ADDA and ADADA systems based on

Tetrahedron Letters 56 (2015) 4607–4612

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ADDA and ADADA systems based on triphenylamine as molecular donors for organic photovoltaics Andreea Diac a,c, Dora Demeter a, Siriporn Jungsuttiwong b, Ion Grosu c, Jean Roncali a,⇑ a

Group Linear Conjugated Systems, CNRS, Moltech-Anjou, University of Angers, 2 Bd Lavoisier, 49045 Angers, France Department of Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani 34190, Thailand c Babes Bolyai University, Center of Supramolecular Organic and Organometallic Chemistry (CSOOMC), Cluj-Napoca, 11 Arany Janos St., 400028 Cluj-Napoca, Romania b

a r t i c l e

i n f o

Article history: Received 27 April 2015 Revised 23 May 2015 Accepted 27 May 2015 Available online 4 June 2015 Keywords: Electron-donor Organic solar cells Triphenylamine

a b s t r a c t Three molecular donor (D) acceptor (A) systems of structure A–D–A–D–A as well as an A–D–D–A compound have been synthesized by spatial extension of reference D–A system containing a triphenylamine donor block (5). UV–Vis absorption spectroscopy, cyclic voltammetry and theoretical calculations show that the presence of a median acceptor group has limited effect on the internal charge transfer while direct dimerization leads to an increase of the effective conjugation length. A cursory evaluation of the new compounds as donor material in bilayer solar cells using fullerene C60 as the acceptor material shows that the presence of a median acceptor has deleterious effect on conversion efficiency while the simple dimerization of the molecule leads to a substantial improvement of the short-circuit current density and efficiency. Ó 2015 Elsevier Ltd. All rights reserved.

Introduction Organic solar cells (OSCs) offer the possibility to develop costeffective new technological applications resorting to the lightness, plasticity and flexibility of organic materials.1–9 Whereas low band gap conjugated polymers have represented the major class of donor materials for solution-processed bulk heterojunction cells (BHJ),1,2 recent years have seen the emergence of intensive research focused on the design and synthesis of molecular donors.3–6 This effort is motivated by the improved reproducibility of the synthesis, purification and properties of the materials associated with well-defined chemical structures and by the possibility to achieve more reliable analyses of structure–properties relationships.3–6 In the past few years a huge number of molecular donors have been synthesized,3–13 and BHJs with power conversion efficiencies (PCE) comparable to those obtained with the best polymer-based cells (10%) have been reported.7,8 These high performances result from a multi-disciplinary research effort in which advances in device fabrication, replacement of C60 by C70 acceptors, morphological control by additives or solvent annealing are associated with new molecular donors.9 Dyes and pigments based on small molecules have been used for a long time in vacuum-deposited bi-layer OSCs,10,11 while ⇑ Corresponding author. E-mail address: [email protected] (J. Roncali). http://dx.doi.org/10.1016/j.tetlet.2015.05.103 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

recent work has shown that high PCE can be achieved with small donors of low molecular weight.6,12 Thus, when combined with C60 as the acceptor compound 5 (Scheme 2) leads to a PCE of 2.53% in a bi-layer junction13 and 4.00% in co-evaporated cell.14 In recent years, this compound has served as a reference and as a starting platform for various modifications of the molecular structure with the double objective of improving the photovoltaic efficiency and deepening the analyses of structure–properties relationships in this class of molecules.15–18 The highest PCE reported so far for molecular BHJ cells have been obtained with extended quasi-linear symmetrical molecules of structure A–D–A8 or D–A–D–A–D.7 In our continuing interest in the manipulation of the electronic properties of type 5 compounds, we report here on a series of donor–acceptor systems obtained by extension of the basic structure of 5 either by simple dimerization leading to the A–D–D–A compound 1, or by insertion of a median acceptor linking group such as carbonyl or dicyanomethylene to form the A–D–A–D–A structures 2–4 (Scheme 1). We describe the synthesis of the molecules, the characterization of their electronic properties by UV–Vis absorption spectroscopy, cyclic voltammetry and theoretical calculations and a preliminary evaluation of their performances in OSCs. Results and discussion The synthesis of compound 5 has already been described.13 Compound 1 was prepared by oxidative coupling of 5 with copper

