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Excitation-Dependent Fluorescence of Triphenylamine-Substituted Tridentate Pyridyl Ruthenium Complexes Lixin Xiao,†,‡ Yongqian Xu,§ Ming Yan,† David Galipeau,† Xiaojun Peng,§ and Xingzhong Yan*,† Center of Applied PhotoVoltaics, Department of Electrical Engineering and Computer Science, South Dakota State UniVersity, Brookings, South Dakota 57007, USA, State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian 116012, People’s Republic of China, and State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: May 3, 2010; ReVised Manuscript ReceiVed: July 30, 2010
Two polypyridyl ruthenium complexes, bis[4-(N,N′-diphenylamino)phenyl-2,2′:6′,2′′-terpyridine]ruthenium(II) (1) and bis[4′-(4-{2-[4-(N,N′-diphenylamino)phenyl]ethylene}phenyl)-2, 2′:6′,2′′-terpyridine]ruthenium(II) (2), have been synthesized. They possess an extended conjugation and strongly coupled electronic states. The features of these compounds were carefully studied from several respects. Steady-state spectroscopy showed that the two compounds had strong excitation dependent emission behaviors caused by mixing features of different electronic states. Femtosecond fluorescence upconversion spectroscopy was also used to investigate the fluorescence dynamics of the compounds. An ultrafast relaxation time of ∼100 fs of the 1MLCT (metalto-ligand charge-transfer) states, which may originate from an ultrafast intersystem crossing to form 3MLCT states, was found in both samples. However, thermal populated states and vibration associated excited state interactions were suggested for 1 with excitation at wavelengths below 400 nm, whereas vibrational energy redistribution with a time scale of few picoseconds was observed in the extended conjugated system of 2. These compounds will have potential application in both artificial photosynthesis systems and photovoltaic devices. Introduction Recently, photovoltaic materials for solar energy conversion have attracted much attention worldwide.1-4 Light harvesting in terms of Fo¨rster-type hoping model has been intensively investigated by time-resolved spectroscopy within many photovoltaic materials.5 Advanced concepts have been also utilized in the interpretation toward the issues of excited-state dynamics, which include how excitons and photoinduced charges migrate and how these processes are controlled in strongly coupled multichromophoric light harvesting systems.6-8 As ideal light absorbers, polypyridyl ruthenium complexes are widely used in photovoltaic cells.9 Some polypyridyl ruthenium complexes have become paradigm complexes for photoinduced redox systems that can convert light energy into chemical energy,10-16 and for sensitized semiconductors that convert solar energy into electricity.9,17-19 The relaxation of excited states through fast electron transfer is the primary step in charge separation in the photovoltaic devices fabricated by these ruthenium complexes.20-22 High conversion efficiency usually results from a fast electron injection from dye sensitizers, for example, polypyridyl ruthenium complexes, to wide band gap semiconductors with little chance for charge transferring backward to the sensitizers.23-25 Long-distance and long-lived charge separated states are critical in both artificial photosynthesis systems and photovoltaic devices.24-28 To mimic nature and create highly efficient light harvesting materials for efficient long-distance energy and * To whom correspondence should be addressed. E-mail: xingzhong.yan@ sdstate.edu. † South Dakota State University. ‡ Peking University. § Dalian University of Technology.
