to experiments. On the other hand, for J[13C1H ] a definitely better prediction is given by MPW1PW91. The analysis aimed to connect the trends in NMR parameter to the properties discussed in the previous sections can be performed on selected carbon chemical shifts. In Figure 13 the measured and calculated differences Δδ between the chemical shift of the carbon atoms directly bonded to the donor and acceptor groups (C1 and C4) are reported against ε for the three probes together with the differences between the corresponding NBO charges. The Δδ values are positive for EPAB and PNA, where C1 is more deshielded than C4, and they increase with increasing ε, since C1 gets more and more deshielded while C4 gets more and more shielded (see pertinent tables in the Supporting Information). On the contrary, 5NI presents slightly negative Δδ values with a much lower sensitivity to the solvent. The measured trends of Δδ are qualitatively reproduced by the calculated differences in NBO charges, Δq[C1C4], both concerning the solvent effects and the differences among the three compounds, thus confirming the validity of the computational analysis used in describing the complex interplay between structures, electronic charge distributions, and solvent effects. We finally note that, concerning the discrepancies in the trends of the calculated Δδ values with respect to those experimentally measured, different factors can play a role: for example, the model we used to simulate the solvent is based on a continuum description, and the solvent response is determined by its macroscopic dielectric constant which is a bulk property and cannot account for eventual specific effects in the first solvation shells. In addition, the same model includes only electrostatic effects, so any other solutesolvent interactions, such as possible dispersion effects, are completely neglected. Both effects are reflected in the shape of the calculated curves for which the increase is steeper and the saturation is reached faster than experimentally. This final correlation between chemical shifts and NBO charges when combined to those reported in the previous sections shows that the differences in the donor and acceptor groups in the three investigated compounds determine their different responses to external perturbations. In particular, the high Δδ values measured in EPAB have to be related to the weaker acceptor character of the COOR group with respect to
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the nitro group present in PNA. Such a weaker electron-attracting capacity will in fact make the C4 atom more shielded than in PNA with a consequent larger Δδ value. This analysis is fully confirmed by the NBO charge differences. As already discussed before, the analysis of 5NI is more difficult as two competitive effects are active, namely, the CT capacity of its donor and acceptor groups and the presence of additional delocalization patterns for the charge. The combined action of these two effects is reflected in the Δδ graph: very small (and negative) values are found in all solvents due to a very effective dispersion of the electronic charge on the entire indole group with consequent shielding of the C1 atom. Once more, the NBO charges correctly reproduce the observed data and confirm that in this system the charge transfer character is weakened due to a more effective delocalization on the indole group. This effect is also responsible for the lower sensitivity to the solvent.
4. CONCLUSIONS In this study we combined QM calculations and NMR measurements to understand at a detailed level the complex interplay of structural and electronic properties with the effects of the solvent in pushpull systems. By using a computational approach in which solvent effects are consistently included in all parts of the QM description (i.e., geometry, electronic density, and response properties) in terms of a polarizable continuum model, we investigated the relation between structures, electronic charge distributions, and NLO properties. The obtained results have finally been integrated with a comparison with experimental NMR parameters which has allowed us to extract the following general rules: • solvent effects on the NLO activity, here quantified in terms of variations of the static hyperpolarizability, have been shown to correlate with the parameter BLA, whereas other parameters, such as IR frequencies and NBO charges, are less effective in revealing solvent-induced changes in the charge transfer character of pushpull probes; • variations of chemical shifts of the carbons involved in the delocalization pattern and in particular of the two carbons directly bonded to the donor and acceptor groups represent an effective index to study the charge transfer character of the probes; • solvent differently affects the NLO activity of probes with different donoracceptor characteristics and, in particular, seems to more effectively increase the NLO properties of probes, such as 5NI, where various patters of charge delocalization are possible. Along this line, a simplified picture in terms of resonance forms can represent a valid tool to rationalize differences among the probes. ’ ASSOCIATED CONTENT
