Characterization of porphyrin supramolecular complexes using NMR diffusion spectroscopy

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1 Journal of Porphyrins and Phthalocyanines J. Porphyrins Phthalocyanines 2002; 6: Published at M Characterization of porphyrin supramolecular complexes using MR diffusion spectroscopy Martin R. Johnston* and Melissa J. Latter Centre for Molecular chitecture, Central Queensland University, Rockhampton, Queensland 4702, Australia Received September 2002 Accepted 30 October 2002 ABSTRACT: The usefulness of diffusion ordered MR spectroscopy (DOSY) as a characterization technique for systems that are non-covalently associated in solution is demonstrated. This study investigated the diffusion properties of a series of porphyrin-based building blocks and host materials which culminated in the characterization of a self-assembled porphyrin-containing capsule. Copyright 2002 Society of Porphyrins & Phthalocyanines. KEYWORDS: diffusion spectroscopy, self-assembly, porphyrins, capsules, DOSY. ITRODUCTIO As a spectroscopic technique, DOSY provides information regarding the size of a molecule (or molecular aggregate) in solution and is emerging as an alternative tool for the characterization of selfassembled systems [-8]. Self-assembled entities are typically constructed using non-covalent interactions which may not be sufficiently strong or well suited to characterization by the traditional techniques of mass spectrometry or X-ray crystallography. DOSY has also been successfully employed for the analysis of covalent combinatorial libraries [9]. Although this technique is not new, advances in MR hardware and associated computational power are making it a more commonplace method, which is reflected in an increasing number of publications occurring in the scientific literature. The underlying theory behind DOSY MR has been discussed in several places, [2, 0] and hence only a brief introduction will be given here. The technique relies on the measurement of a diffusion coefficient D that is related to the average molecular radii of the molecule under examination by the Stokes-Einstein Equation (). R = k b T/6πηD () In this case R is the effective molecular radius, k b is the Boltzmann constant, T is absolute temperature, η is viscosity of solvent and D is the diffusion coefficient. The diffusion co-efficients D are measured from a series of D experiments in which the gradient strengths are increased. Modern DOSY MR sequences utilise pulse field gradient-stimulated echo (PFG-STE) experiments with various modifications to the pulse sequence. The molecular radii are an average of the x, y and z dimensions of the molecule and as such represents an effective diffusional size. In the case of selfassembled systems, these dimensions reflect the overall size of the resultant array rather than the sizes of the individual component molecules from which they are constructed. In this way the diffusion coefficient D allows a simple measurement of molecular size in solution. *Correspondence to: Martin R. Johnston, Martin.johnston@flinders.edu.au, fax: Current address: School of Chemistry, Physics and Earth Sciences, Flinders University, Bedford Park, South Australia, 5042, Australia RESULTS AD DISCUSSIO Our research has been focused on supramolecular systems and in particular the synthesis and use of bis-porphyrin cavities. We have developed a building Copyright 2002 Society of Porphyrins & Phthalocyanines

