pubs.acs.org/Langmuir © 2009 American Chemical Society
Luminescent Alkynylplatinum(II) Terpyridyl Metallogels Stabilized by Pt 3 3 3 Pt, π-π, and Hydrophobic-Hydrophobic Interactions† Anthony Yiu-Yan Tam, Keith Man-Chung Wong, Nianyong Zhu, Guoxin Wang, and Vivian Wing-Wah Yam* Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based Materials Research and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Received December 31, 2008. Revised Manuscript Received March 4, 2009 A series of luminescent alkynylplatinum(II) terpyridyl complexes have been synthesized and characterized by 1H NMR, IR, FAB-mass spectrometry, and elemental analysis; one of the platinum(II) complexes has also been structurally characterized by X-ray crystallography. Their electrochemical and photophysical properties have also been investigated. A majority of the complexes were able to form stable thermoreversible metallogels in organic solvents, tested by the “stable-to-inversion of a test tube” method. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) of the xerogels showed typical fibrous structures on the micrometer scale. Interestingly, whereas 2-OTf, 3-OTf, and 5-PF6 formed thermotropic metallogels mainly through van der Waals’ forces with different emission colors, 1-X (X = OTf, BF4, PF6, and ClO4) showed additional Pt 3 3 3 Pt and π-π interactions to stabilize the resultant metallogels and showed drastic color changes during the gel-to-sol phase transition. The metallogels of 1-X were also different colors, depending on the nature of the counter anions, which governs the degree of aggregation and the extent of Pt 3 3 3 Pt and π-π interactions involved in the gelation process. This property may be utilized to serve as an effective reporter for microenvironmental changes.
Introduction During the past few decades, there was a revival of interest in the spectroscopic study of square-planar platinum(II) complexes, owing to their intriguing spectroscopic and luminescence properties.1-6 The platinum(II) terpyridyl complexes, being one of these classes of platinum(II) complexes, have received much attention because of their propensity to exhibit Pt 3 3 3 Pt and π-π interactions in the solid state.1a-1c,3a-3c,4a-4c,5a,5b Owing to the quenching by the efficient radiationless decay via low-lying triplet ligand field (3LF) excited states, the platinum(II) terpyridyl complexes with weak field ligands as the auxiliary ligand, such as [Pt(tpy) Cl]+ (tpy = terpyridine), are found to be non-emissive in fluid † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author. E-mail:
[email protected].
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solution at ambient temperature.3b-3e,4h,5a-5f Upon incorporation of strong-field σ-donating alkynyl ligands, the 3LF excited state is destabilized and thereby reduces the possibility of such nonradiative deactivation pathways. In addition, this class of alkynylplatinum(II) terpyridyl complexes showed drastic changes in the spectroscopic and luminescence properties upon a change in the microenvironment or in the presence of external stimuli.4a-4h For example, [Pt(tpy)(CC-CCH)]OTf, one of (4) (a) Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Cheung, K. K. Organometallics 2001, 20, 4476–4482. (b) Yam, V. W. W.; Wong, K. M. C.; Zhu, N. J. Am. Chem. Soc. 2002, 124, 6506–6507. (c) Yam, V. W. W.; Chan, K. H. Y.; Wong, K. M. C.; Zhu, N. Chem.-Eur. J. 2005, 11, 4535–4543. (d) Tam, A. Y. Y.; Wong, K. M. C.; Wang, G.; Yam, V. W. W. Chem. Commun. 2007, 2028–2031. (e) Camerel, F.; Ziessel, R.; Donnio, B.; Bourgogne, C.; Guillon, D.; Schmutz, M.; Iacovita, C.; Bucher, J. P. Angew. Chem., Int. Ed. 2007, 46, 2659–2662. (f ) Yu, C.; Chan, K. H. Y.; Wong, K. M. C.; Yam, V. W. W. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 19652–19657. (g) Wong, K. M. C.; Tang, W. S.; Chu, B. W. K.; Zhu, N.; Yam, V. W. W. Organometallics 2004, 23, 3459–3465. (h) Wong, K. M. C.; Tang, W. S.; Lu, X. X.; Zhu, N.; Yam, V. W. W. Inorg. Chem. 2005, 44, 1492–1498. (i) Shikhova, E.; Danilov, E. O.; Kinayyigit, S.; Pomestchenko, I. E.; Tregubov, A. D.; Camerel, F.; Retailleau, P.