Determination of Stability and Degradation in Polysilanes by an

Department of Materials and Metallurgical Engineering, and Samtel Center for Display Technologies, Indian. Institute of Technology Kanpur, Kanpur, 208...
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15860

J. Phys. Chem. B 2005, 109, 15860-15867

Determination of Stability and Degradation in Polysilanes by an Electronic Mechanism Asha Sharma,† U. Lourderaj,‡ Deepak,*,† and N. Sathyamurthy‡ Department of Materials and Metallurgical Engineering, and Samtel Center for Display Technologies, Indian Institute of Technology Kanpur, Kanpur, 208016 India, and Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208016 India ReceiVed: February 20, 2005; In Final Form: May 22, 2005

Polysilanes are potential candidates for active materials in light emitting diodes because of possible emission in the near-ultraviolet to blue region. Unfortunately, they degrade rapidly upon exposure to light because of scission of sigma bonds. Relative stability of four polysilanes, for example, poly(di-n-butylsilane) (PDBS), poly(di-n-hexylsilane) (PDHS), poly(methylphenylsilane) (PMPS), and poly[bis(p-butylphenyl)silane] (PBPS), which have been reported as active materials in light emitting diodes, have been investigated theoretically through semiempirical (AM1) and ab initio (HF/6-31g*) methods and density functional theory using B3LYP parametrization. The AM1 level of calculation predicts the absorption maxima reasonably, but it fails to explain the relative stabilities of the four polysilanes in the excited state. However, calculations based on configuration interaction with single excitation and time-dependent density functional theory suggest additional stabilization in the excited states through intersystem crossing to triplets for PMPS and PBPS, consistent with the experimental observation. In contrast, no such stabilization is predicted for PDBS and PDHS. Furthermore, the existence of a stable triplet state in PMPS may also explain the visible emission observed experimentally in PMPS.

1. Introduction Polysilanes are silicon-based polymers that absorb and emit light in the ultraviolet (UV) or near-ultraviolet (NUV) region. They exhibit nonlinear optical characteristics, thermochromism, and photoconductivity arising from delocalization of σ electrons along the silicon backbone.1-3 They have been used mainly as photoresists in lithography in semiconductor applications.4 However, Suzuki and co-workers5,6 exploited the property of high hole mobility in polysilanes to use them as hole transport materials in organic multilayer light emitting diodes (LEDs). Owing to their optical absorption and emission, attempts have been made recently to utilize dialkyl, monoalkylaryl, and diaryl polysilanes as active materials in LEDs.7-12 Some of the extensively studied polysilanes for LED application are poly(di-n-butylsilane) (PDBS), poly(di-n-hexylsilane) (PDHS), poly(methylphenylsilane) (PMPS), and poly[bis(p-butylphenyl)silane] (PBPS). Bonding in the ground electronic state of polysilanes was initially explained by Sandorfy model C and Sandorfy model H, which uses the sp3 hybrid orbitals on the Si atoms.1 Since then, the polysilanes have been a subject of detailed investigation by various semiempirical and ab initio methodologies. Miller and Michl1 provide a detailed account of the various theoretical and experimental studies on the different properties of polysilanes until 1989. The ground-state properties of linear [R3Si(SiR2)nSiR3] (n ) 1-3; R ) H, CH3) and cyclic [c-(SiR2)n] (n ) 3-6; R ) H, CH3) polysilanes have been studied by various semiempirical (CNDO, MNDO, MINDO, MINDO/3, AM1, PM3, and MNDO) and ab initio (HF) methods.13-16 The excited states of SinH2n+2 * Address correspondence to this author. E-mail: [email protected]. † Department of Materials and Metallurgical Engineering. ‡ Department of Chemistry.

