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Second Generation of Diketopyrrolopyrrole Dyes for NiO based Dye-Sensitized Solar Cells Yoann Farré, Lei Zhang, Yann Pellegrin, Aurelien Planchat, Errol Blart, Mohammed Boujtita, Leif Hammarström, Denis Jacquemin, and Fabrice Odobel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12489 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Second Generation of Diketopyrrolopyrrole Dyes for NiO Based Dye-Sensitized Solar Cells Yoann Farré,a§ Lei Zhangb§ Yann Pellegrin,a Aurelien Planchat,a Errol Blart,a Mohammed Boujtita, a Leif Hammarström,*b Denis Jacquemin,*ac Fabrice Odobel*a a

Université LUNAM, Université de Nantes, CNRS, Chimie et Interdisciplinarité: Synthèse,

Analyse, Modélisation (CEISAM), UMR 6230,2 rue de la Houssinière, 44322Nantes cedex 3, France. E-mail: [email protected]; Tel.: +33 251125429. b

Department of Chemistry – Ångström Laboratory, Uppsala University, Box 523, SE75120

Uppsala, Sweden. E-mail : [email protected]; Tel.: +46 18 471 3648. § These authors contributed equally to the work.

Abstract In this study, four new diketopyrrolopyrrole (DPP) sensitizers, with a dicarboxylated triphenylamine anchoring group for attachment to NiO, were prepared and their electronic absorption, emission and electrochemical properties were recorded. The nature of the electronic excited-states was also modeled with TD-DFT quantum chemistry calculations. The photovoltaic performances of these new dyes were characterized in NiO-based dye-sensitized solar cells (DSCs) with the classical iodide/triiodide and cobaltII/III-polypyridine electrolytes, in which they proved to be quite active. Laser spectroscopy on dye/NiO/electrolyte films gave evidence for ultrafast hole injection into NiO (0.2-10 ps time scales). For the dyes with an c

Institut Universitaire de France, 103 blvd St Michel, 75005 Paris Cedex 5, France. E-

mail: [email protected]; ; Tel: +33 251125564 1 ACS Paragon Plus Environment


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appended naphtalenediimide (NDI) acceptor unit, ultrafast electron transfer to the NDI dramatically prolonged the lifetime of the charge separated state NiO(+)/dye-, from the ps time scale to an average lifetime ≈ 0.25 ms, which is among the slowest charge recombinations ever reported for dye/NiO systems. This allowed for efficient regeneration by Co IIIpolypyridine electrolytes, which translated into much improved PV-performance compared to the DPP dyes without appended NDI. Overall, these results underscore the suitability of DPP as sensitizers for NiO-based photoelectrochemical devices for photovoltaic and photocatalysis.

Introduction The field of p-type dye-sensitized solar cell (p-DSC) has been attracting an increasing scientific interest because these photocathodes can be used for the fabrication of solar cells and particularly tandem DSCs.1, 2, 3, 4 In addition, p-DSC can also be applied in the development of photoelectrocatalytic devices for artificial photosynthesis.1, 5, 6, 7, 8, 9 In order to enhance the solar energy conversion efficiencies of this emerging technology the development of new materials are needed.1, 10, 11 New electrolytes, new p-type semiconductors (p-SCs) and better performing sensitizers are required to achieve higher efficiencies, which have recently reached a record 2.5%.12 The operation principle of p-DSC differs from that of conventional Grätzel cells by the fact that the photoexcited sensitizer injects a hole in the valence band of p-SC rather an electron in the conduction band of TiO2 or ZnO. Accordingly, the electronic properties of optimal sensitizers should be different from those of those used in Grätzel cells.13 Several classes of dyes were reported to sensitize NiO, including push-pull systems,2, 14, 15 perylene imides,3, 16, 17 squaraines,18,

19

BODIPYs,20,

21

porphyrins,22,

23, 24

isoindigos25, ruthenium26,

27, 28, 29

and iridium30

polypyridine complexes. Recently, we have communicated on the great potential of

diketopyrrolopyrrole (DPP) dyes31 for p-DSC because of their numerous interesting features, such as high photostability, electron withdrawing properties well-suited to reach high oxidizing power in the excited state, and straightforward synthesis. DPP derivatives are indeed highly performing sensitizers in conventional TiO2 based DSCs31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 and they were also successfully used as monomer in co-polymers for organic solar cells33, 42, 43, 44, 45 and for organic field effect transistors.46, 47 In the present study, we have modified the first series of dyes, namely by introducing a bis(benzoic acid) aminophenyl moiety as anchoring group instead of a thiophene-2-carboxylic acid (Chart 1). The trisarylamine is an electron rich group and it proved to be particularly suitable to yield quite good performances with several classes of sensitizers for NiO.2,

13, 15, 19

In addition, in these new series of DPP dyes, we have 2

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investigated the influence of the length of the spacer between the amine and the DPP moiety (one or two phenyl units) and the effect of the presence of a naphthalene diimine unit (NDI) as a secondary electron acceptor. The properties of these new dyes were compared to their counterparts DPP-Br and DPP-NDI, previously published and they reveal higher performances. Most importantly, the addition of the NDI acceptor in 3 and 4 led to a very longlived charge separated state after hole injection on NiO compared to 1 and 2, so that the former dyes are compatible with the use of a cobalt-polypyridine electrolyte that gives higher photovoltage than the standard I3-/I- electrolyte.

Chart 1. Structures of the new diketopyrrolopyrrole dyes 1-4 along with those of the reference compounds DPP-Br and DPP-NDI.

Experimental part 1

H, 13C and 31P NMR spectra were recorded on a AVANCE 300 UltraShield BRUKER and AVANCE 400

BRUKER. Chemical shifts for 1H and 13C NMR spectra are referenced relative to residual protium in the deuterated solvent (CDCl3  = 7.26 ppm for 1H and  = 77.16 ppm for 13C; THF-d8  = 3.57, 1.72 ppm for 1H and  = 67.21, 25.31 ppm for 13C; CD3OD  = 3.31 ppm for 1H and  = 49.00 ppm for 13C) or to an internal reference (TMS,  = 0 ppm for both 1H and

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C). NMR spectra were recorded at room

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temperature, chemical shifts are written in ppm and coupling constants in Hz. High-resolution mass (HR-MS) spectra were obtained either by electrospray ionization coupled with high resolution ion trap orbitrap (LTQ-Orbitrap, ThermoFisher Scientific,) or by MALDI-TOF-TOF (Autoflex III de Bruker), working in ion-positive or ion-negative mode. Electrochemical measurements were made under an argon atmosphere in the mixture CH2Cl2/DMF: 95/5 with 0.1 M Bu4NPF6. Cyclic voltammetry experiments were performed by using a SP300 Bio-Logic potentiostat/galvanostat. A standard threeelectrode electrochemical cell was used. Potentials were referred to a saturated calomel electrode as internal reference. All potentials are quoted relative to SCE. The working electrode was a glassy carbon disk and the auxiliary electrode was a Pt wire. In all the experiments the scan rate was 100 mV.s-1. UVvisible absorption spectra were recorded on a Variant Cary 300, using 1 cm path length cells. Emission spectra were recorded on a Fluoromax-4 Horiba Jobin Yvon spectrofluorimeter (1 cm quartz cells). Chemicals were purchased from Sigma-Aldrich or Alfa Aesar and used as received. Thin-layer chromatography (TLC) was performed on aluminium sheets precoated with Merck 5735 Kieselgel 60F254. Column chromatography was carried out either with Merck 5735 Kieselgel 60F (0.040-0.063 mm mesh). The chemicals 5,48 9,49 1048 and 1350 were prepared according to literature procedures.

Di-tert-butyl 4,4'-(benzylazanediyl)dibenzoate: 7 In a sealed tube, tert-butyl 4-bromobenzoate 5 (2.51 g, 9.78 mmol, 3 eq.), benzylamine 6 (0.35 g, 3.26 mmol, 1 eq.) and tBuOK (1.09 g, 9.78 mmol, 3 eq.) were dissolved in dry toluene (20 mL). The mixture was carefully degased by sonification with argon. Pd(dba)2 (37 mg, 0.065 mmol, 0.02 eq.), and XPhos (61 mg, 0.13 mmol, 0.04 eq.) were then added quickly and the solution was stirred at 100°C under argon during 22h. After completion of the reaction, the solution was diluted with water and extracted with dichloromethane (2x). The combined organic layer was washed with brine (3x) and then dried over MgSO4, filtrated and concentrated under reduced pressure. After column chromatography on silica gel (eluent: petroleum ether/dichloromethane), the desired product 7 was obtained as a white powder (1.23 g, 82%). 1

H NMR (300 MHz, CDCl3, 25°C), (ppm): 7.87 (m, 4H), 7.27 (m, 5H), 7.11 (m, 4H), 5.08

(s, 2H), 1.56 (s, 18H,).

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C NMR (75 MHz, CDCl3, 25°C), (ppm): 165.43, 150.60, 137.74,

131.00, 128.79, 127.22, 126.35, 125.33, 119.87, 80.59, 55.93, 28.26. HRMS (ESI+) m/z: [M+H]+ calculated for C29H34NO4, 460.24824; found, [M+H]+ 460.24857. = 0.7 ppm.

