Coplanar indenofluorene-based organic dyes for dye-sensitized solar cells Sumit Chaurasia, Yung-Chung Chen, Hsien-Hsin Chou, Yuh-Shen Wen, Jiann T. Lin * Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan article info Article history: Received 25 May 2012 Received in revised form 4 July 2012 Accepted 13 July 2012 Available online 20 July 2012 Keywords: Dye-sensitized solar cells Indenofluorene Metal-free sensitizers abstract A series of new organic dyes, comprising indenofluorene moiety as a conjugated bridge, with an ex- tended p-groups, such as thiophene and furan, diphenylamine as donor, cyanoacrylic acid group as an electron acceptor and anchoring group, have been synthesized. Photophysical and electrochemical measurements, and theoretical computation were carried out on these dyes. Dye-sensitized solar cells (DSSCs) using these dyes as the sensitizers exhibited photocurrent density (J SC ), open-circuit voltage (V OC ), and fill factor (FF) in the range of 6.95e8.20 mA/cm 2 , 0.70e0.71 V, and 0.69e0.71, respectively, corresponding to an overall conversion efficiency of 3.36e4.05%. The best efficiency reached 56% of the standard cell based on N719. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Sun is an obvious source of clean and cheap energy, therefore, harnessing the power of the Sun with photovoltaic technologies in large scale manners appears to be a promising approach toward the energy challenge. 1 Among different solar cells with the use of organic materials, dye-sensitized solar cells (DSSCs) have attracted consid- erable attention since Gr€ atzel’s seminal report on ruthenium-based sensitizers in 1991. 2 So far, ruthenium(II) polypyridyl complexes still stand out as the most efficient and stable senstitizers for DSSC’s, and the record high conversion efficiency (h) of 12.3% based on Zn- porphyrin complexes have been reached recently. 3 In addition to the more costly ruthenium-based sensitizers, metal-free organic dyes are also intensively exploited in DSSCs because of relatively low cost and flexibility in tailoring molecular structures for manipulation of electronic and photophysical characteristics. 4 Conversion efficiency surpassing 10% has been demonstrated for a metal-free sensitizer- based DSSC. 5 The common skeleton of metal-free sensitizers consists of electron donor, electron acceptor, and conjugated spacer, in which the acceptor also functions as the anchor linked to the photoanode. Such a pushepull type donore(p-spacer)eacceptor motif (DepeA) not only enhances light harvesting at longer wavelength due to the charge transfer transition from the donor to the acceptor, but also facilitates electron injection and charge separation. 6 In DepeA systems, planarization of the conjugated spacer can allow more ef- fective electronic communication between the donor and the ac- ceptor. Some representative planarized spacers for high perfor- mance DSSCs include oligoene, 7 fluorene, 8 spirofluorene, 9 indacenodithiophene, 10 and quinacridone. 11 It is important to note, however, that planarized spacer with sluggish tendency to form quinoid structure may exhibit pep* character dominating over charge-transfer character upon photo-excitation. Consequently, charge separation and electron injection were hampered in spite of intense absorption of the dye. For example, the sensitizer using a ladder-type pentaphenylene segment as the spacer between di(p- tolylamino) donor and 2-cyanoacrylic acceptor absorbs at relatively short wavelength (l max ¼457 nm in CH 2 Cl 2 ), and the cell efficiency reaches only 2.3%, corresponding to 50% of N719-based stand- ard cell (N719¼(cis-di(thiocyanato))bis(2,2 0 -bipyridyl-4,4 0 -dicar- boxylato)ruthenium(II) bis(tetrabutylammonium)). 12 Therefore, we chose (1,2-b)-indenofluorene moiety as a substitute for pentaphe- nylene in order to have a better trade-off between charge transfer and light absorption intensity. (1,2-b)-Indenofluorene was widely used as the building block of functional conjugated polymers for various optoelectronic devices applications. 13 Though there was also report on blue OLED using dispirofluoreneeindenofluorene based DepeA type molecule, 14 to our knowledge, indenofluorene has never been used as a coplanar bridging unit in DepeA systems for DSSCs, possibly due to the synthetic complexity. A broader and stronger absorption band would be obtained for indenofluoene derivatives with respect to corresponding fluorene derivatives, due to the extended p-conju- gation of indenofluorene compared to fluorene. Additionally, better solubility and processability of the dye can be anticipated by in- corporation of four alkyl chains at the indenofluorene moiety. Ap- propriate incorporation of heteroaromatic rings, 15 such as thiophene and furan in the spacer between the donor and the ac- ceptor has been used to tune the electronic and optical properties of the sensitizer, and thus the efficiency of DSSCs. Therefore, we also * Corresponding author. E-mail address: [email protected](J.T. Lin). Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.07.045 Tetrahedron 68 (2012) 7755e7762
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Tetrahedron 68 (2012) 7755e7762
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Tetrahedron
journal homepage: www.elsevier .com/locate/ tet
Coplanar indenofluorene-based organic dyes for dye-sensitized solar cells
Sumit Chaurasia, Yung-Chung Chen, Hsien-Hsin Chou, Yuh-Shen Wen, Jiann T. Lin *
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan
a r t i c l e i n f o
Article history:Received 25 May 2012Received in revised form 4 July 2012Accepted 13 July 2012Available online 20 July 2012
Keywords:Dye-sensitized solar cellsIndenofluoreneMetal-free sensitizers
0040-4020/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.tet.2012.07.045
a b s t r a c t
A series of new organic dyes, comprising indenofluorene moiety as a conjugated bridge, with an ex-tended p-groups, such as thiophene and furan, diphenylamine as donor, cyanoacrylic acid group as anelectron acceptor and anchoring group, have been synthesized. Photophysical and electrochemicalmeasurements, and theoretical computation were carried out on these dyes. Dye-sensitized solar cells(DSSCs) using these dyes as the sensitizers exhibited photocurrent density (JSC), open-circuit voltage(VOC), and fill factor (FF) in the range of 6.95e8.20 mA/cm2, 0.70e0.71 V, and 0.69e0.71, respectively,corresponding to an overall conversion efficiency of 3.36e4.05%. The best efficiency reached 56% of thestandard cell based on N719.
