2,9-Dibenzo[b,def]chrysene as a building block for organic electronics†

In this article, a new series of conjugated polymers based on a low-cost, easily accessible vat dye called Vat orange 1 or 2,9-dibromo-dibenzo[b,def]chrysene-7,14-dione have been prepared. This compound was made electron-donating by reducing and alkylating the ketone groups into alkoxy groups, allowing the introduction of solubilizing, branched alkyl chains at the 7 and 14 positions. Polymerization reactions using Suzuki–Miyaura coupling with 6,60-isoindigo, 3,6-di-2-thienyl-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and 4,7-dithieno-2,1,3-benzothiadiazole (TBT) yielded three donor–acceptor polymers with bandgap values ranging from 1.61 to 1.86 eV. Field-effect transistors (OFETs) and organic solar cells (OSCs) were fabricated and hole mobility values of up to 3.62 10 4 cm V 1 s 1 and a maximum power conversion efficiency of 1.2% were measured, respectively.


Introduction
Dyes possessing a carbon-rich, extended p-conjugated core are promising building blocks for the preparation of semiconductors in organic electronics due to their relatively low cost, stability and ease of functionalization. [1][2][3] In fact, many of them possess synthetic handles such as halogens and ketones that allow easy chemical functionalization and modifications of their electronic properties. Recently, vat orange 3 (4,10dibromoanthanthrone) has been used as the starting material for several p-conjugated molecules and polymers for multiple applications, including field-effect transistors, 4-7 light-emitting diodes, 8,9 aggregation-induced emission (AIE), 10 solar cells [11][12][13][14][15][16][17] and singlet fission. 18,19 This dye showed good versatility as it can be functionalized at the 4, 6, 10 and 12 positions to yield materials with a wide range of properties. [20][21][22][23][24][25][26][27] However, the major drawback of this dye is the presence of two hydrogen atoms at the peri position relative to the two bromine atoms (4 and 10 positions) that cause significant steric hindrance with the neighbouring conjugated units. 28 This structural flaw in regard to the effective conjugation length along with its neutral electronic nature (neither a donor nor an acceptor) makes the synthesis of low bandgap materials based on vat orange 3 very challenging. To overcome this problem, strategies such as the introduction of conjugated spacers such as alkene or alkyne moieties and rigidification through the formation of a carbon bridge to eliminate the peri proton have been explored. 17,28,29 Unfortunately, none of them succeed in significantly decreasing the bandgap of anthanthrone-based molecules and polymers.
Thus, other similar vat dyes have to be explored in order to prepare low bandgap materials that would be suitable for organic electronics applications. Among the commercially available dihalogenated vat dyes, vat orange 1 is particularly appealing since it possesses two bromine atoms at the 2 and 9 positions where virtually no steric hindrance could lead to a high dihedral angle. Moreover, this dye also has two ketones (like vat orange 3), allowing a wide variety of chemical modifications.
Herein, we report the synthesis, characterization and device performances of conjugated polymers based on 2,9-dibenzo [b,def ]chrysene (vat orange 1). In order to ensure the solubility of the polymers, the dye was alkylated at the 7 and 14 positions to give two electron-donor alkoxy groups. This unit was then polymerized with three different electron-accepting units, namely isoindigo (P1), diketopyrrolopyrrole (P2) and bisthiophenylbenzothiadiazole (P3). The polymers were characterized and tested as semiconductors in field-effect transistors and bulk heterojunction solar cells.

