Multiple Interactions in the Self-Association of Porphyrin Discotic

Antoni Segade, Francisco López-Calahorra, and Dolores Velasco*. Departament de Quımica Orga`nica, UniVersitat de Barcelona, Martı i Franque`s 1-11,...
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J. Phys. Chem. B 2008, 112, 7395–7402

7395

Multiple Interactions in the Self-Association of Porphyrin Discotic Mesogens Antoni Segade, Francisco Lo´pez-Calahorra, and Dolores Velasco* Departament de Quı´mica Orga`nica, UniVersitat de Barcelona, Martı´ i Franque`s 1-11, E-08028 Barcelona, Catalunya, Spain ReceiVed: January 14, 2008; ReVised Manuscript ReceiVed: April 3, 2008

The conformational preferences and the self-associational behaviors of two hemin-derived porphyrin compounds, a tetramethyl ester and a liquid crystalline tetrakis(3,5-didodecyloxyphenyl)ester, have been studied by UV/vis and 1H NMR spectroscopy in solution. Results indicate that the 3,5-didodecyloxyphenyl units play an important role in both the conformational and the self-associational behaviors of the mesomorphic tetraester. In the monomeric, nonassociated species, the two propionic 3,5-didodecyloxyphenyl esters establish mutual CH/π interactions that restrict the fluctuative behavior of the chains. In the dimeric, self-associated species, intermolecular CH/π interactions occur in addition to the π-π stacking of the porphyrin cores. The temperature-dependent addition of side CH/π interactions to the π-π stacking of the porphyrin rings accounts for the observed tightening and for the slower dynamics of the dimeric structure. The relationship between the self-associational behavior and the mesomorphism of the hemin-derived porphyrin tetraesters is also discussed. 1. Introduction Noncovalent interactions play a key role in determining the mesomorphic behavior of liquid crystalline compounds.1–7 While the anisometric shape of mesogens is the driving factor for the orientational ordering of the molecules,1 interactions such as π-π stacking,2,3 hydrogen bonding,4–6 or charge-transfer interactions,7 together with the microsegregation of the different substructures of the mesogen,8 improve the positional order of the molecules within the mesophase. Discotic mesogens,9 formed by a flat and rigid aromatic core surrounded by flexible alkyl chains, are illustrative of this level of self-organization: in the mesophase state, the cores tend to stack into long columns that are separated by the liquidlike alkyl chains. Because of this highly ordered structure, the so-called columnar (Col) mesophases display a large anistropy in their bulk properties, a feature of great interest in the design and construction of optoelectronic devices.10–15 We have recently reported the synthesis and mesomorphism of a family of discotic tetraesters derived from natural porphyrin hemin.16 All the mesogens displayed hexagonal columnar mesophases, but the mesophase structure and sequence was found to be dependent on the chemical structure. Free base compounds displayed a low-temperature hexagonal columnar mesophase (noted as Colh1), formed by the stacking of unusual trimolecular associations, and a high-temperature hexagonal columnar mesophase (Colh2), in which single molecules stack in a planar and energetically nonoptimal conformation. Furthermore differential scanning calorimetry (DSC) experiments showed that the transition from the Colh2 to the Colh1 mesophase on cooling was complete only after annealing for several hours. In contrast, the related metalated compounds displayed only the Colh2 arrangement in the whole mesophase range. The mesomorphism was assumed to be the result of a delicate balance between the strength of the intermolecular interactions and the conformational preferences of the molecules. A detailed X-ray * To whom correspondence should be addressed. Phone: + 34 93 4039260. Fax: + 34 93 3397878. E-mail: [email protected].