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N

NC

S

NC

CN

S

CN

N

1 O S

NC

S

CN

N

NC

N

CN

2 O

O

S

NC

S

CN

NC

N

N

CN

3 NC

CN

S

NC

S

CN

NC

N

N

CN

4 Scheme 1. Chemical structure of the target compounds 1–4.

perchlorate.19 A Friedel–Craft reaction between compound 5 and oxalyl chloride gave ketone 2 in 68% yield (Scheme 2). A three-day reflux of 2 with ethylene glycol in the presence of ptoluene sulfonic acid using a Dean-Stark trap gave compound 3 in 46% yield. A Knoevenagel reaction of ketone 2 with malononitrile in the presence of TiCl4 gave the dicyanovinyl derivative 4 in 45% yield. Figure 1 shows the UV–Vis absorption spectra of compounds 1– 4 in dichloromethane (DCM) solutions and those of thin films spun-cast on glass from chloroform solutions. The spectrum of all compounds shows a first transition in the 350–400 nm region followed by a more intense absorption band assigned to an internal charge–transfer (ICT)20 with a maximum (kmax) at 490–500 nm. Comparison with the data for compound 5 shows that except for compound 1 for which a small red shift of kmax is observed, all extended compounds present a small hypsochromic shift of

kmax (Table 1). As generally observed for p-conjugated systems, the spectra of the films present a red shift of kmax and a broadening of the absorption band reflecting intermolecular interactions in the solid state. Further comparison of the solution and solid-state spectra shows that the smallest difference between the two states is observed for compound 2 and the largest one for compound 4, suggesting differences in intermolecular interactions. The intercept of the tangent of the long wavelength absorption edge of the spectra of films with the horizontal axis gave estimated band gaps (Eg) of 2.10 eV for compound 2 and 1.92 eV for compound 4. These results confirm that despite limited effects at the molecular level, the median acceptor group can significantly affect the molecular packing in the solid state. The cyclic voltammogram (CV) of compound 1 shows two reversible one-electron oxidation waves with anodic peak potential Epa1 and Epa2 at 0.86 and 1.05 V (Fig. 2). Comparison with compound 5 reveals a 190 mV negative shift of Epa1 which reflects the extension of the conjugated system, in agreement with UV–Vis data. The second oxidation process can be assigned to either the formation of a dication on the extended system or to the formation of a cation-radical on each part of the molecule. In such case the Epa2–Epa1 difference would indicate that the second process is influenced by the first positive charge on the other part of the molecule. Further ESR experiments are needed to clarify this point. The CV of compound 2 shows a positive shift of Epa to 1.19 V due to the electron-withdrawing effect of the carbonyl group. A similar effect is observed for compound 4 with Epa increasing to 1.21 V. The CV of compound 3 shows some similarities with that of compound 1 with in particular the occurrence of two reversible oxidation steps with Epa1 and Epa2 at 1.04 and 1.18 V. Due to the absence of direct conjugation between the two parts of the molecule, these two waves can be unequivocally attributed to the successive formation of a cation-radical on both parts of the system, the Epa2–Epa1 difference reflecting through-space interactions between positive charges. A particularity of the CV of compound 3 is the presence of a cathodic wave peaking at 0.80 V in the reverse scan. The occurrence of this wave suggests a chemical coupling of the cationradical to form a more extended conjugated system.21–23 Based on the favourable geometry suggested by the optimized structure of the molecule (see below) and a possible activation of the ortho-positions of the phenyl rings, an intramolecular cyclization with formation of a five-membered ring can be proposed (Scheme 3). However, this question is beyond the scope of the present work and will be subjected to more detailed electrochemical investigations. The CV of all compounds exhibits an irreversible reduction wave with a cathodic peak potential (Epc) shifting toward negative potential when an electron-withdrawing carbonyl or dicyanomethylene group is inserted in the middle of the molecule. This effect can be attributed to a weakening of the ICT when two electron acceptor groups are fixed at opposite sides of the TPA donor block. Theoretical calculations

