Article pubs.acs.org/JPCA
Experimental and Theoretical Study on the Photophysical Properties of 90° and 60° Bimetallic Platinum Complexes Jun-Sheng Chen,†,‡ Guang-Jiu Zhao,† Timothy R. Cook,§ Xiao-Fei Sun,† Song-Qiu Yang,† Ming-Xing Zhang,† Ke-Li Han,*,† and Peter J. Stang*,§ †
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States S Supporting Information *
ABSTRACT: The 90° and 60° bimetallic platinum complexes with special structures are widely used in coordination-driven selfassembled metallosupramolecular architectures, and these complexes are the key components of triangular, rectangular, and polygonal metallacycle and metallocage supramolecules. Therefore, spectroscopic techniques and quantum chemistry calculations were employed in this article to investigate the photophysical properties of these bimetallic platinum complexes. Compared with spectra for the ligands, the absorption spectra of these Pt complexes are redshifted, and the fluorescence spectra become wider and are also redshifted. Moreover, the reasons for the low fluorescence quantum yields and short fluorescence lifetimes of these compounds were investigated using quantum chemistry calculations. We demonstrate that the fluorescent states of the bimetallic platinum complexes can be considered as local excited states, and that they possess a ligand-centered π−π* transition feature. Meanwhile, the platinum metals act as perturbation for these transitions, whereas the nonfluorescent states are classified as intramolecular charge-transfer states. Furthermore, a new fluorescence modulation mechanism is developed to explain the different emission processes of these complexes with different ligands.
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property of many platinum complexes.17−27 Goodson and coworkers have employed femtosecond fluorescence upconversion and transient absorption to investigate different platinumcontaining metallacycles.13 We have combined density functional theory (DFT)/time-dependent density functional theory (TDDFT) quantum chemistry calculations and spectroscopic methods to study the photophysical properties of a number of platinum (Pt) complexes.14−16,28 Combining the DFT/ TDDFT method and spectral experiments is an effective technique in studying Pt-containing supramolecular systems.21,29−34 Bimetallic platinum complexes are extensively used in the formation of coordination-driven self-assembled supramolecules. Organoplatinum complexes, such as 0°, 60°, and 90° organoplatinum acceptors and linear units are key components in supramolecular architectures.3,14,35−38 Stang and co-workers have reported numerous supramolecular architectures based on these bimetallic platinum complexes.3 Goodson and co-workers employed femtosecond measurements to investigate the intersystem crossing (ISC) and charge transfer in platinum-
INTRODUCTION Coordination-driven self-assembly employs directional metal− ligand interactions in the assembly process and provides a facile method of assembling a number of molecules into highly organized metallosupramolecular architectures.1 These selfassembled macromolecules have been widely investigated in many fields, such as in synthetic chemistry, medicinal chemistry, molecular devices, chemical sensors, and so on.1,2 Recently, Stang and co-workers have reported a series of supramolecules on the basis of the self-recognition processes.3−5 Therrien et al. discovered that a trigonal prism metallacage host system can deliver drugs into cancer cells.6−9 Chi and co-workers designed a number of two-dimensional polygons and three-dimensional cages that contain transition metals as fluorescent chemosensors for dicarboxylate anions and nitroaromatics, especially for picric acid.2,10 Considering the many special electronic and optical properties of organometallic systems, increasing attentions are focused on the photophysical processes of organometallic materials.11−16 Various experimental techniques, such as the two-photon absorption, steady-state fluorescence, and transient absorption, have been employed to study the photophysical and chargetransfer processes of self-assembled materials.1 Yam, Wong, and others performed systematic investigations on the luminescence © XXXX American Chemical Society
Received: July 22, 2012 Revised: September 11, 2012
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containing supramolecules.13 Zhao et al. studied the intramolecular charge transfer (ICT) in supramolecular systems, and they determined that the ICT process is significantly influenced by intermolecular hydrogen bond and molecular conjugation effect.14,16 Whereas most photophysical investigations largely focused on metallosupramolecules with different structures, discussions on the photophysical nature of the precursor bimetallic platinum complexes are limited. Nevertheless, the features of organoplatinum complexes as key components largely determine the properties of these metallosupramolecules. Detailed spectral experiments and quantum chemistry calculations are aimed not only at understanding the organoplatinum complexes but also for the design and application of supramolecules. In the present work, both steady-state and time-resolved spectroscopic techniques were employed to investigate the photophysical properties of three organoplatinum acceptors: (1) Pt-anthrancene acceptor 1,8-bis(trans-Pt(PEt3)2(NO3)) anthrancene, (2) Pt-phenanthrene acceptor 2,9-bis(trans-Pt(PEt3)2(NO3)) phenanthrene, and (3) Pt-1,4-diethynylbenzene−phenanthrene acceptor 2,9-bis(trans-Pt(PEt3)2(I) diethynylbenzene) phenanthrene. To further understand the nature of these complexes, we compared the results with anthrancene, phenanthrene, and 1,4-diethynylbenzene ligands. Furthermore, DFT and TDDFT were adopted to study the ground and excited states of these molecules.
