Macromolecules 1997, 30, 417-426
`
`417
`
`Fabrication of Quantum Dot/Polymer Composites:
`Phosphine-Functionalized Block Copolymers as Passivating
`Hosts for Cadmium Selenide Nanoclusters
`
`D. E. Fogg,† L. H. Radzilowski,‡ R. Blanski,† R. R. Schrock,*,† and
`E. L. Thomas‡
`Departments of Chemistry and Materials Science and Engineering, Massachusetts Institute
`of Technology, Cambridge, Massachusetts 02139
`Received July 25, 1996; Revised Manuscript Received November 26, 1996X
`
`ABSTRACT: Nearly monodisperse CdSe nanoclusters, surface-passivated with a layer of trioctylphosphine
`and trioctylphosphine oxide, have been sequestered within phosphine-containing domains in a diblock
`copolymer. A convergent approach to fabrication of these composites was adopted via independent
`synthesis of nanoclusters and polymer. Diblock copolymers of phosphine- or phosphine oxide-function-
`alized monomers and methyltetracyclododecene (MTD) were prepared by ring opening metathesis
`polymerization using Mo alkylidene initiators. Nanoclusters were prepared by pyrolysis of CdMe2 and
`SedP(octyl)3 in the presence of P(octyl)3 and OdP(octyl)3. An immediate and sustained increase in
`electronic passivation is found for nanoclusters incorporated into octylphosphine-containing polymers.
`In contrast, nanoclusters in pure hydrocarbon or phosphine oxide-containing polymers rapidly lose
`passivation. Films of nanoclusters in a phosphine-containing polymer matrix were static cast from dilute
`solution. Under suitable conditions, the copolymers underwent microphase separation, and the metal
`chalcogenide clusters were predominantly sequestered within the phosphine-containing microdomains.
`The original, highly uniform cluster size distribution was unaffected.
`
`Introduction
`
`Much recent effort has been directed toward the
`fabrication of polymers containing nanometer-sized
`clusters of inorganic semiconductors.1-6 Recent ad-
`vances in the synthesis of highly monodisperse CdSe
`nanoclusters (“quantum dots”),7 coupled with method-
`ologies for the preparation of ROMP polymers with very
`narrow molecular weight distributions,8-11 offer an
`unprecedented level of control over the molecular ar-
`chitecture of the composite materials. Most of the
`reported approaches to such composites involve in situ
`aggregation of labile metal species within a polymer
`matrix. For example, organometallic-hydrocarbon
`diblock copolymers have been prepared by living ring-
`opening metathesis polymerization (ROMP) of methyl-
`tetracyclododecene (MTD) and metal-functionalized nor-
`bornene monomers.12-14 Microphase separation, fol-
`lowed by hydrogenation or pyrolysis, yielded nanometer-
`sized particles of elemental metal within approximately
`spherical microdomains; cluster size was defined by the
`number of monomers within the metal-functionalized
`block. In a related approach, copolymers of MTD and
`donor-functionalized norbornenes were treated with
`labile metal species.3,15,16 Controlled decomposition of
`the polymer-attached metal complex produced metal
`clusters distributed throughout the polymer matrix. The
`size of the metal clusters formed in lamellar or cylindri-
`cal microdomains typically varied in diameter by (20%,
`although evidence suggests that a single cluster can
`form in a suitably small spherical microdomain. Uni-
`form dispersion of nanoclusters in solid media is fre-
`quently hampered by phase separation and cluster
`aggregation within the matrix.
`In these examples
`dispersion is effected by the affinity of the metal for the
`donor groups uniformly distributed within one block.
`
`† Department of Chemistry.
`‡ Department of Materials Science and Engineering.
`X Abstract published in Advance ACS Abstracts, January 15,
`1997.
`
`An alternative, convergent approach is adopted in the
`present work, in which prefabricated clusters are in-
`corporated within a polymer containing phosphine
`donors attached to the polymer backbone. This ap-
`proach permits us to exploit recently developed methods
`for the preparation of CdSe quantum dots with remark-
`ably narrow size distributions (<5% rms in diameter).7
`Novel optical properties are conferred on the composites
`as a result of the uniform, small particle size. The
`length scale of the clusters (maximum diameter (cid:24)50 Å)
`is smaller than the Bohr diameter of the exciton in bulk
`CdSe (112 Å), leading to three-dimensional quantum
`confinement of photogenerated electron-hole pairs.
`Quantum confinement results in the appearance of
`discrete absorption bands, blue-shifted relative to bulk
`CdSe (for which the excited states form a continuum).
`The extent of the shift is very sensitive to cluster size,
`and visible absorption spectroscopy provides a useful
`probe of both cluster size and the size distribution.
`A further, fundamental advance in these materials
`lies in the luminescent properties of the clusters.