� 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Sun is an obvious source of clean and cheap energy, therefore,harnessing the power of the Sun with photovoltaic technologies inlarge scale manners appears to be a promising approach toward theenergy challenge.1 Among different solar cellswith the use of organicmaterials, dye-sensitized solar cells (DSSCs) have attracted consid-erable attention since Gr€atzel’s seminal report on ruthenium-basedsensitizers in 1991.2 So far, ruthenium(II) polypyridyl complexesstill stand out as the most efficient and stable senstitizers for DSSC’s,and the record high conversion efficiency (h) of 12.3% based on Zn-porphyrin complexes have been reached recently.3 In addition to themore costly ruthenium-based sensitizers, metal-free organic dyesare also intensively exploited in DSSCs because of relatively low costand flexibility in tailoring molecular structures for manipulation ofelectronic and photophysical characteristics.4 Conversion efficiencysurpassing 10% has been demonstrated for a metal-free sensitizer-based DSSC.5 The common skeleton ofmetal-free sensitizers consistsof electron donor, electron acceptor, and conjugated spacer, inwhichthe acceptor also functions as the anchor linked to the photoanode.Such a pushepull type donore(p-spacer)eacceptor motif (DepeA)not only enhances light harvesting at longer wavelength due to thecharge transfer transition from the donor to the acceptor, but alsofacilitates electron injection and charge separation.6 In DepeAsystems, planarization of the conjugated spacer can allow more ef-fective electronic communication between the donor and the ac-ceptor. Some representative planarized spacers for high perfor-mance DSSCs include oligoene,7 fluorene,8 spirofluorene,9
.sinica.edu.tw (J.T. Lin).
All rights reserved.
indacenodithiophene,10 and quinacridone.11 It is important to note,however, that planarized spacer with sluggish tendency to formquinoid structure may exhibit pep* character dominating overcharge-transfer character upon photo-excitation. Consequently,charge separation and electron injection were hampered in spite ofintense absorption of the dye. For example, the sensitizer usinga ladder-type pentaphenylene segment as the spacer between di(p-tolylamino) donor and 2-cyanoacrylic acceptor absorbs at relativelyshort wavelength (lmax¼457 nm in CH2Cl2), and the cell efficiencyreaches only 2.3%, corresponding to 50% of N719-based stand-ard cell (N719¼(cis-di(thiocyanato))bis(2,20-bipyridyl-4,40-dicar-boxylato)ruthenium(II) bis(tetrabutylammonium)).12 Therefore, wechose (1,2-b)-indenofluorene moiety as a substitute for pentaphe-nylene in order to have a better trade-off between charge transferand light absorption intensity.
(1,2-b)-Indenofluorene was widely used as the building block offunctional conjugated polymers for various optoelectronic devicesapplications.13 Though there was also report on blue OLED usingdispirofluoreneeindenofluorene based DepeA type molecule,14 toour knowledge, indenofluorene has never been used as a coplanarbridging unit in DepeA systems for DSSCs, possibly due to thesynthetic complexity. A broader and stronger absorption bandwould be obtained for indenofluoene derivatives with respect tocorresponding fluorene derivatives, due to the extended p-conju-gation of indenofluorene compared to fluorene. Additionally, bettersolubility and processability of the dye can be anticipated by in-corporation of four alkyl chains at the indenofluorene moiety. Ap-propriate incorporation of heteroaromatic rings,15 such asthiophene and furan in the spacer between the donor and the ac-ceptor has been used to tune the electronic and optical properties ofthe sensitizer, and thus the efficiency of DSSCs. Therefore, we also
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e77627756
integrated the indenofluorene with thiophene or furan unit as thespacer of DepeA sensitizer. To the best of our knowledge, this is thefirst report of DSSCs on indenofluorene-based sensitizers.
2. Results and discussion
2.1. Synthesis and characterization
The structures of new metal-free sensitizers are shown in Fig. 1.The synthetic protocols for compound SC-1eSC-3 are illustrated inScheme 1. Indenofluorenewas synthesized in excellent yields using
Fig. 1. Structures of compounds SC-1, SC-2, and SC-3.
300 400 500 6000.0
0.1
0.2
0.3
0.4
0.5
0.6
Abso
rban
ce (a
u)
Wavelength (nm)
SC-1
SC-2
SC-3
Abs (
a.u.
)
Wavelength (mn)
SC-1 SC-2 SC-3
Fig. 2. UVevis absorption spectra of dyes SC-1, SC-2, and SC-3 in THF solutions andinset spectra of dyes SC-1, SC-2, and SC-3 adsorbed on TiO2 (2 mm).
a literature reported method.16 The first step is the alkylation ofmethylene units of indenofluorene for suppressing their tendencyof oxidation and improving solubility.17 The alkylated indeno-fluorene then undergoes double bromination using CuBr2 adsorbed
Scheme 1. Synthesis of organic dyes SC-1, SC-2, and SC-3. Reagents and conditions: (a) (i) n-BuLi, dry THF, �78 �C, N2, (ii) 2-ethylhexyl bromide, �78 �C to rt, then repeat (i) and (ii)again; (b) CuBr2/Al2O3, CCl4, reflux, 8 h, N2; (c) Ph2NH, Pd2(dba)3, rac-2,20-bis(diphenylphosphino)-1,10-biphenyl (BINAP) and sodium tert-butoxide, toluene, 80 �C, 24 h, N2; (d) (i) n-BuLi, dry THF, �78 �C, N2, (ii) dry DMF, �78 �C to rt, N2; (e) cyanoacetic acid, NH4OAc, AcOH, 100 �C, 12 h; (f) (i) (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane/(5-(1,3-dioxolan-2-yl)furan-2-yl)tributylstannane, PdCl2(PPh3)2, dry DMF, N2, (ii) glacial acetic acid, H2O, 50 �C, 6 h.
