Cheng Zhang

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Organization: South Dakota State University

Department: Department of Chemistry and Biochemistry

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TOPICS

Polymer

Polyphenylenevinylene

Chemicals
References

Synthesis and characterization of poly(3,5-didodecyl-cyclopenta[2,1-b;3,4-b′]dithiophen-4-one)

Co-reporter:Cheng Zhang, Jianyuan Sun, Sam-Shajing Sun

Synthetic Metals 2016 Volume 221() pp:275-283

Publication Date(Web):November 2016

DOI:10.1016/j.synthmet.2016.09.012

•The first synthesis of a soluble 4H-cyclopenta[2,1-b;3,4-b′]dithiophene-4-one (CPDTO) homopolymer is reported.•Both Ullmann coupling and oxidation polymerization of didodecyl CPDTO gave oligomers.•A mechanism is provided for oxidation polymerization using FeCl3.•Palladium-catalyzed reaction of C12-diBrCPDTO and hexamethylditin was found effective to polymerize the monomer.•The C12-CPDTO polymer has high dihedral angles by DFT calculation. The poor coplanarity leads to wide band gap.Different methods have been explored to synthesize the first soluble cyclopenta[2,1-b;3,4-b′]dithiophen-4-one (CPDTO) homopolymer, poly(3,5-didodecylCPDTO), C12-PCPDTO. Oxidation of the monomer C12-CPDTO by FeCl3 only gave oligomers. Elemental and NMR analyses indicated that oligomers were chlorinated at the both ends. A mechanism is proposed to explain why chlorination is so problematic for oxidation polymerization of C12-CPDTO in comparison to 3-alkylthiophene. Dedoping of the oligomers was also investigated. Pd-catalyzed reaction of dibromo monomer and hexamethylditin yielded C12-PCPDTO-Sn with an Mw of 22,816. This polymer exhibits a bimodal distribution of molecular weight, which is rationalized by an analysis of relative reactivities of species present in the polymerization process. The optical bandgap is 1.85 eV in both chloroform solution and film, much larger than that of electrochemically synthesized unalkylated PCPDTO film. This is explained by poor backbone planarity as predicted by DFT calculation of C2-CPDTO oligomers. The efficient synthesis of the monomer and the polymer paves the way for future synthesis of soluble planar CPDTO homopolymers with alternating alkylated and non-alkylated CPDTO units.Different methods have been explored to synthesize the first soluble CPDTO homopolymer, poly(3,5-didodecyl-cyclopenta[2,1-b;3,4-b′]dithiophen-4-one), C12-PCPDTO. Chemical oxidation of monomer by FeCl3 only gave oligomers, but Pd-catalyzed reaction of dibromo monomer and hexamethylditin yielded C12-PCPDTO with a Mw of 22,816 and a polydispersity of 2.56. The polymer was characterized by 1H and 13C NMR spectroscopy, gel permeation chromatography, differential thermal analysis, cyclovoltammetry, and UV–vis absorption.

Opposite effects of a singlet oxygen quencher on photochemical degradation of dicyano-substituted poly(phenylenevinylenes) with different side chains

