Photophysical Properties of Coordination-Driven Self-Assembled

Feb 19, 2010 - Guang-Jiu Zhao,† Brian H. Northrop,‡ Peter J. Stang,*,‡ and Ke-Li Han*,† ... surfaces.11–14 Schatz and co-workers have used t...
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Photophysical Properties of Coordination-Driven Self-Assembled Metallosupramolecular Rhomboids: Experimental and Theoretical Investigations Guang-Jiu Zhao,† Brian H. Northrop,‡ Peter J. Stang,*,‡ and Ke-Li Han*,† State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: December 7, 2009; ReVised Manuscript ReceiVed: February 4, 2010

In this work, the photophysical properties of coordination-driven self-assembled metallosupramolecular rhomboids with the donor ligands 1,2-bis(3-pyridyl)ethyne (3a) and 1,4-bis(3-pyridyl)-1,3-butadiyne (3b) are investigated by use of both spectroscopic experiments and quantum chemistry calculations. All the geometric conformations of the chair and boat conformers of 3a and 3b are fully optimized using density functional theory. The time-dependent density functional theory method was also used to study the excited-state properties of these self-assembled metallosupramolecular rhomboids. At the same time, steady-state absorption and fluorescence as well as the time-correlated single photon counting techniques are used to measure their various spectral properties. The fluorescence spectra of these self-assembled metallosupramolecular rhomboids are very wide and show an evident two-peak feature, which can be tuned by different excitation wavelengths. It has been demonstrated that the chair conformers of both 3a and 3b are formed preferentially over their boat conformers due to the close proximity of the chelated bisphosphine platinum groups. Moreover, an additional shoulder observed at 416 nm in the fluorescence spectra of 3b indicates the presence of minor amounts of the boat conformer of 3b. In addition, we have also demonstrated that lengthening the acetylene chain of the donor ligand component of these rhomboids results in a red-shifted and broadened absorption band for these metallosupramolecular rhomboids. Furthermore, the nature of the excited states for these metallosupramolecular rhomboids varies with the acetylene chain length of the donor ligands and with the different conformers. I. Introduction Over the past two decades coordination-driven self-assembly, which combines rigid metal acceptors and organic donors, has provided a facile means of synthesizing metallosupramolecular polygons of predetermined size and geometry.1–4 Stang and coworkers have reported numerous self-assembled molecular polygons through the use of lipophilic phosphine-substituted as well as chelated bisphosphine Pt and Pd complexes.5–10 Wan et al. have employed the scanning tunneling microscopes (STM) to investigate these self-assembled supramolecular polygons on surfaces.11–14 Schatz and co-workers have used theoretical and computational methods to describe various properties of selfassembled nanoscale materials.15–19 To date, most investigations of such systems have focused on the synthesis and structural features of these metallosupramolecular polygons. Reports involving both experimental and theoretical studies of their photophysical properties have been limited except the very recent ultrafast dynamics study by Goodson and co-workers.20 However, a thorough understanding of the photophysical properties of these metallosupramolecules will aid both in recognizing the relationship between metallosupramolecular structure and function and in developing uses of these metallosupramolar polygons in applications such as photoelectronic materials and as photocatalysts.21–27 Zhao et al. have used density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods to investigate the ground and excited * To whom correspondence should be addressed, [email protected] (K.L. Han) and [email protected] (P. J. Stang). † State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Department of Chemistry, University of Utah.

states of aquo palladium(II) complexes cis-[(dppp)Pd(H2O)2]2+, cis-[(dppp)Pd(H2O)(OSO2CF3)]+(OSO2CF3)-, and cis-[(dppp)Pd(H2O)2]2+(OSO2CF3)-2.28 Insights into the very important influence of hydrogen bonding on the structural and spectral properties of these three aquo Pd(II) and other hydrogen-bonded complexes were presented for the first time.29–31 It was demonstrated that the DFT/TDDFT method is a reliable theoretical tool to study complex supramolecular systems in combination with their spectroscopic experiments.32–34 Therefore, in the present work, we have studied the photophysical properties of metallosupramolecular polygons through both spectral experiments and quantum chemistry calculations. Chi et al. have self-assembled metallosupramolecular rhomboids by use of flexible donor ligands like 1,2-bis(3-pyridyl)ethyne and 1,4-bis(3-pyridyl)-1,3-butadiyne upon combination with organoplatinum 90° acceptor units.35 These systems are unique examples of versatile pyridine donors adjusting their bonding directionality to accommodate rigid platinum acceptors in the formation of closed macrocycles.35 They found that the two self-assembled metallosupramolecular rhomboids with different ligands have different conformational properties in their NMR spectra. Both the chair and boat conformers of metallosupramolecular rhomboid with 1,4-bis(3-pyridyl)-1,3-butadiyne ligand were detected in the NMR spectra.35 However, the boat conformer of metallosupramolecular rhomboid with 1,2-bis(3pyridyl)ethyne was not observed in the NMR spectra. The closer proximity of the phosphine groups in a boat conformation may make this structure less energetically favorable than its chair counterpart.35 In the present work, we study the photophysical properties of the two self-assembled metallosupramolecular rhomboids

