Triazine-branched mono- and dianchoring organic dyes: Effect of acceptor arms on optical and photovoltaic properties

Accepted Manuscript Triazine-branched mono- and dianchoring organic dyes: Effect of acceptor arms on optical and photovoltaic properties K.R. Justin Thomas, Addanki Venkateswararao, Rajendiran Balasaravanan, ChunTing Li, Kuo-Chuan Ho PII:

S0143-7208(18)32556-7

DOI:

https://doi.org/10.1016/j.dyepig.2019.02.013

Reference:

DYPI 7347

To appear in:

Dyes and Pigments

Received Date: 20 November 2018 Revised Date:

6 February 2019

Accepted Date: 9 February 2019

Please cite this article as: Thomas KRJ, Venkateswararao A, Balasaravanan R, Li C-T, Ho K-C, Triazine-branched mono- and dianchoring organic dyes: Effect of acceptor arms on optical and photovoltaic properties, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.02.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Triazine-Branched Mono- and Dianchoring Organic Dyes: Effect of Acceptor Arms on Optical and Photovoltaic Properties and Kuo-Chuan Ho# †

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K. R. Justin Thomas,*† Addanki Venkateswararao,† Rajendiran Balasaravanan,† Chun-Ting Li#

Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247 667, India

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

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#

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*Corresponding author. E-mail: [email protected]. Web: http://faculty.iitr.ac.in/~krjt8fcy/. Abstract

Three triazine-cored organic dyes containing triarylamine donor and cyanoacrylic acid acceptor were synthesized and used as sensitizers in dye-sensitized solar cells. Mono- and dianchoring

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dyes were designed by varying the number of donor/acceptor arms and conjugation length of the acceptor arm to study the effect of molecular structure on charge transfer characteristics and photovoltaic performance. They exhibited intense longer wavelength absorption with high molar

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extinction coefficients tunable depending on the number of acceptor arms and the nature of its composition. The electron accepting triazine linker favorably alters the energies of the frontier

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molecular orbitals which facilities the electron injection into the conduction band of TiO2 and dye regeneration by the electrolyte. A dye containing two donor arms and a acceptor composed of bithiophene spacer displayed promising power conversion efficiency (4.29%) in the series owing to high photocurrent density and open circuit voltage arising from the comparatively low charge transfer resistance and high recombination resistance in the device.

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Keywords: Triazine; triarylamine; organic dyes; donor-acceptor compounds; optical properties; dye-sensitized solar cells.

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1. Introduction Dye sensitized solar cells (DSSCs) also termed as Grätzel cells are fabricated using organic dyes as sensitizers are attractive due to their structural simplicity, cheap ingredients and tunable

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efficiency by chemical modification of sensitizers. [1] The power conversion efficiency (PCE) of DSSC is mainly dependent on the functional properties of the sensitizer. Besides their absorption

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properties, their three-dimensional structure also influence the device outcome. Functional and structural optimization of organic dyes have been pursued as a major research theme in the recent years in an attempt to realize high PCE in DSSC. [2] Various light absorbers such as Ru-, [3] Zn-, [4] Pb-complexes, [5] quantum dots [6], perovskites and organic dyes [7] were

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demonstrated as efficient sensitizers. Particularly, organic dyes are attractive than the other sensitizers due to their high molar extinction coefficients in visible region, negligible toxicity, low cost, simple and facile synthetic and purification pathways and versatile functional response

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for chemical modifications. [8] Organic dyes enjoy high molar extinction coefficients in visible region and structural diversity to tune functional properties such as optical and electrochemical

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properties. [2] Functional optimization of organic dyes were performed by adoption of several strategies such as (1) incorporation of star-shaped molecular geometry to impede aggregation [2e,9]; (2) introduction of electron-rich planar linker or auxiliary donor to manipulate donoracceptor interactions and to realize intense absorption [10,11] (3) addition of auxiliary acceptor at the donor or acceptor side to push absorption to near-IR (NIR) region [2d,12] (4) tethering auxiliary donor on the donor or linker to suppress back electron transfer [13] (5) alkyl chains on donor site or linker to increase recombination resistance [14] (6) multi-anchoring units to 2

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suppress the charge recombination between the injected electrons and dye cations. [15] Dyes containing multi-anchoring sites present advantages such as multiple electron transfer pathways, better light-harvesting and improved stability than the mono-anchoring dyes. [16]

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Another important factor is electron recombination, which severely deteriorates the PCE of DSSC. Modification of organic dyes by introducing new functional units is essential to suppress the recombination losses and boost the PCE. Dyes containing bulky donors with alkoxy units

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were found to efficiently suppress dye aggregation and charge recombination resulting in high PCE. [17] However, they suffered due to low-lying highest occupied molecular orbital (HOMO)

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which requires cobalt electrolytes to accelerate dye regeneration. Incorporation of imidazole, phenanthroimidazole or benzimidazole on donor unit led to high PCE arising from photocurrent density and retardation of back electron transfer. [18] Zheng and co-workers used heterocylic auxiliary chromophores on donor and found it helps to red shifted absorption and stabilize the

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oxidized dye by delocalization of positve charge. [19] Fluorenylidine and imidazole units have also been demonstrated as successful auxiliary chromophore. [20] Lin and coworkers found suppression of recombination and enhancement of short circuit current (JSC) in di-anchoring