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2007, 46, 3038–3048. ( j) Lu, W.; Law, Y. C.; Han, J.; Chui, S. S. Y.; Ma, D. L.; Zhu, N.; Che, C. M. Chem. Asian J. 2008, 3, 59–69. (k) Chakraborty, S.; Wadas, T. J.; Hester, H.; Flaschenreim, C.; Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6284–6293. (l) Rachford, A. A.; Goeb, S.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2008, 47, 4348–4355. :: (5) (a) Buchner, R.; Cunningham, C. T.; Field, J. S.; Haines, R. J.; McMillin, D. R.; Summerton, G. C. J. Chem. Soc., Dalton Trans. 1999, 711–717. (b) Wadas, T. J.; Wang, Q. M.; Kim, Y. J.; Flaschenreim, C.; Blanton, T. N.; Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 16841–16849. (c) Crites, D. K.; Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta 1998, 346–353. (d) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem. 2000, 39, 2708. (e) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.; Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg. Chem. 2001, 40, 2193–2709. (f ) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-W.; Zhang, L.-P.; Tung, C.-H. Inorg. Chem. 2002, 41, 5653–5655. (g) Han, X.; Wu, L. Z.; Si, G.; Pan, J.; Yang, Q. Z.; Zhang, L. P.; Tung, C. H. Chem.-Eur. J. 2007, 13, 1231–1239. (6) (a) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro, M. L.; Rajapakse, N. Coord. Chem. Rev. 2006, 250, 1819-1828. (b) Lai, S. W.; Che, C. M. Top. Curr. Chem. 2004, 241, 27–63.
Published on Web 06/05/2009
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the alkynylplatinum(II) terpyridyl complexes reported by our group, was found to exhibit drastic color changes from yellow to blue through green in solution upon the variation of diethyl ether composition in acetonitrile.4b The reason behind this observation is suggested to be a result of the formation of aggregate species from monomeric species through intermolecular Pt 3 3 3 Pt and π-π interactions.4b An extension of this work with various counter anions and their effect on the degree of aggregation has also been reported.4c,4d Low-molecular-weight organogels have received much attention in the past decade, mainly because of their ability to construct distinct supramolecular structures from gelator molecules in organic solvents through spontaneous as well as controlled selfassembly processes.7-9 The organogel is simply prepared by heating a mixture of a gelator and organic solvent until a clear solution is obtained. Upon cooling to room temperature, the mixture forms a thermoreversible viscoelastic liquidlike or solidlike material called an organogel, which is stable to inversion. Non-covalent interactions, such as hydrogen bonding, van der Waals forces, and π-π interactions, are the common driving force for such self-assembly processes. In the process of organogelation, the gelator molecules self-assemble through non-covalent interactions to form fibrous architectures on the nanometer scale, which in turn builds up a micrometer-scale entangled 3D network entrapping/immobilizing organic solvent molecules. This in turn prevents the organic solvent from flowing. With slight modifications of the chemical composition of the gelator molecules such as chirality and functional groups, the microscopic and macroscopic properties can be significantly influenced. This could be a simple but versatile method for fabricating nanoscale materials from such a bottom-up approach.7 Parallel to the growing interest in the development of organogels, there has also been an increasing interest in the investigation of transition-metal complexes10,11 as metallogelators, with examples including Au(I),11a Pt(II),4d,4e,4j,11c,11h Cu(I),11b and Re(I).11i The reason for this interest stems from the availability and the diversity of metal-ligand chromophores and their associated rich spectroscopic and luminescence properties.4d,4e,10,11 Aida and co-workers have successfully synthesized an interesting class
