Excited State Absorption Dynamics in Metal Cluster Polymer

Jun 23, 2007 - (818J-09B energy probe, Newport Corporation) linked to the energy meter .... C60 exhibits excited-state nonlinearity.29 Compared with C...
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J. Phys. Chem. B 2007, 111, 7987-7993

7987

Excited State Absorption Dynamics in Metal Cluster Polymer [WS4Cu3I(4-bpy)3]n Solution Junyi Yang,† Jihua Gu,† Yinglin Song,*,†,‡,| Shi Guang,‡ Yuxiao Wang,† Wenhua Zhang,§ and Jianping Lang*,§ School of Physical Science and Technology, Suzhou UniVersity, Suzhou 215006, P.R. China, Physics Department, Harbin Institute of Technology, Harbin 15001, P.R. China, School of Chemistry and Chemical Engineering, Suzhou UniVersity, Suzhou 215006, P.R. China ReceiVed: January 26, 2007; In Final Form: May 10, 2007

The nonlinear absorptive property of a novel metal cluster [WS4Cu3I(4-bpy)3]n in DMF solution is studied by using an open-aperture Z-scan technique with picosecond and nanosecond laser pulses at the wavelength of 532 nm. The experimental results show that the cluster has strong nonlinear absorption under the 8 ns pulse excitation and a relatively weak nonlinear absorptive property under the picosecond pulse excitation. The picosecond pump-probe response of the metal cluster is similar to that of C60 solution, which implies that the nonlinear mechanisms are the same for the two materials. By using the rate-equation model, the experimental data are theoretically simulated; several optical parameters of the cluster, especially the lifetime of the higher excited singlet state of the cluster, are obtained.

Introduction Materials that possess nonlinear optical (NLO) properties have been investigated extensively for their potential applications in optical phase conjugation, optical switches, optical bistability, optical limiting devices, and so on.1-5 Recently, metal clusters6-19 have received much attention and been demonstrated to be a new kind of excellent optical limiting (OL) molecule. Such metal clusters can have various kinds of architectures, which may change or enhance the optical nonlinearities and thus can be optimized for each photonic application. Most of the studies show that the NLO properties of the metal clusters are due to excited-state nonlinearity, but some study results also show that they arise from thermal nonlinear scattering.20 In this paper, the syntheses and characterization of a novel cluster compound, [WS4Cu3I(4-bpy)3]n, is reported. The nonlinear absorption of the cluster solution was investigated by using a Z-scan technique21 with nanosecond and picosecond pulses. The nonlinear optical mechanics of the cluster is studied by the picosecond time-resolved pump-probe measurement. By comparing the results of pump-probe and Z-scan measurements to the rate-equation model, the values for the excited-state lifetime and the excited-state absorption cross section are obtained. The lifetime of the higher excited singlet state is also obtained in this paper. Sample Preparation and Experiment General. (NH4)2[WS4]22 was prepared as reported in the literature. Solvent Et2O (ether) and 4-tert-butylpyridine (4-tpy) were pre-dried over activated molecular sieves and refluxed over the appropriate drying agents under argon. Acetone and CuI were obtained from commercial sources and were used as * Corresponding authors. † School of Physical Science and Technology, Suzhou University. ‡ Harbin Institute of Technology. § School of Chemistry and Chemical Engineering, Suzhou University. | E-mail: [email protected].