Figure 1. Anisotropic ∆δ effect of an aromatic ring current. For the benzene ring, the intensity factor I has a value of 27.6 ppm Å3.

diffraction (XRD) characterization of the crystalline phase might provide further insight into the structural factors responsible for the unusual mesomorphsim of the free base compounds. Unfortunately, the mesogens of interest are liquid crystalline at room temperature and present glassy states rather than crystals upon cooling at lower temperatures, thus precluding the use of this technique. Nuclear magnetic resonance (NMR) is a powerful, friendly and readily available tool for the elucidation of macromolecular and supramolecular structures and has become an alternative to XRD.17,18 The simple analysis of phenomena such as the nuclear Overhauser effect (NOE)19 or the value of the escalar proton-proton coupling constants20 has been shown to provide useful information about the structural features of complex molecules. For the study of the hemin-derived columnar tetraesters, the presence of several aromatic rings in their structure focused our attention on the changes in the chemical shift (∆δ) that are induced by aromatic ring currents on the spatially close nuclei. As schematized in Figure 1, the anisotropic ∆δ depends on the distance (r) as well as on the angular deviation (θ) of the nucleus with respect to the centroid of the ring current.21 The experimental determination of ∆δ values as well as their comparison with theoretically estimated values allows qualitative and quantitative conclusions about the structure and the conformational preferences of molecules.22 Moreover,18 when aromatic compounds form hetero- or selfassociates, their ring currents also promote an easily observable effect, termed CIS (complexation-induced shift) in host-guest chemistry.18 The self-association of compounds in solution has been widely exploited in the research on the geometry and the

10.1021/jp800475f CCC: $40.75  2008 American Chemical Society Published on Web 05/30/2008

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CHART 1: Structures of the Hemin-Derived Tetraesters 1 and 2 and Labeling of the Selected Nuclei

Figure 2. Region of the 1H NMR spectrum of compound 2 displaying the splitting of the 3,5-didodecyloxyphenyl ester signals.

CHART 2: Reference Compounds and Chemical Shifts of the Nuclei Used in the Determination of the Porphyrin-Induced ∆δ Values of Compounds 1 and 2a

strength of the intermolecular interactions in porphyrins23–29 as well as in other compounds.30–35 In the case of porphyrins, most self-association studies deal with metalated compounds, while interactions of free base compounds have received little attention26–29 because of their weaker nature.36 The use of organic solvents such as dichloromethane or chloroform in this kind of studies provides a good solubilization of the aromatic rings but at the same time it does not interfere with the interactions established by the compounds. In the present paper we report the spectroscopic study of the conformational and self-associational behaviors of two heminderived porphyrin compounds, whose structure is shown in Chart 1: tetramethyl ester 1 and the liquid crystalline tetrakis(3,5didodecyloxyphenyl) ester 2. The former can be regarded as an “isolated” porphyrin core of the more complex latter structure, allowing the observation and identification of the effects associated with each type of aromatic unit. As discussed in the last part of the article, the analysis of the self-associative behaviors of 1 and 2 provides a key insight into the columnar mesomorphism of the hemin-derived tetraesters. 2. Experimental Section 2.1. Materials. The synthesis and characterization of porphyrin compounds 1 and 2 has been previously reported.16 Methyl acrylate (3) and methyl propionate (4) were purchased from Aldrich. The synthesis of 3,5-didodecyloxyphenyl acrylate (5) and propionate (6) is described in the Supporting Information. 2.2. Spectroscopic Measurements. UV/vis spectra were recorded in a Varian Cary 500 spectrophotometer using 1-cm quartz cubettes. Spectroscopic grade solvents were purchased from Scharlau. The cast film of porphyrin 2 was prepared by dropping a CH2Cl2 solution of the compound onto a quartz plate and drying the film in a vacuum desiccator with P2O5. Routine 1H, 13C, HSQC, COSY, and 2D NOESY spectra were recorded in a Varian Mercury 400-MHz spectrometer. The temperature-variable 1H spectra were recorded in a Varian Unity 300 MHz spectrometer. Samples of known milimolar concentration in were prepared as follows: the appropriate amount of the corresponding compound was weighed in a Mettler Toledo AG245 scale (accuracy of (0.01 mg) and dissolved in 1000

a

δ values of the signals of interest at 298 K are in brackets.