Co-reporter:Logan P. Sanow, Cheng Zhang

Polymer Degradation and Stability 2015 Volume 122() pp:146-152

Publication Date(Web):December 2015

DOI:10.1016/j.polymdegradstab.2015.10.018

RO-diCN-PPV and C8-diCN-PPV, poly(1,4-phenylene-1,2-dicyanovinylene) with alkoxy and octyl side chains, have recently been shown to photodegrade via a singlet oxygen mechanism, and RO-diCN-PPV is seven times more stable. To improve photostability, 1,4-diazabicyclo[2.2.2]octane (DABCO), a singlet oxygen quencher, was used as a dopant. To our surprise, DABCO exhibited opposite effects on their photodegradation. With 15 mol% DABCO, degradation rate of C8-diCN-PPV decreased by 65%, while that of RO-diCN-PPV increased by 246%. The DABCO content in C8-diCN-PPV film remained unchanged during 20 min of illumination, but mostly disappeared in RO-diCN-PPV in only 5 min due to decomposition. IR and MW analysis results suggest that DABCO slowed down degradation of C8-diCN-PPV without altering the mechanism, but accelerated RO-diCN-PPV photodegradation by initiating a radical process. C8-diCN-PPV's HOMO energy is lower than that of DABCO by 1.78 eV, a gap too wide for efficient electron transfer to happen. On the other hand, the HOMO of RO-diCN-PPV is only lower by 1.14 eV, allowing DABCO to donate electron to photoexcited RO-diCN-PPV to initiate a radical process that damaged the polymer and destroyed DABCO itself. It was also found that, in RO-diCN-PPV, radical decomposition takes very different paths from those of RO-PPVs and produce very different products.RO-diCN-PPV and C8-diCN-PPV photodegrade via singlet oxygen mechanism and RO-diCN-PPV is seven times more stable. 1,4-diazabicyclo[2.2.2]octane (DABCO), a well-known singlet oxygen quencher, exhibits opposite effects on photodegradation of the polymers. With 15 mol% DABCO, degradation rate of C8-diCN-PPV decreased by 65%, while that of RO-diCN-PPV increased by 246%. Electron donation from DABCO to RO-diCN-PPV can happen upon photoexcitation and results in generation of DABCO cationic radicals which initiate a radical process that damages the polymer and destroys DABCO.

Photochemical stability of dicyano-substituted poly(phenylenevinylenes) with different side chains

Co-reporter:Logan P. Sanow;Jianyuan Sun

Journal of Polymer Science Part A: Polymer Chemistry 2015 Volume 53( Issue 24) pp:2820-2828

Publication Date(Web):

DOI:10.1002/pola.27755

ABSTRACT

The photodegradation mechanism study has been conducted on poly(2,5-dioctyl-1,4-phenylene-1,2-dicyanovinylene) (C8-diCN-PPV) and poly[2,5-bis(decyloxy)−1,4-phenylene-1,2-dicyanovinylene] (ROdiCN-PPV) to understand the reason behind the faster photodegradation of C8-diCN-PPV which has a lower LUMO. In both polymers, radical superoxide anion mechanism, which is responsible for electron-rich RO-PPVs, is found to be energetically unfavorable for both diCN-PPVs due to diCN substitution. The IR analysis results confirm this and suggest that singlet oxygen (O₂) is the main culprit for photodegradation of both polymers, which cleaves the CC bonds into carboxylic acids. The rates of MW reduction (by GPC) and increase in carbonyl IR absorption intensity are in excellent agreement for both polymers. Phosphorescence study indicates that the faster photodegradation of C8-diCN-PPV is due to intersystem crossing, which helps generate singlet O₂ upon photoexcitation. No phosphorescence was detected in RO-diCN-PPV, suggesting that inefficient intersystem crossing makes RO-diCN-PPV photochemically more stable. This work shows that a small difference in side chain structure can lead to a significant difference in photochemical stability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2820–2828

Mitsunobu reactions of aliphatic alcohols and bulky phenols

Co-reporter:Dan Liu, Logan P. Sanow, Cheng Zhang

Tetrahedron Letters 2014 Volume 55(Issue 19) pp:3090-3092

Publication Date(Web):7 May 2014

DOI:10.1016/j.tetlet.2014.03.138

Alkylation of hydrazinedicarboxylate (a Mitsunobu by-product) is not a notable problem in common Mitsunobu alkyl aryl etherification reactions. Good yields can be obtained with a wide range of solvents. However, this side reaction can cause yield reduction for the reactions of sterically hindered phenols and primary alcohols. To suppress the side reaction, solvent effect was investigated. It was found that hydrazinedicarboxylate is about five times less soluble in diethyl ether than in THF, and the yields are improved for ortho-substituted phenols of a wide range of steric hindrance using diethyl ether as the solvent instead of THF which is the more commonly used for Mitsunobu reactions.Alkylation of hydrazinedicarboxylate (a Mitsunobu by-product) could be a side reaction in Mitsunobu alkyl aryl etherification reactions. For most of such reactions in the literature, this side reaction is not a notable problem and good yields can be obtained with a wide range of solvents. However, this side reaction can cause yield reduction for the reactions of sterically hindered phenols and primary alcohols. To suppress the side reaction, solvent effect was studied and it was found that the yields are improved by using a weaker solvent (diethyl ether), instead of THF.