10.1021/jp911597z  2010 American Chemical Society Published on Web 02/19/2010

Metallosupramolecular Rhomboids SCHEME 1: Self-Assembly of Metallosupramolecular Rhomboids 3a and 3b

with the 1,2-bis(3-pyridyl)ethyne and 1,4-bis(3-pyridyl)-1,3butadiyne ligands by use of spectroscopic methods and theoretical computations. The geometric conformations of both chair and boat conformers of these self-assembled metallosupramolecular rhomboids have been theoretically optimized and discussed. The nature of the low-lying electronic excited states of these metallosupramolecular rhomboids have been studied by the time-dependent density functional theory (TDDFT) method and compared with their steady-state absorption and fluorescence spectral results. Furthermore, the presence of the boat conformer of the self-assembled metallosupramolecular rhomboid with the 1,4-bis(3-pyridyl)-1,3-butadiyne ligand has been confirmed by the steady-state absorption and fluorescence spectra. In addition, the time-correlated single photon counting (TCSPC) method was used to measure time-resolved fluorescence decays. II. Experimental and Theoretical Methods The conformationally flexible donor ligands 1,2-bis(3-pyridyl)ethyne (1a), and 1,4-bis(3-pyridyl)-1,3-butadiyne (1b) were used to self-assemble metallosupramolecular rhomboids 1,2bis(3-pyridyl)ethyne (3a) and 1,4-bis(3-pyridyl)-1,3-butadiyne (3b), respectively, when combined with chelated bisphosphine platinum complex 2 (Scheme 1).35 Both 3a and 3b are soluble in CH2Cl2 at room temperature. The UV-vis absorption spectra were measured on an HP 8453 spectrophotometer. Steady-state fluorescence and timeresolved fluorescence decays were determined on a HORIBA JOBIN YVON FluoroMax-4 spectrofluorometer. Time-resolved fluorescence decays were recorded using the time-correlated single photon counting (TCSPC) method.36–41 Data analysis was carried out on commercial software provided by Horiba Instruments. The quality of the fits was measured by the χ2 values and distribution of residuals. The ground states of 3a and 3b were studied by the density functional theoretical (DFT) method with the BP86 functional and RI approximation.42 Furthermore, their excited states were investigated by the time-dependent density functional theory (TDDFT) method with the B3LYP functional, which have been widely used in the previous studies on large molecules.43 All quantum chemical calculations were performed using the Turbomole program suite along with the def-SV(P) basis set for nonmetal elements as well as the def-TZVP and pseudopotential def-ecp basis sets for the platinum atoms.44–46 III. Results and Discussion The fully optimized geometries of the chair and boat conformers of 3a and 3b are shown in Table 1. One can note that the N1-N2 distance between the pyridyl ligands of 3a is longer in its boat conformer than in the chair conformer. At the same time, the N1-Pt-N2 angle of the boat conformer of 3a increases by comparison with that of the chair conformer of 3a. While the N1-N2 distance and N1-Pt-N2 angle of 3b are

J. Phys. Chem. A, Vol. 114, No. 10, 2010 3419 TABLE 1: Calculated Important Bond Distances (in Å) as Well as Bond Angles and Dihedral Angles (in deg) for the Fully Optimized Structures of Both Chair and Boat Conformers of 3a and 3b