µL of CDCl3. A convenient aliquot (ca. 0.8 mL) was then withdrawn for the recording of the spectra. The residual CHCl3 peak was used as the reference δ value at 7.260 ppm. In the characterization of the self-associative behavior, dimerization constants (Kdim) were obtained by fitting a monomer-dimer model to the experimental data sets, using the nonlinear advanced fitting tool contained in the OriginPro 7.0 software. The following observed chemical shift (δobs) vs concentration (C) relationship37 was fitted

(

δobs ) δmonomer + CIS 1 +

1 - √8KdimC + 1 4KdimC

)

(1)

Uncertainties are expressed as twice the standard deviation (95% confidence). CIS values for compounds 1 and 2 were determined by fitting the monomer-dimer model to the data sets of each individual signal, previously identified with the aid of the corresponding 2D NOESY spectrum. The Kdim value, previoulsy determined from the average meso data set at the studied temperature, was set as a fixed parameter within the fitting algorithm. 3. Results and Discussion 3.1. Intramolecular Interactions: CH/π Bridges and Conformational Behavior. Porphyrin compounds 1 and 2 were characterized through their 1H, 13C, COSY, HSQC, and 2D NOESY NMR spectra. Groups attached to the porphyrin meso and β-positions such as the H5, H10, H15, and H20 protons as

Self-Association of Porphyrin Discotic Mesogens

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TABLE 1: Porphyrin-Induced ∆δ Values for Compounds 1 and 2 ∆δ/ppm porphyrin compound 1 2

c

nucleia

reference compoundb

298 K

243 Kc

acrylate COOCH3 propionate COOCH3 acrylate Hortho acrylate Hpara acrylate ether CH2 propionate Horto propionate Hpara propionate ether CH2

3 4 5 5 5 6 6 6

+0.33 -0.01 +0.29 +0.12 +0.13 -0.62 -0.27 -0.76

+0.35 -0.03 +0.26 +0.12 +0.10 -0.80 -0.34 -1.01

a Labeling shown in Chart 1. b Chemical shifts shown in Chart 2. From the reference δ values at 298 K.

well as the 2-, 7-, 12-, and 18-methyl groups are magnetically nonequivalent and displayed individual signals in the 1H NMR spectrum. The signals of the esters were also split in two sets belonging either to the acrylate or to the propionate esters (Figure 2). This differenttiation was attributed to a different magnetic ring current effect (∆δ) exerted by the porphyrin ring, and although the several simultaneous currents within a porphyrin ring demand more refined methods to accurately predict induced ∆δ values,38–40 the expression in Figure 1 still allows qualitative estimations. Hence negative values must be expected for nuclei located above the porphyrin ring and positive values for nuclei next to it. To quantify the magnetic influence of the porphyrin ring, ∆δ values of compound 1 were determined by comparing the 1H chemical shifts of its methyl esters with those of reference compounds 3 and 4 (see Chart 2), which resemble the porphyrinfree acrylate and propionate side structures. To diminish any self-associative effect, the NMR spectrum of 1 was recorded at a low concentration (1.06 mM). The porphyrin-induced ∆δ values were then estimated to be +0.33 ppm for the acrylate esters and -0.01 ppm for the propionate esters (see Table 1), in agreement with the expectations: the rigid acrylic esters are coplanar with the porphrin ring, while the fluctuating propionic esters present a time-averaged position consistent with an outof-plane and extended conformation pointing outside the porphyrin ring. Likewise, the ∆δ values for the ester units in compound 2 (see Table 1) were determined from the references 3,5didodecyloxyphenyl acrylate (5) and propionate (6). In the acrylate units, the value for the phenyl H2 and H6 protons (Hortho) is similar to that of the acrylate methyls of 1 and the effects on the farther phenyl H4 (Hpara) and the dodecyl methylenes (ether CH2) are smaller. On the other hand, significant shieldings ranging from -0.27 to -0.76 ppm were found for the propionate units instead of the negligible effect in compound 1. An additive time-averaged shielding exerted by one phenyl unit on the other, as judged from the spectral data reported for mesoporphyrin dibenzyl ester, can be estimated as -0.2 ppm.41 The higher upfield shift therefore suggests the establishment of some type of interactions bringing both propionate 3,5-didodecyloxyphenyl rings into proximity. The π-π stacking of a phenyl ring either with the central porphyrin ring, giving rise to “folded” conformations, or with the other phenyl ring seem unlikely, given the electron-rich character of all the rings involved.41 The comparison of the ∆δ values shows that the ether CH2 protons are the most shielded nuclei, pointing to a location centered above the neighboring phenyl ring, typical of CH/π bridges.42,43 These noncovalent interactions play an important

