Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of zinc chlorophyll materials for dye-sensitized solar cell applications Sule Erten-Ela a,⇑, Olena Vakuliuk b, Anna Tarnowska c, Kasim Ocakoglu d,e,⇑, Daniel T. Gryko b,c,⇑ a
Solar Energy Institute, Ege University, Bornova 35100, Izmir, Turkey Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland c Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland d Advanced Technology Research & Application Center, Mersin University, Ciftlikkoy Campus, TR-33343 Yenisehir, Mersin, Turkey e Department of Energy Systems Engineering, Mersin University, Tarsus Faculty of Technology,33480 Mersin, Turkey b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Zinc chlorophyll materials have been
Chlorophyll dyes for dye sensitized solar cell.
synthesized. Zinc chlorophyll materials have been characterized. Dye sensitized solar cells were fabricated and characterized.
OH
OH CH 3
OH CH3
CH 3
H 3C
H3C N
N Zn
N
O
a r t i c l e
i n f o
Article history: Received 23 March 2014 Received in revised form 3 July 2014 Accepted 15 July 2014 Available online 1 August 2014 Keywords: Zinc chlorophyll Dye sensitized solar cell Electrolyte Organometallic synthesis
CH 3
N
O
N CH3
H3C
O
O
O C3H7
N
N
CH 3
O O
CH 3
CH3
Zn
H 3C
O
O
N
Zn N
CH3
H3C
CH3
H3C
N
Br N
N CH 3
O
N
N
Zn N
OH 31
CH 3
H 3C
N
H 3C
CH 3
CH3
O
131
O CH3
a b s t r a c t To design sensitizers for dye sensitized solar cells (DSSCs), a series of zinc chlorins with different substituents were synthesized. Novel zinc methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (ZnChl-1), zinc methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (ZnChl-2), zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (ZnChl-3), zinc propyl 3-devinyl-3hydroxymethyl-pyropheophorbide-a (ZnChl-4) were synthesized and their photovoltaic performances were evaluated in dye-sensitized solar cells. Photoelectrodes with a 7 lm thick nanoporous layer and a 5 lm thick light-scattering layer were used to fabricate dye sensitized solar cells. The best efficiency was obtained with ZnChl-2 sensitizer. ZnChl-2 gave a Jsc of 3.5 mA/cm2, Voc of 412 mV, FF of 0.56 and an overall conversion efficiency of 0.81 at full sun (1000 W m 2). Ó 2014 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors. Tel.: +90 2323111231; fax: +90 2323886027 (S. ErtenEla). Address: Advanced Technology Research & Application Center, Mersin University, Ciftlikkoy Campus, TR-33343 Yenisehir, Mersin, Turkey. Tel.: +90 3243610001/4961; fax: +90 3243610153 (K. Ocakoglu). Address: Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland (D.T. Gryko). E-mail addresses: [email protected], [email protected] (S. Erten-Ela), [email protected], [email protected] (K. Ocakoglu), dtgryko @icho.edu.pl (D.T. Gryko). http://dx.doi.org/10.1016/j.saa.2014.07.026 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Over the past two decades dye sensitized solar cells have emerged as an important technology for the development of lowcost, easy processable, clean energy production [1–5]. A typical DSSC is composed of a dye molecule that is anchored to a mesoporous TiO2 surface. Efficient charge separation in dye sensitized solar cells is achieved by photoinduced electron injection from organic sensitizer into the conduction band of a metal oxide electrode.
S. Erten-Ela et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
The dye cation radical is regenerated by a redox electrolyte. Under simulated AM1.5G illumination (100 mW cm 2), the power conversion efficiency of a cell, g is defined as the product of the generated photocurrent density (Jsc), the open-circuit photovoltage (Voc), and the fill factor (FF), as follows: g = JscVocFF [4,6]. The efficient light harvesting potential of chlorophyll dyes, exemplified by their primary role in photosynthesis, makes them promising candidates for light harvesting antennas in dye sensitized solar cells [7,8]. Porphyrin based dyes have been evaluated as photosensitizers for DSSC due to their strong Soret (400– 450 nm) and moderate Q bands (550–600 nm) absorption properties as well as their primary role in photosynthesis mechanism [9–11]. They have demonstrated charge transfer kinetics indistinguishable from those of ruthenium polypyridyl complexes [12], which are the most efficient dyes ever reported. Moreover, optical, photophysical and electrochemical properties can be systematically evaluated. However, they generally show inferior performances to ruthenium polypyridyl complexes as photosensitizers. Main reason for such inferiority would be attributed to the limited light absorption, poor matching to solar light distribution [13,14]. A major challenge in designing molecular based solar cells lies in the choice and organization of the light absorbing pigments. Efficient solar energy conversion requires light absorption over a broad spectral range in the visible and near infrared regions. Chlorophyll photosynthetic pigments are promising light absorbers for solar cells, owing to their high molar absorption coefficients, tuneable photophysical properties and light harvesting properties [15]. In this study, four different chlorophyll derivatives were synthesized according to the slightly modified conditions. Obtained products were characterized by 1H NMR and mass spectrometry. Their optical properties and cyclic voltammograms were also investigated. Zinc complexes of these tetrapyrrolic species were selected to perform the studies in dye sensitized solar cell. Zinc chlorophyll dyes with different substituents show different performances. Under the optimized conditions, ZnChl-2 sensitizer gave the best results with the overall conversion efficiency of 0.81% (g), short-circuit photocurrent of 3.5 mA/cm2 (Jsc), open circuit photovoltage of 412 mV (Voc), fill factor of 0.56 (FF). It is found that ZnChl-2 gave high efficiency within these classes of dye sensitizers.
31
H3C
OH
N
OH
CH3
H3C
N Zn CH3
H
O
H3C
N
H H3C
N
N CH3
H O
H3CO
CH3
O
ZnChl-2 OH
H3C
N
N
N
N N
N CH3
H
O
CH3
Zn H H3C
CH3
H
O H3CO
CH3
CH3
Zn H H3 C
N
1
ZnChl-1 OH
CH3
Zn N
O13 H3CO
N
Br N
N H H3 C
CH3
CH3
ZnChl-3
O H7C3O
O
ZnChl-4
Fig. 1. Molecular structure of the complexes.
