J Inorg Organomet Polym (2015) 25:27–36 DOI 10.1007/s10904-014-0093-1

Organometallic Conjugated Polyelectrolytes: Synthesis and Applications Xiaolei Cai • Ruoyu Zhan • Guangxue Feng Bin Liu

•

Received: 15 August 2014 / Accepted: 23 September 2014 / Published online: 5 October 2014 Ó Springer Science+Business Media New York 2014

Abstract Conjugated polyelectrolytes (CPEs) have gain great research interest during the past decades. The incorporation of phosphorescent transition-metal complexes into CPEs backbones could yield organometallic CPEs (OMCPEs), which exhibit unique physical and chemical properties, enabling their applications in electroluminescence device, sensing, bioimaging and photodynamic therapy fields. This review begins with a brief introduction of synthetic approaches towards OMCPEs, followed by summary of the recent advances for applications. Some outlooks are also highlighted at the end. Keywords Conjugated polyelectrolytes  Organometallic conjugated polyelectrolytes  Electroluminescence devices  Sensing  Bioimaging  Photodynamic therapy

1 Introduction Conjugated polyelectrolytes (CPEs) are organic macromolecules composed of highly delocalized p-conjugated backbones and charged side chains [1, 2]. They combine the semiconducting and light-harvesting properties of X. Cai  R. Zhan  G. Feng  B. Liu (&) Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore e-mail: [email protected] R. Zhan School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China B. Liu Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore

conjugated polymers (CPs) as well as good solubility and processability in polar solvents (e.g. in alcohol and water) and ionic nature of polyelectrolytes. CPEs are developed after neutral CPs for electroluminescence (EL) devices, motivated by the potential of using environmentally friendly, solution-based deposition techniques for multilayer device fabrication [3, 4]. By utilizing the tunable band gap and the multiple ionic side chains that can be function as anchoring groups, CPEs are capable to be used as light harvesting and sensitizer materials for solar cells [5–8]. Due to their highly p-electron delocalized backbone structures, CPEs also enable fast intra- and inter-chain energy transfer and result in amplified signal output upon interaction with targeted molecules or stimuli, which are superior over small molecular organic dyes in the field of biosensing [8–13]. Moreover, the existence of charged or ionic side chains endows CPEs with good solubility in aqueous solution, further expanding their applications in bioimaging and therapy [2, 14–18]. Recently, the phosphorescent transition-metal complexes composed of transition metal centers and chelating ligands have attracted great attention due to their unique photophysical properties, which include high quantum efficiency, long phosphorescence lifetime, tunable emission color from blue to near infrared region, and large Stokes shifts. Taking the advantages of both CPEs and transition-metal complex, organometallic CPEs (OMCPEs) have been developed by introduction of phosphorescent transition-metal complexes to either the backbone of CPEs via polymerization processes or to CPE side chains through physically electrostatic interactions [19]. Currently, the most commonly used metal complexes of the OMCPEs developed for EL devices, biosensor, bioimaging applications are iridium (Ir(III)) and platinum (Pt(II)) complexes due to their intense phosphorescence and good photostability [20].

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X Ar X

+ Y

O O B Ar' B O O

Y

+ X

Ligand

X

Suzuki coupling reaction

Ar Ar'

Pd catalyst

Ligand

Ar'

x

y n

Metal

Metal

X = Cl or Br Ar

+

X

Ligand

X

Sonogashira coupling reaction Pd catalyst

Ar

Metal

Ligand n Metal

Y

alkyl group

Scheme 1 Synthetic routes towards OMCPEs

The introduction of transition-metal complexes to CPEs is able to reduce the triplet–triplet annihilation, leading to high current density of the blended devices [21]. In addition, CPEs act as energy donors and metal complexes act as energy acceptors in OMCPEs, and the fluorescence resonance energy transfer (FRET) between the CPEs and metal complexes could be controlled, which improves the sensitivity and selectivity for specific sensing. Due to the energy transfer from CPEs to metal complexes, OMCPEs exhibit narrow emission, which reduces the possibility of spectral overlap and thus potentially favors multiplex detection [22]. By taking advantages of the long lifetime of the metal complex, OMCPEs could also provide an effective means to reduce the background interference by using time-resolved photoluminescence technique (TRPT) [23]. Moreover, the energy transfer between the triplet state of the metal complex and the ground state of molecular oxygen could be triggered by photo-excitation, which generates singlet oxygen (1O2) and allows its potential application in photodynamic therapy (PDT) [24]. In this review, we summarize the recent advances of OMCPEs.The article starts with the synthetic strategies of OMCPEs, followed by their applications, including EL device development, biomolecule sensing, metal cation detection, bioimaging and PDT. Outlooks of OMCPEs are also discussed at the end of the review.