have also been investigated by various semiempirical methods.17,18 Liu et al.19 have investigated the geometry, the electronic structure, and linear optical properties of methylsubstituted trans-oligosilanes up to 16 units using local density approximation (LDA) in density functional theory (DFT) to explain the molecule to polymer transition. Furthermore, ab initio-calculated excited states of SinH2n+2, n ) 2-5, have been reported using configuration interaction (CI) method with single and double excitations.20,21 These excited-state calculations have shown that the excitation energy decreases as the chain length increases. In addition, the band gap of organopolysilanes increases with a decrease in the dihedral angle from all trans (180°) to 148° (7/3 helix) and 62.7° (4/1 helix).22 The orbitals in the Sandorfy model are characterized by the symmetry with respect to the plane containing the Si atoms. A σ orbital is symmetrical, and a π orbital is nonsymmetrical with respect to this plane.23 Accordingly, Kishida et al.24 have explained the electronic transition in these materials by taking into account the semiempirical model Hamiltonian (Sandorfy C), in which the sp3 orbitals of Si along the backbone chain are included. The first absorption peaks in shorter oligomers (n ) 2) differ from that of the longer one; the transitions in the former are from HOMO (σ) to the second LUMO (π*), and in the latter, it is HOMO (σ) f LUMO (σ*). To understand the behavior of polysilanes as one-dimensional (1-D) conductors, Tachikawa25,26 has attempted to elucidate the ground-state and excited-state properties of the permethyloligosilane radical cation and anion for Sin(Me)2n+2 (n ) 4-20) using the semiempirical PM3-CI method. The localization mechanism for the disordered conformation has been investigated by molecular dynamics and extended Hu¨ckel molecular orbital (EHMO) calculations.27 Tada and Yoshimura28 studied the excited states of Si2H6--Si5H14- by the CIS method and suggested that the polysilane radical anions can exist as bound

10.1021/jp0508756 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/30/2005

Stability and Degradation of Polysilanes

J. Phys. Chem. B, Vol. 109, No. 33, 2005 15861 TABLE 1: Geometrical Parameters and Absorption Maxima for the Model Compounds as Obtained from AM1 Calculations (experimental values are given in parentheses) Bond Length (Å) terminal Si-C molecule Si(2)-Si(3) (methyl) PDBS PDHS PMPS PBPS a

Figure 1. (a) Chemical structure of polysilanes investigated. (b) Model structures used in calculations, where R corresponds to the side groups appearing in (a).

species with σ-type ground state with an unpaired electron in the antibonding σ* orbital. 1.1. Polysilanes in LED Applications. The emission from polysilanes can be used as an active source for electroluminescence (EL) devices, mainly suitable for blue emission. However, despite potential emission in UV or NUV, a major difficulty with polysilanes is that they degrade upon exposure to UV light by scission of Si-Si σ bonds.29,30 This is a bottleneck in the utilization of polysilane-based LEDs. Therefore, to harvest UVNUV emission from polysilanes, greater stability of these materials needs to be achieved. Four polysilanes, PDBS, PDHS, PMPS, and PBPS, have been reported as active materials (see Figure 1a for chemical structures) for LEDs.9,10 Among these, the PBPS-based LEDs show the greatest durability of the device at room temperature, and NUV EL is observed continuously for over 12 h.9,10 For PDBS and PDHS, the room temperature EL is not observable, and for PMPS, the device lifetime is shorter than that of PBPS.9,10 However, to fabricate useful devices based on a polysilane, the device lifetime would have to be in thousands of hours. The four polysilanes listed here provide an opportunity to investigate the relative differences in life-times of devices based on them. By understanding the mechanism of degradation, it may be possible to design suitable long-lasting polysilane-based LEDs. 2. Methodology There are several ways in which the degradation of polysilanes may occur. For example, once excited, the polysilanes may not return to their ground states before scission of the SiSi bond. In addition, the degradation may occur due to

2.46 2.46 (2.4)a 2.47 2.48

1.83 1.83 1.83 1.83

Si-C (R)

Si-C (phenyl)

λmax (nm)

1.86 1.85 (1.87)a 1.85 -

1.77 1.79

426 (314)b 426 (316)b 423 (337)c 427 (390)c

From ref 33. b From ref 34. c From ref 1.