Compound 8


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In an autoclave, was introduced arylamine 7 (1.84 g, 4.0 mmol, 1 eq.) in a solution of EtOH/THF (1:1) (40 mL). Palladium on carcoal (10%) (0.43 g, 0.4 mmol, 0.1 eq.) was added to the solution and the autoclave was set under pressure of hydrogen (35 bars) after 3 purges. The solution was stirred under hydrogen (38 bars) at 50°C during 3 days until total conversion. The solution was then filtrated on a small column of silica gel and washed abundantly with CH2Cl2. The filtrate was concentrated under reduced pressure and the arylamine 8 was obtained as a white solid in 90% yield (1.32 g). 1H NMR (300 MHz, CDCl3, 25°C), (ppm): 7.92 (m , 4H), 7.09 (m, 4H), 6.19 (br s, 1H), 1.59 (s, 18H). 13C NMR (75 MHz, CDCl3, 25°C), (ppm): 165.47, 145.51, 131.23, 124.95, 116.73, 80.59, 28.27. HRMS (TOF MS ES-) m/z: [M-H]calculated for C22H26NO4, 368.1862; found, [M-H]- 368.1866. = 1.1 ppm.

Compound 11 In a sealed tube, were weighted under air the arylamine 10 (53 mg, 0.093 mmol, 1 eq.) and DPP-Br2 9 (127 mg, 0.19 mmol, 2 eq.). THF (without stabilizer) (4 mL) and water (1 mL) was introduced and the mixture was degassed by sonification with argon. K2CO3 (39 mg, 0.28 mmol, 3 eq.) and Pd(PPh3)4 (5 mg, 0.0047 mmol, 0.05 eq.) were then added and the solution was stirred at 80°C under argon atmosphere for 18h. The mixture was then diluted with water and extracted with CH2Cl2 (4x). The combined organic layer was washed with brine (2x) and dried over MgSO4, filtrated and concentrated under reduced pressure. The crude product was purified by chromatography on silica gel (eluent: petroleum ether/dichloromethane and dichloromethane). The desired product was obtained as an orange/red solid in 50% yield (48 mg). 1H NMR (300 MHz, CDCl3, 25°C), (ppm): 7.90 (d, J=8.73 Hz, 4H), 7.85 (d, J=8.45 Hz, 2H), 7.70 (d, J=8.42 Hz, 2H), 7.63 (m, 4H), 7.59 (d, J=8.57 Hz, 2H), 7.20 (d, J=8.57 Hz, 2H), 7.13 (d, J=8.74 Hz, 4H), 3.78 (d, J=7.47 Hz, 2H), 3.72 (d, J=7.47 Hz, 2H), 1.59 (s, 18H), 1.401.55 (m, 2H), 1.35-0.97 (m, 16H), 0.65-0.84 (m, 12H).

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C NMR (75 MHz, CDCl3, 25°C),

(ppm): 165.32, 162.74, 162.58, 150.48, 148.86, 147.03, 146.36, 142.68, 135.00, 132.09, 130.91, 130.11, 129.27, 128.29, 127.43, 127.14, 126.95, 126.53, 125.94, 125.36, 122.90, 110.09, 109.69, 80.80, 45.06, 44.95, 38.54, 38.48, 30.30, 28.24, 23.75, 22.85, 13.98, 13.96, 10.43. HRMS (ESI+) m/z: [M+H]+ calculated for C62H73O6N3Br, 1034.46773; found, [M+H]+ 1034.47278. = 4.9 ppm.



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Compound 1 In a round bottom flask containing compound 11 (48 mg, 0.046 mmol, 1 eq.) under argon atmosphere was introduced dichloromethane (1.5 mL) and then trifluoroacetic acid (1.5 mL) was slowly added. The resulting solution was then stirred for 3 h at room temperature. A slight color change from orange to red was observed. After completion of the reaction monitored by TLC, the crude reaction mixture was concentrated under reduced pressure and purified by column chromatography on silica gel (CH2Cl2/MeOH [90:10] and then CH2Cl2/MeOH/AcOH [90:9:1]). The desired product was obtained as an orange powder in 99% yield (43 mg). 1H NMR (400 MHz, CDCl3/TFA [9:1], 25°C), (ppm): 8.03 (d, J=8.76 Hz, 4H), 7.79 (s, 4H), 7.71 (d, J= 8.32 Hz, 2H), 7.68 (d, J=8.55 Hz, 2H), 7.55 (d, J=8.09 Hz, 2H), 7.29 (d, J=8.38 Hz, 2H), 7.21 (d, J=8.81 Hz, 4H), 3.78 (d, J=6.12 Hz, 2H), 3.72 (d, J=6.12 Hz, 2H), 1.46 (m, 2H), 0.961.39 (m, 16H), 0.75-0.93 (m, 6H), 0.70 (t, J=7.17 Hz, 6H). 13C NMR (75 MHz, CDCl3/MeOD [9:1], 25°C), (ppm): 167.41, 162.54, 162.36, 150.30, 149.12, 147.19, 145.95, 135.64, 131.64, 130.80, 129.72, 128.85, 127.90, 126.80, 126.48, 125.68, 125.03, 122.37, 109.47, 109.07, 44.45, 44.30, 38.14, 38.04, 29.76, 29.09, 27.68, 26.31, 23.28, 22.21, 13.17, 13.04, 9.57. HRMS (MALDI-TOF) m/z: [M+H]+ calculated for C54H57BrN3O6, 922.3425; found, [M+H]+ 922.3459. = 3.7 ppm.

Compound 12 In a sealed tube, arylamine 5 (50 mg, 0.14 mmol, 1.0 eq.), compound 9 (100 mg, 0.15 mmol, 1.1 eq.) and tBuOK (30 mg, 0.27 mmol, 1.5 eq.) were dissolved in dry toluene (2 mL). The mixture was carefully degassed by sonification with argon. Pd(dba)2 (6 mg, 0.01 mmol, 0.07 eq.), and XPhos (10 mg, 0.02 mmol, 0.14 eq.) were then added quickly and the solution was stirred at 60°C under argon during 16h. After completion of the reaction, the solution was diluted with water and extracted with dichloromethane (3x). The combined organic layer was washed with brine (3x) and then dried over MgSO4, filtrated and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (pure CH2Cl2 and then CH2Cl2/MeOH [99:1]). The desired orange product was obtained in 17% yield (22 mg). 1H NMR (300 MHz, CDCl3, 25°C), (ppm): 7.92 (d, J=8.78 Hz, 4H), 7.74 (d, J=8.81 Hz, 2H), 7.65 (s, 4H), 7.18 (d, J=8.81 Hz, 2H), 7.14 (d, J=8.78 Hz, 4H), 3.73 (m, 4H), 1.59 (s, 18H), 1.43-1.56 (m, 2H), 1.02-1.28 (m, 16H), 0.67-0.87 (m, 12H). 13C NMR (75 MHz, CDCl3, 25°C), (ppm): 165.16, 162.87, 162.55, 149.88, 148.79, 148.63, 146.41, 132.14, 131.05, 130.19, 6 ACS Paragon Plus Environment

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130.06, 127.54, 125.29, 123.94, 123.57, 123.30, 110.18, 109.35, 80.00, 45.06, 44.99, 38.66, 38.57, 30.37, 30.28, 28.40, 28.22, 23.71, 22.86, 14.00, 13.94, 10.41. HRMS (TOF MS ES+) m/z: [M+Na]+ calculated for C56H68BrN3O6Na, 980.4189; found, [M+Na]+ 980.4224. = 3.6 ppm.

Compound 2 In a round bottom flask containing compound 12 (25 mg, 0.026 mmol, 1 eq.) under argon was introduced dichloromethane (1.5 mL) and then trifluoroacetic acid (1.5 mL) was slowly added. The resulting solution was then stirred for 3 h at room temperature. A slight color change from orange to red was observed. After completion of the reaction monitored by TLC, the crude was concentrated under reduced pressure and purified by precipitation in a mixture of chloroform/methanol by addition of hexane. The product was obtained as an orange powder in 99% yield (24 mg). 1H NMR (300 MHz, CDCl3/MeOD [5:1], 25°C), (ppm): 7.86 (d, J=8.72 Hz, 4H), 7.63 (d, J=8.45 Hz, 2H), 7.55 (d, J=8.80 Hz, 2H), 7.51 (d, J=8.80 Hz, 2H), 7.11 (d, J=8.45 Hz, 2H), 7.05 (d, J=8.69 Hz, 4H), 3.61 (m, 4H), 1.37 (m, 2H), 0.91-1.20 (m, 16H), 0.550.73 (m, 12H).

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C NMR (75 MHz, CDCl3/MeOD [5:1], 25°C), (ppm): 167.72, 162.60,

162.28, 149.79, 148.66, 148.45, 146.57, 131.63, 130.96, 129.79, 125.54, 124.92, 123.44, 122.81, 109.50, 108.76, 44.42, 44.33, 38.13, 38.09, 29.78, 29.68, 29.03, 27.77, 27.62, 23.15, 22.19, 13.10, 13.03, 9.56. HRMS (MALDI-TOF) m/z: [M+H]+ calculated for C48H53BrN3O6, 846.3112; found, [M+H]+ 846.3132. = 2.4 ppm.

Compound 14 In a sealed tube, were introduced successively under air, DPP-Br2 9 (470 mg, 0.70 mmol, 2 eq.), the building block 10 (168 mg, 0.35 mmol, 1 eq.), triethylamine (1.0 mL) in dry toluene (15 mL). The solution was then degassed by sonification. Copper iodide (7 mg, 0.035 mmol, 0.1 eq.) and Pd(Ph3)4 (20 mg, 0.0018 mmol, 0.05 eq.) were then quickly added and the resulting mixture was stirred under argon at 50°C for 18h. The crude was concentrated under reduced pressure and directly purified by chromatography on silica gel (CH2Cl2/MeOH [99:1]). 220 mg (59% yield) of the desired compound 14 was obtained as a red solid. 1H NMR (400 MHz, CDCl3, 25°C), (ppm): 8.80 (s, 4H), 7.80 (d, J=8.41 Hz, 2H), 7.75 (d, J=8.41 Hz, 2H), 7.67 (d, J=8.44 Hz, 2H), 7.64 (m, 4H), 7.34 (d, J=8.49 Hz, 2H), 4.22 (m, 2H), 3.74 (m, 4H), 1.77 (m, 7 ACS Paragon Plus Environment


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2H), 1.01-1.57 (m, 28H), 0.88 (t, J=7.02 Hz, 3H), 0.80 (m, 6H), 0.72 (t, J=7.41 Hz, 6H).