received. FT-IR spectra in the 400-4000 cm-1 range were recorded on a Nicolet MagNa-IR 550 spectrophotometer using KBr pellets. The elemental analyses for C, H, and N were performed on an EA1110 CHNS elemental analyzer. UV-vis spectra were measured on a HITACHI U-2810 spectrophotometer. Preparation of [WS4Cu3I(4-bpy)3]n. To a 4 mL Me2CO (acetone) solution containing CuI (0.0457 g, 0.24 mmol) was added solid (NH4)2[WS4] (0.0276 g, 0.08 mmol), and the mixture was stirred. 4-tert-Butylpyridine (0.1082 g, 0.8 mmol) was added dropwise to give a homogeneous solution, which was layered with Et2O to afford red needles one week later. The crystals were collected and washed with Et2O and dried in vacuo. Yield: 0.0331 g (40% based on W). Anal. Calcd for C27H39Cu3IN3S4W (%): C, 31.32; H, 3.80; N, 4.06. Found (%): C, 31.03; H, 3.75; N, 4.10. IR (KBr, cm-1): 2964 (s), 2868 (w), 1611 (s), 1498 (m), 1461 (m), 1418 (s), 1367 (w), 1273 (m), 1227 (m), 1070 (m), 1017 (m), 828 (s), 723 (w), 569 (s), 438 (m). 1H NMR ((CD3)2SO): δ (ppm) 8.10-8.72 (d, 2H, PyH), 7.69-7.88 (d, 2H, PyH), 1.27 (s, 9H, CH3). X-ray Crystallography. X-ray quality crystals of [WS4Cu3I(4-bpy)3]n were obtained directly from the above preparation. All measurements were made on a Rigaku Mercury CCD X-ray diffractometer (3 kV, sealed tube) using graphite monochromated Mo KR radiation (λ ) 0.71070 Å). A single crystal of [WS4Cu3I(4-bpy)3]n was mounted at the top of a glass fiber and cooled at 193 K in a stream of gaseous nitrogen. Cell parameters were refined on all observed reflections by the program CrystalClear (Rigaku and MSc, Ver. 1.3, 2001). The collected data were reduced by the program CrystalClear, and an absorption correction (multiscan) was applied. The reflection data were also corrected for Lorentz and polarization effects. The structure of [WS4Cu3I(4-bpy)3]n was solved by direct methods and refined against F2 for all independent reflections. All non-hydrogen atoms, except for the disordered 4-tpy part, were refined anisotropically. All hydrogen atoms for the nondisordered 4-tpy part were located on their calculated positions.

10.1021/jp070705o CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007

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TABLE 1: Crystal Data and Structure Refinement for [WS4Cu3I(4-bpy)3]n Crystallographic Information empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) β (°) V (Å 3) Z Dc (g cm-3) F(000) µ (Mo KR) (cm-1) 2θmax (deg) reflections collected independent reflections observed reflections R (RW)a GOF no. of parameters a

C27H39Cu3IN3S4W 1035.22 monoclinic C2/c 35.89(13) 5.521(14) 21.76(13) 115.78(10) 3883(29) 4 1.771 2000 5.604 51.2 2467 2467 (Rint ) 0.0820) 2412 0.1160 (0.2838) 1.149 145

R ) Σ||Fo|-|Fc||/Σ|Fo|; RW ) [Σw(|Fo|-|Fc|)2/Σw|Fo|2]1/2.

The iodine atom and one 4-tpy moiety share the same occupancy site, and their decimal factors were fixed at 0.5:0.5; furthermore, C17 and C18 in the tert-butyl part of 4-tpy displays a continuous positional disorder and their occupancy ratios (C17,C18/ C17A,C18A) were refined to 0.74/0.26. All calculations were performed on a DELL PC computer by using the SHELXTL-9723 software package. Pertinent crystallographic data for [WS4Cu3I(4-bpy)3]n is given in Table 1. Selected bond lengths and angles are listed in Table 2. Nonlinear Optical Measurements. The nonlinear optical absorption of the cluster solution is measured by the openaperture Z-scan technique using a Nd:YAG 532 nm laser (Continuum) with a pulse width of 8 ns (fwhm) and a Q-switched Nd:YAG 532 nm laser (EKSPLA, PL2143A) with a pulse width of 30 ps (fwhm), repetition rate of 1 Hz, respectively. The spatial distribution of the pulse is nearly a Gaussian profile. The sample solution was placed in quartz cells of 2 mm thickness. The quartz cell with the sample was placed on a translation stage controlled by a computer that moved the sample along the z-axis with respect to the focal point of a 308 mm focal lens. The laser pulses adjusted by an attenuator were separated into two beams by using a splitter. The two beams were simultaneously measured by using two energy detectors (818J-09B energy probe, Newport Corporation) linked to the energy meter (model 2835-C, Newport). A personal computer was used to collect data coming from the energy meter through the RS-232C interface. In the picosecond pump-probe experiments, a Q-switched Nd:YAG laser (EKSPLA, PL2143A) was used to produce 30 ps (fwhm) laser pluses at 532 nm with a repetition rate of

Figure 1. Perspective view of the repeating units of [WS4Cu3I(4-bpy)3]n with 50% thermal ellipsoids; the disordered parts are presented and all the hydrogen atoms are omitted for clarity.