environmental reasons and thermal or conformational instability in thin film form. In the present investigation, electronic structure calculations have been carried out to examine the relative stability of PDBS, PDHS, PMPS, and PBPS, whose chemical structures are shown in Figure 1a. Since these polymers differ primarily in their side groups, their relative stability must relate to the side groups. To evaluate the effect of the side group on the stability, the model system chosen for investigation has four Si atoms with methyl groups at the terminal Si atoms and different substituents corresponding to the four polysilanes at the two middle silicon atoms, as shown in Figure 1b. The four model systems have been studied using semiempirical, ab initio, and density functional theoretical methods. The AM1 Hamiltonian was used for semiempirical calculations for both the ground and the excited states. The excited states were investigated by partial CI, including electron-pair excitations (AM1/PECI ) 8). It includes single and double excitations involving electron pairs over a limited active space consisting of four occupied and four virtual orbitals. Hartree-Fock and DFT methods were also employed for the ground-state calculations. The excited states were studied using configuration interaction singles (CIS) and the time-dependent density functional theory (TDDFT) approach. Both 3-21g* and 6-31g* basis sets were employed in the calculations, and for the DFT, B3LYP parametrization was used. The semiempirical AM1 calculations were performed using MOPAC,31 and the ab initio and TDDFT calculations were performed using Gaussian 9832 suite of programs. 3. Results and Discussion 3.1. AM1 Calculations. The ground state optimized geometrical parameters as obtained for the four model compounds from AM1 calculations are listed in Table 1. The Si(2)-Si(3) bond lengths are 2.46 Å for PDBS and PDHS, 2.47 Å for PMPS, and 2.48 Å for PBPS. The Si-Si bond length of 2.46 Å for PDHS is in good agreement with the experimental value of 2.4 Å.33 The Si-C (R) bond length is 1.85 Å, in accord with the experimental value of 1.87 Å for PDHS.33 Figure 2 shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) diagrams for the four model systems. The HOMO is clearly a σ-type bonding orbital of the Si-Si backbone, and the LUMO is an antibonding σ* orbital. Both the HOMO and the LUMO do not have any significant contribution from the side groups, as can be seen from Figure 2. It is found that the first excited singlet (S1) states arise from σ-σ* (HOMO-LUMO) transitions in all four cases. Hence, it can be anticipated that the transitions involving these orbitals are of similar nature for all four cases. The absorption maximum (λmax) values corresponding to the vertical excitation energies for the first singlet state (S1) listed in Table 1 are comparable for all four systems (423-427 nm).

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Figure 2. Schematic diagrams of the HOMO and the LUMO of PDBS, PDHS, PMPS, and PBPS model structures as obtained from AM1 calculations.

However, the observed (in solution) experimental values differ significantly: 314, 316, 337, and 390 nm for PDBS, PDHS, PMPS, and PBPS, respectively.1,34 It has been observed that polysilanes undergo substantial photodegradation upon UV exposure with the scission of the Si-Si chain, resulting in a decrease in the molecular weight of the polysilane.1 To understand the degradation mechanism of polysilanes upon UV exposure, potential energy curves (PEC) for the ground and the excited states for these four models systems were generated as a function of the Si(2)-Si(3) bond distance, as shown in Figure 3. In these calculations, after determining the ground-state equilibrium geometry, the Si(2)Si(3) bond distance was varied to generate the potential energy curves. When the Si(2)-Si(3) bond is varied from its equilibrium value discretely, the rest of the geometry is optimized, and the excited-state energies were calculated for these partially optimized structures. The ground- and excited-state energies thus obtained are depicted in Figure 3. In all four cases, as expected, the PEC has a minimum for S0. In addition, the S1 state appears stable. However, the first

