Aminobenzodione-based polymers with low bandgaps and solvatochromic behavior†

Pd-catalyzed amination polymerization of deep blue aminobenzodifuranonemonomer 3-(4-bromo-phenyl)7-(4-octylaminophenyl)-benzo[1,2-b:4,5-b0]-difuran-2,6-dione (M1) is described, as well as polymerization of symmetric dibromophenyl-benzodifuranone (M2), or the corresponding dibromophenylbenzodipyrrolidone (M3) with N,N0-dialkylated phenylenediamines (M4a,b). The resulting polymers P1–3 exhibit low bandgaps (1.08–1.47 eV), broad UV/vis absorption bands (400–900 nm), and a large solvatochromic shift up to 3140 cm 1 from n-hexane to hexamethylphosphoramide. Multiple linear regression analyses of ~nmax of the solvent-dependent solvatochromic UV/vis absorption bands of M1, P1 and P2a are presented, from which Kamlet–Taft and Catalán solvent parameters were determined. All monomers and polymers exhibit high extinction coefficients up to 8.6 10 L mol 1 cm 1 and high photostability, and might be suitable for electronic applications.


Introduction
Molecular D-A (donor-acceptor) systems are of current interest owing to their potential applications in molecular electronic devices, 1 nonlinear optics, 2 and articial photosynthetic systems. 3mong all kinds of electron acceptor moieties, benzodifuranone (BDF), benzodipyrrolidone (BDP) and their derivatives because of their quinonoid structure have received much attention recently as promising candidates for electron-decient materials. 4,5BDF and BDP are high-performance pigments serving as important building blocks for near-infrared (NIR) absorption.They were rst developed in the mid-1970s and commercialized as disperse dyes owing to their deep color and high photochemical stability. 6epending on the substitution pattern, BDFs exhibit red to blue colors. 7onomeric aminobenzodifuranone (ABDF) is a deep blue colored dye with interesting solvatochromic behavior, 8 which has attracted our attention.The aminobenzodifuranone chromophore is a typical D-A system, in which aniline as an electron donor is combined with benzodifuranone as an acceptor unit.To our knowledge, only a few articles reported on derivatives of ABDF, 8 while polymers based on ABDF are completely unknown.
Polymer P2b.In a Schlenk ask, M2 (137.6 mg, 0.28 mmol), N,N-bis(2-octyldodecyl)benzene-1,4-diamine dihydrochloride salt (M4b) (205 mg, 0.28 mmol), Pd 2 (dba) 3 (7.6 mg, 0.008 mmol) and X-Phos (25 mg, 0.053 mmol) are dissolved in a mixture of DMF-toluene (2 : 1) (3.2 ml) under nitrogen.Then, cesium carbonate (198 mg, 0.60 mmol) is added and the mixture is allowed to stir for 48 hours under nitrogen at 90 C. Aer completion of the reaction, the dark solution is treated with DCM, washed three times with water and once with brine.Then the organic layer is dried over anhydrous magnesium sulfate and the solvent is removed at reduced pressure.Subsequently, the crude product is dissolved in a minimal amount of DCM and precipitated in methanol.The product is obtained as a dark blue solid (156.5 mg, yield: 54%). 1 H NMR (300 MHz, CDCl 3 ) d ppm: 7.70-7.76 (d, 4), 7.28  (s, 2H), 6.96-7.02 (d, 4H), 6.61-6.66 (d, 2H), 6.51-6.59 (d, 2H Polymer P3.In a Schlenk ask, M3 (100.0 mg, 0.095 mmol), M4b (38.4 mg, 0.095 mmol), Pd 2 (dba) 3 (2.6 mg, 0.029 mmol) and X-Phos (8.2 mg, 0.017 mmol) are dissolved in dry toluene (3 ml) under nitrogen.Then, t-BuOK (45 mg, 0.44 mmol) is added and the mixture is allowed to stir for 24 hours under nitrogen at 90 C. Aer completion of the reaction, the dark blue solution is treated with DCM, washed three times with water and once with brine.Then the organic layer is dried over anhydrous magnesium sulfate and the solvent is removed at reduced pressure.Aer that, the crude product is dissolved in a minimal amount of DCM and precipitated in methanol.The product is obtained as a dark blue solid (65.7 mg, yield: 57%).7.59-7.75(br, 4H), 7.17