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13

C

NMR (100 MHz, CDCl3, 25°C), (ppm): 161.83, 161.67, 161.62, 161.49, 147.07, 146.58, 133.65, 131.80, 131.12, 130.99, 130.38, 129.97, 129.11, 127.79, 127.64, 127.24, 126.34, 126.10, 126.06, 125.84, 125.54, 124.70, 124.54, 122.97, 109.08, 90.17, 89.11, 44.13, 44.01, 40.10, 37.57, 37.55, 30.80, 29.33, 28.68, 28.28, 28.18, 27.27, 27.09, 26.10, 22.77, 21.85, 21.62, 13.06, 12.91, 9.41. HRMS (MALDI-TOF) m/z: [M+H]+ calculated for C64H68BrN4O6, 1067.4317; found, [M+H]+ 1067.4350. = 3.1 ppm.

Compound 15 In a sealed tube, arylamine 5 (20 mg, 0.052 mmol, 1.1 eq.), compound 14 (50 mg, 0.047 mmol, 1 eq.) and tBuOK (8 mg, 0.071 mmol, 1.5 eq.) were dissolved in dry toluene (4 mL). The mixture was carefully degassed by sonification with argon. Pd(dba)2 (2 mg, 0.002 mmol, 0.05 eq.), and XPhos (3 mg, 0.005 mmol, 0.10 eq.) were then added quickly and the solution was stirred at 90°C under argon during 18h. After completion of the reaction, the solution was diluted with water and extracted with dichloromethane (2x). The combined organic layer was washed with brine (3x) and then dried over MgSO4, filtrated and concentrated under reduced pressure. After precipitation in MeOH from CH2Cl2, the desired product 15 was obtained as a red product (63 mg, 98%). 1H NMR (300 MHz, CDCl3, 25°C), (ppm): 8.81 (s, 4H), 7.92 (d, J=8.60 Hz, 4H), 7.81 (d, J=8.28 Hz, 2H), 7.76 (d, J=8.22 Hz, 4H), 7.69 (d, 8.58 Hz, 2H), 7.34 (d, J=8.58 Hz, 2H), 7.19 (d, J=8.58 Hz, 2H), 7.14 (d, J=8.67 Hz, 4H), 4.21 (t, J=7.79 Hz, 2H), 3.76 (m, 4H), 1.76 (m, 2H), 1.60 (s, 18H), 1.05-1.50 (m, 28H), 0.66-0.92 (m, 15H). 13C NMR (75 MHz, CDCl3, 25°C), (ppm): 165.17, 162.87, 162.72, 149.90, 148.75, 148.57, 146.83, 134.63, 132.83, 132.02, 131.46, 131.04, 130.23, 128.78, 128.63, 128.48, 127.51, 127.09, 126.52, 125.44, 123.95, 123.59, 110.38, 109.53, 80.99, 45.14, 41.10, 38.70, 38.54, 31.82, 30.38, 30.31, 29.70, 29.29, 29.20, 28.41, 28.23, 28.09, 27.10, 23.72, 22.88, 22.64, 14.10, 14.02, 13.96, 10.43. HRMS (MALDI-TOF) m/z: [M+H]+ calculated for C86H94N5O10, 1356.6995; found, [M+H]+ 1356.7088. = 3.2 ppm.

Compound 4 In a round bottom flask containing compound 15 (18 mg, 0.013 mmol, 1 eq.) under argon was introduced dichloromethane (4 mL) and then trifluoroacetic acid (0.2 mL) was slowly added. 8 ACS Paragon Plus Environment

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The resulting solution was then stirred for 3h at room temperature. After completion of the reaction monitored by TLC, the crude was concentrated under reduced pressure and purified by precipitation in a mixture of dichloromethane/methanol by addition of hexane. The product was obtained as a red powder in 73% yield (12 mg). 1H NMR (400 MHz, THF-d8 , 25°C), (ppm): 8.71 (s, 4H), 7.97 (d, J=7.96 Hz, 4H), 7.93 (m, 4H), 7.69 (m, 4H), 7.39 (d, J=8.48 Hz, 2H), 7.24 (d, 8.78 Hz, 2H), 7.19 (d, J=8.78 Hz, 4H), 4.16 (m, 2H), 3.83 (m, 4H), 1.75 (m, 2H), 1.10-1.50 (m, 28H), 0.89 (t, J=6.97 Hz, 3H), 0.71-0.87 (m, 12H). 13C NMR (100 MHz, THF-d8 , 25°C), (ppm): 164.22, 160.62, 160.60, 160.21, 160.01, 147.15, 146.91, 145.88, 144.16, 134.14, 130.08, 129.69, 129.32, 128.53, 128.41, 128.29, 127.42, 126.95, 125.26, 125.20, 124.96, 124.59, 123.37, 121.98, 121.72, 121.20, 108.45, 107.52, 89.11, 87.63, 42.56, 38.48, 36.88, 36.84, 29.96, 28.49, 28.47, 27.78,27.42, 27.33, 26.46, 26.35, 26.07, 25.56, 25.17, 23.01, 22.80, 21.79, 21.74, 20.90, 20.70, 11.58, 11.52, 11.45, 7.91. HRMS (MALDI-TOF) m/z: [M+H]+ calculated for C78H78N5O10, 1244.5743; found, [M+H]+ 1244.5769. = 2.1 ppm.

Compound 16 In a sealed tube, were weighted under air the arylamine 10 (30 mg, 0.052 mmol, 1.1 eq.), compound 14 (50 mg, 0.047 mmol, 1 eq.), K2CO3 (20 mg, 0.14 mmol, 3 eq.).THF (without stabilizer) (4 mL) and water (1 mL) was introduced and the mixture was degassed by sonification with argon. Pd(PPh3)4 (3 mg, 0.0024 mmol, 0.05 eq.) were then added and the solution was stirred at 80°C for 18h. The mixture was then diluted with water and extracted with CH2Cl2 (2x). The combined organic layer was washed with brine (2x) and dried over MgSO4, filtrated and concentrated under reduced pressure. The crude product was precipitated in CH2Cl2/MeOH and recovered by filtration as a dark red solid (65 mg, 96% yield). 1H NMR (400 MHz, CDCl3, 25°C), (ppm): 8.80 (s, 4H), 7.90 (d, J=8.75 Hz, 4H), 7.87-7.92 (m, 2H), 7.83 (d, J=8.50 Hz, 2H), 7.74 (m, 4H), 7.70 (d, J=8.48 Hz, 2H), 7.60 (d, J=8.64 Hz, 2H), 7.35 (d, J=8.56 Hz, 2H), 7.21 (d, J=8.61 Hz, 2H), 7.13 (d, J=8.83 Hz, 4H), 4.22 (m, 2H), 3.80 (m, 4H), 1.77 (m, 2H), 1.60 (s, 18H), 1.55-1.60 (m, 2H), 1.05-1.55 (m, 28H), 0.89 (t, J=7.00 Hz, 3H), 0.70-0.85 (m, 12H). 13C NMR (100 MHz, CDCl3, 25°C), (ppm): 165.32, 162.86, 162.79, 162.76, 162.71, 150.51, 148.77, 147.42, 146.38, 142.70, 136.07, 134.66, 132.81, 132.03, 131.43, 131.01, 130.92, 129.30, 128.79, 128.66, 128.45, 128.29, 127.27, 127.12, 127.09, 127.00, 126.88, 126.59, 126.56, 125.94, 125.55, 124.02, 122.93, 110.40, 109.98, 91.09, 80.80, 45.20, 41.11, 38.61, 38.56, 31.80, 30.36, 29.70, 29.28, 29.19, 28.25, 28.10, 27.10, 23.80, 22.88, 9 ACS Paragon Plus Environment


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22.86, 22.63, 14.07, 13.96, 13.94, 10.45 HRMS (MALDI-TOF) m/z: [M+H]+ calculated for C92H98N5O10, 1432.7308; found, [M+H]+ 1432.7337. = 2.0 ppm.

Compound 3 In a round bottom flask containing compound 16 (35 mg, 0.024 mmol, 1 eq.) under argon was added dichloromethane (2 mL) and then trifluoroacetic acid (0.1 mL, 50 eq.) was added slowly. The resulting solution was then stirred for 6 h at room temperature. After completion of the reaction monitored by TLC, the crude was concentrated under reduced pressure and purified by precipitation in a mixture of dichloromethane/methanol by addition of hexane. The product was obtained as a red powder in 88% yield (28 mg). 1H NMR (400 MHz, CDCl3/TFA [9:1], 25°C), (ppm): 8.90 (s, 4H), 8.04 (d, J=8.79 Hz), 7.71-7.82 (m, 8H), 7.77 (d, J=8.52 Hz, 2H), 7.68 (d, J=8.43 Hz, 2H), 7.36 (d, 8.40 Hz, 2H), 7.29 (d, J=8.10 Hz, 2H), 7.22 (d, J=8.75 Hz, 4H), 4.25 (m, 2H), 3.79 (m, 4H), 1.78 (m, 2H), 0.99-1.55 (m, 28H), 0.89 (t, J=7.13 Hz, 3H), 0.77-0.85 (m, 6H), 0.69-0.76 (m, 6H). 13C NMR (100 MHz, CDCl3/TFA [9:1], 25°C), (ppm): 172.54, 163.90, 163, 55, 152.10, 145.66, 137.09, 133.72, 133.20, 132.40, 132.31, 131.87, 129.29, 129.27, 128.80, 128.56, 127.53, 127.02, 126.78, 126.30, 124.74, 122.81, 122.15, 45.72, 45.68, 41.76, 38.60, 31.71, 30.18, 29.15, 29.08, 28.11, 27.92, 26.96, 23.55, 22.71, 22.55, 13.90, 13.75, 13.70, 10.13. HRMS (MALDI-TOF) m/z: [M+H]+ calculated for C84H82N5O10, 1320.6056; found, [M+H]+ 1320.6069. = 1.0 ppm.