2 758 M.R. JOHSTO AD M.J. LATTER block approach to the production of these cavities which utilises different coupling protocols based on cycloaddition reactions [, 2], and in this way bis-porphyrin cavities of different sizes may be constructed [3-5]. Initially investigations focussed upon the complexation of guest molecules, such as difunctional porphyrins and aromatic diimides, within these cavities and were examined as artificial photosynthetic systems [6-9]. The ideas were expanded with the use of a tetrapyridyl substituted porphyrin guest to yield a molecular universal joint [20]. All of the above systems were sufficiently stable so as to be characterized by familiar procedures eg. H MR, high resolution mass spectrometry and solution photophysical studies. More recently however this research has been extended into the arena of molecular capsules. Capsules are three dimensional hosts formed as a result of intermolecular interactions between components that may not be hosts in their own right [2]. Capsule systems are held together by noncovalent interactions with hydrogen bonding [22-24] and metal coordination [25-27] most frequently employed. Using metal mediated self-assembly we have produced a capsule from the dimerisation of self-complementary bis-porphyrin cavities [28]. Although we have obtained positive evidence for capsule formation using FAB mass spectrometry we have been unable to observe directly the encapsulation of guests by MR spectroscopy in solution. However, in the literature the observation of upfield shifted signals for encapsulated guests has been used to unequivocally prove that a capsule structure has indeed formed in solution [2]. The need for a determination of a capsule structure in solution from the dimerisation of our self-complementary bisporphyrin led us to examine DOSY experiments since the usual MR techniques were not conclusive. Instrumentation and methods The PFG-MR experiments were acquired on Bruker DRX400 MR (400 MHz) spectrometer, using the bipolar pulse-longitudinal eddy current delay (BPP-LED) pulse sequence [29] as supplied with XWIMR. The software generates an array of experiments containing increasing gradient strengths. The probe was 5 mm inverse detection quad probe (H, C, P, ) fitted with z-gradient facility capable of delivery up to 50 G/cm. Various gradient pulse delays (-5 ms) and diffusion times in the range ( ms) were used so as to give a variety of experimental conditions. After Fourier transformation of the various D H spectra the intensity of a resonance was measured in each spectrum and the overall curve of intensity versus gradient strength fitted using standard XWIMR algorithms to yield a diffusion constant. The resonances followed were for tertbutyl and meso-aromatic protons of the porphyrin building blocks and cavities and the results averaged. The results of experiments with different gradient pulse sizes and diffusion times were averaged for the resonances under examination to give a final diffusion constant. All spectra were recorded in CD 2 Cl 2 at 303 K and each sample was calibrated to external CHCl 3. The average molecular radii were calculated from the experimentally determined diffusion coefficients by the Stokes-Einstein equation. Molecular modelling was carried out on Silicon Graphics workstation (O 2 ) using SPARTA version at the semi-empirical level of theory (AM). The theoretical average molecular radii were calculated from an average of the molecular dimensions in the x, y and z directions for gasphase minimised structures. These dimensions were then used as an estimate of the size of molecules in solution. Results To establish the validity of our experimental technique a series of porphyrin molecules of increasing size were examined in DOSY experiments so as to determine their diffusion coefficients and hence molecular radii in solution. These components employed in this study include: porphyrin block and bis-porphyrin cavity 2 and are shown in Fig.. Once these covalently linked molecules had been studied, our attention was directed to complexes 2.3 and 5 (Fig. ) formed using metal-ion coordination. The synthesis and characterization of these blocks, cavities and capsule have been reported previously [4, 5, 28]. In all cases molecular modelling was employed to determine the theoretical size of each of the components used in this study, and the space filling representation of these are presented in Fig. 2. MR spectroscopy The porphyrin-containing components and 2 have been thoroughly characterized by H MR [4, 5]. In addition, H MR spectroscopy has proved to be invaluable in characterizing the host/guest complex 2.3 since coordination to the metalloporphyrin results in considerable upfield shifts for all proton resonances of the guest [28]. In particular, the proton resonance to undergo the largest shift was the α-pyridyl (Av. Δδ = 5.83 ppm) of the guest due to the large magnetic anisotropy of the host porphyrin macrocycle. Similar shifts, but of a smaller magnitude, were also observed for other protons resonances of the guest as the distance from the porphyrin shielding increased. These shifted resonances were used to determine the diffusion characteristics of the guest molecule within the complex. The lack of any magnetic anisotropy