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Experimental section All materials were reagent grade and were used as received unless otherwise noted. Molecular structures of Zinc chlorophyll materials were supplied in Fig. 1. Materials characterization The UV–Vis absorption spectra of synthesized dyes were recorded in a 1 cm path length quartz cell by using Analytic JENA S 600. The infrared (IR) spectra were obtained by using PerkinElmer, FT-IR/MIR-FIR (ATR) spectrophotometer. 1H NMR spectra were measured on Varian 500 MHz spectrometer. The mass spectra were obtained via electrospray ionization (ESI-MS), and matrix-assisted laser desorption/ionization (MALDI). Cyclic voltammetry measurements of synthesized dye were taken by using CH-Instrument 660 B Model Potentiostat equipment. Dye sensitized solar cells were characterized by current–voltage (J V) measurement. All current–voltage (J V) were done under 100 mW/cm2 light intensity and AM 1.5 conditions. 450 W Xenon light source (Oriel) was used to give an irradiance of various intensities. J V data collection was made by using Keithley 2400 Source-Meter and LabView data acquisition software. Synthesis of zinc complexes Synthetic routes were presented in Fig. 2. And 1H NMR spectra were shown in Figs. 3 and 4 for all Zinc chlorophyll materials. 3-Devinyl-3-formyl-pyropheophorbide-a (2) Methyl pyropheophorbide-a (1) was converted to 2 according to the literature procedure [16] with 55% yield. Spectral properties correlate with reported data. 20-Bromo-3-devinyl-3-formylpyropheophorbide-a (3) 3-devinyl-3-formyl-pyropheophorbide-a (2) (850 mg, 1.55 mmol) was dissolved in 500 ml of CH2Cl2, pyridiniumhydrobromide perbromide (595 mg, 1.86 mmol, 1.2 eq.) was added and obtained mixture was stirred for 40 min at RT. Subsequently, reaction mixture was poured into 2% HCl(aq), extracted with CH2Cl2, dried over anhydrous Na2SO4 and concentrated in vacuo. Crude 3 was purified by means of dry column vacuum chromatography (DCVC, silica gel, THF:CH2Cl2 = 2:96) to afford 753 mg (77.2%) of product 3. 1H NMR (CDCl3): 11,49, 10,48, 9.46 (s, each 1H, CHO, 5-H, 10-H), 5.26 (2H, d, J1 = 10 Hz, 131-CH2), 5.21 (1H, m, OH),4.89 (2H, dd, J1 = 7.1 Hz, J2 = 1 Hz, 17-H), 4.81 (1H, m, 18-H), 4.26 (1H, dd, J1 = 9.2 Hz, J2 = 3.1 Hz, 3-CH2), 3.91 (3H, s, 172-CO2 CH3), 3.86 (2H, q, J = 7.5 Hz, 8-CH2), 3.63, 3.60, 3.23, (3H each, s, 2-CH3, 7-CH3, 12-CH3), 2.71–2.53 (2H, m, 17-CH2CH2), 2.50 (3H, bs, 18-CH3), 2.35–2.19 (2H, m, 17-CH2CH2) 1.25 (3H, m, 81-CH3), 1.30 (1H, s, NH), 1.93 (1H, s, NH). MS (ESI) found: m/z 628.1701 [M+]; calcd. for C33H33N4O4Br (628.1696). Rf = 0.61 (silica gel, THF: CH2Cl2 = 4:96). Methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (Chl-2) 3 (755 mg, 1.2 mmol) was dissolved in the dry CH2Cl2 and borane tert-butylamine complex (150 mg, 1.68 mmol, 1.4 eq.) was added. After been stirred overnight at 0 °C, resulted mixture was washed with 5% HCl, H2O, and brine. Combined organic layers were dried over Na2SO4 and concentrated on vacuo. Crude product was purified by means of dry column vacuum chromatography (DCVC, silica gel, THF:CH2Cl2 = 5:95) to afford 696.6 mg (92%) of clean Chl2. 1H NMR (CDCl3): 9.600, 9.30 (s, each 1H, 5-H, 10-H), 5.92 (2H, dd, J1 = 11 Hz, J2 = 4 Hz, 131-CH2), 5.61 (1H, bs, OH), 5.08–4.91 (2H, m,
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O NH N
O
OsO4 NaIO4
N
NH
Dioxane RT, 18 h
HN
H
H3C
O
N
N
HN
H H
O O
RT, 20 min
HN
N
NH Br
H
H H3C
Py.HBr.Br2 DCM
N
H O
O
O
H3C
2
O O
O
3
.
BH3 t-BuNH2 DCM(dry) 0 oC, 18 h
HO
HO NH N
H Ph (AllylPdCl)2
N
NH
P(t-Bu)3
HN
N
DABCO
H
H O
O
H3C
O Chl-1
Zn(OAc)2
O
Chl-2
DCM/MeOH reflux
HO
HO N
N
N
N
N Zn
Br
Zn
N
N
N
H
H
H
H O O
O O
Zn(OAc)2
DCM/MeOH reflux
H 3C
HN
H
H H3 C
N
Br
O ZnChl-1
H3C
O O
O ZnChl-2
Fig. 2. Synthetic routes for the complexes, ZnChl-1 and ZnChl-2.
3-CH2), 4.79 (1H, t, J = 7 Hz, 18-H), 4.06 (1H, dd, J1 = 9.3 Hz, J2 = 2.7 Hz, 17-H), 3.72–3.61 (8H, m, 8-CH2, 172-CO2CH3, 2-CH3,), 3.49, 3.28 (3H each, s, 7-CH3, 12-CH3), 2.66–2.55, 2.33–2.21 (2H each, m, 17-CH2CH2), 1.70 (3H, bs, 18-CH3), 1.51–1.43 (3H, m, 81CH3), 2.14 (s, 2H, NH). MS (ESI) found: m/z 630.1763 [M+]; calcd. for C33H35N4O4Br (631.1844). Rf = 0.48 (silica gel, MeOH/CH2Cl2, 2:98). Zinc methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbidea (ZnChl-2) Methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (Chl-2, 50 mg, 79.2 lmol) was dissolved in the mixture of MeOH (10 ml) and CH2Cl2 (20 ml). Subsequently, Zn(OAc)2 (anhydrous) (17.5, mg, 79.2 lmol, 1 eq.) was added to the resulted solution and mixture was refluxed for 3.5 h. After reaction completion, 4% NaHCO3(aq) was added and stirred for additional 15 min. Reaction mixture then was washed with water, dried over Na2SO4and evaporated. The crude product was recrystallized from CH2Cl2/MeOH mixture to afford corresponding zinc complex ZnChl-2 (46.6 mg, 85%) as a dark green solid. 1H NMR (THF-d8): 9.70, 9.69 (s, each 1H, 5-H, 10-H), 5.76(2H, dd, J1 = 10 Hz, J2 = 4.6 Hz, 131-CH2), 5.07(2H, bs, 3-CH2), 4.98 (1H, m, OH), 4.42 (1H, t, J = 6 Hz, 18-H), 4.14 (1H, dd, J1 = 8.5 Hz, J2 = 4 Hz, 17-H), 3.83 (2H, q, J = 7.5 Hz,
8-CH2), 3.62, 3.55, 3.52, 3.34 (3H, s, 172-CO2CH3, 2-CH3, 7-CH3, 12-CH3), 2.62–2.51, 2.25–2.15 (each 2H, m, 17-CH2CH2), 2.40 (3H, s, 18-CH3), 1.61 (3H, m, 81-CH3). MS (ESI) found: m/z 692.1 [M+]; calcd. for C33H33N4O4ZnBr (692.0977). Rf = 0.50 (silica gel, MeOH/ CH2Cl2, 5:95). Methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (Chl-1) Methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (Chl-2) (200 mg, 0.316 mmol) and (AllylPdCl2)2 were placed in the Schlenk tube. Subsequently CH3CNdry (20 ml), t-Bu3P (320 lL of 10% w/v solution in hexanes, 10 mol%), phenylacetylene (1.6 g, 15.8 mmol, 50 eq.) and DABCO (142 mg, 1.264 mmol, 4 eq.) were added in that order under positive pressure of argon. The resulting mixture was stirred overnight at RT. After reaction completion, it was evaporated with Celite and purified by means of DCVC (silica gel, hexane, then MeOH: CH2Cl2 = 1:99) to afford 37 mg (18%) of pure Chl-1. 1H NMR (CDCl3): 9.28, 9.02 (s, each 1H, 5-H, 10-H), 7.82 (2H, d, J = 7.6 Hz, Ph), 7.56 (2H, t, J = 7.5 Hz, Ph), 7.48 (1H, t, J1 = 7.5 Hz, Ph), 5.71 (2H, d, J = 6 Hz, 3-CH2), 4.81 (2H, d, J = 6 Hz, 131-CH2), 4.71 (1H, m, OH), 3.56–3.51 (2H, m, 18-H, 17-H), 3.38 (2H, bs, 8-CH2), 3.36, 3.14, 3.13, 3.12 (s, 3H each, 172-CO2CH3, 2CH3, 7-CH3, 12-CH3), 2.60–2.52, 2.26–2.20 (each 2H, m, 17-CH2
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679
Fig. 3. 1H NMR Spectrum of (ZnChl-1) and 1H NMR Spectrum of (ZnChl-2).