Pt-P1 Pt-P2 Pt-N1 Pt-N2 Pt-Pt′ P1-P2 N1-N2 C1-C1′ C2-C2′ C3-C3′ C4-C4′ C5-C5′ C6-C6′ C7-C7′ C8-C8′ C9-C9′ C10-C10′ P1-Pt-P2 P1-Pt-N1 N1-Pt-N2 N2-Pt-P2 Pt-N1-C1 Pt-N1-C5 C3-C4-C6 C5-C4-C6 C4-C6-C7 C6-C7-C8 C7-C8-C9 C8-C9-C10 P1-Pt-N1-C1 P2-Pt-N2-C1′

3a chair

boat

2.356 2.356 2.210 2.212 11.05 3.688 2.867 3.786 5.211 5.631 4.630 3.310 4.807 4.813 4.644

2.358 2.357 2.206 2.208 10.30 3.689 2.873 3.847 5.341 5.769 4.691 3.309 4.848 4.848 4.688

103.0 87.96 80.85 88.20 120.6 120.0 122.1 120.2 176.1 176.1

103.0 87.85 81.23 87.95 120.6 120.1 121.5 120.9 174.4 174.4

88.81 89.24

87.37 87.18

3b chair

boat

2.356 2.355 2.201 2.202 13.31 3.692 2.869 3.792 5.290 5.793 4.782 3.368 5.051 5.147 5.142 5.038 4.763 103.2 87.73 81.31 87.79 120.4 120.7 122.4 120.0 176.2 177.5 177.4 176.1 89.94 89.00

2.356 2.356 2.200 2.199 12.79 3.691 2.869 3.813 5.329 5.827 4.789 3.360 5.047 5.143 5.141 5.039 4.779 103.1 87.73 81.40 87.73 120.8 120.3 121.9 120.6 175.2 176.9 176.9 175.3 88.60 88.77

nearly the same in its chair and boat conformers. It can be demonstrated that the interaction between the chelated bisphosphine platinum groups is significantly dependent on the acetylene chain length of the donor ligand. In the boat conformer of 3a, the interaction between the phosphine groups is strong, since the phosphine groups are very close. As a result, the geometric conformation of the boat conformer of 3a is markedly influenced by the close proximity of the phosphine groups. It is also noted that the calculated energies of the chair conformers of both 3a and 3b are always lower than those of the boat conformers. This is consistent with the NMR results and suggests that the chair forms of 3a and 3b are preferred over the boat conformations, particularly in the case of 3a in addition to the close proximity of the phosphine groups.35 Hence, the boat conformer of 3a cannot be observed in the NMR. Moreover, one can note that the acetylene chain of the boat conformer is curved more than that of the chair conformer. The calculated dipole moments of the boat conformers are larger than their respective chair conformers for both 3a and 3b (388 D for the chair conformer and 429 D for the boat conformer). The large dipole moments of the different conformers for both 3a and 3b are ascribed to the more electropositive P atoms and more electronegative N atoms. Figure 1 shows the steady-state absorption and fluorescence spectra of 3a and 3b in CH2Cl2 solution. It is observed that the absorption band of 3b is significantly red-shifted and becomes wider in comparison with that of 3a. Hence, lengthening the acetylene chain of the donor ligands can red-shift and broaden the absorption band of these metallosupramolecular rhomboids. At the same time, one can note that the fluorescence spectra of

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Figure 1. Steady-state absorption and fluorescence spectra at different excitation wavelengths for 3a and 3b in CH2Cl2 solution.

3a and 3b are very wide and show evidence for two peaks. Furthermore, the relative intensity of the peaks can be tuned when the excitation wavelength is changed. The calculated electronic transition energies and the corresponding oscillator strength of the low-lying singlet excited states are listed in Table 2. Since the boat conformer of 3a cannot be observed due to the close proximity of the phosphine groups, the calculated results of the boat conformer of 3a are not shown. From Table 2, the calculated longest absorption for the chair conformer of 3a is at 322 nm, and at 381 nm for the chair conformer of 3b, which are consistent with the red-shifted experimental spectra of 3b in comparison with 3a. Furthermore, the calculated wavelengths are in good agreement with the red side of their experimental absorption band. The longest absorption for the boat conformer of 3b is calculated to be at 406 nm with an oscillator strength of zero. Hence, the 406 nm peak cannot be observed in the absorption spectra. However, an additional shoulder at around 416 nm can be observed in the