Figure 3. Intramolecular CH/π interaction mode proposed for compound 2. The conversion between the different interacting pairs (see text) may be achieved through the rotation of the OestersCphenyl bond in a noninteracting “open” state.

role in crystal packing, but their participation in the control of the conformational44–50 and the associational51–53 behavior of molecules in solution is also well-known. Specifically, in the structure of a ferulic acid derivative, two methoxyphenyl units have been shown to establish a double interaction through the methoxy group of each unit and the benzene ring of the other one.45 As proposed in Figure 3, the interaction geometry in 2 can be similar, although the 3,5-disubstituted pattern of the phenyl rings would lead to four possible pairs of interacting chains. The unique signal observed for all the equivalent protons of the 3,5-didodecyloxyphenyl propionate esters indicates that the different interacting pairs (or “closed” modes) are in fast exchange within the time scale of the NMR experiment. Moreover, as the chemical shifts of the signals are temperature dependent (see Table 1), the exchange must proceed through the equilibrium of the “closed” modes with a noninteracting or “open” state, in which the propionate esters freely fluctuate as those of compound 1. The flexibility of the propionate chains provides a certain variability in the structure of both the “open” and the “closed” modes, thus precluding any rigorous estimation of the thermodynamic parameters associated to the interactions from the 1H NMR data.49,50 Recent theoretical calculations estimate the enthalpic stabilization of each CH/π interaction in ca. -11 kJ mol-1,54,55 but on the other hand the entropy change by the conformational locking of the propionate chains in the “closed” geometries must be similar to the enthalpic benefit. Larger ∆δ values are observed at lower temperatures (243 K values in Table 1), indicating that the CH/π interacting modes are stabilized when the contribution of the entropic term is lowered. 3.2. Intermolecular Interactions: Porphyrin π-π Stacking, Side CH/π Bridges and Structural Tightening. The selfassociation of compounds 1 and 2 was first examined by UV/ vis spectroscopy. A relatively sharp and invariable Soret band indicated that self-association of both compounds was negligible for CH2Cl2 solutions in the 10-6-10-4 M range. But the addition of CH3OH, known to decrease the solute-solvent interactions,56 caused a hypsochromic shift of the Soret band and a bathochromic shift of the Q bands of compound 2, along with a general broadening of the spectrum (top part of Figure 4). This behavior is commonly associated with the excitonic coupling between neighboring porphyrin units, caused by the formation of cofacial aggregates of undefined geometry.57 Furthermore the spectrum of the solution with the highest CH3OH content is almost identical to the spectrum of cast film of 2 in the Colh1 mesophase (bottom part of Figure 4), indicating a similarity between the associated species in solution and the arrangement of the molecules within the mesophase.58,59 A concentration- and temperature-variable NMR study of the self-association of porphyrin compounds 1 and 2 was then

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Segade et al. TABLE 3: Enthalpies and Entropies for the Dimerization of Compounds 1 and 2 -1

∆H/kJ mol ∆S /J mol-1K-1

Figure 4. UV/vis spectra of compound 2 in different CH2Cl2:MeOH mixtures (upper). Normalized spectra of an associated solution vs cast film (lower).