3 MR DIFFUSIO SPECTROSCOPY 759 H H R R R R associated with the guest 3 meant that the resonances of host 2 were essentially unchanged within the complex 2.3 [28]. DOSY MR spectroscopy 2 R R 4 (R = 4-Py) As systems of different size diffuse through solution at different rates, a good correlation between theoretical and experimental diffusion coefficients or molecular radii is sufficient evidence to confirm the presence of the assembled system [2]. The molecular models used to determine the theoretical molecular radii are shown in Fig. 2. Several important factors emerge from comparisons between the various blocks and cavities shown in Fig. 2: Between block and the host 2 there is an effective tripling in size of the molecule when viewed from the x-direction (0 to 37 Å). However from the y direction there is no change (6 Å) in the dimensions of the molecule. Thus, when combined the result is only a fifty percent increase in the average molecular Fig.. Porphyrin-containing systems used in the DOSY study ( = 3,5-ditertbutylphenyl) radius between and 2. A decrease in molecular radius occurs between the cavity 2 and complex 2.3. This is due to a collapse in the cavity walls of 2 to accommodate the small guest effectively reducing the size of the host [30]. In the capsule structure 5, even though there are now two bis-porphyrin units present, the extremities of one bis-porphyrin does not protrude outside the dimensions of the second bis-porphyrin. This fact when combined with the collapse of both cavities, gives only a marginal increase in radii when compared with the free host 2. The building block and bis-porphyrin cavity 2 were used in this study as standards to evaluate the accuracy of the DOSY technique. The radii shown in Table for these components show good agreement between theoretical and experimental values indicating that although calculations were done in the gas-phase they are a good representation of the average solution structure of the molecules. In general the experimentally determined results are smaller, 5

4 760 M.R. JOHSTO AD M.J. LATTER a) 26 Å z y 0 Å 6 Å x R R c) b) 2 R R Å 37 Å 25 Å 6 Å 6 Å 26 Å d) 33 Å 5 26 Å 26 Å Fig. 2. Space filling models of components used in DOSY study. Molecular modelling (AM) allowed dimensions shown to be measured and were used to calculate the theoretical molecular radii (Å) ( = 3,5-ditertbutylphenyl)

5 MR DIFFUSIO SPECTROSCOPY 76 but within uncertainty range, of the theoretical calculations. The experiments were carried out in dichloromethane as this allowed a common solvent to be used for all experiments in Table since capsule formation is solvent dependant [28]. Once a correlation between theoretical and experimental data had been established for the porphyrin components and 2 we turned our attention to the non-covalent host-guest complex 2.3. The complex 2.3 was previously confirmed by the H MR chemical shift changes in resonances of 3, as well as high resolution electrospray mass spectroscopy. An examination of the proton resonances for the complexed guest within 2 using DOSY MR gave an experimental molecular radii of 0.0 Å which compared favourably with the theoretical value (Table ). These results revealed that the guest did in fact diffuse through solution as a complex with a molecular size corresponding to that of a collapsed host. This is to be expected since the association constant for 2.3 was measured to be large at 0 8 M - (toluene) [7]. Further, examination of the tert-butyl resonances of the porphyrin host yielded a value consistent with a collapsed geometry rather than that of 2. The theoretical radii of the Table. Experimental and theoretical molecular radii determined from DOSY experiments and molecular modelling respectively Porphyrin component Experimental (Å) a (303 K in CH 2 Cl 2 ) Theoretical (Å) (AM gas phase) porphyrin block 7.9 ±. 8.7 bis-porphyrin cavity ± host-guest complex b ±.6.2 capsule system ± a Errors are stated as standard deviations. b This study was conducted in deuterated chloroform due to solubility problems of the guest in CD 2 Cl 2. uncomplexed host 2 and guest 3 were calculated to be 2.8 and 4. Å, respectively and were not observed in the experiment. Due to the success of the preceeding study, the diffusion of the non-covalently assembled capsule system was thus investigated. Although the shifts observed in the H MR spectra indicated that complexation was definitely occurring in solution, there was not sufficient evidence to unequivocally eliminate the alternative complexation mode (linear stack, Fig. 3) since similar shifts are expected for each assembly mode. Molecular modelling of possible linear stacks was conducted and the theoretical molecular radii were calculated for several of these assemblies (Fig. 3). As there is no simple way to determine how many cavities would be assembled in forming a stack, the theoretical values for stacks containing up to 6 cavities are tabulated. Comparing the experimentally determined radii for 5 (Table ) with those from Fig. 3 it is clear that there is no correlation and that the most likely configuration of the complexed bis-porphyrins in solution is that of a capsule. If a stack assembly was occurring the molecular radius would be expected to be much larger than that obtained as the polymeric structure would diffuse through solution at a slower rate. This result provides additional evidence for the presence of the capsule structure in solution along with the previously observed chemical shift changes and mass spectral data. COCLUSIO PFG-DOSY MR has proven to be an extremely powerful tool to observe self-assembled systems in solution that were difficult to characterize by normal spectroscopic techniques. The application o. of bis-porphyrin cavities in stack Theoretical molecular radii (Å) Fig. 3. Space filling representation of the two possible modes of assembly a) capsule and b) linear stack and theoretical molecular radii for possible assembled structures determined from molecular modelling. Porphyrin substituents have been omitted for clarity

