PolymerChemistry

PAPER

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aCenter for Molecular Systems and Organi

Organic Electronics & Information Display

Nanjing University of Posts & Telecommuni

iamlhxie@njupt.edu.cn; iammdyi@njupt.ed

25 8586 6533; Tel: +86 25 8586 6396bJiangsu-Singapore Joint Research Center fo

Displays and Institute of Advanced Mater

Nanjing 211816, China. E-mail: huangwei

Tel: +86 25 5813 9001

† Electronic supplementary informationstructural characterizations for th10.1039/c3py21154a

Cite this: DOI: 10.1039/c3py21154a

Received 28th December 2012Accepted 7th February 2013

DOI: 10.1039/c3py21154a

www.rsc.org/polymers

This journal is ª The Royal Society of

Hindrance-functionalized p-stacked polymer based onpolystyrene with pendent cardo groups for organicelectronics†

Cheng-Rong Yin,a Yuan Han,a Lu Li,a Shang-Hui Ye,a Wei-Wei Mao,a Ming-Dong Yi,*a

Hai-Feng Ling,a Ling-Hai Xie,*a Guang-Wei Zhanga and Wei Huang*ab

A hindrance-functionalized p-stacked polymer, poly(4-(9-phenyl-fluoren-9-yl)styrene) (PPFS), was designed

and synthesized by the introduction of bulky 9-phenylfluorenyl moieties (PFMs) with 3D cardo structure

into all the phenyl units of polystyrene (PS) through the sp3 carbon linkage. The PFMs obviously alter p-

stacked conformation and interchain interactions with respect to the precursor PS. The suitable HOMO–

LUMO energy gap, high triplet energy, and excellent thermal and morphological stabilities enable PPFS

to serve as not only electrically bistable materials but also phosphorescent hosts. Memory devices using

PPFS as an active layer exhibited nonvolatile flash memory performance with ON/OFF current ratio up

to 104, high stability in retention time up to 104 s and a number of read cycles up to 105 under a read

voltage of 1.5 V in both ON and OFF states, while PS based devices exhibited no memory performance.

Polymer light-emitting devices (PLEDs) using PPFS as the host and FIrpic as the guest exhibited blue

emission with CIE chromaticity coordinates (x, y) of (0.146, 0.349). These results indicate that hindrance-

functionalization opens a way to transform commercially available general purpose polystyrenes into

functional p-stacked macromolecules.

Introduction

It is well known that noncovalent p–p interactions play a crucialrole in expressing molecular and self-assembled complexstructures for the realization of functions in biology, chemistryand organic materials.1–6 DNA,7 polystyrenes,8 poly-(dibenzofulvene) (PDBF),9–12 poly(vinylcarbazole) (PVK)13–15 andother synthetic polymers16–19 were identied with intra-molecular supramolecular p-stacked motifs, resulting invarious intriguing properties. With regard to conjugated poly-mers, the p-stacked polymers offer alternative p-orbital chan-nels for charge transport, which created state-of-the-artsupramolecular polymer semiconductors (SPSs) in consider-ation of the central roles of weak supramolecular p–p interac-tions.20–22 Typical vinyl-type p-stacked polymers exhibit unique

c Devices (CMSOD), Key Laboratory for

s and Institute of Advanced Materials,

cations, Nanjing 210046, China. E-mail:

u.cn; huangwei@cashq.ac.cn; Fax: +86

r Organic/Bio-Electronics & Information

ials, Nanjing University of Technology,

@cashq.ac.cn; Fax: +86 25 5813 9988;

(ESI) available: Synthetic methods,e new compounds. See DOI:

Chemistry 2013

dynamic conformations in comparison to concurrent versatilep-conjugated polymers. The strategy of hindrance functionali-zation has been demonstrated to control the electronic struc-tures and conformational change of p-stacked polymers by ourgroup.22 Incorporating 9-phenyluorenyl moieties (PFMs) withsteric hindrance effect into PVK succeeded in creating new-generation electrically bistable materials for ash memorydevices and polymer hosts for blue electrophosphorescentpolymer light-emitting devices (PPLEDs).23 The functionalizedPVK shows improved electron-transporting ability, thermal andmorphological stabilities as compared to PVK, which expandedthe applications of p-stacked polymers in organic electronicdevices.24–28 Thus, the designing hindrance-functionalized p-stacked polymers is of great interest for organic electronics.