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dyes. [21] Wang and coworkers reported nearly double PCE for cross-shaped dianchoring dyes when compared with corresponding mono-anchoring dye due to significant improvement of

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electron life time and open circuit voltage. [22] Recently, Cao and coworkers also made a similar observation with phenothiazine-based di-anchoring dyes, which exhibited better JSC and opencircuit voltage (VOC) when compared to mono-anchoring analogues. [23] Triazine-based organic materials have been extensively used as electron transport or bipolar materials in organic light emitting diodes (OLED), [24] thermally active delayed fluorescence (TADF) emitter, [25] donors in bulk hetero junction solar cells (BHJ) [26] and building blocks in

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covalent organic frameworks (COFs) [27]. It can be easily synthesized by trimerization of nitriles [28] and the alteration of π-conjugate arms at 2,4,6 positions of triazine core which are most commonly substituted with phenyl, [29] thiophene, [30] pyridine [31] and fluorene [32]

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units. The introduction of thiophene instead of phenyl favors strong delocalization of electrons throughout the conjugation and benefits the optical properties. In 2004, Tan et al. first time synthesized fluorene-substituted triazine and demonstrated to possess large two-photon

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absorption. [33] Later, Li et al. reported the synthesis of triazine with phenyl linker based sensitizers and utilized in dye-sensitized solar cells to achieve PCE of 1.81%. Replacement of

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phenyl with thiophene enhanced the PCE to 3.69%. [34] It is interesting to note that π-spacer in triazine derivatives plays a vital role to improve the JSC and PCE. It is our hypothesis that the incorporation of fluorene and thiophene on the triazine core may help to extend the light harvesting properties. A new set of star-shaped triazine dyes possessing triarylamine donor,

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fluorene conjugation and cyanoacrylic acid acceptor were designed and synthesized (Fig. 1). The dyes exhibited visible region absorption with high molar extinction coefficients (> 96000 M-1cm1

). A dye with bithiophene unit in conjugation showed high PCE (4.29%) in the series

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attributable to good absorption properties. The reason for the enhancement in PCE is scrutinized

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by electrochemical impedance spectroscopy measurements.

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Fig. 1. Structures of the triazine-based dyes. 2. Experimental Section

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2.1. General methods and materials Chemicals were purchased from commercial sources and used without further purification. Solvents were dried using standard procedures prior to use. Column chromatography purification was performed with silica gel (230-400 mesh) as a stationary phase in a column of 40.0 cm long

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C NMR spectra were recorded in a Bruker (Bruker

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nicolet) by using KBr pellets. The 1H and

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and 3.0 cm diameter. The IR spectra were recorded on a NEXUS FT-IR spectrometer (Thermo

Avance 3) spectrometer operating at 500.13 and 125.77 MHz respectively. Deuterated chloroform (CDCl3) and dimethyl sulfoxide (DMSO-d6) were used as solvent with their residual peak as chemical shift reference at δ 7.26 and 2.52 ppm for 1H; 77.0 and 39.5 ppm for

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C,

respectively. UV-Vis spectra were recorded in toluene (Tol), dichloromethane (DCM), N,N-

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dimethyl formamide (DMF), acetonitrile (ACN), tetrahydrofuran (THF) at room temperature in quartz cuvettes using a Cary spectrophotometer (Cary 100). Emission spectra were recorded on a Shimadzu spectrofluorophotometer at room temperature. The cyclic voltammetry (CV) and

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differential pulse voltammetry (DPV) were performed on epsilon (Basi epsilon) electrochemical

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analyzer using a glassy carbon working electrode, a non-aqueous Ag/AgNO3 reference electrode. The experiments were performed at room temperature under nitrogen atmosphere in dichloromethane, Bu4NClO4 as supporting electrolyte (0.1 M). The high-resolution mass spectra were obtained from a Bruker Daltoniks GmBH (micrOTOF-QII) ESI mass spectrometer in the positive ion mode. 2.2. Synthesis

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7-Bromo-9,9-dipropyl-9H-fluorene-2-carbonitrile [35] and 2,4,6-tris(7-bromo-9,9-dipropyl9H-fluoren-2-yl)-1,3,5-triazine [32] were prepared according to literature procedures.

7-Bromo-9,9-dipropyl-9H-fluorene-2-carbonitrile

(14.17

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2.2.1. 2,4,6-Tris(7-bromo-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazine, (1) g,

40

mmol)

and

trifluoromethanesulfonic acid (6.00 g, 40 mmol) were dissolved in dichloromethane (40 mL).

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The reaction mixture was stirred at 0 °C for 1 h and then at room temperature for 24 h. The organic soluble content was extracted with dichloromethane, dried over anhydrous sodium

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sulfate and evaporated to leave the crude product. Finally, the crude product was recrystallized from methanol to obtain pure compound. White solid; Yield: 10.76 g (76%); mp 122-124 °C; 1H NMR (CDCl3, 500.13 MHz) δ 8.83 (d, J = 8.0 Hz, 3H), 8.74 (s, 3H), 7.91 (d, J = 8.0 Hz, 3H), 7.69 (d, J = 8.0 Hz, 3H), 7.57 (s, 3H), 7.53 (d, J = 8.0 Hz, 3H), 2.13-2.18 (m, 6H), 2.01-2.06 (m, 13

C NMR (CDCl3, 125.77 MHz) δ 172.1, 154.5, 151.2, 144.8, 139.6,

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6H), 0.81-0.73 (m, 30H);

136.0, 130.6, 129.7, 128.9, 126.8, 123.6, 122.2, 121.9, 120.3, 56.1, 42.9, 17.6, 14.8; HRMS calcd for C60H60Br3N3 [M]+ m/z 1059.2331, found 1059.2314.