CH2), 1.86 (3H, s, 18-CH3), 1.26 (3H, s, 81-CH3), 1.42(bs,1H, NH), 1.94 (bs, 1H, NH). MS (ESI) found: m/z 652.3079 [M]+; calcd. for C41H41N4O4 (652.3050). Rf = 0.48 (silica gel, MeOH/CH2Cl2, 2:98). Zinc methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (ZnChl-1) Methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (Chl-1, 30.0 mg, 46.1 lmol) was dissolved in the mixture of MeOH (6 ml) and CH2Cl2 (12 ml). Subsequently, Zn(OAc)2 (anhydrous) (8.5 mg, 46.1 lmol) was added to the resulted solution and mixture was refluxed for 3.5 h. After reaction completion, 4% NaHCO3(aq) was added and stirred for additional 15 min. Reaction mixture then was washed with water, dried over Na2SO4and evaporated. The crude product was recrystallized from CH2Cl2/ MeOH to afford corresponding zinc complex (27.1 mg, 82%) as a dark green solid. 1H NMR (THF-d8): 9.55, 9.53 (s, each 1H, 5-H,
10-H), 7.84 (2H, d, J = 7.8 Hz, Ph), 7.51 (2H, t, J = 7.5 Hz, Ph), 7.40 (1H, dt, J1 = 7.4 Hz, J2 = 1 Hz, Ph), 5.71 (2H, d, J = 6 Hz, 3-CH2), 5.07 (2H, d, J = 6 Hz, 131-CH2), 4.97 (1H, m, OH), 4.42 (1H, t, J = 6 Hz, 18-H), 4.20 (1H, m, 17-H), 3.77 (2H, m, 8-CH2), 3.72, 3.65, 3.52, 3.27 (3H, s, 172-CO2CH3, 2-CH3, 7-CH3, 12-CH3), 2.60– 2.50, 2.26–2.15 (each 2H, m, 17-CH2CH2), 1.86 (3H, s, 18-CH3), 1.61 (3H, m, 81-CH3). MS (ESI) found: m/z 715.23 [M + H]+; calcd. for C41H39N4O4Zn (715.2250). Rf = 0.54 (silica gel, MeOH/CH2Cl2, 5:95). Zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (ZnChl-3) ZnChl-3 was synthesized according to previously reported method [17].Spectral properties correlates with the reported data. 1 H NMR (DMSO-d6): 9.59, 9.35, 8.47 (each 1H, s, 5, 10, 20-H), 5.61 (2H, d, J = 5 Hz, 3-CH2), 5.55 (1H, bs, 31-OH), 5.11, 5.03 (each 1H, d, J = 20 Hz, 131-CH2), 4.51 (1H, m, 18-H), 4.25 (1H, m, 17-H), 3.77
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S. Erten-Ela et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
Fig. 4. 1H NMR Spectrum of (ZnChl-3) and 1H NMR Spectrum of (ZnChl-4).
(2H, m, 8-CH2), 3.58 (3H, s, 172-COOCH3) 3.54 (3H, s, 12-CH3), 3.35 (3H, s, 2-CH3), 3.25 (3H, s, 7-CH3), 2.65–2.56, 2.32–2.15 (each 2H, m, 17-CH2CH2), 1.74 (3H, d, J = 7 Hz, 18-CH3), 1.66 (3H, t, J = 7.5 Hz, 81-CH3). MS (ESI) found: m/z 616.04. Calcd. for C33H34N4O4Zn:[M+] = 616.3.
1.75 (3H, d, J = 6.5 Hz, 18-CH3), 1.67 (3H, t, J = 7.5 Hz, 81-CH3), 1.52–1.41 (8H, m, 172-COOCCH2), 0.79 (3H, t, J = 7,5 Hz, 172COOC2CH3). MS (ESI) found: m/z 644.09. Calcd. for C35H38N4O4Zn: [M+] = 644.3.
Zinc propyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (ZnChl-4) ZnChl-4 was synthesized according to previously reported method [17]. Spectral properties correlate with the reported data. 1 H NMR (DMSO): 9.59, 9.35, 8.47 (each 1H, s, 5, 10, 20-H), 5.62 (2H, d, J = 5,5 Hz, 3-CH2), 5.54 (1H, t, J = 5.5 Hz, 31-OH), 5.11, 5.03 (each 1H, d, J = 20 Hz, 131-CH2), 4.55–4.80 (1H, m, 18-H), 4.26 (1H, m, 17-H), 3.95–3.87 (2H, m, 172-COOCH2), 3.77 (2H, q, J = 7 Hz, 8-CH2), 3.59 (3H, s, 12-CH3), 3.26 (3H, s, 2-CH3), 3.24 (3H, s, 7-CH3), 2.64–2.54, 2.28–2.17 (each 2H, m, 17-CH2CH2),
Electrochemical studies The cyclic voltammograms were collected using a CH-Instrument 660 B Model electrochemical analyzer. The redox potentials of the complexes were measured using a three-electrode apparatus comprising a platinum wire counter electrode, working electrode, and an Ag/AgCl reference electrode. DMF was used as a solvent and the supporting electrolyte is tetrabutylammonium hexafluorophosphate (TBAPF6), 0.1 M. Ferrocene was added to each sample
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Fig. 6. Cyclic voltammogram of zinc chlorophyll derivatives and J–V curves of dye sensitized solar cells.
Fig. 5. UV–Vis absorption and steady-state fluorescence spectra of the complexes measured in THF. kex: 450 nm.
pre-drilled holes in the counter electrode. The filling holes were sealed using Surlyn and a microscope cover glass. Current–voltage (J–V) curves are obtained using Keithley measurement unit and the light source consisted of an Oriel Xe-lamp.
