10696
J. Phys. Chem. 1996, 100, 10696-10700
Influence of Structure and Composition on Linear and Nonlinear Optical Properties of Clusters: A Case Study on Clusters (n-Bu4N)3[MS4M′3XX′3], MS4M′3X(PPh3)3, and (n-Bu4N)2[MOS3M′3X′3] (M ) Mo, W; M′ ) Cu, Ag; X ) Cl, Br; X′ ) Cl, Br, I, SCN) S. Shi* SS-ALL Enterprise, 33-17 International Plaza, Singapore
Z. Lin and Y. Mo Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
X. Q. Xin Department of Chemistry, Nanjing UniVersity, Nanjing, China ReceiVed: January 24, 1996; In Final Form: April 16, 1996X
Linear and nonlinear optical properties of three families of inorganic clusters (n-Bu4N)3[MS4M′3XX′3], MS4M′3X(PPh3)3, and (n-Bu4N)2[MOS3M′3X′3] (M ) Mo, W; M′ ) Cu, Ag; X ) Cl, Br; X′ ) Cl, Br, I, SCN) were compared. Synthetic routes leading to these clusters are discussed along with the structural and compositional aspects that influence clusters’ optical performance. A scheme to understand these influences is proposed based on qualitative molecular orbital considerations.
Introduction Encouraged by application potentials of nonlinear optical devices, much effort has been made in the last 15 years in inventing new nonlinear optical (NLO) devices and circuits and in demonstrating their logic operation, signal switch, and energy (or power) limiting functions.1-5 Despite the progress made, discovery of still better NLO materials remains the bottleneck that slows the speed of further development of nonlinear optics. In the field of optical limiting (OL), attention has been drawn to reverse saturable absorption (RSA) in the visible portion of spectrum. The materials most extensively studied include semiconductors,6,7 organic dyes,8,9 metallophthalocyanines,10,11 and fullerenes.12-14 Recently, interesting NLO properties were observed in a variety of inorganic clusters.15-23 The clusters’ linear extinction coefficients vary from 103 to 100 M-1 cm-1 depending on structural details. Some clusters exhibit rather large NLO effects (χ(3) ≈ 10-10 esu) even in very diluted solutions (10-4-10-5 M). The relative contributions of different NLO mechanisms change with the type of cluster. In this paper we compare both linear and nonlinear optical properties of members in the families of cubic cage shaped ionic clusters (n-Bu4N)3[MS4M′3XX′3], cubic cage shaped neutral clusters MS4M′3X(PPh3)3, and nest-shaped ionic clusters (nBu4N)2[MOS3M′3X′3] (M ) Mo, W; M′ ) Cu, Ag; X ) Cl, Br; X′ ) Cl, Br, I, SCN). We also address structural and compositional factors that determine linear and nonlinear optical properties of these clusters. Molecular orbital analysis was used to reach a certain qualitative understanding of underlying physics that gives rise to the observed NLO phenomena. Experiments and Calculations Synthesis. The following clusters used in this study were synthesized according to known methods. They are (1) cubic * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00238-9 CCC: $12.00
cage shaped ionic clusters, (n-Bu4N)3[MoS4Ag3BrCl3] (I),15 (nBu4N)3[MoS4Ag3Br4] (II),16 (n-Bu4N)3[MoS4Ag3BrI3] (III),15,24 (n-Bu4N)3[WS4Ag3Br4] (IV),17 and (n-Bu4N)3[WS4Cu3Br4] (V);17 (2) cubic cage shaped neutral clusters, MoS4Cu3Cl(PPh3)3 (VI),25 MoS4Ag3Cl(PPh3)3 (VII),26 WS4Ag3Cl(PPh3)3‚0.5P(S)Ph3‚3H2O (VIII),27 and WS4Ag3Br(PPh3)3‚H2O (IX);28 and (3) nest-shaped ionic clusters, (n-Bu4N)2[MoOS3Cu3(NCS)3] (X),18 (n-Bu4N)2[MoOS3Cu3BrCl2] (XI),19 and (Et4N)4[Mo2Cu6O2S6Br2I4] (XII).20 Results of elemental analysis of these clusters agree satisfactorily with their individual chemical formulas. Carbon, hydrogen, and nitrogen analyses were performed on a Perkin-Elmer 240C elemental analyzer. Metal, halogen, phosphorus, and sulfur atoms were analyzed with a JA 1100 + 2000 ICP quatometer. Linear and Nonlinear Optical Data. Linear extinction coefficients of the clusters were determined with a Shimadzu UV-240 spectrophotometer. NLO data of clusters I-V were taken from the literature.15-23 NLO data of clusters VI and IX were measured in this study under conditions similar to that described earlier.15,17-22 The optical excitation pulses were generated from a Q-switched frequency-doubled Nd:YAG laser. The spatial profiles of