2 HZ. The pump-probe experimental setup was a standard one, and the probe peak irradiance was approximately 8% of the pump irradiance.20 A variable delay is introduced into the probe path, and the two pulses are recombined at the sample cell (2 mm thickness) at a small angle (8°). The probe waist is three times smaller than the pump waist. The small angle between the beams and the fact that the probe spot size is considerably smaller than the pump, ensure that the probe could test a uniformly excited region of material in the 2 mm cell. The polarization of the pump beam was rotated by 90 ° with respect to that of the probe beam with polarizer to avoid interference. The change of the probe beam intensity versus the delay time is recorded after the pump beam. Results and Discussion Chemical Properties of [WS4Cu3I(4-bpy)3]n. The reaction of (NH4)2[WS4] with 3 equiv of CuI and excess 4-tpy in acetone solution produced [WS4Cu3I(4-bpy)3]n as red needles in 40% yield. [WS4Cu3I(4-bpy)3]n is air-stable and soluble in common solvents such as MeCN (acetonitrile), CH2Cl2, DMF (N,N′dimethylformamide), and DMSO (dimethyl sulfoxide). The oxidation states of the W and Cu atoms in [WS4Cu3I(4-bpy)3]n are assumed to be +6 and +1, respectively. The IR spectra show W-S stretching vibration at 438 cm-1, and 2964, 1611, 1498, 1418 cm-1 indicate the presence of 4-tpy. The 1H NMR spectra in DMSO-d6 show the correct aromatic/methyl proton ratios of 4-typ for [WS4Cu3I(4-bpy)3]n.

TABLE 2: Selected Bond Lengths (Å) and Bond Angles (°) for [WS4Cu3I(4-bpy)3]n bond lengths (Å) W(1)-S(1) Cu(1)-S(1) Cu(2)-S(2)a

2.207(8) 2.309(12) 2.325(10)

bond lengths (Å) W(1)-S(2) Cu(1)-S(2) Cu(1)-N(1)

bond angles (deg) S(1)-W(1)-S(2) S(2)b-W(1)-S(2) S(1)-Cu(2)-S (2)a a

109.8(2) 108.9(4) 113.3(2)

2.229(11) 2.292(8) 2.081(13)

bond lengths (Å) I(1)-Cu(1) Cu(2)-S(1) Cu(1)-N(2)

bond angles (deg) S(1)-W(1)-S( 1)b S(2)-Cu(1)-S(1) S(1)-Cu(2)-S(2)c

x, y - 1, z. b -x + 1, y, -z + 5/2. c -x + 1, y - 1, -z + 5/2.

108.0(4) 104.1(2) 113.7(3)

2.692(10) 2.315(8) 1.95(3) bond angles (deg)

S(1)-W(1)-S(2)b S(1)b-Cu(2)-S(1)

110.2(3) 101.0(4)

Metal Cluster Polymer [Cu3IWS4(4-bpy)3]n Solution

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Figure 2. Perspective view of the one-dimensional chain of [WS4Cu3I(4-bpy)3]n, extended along the crystallographic b-axis and with 50% thermal ellipsoids; the disordered parts are presented and all the hydrogen atoms are omitted for clarity.

Figure 4. Absorption spectra of [WS4Cu3I(4-bpy)3]n in DMF solution in 1 cm optical path, with concentration 2.1 × 10-4 mol L-1.

infinite chain, by sharing one copper atom in the crystallographic b direction. (Figure 2). The 1D chains are about 18.16 and 11.61 Å apart in the crystallographic a-and c-axis, respectively. There are no obvious interactions observed according to the crystal packing diagram (Figure 3). Figure 3. A packing diagram of [WS4Cu3I(4-bpy)3]n looking down in the Ob axis; all hydrogen atoms are omitted for clarity.