bS
Supporting Information. Selected NMR spectra, tables of experimental and calculated 13C and 1H chemical shifts referred to TMS, and 1H1H and 1H13C scalar couplings; calculated IR vibrational frequencies for selected modes. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (D.C.);
[email protected] (B.M.). 10043
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’ ACKNOWLEDGMENT This paper is dedicated to the memory of its first author, Alberto Marini, enthusiastic young scientist, cheerful and unforgettable friend. ’ REFERENCES (1) (a) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (b) Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: New York, 1987. (c) Nalwa, H. S.; Miyata, S. Nonlinear Optics of Organic Molecules and Polymers; CRC Press: Boca Raton, FL, 1997. (2) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994, 94, 195. (3) Bishop, D. M. Adv. Chem. Phys. 1998, 104, 1. (4) (a) Marder, S. R.; Kippelen, B.; Jen, A. K.-Y.; Peyghambarian, N. Nature 1997, 388, 845. (b) Marder, S. R.; Cheng, L. T.; Tiemann, B. G.; Friedli, A. C.; Blanchard-Desce, M.; Perry, J. W.; Skindhoej, J. Science 1994, 263, 511. (c) Marder, S. R.; Gorman, C. B.; Tiemann, B. G.; Cheng, L. T. J. Am. Chem. Soc. 1993, 115, 2524. (d) Marder, S. R.; Beratan, D. N.; Cheng, L.-T. Science 1991, 252, 103. (5) Wolff, J. J.; Wortmann, R. Adv. Phys. Org. Chem. 1999, 32, 121. (6) Marder, S. R. Chem. Commun. 2006, 131, 131. (7) (a) Bossi, A.; Licandro, E.; Maiorana, S.; Rigamonti, C.; Righetto, S.; Stephenson, G. R.; Spassova, M; Botek, E.; Champagne, B. J. Phys. Chem. C 2008, 112, 7900. (b) Sergeyev, S.; Didier, D.; Boitsov, V.; Teshome, A.; Asselberghs, I.; Clays, K.; Vande Velde, C. M. L.; Plaquet, A.; Champagne, B. Chem.—Eur. J. 2010, 16, 8181. (c) Coe, B. J.; Fielden, J.; Foxon, S. P.; Helliwell, M.; Asselberghs, I.; Clays, K.; De Mey, K.; Brunschwig, B. S. J. Org. Chem. 2010, 75, 8550. (8) (a) Abbotto, A.; Beverina, L.; Bradamante, S.; Facchetti, A.; Klein, C.; Pagani, G. A.; Abshiro, M. R.; Wortmann, R. D. Chem.—Eur. J. 2003, 9, 1991. (b) Archetti, G; Abbotto, A.; Wortmann, R. D. Chem.— Eur. J. 2006, 12, 7151. (9) Bogdan, E.; Plaquet, A.; Antonov, L.; Rodriguez, V.; Ducasse, L.; Champagne, B.; Castet, F. J. Phys. Chem. C 2010, 114, 12760. (10) (a) Willetts, A.; Rice, J. E. J. Chem. Phys. 1993, 99, 426. (b) Yu, J.; Zerner, M. C. J. Chem. Phys. 1994, 100, 7487. (11) (a) Sylvester-Hvid, K. O.; Mikkelsen, K. V.; Jonsson, D.; Norman, P.; Å gren, H. J. Chem. Phys. 1998, 5576, 109. (b) Kongsted, J.; Osted, A.; Mikkelsen, K. V.; Christiansen, O. J. Chem. Phys. 2003, 119, 10519. (c) Sylvester-Hvid; Mikkelsen; Norman; Jonsson; Ågren J. Phys. Chem. A 2004, 108, 8961. (12) (a) Cammi, R.; Cossi, M.; Mennucci, B.; Tomasi J. Chem. Phys. 1996, 105, 10556. (b) Cammi, R.; Mennucci, B.; Tomasi, J. J. Am. Chem. Soc. 1998, 120, 8834. (c) Mennucci, B.; Cammi, R.; Tomasi, J. Int. J. Quantum Chem. 1999, 75, 767. (d) Cammi, R.; Mennucci, B.; Tomasi, J. J. Phys. Chem. A 2000, 104, 4690. (e) Corozzi, A.; Mennucci, B.; Cammi, R.; Tomasi, J. J. Phys. Chem. A 2009, 113, 14774. (13) (a) Munn, R. W.; Luo, Y.; Macak, P.; Å gren, H. J. Chem. Phys. 2001, 114, 3105. (b) Ray, P. C.; Leszcynski, J. Chem. Phys. Lett. 2004, 399, 162. (c) Frediani, L.; Å gren, H.; Ferrighi, L.; Ruud, K. J. Chem. Phys. 2005, 123, 144117. (d) Ferrighi, L.; Frediani, L.; Cappelli, C.; Salek, P.; Å gren, H.; Helgaker, T.; Ruud, K. Chem. Phys. Lett. 2006, 425, 267. (g) Jensen, L.; van Duijnen, P. T.; Snijders, J. G. J. Chem. Phys. 2003, 119, 12998. (f) Jensen, L.; van Duijnen, P. T. J. Chem. Phys. 2005, 123, 074307. (14) Matichak, J. D.; Hales, J. M.; Barlow, S.; Perry, J. W.; Marder, S. R. J. Phys. Chem. A 2011, 115, 2160. (15) Molinos-Gomez, A.; Vidal, X.; Maymo, M.; Velasco, D.; Martorell, J.; Lopez-Calahorra, F. Tetrahedron 2005, 61, 9075. (16) (a) Kleinpeter, E.; Koch, A.; Mikhova, B.; Stamboliyska, B. A.; Kolev, T. M. Tetrahedron Lett. 2008, 49, 1323. (b) Kleinpeter, E.; Klod, S.; Rudorf, W.-D. J. Org. Chem. 2004, 69, 4317. (17) (a) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999.(b) In Continuum solvation models in chemical physics, from theory to applications; Mennucci, B., Cammi, R., Eds.; Wiley: Chichester, 2007.