`Electronic passivation of the nanoclusters is effected by
`saturating the cluster surface with bulky trioctylphos-
`phine/trioctylphosphine oxide groups, thus preventing
`localization of the carriers in trap states, and sterically
`enforcing segregation of the clusters. The resulting
`overlap between the electron and hole wavefunctions
`of the exciton leads to a high rate of radiative recom-
`bination, and quantum yields of up to 10% are observed
`in room-temperature fluorescence experiments.7
`Our principal interest to date has focused on the
`development of suitable donor-functionalized monomers
`and optimization of their polymerization, as well as the
`cluster-sequestering ability of the resulting polymers.
`In principle, the properties of the host polymer (either
`in a random or block copolymer) can be tailored for the
`intended application. For example, the photoelectronic
`properties of uniformly dispersed nanoclusters could be
`exploited to provide electronic devices within a conduc-
`tive polymer matrix.
`In this work we report the
`
`S0024-9297(96)01103-5 CCC: $14.00 © 1997 American Chemical Society
`
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`418 Fogg et al.
`
`development of copolymers that contain phosphine- and
`phosphine oxide-functionalized norbornene blocks. Quan-
`titative uptake of trioctylphosphine/phosphine oxide-
`capped CdSe quantum dots by these materials is
`described. In the case of the phosphine-functionalized
`polymers, excellent retention of the narrow size distri-
`bution of the clusters is found. Fluorescence is en-
`hanced, indicating an increase in electronic passivation
`in the donor matrix. Transmission electron microscopy
`(TEM) examination of thin films of the composites
`reveals segregation of the clusters within an extended
`network of phosphine-rich domains. Samples micro-
`tomed from the bulk show segregation into spherical
`microdomains. We are currently exploring the potential
`of these materials for exciton delocalization and charge
`transport. Future work will focus on synthesis of
`copolymers containing electron- and hole-carrier seg-
`ments, for the development of new materials for elec-
`troluminescence applications.
`
`Experimental Section
`General. All experiments, unless otherwise noted, were
`performed under a nitrogen atmosphere in a Vacuum Atmo-
`sphere drybox or by using standard Schlenk/vacuum line
`techniques. Hexanes, diethyl ether, and tetrahydrofuran
`(THF) for polymerization were distilled from purple sodium
`benzophenone ketyl under nitrogen and then stored in the
`drybox over activated 4 Å molecular sieves. Pentane was
`washed with 5% nitric acid in sulfuric acid, stored over calcium
`chloride, and then distilled from sodium benzophenone ketyl
`under nitrogen. n-Butanol, cyclohexane, and methylene chlo-
`ride were used as received. Tri-n-octylphosphine oxide (92%)
`and tri-n-octylphosphine were obtained from Johnson Matthey.
`The former was used as received, the latter was distilled under
`Ar prior to use. Selenium shot (1.6 mm, 99.999+%) was used
`as received from Johnson Matthey. Dimethylcadmium was
`obtained from Organometallics, Inc., purified by filtration and
`distillation, and stored under N2 at -40 °C. Methyltetra-
`cyclododecene (MTD) was provided by B. F. Goodrich and was
`purified by vacuum distillation from sodium. 5-Norbornen-
`2-yl-methanol and 1,5-dibromopentane were obtained from
`Aldrich Chemical Co. and used as received. ROMP initiators
`Mo(CHCMe2Ph)(NAr)(O-t-Bu)2 (Ar ) 2,6-C6H3-i-Pr2; Ar¢)2,6-
`C6H3-Me2),17 racemic NORPHOS18 (NORPHOS ) 2-exo-3-endo-
`bis(diphenylphosphino)bicyclo[2.2.1]heptene), and P(NEt2)-
`19 were synthesized according to literature procedures.
`(oct)2
`The synthesis of 2-(chloromethyl)-5-norbornene was carried out
`as described in the literature,20 except at room temperature
`instead of reflux temperature.
`Nuclear magnetic resonance (NMR) spectra were recorded
`on a Varian XL301 (300.0 MHz for 1H, 121.4 MHz for 31P) FT-
`NMR spectrometer. Proton spectra were measured in C6D6
`unless otherwise noted; data are listed in parts per million
`downfield from tetramethylsilane and referenced against the
`residual proton signals from the deuterated solvent ((cid:228) 7.16,
`C6D6) as internal standard. 31P NMR spectra are referenced
`against 85% H3PO4 as an external standard. Downfield shifts
`are taken as positive for all nuclei. Gel permeation chromato-
`graphic (GPC) analysis was carried out at room temperature
`employing a Rheodyne Model 7125 sample injector, a Kratos
`Spectroflow 408 pump, two Jordi-Gel DVB mixed bed columns
`in series, and a Viscotek differential refractometer/viscometer
`H-500 on samples 0.1-0.3% (w/v) in THF, which were filtered
`through a Millex-SR 0.5 (cid:237)m filter in order to remove particu-
`lates. GPC columns were calibrated versus commercially
`available polystyrene standards (Polymer Laboratories Ltd.)