Sharma et al. excited triplet state (T1) is repulsive, leading to the dissociation of the molecule along the Si-Si bond. It is clear from Figure 3 that there are other excited triplet states close to the S1 state. From the nature of the excited states, it can be anticipated that the dissociation of the molecule can take place in the T1 state, preceded by an intersystem crossing (ISC) between the S1 state and the higher triplet states that are close in energy to the S1 state. Although polysilanes degrade upon UV irradiation, it has been observed that PBPS has a greater stability as an EL device, emitting for more than 12 h at room temperature.9 The factors that can account for the stability of these molecules under investigation are the singlet-triplet energy gap and the nature of the T1 state. The S1-T1 energy gaps for the model systems of PDBS, PDHS, PMPS, and PBPS are comparable in magnitude, and the T1 state is repulsive with similar slopes in all of the cases. Hence, a comparison of the S1-T1 energy gap as well as the slope of the T1 states would not explain the observed additional stability of PBPS.9 In short, AM1 level of calculations do not explain the relative instability of the three polysilanes in comparison to that of PBPS. Furthermore, it is evident that the λmax values computed for all four molecules are close to each other, in keeping with the HOMO-LUMO picture in Figure 2, where the side group seems to have no effect. Since, experimentally, we do know that the side groups have an influence on λmax, we conclude that the AM1 calculations may not be adequate. Therefore, we have also performed ab initio calculations for the four model systems. 3.2. Ab initio and DFT Calculations. Ab initio calculations were performed for the four model systems at the HF level of theory using 3-21g* and 6-31g* basis sets. It was found that the results obtained from the 3-21g* basis set were similar to those obtained using 6-31g*. To evaluate the effect of electron correlation, DFT(B3LYP) calculations were also carried out using 3-21g* and 6-31g* basis sets. Table 2 lists the groundstate parameters for PDBS, PDHS, PMPS, and PBPS as obtained from these calculations. The ground state optimized Si(2)-Si(3) bond lengths for PDBS, PDHS, PMPS, and PBPS at the HF/6-31g* level of theory are 2.41, 2.41, 2.39, and 2.41 Å, respectively. The Si-Si bond length of 2.41 Å for PDHS is in good agreement with the experimental value of 2.4 Å.33 The Si-Si-Si-Si dihedral angle (ω) was found to be 163, -163, -171, and -169° for PDBS, PDHS, PMPS, and PBPS, respectively. The ground-state parameters for the four model systems obtained from the DFT calculations are also in good agreement with the available experimental values. In fact, there is no significant difference between the ground-state parameters obtained by HF and DFT calculations. The vertical excitation energies calculated using CIS and TDDFT methodologies for the four model systems are reported in Table 3. Both CIS and TDDFT results show comparable values and are in agreement with the experimentally observed trends for the four polymers. However, the theoretically predicted λmax values are much lower than the experimental values. This is not surprising since the experimental λmax values were obtained from solutions of polymers, whereas we are only considering a smaller model system in the gas phase.1,34 It is expected that the λmax value would increase with an increase in chain length. The effect of chain length on the HOMO-LUMO gap as well as the λmax values is illustrated using SinH2n+2 (n ) 2-20) as a model using 6-31g* basis set in Figure 4. The HOMO-LUMO energy gap decreases with an increase in the number of Si atoms until n ) 20, beyond which the gap levels

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Figure 3. Potential energy curves of the model systems as a function of the Si(2)-Si(3) bond, as obtained from AM1/PECI ) 8 calculations.

TABLE 2: Geometrical Parameters for the Ground State of the Model Polymers as Obtained from Ab initio (HF) and DFT(B3LYP) Calculations (experimental values are given in parentheses) Bond length (Å) Si(2)-Si(3) HF

Dihedral angle (degree) Si(1)-Si(2)-Si(3)-Si(4)

DFT (B3LYP)

HF

DFT (B3LYP)

molecule

3-21g*

6-31g*

3-21g*

6-31g*

3-21g*

6-31g*

3-21g*

6-31g*

PDBS PDHS PMPS PBPS

2.38 2.38 2.36 2.37

2.41 2.41 2.39 2.41

2.37 2.37 2.35 2.36

2.40 2.40 (2.4)a 2.38 2.40

161 -161 -169 -172

163 -163 -171 -169

157 -162 -177 -172

163 (154)b -165 (180)b -171 -172

a

From ref 33. b From ref 34.