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of the trinuclear Au(I) pyrazolate metallacyclic complexes, which showed gelation properties through aurophilic (Au 3 3 3 Au) interactions as well as van der Waals’ forces. Upon doping or dedoping of Ag(I) ions, the sol-gel phase transition showed reversible switching of red-green-blue luminescence.11a Recently, our group and Ziessel’s group have demonstrated that alkynylplatinum(II) terpyridyl complexes are capable of forming stable metallogels and showed drastic color changes during the gel-to-sol phase transition.4d,4e These remarkable spectroscopic changes are believed to originate from aggregation/deaggregation processes of the square-planar platinum(II) complexes upon heating. We also communicated the preliminary result on the effect of counter anions on the gelation properties.4d As an extension of our previous studies, we herein report the preparation, electrochemistry, and photophysical properties of a series of alkynylplatinum(II) terpyridyl complexes (Scheme 1). The majority of the complexes are found to exhibit stable thermotropic metallogels in organic solvents. Their gelation properties have been investigated by electron microscopy, UV-vis absorption spectroscopy, and luminescence spectroscopy. The metallogel of 1-OTf in DMSO showed drastic color changes during the gel-to-sol phase transition. Interestingly, the colors of metallogel 1-X in DMSO and their near-IR emissions are found to be influenced by the nature of the counter anions. In addition, with modifications of the electronic properties of the ligands as well as the organic solvents, the electronic absorption and luminescence properties of metallogels (2-OTf, 3-OTf, and 5-PF6) could be readily tuned. X-ray Crystal Structure. The crystal structure of the complex cation of 6-OTf in the red form is depicted in Figure 1. An attempt to obtain the single crystal of its dark-brown form for X-ray crystallography was not successful. Selected bond distances and angles are tabulated in Table S1. Similar to the platinum(II) terpyridyl complexes reported in the literature,3a-3f,4a-4c,4g-4k,5a,5b the platinum(II) metal center Scheme 1
(7) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3160. (b) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237–1247. (c) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem. Rec. 2003, 3, 212–224. (d) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980–999. (8) (a) Ikeda, M.; Takeuchi, M.; Shinkai, S. Chem. Commun. 2003, 1354–1355. (b) Shirakawa, M.; Kawano, S.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem. 2003, 68, 5037–5044. (c) An, B.-K.; Lee, D.-S.; Lee, J.-S.; Park, Y.-S.; Song, H.-S.; Park, S. Y. J. Am. Chem. Soc. 2004, 126, 10232–10233. (d) Ryu, S. Y.; Kim, S.; Seo, J.; Kim, Y.-W.; Kwon, O.-H.; Jang, D.-J.; Park, S. Y. Chem. Commun. 2004, 70– 71. (e) Mukhopadhyay, P.; Iwashita, Y.; Shirakawa, M.; Kawano, S.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2006, 45, 1592–1599. (f ) Kato, T.; Kamikawa, Y. Langmuir 2007, 23, 274–278. (g) Srinivasan, S.; Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5746–5749. (9) (a) Lee, S. J.; Lee, S. S.; Kim, J. S.; Lee, J. Y.; Jung, J. H. Chem. Mater. 2005, 17, 6517–6520. (b) Jung, J. H.; Lee, S.-H.; Yoo, J. S.; Yoshida, K.; Shimizu, T.; Shinkai, S. Chem.-Eur. J. 2003, 9, 5307–5313. (c) Kawano, S.; Tamaru, S.; Fujita, N.; Shinkai, S. Chem.-Eur. J. 2004, 10, 343–351. (d) George, S. J.; Ajayaghosh, A.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 3422–3425. (e) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644–654. (f) Nam, S. R.; Lee, H. Y.; Hong, J.-I. Chem.-Eur. J. 2008, 14, 6040–6043. (10) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680–1682. (11) (a) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179–183. (b) Kawano, S.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592–8593. (c) Shirakawa, M.; Fujita, N.; Tani, T.; Kaneko, K.; Shinkai, S. Chem. Commun. 2005, 4149–4151. (d) Camerel, F.; Bonardi, L.; Schmutz, M.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 4548–4549. (e) Naota, T.; Koori, H. J. Am. Chem. Soc. 2005, 127, 9324–9325. (f ) Weng, W.; Beck, J. B.; Jamieson, A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 127, 11663–11672. (g) Tu, T.; Aseenmacher, W.; :: Peterlik, H.; Weisbarth, R.; Nieger, M.; Dotz, K. H. Angew. Chem., Int. Ed. 2007, 46, 6368–6371. (h) Cardolaccia, T.; Li, Y.; Schanze, K. S. J. Am. Chem. Soc. 2008, 130, 2535–2545. (i) Lam, S. T.; Wang, G.; Yam, V. W. W. Organometallics 2008, 27, 4545–4547.
