Rationalized Approach to Molecular Tailoring of Polymetallocenes

Nov 10, 2004 - Rationalized Approach to Molecular Tailoring of Polymetallocenes with Predictable Optical Properties. Chantal Paquet,Paul W. Cyr,Eugeni...
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Chem. Mater. 2004, 16, 5205-5211

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Rationalized Approach to Molecular Tailoring of Polymetallocenes with Predictable Optical Properties Chantal Paquet, Paul W. Cyr, Eugenia Kumacheva,* and Ian Manners* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Received April 11, 2004. Revised Manuscript Received September 10, 2004

A rationalized approach to tuning the optical properties of metallocenes is reported. The optical properties of a series of high molecular weight polymetallocenes with main chain ferrocene or ruthenocene units were examined. The refractive indices of polymetallocenes were tuned from 1.599 to 1.747 by modifying the metal, the substituent group, or the spacer element. The molar refraction of the backbone repeating unit, RfcE, {where fc ) [(η5-C5H4)2Fe] and E ) Si, Ge, Sn, or P} was calculated to range from 57.2 to 63.6 cm3/mol and demonstrated to comply to the additivity principle of molar refraction. The value of the molar refraction of the backbone repeating unit, RfcE, was used to predict the refractive index of molecularly tailored polyferrocenes. The optical dispersion of polyferrocenes, characterized by the Abbe´ number, was demonstrated to be lower than that of organic polymers with similar refractive indices.

Introduction The use of polymers in optical devices offers important advantages such as low cost, low weight, and ease of processing. Unfortunately, polymers have a narrow range of refractive indices (RIs), with most polymeric materials having RI values from 1.35 to 1.65.1 This drawback limits the application of polymers in many optical devices designed to have microcomponents with a large refractive index contrast.2,3 A common approach to increasing the refractive index (RI) of polymers employs molecular tailoring. Rationalizing the RI with the molecular structure of polymers is realized by applying the Lorentz-Lorenz equation (Equation 1), which relates molar refraction (RM) and molar volume (VM) of the polymer repeating unit to the refractive index (n) of the polymer.4

n2 - 1

VM ) RM )

n2 + 2

∑i Ri

(1)

Molar refraction is an additive term. Therefore the summation of the molar refractions of functional groups (Ri) and the molar refraction of the backbone repeating unit can give the total molar refraction of the polymer repeat unit. By molecular tailoring of the polymer backbone with appropriately chosen high molar refraction functional groups, high RI polymers can be obtained. For instance, substituents such as bromine have been used successfully to yield high RI pentabromophen* To whom correspondence should be addressed. E-mail: [email protected] and [email protected]. (1) Dislich, H. Angew. Chem., Int. Ed. Engl. 1979, 18, 49. (2) Ma, H.; Jen, A. K. Y.; Dalton, L. R. Adv. Mater. 2002, 14, 1339. (3) Urbas, A.; Fink, Y.; Thomas, E. L. Macromolecules 1999, 32, 4748. (4) VanKrevelen, D. W. Properties of Polymers; 3rd ed.; Elsevier: Amsterdam, 1990.

yl methacrylates (n ) 1.71) while high molar refraction π-conjugated functional groups, such as naphthyl, have been incorporated into polymers, yielding RI up to 1.63.5 In a similar way, π conjugation in the backbone increases polymer refractive indices to 1.75 ( 0.15.6 Unfortunately, the use of π conjugation as a route to increasing the RI of polymers has two serious drawbacks: high optical dispersion and significant absorption. In addition, due to their propensity for π stacking, π-conjugated polymers tend to crystallize. This feature leads to birefringence and scattering by the polymer material. Polymer composites with high refractive index inorganic components have been employed as an alternative approach to using high molar refraction functional groups. For example, TiO2 and ZnS nanoparticles have been successfully incorporated into polythiourethanes to give a composite material with refractive indices of up to 1.88.7,8 The drawback of this approach is the increase in Rayleigh scattering when the size of the inorganic particles or their aggregates approaches ca. 25 nm. In addition, nanocomposite materials may feature optical inhomogeneities. Furthermore, due to the formation of nanoparticle network structures, polymer composites can present problems in processing. Combining the advantages of molecular tailoring of polymers with the high molar refraction of inorganic elements is a promising way of increasing their refractive index while maintaining low optical dispersion. For instance, sulfide functionalized polymers have been reported to have a RI as high as 1.68, along with low (5) Olshavsky, M.; Allcock, H. R. Macromolecules 1997, 30, 4179. (6) Yang, C. J.; Jenekhe, S. A. Chem. Mater. 1995, 7, 1276. (7) Lu, C. L.; Cui, Z. C.; Guan, C.; Guan, J. Q.; Yang, B.; Shen, J. C. Macromol. Mater. Eng. 2003, 288, 717. (8) Lu, C. L.; Cui, Z. C.; Wang, Y.; Li, Z.; Guan, C.; Yang, B.; Shen, J. C. J. Mater. Chem. 2003, 13, 2189.