2 Synthetic Strategies for OMCPEs Generally, OMCPEs are formed by introducing transitionmetal complexes into the CPEs backbone by chemical reactions. Two typical methods are usually applied, which include Suzuki coupling polymerization and Sonogashira coupling reaction (Scheme 1). Besides this, it is also able to introduce the transition-metal complex to the side chains of CPEs, where the CPE-metal hybrid can be formed by the electrostatic interaction between oppositely charged CPE side chains and the transition-metal complexes.

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2.1 Suzuki Coupling In 2011, Huang et al. reported the synthesis of P1 using Suzuki coupling reaction. They firstly synthesized the Iridium (III) complex monomers, after which 6 or 12 mol% Iridium (III) complex were polymerized with 2,7-Dibromo9,9-bis(6-bromohexyl)-9H-fluorene and 2,20 -(9,9-bis(6bromohexyl)-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl1,3,2-dioxaborolane) by Pd-catalyzed Suzuki coupling reactions to generate the target polymer as shown in Scheme 2. Finally the quaternization of the polymer was conducted to obtain P1 [22]. 2.2 Sonogashira Coupling In 2010, Wang et al. reported the synthesis of platinum(II) containing OMCPEs (P2) by Sonogashira coupling reaction. They prepared the diethynyl monomer, which was polymerized with [Pt(PMe3)2Cl2] ligand to yield P2 (Scheme 3). P2 has an alternating fluorene ethynylene and platinum (II) complex backbone and aspartic acid containing side chains. [25]. 2.3 CPE-Metal Hybrid For the formation of CPE-metal hybrid, typically, the metal complex is cationic, which can be mixed with anionic CPEs to form hybrid by electrostatic interaction or hydrogen bonding. Compared with the chemical strategy, this method avoids the complicated chemical synthetic routes, which provides a simple alternative for the preparation of new materials containing both transition-metal and CPE functionalities. As shown in Scheme 4, through simple mixing, an anionic CPE poly(phenylene ethynylene) with sulfonate groups in the side chains (PPE-SO3-) and a cationic phosphorescent Pt(II) oligomer can form PPE-SO3-–Pt(II) hybrid (P3) via electrostatic interaction [19].

J Inorg Organomet Polym (2015) 25:27–36 C8H17

29

C8H17

Br

Br N

+

+

Br

Br

O B

BrC6H12

N

BrC6H12

C6H12Br

1. Pd(PPh3)4 Toluene, K2CO3

O

2.

B

3.

Br

B

O

Ir

O

C6H12Br

OH OH

2

C8H17

C8H17

C8H17

C8H17

N(CH3)3 x BrC6H12

N

y n

x

Ir

C6H12Br

R

N 2

P1

n

y

N Ir

R

N 2

R = C6H12N(CH3)3Br

Scheme 2 Synthetic scheme of P1 by Suzuki coupling reaction

P Pt n P

P Cl Pt Cl P O HN

O O

O

O

O

O O

O

O

1,4-Dioxane

NH O

O

KOH

O

HN

NH O

O

O

O

P Pt n P O

HN

O

NH O

O

O

O

O

OK

P2

O OK

OK

O OK

Scheme 3 Synthetic scheme of P2 by Sonogashira coupling reaction

Scheme 4 Assembly of CPEmetal hybrid P3 via electrostatic interaction

2+

N N

Pt N

NMe 3

O(CH 2 )3 SO3Na

(OTf) 2 O(CH 2 )3 SO3 Na

n

P3

3 Applications 3.1 Device During the past decades, the development of polymer lightemitting devices (PLED), whose light emission originates from neutral CPs, is growing rapidly. Compared to OLED based on small molecules, PLED could operate at lower voltages and is more power-efficient [26]. In addition, the manufacturing technologies including solution based ink-jet printing and spin coating, are much cheaper than traditional evaporation techniques used for OLEDs. In addition, multilayer devices with balanced electron and hole injection could lead to further improved device performance. One of the elegant strategies to realize multilayer films is to take advantage of the different solubility of organic-soluble CPs