ARTICLE
(18) Bohmann, J. A.; Weinhold, F.; Farrar, T. C. J. Chem. Phys. 1997, 107, 1173. (19) Frisch, M. J. et al. Gaussian 09 ,Revision A01; Gaussian, Inc.: Wallingford, CT, 2009. (20) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (21) (a) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393, 51. (b) Chiba, M.; Tsuneda, T.; Hirao, K. J. Chem. Phys. 2006, 124, 144106. (22) (a) Adamo, C.; Barone, V. J. Chem. Phys. 199, 110, 6158. (b) Ernzerhof, M.; Scuseria, G. E. J. Chem. Phys. 1999, 110, 5029. (23) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664. (24) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (25) (a) Kupka, T.; Stachow, M.; Nieradka, M.; Kaminsky, J.; Pluta, T. J. Chem. Theory Comput. 2010, 6, 1580. (b) Zhao, Y.; Truhlar, D. G. J. Phys. Chem. Lett. 2008, 112, 6794. (26) (a) Hameka, H. F. Rev. Mod. Phys. 1962, 34, 87. (b) Ditchfield, R. Mol. Phys. 1974, 27, 789. (c) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251. (27) (a) Cances, E.; Mennucci, B. J. Math. Chem. 1998, 23, 309. (b) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (c) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506. (28) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (29) (a) Willetts, A.; Rice, J. E.; Burland, D. M.; Shelton, D. P. J. Chem. Phys. 1992, 97, 7590.(b) Shi, R. F.; Garito, A. F. In Characterization Techniques and Tabulations for Organic Nonlinear Optical Materials; Kuzyk, M. G., Dirk, C. W., Eds.; Dekker: New York, 1998; p 128. (c) Reis, H. J. Chem. Phys. 2006, 125, 014506. (30) (a) Ferrighi, L.; Frediani, L.; Cappelli, C.; Sazek, P.; Ågren, H.; Helgaker, T.; Ruud, K. Chem. Phys. Lett. 2006, 425, 267. (b) Peach, M. J. G.; Helgaker, T.; Sazek, P.; Keal, T. W.; Lutnaes, O. B.; Tozer, D. J.; Handy, N. C. Phys. Chem. Chem. Phys. 2006, 8, 558. (c) Jacquemin, D.; Perpete, E. A.; Scalmani, G.; Frisch, M. J.; Kobayashi, R.; Adamo, C. J. Chem. Phys. 2007, 126, 144105. (d) Jacquemin, D.; Perpete, E; Medved, M.; Scalmani, G.; Frisch, M. J.; Kobayashi, R.; Adamo, C. J. Chem. Phys. 2007, 126, 191108. (31) (a) Helgaker, T.; Jaszunski, M.; Ruud, K. Chem. Rev. 1999, 99, 293.(b) Calculation of NMR and EPR Parameters. Theory and Applications; Kaupp, M., Buhl, M., Malkin, V. G., Eds.; Wiley-VCH: Weinheim, 2004. (32) Kupka, T.; Stachow, M.; Nieradka, M.; Kaminsky, J.; Pluta, T. J. Chem. Theory Comput. 2010, 6, 1580. (33) Deng, W.; Cheeseman, J. R.; Frisch, M. J. J. Chem. Theory Comput. 2006, 2, 1028. (34) Jensen, F. J. Chem. Theory Comput. 2006, 2, 1360.
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