Photophysical study Samples were prepared on fluorine-doped SnO2 conducting glass slides with a microscope glass cover, and filled with a solvent with or without a redox couple. For the time-correlated single photon counting experiment the sample was excited with a picosecond diode laser (Edinburgh Instruments, EPL470) at 470 nm (77.1 ps pulses, 15 pJ laser power), a 530 nm long pass filter was used between sample and detector. The instrument response function (IRF: 60 ps), determined with a scattering sample, was free to move relative to the decay during analysis; the time scale presented in the graphs is arbitrarily chosen and does not affect the fitting. All decays were analyzed by global fitting in Igor Pro 6. 10 ACS Paragon Plus Environment

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Nanosecond and femtosecond transient spectroscopy methods have been described previously 51, 52

.In brief, nanosecond transient absorption and emission were measured with a Q-switched

YAG laser (Quanta Ray, Spectra Physics) that delivered ca. 10 ns pulses at 10 pulses/second. Pulse energies were 2-10 mJ/pulse at the sample. The spectrometer (Edinburgh Instruments LP920-K) used a 450 W Xe-lamp to generate probe light. For single wavelength traces the probe signal was detected using a monochromator, a P928-type PMT and a digital oscilloscope while transient spectra were collected using a spectrograph and a CCD (ANDOR). A 500 nm short pass filter was used to help the monochromator to reject scattered laser light when measuring the kinetic traces at 480 nm of each dye on the NiO. The femtosecond transient absorption spectrometer consists of a 1 kHz Ti:Sapphire amplifier (Legend-HE-Cryo, Coherent) pumped by a frequency doubled Q-switched Nd:YLF laser and seeded by a modelocked Ti:Sapphire oscillator (Vitesse-800, Coherent). The output is 800 nm pulses with a temporal width of about 100 fs. The output is split to form pump and probe beams. Desired pump wavelength was obtained with a TOPAS-white, and with neutral density filters the energy of each pulse was kept between 200 and 400 nJ. The white light continuum probe was obtained by focusing part of the 800 nm light on a moving CaF2 plate. Polarization of the pump was set at magic angle, 54.7° relative to the probe. Instrumental response time depends on pump and probe wavelengths, but are typically about 150 fs. The NiO samples were translated to avoid photodegradation.

Solar cells fabrication and photovoltaic measurements: Conductive glass substrates (F-doped SnO2) were purchased from Solaronix (TEC15, sheet resistance 15 Ω/square). Conductive glass substrates were successively cleaned by sonication in soapy water, then acidified ethanol for 10 min before being fired at 450 °C for 30 min. Once cooled down to room temperature, FTO plates were screen printed with NiO using a homemade paste. The NiO screen-printing paste was produced by preparing a slurry of 3 g of NiO nanopowder (Inframat) suspended in 10 mL of distilled ethanol and ball-milled (500 rpm) for 11 ACS Paragon Plus Environment


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24 h. The resulting slurry was mixed in a round-bottom flask with 10 ml of 10 wt% ethanolic ethyl cellulose (Sigma Aldrich) solution and 20 ml terpineol, followed by slow ethanol removal by rotary evaporation. The dried film was calcined in air at 400 °C for 0.5 h. The prepared NiO electrodes were soaked while still hot (80 °C) in a 0.2 mM solution (CH2Cl2/EtOH: 1/1) for each dye during 16 h. Electrolytes used are composed of: 1 M lithium iodide and 0.1M diiodine in acetonitrile for I3/I- electrolyte and 0.1 M CoII(dtb-bpy)3, 0.1 M CoIII(dtb-bpy)3 and 0.1M LiClO4 in propylene carbonate for cobalt complex as redox shuttle. Platinum counter electrodes were prepared by depositing a few drops of an isopropanol solution of hexachloroplatinic acid in distilled isopropanol (2 mg per mL) on FTO plates (TEC7, Solaronix). Substrates were then fired at 375°C for 30 mn. The photocathode and the counter electrode were placed on top of each other and sealed using a thin transparent film of Surlyn polymer (DuPont, 25 µm) as spacer. A drop of electrolyte was introduced through a predrilled hole in the counter electrode by vacuum backfilling, the hole was then sealed by a glass stopper with surlyn. The cell had an active area of 0.25 cm2. The current-voltage characteristics were determined by applying an external potential bias to the cell and measuring the photocurrent using a Keithley model 2400 digital source meter. The solar simulator is an Oriel Lamp calibrated to 100 mW/cm². The dye loading was determined by desorption experiments by dipping the NiO photocathodes, into a DMF solution containing phenylphosphonic acid (2 ml, 50 mg.ml-1). After about one minute, the color of the NiO film vanished and the solution turned red indicative of desorption of the dyes and the quantity of the dye was estimated from the absorbance of the solution taking  = 1.91x104 M-1xcm-1 at 495 nm for DPP-NDI,  = 2.12x104 M-1xcm-1 at 502 nm for 4 and  = 2.31x104 M-1xcm-1 at 492 nm for 3.

Methods Theory We have followed the protocol successfully used for DPP-Br and DPP-NDI.31 All simulations have been achieved with the Gaussian09 program, using Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT), for the ground and excited state properties, respectively. The computational protocol proceeds through a four step strategy that is efficient to determine the charge transfer features of rod-like organic dyes: 1) the ground-state geometrical parameters 12 ACS Paragon Plus Environment

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have been determined at the PBE0/6-311G(d,p),53 via a force-minimization process using a SCF convergence threshold of 10-10 a.u.; 2) the vibrational spectrum of each derivative has been determined analytically at the same level of theory, that is PBE0/6-311G(d,p), and it has been checked that all structures correspond to true minima of the potential energy surface; 3) the first fifteen low-lying excited-states have been determined within the vertical TD-DFT approximation using the CAM- B3LYP/6-311++G(d,p)54 level of approximation with a tight SCF convergence threshold (at least 10-7 a.u.) ; 4) the charge-transfer parameters have been estimated with the procedure defined by Ciofini, Le Bahers and co-workers55, 56 using the CAMB3LYP electronic densities. It proposes to evaluate the distance separating the barycenters of the electron density gain/depletion upon electron transition. All calculations systematically include a modelling of bulk solvent effects (here CH2Cl2) through the Polarizable Continuum Model (PCM).57 During the simulations, the long alkyl chains such as alkyl moiety have been replaced by methyl groups in order to lighten the computational burden. The exciton binding energies were computed by determining the difference between the electronic and optical gap. The former was computed as IP-EA, where the IP and EA were determined with the SCF method at the PCM-CAM-B3LYP/6-311++G(d,p) level, whereas the latter was determined with TD-DFT as described above.

Results and discussion Synthesis of the dyes The synthesis of dyes 1-4 is rather straightforward as it is based on four known building blocks namely the dibromo-diketopyrrrolopyrrole 9,49 the N,N-di(4-benzoic acid tert-butyl ester)-4(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)amine 10,48 N-(4’-ethynylphenyl)-N’(octyl)naphthalene-1,8:4,5-tetracarboxydiimide 1350 and the N,N-di(4-benzoic acid tert-butyl ester)-amine 8. Compounds 9,49 1048and 1350 were prepared according to already described methods published in the literature, while that of 8 is described in Scheme 1. Starting with a double Buchwald-Hartwig coupling between tert-butyl 4-bromobenzoate 5 and benzylamine 6, the tertiary amine 7 was obtained in 82% yield employing Pd(dba)2 as precatalyst in presence of the phosphine ligand XPhos. The debenzylation of 7 into the secondary diaryl amine 8 was accomplished by hydrogenolysis. The reaction conditions are significantly hasher than usual (38 bars pressure of hydrogen at 50°C for 3 days with palladium on charcoal) but 8 was isolated pure and with a good yield (90%) without purification by column chromatography. 13 ACS Paragon Plus Environment


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Scheme 1. Synthesis of N,N-di(4-benzoic acid tert-butyl ester)-amine 8. Reagents and conditions: a) Pd2(dba)3-CHCl3, XPhos, tBuOK, toluene, 100°C, 22h; b) Pd/C, 38 bars H2, THF/EtOH (1:1), 50°C, 3 days.

The synthesis of the bromo-substituted DPP dyes 1 and 2 is shown in Scheme 2. It starts with a Suzuki cross-coupling reaction between bis-bromo DPP 9 and the N,N-Di(4-benzoic acid tertbutyl

ester)-4-trimethylsilanylethynyl-phenylboronate

ester

10

using

the

palladium

tetrakis(triphenylphosphine) complex as catalyst with potassium carbonate as a base and it afforded 11 in 50% yield. The formation of carbon-nitrogen bond in 12 was achieved with a Buchward-Hartwig cross-coupling reaction between 9 and the secondary amine 8 with palladium(0) and the phosphine X-Phos as catalytic system. In our hands, we have observed that the classically used ligand tert-butylphosphane gave much lower yields than reported.48 The tertbutyl ester group of the dyes 11 and 12 was hydrolyzed with trifluoroacetic acid to afford the sensitizers 1 and 2 respectively in almost quantitative yields (>90%).