Methods
Instrumentation.UV/vis absorption spectra were recorded using a Perkin-Elmer Lambda 14 spectrometer.Photoluminescence spectra were recorded using a Perkin-Elmer LS50B spectrometer.The photostability was recorded using a 200 W Hg lamp (Oriel Instruments 6283) at a distance of 20 cm at room temperature. 1H NMR spectra were recorded using a Bruker DPX 300 spectrometer, which operates at 300 MHz.Molecular weights were determined by size exclusion chromatography (SEC) using a Water/Millipore UV detector 481 and a mixed gel column (Latek/Styragel 50/1000 nm pore size).All measurements were carried out in tetrahydrofuran at 45 C. The column was calibrated using commercially available polystyrene standards.Cyclic voltammograms were recorded using a potentiostat PG 390 from Heka Company.The thin lms of the polymers were cast on an ITO electrode and cycled in acetonitrile (saturated with nitrogen) containing 0.1 M tetrabutylammonium hexa-uorophosphate (TBAPF 6 ) as the electrolyte salt.Platinum was used as reference and counter electrodes.The voltage data were calculated for the ferrocene/ferrocenium redox couple.The scan rate was 100 mV s À1 and the temperature was 20 C.

Results and discussion
In this article, we report on the novel monomer M1 and four aminobenzodione-based p-conjugated polymers.M1 was prepared according to Scheme 1 which is very soluble in common organic solvents and suitable for Buchwald-Hartwig amination and Stille, Heck and Suzuki coupling because of the bromine and alkylamino end groups.
The polymers consist of amino groups as electron-donating units, and BDF or BDP as electron-accepting units causing low bandgaps, broad UV/vis absorption and high photostability.BDF/BDP-based monomers M2 and M3, and the N,N 0 -dialkylaminobenzene monomer M4 were prepared according to the literature.4a,5a,9 The polymers were synthesized by Buchwald-Hartwig amination using Pd 2 (dba) 3 as the catalyst and X-Phos as the ligand (Scheme 2). 10 The polymers were characterized by 1 H NMR spectroscopy (Fig. 1) and gel permeation chromatography (GPC).The signals around 0.8-3.3ppm originate from the alkyl groups, while the signals around 6.72-7.96are typical for the protons of the core of BDF and the phenyl groups attached to the core.For M1, the chemical shis at 4.37 ppm originate from the protons attached to the amino group while these signals disappear for the polymers due to polymerization.The number-average molecular weights were found to be 5.9 kDa (PDI: 2.2, P1), 5.7 kDa (PDI: 1.5, P2a), 9.7 kDa (PDI: 1.7, P2b) and 10.1 kDa (PDI: 1.5, P3), respectively.

Optical properties
In Fig. 2, the UV/vis spectrum of M1 in dichloromethane solution is shown.M1 exhibits a broad UV/vis absorption band with a maximum at 628 nm, the extinction coefficient of the maximum being 6.5 Â 10 4 L mol À1 cm À1 .The maximum of M1 is 121 nm red-shied compared with M2 in dichloromethane since the alkylamino unit is a powerful donor, which shis the UV/vis absorption to a longer wavelength.M1 is readily soluble in common organic solvents such as toluene, N,N-dimethylformamide and so on, in contrast to M2, which is not very soluble in toluene or dichloromethane, and therefore not suitable for Suzuki coupling.This is rstly due to the presence of the amino group, which can form a highly polar resonance structure and interact with adjacent solvent molecules (see a suggested mechanism in S4 †), and secondly the alkyl substituent of the amino group favors the solubility.
In Fig. 3, the UV/vis absorption spectra of the polymers in dichloromethane solution and as thin lms are shown.All polymers are deep colored, the extinction coefficients of the strongest bands being 1.7-3.1 Â 10 4 L mol À1 cm À1 .The UV/vis absorption spectra of P2a and P2b in dichloromethane exhibit broad bands with maxima at 709 and 724 nm.The maximum of P2b is slightly red-shied compared with P2a, since P2b has a higher molecular weight and a more strongly extended p-system.The UV/vis absorption spectrum of P3 in dichloromethane exhibits a strong maximum at 623 nm.Compared with P2a and P2b, the maximum of P1 is blue-shied (645 nm), giving rise to weaker D-A interactions between the alkylamino groups and the benzodifuranone core.In P2a and P2b enhanced intramolecular charge transfer (ICT) is due to the presence of the N,N 0 -dialkyl-1,4phenylenediamine unit with a more powerful donor ability favoring the red shied absorption.The red shi is stronger than those of the polymers containing the thienyl-benzodione chromophore (88 nm for P1, 152 nm for P2a, 167 nm for P2b and 58 nm for P3 in dichloromethane, respectively) since the alkylamino group is a more powerful electron donor than the thienyl group.4a,5a All polymers show a very broad UV/vis absorption from 400 to 900 nm, which matches the solar photon most intense ux (that is in the 400-800 nm range) 11 in thin lms with maxima between 638 and 725 nm (Fig. 2 and Table 1).All UV/vis absorption spectra of lms are red shied when compared to dichloromethane solution spectra.The strongest red-shi was found for P1 with 63 nm.This indicates a gain of planar conformation and/or the presence of p-p interchain association in the solid state.The optical HOMO-LUMO energy gaps estimated from the onset of absorption for lms are 1.19-1.47eV.