and water-soluble CPEs, where there is no solubility issue at the interface. In addition, CPEs have also been found as effective interface modifiers, which can significantly improve electron injection and allow the use of more air stable cathode materials [27]. However, initial trial of using CPEs as the light emitting materials in PLEDs did not lead to satisfactory results [3]. By introducing phosphorescent metal complex into the polyelectrolytes backbones or side chains, the electroluminescent efficiency could be improved since both the singlet and triplet emission could be realized and utilized to improve the electroluminescent efficiency [28]. Several studies have reported the synthesis of polyelectrolytes with charged metal (such as Iridium) complexes on polyelectrolyte backbones [29, 30]. But they exhibit a delay between switch-on and turn-on of the emission due to slow ion movement in the electrochemical process [31].

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Scheme 5 Chemical structures of P4 and P5

N

1-x C8 H17

C 8H 17

C 8H 17

x = 0, 0.5, 1, 2, 5, 10% PFN-DppyIrppyx N Br

Br N

C 8H 17

N

x n

Ir N

P4

N C 8H 17

C 8H 17

O O

Ir N 1-x n

x C 8H 17 x = 0, 1, 2, 5, 8% PFNBr-MeNaPyIr

Recently, Cao et al. and Heeger et al. proved that devices made from polyelectrolyte containing quaternized aminoalkyl groups have no electrochemical process during the charge injection and transport of such devices [27, 32]. Subsequently, Cao et al. synthesized P4 (Scheme 5) with aminoalkyl-substituted polyfluorene main chain and neutral Ir(ppy)3 (up to 10 %) incorporated into the polymer backbone by Suzuki coupling reaction [33]. The quaternized ammonium groups endow P4 with good solubility in polar solvents, which enables easier fabrication of multilayer polymer phosphorescent PLEDs. Devices made from P4 and its neutral precursor emit orange-red light in the region of 598–602 nm. The fast response in light turn-on indicates that there is no sign of electrochemical involved, which means that the effect of slow ion movement is avoided. The best device performances are achieved with external quantum efficiency (EQE) 0.69 and 0.54 % and luminance efficiency (LE) 1.42 and 1.10 cd A-1 for Al and Au as the cathode, respectively. Most importantly, it was proved that the performance of the devices with air-stable high work-function metals such as Al and Au as the cathode is comparable with low work-function metals, such as Ba. This study is also the first to report that Au could be used as cathode in the phosphorescent OLEDs. Followed by previous work, P5 (Scheme 5) was synthesized using fluorene and iridium acetylacetonate complexes containing two bromine atoms in different ligands (naphthinatopyridine) as co-monomer by Suzuki coupling reaction, resulting in chelating copolymers with Ir atoms in aminoalkyl-substituted polyfluorene main chain with ionic

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P5

C 8H 17 N Br

Br N

groups in the side chains [34]. The devices made from P5 displayed a saturated red emission around 640 nm. Similarly, devices using air-stable high work function metals, such as Al, as cathode also showed comparable performance with low work function metals. 3.2 Biomolecular Sensing 3.2.1 Heparin Sensing Heparin is a naturally occurring highly sulfated glycosaminoglycan that has long been used as an anticoagulant drug to prevent blood clots clinically. Heparin overdoes may lead to serious problems, such as hemorrhage and thrombocytopenia [36]. Therefore, close monitoring and quantification of heparin is very important. Huang et al. reported a OMCPE probe P6 (Fig. 1a) for direct visual sensing of heparin in both water and biological media [35]. P6 was synthesized by incorporating a fraction of phosphorescent Ir(III) complex into cationic polyfluorene main chains via Suzuki coupling reaction. The mole percentage of Ir(III) complex was optimized to balance a low background signal (wide analyte quantification range) and a relatively high Ir(III) complex content (significant spectrum change after adding analyte), and was determined to be 4 mol%. In the absence of analyte, P6 emits blue fluorescence with emission maximum centered at 443 nm (Fig. 1b). Upon addition of heparin, the red emission intensity at 632 nm increases progressively at the expense of blue emission intensity and the final solution color is red.