charge transfer, new methods need to be developed for controlling the strongly coupled dynamics within photovoltaic materials.21,3 Also, in order to understand the interfacial charge transfer processes in dye-sensitized wide-band semiconductors, the relaxation dynamics of the excited states with the ruthenium sensitizers has to be investigated because that is crucial for controlling electron injection rates.21,3 Investigations on the dynamics of excited states of polypyridyl ruthenium complexes are very imperative for understanding their photophysical aspects toward high conversion efficiency of solar energy. This will consequentially bring out a variety of applications of the ruthenium complexes in the design of nonlinear optical, electrontransfer assemblies, artificial photosynthesis systems, and photochemical energy storage devices. Polypyridyl ruthenium complexes show strong charge-transfer transitions between the metal-centered d orbital and the ligandcentered π* orbital, normally named as metal-to-ligand chargetransfer (MLCT) transitions.10-16 These MLCT states are typically the lowest lying excited states since the highest occupied orbitals mainly possess the character from the 4d orbital, and the lowest unoccupied states have that from the pyridine π* orbitals.10,6 For the dynamics investigation of these excited states,29 femtosecond time-resolved transient absorption (TA) is a conventional method, whereas the time-resolved fluorescence technique is seldom reported.10,30 The TA measurements of the paradigm compound, tris(2,2′-bipyridyl)ruthenium(II), showed a large dipole moment in the excited state. This dipole moment was oriented along the axis from metal to the center of one of the bipyridine units. These behaviors suggested that the excited electron is likely localized on one of the three bipyridine ligands, which is the topic of hot debate.10,29 Moreover, it is hard to directly observe the relaxation of the
10.1021/jp1040234 2010 American Chemical Society Published on Web 08/11/2010
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Figure 1. Structures of the complexes studied.
singlet state 1MLCT due to extremely weak emission and a rapid intersystem crossing (ISC) for forming a triplet state 3MLCT within 1 ps. Recently, picosecond-time-resolved streak camera technique has provided dynamics information in the tens of picoseconds time-scale which encompasses some sub-picoseconds relaxations of the vibronic or electronic levels in the triplet manifold of ruthenium complexes.30 Thus, femtosecond timeresolved fluorescence appears to be more suitable to study the excitation relaxation process for the ultrafast response time within sub-picoseconds. However, the reported investigations of the photophysics and photochemistry have been mostly studied in the polypyridyl complexes with bidentate ligands.10 There is scarce investigation regarding to polypyridyl complexes with tridentate ligands, and the assignment of the origin of the observed femtosecond emission has not proven straightforward.10,31 To explore charge transfer and its role for achieving efficient sunlight conversion in the polypyridyl complexes, new polypyridyl ruthenium complexes with triphenylamine modified tridentate ligands have been synthesized, and their early photophysical processes have been investigated by femtosecond fluorescence up-conversion technique. As a result, there are extended conjugation and strongly coupled electronic states within these new bis-tridentate ruthenium complexes. Their emission behaviors are highly dependent on excitations. These compounds will have potential application in both artificial photosynthesis systems and photovoltaic devices. Experimental Section Materials. Two polypyridyl ruthenium complexes, bis[4(N,N′-diphenylamino)phenyl-2,2′: 6′,2′′-terpyridine]ruthenium (1) and bis[4′-(4-{2-[4-(N,N′-diphenylamino)phenyl]ethylene}phenyl)-2,2′:6′,2′′-terpyridine]ruthenium (2) (Figure 1), were synthesized in this work. Ligands, 4-(N,N′-diphenylamino)phenyl-2,2′:6′,2′′-terpyridine (L1) and 4′-(4-{2-[4-(N,N′-diphenylamino)phenyl]ethylene}phenyl)-2,2′:6′,2′′-terpyridine (L2), were synthesized by using the methods in literatures.32,33 Their spectral data were list in Table 1. Common solvents were purified by redistillation. The synthesis protocols of the two polypyridyl complexes are described below. RuCl3 · 3H2O (200 mg, 0.77 mmol) was added into EtOH/ CHCl3 (3:1, v/v) solution with 0.77 mmol of ligands. After continuous stirring and refluxing for 2 h under nitrogen, the mixture was cooled down, and the solvents were removed by vacuum evaporation. Brown powder was obtained. A 200 mg portion of this brown powder and 0.45 mmol of ligand were then mixed together and redissolved in 50 mL of ethanol. This solution was stirred and refluxed for 24 h. After filtering out the undissolvable substances in the cool solution, the solvents were removed by rotating evaporation, and solid residuals were
TABLE 1: UV-Vis Absorption Maximum, Molecular Extinction Coefficient, Emission Maximum, and Emission Quantum Yield (O) of the Ligands in CH2Cl2 λabsmax (ε, 104/cm-1 M-1)a compounds L1 L2
πtpy-πtpy* 288 (2.09) 282 (5.83)
πph-πtpy* 356 (1.82) 385 (1.84)
em λmax (nm)
472 514
b
φ 0.58b 0.55
a tpy, denotes trispyridine moiety and ph, denotes the phenyl moiety linked to the trispyridine moiety in the ligand. b Data from ref 32b.