Compound [WS4Cu3I(4-bpy)3]n crystallized in the monoclinic space group C2/c, and the asymmetric unit contains one-half of the neutral [WS4Cu3I(4-bpy)3] molecule. As shown in Figure 1, the cluster core can be viewed as one [WS4]2- captured three Cu(I) atoms. Within the repeating unit, all three Cu atoms display distorted tetrahedral coordination geometries, but their coordination spheres are somewhat different; Cu(1) and Cu(1A) are, respectively, coordinated by one 4-tpy, and one coordination site shared by half an iodine atom and half a 4-tpy molecule, Such a static substitutional disorder is extremely rare.24 Cu(2) is tetrahedrally coordinated by four S atoms from two adjacent [WS4Cu3I(4-bpy)3] units. Because of the similar coordination geometries for Cu(1) and Cu(2), the average Cu-S distances are similar (2.301 and 2.325 Å for Cu(1) and Cu(2), respectively). The average Cu-S distances are similar to those found in [PPh4]2[(Cp*WS3Cu3Br2)2(µ2-Br)2] (av. 2.234 Å)25 and [Et4N]3[WOS3(CuBr)3(µ2-Br)]‚2H2O (av. 2.293 Å).26 The Cu(1)-I(1) (2.692(10) Å) was comparable to those found in [Et4N]3[WOS3(CuI)3(µ2-I)]‚H2O (av. 2.680 Å),27 but longer than that reported in [Et4N]4[Mo2O2S6Cu6I6] (av. 2.448 Å).28 The adjacent [WS4Cu3I(4-bpy)3] units are fused to a unique 1D

Optical Properties of [WS4Cu3I(4-bpy)3]n. Figure 4 shows the UV-vis spectra of [WS4Cu3I(4-bpy)3]n in DMF. It was characterized by two bands at 296 and 435 nm. The absorption band at 435 nm was probably dominated by S-WVI chargetransfer transitions of the [WS4] moiety. Compound [WS4Cu3I(4-bpy)3]n has a low absorbance at 532 nm. This promises lowintensity loss and small temperature changes. Figure 5 gives the nonlinear absorptive properties of the cluster solution with an 8 ns and 30 ps pulse under an openaperture Z-scan configuration. It shows that the cluster has strong nonlinear absorption under the 8 ns pulse excitation. However, under the picosecond pulse excitation, the cluster exhibits a relatively weak nonlinear absorptive property. Because the solvent DMF shows no nonlinear absorption under the identical experimental conditions, the observed nonlinear absorption effect should originate from the solute [WS4Cu3I (4-bpy)3]n. The valley of the normalized transmittance indicates that the laser pulses experience strong reverse saturable absorption (RSA). Moreover, we note that the intensity of an 8 ns pulse is smaller than the intensity of a 30 ps pulse. This phenomenon indicates that the mechanism of the nonlinear absorption may be excitedstate absorption. The picosecond nonlinear absorption may be due to the singlet excited states, but the nanosecond one may

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Figure 5. (a) Open-aperture Z-scans curve of the cluster solution for 8 ns pulse width. (b) Open-aperture Z-scans curve of the cluster solution for 30 ps pulse width. The squares represent experimental data, and the solid lines are theoretical fitting curves.

be related to not only the singlet excited states but also the triplet excited states, where the absorption cross sections of the excited states are large than that of the ground states.

theoretical fit to the experimental results of C60 based on the five-level model rate equations and eqs 6-9:33