Cl

O , AlCl3

N

O

S

Cl

2

DCE, rflx 5 Cu(ClO4 )2 1

NC

CN

CH 2(CN)2

(-CH2 OH)2 TiCl4 Tol, rflx 3

Scheme 2. Synthesis of compounds 1–4.

TiCl4 DCE rflx 4

Figure 3 shows the optimized geometries of the four molecules and the molecular orbital surfaces. As expected the HOMO is essentially distributed over the TPA blocks and the LUMO on the dicyanovinyl acceptor group. The introduction of a median electron-acceptor linking group does not significantly modify the distribution except for compound 4 for which some LUMO coefficients also appear on the DCM groups. The calculated values of the energy levels of the frontier orbitals and HOMO–LUMO gap agree well with experiments and confirm a slight increase of the HOMO for compounds 1 and 3 and a small decrease of HOMO and LUMO for 2 and 4 (Table 1).

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1.0

1

1.0

Absorbance (a.u.)

Absorbance (a.u.)

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2

0.8 0.6 0.4 0.2

0.2 0.0 300

400

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700

0.0 300

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Wavelength (nm)

1.0

3

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800

4

0.8

0.8 Absorbance (a.u.)

Absorbance (a.u.)

1.0

500

Wavelength (nm)

0.6 0.4 0.2

0.6 0.4 0.2

0.0 400

500

600

700

0.0 300

800

400

500

600

Wavelength (nm)

Wavelength (nm)

Figure 1. UV–Vis absorption spectra of compounds 1–4. Dashed lines: in DCM. Solid line: as thin film spin-cast on glass from chloroform solutions. Table 1 Data of UV–Vis absorption spectroscopy and cyclic voltammetry (0.10 M Bu4NPF6/CH2Cl2, scan rate 100 mV s1, Pt electrodes, ref SCE) for compounds 1–5

a b c d e

Donor

kmaxa [nm]

emax [M1cm1]

kmaxb [nm]

Eg [eV]

Epa1, Epa2 [V]

Epc [V]

EHOMOc [eV]

ELUMOd [eV]

EHOMOe [eV]

ELUMOe [eV]

1 2 3 4 5

510 487 501 489 502

63000 54000 54000 78000 36300

529 492 515 524 520

1.97 2.09 2.12 1.92 2.00

0.86, 1.05 1.19 1.04, 118 1.21 1.04

1.11 1.16 1.11 1.34 1.03

5.74 5.96 5.89 6.01 5.83

3.31 3.42 3.42 3.48 3.36

5.10 5.35 5.20 5.45 5.23

2.83 2.88 2.82 2.94 2.80

In CH2Cl2. Thin films spun-cast on glass. Using E0ox with an offset of 4.99 eV for SCE versus the vacuum level. Determined by EHOMO-DE. Values calculated by DFT.

1

2 5 µA

3

4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 E (V vs SCE) E (V vs SCE) Figure 2. Cyclic voltammograms of compounds 1–4 in 0.10 M Bu4NPF6/DCM, Pt electrodes, scan rate = 100 mV s1.