Zhao et al. fluorescence spectra of 3b. This fluorescence shoulder can be attributed to the boat conformer of 3b, since the calculated wavelength of the boat conformer of 3b is in accordance with the wavelength of the spectral shoulder. At the same time, this spectral shoulder at 416 nm also confirms the presence of a minor boat conformer for 3b. The nature of the low-lying singlet excited states for the chair conformer of 3a as well as the chair and boat conformers of 3b are also listed in Table 2. At the same time, the related molecular orbitals (MOs) are calculated and shown in Table 3. One can note that the low-lying excited states of the chair conformer for 3a are metal-centered (MC), intraligand (IL), intraligand charge transfer (ILCT), and metal-to-ligand charge transfer (MLCT) states, respectively. However, the excited states of the chair conformer for 3b are of IL, ILCT, and have metal-tometal charge transfer (MMCT) characters. In addition, the boat conformer of 3b has excited states of IL, ILCT, and MC features. Therefore, it is demonstrated that the nature of the lowlying excited states for these metallosupramolecular rhomboids varies with the acetylene chain length of the donor ligands as well as their different conformers. The different nature of excited states may play a vital role in the possible photophysical and photochemical applications of these coordination-driven selfassembled metallosupramolecular rhomboids. Figure 2 depicts the fluorescence decays of the 382 nm fluorescence peak for both 3a and 3b with an excitation wavelength of 295 nm. The four-exponential features are examined by nonlinear least-squares calculations to determine the lifetimes for each decay process. For 3a, the four lifetime values are 4.1 ( 0.2, 9.6 ( 0.3, 27.5 ( 1.0, and 0.83 ( 0.01 ns with the corresponding relative amplitude of 0.36, 0.29, 0.05, and 0.30, respectively. For 3b, the four lifetime values are 4.0 ( 0.3, 9.5 ( 0.5, 22.0 ( 1.0, and 0.24 ( 0.004 ns with the corresponding relative amplitude of 0.17, 0.23, 0.06, and 0.54, respectively. These differences between the fluorescence life-

TABLE 2: Calculated Electronic Transition Energies (nm) and Corresponding Oscillator Strength (in Parentheses) of the Low-Lying Singlet Excited States.a 3a S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 a

3b

chair

chair

boat

322.8 (0.0004) H-2 f L + 5 MC 322.3 (0.0116) H-2 f L + 7 MC 321.4 (0.0009) HfL IL 308.5 (0.0631) H-1 f L MLCT 307.8 (0.0008) H-2 f L MLCT 300.7 (0.2030) H-3fL MLCT 296.4 (0.5637) HfL+1 ILCT 294.2 (0.0008) H-4 f L IL 291.4 (0.0003) H-1 f L + 1 MLCT 290.7 (0.0004) H-5 f L MLCT 290.2 (0.0090) H f L + 2 ILCT 290.0 (0.0041) H-2 f L + 2 MLCT

380.6 (0.0038) H f L IL 370.0 (0.0007) H-2 f L ILCT 367.7 (0.0000) H-3fL ILCT 363.1 (0.4683) H-1 f L ILCT 347.6 (0.0161) H f L + 1 IL 340.9 (0.0019) H-1fL+1 ILCT 334.0 (0.0099) HfL+2 IL 329.0 (0.0004) H f L + 3 IL 321.2 (0.0002) H-2 f L + 1 ILCT 319.7 (0.0014) H-5 f L + 6 MMCT 319.5 (0.0060) H-4 f L + 7 MMCT 318.3 (0.0044) H f L + 2 IL

406.3 (0.0000) H f L IL 377.7 (0.1350) H-1 f L ILCT 373.3 (0.0000) H-2fL ILCT 372.7 (0.0029) H-3 f L ILCT 362.8 (0.1470) H f L + 1 IL 360.3 (0.0001) H-1fL+1 ILCT 327.0 (0.0027) HfL+3 ILCT 326.7 (0.0065) H f L + 2 ILCT 318.9 (0.0036) H-5 f L + 4 MC 318.8 (0.0055) H-5 f L + 4 MC 318.6 (0.0036) H-1 f L + 3 ILCT 315.4 (0.0012) H-2 f L + 1 ILCT

The dominative orbital contribution and the nature of various excited states are also listed: H, HOMO; L, LUMO; MC, metal-centered; IL, intraligand; ILCT, intraligand charge transfer; MLCT, metal-to-ligand charge transfer; MMCT, metal-to-metal charge transfer.