10.1021/cm049403e CCC: $27.50 © 2004 American Chemical Society Published on Web 11/10/2004

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optical dispersion.9,10 Lead dimethacrylate copolymerized with styrene and methacrylic acid afforded a material with an increasing refractive index (up to 1.63) concurrent with decreasing optical dispersion as the Pb2+ loading was increased.11 Polyphosphazenes possessing a -PdN- backbone showed a refractive index up to 1.75 when tailored with brominated naphthyl groups.12 Polymetallocenes are a class of inorganic polymers consisting of an alternating ferrocene unit and a main group element (E) substituted with aryl or alkyl groups. These polymers are readily available by ring-opening polymerization (ROP) techniques.13 It can be expected that the high concentration of inorganic elements in the backbone of polymetallocenes would lead to their high refractive indices.14 The possibility of coupling their optical properties with the redox switching properties of the Fe center is a promising feature in the fabrication of polyferrocene responsive optical devices. For instance, a photonic crystal fabricated with polyferrocenylsilane (PFS) as the matrix material demonstrated a rapid chemomechanical response to applied potential, thereby producing a tunable optical stopband.15 The refractive index response of PFS to NH3 and CO2 has also been used to develop tapered optical fibers with sensing capabilities.16 Despite the potential of using polyferrocenes for optical applications, their optical properties have not yet been investigated in detail. Herein, we report the optical properties of polymetallocenes, such as the refractive index, molar refraction, and optical dispersion. (Polymetallocenes exhibit Laporte forbidden d-d transition near λ ) 440 nm for polyferrocenes and λ ) 320 nm for polyruthenocenes. This transition is very weak and it does not significantly change the polymer refractive index at the wavelength 589 nm.) Since the synthetic route to preparing polymetallocenes allows easy tailoring of the spacer element (E) and the substituent groups (R) by choosing suitable organometallic precursors as described by Scheme 1, we devised our study to explore the role of E and R on the optical properties of these polymers.14 Our purpose was twofold: to show the tunability of the optical properties achieved by molecular tailoring and to elucidate the relationship between molecular structure and optical properties of polymetallocenes. The latter provides a basis for designing new polymetallocene materials for optical applications. Experimental Section Synthesis. All reactions were performed in a N2-atmosphere using either standard Schlenk techniques or a N2(9) Lu, C. L.; Cui, Z. C.; Wang, Y. X.; Yang, B.; Shen, J. C. J. Appl. Polym. Sci. 2003, 89, 2426. (10) Okubo, T.; Kohmoto, S.; Yamamoto, M. Pure Appl. Chem. 1998, A35, 1819. (11) Lin, Q.; Yang, B.; Li, J.; Meng, X.; Shen, J. Polymer 2000, 41, 8305. (12) Olshavsky, M. A.; Allcock, H. R. Macromolecules 1995, 28, 6188. (13) Kulbaba, K.; Manners, I. Macromol. Rapid Commun. 2001, 22, 711. (14) for preliminary results, see our communication: Paquet C.; Cyr P. W.; Kumacheva E.; Manners I. Chem. Commun. 2004, 2, 234. (15) Arsenault, A.; Mı´guez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. Adv. Mater. 2003, 15, 503. (16) Espada, L. I.; Shadaram, M.; Robillard, J.; Pannell, K. H. J. Inorg. Organomet. Polym. 2000, 10, 169.

Paquet et al. Scheme 1. Synthesis of Polyferrocenes by Ring Opening Polymerization (ROP) (a) for Polyferrocenes Containing Group 14 Spacer Element (where E)Si, Ge, or Sn and R′ and R”) Alkyl or aryl) and (b) Polyferrocenylphosphines