values were measured in dichloromethane (DCM) solvent, and all the solvents used in the experiment are of high-performance liquid chromatography grade. The UV−vis absorption spectra were obtained using an HP 8453 spectrophotometer. The steady-state fluorescence and time-resolved fluorescence decays were recorded using a Horiba Jobin Yvon FluoroMax-4 spectrofluorometer. The time-resolved fluorescence decays were recorded using the time-correlated single photon counting (TCSPC) method. Data analysis was conducted via commercial software provided by Horiba Instruments. In the present work, all theoretical calculations were done using the Turbomole program suite.39−45 The ground-state optimized geometries of compounds 1−3 were studied theoretically by DFT using the BP86 functional and RI approximation.46,47 The TDDFT method with BP86 functional, widely used in large molecule systems, was applied to the electronic excited states. Throughout the ground-state optimizations and excited-state studies, the SVP basis set was chosen for nonmetallic elements, and the def-TZVP and pseudopotential def-ecp basis sets were selected for the platinum atom.39,48,49
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RESULTS AND DISCUSSION Steady-State Absorption and Emission. Figure 2 shows the steady-state optical absorption and emission data for anthrancene, phenanthrene, 1,4-diethynylbenzene, and organometallic compounds 1−3. The absorption, emission, and fluorescence quantum yields are listed in Table 1. The absorption band of compounds 1−3 are significantly redshifted, and the absorption bands of compound 3 become wider compared with compound 2. Thus, the introduction of 1,4diethynylbenzene increases the conjugation effect, which leads to the red shifting and broadening of the absorption band of the bimetallic platinum complexes. The absorption spectra of anthrancene and compound 1 and phenanthrene and compound 2 show a similar vibronic progression. This phenomenon is ascribed to the π−π* transition of anthrancene and phenanthrene, and their conjugation spreads to the Pt complex.13 Compared with the vibronic progressions for anthrancene and compound 1, the vibronic progressions of phenanthrene and compound 2 are relatively weak. Vibronic progression is absent in the absorption spectra of compound 3 primarily because of the weaker vibronic progression in the phenanthrene complex, which has a larger conjugation effect due to the introduction of 1,4-diethynylbenzene. The emission spectra of compounds 1−3 are shown in Figure 2. For compound 1, the maximum fluorescence occurs at approximately 428 nm, and three relatively small peaks are observed at 385, 404, and 453 nm, respectively. Compared with the fluorescence spectra of anthrancene showen in Figure 2a, the emission spectra of compound 1 become wider and redshifted. For compound 2, the emission maximum occurs at 388 nm, and a small peak is observed at 480 nm. On the other hand, compound 3 shows peak fluorescence at 410 nm and a small shoulder peak at 425 nm. The emission spectra of compounds 2 and 3 are notably red-shifted compared with their ligands phenanthrene and 1,4-diethynylbenzene, as shown in Figure 2c−f. Likewise, the absorption and maximum emission of compound 3 are red-shifted. The emission spectra of compound 3 and 1,4-diethynylbenzene have similar shape, which demonstrate that the emission states possess ligandcentered state features.