TABLE 3: Spectral Parameters for the Model Polymers as Obtained from Ab initio Calculations CIS 3-21g* molecule

λmax (nm)

PDBS PDHS PMPS PBPS

185 185 196 200

a

TDDFT (B3LYP)

6-31g* f

λmax (nm)

0.001 0.0009 0.4120 0.1961

187 187 203 208

3-21g* f

major transition

λmax (nm)

0.001 0.0008 0.3436 0.1339

σ f π* σ f π* σ + πb f π* b σ + πb f σ* + π* b

227 227 263 268

6-31g* f

λmax (nm)

0.0009 0.0012 0.2794 0.0047

229 228 262 270

Expt. f

major transition

λmax (nm)

0.0013 0.0014 0.2898 0.0048

σ f π* σ f π* σ + πb f π* b σ + πb f π* b

314a 316a 337c 390c

From ref 34. b Refers to the π and π* orbitals of the phenyl group. c From ref 1.

off to 10.4 eV, as illustrated in Figure 4a. Correspondingly, the λmax values vary from 105 to 212 nm as the number of Si atoms in the chain increases from 2 to 20 (Figure 4b). This observation is in agreement with the experimentally observed trends.1 The HOMO, LUMO, and LUMO + 1 for the four model systems as obtained from HF/6-31g* calculations are shown schematically in Figure 5. Analyzing these in the framework of the Sandorfy model, the HOMOs and the LUMOs of PDBS and PDHS are of σ- and σ*-type, respectively, and do not have noticeable contributions from the side groups. The HOMOs for PMPS and PBPS are essentially of σ-type, but they do get

contributions from the πb orbitals of the phenyl groups that are attached to the Si chain. The LUMO of PMPS is dominated by the π*b orbital from the aromatic ring of the side group, and there is some delocalization over the silicon backbone. The LUMO of PBPS is also composed of the π*b orbital of the phenyl ring and the delocalized σ* orbitals of Si. Similarly, an examination of LUMO + 1 reveals that it is of π*-type for PDBS and PDHS. The LUMO + 1 of PMPS is σ*-type, with some π*b character from the benzene group. The LUMO + 1 of PBPS is of π*b-type, with delocalization from the two benzene groups.

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Sharma et al.

Figure 4. (a) The HOMO-LUMO energy gap and (b) the λmax value as a function of chain length for polysilanes.

Finally, we observe that the S1 state arises from σ f π* (HOMO to LUMO + 1) transition for PDBS and PDHS. For PMPS and PBPS, on the other hand, the S1 state arises from σ + πb f π*b and σ + πb f σ* + π*b transitions, respectively. The predicted transition energies are identical (λmax ) 187 nm) for PDBS and PDHS. In comparison, the λmax for PMPS and PBPS is larger because of the delocalization between the SiSi σ orbitals and the πb orbitals of phenyl ring that are involved in the transition. A similar observation was reported by Nesˇpu˚rek et al.35 for PMPS, for which the transition was related to a charge transfer (CT). The oscillator strengths (f) calculated for the S1 state for the four model systems are reported in Table 3. Larger oscillator strengths observed for PMPS and PBPS in comparison to those for PDBS and PDHS are a result of πb-π*b transition. A similar observation of higher f for transition in phenyl-substituted polysilanes (PBPS ) 0.11, PMPS ) 0.091) in comparison to alkyl-substituted polysilanes (PDBS ) 0.053, PDHS ) 0.056) has been reported by Seki et al.36 Although, TDDFT calculations yield oscillator strengths that are comparable in magnitude with CIS predictions for PDBS, PDHS, and PMPS, for some inexplicable reason, it (TDDFT) underestimates f for PBPS. Similar to AM1 results reported in Figure 3, the ground- and excited-state PECs for PDBS, PDHS, PMPS, and PBPS, generated by CIS/6-31g* calculations, are reported in Figure 6. As was described earlier, these potential energy curves were generated by varying the Si(2)-Si(3) bond distance and carrying out CIS calculations for these geometries, while keeping all other geometric parameters constant. To evaluate the reliability of such an approach of keeping some geometric parameters constant, we also computed the PECs for PMPS by allowing the optimization of all the geometric parameters, except the Si(2)-Si(3) bond distance. The results plotted as broken lines in

Figure 5. Schematic diagrams of the HOMO, LUMO, and LUMO + 1 of PDBS, PDHS, PMPS, and PBPS model structures as obtained from HF/6-31g* calculations.

