Low-Symmetrical Zinc(II) Benzonaphthoporphyrazine Sensitizers for

Apr 27, 2016 - Two ring-expanded naphthalocyanine-based sensitizers NcS1 and NcS2 have been designed and synthesized to harvest near-IR light energy i...
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Low-Symmetrical Zinc(II) Benzonaphthoporphyrazine Sensitizers for Light-Harvesting in Near-IR Region of Dye-Sensitized Solar Cells Takuro Ikeuchi, Shogo Mori, Nagao Kobayashi, and Mutsumi Kimura* Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan S Supporting Information *

ABSTRACT: Two ring-expanded naphthalocyanine-based sensitizers NcS1 and NcS2 have been designed and synthesized to harvest near-IR light energy in dye-sensitized solar cells. Lowsymmetrical “push-pull” structures of NcS1 and NcS2 enable the red-shift of absorption spectrum as well as the defined Q-band splitting. The zinc benzonaphthoporphyrazine sensitizer NcS1 possessing one carboxylic acid and six 2,6-diisopropylphenoxy units showed a PCE value of 3.2% when used as a lightharvesting dye on a TiO2 electrode under one sun condition. The NcS1 cell showed a broad photoresponse at wavelengths from 600 to 850 nm.

1. INTRODUCTION Phthalocyanines (Pcs) and their analogues have attracted attention as red/near-infrared (IR) light absorbing dyes, and their electronic modulation has opened the door to many applications in organic-based electronic and photonic devices.1 A two-dimensional conjugated aromatic chromophoric system of Pcs generated intense Q bands in the red/near-infrared light region with high molar absorptivity above 105 L mol−1 cm−1. Recently, Pcs and their metal complexes have been investigated as light-harvesting sensitizers of dye-sensitized solar cells (DSSCs), and the power conversion efficiencies (PCE) of Pc-sensitized DSSCs have significantly improved by systematic molecular engineering such as asymmetric electronic structural design, steric suppression of aggregation, and optimization of adsorption sites.2−4 While the Pc sensitizers displayed above 80% of incident photon-to-current efficiency (IPCE) from 600 to 720 nm, the IPCE value steeply decreased in the far red and near-IR positions. The Q-band position can shift into the near-IR light region by the extension of the π-conjugated system, the change of central metal, and the peripheral substitution. Since the first synthesis of naphthalocyanines (Ncs) by de Diesbach in 1927,5a Ncs have been used as near-IR absorbing dyes in recordable CD-R and photodynamic cancer treatment.5b,c The attachment of naphthalene units with a porphyrazine core leads to redshifted Q-band positions by ca. 100 nm to longer wavelengths than those of Pcs.6 Silicon Ncs-based sensitizers possessing axial substituents were synthesized to expand the light-harvesting area of DSSCs by several research groups.7 However, the reported PCE values of Nc-sensitized DSSCs were less than 1%.7,8 This is mainly due to the unfavorable energy levels of narrow energy-gap Ncs sensitizers for photoinduced electron transfer reactions in DSSCs. In this study, we design zinc © XXXX American Chemical Society

benzonaphthoporphyrazine-based sensitizers NcS1 and NcS2 according to the molecular design rule of efficient ZnPc-based sensitizers PcS reported by our group.4 The DSSC cell sensitized by NcS1 showed a PCE value of 3.2% under simulated air mass 1.5 global sunlight, and the IPCE of the NcS1 cell at 760 nm reached over 60%.

2. RESULTS AND DISCUSSION Two low-symmetrical benzonaphthoporphyrazines NcS1 and NcS2 possessing six bulky phenoxy substituents and one carboxylic acid were synthesized as the sensitizers of DSSCs. Bulky phenoxy substituents can prevent the aggregation of macrocyclic ligands through intermolecular π−π interaction and act as electron donating units to enable directional electron transfer in excited states.4 The carboxylic acid in NsS1 and NcS2 can form an ester linkage with the surface of TiO2 to make a pathway of electron injection from the excited dye to Received: March 7, 2016