Results and discussion
The synthesis of P1-P3 is described in Scheme 1. In order to ensure the solubility of P1-P3, 2,9-dibromo-dibenzo[b,def ]chrysene-7,14-dione (vat orange 1) was first alkylated using a branched alkyl chain using an adapted protocol developed for vat orange 3 to obtain compound 1. 20 Then, a two-fold Miyaura borylation reaction under standard conditions obtained compound 2 in 53% yield. P1-P3 were prepared by Suzuki coupling using the same procedure by coupling compound 2 with isoindigo, DPP and TTBT, respectively. After the synthesis, the polymers were purified by precipitation in methanol, followed by washing with methanol, hexanes and chloroform using a Soxhlet apparatus to remove the oligomers and the catalyst residues. The reaction yield for each polymer was calculated using the soluble fraction only. The molecular weight values (M n and M w ) of P1-P3 were measured by sizeexclusion chromatography using polystyrene standards and 1,2,4-trichlorochlorobenzene as the eluent at 110 1C (Table 1).
Number average molecular weight (M n ) values ranging from 33 to 46 kg mol À1 were obtained, corresponding to the degree of polymerization (X n ) values between 21 and 28, providing good film-forming properties.
Thermal properties were measured using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) and the results are shown in Fig. S9-S12 (ESI †). In all cases, the measured glass transition temperature (T g ) values were between 100 and 120 1C and no melting transition was observed, meaning that P1-P3 are amorphous. These observations agree with the nature of the long, branched alkyl chain attached to the 2,9-dibenzo[b,def ]chrysene unit. 30 TGA analysis was performed under a nitrogen atmosphere at a heating rate of 10 1C per minute. Decomposition temperature (T d ) values between 280 and 290 1C (taken a 5% weight) were measured for all three polymers, indicating that the most sensitive part of the polymers is likely the 2,9-dibenzo[b,def ]chrysene unit. These T d values are similar to those measured for the conjugated polymers based on anthanthrene (vat orange 3). 29 Although these values are lower than those of most conjugated polymers, they are high enough to ensure good thermal stability of the polymers for device fabrication and operation.
The optical properties in both the solution and solid states of P1-P3 are shown in Fig. 1 and are summarized in Table 2. All the polymers showed a broad absorption band in the UV-visible region with l max values ranging from 478 to 700 nm in solution. For P1, the band in the high energy region centered at 478 nm can be attributed to the p-p* transition while the broad band centered at 572 nm is attributed to the Scheme 1 Synthesis of P1-P3.  presence of a charge transfer complex between the dibenzo[b,def ]chrysene and isoindigo units. 31 Interestingly, the solid state spectrum exhibits only a slight bathochromic shift compared to that in solution, indicating that the polymer backbone conformation is very similar in both states. The same behavior was also observed for P2 and P3.
As reported previously for BTD-based conjugated polymers, 32,33 P3 exhibits two sets of absorption bands centered at 384 and 492 nm. Interestingly, the shoulder at 540 nm in the solution UVvisible spectrum, associated to the presence of an intramolecular donor-acceptor complex, increased in intensity in the solid state while becoming broader towards the low energy region of the spectrum. This can be attributed to a planarization of the polymer backbone in the solid state, making the charge transfer complex between the 2,9-dibenzo[b,def ]chrysene and BTD units more efficient. All the polymers exhibit a bandgap value below 1.9 eV (see Table 2), the lowest value being obtained for P2 (1.61 eV). These values correlate well with those measured using electrochemistry in thin films. As expected, the HOMO energy values for P1-P3 are very similar since they are governed by the donor unit that is the same for all three polymers (2,9-dibenzo[b,def ]chrysene unit). For the LUMO, P2 exhibits the lowest energy value at À3.73 eV, which is suitable to undergo charge transfer upon excitation with most of the electron-acceptor molecules used in the fabrication of solar cells.

Theoretical calculations
DFT calculations were performed on the dimers of the three polymers to assess their conformation, the energy levels, and the frontier orbitals distribution using the Gaussian 09 software. 34 Research on the conformation of the minimum energy was first carried out. For this, each part, the vat orange derivative with the bromine atoms substituted by ethyl chains, and the three groups, were first optimized. To find the minimum of energy for the final molecule, the bond between the two parts was rotated by 10 degrees, and the potential energy was computed for each dihedral angle. The density functional theory (DFT) approach was considered using 6-31G** as the basis set and B3LYP as the functional. The dihedral angle corresponds to four consecutive carbon atoms.
Of the three polymers, P2 is the one presenting the lowest dihedral angle (y = 30.31) between the vat orange 1 and the comonomer (see Table S3 and Fig. S23 in the ESI † section). For P1 and P3, the presence of phenyl and an alkyl chain at the 3 position of thiophene induces a wider dihedral angle (y = 40.61 and 44.91, respectively). The low y value in P2 can partially explain the lower bandgap value (E g = 1.61 eV).
For all three polymers, the HOMO orbital is mainly localized on the 2,9-dibenzo[b,def ]chrysene unit, except for P2 in which the HOMO is delocalized over the entire dimer (Fig. 2). As expected, the LUMO orbital is located almost exclusively on the electron-withdrawing unit. The bandgap values calculated for the dimers of P1, P2 and P3 are 2.13, 2.30 and 2.31, respectively.