TABLE 2: Dimerization Constants (Kdim) of Porphyrin Compounds 1 and 2 T/K 323 298 283 263 243

porphyrin 1/M-1 27 ( 6 38 ( 8 48 ( 9 69 ( 15 94 ( 24

porphyrin 2/M-1 14 ( 8 20 ( 12 27 ( 6 50 ( 17 92 ( 28

carried out by recording the 1H spectra of milimolar (10-3 M) solutions in CDCl3 at five different temperatures between 243 and 323 K. Ring current effects could be clearly observed, and for compound 2 the largest changes were displayed by the protons of the porphyrin core while the 3,5-didodecyloxyphenyl units displayed only small shifts. Moreover, significant broadening and overlapping of the porphyrin ring signals were observed at low temperatures for 2 but not for 1 (see Supporting Information). The average value of the four meso chemical shifts, instead of the NH or the individual meso signals, was therefore used in the determination of the self-association constants, providing the same results within the experimental error (see Supporting Information). The usual monomer-dimer model37 yielded the dimerization constants (Kdim) presented in Table 2. The agreement of the fitted model and the data indicates that neither 1 nor 2 self-associate beyond dimeric species within the experimental conditions,25,28,34 and the lower self-association displayed by 2 at high temperatures is noteworthy. The 298 K values of Kdim are of the same order as those reported for other free base porphyrins not bearing strong electron-withdrawing groups.27,29 As expected, the acrylate side chains induce porphyrin self-association only in a weak manner.

porphyrin 1

porphyrin 2

-10.3 ( 0.7 -4.3 ( 2.6

-15.7 ( 1.7 -27.5 ( 6.0

The thermodynamic parameters associated with the selfassociation of compounds 1 and 2 were estimated from the temperature-dependent values of Kdim (Table 3). The determined enthalpies are much smaller than those of metalated porphyrins36,60 but are strong enough to drive the self-association process. However, while in porphyrin 1 the low enthalpy is accompanied by a low entropy, as expected for such a weak interaction,61 in compound 2 the slight change of the enthalpy (ca. -5 kJ mol-1) is associated to a larger entropic change. These two different balances suggest that the enthalpic and entropic data for 1 are only related to the π-π stacking between free base porphyrins, while the changes observed for 2 display a further stabilization that might be afforded by intermolecular CH/π interactions between neighboring 3,5-didodeycloxyphenyl rings. The addition of these interactions to the π-π stacking of the porphyrin cores would afford the higher association enthalpy but would also demand a larger entropic cost due to the conformational locking of the involved propionate chains. To examine this issue, the geometries of the porphyrin associations were examined through the changes in the proton chemical shift induced by the dimerization process (CIS values). Data for both the compounds 1 and 2 are presented in Table 4.62 Derivatives of natural porphyrins stack through an alternating arrangement of the porphyrin rings, with a vertical separation of approximately 3.5 Å. In solution, the dynamic sliding and rotation of one ring with respect to the other yields a timeaveraged structure where the rings are offset along both the H5-H15 and the H10-H20 axes of one molecule.25,36 The analysis of the CIS values of porphyrin 1 is consistent with this timeaveraged geometry (Figure 5). The upfield shift of the NH signal is in agreement with an interporphyrin separation around 3.5 Å,28,40 and the values of the meso protons and the porphyrin β-methyls indicate a large offset along the H5-H15 axis that locates the C2-C8 fragment of each porphyrin skeleton above the other porphyrin ring. This large offset may be provided by the additional π-interacting surface of the 3-acrylate chain. If an additional shielding exerted by the acrylate chains is taken into consideration,63 the offset along the H10-H20 axis also becomes apparent, with each H10 proton above the neighboring ring. The propionic ester chains are progressively less affected as the observed nuclei separate from their own porphyrin ring, indicating that their fluctuation takes place mostly in the outer and unhindered faces of the dimer. Upon lowering the temperature to 243 K, the CIS values reflect a reduced dynamic sliding that decreases the access of H5 and of the 7-methyl group to the more shielding locations. The NH signal, which can be considered a good indicator of the relative position of the porphyrin rings, remains with a -4.0 ppm shielding. The CIS values of compound 2 at 298 K display the same internal relative magnitudes as in 1, being indicative of a similar stacking geometry of the porphyrin rings. But the absolute values, except that of H15, are smaller. The steric bulk of the acrylate phenyl rings, which are not expected to be coplanar but rather perpendicular to the porphyrin ring,64,65 may increase the interporphyrin separation and thus concomitantly decrease the induced shielding. The most outstanding changes are observed for the propionic esters: in contrast to porphyrin 1, the β-CH2 protons are more shielded than their R-CH2 partners