A preliminary evaluation of the photovoltaic efficiency of compounds 1–4 has been carried out on bi-layer cells of structure. ITO/PEDOT:PSS/donor/C60/Al. Donor films (20 nm) were spuncast from chloroform solutions. After fabrication the cells are subjected to a ten minute thermal annealing at 120–140 °C. Although less efficient than BHJs, these simple devices are more appropriate for investigating structure–properties relationships. Furthermore this standard procedure allows a direct comparison with the large library of molecules already evaluated in identical conditions.6,13–18 The data in Table 2 show that the introduction of a median carbonyl or DCM acceptor group between the two blocks leads to a net decrease of PCE from 2.53% for 5 to 0.55% and 1.38% for 2 and 4, respectively. This effect is due in particular to a decrease of the short-circuit current density (Jsc) and before all to low fill-factors FF. A lesser decrease of PCE is observed for 3 which is anyway less efficient than the reference compound 5. On the other hand, a significant improvement of Jsc from 5.77 to 8.41 mA cm2 is observed for donor 1 which results in a slight increase of PCE in spite of a large decrease of open-circuit voltage (Voc). This reduced voltage is related to the increase of the HOMO level24 caused by the extension of conjugation from 5 to 1 (Fig. 4).

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O

3

Eox

O

S

NC

S

CN

N

N

NC

CN

Scheme 3. Possible electrochemically induced intramolecular cyclization of compound 3.

Figure 3. Molecular orbital surface of molecules 1–4 (from top to bottom) calculated by DFT/B3LYP/6-31G(d,p) with CH2Cl2 solvent (C-PCM model). Left: optimized geometry, middle HOMO, right: LUMO.

Conclusion Table 2 Photovoltaic Characteristics of OPV cells based on donors 1–4 under AM 1.5 simulated solar light conditions with an incident power light of 90 mW cm2

a

Compd

Jsc (mA cm2)

Voc (V)

FF (%)

PCE (%)

1 2 3 4 5

8.41 2.66 5.10 4.88 5.77

0.63 0.69 0.98 0.79 0.92

44.4 27.8 39.9 32.9 52.0

2.62 0.55 2.16 1.38 2.53a

From Ref. 13.

To summarize the extension of a basic push-pull molecule by direct dimerization of by introduction of a median acceptor group has been investigated. These structural modifications have limited effects on the electronic properties of the molecules but they seem to exert a larger influence on the molecular packing in the material. Preliminary results on bi-layer OSCs show that the introduction of a median acceptor group has deleterious consequence for the conversion efficiency while in contrast, the direct dimerization leads to a significant increase of the short-circuit current density with a moderate improvement of conversion efficiency.

A. Diac et al. / Tetrahedron Letters 56 (2015) 4607–4612

2,20 -((5,50 -((((1,3-Dioxolane-2,2-diyl)bis(4,1-phenylene))bis(phenylazanediyl))bis(4,1-phenylene))bis(thiophene-5,2-diyl))bis(methanylylidene))dimalononitrile (3)

0.020

Current density (A cm-2)

0.015 0.010 0.005 0.000 -0.005 -0.010

-0.2

4611

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V) Figure 4. Current density versus voltage curves of bilayer cell ITO/PEDOT:PSS/1/ C60/Al under AM 1.5 simulated solar irradiation at 90 mW cm2.