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Figure 1. Perspective drawing of the complex cation of 6-OTf with atomic numbering. Hydrogen atoms and the counter anion are omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
adopts a distorted square-planar geometry, in which the N-Pt-N angles (N(1)-Pt(1)-N(2) 80.4°, N(2)-Pt(1)-N(3) 80.64°, and N(1)-Pt(1)-N(3) 161.0°) were found to deviate from the idealized values of 90 and 180°. This could be attributed to the steric demand of the terpyridine ligands. The bond lengths of Pt-C and CtC are 1.979 and 1.197 A˚, respectively, which are comparable to those found in the related alkynylplatinum(II) complexes.4a-4c,4g-4k The interplanar angle between the plane of the aryl ring of the arylalkynyl ligand and that of the [Pt(tpy)] unit is 4.1°, indicative of a coplanar arrangement. The crystal packing of 6-OTf shows that the cationic molecules are stacked in a head-to-tail orientation (Figure S1a), as revealed by the rotation of the individual molecules with respect to their neighbors with a C-Pt-Pt-C torsion angle of 180°. The “short” (3.87 A˚) and “long” (5.37 A˚) Pt 3 3 3 Pt bonds exhibit a zigzag arrangement with a Pt-Pt-Pt angle of 119.1° (Figure S1b). In view of the previous study in which [Pt(tpy) (CtC-CtCH)]OTf in its dark-green form with a Pt-Pt-Pt angle of 179.2° showed stronger Pt 3 3 3 Pt and π-π interactions than its red form with a Pt-Pt-Pt angle of 154.3°,4b it is suggested that the deviation of the Pt-Pt-Pt angle from 180° would lead to a decrease in the extent of Pt 3 3 3 Pt and π-π interactions as a result of poorer overlapping of the dz2 orbitals between the two adjacent platinum(II) complex cations. A similar phenomenon was also observed for the crystals of [Pt (tpy)(CtC-C6H5)]PF6 (dark-brown crystals) and [Pt(tpy) (CtC-C6H5)]ClO4 (orange crystals).4a,4c On the basis of this correlation, it is believed that the dark-brown form of 6-OTf should form stronger Pt 3 3 3 Pt and π-π interactions with a smaller deviation from 180° in the Pt-Pt-Pt angle than its red form (vide infra). Electrochemistry. In general, the cyclic voltammograms of complexes 1+-6+ in dichloromethane or acetonitrile solutions in the presence of 0.1 M nBu4NPF6 displayed two quasireversible couples at approximately -0.83 to -1.63 V and an irreversible anodic wave at approximately +1.02 to +1.21 V versus SCE. The electrochemical data are summarized in Table S2. Because the reduction couples are found to be insensitive to the nature of the alkynyl ligands but sensitive to the substituents on the terpyridine ligand, they are believed to arise from the terpyridine-based reductions.3g,4a,4c,4g,4i,4k,5c,5f,6 Such an assignment is in line with the observation that 2+, 3+, and 4+ showed a more negative reduction potential than 1+ in CH2Cl2 (0.1 mol dm-3 nBu4NPF6) because the electrondonating tert-butyl groups on the terpyridine ligand would Langmuir 2009, 25(15), 8685–8695
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render the π*(tBu3tpy) orbitals higher-lying in energy and hence a more negative reduction potential. Interestingly, the difference in reduction potentials between 1+ and 5+ is smaller than expected because the aryl and terpyridyl groups in 5+ are not perfectly coplanar to each other as a result of steric repulsion between hydrogen atoms at the ortho positions of the aryl ring and those at the 30 ,50 -positions of the terpyridine, resulting in the stabilization of the π*(tpy) orbitals in 5+ to a much lesser extent than expected. In addition, the relatively readily