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TABLE 4: Selected CIS Values of Porphyrins 1 and 2 porphyrin nucleia Meso Protons H5 H10 H15 H20 Porphyrin CH3 2-CH3 7-CH3 12-CH3 18-CH3 Acrylate Esters COOCH3 Hortho Hpara ether CH2 Propionate Esters R-CH2 β-CH2 COOCH3 Hortho Hpara ether CH2 NH a

298 K (Kdim ) 38 M ) -1

1

porphyrin

243 K (Kdim ) 94 M ) -1

298 K (Kdim ) 20 M ) -1

2 243 K (Kdim ) 92 M-1)

-3.33 ( 0.07 -1.87 ( 0.03 -1.01 ( 0.02 -1.92 ( 0.04

-3.08 ( 0.06 -1.91 ( 0.04 -0.98 ( 0.02 -1.74 ( 0.04

-2.07( 0.08 -1.23 ( 0.05 -1.02 ( 0.07 -1.36 ( 0.10

-1.29 ( 0.03 -1.94 ( 0.04 -0.55 ( 0.01 -0.60 ( 0.02

-1.26 ( 0.04 -1.72 ( 0.08 -0.47 ( 0.03 -0.61 ( 0.04

-0.57 ( 0.02 -1.02 ( 0.04 -0.30 ( 0.01 -0.38 ( 0.02

-0.42 ( 0.01 -0.73 ( 0.02 -0.27 ( 0.01 -0.37 ( 0.02

+0.16 ( 0.01

+0.14 ( 0.01

+0.17 ( 0.01 +0.09 ( 0.01 +0.06 ( 0.01

+0.14 ( 0.01 +0.07 ( 0.01 0.00 ( 0.01

-0.39 ( 0.01 -0.22 ( 0.01 +0.03 ( 0.01

-0.35 ( 0.02 -0.23 ( 0.01 -0.01 ( 0.01

-0.20 ( 0.01 -0.26 ( 0.02

-0.19 ( 0.01 -0.36 ( 0.01

-4.00 ( 0.10

-3.90 ( 0.12

-0.15 ( 0.04 -0.08 ( 0.01 -0.15 ( 0.04 -3.39 ( 0.16

-0.23 ( 0.02 -0.12 ( 0.01 -0.20 ( 0.02 -3.89 ( 0.09

b b b b

Labeling shown in Chart 1. b Not determined due to signal broadening and overlapping (see text).

Figure 6. Plot of the NH CIS value vs temperature for porphyrin compounds 1 and 2.

Figure 5. Schematic top and side views of the time-averaged dimer structure of porphyrin 1. The locations of the meso H5 and H15 protons are shown for orientative purposes.

and the 3,5-didodecyloxyphenyl units are shifted upfield. These changes are consistent with the proposed intermolecular CH/π interactions, by the approach of a propionate chain to one acrylate chain of the neighboring molecule in the dimeric structure. Upon examining the CIS values of porphyrin 2 at 243 K, several variations are observed with respect to those at 298 K: around the region of the acrylic esters the 2- and the 7-CH3 protons are further deshielded, and for the propionic esters the β-CH2 and the 3,5-didodecyloxyphenyl units are further shielded. Furthermore the CIS of the NH signal displays a change of -0.50 ppm. In fact, if the NH CIS values of 1 and 2 are determined for every temperature studied (Figure 6): for 1 values remain rather constant around -4 ppm, while for 2 there is a continuous increase of the porphyrin-induced shielding as the