A

DOI: 10.1021/acs.inorgchem.6b00562 Inorg. Chem. XXXX, XXX, XXX−XXX

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the normalized absorption and fluorescence spectra (Figure S1). Furthermore, the spectrum of NcS1 revealed a well-resolved split Q-band at 768 and 731 nm because of the lower symmetry of the aromatic macrocycles.10 While the split Q bands have also been observed in the low-symmetrical benzonaphthoporphyrazine complexes,11 the widths of the split Q bands for the reported complexes were smaller than that of NcS1. The “push-pull” structure of NcS1 possessing donor and acceptor substituents also contributes to increasing the splitting width of the Q bands. The Q bands of NcS2 were red-shifted as compared with those of NcS1, which is ascribed to the enhancing of the electron pushing abilities of peripheral units by the introduction of electron donating octyloxy substituents. The positions of the MCD peaks and troughs nearly correspond to the absorption spectra and are composed of superpositions of Faraday B-terms. This suggests the splitting of the lowest unoccupied molecular orbital (LUMO) energy levels of the low-symmetrical complexes.12 With a view to evaluate the highest occupied molecular orbital (HOMO) energy levels of NcS1 and NcS2, we have carried out the electrochemistry by using a differential pulse voltammetric (DPV) technique (Figure S2). The HOMO levels of NcS1 and NcS2 adsorbed onto the TiO2 nanoparticles were 0.70 and 0.64 V vs normal hydrogen electrode (NHE), which are more negative than that of PcS18.4c The HOMO level of NcS2 was lower than that of NcS1, also indicating the high electron donating ability of octyloxy units. The LUMO energy levels of NcS1 and NcS2 were −0.89 and −0.93 V vs NHE calculated from HOMO and E0−0 values. These LUMO levels are almost the same as that of PcS18,4c implying that the ring expansion resulted in a narrowing of Eg owing to the destabilization of the HOMO energy level. Density functional theory (DFT) calculations at the B3LYP/ 6-31G* level (Gaussian 09) were conducted for NcS1 and NcS2 to enhance our understanding of the spectroscopic and electrochemical data. The trend in the absorption spectra and relative oscillator strength of NcS1 and NcS2 calculated by time-dependence DFT were the same as the trends observed in the experimental absorption spectra (Figure 2 and Table 2). The split Q bands can be assigned to the HOMO → LUMO and HOMO → LUMO+1 transitions of the macrocycles. The lowest-lying electronic transition of NcS2 was red-shifted by 12 nm compared with that of NcS1 through the upper shift of the HOMO energy level. The HOMOs are localized over the porphyrazine ring and the fused benzene rings within the macrocyclic ligand (Figure 2). The LUMO and LUMO+1 are delocalized along the x and y axis of the macrocyclic ligand with sufficient electron densities on the carboxylic acid moiety, suggesting an electronically good connection between the π-conjugated macrocyclic ligand and TiO2 by linking of carboxylic acid in dyes with the TiO2 surface. Porous TiO2 films on quartz substrates were immersed into the toluene solution of NcS1 and NcS2 to obtain absorption spectra of dye-stained films. The Q-band of NcS1 adsorbed onto the TiO2 surface was slightly broadened compared with that in toluene (Figure 1a). According to the molecular model of NcS1, two isopropyl groups in the peripheral phenoxy units partially cover around the benzonaphthoporphyrazine ring. Thus, the uncovered portions of rings come close with each other within the monolayer adsorbed on the TiO2 surface, and the intermolecular interaction among rings results in the broadening of the Q-band.13 The absorption intensity of the Q-band peak at 731 nm decreased by the electronic interaction between NcS1 and the TiO2.4d In contrast, NcS2 adsorbed on

TiO2. 2,3-Dicyanonaphthalene precursors were prepared by the reaction of 2,3-dibromo-6,7-dicyanonaphthalene with phenols in dry DMF in the presence of K2CO3 and 18-crown-6-ether.9 Low-symmetrical NcS1 and NcS2 were synthesized by the statistical tetramerization of 2,3-dicyanonaphthalene precursors with methyl 3,4-dicyanobenzoate, and the target compounds were separated from the product mixture by the column chromatography. The ester group was hydrolyzed with an aqueous alkaline solution to afford NcS1 and NcS2. Figure 1a,b shows absorption and magnetic circular dichroism (MCD) spectra of NcS1 and NcS2 in toluene. The absorption