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TABLE 3: Calculated Frontier Molecular Orbitals and Corresponding Orbital Energies (in eV) for Different Conformers of 3a and 3b

Figure 2. Fluorescence decays of the 382 nm fluorescence at the excitation wavelength of 295 nm for both the 3a and 3b.

times of 3a and 3b may be related to the differing nature of their excited states. This means that the radiative and nonradiative rates of these excited states are different from each other. More experimental and theoretical studies on the excited-state ultrafast dynamics of these metallosupramolecular rhomboids are needed and will be performed in the future to further understand the interesting properties of their electronic excited states. IV. Conclusion In summary, we report the photophysical properties of the coordination-driven self-assembled metallosupramolecular rhomboids with the donor ligands 1,2-bis(3-pyridyl)ethyne (3a) and 1,4-bis(3-pyridyl)-1,3-butadiyne (3b) by use of both spectroscopic experiments and quantum chemistry calculations. All geometric conformations of the chair and boat conformers of 3a and 3b were fully optimized. It is demonstrated that the chair conformers of both 3a and 3b are formed preferentially over their boat conformers due to the close proximity of the chelated bisphosphine platinum groups. In particular, the boat conformer of 3a cannot be formed due to the strong interaction between the close phosphine groups. In addition, we have also demonstrated that lengthening the acetylene chain of the donor ligand component of these rhomboids results in a red shifted and broadened absorption band for these metallosupramolecular rhomboids. At the same time, their fluorescence spectra are very wide and show evidence of two peaks, which can be tuned by different excitation wavelengths. Moreover, an additional shoulder observed at 416 nm in the fluorescence spectra of 3b indicates the presence of minor amounts of the boat conformer of 3b. Furthermore, the nature of the excited states for these metallosupramolecular rhomboids varies with the acetylene chain length of the donor ligands and with the different

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conformers. All of the theoretical results are in agreement with the spectroscopic experiments. Additional experimental and theoretical studies of the excited state properties of these metallosupramolecular rhomboids will be performed to further explore their potential uses in various photophysical and photochemical applications. Acknowledgment. K.L.H. and G.J.Z. thank the NSFC (20903094 and 20833008) and NKBRSF (2007CB815202 and 2009CB220010) for financial support. P.J.S. thanks the NSF(CHE-306720 and CHE-0820955) for financial support. B.H.N. thanks the NIH (GM-080820) for financial support. Supporting Information Available: Cartesian coordinates of the optimized geometries for various conformers of the coordination-driven self-assembled metallosupramolecular rhomboids 3a and 3b. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (2) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853. (3) Stang, P. J. J. Org. Chem. 2009, 74, 2. (4) Maksymovych, P.; Sorescu, D. C.; Jordan, K. D.; Yates, J. T. Science 2008, 322, 1664. (5) Stang, P. J.; Cao, D. H.; Saito, S.; Arif, A. M. J. Am. Chem. Soc. 1995, 117, 6273. (6) Stang, P. J.; Olenyuk, B. Angew. Chem., Int. Ed. Engl. 1996, 35, 732. (7) Muller, C.; Whiteford, J. A.; Stang, P. J. J. Am. Chem. Soc. 1998, 120, 9827. (8) Schweiger, M.; Seidel, S. R.; Arif, A. M.; Stang, P. J. Inorg. Chem. 2002, 41, 2556. (9) Addicott, C.; Das, N.; Stang, P. J. Inorg. Chem. 2004, 43, 5335. (10) Zhao, L.; Northrop, B. H.; Stang, P. J. J. Am. Chem. Soc. 2008, 130, 11886. (11) Yuan, Q. H.; Wan, L. J.; Jude, H.; Stang, P. J. J. Am. Chem. Soc. 2005, 127, 16279. (12) Li, S. S.; Northrop, B. H.; Yuan, Q. H.; Wan, L. J.; Stang, P. J. Acc. Chem. Res. 2009, 42, 249. (13) Gong, J. R.; Wan, L. J.; Yuan, Q. H.; Bai, C. L.; Jude, H.; Stang, P. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 971. (14) Li, S. S.; Yan, H. J.; Wan, L. J.; Yang, H. B.; Northrop, B. H.; Stang, P. J. J. Am. Chem. Soc. 2007, 129, 9268. (15) Schatz, G. C. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6885.

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