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General method for C-N coupling

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A mixture of 1 (10.61 g, 10 mmol), diphenylamine (2, 3.72 g, 22 mmol), Pd(dba)2 (0.11 g, 0.22 mmol), dppf (0.12 g, 0.22 mmol), sodium tert-butoxide (5.28 g, 55 mmol), and toluene (25 mL) was taken in a pressure tube and heated at 80 °C for 48 h under N2 atmosphere. After completion of the reaction, the volatiles were removed under vacuum, and the resulting solution extracted with dichloromethane (3 × 60 mL). The combined organic extract was washed with brine solution, dried over Na2SO4, and concentrated to produce a yellow solid. Further, the crude

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product was purified by silica gel column chromatography by using hexanes/dichloromethane mixture (2:1) as an eluent. 7-(4,6-Bis(7-bromo-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazin-2-yl)-N,N-diphenyl-9,9-

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2.2.2.

dipropyl-9H-fluoren-2-amine (3a)

Yellow solid; Yield: 3.10 g (27%); mp 135-137 °C; 1H NMR (CDCl3, 500.13 MHz) δ 8.80-

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8.84 (m, 3H), 8.72 (dd, J = 4.5 Hz, J = 1.0 Hz, 2H), 7.83 (d, J = 8.0 Hz, 2H), 7.66-7.69 (m, 2H), 7.27-7.30 (m, 6H), 7.17-7.18 (m, 5H), 7.16 (s, 3H), 7.04-7.08 (m, 5H), 2.01-2.13 (m, 8H), 1.89-

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1.94 (m, 4H), 0.77-0.85 (m, 12H), 0.73 (t, J = 7.0 Hz, 18H); 13C NMR (CDCl3, 125.77 MHz) δ 172.16, 172.12, 154.5, 153.8, 152.3, 151.5, 151.1, 148.6, 148.2, 145.9, 144.7, 143.4, 140.6, 139.6, 136.1, 135.9, 135.4, 130.6, 129.7, 129.6, 128.8, 128.6, 128.4, 127.3, 126.8, 124.5, 123.6, 123.4, 123.2, 122.6, 122.2, 121.6, 121.3, 120.9, 120.2, 119.5, 119.1, 118.1, 56.1, 55.8, 55.7, 43.0,

found 1148.3977.

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42.9, 17.7, 17.60, 17.55, 14.89, 14.86, 14.8; HRMS calcd for C72H70Br2N4 [M]+ m/z 1148.3961,

2.2.3. 7,7'-(6-(7-Bromo-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazine-2,4-diyl)bis(N,N-diphenyl-

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9,9-dipropyl-9H-fluoren-2-amine) (3b)

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Yellow solid; Yield: 3.59 g (29%); mp 164-166 °C; 1H NMR (CDCl3, 500.13 MHz) δ 8.808.84 (m, 3H), 8.75-8.77 (m, 2H), 8.71-8.73 (m, 1H), 7.93-7.95 (m, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.84 (d, J = 7.5 Hz, 2H), 7.67-7.69 (m, 2H), 7.57 (s, 1H), 7.52 (d, J = 8.0 Hz, 1H), 7.43-7.45 (m, 1H), 7.39-7.41 (m, 2H), 7.27-7.31 (m, 7H), 7.25 (s, 2H), 7.17 (d, J = 9.0 Hz, 4H), 7.04-7.09 (m, 6H), 6.93 (t, J = 7.5 Hz, 2H), 2.12-2.19 (m, 4H), 2.00-2.09 (m, 6H), 1.89-1.95 (m, 2H), 0.74-0.85 (m, 30H); 13C NMR (CDCl3, 125.77 MHz) δ 172.16, 172.12, 172.06, 171.9, 154.5, 153.8, 151.4, 151.3, 151.1, 148.6, 148.5, 148.2, 145.8, 145.82, 144.75, 140.7, 139.7, 136.2, 135.42, 135.37, 7

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134.8, 134.7, 130.5, 129.6, 128.8, 128.4, 127.3, 126.7, 124.5, 123.6, 123.4, 123.1, 121.5, 120.2, 119.5, 119.1, 56.1, 55.8, 55.7, 43.0, 42.9, 17.7, 17.6, 17.6, 14.9, 14.8; HRMS calcd for

2.2.4.