left. A little acetone was then added to redissolve the residues. After adding saturated NH4PF6 aqueous solution into the acetone solution, precipitations were formed. These precipitations, which are crude products, were collected by filtering and were washed with water. The crude products were dried and then purified by silica gel column chromatography with an eluent, CH3CN/H2O/ KNO3 saturated aqueous solution (30/1/1, v/v/v). A developed red portion was collected and concentrated. To remove the KNO3, saturated NH4PF6 aqueous solution was added into the concentrated solution again. Red precipitations were collected and washed with water. These dried red precipitations are pure products. The 1H NMR and mass spectroscopy data were listed below. 1 (yield 57%): 1H NMR (400 MHz, δppm, CD3COCD3): 6.10 (d, J ) 8.4 Hz, 2H), 6.40 (d, J ) 7.6 Hz, 2H), 7.05 (d, J ) 7.6 Hz, 6H), 7.17 (m, 10H), 7.37 (m, 8H), 7.51 (m, 1H), 7.59 (s, 1H), 7.89 (d, J ) 6.0 Hz, 6H), 8.08 (d, J ) 8.4 Hz, 2H), 8.25 (d, J ) 7.6 Hz, 1H), 8.44 (dd, J ) 6.0 Hz, J ) 6.0 Hz, 1H), 8.61 (d, J ) 7.2 Hz, 2H), 8.72 (m, 2H), 9.17 (d, J ) 8.0 Hz, 2H), 9.26 (s, 2H), 13C NMR (100 MHz, δppm, CD3COCD3): 118.27, 119.37, 122.44, 123.55, 124.38, 124.91, 125.64, 125.92, 126.99, 127.30, 128.75, 129.51, 130.58, 137.54, 138.51, 145.80, 147.30, 149.86, 152.55, 154.40, 159.32, 161.06, 168.04, ESIMS (positive): m/z [M2+/2] ) 527 (cal. 1054). 2 (yield 50%): 1H NMR (400 MHz, δppm, DMSO-d6): 7.01 (d, J ) 8.8 Hz, 4H), 7.10 (m, 12H), 7.29 (m, 12H), 7.36 (dd, J ) 7.2 Hz, J ) 8.4 Hz, 8H), 7.56 (d, J ) 5.6 Hz, 2H), 7.61 (d, J ) 8.0 Hz, 2H), 7.95 (d, J ) 8.0 Hz, 4H, H5, H5′′), 8.08 (dd, J ) 6.4 Hz, J ) 5.6 Hz, 4H, H4, H4′′), 8.53 (d, J ) 7.2 Hz, 4H, H3, H3′′), 9.18 (d, J ) 8.4 Hz, 4H, H6, H6′′), 9.55 (s, 4H, H3′, H5′), ESI-MS (positive): m/z [M2+/2] ) 629 (cal. 1258). Steady-State Absorption and Emission. Steady-state absorption was recorded by an Agilent 8352 UV-visible spectrophotometer. Steady-state emission of the polypyridyl ruthenium solutions was measured by an Edinburgh FL920 spectrofluorophotometer. The solutions used in these measurements were with concentrations in the range from 0.01 to 0.1 mM. Electrochemistry Measurements. Electrochemical studies of the ruthenium complexes were conducted in anhydrous
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CH3CN, with 0.1 M Bu4NPF4 as the supporting electrolyte using a BAS-100W electrochemical workstation at a scan rate of 100 mV/s. A glassy carbon electrode was used as the working electrode, which was polished with an alumina paste before analysis. Counter and reference electrodes were composed of platinum wire and a non aqueous Ag/Ag+ electrode (0.01 M AgNO3/0.1 M n-Bu4-NPF6 in CH3CN), respectively. The halfwave potential of ferrocene/ferrocenium redox couple vs the Ag/Ag+ electrode is 80 mV in CH3CN.34 Fluorescence Up-Conversion Measurements. Time-resolved fluorescence measurements for the polypyridyl ruthenium solutions were carried out by using a femtosecond up-conversion technique. These solutions used in the measurements were prepared by dry and