`ranging from 1206 to 1.03 (cid:2) 106 MW. Fluorescence experi-
`ments were performed on a SPEX Fluorolog-2 spectrometer,
`using front face collection with 0.4 mm slits. Fluorescence
`spectra for (cid:24)37 Å clusters (absorbance (cid:236)max 536 nm) were
`collected between 480 and 600 nm with 436 nm excitation, and
`for (cid:24)45 Å clusters (absorbance (cid:236)max 578 nm), between 530 and
`640 nm with 478 nm excitation. Optical absorption spectra
`
`Macromolecules, Vol. 30, No. 3, 1997
`
`were obtained at room temperature on a Hewlett-Packard
`8452A diode array spectrometer, using 1 cm quartz cuvettes.
`Transmission electron microscopy (TEM) was performed on a
`JEOL 200 CX in bright field at 100 kV.
`Nanocluster Synthesis. CdSe quantum dots were pre-
`pared by pyrolysis of CdMe2 and SedP(oct)3 (oct ) octyl) in
`the presence of P(oct)3 and its oxide, as described by Murray
`et al.7 Slightly higher temperatures (330 °C) were employed.
`Size-selection was effected by three cycles of reprecipitation
`from nBuOH-hexanes (5:1) by dropwise addition of MeOH.
`Clusters were dried in vacuo and stored under N2 in a drybox.
`PCl(oct)2. Ethereal HCl (1 M, 335 mL) was added dropwise
`to an ice-cold solution of P(NEt3)(oct)2 (50 g, 0.152 mol) in
`diethyl ether (300 mL). The mixture was allowed to warm to
`room temperature. It was checked for acidity, filtered through
`Celite in order to remove diethylamine hydrochloride, and then
`stripped to dryness. The crude product was distilled to yield
`27.5 g (62%) of a colorless oil (bp 128-135 °C, 0.4 Torr): 1H
`NMR (cid:228) 1.21-1.51 (m, 28H, CH2), 0.87-0.92 (t, J ) 7 Hz, 6H,
`CH3); 31P{1H} NMR (cid:228) 113.5 (s); 13C{1H} NMR (cid:228) 35.5 (d, JPC )
`28 Hz), 32.2 (s), 31.2 (d, JPC ) 10 Hz), 29.6 (s), 29.57 (s), 24.9
`(d, JPC ) 12 Hz), 23.1 (s), 14.3 (s).
`5-Norbornen-2-yl-CH2P(oct)2 [NBE-CH2P(oct)2, 1]. A
`solution of NBE-CH2Cl (5.60 g, 39.3 mmol) in THF (2 mL) was
`added dropwise to a suspension of Mg powder (1.15 g, 47.1
`mmol) in THF (50 mL). The solution was refluxed for 3 h,
`cooled, and filtered through Celite in the drybox. Titration
`against MeOH (1,10-phenanthroline indicator) indicated 28.3
`mmol of active Grignard, which was added to a chilled solution
`of PCl(oct)2 (8.285 g, 28.3 mmol) in THF (40 mL). The solution
`was stirred for 12 h and then stripped of solvent and extracted
`with pentane. The pentane extract was filtered, stripped, and
`distilled to yield 9.1 g (64%) of 1 as a colorless oil (bp 150 °C,
`0.1 Torr): 1H NMR (cid:228) (endo isomer, (cid:24)80%) 6.11-6.13 (m, 1H,
`olefinic), 6.04-6.07 (m, 1H, olefinic), 2.94 (br s, 1H, bridgehead
`CH), 2.67 (br s, 1H, bridgehead CH), 2.1-2.23 (m, 1H, exo CH),
`1.94-1.98 (m, 1H, exo CH of CH2), 1.17-1.57 (m, 32H, CH2),
`0.88-0.92 (m, 6H, CH3), 0.68-0.78 (m, 1H, endo CH of CH2);
`(exo isomer, partial) 5.94-6.0 (m, 1H, olefinic), 2.71 (br s, 1H,
`bridgehead CH); 31P{1H} NMR (cid:228) -35.3 (s, exo), -35.7 (s, endo).
`5-Norbornen-2-yl-CH2P(O)(oct)2 [NBE-CH2P(O)(oct)2,
`2]. A solution of 30% aqueous H2O2 (3.7 mL, 33 mmol) was
`added dropwise to ice-cooled 1 (6.0 g, 16.5 mmol) in CH2Cl2
`(50 mL) in air. The solution was stirred for 2 h and then
`stripped of solvent. The crude, pale yellow product was
`distilled to give 5.6 g (90%) of 2 as a colorless oil (bp 180 °C,
`1H NMR (cid:228) (endo isomer, (cid:24)80%) 6.09 (m, 1H,
`0.03 Torr):
`olefinic), 6.03-6.04 (m, 1H, olefinic), 2.99 (br s, 1H, bridgehead
`CH), 2.63 (br s, 1H, bridgehead CH), 2.34-2.50 (m, 1H, exo
`CH), 1.88-1.95 (m, 1H, exo CH of CH2), 1.1-1.6 (m, 32H, CH2),
`0.87-0.92 (m, 6H, CH3), 0.55-0.63 (m, 1H, endo CH of CH2);
`(exo isomer, partial) 5.91-5.98 (m, 1H, olefinic), 2.71 (br s,
`1H, bridgehead CH), 2.74 (br s, 1H, bridgehead CH); 31P{1H}
`NMR (121 MHz, C6D6) (cid:228) 49.6 (s, exo), 49.8 (s, endo); 13C{1H}
`NMR (cid:228) (endo isomer) 138.4, 133.1 (C olefinic), 50.2, 48.1, 43.6,
`43.1, 34.8, 32.6, 31.9, 30.0, 29.9, 23.5, 22.4, 14.7; (exo isomer,
`partial) 137.5, 137.1 (C olefinic), 50.1, 49.4, 45.9, 39.9, 34.3,
`33.4, 30.5, 29.6, 29.4, 28.7, 28.5, 27.0.