solution at the end of the experiments and ferrocenium/ferrocene redox couple was used as an internal potential reference. Results and discussion Absorption and emission properties Photovoltaic characterization Solar cell fabrication and characterization The counter electrode consisted platinum (Platisol, Solaronix) coated FTO glass (TEC 8; Hartford Glass). TiO2 coated FTO substrates were immersed in a 0.5 mM solution of the chlorophyll complexes in CH3CN: t-BuOH: MeOH (1:1:1) at room temperature overnight, and dried under a flow of nitrogen. The active solar cell area was 0.16 cm2. The cell was sealed using a Surlyn (60 lm, Solaronix) and the 0.6 M N-methyl-N-butyl-imidazolium iodide (BMII) + 0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile as redox electrolyte solution was introduced through
The UV/Vis absorption and emission spectra of the complexes measured in THF were shown in Fig. 5, and the energy maxima and absorption coefficients were summarized in Table 1. The UV/ Vis absorption spectra in Fig. 5 clearly showed that all complexes have characteristic sharp absorption peaks, which belong to the monomer form of zinc chlorophyll derivatives in the 350–680 nm region. For ZnChl-2, ZnChl-3, ZnChl-4, Soret and Qy bands were observed at 424 and 646 nm, respectively. For ZnChl-1, Soret and Qy bands were observed at 434 nm and 664 nm, respectively because of substituent group. The high-energy and lower energy bands of the absorption spectra of the complexes was dominated
Table 1 UV–Vis absorption and emission properties of the complexes measured in THF. kex: 450 nm. Dye
Absorption kmax (nm)(e/104 M cm
ZnChl-1 ZnChl-2 ZnChl-3 ZnChl-4
664(10.5) 646(13) 646(12.4) 646(12.2)
628(2.7) 602(1.9) 601(1.9) 601(1.9)
1
)
Emission kmax (nm) 578(1.6) 566(1.1) 565(1.2) 565(1.1)
538(0.6) 520(0.8) 520(0.8) 520(0.8)
434(17.3) 424(18) 424(16.7) 424(16.1)
408(9.0) 404(10.4) 404(9.6) 403(9.2)
671 656 654 654
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Table 2 Redox potentials and EHOMO and ELUMO levels of zinc chlorophyll derivatives. Eoxidationa (V) ZnChl-1 ZnChl-2 ZnChl-3 ZnChl-4 a b c d e
Ereductionb (V)
0.63 0.60 0.56 0.53
0.74 0.85 0.61 0.71
EHOMOc(eV)
ELUMOd(eV)
EBand
5.00 4.97 4.93 4.90
3.63 3.52 3.76 3.66
1.37 1.45 1.17 1.24
Gap
e
First oxidation potentials of ZnChl materials. Reduction potentials of ZnChl materials. HOMO energy level of ZnChl materials. LUMO energy level of ZnChl materials. Energy band gap of ZnChl materials.
gave different efficiencies in dye sensitized solar cells. Bromosubstituted complex showed best efficiency in these classes of dyes. It has been reported that DSSCs based on chlorophyll derivatives especially free base compounds bearing carboxyl anchoring group exhibit a promising results [18,19]. However, it should be take into the account that free base and magnesium chlorophyll derivatives are less stable than zinc analogs. Almost all of these complexes which show high conversion efficiencies have a carboxyl anchoring group and a vinylene space between the main structure and carboxy group [18,19]. By using these groups, it can be increased the efficient electron transfer from the chromophore to TiO2. Conclusion
Table 3 DSSC performances of chlorin dyes.a
by intraligand p–p transitions and MLCT transitions, respectively. The emission spectra of the chlorophylls were obtained in an airequilibrated THF solution at room temperature (Fig. 5). When exciting at 450 nm, all four derivatives exhibited an intense maximum at 656 or 671 for different chlorophyll dyes nm for all samples. Results are summarized in Table 1.
Zinc Chlorophyll sensitizers were synthesized and characterized by 1H NMR, UV–Vis, cyclic voltammetry. The performances of zinc chlorin dyes have been tested in dye sensitized solar cells. The performances of the photovoltaic devices depend significantly on the substituent groups. Among the dyes examined, DSSCs based on ZnChl-2 exhibited the best overall light to electricity conversion efficiency of 0.81% under AM 1.5 irradiation (100 mW cm 2). Molecular design based on these semi-synthetic chlorophyll derivatives can lead to further development of better sensitizers [18]. One of the strategies is to use a carboxyl anchoring group instead of OH and a vinylene space between the main structure and carboxy group [18]. In this way, it can be increased the efficient electron transfer from the chromophore to TiO2. The low photocurrent efficiencies obtained from the complexes reported in this study can be attributed to lack of these groups.
Electrochemical properties of dye
Acknowledgements
Cyclic voltammetry was employed to determine the redox potentials of chlorophyll sensitizers in Fig. 6. The electrochemical properties of the complexes have been studied in CH2Cl2. All redox potentials were calibrated vs SCE. All chlorophyll sensitizers showed reversible oxidations and reversible reduction peaks. The first oxidation potentials vs Fc/Fc + were listed and all results were summarized in Table 2. Their first reduction potentials corresponding to LUMO energy of the chlorophyll dyes were all higher than that of the conduction band of the TiO2 semiconductor. Also their first oxidation potentials corresponding to HOMO energy of the chlorophyll dyes were all lower than that of I /I3 redox couple. These results also ensured the generation of current in the presence of light.
We acknowledge financial support from The Scientific and Technological Research Council of Turkey (TUBITAK) Grants: 112M809 and 110M803 and Polish National Science Center (844/ N-ESF-EuroSolarFuels/10/2011/0) in the framework of European Science Foundation (ESF-EUROCORES-EuroSolarFuels-10-FP-006), and especially to Alexander von Humboldt Foundation (AvH).
Dye
Voc (mV)
Jsc (mA cm
ZnChl-1 ZnChl-2 ZnChl-3 ZnChl-4
416 412 395 418
3.06 3.50 3.75 2.53
2
)
ff
g (%)
0.61 0.56 0.51 0.64
0.78 0.81 0.76 0.68
a Voc is the open-circuit potential, Jsc, short circuit current, ff is the fill factor, and g is the overall efficiency of the cell under standard condition.
Photovoltaic performance of dye sensitized solar cells Under the solar irradiation, chlorophyll sensitizers were electronically excited, and as a result of this, electrons being injected into the conduction band of TiO2 semiconductor. The oxidized chlorophyll sensitizer was reduced back to the ground state by electron donor redox couple in the electrolyte. Following the general mechanism for dye sensitized solar cells, collected electrons in the conduction band pass through the external circuit to arrive at the counter electrode. Photovoltaic data were summarized in Fig. 6 and Table 3. ZnChl-2 sensitizer gave the highest efficiency of 0.81% with a short circuit photocurrent density of 3.5 mA/cm2 an open circuit voltage of 412 mV and a fill factor of 0.56 under the standard illumination test conditions. Photovoltaic performances were determined in the order of ZnChl-2 > ZnChl-1 > ZnChl-3 > ZnChl-4 from photovoltaic characterizations. Zinc chlorophyll dyes with different substituted groups
References [1] B. O’Regan, M. Gratzel, Nature 353 (1991) 737. [2] M. Graetzel, Nature 414 (2001) 338. [3] P.A. Bouit, M. Marszalek, R. Humphry-Baker, R. Viruela, E. Ortí, S.M. Zakeeruddin, M. Graetzel, J.L. Delgado, N. Martín, Chem. Eur. J. 18 (2012) 11621. [4] K. Ocakoglu, E. Harputlu, P. Guloglu, S. Erten-Ela, Synth. Met. 162 (2012) 2125. [5] S. Erten-Ela, S.G. Colak, K. Ocakoglu, Inorg. Chim. Acta 405 (2013) 252. [6] T. Miyatake, H. Tamiaki, A.R. Holzwarth, K. Schaffner, Photochem. Photobiol. 69 (1999) 448. [7] K. Ocakoglu, K.S. Joya, E. Harputlu, A. Tarnowska, D.T. Gryko, Nanoscale (2014), http://dx.doi.org/10.1039/C4NR01661K. [8] A. Pandit, K. Ocakoglu, F. Buda, T. van Marle, A.R. Holzwarth, H.J.M. de Groot, J. Phys. Chem. B 117 (2013) 11292. [9] A. Kay, M. Graetzel, J. Phys. Chem. 97 (1993) 6272. [10] Y. Tachibana, S.A. Haque, I.P. Mercer, J.R. Durrant, D.R. Klug, J. Phys. Chem. B 104 (2000) 1198. [11] T.L. Ma, K. Inoue, H. Noma, K. Yao, E. Abe, J. Photochem. Photobiol., A 152 (2002) 207. [12] F. Odobel, E. Blart, M. Lagree, M. Villieras, H. Boujtita, N.E. Murr, S. Caramori, C.A. Bignozzi, J. Mater. Chem. 13 (2003) 502. [13] S. Cherian, C.C. Wamser, J. Phys. Chem. B 104 (2000) 3624. [14] J. Jasieniak, M. Johnston, E.R. Waclawik, J. Phys. Chem. B 108 (2004) 12962. [15] G.M. Hasselman, D.F. Watson, J.R. Stromberg, D.F. Bocian, D. Holten, J.S. Lindsey, G.J. Meyer, J. Phys. Chem. B 110 (2006) 25430. [16] D.G. Johnson, W.A. Svec, M.R. Wasielewski, Israel J. Chem. 28 (1988) 193. [17] S. Shoji, T. Hashishin, H. Tamiaki, Chem. Eur. J. 18 (2012) 13331. [18] X.F. Wanga, H. Tamiaki, Energy Environ. Sci. 3 (2010) 94–106. [19] X.F. Wang, H. Tamiaki, O. Kitao, T. Ikeuchi, S.I. Sasaki, J. Power Sources 242 (2013) 860–864.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of zinc chlorophyll materials for dye-sensitized solar cell applications Sule Erten-Ela a,⇑, Olena Vakuliuk b, Anna Tarnowska c, Kasim Ocakoglu d,e,⇑, Daniel T. Gryko b,c,⇑ a
Solar Energy Institute, Ege University, Bornova 35100, Izmir, Turkey Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland c Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland d Advanced Technology Research & Application Center, Mersin University, Ciftlikkoy Campus, TR-33343 Yenisehir, Mersin, Turkey e Department of Energy Systems Engineering, Mersin University, Tarsus Faculty of Technology,33480 Mersin, Turkey b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Zinc chlorophyll materials have been
Chlorophyll dyes for dye sensitized solar cell.