temperature is decreased. This change may be attributable to a shorter time-averaged distance between the porphyrin cores in the structure of the dimeric species. The approach of the interacting molecules is usually considered indicative of structural tightening,66 and the broadening of the signals in the NMR spectrum of 2 at low temperatures, which has also been reported on shortening the linkers of covalently bound porphyrin dimers,67 is also consistent with this feature. The structural tightening by the establishment of multiple interactions is an issue that has important implications in complex systems such as ligand-protein interfaces.68,69 Williams and co-workers have proposed that, in addition to the classical “entropic chelate effect”, the establishment of successive interactions causes an overall motional decrease that enthalpically improves each individual interaction.70 However Hunter and co-workers have pointed out that this “enthalpic chelate effect” is related to the conformational flexibility of the interacting interfaces, and the addition of interactions does not necessarily provide an improvement in the free energy of association if its entropic cost overcomes the enthalpic benefit. Then not all the possible intermolecular interactions are

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Figure 7. Representation of the equilibria between different dimeric structures proposed for the self-association of porphyrin tetraester 2. The arrows indicate the conformational and fluctuational freedom of the dimeric species upon establishing additional CH/π interactions.

established at the same time and the associated species are a mixture of “partially bound states”.71 The weak interactions involved in the self-association of 2 seem to provide a limiting case where this behavior is observed. As presented in Figure 7, a series of equilibria between species with a different number of intermolecular CH/π bridges is proposed for the self-association of compound 2. The determined Kdim constants would therefore be a statistical combination of the involved K0, K1, and K2 values. The association begins through the π-stacking of the porphyrin cores. As judged from both the lower Kdim and CIS values of 2 with respect to 1, at high temperatures (323 K) the side phenyl esters would hinder an optimal porphyrin π-stacking (K0 < Kdim,1) and indeed the CIS of the NH signal at 323 K (-2.90 ppm) can be related to an interporphyrin distance around 4.0 Å,40,72 in contrast to the usual 3.5 Å. The formation of a first side interaction would play an important role in stabilizing the dimeric structures by improving the central porphyrin-porphyrin stacking in virtue of the “enthalpic chelate effect”. The presence of four propionate esters in the dimeric structure allows this first CH/π bridge to be established at four different points. Hence the population of single CH/π bridged species is not regulated by K1 but by 4K1 and becomes rapidly predominating over the species featuring only the π-stacking. The resulting closer position of the porphyrin rings would be reflected by a more shielded CIS of the NH signal. The formation of another simultaneous CH/π interaction requires the almost complete abolishment of the relative movement of the molecules, and hence its associated entropic cost is rather high. As the enthalpic benefit of the second CH/π bridge is not expected to be very different from that of the first one, K2 should be lower than K1 and the double CH/π bridged species would be significantly formed only at low temperatures. Such tight species would cause a further shielding of the NH signal and a general broadening of the NMR spectrum due to the slowing of the exchange between species, as is indeed observed experimentally. 3.3. Intermolecular Interactions in the Mesophases of the Hemin-Derived Tetraesters. Although CH/π interactions cannot be directly observed in the mesophase state, several published data relate their observation in the crystal phase to their involvement in the mesomorphism of mesogenic compounds. In the 5-alkoxy-2-benzoylaminotropones developed by Mori and co-workers,73 they have an important influence on the positional ordering of the molecules within the mesophase. More interestingly, in the nickel(II) bis(5,5′-alkoxy)salen complexes, the transition from a low-temperature SmE mesophase to a high-temperature SmA mesophase is attributed to the breaking of dimeric species, held by side CH/π interactions, to

Figure 8. Schematic representation of the structures of the Colh1 and Colh2 mesophases observed for the hemin-derived liquid crystalline tetraesters. Dashed bonds indicate the intermolecular interactions within the oligomolecular associations.