`5-Norbornen-2-yl-CH2O(CH2)5Br [NBE-CH2O(CH2)5Br,
`3]. Aqueous NaOH (50% w/w, 195 g) was added to a mixture
`of 5-norbornene-2-methanol (50.7 g, 0.408 mmol) and 1,5-
`dibromopentane (169.1 g, 0.735 mol) in cyclohexane (400 mL)
`+Cl- (9.5 g, 23.5 mmol), was
`in air. Aliquat, CH3N[(CH2)7CH3]3
`added as phase-transfer catalyst, and the mixture stirred for
`9 days, or until no unreacted alcohol was observable by 1H
`NMR. The organic layer was then separated, washed with
`H2O (3 (cid:2)100 mL), dried over MgSO 4, and stripped of solvent.
`Fractional distillation under vacuum gave 62.3 g (56%) of a
`clear, colorless oil (bp 125-130 °C, 0.15 Torr) after removal of
`unreacted dibromopentane (bp 65-70 °C, 1 Torr): 1H NMR (cid:228)
`(endo isomer, (cid:24)80%) 6.04-6.09 (m, 1H, olefinic), 5.86-5.88
`(m, 1H, olefinic), 2.93-3.42 (m, 6H, CH2), 2.86 (br s, 1H,
`bridgehead CH), 2.74 (br s, 1H, bridgehead CH), 2.26-2.31
`(m, 1H, exo CH), 1.18-1.89 (m, 9H, CH2), 0.41-0.47 (m, 1H,
`endo CH of CH2); (exo isomer, partial) 6.01-6.02 (m, 1H,
`olefinic), 2.70 (br s, 1H, bridgehead CH); 13C{1H} NMR (cid:228) (endo
`
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`Macromolecules, Vol. 30, No. 3, 1997
`
`Fabrication of Quantum Dot/Polymer Composites 419
`
`polymera
`
`[MTD]300
`[MTD]300[NORPHOS]20
`[MTD]300[NBE-CH2P(O)(oct)2]20
`
`[MTD]300[NBE-CH2O(CH2)5P(oct)2]20
`
`[MTD]300[NBE-CH2O(CH2)5P(O)(oct)2]20
`
`PDIb
`Mn (found)
`1.06
`70 800
`1.16
`35 700
`1.93
`75 000c
`1.59
`2 034 000
`1.05
`53 000d
`1.03
`795 200
`1.07
`81 000
`135 000e
`1.03
`a All polymers were prepared in THF, using Mo(CHMe2Ph)(NAr)(O-t-Bu)2 as initiator. Theoretical MW values are based on 100%
`conversion of monomer and complete consumption of initiator, allowing for PhMe2CCHd and dCHPh end groups. Mn (found) was
`determined by viscometry relative to polystyrene standards. b Polydispersity index, Mw/Mn. c Main peak of bimodal trace; approximate
`relative area 6.5:1. d Main peak of bimdoal trace; approximate relative area 12:1. e Main peak of bimodal trace; approximate relative area
`2:1.
`isomer) 136.8, 132.2 (C olefinic), 49.3, 43.9, 42.1, 38.7, 33.7,
`32.6, 29.1, 28.8, 24.9; (exo isomer, partial) 136.3 (C olefinic),
`44.9, 43.6, 41.5, 38.8, 29.7. Anal. Calcd for C13H21BrO; C,
`57.15; H, 7.75. Found: C, 57.16; H, 8.01.