Experimental and computational studies of 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDTO)-oligomers

Co-reporter:Cheng Zhang, Jianyuan Sun, Qiquan Qiao, Jing Li

Polymer 2014 Volume 55(Issue 18) pp:4677-4683

Publication Date(Web):2 September 2014

DOI:10.1016/j.polymer.2014.07.023

4H-Cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDTO), CPDTO ketal (CPDTO-k) and their oligomers have been synthesized and their optical absorption, fluorescent, electrochemical properties are characterized. Structure optimization and time-dependent energy calculation have been carried out for (CPDTO-k)1,2,3,6 and (CPDTO)1,2,3,6 using the B3LYP functional and standard split valence plus polarization basis set 6-31G(d,p) in the DFT formalism. The pi conjugation between carbonyl oxygen and thiophene, which is orthogonal to the backbone conjugation, is evident from the DFT calculation and analysis, and is responsible for the observed slower narrowing of bandgaps (Eg) of CPDTO oligomers with increasing number of repeat units. The orthogonal conjugation also leads to different distribution of LUMO from that of HOMO and makes CPDTO oligomers weak in the lowest energy absorption. Enhanced intensity and red-shifted peak wavelength of the lowest energy UV–vis absorption of CPDTO film suggest the presence of strong intermolecular interactions among CPDTO molecules in the solid state, which may explain why the bandgap (1.1–1.2 eV) reported for the electrochemically deposited CPDTO homopolymer film is significantly lower than the bandgap (1.5–1.7 eV) estimated in this study for PCPDTO chloroform solution from the Egs of CPDTO oligomers.

Design, synthesis, and characterization of a novel c-donor-nc-bridge-c-acceptor type block copolymer for optoelectronic applications

Co-reporter:Sam-Shajing Sun;Jaleesa Brooks;Thuong Nguyen

Journal of Polymer Science Part A: Polymer Chemistry 2014 Volume 52( Issue 8) pp:1149-1160

Publication Date(Web):

DOI:10.1002/pola.27098

ABSTRACT

A novel conjugated block copolymer system containing a donor-type conjugated block (c-D) covalently connected to an acceptor type conjugated block (c-A) via a nonconjugated and flexible bridge chain (nc-B), also called a DBA type block copolymer, has been designed, synthesized, and characterized for potential cost-effective and high-efficiency optoelectronic applications such as solar cells. Specifically, D is a regio-regular para-2-ethylhexyloxy-substituted polyphenylenevinylene (or EH-RO-PPV), A is a regio-regular polyphenylenevinylene with sulfone (SO₂) acceptor moiety and a linear oxydecane (-OC₁₀H₂₁) group substituted on every phenylene unit, and B contains an aliphatic chain with two or four methylene units. The size of each block can be controlled via synthetic feed ratio of the monomer and the terminator. The measured average molecular weights of D, A, and DBA based on gel permission chromatography are in good agreements with the molecular weights calculated using the monomer:terminator synthetic feed ratios. Preliminary optoelectronic device studies revealed an order of magnitude better improvement in photoelectric power conversion efficiency of DBA over the corresponding D/A blend under identical fabrication and testing conditions. Such improvements could be attributed to more efficient photo induced charge separation and charge transport in DBA versus in D/A blends. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1149–1160

Dicyano-Substituted Poly(phenylenevinylene) (DiCN–PPV) and the Effect of Cyano Substitution on Photochemical Stability