Electrochemical properties
The electrochemical properties of the four polymers were investigated by cyclic voltammetry.The conditions of the measurement are described in the Experimental part.The HOMO and LUMO energy levels of the polymers were estimated from the onset of the oxidation and reduction curves, respectively (see Fig. 4).It can be seen that anodic oxidation of polymers sets in at low potentials of 0.32 V (P1), 0.17 V (P2a), 0.15 V (P2b) and 0.89 V (P3), respectively.Two to three anodic waves with maxima between 0.5 1.2 V occur, which can be ascribed to the formation of cation radicals and dications.The reductive cycles of all polymers exhibit two reversible cathodic waves, which originate from the reduction of the quinonoid to a benzoic structure.This can possibly be explained with a stabilizing negative charge of the oxygen atoms in the carbonyl groups of both the lactone groups in the benzodione units.
All polymers show low LUMO (À3.79 to À4.24 eV) and HOMO levels (À4.95 to À5.81 eV).Due to the low LUMO energy level of the polymers, good electron injection and ambient stabilities of OFET devices can be expected.5b The polymers exhibit quite low HOMO-LUMO bandgaps (1.08 to 1.45 eV).In Table 1, optical and bandgap data of the polymers are compiled.

Photostability properties
The photostability was studied by exposing toluene solutions of the polymers to a 200 W Hg-lamp at a distance of 20 cm and measuring the decrease of the optical absorption vs. time.In Fig. 5a, UV/vis absorption spectra of P2b in toluene are shown before and aer irradiation for different time periods.Corresponding spectra of M2, M3, P1, P2a and P3 are shown in Fig. S2 and S3.† The plots of ln(A t /A 0 ) (A t,0 ¼ absorbance at time t and time t ¼ 0) vs. time for irradiation of polymers lead to nearly straight lines.From the initial slope the rate constants (k) of the photoreaction were derived (Fig. 5 and Table 1).As can be seen from Fig. 5, P2a and P2b are more stable than P1 since the N,N 0 -dialkyl-1,4-phenylenediamine unit exhibits a more powerful donor ability than the alkylamino unit.Probably the strong donor-acceptor character of these polymers prevents energy transfer from the amino groups to the BDF core and thus contributes to a higher photostability.Compared with P2a, P3 is less stable, which can be ascribed to the fact that the monomer BDP chromophore is less stable than BDF in UV-light (see S2 †).In general, polyiminobenzodiones are less stable than isoDPPbased conjugated polymers 12 or 1,10-naphthodifuranonebased polymers, 4d,13 but more stable than benzodifuranonebased polymers 4d or DPP-14 and DTPP-based polymers.Until now, empirical polarity scales have always described the polarizability and dipolarity of the solvent together in one parameter.The rst and only successful attempt to separate polarizability and dipolarity was suggested by Catalán with the introduction of solvent acidity (SA), 18 solvent basicity (SB), 19 solvent polarizability (SP) 20 and solvent dipolarity (SdP) 21 (eqn (2)).
ñmax ¼ ñmax,0 + aSA + bSB + dSP + eSdP (2) The three compounds show the shortest wavelength UV/ vis absorption band in n-hexane, and exhibit the strongest bathochromic shi in DMSO (for M1, P1 and P2a, D values (l max,pol À l max,nonpol ) of 1700 cm À1 , 2400 cm À1 , and 1180 cm À1 , respectively).Both equations show good accuracy for M1 with r > 0.9 (eqn (2) in Table 2 and eqn (1) in Table S2 †).It is shown that especially the HBA ability of the solvents (b or SB) causes a strong bathochromic shi (b < 0).This can be easily explained by H-bonding between the NH-group in the main chain and HBA-solvents.The inuence of the b-term of the solvents on the bathochromic shi of M1 represents the interactions of HBA capacity solvents with the proton of the N-H group, which increases the +M-effect and therefore strengthening of the aromatic push-pull system occurs.HBD-solvents only exhibit a small effect typical for benzodifuranone-based dyes.8b-d A weak bathochromic shi can be recognized due to the interaction with the carbonyl groups (a < 0).Polarizability and dipolarity cause a bathochromic shi (p*, SP and SdP < 0), respectively.The solvatochromic range of M1 is 1700 cm À1 from n-hexane to DMSO.Hexamethylphosphoramide (HMPA) has not been included in the correlation, since it causes an unexpectedly high bathochromic shi of the UV/vis absorption maximum.