deoxygenated dichloromethane (CH2Cl2). The femtosecond fluorescence up-conversion system used in this work is a FOG 100 system (CDP, Russia) with a modelocked Ti-sapphire laser (Tsunami, Spectra Physics) source pumped by a 10 W CWNd:YVO4 laser (Millennia, SpectraPhysics). The solutions were excited by the second harmonic light generated by doubling the fundamental light from the mode-locked Ti-sapphire laser with a pulse width of ∼57 fs at a repetition rate of 86 MHz. The center wavelength of the light was tunable from 730 to 860 nm, and the laser spectrum was monitored by a spectrum analyzer (Ocean Optics). The polarization of the excitation (second harmonic) beam for the anisotropy measurements was controlled with a Berek’s plate, as illustrated widely in literature.36 Parallel (Fpar) and perpendicular (Fper) polarized fluorescence signals were generated by vertically or horizontally polarized excitation, respectively. The isotropic fluorescence was calculated by: Fpar+ 2GFper, and fluorescence anisotropy (γ) was then given by: γ) (Fpar - GFper)/(Fpar+ 2GFper). Here, the G factor was calibrated by measuring the polarized fluorescence decay of perylene in toluene,37 which gave equal polarized fluorescence intensity after a complete rotation diffusion in tens of picoseconds. The G value was found to be ∼1.004 for the system. A rotating sample cell was used to avoid possible photodegradation and other accumulative effects. For all the measurements, a concentration of ∼0.1 mM of the two samples in CH2Cl2 has been used. The fluorescence emitted from the sample was collected with an achromatic lens and directed onto a β-barium borate crystal (BBO). The fundamental light passed through a motorized optical delay line and then mixes with the sample emission in a nonlinear crystal to generate a sum frequency signal. The signal was dispersed by using a monochromator and detected by a photomultiplier tube (Hamamatsu R1527P). The instrument response function (IRF) for the system was estimated to be 188 fs at full width at half-maximum (fwhm).34 The fluorescence data have been deconvoluted with the IRF by using the vFit program (CDP, Russia). The smallest χ2 value and a residual fluorescence value less than 1% were controlled during the fitting and simulation.38 Results and Discussion Steady-State Spectroscopy. Figure 2 shows the absorption of the two polypyridyl complexes. The very intense bands below 400 nm originate from intraligand πL-πL* transitions (Table 1). The absorption shoulder at 405 nm and the peak at 532 nm of complex 1 can be assigned to MLCT transitions, where an electron is promoted from the metal-centered t2g orbital into a ligand-centered π* orbital.39 In comparison to the complex 1, there is a MLCT peak at 421 nm with shoulders at 457 and 549 nm in the absorption spectrum of the complex 2. These shoulders in longer wavelength indicate the absorption features from the increase of conjugation with the ligands. Their
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Figure 2. Absorption spectra in CH2Cl2 (inset: an expansion spectrum for 2).