J Inorg Organomet Polym (2015) 25:27–36

31

(a)

(b) 0.96 R

Ir

O

2

R

0.04 n O N

R = C6H12N(CH3)3Br

S

Fig. 1 a Chemical structure of P6. b PL spectra of P6 solution in the presence of 0–70 lM heparin upon excitation at 380 nm. The insets show the photographs of P6 solutions in the presence of 0, 15, 30, 45

y

S

2

Ar

Ar

x

Ar

Ar

a

N Ir O O

m

b

Ar = Ar c

R

Ar

R

n

R = C6H12N(CH3)3Br

P7

P6

and 60 lM heparin under UV lamp illumination [35]. Reproduced with permission. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

solutions containing interfering dye rhodamine B (RB) (emission lifetime: 2.1 ns). Almost no PL emission change can be observed due to the presence of strong RB background signals. In contrast, distinguishable spectra change with a *3.1-fold increase in intensity is observed in TRES after a delay of 99 ns. These results indicate TRPT can be used for further improving assay signal-to-noise (S/N) ratio. Based on the same sensing mechanism, the same group also developed a hyperbranched OMCPE containing fluorene and 4 mol% Ir(III) complex units (P7, Scheme 6) for heparin sensing [37]. Addition of heparin leads to a significant solution color change from blue to finally red. Heparin can be quantified in the range of 0–44 lM in buffer, and the detection limit reaches as low as 50 nm.

Scheme 6 Chemical structure of P7

3.2.2 Histone Sensing Based on the dynamic light scattering (DLS) and transmission electron microscopy (TEM) results, the sensing mechanism was proposed. P6 self-assembles into nanoparticles (NPs) (mean diameter *50 nm) prior to heparin addition. Electrostatic interactions between heparin and P6 NPs induce P6 NPs to be more compact, which favors both intra- and inter- FRET from donor fluorene segments to acceptor Ir(III) complex. Heparin can be linearly quantified in the range of 0–70 and 0–5 lM in buffer and 5 % serum, respectively. In addition, heparin analogies hyaluronic acid and chondroitin sulfate and six interfering biomolecules cannot induce significant red emission increase of P6, indicating its good sensitivity and selectivity. One particular merit of P6 is its long emission lifetime of phosphorescent signal (223 ns), which allows for eliminating undesirable background fluorescence from the complicated environment. Both steady state and timeresolved emission spectra (TRES) were recorded for

Protein sensing by fluorescent materials has attracted significant scientific and economic interests due to its wide applications in the area of proteomics, medical diagnostics and pathogen detection [38]. CPs with donor–acceptor architectures have been reported for protein detection [39, 40]. However, due to the undesired FRET caused by poor water solubility and self-aggregation of the polymer before analyte addition [41], the utilization of self-aggregation induced FRET for protein sensing is still a challenge. By taking the advantage of solubility limitation of CPEs as well as hydrophobic interaction between protein and OMCPE backbones, Huang et al. developed an assay for protein histone sensing through a unique weakened FRET process using P1 [22]. The high content of hydrophobic Ir(III) complex (12 mol%) leads to the poor solubility and self-aggregation of P1, resulting in efficient intrinsic FRET from fluorene to Ir(III) complex, and the solution emits red fluorescence in the absence of proteins (Fig. 2a). The strong hydrophobic interaction between histone and P1