on Al2O3.18 One of the bromo substituent is replaced by dipheny-lamino entity via palladium catalyzed BuchwaldeHartwig CeNcoupling reaction of the dibromo intermediate with di-phenylamine.19 The arylamine intermediate, 4, is then formylated.
Subsequent condensation of the thus formed aromatic aldehydewith cyanoacetic acid results in formation of SC-1. For the synthesesof SC-2 and SC-3, (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributyl-stannane or (5-(1,3-dioxolan-2-yl)furan-2-yl)tributylstannane isallowed to undergo palladium catalyzed Stille coupling reactionwith 4,20 followed by condensation of thus formed aldehyde withcyanoacetic acid. The dyes are obtained as red powders, and aresoluble in common organic solvents, such as THF, CH2Cl2, andCHCl3.
2.2. Photophysical properties
The UVevis absorption spectra of the newdyes in THFare shownin Fig. 2 and the data are collected in Table 1. The absorption band ata shorter wavelength (w300 nm) is attributed to localized pep*transition of basic chromophore, indenofluorene, as evidenced from
the absorption spectra of indenofluorene reported in literature,21
ranging from 290 to 350 nm. The broad band ranging from 408 to430 nm and possessing a high extinction coefficient (>3.7�104 M�1 cm�1) is attributed to the more delocalized pep*
a Recorded in THF (solution).b Absorption maxima of film (adsorbed on TiO2).c Recorded in CH2Cl2. scan rate¼100 mV s�1; electrolyte¼(n-C4H9)4NPF6; potentials are quoted with reference to the internal ferrocene standard (E1/2¼þ147eþ166 mV vs
Ag/AgNO3).d The HOMO and LUMO energies are calculated using formula HOMO¼5.1þ(E1/2�EFc).e The optical bandgap energy gap, E0e0, was derived from the intersection of absorption and emission spectra.f The bandgap E0e0*: the excited-state oxidation potential versus NHE.
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4-0.00002
-0.00001
0.00000
0.00001
0.00002
0.00003
Fc
Cur
rent
(au)
Potential (V)
SC-1
SC-2
SC-3
Fc
Fig. 3. Cyclic voltammograms of SC-1, SC-2, and SC-3 recorded in CH2Cl2.
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762 7757
transition with charge-transfer character. The charge-transfercharacter of the band is also confirmed by theoretical computation(vide infra). The absorption spectra of SC-2 (lmax¼426 nm) and SC-3(lmax¼430 nm) show considerable red shifts as compared with thatof SC-1 (lmax¼408 nm). This result clearly indicates the importanceof an extended p conjugation via incorporation of a heteroaromaticmoiety. The molar extinction coefficient is also increased signifi-cantly in SC-2 and SC-3. TheUVevis absorption spectramaximum inCH2Cl2 solution was measured to be 448, 462, and 472 nm for SC-1,SC-2, and SC-3, respectively. Compared with the pentaphenylenedye (lmax¼457 nm),12 a red shift of the absorption for SC-2 and SC-3was observed. Thismay be due toweaker electronic communicationbetween the donor and the acceptor in the pentaphenylene dye.Fig. 2 (Inset) depicts the absorption spectra of the organic dyesadsorbed on TiO2 surface. They are slightly red-shifted comparedwith the absorption spectra in THF solution, and may be attributedto J-aggregation of the dyemolecules. The dyes are weakly emissivewith the emission maximum ranging from 504 to 534 nm, whichfollow the trend SC-1<SC-3<SC-2 (Fig. S1 see Supplementary data).The larger dihedral angle between the indenofluorene and thethiophene rings in the ground state may be the cause of its largerStokes shift of the emission spectra.
2.3. Electrochemical properties
The electrochemical properties of the dyes were studied bycyclic voltammetry (CV) and differential pulse voltammetry (DPV).The relevant CV data are collected in Table 1 and the cyclic vol-tammograms are shown in Fig. 3. All the dyes are redox-stable,exhibiting reversible oxidation couples. Only one quasi-reversibleredox wave was observed for SC-1, while two quasi-reversible re-dox waves were observed for SC-2 and SC-3. The first redox wavemay be attributed to the oxidation of the arylamine, while thesecond quasi-reversible redox wave is most likely stemmed fromthe oxidation of the conjugated segment. The first oxidation po-tential of the compounds decreases in the order of SC-1>SC-2>SC-3. This can be rationalized by the electronic influence of electronexcessive thiophene (electron density of carbon: �0.011; �0.074)or furan (electron density of carbon: �0.019; �0.105)22 on thearylamine. The energy levels of the HOMOs (highest occupiedmolecular orbitals) in these materials were calculated with refer-ence to ferrocene (5.1 eV)23 and ranged from 5.46 to 5.51 eV. Thistogether with the energy gap (E0e0, 2.56e2.71 eV) obtained fromthe edge of absorption spectra were utilized to derive the LUMO(lowest unoccupied molecular orbital) energy.