`5-Norbornen-2-yl-CH2O(CH2)5P(oct)2 [NBE-CH2O(CH2)5-
`P(oct)2, 4]. Neat 3 (10.0 g, 36.6 mmol) was added dropwise
`to a suspension of powdered Mg (1.20 g, 49.4 mmol) in THF
`(50 mL) over a period of 1 h and the addition funnel was rinsed
`with a small quantity of THF. The solution was gently
`refluxed for 12 h and then cooled to room temperature, brought
`into the glovebox, and filtered through Celite. The resulting
`yellow-brown Grignard reagent was titrated as for 1. The
`solution was cooled to -40 °C, and neat PCl(oct)2 (10.72 g, 36.6
`mmol) was added dropwise. The solution was stirred for 3 h,
`filtered through Celite to remove MgCl2, and stripped of
`solvent. The residue was taken up in pentane, refiltered, and
`stripped to a colorless oil (15.6 g, 94%). The product was
`purified by chromatography on neutral Al2O3 (hexanes eluant)
`and distillation (bp 220 °C, 0.03 Torr) to yield 11.4 g (69%):
`1H NMR (cid:228) (endo isomer, (cid:24)80%) 6.03-6.06 (m, 1H, olefinic),
`5.93-5.98 (m, 1H, olefinic), 2.95-3.34 (m, 7H, CH2 and
`bridgehead CH), 2.64 (br s, 1H, bridgehead CH), 2.36-2.38
`(m, 1H, exo CH), 1.11-1.74 (m, 37H, CH2), 0.87-0.91 (m, 6H,
`CH3), 0.41-0.47 (m, 1H, endo CH of CH2); (exo isomer, partial)
`2.82 (br s, 1H, bridgehead CH), 2.68 (br s, 1H, bridgehead CH);
`31P{1H} NMR (cid:228) -31.9 (s); 13C{1H} NMR (cid:228) (endo isomer) 136.9,
`132.0 (C olefinic), 49.2, 49.1, 43.7, 43.3, 41.9, 41.3, 38.5, 33.5,
`33.1, 32.3, 31.6, 28.8, 28.6, 28.5, 26.5, 24.6, 23.8, 19.4, 13.4;
`(exo isomer, partial) 136.3 (C olefinic), 44.7, 44.6, 43.4, 43.0,
`41.5, 41.2, 38.6, 31.7, 31.5, 31.4, 29.5, 29.3, 29.2, 26.6, 12.3.
`5-Norbornen-2-yl-CH2O(CH2)5P(O)(oct)2 [NBE-CH2O-
`(CH2)5P(O)(oct)2, 5]. Aqueous H2O2 (7.5 g, 33 mmol, 30% (w/
`w)) was added drop-wise to a solution of crude 4 (15.0 g, 33.3
`mmol) in CH2Cl2 (50 mL) at room temperature. The mixture
`was stirred for 5 h and then partitioned between CH2Cl2 and
`water. The organic layer was dried (MgSO4), stripped, and
`chromatographed on neutral Al2O3, eluting initially with neat
`hexanes and then with 5% acetone-hexanes. The colorless
`product was distilled under vacuum (235 °C, 0.03 Torr) to yield
`8.6 g (55%): 1H NMR (cid:228) (endo isomer, (cid:24)80%) 6.0-6.1 (m, 1H,
`olefinic), 5.93-5.98 (m, 1H, olefinic), 2.90-3.32 (m, 7H, CH2
`and bridgehead CH), 2.63 (br s, 1H, bridgehead CH), 2.26-
`2.38 (m, 1H, exo CH), 1.1-1.8 (m, 37H, CH2), 0.8-0.95 (m,
`6H, CH3), 0.4-0.5 (m, 1H, endo CH of CH2); (exo isomer,
`partial) 2.8 (br s, 1H, bridgehead CH), 2.68 (br s, 1H,
`bridgehead CH); 31P{1H} NMR (cid:228) 44.2 (s); 13C{1H} NMR (cid:228) (endo
`isomer)136.6, 132.3 (C olefinic), 49.2, 44.0, 42.1, 38.8, 31.7,
`31.1, 31.0, 29.3, 29.1, 28.4, 27.5, 22.6, 21.7, 13.8; (exo isomer,
`partial) 136.4 (C olefinic), 44.9, 43.8, 41.5, 39.0, 29.5, 29.0, 27.9,
`27.7, 21.6, 21.5. Anal. Calcd for C29H55O2P; C, 74.63; H, 11.88.
`Found: C, 74.49; H, 11.98.
`Reaction of Phosphine Monomers with Mo Initiators.
`(a) NBE-CH2P(oct)2 (1, 5 equiv) + Mo(NAr)(CHCMe2Ph)-
`(O-t-Bu)2 (6). A solution of 6 (5 mg, 9 (cid:237)mol) in C6D6 (0.5 mL)
`was added to a solution of 1 (17 mg, 47 (cid:237)mol) in C6D6 (0.5
`mL): 1H NMR (cid:228) 13.2 (m), 12.83 (m), 12.6 (m), 6.04-6.13 (m,
`olefinic, 1), 5.6 (m, ring-open olefin); 31P{1H} NMR (cid:228) 31.4 (s),
`28.4 (s), 21.9 (s), 21.4 (s), -34.9 (s, exo 1), -35.8 (s, endo 1).
`(b) NBE-CH2P(O)(oct)2 (2, 5 equiv). A solution of 6 (5
`mg, 9.1 (cid:237)mol) in C6D6 (0.5 mL) was added to 2 (18 mg, 48
`
`Table 1. Physical Data for Polymers
`% yield
`MW (theory)
`96
`52 500
`98
`61 800
`81
`59 800
`
`98
`
`87
`
`61 500
`
`61 800
`
`(cid:237)mol): 1H NMR (cid:228) 5.6 (br m, ring-open olefin); 31P{1H} NMR (cid:228)
`49.6 (s), 50 (s).