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Scheme 2. Synthesis of sensitizers 1 and 2. Reagents and conditions: a) Pd(PPh3)4, K2CO3, THF/water (4 :1), 80°C, 18h ; b) TFA/CH2Cl2 (1 :1), RT, 3h ; c) Pd(dba)2, XPhos, tBuOK, 90°C, 18h.

Finally, the preparation of the dyads 3 and 4, which are the parent compound of DPP-NDI, are outlined in Scheme 3. The synthesis starts with a first Sonogashira coupling between dibromoDPP 9 and N-(4-Ethynyl-phenyl)-N′-(n-octyl)-naphthalene-diimide 13 which afforded the monocoupled adduct 14 with a slightly better yield (59%) than the Suzuki reaction leading to compound 11 (50% see Scheme 2). Subsequently, the compound 14 was involved either in a Buchward-Hartwig cross-coupling reaction with 8 to afford the dye 15 with 98% yield or in a Suzuki cross-coupling reaction with the boronate ester 10 to give the dye 16 with 96% yield. Finally, the tertbutyl ester group of the dyes 15 and 16 was hydrolyzed in almost quantitative yield with trifluoroacetic acid to furnish the sensitizers 4 and 3 respectively.



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Scheme 3. Synthesis of dyads 3 and 4. Reagents and conditions: a) Pd(PPh3)4, CuI, Et3N, toluene, 50°C, 18h b) Pd(dba)2, XPhos, tBuOK, 90°C, 18h ; c) Pd(PPh3)4, K2CO3, THF/water (4 :1), 80°C, 18h ; d) TFA/CH2Cl2 , RT, 3h.

Electronic UV-visible absorption and emission spectroscopy The absorption and emission spectra of the dyes 1-4 along with those of the references DPPBr and DPP-NDI, recorded in dichloromethane solution, are shown in Figure 1 and the spectroscopic data are collected in Table 1. All these dyes exhibit an intense absorption band around 500 nm attributed to a -* transition which is mostly localized on the lactame unit of DPP (cf. TD-DFT calculations below). In the spectra of the DPP-NDI dyads supplementary absorption bands can be seen at 359 and 380 nm, which are attributed to vibronic-* transitions on NDI unit. Surprisingly, the attachment of a trisarylamine moiety to the DPP chromophore does not induce significant bathochromic effect relative to DPP-Br, while similar systems used for organic solar cell display red shifted absorption spectra.58, 59 This is certainly due to the presence carboxylic acid anchoring groups, whose electron withdrawing character 16 ACS Paragon Plus Environment

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reduces the electron releasing ability of the amine. In addition, the presence of an additional phenyl unit between the DPP core and the nitrogen amine in dyes 1 and 3 decreases the electronic interaction, which can be clearly witnessed by the hypsochromic shift of about 10 nm of the lowest energy transition attributed to a charge transfer band between these groups (Table 1). The DPP dyes substituted with bromide are fluorescent, while the addition of the NDI unit almost completely quenches the luminescence. This is in line with previously described similar dyads, in which the singlet-excited state of the dye is quenched in solution by photoinduced electron transfer to form +Dye-NDI- (see photophysical study below).17, 24, 31 However, on NiO, hole injection is a very fast process (in the short picosecond time scale and even faster, see the results from time-resolved spectroscopy below) so that it can outcompete the direct intramolecular electron transfer to NDI, more certainly because electronic coupling with NDI is weak.3, 17, 60, 61 Indeed, it is well accepted that for NDI derivatives there is a node for both HOMO and LUMO orbitals on the nitrogen of the bisimide groups limiting thus the electronic coupling with any subunit attached via these positions on NDI.50 When attached to NiO, the visible absorption band shifts only slightly to the red (Figure S1) compared to solution. A weak fluorescence is still detectable, and the shoulder visible on the red side for 1 and 2 in solution is absent when on NiO (see transient absorption data below).



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Figure 1. Electronic absorption (straight line) and normalized emission (dashed line) spectra of the dyes recorded in dichloromethane solution.

Table 1. Wavelengths of maximal absorption (abs) with extinction coefficient (), wavelength of maximal emission (em) recorded at room temperature in dichloromethane and zero-zero energy level of the lowest singlet excited state (E00) of the DPP moiety.

a

Dyes

abs /nm (/M-1cm-1)

em /nm

1

355 (27100); 487 (17800)

563

2.33

2

352 (14600); 404 (7100); 500 (15900)

572

2.28

DPP-Br

490 (16900)

571

2.31

3

360 (50800); 380 (41500); 492 (22000)

575

2.29

4

359 (39400); 380(32200); 502(20500)

586

2.26

DPP-NDI

359 (44400); 380 (38600); 495 (22400)

577

2.28

a

E00 / eV

calculated according to the equation: E00 = 1240/inter, with inter wavelength at the intersection

of the normalized absorption and emission spectra.

Electrochemical study and electron transfer driving forces


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An electrochemical study by cyclic voltammetry was undertaken to determine the redox potentials of the dyes as well as to calculate the hole injection (G°inj) and dye regeneration (G°reg) driving forces and these data are collected in Table 2. The first oxidation occurs on the trisaryl amine moiety around 1.1 V vs. SCE with a generally non-reversible or weakly reversible wave while the DPP core gets reduced with a non-reversible wave at around -1.2 V. This process arises in same potential range as that reported with other dyes functionalized with this arylamine anchoring group.20, 21, 62, 63, 64 In the dyads 3 and 4, the first reversible reduction takes place on the NDI moiety around -0.6 V vs. SCE and a second reversible reduction occurs at around-1.0V. This is followed by the reduction of the DPP at around -1.2V with near zero cathodic shift of the reduction potential indicating that the two electrogenerated electrons poorly interacts through the phenyl spacers. The large separation of the reduction potentials of these two units imparts that there is a large driving force for the electron shift reaction in these dyads (around -0.6 eV for DPP--NDI → DPP-NDI-). The calculated Gibbs free energies of the photoinduced hole injection into NiO valence band and the regeneration reaction clearly indicated that all these dyes display sufficient exergonicity for these processes to spontaneously occur (Table 2).

Table 2. Half-wave potentials recorded by cyclic voltammetry at room temperature in dichloromethane/DMF [95/5] solution with Bu4NPF6 (0.15 M) as supported electrolyte and referenced versus saturated calomel electrode (SCE).

eV

cG° reg with I3eV

Greg with CoIII eV

1.07

-0.77

-0.94

-1.47

-

1.03

-0.73

-0.93

-1.46

-1.24

-

1.07

-0.77

-0.92

-1.45

1.15 (60)

-1.22 (90)

-0.60 (110)

1.07

-0.77

-0.28

-0.81

1.28 (90)

0.99 (100)

-1.27 (70)

-0.61 (100)

0.99

-0.69

-0.29

-0.82

-

1.16

-1.23

-0.60

1.05

-0.75

-0.28

-0.81

E1/2(TPA+/TPA) (V) E (mV)

E1/2(DPP+/DPP) (V) E (mV)

E1/2(DPP/DP P-)(V) E (mV)

E1/2(NDI/NDI-) (V) E (mV)

1

1.19 (90)

1.13 (120)

-1.26e

-

2

1.27 (130)

1.01 (110)

-1.25e

DPP-Br

-

1.20

3

1.15 (60)

4

Dyes

DPPNDI a

aE

1/2(DPP*/ DPP-)(V)

bG°

inj

Calculated according to the equation: E1/2(DPP*/DPP-) = E1/2(DPP/DPP-) + E00. bCalculated

according to the equation: G°inj=EBV(NiO)- ERed(DPP*/DPP-) with EBV(NiO) = 0.30 V vs SCE.65 cCalculated according to the equation: Greg= ERed(S/S-) - E(I3-/I2-●) with E(I3-/I2-●) = -



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0.32 V.66 dCalculated according to the equation: G°reg= ERed(S/S-) - E(CoIII/CoII) with E(CoIII/CoII) = 0.21 V.67 eNon reversible process, cathodic peak potential.

Quantum chemical calculations To rationalize deeper the electronic properties DFT quantum chemical calculations were performed in these new series of dyes. The key results are collected in Table 3. Let us start by a comparison of 1, 2 and DPP-Br. Consistently with the experimental spectra, all these dyes present similar absorption wavelengths and absorption intensities. The computed ordering for the abs (2 > DPP-Br > 1) is also consistent with the measured values (Table 1). However, the examination of the nature of the excited-states related to these absorption bands, substantial differences appear. This statement holds for both the charge-transfer (CT) intensities and the density difference plots shown in Figure 2. In DPP-Br, the excitation is, as expected, mainly localized on the DPP moiety and the two vicinal phenyl rings and a moderate CT occurs. Replacing the terminal thiophene group by a trisarylamine (2) increases the CT by a factor of two, the new group adding as a donor (mostly in blue in Figure 2). We note that this increase is moderate as the computed charge separation remains limited to less than 1.5 Å. As stated above, we attribute this effect to the presence of two carboxylic groups that diminish the donating power of the amine. When increasing the linker length (going from 2 to 1) the large dihedral angle of the biphenyl units prevents an efficient direct electronic communication between the triphenylamine and the DPP, and the excited-state is mostly localized on the DPP unit with a negligible CT effect. This lack of CT may be detrimental for DSC, but the excited-state is now well separated from the anchoring group, which should limit the charge recombination that is known to be the key limiting parameter in p-DSC.1, 10, 11 Let us now turn to the dyads 3 and 4 which contains a NDI unit. The geometry optimization shows that the NDI unit is perpendicular to the closest phenyl ring, i.e., there is not direct communication between the DPP dye and the appended NDI. The evolution of the spectra with respect to the previous structures (a slight bathochromic shift and an increase of the intensity) is well reproduced by theory (Table 3). The NDI-centered band is present in the theoretical spectrum, though at slightly too high energy (347 instead of 380 nm). Due to the decoupling between NDI and DPP moieties, the main conclusions obtained above (ordering of the CT distance, role of the trisarylamine group in the excited state...) still hold when comparing 3, 4 and DPP-NDI. As could be expected, theory foresees that the LUMO of the dyads is exclusively localized on the NDI (the transitions at ca. 20 ACS Paragon Plus Environment