2.4. Theoretical approach
Density functional calculations at the B3LYP/6-31G* level wasconducted for the compounds to correlated between dye structuresand corresponding physical properties, The excessive 2-ethylhexyl
chains were replaced by isobutyl groups to save computationalefforts. The computed frontier orbitals of the compounds and theircorresponding energy states are included in Figs. S2 and S3 (seeSupplementary data), respectively. The configurations of transi-tions in gas-phase were also conducted using time-dependent DFTand the results were summarized in Table S1 (see Supplementarydata). Fig. 4 displays the HOMO and LUMO orbitals of SC-1eSC-3.The HOMO orbitals mainly reside on the arylamine extending to theconjugated spacer, and LUMO orbitals mainly reside on 2-cyanoacrylic acid extending to the conjugated spacer. Therefore,charge-transfer character is evident for the S0/S1 transition,which solely comes from HOMO to LUMO. Fig. 5 displays the di-hedral angles between the neighboring groups in the conjugatedspacer. It is interesting to note that the dihedral angle between thethiophene and the indenofluorene rings in SC-2 is significantlylarger than that between the furan and the indenofluorene rings inSC-3 possibly due to more pronounced steric hindrance arose fromlarger sulfur atom. The planarity of SC-3 was further establishedfrom their crystal structure (Fig. S4, see Supplementary data),which shows that the bridging unit (indenofluorene) and furan arealmost coplanar, also confirming the validity of computation.
2.5. Photovoltaic properties
Dye-sensitized solar cells were fabricated with the use of thesenew dyes and nanocrystalline anatase TiO2. The cells had an ef-fective area of 0.25 cm2, and the electrolyte used was composed of0.05 M I2/0.5 M LiI/0.5 M tert-butylpyridine (TBP) in acetonitrilesolution. The device performance statistics under standard globalAM 1.5 solar condition are presented in Table 2. The photo-currentevoltage (JeV) curves, dark current and the incident
Fig. 5. Dihedral angles between the neighboring units of the dyes.-0.2 0.0 0.2 0.4 0.6 0.8 1.0
-10-8-6-4-202468
1012141618
Cur
rent
den
sity
(mA/
cm2 )
Voltage (V)
SC-1
SC-2
SC-3
N719
Fig. 6. The current densityevoltage and dark-current curves of DSSCs based onSC-1eSC-3.
400 500 600 700 8000
10
20
30
40
50
60
70
80
90
100
EQE
(%)
Wavelength (nm)
SC-1
SC-2
SC-3
N719
Fig. 4. HOMOs and LUMOs of the dyes.
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e77627758
monochromatic photo-to-current conversion efficiency (IPCE) plotsof the cells are shown in Fig. 6 and Fig. 7, respectively. The short-circuit photocurrent density (JSC), open-circuit voltage (VOC) andfill factor (FF) of the devices are in the range of 6.95e8.20 mA/cm2,0.70e0.71 V and 0.69e0.71, respectively, corresponding to anoverall conversion efficiency of 3.36e4.05%. DSSC of SC-3 has thebest power conversion efficiency (4.05%) among all, reaching 56% ofN719-based standard cell (conversion efficiency¼7.18%) fabricatedand measured under similar conditions. Action spectra of the in-cident photon-to-current conversion efficiency (IPCE) as a functionof wavelength were measured to evaluate the photoresponse in thewhole region, and the data are plotted in Fig. 7. High IPCE perfor-mance (above 80%) was observed in the range from 400 to 550 nmfor dyes SC-2 and SC-3, withmaximumvalues of 80% at 480 nm and88% at 460 nm, which is much higher than the pentaphenylenebased dye (w70%).12 The higher efficiency of SC-2 and SC-3 is at-tributed to the better light harvesting due to extended p-conju-gation and/or the higher dye density of SC-2 (4.02�10�7 mol/cm2)and SC-3 (3.84�10�7 mol/cm2) than SC-1 (3.34�10�7 mol/cm2) onTiO2 surface.
The slightly lower cell efficiency (47% of N719 standard cell) ofSC-1 compared to pentaphenylene based dye (50% of N719 standardcell) can be attributed to the inferior light harvesting of the former.12
The higher cell efficiency of SC-2 and SC-3 (56% of N719 standardcell) than pentaphenylene based dye may be also due to the betterlight harvesting of the former (vide supra). It is interesting tonote that a ladder-type dye with thiophene-implanted planarized
(4,9-dihydro-4,4,9,9-tetrakis(4-methylphenyl)-s-indaceno[1,2-b:5,6-b0]dithiophene) as the spacer was reported to have a low cell ef-ficiency of 2.31%, though the dye has longer absorption maximum(500 nm) and larger extinction coefficient (84,000 M�1 cm�1) thanSC-2 and SC-3.24 Possibly the bulkiness of the molecule results ina lower dye density of the former on TiO2 surface.
The electrochemical impedance measurement under illumina-tion was shown in Fig. 8. In general, electrochemical impedancemeasured under illumination can provide information on electroninjection and transport inside TiO2.25 Upon illumination of100 mW cm�2 under open circuit conditions, the radius of the in-termediate frequency semicircle in the Nyquist plot (Fig. 8) repre-sents the electron transport resistance. The lower electron
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762 7759
transport resistance (Rct) will assist electron collection and improvethe cell efficiency. Therefore, the order of decreasing Rct (Table 2),SC-1>SC-2>SC-3, is consistent with the trend of the cellperformance.
10 20 30 40 50 600
2
4
6
8
10
12
14
16
18
20
22
-Z
'' (O
hm
)
Z' (Ohm)
SC-1 SC-2 SC-3 N719
Fig. 8. Electrochemical impedance spectra (Nyquist plots) of DSSC for dyes measuredunder illumination (AM 1.5).