Experimental 2,20 -((5,50 -(([1,10 -Biphenyl]-4,40 -diylbis(phenylazanediyl))bis(4, 1-phenylene))-bis(thiophene-5,2-diyl))bis(methanylylidene)) dimalononitrile (1) A solution of copper(II) perchlorate hexahydrate (140 mg, 0.38 mmol) in 5 mL acetonitrile was added dropwise to a solution of 5 (100 mg, 0.25 mmol) in 10 mL acetonitrile. The reaction mixture was stirred for 5 days at room temperature under argon atmosphere. Solid K2CO3 (310 mg, 2.25 mmol) and water (0.6 mL) were added and the solution was further stirred for 30 min. The mixture was filtered and the solids were washed with MC. The combined organic layers were filtered through a short pad of alumina. The solvent was removed and the residue was chromatographed on silica gel using dichloromethane as eluent to yield 80 mg (40%) of a deep-violet powder. Mp: 114–116 °C. 1H NMR (300 MHz, CDCl3), d (ppm): 7.75 (s, 2H), 7.68 (d, 2H, J = 4.2 Hz), 7.57–7.51 (m, 8H), 7.37–7.32 (m, 6H), 7.21–7.15 (m, 10H), 7.10 (d, 4H, J = 8.7 Hz). 13 C NMR (75 MHz, CDCl3), d (ppm): 157.1, 150.4, 149.8, 146.6, 145.9, 140.5, 136.1, 133.3, 129.8, 127.9, 127.8, 125.8, 125.5, 125.3, 124.7, 123.5, 122.2, 114.7, 113.8, 75.3. HRMS (MALDITOF): calculated: 804.2130; found: 804.2125. 2,20 -((5,50 -(((Carbonylbis(4,1-phenylene))bis(phenylazanediyl)) bis(4,1-phenylene))-bis(thiophene-5,2-diyl))bis(methanylylidene))dimalononitrile (2) To a solution of 5 (250 mg, 0.62 mmol) in 50 mL of 1,2-dichloroethane at 0 °C, oxalyl chloride (0.13 mL, 1.49 mmol) and AlCl3 (165 mg, 1.24 mmol) were added. The mixture was stirred 1 h at room temp. under argon. Another equivalent of 5 (250 mg, 0.62 mmol) was added and the reaction was stirred at 90 °C for five days. The solution was diluted with MC, washed with water and brine and dried over MgSO4. Solvent removal and column chromatography on silica gel, using MC as eluent gave 350 mg (68%) of deep-red powder. Mp: 128–130 °C. 1H NMR (300 MHz, CDCl3), d (ppm): 7.77 (s, 2H), 7.75 (d, 4H, J = 8.7 Hz), 7.70 (d, 2H, J = 4.2 Hz), 7.60 (d, 4H, J = 8.7 Hz), 7.40–7.35 (m, 6H), 7.21–7.18 (m, 6H), 7.17 (d, 4H, J = 8.7 Hz), 7.13 (d, 4H, J = 8.7 Hz). 13C NMR (75 MHz, CDCl3), d (ppm): 156.4, 150.7, 150.5, 148.9, 146.1, 140.4, 133.8, 132.0, 131.9, 130.1, 127.9, 127.0, 126.6, 125.6, 124.2, 124.0, 122.0, 114.5, 113.7, 77.4, 76.0. HRMS (MALDI-TOF): calculated: 832.2079; found: 832.2081.