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C84H80BrN5 [M + Na]+ m/z 1260.5489, found 1260.5497. 5-(7-(4,6-Bis(7-(diphenylamino)-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazin-2-yl)-9,9-

dipropyl-9H-fluoren-2-yl)thiophene-2-carbaldehyde (4a)

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A mixture of 3b (0.62 g, 0.5 mmol), (5-(1,3-dioxolan-2-yl)thiophen-2-yl)tributylstannane (0.24 g, 0.6 mmol) and dry DMF (5 mL) was maintained under nitrogen atmosphere and

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Pd(PPh3)2Cl2 (0.01 g, 0.01 mmol) added to it. The resulting reaction mixture was heated at 80 °C for 15 h. After the completion of the reaction, it was poured into cold water and extracted with dichloromethane (3 × 40 mL). Removal of volatiles from the dichloromethane extract by rotary evaporation produced a solid residue. It was dissolved in glacial acetic acid (4 mL) and heated at

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60 °C for 1 h to form a clear solution. After the addition of water (3 mL) it was maintained at 60 °C for additional 1 h. At the end, the reaction was extracted with dichloromethane (3 × 40 mL). The dichloromethane extract was washed thoroughly with brine solution and dried over

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anhydrous sodium sulfate. The solid residue obtained on evaporation of the dichloromethane extract was purified by silica gel column chromatography using hexanes/dichloromethane

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mixture (1:1) as an eluent. Yellow solid; Yield: 0.52 g (82%); mp 177-179 °C; IR (KBr, cm-1) 1667 (νC=O); 1H NMR (CDCl3, 500.13 MHz) δ 9.93 (s, 1H), 8.85 (dd, J = 8.0 Hz, J = 1.5 Hz , 1H), 8.81 (dd, J = 8.0 Hz, J = 1.5 Hz, 2H), 8.78 (s, 1H), 8.72 (d, J = 1.0 Hz, 2H), 7.95 (d, J = 8.0 Hz, 1H), 7.87 (d, J = 7.5 Hz, 1H), 7.83 (d, J = 8.0 Hz, 2H), 7.79 (d, J = 4.0 Hz, 1H), 7.72-7.75 (m, 2H), 7.67 (d, J = 2.0 Hz, 2H), 7.53 (d, J = 4.0 Hz, 1H), 7.29 (t, J = 7.5 Hz, 8H), 7.16-7.18 (m, 10H), 7.08 (d, J = 2.0 Hz, 1H), 7.05-7.08 (m, 4H), 7.04 (s, 1H), 2.17-2.23 (m, 2H), 2.07-2.13 (m, 6H), 1.89-1.95 (m, 4H), 0.79-0.87 (m, 12H), 0.72-0.76 (m, 20H); 13C NMR (CDCl3, 125.77 8

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MHz) δ 183.1, 172.2, 171.9, 155.2, 153.8, 153.4, 151.9, 151.3, 148.6, 148.2, 145.9, 144.6, 142.6, 142.2, 137.9, 136.4, 135.4, 134.6, 133.0, 129.6, 128.8, 127.5, 126.0, 124.5, 123.7, 123.4, 123.2,

C89H83N5OS [M]+ m/z 1269.6312, found 1269.6319. 2.2.5.

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121.6, 121.2, 120.5, 119.5, 119.1, 56.0, 55.7, 43.0, 42.9, 17.7, 17.6, 14.9, 14.8; HRMS calcd for

5'-(7-(4,6-Bis(7-(diphenylamino)-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazin-2-yl)-9,9-

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dipropyl-9H-fluoren-2-yl)-2,2'-bithiophene-5-carbaldehyde (4b)

It was synthesized from 3b (0.62 g, 0.5 mmol) and (5-(5-(1,3-dioxolan-2-yl)thiophen-2-

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yl)thiophen-2-yl)tributylstannane (0.29 g, 0.6 mmol) by following a procedure similar to that described above for 4a. Orange solid; Yield (0.41 g, 60%); mp 190-192 °C; IR (KBr, cm-1) 1665 (νC=O); 1H NMR (CDCl3, 500.13 MHz) δ 9.88 (s, 1H), 8.79-8.86 (m, 3H), 8.78 (s, 1H), 8.72 (s, 2H), 7.93-7.96 (m, 1H), 7.83-7.85 (m, 3H), 7.72 (d, J = 6.5 Hz, 1H), 7.67-7.71 (m, 3H), 7.65 (s,

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1H), 7.40 (s, 2H), 7.36-7.37 (m, 1H), 7.27-7.31 (m, 8H), 7.16-7.18 (m, 10H), 7.04-7.08 (m, 6H), 2.18-2.24 (m, 2H), 2.07-2.11 (m, 6H), 1.89-1.95 (m, 4H), 0.83 (t, J = 7.0 Hz, 12H), 0.75 (t, J = 7.0 Hz, 18H); 13C NMR (CDCl3, 125.77 MHz) δ 182.9, 172.2, 172.0, 153.9, 151.7, 151.3, 148.6,

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148.2, 147.6, 147.1, 145.8, 114.9, 142.0, 141.1, 137.8, 136.2, 135.4, 134.8, 133.5, 128.8, 128.7, 127.6, 127.4, 126.5, 125.4, 124.7, 124.60, 124.56, 124.4, 123.51, 123.47, 123.2, 121.6, 121.5,

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119.5, 119.2, 56.0, 55.8, 55.73, 55.65, 55.6, 43.0, 42.9, 17.9, 17.8, 17.7, 14.90, 14.88; HRMS calcd for C93H85N5OS2 [M]+ m/z 1351.6190, found 1351.6195. 2.2.6.