6 762 M.R. JOHSTO AD M.J. LATTER of our study to a series of porphyrin components and complexes has shown that reliable data can be obtained for a range of different sized systems. Due to the relative ease of obtaining useful information regarding molecular size DOSY MR will be an appealing option for the characterization of systems held together by non-covalent interactions. REFERECES. Michelsen U and Hunter CA. Angew. Chem., Int. Ed. 2000; 39: Timmerman P, Weidmann JL, Jolliffe KA, Prins LJ, Reinhoudt D, Shinkai S, Frish L and Cohen Y. J. Chem. Soc., Perkin Trans : Cameron KS and Fielding L. J. Org. Chem. 200; 66: Keresztes I and Williard PG. J. Am. Chem. Soc. 2000; 22: Hori A, Kumazawa K, Kusukawa T, Chand DK, Fujita M, Sakamoto S and Yamaguchi K. Chem.--Eur. J. 200; 7: Frish L, Matthews SE, Bohmer V and Cohen Y. J. Chem. Soc., Perkin Trans : Cain JB, Zhang K, Betts DE, DeSimone JM and Johnson CS, Jr. J. Am. Chem. Soc. 998; 20: Frish L, Vysotsky MO, Matthews SE, Bohmer V and Cohen Y. J. Chem. Soc., Perkin Trans ; : Hodge P, Monvisade P, Morris GA and Preece I. Chem. Commun. (Cambridge, U. K.) 200: Johnson CS. J. Magn. Reson. 993; 02: Warrener R, Margetic D and Russell RA. Synlett 998: Warrener R, Margetic D, Amarasekara AS, Butler D, Mahadevan IB and Russell RA. Org. Lett. 999; : Johnston MR. Molecules 200; 6: Warrener R, Johnston MR and Gunter MJ. Synlett 998: Johnston MR, Gunter MJ and Warrener R. Tetrahedron 2002; 58: Flamigni L, Marconi G and Johnston MR. Phys. Chem. Chem. Phys. 200; 3: Flamigni L, Talarico AM, Barigelletti F and Johnston MR. Photochem. Photobiol. Sci. 2002; : Flamigni L and Johnston MR. ew J. Chem. 200; 25: Flamigni L, Johnston MR and Giribabu L. Chem.--Eur. J. 2002; 8: Johnston MR, Gunter MJ and Warrener R. Chem. Commun. (Cambridge, U. K.) 998: Hof F, Craig SL, uckolls C and Rebek J. Angew. Chem., Int. Ed. 2002; 4: Rebek J. Chem. Commun. (Cambridge, U. K.) 2000: Rebek J, Jr. Acc. Chem. Res. 999; 32: Bohmer V and Vysotsky MO. Aust. J. Chem. 200; 54: Fujita M. Chem. Soc. Rev. 998; 27: Leininger S, Olenyuk B and Stang PJ. Chem. Rev. (Washington, DC, U. S.) 2000; 00: Jones CJ. Chem. Soc. Rev. 998; 27: Johnston MR, Latter MJ and Warrener R. Org. Lett. 2002; 4: Wu D, Chen A and Johnson CS, Jr. J. Magn. Reson. 995; 5: Johnston MR, Latter MJ and Warrener R. Aust. J. Chem. 200; 54:

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