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Synthesis of Carbon-bridged Oligo(phenylenevinylene)s (COPV) Xiaohan Sun1,2

1 Department of Chemical Engineering, Columbia University, New York, NY, USA 10027 2 UTRIP Program, Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan n ABSTRACT Carbon-bridged oligo(phenylenevinylene)s, specifically, PV monomer and dimer (COPV-1 and COPV-2) was successfully synthesized using a general protocol developed for COPV synthesis, involving a reductive cyclization followed by a Friedel-Crafts cyclization. 2 grams of each COPV compound was obtained with a high percent yield of 96%. The compound was confirmed by NMR analysis. n INTRODUCTION Organic electronics deal with electrically-conductive polymers and small molecules that are carbon-based, which is in contrast to traditional electronics that relies on inorganic conductors and semiconductors, such as copper and silicon. They are lighter, more flexible, and less expensive than inorganic conductors, making them a desirable alternative in optoelectronic applications. Since π-conjugated organic materials are highly responsive to external stimuli such as electrons and photons,

they have been reported to exhibit great potentials for organic optoelectronic devices. For example, phenylenevinylenes, which is structurally flexible, has been synthesized and studied previously, and its useful applications such as solar cells and organic LEDs have been reported years ago.1 However, the problem in practical application is short effective conjugation length due to non-planarity, and low stability due to free rotations. Therefore, in order to achieve high performance and high stability at the same time, we wanted to synthesize carbon bridged OPVs, (COPVs), which is supposed to be a rigid planar structure that is highly conjugated and highly stable. It could be a very powerful building block for optoelectronic applications due to high responsiveness to doping and photoexcitation, and higher stability than conventional

phenylenevinylenes. In our lab, COPV-1 to COPV-6 have been synthesized and studied by Dr.

C

C

C

C

ArAr

ArAr

ArAr

ArAr

C

CArAr

ArAr

C

C

C

C

ArAr

PhPh

PhPh

ArAr

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C

C

C

PhPh

PhPh

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C

PhPhC

CPhPh

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C

Ar Ar

Ar Ar

ArAr

ArAr

ArAr

ArAr

C

C

C

C

ArAr

ArAr

C

C

ArArC

CArAr

ArAr

ArAr

ArAr

ArAr

C

C

Ar Ar

Ar Ar

COPV-1

COPV-2

COPV-3

COPV-4

COPV-5

COPV-6Ar

C

C

C

C

Ar Ar

Ph Ph

Ph Ph

Ar ArC

C

C

C

Ar

Ph Ph

Ph Ph

Ar Ar

C

C

C

C

Ar Ar

Ph Ph

Ph Ph

Ar Ar

Ar = C8H17

Ar2C

CAr'2 n

n = 1– 6

H

H

          Figure 1. Structure of COPV-1 to COPV-6

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Xiaozhang Zhu previously, structures shown in Figure 1. Following a similar protocol, the aim of this program is to synthesize COPV-1 and COPV-2, and confirm its structure by NMR. n RESULTS AND DISCUSSION Synthesis of COPV-1 COPV-1 was synthesized following the procedures from literature2, shown in Scheme 1.

                          1                                                                         2   Scheme  1.  Synthetic  Pathway  for  COPV-­‐1  

Substrate 1 was initially treated with lithium naphthalenide, and then quenched with Ar2CO to obtain substrate 2. Substrate 2 was then treated with boron trifluoride to obtain COPV-1 (96% yield), which is a white solid, shown in Figure 2. The structure of the compound was confirmed by 1H NMR analysis.  

                      a b c Figure 2. (a) Photograph of COPV-1 solid under ambient light (b) Photograph of COPV-1 (left) and COPV-2 dissolved in dichloromethane under ambient light. (c) Photograph of COPV-1 (left) and COPV-2 dissolved in dichloromethane under UV light.

Synthesis of COPV-2 COPV-2 was synthesized following a similar procedure from literature2, shown in Scheme 2.