Co-reporter:Jianyuan Sun, Logan P. Sanow, Sam-Shajing Sun, and Cheng Zhang

Macromolecules 2013 Volume 46(Issue 11) pp:4247-4254

Publication Date(Web):May 23, 2013

DOI:10.1021/ma400661f

A didecyloxy-substituted poly(phenylenedicyanovinylene), DiCN–PPV, has been synthesized. The dicyano-substituted vinylene units exist in both trans and cis (∼65:35) conformations as determined by 1H NMR analysis, and cannot be converted to all trans due to the presence of a thermodynamic equilibrium of the two conformations, in contrast to the vinylene units in regular PPVs. The unusually high cis content makes this polymer highly amorphous, very soluble in organic solvent, and highly fluorescent in the solid state with an estimated quantum yield up to 0.34, four times more fluorescent than its chloroform solution. The LUMO and HOMO energies of the new polymer were measured by cyclovoltammetry. The cyano groups in DiCN–PPV brings a decrease in LUMO energy by 0.79 eV, and makes the polymer more stable to intense white light (>20 times as strong as the sunlight) than poly(2,5-didecyloxy-1,4-phenylenevinylene), C10O–PPV, by more than 2 orders of magnitude. The excellent photochemical stability and high fluorescence quantum yield in the solid state make DiCN–PPV a good candidate for outdoor fluorescent applications such as remote optical sensing.

Regioregularity and solar cell device performance of poly(3-dodecylthienylenevinylene)

Co-reporter:Jianyuan Sun;Swaminathan Venkatesan;Rui Li;Sam-Shajing Sun;Qiquan Qiao

Journal of Polymer Science Part B: Polymer Physics 2012 Volume 50( Issue 13) pp:917-922

Publication Date(Web):

DOI:10.1002/polb.23078

Abstract

Influence of side chain regioregularity on photovoltaic performance has been investigated in bulk heterojunction solar cells based on a series of poly(3-dodecylthienylenevinylene)s (C₁₂-PTV) and 6,6-phenyl-C₆₁-butyric acid methyl ester (PCBM). Performance of each C₁₂-PTV is optimized for fair comparison. It is found that regiorandomness has no detrimental effect on device performance, in sharp contrast to poly(3-hexylthiophene) (P3HT). Fully regioregular C₁₂-PTV performs slightly poorer than less regioregular ones mainly due to its fast crystallization behavior. The results suggest that introduction of side chain regiorandomness is an effective strategy to enhance processability of certain types of polymers without a reduction in photovoltaic performance. The better polymer:PCBM weight ratio, found to be 3:7 for all C₁₂-PTVs, and improved device performance, as compared with the literature work on the same polymer synthesized by a different method, demonstrate again the importance of the integrity of polymer main chain structure. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Poly(3-dodecylthienylenevinylene)s: Regioregularity and Crystallinity

Co-reporter:Cheng Zhang, Jianyuan Sun, Rui Li, Sam-Shajing Sun, Evan Lafalce, and Xiaomei Jiang

Macromolecules 2011 Volume 44(Issue 16) pp:6389-6396

Publication Date(Web):July 25, 2011

DOI:10.1021/ma200434v

A series of poly(dodecylthienylenevinylenes), C12-PTVs-a:b, with tunable regioregularity and solubility have been synthesized using two isomeric comonomers with molar ratios from 0:10 to 5:5, and characterized by NMR spectroscopy, differential scanning calorimetry, absorption spectroscopy, cyclovoltammetry, and X-ray diffraction. For the first time, the aromatic and olefinic 1H NMR peaks of regioregular head–tail Cn-PTVs are clearly assigned. No major difference in XRD peak intensity is observed among regiorandom and fully regioregular C12-PTVs. Voc of C12-PTVs-a:b and PC60BM blend solar cells are in the range of 0.43 to 0.51 V and consistently decrease as regioregularity is reduced. C12-PTV-3:7:PC60BM solar cells give the highest short-circuit current and practically same efficiency as that of C12-PTV-1:9:PC60BM devices. The results suggest a new strategy to enhance processability of optoelectronic polymers without sacrificing their crystallinity and device performance.