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EXPERIMENTAL AND THEORETICAL METHODS The bimetallic platinum compounds 1,8-bis(trans-Pt(PEt3)2(NO3)) anthrancene, 2,9-bis(trans-Pt(PEt3)2(NO3)) phenanthrene, and 2,9-bis(trans-Pt(PEt3)2(I) diethynylbenzene) phenanthrene were synthesized according to the procedures outlined in the literature.36−38 The phenanthrene and 1,4-diethynylbenzene (≥97%) were purchased from J&K Chemical. Anthrancene (≥97%) was purchased from Tianjin Guangfu Fine Chemical Research Institute and used without further purification. Figure 1 shows all the investigated structures. All spectral experimental and fluorescence lifetime
Figure 1. Molecular structures of anthrancene, phenanthrene, 1,4diethynylbenzene, and organometallic compounds 1−3. B
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27% in ethanol)50−52 and phenanthrene (ΦF = 12% in ethanol),50 respectively. The fluorescence quantum yields of compounds 1−3 are 4.3%, 1.1%, and 8.9%, respectively. The fluorescence quantum yields of these Pt complexes are smaller than those of the ligand chromophores, due to the heavy atoms enhancing the spin−orbital coupling effect of these ligands, thereby increasing the ISC rate13,53,54 or the ICT effect,16 and leading to fluorescence quenching. Furthermore, the quantum yield of compound 2 is smaller than that of compound 1 due to the different ligands, whereas compound 3, together with the larger conjugation effect of 1,4-diethynylbenzene, has a larger quantum yield than compound 2. Consequently, a substituent with a large conjugation effect can enhance the fluorescence of these bimetallic platinum complexes. However, the absorption bands and emission spectra of these complexes have a shape similar to that of the ligands, which indicate that these complexes have ligand-centered irradiative excited states, and the platinum atoms act as perturbation for these excited states. Time-Resolved Fluorescence Spectra. The time-resolved fluorescence decays of the different ligands and of compounds 1−3 at excitation wavelength of 295 or 376 nm under different emission wavelengths were measured. Figure 3 shows the fluorescence decays and the fluorescence peaks for anthrancene, phenanthrene, and 1,4-diethynylbenzene and for compounds 1−3 at different excitation and emission wavelengths. The fluorescence decay process was fitted by single, double, or triplet exponential and fluorescence lifetime values. The corresponding relative amplitudes (RAs) and their average lifetime values are listed in Tables 2−4 and Table S1 for compounds 1−3, anthrancene, phenanthrene, and 1,4-diethynylbenzene, respectively. For compound 1, the fluorescence decay processes were fitted by a triplet exponential. The three lifetime values of compound 1 at excitation wavelength of 376 nm and emission wavelengths of 400, 430, and 450 nm are listed in Table 2, and the fluorescence decays are shown in Figure 3b. The increase in emission wavelength is shown to induce a corresponding RA increase in the shortest lifetime τ1 and an RA decrease in τ2, leading to reduction in the average lifetime. The lifetime values of anthrancene were almost constant with the increase in emission wavelength at an excitation wavelength of 376 nm, as shown in Table S1 (Supporting Information) and Figure 3a. The fluorescence decay processes of compound 2 were fitted by a triplet exponential at excitation wavelength of 295 nm and emission wavelengths of 345, 385, and 415 nm; the lifetime values and fluorescence decays are shown in Table 3 and Figure 3d. The RA of τ2 obviously increased with the increase in emission wavelength, and the RA of τ3 decreased. Therefore, the average fluorescence lifetime values of compound 2 decreased with the increase in emission wavelength. For the ligand phenanthrene, the fluorescence decay processes were fitted by a single exponential, and the lifetime values remained constant with the increase in emission wavelength. The lifetime values and fluorescence decays for phenanthrene are shown in Table S1 (Supporting Information) and Figure 3c. For compound 3, the fluorescence decays were fitted by a double exponential at an excitation wavelength of 295 nm and different emission wavelengths, as shown in Table 4 and Figure 3f. The shortest lifetime value τ1 was less than 0.1 ns, and RA became larger with an increase in emission wavelength. The lifetime values of ligand 1,4-diethynylbenzene remained constant. The lifetime values are listed in Table S1 (Supporting Information), and the fluorescence decays for compound 3 are
Figure 2. Steady-state absorbance and emission spectra of anthrancene, phenanthrene, 1,4-diethynylbenzene, and compounds 1−3: (a) spectra of anthrancene, (b) spectra of compound 1, (c) spectra of phenanthrene, (d) spectra of compound 2, (e) spectra of 1,4-diethynylbenzene, (f) spectra of compound 3. The solid line denotes the absorption spectra. The dotted line denotes the emission spectra at excitation wavelengths of 340, 360, 295, 323, 273, and 349 nm for (a)−(f), respectively.