synthesized. Zinc chlorophyll materials have been characterized. Dye sensitized solar cells were fabricated and characterized.
OH
OH CH 3
OH CH3
CH 3
H 3C
H3C N
N Zn
N
O
a r t i c l e
i n f o
Article history: Received 23 March 2014 Received in revised form 3 July 2014 Accepted 15 July 2014 Available online 1 August 2014 Keywords: Zinc chlorophyll Dye sensitized solar cell Electrolyte Organometallic synthesis
CH 3
N
O
N CH3
H3C
O
O
O C3H7
N
N
CH 3
O O
CH 3
CH3
Zn
H 3C
O
O
N
Zn N
CH3
H3C
CH3
H3C
N
Br N
N CH 3
O
N
N
Zn N
OH 31
CH 3
H 3C
N
H 3C
CH 3
CH3
O
131
O CH3
a b s t r a c t To design sensitizers for dye sensitized solar cells (DSSCs), a series of zinc chlorins with different substituents were synthesized. Novel zinc methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (ZnChl-1), zinc methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (ZnChl-2), zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (ZnChl-3), zinc propyl 3-devinyl-3hydroxymethyl-pyropheophorbide-a (ZnChl-4) were synthesized and their photovoltaic performances were evaluated in dye-sensitized solar cells. Photoelectrodes with a 7 lm thick nanoporous layer and a 5 lm thick light-scattering layer were used to fabricate dye sensitized solar cells. The best efficiency was obtained with ZnChl-2 sensitizer. ZnChl-2 gave a Jsc of 3.5 mA/cm2, Voc of 412 mV, FF of 0.56 and an overall conversion efficiency of 0.81 at full sun (1000 W m 2). Ó 2014 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors. Tel.: +90 2323111231; fax: +90 2323886027 (S. ErtenEla). Address: Advanced Technology Research & Application Center, Mersin University, Ciftlikkoy Campus, TR-33343 Yenisehir, Mersin, Turkey. Tel.: +90 3243610001/4961; fax: +90 3243610153 (K. Ocakoglu). Address: Institute of Organic Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland (D.T. Gryko). E-mail addresses: [email protected], [email protected] (S. Erten-Ela), [email protected], [email protected] (K. Ocakoglu), dtgryko @icho.edu.pl (D.T. Gryko). http://dx.doi.org/10.1016/j.saa.2014.07.026 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Over the past two decades dye sensitized solar cells have emerged as an important technology for the development of lowcost, easy processable, clean energy production [1–5]. A typical DSSC is composed of a dye molecule that is anchored to a mesoporous TiO2 surface. Efficient charge separation in dye sensitized solar cells is achieved by photoinduced electron injection from organic sensitizer into the conduction band of a metal oxide electrode.
S. Erten-Ela et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
The dye cation radical is regenerated by a redox electrolyte. Under simulated AM1.5G illumination (100 mW cm 2), the power conversion efficiency of a cell, g is defined as the product of the generated photocurrent density (Jsc), the open-circuit photovoltage (Voc), and the fill factor (FF), as follows: g = JscVocFF [4,6]. The efficient light harvesting potential of chlorophyll dyes, exemplified by their primary role in photosynthesis, makes them promising candidates for light harvesting antennas in dye sensitized solar cells [7,8]. Porphyrin based dyes have been evaluated as photosensitizers for DSSC due to their strong Soret (400– 450 nm) and moderate Q bands (550–600 nm) absorption properties as well as their primary role in photosynthesis mechanism [9–11]. They have demonstrated charge transfer kinetics indistinguishable from those of ruthenium polypyridyl complexes [12], which are the most efficient dyes ever reported. Moreover, optical, photophysical and electrochemical properties can be systematically evaluated. However, they generally show inferior performances to ruthenium polypyridyl complexes as photosensitizers. Main reason for such inferiority would be attributed to the limited light absorption, poor matching to solar light distribution [13,14]. A major challenge in designing molecular based solar cells lies in the choice and organization of the light absorbing pigments. Efficient solar energy conversion requires light absorption over a broad spectral range in the visible and near infrared regions. Chlorophyll photosynthetic pigments are promising light absorbers for solar cells, owing to their high molar absorption coefficients, tuneable photophysical properties and light harvesting properties [15]. In this study, four different chlorophyll derivatives were synthesized according to the slightly modified conditions. Obtained products were characterized by 1H NMR and mass spectrometry. Their optical properties and cyclic voltammograms were also investigated. Zinc complexes of these tetrapyrrolic species were selected to perform the studies in dye sensitized solar cell. Zinc chlorophyll dyes with different substituents show different performances. Under the optimized conditions, ZnChl-2 sensitizer gave the best results with the overall conversion efficiency of 0.81% (g), short-circuit photocurrent of 3.5 mA/cm2 (Jsc), open circuit photovoltage of 412 mV (Voc), fill factor of 0.56 (FF). It is found that ZnChl-2 gave high efficiency within these classes of dye sensitizers.
31
H3C
OH
N
OH
CH3
H3C
N Zn CH3
H
O
H3C
N
H H3C
N
N CH3
H O
H3CO
CH3
O
ZnChl-2 OH
H3C
N
N
N
N N
N CH3
H
O
CH3
Zn H H3C
CH3
H
O H3CO
CH3
CH3
Zn H H3 C
N
1
ZnChl-1 OH
CH3
Zn N
O13 H3CO
N
Br N
N H H3 C
CH3
CH3
ZnChl-3
O H7C3O
O
ZnChl-4
Fig. 1. Molecular structure of the complexes.