The Nyquist plots of DSSCs under a forward bias of �0.50 V inthe dark are shown in Fig. 9. The semicircle in the Nyquist plots canbe used to derive the charge recombination resistance on the TiO2surface (Rrec) by fitting curves using Z-view software. The larger Rrecvalue implies the smaller dark current, and the smaller Rrec valuemeans more facile recombination of the electron in the conductionband with the oxidized electrolyte, and thus a lower VOC. The Rrecvalues of SC-3 and SC-2 are significantly higher than that of SC-1,implying that there is more effective suppression of the electronrecombination with the electrolyte. However, only minimal de-crease of dark current and increase of VOC values were observed forSC-3 and SC-2 compared to SC-1. We speculate that the Fermi levelof TiO2 adsorbed with SC-3 or SC-2 was shifted in a downwarddirection compared to that adsorbed with SC-1.
0 500 1000 1500 2000 2500 30000
500
1000
1500
2000
-Z
'' (O
hm
)
Z' (Ohm)
SC-1
SC-2
SC-3
N719
Fig. 9. Electrochemical impedance spectra (Nyquist plots) of DSSC for SC-1eSC-3 dyesmeasured in the dark under �0.50 V bias.
3. Conclusion
In summary, we have successfully synthesized a series of metal-free organic DSSC sensitizers by exploiting indenofluorene as a co-planar bridging unit, thiophene and furan as extended p-spacergroup in the DepeA structure, using the conventional diarylamineas donor and cyanoacetic acid as acceptor. DSSCs using these dyesas the sensitizers exhibited high IPCE performance (above 80%) inthe range from 400 to 550 nm. The highest conversion efficiency
reaches 4.05%, which is 56% of the efficiency of the reference cellbased on ruthenium dye N719, and surpasses that of ladder-typepentaphenylene- or indaceno[1,2-b:5,6-b0]dithiophene-based dye.
4. Experimental section
4.1. General information
1H NMR and 13C NMR spectra were taken on a Bruker AMX-400or Bruker AV-400 spectrometer using chloroform-d1 (CDCl3), oracetone-d6 as the solvent. Cyclic Voltammetric (CV) measurementwas performed on BAS-100 using CH2Cl2 as solvent. UVevis (Ul-traviolet-Visible) spectra were recorded on a Dynamica DB-20Spectrophotometer. PL (Fluorescence) spectra were recorded ona Hitachi F-4500 Spectrophotometer. Elemental analysis was per-formed on a PerkineElmer 2400. Fast atom bombardment massspectrometry (FABMS) analysis was performed on a JEOL TokyoJapan JMS-700 mass spectrometer equipped with the standard FABsource. The photoelectrochemical characterizations on the solarcells were carried out using an Oriel Class A solar simulator (Oriel91195A, Newport Corp.). Photocurrentevoltage characteristics ofthe DSSCs were recordedwith a potentiostat/galvanostat (CHI650B,CH Instruments, Inc.) at a light intensity of 100mW cm�2 calibratedby an Oriel reference solar cell (Oriel 91150, Newport Corp.). Themonochromatic quantum efficiency was recorded througha monochromator (Oriel 74100, Newport Corp.) at short circuitcondition. The intensity of each wavelength was in the range of1e3 mW cm�2. Electrochemical impedance spectra (EIS) wererecorded for DSSC under illumination at open-circuit voltage (VOC)or dark at �0.55 V potential at room temperature. The frequenciesexplored ranged from 10 mHz to 100 kHz. The TiO2 nanoparticlesand the reference compound, N719, were purchased from Solar-onix, S.A., Switzerland.
4.2. Synthetic details and characterization
4.2.1. 6,6,12,12-Tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluo-rene (2). To a stirred solution of indenofluorene 115 (2.54 g,10 mmol) in dry THF (30 mL) under nitrogen, 15 mL of n-butyl-lithium (n-BuLi) (24 mmol, 1.6 M in hexane) was added dropwise at�78 �C. The solution turned to deep purple. The solution was stir-red for an additional 1.5 h at �78 �C and 1-bromo-2-ethylhexane(5.4 mL, 30 mmol) was added dropwise and the solution waswarmed to room temperature. After stirring for 1.5 h, the solutionwas cooled again to �78 �C and second portion of n-BuLi (15 mL,24 mmol) was added dropwise and stirred again for an additional1.5 h before the second portion of 1-bromo-2-ethylhexane (6 mL,33 mmol) was added. The solution was warmed to room temper-ature and stirred for 12 h. It was quenched by adding water. Themixture was extracted using EtOAc and washed with H2O. The or-ganic layer was dried over anhydrous MgSO4 and then concen-trated under reduced pressure to afford the crude product. Furtherpurification was performed by column chromatography to providea thick oil (excess 2-ehthylhexylbromide was removed under vac-uum at 80 �C for 8e10 h). Yield¼6.35 (90%). 1H NMR (chloroform-d1, 400 MHz): d 0.41e0.93 (m, 60H), 1.95e2.10 (m, 8H), 7.22e7.26(m, 2H), 7.31 (t, J¼7.4 Hz, 2H), 7.37 (d, J¼7.4 Hz, 2H), 7.65 (s, 2H),7.68e7.72 (m, 2H); 13C NMR (chloroform-d1, 100 MHz): d 10.25,10.46, 10.60, 14.20, 14.32, 22.91, 22.97, 26.89, 27.05, 27.17, 28.20,28.39, 28.53, 33.81, 33.91, 34.03, 34.10, 34.74, 44.56, 44.70, 44.99,45.14, 45.32, 54.68, 115.10, 119.34, 124.21, 126.20, 126.82, 140.58,141.92, 149.90, 151.04. FABMS (m/z): 703.0 (Mþþ1).