Figure 1. Absorption and MCD spectra of (a) NcS1 and (b) NcS2 in toluene (solid line). Dotted lines are absorption spectra of dye-stained TiO2 films of NcS1 and NcS2.

maximum of the Q-band for NcS1 in toluene red shifts by 79 nm (1493 cm−1) relative to that of previously reported ZnPcbased sensitizer PcS18 decorated with the same peripheral substituents (λmax = 689 nm),4c indicating the expansion of the π-conjugated system (Table 1). The optical energy gap (E0−0) of NcS1 was determined to be 1.59 eV from the cross point of Table 1. Photophysical and Electrochemical Data for NcS1 and NcS2 dyes NcS1 NcS2

λmax/nma f

768 (4.93 ) 780 (4.98f)

λem/nmb

E0−0/eVc

HOMO/Vd

LUMO/Ve

772 782

1.59 1.57

0.70 0.64

−0.89 −0.93

Absorption and fluorescence peaks were measured in toluene. Optical bandgap were calculated from the intersection of normalized absorption and fluorescence spectra. dHOMO energy levels were determined from first oxidation potential from DPV (vs NHE). e LUMO energy levels were calculated from LUMO = HOMO-E0−0 (vs NHE). flog ε. a,b c

B

DOI: 10.1021/acs.inorgchem.6b00562 Inorg. Chem. XXXX, XXX, XXX−XXX

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for NcS1 and NcS2, respectively. The adsorption density of NcS1 was lower than that of PcS18 (1.3 × 10−4 mol/cm3) due to the enlargement of ligand size.4c The steric crowding of alkyl chains around the ring in NcS2 decreased the dye adsorption density on the TiO2 surface. We fabricated DSSCs using double-layered TiO2 electrodes with electrolytes containing 0.6 M dimethylpropylimidazolium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M tertbutylpyridine (tBP) in acetonitrile. Figure 3a shows the photo-

Figure 3. (a) Photocurrent voltage curves obtained with DSSCs based on NcS1 (black line) and NcS1/CDCA (red line) under a standard global AM 1.5 solar condition (solid line) and dark current (dotted line). (b) Incident photon-to-current conversion efficiency spectrum for DSSC based on NcS1 (black line) and NcS1/CDCA (red line). Figure 2. (a) Simulated absorption spectra and oscillator strength of NcS1 (solid line) and NcS2 (dashed line) obtained by TD-DFT at the B3LYP/6-31G(d) level. (b) Molecular orbitals and energy diagrams of NcS1 and NcS2 obtained by TD-DFT at the B3LYP/6-31G(d) level.

current density−voltage curves of the DSSCs using the NcS1 under global AM 1.5 simulated solar conditions. The PCE of the NcS1 cell was 2.4% with a short current photocurrent density (Jsc) of 6.2 mA cm−2, an open-circuit voltage (Voc) of 535 mV, and a fill factor (FF) of 0.73 (Table 3). The NcS1 cell

Table 2. Selected Transition Energies and Wave Functions of NcS1 and NcS2 Based on TD-DFT Calculations dyes

λ/nm

f

NcS1

702 662 450 426

0.53 0.79 0.18 0.42

396

0.31

715 666 466 444

0.51 0.77 0.23 0.44

408

0.24

NcS2

Table 3. Photovoltaic Performance of DSSCs Based on NcS1 and NcS2a

wave functions HOMO→LUMO(98%) HOMO→LUMO+1(97%) HOMO−2→LUMO(82%) HOMO−5→LUMO(40%), LUMO(37%) HOMO−5→LUMO(53%), LUMO(20%) HOMO→LUMO(98%) HOMO→LUMO+1(97%) HOMO−2→LUMO(93%) HOMO−5→LUMO(46%), LUMO(20%) HOMO−5→LUMO(58%), LUMO(28%)

dyes

CDCA/mM

Voc/mV

Jsc/mA cm−1

FF

PCE/%

NcS1

0 2.5 0

535 545 512

6.2 8.2 1.7

0.73 0.71 0.73

2.4 3.2 0.6

HOMO−3→ NcS2 a

HOMO−6→

Performance data are top values of five DSSC cells.