677
Experimental section All materials were reagent grade and were used as received unless otherwise noted. Molecular structures of Zinc chlorophyll materials were supplied in Fig. 1. Materials characterization The UV–Vis absorption spectra of synthesized dyes were recorded in a 1 cm path length quartz cell by using Analytic JENA S 600. The infrared (IR) spectra were obtained by using PerkinElmer, FT-IR/MIR-FIR (ATR) spectrophotometer. 1H NMR spectra were measured on Varian 500 MHz spectrometer. The mass spectra were obtained via electrospray ionization (ESI-MS), and matrix-assisted laser desorption/ionization (MALDI). Cyclic voltammetry measurements of synthesized dye were taken by using CH-Instrument 660 B Model Potentiostat equipment. Dye sensitized solar cells were characterized by current–voltage (J V) measurement. All current–voltage (J V) were done under 100 mW/cm2 light intensity and AM 1.5 conditions. 450 W Xenon light source (Oriel) was used to give an irradiance of various intensities. J V data collection was made by using Keithley 2400 Source-Meter and LabView data acquisition software. Synthesis of zinc complexes Synthetic routes were presented in Fig. 2. And 1H NMR spectra were shown in Figs. 3 and 4 for all Zinc chlorophyll materials. 3-Devinyl-3-formyl-pyropheophorbide-a (2) Methyl pyropheophorbide-a (1) was converted to 2 according to the literature procedure [16] with 55% yield. Spectral properties correlate with reported data. 20-Bromo-3-devinyl-3-formylpyropheophorbide-a (3) 3-devinyl-3-formyl-pyropheophorbide-a (2) (850 mg, 1.55 mmol) was dissolved in 500 ml of CH2Cl2, pyridiniumhydrobromide perbromide (595 mg, 1.86 mmol, 1.2 eq.) was added and obtained mixture was stirred for 40 min at RT. Subsequently, reaction mixture was poured into 2% HCl(aq), extracted with CH2Cl2, dried over anhydrous Na2SO4 and concentrated in vacuo. Crude 3 was purified by means of dry column vacuum chromatography (DCVC, silica gel, THF:CH2Cl2 = 2:96) to afford 753 mg (77.2%) of product 3. 1H NMR (CDCl3): 11,49, 10,48, 9.46 (s, each 1H, CHO, 5-H, 10-H), 5.26 (2H, d, J1 = 10 Hz, 131-CH2), 5.21 (1H, m, OH),4.89 (2H, dd, J1 = 7.1 Hz, J2 = 1 Hz, 17-H), 4.81 (1H, m, 18-H), 4.26 (1H, dd, J1 = 9.2 Hz, J2 = 3.1 Hz, 3-CH2), 3.91 (3H, s, 172-CO2 CH3), 3.86 (2H, q, J = 7.5 Hz, 8-CH2), 3.63, 3.60, 3.23, (3H each, s, 2-CH3, 7-CH3, 12-CH3), 2.71–2.53 (2H, m, 17-CH2CH2), 2.50 (3H, bs, 18-CH3), 2.35–2.19 (2H, m, 17-CH2CH2) 1.25 (3H, m, 81-CH3), 1.30 (1H, s, NH), 1.93 (1H, s, NH). MS (ESI) found: m/z 628.1701 [M+]; calcd. for C33H33N4O4Br (628.1696). Rf = 0.61 (silica gel, THF: CH2Cl2 = 4:96). Methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (Chl-2) 3 (755 mg, 1.2 mmol) was dissolved in the dry CH2Cl2 and borane tert-butylamine complex (150 mg, 1.68 mmol, 1.4 eq.) was added. After been stirred overnight at 0 °C, resulted mixture was washed with 5% HCl, H2O, and brine. Combined organic layers were dried over Na2SO4 and concentrated on vacuo. Crude product was purified by means of dry column vacuum chromatography (DCVC, silica gel, THF:CH2Cl2 = 5:95) to afford 696.6 mg (92%) of clean Chl2. 1H NMR (CDCl3): 9.600, 9.30 (s, each 1H, 5-H, 10-H), 5.92 (2H, dd, J1 = 11 Hz, J2 = 4 Hz, 131-CH2), 5.61 (1H, bs, OH), 5.08–4.91 (2H, m,
678
S. Erten-Ela et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
O NH N
O
OsO4 NaIO4
N
NH
Dioxane RT, 18 h
HN
H
H3C
O
N
N
HN
H H
O O
RT, 20 min
HN
N
NH Br
H
H H3C
Py.HBr.Br2 DCM
N
H O
O
O
H3C
2
O O
O
3
.
BH3 t-BuNH2 DCM(dry) 0 oC, 18 h
HO
HO NH N
H Ph (AllylPdCl)2
N
NH
P(t-Bu)3
HN
N
DABCO
H
H O
O
H3C
O Chl-1
Zn(OAc)2
O
Chl-2
DCM/MeOH reflux
HO
HO N
N
N
N
N Zn
Br
Zn
N
N
N
H
H
H
H O O
O O
Zn(OAc)2
DCM/MeOH reflux
H 3C
HN
H
H H3 C
N
Br
O ZnChl-1
H3C
O O
O ZnChl-2
Fig. 2. Synthetic routes for the complexes, ZnChl-1 and ZnChl-2.
3-CH2), 4.79 (1H, t, J = 7 Hz, 18-H), 4.06 (1H, dd, J1 = 9.3 Hz, J2 = 2.7 Hz, 17-H), 3.72–3.61 (8H, m, 8-CH2, 172-CO2CH3, 2-CH3,), 3.49, 3.28 (3H each, s, 7-CH3, 12-CH3), 2.66–2.55, 2.33–2.21 (2H each, m, 17-CH2CH2), 1.70 (3H, bs, 18-CH3), 1.51–1.43 (3H, m, 81CH3), 2.14 (s, 2H, NH). MS (ESI) found: m/z 630.1763 [M+]; calcd. for C33H35N4O4Br (631.1844). Rf = 0.48 (silica gel, MeOH/CH2Cl2, 2:98). Zinc methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbidea (ZnChl-2) Methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (Chl-2, 50 mg, 79.2 lmol) was dissolved in the mixture of MeOH (10 ml) and CH2Cl2 (20 ml). Subsequently, Zn(OAc)2 (anhydrous) (17.5, mg, 79.2 lmol, 1 eq.) was added to the resulted solution and mixture was refluxed for 3.5 h. After reaction completion, 4% NaHCO3(aq) was added and stirred for additional 15 min. Reaction mixture then was washed with water, dried over Na2SO4and evaporated. The crude product was recrystallized from CH2Cl2/MeOH mixture to afford corresponding zinc complex ZnChl-2 (46.6 mg, 85%) as a dark green solid. 1H NMR (THF-d8): 9.70, 9.69 (s, each 1H, 5-H, 10-H), 5.76(2H, dd, J1 = 10 Hz, J2 = 4.6 Hz, 131-CH2), 5.07(2H, bs, 3-CH2), 4.98 (1H, m, OH), 4.42 (1H, t, J = 6 Hz, 18-H), 4.14 (1H, dd, J1 = 8.5 Hz, J2 = 4 Hz, 17-H), 3.83 (2H, q, J = 7.5 Hz,
8-CH2), 3.62, 3.55, 3.52, 3.34 (3H, s, 172-CO2CH3, 2-CH3, 7-CH3, 12-CH3), 2.62–2.51, 2.25–2.15 (each 2H, m, 17-CH2CH2), 2.40 (3H, s, 18-CH3), 1.61 (3H, m, 81-CH3). MS (ESI) found: m/z 692.1 [M+]; calcd. for C33H33N4O4ZnBr (692.0977). Rf = 0.50 (silica gel, MeOH/ CH2Cl2, 5:95). Methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (Chl-1) Methyl 20-bromo-3-devinyl-3-hydroxymethylpyropheophorbide-a (Chl-2) (200 mg, 0.316 mmol) and (AllylPdCl2)2 were placed in the Schlenk tube. Subsequently CH3CNdry (20 ml), t-Bu3P (320 lL of 10% w/v solution in hexanes, 10 mol%), phenylacetylene (1.6 g, 15.8 mmol, 50 eq.) and DABCO (142 mg, 1.264 mmol, 4 eq.) were added in that order under positive pressure of argon. The resulting mixture was stirred overnight at RT. After reaction completion, it was evaporated with Celite and purified by means of DCVC (silica gel, hexane, then MeOH: CH2Cl2 = 1:99) to afford 37 mg (18%) of pure Chl-1. 1H NMR (CDCl3): 9.28, 9.02 (s, each 1H, 5-H, 10-H), 7.82 (2H, d, J = 7.6 Hz, Ph), 7.56 (2H, t, J = 7.5 Hz, Ph), 7.48 (1H, t, J1 = 7.5 Hz, Ph), 5.71 (2H, d, J = 6 Hz, 3-CH2), 4.81 (2H, d, J = 6 Hz, 131-CH2), 4.71 (1H, m, OH), 3.56–3.51 (2H, m, 18-H, 17-H), 3.38 (2H, bs, 8-CH2), 3.36, 3.14, 3.13, 3.12 (s, 3H each, 172-CO2CH3, 2CH3, 7-CH3, 12-CH3), 2.60–2.52, 2.26–2.20 (each 2H, m, 17-CH2
S. Erten-Ela et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
679
Fig. 3. 1H NMR Spectrum of (ZnChl-1) and 1H NMR Spectrum of (ZnChl-2).