Figure 6 show that such an approach is indeed reliable. Therefore, for the other three model systems, the PECs were computed by varying the Si(2)-Si(3) distance only. The ground state (S0) has a deep well, while the S1 state is almost flat and has similar behavior in all cases. Most strikingly, the PECs for T1 exhibit a minimum for PMPS and PBPS, whereas they are purely repulsive for PDBS and PDHS. Experimentally, as has been noted earlier, PMPS and PBPS are more stable materials.9 Therefore, on the basis of these calculations, it appears that this additional stabilization is the result of a stable T1 state. Nonetheless, even in PMPS and PBPS, like in the other two polymers, scission of the Si-Si chain may occur via the S1 state, unless ISC is fast enough. Although PMPS and PBPS do not have the kind of stability exhibited by other organic molecules used in organic LEDs, these two polysilanes are more stable than PDBS and PDHS because of the stable triplet state. The molecule upon irradiation is normally excited to the S1 state. Therefore, to stabilize the molecule in the T1 state, an ISC must occur. Although the S1-T1 gap (2.6 eV) in PMPS and PBPS is greater than that for PDBS and PDHS (1 eV), it is apparent from Figure 6 that there is a multitude of triplet states

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Figure 6. Potential energy curves for the ground and excited states of the model systems generated using CIS/6-31g*. The solid lines represent the results obtained by varying the Si(2)-Si(3) bond distance while keeping the other geometric parameters fixed. The dotted lines in the figure for PMPS represent results obtained by optimizing the other geometric parameters for each value of Si(2)-Si(3) bond distance.

available in the vicinity of the S1 state. Therefore, the presence of the close-lying triplet states could facilitate ISC to an excited triplet state before the system decays to the T1 state. In contrast, PDBS and PDHS do not have any stabilization mechanism. Accordingly, consistent with the experimental observation, they degrade rapidly. To further verify the computed CIS results, we have obtained these curves from TDDFT calculations also. The latter show that the nature of the PECs is not changed as we go from the CIS to TDDFT level of theory. However, it can be seen from Table 3 that the λmax values as estimated by TDDFT calculations are significantly larger and closer to the experimental results than those predicted from the CIS calculations. It must be added that the relative values of λmax among the four molecules remain the same in both methods. 3.3. On the Origin of Visible Emission in Polysilanes. Although the subject of this paper is determination of stability in polysilanes, the presence of a stable triplet state in aryl polysilanes raises the possibility of explaining the origin of visible emission and phosphorescence observed in a variety of polysilanes,37 especially at low temperatures. For example, photoluminescence from PMPS shows a narrow excitonic peak at 3.5 eV and a broad visible emission centered at 2.7 eV. The origin of this visible emission has been explained largely by (a) a charge transfer state, (b) phosphorescence from a triplet state, or (c) a defect-based model. No conclusive evidence has come in favor of any one of these three mechanisms so far. However, a general inclination seems to be toward explaining the visible emission by a defect based model. (a) Charge Transfer State. The visible emission in PMPS was first assigned by Kagawa et al.38 to an intruder band arising

from the phenyl side group and then reassigned by Ito et al.39 to a charge transfer state between Si σ and phenyl π*b based on an updated band calculation.40 However, the observation of visible emission from dialkyl polysilanes precludes this as a general mechanism for visible emission. (b) Phosphorescence in Polysilanes. Evidence of phosphorescence is available for a variety of polysilanes. For example, Harrah and Ziegler,41 and Michl et al.42 demonstrated phosphorescence from poly(methylpropylsilane) (PMPrS) in the visible range, ascribing it to a localized triplet state. In contrast, Ito et al.43 reported long-lived emission (1-4 ms after excitation) at a significantly lower temperature (4.2 K) than that in the previous two reports. At this temperature, in addition to the broad visible phosphorescence, they also demonstrated a narrow intense peak at approximately 360 nm, only slightly red shifted from the fluorescence peak, but they attributed the visible emission to the presence of impurities on the basis of differences between the absorption and excitation spectra. When Maeda et al.44 prepared the same polymer by anion polymerization (as opposed to Wurtz coupling reaction), which is thought to yield defect/impurity-free polymer, the visible phosphorescence disappeared. However, the NUV phosphorescence persisted, corresponding to a triplet state separated by 0.17 eV from the singlet state. It is for this reason that the visible emission has been ascribed to defects, rather than triplet phosphorescence. Similarly, Walsh et al.45 also observed phosphorescence in PDHS, again arising from a triplet state located only 0.12 eV below the lowest singlet excited state. In PMPS, where originally the visible emission was assigned to a charge transfer state, Harrah and Zeigler46 showed spectrally overlapping prompt fluorescence and phosphorescence in the