the linearly polarized optical pulses (λ ) 532 nm) were nearly Gaussian. The pulse duration (full width at 1/e maximum) was measured at 7 ( 1 ns. The light was focused onto the sample with a 25 cm focal length focusing mirror. The spot radius of the laser beam was measured at 35 ( 5 µm (half-width at 1/e2 maximum). The interval between the laser pulses was set at 20 s for operational convenience. Energies of both the incident and transmitted signals were measured simultaneously by two Laser Precision detectors (RjP735 and RjP-765 energy probes) that were linked to a computer by an IEEE interface. The Z-scan29 data were obtained by keeping the incident laser energy constant. The light transmittance through a sample was measured with respect to the sample’s Z-position, with (and without) a 1.8 mm diameter aperture placed in front of the transmission detector. The acetonitrile solutions of the clusters were placed in a 1 mm © 1996 American Chemical Society
Influence of Structure on Optical Properties of Clusters
J. Phys. Chem., Vol. 100, No. 25, 1996 10697
quartz cuvette for optical measurements. The liability of the optical system is satisfactory as described before.17-22 Molecular Orbital Analysis. Molecular orbitals were calculated using the extended Hu¨ckel molecular orbital (EHMO) method.30 The relevant EHMO parameters are taken from the literature30b except for S,30c Br,30d W,30e and Ag.30f Results of the EHMO calculation were graphically plotted employing the plotting program of CACAO 4.0 (Computer Aided Composition of Atomic Orbitals) developed by C. Mealli and D. M. Proserpio.31 Cluster bond lengths and bond angles used for the EHMO calculations were symmetrized (average bond lengths and bond angles were used to replace individual bond lengths and bond angles reported in literature15-28) to assist assignment of symmetry representations of molecular orbitals. For the purpose of qualitative comparison, structures of clusters VII and VIII were assumed on the basis of known structures of other members of their family. Preliminary results show that coordination of PPh3 or PH3 to metal atoms in the clusters introduces little difference to the molecular orbitals of interest. PH3 was, therefore, used instead of PPh3 for all the members of the neutral cage family in the calculations. This paper simply aims at mapping out qualitative influences of structural alternations on the NLO properties of inorganic clusters. The orbital analysis in the following sections is therefore very qualitative in nature and remains to be tested by further theoretical and experimental work. Results and Discussion Synthetic Routes. Clusters belonging to the families of (nBu4N)3[MS4M′3XX′3], MS4M′3X(PPh3)3, and (n-Bu4N)2[MOS3M′3X′3] (M ) Mo, W; M′ ) Cu, Ag; X ) Cl, Br; X′ ) Cl, Br, I, SCN) can be synthesized, in general, either by solid state reactions or by solution reactions. Reactions of tetrathiometallates MS42- (M ) Mo, W) with corresponding metal halides M′X′ (M′ ) Cu, Ag, X′ ) Cl, Br, I) in the presence of extra halides (X- ) Cl-, Br-) yield desired anionic cubic cage shaped clusters.
MS42- + 3M′X′ + X- f [MS4M′3XX′3]3-
(1)
Corresponding neutral clusters can be synthesized through either ligand substitution,
[MS4M′3XX′3]3- + 3PPh3 f MS4M′3X(PPh3)3 + 3X- (2) or one-pot reaction in the presence of PPh3,
MS42- + 3M′X′ + 3PPh3 + X- f MS4M′3X(PPh3)3 + 3X- (3) where the substitution of PPh3 for X′ may precede the assembly of the clusters. Neutral ligands such as AsPh3 and py have also been used to synthesize analogous cage clusters.32,33 The neutral clusters are generally less soluble in organic solvents like acetone and acetonitrile.
The nest-shaped clusters can be synthesized using trithiometallates MOS32- as the starting material,
MOS32- + 3M′X′ f [MOS3M′3X′3]2-
(4)
Attempts to use MS42- as starting material to synthesize nestshaped clusters [MS4M′3X′3]2- in the absence of extra X- have been unsuccessful. Such reactions yield primarily more flattened [MS4M′4X′4]2- and [MS4M′3X′3]2-, which often further polymerize during the course of the reactions.34,35 Small amounts of cage-shaped clusters [MS4M′3X′4]3- were occasionally found as side products.