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490 nm promote the electron to the LUMO+1 located on the DPP), that can therefore effectively act as a secondary acceptor in agreement with the reduction potentials measured by cyclic voltammetry (Table 2). For 1, 2 and DPP-Br, we have calculated the exciton binding energies of 1.27 eV, 1.26 eV and 1.25 eV. This parameter does not vary significantly within the series and cannot explain the differences of photovoltaic performances as it was done with other dyes.68

Table 3. Computed wavelength of maximal absorption (abs) with oscillator strengths (f), ground-state dipole moment (), charge-transfer distance (dCT) and amount of transferred charge (qCT). The results for DPP-Br and DPP-NDI are taken from ref.31. Dyes

abs /nm (f/au)

/ D

dCT / Å

qCT / e

1

466 (0.87)

2.61

0.51

0.38

2

475 (0.89)

2.72

1.34

0.40

DPP-Br

472 (0.86)

3.45

0.61

0.38

3

482 (1.25), 347 (0.78)

3.68

0.92

0.39

4

492 (1.27), 347 (1.04)

3.17

1.56

0.41

DPP-NDI

490 (1.24), 347 (0.81)

3.60

0.25

0.39



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Figure 2. Computed density difference plots for dyes 1 (top) 2 (center) and DPP-Br (bottom). The blue (red) regions correspond to decrease (increase) of the density upon photon absorption.

Electron Transfer studies by Transient Absorption Spectroscopy The electron transfer dynamics of the dyes in CH2Cl2 solution and on NiO films were studied by femtosecond and nanosecond transient absorption spectroscopies. The solution phase dynamics of the singlet excited state of all dyes were investigated, also with time-correlated single-photon counting (TCSPC), as reference for the dynamics on NiO. The transient absorption spectra and summary kinetic data is shown in the ESI (Table S1, Figure S3-S6). 1 and 2 exhibit similar transient spectra at early times, where the excited state shows a broad band around 400 nm with a small shoulder at 440 nm, and another absorbance band at >660 nm. The ground state bleach around 500 nm agrees with the steady state absorption spectra. Stimulated emission shows initially at ~560 nm and is then dynamically Stokes shifted to the red on a time scale of several ps. For 2 the subsequent decay of the relaxed excited state back to the ground state could be fitted with a sum of two exponentials with  = 730 ps (75% of the amplitude) and > 2 ns (25%), respectively. This is in fair agreement with the TCSPC measurements, where the longer component was determined to 4.2 ns (Figure S2). For comparison the DPPBr dye, without the appended TPA group, showed a fluorescence lifetime of 10 ns (not shown). We tentatively attribute the much shorter, 730 ps component, observed for 2 (also seen in TCSPC) to partial self-quenching/aggregation in solution, related to the shoulder in the fluorescence spectra (Figure 1). Note that the shoulder is absent for the dyes absorbed on NiO


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(Figure S1). An alternative assignment for the short excited state lifetime would be intramolecular charge separation to form the TPA●+-DPP●- state that then recombines to the ground state much faster than it is formed (as seen for 1 below); however, this would require an explanation why only 75% of the dyes show the same reactivity. Surprisingly, the excited state of 1 decays much faster than that of 2,  = 26 ps, in agreement with the TCSPC result (cf. Figures S2 and S3). This result may appear to contrast with the DFT calculation results, where the addition of one phenyl group in 1 makes the excited state mainly localized on the DPP moiety and prevents the intramolecular CT character of the excited state. However, the intramolecular charge separated state TPA●+-DPP●- is distinct from the local excited state and stabilized at a quite different solvent polarization coordinate. Indeed, the resulting spectral shape of 1 at 60 ps (Figure S3) shows features that qualitatively resemble the sum of a TPA●+ radical (strong peak around 680 nm) and DPP●- (430 nm absorption and ground state bleach). The TPA●+-DPP●- state is formed with  = 26 ps, and decays by recombination to the ground state with  ≈ 140 ps (Table S1; for decay-associated spectra (DAS) in solution, see Figures S3-S6). The energy of the charge separated state of 1, as estimated from the difference between the E1/2(TPA+/TPA) and E1/2(DPP/DPP-) potentials (Table 2), is 2.45 eV. This is close to the excited state energy E00 = 2.32 V, and should be further lowered by coulombic attraction. Given the approximate nature of these estimates, it is reasonable that the charge separated state lies below the initial excited state in 1. For 2 the excited state energy of the dye is 50 meV lower and the TPA unit 80 mV more difficult to oxidize than in 1 (see Table 2), thus energetic differences can probably explain the slower electron transfer reactivity of 2, in spite of the shorter TPA-DPP distance.



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Figure 3. Transient absorption spectra for 3 and 4 in DCM. Both spectra, 3 (solid) and 4 (dashed) are recorded at a 10 ps delay after excitation with 525 nm (500 nJ). Inset is the kinetic traces at 480 nm probe for 3 (red) and 4 (blue).

For 3 and 4, the dyes have a secondary electron NDI acceptor in the molecular structure. Initially they show similar transient absorption spectra as for the 1 and 2 singlet excited state, but 4 rapidly develops spectral signatures of an intramolecular charge separated state where DPP is instead oxidized by NDI: DPP●+-NDI●-. Figure 3 shows the transient spectra of 3 and 4 in DCM 10 ps after excitation, where clearly a charge separated state can be observed for 4, coexisting with the remaining initial excited state: a DPP ground state bleach accompanied by one NDI●- absorption peak at 480 nm and a smaller one at ~610 nm, overlapping with the strong remaining stimulated emission, together with the absorption peak at ~440 nm from oxidized DPP radical cation. An absorption trace at 480 nm shows how the initial bleach recovers and is converted to a net absorption ( = 12 ps) from NDI●-, which subsequently decays to base line with  = 63 ps (Figure 3, inset). The concomitant bleach of DPP and absence of TPA●+ absorption at 680 nm suggests that the intramolecular charge separated state stays on the DPP●+NDI●- units; for 4 this is expected from the relative TPA+/0 and DPP+/0 potentials (Table 2). For 24 ACS Paragon Plus Environment

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3 with similar TPA+/0 and DPP+/0 potentials the result can be explained by greater coloumbic stabilization by the NDI unit at shorter distance, thus favoring DPP over TPA oxidation. Analyzing the data in the region dominated by stimulated emission it is clear that most of that negative signal disappears with the slower component ( = 63 ps; see traces and DAS in Figure S6). Thus, we can assign the 63 ps component to intramolecular charge separation, forming DPP●+-NDI●- followed by a faster recombination  = 12 ps (so-called inverted kinetics). This explains that the NDI●- signal is never strong compared to that from the initial excited state. Also 3 shows a rapid excited state decay, but with less clear signs of NDI●-, these are seen in the decay associated spectra (DAS) only (Figure S5). The 480 nm bleach recovery parallels that of stimulated emission at 620 nm (Figure S5), occurring on the 140 ps time scale. Therefore, we again assign the slow component ( = 140 ps) to charge separation forming DPP●+-NDI●-, but the latter has a short lifetime of 16 ps and never accumulates to a clearly detectable degree (see Table S1 and Figure S5). The studies of 1-4 in solution show that intramolecular charge separation and subsequent recombination to the ground state can occur on a ps timescale in all dyes (except possibly 2). This has to be considered also for the dyes on NiO. However, as discussed below, hole transfer to NiO dominated excited state reactivity in all cases. Transient absorption spectroscopy with 120 fs, 500 nJ laser pulse excitation at 525 nm was performed on the dye-sensitized NiO films in the presence of LiClO4 propylene carbonate solution. Both transient spectra of 1 and 2 gave initially a similar excited state signal as in DCM, but the signals decrease on a 1-10 ps time scale, and the spectrum blue-shifts as seen most clearly by the shift of the isosbestic point from ~640 nm to ~600 nm (Figure S7 and S8). From the DAS (Figure S3-S4) it is seen that most of the DPP bleach and stimulated emission disappears on this time scale, so that the excited state is rapidly quenched as compared to the case in solution. Instead a more long-lived spectrum with an absorption band around 640 nm 25 ACS Paragon Plus Environment


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and bleach around 500 nm is formed. This can be attributed to DPP●- formed after hole injection to NiO. Multi-exponential hole injection and recombination kinetics is typical for dye-sensitized NiO and the processes often overlap in time.11, 60, 61, 67 Each time constant derived does not reflect an individual process or sub-population, but represents the time scales of a heterogeneous process, and the precise values should not be over-interpreted. The number of exponents used to fit data is somewhat arbitrarily chosen as the minimum number of exponents needed to give representative fit.

Figure 4. Kinetic traces of dye 2 on NiO probed at 570 nm (open diamond) and 625 nm (open circle), solid line is the multi-exponential fit.