`(c) NBE-CH2O(CH2)5P(oct)2 (4, 5 equiv). A solution of 6
`(1.0 mg, 1.8 (cid:237)mol) in C6D6 (0.5 mL) was added to a solution of
`4 (4 mg, 9 (cid:237)mol) in C6D6 (0.2 mL). Monitoring of the reaction
`by 1H NMR indicated complete consumption of monomer
`within 1 h: 1H NMR (cid:228) 5.6 (br m, ring-open olefin); 31P{1H}
`NMR (cid:228) -31.9 (s).
`General Procedure for Polymer Synthesis. The syn-
`thesis of the [MTD]300[NBE-CH2O(CH2)5P(oct)2]20 diblock co-
`polymer (see Table 1), is given as an example. (The numerical
`subscripts following the monomer name indicate the number
`of equivalents of a monomer that are added to 1 equiv of the
`alkylidene initiator. Previous studies with these initiators
`have shown that in many cases the number of equivalents
`added approximately equal the actual degree of polymerization
`of the individual blocks21). A solution of Mo(CHCMe2Ph)(NAr)-
`(O-t-Bu)2 6 (1.0 mg, 1.82 (cid:237)mol) in THF (1 mL) was added all
`at once to a rapidly stirred solution of MTD (95 mg, 0.545
`mmol) in THF (7 mL). After 1 h, a solution of NBE-CH2O-
`(CH2)5P(oct)2 (16.4 mg, 36.4 (cid:237)mol) in THF (2 mL) was added,
`and the mixture stirred for 2 h before quenching by addition
`of 3 drops of PhCHO. The solution was stirred for 2 h and
`then reduced in volume to 1 mL and added dropwise to
`degassed MeOH (20 mL). The white solid was collected by
`filtration, washed with MeOH, and dried under high vacuum
`for 24 h to yield 104 mg (93%).
`Extent of Incorporation of CdSe Nanoclusters into
`Polymers.
`In a representative experiment, a solution of
`[MTD]300 (50 mg) in THF (2.0 mL) was added to a solution of
`the nanoclusters (10 mg) in THF (1 mL). The mixture was
`stirred for 1 h and the solvent was removed in vacuo.
`Extraction of the residue with 10 mL of pentane yielded a pale
`orange solution, which was diluted to 2.0 mL. The UV-visible
`spectrum of the solution was measured and the concentration
`of nanoclusters in solution calculated by interpolation against
`a Beer’s law plot for the polymer-free nanoclusters.
`Preparation of Samples for TEM Analysis. Samples
`were prepared from solution-cast films in two different ranges
`of thickness: thin films (< 100 nm) and bulk films (0.1 to 1
`mm). For thin films, typically 0.84 mg (42 (cid:237)L of a 20 mg/mL
`solution) of copolymer in THF was diluted with 1.8 mL of THF.
`A solution of the nanoclusters (170 (cid:237)L of a 1 mg/mL solution
`in THF) was added to give 20% w/w cluster to polymer, and a
`final concentration of 0.05 wt % polymer relative to THF. The
`solution was allowed to equilibrate for 24 h. Films were then
`cast by dropping (cid:24)100 (cid:237)L of the solution onto cleaved mica
`coated with (cid:24)10 nm of evaporated carbon. The rate of solvent
`evaporation from the cast solution was reduced by addition of
`3 mL of THF to the casting chamber, which was covered and
`left to stand. After drying (typically within 24 h) and removing
`from the drybox, pieces of the carbon/polymer composite films
`were floated onto deionized water and picked up with 200 mesh
`copper TEM grids. Film thicknesses were estimated to be in
`the range of 30-50 nm based on their silver interference color
`when floating on water. Thin films were also studied in cross
`section. Sample-containing grids (as prepared above) were
`first coated with approximately 2 nm of evaporated carbon on
`both sides, embedded in MedCast epoxy, and then cured at
`70 °C overnight. The carbon coating simultaneously prevents
`
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`420 Fogg et al.
`
`Macromolecules, Vol. 30, No. 3, 1997
`
`Scheme 1. Routes to Dioctylphosphine- and Phosphine Oxide-Functionalized Norbornene Monomers
`
`swelling of the copolymer film by the uncured epoxy and
`enhances adhesion at the epoxy-specimen interface. Cross
`sections were made by cutting directly through the embedded
`sample/grid in a direction normal to the grid plane using a
`Reichert Ultracut E ultramicrotome and a diamond knife at
`room temperature. The resulting sections were approximately
`30-50 nm thick. Bulk films were prepared by casting a
`solution of 5 wt % polymer relative to THF into a ceramic
`crucible, covering, and keeping in a drybox until all solvent
`had evaporated (approximately 7 d). The resulting (cid:24)0.5 mm
`thick films were ultramicrotomed to give 50 nm thick sections.