4.2.2. 2,8-Dibromo-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroind-eno[1,2-b]fluorene (3). To a stirred solution of 2 (5.0 g, 7.13 mmol) incarbon tetrachloride (50 mL) at room temperature was added
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e77627760
CuBr2/Al2O3 (20 g) and mixture was stirred at 80 �C under N2 at-mosphere and was monitored through 1H NMR. After completeconsumption of starting material (after 6e8 h), the reaction mix-ture was filtered, and the precipitate was washed with dichloro-methane (100 mL�3). The combined organic extracts wereconcentrated under reduced pressure, and the crude product waspurified by flash chromatography to give compound 3 as colorlessoil. Yield: 6.0 g (98%). 1H NMR (chloroform-d1, 400 MHz):d 0.40e0.95 (m, 60H), 1.92e2.10 (m, 8H), 7.45 (d, J¼8.0 Hz, 2H), 7.50(s, 2H), 7.55e7.59 (m, 2H), 7.61 (m, 2H); 13C NMR (chloroform-d1,100 MHz): d 10.29, 10.33, 10.49, 14.16, 14.23, 22.85, 22.91, 26.97,27.04, 27.12, 28.07, 28.19, 28.37, 33.65, 33.76, 33.88, 34.72, 34.79,44.29, 44.43, 44.62, 44.83, 44.98, 54.93, 115.17, 120.39, 120.68,127.46, 127.57, 129.99, 139.78, 140.51, 149.80, 153.22, 153.30. FABMS(m/z): 861 (Mþþ1).
4.2.3. 8-Bromo-6,6,12,12-tetrakis(2-ethylhexyl)-N,N-diphenyl-6,12-dihydroindeno[1,2-b]fluoren-2-amine (4). A mixture of di-phenylamine (0.591 g, 3.5 mmol), 3 (3.0 g, 3.5 mmol), Pd2(dba)3(0.064 g, 0.070 mmol), rac-2,20-bis(diphenylphosphino)-1,10-bi-phenyl (BINAP) (0.043 g, 0.14 mmol), and sodium tert-butoxide(0.504 g, 5.25 mmol) in toluene was stirred at 80 �C under N2 for24 h. After completion of the reaction toluene was evaporatedunder vacuum and reaction mixture was poured into water,extracted with ether and the combined organic phase was washedwith brine (30 mL�2) and then dried over MgSO4. After removal ofsolvent, the crude product was purified through column chroma-tography on silica gel using hexane as the eluent to give the purifiedproduct 4 as colorless oil in 64% yield (2.13 g). 1H NMR (chloroform-d1, 300 MHz) d 0.4e0.10 (m, 60H), 1.72e2.10 (m, 8H), 6.91e7.15 (m,8H), 7.18e7.25 (m, 4H), 7.39e7.60 (m, 6H); 13C NMR (chloroform-d1,100 MHz) d 10.05, 10.46, 10.78, 10.87, 14.25, 14.34, 22.97, 23.03,23.13, 23.21, 26.56, 26.93, 27.12, 27.25, 27.38, 27.54, 28.21, 28.26,28.33, 28.49, 29.01, 29.92, 30.57, 33.69, 33.95, 34.40, 34.84, 44.77,44.87, 45.33, 54.83, 55.04, 114.74, 115.14, 120.05, 120.11, 120.31,120.54, 121.24, 121.31, 122.45, 122.53, 123.76, 123.92, 124.62, 127.50,129.34, 129.99, 138.84, 140.86, 140.93, 146.75, 148.35, 149.75,150.25, 150.30, 152.66, 152.76, 153.31; HRMS (FAB): (m/z) Mþ calcdfor C64H86BrN, 948.6022; found, 948.6045.
4.2.4. 8-(Diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluorene-2-carbaldehyde (5). Under a nitrogenatmosphere and at �78 �C, n-BuLi (0.500 mL, 1.6 M in hexane)was added dropwise to a dry THF (30 mL) solution containing 4(0.50 g, 0.58 mmol). After 1 h stirring, 0.1 mL of N-methyl-formamide was added slowly to the reaction solution. The tem-perature of the solution was brought back to room temperature,and stirred over night. The reaction was then quenched with 2 NHCl. The solution was extracted with ethyl acetate and subjectedto flash column chromatography. A light yellow thick oil wasobtained in 41% yield (368 mg). 1H NMR (chloroform-d1,400 MHz) d 0.44e1.05 (m, 60H), 1.80e2.11 (m, 8H), 6.95e7.01 (m,2H), 7.04e7.11 (m, 6H), 7.19e7.25 (m, 4H), 7.57e7.63 (m, 2H), 7.67(s, 1H), 7.79e7.86 (m, 2H), 7.87e7.90 (m, 1H), 10.03 (s, 1H). FABMS(m/z): 898 (Mþþ1).
4.2.5. 2-Cyano-3-(8-(diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluoren-2-yl)acrylicacid (SC-1). Amixture of8-(diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroin-deno[1,2-b]fluorene-2-carbaldehyde 5 (200 mg, 0.223 mmol), cya-noacetic acid (28 mg, 0.334mmol), ammonium acetate (5.0 mg), andacetic acid (8 mL) was heated at 100 �C for 12 h. The solution wasthen cooled using crushed ice, resulting solid was filtered, washedthoroughly with water, followed by flash chromatography usingdichloromethane (with 2% methanol) and re-precipitated fromdichloromethane by pouring into methanol to give a red solid
4.2.6. 5-(8-(Diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluoren-2-yl)thiophene-2-carbaldehyde(6a). To a flame dried two neck flask fitted with condenser chargedwith 4 (0.50 g, 0.53 mmol), (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane (0.353 g, 0.79 mmol), Pd(PPh3)2Cl2 (8.0 mg,0.01 mmol), and dry DMF (15 mL) were added under N2 atmo-sphere. The mixture was heated to 80 �C for 24 h. After completionthe reaction mixture was cooled and extracted with ether/brineand the organic layers were combined and dried over Na2CO3.Evaporation of the solvent gave crude dioxolane derivative.