A mixture of 1 (100 mg, 0.12 mmol), ethylene glycol (0.06 mL, 1.02 mmol) and p-toluene sulfonic acid (2 mg, 0.012 mmol) in 30 mL of toluene was refluxed for 5 days using a Dean-Stark trap. After solvent removal the residue was dissolved in MC, washed with water and brine, dried over MgSO4 and concentrated. Column chromatography on silica gel using MC as eluent yielded 48 mg (46%) of a red solid. Mp: 140–142 °C. 1H NMR (300 MHz, CDCl3), d (ppm): 7.74 (s, 2H), 7.68(d, 2H, J = 4.2 Hz), 7.53 (d, 4H, J = 8.7 Hz), 7.43 (d, 4H, J = 8.4 Hz), 7.34–7.29 (m, 6H), 7.16–7.13 (m, 6H), 7.10 (d, 4H, J = 8.4 Hz), 7.06 (d, 4H, J = 8.7 Hz), 4.01 (s, 4H). 13C NMR (75 MHz, CDCl3), d (ppm): 157.1, 150.4, 149.8, 146.7, 146.6, 140.5, 137.6, 133.3, 129.8, 127.7, 125.9, 125.4, 124.8, 124.5, 123.5, 122.3, 114.7, 113.8, 109.3, 77.4, 75.4, 65.1. HRMS (MALDI-TOF): calculated: 876.2341; found: 876.2335. 2,20 -((5,50 -((((2,2-Dicyanoethene-1,1-diyl)bis(4,1-phenylene))bis(phenylazanediyl)) bis(4,1-phenylene))bis(thiophene-5,2-diyl)) bis(methanylylidene))dimalononitrile (4) To a mixture of 2 (250 mg, 0.3 mmol) and malononitrile (80 mg, 1.2 mmol) in 30 mL 1,2-dichloroethane cooled to 0 °C, titanium tetrachloride (0.24 ml, 2.2 mmol) and pyridine (0.48 ml, 6 mmol) were slowly added under argon. The solution was refluxed for three days. The reaction mixture was filtered through Celite and the solids were washed several times with MC. The filtrate was washed with HCl (aq) and with NaHCO3 (aq). The organic layer was dried over MgSO4 and concentrated. Chromatography on silica gel using dichloromethane as eluent, afforded 120 mg (45%) of a dark-red powder. Mp: 314–315 °C. 1H NMR (300 MHz, CDCl3), d (ppm): 7.78 (s, 2H), 7.71 (d, 2H, J = 3.9 Hz), 7.63 (d, 4H, J = 8.7 Hz), 7.42–7.36 (m, 10H), 7.27–7.18 (m, 10H), 7.09 (d, 4H, J = 8.7 Hz). 13C NMR (75 MHz, CDCl3), d (ppm): 207.0, 172.6, 155.9, 151.2, 150.5, 148.2, 145.5, 140.3, 134.1, 132.8, 130.3, 129.1, 128.0, 127.9, 126.9, 126.2, 125.1, 124.3, 120.8, 155.5, 114.4, 113.6, 77.4, 76.4. HRMS (MALDI-TOF): calculated: 880.2191; found: 880.2189. Theoretical calculations have been performed using the Gaussian 09 program package.25 The ground state geometries (S0) and electronic structure of each molecule have been determined by a full optimization of its structural parameters. There is no symmetry constraint on the geometric optimization. Calculations on the electronic ground state of all molecules were carried out using density functional theory (DFT) method combined with Becke’s three-parameter hybrid functional and Lee–Yang–Parr’s gradientcorrected correlation functional (B3LYP),26 with a 6-31G(d,p) level of theory. The HOMO–LUMO energy gap (DH–L) was evaluated by B3LYP, while the lowest excitation energy was evaluated by using TD-B3LYP level of theory. Both methods were evaluated based on the S0 geometries together with the consideration of solvation effect of dichloromethane (CH2Cl2) by the Conductor-Polarizable Continuum Model (C-PCM). Device fabrication and testing Indium-tin oxide coated glass slides of 27  28 mm with a surface resistance of 10 O/sq were purchased from Kintec. Part of the ITO layer was etched away with zinc powder and 37% HCl. The electrodes were then cleaned in ultrasonic bath successively with Deconex (VWR international GmbH), distilled water, acetone and isopropanol for 15 min each and dried by nitrogen. The electrodes were then modified by a spin-cast layer of PEDOT:PSS (HC Starck)

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filtered through a 0.45 lm membrane prior use) and the electrodes was then dried at 110 °C for 15 min. The donor layers were spuncast from chloroform (5 mg mL1). After film deposition the devices were introduced in a glovebox and 40 nm of C60 and 100 nm of aluminum were successively thermally evaporated under a pressure of 2  106 mbar through a mask defining two circular cells of 6.0 mm on each ITO electrode, each batch consists of eight cells. Current versus voltage curves were recorded using a Keithley 236 source-measure unit. The light source was an AM1.5 Solar Constant 575 PV simulator (Steuernagel Lichttechnik. The light intensity was measured by a broad-band power meter (13PEM001, Melles Griot). Acknowledgments The authors thank the French ‘Minisère des affaires étrangères’ for the attribution of an Eiffel Grant to A.D. References and notes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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