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VIII. Concluding Remarks on the Importance of Radical-Radical Recombination on the Efficiency of Polymer Photochemical Degradation

VIII. Concluding Remarks on the Importance of Radical-Radical Recombination on the Efficiency of Polymer Photochemical Degradation

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References



285



two radicals in close proximity explains: (1) the effect of tensile stress on the

rate of polymer degradation, (2) the effect of temperature on photodegradation

rates, and (3) the effect of polymer curing on degradation rates. In each of these

situations, the efficiency of radical diffusion affects the net efficiency of radicalradical recombination, which affects the efficiency of photodegradation. The

concept of radical-radical recombination also manifests itself in the biphasic

kinetics observed for polymers in many photodegradations. In this situation,

the initial high rate of decay is interpreted as being due to radical capture

within a matrix cage.



IX. ACKNOWLEDGMENTS

Acknowledgment is made to the National Science Foundation and to the

Petroleum Research Fund, administered by the American Chemical Society,

for the support of the authors’ work described in this chapter.



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CHAPTER 8



Optical Properties and Photophysics

of Platinum-Containing

Poly(aryleneethynylene)s

Wai-Yeung Wong

Department of Chemistry and Centre for Advanced Luminescence

Materials, Hong Kong Baptist University, Waterloo Road,

Kowloon Tong, Hong Kong, P. R. China



CONTENTS

I. INTRODUCTION

II. SYNTHETIC METHODS AND MATERIALS

CHARACTERIZATION

III. OPTICAL AND PHOTOPHYSICAL PROPERTIES

A. Energy Gap Law for Triplet States

i. Effect of π-Conjugation and Interruption

ii. Effect of Fused Ring

iii. Effect of Ring Substitution

iv. Effect of Donor-Acceptor Interaction

v. Effect of Temperature

B. Phosphorescence Color Tuning of Metallopolyynes

C. Roles of Metallopolyynes in Optoelectronic

and Photonic Devices

i. Light-Emitting Devices

ii. Photovoltaic Cells

iii. Optical Power Limiters



290

291

298

298

300

309

309

310

312

312

314

314

315

317



Macromolecules Containing Metal and Metal-like Elements,

Volume 10: Photophysics and photochemistry of metal-containing polymer,

Edited by Alaa S. Abd-El Aziz, Charles E. Carraher Jr., Pierre D. Harvey, Charles U. Pittman Jr., Martel Zeldin.

Copyright r 2010 John Wiley & Sons, Inc.



289



290



Optical Properties and Photophysics



IV. SUMMARY

V. ACKNOWLEDGMENTS

VI. REFERENCES



320

320

321



I. INTRODUCTION

Incorporation of transition metal elements into macromolecular organic

structures allows the hybridization of the interesting physical characteristics of

metals such as electronic, optical, and magnetic properties with the solubility

and processability of traditional carbon-based polymers.1À5 Transition metal

centers with a large variety of ligand environments, oxidation states, and

structural geometries may offer distinct physical, optoelectronic, and structural

properties on these purely organic polymers. Within the framework of

synthetic metal-containing polymers, polymers with metalÀcarbon σ-bonds

in the main chain represent one of the attractive and important subsets of

these materials.1À4 Rigid-rod transition metal acetylide polymers, or polymetallaynes in short, have spurred tremendous worldwide interest at the

forefront of many metallopolymer investigations.6

The development of synthetic methodologies toward transition metal

acetylide oligoynes and polyynes of the form trans-[2M(L)2CC(R)

CC2]n (L5auxiliary ligands, R5spacer unit) has shown much progress following the initial reports by the Japanese group in the 1970s on the synthesis of

polymeric Pt and Pd acetylides,7À10 and the interest has been principally stimulated from their applications in molecular electronics and materials science