The resulting dioxolane derivative was suspended in glacialacetic acid and heated to 50 �C. After a clear solution is formed,1 mL of water was added and the temperature of the reactionmixture was maintained at 50 �C for 5e6 h. It was cooled andpoured into crushed ice, orange precipitate formedwas filtered andwashed with H2O. The aldehyde product (6a) was further purifiedby flash chromatography using CH2Cl2/hexane (60:40 by vol) as theeluent to yield an orange solid 370 mg (71%). 1H NMR (chloroform-d1, 400 MHz): d 0.45e1.05 (m, 60H), 1.78e1.87 (m, 2H), 1.97e2.11(m, 6H), 6.94e7.02 (m, 2H), 7.03e7.13 (m, 6H), 7.19e7.24 (m, 4H),7.42 (s, 1H), 7.56e7.66 (m, 5H), 7.68e7.76 (m, 2H), 9.88 (s, 1H). 13CNMR (chloroform-d1, 100 MHz): d 9.94, 10.39, 10.67, 10.76, 14.02,14.24, 22.82, 23.02, 23.10, 26.43, 26.83, 27.07, 27.23, 27.41, 28.08,28.22, 28.39, 28.88, 29.81, 33.44, 33.57, 33.89, 33.98, 34.27, 34.78,44.64, 44.74, 45.17, 54.79, 68.34, 99.71, 114.89, 116.30, 119.89,120.73, 120.92,121.26,122.69,122.78,123.99,124.14, 124.39,124.72,126.82, 129.04, 129.12, 129.40, 131.09, 132.14, 136.58, 136.71, 138.51,142.78, 142.83, 147.39, 148.23, 150.65, 150.69, 152.13, 152.96, 153.06,156.79, 168.01, 168.23. HRMS (FAB) m/z [Mþ] calcd for C69H89N2OS:980.6743; found: 980.6758.
4.2.7. 2-Cyano-3-(5-(8-(diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluoren-2-yl)thiophen-2-yl)acrylic acid (SC-2). A mixture of 6a (300 mg, 0.31 mmol), cyano-acetic acid (40 mg, 0.46 mmol), ammonium acetate (8.0 mg) andacetic acid (10 mL) was heated at 100 �C for 12 h. The solution wascooled by using crushed ice, the resulting solid was filtered, washedthoroughly with water, dried and purified through flash chroma-tography using 2% methanol in dichloromethane and further re-precipitated from dichloromethane by pouring into methanol togive a red solid (SC-2) (248 mg, 78%). 1H NMR (acetone-d6,400 MHz): d 0.50e1.10 (m, 60H), 1.90e2.00 (m, 2H), 2.15e2.25 (m,6H), 7.02e7.15 (m, 7H), 7.19e7.24 (m,1H), 7.29e7.35 (m, 4H), 7.76 (s,1H), 7.81e7.86 (m, 2H), 7.92e8.04 (m, 5H), 8.46 (s, 1H). 13C NMR(100 MHz, CDCl3): d 9.84, 10.27, 10.31, 10.66, 13.64, 13.96, 14.16,16.96, 22.71, 22.77, 26.33, 26.75, 26.97, 27.12, 27.32, 27.76, 27.97,28.10, 28.28, 28.77, 29.70, 33.32, 33.46, 33.76, 34.56, 34.67, 44.59,44.76, 45.07, 54.67, 114.59, 115.30, 119.67, 120.21, 120.91, 120.99,121.32, 121.91, 122.58, 123.73, 123.84, 123.88, 125.70, 129.14, 130.28,134.50, 136.93, 137.06, 138.65, 139.71, 141.19, 143.77, 146.65, 148.08,150.15,150.20,150.52,151.97,152.04,152.53,152.63. HRMS (FAB)m/z [Mþ] calcd for C72H90N2O2S: 1048.6880; found: 1048.6876.
S. Chaurasia et al. / Tetrahedron 68 (2012) 7755e7762 7761
Elemental analysis calcd (%) for C72H90N2O2S$H2O: C, 81.16; H, 8.70;N, 2.63; found: C, 80.73; H, 8.60; N, 2.79.
4.2.8. 5-(8-(Diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluoren-2-yl)furan-2-carbaldehyde(6b). To a flame dried two neck flask fitted with condenser chargedwith 4 (0.50 g, 0.53 mmol), (5-(1,3-dioxolan-2-yl)furan-2-yl)tribu-tylstannane (0.340 g,0.79 mmol), Pd(PPh3)2Cl2 (8.0 mg,0.010 mmol)and dry DMF 15 mL was added under N2 atmosphere. The mixturewas heated to 80 �C for 24 h. After completion the reaction mixturewas cooled and extracted with ether/brine, and the organic layerswere combined and dried over Na2CO3. Evaporation of the solventgave crude dioxolane derivative.