5,5'-(7,7'-(6-(7-(Diphenylamino)-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazine-2,4-

diyl)bis(9,9-dipropyl-9H-fluorene-7,2-diyl))dithiophene-2-carbaldehyde (4c) It was synthesized from 3a (0.58 g, 0.5 mmol) (5-(1,3-dioxolan-2-yl)thiophen-2yl)tributylstannane (0.48 g, 1.2 mmol) by following a procedure similar to that described above 9

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for 4a. Yellow solid; Yield (0.44 g, 72%); mp 142-144 °C; IR (KBr, cm-1) 1659 (νC=O); 1H NMR (CDCl3, 500.13 MHz) δ 9.93 (s, 1H), 9.92 (s, 1H), 8.86 (dd, J = 8.0 Hz, J = 1.5 Hz, 2H), 8.82 (dd, J = 8.0 Hz, J = 1.5 Hz, 1H), 8.79 (s, 2H), 8.73 (d, J = 1.0 Hz, 2H), 7.97 (d, J = 8.0 Hz, 1H),

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7.88 (d, J = 7.5 Hz, 1H), 7.84 (d, J = 8.0 Hz, 1H), 7.80 (d, J = 4.0 Hz, 2H), 7.54 (d, J = 1.5 Hz, 1H), 7.74 (d, J = 2.0 Hz, 2H), 7.72-7.73 (m, 3H), 7.68 (d, J = 8.0 Hz, 1H), 7.53 (d, J = 4.0 Hz, 2H), 7.43 (d, J = 4.0 Hz, 2H), 7.27-7.31 (m, 4H), 7.16-7.19 (m, 4H), 7.04-7.09 (m, 3H), 2.18-

Hz, 18H);

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2.22 (m, 2H), 2.09-2.14 (m, 6H), 1.90-1.96 (m, 2H), 0.80 (t, J = 2.0 Hz, 12H), 0.75 (t, J = 6.5 C NMR (CDCl3, 125.77 MHz) δ 182.9, 182.8, 172.2, 172.0, 153.8, 153.4, 151.7,

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151.3, 148.6, 148.2, 147.5, 147.1, 145.8, 144.9, 141.9, 141.1, 137.7, 137.6, 136.1, 134.7, 133.5, 129.6, 128.8, 128.7, 127.6, 127.4, 126.5, 125.4, 124.7, 124.58, 124.55, 124.50, 124.4, 123.7, 123.49, 123.45, 123.4, 123.21 121.54 121.4, 120.53, 120.50, 120.3, 119.5, 119.1, 56.0, 55.7, 43.0, 42.9, 17.9, 17.8, 17.73, 17.66, 14.88, 14.86; HRMS calcd for C82H76N4O2S2 [M + Na]+ m/z

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1235.5301, found 1235.5314.

2.2.7. (E)-3-(5-(7-(4,6-Bis(7-(diphenylamino)-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazin-2-yl)-

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9,9-dipropyl-9H-fluoren-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (T1) The aldehyde 4a (0.25 g, 0.20 mmol), 2-cyanoacetic acid (0.03 g, 0.4 mmol), ammonium

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acetate (0.02 g, 0.20 mmol), and acetic acid (5 mL) were mixed and refluxed for 24 h. The resulting red precipitate was filtered and washed several times with water and dried. It was crystallized from dichloromethane/hexanes mixture to obtain the analytically pure sample. Yellow solid. Yield: 0.24 g (91%); mp 270-272 °C; IR (KBr, cm-1) 2216 (νC≡N); 1H NMR (CDCl3, 500.13 MHz) δ 8.86 (dd, J = 7.5 Hz, J = 1.5 Hz, 1H), 8.79-8.83 (m, 3H), 8.72 (s, 2H), 8.40 (s, 1H), 7.96-7.98 (m, 1H), 7.87-7.89 (m, 1H), 7.83-7.85 (m, 3H), 7.78 (dd, J = 8.0 Hz, J = 2.0 Hz, 1H), 7.74 (d, J = 3.5 Hz, 1H), 7.67-7.68 (m, 2H), 7.57-7.58 (m, 1H), 7.27-7.29 (m, 8H), 10

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7.16 (d, J = 8.0 Hz, 10H), 7.04-7.08 (m, 6H), 2.20-2.26 (m, 2H), 2.07-2.14 (m, 6H), 1.90-1.95 (m, 4H), 0.83 (t, J = 7.0 Hz, 12H), 0.75 (t, J = 6.5 Hz, 18H); 13C NMR (CDCl3, 125.77 MHz) δ 172.2, 171.9, 156.4, 156.1, 153.9, 153.6, 152.0, 151.4, 148.6, 148.2, 145.9, 142.6, 140.4, 137.8,

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137.3, 136.7, 136.6, 136.5, 135.7, 135.6, 135.4, 135.1, 134.7, 132.6, 130.5, 129.6, 128.8, 126.2, 124.6, 123.52, 123.47, 123.2, 121.59, 121.57, 121.2, 120.6, 119.4, 119.2, 56.2, 55.7, 43.0, 42.9, 17.8, 17.7, 17.7, 14.9, 14.8; HRMS calcd for C92H84N6O2S [M + Na]+ m/z 1359.6268, found

(E)-3-(5-(5-(7-(4,6-Bis(7-(diphenylamino)-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazin-2-

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2.2.8.