displayed the IPCE response in the region from 600 to 850 nm, and the IPCE maximum was 41% at 760 nm (Figure 3b). The IPCE spectrum of the NcS1 cell was red-shifted by 50 nm compared with that of the PcS18 cell. We also fabricated the NcS1 cell with a mixing of coadsorbant 3α,7α-dihydroxy-5βcholic acid (CDCA) to diminish the dye interaction on the TiO2 surface.3a−c The TiO2 electrode was stained with NcS1 solution ([NcS1] = 0.05 mM) containing 2.5 mM CDCA. The absorption spectrum of NcS1/CDCA stained TiO2 film exhibited sharp Q-band peaks relative to that of NcS1 film (Figure S3). The dye adsorption density of NcS1 onto the

HOMO−3→ HOMO−6→

TiO2 kept sharp Q bands, indicating that alkyl chains can prevent the formation of the aggregation of rings on the TiO2 surface (Figure 1b). The dye adsorption density onto the porous TiO2 films were determined to be 9.4 × 10−5 and 2.4 × 10−5 mol/cm3 C

DOI: 10.1021/acs.inorgchem.6b00562 Inorg. Chem. XXXX, XXX, XXX−XXX

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(thickness: 4 μm) for a working electrodes were prepared by applying pastes of TiO2 nanoparticles having 15−20 nm diameter onto transparent conducting glass substrates (SnO2:F, on 1.8 mm thick glass substrate, Asahi Glass). The TiO2 electrodes were immersed into 0.05 mM toluene solutions of NcS1 or NcS2 for 3 h, and the dyestained electrodes were used as a working electrode. A reference electrode was Ag/AgCl, corrected for junction potentials by being referenced to the ferrocenium/ferrocene (Fc+/Fc) couple. DFT and TD-DFT calculations were performed using Becke’s hybrid exchange functional B317 with the Lee−Yang−Parr correlation functional LYP (B3LYP)18 and 6-31G*19 basis set as implemented in the Gaussian 09 software suit.20 All chemicals were purchased from commercial supplies used without further purification. Column chromatography was performed with activated alumina (Wako, 200mesh) or silica gel (Wakogel C-200). Recycling preparative gel permeation chromatography was carried out by a JAI recycling preparative HPLC using CHCl3 as an eluent. Analytical thin layer chromatography was performed with commercial Merck plates coated with silica gel 60 F254 or aluminum oxide 60 F254. 4.2. Synthesis of 2,3-Dicyanonaphthalene Precursors 1 and 2. 1: Naphthalonitrile 1 was synthesized from 2,6-diisopropylphenol

TiO2 was significantly decreased by the coadsorption with CDCA (1.2 × 10−5 mol/cm3). These suggest that the immobilization of NcS1 within the CDCA layer on the TiO2 surface can diminish the interaction among NcS1. The PCE value for the NcS1 cell was improved to 3.2%, and the IPCE maximum at 760 nm was enhanced to 61% by the coadsorption with CDCA. To the best of our knowledge, the IPCE maximum of the NcS1 cell is the highest value reported for Nc dye-based DSSCs.7,8 The NcS2 cell showed an inferior PCE value relative to the NcS1 cell due to an insufficient offset (0.24 V) between the I−/I3− redox potential (0.40 V vs NHE) and the HOMO energy level of NcS2 for efficient regeneration of oxidized dyes through electron donation from iodide as well as the poor dye adsorption density on the TiO2 (Table 3 and Figure S4).