unimolecular species.74 These molecules have also been shown to self-associate into dimers in the solution state.75 At this point, it must be noted that the forces governing the solid state are much stronger than the dispersion forces that stabilize the liquid crystal state, and therefore a structural correlation between the crystal and the mesophase state may not be straightforward. On the other side, the weak interactions in self-association are expected to share a high degree of similarity with those of the liquid crystal state, suggesting that the observed behavior of mesogen 2 in solution can be translated to the mesophase state. To our knowledge, this is the first example of the involvement of CH/π interactions in the mesomorphism of a discotic mesogen. With the results here reported in hand, the mesomorphsim of the familiy of hemin-derived tetraesters16 can be more completely explained. As schematized in Figure 8, in the hightemperature mesophase (Colh2) molecules have a planar conformation with an optimal π-π stacking of the porphyrin rings. The stronger interaction in the metalated compounds allows the molecules to maintain this conformation in the whole mesomorphic range. But in the free base compounds, the porphyrin π-π stacking is not strong enough to afford the energetical cost of the planar structure, which can only be achieved when an additional thermal energy, above 60 °C, is provided to the system. At lower temperatures, the propionate chains and the phenyl rings tend toward their energetically optimal out-of-plane geometry, hindering an efficient porphyrin-porphyrin stacking. A new mesophase structure (Colh1), with a columnar organiza-

Self-Association of Porphyrin Discotic Mesogens tion of molecular associations resembling the self-associated species of 2 in solution, is therefore formed. The molecules in the associations are held together by the establishment of side intermolecular CH/π bridges, and the overall structure displays a higher order and a higher stability when compared to the Colh2 mesophase. The molecular stoichiometry in the associations, usually higher than two, would be the result of the higher “concentration” of the molecules in the mesophase with respect to the solution state.25,58 When the Colh1 mesophase is heated, the intermolecular interactions become less stabilizing until the Colh2 mesophase, with the planar conformation of the molecules, becomes the most favorable structure. However, since neither the metalated nor the free base compounds bearing short butyl chains display this type of mesophase, an additional factor must be taken into consideration for the stabilization of the Colh2 structure. The interdigitation of the alkyl chains of neighboring columnar stacks, together with the efficient occupation of the space between the acrylate chains of a molecule in order to attain the most packed structure, might provide the explanation. 4. Conclusions The NMR studies performed here show that the mesomorphic behavior of the columnar hemin-derived tetraesters is clearly influenced by the presence of the 3,5-dialkoxyphenyl units in their structure. In solution, these units establish both intramolecular and intermolecular CH/π interactions that control the conformational and self-associational behavior of the molecules. Furthermore the linkage of the phenyl units to the porphyrin core through flexible propionate chains leads to the formation of different dimeric structures, depending on the relevance of the entropic term on the self-association process. At room temperature, the π-π stacking between the porphyrin rings and a unique side CH/π bridge form the mainly observed structure, while at lower temperatures, where the entropic opposition is lower, a second CH/π interaction is added to the dimeric species. The conclusions drawn from the solution studies can be extrapolated to the mesophase state, clarifying the issues regarding the intracolumnar structure and the mesomorphic behavior of the hemin-derived tetraesters. Acknowledgment. We thank the Unitat de RMN dels Serveis Cientı´fico-Te`cnics (SCT) de la Universitat de Barcelona for their help in the recording of the temperature-variable 1H NMR spectra. Financial support from the Ministerio de Educacio´n y Ciencia (project CTQ 2006-15611-C02-02) is acknowledged. Supporting Information Available: Synthetic procedures for 3,5-didodecyloxyphenyl acrylate (5) and propionate (6), DSC and XRD data of the mesomorphic behavior of compound 2 and NOESY spectra, δ vs concentration and temperature data sets, and curve fittings for both compounds 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Demus, D. In Handbook of Liquid Crystals; Demus, D., Goodby, J. W., Gray, G. W., Spiess, H. W., Vill, V., Eds; VCH Verlag: Weinheim, 1998; Vol. 1, p 133. (2) Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1994, 116, 2655. (3) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater. 1996, 8, 510. (4) Gearba, R. I.; Lehmann, M.; Levin, J.; Ivanov, D. A.; Koch, M. H. J.; Barbera´, J.; Debije, M. G.; Piris, J.; Geerts, Y. H. AdV. Mater. 2003, 15, 1614. (5) Bushey, M. L.; Nguyen, T.-Q.; Zhang, W.; Horoszewski, D.; Nuckolls, C. Angew. Chem., Int. Ed. 2004, 43, 5446.

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