Besides PVK, polystyrenes (PS) are one kind of conventionalvinyl-type p-stacked polymers with characteristics of chemicalinertness and optical stability. However, the conformationaldynamics of PS are too fast to maintain stable p-stackedconformation, and the broad energy gap of 6.55 eV limited thePS serving as semiconductor in plastic electronics.29 In thiscontext, the functionalization of PS to achieve novel p-stackedpolymers with narrower energy gap and more stable p-stackedconformation have been developed for organic thin-lmdevices, especially for nonvolatile organic memory devices.Some functionalized PS with exible spacers were reported toexhibit ash memory performance.30,31 However, these poly-mers show unsatisfactory thermal stability with relative low

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Polymer Chemistry Paper

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glass transitions temperature (Tg), which is unfavorable for thestability and life-time of devices. Some functionalized PS withrigid spacers showing improved thermal stability were reportedto exhibit only write-once-read-many-times (WORM) memoryperformance.32,33 Thus, the functionalized PS with satisfactorythermal stability achieving ash memory performance stillneed to be investigated. In order to obtain such functionalizedPS, our group introduced bulky PFMs as rigid cardo spacers intoall the phenyl units of PS through the sp3 hybridized C-9 carbonatom to obtain novel hindrance functionalized PS, poly(4-(9-phenyl-uoren-9-yl)styrene) (PPFS). PPFS shows excellentthermal and morphological stability. Memory devices usingPPFS as active layers exhibited ash memory performance,while PS based device exhibited no memory performance.Furthermore, PPFS shows high triplet energy (ET) of 2.80 eV andsuitable energy gap of 4.18 eV, which could act as efficient hostmaterials for typical blue electrophosphorescent emitters, bis-[(4,6-diuorophenyl)pyrid-inato-N,C2(picolinato)iridium(III)(FIrpic) (ET ¼ 2.65 eV). The PPLEDs using PPFS as host andFIrpic as guest exhibited blue emission with CommissionInternationale de l'Eclairage (CIE) chromaticity coordinates(x, y) of (0.146, 0.349). The PPFS being applied not only aspolymeric memory material for memory but also as a polymerhost for PPLEDs indicated the versatility of hindrance-func-tionalized p-stacked polymers.

Results and discussion

Synthetic routes for themonomer 5 and target PPFS are depictedin Scheme 1. PFM-functionalized toluene 2 was synthesizedthrough Friedel–Cras reaction using toluene/9-phenyl-uoren-9-ol as the reactant and CH3SO3H as a Lewis acid catalyst in ayield of 55%. 2 andNBSwere brought to reux for a total of 4 h inCCl4 by weight AIBN as initiator to give 3 in a yield of 98%. Thefollowing two oxidations of compound 3 were carried out underthe catalysis of CaCO3 and PCC, respectively, to give compound 4in a yield of 90%.Monomer 5 in the yield of 98%was obtained bythe vinylation of 4 under the conditions CH3PPh3Br/n-BuLi.Monomer 5 was allowed to polymerize in toluene using AIBN asthe initiator to prepare the target polymer PPFS. The polymerPPFS was readily soluble in common organic solvents such aschloroform, THF, and toluene, chlorobenzene. The number-

Scheme 1 Synthetic route for PPFS. Reagents and conditions: (a) toluene,CH3SO3H, 140 �C, reflux, 12 h; (b) NBS/AIBN, CCl4, 80 �C, reflux, 4 h; (c) CaCO3, 1,4-dioxane/H2O, 101 �C, reflux, 48 hours; (d) PCC, CH2Cl2, reflux, 4 h; (e) n-BuLi/THF,CH3PPh3Br, reflux, 1.5 h; and (f) AIBN, toluene, 85 �C, reflux, 48 h.

Polym. Chem.

average molecular weight (Mn) and polydispersity index (PDI) ofPPFS, as determined by GPC against a PS standard in THF, werearound 1.1 � 104 and 1.80. The conductivity of PPFS was 2.4 �10�6 S m�1. DSC and TGA were performed for the evaluation ofthermal properties of PPFS (Fig. 1). PPFS exhibited outstandingthermal stability, with the decomposition temperature (Td) (5%weight loss) up to 434 �C. PPFS also showed excellent morpho-logical stability with distinct Tg appeared at 242 �C. The highthermal stability of PPFS may be attributed to the rigid 3D cardostructures hindering the crystallization process of the polymers,which is greatly favorable for the stable polymer memory and/orelectroluminescent host materials in devices.