atmosphere glovebox. Solvents were dried and deoxygenated by standard methods. The [1]ferrocenophane monomers of polymers 1a-1d, 1f, 2, 3a-3b, 4b, 5b, and 7 were prepared as described elsewhere.13,17-19 Preparation of other [1]ferrocenophane monomers is described below. Polyferrocenylsilane polymers 1a-1d were prepared by transition metal catalyzed ROP of [1]silaferrocenophane monomers, whereas 1f underwent ROP for 90 min at 150 °C in a sealed evacuated Pyrex tube. The polyferrocenylgermanes were prepared by thermal polymerization of [1]germaferrocenophane in toluene at 80 °C.17,20 The [1]stannaferrocenophanes underwent ROP in the melt in a sealed evacuated Pyrex tube for 30 min at 150 °C to give polyferrocenylstannane 3a and 3b.18,19 Polyferrocenylphosphine, 4b, was prepared by anionic ROP with n-BuLi in THF followed by quenching with H2O. Polyferrocenylphosphinesulfides were prepared by sulfurizing polyferrocenylphosphine with S8.21 The poly(ruthenocenyldimesitylstannane), 7, was polymerized by thermal ROP as described elsewhere.22 Monomer Characterization. Elemental analyses were performed by Quantitative Technologies Inc. (Whitehouse, NJ) or in the Analest facility in the Department of Chemistry at the University of Toronto. NMR spectra were recorded on either a Varian Unity 400 MHz (399.76 MHz for 1H, 100.52 MHz for 13C, 79.42 MHz for 29Si), or a Mercury 300 MHz (300.05 MHz for 1H, 75.46 MHz for 13C, 121.46 MHz for 31P) spectrometer. Chemical shifts are reported relative to residual proton solvent peaks (1H, 13C) and relative to external TMS (29Si) and P(OMe)3 (31P: δ 141.0 vs external 85% aqueous H3PO4). Mass spectra were obtained with a VG 70-250S mass spectrometer operating in electron impact (EI) mode. Synthesis of New Monomers. Methyl(p-Bromophenoxy)[1]Silaferrocenophane (fcSiMe(p-OC6H4Br)). Triethylamine (NEt3) (4.80 mL, 34.44 mmol) was added to a bright orange solution of fcSiMeCl (1.13 g, 4.29 mmol) and p-bromophenol (0.75 g, 4.33 mmol) in Et2O (75 mL) leading to rapid formation of a light orange-red suspension. The suspension was stirred for 4 h followed by removal of the solvent in vacuo. The red residue was extracted with hexanes, filtered through Celite, concentrated, and then cooled to -55 °C to yield fine orange crystals that were collected and dried under vacuum. Yield: 1.06 g (62%). 1H(C6D6): δ 0.41, s (3H, Si-CH3), 3.79, m (2H, Cp-H), 4.07, m (2H, Cp-H), 4.35, m (2H, Cp-H), 4.39, m (2H, Cp-H), 6.87, d JHH ) 7.98 Hz (2H, Ph-H), 7.18, d JHH ) 7.98 Hz (2H, Ph-H). 13C{1H} (C6D6): δ -2.5 (Si-CH3), 36.2 (ipso Cp), 74.8 (Cp), 76.2 (Cp), 78.4 (Cp), 79.1 (Cp), 115.3 (Ph), 122.6 (Ph), 133.3 (Ph), 154.4 (Ph). Anal. Calcd for C17H15BrFeOSi: (17) Peckham, T. J.; Massey, J. A.; Edwards, M.; Manners, I.; Foucher, D. A. Macromolecules 1996, 29, 2396. (18) Ja¨kle, F.; Rulkens, R.; Zech, G.; Foucher, D. A.; Lough, A. J.; Manners, I. Chem. Eur. J. 1998, 4, 2117. (19) Baumgartner, T.; Ja¨kle, F.; Rulkens, R.; Zech, G.; Lough, A. J.; Manners, I. J. Am. Chem. Soc. 2002, 124, 10062. (20) Foucher, D. A.; Edwards, M.; Burrow, R. A.; Lough, A. J.; Manners, I. Organometallics 1994, 13, 4959. (21) Peckham, T. J.; Massey, J. A.; Honeyman, C. H.; Manners, I. Macromolecules 1999, 32, 2830. (22) Vogel, U.; Lough, A. J.; Manners, I. Angew. Chem., Int. Ed. 2004, 43, 3321.