Figure 4 shows the kinetic traces and a sum of multi-exponential fitting for dye 2 probed at 570 nm and 625 nm, the time constants obtained from global fit of the transient data are reported in ESI (Table S2). Hole injection occurs on a time scale of 1-10 ps, but rapid charge recombination (~50 ps) depletes the charge separated state. The signals for 1/NiO are smaller and therefore the dynamics is harder to analyze but essentially agrees with that for 2/NiO (see ESI). To conclude, while the excited dyes 1 and 2 are quenched by hole transfer to NiO on the 1-10 ps time scale, the subsequent recombination of the reduced DPP anion with the injected hole in NiO is also


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rapid, which most likely is the reason for the poor photovoltaic performance of dyes 1 and 2 as sensitizers in p-type DSCs (see below). Given the multi-exponential kinetics, we see no significant difference in the dynamics of either hole injection or recombination between 1 and 2, in spite of the extra phenyl spacer in 1. However, compared to DPP-Br that has a different and shorter anchoring group, the majority of recombination for 1 and 2 occurs on almost an order of magnitude shorter time scale.

b)

a)

Figure 5. Transient absorption spectra of NiO/3 (a) and NiO/4 (b) in presence of 0.1 M LiClO4.

a)



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b)

Figure 6. Normalized kinetic traces comparison of NiO/3 (green circle) and NiO/4 (blue diamond) at 481 nm (a) and 620 nm (b) in the presence of 0.1 M LiClO4. Solid line is the multi-exponential fit for each kinetic trace.

Appending a secondary electron acceptor to replace the bromine group, dye 3 and 4 sensitized NiO films were excited at 525 nm in presence of 0.1 LiClO4, covered with a thin glass. Both the initial singlet excited state of NiO/3 and NiO/4 show the broad absorbance and stimulated emission (Figure 5 a-b and see ESI, DAS analysis) as was also the case in solution. Already after some ps, however, the excited state features have been replaced by those of the reduced NDI●- unit: a strong peak around 480 nm and a weaker one at 605 nm (Figure 5a-b). The stimulated emission decay kinetics at 620 nm for 3 is indistinguishable from that of the NDI●rise at 480 nm, for 4 the NDI●- formation is possibly slightly slower (Figure 6a-b). Global fits to the data showed excited state decay with ultra-fast (0.2 ps) up to 15 ps components (Table S2). First, this suggests that hole injection from excited DPP is faster than intramolecular charge separation ( = 60-140 ps in solution, see above). Second, the subsequent electron transfer from DPP●- to the NDI unit must be at least as fast as hole injection, as NDI●- is formed without any clear accumulation of a DPP●- intermediate. Third, the NDI●- signal remains almost constant 28 ACS Paragon Plus Environment

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until the end of the optical delay (2 ns), giving evidence for the strong increase in charge separation lifetime of 3 and 4 compared to 1 and 2, afforded by the appended NDI unit. To monitor the recombination time scale and regeneration by the redox couple, the sensitized NiO/3 and NiO/4 samples were investigated on a longer time scale, by nanosecond transient absorption spectroscopy. On a ns-ms time scale after excitation with a 532 nm, 10 ns laser pulse the NiO/3 and NiO/4 samples show a clear signature of the NDI●- radical anion (Figure 7a). The NiO hole absorption is comparably weak and cannot be distinguished.69 If any DPP●- anion would be formed, its lifetime should be on the ps time scale and not be observed here; the spectral difference in Figure 7a is attributed to the different ground state absorption of NiO/3 and NiO/4. The signal is very long-lived compared to other dye/NiO systems11, 30 and decays on a time scale of up to 1 ms, very similar to the results of the best performing NiO DSCs70 (Figure 7b-c). A double exponential fit gave the time constants of ≈ 50 μs and ≈ 500 μs and equal amplitudes, for both 3 and 4 (Table S3). A stretched exponential function (KWW) did not fit the data as well (see Figure S10) Addition of 0.1 M CoIII(dtb)3 rapidly regenerated the dye, as seen from the much faster NDI●- decay kinetics; > 85% of the signal decayed with  = 20-25 μs. The result is that dye regeneration is rather efficient, >60% as judged from the kinetics, which is most likely the main reason for improvement of the photovoltaic properties for those two dyes compared to 1 and 2. The slightly better performance of NiO/3 than NiO/4 could be related to a slower (570 s) component in the absence of CoIII(dtb)3 and more proportion of this component than for NiO/4 (470 s) (Table 4), leading to a higher yield of regeneration. The difference is small, however, showing that the additional phenyl spacer in 3 has only marginal impact on the recombination kinetics. The recombination kinetics is also very similar to that for DPP-NDI, however. Thus, in contrast to the comparison between 1, 2 and DPPBr, the TPA anchoring group of 3 and 4, does not give a slower charge recombination than in DPP-NDI. A possible reason could be that recombination for these longer dyes does not 29 ACS Paragon Plus Environment


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occur through the anchoring group, but by direct tunneling contact between the NDI group and NiO. Similar effects have been suggested for other rod-like molecules on mesoporousTiO2 films.71 Table 4. Time constants from ns transient absorption measurements of NiO/3 and NiO/4 with and without electrolyte.

LiClO4

CoIII

Lifetime

NiO/3

NiO/4

1

49 s/51%

47 s/58%

2

570 s/49%

476 s/42%

1

21 s/87%

24 s/89%

2

445 s/13%

410s/11%


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a)

b) a)

)

c)

Figure 7. Normalized ns transient absorption spectra of NiO/3 and NiO/4 (a) and the recombination, regeneration of each sample with or without CoIII(dtb)3 electrolyte (panel b for dye 3 and panel c for dye 4) (excitation at 532 nm). Note that CoII(dtb)3 absorbs more than CoIII(dtb)3 does at 480 nm, which explains the positive signal at the end of the observed decay.



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Figure S10 presents the dye/NiO recombination traces on a shorter time scale. Here we do not observe the fast ≈15 ns recombination component that was recently reported by D’Amario et al.72 This was attributed to rapid recombination in parallel to hole relaxation, i.e. observation of this phenomenon requires significant recombination on a similar time scale as the putative relaxation (15 ns). With the present dyes 3 and 4 we see no significant recombination on a time scale < 1 μs so any hole relaxation would have been completed before recombination. At the same time, recombination with NiO/1 and NiO/2 was complete on a < 2 ns time scale. Thus, the fact that we do not observe any 15 ns component with the present dyes does not give evidence against the interpretation of D’Amario et al.72 To conclude, the time-resolved studies have revealed a large effect of the appended NDI unit in 3 and 4, as it is rapidly attracting the electron ejected from NiO (1-10 ps time scale) and thus retard recombination with the hole from the 50 ps time scale observed for 1 and 2 to the 50-500 μs time scale (Table 4). The dye is also regenerated by the CoIII(dtb)3 electrolyte faster than recombination, allowing for an efficient regeneration step.

Photovoltaic measurements All these new dyes 1-4 were tested and compared to the reference dyes DPP-Br and DPP-NDI in sandwich dye-sensitized solar cells. These cells were fabricated with 1.2 m thick mesoporous NiO films and assembled to a platinum counter electrode (see experimental part for details). In this study, two electrolytes were investigated: the classical iodide/triiodide electrolyte and the cobalt electrolyte using the tris(4,4’-ditert-butyl-2,2-bipyridine)cobalt complexes as redox mediators.67, 73 The latter is a slower dye regenerator and therefore only compatible with long-lived charge separated state (in the range of microsecond timescale) but enables much higher open circuits voltage (Voc) than iodide electrolyte.3 The characteristic parameters of the cells are gathered in Table 5 and the photoaction spectra of the cells are shown in Figure 8. First, the photovoltaic performances of the series of bromo substituted DPP increases in the following order 1 >> 2 > DPP-Br independently of the electrolyte composition. Clearly, the DPP sensitizers with the longer spacer (dye 1) performs better than those with shorter linker (DPP-Br and dye 2) showing a higher Jsc. However, the sensitizer 1 does not exhibit higher extinction coefficient or red shifted absorption bands compared to sensitizers 2 or DPP-Br. In addition, the dye loading, measured by desorption experiments is, within experimental error, 32 ACS Paragon Plus Environment

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similar for all the dyes (around 15 nmol/cm2). Therefore, the light harvesting efficiency (LHE) cannot explain the differences in photovoltaic performances. The higher IPCE value measured with 1 cannot be ascribed to a larger injection quantum yield, firstly because the hole injection driving force is very large and quite similar for all these dyes (Table 2), and secondly because the photophysical study has shown that the rate constants for hole injection is much faster than the intrinsic decay of the dye singlet excited state and have similar values for all the dyes. Consequently, we conclude that the higher performances of dyes 1 and 2 relative to DPP-Br probably stem either from a bit higher charge collection efficiency, which may arise from a slightly slower charge recombination with the injected hole in NiO or a lower charge recombination of the hole with the electrolyte, owing to a different packing on the NiO surface decreasing thus the access of the redox mediator to the surface. The latter explanation is consistent with the slightly higher Voc measured with dyes 1 and 2.