oxidizable aryl groups in 5+ could be electron-donating and thereby could raise the π*(tpy) orbital energy. Because of the compensating effect, the reduction potentials of terpyridine and phenylterpyridine are fairly close to each other. A similar finding was also observed by McMillin and co-workers.5e In contrast to the reduction couples, the oxidation waves are sensitive to the variation of substituents on the phenylethynyl ligands and are attributed to the alkynyl-based ligand-centered oxidation, mixed with some metal-centered character.4a,4c,4g,4i,4k,6 For 2+ and 3+ having the same terpyridine ligand, it was found that 2+ (+1.03 V) showed a less anodic potential for its oxidation than 3+ (+1.21 V). This could be ascribed to the presence of the three electron-donating alkoxy groups in 2+, which would raise the dπ(Pt) and π(CtC) orbital energy with respect to the less electron-rich trialkoxybenzamide groups in 3+. The almost identical potential for the first oxidation of 3+ and 4+ is not unreasonable because one would not expect the chain length of the alkoxy groups to have any significant effect on the potential of the oxidation couples. Apart from the first oxidation wave, 3+, 4+ and 5+ showed an additional irreversible oxidation wave at ca. +1.46 to +1.57 V vs SCE. In view of the observation of an oxidation wave at +1.61 V vs SCE for the free alkyne, this additional oxidation wave has been ascribed to the trialkoxybenzamide-based oxidation.4k UV-Vis Absorption Spectroscopy. The electronic absorption spectra of 1+-6+ in solutions at 298 K showed intense absorption bands at 267-376 nm and moderately intense absorptions at 402-476 nm. The photophysical data of 1+-6+ are summarized in Table 1. The high-energy intense absorption bands are assigned as the intraligand (IL) [πfπ*] transitions of the alkynyl and terpyridyl ligands, whereas the low-energy and less intense absorption bands are tentatively assigned as an admixture of metal-to-ligand charge transfer (MLCT) [dπ(Pt)fπ*(tpy)] and ligand-to-ligand charge transfer (LLCT) [π(CtC)fπ*(tpy)] transitions.4,5f,5g In 5-PF6, the mixing of an intraligand charge transfer (ILCT) [π(Ph)fπ*(tpy)] character into the low-energy MLCT/LLCT bands is possible.5f,5g As with other platinum(II) polypyridine complexes,1b,2c,3e,4h,5e negative solvatochromism was observed for 1-OTf, in which the MLCT/LLCT absorption bands showed a blue shift in energy (from 500 nm in CH2Cl2 to 476 nm in DMSO) upon an increase in the solvent polarity. This could be attributed to the decrease in the dipole moment of the excited state relative to its ground state. 3-OTf and 4-OTf in CH3CN showed essentially identical UV-vis absorption patterns with similar extinction coefficients, suggesting that the chain length of the alkoxy groups did not have any significant effect on the MLCT/LLCT absorption. This also suggests that the MLCT/LLCT absorption band in 6-OTf, which occurs at lower energy (464 nm) than that in 2-OTf (458 nm) in CH3CN, is not due to the difference in the chain length of the alkoxy groups but rather to the presence of the electron-donating tert-butyl groups on the terpyridine ligands in 2-OTf, which would render the π*(tBu3tpy) orbital higher-lying in energy and therefore increasing the MLCT/ LLCT energy. DOI: 10.1021/la804326c
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Tam et al. Table 1. Photophysical Data absorption
complex
medium (T/K)
emission -1
-1
λmax /nm (εmax /dm mol cm ) 3
λem /nm (τo /μs)
φema
DMSO (298) 267 (36 820), 334 sh (13 280), 349 (12 225), 476 (4435) 780 (