`
`Results
`Synthesis of CdSe Nanoclusters. Nanoclusters
`with a narrow size distribution, electronically passivated
`by a surface layer of P(oct)3 and its oxide, were prepared
`by the method of Murray et al. (eq 1).7 Pyrolysis of
`
`unclear, but the high oxygen-sensitivity of these meas-
`urements requires rigorous exclusion of air in all
`comparative fluorescence experiments conducted under
`initially anaerobic conditions, such as the passivation
`and some of the polymer-incorporation experiments
`described below. Experiments conducted under aerobic
`conditions need only be corrected for volume before
`emission is remeasured, but are limited to immediate
`comparative measurements.
`Monomer Synthesis and Properties. Three classes
`of phosphine-substituted norbornene monomers were
`investigated. One is the commercially available diphos-
`phine NORPHOS, while the other two are dioctylphos-
`phine derivatives of 5-norbornene-2-methanol. (All such
`2-substituted-5-norbornenes will be abbreviated as
`NBE-R throughout the rest of this paper.) The phos-
`phine-containing monomers are the following:
`
`dimethylcadmium and SedP(oct)3 at (cid:24)330 °C in the
`presence of P(oct)3 and molten OdP(oct)3 induces rapid
`and homogeneous nucleation, as indicated by an instan-
`taneous color change from colorless to orange.
`A sharp drop in temperature ((cid:24)100 °C) accompanies
`initial nucleation. Subsequent growth and annealing
`of the initially formed clusters are effected by reheating,
`during which the size of the nanoclusters is monitored
`by visible spectroscopy. The rate of reheating is critical
`to maintaining a narrow size distribution. ˘oo rapid a
`temperature rise permits formation of new nucleation
`sites, while too slow a rate permits aggregation of
`smaller clusters. In either case the principal absorption
`band broadens, and secondary absorption features lose
`definition or disappear altogether. Refinement of the
`size distribution and removal of solvating phosphine/
`phosphine oxide is effected by three cycles of reprecipi-
`tation (beyond which loss of the passivating layer occurs,
`and fluorescence is diminished). Clusters were dried
`in vacuo and stored under N2 in a drybox, as decreased
`fluorescence was found on prolonged exposure of the
`clusters to air in the solid state, or overnight in solution.
`Cluster degradation by oxidation of Se surfaces to SeO2
`on exposure to air has been described in related work.22
`Excellent retention of fluorescence is found for THF
`solutions of CdSe[OdP(oct)3,P(oct)3] nanoclusters made
`up and stored in the drybox for up to 1 week. An
`unexpected complication arising from this protocol is an
`observed increase in fluorescence intensity as air dif-
`fuses into solution. The cause of this increase is still
`
`Routes to 1 and 4 are shown in Scheme 1. NBE-
`CH2Cl and NBE-CH2O(CH2)5Br (3), prepared by reac-
`tion of the norbornyl alcohol precursor with SOCl2 or
`1,5-dibromopentane, respectively, were converted to
`their Grignard derivatives and then treated with
`PCl(oct)2 to generate the desired phosphines. The
`chlorophosphine itself is prepared by acid deamination
`of Et2NP(oct)2.19 All norbornene derivatives are ob-
`tained as mixtures of endo and exo isomers, which are
`not separated. For 1, two singlets are observed by 31P
`NMR in a ratio of (cid:24)4.5:1. The upfield singlet ((cid:228) -35.3)
`is assigned to the exo isomer; the downfield signal ((cid:228)
`-35.7) to the predominant endo species. For 4, in which
`the phosphorus atom is further from the endo/exo
`center, only one 31P NMR signal is observed. The
`phosphine oxides [NBE-CH2P(O)(oct)2, 2; NBE-CH2O-
`(CH2)5P(O)(oct)2, 5], which are readily obtained by H2O2
`oxidation of the corresponding phosphines, generate 31P
`NMR resonances (cid:24)70 ppm downfield from the phos-
`phine signals.
`
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`Macromolecules, Vol. 30, No. 3, 1997
`
`Fabrication of Quantum Dot/Polymer Composites 421
`
`Scheme 2. Sequential Polymerization of MTD and Phosphine- or Phosphine Oxide-Functionalized Norbornene
`Derivatives
`
`Polymer Synthesis. Syntheses of MTD homopoly-
`mers and MTD-NORPHOS diblock copolymer have been
`described.3,15 Homopolymers of MTD are written as
`[MTD]n, where n represents the number of equivalents
`of monomer used per equivalent of initiator. Almost
`certainly they do not have regular structures, but
`probably a mixture of cis and trans double bonds and
`head-to-head, head-to-tail, and tail-to-tail arrangement
`of the repeat unit.8,21 The structure of poly(norbornene)
`blocks derived from an endo/exo mixture of monosub-
`stituted norbornenes is expected to be similarly complex.
`The structures given for the polymers therefore are
`simplified representations.