Model Interpretation. To gain an insight into the nonlinear origin of the cluster, picosecond time-resolved pump-probe experiments have been performed at 532 nm on the metal cluster [WS4Cu3I(4-bpy)3]n. Experimental results are shown in Figure 6. The optical nonlinearity of the cluster, CBS (carbon black suspension), and C60 are studied by using the same experimental setup, respectively. Previous studies of CBS indicate that nonlinear scattering dominates the nonlinear process,20 while C60 exhibits excited-state nonlinearity.29 Compared with CBS, the transmittance of the cluster and C60 decreased rapidly. This indicated that the cluster has rapid optical response, which is similar to C60. In general, the electronic structure of the metal cluster and C60 can be described by a five-level model (Figure 7).18,29,30 To explain the experimental data, we make use of the five-level model. The process of absorption in a five-level system is as follows. It is assumed that linear absorption promotes electrons from the ground state S0 to the first excited singlet state S1. From this state, one of three things may happen. The electron can relax to the ground state by a radiative or nonradiative transition. Another possibility is for the electron to undergo a spin-flip transition to the lowest triplet state T1. This process is called intersystem crossing. The third possibility is that the molecule may absorb another photon, which promotes the electron to a higher excited singlet state S2, from which it then relaxes very rapidly back to the first excited singlet state. For an electron in the lowest triplet state T1, two possibilities exist. It may relax by phosphorescence to the ground state S0. The other possibility is that the molecule absorbs another photon, promoting the electron to a higher-lying triplet state T2. The electron then relaxes very rapidly back to the lowest triplet state T1. We think that the contrast of the results between the cluster and C60 is important due to the difference in the relaxation time of the first excited singlet state τS1, intersystem crossing time τISC, the lifetime of the first excited triplet state τT0, the excitedstate absorption cross sections from the first excited singlet state S1, and the first excited triplet state T1. The photophysical parameters of the C60, σ0, σ1, σ2, τISC, τS1, and τT0, were taken from the literature29,31,32 to be 3.1 × 10-18 cm2, 1.6 × 10-17 cm2, 9.5 × 10-18 cm2, τISC ) 1.2 ns, τS1 ) 30 ns, and τT1 ) 40 µs, respectively. The relaxation times of the S2 and T2 states are very short and taken as 100 fs.32 The solid line is the

σ0IeN0 N1 N3 dN0 )+ + dt pω τS1 τT1

(1)

σ1IeN1 σ0N0Ie N1 N1 N2 dN1 )+ + dt pω pω τS1 τISC τS2

(2)

dN2 σ1IeN1 N2 ) dt pω τS2

(3)

σ2IeN3 N1 N3 N4 dN3 )+ + dt pω τISC τT1 τT2

(4)

dN4 σ2IeN3 N4 ) dt pω τT2

(5)

dIe ) -(σ0N0 + σ1N1 + σ2N3)Ie dz′

(6)

dIp ) -(σ0N0 + σ1N1 + σ2N3)Ip dz′

(7)

Ie(z ) 0) ) I0e

ω0e2 ωe2

Ip(z ) 0) ) I0p

[

exp -

ω0p2 ωp2

2r2 (t - td) ωe2 τ2

(

exp -

2r2 t2 ωe2 τ2

)

2

]

(8)

(9)

where Ie and Ip are the irradiance of the pump beam and probe beam, respectively. ωi(z) ) ω0i[1 + (z/z0i)2]1/2 ( where i ) p,e) is the probe-beam and pump-beam radius at z, respectively. z is the sample position away from the focus on the axis, z0i ) πω0i2/λ is the diffraction length of the beam, ω0i (where i ) e,p) is the beam radius at the focus, I0i is the on-axis peak irradiance at the focus and can be calculated from I0i ) 2Ei/ (π3/2ω0i2τ), Ei (where i ) p,e) is the pulse energy, and τ is the laser pulse width (HW1/e). td is the time delay between the pump and probe. σ0 is the ground-state absorption cross section, σ1 and σ2 are the excited-state absorption cross sections from states S1 and T1, respectively; N0, N1, N2, N3, and N4 represent the

Metal Cluster Polymer [Cu3IWS4(4-bpy)3]n Solution

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Figure 7. . Five-level model for the nonlinear absorption behavior of the sample. S: singlet states; T: triplet states.

Figure 6. (a) Pump-probe results for CBS. (b) Pump-probe results for C60. (c) Pump-probe results for the cluster [WS4Cu3I(4-bpy)3]n. The squares represent experimental data, and the solid lines are theoretical fitting curves.

number densities of states S0, S1, S2, T1, and T2, respectively; τISC is the intersystem crossing time; z′ is the propagation length in the sample. Lifetime and Absorption Cross-Section Determination for S1. The excellent agreement between the numerical simulations and the data reported here indicates that this five-level model can be accurate to describe the photophysical dynamics of the sample. As shown in Figure 6c, the experimental curve character of the cluster solution is very similar to that of the C60 solution. Initially, the absorption of the solution increases as a function of time in a manner consistent with the temporal integration of the pump pulse; the instant drop of the probe is dominant due

