Optical bandgaps and fluorescence resonance energy transfer studies of a series of poly(phenyleneethynylene) derivatives

doi:10.1016/j.reactfunctpolym.2011.07.003

Reactive and Functional Polymers

Volume 71, Issue 10, October 2011, Pages 1008-1015

https://doi.org/10.1016/j.reactfunctpolym.2011.07.003 Get rights and content

Abstract

In this paper, research work focuses on the synthesis of a series of PPE-based polymers with commonly used conjugated units, including thiophene, benzo[c][1,2,5]thiadiazole (BT), benzo[c][1,2,5]selenadiazole (BSe), etc. The optical bandgaps of these polymers were tuned in the range of 2.10–2.76 eV. The order of bandgap-lowering ability of these units in PPE-derivatives is: M-3 > M-6 > M-5 > M-4 > M-9 ⩾ M-7, M-8 > M-2. Their FRET applications in polymer solar cell and TNT detection were studied respectively, and the results indicated that all these PPE-derivatives were good candidate materials for polymer solar cells or detecting TNT in solution. Furthermore, if electron-acceptor units had structures similar to the diphenylquinoxaline in the PPE-derivatives chain, the polymers would give a better fluorescence quenching in response to TNT compound. Polymers PPE-7 and PPE-8 were chosen as representative samples to investigate their photo-oxidative stability compared with that of PPVs or PTs. The results demonstrated that both polymers PPE-7 and PPE-8 were more photo-oxidatively stable than MEH-PPV or P3HT.

Introduction

Conjugated polymers have attracted considerable attention due to their versatile applications in the fields of conducting polymers and optoelectronic devices. As optoelectronic materials, conjugated polymers have been broadly used in polymer light-emitting diodes (PLED) [1], polymer field-effect transistors (PFETs) [2], chemical sensors [3], photodetectors [4], polymer solar cells (PSCs) [5], etc.

Among these applications, conjugated polymers for implementation in PSCs have gained the most interest in recent years [6]. Until now, the photoactive layer of polymer photovoltaic devices mainly consists of conjugated polymers, such as poly(p-phenylenevinylene) derivatives (PPVs) [7], [8], [9], [10], polythiophene derivatives (PTs) [11], [12]. Poly(phenyleneethynylene)s (PPEs) type polymers have similar structures compared to the PPVs type polymers, but they have attracted less attention because of their higher bandgaps. The mismatch between the absorption spectrum of the PPEs and the solar emission will lead to lower power conversion efficiency of PSCs. However, in comparison to PPVs, PPEs are well known to be more photo-oxidatively stable than that of PPVs ascribe to PPEs with a higher oxidation potential [13]. Hence, PPEs would be still promising photovoltaic materials if their bandgaps were decreased.

In this paper, a series of PPE-based polymers with commonly used conjugated units, including thiophene, benzo[c][1,2,5]thiadiazole (BT), benzo[c][1,2,5]selenadiazole (BSe), etc., were synthesized in order to tune their bandgaps, and the results provided some useful reference to control bandgaps of PPE-derivatives. Additionally, their fluorescence resonance energy transfer (FRET) applications in PSCs and TNT detection were studied respectively, and the results indicated that all these PPE-derivatives were good candidate materials for PSCs or detecting TNT in solution. Furthermore, if electron-acceptor units had structures similar to the diphenylquinoxaline in the PPE-derivatives chain, the polymers would give a better fluorescence quenching in response to TNT compound. Polymers poly[(dioctyloxyphenylene-ethynylene)-alt-(5,8-dibromo-2,3-diphenylquinoxaline)] (PPE-7) and poly[(dioctyloxyphenylene-ethynylene)-alt-(10,13-dibromodibenzo[a,c]phenazine)] (PPE-8) were chosen as representative samples to investigate their photo-oxidative stability compared with that of PPVs or PTs. The results demonstrated that both polymers PPE-7 and PPE-8 were more photo-oxidatively stable than MEH-PPV or P3HT.

Section snippets

Materials

2,5-Dibromothiophene (M-2) and all starting materials were purchased from Sinopharm Chemical Reagent Co. Ltd. All these agents were analytical purity and solvents were purified according to standard methods prior to use. Monomers 1,4-diethynyl-2,5-dioctyloxbenzene (M-1) [14], 2,5-dibromo-3,4-dinitrothiophene (M-3) [15], 4,7-dibromo-2,1,3-benzothiadiazole (M-4) [16], 4,7-dibromo-5,6-dinitryl-2,1,3-benzothiadiazole (M-5) [17], 4,7-dibromoselenole (M-6) [18], 5,8-dibromoquinoxaline (M-7) [19],

Thermal stability and thermal transitions

All of these PPE-based polymers exhibit very similar thermal stability, and Fig. 1 shows the thermogravimetric analysis (TGA) plots of PPE-7 and PPE-8 as representative samples. The onset decomposition temperatures of the polymers are around 260 °C with protection of nitrogen. Obviously, the thermal stability of these PPE-based polymers is adequate for their applications in optoelectronic devices.

Differential Scanning Calorimetry (DSC) has been used to detect thermal transitions for each

Conclusion

In this paper, eight PPE-based conjugated polymers (Scheme 2) were synthesized, and eight commonly used aromatic units (Scheme 1) were used to tailor the bandgaps of the polymers. The effects of these units on bandgaps were investigated systematically by absorption spectroscopy. The results show that bandgaps of these polymers could be tuned effectively by copolymerizing with different units. The order of bandgap-lowering ability of these units in PPE-derivatives is: M-3 > M-6 > M-5 > M-4 > M-9 ⩾ M-7, M-8

Acknowledgements

This work was supported by the ‘Doctorate Foundation of Northwestern Polytechnical University (Grant No.: CX201118), National Program on Key Basic Research Project (973 Program) (Grant No.: 2010CB635111), ChunHui Project of The Ministry of Education (Grant No.: Z2009-1-71003)’. The authors acknowledge Hongling Yan for assisting the synthesis of partial PPE-derivatives.

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Optical bandgaps and fluorescence resonance energy transfer studies of a series of poly(phenyleneethynylene) derivatives

Abstract

Introduction

Section snippets

Materials

Thermal stability and thermal transitions

Conclusion

Acknowledgements

J. Appl. Polym. Sci.

Rev. Sci. Instrum.

Nature

Nature

Nature

Science

Chem. Mater.

Proc. Natl. Acad. Sci.

Adv. Mater.

Conjugated Polymers: Processing and Applications

Macromolecules

Thin Solid Films

Macromol. Chem. Phys.

Thin Solid Films

Macromolecules

Proc. SPIE

Chem. Mater.

Thin Solid Films

Macromol. Chem. Phys.

Synthesis

J. Heterocycl. Chem.

Chem. Pharm. Bull.