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The interactions could induce the quenching of PPE-SO3fluorescence due to the ground state static quenched by extremely rapid excitation diffusion along the PPE-SO3chain to alkynylplatinum(II) terpyridine complexes as well the enhanced FRET from PPE-SO3- to alkynylplatinum(II) terpyridine complexes, resulting in growth of triplet metal– metal-to-ligand charge transfer (3MMLCT) emission in the near-infrared (NIR) region. After adding HSA, the PPESO3- could bind to HSA by electrostatic and hydrophobic interactions, leading to the de-aggregation of the PPESO3-–Pt(II) hybrid and the decrease in FRET. As a result, there is a drop of the 3MMLCT emission intensity at 800 nm, and an increase of the p–p* fluorescence of PPESO3- centered at 430 nm (Fig. 3). By combing the intensity changes at 800 and 430 nm, selective detection of HSA could be achieved with a detection limit of 1.25 nM. 3.3 Metal Cations Detection

Fig. 2 a Photographs of P1 solutions in the absence and presence of different proteins under UV lamp illumination. b Schematic illustration of histone detection [22]. Reproduced with permission. Copyright (2011) American Chemical Society

leads to the breakage of P1 self-formed aggregates, FRET between fluorene and Ir(III) complex is reduced and the solution color changes from red to lilac (Fig. 2). In contrast, other eight proteins have negligible influence on the OMCPE aggregates, and the emission of P1 remains in red (Fig. 2a), which allows visual sensing of histone. Linear quantification of histone is realized in the range of 0-10 lM and the detection limit is 0.06 lM. 3.2.3 Human Serum Albumin Detection Human serum albumin (HSA) is the most abundant protein in the blood plasma and serves as physiological carrier in many important biological compartment and transporter of various endogenous and exogenous substances [42]. Abnormal level of HSA in human body is a sign for several diseases, e.g., low HSA level in the blood plasma indicates liver failure while excess amount of HSA in urine is an early sign of incipient renal disease [43, 44]. Thus, there is a significant importance to develop HSA detection probe with high sensitivity for clinical applications. A CPE-metal hybrid with a high sensitivity for HSA detection was reported in 2011 [45]. As shown in Scheme 4, PPE-SO3- and alkynylplatinum(II) terpyridine complexes could form PPE-SO3-–Pt(II) hybrid (P3) through the Pt–Pt, electrostatic and/or p–p interactions.

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Heavy metal pollutions have attracted great public attention due to its serious effects on the environment and human health, especially in developing countries. The heavy metal ions are easily to spread in the environment and accumulate in human body and cause severe health problems [46]. Thus, reliable and efficient methods for heavy metal ions detection are required. 3.3.1 Ag? Detection Silver ions (Ag?), which can cause environmental pollution as well as adverse health effects on human, have attracted great public attention [47]. Development of rapid and selective detection method of Ag? in water and food resource has gained lots of interest. Current detection methods are mature but expensive and intricate, so more efficient, cheap and reliable method is required. In 2010, Wang et al. first reported a water-soluble platinum (II) acetylide-based CPE for the direct sensing of Ag? in aqueous media [25]. P2 with an alternating fluorene ethynylene and platinum (II) complex backbone and aspartic acid containing side chains was specially designed and synthesized via Sonogashira coupling reaction (Scheme 2). In degassed water, P2 has two emission bands with maxima centered at 405 and 540 nm, corresponding to fluorescence and phosphorescence through the efficient intersystem crossing (ISC) from the singlet to the triplet states, respectively. Due to the specific binding between aspartic acid and Ag?, only the addition of Ag? into P2 solution could lead to the red-shift of the absorptionmaximum, and as a consequence, the solution color changes from colorless to yellow (Fig. 4), making the naked-eye detection possible. The addition of Ag? could also further promote efficient ISC, as evidenced by the increase of the

J Inorg Organomet Polym (2015) 25:27–36

Fig. 3 Relative emission intensity of P3 hybrid at 800 nm (a) and 430 nm (b) in the presence of different substrates. Substrate tested: N no substrate added, H HSA, B bovine serum albumin, dA poly(dA)25, dC poly(dC)25, dG poly(dG)25, dT poly(dT)25,

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R arginine, G glycine, PD poly(aspartic acid), PK poly(lysine bromide), S spermine, PY poly-(tyrosine), L lysozyme, T trypsin [45]. Reproduced with permission. Copyright (2011) American Chemical Society