Table 5. Mean photoelectrochemical metrics (within 6 different cells) of the p-DSCs sensitized with DPP dyes and employing either the iodide/triiodide (up) or cobalt (below) electrolytes recorded under AM1.5 G simulated sunlight (1000 W/m2). Dye

1

2 a

DPP-Br

3

4 a

DPP-NDI

electrolyte

Jsc

Voc

ff

PCE

mA/cm2

mV

%

%

I3-/I-

1.89 ± 0.30

100 ± 12

33 ± 2.9

0.063 ± 0.020

CoIII/II

0.49 ± 0.12

198 ± 22

24 ± 0.3

0.024 ± 0.008

I3-/I-

1.44 ± 0.30

84 ± 7

33 ± 2.1

0.040 ± 0.009

CoIII/II

0.41 ± 0.020

134 ± 20

24 ± 0.3

0.013 ± 0.001

I3-/I-

0.88 ± 0.13

70 ± 4

33 ± 1.1

0.020 ± 0.005

CoIII/II

0.26 ± 0.049

103 ± 27

28 ± 5.8

0.007 ± 0.002

I3-/I-

2.03 ± 0.48

90 ± 9

33 ± 0.5

0.062 ± 0.021

CoIII/II

2.06 ± 0.12

330 ± 27

30 ± 1.2

0.205 ± 0.026

I3-/I-

1.72 ± 0.13

76 ± 3.5

32 ± 0.3

0.041 ± 0.004

CoIII/II

1.95 ± 0.053

370 ± 17

29 ± 0.3

0.21 ± 0.014

I3-/I-

1.79 ± 0.047

81 ± 0.8

34 ± 0.7

0.048 ± 0.001

CoIII/II

1.56 ± 0.042

292 ± 5.0

29 ± 0.1

0.13 ± 0,006

Voc= open circuit voltage, Jsc = short circuit current density, ff= fill factor and PCE = Jsc x Voc x ff/P(light) 33 ACS Paragon Plus Environment


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Concerning, the dyads 3, 4 and DPP-NDI the photoconversion efficiency (PCEs) increases in the following order 3 ≈ 4 > DPP-NDI and is much higher than that of dyes 1, 2 and DPP-Br independently of the electrolyte compositions. Again, we can rule out both LHE and hole injection efficiency differences to explain this result, because the dye loading and the hole injection rate constants are quite similar within these second series. Nevertheless, the photophysical study points to a slightly, but noticeable, longer lived charge separated state for dye 3 than 4. As a consequence, the dye regeneration by the electrolyte might be slightly higher for dye 3. Interestingly, dye 4 shows a slightly higher Jsc value with cobalt electrolyte than that obtained with iodine electrolyte. First, in the dyads systems (3, 4 and DPP-NDI) the electrolyte is not reduced by DPP but rather by NDI, whose reduction potential is much less negative than that of DPP (Table 2). The reduction potential of triiodide into diiodide radical anion (E(I3-/I2●

) = - 0.32 V vs. SCE)66 is more negative than that of CoIII/II (E(CoIII/CoII) = 0.21 vs SCE),67

therefore the regeneration driving force is much higher with cobalt electrolyte than with iodide explaining thus the higher Jsc measured the former electrolyte. We have already reported such behavior before.17 Second, the transient absorption spectroscopy study indicates that the lifetime of the reduced dye in 4 is longer than that of 3 (Table 4). Therefore, the regeneration of reduced 4 by Co(III) competes more efficiently with the charge recombination reaction, which is particularly important as this cobalt(III) complex is known to be a slow electron acceptor.67 In short, all the new dyes prepared in this work are better performing than the first generation (DPP-Br and DPP-NDI) as their photovoltaic performances are higher irrespective of the selected electrolytes (Table 5), and this is probably due to small variations of kinetics of dye regeneration or lower interfacial charge recombinations with the electrolyte.


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a)

b)

Figure 8. IPCE spectra of NiO based DSCs sensitized with the dyes with iodide/triiodide electrolyte (a) and cobalt electrolyte (b).



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The IPCE spectra of the dyads with the cobalt electrolyte show a real enhancement of the photoactivity with regard to the parent dyes 1, 2 and DPP-Br (Figure 8b). This indicates that the presence of a NDI secondary acceptor is crucial to exploit this cobalt electrolyte. In addition, the photoactivity is significantly higher on both the DPP absorption band (around 500 nm) but also on the NDI transition (around 380 nm) meaning that photoexcitation of the latter probably triggers an electron transfer from DPP to NDI, which is followed by hole injection into NiO from the radical cation of DPP (Figure 8). With the iodide electrolyte, the intense IPCE around 380 nm is certainly a combination of the photoactivity of NDI along with that of the triiodide since it is known that the latter absorbs in this region and leads to a long-lived excited state with strong oxidizing power (Figure 8a).73, 74 However, the IPCE spectra of all these dyes reveal that there is a valley around 420 nm, where the photon flux of solar spectrum is very large (Figure 8a-b). As a consequence, the design of new DPP dyes with higher absorbance in this region would significantly boost the Jsc and therefore the overall PCE. These latter aspects should be taken into consideration for further engineering of new sensitizers for NiO with DPP moiety.

Electrochemical impedance spectroscopy The solar cells were investigated by electrochemical impedance spectroscopy (EIS) recorded under illumination to characterize further the properties of these dyes. The transmission line model, developed by Bisquert and co-workers75, 76 and adapted by other groups77, 78, 79, 80 to NiO based p-DSCs, was used here. The Nyquist plots display the classical two loops: a smaller one in high frequency region attributed to the charge transfer resistance at the interface electrolyte/counter-electrode and a larger one in lower frequency part, which is assigned to the recombination resistance RREC at the interface NiO/dye/electrolyte (Figure S11). The RREC represents the combination of the hole injection in NiO from the dye excited-state, the regeneration of the reduced dye by the electrolyte minus the charge recombination processes (recombination of the holes in NiO with iodide or with the reduced dyes). As a consequence, a lower RREC accounts for a higher density of hole injection flux of the system and therefore a higher Jsc. The measured RREC are consistent with the values of Jsc as they are ranked in the following order DPP-Br > 2 > 3 > 4 ≈DPP-NDI ≈ 1 for iodide electrolyte and in the order DPP-Br > 2 ≈ 1 > DPP-NDI > 4 ≈ 3 for cobalt electrolyte (Table 6). Accordingly, the best dyes are those for which the injection and regeneration efficiencies are the highest. With 36 ACS Paragon Plus Environment

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iodide/triiodide electrolyte, there is no large change in photocurrent densities between the dyads and simple dyes as the latter exhibit higher driving force (see above and Table 2) and a preassociation of the dye with triiodide may facilitate the regeneration reaction.

Table 6. Recombination resistance (RREC) and hole lifetimes (h+) of the p-DSCs measured by EIS under illumination at open circuit voltage. Iodide electrolyte

Cobalt electrolyte

RREC

h+

RREC

h+

()

(ms)

()

(ms)

1

49

50

1319

785

2

65

83

1321

841

DPP-Br

88

67

2503

1230

3

60

85

365

157

4

52

73

384

177

DPP-NDI

51

63

494

252

Dye

The hole lifetime (h+) is the average time during which the injected holes in NiO survive before recombining with the reduced dye or with iodide anion in the electrolyte. Usually a longer h+ indicates lower charge recombinations and therefore a higher concentration of hole carriers in the valence band of NiO. Within experimental error, with iodide electrolyte the Voc is quite similar for all the dyes. For dyad sensitizers (3, 4 and DPP-NDI) with the cobalt electrolyte, the higher Voc measured with dye 4 is consistent with the longer hole lifetime in line with slower charge recombination measured by transient absorption spectroscopy. (see above). With the simple dyes (1, 2 and DPP-Br) there is no correlation between the Voc and the h+ suggesting that the Voc is not mostly controlled by charge recombination processes. We can observe that the dyes with two anchoring groups (1, 2, 3 and 4) give substantially higher Voc than reference dyes (DPP-Br and DPP-NDI having a single CO2H anchoring group). This suggests either the occurrence of a higher upward valence band bending due to the release of two protons upon binding on NiO, or these dyes have a dipole moment more oriented towards NiO surface. Assuming a perpendicular orientation of the dye relative to NiO surface, the computed dipole moment vectors of dyes 1, 2 are indeed more oriented towards the NiO surface than that of DPP-Br (Figure S12). 37 ACS Paragon Plus Environment


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In summary, the difference of Voc within these dyes are most certainly the consequence of a valence band bending (induced by energy level determining protons released by the binding groups or by the dipole moment which is more oriented to the surface) rather than differences in the charge recombination reactions.

Conclusions We have presented the synthesis of four new DPP-based sensitizers and their detailed characterizations by different techniques including transient absorption spectroscopy in solution and on NiO films and they were modeled with TD-DFT quantum chemical calculations. The photovoltaic performances were also determined in NiO based p-DSCs. This series exhibits suitable photoredox properties to act as efficient sensitizers on NiO because they absorb light with a quite good efficiency, the energetics for hole injection and charge recombination are sufficiently high to expect fast processes. This is indeed supported by the photophysical study, which indicates a very fast hole injection reaction leading to a hole injection quantum yield probably close to unity for all the dyes. The charge recombination rate constants, however, differ a lot from the dyes substituted by bromo compared to those substituted with NDI. The latter exhibit a long lived charge separated state which lies in the microsecond timescale. These new DPP-NDI dyes display the longest charge separated state lifetimes ever recorded for p-DSC materials. As a result, the dyes containing NDI electron acceptor give higher photovoltaic performances and are compatible with the cobalt electrolyte. Overall, they lead to higher photoconversion efficiencies than the previously reported first generation of DPP dyes.

Supporting Information Available: UV-vis absorption and emission of the dyes grafted on NiO, transient absorption spectra (up) with kinetic traces and time constants; Nyquist plots of NiO based p-DSSC and representation of the permanent dipole moment of the dyes.

Acknowledgements ANR is gratefully acknowledged for the financial support of these researches through the program POSITIF (ANR-12-PRGE-0016-01) and Région des Pays de la Loire for the project LUMOMAT and Europe for COST CM1202 program (PERSPECT H2O). D.J. acknowledges 38 ACS Paragon Plus Environment

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both the European Research Council (ERC) and the Région des Pays de la Loire for financial support in the framework of a Starting Grant (Marches - 278845) and a recrutement sur poste stratégique, respectively. This research has used resources of the GENCI-CINES/IDRIS and the CCIPL (Centre de Calcul Intensif des Pays de Loire). L.H and L.Z. gratefully acknowledge the Knut and Alice Wallenberg Foundation, the Swedish Energy Agency, The Swedish Research Council for support, and the Chinese Scolarship Council for a doctoral fellowship to L.Z. Nadine Szuwarski is acknowledged for the screen printing of the NiO films on FTO substrates.

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