Carbazole and Diketopyrrolopyrrole-Based D-A π-Conjugated Oligomers Accessed via Direct C–H Arylation for Opto-Electronic Property and Performance Study

Five carbazole and diketopyrrolopyrrole-based donor-acceptor (D-A) new π-conjugated oligomers (π-COs) with gradually elongated lengths are facilely synthesized via a single pot of direct C–H arylation with merits of atom- and step-economy. The structure-property-performance correlations of these π-COs and their parent polymer are studied in detail by opto-electronic characterizations and bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. It is found that the π-COs having longer lengths enable better performance in OPVs owing to the enhanced intermolecular interaction with the elongation of the conjugations. The above results not only highlight the powerful synthetic strategy here provided, but also reveal that π-COs with unique properties might find promising application in OPVs.

Despite the abovementioned potentials, the synthetic tools for efficient access of π-COs, especially for the long-chain ones, are still very limited. Until now, approaches to π-conjugated materials mainly rely on the Pd-catalyzed C-M/C-X (M = B or Sn, X = Br or I) Suzuki or Stille cross couplings involving the pre-functionalization of precursors with C-M bonds under harsh conditions, such as anhydrous strong bases, and low reaction temperatures [29]. Meanwhile, the poor compatibility of active C δ− -M δ+ bonds with the electrophilic groups will be unfavorable to the structural diversity, and thus the controllable temperatures [29]. Meanwhile, the poor compatibility of active C δ--M δ+ bonds with the electrophilic groups will be unfavorable to the structural diversity, and thus the controllable and scalable synthesis of π-functional materials. It is known that C-H represent the most widely distributed bond among the organic compounds. The direct transformation of C-H bonds [30,31] to the desired functional groups (i.e., direct functionalization of C-H bonds) is thus considered as the preferential method to shorten synthetic steps, reduce side-products, and improve the atom economy. In recent years, direct C-H arylation (i.e., C-H/C-X couplings) has emerged as one of the most promising alternatives to the classical C-M/C-X (M = B or Sn) cross couplings for the synthesis of π-conjugated materials, owing to its outstanding merits of functional group compatibility, atom economy, and cost-effectiveness [32][33][34][35][36][37][38][39][40].

Opto-Electrochemical Property Study
Typically, the opto-electronic properties of π-conjugate materials can be finely tuned via introducing D-A architecture to modulate the electron push-pull effect. Here, the DPP-Cz-based D-A π-COs (Scheme 1) with gradually elongated conjugations will be ideal models for a structure-property-performance correlation study. The opto-electrochemical properties of Os1~5 and P1 were investigated by UV-vis absorption and photoluminescence (PL) spectroscopies and a cyclic voltammetry (CV) test ( Figure 2 and Table 1). As shown in Figure 2a,b, these five π-COs with A.A-D-A.A backbones exhibit broad light absorption ranging from 450 to 800 nm both in diluted chloroform (CF) solutions and solid state films. The structural evolution from O1 to O5 and P1 leads to gradual red-shifts in light absorption, implying the enhanced intramolecular charge transfer (ICT) with the elongation of π-conjugation. All π-COs and P1 exhibit similar spectral profiles involving two bands at 320-500 and 500-700 nm, which are due to the localized π-π* transition and intermolecular charge transfer associated with interchain π-π transition, respectively.