Rationalized Approach to Molecular Tailoring C, 51.16; H, 3.79. Found: C, 50.81; H, 3.87. HRMS: Calcd for C17H15Br56FeOSi: 397.942 494. Found: 397.941 588. p-t-Butylphenyl[1]Phosphaferrocenophane (fcP(p-C6H4tBu)). To a suspension of dilithioferrocene‚2/3TMEDA (10.0 g, 36.3 mmol) in hexanes (250 mL) at -78 °C, dichloro(p-tbutylphenyl)phosphine (8.51 g, 36.2 mmol) was added dropwise. The mixture was slowly warmed to room temperature and stirred vigorously for 4 h. The resulting red suspension was filtered to remove LiCl, and then the solvent was removed from the clear red filtrate under vacuum. The red residue was extracted with toluene, filtered, and the product crystallized at -55 °C. After recrystallization from toluene, the product was isolated and dried in vacuo. Yield: 6.3 g (50%). 31P{1H} (C6D6): δ 12.8, s. 1H (C6D6): δ 1.23, s (9H, (CH3)3), 4.18, s (4H, Cp-H), 4.35, s (2H, Cp-H), 4.55, s (2H, Cp-H), 7.27, dd (2H, 3 JHH ) 8.40 Hz, 4JPH ) 1.65 Hz, m-Ph protons), 7.63, dd (2H, 3 JHH ) 8.40 Hz, 3JPH ) 6.30 Hz, o-Ph protons). 13C{1H}(C6D6): δ 19.2, d (JPC ) 54.9 Hz, ipso-Cp), 35.0, s (C(CH3)3), 77.5, d (JPC ) 7.6 Hz, Cp), 77.6, d (JPC ) 77.6 Hz, Cp), 78.0, d (JPC ) 6.8 Hz, Cp), 78.5, d (JPC ) 1.5 Hz, Cp), 126.3, d (3JPC ) 13.7 Hz, m-Ph), 131.0, d (2JPC ) 13.7 Hz, o-Ph), 134.8, d (JPC ) 9.9 Hz, ipso-Ph), 151.0, s (p-Ph), 31.7,s (C(CH3)3). Anal. Calcd for C20H21FeP: C, 68.99; H, 6.08. Found: C, 68.95; H, 6.01. HRMS: Calcd for C20H2156FeP: 348.073 029. Found: 348.072 857. Synthesis of New Polymers. Poly(Ferrocenylmethyl(pBromophenoxy)Silane) (1e). A solution of fcSiMe(p-OC6H4Br) (0.5 g, 1.25 mmol) and Karstedt’s catalyst (0.15 mol % of Pt) in toluene (10 mL) was stirred for 18 h at room temperature. The polymer was then collected by precipitation into hexanes and dried in vacuo. Yield: 0.35 g (70%). 1H (CD2Cl2): δ 0.68, s (3H, CH3), 4.07, br s (Cp-H), 4.12, m (Cp-H), 4.18, br s (CpH), 4.30, m (Cp-H), 6.15, d JHH ) 8.85 Hz (2H, Ph-H), 7.27, d JHH ) 8.85 Hz (2H, Ph-H). 13C{1H) (CD2Cl2): δ -1.2 (CH3), 68.9 (ipso-Cp), 72.9 (Cp), 74.5 (Cp), 114.1 (Ph), 122.4 (Ph), 132.7 (Ph), 155.0 (Ph). Poly(Ferrocenylmethylthienylsilane) (1g). To a solution of poly(ferrocenylchloromethylsilane) (108 mg, 0.411 mmol) in THF (8 mL) was added 0.5 mL of a 1 M 2-thienyllithium solution in THF at -78 °C. The reaction mixture was then allowed to warm to room temperature and stirred for 18 h. Methanol (0.1 mL) was then added, the mixture was stirred for a further 1 h, then NEt3 (0.1 mL) was added. After stirring the reaction for another 2 h, the solvent was removed in vacuo. The residue was redissolved in ca. 3 mL of THF, filtered, and then precipitated into hexanes, collected, and dried. The product was isolated after reprecipitation from THF/hexanes and dried under vacuum. Yield: 100 mg (78%). 1H (CDCl3): δ 0.73, br s (3H, CH3), 4.33-3.97, br m (Cp-H), 7.21, s (1H, thienyl), 7.33, s (1H, thienyl), 7.62, s (1H, thienyl). 13C (CDCl3): δ -1.6 (CH3), 70.1 (Cp), 72.3 (Cp), 72.6 (Cp), 74.3 (Cp), 128.0 (thienyl), 130.8 (thienyl), 135.4 (thienyl), 137.6 (thienyl). 29Si (CDCl3): δ -14.1, s. Poly(Ferrocenyl(p-t-Butylphenyl)Phosphine) (4a). Anionic ring-opening polymerization of fcP(p-C6H4t-Bu) was achieved using butyllithium as initiator. Thus, after addition of 30 µL of 1.6 M BuLi to a THF solution of fcP(p-C6H4t-Bu) (0.46 g, 1.32 mmol), the solution was stirred for 30 min, then H2O was added to quench the reaction. The polymer was isolated by precipitation into hexanes. Yield: 0.44 g (95%). 31P{1H} (C6D6): δ -32.6, s. 1H (C6D6): δ 1.23, s (9H, (CH3)3), 3.804.60, br m (8H, Cp-H), 7.24, s (2H, Ph-H), 7.68 (2H, Ph-H). Poly(Ferrocenyl(p-t-Butylphenyl)Phosphine Sulfide) (5a). A solution of 4a (0.20 g, 0.57 mmol) and S8 (0.02 g, 0.62 mmol) in CH2Cl2 (15 mL) was stirred for 24 h at RT. The solvent was then removed at the pump and the residue was extracted with THF and filtered. Addition of the resulting filtrate to hexanes afforded the sulfurized polymer as a fine yellow powder in nearly quantitative yield. 31P{1H} (C6D6): δ 37.3, s. 1H (C6D6): δ 1.23, s (9H, (CH3)3), 3.80-5.20, br m (8H, Cp-H), 7.38, s (2H, Ph-H), 7.97 (2H, Ph-H). Poly(ferrocenyl(p-t-Butylphenyl)Phosphine Selenide) (6a). Polymer 4a (0.07 g, 0.20 mmol) and selenium powder (0.36 g, 0.45 mmol) in CH2Cl2 (15 mL) were stirred for 48 h at RT, and then the solvent was removed in vacuo. The residue was

Chem. Mater., Vol. 16, No. 24, 2004 5207 Table 1. Molecular Weight and Polydispersity Index (PDI) of Polymetallocenes 1a 1b 1c 1d 1e 1f 1g 2

Mn (g/mol)

PDI

59 300 143 000 54 600 147 800 97 700 522 900 247 300 161 900

7.2 2.1 3.3 1.9 2.6 1.7 5.3 2.2

3a 3b 4a 4b 5a 5b 6a 6b 7

Mn (g/mol)