absorption is weaker than the peak at 532 nm for 1. This is very likely due to intramolecular electron transfer from the conjugated triphenylamine groups to the metal center, as might impede metal to ligand charge transfer. This was consistent with the results from electrochemistry studies below. Figure 3 shows the cyclic voltammograms of the two compounds. Two oxidation potentials (0.86 and 0.49 V) and one reduction potential (-1.60 V) were found for 1. Interestingly, both the oxidation (0.69 and 0.48 V) and reduction (-0.34 and -1.24 V) potentials were found to be lower for 2 than those for 1. Such a behavior may be due to the increase of the conjugation in 2. During the oxidation of 2, the conjugated triphenyl electron donor could increase the energy level of the t2g orbital of the metal center and lower the oxidation potentials. Also, this delocalization feature could present at lower reduction potentials. In comparison to 1, two low reduction potentials were found for 2. The lowest one at -0.34 V was very likely due to intramolecular charge transfer; the triphenyl donors could transfer electrons to the metal center through the conjugated structure. It seems that electrons are delocalized in 2 at the ground state. This is consistent to the observation in the absorption spectrum of 2. These behaviors will help to create long-distance charge separated states, which may be used to storage the (light) energy in a chemistry form. In addition, for both complexes, the absorption spectral features relevant to the MLCT transitions are different from the previous observation with tris(2,2′bipyridyl)ruthenium(II) which appears nonseparated broad double-peak from 400-500 nm (in MeCN).10 This is probably due to the change of the coordinate field, which is associated with the geometry change from these tridentate ligands. Figure 4 shows emission spectra of 1 at different excitation wavelengths. The emission was found to be highly excitation dependent. For an excitation at 375 nm, there was a strong emission at ∼467 nm, which was mainly from the ligandcentered πL* states (Table 1). However, the emission from 1 MLCT states might not be completely excluded in this case because their higher Franck-Condon states could be excited when the excitation is at 375 nm. The excitation of Franck-Condon states was observed for tris(2,2′-bipyridyl)ruthenium(II).10,13 The excitation spectrum revealed that the emission at 467 nm was originated from the excitation at wavelengths below 420 nm (cutoff in Figure 4b). When moving the excitation to 400 nm, the intensity of the emission at ∼467 nm was observed to be significantly suppressed [intensity dropped from 2.74 × 105 counts/s (cps) to 4.0 × 104 cps], and a new emission band at ∼633 nm appeared. Further shifting the excitation to a higher wavelength (lower energy) at 432 nm, the emission band at
Fluorescence of Tridentate Pyridyl Ruthenium Complexes
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Figure 3. Cyclicvoltammograms of the two complexes.
Figure 4. Steady-state emission spectra of 1: (a) excitation depended emission, and (b) excitation spectra.
∼467 nm disappeared and only the emission band at ∼633 nm was left with a substantial intensity (1.30 × 105 cps). The emission at 467 nm was mainly originated from the ligandcentered πL* states; and it was rather possible to attribute the observed 633-nm emission band to the emission of the triplet metal-to-ligand charge transfer (3MLCT) states, as was confirmed by Figure 4b. The excitation spectrum with emission at 633 nm showed an excitation range from 350 to 550 nm (Figure 4b), as indicates that this emission could not completely exclude the contribution from the excitation below 432 nm (Figure 4b). In such a case, the ligand-centered πL* states, which can also be considered as higher-energy 1MLCT states, may take diabatic processes to relax to lower-energy 1MLCT states, and then take ISC to relax to 3MLCT states. This was accompanied by a large distortion in geometry.16 As a result, a large energy difference7350 cm-1 (201 nm) between the excitation and the emission was found. This is related to the exchange energy between the singlet and triplet states. Also, large reorganization energy due to large distortion for the excited states may not be excluded. There was extremely weak emission from the excitation at 532 nm, that is, the 1MLCT excitation. These observed behaviors above suggest that the electronic system for both the ligands and the metal center is somehow isolated, as may be associated by the distortion in geometry of the complex 1. This agreed with the observation of the high redox potentials and was also proven in geometry optimization by using the Dmol3 software package at the GGA-PBE level (Figure 5). The phenyl group that connects to the polypyridyl ligand is positioned at a twisted angle to the plane of the three nitrogen atoms in the ligand. This angle is smaller in 2 due to the more flexibility of a double bond in the ligand. A twisted geometry with 1 could be much easier to be formed after the 1MLCT excitation, and the large energy difference should include the contribution from the structure distortion. The emission spectrum of 2 is shown in Figure 6. A single emission band was observed with a maximum at 517 nm. The
Figure 5. Optimal structures in CH2Cl2 of the two polypyridyl ruthenium complexes (Geometry optimization was carried out by Dmol3 software package at the GGA-PBE level with a DNP base set and a PCM description of solvent).