to the excited single state absorption, which has larger cross sections than the ground state. Once the pump pulse has passed through the sample, the initial response is followed by recovery and a long low transmittance tail has appeared. This behavior is consistent with induced absorption in the first excited singlet state that increases as the excited state is populated and then diminishes as the population in that state relaxes to the ground state S0 and the lowest triplet state T1. The long low transmittance tail for the cluster solution is attributed to the absorption and the long lifetime of excited triplet state which also has larger cross sections than the ground state. The recovery of the transmittance is primarily determined by the lifetime of the excited singlet state τS1 and the valley bottom of the transmission near zero delay is primarily sensitive to the first excited singlet state cross section σ1. Moreover, the intersystem crossing time τISC and the absorption cross section σ2 have little influence on it for the recovery time is too short for the triplet-state transitions. The long low tail of the curve is determined by two key parameters: the absorption cross section of the excited triplet state σ2 and the intersystem crossing time τISC. The relaxation time of the singlet state S2 and the triplet state is considerably shorter than the pulse duration, and we estimate τS2,τT2 e 1 ps. Here, σ0 ) 6.45 × 10-18 cm2, which is provided by the linear absorption measurements; the linear transmittance measurement is 78%. The most difficult part of the work is the uncertainty of σ2 and τISC. However, as in the above discussion, the parameter σ2 and τISC have no influence on the valley and the recovery of the curve. We can adjust the two parameters properly when we fit the theoretical curve to the experimental results. For these conditions, to solve the rate equations and fit the data allows us to uniquely extract τS1 and σ1. The theoretical curve superposed on the data in Figure 6c is obtained using a single set of parameters, σ1 ) 12.1 × 10-18 cm2 and τS1 ) 80 ps. Here, I0e ) 3.1 × 1013 W/m2. It is confirmed that the theoretical fitting is in good agreement with the experimental result. The first singlet state S1 has a larger absorption cross section than the ground state S0, which results in RSA as shown in Figure 5b under the 30 ps laser pulse excitation. To Determine Intersystem Crossing Time and Absorption Cross Section of T1. The long lifetime and the large absorption of the first triplet state T1 result in the long low transmittance tail. The lifetime of T1 is assumed to be several microseconds. The degree of the transmittance decrease is dependent on τISC and σ2 because the absorption of the triplet state is determined not only by the absorption cross section but also by the population of this state. All the proper combinations of these two parameters can fit the experimental data very well. So these two parameters cannot be ascertained only by the pump-probe experiment. However, the problem can be overcome by the nanosecond Z-scan results. Because the lifetimes of energy levels S2 and T2 are very short, the populations of these two levels can be neglected in the nanosecond case. With 8 ns > τISC, there should be a significant population transfer from the singlet states S1 to the triplet state T1, and because of its long

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Figure 8. Population density calculations as a function of time at the exit plane of the sample for an incident intensity of (a) 1.04 GW/cm2 (8 ns laser pulses) and (b) 22 GW/cm2 (30 ps laser pulses).

TABLE 3: Photophysical Parameters Calculated from the Rate Equation Analysis cluster

σ0 (cm2)

σ1 (cm2)

σ2 (cm2)

τS1 (ps)

τS2 (fs)

τISC (ns)