Fig. 4 Photographs of P2 solutions in the absence and presence of different metal ions: 0 blank, 1 Ag?, 2 Hg2?, 3 Pb2?, 4 Mg2?, 5 Cs?, 6 Li?, 7 Cd2?, 8 Ca2?, 9 Fe3?, 10 Ni2?, 11 Cu2?, 12 Zn2?, 13 Co2? [25]. Reproduced with permission. Copyright (2011) American Chemical Society

intensity ratio between phosphorescence and fluorescence signals. Besides, both emission peaks undergo obvious red shift in response to Ag?. Most other metal ions could only quench the fluorescence and phosphorescence of P2 to some extent and could not induce the position change of emission peaks. P2 has a very broad Ag? linear quantification range (1 lM to 2 mM) and its limit of detection could be up to ppb level. It is also significant that this method could achieve the Ag? detection in aqueous solution due to the good solubility of P2, while other detection methods need to be operated in organic or mixed aqueousorganic solutions [48–51]. Besides this study, Schanze’s group has reported the luminescence quenching of platinum(II) acetylide based OMCPEs by viologens [52] and phosphorescence quenching of platinum acetylide based CPs by different metal ions [53]. 3.3.2 Hg2? Detection Mercury could lead to serious health problems even at low concentrations. The inorganic mercury could be converted into neurotoxic methylmercury [54], which could accumulate in human body and eventually cause brain damage and serious cognitive and motion disorders [55].

A CPE-metal hybrid P8 (Fig. 5a) composed of an anionic CPE (PFB-SO3-) and a cationic phosphorescent Ir(III) oligomer has been reported by Huang and coworkers for selective and sensitive sensing of mercury ion (Hg2?) [19]. The Ir(III) complex has an intense emission peak at 635 nm while PFB-SO3- has absorption and emission peaks centered at 370 and 422 nm, respectively. FRET from the blue-emissive PFB-SO3- to the red-emissive phosphorescent Ir(III) complex results in an enhanced red emissive hybrid complex. Based on Pearson’s soft and hard acids and bases theory, the sulfur atoms (as a soft base) on the C–N ligand of Ir(III) complex is able to coordinate with Hg2? (as a soft acid) [56]. Addition of Hg2? triggers fast decomposition of P8 hybrid, which reduces the FRET between PFB-SO3- and Ir(III) complex and causes obvious decrease of the red emission from phosphorescent Ir(III) complex while the blue emission from PFB-SO3- changes slightly. It enables the ratiometric and multicolor detection for Hg2? as the emission color of the hybrid changes from red to purple upon addition of Hg2? (Figs. 5b and 5c). The sensing sensitivity is as low as 0.2 lM. Making use of the long emission lifetime of Ir(III) complex, TRPT was applied to eliminate the short-lived background interference in complicated environment,

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(a) S N

2 O

R

Ir

n

O R

R

R

SO3Na

NaO3S

R = C6H12N(CH3)3Br

P8

(c) (b)

Fig. 5 a Chemical structure of P8 hybrid. b PL titration spectra of P8 hybrid. The insets show the photographs of P8 hybrid in the absence and presence of Hg2? under UV lamp illumination. c PL response of P8 hybrid in the presence of different metal ions: 1 K?, 2 Na2?, 3

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(a) x R

R

P9 : x : y = 4 : 96

y n O

Ir

N S

P10 : x : y = 8 : 92 R = (CH2CH2O)2CH2CH2N(CH3)3Br

(c)

O

2

Fig. 6 a Chemical structures of P9 and P10. b PL spectra of P10 in 10 mM PBS under different oxygen pressures. The insets show the photographs of P10 solutions saturated with N2, air and O2. Confocal images of HeLa cells after incubation with P9 dots (c) merged channels of 420–460 and 600–650 nm channels (d) fluorescent lifetime imaging [57]. Reproduced with permission. Copyright 2014 WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

Ca2?, 4 Mg2?, 5 Ni?, 6 Zn2?, 7 Cd2?, 8 Co2?, 9 Cr3?, 10 Cu2?, 11 Fe3?, 12 Pb2?, 13 Hg2? [19]. Reproduced with permission. Copyright 2013 WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

(d)