Field-effect transistors (OFETs)
To investigate the influence of the acceptor moiety on the fieldeffect mobility and charge transport, bottom-gate, top-contact organic field-effect transistors (OFETs) were fabricated. = 1240/l onset ). d HOMO and LUMO energy levels were calculated using the formula E HOMO = À(4.71 + E ox onset ) eV and E LUMO = À(4.71 + E re onset ) eV, in which E ox onset and E re onset are the oxidation onset potential (the CV thin film) and the reduction onset potential (the CV thin film), respectively, versus SCE.

Materials Advances Paper
Open Access Article. Published on 10 November 2021. Downloaded on 6/28/2023 10:28:21 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
The device parameters and the extrapolated figures of merit are shown in Table 3 and fabrication procedures and device characteristics are detailed in the ESI. † The devices prepared from P1, containing the isoindigo acceptor, exhibited a relatively low mobility (2.45 Â 10 À5 cm 2 V À1 s À1 ) and a low I ON/OFF current ratio. The devices prepared from P1 also possess the highest threshold voltage of À14.46 V. In contrast, P2, which contains the diketopyrrolopyrrole acceptor, exhibits a higher charge mobility and better charge transport (an average mobility of 2.99 Â 10 À4 cm 2 V À1 s À1 ), and overall device characteristics. For P3, which contains benzothiadiazole, an average field-effect mobility of 3.22 Â 10 À4 cm 2 V À1 s À1 , an I ON/OFF of 10 3 and a threshold voltage of À9.53 V were measured. Despite the important differences in charge transport between the three semiconducting polymers, all devices showed typical transfer and output characteristics as depicted in Fig. S22 (ESI †).

Organic solar cells (OSCs)
P1-P3 were tested in organic solar cells (OSCs) as p-type materials and the results are summarized in Table 4. The devices were prepared in an inverted geometry using [6,6]phenyl-C61-butyric acid methyl ester (PC 61 BM) as the electron acceptor in the heterojunction blend. The geometry studied was ITO/ZnO/Polymer:PC 61 BM/MoO 3 /Ag. Different ratios of diphenyl ether (DPE) as an additive in the active layer were tested (0, 1 and 2% V/V) to achieve the highest power conversion efficiency (PCE) for each polymer. 35 P1 obtained a PCE of 0.25% without any additive. Under these conditions, P1 achieved the highest open-circuit voltage (V OC = 0.86 V) of the series, which is in accordance with the HOMO level of the polymer compared to P2 and P3. P2 obtained a quite a bit better PCE of 0.66% using 2% V/V DPE. This polymer gave a shortcircuit current density ( J SC ) four times better than that of P1 (2.30 mA cm À2 vs. 0.577 mA cm À2 ), but loses its gain because of a poor fill factor (FF) of 0.387 (P1:0.51). P3 achieved the highest J SC , FF and PCE of the series with 2.8 mA cm À2 , 0.53 and 1.2%, respectively, using 1% V/V DPE.
In the case where no additive was used, P2 performances were very low, worse than P1, probably because the LUMO of P2 (À3.73 eV) is similar to PC 61 BM's. 36 For P1 and P3, their differences in performance may be due to the dihedral angle between the donor and acceptor units.

Conclusions
In conclusion, three donor-acceptor polymers based on vat orange 1 were synthesized using Suzuki coupling. All three polymers are highly soluble in common organic solvents and exhibit excellent film-forming properties. As shown by DFT calculations, the dialkoxy form of vat orange 1 is an electrondonor on which most of the HOMO frontier orbital is localized when coupled to the electron-deficient p-conjugated units. All polymers possess a bandgap value of under 1.9 eV, making them suitable for applications in solar cells. Moreover, they all exhibit hole-transporting properties in field-effect transistors. Thus, vat orange 1 proved to be an efficient, low-cost building block for the preparation of the conjugated polymers for organic electronics.

Conflicts of interest
There are no conflicts to declare.