3. CONCLUSIONS Two low-symmetrical zinc benzonaphthoporphyrazine dyes NcS1 and NcS2 were synthesized as sensitizers for the lightharvesting in the near-IR region of DSSCs. The ring expansion of the porphyrazine ligand resulted in the red-shift of the Q-band into the near-IR region. The low-symmetrical “pushpull’ structures in NcS1 and NcS2 produced the large splitting of Q bands through the energy difference of LUMO and LUMO+1 energy levels as confirmed by MCD and DFT calculation. Whereas NcS1 exhibited the moderate PCE around 3% under one sun condition, the light-harvesting area was expanded to 850 nm with the IPCE maxima over 60% at 760 nm. On the other hand, NcS2 showed low performance due to the small offset between the HOMO level of NcS2 and the I−/I3− redox potential. Although the ring expansion and the introduction of electron donating substituents are effective for the red-shifting of the Q band, the HOMO energy levels of ring-expanded dyes are usually destabilized. The destabilization of HOMO levels leads to the reduction of the thermodynamic driving force for the regeneration of the dye by the I−/I3− based redox electrolyte in DSSCs. Since the driving force for the photoinduced injection of electrons into TiO2 in NcS1 and NcS2 is still above 300 mV,14 the enhancement of PCE in the near-IR harvesting dyes requires the fine-tuning of energy levels while keeping narrow band gaps. Furthermore, the bulkiness of 2,6-diisopropylphenoxy units in NcS1 was not enough to prevent the molecular aggregation among large benzonaphthoporphyrazine ligands on the TiO2 surface. We are now continuing the molecular engineering of Nc-based sensitizers on the fine-tuning of energy levels and the structural optimization of peripheral units. The highly-efficient Nc-based sensitizers possessing the IPCE values above 80% up to 900 nm will enable more efficient utilization of whole solar light energy through co-sensitization DSSCs,3b,4b,e energy relay devices,15 and tandem DSSCs.16

(0.55 mL, 2.97 mmol) and 2,3-dibromo-6,7-dicyanonaphthalene (0.2 g, 0.6 mmol) (Toxic by inhalation) in the presence of K2CO3 (1.23 g, 8.93 mmol) and 18-crown 6-ether (10 mg, 0.04 mmol) according to the reported method.4c,9 Yield 0.13g (41%). 1H NMR(CDCl3, 400.13 MHz): δ/ppm =7.96 (2H, s, ArH), 7.33 (6H, m, ArH), 6.81 (2H, s, ArH), 3.09 (4H, m, −CH2-), 1.21 (24H, m, −CH3) ; 13C NMR (CDCl3, 100.61 MHz): δ/ppm =22.99, 24.65, 27.87, 108.91, 110.13, 116.51, 125.41, 127.35, 130.26, 133.96, 141.43, 148.26, 152.57. FT-IR (ATR): υ = 2230 (−CN) cm−1. ESI-TOF HRMS (APCI): m/z 531.3043 [M+H+], calcd. for C36H38N2O2: m/z 530.2933. 2: Naphthalonitrile 2 was synthesized from 2,6-bis(octyloxy)phenol (1.1 g, 3.13 mmol)4d and 2,3-dibromo-6,7-dicyanonaphthalene (0.21 g, 0.63 mmol) (Toxic by inhalation) according to the same method of 1. Yield: 16%. 1H NMR (CDCl3, 400.13 MHz): δ /ppm =7.95 (2H, s, ArH), 7.18 (2H, t, J = 8.0 Hz, ArH), 6.91 (2H, s, ArH), 6.69 (4H, d, J = 8.0 Hz, ArH), 3.97 (8H, t, J = 8.0 Hz, -OCH2-), 1.20 (48H, m, −CH2-), 0.81 (12H, m, −CH3); 13C NMR (CDCl3, 100.61 MHz): δ/ppm =14.43, 14.49, 23.02, 26.15, 26.42, 29.50, 29.58, 29.61, 29.78, 32.03, 32.14, 32.22, 69.59, 69.96, 107.38, 108.03, 109.96, 116.75, 126.53, 130.33, 132.57, 133.98, 152.38, 153.02. FT-IR (ATR): υ = 2230 (−CN) cm−1. ESI-TOF HRMS (APCI): m/z 875.5948 [M+H+], calcd. for C56H78N2O6: m/z 874.5860. 4.3. Synthesis of Sensitizers NcS1 and NcS2. NcS1: NcS1 was synthesized from 1 (120 mg, 0.23 mmol), methyl 3,4-dicyanobenzoate (10 mg, 53.8 μmol), and Zn(CH3COO)2 (15 mg, 80.7 μmol) according to the reported method.4c Yield: 20 mg (83%). UV−vis in toluene λmax/nm (log ε): 768 (4.93), 731 (4.90), 694 (4.38), 657 (4.26), 344 (4.54). 1 H NMR (CDCl 3 , 400.13 MHz): δ/ppm =10.2 (1H, br, COOH), 9.4−9.6 (6H, br, ArH), 7.2−7.6 (27H, m, ArH), 3.2 (12H, br, -CH-), 1.2 (72H, m, −CH3). IR (ATR): υ = 1681 (−COOH) cm−1. MALDI-TOF Ms (dithranol): m/z 1826.26 (M+H), Calcd for C117H118N8O8Zn: m/z 1826.84. NcS2 was synthesized from 2 and methyl 3,4-dicyanobenzoate. Yield: 8%. UV−vis in toluene λmax/nm (log ε/L mol−1 cm−1): 780 (4.98), 740 (4.71), 710 (4.36), 662 (4.15), 340 (4.66). 1H NMR (CDCl3, 400.13 MHz): δ/ppm =10.1 (1H, br, COOH), 9.4−9.6 (6H, br, ArH), 7.5−7.7 (6H, m, ArH), 7.3−7.4 (9H, m, ArH), 6.8 (12H, d, J = 6.8 Hz, ArH), 4.1 (24H, br, -OCH2-), 1.2−1.7 (144H, m, −CH2−),

