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Photoinduced Step-Growth Polymerization of N-Ethylcarbazole Erdem Sari, Gorkem Yilmaz, Sermet Koyuncu, and Yusuf Yagci J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08668 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018
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Journal of the American Chemical Society
Photoinduced Step-Growth Polymerization of N-Ethylcarbazole Erdem Sari,ξ Gorkem Yilmaz,ξ Sermet Koyuncu,℘ Yusuf Yagciξ,* ξ
Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey Canakkale Onsekiz Mart University, Department of Chemical Engineering, TR-17020 Canakkale, Turkey
℘
Supporting Information ABSTRACT: A novel method for photochemical step-growth synthesis of poly(N-ethylcarbazole) (PEC) via consecutive diphenyl iodonium hexafluorophospate (Ph2I+PF6-) mediated electron transfer and coupling reactions is reported. The photoinduced electron transfer reaction of the excited N-ethylcarbazole (EC) in the presence of Ph2I+PF6- as the oxidizing salt proceeded efficiently giving EC radical cations (EC+.). Subsequently, the protons released concomitantly with coupling of two EC radical cations. The successive reactions involving excitation, electron transfer, proton release, and coupling lead to the formation of PEC. The electrochemical properties and surface morphology of the thin films of the formed polymers before and after dedoping were investigated by cyclic voltammetry, differential pulse voltammetry and atomic force microscopy techniques, respectively.
Scheme 1. Synthesis of PEC by Yamamoto cross-coupling reactions and photoinduced step growth polymerization.
Br
Br N R
Ni cat. n
N R
R: alkyl chains
Previous strategies: i.e. Yamamoto coupling Ph2I+PF6N
hv
N
n
This work: Photoinduced step-growth polymerization
Conjugated polymers receive constant interest because of their applicability in the fabrication of various electronics and electrooptic devices such as organic photovoltaics,1 light emitting diodes,2 batteries,3 electrochromic devices4 and sensors5 arising from their characteristic electronic and optical properties. The widely applied strategies for the synthesis of these polymers include oxidative polymerization,6 electropolymerization7 and transition metal mediated processes such as Suzuki8 and Yamamoto9 coupling reactions. Depending on the synthetic methodology applied, polymers with different morphologies and consequently different physical and chemical properties can be obtained. They can also be combined with other synthetic polymers to improve their processability and properties.10 Polycarbazoles (PCs) represent a class of conjugated polymers having structures differently constructed via functionalization of the monomer.11 PCs are of vital importance as they possess increased biological potency often coupled with reduced side effects. The homo and copolymers can most commonly be prepared by coupling reactions.12 Because of the poor solubility characteristics caused by the tendency of the conjugated planar π-system toward stacking, generally monomers with long alkyl chain functions are preferred. Typical preparation methodology for a highly conjugated PC is based on Yamamoto coupling13 and presented in Scheme 1.
Photoinduced reactions are attractive processes for the production of macromolecular structures due to the advantages offered by low energy requirements, high reaction rates, and temporal and spatial control.14 Recent developments in photoinduced controlled/living polymerizations and ligation processes have also provided the possibility for structural control.15 Free radical,16 cationic,17 and anionic18 polymerizations can be realized in the broad wavelength range covering UV, visible and near-IR region of the electromagnetic spectrum. Although the photoinduced chain processes are in more advanced state, the corresponding step-growth polymerizations can also be performed by photochemical means. In previous studies from our laboratory, we have shown that polythiophenes can be prepared by photoinduced electron transfer reactions.19 As cationic and radical species are formed as intermediates, the described photoinduced electron transfer process was shown to efficiently initiate radical and cationic polymerizations in the near UV region.15k,20 Given our long-standing interest in the development and exploitation of new photochemical approaches for fabrication of macromolecular structures with different topologies, we envisaged a novel platform to facilitate rapid synthesis of PCs. We hypothesized that the synthesis can be successful through photoinduced electron transfer reactions that have been capitalized on polythiophene and drivatives. By virtue of the carbazole-type radical cation formation and the crucial role of these species in the electropolymerization, the described photoinduced process has the potential to form PCs (Scheme 1).
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Initially, in order to confirm the related photoinduced electron transfer process through interaction of the excited state of Nethylcarbazole (EC) with iodonium salt, real-time UV analysis under diluted concentrations were performed. When the spectrum of the mixture of EC and the onium salt is compared with that of EC alone, it has been observed that an interaction in the ground state can be excluded (Supporting Information, Figure S1). As seen in Figure 1a, broad peaks at around 500-750 nm rapidly raised indicating the realization of conjugation under oxidative conditions (Figure 1a). The high wavelength bands are attributed to the polaronic form of ethyl carbazole polymer (PEC) (Inset Figure 1b).21 In order to break the polaronic form, the material obtained was treated with hydrazinium hydroxide:methanol mixture (1:1; v:v), which is known to remove the PF6- dopant ions on the PEC polymer. Optical band gap as calculated from the absorption edge was found to be 3.22 eV (Figure 1b).