The resulting dioxolane derivative was suspended in glacialacetic acid and heated to 50 �C. After a clear solution is formed,1 mL of water was added and the temperature of the mixture wasmaintained at 50 �C for 5e6 h. It was then cooled and poured intocrushed ice, orange precipitate formed was formed, filtered andwashed with H2O. The aldehyde product 6bwas further purified byflash chromatography using CH2Cl2/hexane (60:40 by vol) as theeluent to form an orange solid (326 mg, 64%). 1H NMR (acetone-d6,400 MHz) d 0.50e1.10 (m, 60H), 1.94e2.01 (m, 2H), 2.18e2.28 (m,6H), 7.05e7.16 (m, 7H), 7.21e7.28 (m, 2H), 7.31e7.37 (m, 4H),7.59e7.60 (m, 1H), 7.82e7.88 (m, 1H), 7.93e8.04 (m, 4H), 8.10e8.15(m, 1H), 9.71 (s, 1H). 13C NMR (chloroform-d1, 100 MHz) d 10.07,10.38, 10.52, 10.80, 10.89, 13.91, 14.20, 23.11, 23.19, 23.33, 27.42,27.59, 27.65, 27.74, 27.85, 28.05, 28.55, 28.78, 34.11, 34.37, 34.46,34.78, 34.92, 35.29, 35.35, 44.59, 44.80, 44.92, 45.31, 45.45, 55.32,55.39, 108.20, 115.61, 115.66, 116.37, 120.48, 121.13, 121.51, 121.63,121.68, 123.09, 123.19, 123.27, 124.11, 124.29, 124.41, 124.91, 129.87,139.82, 141.81, 144.34, 147.45, 148.81, 150.88, 152.90, 177.29. FABMS(m/z): 963 (Mþ).
4.2.9. 2-Cyano-3-(5-(8-(diphenylamino)-6,6,12,12-tetrakis(2-ethylhexyl)-6,12-dihydroindeno[1,2-b]fluoren-2-yl)furan-2-yl)acrylicacid (SC-3). A mixture of 6b (250 mg, 0.26 mmol), cyanoacetic acid(33 mg, 0.34 mmol), ammonium acetate (6 mg), and acetic acid(10 mL) was heated at 100 �C for 12 h. The solution was cooled byusing crushed ice, the resulting solid was filtered, washed thor-oughly with water, dried, and purified through flash chromatog-raphy using 2% methanol in dichloromethane and further re-precipitated from dichloromethane by pouring into methanol togive a red solid (190 mg, 72%). 1H NMR (chloroform-d1, 400 MHz):d 0.50e1.10 (m, 60H), 1.80e1.90 (m, 2H), 2.00e2.20 (m, 6H),6.96e7.12 (m, 3H), 7.05e7.13 (m, 6H), 7.20e7.26 (m, 5H), 7.58e7.64(m, 3H), 7.77 (d, J¼8.0 Hz, 1H), 7.87 (d, J¼8.0 Hz, 1H), 7.92 (s, 1H),8.04 (s, 1H). 13C NMR (chloroform-d1, 100 MHz): d 10.06, 10.40,10.53, 10.79, 10.88, 14.11, 14.34, 22.93, 22.99, 23.13, 23.21, 26.57,26.97, 27.18, 27.33, 27.38, 27.55, 28.18, 28.25, 28.45, 29.01, 33.56,33.66, 33.78, 33.98, 34.03, 34.39, 34.82, 34.92, 44.76, 44.88, 54.79,54.83, 54.98, 95.03, 109.16, 114.84, 115.63, 115.96, 119.93, 120.49,121.21, 121.53, 122.52, 122.59, 123.82, 123.97, 124.56, 124.98, 126.09,129.36, 138.95, 139.16, 141.50, 141.56, 144.54, 146.92, 147.76, 148.32,150.36, 150.41, 151.08, 152.13, 152.81, 152.90, 162.31, 168.31. HRMS(FAB)m/z [Mþ] calcd for C72H90N2O3: 1031.7030; found: 1031.7063.Anal. Calcd for C72H90N2O3: C, 83.84; H, 8.79; N 2.72; found: C,83.45; H, 8.96; N, 2.37.
4.3. Devices fabrication
The photoanode used was the TiO2 thin film (12 mm of 20 nmparticles as the absorbing layer and 6 mm of 400 nm particles as thescattering layer) coated on FTO glass substrate with a dimension of0.5�0.5 cm2.26 The film thickness measured by a profilometer(Dektak3, Veeco/Sloan Instruments Inc., USA). A platinized FTOproduced by thermopyrolysis of H2PtCl6 was used as a counter
electrode. The TiO2 thin film was dipped into the THF solutioncontaining 3�10�4 M dye sensitizers for at least 12 h. After rinsingwith THF, the photoanode adhered with a polyester tape of 60 mmin thickness and with a square aperture of 0.36 cm2 was placed ontop of the counter electrode and tightly clipping them together toform a cell. Electrolyte was then injected into the space and thensealing the cell with the Torr Seal cement (Varian, MA, USA). Theelectrolyte was composed of 0.5 M lithium iodide (LiI), 0.05 M io-dine (I2), and 0.5 M 4-tert-butylpyridine that was dissolved inacetonitrile.
4.4. Quantum chemistry computation
The computation were performed with Q-Chem 4.0 software.27
Geometry optimization of the molecules were performed usinghybrid B3LYP functional and 6-31G* basis set. For each molecule,a number of possible conformations were examined and the onewith the lowest energy was used. The same functional was alsoapplied for the calculation of excited states using time-dependentdensity functional theory (TD-DFT). There exist a number of pre-vious works that employed TD-DFT to characterize excited stateswith charge-transfer character.28 In some cases underestimation ofthe excitation energies was seen.29 Therefore, in the present work,we use TD-DFT to visualize the extent of transition moments aswell as their charge-transfer characters, and avoid drawing con-clusions from the excitation energy.
Acknowledgements
We thank the Institute of Chemistry, Academia Sinica and Na-tional Science Council, Taiwan, for financial support.
Supplementary data
This material can be found in the online version. Containedwithin are the details of quantum chemistry computation and the1H and 13C NMR spectra. Supplementary data related to this articlecan be found online at http://dx.doi.org/10.1016/j.tet.2012.07.045.
References and notes
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