Photophysical properties

Fig. 2 displays the normalized absorption and photo-luminescence (PL) spectra of PPFS and PS in solution and inthin lms spin-coated from toluene solution. PPFS exhibitalmost the same UV with absorption bands located at 273 nm,298 nm and 310 nm in dilute solution and in thin lms. Theabsorption band at 273 nm can be assigned to p–p* transitionof benzene, while the peaks at 298 and 310 nm can be attributedto the p–p* transition of the uorene. Compared with theabsorption band of PS (labs,max ¼ 260 nm), a bathochromic shiof 15 nm for absorption band of benzene in PPFS is probablydue to the supra-conjugation effect of bulky PFMs. PPFS insolution exhibit emission spectra with peaks at 316 nm and327 nm. PPFS in the thin lm exhibits emission peaks at 346and 357 nm, which is 30 nm red-shied compared with that ofthe solution spectrum, suggesting that intrachain partial face-to-face overlaps of uorene groups in the lm of PPFSincreased. The phosphorescence spectra of PPFS was measuredin CH2Cl2 glass at 77 K (Fig. S1†). The value of ET level of PPFSwas estimated from the highest energy 0–0 phosphorescentemission located at 443 nm to be 2.80 eV. The ET level of 2.80 eVis high enough for PPFS to serve as a host material for bluetriplet emitters, such as FIrpic.

Electrochemical properties

Cyclic voltammogram (CV) measurement was carried out toinvestigate the oxidation and reduction behavior of PPFS and

Fig. 1 TGA curve of PPFS recorded at a heating rate of 20 �C min�1 (decom-position temperature from the position of 5% weight loss). Inset: DSC trace ofPPFS recorded at a heating rate of 20 �C min�1.

This journal is ª The Royal Society of Chemistry 2013

Fig. 2 Normalized UV absorption and PL of PPFS and PS in film and in dilutesolution.

Fig. 4 (a) Typical I–V curves of PS thin film devices. (b) Typical I–V curves of PPFSthin film memory devices.

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estimate the HOMO and LUMO energy levels (Fig. 3). Theoxidation and reduction onset potentials of PPFS were recor-ded at 1.32 V and �2.86 V versus Ag/Ag+, respectively. As aresult, the HOMO and LUMO energy levels were estimated tobe �6.11 eV and �1.93 eV for PPFS according to the empiricalformula HOMO/LUMO ¼ �(Eox/red � EFOC) � 4.8 eV, where thevalue of EFOC for ferrocene is 0.01 V vs. Ag/Ag+ and 4.8 eV is theenergy level of ferrocene under vacuum. Compared with theenergy levels of PS (HOMO ¼ �6.95 eV, LUMO ¼ �0.4 eV),34

PPFS shows higher HOMO and lower LUMO energy levels,indicating that PPFS has improved hole and electron-trans-porting abilities compared with PS owing to the introductionof PFMs. The wider energy gap of PPFS as compared to FIrpic(HOMO ¼ �5.8 eV, LUMO ¼ �3.1 eV) is expected to favorcarrier collection and recombination and exciton connementwhen PPFS acts as the host and FIrpic acts as the guest inPPLEDs.35

Memory device characteristics

Fig. 4 shows the electrical characteristics of ITO/PS or PPFS/Alsandwich devices. For PS, no conductance switching behaviorwas observed with an increase in the applied voltage conrmingthe insulation of PS, which was in accordance with the reportedresults.36 For PPFS, non-volatile ash memory behavior wasexhibited with the change of applied voltage. The device wasinitially at a low-conductivity state when fabricated. When thenegative voltage was applied from 0 to �6 V (Sweep 1), the

Fig. 3 Cyclic voltammograms of PPFS film on a glassy carbon electrode (0.1MBu4NBF4 in acetonitrile, scanning rates 20 mV s�1).

This journal is ª The Royal Society of Chemistry 2013

device switched from a low-conductivity state (OFF state) to ahigh-conductivity state (ON state) at a switching thresholdvoltage about �0.5 V. The device remains in the ON state whenthe voltage sweep was repeated from 0 to �6 V (Sweep 2). As apositive voltage sweeps from 0 to 6 V (Sweep 3), the deviceswitches back to OFF state at 3.5 V, indicating the erasingoperation for the memory device. When the positive voltagesweep is subsequently applied (Sweep 4), the device remains at alow conductivity. The retention time test on PPFS basedmemory device for the ON and OFF states is shown in Fig. 5a.The ON/OFF current ratio maintained at approximately 104 at1.5 V is stable for at least 104 s. As shown in Fig. 5b, no degra-dation was observed for both the ON and OFF states up to 105

read cycles. Fig. 5c shows the programmable cyclic duration ofPPFS-based memory device under the action of repeated write–read–erase–reread (WRER) cycles. The voltages of write, read,erase, and reread for the PPFS-based memory device are�2, 1.5,5, 1.5 V, respectively.