CH2), 1.86 (3H, s, 18-CH3), 1.26 (3H, s, 81-CH3), 1.42(bs,1H, NH), 1.94 (bs, 1H, NH). MS (ESI) found: m/z 652.3079 [M]+; calcd. for C41H41N4O4 (652.3050). Rf = 0.48 (silica gel, MeOH/CH2Cl2, 2:98). Zinc methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (ZnChl-1) Methyl 3-devinyl-3-hydroxymethyl-20-phenylacetylenylpyropheophorbide-a (Chl-1, 30.0 mg, 46.1 lmol) was dissolved in the mixture of MeOH (6 ml) and CH2Cl2 (12 ml). Subsequently, Zn(OAc)2 (anhydrous) (8.5 mg, 46.1 lmol) was added to the resulted solution and mixture was refluxed for 3.5 h. After reaction completion, 4% NaHCO3(aq) was added and stirred for additional 15 min. Reaction mixture then was washed with water, dried over Na2SO4and evaporated. The crude product was recrystallized from CH2Cl2/ MeOH to afford corresponding zinc complex (27.1 mg, 82%) as a dark green solid. 1H NMR (THF-d8): 9.55, 9.53 (s, each 1H, 5-H,
10-H), 7.84 (2H, d, J = 7.8 Hz, Ph), 7.51 (2H, t, J = 7.5 Hz, Ph), 7.40 (1H, dt, J1 = 7.4 Hz, J2 = 1 Hz, Ph), 5.71 (2H, d, J = 6 Hz, 3-CH2), 5.07 (2H, d, J = 6 Hz, 131-CH2), 4.97 (1H, m, OH), 4.42 (1H, t, J = 6 Hz, 18-H), 4.20 (1H, m, 17-H), 3.77 (2H, m, 8-CH2), 3.72, 3.65, 3.52, 3.27 (3H, s, 172-CO2CH3, 2-CH3, 7-CH3, 12-CH3), 2.60– 2.50, 2.26–2.15 (each 2H, m, 17-CH2CH2), 1.86 (3H, s, 18-CH3), 1.61 (3H, m, 81-CH3). MS (ESI) found: m/z 715.23 [M + H]+; calcd. for C41H39N4O4Zn (715.2250). Rf = 0.54 (silica gel, MeOH/CH2Cl2, 5:95). Zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (ZnChl-3) ZnChl-3 was synthesized according to previously reported method [17].Spectral properties correlates with the reported data. 1 H NMR (DMSO-d6): 9.59, 9.35, 8.47 (each 1H, s, 5, 10, 20-H), 5.61 (2H, d, J = 5 Hz, 3-CH2), 5.55 (1H, bs, 31-OH), 5.11, 5.03 (each 1H, d, J = 20 Hz, 131-CH2), 4.51 (1H, m, 18-H), 4.25 (1H, m, 17-H), 3.77
680
S. Erten-Ela et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
Fig. 4. 1H NMR Spectrum of (ZnChl-3) and 1H NMR Spectrum of (ZnChl-4).
(2H, m, 8-CH2), 3.58 (3H, s, 172-COOCH3) 3.54 (3H, s, 12-CH3), 3.35 (3H, s, 2-CH3), 3.25 (3H, s, 7-CH3), 2.65–2.56, 2.32–2.15 (each 2H, m, 17-CH2CH2), 1.74 (3H, d, J = 7 Hz, 18-CH3), 1.66 (3H, t, J = 7.5 Hz, 81-CH3). MS (ESI) found: m/z 616.04. Calcd. for C33H34N4O4Zn:[M+] = 616.3.
1.75 (3H, d, J = 6.5 Hz, 18-CH3), 1.67 (3H, t, J = 7.5 Hz, 81-CH3), 1.52–1.41 (8H, m, 172-COOCCH2), 0.79 (3H, t, J = 7,5 Hz, 172COOC2CH3). MS (ESI) found: m/z 644.09. Calcd. for C35H38N4O4Zn: [M+] = 644.3.
Zinc propyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (ZnChl-4) ZnChl-4 was synthesized according to previously reported method [17]. Spectral properties correlate with the reported data. 1 H NMR (DMSO): 9.59, 9.35, 8.47 (each 1H, s, 5, 10, 20-H), 5.62 (2H, d, J = 5,5 Hz, 3-CH2), 5.54 (1H, t, J = 5.5 Hz, 31-OH), 5.11, 5.03 (each 1H, d, J = 20 Hz, 131-CH2), 4.55–4.80 (1H, m, 18-H), 4.26 (1H, m, 17-H), 3.95–3.87 (2H, m, 172-COOCH2), 3.77 (2H, q, J = 7 Hz, 8-CH2), 3.59 (3H, s, 12-CH3), 3.26 (3H, s, 2-CH3), 3.24 (3H, s, 7-CH3), 2.64–2.54, 2.28–2.17 (each 2H, m, 17-CH2CH2),
Electrochemical studies The cyclic voltammograms were collected using a CH-Instrument 660 B Model electrochemical analyzer. The redox potentials of the complexes were measured using a three-electrode apparatus comprising a platinum wire counter electrode, working electrode, and an Ag/AgCl reference electrode. DMF was used as a solvent and the supporting electrolyte is tetrabutylammonium hexafluorophosphate (TBAPF6), 0.1 M. Ferrocene was added to each sample
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Fig. 6. Cyclic voltammogram of zinc chlorophyll derivatives and J–V curves of dye sensitized solar cells.