Table 1. Absorption bands, λemiss, and Fluorescence Quantum Yield for Each Ligands and Pt Complexes compounds anthrancene 1 phenanthrene 2 1,4diethynylbenzene 3
absorption bands λmax (nm) 326, 341, 359, 378 356, 375, 396, 418 276, 283, 295
λex (nm) 340 360 295
291, 309, 323, 347, 364 260, 273
323
349, 377
349
273
λemis (nm)
quantum yield
381, 402, 426, 452 385, 404, 428, 453 335, 353, 369, 390 388, 480
0.17 ± 0.01
300, 310, 320 410, 425
0.043 ± 0.008 0.13 ± 0.01 0.011 ± 0.002 0.43 ± 0.03 0.089 ± 0.005
On the other hand, the fluorescence quantum yields of the organoplatinum complexes are smaller than those of the ligand anthrancene, 1,4-diethynylbenzene, and phenanthrene, as listed in Table 1. The fluorescence quantum yields of compound 1 and compounds 2 and 3 in DCM solution were determined relative to the equiabsorbing solutions of anthracene (ΦF = C
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Figure 3. Time-resolved fluorescence decays of ligands and complexes: (a) anthrancene, (b) compound 1, (c) phenanthrene, (d) compound 2, (e) 1,4-diethynylbenzene, and (f) compound 3 at excitation wavelengths of 376 or 295 nm with different emission wavelengths.
Table 2. Triplet Exponential Fit Result with Three Lifetime Values (τ/ns) and the Corresponding RAs of the Fluorescence Decays of Compound 1 at an Excitation Wavelength of 376 nm with Different Emission Wavelengths triplet exponential fit emission wavelength/nm
τ1
RA1
τ2
RA2
τ3
RA3
τa
χ2
400 430 450
0.711 ± 0.011 0.120 ± 0.007 0.196 ± 0.009
11.50 43.85 67.98
2.149 ± 0.010 2.001 ± 0.012 1.962 ± 0.020
86.73 52.89 34.45
9.004 ± 0.330 8.016 ± 0.197 7.824 ± 0.019
1.77 3.26 3.58
2.105 ± 0.016 1.372 ± 0.016 1.089 ± 0.014
1.093 1.127 1.195
Table 3. Triplet Exponential Fit Result with Three Lifetime Values (τ/ns) and the Corresponding RAs of the Fluorescence Decays of Compound 2 at an Excitation Wavelength of 295 nm with Different Emission Wavelengths triplet exponential fit emission wavelength/nm
τ1
RA1
τ2
RA2
τ3
RA3
τa
χ2
345 385 415
0.387 ± 0.018 0.336 ± 0.033 0.595 ± 0.043
7.30 6.19 6.50
3.686 ± 0.092 5.081 ± 0.025 4.914 ± 0.028
26.17 76.82 82.83
10.452 ± 0.028 15.130 ± 0.124 13.089 ± 0.185
66.53 16.99 10.67
7.947 ± 0.044 6.495 ± 0.042 5.506 ± 0.046
1.016 1.118 1.066
shown in Figure 3e. For compound 3, at excitation wavelength of 376 nm, the fluorescence decays at 385, 407, and 450 nm were fitted by a single exponential. The decays are shown in Figure S1 (Supporting Information), and the lifetime values are
less than 0.1 ns, which demonstrate the low emission state of compound 3 under a short lifetime. On the basis of the above results, multiple emitting species55 or multiple emissive states may have existed in these bimetallic D
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Table 4. Double Exponential Fit Result with Two Lifetime Values (τ/ns) and the Corresponding RAs of the Fluorescence Decays of Compound 3 at an Excitation Wavelength of 295 nm with Different Emission Wavelengths double exponential fit emission wavelength/nm
τ1
RA1
τ2
RA2
τa
χ2
385 407 415
0.102 ± 0.003