15866 J. Phys. Chem. B, Vol. 109, No. 33, 2005 visible region. The prompt visible fluorescence in a thick PMPS film grew with an increasing exposure to the excitation radiation, and thus this fluorescence was attributed to the presence of products of photolysis (defects). In short, the visible emission was assigned to both the defects and phosphorescence. (c) Defect Model. As discussed above, in view of the absence of visible emission in PMPrS prepared by anion polymerization, the visible emission was attributed to presence of defects, such as branching points and impurities in polysilanes. To strengthen this explanation, the subsequent effort was to deliberately introduce branching points in a polymer and observe the changes in the yield of visible emission. For example, in PMPS, where the visible emission is obtained up to reasonably high temperatures, a controlled amount of a trichlorosilane was added to the polymer precursor of methylphenyldichlorosilane to create the branching point defects.47-49 The resulting density of defects/ branched Si units determined was from an analysis of NMR and EPR spectra. In understanding the origin of visible emission in PMPS, of particular importance are observations of Kamata et al.51 and Fujiki.48 They noted that the absorption spectrum of PMPS is coincident with the excitation spectrum of NUV emission, but the two are significantly different for the visible emission. This suggested energy transfer from the main chain to levels that are responsible for visible emission. Since the yield of visible emission increased in the polymers synthesized with a greater density of defects, it was assumed that the energy level to which energy transfer occurs is due to defects in PMPS. Then, by extrapolation of the measured data for yield of visible emission, it was determined to subside below 1% defects in the polymer. In short, since evidence generated by intentionally adding defects in polymer points to emission from defect levels, even in polymers where defects are not deliberately added, but may be present at a level of approximately 3% due to the nature of synthesis, by empirical extension, the defects are thought to be responsible for visible emission. To further elucidate the role of defects in several polysilanes, subsequently, several investigations have been reported to demonstrate a substantial reduction of hole mobility,49 increased stability of neutral silyl radicals,49 increased polaron binding energy, and decreased extinction coefficients47 as the defect density in PMPS increases. In the context of present investigation, the triplet state in dialkyl polysilanes is dissociative. Therefore, phosphorescence, if any, may be short-lived or may be observed only at very low temperatures, 1.5 K,45 when the nuclear motion is almost frozen. Also, because our calculations are for a model system, a quantitative estimate is difficult. However, if the S1-S0 gap reported in Figure 6 is scaled with the measured absorption in PDHS, the S1 to T1 gap is between 0.5 and 0.6 eV, consistent with that observed experimentally (below 1 eV).45 In contrast, in aryl alkyl polysilanes, such as PMPS, the triplet state is stable (see Figure 6). Thus, consistent with the difference in the absorption and the excitation spectrum of PMPS, the energy transfer may occur from the main chain to a localized triplet level, which in turn may be responsible for visible emission. Again, if the S1-S0 gap in Figure 6 is scaled with absorption in PMPS at 3.7 eV, the T1-S0 gap would be 2.1 eV, which is in the visible region. Correspondingly, the observed broad peak in PMPS is at 2.7 eV,48 a good agreement considering that the calculations are on a model molecule. Also notice that the visible emission in PMPS is observable even at temperatures only slightly below room temperature. Thus, the stable triplet state in PMPS in comparison to a dissociative state in PDHS may