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1359.6281.

yl)-9,9-dipropyl-9H-fluoren-2-yl)thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (T2) It was prepared form 4b (0.28 g, 0.20 mmol) by following a procedure described above for T1. Red solid. Yield: 0.24 g (83%); mp 245-247 °C; IR (KBr, cm-1) 2219 (νC≡N); 1H NMR

TE D

(CDCl3, 500.13 MHz) δ 8.74-8.80 (m, 6H), 8.51 (s, 1H), 8.11-8.12 (m, 1H), 8.00 (s, 4H), 7.92 (d, J = 1.0 Hz, 1H), 7.84-7.85 (m, 2H), 7.70 (d, J = 1.0 Hz, 2H), 7.31-7.33 (m, 8H), 6.98-7.14 (m, 18H), 2.14-2.15 (m, 4H), 2.02-2.04 (m, 4H), 1.88 (m, 4H), 0.66-0.86 (m, 30H);

13

C NMR

EP

(CDCl3, 125.77 MHz) δ 172.2, 172.0, 171.9, 153.8, 151.7, 151.3, 148.6, 148.2, 147.6, 147.1, 145.8, 144.9, 141.9, 141.1, 137.7, 137.6, 136.1, 135.4, 134.7, 130.0, 128.8, 128.7, 127.6, 127.4,

AC C

126.5, 124.7, 124.6, 124.4, 123.7, 123.6, 123.5, 123.46, 123.43, 123.2, 121.5, 121.49, 121.44, 121.42, 120.53, 120.51, 120.3, 119.5, 119.1, 56.0, 55.8, 43.03, 42.96, 42.90, 17.74, 17.67, 14.89, 14.87; HRMS calcd for C96H86N6O2S2 [M + Na]+ m/z 1441.6145, found 1441.6167. 2.2.9.

(2E,2'E)-3,3'-(5,5'-(7,7'-(6-(7-(Diphenylamino)-9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-

triazine-2,4-diyl)bis(9,9-dipropyl-9H-fluorene-7,2-diyl))bis(thiophene-5,2-diyl))bis(2cyanoacrylic acid) (T3)

11

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It was prepared form 4c (0.24 g, 0.20 mmol) by following a procedure described above for T1. Yellow solid. Yield: 0.23 g (85%); mp 245-247 °C; IR (KBr, cm-1) 2215 (νC≡N); 1H NMR (CDCl3, 500.13 MHz) δ 8.76-8.85 (m, 6H), 8.38-8.40 (m, 1H), 8.18 (s, 1H), 8.12-8.18 (m, 5H),

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7.84-7.89 (m, 4H), 7.72 (d, J = 2.5 Hz, 2H), 7.16-7.34 (m, 4H), 7.08-7.09 (m, 5H), 7.00 (dd, J = 3.0 Hz, J = 1.0 Hz, 1H), 2.18-2.19 (m, 8H), 2.04-2.09 (m, 2H), 1.90-1.93 (m, 2H), 0.69-0.89 (m, 30H); 13C NMR (CDCl3, 125.77 MHz) δ 172.2, 171.9, 156.8, 153.9, 153.6, 152.0, 151.4, 149.0,

SC

148.3, 148.2, 145.9, 144.5, 142.7, 140.6, 140.5, 136.6, 135.4, 135.0, 134.7, 132.5, 129.6, 128.9, 128.8, 126.3, 125.1, 124.6, 123.5, 123.5, 123.3, 121.62, 121.57, 121.2, 120.6, 119.5, 119.1, 56.2,

1347.5598, found 1347.5579. 2.3. Computational methods

M AN U

55.7, 42.98, 42.94, 42.90, 17.8, 17.7, 14.9, 14.8; HRMS calcd for C88H78N6O4S2 [M + H]+ m/z

All the density functional theoretical calculations were performed using Gaussian 09

TE D

program package. The geometries of the dyes in the ground state were fully optimized without any symmetry constraints employing Becke’s hybrid correlation functional B3LYP [36] with 631g (d, p) basis set for all atoms. Vibrational analysis on the optimized structures was performed

EP

to confirm the structure. The excitation energies and oscillator strengths for the lowest ten singlet-singlet transitions were obtained by time-dependent density functional theory (TD-DFT)

AC C

at the same theoretical level. 2.4. Device fabrication and characterization The dye-sensitized solar cells were fabricated as described in our earlier paper. [20a] The photoanodes were composed of a 14 µm thick transparent TiO2 layer and a scattering layer of 4.5 µm thickness. The TiO2 film was immersed in a 3 × 10-4 M solution of dye in a solvent mixture containing ACN, tert-butyl alcohol and dimethyl sulfoxide (DMSO) (volume ratio of 1:1:3) at

12

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room temperature for 24 h to allow the adsorption of the dye. The illuminated area of the DSSC was restricted to 0.16 cm2 by using a mask.

RI PT

3. Results and discussion 3.1. Synthesis and characterization

The synthetic scheme of the target dyes (T1-T3) is displayed in Scheme 1. The synthesis of the dyes involves different protocols such as Hartwig–Buchwald C–N coupling [37] and Stille

SC

coupling [38] reactions. Cyclization of 7-bromo-9,9-dipropyl-9H-fluorene-2-carbonitrile [35] promoted by trifluoromethanesulfonic acid (CF3SO3H) in dichloromethane led to the formation

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of 2,4,6-tris(9,9-dipropyl-9H-fluoren-2-yl)-1,3,5-triazine (1). It was subjected to C–N coupling reaction with diphenylamine (2) to obtain a mixture of (3a) and (3b) in moderate yields. Stille coupling reaction of 3a and 3b with corresponding tin derivatives of thiophene or bithiophene followed by acidic hydrolysis gave the required aldehydes (4a-4c) in good yields. Finally, the

TE D

aldehydes were converted to the target dyes (T1-T3) via Knoevenagel condensation [39] in excellent yields. All these compounds were thoroughly examined by IR, NMR and HR Mass spectral methods and the data is consist with the proposed chemical structures. The dyes are

EP

orange to red in color and soluble in common organic solvents.