(b)

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resulting in high S/N ratio. The interference from RB (emission lifetime: 2.1 ns) was eliminated after a delay time of 99 ns. 3.4 Imaging and Photodynamic Therapy As the metal complex is sensitive to oxygen, recently, a series of new OMCPEs (P9 and P10, Fig. 6a) containing Ir(III) complexes have been developed for oxygen sensing, bioimaging, and PDT [57]. PDT utilizes photosensitizers to generate reactive oxygen species to kill cancer cells, which has attracted great attention and becomes one of the most powerful methods for cancer therapy in recent years [58]. It was found that phosphorescent Ir(III) complexes are able to detect the oxygen as well as act as photosensitizers for PDT [59]. P9 and P10 were synthesized through Suzuki coupling reaction of oxygen-sensitive Ir(III) complexes and an oxygen-insensitive fluorene units followed by quaternization. The amphiphilic structures could lead P9 and P10 self-assemble to NPs with ultra-small size (*6 nm) in aqueous solution. Efficient FRET occurs between the blue emissive (450 nm) fluorene units to red emissive (630 nm) Ir(III) complexes, resulting in the red emission of P9 and P10 solution without analyte addition. The red emission intensity of P10 is higher than that of P9 due to more efficient FRET. The oxygen could reduce FRET and quench the red emission from Ir(III) complexes at 630 nm and lead to color change from red to blue for oxygen sensing (Fig. 6b). The bioimaging ability was also explored by a confocal laser scanning microscopy. After incubation the HeLa cells with P9 NPs, both blue and red luminescence were shown in cell cytoplasm, indicating that the P9 NPs can pass across the cell membrane and internalize into the cytoplasm. Fluorescence lifetime imaging microscopy was applied to investigate the long emission lifetime of P9. In Fig. 6d, a high-quality long emission lifetime signal (s = 150 ns) was observed, which proves that P9 could help to eliminate the effect of short-lived background interferences for complicated biological cell imaging. Besides, P9 and P10 can also serve as photosentitizers for PDT. The energy transfer between the triplet state of the Ir(III) complex in the OMCPEs and the ground state of molecular oxygen can be triggered by photoexcitation, generating electronically excited state of molecular oxygen, i.e., singlet oxygen (1O2). As a result, the cell apoptosis and cell death through the PDT process can be induced. The morphologies of HeLa cells in the dark control and light control without any P9 treatment remain normal while the morphologies of HeLa cells incubated with P9 NPs upon light irradiation change dramatically with a large amount of dead cells floating in the media.

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4 Conclusion and Outlook In conclusion, this article summarizes current development and applications of OMCPEs in EL devices, biomolecule sensing, metal ion detection, cell imaging and PDT. The synthetic strategies for OMCPEs generally include the chemical polymerization via Suzuki coupling and Sonogashira coupling. CPE-metal hybrid could also be formed through physical interactions. The photophysical properties and specific responses to different analytes of these OMCPEs and CPE-metal hybrid are highly dependent on FRET between the CPE backbones and the metal complex and the properties of analytes. By altering the type and ratio of CPE backbones and metal complexes, the photoluminescence properties and color emission can be tuned and utilized to adapt different applications. Future development of OMCPES should focus on the development of OMCPEs with different emission colors. Highly sensitive and selective sensing and detection of various molecules or ions by OMCPEs are still in high demand due to their unique time delayed fluorescence which can eliminate background interference in real applications. As the demonstrated OMCPE fluorescent assays are largely based on electrostatic interactions with the analyte, further development of targeted specific OMCPEs could be achieved by attaching specially designed recognition elements to CPEs to realize lock-key based detection. The integration of more metal complexes with different CPEs will provide more potential candidates for PLED and biomedical applications. The rapid development of polymer chemistry, materials science, and biological science will provide deeper understanding of the structures and inter/intra-molecular interactions of OMCPEs and offer more research opportunities for the exploration of their synthesis and applications. Acknowledgments We thank the Singapore National Research Foundation (R-279-000-390-281), the SMART (R279-000-378-592), and the Institute of Materials Research and Engineering of Singapore (IMRE/12-8P1103) for financial support.

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