According to the operating mechanism of the PPFS basedmemory device, our previous suggested conformation inducedelectrical switching mechanism could explain the electricalswitching behavior of PPFS appropriately.22 The unstableconformational dynamics of PS has been improved by theintroduction of bulky PFMs. Different stable conformations ofPPFS with high or low conductivity states could be obtainedunder the action of electric eld through the p–p stackingalignment of pendent groups in PPFS (Fig. 6). The conforma-tion change of PPFS induced the electrical switchingperformance.

Polym. Chem.

Fig. 5 (a) Retention time test of the PPFS device in ambient conditions. (b)Stimulus effect of read pulses on the PPFS device current in the OFF and ON states.(c) Write–read–erase–read operations of the PPFS device with the correspondingapplied voltages.

Fig. 6 The front (a) and side (b) view of the molecular simulation model of PPFS.

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Electroluminescence properties of PPLEDs

To evaluate the potential of PPFS as host materials, we fabri-cated a device using PPFS as blue phosphorescent hosts andFIrpic as the phosphorescent emitter. The device was fabri-cated using the device structure ITO/PEDOT:PSS (10 nm)/PPFS:30% FIrpic (65 nm)/TPBI (40 nm)/Ca:Ag. The emittinglayer was spin-coating from chlorobenzene solution onto thePEDOT:PSS smoothed ITO glass substrate, and the electron-transporting layer of TPBI was vapor deposited under vacuum.As for the EL spectra of the blue PPLEDs, the devices showedpure blue emission with emission peak at 476 nm and a

Fig. 7 (a) EL spectra of PPFS device doped with FIrpic at a mass ratio of 30 wt%.(b) Current density–voltage (J–V) and luminance–voltage (L–V) curves of PPFSdevices. (c) Luminance and power efficiency characteristic curves as a function ofluminance of PPFS devices.

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shoulder at around 500 nm, and CIE chromaticity coordinates(x, y) of (0.146, 0.349) (for details, see Fig. 7a). The currentdensity–voltage–luminance (J–V–L) curves and the luminance–efficiency curves are presented in Fig. 7b and c, respectively.The device exhibits maximum luminance of 793 cd m�2,maximum luminance efficiency of 3.0 cd A�1 and themaximum power efficiency of 0.54 lm W�1. The EL perfor-mance of PPFS based device may be further improved inoptimized devices.

Conclusions

In conclusion, we report the synthesis and properties of a novelvinyl-type p-stacked polymer based on PS with pendent cardogroup, PPFS. Steric hindrance design of PPFS with cardo group(PFMs) shows excellent thermal, morphological stabilities,stable p-stacked conformation, high triplet energy level of2.80 eV and suitable energy gap of 4.18 eV. Memory devicesusing PPFS as active layers exhibited ash memory perfor-mance with ON/OFF current ratio up to 104, high stability inretention time up to 104 s and the number of read cycles up to105 under a read voltage of 1.5 V in both ON and OFF states,while the PS based device exhibited no memory performance.The PPLEDs using PPFS as the host and FIrpic as the guestexhibited blue emission with CIE chromaticity coordinates(x, y) of (0.146, 0.349). The results indicate the versatileapplications of our hindrance-functionalized p-stacked poly-mers based on PS.

Acknowledgements

The project was supported by the National Key Basic ResearchProgram of China (973) (2009CB930601), National NaturalScience Foundation of China (21274064, 21144004, 20974046,20774043, 20704023, 51173081, 50428303, BZ2010043,61177029, 61136003, 60876010), The Program for New CenturyExcellent Talents in University (NCET-11-0992), Key Project ofthe Ministry of Education of China (707032, 208050), NaturalScience Foundation of Jiangsu Province, China (SBK201122680,BK2011761, BK2008053, BK2009025, 10KJB510013, SJ2009003),and Program for Postgraduates Research Innovation inUniversity of Jiangsu Province (CXZZ11_0410), L. H. X. thanksJiangsu Overseas Research & Training Program for UniversityProminent Young & Middle-aged Teachers and Presidents.

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