`Polymerizations were carried out in THF, using
`Mo(CHCMe2Ph)(NAr)(O-t-Bu)2 (6) as the initiator
`(Scheme 2). Diblock copolymers were prepared by
`sequential addition of MTD and the appropriate phos-
`phine-functionalized monomer to the ROMP initiator.
`Polymerizations were terminated by addition of benz-
`aldehyde, cleaving the polymer chain from the metal
`in a Wittig-like reaction. Polymers were precipitated
`by dropwise addition of the concentrated reaction mix-
`ture to degassed methanol and analyzed by GPC. As
`GPC analysis of [MTD]300 typically shows a unimodal
`molecular weight distribution, with a polydispersity
`index (PDI) of 1.03-1.07, this reaction was used as a
`periodic check on initiator stability and performance.
`NMR studies of the reaction of 6 with several equiva-
`lents of NBE-CH2P(oct)2 (1) revealed that 6 was con-
`sumed completely, but 1 was not. Chelation of the
`initially formed alkylidene to give a coordinatively
`saturated, inactive species, one possible structure of
`which is shown in eq 2, was inferred from the appear-
`ance of 31P NMR singlets due to coordinated phosphorus
`
`((cid:228) (cid:24)22, 28). Addition of PMe3 regenerated the two
`singlet resonances near -35 ppm due to free 1 as a
`consequence of displacement of the chelated phosphorus
`(eq 2). In contrast, NMR studies showed that monomers
`2 and 4 (5 equiv) are rapidly and quantitatively poly-
`merized by 6 in C6D6. Alkylidene R proton resonances
`(11.5-11.7 ppm) completely disappeared upon addition
`of monomer, while olefinic norbornene proton reso-
`nances at (cid:24)6.0 ppm were replaced by resonances at
`(cid:24)5.6 ppm for olefinic protons in the olefin formed upon
`opening the norbornene ring. No 31P NMR signals for
`coordinated phosphine or phosphine oxide were ob-
`served.
`The diminished Lewis basicity of the phosphine oxide
`functionality, relative to phosphine, probably prevents
`chelation in the case of 2. For the long-chain phosphine
`monomer 4, entropic factors disfavor chelate formation.
`
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`422 Fogg et al.
`
`Macromolecules, Vol. 30, No. 3, 1997
`
`Table 2. Summary of Data for Clusters Incorporated into Polymers
`
`polymer sample
`
`¢ emissionc
`37 Å clusters
`45 Å clusters
`% uptakea
`b
`¢(cid:236)abs
`4
`3
`2
`33
`[MTD]300
`[MTD]300[NORPHOS]20
`116d
`0
`100
`7
`2
`6
`100
`[MTD]300[NBE-CH2P(O)(oct)2]20
`[MTD]300[NBE-CH2O(CH2)5P(oct)2]20
`180
`230
`0
`100
`[MTD]300[NBE-CH2O(CH2)5P(O)(oct)2]20
`8
`2
`6
`100
`a Weight fraction of cluster taken up by polymer; initial loading level 20 wt %. b Changes in position of (cid:236)max(nm) relative to free clusters
`given for small (37 Å) clusters. Larger (45 Å) clusters exhibit no shift in any of the polymers examined. c As percent of initial fluorescence
`intensity (of free clusters); measured within 30 min of mixing. d Drops to 5% of initial value within 12 h.
`
`Copolymers of MTD with monomers 2, 4, or5were
`readily prepared by sequential addition of the monomers
`to initiator 6 (Scheme 2). The timescale of polymeri-
`zation for 4 was assessed both from in situ NMR
`experiments and by quenching successive aliquots of a
`reaction with PhCHO for subsequent 1H NMR analysis.
`Formation of the P20 block was found to be complete
`within 1 h.
`Polymerization Results. GPC analysis of [MTD]300
`and [MTD]300[NORPHOS]20 indicated narrow, unimodal
`molecular weight distributions. An unexpected bi-
`modality was observed for the MTD copolymers of 2, 4,
`and 5, with a variable peak ratio, and an Mn ratio of
`(cid:24)5-10:1.
`(Peak ratios and Mn values cannot be pre-
`cisely determined, owing to overlap between the two
`peaks.) The very narrow PDI values for the peaks
`(typically <1.06) imply that two distinct initiator species
`are formed, each giving rise to a living polymer. The
`observed bimodality is not due to an intrinsic rate
`difference between Mo-MTD and Mo-phosphine propa-
`gating species reacting with the phosphine-containing
`monomer, as bimodality is retained on reversing the
`usual order of addition (i.e. adding 4 first, then MTD).
`The involvement of monomer phosphine as a donor
`ligand23 is also improbable; not only would a diminished
`effect be expected for copolymers of 2 and 5 (in which
`the Lewis basicity of the auxiliary donor is much
`attenuated), but a bimodal GPC trace is also found for
`copolymers of MTD and NBE-CH2O(CH2)5Br. Binding
`of Br to the metal is unlikely to be significant, as GP