[WS4Cu3I(4-bpy)2]n

6.45 × 10-18

12.1 × 10-18

39.6 × 10-18

80

60

2.5

lifetime the population of the triplet state will accumulate more and more, as shown in Figure 8a. Thus we can obtain σ2 and τISC by adjusting these two parameters to agree with the pumpprobe and nanosecond Z-scan experiments simultaneously. Good theoretical fitting lines are shown in Figures 5a and 6c, and they are obtained using a single set of parameters σ2 ) 39.6 × 10-18 cm2, τISC ) 2.5 ns. The first triplet excited state T1 has a larger absorption cross section than the ground state S0, which results in RSA. In the nanosecond case, ω0 is about 28 µm, I0 is 1.04 × 1013 W/m2, and the linear transmittance measurement is 69%. Lifetime Determination for Higher Excited Singlet State S2. Now the obtained parameters can be used to fit the observed picosecond nonlinear absorption behavior and to obtain the lifetime of the higher excited singlet state S2. Because the picosecond pulse duration is rather shorter than the temporal duration of the population transfer from the singlet excited states to the triplet states, that is, τISC . 30 ps, the contributions of the triplet states are very small and can be neglected, as shown in Figure 8b. In the picosecond case, ω0 is about 24 µm, I0 is 2.2 × 1014 W/m2, and the linear transmittance is measured to be 74%. The lifetime of the higher excited singlet state S2 can be acquired according to the good theoretical fitting as shown in Figure 5b, τS2 is obtained to be about 60 fs. We must point out that the relaxation of the higher excited singlet state S2 is assumed to be τS2 e 1 ps at first, but actually it is obtained to be 60 fs. This has little influence on the value of the excitedstate absorption cross sections σ1, because the valley bottom of the pump-probe result is behind the zero delay time by about 30 ps, which gives the population of the higher excited state enough time to relax to the first excited state after the pump beam has passed though the sample. Last, the photophysical parameters obtained through the rate-equation analysis are listed in Table 3. The above discussions indicate that the results of the pumpprobe experiment are consistent with the results of the Z-scan measurement. We can make a confirmation that the dominant nonlinear absorption mechanism of the cluster is excited-state absorption. The nanosecond nonlinear absorption can be attributed mostly to triplet-triplet state absorption, while the picosecond absorption data is almost completely attributable to the excited singlet state absorption. The ratio of the absorption

cross section of the excited states to that of the ground state σe/σ0 for the cluster is about 1.8 for the picosecond pulse and about 6 for the nanosecond pulse, respectively. So the cluster has strong nonlinear absorption under the 8 ns pulse excitation. Under the picosecond pulse excitation, the cluster exhibits a relatively weak nonlinear absorptive property. Conclusion In summary, we have synthesized and characterized a novel cluster [WS4Cu3I(4-bpy)3]n. We use the Z-scan technique with 8 ns and 30 ps laser pulses to investigate the nonlinear absorption of the cluster. A time-resolved pump-probe experiment was conducted on the cluster to give direct evidence on the physical origin of the observed nonlinear absorption property, which is excited-state absorption. The key photophysical parameters of the cluster were obtained by the pump-probe and Z-scan measurements Acknowledgment. We gratefully acknowledge the National Natural Science Fund of China (Grant No. 10104007, the Program for New Century Excellent Talents in University (NCET-04-0333), and the Excellent Youth Fund of Heilongjiang Province (Grant No. JC-04-04). References and Notes (1) Lanzerotii, M. Y.; Schirmer, R. W.; Gaeta, A. L. Appl. Phys. Lett. 1996, 69, 1199. (2) Lidorikis, E.; Li, Q. M.; Soukoulis, C. M. Phys. ReV. E 1997, 55, 3613. (3) John, S.; Quang, T. Phys. ReV. A 1996, 54, 4479. (4) Zhu, X. H.; Chen, X. F.; Zhang, Y.; You, X. Z.; Tan, W. L.; Ji, W. Chem. Lett. 1999, 11, 1211. (5) Zhang, H.; Zelmon, D. E.; Deng, L. G.; Liu, H. K.; Teo, B. K. J. Am. Chem. Soc. 2001, 123, 11300. (6) Hou, H. W.; Xin, X. Q.; Liu, J.; Chen, M. Q.; Shu, S. J. Chem. Soc., Dalton Trans. 1994, 32, 11. (7) Ji, W.; Shi, S.; Du, H. J.; Ge, P.; Tang, S. H. J. Phys. Chem. 1995, 99, 17297. (8) Ji, W.; Du, H. J.; Tang, S. H.; Shi, S. J. Opt. Soc. Am. B 1995, 12, 876. (9) Shi, S.; Ji, W.; Lang, P.; Xin, X. Q. J. Phys. Chem. 1994, 98, 3570. (10) Shi, S.; Ji, W.; Tang, S. H.; Lang, J. P.; Xin, X. Q. J. Am. Chem. Soc. 1994, 116, 3615. (11) Shi, S.; Hou, H. W.; Xin, X. Q. J. Phys. Chem. 1995, 99, 4050. (12) Hou, H. W.; Long, D. L.; Xin, X. Q.; Huang, X. X.; Kang, B. S.; Ge, P.; Ji, W.; Shi, S. Inorg. Chem. 1996, 35, 5363.

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