PDI

71 200 116 900 10 000 11 000 9,700 19 200 8,700 3,900 270 000

2.0 2.4 1.3 1.3 1.2 1.0 1.3 1.9 2.3

extracted with THF and then filtered twice through Celite. The product was isolated by precipitation of the filtrate into hexanes and dried under vacuum. Yield: 0.04 g (47%). 31P{1H} (CDCl3): δ 26.0, s. 1H (C6D6): δ 1.20, m (9H, (CH3)3), 4.15-5.00, br m (8H, Cp-H), 7.30, br m (2H, Ph-H), 8.03, br m (2H, Ph-H). Poly(Ferrocenylphenylphosphine Selenide) (6b). Polymer 4b (0.14 g, 0.48 mmol) and selenium powder (0.04 g, 0.51 mmol) in CH2Cl2 (50 mL) were stirred for 18 h at RT, and then the solvent was removed in vacuo. The residue was extracted with THF and then filtered twice through Celite. The product was isolated by precipitation of the filtrate into hexanes, and dried under vacuum. Yield: 0.10 g (58%). 31P{1H} (CDCl3): δ 27.3, s. 1H (C6D6): δ 3.98-5.73, br m (8H, Cp-H), 7.46, br s (3H, Ph-H), 7.80, br s (2H, Ph-H). Polymer Characterization. All polymers were characterized by 1H NMR and gel permeation chromatography (GPC). Molecular weights were determined by GPC using a Viscotek GPC MAX liquid chromatograph equipped with a Viscotek Triple Detector Array. The triple detector array consisted of a deflection refractometer, a four-capillary differential viscometer, and a right angle laser light scattering detector (λ0 ) 670 nm). For polyferrocenylsilane homopolymers, it has been shown that there is excellent agreement between Mw obtained from GPC with triple detection and absolute Mw obtained from low angle laser light scattering.24 Conventional calibration was also used, and molecular weights were determined relative to polystyrene standards purchased from American Polymer Standards. In both cases, a flow rate of 1.0 mL/min was used with ACS grade THF as the eluent. The molecular weights (Mn) of polymetallocenes studied herein are listed in Table 1. Film Preparation. Polymetallocene films were prepared by spin coating 3.5 ( 1.5 wt % polymer solutions in chlorobenzene or toluene on single-side polished silicon substrates. The films were processed in an oven at 150 °C for 1 h to eliminate residual solvent and then were rapidly cooled to avoid potential crystallization of the symmetrically substituted polyferrocenes, (1c, 2, and 3a).13,23 The film thickness and surface roughness were determined using a Dektak profilometer. Film thickness was from 80 to 400 nm with RMS roughness of approximately 10 nm. Refractive Index Measurement. A Sopra (GES-5) spectroscopic ellipsometer with a wavelength tunability from 250 to 2000 nm acquired data in units of electronvolts, using the analyzer in a previous tracking mode and with the integration time for each data point being determined by either the minimum threshold of 2 000 000 detector counts or 10 s. The Levenberg-Marquardt regression algorithm was used to find the minimum difference between the acquired Ψ and ∆ curves and the data generated from an optical model. The model consisted of three layers: a homogeneous crystalline Si substrate, a 2 nm SiO2 layer, and the polymer film. The polymer layer was described by a Cauchy dispersion law with one Lorentz peak superimposed. Starting parameters included (23) Foucher, D. A.; Ziembinski, R.; Tang, B.-Z.; Macdonald, P. M.; Massey, J.; Jaeger, C. R.; Vancso, G. J.; Manners, I. Macromolecules 1993, 26, 2878. (24) Massey, J. A.; Kulbaba, K.; Winnik, M. A.; Manners, I. J. Polym. Sci., Polym. Phys. 2000, 38, 3032.

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the thickness of the film found using profilometry, the peak position and the width for the d-d transition of the polymer.23 For each polymetallocene derivative, three samples of different thickness were analyzed. Absorbance Measurement. Absorbance spectra of polymetallocenes in chlorobenzene were measured using a Varian Cary 5000 UV-vis-near-IR spectrophotometer with a spectral bandwidth of 2 nm. Calculations of Optical Parameters. Refractive Index. For each polyferrocene, the value of RI, quoted at λ ) 589 nm, was the average of the refractive indices found from the regression of three separately acquired Ψ(λ) and ∆(λ) curves. Molar Volume. Provided that the end-group effect of polymers is insignificant, the sequential nature of polymers allows the density to be defined in terms of the molar mass (MM) and volume (VM) of the repeating unit.4

F)

MM VM

Table 2. Refractive Index and Standard Deviation of Polymetallocenes Studied

(2)

The theoretical prediction of density (F) of polymers therefore involves calculations using the molar mass and the molar volume of the repeating unit from the constituent atoms. However, calculation of molar volumes by the addition of volumes of the constituent atoms is invalid, since it does not take into account the reduction in volume during bonding. A corrected van der Waals volume (VW,A), accounting for the reduction in atomic volume upon bonding is described by eq 3 for an atom A with radius R:

VW,A ) NA

[

4π 3 R 3

hi ) R -

∑ πh

2 i

(

R-

)]

hi 3

li R2 ri2 + 2 2li 2li

(3)

(4)

where NA is Avogadro’s number, r is the radius of atom i, covalently bonded to A, and li is the bond distance between the atoms A and i. A tabulation of VW is provided by Van Krevelen for common organic structural groups.4 The correlation between VW and VM of repeating units of polymers (calculated from density data) demonstrates a reproducible linear relationship where VM/VW is 1.60(0.045.4 The molar volumes of polyferrocenes were calculated using the principle described above. As a starting point, the density of poly(methylphenylferrocenylsilane) (1c) was determined by Porous Material Inc. using pycnometry and found to be 1.305 ( 0.018 g/cm3. The Vw of 1c was then calculated using eq 2 and the relationship VM/VW ) 1.60. Next, Vw,1c was reduced to the molar volume of the ferrocene unit (fc), VW,fc, by subtracting the Vw of a phenyl group, a methyl group, and a Si atom bonded to four carbon atoms (eqs 3 and 4).25 Using VW,fc, VW,E (determined from eqs 3 and 4), and VW,R (tabulated by Van Krevelen),4 the VW of repeating units of polyferrocene were found and converted to VM. Molar Refraction. Molar refraction is a molecular parameter describing the refractive power of a structural group. Using the Lorentz-Lorenz equation, Van Krevelen demonstrated the validity of using Goedhart’s molar refraction values of organic structural groups for amorphous polymers.4 The molar refraction for polyferrocene repeating units (RM) was therefore determined using eq 1, VM, and the value of the refractive index at λ ) 589 nm (see Table 1). Since molar refractions are additive terms (eq 1), the molar refraction of the fcE backbone unit (RfcE) was calculated by subtracting molar refractions of the substituent groups (tabulated by Van Krevelen) from the molar refraction of the repeating unit (RM).4 Abbe´ Number. To examine the optical dispersion of polyferrocenes, the Abbe´ number, a parameter characterizing the changes in the refractive index with wavelength, was calcu(25) Batsanov, S. S. J. Mol. Struc.-Theochem. 1999, 468, 151.

lated. The value of the Abbe´ number (vD), calculated by taking the refractive index of the material at the wavelengths 486, 656, and 589 nm (nF, nC, and nD, respectively), increases with decreasing optical dispersion.1

vD )

nD - 1 nF - nC

(5)

Results and Discussions Molecular Tuning of RI of Polyferrocenes. Table 2 gives the structures of polyferrocenes, their refractive indices at λ ) 589 nm, and the standard deviation. The polymers are classified according to the spacer element (E) in the backbone. For polyferrocenylsilanes (PFSs) (1a-1g), the variation of the refractive indices of 1.599 to 1.696 can be attributed to the variation in substituent groups. According to eq 1, both the molar refraction (RM) and molar volume (VM) of the substituent contribute to the RI values; the larger the RM or the smaller the VM, the higher a RI. Therefore, the lower RI value of the ethyl-substituted PFS (1b) in comparison to the methylsubstituted (1c) PFS, that is n ) 1.663 versus n ) 1.678, respectively, shows that the effect of molar volume is larger than the effect of molar refraction. The PFS series therefore demonstrate that to effectively increase the refractive index, high molar refraction and low molar volume substituent groups are required. This effect is also clearly demonstrated by comparing the refractive indices of the polyferrocenylphosphines 4a (n ) 1.663) and 4b (n ) 1.737), where the additional bulky tert-butyl group of 4a lowers the RI by 4% in comparison to 4b. The effects of the spacer element on the refractive index were examined by comparing polyferrocene polymers with the same substituent groups but a different

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Table 3. Molar Volumes (VM) and Molar Refraction (RM) of the Polyferrocenes Repeating Unit and the Molar Refraction of the Backbone Repeating Unit (RfcE)

1a 1b 1c 1d 2 3a 3b 4a 4b

VM (cm3/mol)

RM (cm3/mol)

RfcE (cm3/mol)a

average RfcE (cm3/mol)

227 198 182 233 186 289 401 284 213

74.4 73.3 68.5 88.2 71.1 104 148 105 85.5

57.2 57.4 57.2 57.1 59.0 64.3 62.8 59.1 59.6

57.2

59.0 63.6 59.3

a Our estimation of the error in the values of V gives variation M in the values of RfcE on the order of ∼1.1%.