AC C

3.2. Photophysical properties

The absorption spectra of dyes recorded in DCM are displayed in Fig. 3 and pertinent data compiled in Table 1. All the dyes (T1-T3) showed two absorption peaks. Less intense shorter wavelength absorption (300 nm) may originate from the fluorenylamine chromophore while the longer wavelength absorption (~425 nm) from the acceptor arm including triazine unit. [40] The later absorption may contain a weak charge transfer component from donor to acceptor (vide supra). 13

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T1 T2 T3

-1

Molar Extinction Coefficient, M cm

-1

110000

RI PT

80000

20000

400

500

M AN U

300

SC

50000

600

Wavelength, nm

Fig. 2. Absorption spectra of the dyes recorded in dichloromethane. In order to find out the origin and nature of the peaks, the absorption spectra of the parent

TE D

triazine (1), amine (3a and 3b) and aldehyde derivatives (4a-4c) were recorded in dichloromethane. As the three arms in the triazine core are not expected to electronically interact with one another, the donor-acceptor interactions are largely restricted to amine and triazine. So

EP

the dominant chromophores in the dyes give rise to similar absorption. The dyes showed red shifted absorption when compared to their bromo and aldehyde precursors with the absorption

AC C

wavelength order, Br < aldehyde < dye (Table 1). This supports the enhancement of conjugation length or charge transfer propensity in the dyes due to the presence of cyanoacrylic acid segment. The absorption wavelength of all the dyes are similar, except for T2, which showed an additional shoulder at ~490 nm. This peak probably is originating due to the charge transfer from the πspacer to cyanoacrylic acid acceptor. It is interesting to note that organic dyes containing bithiophene segment showed additional oxidation confirming the electron richness of the conjugation pathway. [13] 14

TE D

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SC

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ACCEPTED MANUSCRIPT

AC C

EP

Scheme 1. Synthetic scheme of the dyes T1-T3.

15

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Table 1. Optical and electrochemical data of the dyes and precursors recorded in DCM

a

Ereda, V (∆EP, mV) ‒2.02 (75) ‒2.14 (148) ‒2.07 (122) ‒2.07 (59) ‒2.02 (111) ‒2.09 (53) ‒2.04 (42) ‒2.06 (42) ‒2.04 (58)

HOMO,b eV ‒6.11 ‒5.29 ‒5.29 ‒5.28 ‒5.29 ‒5.28 ‒5.27 ‒5.27 ‒5.28

LUMO, eV ‒2.78 ‒2.73 ‒2.66 ‒2.73 ‒2.78 ‒2.71 ‒2.76 ‒2.74 ‒2.76

c

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356 (95.9), 348 (92.7) 409 (31.8), 353 (72.2) 414 (72.3), 309 (66.9) 405 (87.6), 306 (50.0) 418 (115.5), 305 (54.8) 385 (116.0), 310 (35.3) 423 (96.5), 305 (54.1) 490 (sh), 423 (88.0), 302 (59.8) 429 (57.0), 354 (32.4), 303 (27.9)

Eoxa, V (∆EP, mV) 0.49 (105) 0.49 (86) 0.48 (82) 0.49 (81) 0.48 (58) 0.47 (73) 0.47 (74) 0.48 (56)

SC

1 3a 3b 4a 4b 4c T1 T2 T3

λabs, nm (εmax × 103 M‒1 cm-1)

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compd.

E0-0,d eV 3.33 2.56 2.63 2.55 2.51 2.57 2.51 2.53 2.52

Eox,e V (vs NHE) 1.26 1.26 1.25 1.26 1.25 1.24 1.24 1.25

Eox*,f V (vs NHE) -1.30 -1.37 -1.30 -1.25 -1.32 -1.27 -1.29 -1.27

Redox potentials vs ferrocene internal standard. b Deduced from the oxidation potential using the formula HOMO = ‒(4.8 + Eox). c Deduced using the

formula LUMO = ‒(4.8 ‒ Ered). d Calculated from HOMO and LUMO gap. e Ground state oxidation potential vs NHE by adding solvent correction of 0.77 V

AC C

EP

TE D

to EOX. f calculated from Eox* = Eox - E0-0.

16

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Molar Extinction Coefficient, M cm

-1

130000 Tol THF CHCl3

(a)

DCM DMF ACN

RI PT

70000

40000

10000 280

330

380

430

Wavelength, nm

530

580

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DCM DCM + TFA DCM + TEA

90000

(b)

60000

30000

TE D

-1

Molar Extinction Coefficient, M cm

-1

120000

480

SC

-1

100000

0

270

380

490

600

710

820

Wavelength, nm

EP

Fig. 3. Absorption spectra of the dye T2 recorded in (a) different solvents and (b) in the presence of TFA and TEA.