4. EXPERIMENTAL SECTION 4.1. General. NMR spectra were recorded on a Bruker AVANCE 400 FT NMR spectrometer at 399.65 and 100.62 MHz for 1H and 13C in CDCl3 solution. Chemical shifts are reported relative to internal tetramethylsilane. Absorption spectra were measured on a SHIMAZU UV-2600. Fluorescence spectra were recorded on a JASCO spectrophotometer FP-8600. MCD spectra were recorded on a JASCO J-830 spectrodichrometer with a magnetic field of up to 1.6 T. MALDI-TOF mass spectra were obtained on a Bruker Autoflex spectrometer with dithranol as matrix. Mass spectra with electrospray ionization were obtained on a Bruker Daltonics micrOTOF II. DPV data were recorded with an ALS 720C potentiostat, and electrochemical experiments were performed under purified nitrogen gas. Nanoporous TiO2 electrodes D

DOI: 10.1021/acs.inorgchem.6b00562 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 0.9 (36H, br, −CH3). IR (ATR): υ = 1681 (−COOH) cm−1. MALDI-TOF Ms (dithranol): m/z 2859.24 (M+H), Calcd for C177H238N8O20Zn: m/z 2859.71. 4.4. Fabrication of DSSCs using NcS1 and NsS2.4 Nanoporous TiO2 electrodes (apparent surface area: 0.25 cm2 (0.5 × 0.5 cm)) were printed onto transparent conducting glass substrates by the screen printing technique using two TiO2 nanoparticle pastes having different diameters of 15−20 and 400 nm. After sintering at 550 °C for 30 min in air, the TiO2 electrodes were treated with TiCl4. The dye adsorption of NcS1 or NcS2 onto TiO2 films was achieved by immersion of electrodes in 0.05 mM toluene solutions of the dyes for 32 h at 25 °C. Dye-adsorbed TiO2 electrode and Pt counter electrode were separated by a 50 μm thick hot melt ring (Surlyn, DuPont) and sealed by heating. Redox electrolytes (0.1 M LiI, 0.6 M DMPImI, 0.5 M tBP, and 0.05 M I2 in dehydrated acetonitrile) were injected to the space between two electrodes, and then measured the photovoltaic performance by applying black mask (0.16 cm2) for the reduction of diffusive light under one sun conditions (AM 1.5, 100 mW/cm2) by a solar simulator (Otenso-Sun 3SD, Bunko Keiki).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00562. Absorption and fluorescence spectra of NcS1, DPV of NcS1-stained TiO2 electrode, absorption spectrum of dye-stained TiO2 films of NcS1/CDCA, and DSSC results of NcS2. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81 268 21 5499. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially supported by Grants-in-Aid for Scientific Research (A) (No. 15H02172) and (B) (No. 22350086) from the Japan Society for the Promotion of Science of Japan.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00562 Inorg. Chem. XXXX, XXX, XXX−XXX