Since, in P1, the N-H function is substituted, the effect of HBA-ability (b) should be decreased, but this was not observed.The remaining inuence of the HBA-ability of the solvent on the polymeric dye might be ascribed to an N-H end group, for example.Especially for short-chain polymers a visible inuence of end groups is conceivable.The inuences of H-bonding (a + b) are almost unchanged.The most dominant effect on the solvatochromic behavior is caused by interaction with solvents of different dipolarity/polarizability which is reected in the large coefficient s in eqn (1) or coefficients d and e in eqn (2).8d The dipolar structures of dyes with an enlarged p-system usually are stabilized within the molecule rather than by interaction with a solvent molecule.To our surprise, the effect of polarizability and dipolarity of P1 increases signicantly compared with M1 (the rate SP/SdP remains unchanged).This could rstly be ascribed to the fact that P1 exhibits poor solubility in some solvents, if the chain length gets larger.In those solvents only very short chains, probably dimers or trimers, are measured by UV/vis spectroscopy.Secondly, the presence of N-H end groups found by the unchanged high inuence of SB, speaks for such short chains.The solvatochromic shi of P1 from n-hexane to HMPA is 3140 cm À1 , and from n-hexane to DMSO is 2400 cm À1 , which are much larger than that of M1.
For P2a, the inuence of HBA-solvents is smaller, since the N-H-functionality is not present anymore.The residual effect may point to an N-H-end group again.The inuence of polarizability and dipolarity remains unchanged compared with M1.The solvatochromic shi is decreased compared with those of M1 and P1a.From n-hexane to HMPA it is 1200 cm À1 , and from n-hexane to DMSO it is 1180 cm À1 .

Conclusions
In this article, the deep blue monomer M1 and four p-conjugated polymers based on aminobenzodione are described.M1 is very soluble in common organic solvents and suitable for Buchwald amination and Stille and Suzuki coupling because of the bromine and alkylamino substituent groups.The polymers exhibit quite low bandgaps (1.08-1.47eV), high photostability and a large solvatochromic shi up to 3140 cm À1 .Furthermore, the polymers show broad UV/vis absorption bands in a range from 400 to 900 nm with high extinction coefficients of 1.7 to 3.1 Â 10 4 L mol À1 cm À1 .This matches well with the solar photon most intense ux.The broad absorption in the visible region of the spectra, combined with high color depth, high photostability and a low bandgap render aminobenzodionebased polymers interesting as building blocks for optoelectronic materials.

Table 1 a
Optical, band gap and photostability data of the polymers Polymers l max [nm] Extinct.coeff.3 (l max ) [L mol À1 cm À1 ] Photostability was determined upon irradiation with a 200 W Hg lamp.b Optical bandgap E opt g was measured at the onset of absorption of the polymer lm (E opt g ¼ 1240/l abs,onset eV).c E ec g ¼ E HOMO À E LUMO , ÀE LUMO ¼ E onset(red) + 4.8 eV and ÀE HOMO ¼ E onset(ox) + 4.8 eV, where E onset(red) and E onset(ox) are the onset potentials of the oxidation and reduction processes vs. ferrocene.

Fig. 5
Fig. 5 UV/vis absorption spectra of P2b before and after irradiation in toluene with a 200 W Hg-lamp and ln(A t /A 0 ) vs. time for determination of rate constants (k) for polymers.

Table 2
Solvent-independent correlation coefficients a, b, d and e of the Catal án parameters SA, SB, SP and SdP, solute property of the reference system ñmax,0 cyclohexane, number of solvents (n), correlation coefficient (r), standard deviation (sd), and significance (f) of the calculated solvatochromism of the model compounds M1, P1 and P2a