Opto-Electrochemical Property Study
Typically, the opto-electronic properties of π-conjugate materials can be finely tuned via introducing D-A architecture to modulate the electron push-pull effect. Here, the DPP-Cz-based D-A π-COs (Scheme 1) with gradually elongated conjugations will be ideal models for a structure-property-performance correlation study. The opto-electrochemical properties of Os1~5 and P1 were investigated by UV-vis absorption and photoluminescence (PL) spectroscopies and a cyclic voltammetry (CV) test ( Figure 2 and Table 1). As shown in Figure 2a,b, these five π-COs with A.A-D-A.A backbones exhibit broad light absorption ranging from 450 to 800 nm both in diluted chloroform (CF) solutions and solid state films. The structural evolution from O1 to O5 and P1 leads to gradual red-shifts in light absorption, implying the enhanced intramolecular charge transfer (ICT) with the elongation of π-conjugation. All π-COs and P1 exhibit similar spectral profiles involving two bands at 320-500 and 500-700 nm, which are due to the localized π-π* transition and intermolecular charge transfer associated with interchain π-π transition, respectively. two absorption peaks at 550~590 nm and 605~660 nm, respectively. As shown in Figure  2b and Figure S4 (SI), the relative intensity of the peaks at longer wavelengths compared to the peak at shorter wavelengths increase gradually as the π-COs evolved from O1 to O5 and P1 due to the increased interchain interaction with the elongation of π-conjugations.   The thin film of Os1~5 and P1 show a broadened absorption region with the λmax f at 575, 614, 644, 638, 704.5 and 696 nm, respectively. Compared to solutions, the light absorption of thin films (λmax f ) of the π-COs and P1 exhibit a distinct broadened shift and peaks at the ICT band (Figure 2b), indicating the strong intermolecular π-π stacking in the solidstate films. Owing to the aggregated solid state, the thin films of π-COs and P1 have two more pronounced vibration peaks with a broader spectrum compared to their corresponding solutions. The PL spectroscopy test (Figure 2c) was employed to reveal the reorganization energy between the ground and excited-state transition. The CF solutions of Os1~5 excited by 500, 520, 546, 547, and 548 nm gave emission peaks (λem) at 646, 688, 692, 698 The maximum absorption peaks of Os1~5 and P1 in solutions (λ max s ) are located at 598, 603, 645, 650, 652.5 and 658 nm, respectively. The red-shifts of Os2~5 and P1 compared to O1 are 5, 47, 52, 54.5 and 60 nm, respectively, as a result of the increase of π-conjugation length. Corresponding to the gradual red-shifts of light absorption, the colors of Os1~5 and P1 solutions evolved from purple, blue-purple, blue, blue-green to green (Scheme 1). The extinction coefficients (ε) of Os1~5 solutions at λ max s are 6.3 × 10 4 , 1.2 × 10 5 , 1.9 × 10 5 , 2.8 × 10 5 and 3.8 × 10 5 M −1 ·cm −1 respectively, showing a near linear-correlation with the numbers of the repeating units (i.e., DPP-Cz) involved. The π-COs and P1 have two absorption peaks at 550~590 nm and 605~660 nm, respectively. As shown in Figure 2b and Figure S4 (SI), the relative intensity of the peaks at longer wavelengths compared to the peak at shorter wavelengths increase gradually as the π-COs evolved from O1 to O5 and P1 due to the increased interchain interaction with the elongation of π-conjugations. The light absorption onsets (λ onset ) of Os1~5 and P1 are 708, 725, 749, 756, 761 and 776 nm, respectively. Accordingly, the optical bandgaps (E g opt ) of Os1~5 and P1 calculated from 1240/λ onset are 1.75, 1.71, 1.66, 1.64, 1.63 and 1.60 eV, respectively.
The thin film of Os1~5 and P1 show a broadened absorption region with the λ max f at 575, 614, 644, 638, 704.5 and 696 nm, respectively. Compared to solutions, the light absorption of thin films (λ max f ) of the π-COs and P1 exhibit a distinct broadened shift and peaks at the ICT band (Figure 2b), indicating the strong intermolecular π-π stacking in the solid-state films. Owing to the aggregated solid state, the thin films of π-COs and P1 have two more pronounced vibration peaks with a broader spectrum compared to their corresponding solutions. The PL spectroscopy test (Figure 2c) was employed to reveal the reorganization energy between the ground and excited-state transition. The CF solutions of Os1~5 excited by 500, 520, 546, 547, and 548 nm gave emission peaks (λ em ) at 646, 688, 692, 698 and 702 nm, respectively. The Stokes shifts (SS) of Os1~5 are 43, 43, 42, 43, and 44 nm, respectively.
The frontier molecular orbital (FMO) levels were evaluated by CV test. The highest occupied molecular orbital (HOMO) levels of π-COs and P1 can be estimated from CV curves ( Figure S5, SI). The corresponding lowest unoccupied molecular orbital (LUMO) levels were calculated from E LUMO = E HOMO + E g opt . The HOMO/LUMO levels of Os1~5 and P1  (Figure 2d). The HOMO levels display an up-shift trend from O1 to O5, owing to the increasing ratios of electronic-donating Cz with the increase of π-conjugation lengths, i.e., the DPP/Cz ratios for O1, O2, O3, O4 and O5 are 2/1, 3/2, 4/3, 5/4 and 6/5 (Scheme 1), respectively, which also accounts for the deepest LUMO level of O1.

BHJ OPV Performance Study
The OPV performances of the π-COs and P1 were studied by a device architecture of glass/ITO/PEDOT:PSS/BHJ layer/PFN-Br/Ag, wherein the BHJ layers consist of π-COs (or P1) and PC 70 BM as donor and acceptor, respectively. All BHJ layers were fabricated by spin-coating with π-COs/PC 70 BM mixed solution on unheated substrates. The current density-voltage (J-V) curves and the photovoltaic parameters of the BHJ devices are depicted in Figure 3 and Table 2. The power conversion efficiencies (PCE) π-COs increases with the increase of π-conjugation length, and the PCE of O5 is the closest to that of P1. Despite the fact that π-COs and P1 have low PCEs, the J SC of O4 and O5 are very close to that of P1, which can be attributed to O4 and O5, which are close to the effective conjugation length. In addition, the fill factor (FF) of Os1~5 increases with the increase of chain length. The FF of O5 even exceeds that of P1. Although the π-COs decreases in V OC s with the increase of chain length, however, all π-COs exhibit higher V OC s compared to P1 (0.75 V), which can be attributed to a deeper HOMO level of the π-COs (Figure 2d) that have larger offsets with the LUMO of PC 70 BM. and 702 nm, respectively. The Stokes shifts (SS) of Os1~5 are 43, 43, 42, 43, and 44 nm, respectively.