Figure 1. Variation in the molar refraction of the backbone repeat unit with the polarizability of the spacer element E (RE). The circles represent group 14 spacer elements and the square represents the group 15 element, P.

spacer element such as 1c and 2, 5a and 6a, or 5b and 6b. Comparison of the RI of these pairs shows that n1c < n2, n5a < n6a, and n5b < n6b. These series demonstrates that the larger the polarizability of the spacer group (RE), (RSi < RGe and RPdS < RPdSe), the higher the refractive index.26 We stress that polyferrocenylphosphines show particularly high refractive indices for nonconjugated polymers. For instance, a refractive index value of 1.747 was measured for 6b. The results shown in Table 2 indicate that two approaches exist for tuning the refractive index of polyferrocenes: either by varying the substituent groups and/or varying the spacer element. In the first approach, molecular tailoring by varying the substituent groups requires a consideration of both their molar volumes and their molar refractions. The second approach involves modifying the spacer element where increasing atomic polarizability of the spacer element correlates with an increase in the refractive index. Molar Refraction. Molar volumes (VM) and molar refractions (RM) for each repeating unit, calculated for polyferrocene derivatives as described in the Experimental section, are given in Table 3 (columns 1 and 2, respectively). The molar refraction of the backbone repeating unit (RfcE) (column 3, Table 3) was obtained by subtracting the molar refraction of the substituent groups from RM. We calculated values of RfcE of 57.2, 59.0, 63.6, and 59.3 cm3/mol for fcSi, fcGe, fcSn, and fcP, respectively. In comparison to other inorganic polymers, the two atom backbone repeating unit of polyphosphazenes, -PdN-, has a molar refraction of 14.36 cm3/ mol.5 It is also noteworthy that the values of molar refractions of the fcE units are approximately in the same range as for repeating units of conjugated polymers (R ∼30-80 cm3/mol). 6 The usefulness of RfcE values lies in the ability to predict the refractive index of polyferrocenes tailored with various substituent groups. Inspection of RfcSi values demonstrates that the additivity principle, expressed by eq 1, is maintained for polyferrocenes. The accuracy of RfcSi of 57.2 ( 0.2 cm3/mol, obtained by subtracting molar refractions of the substituent groups from RM of four different polyferrocenylsilanes, validates compliance with the additivity principle of molar refraction. The closeness in the values of RfcSn calculated from 3a and 3b (likewise for RfcP from 4a and 4b) indicates

that the additivity principle is also upheld for these families of polyferrocenes. In view of the above, it is instructive to consider polymers 3a and 3b, which show a relatively low refractive index value (1.639 and 1.662, respectively) because of the bulky substituents (high VM) on the Sn atom despite the fact that the value of RfcSn is high (63.6 cm3/mol). Thus, on the basis of our findings, we predict that a higher value of RI of 1.73 can be achieved by tailoring polyferrocenylstannes with two methyl substituents (low molar volumes), or a RI of 1.82 can be achieved by tailoring with two naphthyl substituents (high molar refraction). Polarizability of E versus the RfcE. To further explore the role of the spacer element on the molar refraction, we examined the relationship between RfcE and atomic polarizibility of E (RE). We used the relationship between the polarizability and molar refraction of a medium, expressed by the Lorentz-Lorenz equation,

(26) Lide, D. R. CRC Handbook of Chemistry and Physics; 76th ed.; CRC Press: Boca Raton, FL, 1995.

(27) Aroney, M.; LeFe`vre, R. J. W.; Somasundaram, K. M. J. Chem. Soc. 1960, 1812.

4π 3

N AR ) R M )

∑i Ri

(6)

where NA is Avogadro’s number, RM is the molar refraction of the repeating unit, and Ri is the molar refraction of the functional groups.4 In eq 7, the molar refraction of the fcE unit, RfcE, is expressed as a function of the polarizability of the fc (Rfc) and E (RE) moieties. In Figure 1, the variation of RfcE as a function of RE changes linearly for group 14 elements, giving a straight line regression slope of 2.77 × 1024 mol-1. The experimental slope is similar to the expected slope of 4/3πNA ) 2.52 × 1024 mol-1.

RfcE ) Rfc + RE )

4π N (R + RE) 3 A fc

(7)

The y-intercept gives a Rfc value of 42.3 cm3/mol for ferrocene in polyferrocenes; this value is comparable to a previous value of 50.4 cm3/mol found for molecular ferrocene by Aroney et al.27 The strong deviation of RfcP from group 14 linearity points to an error in using RE that assumes that the spacer elements are in an unbound state.26 In contrast with group 14 elements, the P-center in polyferrocene series 4 is trivalent with a nonbonding pair of electrons. Therefore, for P the higher molar refraction per polar-

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Paquet et al.

Figure 2. Variation in the Abbe´ number (vD) with the RI of selected polyferrocenes. The hatched area marks the region of Abbe´ numbers and RIs for inorganic glass and the dotted area for organic polymers. Table 4. Comparison of the RI and Abbe´ Number of a Polyruthenocene and a Polyferrocene 7 3b

n589 nm

std. dev.

v589 nm

std. dev.