Electronic Transitions. The intrinsic electronic spectra of both cubic cage shaped and nest-shaped Mo(W)-S(O)-Cu(Ag) clusters discussed in this paper are characterized by two absorption regions: (1) λ1 ) 200-350 nm; (2) λ2 ) 400-500 nm.35 Peak positions and extinction coefficients are listed in Table 1. Transitions in the Region of 200-350 nm. The intense absorption peaks in the region of 200-350 nm are attributable to sulfur-based transitions. Similar features were observed with simple salts of tetrathiomolybdate and tetrathiotungstate,36 where S f S* (t1 f 4t2 and 3t2 f 2e) transitions were reported to occur at 241 and 316 nm in (Et4N)2[MoS4].37 In the Mo(W)S(O)-Cu(Ag) clusters discussed in this paper, the electronic absorption of the MS4 (M ) Mo, W) units still has the characteristics of their parent complexes (MoS42- and WS42-). The “3t2 f 2e” transition carries certain ligand-to-metal charge transfer character. The “t1” molecular orbitals are derived exclusively from the p orbitals of sulfur atoms. The “3t2” orbitals are the M-S bonding orbitals that are mainly sulfur’s p orbitals in character. The “4t2” and “2e” orbitals are mainly derived from metal d orbitals with M-S antibonding character. Transitions in the Region of 400-500 nm. The S f Mo charge transfer transition (corresponding to t1 f 2e transition of the MoS42-) occurs in the region of 473-491 nm in the clusters. In comparison, the S f Mo charge transfer occurs at ∼460 nm in MoS42-.36,37 The red shift of the S f Mo transition in the clusters with respect to the MoS42- is attributable to the coordination of the M′X′ groups to the MoS42- unit, which reduces significantly the antibonding character of the LUMOs (corresponding to the 2e orbitals of the parent MoS42-). Figure 1 shows the LUMOs and HOMOs of some typical anionic clusters. The LUMOs of the clusters are mainly Mo’s d orbitals that possess Mo-S antibonding and S-M′X′ bonding character. The HOMOs of the clusters are mainly p orbitals of S in the MoS42- unit, which mix with the d and p orbitals from the M′X′ units in the clusters. The S f M charge transfer transition peaks are obviously blue shifted when Mo is replaced by W in the clusters. Peaks at ∼490 nm were shifted by ∼70 nm to shorter wavelengths, Table 1. Similar hypsochromic shifts were also observed on moving from MoS42- to WS42-.33 The molecular orbitals of all the cubic cage shaped clusters, both anionic [MS4M′3XX′3]3- and neutral MS4M′3X(PPH3)3, can be categorized into four groups: (1) mainly the s orbital of the sulfur atoms and the p orbitals of the metal (M′) atoms; (2) delocalized M-S bonding orbitals with mainly sulfur’s p orbitals
10698 J. Phys. Chem., Vol. 100, No. 25, 1996
Shi et al.
TABLE 1: Selected Linear and Nonlinear Optical Parametersa no.
cluster
I II III IV V VI VII VIII IX X XI XII
[MoS4Ag3BrCl3]3[MoS4Ag3BrBr3]3[MoS4Ag3BrI3]3[WS4Ag3BrBr3]3[WS4Cu3BrBr3]3MoS4Cu3Cl(PPh3)3 MoS4Ag3Cl(PPh3)3 WS4Ag3Cl(PPh3)3 WS4Ag3Br(PPh3)3 [MoOS3Cu3(NCS)3]2[MoOS3Cu3BrCl2]2[MoOS3Cu3BrI2]24-
λ1 (nm)b
1 λ2 2 3 ∆E F1/2 Fs n2′ R2′ (M-1 cm-1)c (nm)b (M-1 cm-1)c (M-1 cm-1)d (eV)e (J cm-2)f (J cm-2)f (m2 W-1 M-1)g (m W-1 M-1)g
318(sh) 320 327(sh) 304 316 303
8.9 × 103 9.3 × 103 1.8 × 104 1.9 × 104 1.8 × 104 1.5 × 104
473 483 491 413 431 418
4.6 × 103 4.8 × 103 1.2 × 104 5.2 × 103 6.7 × 103 4.9 × 104
2.8 × 103 3.8 × 103 1.1 × 104 8.7 × 102 2.6 × 102 3.7 × 10
296(sh) 404 408 410
1.9 × 104 3.8 × 103 7.3 × 103 1.6 × 104
405 495 500 502
5.5 × 103 9.6 × 102 1.7 × 103 4.6 × 103
1.5 × 10 5.7 × 102 1.2 × 103 5.0 × 102
2.08 2.07 2.07 2.47 2.54 2.29 2.15 2.53 2.56 2.25 2.26 2.18
0.6 0.6 0.5 0.7 1.3 ndh nd nd nd 7h 10h 2h
0.3 0.3 0.3 0.5 0.7 nd nd nd nd 2h 3h 0.2h
nd nd nd nd -1.8 × 10-8