On the other hand, photoluminescence behavior of the polymer solution in dichloromethane was investigated by using excitation and emission spectra (Figure 1c). When PEC was excited at 350 nm, it exhibited an emission band centered at 402 nm with a Stokes’ shift of 52 nm corresponding to blue regime in the visible spectrum. It is observed that the fluorescence emission was slightly intensified after the dedoping process. Thus, fluorescence emission of dedoped PEC is slightly quenched as a result of interaction between PF6- ions and conjugated PEC backbone. The electrochemical behavior of the product before and after the dedoping process was also investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The positive scan in CV displayed peaks at Eoxp,a= 1.5V, Eoxp,c=1.1 V and Eoxp,1/2= 1.3 V, which can be attributed to the conjugated carbazole repeating units (Figure 2a). After the dedoping process, relatively lower oxidation potentials were obtained (Eoxp,a= 1.4 V, Eoxp,c=1.2 V and Eoxp,1/2= 1.3 V) due to the more rich electron density of the oxidative N-position of carbazole backbone. The onset potentials obtained from the DPV measurements were used to calculate the HOMO levels of PEC before and after the dedoping process. The HOMO levels were reduced from -5.28 eV to -5.44 eV after dedoping.
Figure 2. Redox behavior (a) and DPV measurements and HOMO levels (b) of PEC before and after dedoping process in 0.1 M TBAPF6/acetonitrile electrolyte solution at scan rate 100 and 5 mV/s, respectively vs. Ag wire. Figure 1. UV-vis spectral change during irradiation of EC (2.5 x 10-4 mol·L-1) in the presence of Ph2I+PF6- (5 x 10-4 mol·L-1) in CH2Cl2 (a) UV-vis, Inset: decrease of the polaron band (b) and fluorescence spectra of PEC before and after removal of PF6- (c).
Figure 3 shows the 1H NMR spectra of PEC and its monomer EC in CDCl3. The significant broadening of the corresponding signals further substantiates the conclusion drawn from the UV-vis spectral analysis.
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Journal of the American Chemical Society roughness of the polymer was found to be 4.2 nm according to the topography image. As expected, the dedoping process gives rise to the formation of more perforated surfaces due the absence of the above-mentioned attractions. The AFM image exhibits agglomerated and partly uniform grains arising from helical 3,6linkage between carbazole moieties of the polymer backbone.22 The grains growth occurs with changing the orientation of carbazole moieties on the polymer backbone. Due to these interactions, the roughness increased to 8.4 nm after the dedoping process. Increased surface roughness may be due to the better π stacking between carbazole units on the polymer backbone after removing the PF6- ions. Apparently, the morphologies of two polymer films are quite different from each other and its properties can be affected by the 3D molecular shape of PEC. Figure 3. 1H NMR spectra of N-ethylcarbazole and poly(Nethylcarbazole) Inspired by our studies with thiophene derivatives and the spectroscopic evidences obtained in this study, the polymerization is considered to follow a step-growth mechanism by successive cycles of photoinduced electron transfer and coupling of radical cations as depicted in Scheme 2. By virtue of the EC type radical cation formation by electron transfer in the exciplex and the crucial role of these species in electro and oxidative polymerization, the described photoinduced process afforded to form such conjugated polymeric molecules.
Scheme 2. Polymerization of N-ethylcarbazole by photoinduced step-growth polymerization using diphenyliodonium hexafluorophosphate Figure 4. AFM images of PEC thin film on the glass surface before (a) and after dedoping process (b).
* N
N hv
In summary, we have accomplished the first photoinduced stepgrowth synthesis of EC. The key steps included photoinduced formation of exciplex from EC and the iodonium ions, electron transfer reaction within the exciplex to form radical cations of EC and proton release. An efficient sequence proceeded for the laststage construction of the PEC by the coupling process. This work paves the way for the photochemical synthesis of carbazole type conjugated polymers and may facilitate the electronic studies of this fascinating class through formation on specific surfaces with spatial control. Future studies will pursue the incorporation and effects of other structurally related molecules, and synthetic manipulations to combine the described step-growth polymerization with chain polymerizations by taking advantage of the radical and ionic species thus formed.
*
* N
N
Ph2I+ PF6-
Ph2I+ PF6(exciplex)
* N Ph 2I+ PF6-
e- transfer
N
PF6+ Ph2I
(exciplex)
N
PF 6-
N
N
dimerization
2
- 2 H+PF6-
ASSOCIATED CONTENT
N N
N N hv Ph2I+ PF6-
Supporting Information n
As stated, carbazole based polymers are essential components in optoelectronic applications such as organic light emitting diodes, photovoltaic and electrochromic devices as hole transport materials. Thin film morphology of the polymer is of great importance to meet the physical properties demanded for such applications. Therefore, the surface morphology of PEC films was investigated. PEC films were prepared on glass surface by spin casting process using 2% polymer solution in THF. Surface morphology of PEC films prepared before and after the dedoping process was examined by Atomic Force Microscopy (AFM) (Figure 4). The AFM image of the PEC film before dedoping exhibits uniform structure due to PF6- ions, which have ion-dipole attractions with the glass surface. The RMS (root mean surface)
The Supporting Information is available free of charge on the ACS Publications website. List of the materials, instrumentation, experimental details and UV analysis can be found in Supporting Information (PDF).
AUTHOR INFORMATION Corresponding Author
[email protected] ORCID Yusuf Yagci: 0000-0001-6244-6786
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Gorkem Yilmaz: 0000-0002-8638-4548 Sermet Koyuncu: 0000-0001-8352-8326 Erdem Sari: 0000-0002-8259-2032
Author Contributions All authors contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT The Authors would like to thank Istanbul Technical University for financial support.
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