Fig. 5. UV–Vis absorption and steady-state fluorescence spectra of the complexes measured in THF. kex: 450 nm.
pre-drilled holes in the counter electrode. The filling holes were sealed using Surlyn and a microscope cover glass. Current–voltage (J–V) curves are obtained using Keithley measurement unit and the light source consisted of an Oriel Xe-lamp.
solution at the end of the experiments and ferrocenium/ferrocene redox couple was used as an internal potential reference. Results and discussion Absorption and emission properties Photovoltaic characterization Solar cell fabrication and characterization The counter electrode consisted platinum (Platisol, Solaronix) coated FTO glass (TEC 8; Hartford Glass). TiO2 coated FTO substrates were immersed in a 0.5 mM solution of the chlorophyll complexes in CH3CN: t-BuOH: MeOH (1:1:1) at room temperature overnight, and dried under a flow of nitrogen. The active solar cell area was 0.16 cm2. The cell was sealed using a Surlyn (60 lm, Solaronix) and the 0.6 M N-methyl-N-butyl-imidazolium iodide (BMII) + 0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile as redox electrolyte solution was introduced through
The UV/Vis absorption and emission spectra of the complexes measured in THF were shown in Fig. 5, and the energy maxima and absorption coefficients were summarized in Table 1. The UV/ Vis absorption spectra in Fig. 5 clearly showed that all complexes have characteristic sharp absorption peaks, which belong to the monomer form of zinc chlorophyll derivatives in the 350–680 nm region. For ZnChl-2, ZnChl-3, ZnChl-4, Soret and Qy bands were observed at 424 and 646 nm, respectively. For ZnChl-1, Soret and Qy bands were observed at 434 nm and 664 nm, respectively because of substituent group. The high-energy and lower energy bands of the absorption spectra of the complexes was dominated
Table 1 UV–Vis absorption and emission properties of the complexes measured in THF. kex: 450 nm. Dye
Absorption kmax (nm)(e/104 M cm
ZnChl-1 ZnChl-2 ZnChl-3 ZnChl-4
664(10.5) 646(13) 646(12.4) 646(12.2)
628(2.7) 602(1.9) 601(1.9) 601(1.9)
1
)
Emission kmax (nm) 578(1.6) 566(1.1) 565(1.2) 565(1.1)
538(0.6) 520(0.8) 520(0.8) 520(0.8)
434(17.3) 424(18) 424(16.7) 424(16.1)
408(9.0) 404(10.4) 404(9.6) 403(9.2)
671 656 654 654
682
S. Erten-Ela et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 676–682
Table 2 Redox potentials and EHOMO and ELUMO levels of zinc chlorophyll derivatives. Eoxidationa (V) ZnChl-1 ZnChl-2 ZnChl-3 ZnChl-4 a b c d e
Ereductionb (V)
0.63 0.60 0.56 0.53
0.74 0.85 0.61 0.71
EHOMOc(eV)
ELUMOd(eV)
EBand
5.00 4.97 4.93 4.90
3.63 3.52 3.76 3.66
1.37 1.45 1.17 1.24
Gap
e
First oxidation potentials of ZnChl materials. Reduction potentials of ZnChl materials. HOMO energy level of ZnChl materials. LUMO energy level of ZnChl materials. Energy band gap of ZnChl materials.
gave different efficiencies in dye sensitized solar cells. Bromosubstituted complex showed best efficiency in these classes of dyes. It has been reported that DSSCs based on chlorophyll derivatives especially free base compounds bearing carboxyl anchoring group exhibit a promising results [18,19]. However, it should be take into the account that free base and magnesium chlorophyll derivatives are less stable than zinc analogs. Almost all of these complexes which show high conversion efficiencies have a carboxyl anchoring group and a vinylene space between the main structure and carboxy group [18,19]. By using these groups, it can be increased the efficient electron transfer from the chromophore to TiO2. Conclusion
Table 3 DSSC performances of chlorin dyes.a
by intraligand p–p transitions and MLCT transitions, respectively. The emission spectra of the chlorophylls were obtained in an airequilibrated THF solution at room temperature (Fig. 5). When exciting at 450 nm, all four derivatives exhibited an intense maximum at 656 or 671 for different chlorophyll dyes nm for all samples. Results are summarized in Table 1.
Zinc Chlorophyll sensitizers were synthesized and characterized by 1H NMR, UV–Vis, cyclic voltammetry. The performances of zinc chlorin dyes have been tested in dye sensitized solar cells. The performances of the photovoltaic devices depend significantly on the substituent groups. Among the dyes examined, DSSCs based on ZnChl-2 exhibited the best overall light to electricity conversion efficiency of 0.81% under AM 1.5 irradiation (100 mW cm 2). Molecular design based on these semi-synthetic chlorophyll derivatives can lead to further development of better sensitizers [18]. One of the strategies is to use a carboxyl anchoring group instead of OH and a vinylene space between the main structure and carboxy group [18]. In this way, it can be increased the efficient electron transfer from the chromophore to TiO2. The low photocurrent efficiencies obtained from the complexes reported in this study can be attributed to lack of these groups.
Electrochemical properties of dye
Acknowledgements
Cyclic voltammetry was employed to determine the redox potentials of chlorophyll sensitizers in Fig. 6. The electrochemical properties of the complexes have been studied in CH2Cl2. All redox potentials were calibrated vs SCE. All chlorophyll sensitizers showed reversible oxidations and reversible reduction peaks. The first oxidation potentials vs Fc/Fc + were listed and all results were summarized in Table 2. Their first reduction potentials corresponding to LUMO energy of the chlorophyll dyes were all higher than that of the conduction band of the TiO2 semiconductor. Also their first oxidation potentials corresponding to HOMO energy of the chlorophyll dyes were all lower than that of I /I3 redox couple. These results also ensured the generation of current in the presence of light.
We acknowledge financial support from The Scientific and Technological Research Council of Turkey (TUBITAK) Grants: 112M809 and 110M803 and Polish National Science Center (844/ N-ESF-EuroSolarFuels/10/2011/0) in the framework of European Science Foundation (ESF-EUROCORES-EuroSolarFuels-10-FP-006), and especially to Alexander von Humboldt Foundation (AvH).
Dye
Voc (mV)
Jsc (mA cm
ZnChl-1 ZnChl-2 ZnChl-3 ZnChl-4
416 412 395 418
3.06 3.50 3.75 2.53
2
)
ff
g (%)
0.61 0.56 0.51 0.64
0.78 0.81 0.76 0.68
a Voc is the open-circuit potential, Jsc, short circuit current, ff is the fill factor, and g is the overall efficiency of the cell under standard condition.
Photovoltaic performance of dye sensitized solar cells Under the solar irradiation, chlorophyll sensitizers were electronically excited, and as a result of this, electrons being injected into the conduction band of TiO2 semiconductor. The oxidized chlorophyll sensitizer was reduced back to the ground state by electron donor redox couple in the electrolyte. Following the general mechanism for dye sensitized solar cells, collected electrons in the conduction band pass through the external circuit to arrive at the counter electrode. Photovoltaic data were summarized in Fig. 6 and Table 3. ZnChl-2 sensitizer gave the highest efficiency of 0.81% with a short circuit photocurrent density of 3.5 mA/cm2 an open circuit voltage of 412 mV and a fill factor of 0.56 under the standard illumination test conditions. Photovoltaic performances were determined in the order of ZnChl-2 > ZnChl-1 > ZnChl-3 > ZnChl-4 from photovoltaic characterizations. Zinc chlorophyll dyes with different substituted groups
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