OMeAr

Ph

C

Ar

Ar Ar

PhAr

C

CAr Ar

Ar Ar

1)LiNaph (2eq)Ar

OH

benzophenone derivativesBF3 Et2O

COPV-1 (2 steps 96%)

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                                                              Scheme  2.  Synthetic  Pathway  for  COPV-­‐2    

All the reagents are almost the same as that in the synthesis of COPV-1, only the starting material is different. This general reaction pathway is very efficient, with a reductive cyclization as the first step, followed by a Friedel-Crafts cyclization as the second step. A carbon bridge can be successfully buttressed within the framework of a PV unit in this way. Starting from the synthesis of a PV monomer and a dimer, we are able to extend to that of COPV-6 in a similar fashion. Stability and Photophysical properties This synthesis of COPV is of great importance due to many good properties of COPVs, such as high thermal and photochemical stability. According to previous research, COPVs can be kept in air without decomposition for at least one year. In particular, COPV-6 melts at a temperature as high as 220℃.3

Such stability of COPVs is much higher than the unsubstituted PVs with several repeating units. Photostability is also an important aspect. Researchers found that with irradiation of Xenon lamp (300W, intensity of 1 x 106 W/m2) in air-saturated methylcyclohexane solution at 30 °C, COPV-4 survives with a half-life of 1578 mins. In comparison, P3HT decomposes 8.3 times faster, and 1,4-bis[(E)-styryl]benzene decomposes over 100 times faster with a half-life of only 12 mins.4 COPVs exhibit very unique photophysical properties as well. As shown in Figure 3, UV-vis spectra of both absorption and emission exhibited intense and well-defined bands.5 From COPV1 to COPV6, increasing conjugation length results in decreasing separation between adjacent energy levels, which corresponds to increasing absorption wavelength. In particular, the absorption spectra show very high extinction coefficient, indicating strong ability to absorb light at a given wavelength. The emission spectra give high quantum yield (up to 1) irrespective of the chain length (number of unit), indicating high emission efficiency. Therefore, we can easily adjust the photophysical properties of COPVs by changing the chain length.

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PhPh

Li

Li

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C

Ph Ph

PhPh

ArOH

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ArAr

HO C

C

Ph Ph

PhPh

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Ar Ar

ArAr

LiNaph(4 equiv)

THFrt, 30 min

BF3 Et2O

CH2Cl2, rt15 min

Ar2CO

"synthetic module"

COPV2

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a b

c

Figure 3. Spectral properties of COPV-1 to COPV-6 in dichloromethane. (a) UV-vis absorption; (b) emission; (c) Photographs of colors under ambient light (left) and under UV light.

n CONCLUSION In this project, COPV-1 and COPV-2 have been successfully synthesized, and structures confirmed by NMR analysis. COPVs exhibited high stability as well as unique photophysical properties. Researchers found that the COPVs we have synthesized can be used for dye-sensitized solar cell (DSSC).6 Such metal-free organic dyes are potentially cheaper and less toxic, therefore indicating the importance of this synthesis work. n ACKNOWLEDGEMENTS I would like to thank Dr. Naiti Lin and Tsuji san for all their help and guidance. I would also like to thank Professor E. Nakamura for insightful advice and lesson. The experiment is financially supported by Nakamura Lab, and all the chemicals were purchased commercially and used without further purification. n REFERENCES (1) X. Zhu, H. Tsuji, J. Teodomiro, L. Navarrete, J. Cordon, E. Nakamura, J.Am. Chem. Soc., 2012,

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134, 19253. (2) Zhu, X.; Tsuji, H.; Nakamura, E. et al., J. Am. Chem. Soc. 2009, 131, 13596. (3) X. Zhu, H. Tsuji, J. Teodomiro, L. Navarrete, J. Cordon, E. Nakamura, J.Am. Chem. Soc., 2012, 134, 19253. (4) X. Zhu, H. Tsuji, J. Teodomiro, L. Navarrete, J. Cordon, E. Nakamura, J.Am. Chem. Soc., 2012, 134, 19253. (5) X. Zhu, H. Tsuji, J. Teodomiro, L. Navarrete, J. Cordon, E. Nakamura, J.Am. Chem. Soc., 2012, 134, 19253. (6) X. Zhu, H. Tsuji, A. Yella, A.-S. Chauvin, M. Grätzel, E. Nakamura, Chem. Commun., 2013, 49, 582.