1.662 1.661

0.004 0.008

25.33 24.44

0.19 0.83

izability presumably expresses the stronger propensity of nonbonding electrons to be polarized by an applied electric field. Previous studies of phosphorus compounds showed that their molar refractivity strongly depends on P valency.28,29 Abbe´ Number. The optical dispersion of polyferrocenes 1-6 was expressed as the Abbe´ number, vD (eq 5). To demonstrate the optical merits of polyferrocenes, we adapted the data of Dislich1 for the relationship between the Abbe´ number (optical dispersion) and the refractive indices for inorganic glasses and organic polymers. In Figure 2, the dotted and the hatched areas show the range of Abbe´ numbers and the corresponding refractive indices for purely organic polymers and for inorganic glasses, respectively. The dotted area illustrates the relationship between RI and optical dispersion for organic polymers: the higher the RI, the lower the value of vD (i.e., higher optical dispersion). We were able to compare polyferrocenes with organic polymers in two ways. First, for the same RI value of n ) 1.66 shown by the vertical dashed line, polyferrocenes generally have a substantially higher Abbe´ number (lower dispersion) than organic polymers. Second, for the same optical dispersion, shown by the horizontal dashed line at vD ) 23, polyferrocenes possess significantly higher values of RI than organic polymers. Thus, the main group spacer elements prove to be efficient in increasing the refractive index of a polymer without increasing its optical dispersion. Overall, on the basis of comparison of refractive indices and optical dispersion, polyferrocenes have an intermediate position between organic polymers and inorganic glasses. Polyruthenocenes versus Polyferrocenes. We have further examined the optical properties of poly(ruthenocenyldimesitylstannane),22 the ruthenocene analogue of poly(dimesitylferrocenylstannane) (Table 2, 7 and 3b, respectively). Table 4 compares the RI and Abbe´ numbers of 7 and 3b. Similar refractive indices for 7 and 3b indicate that the molar volume increase of ruthenocene in comparison to ferrocene is balanced by (28) Sayre, R. J. Chem. Soc. 1958, 80, 5438. (29) Jones, W. J.; Davis, W. C.; Dyke, W. J. C. J. Phys. Chem. 1933, 37, 583.

Figure 3. Comparison of the refractive index (left axis), measured by ellipsometry, and the absorbance (right axis), measured by spectrophotometry, of 7 (light line) and 3a (heavy line) as a function of wavelength.

the increase in molar refraction of ruthenocene. The Abbe´ numbers of 3b and 7 are also similar, suggesting that optical dispersion of polymetallocenes originates from electronic transitions that are independent of the metal center, such as the π f π* of the cyclopentadienyl rings. This effect is expected since the lowest allowed electronic transition of metallocenes possessing high absorptivity is the π f π* excitation. A critical advantage of polyruthenocenes in the context of their optical application originates from their absorption properties. Figure 3 shows the absorbance spectra and the RI spectra as functions of wavelength for 7 and the polyferrocene analogue 3b. While the difference in RI and optical dispersion is insignificant, the absorbance spectrum of 7 shows that the polymer does not possess electronic transitions in the visible range.22 The lowest energy transition of d6-metallocene, a spin-allowed Laporte forbidden d f d transition, occurs at λ ∼ 320 nm for ruthenocene,30 making polyruthenocenylstannanes colorless. However, the d f d transition of polyferrocenes, although very weak ( ) 88 M-1 cm-1),23 occurs at λ ∼ 440 nm, giving the polymers an orange color and potentially leading to optical loss in optical applications using the visible spectral range. Conclusion We have explored the optical properties of polymetallocenes and their relation to polymer molecular structure. The characterization of the refractive index of polyferrocenes with various substituent groups and spacer elements showed that polyferrocenes have exceptionally high refractive indices with relatively low optical dispersion. Using values of refractive indices at λ ) 589 nm, we were able to determine the molar refraction of the polymer repeating units, as well as the molar refraction of the fcE unit. The RfcE was found to conform to the additivity principle of molar refraction and therefore can be used to predict the refractive index of molecularly tailored polyferrocenes using the LorentzLorenz equation. The structure-property relationship of the fcE unit was elucidated by exploring the correlation between RfcE and RE. The lone pairs present in trivalent phosphorus centers in polyferrocenylphos(30) Sohn, Y. S.; Hendrickson, D.; Gray, H. B. J. Am. Chem. Soc. 1971, 93, 3603.

Rationalized Approach to Molecular Tailoring

phines were found to result in higher molar refractions than those for analogous materials based on group 14 elements and therefore afforded polyferrocenylphosphines with higher refractive indices. A study of the Abbe´ numbers of polymetallocenes showed that they possess lower optical dispersion than organic polymers for the same values of refractive index. We also demonstrated a potential advantage of polyruthenocenes as materials for optical applications. Thus, poly(ruthenocenyldimesitylstannane) possesses a high refractive

Chem. Mater., Vol. 16, No. 24, 2004 5211

index (n ) 1.66) and also the absence of electronic transitions in the visible range. Acknowledgment. E.K. and I.M. thank the Canadian Government for Canada Research Chairs. C.P. thanks NSERC for a PGS A and is grateful to Sara Bourke for providing polymer 2, Dr. Thomas Baumgartner for polymers 3a and 3b, Dr. Ulf Vogel for polymer 7, and Hong Zhang for GPC measurements. CM049403E