Sharma et al. be responsible for observing phosphorescence in PMPS at temperatures higher than in PDHS. However, in PMPS, Harrah and Ziegler46 observed both prompt (ns) and delayed (ms) visible emission. Thus, if visible emission would be assigned to triplet levels, then there could be observation of two sets of energy transfer kinetics. Alternatively, both defect emission and triplet phosphorescence may be present. Furthermore, although calculations for PMPrS are not a part of this paper, to propose triplet emission as a general mechanism of phosphorescence, the observation of missing visible emission in anion-polymerized and subsequently purified PMPrS44 would have to be revisited. 4. Conclusions Attempts were made to understand the relative stability and the mechanism of degradation of polysilanes, by taking four model systems (PDBS, PDHS, PMPS, and PBPS) and performing semiempirical (AM1), ab initio (HF, CIS), and DFT (TDDFT) calculations. AM1 calculations do not discern between the different substituents in the four model polysilanes, yielding similar λmax values. Therefore, AM1 calculations are inadequate to explain the relative stability of the four polysilanes. On the other hand, ab initio and DFT calculations on the model polysilanes provide meaningful explanations, with striking correlation between the calculations and experimental observations. Ab initio (CIS) calculations show that PDBS and PDHS have similar transition energies attributed to similar orbitals involved in the transition. In contrast, the observed higher λmax values in PMPS and PBPS are due to delocalization of the σ orbitals to the π orbitals of the benzene side chains. The CIS-calculated PECs demonstrate the relative stability of the four polymers. The presence of a local minimum in the T1 state increases the stability of PMPS and PBPS. This observation is further confirmed by TDDFT calculations. While we recognize that there can be many pathways for degradation in polysilanes, the correlation of our calculated results with experimental observations suggests that one of the causes of degradation in polysilanes can be the scission of SiSi bonds in the excited state, and that some stabilization is possible when a stable excited state exists, such as in PMPS and PBPS. Therefore, these calculations can be a basis to design new, more stable polysilanes. References and Notes (1) Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359. (2) Kepler, R. G.; Zeigler, J. M.; Harrah, L. A.; Kurtz, S. R. Phys. ReV. B 1987, 35, 2818. (3) Abkowitz, M.; Knier, F. E.; Yuh, H. J.; Weagley, U. J.; Stolka, M. Solid State Commun. 1987, 62, 547. (4) Hayase, S. Prog. Polym. Sc. 2003, 28, 359. (5) Suzuki, H.; Meyer, H.; Hoshino, S. J. Appl. Phys. 1995, 78, 2684. (6) Hoshino, S.; Suzuki, H. Appl. Phys. Lett. 1996, 69, 224. (7) Fujii, A.; Yoshimoto, K.; Yoshida, M.; Ohmori, Y.; Yoshino, K. Jpn. J. Appl. Phys. 1995, 34, L1365. (8) Hattori, R.; Sugano, T.; Shirafuji, J.; Fujiki, T. Jpn. J. Appl. Phys. 1996, 35, L1509. (9) Suzuki, H.; Hoshino, S.; Yuan, C. H.; Fujiki, M.; Toyoda, S.; Matsumoto, N. IEEE J. Quantum Electron. 1998, 4, 1, 129. (10) Suzuki, H.; Hoshino, S.; Yuan, C. H.; Fujiki, M.; Toyoda, S.; Matsumoto, N. Thin Solid Films 1998, 331, 64. (11) Xu, Y.; Fujino, T.; Watase, S.; Naito, H.; Oka, K.; Dohmaru, T. Jpn. J. Appl. Phys. 1999, 38, 2609. (12) Suzuki, H.; Hoshino, S.; Furukawa, K.; Ebata, K.; Yuan, C. H.; Bleyl, I. Polym. AdV. Technol. 2000, 11, 460. (13) Takeda, K.; Matsumoto, N.; Fukuchi, M. Phys. ReV. B 1984, 30, 5871. (14) Mintmire, J. W.; Ortiz, J. V. Macromolecules 1988, 21, 1189. (15) Nelson, J. T.; Pietro, W. J. J. Phys. Chem. 1988, 92, 1365. (16) Apeloig, Y.; Danovich, D. Organometallics 1996, 15, 350

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