BHJ OPV Performance Study
The OPV performances of the π-COs and P1 were studied by a device architecture of glass/ITO/PEDOT:PSS/BHJ layer/PFN-Br/Ag, wherein the BHJ layers consist of π-COs (or P1) and PC70BM as donor and acceptor, respectively. All BHJ layers were fabricated by spin-coating with π-COs/PC70BM mixed solution on unheated substrates. The current density-voltage (J-V) curves and the photovoltaic parameters of the BHJ devices are depicted in Figure 3 and Table 2. The power conversion efficiencies (PCE) π-COs increases with the increase of π-conjugation length, and the PCE of O5 is the closest to that of P1. Despite the fact that π-COs and P1 have low PCEs, the JSC of O4 and O5 are very close to that of P1, which can be attributed to O4 and O5, which are close to the effective conjugation length. In addition, the fill factor (FF) of Os1~5 increases with the increase of chain length. The FF of O5 even exceeds that of P1. Although the π-COs decreases in VOCs with the increase of chain length, however, all π-COs exhibit higher VOCs compared to P1 (0.75 V), which can be attributed to a deeper HOMO level of the π-COs (Figure 2d) that have larger offsets with the LUMO of PC70BM.

Measurements and Reagents
Unless otherwise specified, all of the conventional chemicals were purchased from Energy Chemical. Cz and DPP were purchased from SunaTech Inc., Soochow. The anhydrous toluene was obtained by distillation after calcium hydride treatment. The 1 H and 13 C NMR spectra were obtained from the samples dissolved in CDCl 3 using a Bruker 400 spectrometer ( 1 H NMR 400 MHz and 13 C NMR 101 MHz). MALDI-TOF MS were performed on a Bruker Auto flex II using 2,5-dihydroxybenzoicacid or α-cyano-4-hydroxycinnamicacid as the matrixes. Elemental analyses were conducted on a Flash EA 1112 elemental analyzer. The Mn, Mw and PDI of all oligomers and polymers were measured with the GPC (Viscotek TDA302 triple detector) using THF as the eluent. The UV-vis absorption spectra were measured by a Shimadzu UV-2450 spectrophotometer. Cyclic voltammetry (CV) was done on a CHI 661C electrochemical workstation with a Pt disk, Pt plate, and standard 10 calomel electrode (SCE) as the working electrode, counter electrode, and reference electrode, respectively, in a 0.1 mol L −1 tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) CH 2 Cl 2 solution.

Synthesis of P1 ((Cz-DPP) n )
DPP (139.71 mg, 1 equiv) and Cz (150 mg, 1 equal) were used as reactants. Other additives are the same as the synthesis of π-COs. After polymerization for 48 h, the crude product was purified by precipitation in methanol, filtered, and washed on Soxhlet with methanol acetone, hexanes, and chloroform successively. The chloroform fraction was condensed under reduced pressure, and obtained the polymer P1: (238 mg, yield 83%). 1

BHJ Device Fabrication
The conventional glass/ITO/PEDOT:PSS/BHJ layer/PFN-Br/Ag structure was employed to fabricate the OPV devices. The indium tin oxide (ITO) substrates were cleaned sequentially with deionized water and isopropyl alcohol by ultrasonication, and then dried in an oven at 60 • C overnight. The dried ITO substrates were treated with oxygen plasma for 5 min and then coated with PEDOT:PSS at 3000 rpm for 60 s. The film was annealed on a hot dish at 150 • C in air for 15 min to obtain a thickness of about 40 nm. The substrates were then transferred to a N2-protected glove box. The active layer was obtained by spincoating a chlorobenzene solution containing D: A (1:1) and 0.5% (v/v) 8-diiodooctane (DIO) additive at a total concentration of 20 mg mL −1 . The optimum film thickness of about 100 nm was obtained by controlling the rotation speed of about 2500 rpm. The substrate coated with the active layer was then placed on a hot plate and thermally annealed for 10 min. Subsequently, about 6 nm of PFN-Br was spin-coated onto the active layer as a cathode interfacial layer. Finally, 100 nm of silver was thermally deposited on top of the interface by a mask in a vacuum chamber at 1 × 10 −7 Torr pressure. The effective area of the device is 0.04 cm 2 .

Conclusions
A series of DPP-Cz based π-COs Os1~5 with gradually increasing π-conjugation lengths were designed and facilely synthesized via a one-pot direct C-H arylation for opto-electronic property and OPV performance study. All π-COs involve the same building blocks (i.e., DPP and Cz), but have different conjugation lengths. The Os1~5 and their parent polymer P1 were employed as donor materials to pair with PC 71 BM for BHJ OPV device study. The results obtained here demonstrate that D-A backboned π-COs with progressively increased conjugation chain lengths via one pot C-H/C-Br coupling is a promising way to design and synthesize π-COs for BHJ OPV device applications and expand the horizons of π-functional materials.