shapes of the emission spectra at an excitation below 400 nm were similar to each other, whereas the maximum emission was observed to be suppressed and a shoulder around 589 nm appeared for an excitation at 432 nm. The emission spectrum with 432 nm excitation was fitted well by a two-peak Gaussian function, which are at 517 nm (19350 cm-1) and 589 nm (16970 cm-1), respectively (Figure 6b). However, ones should not simply assign the emission band at 517 nm to the emission of the ligands and the emission band at 589 nm to the emission of the 3MLCT states. The emission band at 517 nm may consist of the contributions from both the ligand (πL*) excited states (Table 1) and the 3MLCT states, as the excitation spectrum was observed covering the absorption range of the πL-πL* transitions and the MLCT transition around 421 nm (Figure 6c). With an increased conjugation in the complex 2, there should be a large delocalization and a strong electronic coupling between the ligands and metal center in comparison to 1. Thus, the emission at 517 nm shows a shoulder at 589 nm, as enjoying a
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Figure 6. Steady-state emission spectra of 2: (a) emission spectra at different excitation, (b) fitted spectra (violet) for the emission spectrum for the excitation at 432 nm, and (c) excitation spectrum with emission at 517 nm.
Figure 7. Fluorescence decay curves of 1 (royal) and 2 (wine) with excitation at 375 nm and emission 480 nm: (a) fluorescence decay within 18 ps; (b) decay within 1.8 ps [The thick lines were generated via deconvolution with the IRF with two or three exponential decay.]
TABLE 2: Decay Constants for the Fluorescence Decay at Different Wavelengths with an Excitation of 375 nm emission at 480 nm long-term decay τ1/fs (A1)
τ2/ps (A2)
initial decay τ3/ps (A3)
′
τ1 /fs (A1′)
′
τ2 /ps (A2′)
emission at 525 nm initial decay ′
τ3 /ps (A3′)
τ1′′/fs (A1′′)
τ2′′/ps (A2′′)
1 176 ( 30(0.74) 1.4 ( 0.2 (-0.05) 208 ( 7 (0.21) 100 ( 40 (0.78) 1.0 ( 0.3 (-0.07) 46 ( 2 (0.15) 154 ( 40 (0.75) 228 ( 10 (0.25) 2 100 ( 40 (0.96) 1.0 ( 0.2 (0.03) 52 ( 2 (0.01) 107 ( 40 (0.86) 1.9 ( 0.3 (0.14) N/A 102 ( 30 (0.86) 2.86 ( 0.3 (0.14)
mixing feature of the relaxation from the πL*, 1MLCT, and 3 MLCT states. Time-Resolved Fluorescence Dynamics. Figure 7 illustrates the fluorescence dynamics at 480 nm of the complexes 1 and 2 with an excitation at 375 nm. These transients can be deconvoluted with the IRF by using a sum of multiple exponential functions: I(t))[∑i Ai exp(-t/τi)]XF(t), in which F(t) is a constant term convoluted with the IRF, A is the amplitude, and τ is the time constant. The IRF is ∼188 fs (full width at halfmaximum) for the measurements. The deconvoluted data by using two- or three-exponential functions were listed in Table 2. Ultrafast decay components within ∼100 fs were found in the fluorescence dynamics for the both complexes. This ultrafast component can be assigned to the decay of the 1MLCT emissions due to an ultrafast intersystem crossing to form 3 MLCT states, as is consistent with the observation of the timescale in 100-300 fs for tris(2,2′-bipyridyl)ruthenium(II).10,29c,e However, there is significant difference between the two complexes. For the complex 1, this ultrafast decay was followed by a rebounding time with a time scale of