(Fig. 1).1,2 In the early 1990s, there were also a couple of reports on the

synthesis of rigid-rod metal-containing polyynes of groups 8 and 10 by Lewis

and co-workers based on bis(trimethylstannylacetylide) synthons.11À14 Since

then, these organometallic-based oligomeric and polymeric functional materials have become famous for their unique properties, such as electrical conductivities, rich luminescence and nonlinear optical properties, liquid

crystallinity and photovoltaic effect.6 The prototypical polymer for much of

this work is trans-[2Pt(PBu3)2CC(p-C6H4)CC2]n (1). To date, a large

series of derivatives containing various conjugated carbocyclic and heteroaromatic ring systems are known.1,6 All of these materials are organic soluble,

and the solubility and polymer length can be adjusted by an appropriate

selection of Ar or L units (Fig. 1). The advances in chemical synthesis have also

resulted in the preparation of a library of application-oriented conjugated

polymers of this kind that can display diverse optical and photophysical

properties. This chapter provides a comprehensive overiew on this topic for

platinum-containing poly(aryleneethynylene)s.



Synthetic Methods and Materials Characterization



291



Chemical/optical sensors



L

Pt

L



Nonlinear

optics



Photovoltaics



Luminescence



Liquid

crystals



Ar

n



Light-emitting devices



FIGURE 1. Polyplatinynes in different areas of material applications.



II. SYNTHETIC METHODS AND MATERIALS

CHARACTERIZATION

Since the pioneering work by Hagihara et al. on the prototype compound

of group 10 metals spaced by the phenylene ring in polymer 1 followed by the

synthetic extension of the method to group 8 metals,7À10 there has been a large

body of literature on the chemical syntheses, spectroscopic and structural

aspects, material properties as well as potential applications of organometallic

polyyne polymers of the late transition metals. Long and others have provided

leading references to the modern literature of the synthesis and mechanism of

metal polyynes, and these topics will not be included here.1,2,4,6 In general, a

series of diethynyl-derived spacers can be used as versatile precursors to produce dinuclear and polymeric compounds of platinum by the most common

copper(I)-catalyzed dehydrohalogenative coupling procedures in an amine

solvent (e.g., diethylamine or diisopropylamine) at room temperature. The

Me3SiCCRCCSiMe3 derivatives can be obtained in good yields from

the Sonogashira coupling of Me3SiCCH with the appropriate dibromo

species, which upon deprotection with a suitable base such as K2CO3 in

MeOH or Bu4NF in THF readily afford the diterminal alkyne compounds

HCCRCCH (Fig. 2). The feed mole ratios of the platinum chloride precursors and the active diethynyl ligands are 2:1 and 1:1 for the model complex

and polymer syntheses, respectively. As summarized in Figure 3, the spacers

adopted can range widely from common organic carbocyclic rings (1À27 and

75) to various heterocyclic moieties (28À51) and even the inorganic units, such

as some main group (52À74) and transition metal elements (17 and 27).

Purification of the polymers was effected by column chromatography on silica



292



Optical Properties and Photophysics

Br



R



Br

(i)



Me3Si



SiMe3



R

(ii)



H



H



R



(iii)



PBu3



(iv)

PEt3



Pt



R



PEt3



Pt



Pt



R



n



PBu3



PEt3

Polyyne



PEt3

Diyne



FIGURE 2. Synthetic schemes for polyplatinynes and their molecular model compounds.

Reagents and conditions: (i) Trimethylsilylacetylene, Pd(OAc)2, CuI, PPh3, amine;

(ii) K2CO3 in MeOH or Bu4NF in THF; (iii) trans-[PtCl2(PBu3)2] (1 equiv.), CuI, amine;

(iv) trans-[PtCl(Ph)(PEt3)2] (2 equiv.), CuI, amine.



PBu3

Pt



R



PBu3



n



Carbocyclic spacers

none

1



15



16, 17



18



2



NH2



615



820

715



F



F



419



3



O



OCH3



H3CO



920



519



O



O



H3CO



10



20



1120



F



F



PEt3

F



F

12



20



13



20



C

H2



F

20



16



14



22



Pt

PEt3

1723



1521



FIGURE 3. Chemical structures of platinum(II) polyyne polymers 1À75.



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