AC C

The absorption spectra of the dyes were examined in solvents of different polarity to ascertain the interaction of the solvent with the dyes. All the dyes showed a slight blue shift for longer wavelength absorption in polar solvents such as THF and DMF owing interaction of solvent with carboxylic acid unit. [41, 42] A representative example of solvatochromism observed for the dye T2 is displayed in Fig. 3. For the dye T2, the shoulder peak completely disappeared in THF and DMF. This observation further corroborates the origin of the absorption; the charge transfer

17

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between electron-rich π-spacer comprising fluorenylbithiophene segment and cyanoacrylic acid unit. THF and DMF probably deprotonate carboxylic acid unit and diminish the charge transfer.

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[41,42] Interestingly on addition of TFA, orange color of the dye solutions turns to blue due to the formation of new peak at longer wavelength region. It is attributed to the charge transfer between

SC

the donor and protonated triazine unit. [43] To further elucidate the nature of the new peak, the absorption spectra of precursor (1), bromo and aldehyde derivatives in the presence of TFA were

M AN U

recorded. All compounds showed new red-shifted peak arising from charge transfer between triarylamine donor and protonated triazine unit. 1.1

0.9

TE D

Absorbance

0.7

T1 T2 T3

0.5

EP

0.3

(b)

AC C

0.1

400

450

500

550

600

650

Wavelength, nm

Fig. 4. Absorption spectra of the dyes anchored on TiO2 films.

The higher molar extinction coefficients of the dyes range from 57,000-97,500 M-1 cm-1, which is far superior to those of the ruthenium dyes, N3 and N719. [44] The dyes showed red shifted absorption and higher molar extinction coefficients when compared to known fluorene 18

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dyes [20a,45] containing similar donor and acceptor but lacks triazine unit. Some interesting observations are noteworthy. The dyes containing two donor arms (T1 and T2) exhibited high molar extinction coefficients suggesting that the absorption at 420 nm have more contribution

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from the donor arm. Similarly introduction of bithiophene broadens the absorption with additional shoulder at low energy region pointing the beneficial role of electron-richness close to the acceptor end. Further, the absorption spectra of all the dyes recorded thin film (Fig. 4)

SC

displayed slight red shift (36 nm to 69 nm) when compared to their solution spectra typical of most organic dyes. [46] The dye T2 showed most red shifted absorption in thin film when

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compared to other dyes indicating the elongated conjugation assisting the aggregation in the solid state. [47]

All the dyes (T1-T3) are weakly emitting in dichloromethane with green to yellow emission as shown in Fig. 5a. The excited state of the dyes are stabilized by polar solvents due to effective

TE D

solvation of the dye molecules in more polar solvents and resulted in longer wavelength emission. The representative solvatochromism is displayed in Fig. 5b for the dye T1. The solvent dependent photophysical parameters of the dyes were analyzed by Lippert-Mataga plot [48] and

EP

Stokes shift vs ET(30) parameter [49] correlation. The dye T1 displayed linear variation in the

AC C

plots as displayed in Fig. 6. This indicates that the dye T1 possess charge transfer excited state which is effectively solvated by polar solvents.

19

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1.1

T1 T2 T3

Emission Intensity

Emission Intensity

0.9

0.7

0.5

0.3

0.9

Tol THF CHCl3

0.7

DCM DMF ACN

0.5

0.3

0.1

0.1 450

500

550

600

650

430

700

510

Wavelength, nm

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1.1

590

670

750

SC

Wavelength, nm

Fig. 5. Emission spectra of the dyes (a) T1-T3 recorded in dichloromethane and (b) T1 recorded

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in different solvents. 9000

9000

DMF

-1

-1

DCM

R = 0.94 CHCl3 3000 Tol 1000

0.0

0.1

0.2

0.3

EP

-0.1

THF

TE D

Stokes Shift, cm

5000

DMF

7000

ACN

Stokes Shift, cm

7000

0.4

ACN

5000

DCM

THF

R = 0.95

CHCl3 3000 TOL 1000 30

35

40

45

50

ET(30) , Kcal/mol

Orientation Polarizability, ∆f

AC C

Fig. 6. Correlation of Stokes shift for dye T1 with (a) orientation polarizability and (b) ET(30) parameter of the solvents.

3.3. Electrochemical properties The cyclic voltammograms (Fig. 7) of the dyes and the precursors were recoded. All the compounds showed a quasi-reversible oxidation arising from the oxidation of triarylamine and a reduction originating from triazine unit. The dyes T1 and T2 have the same oxidation potential at

20

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0.47 V and dye T3 showed oxidation at 0.48 V due to two acceptors attached to central core. The precursor bromo and aldehyde derivatives oxidized at 0.49 V and 0.48 V respectively. The dyes undergo relatively facile oxidation attributable to the elongated conjugation pathway composed

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to electron rich thiophene unit. The lowest unoccupied molecular orbital (LUMO) of the dyes (T1-T3) are lowered and thermodynamically favorable to inject the electrons into the conduction

33

1 3a 3b

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Current (µ A)

14

-5

Fc

4a 4b 4c

Fc

T1 T2 T3

TE D

-24

-43

SC

band of TiO2.

1.0

0.1

-0.8

-1.7

-2.6

Potential vs Fc/Fc+ (V)

AC C

EP

Fig. 7. Cyclic voltammograms of the triazine compounds recorded in dichloromethane.

21

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Fig. 8. Energy level diagram of the materials used in the DSSCs.

TE D

Energy level diagram of the dyes, CB of TiO2 and redox electrolyte are displayed in Fig. 8. Ground state oxidation potential of all the dyes located at ~ 1.24 V and excited state potential of the dyes positioned at ~ -1.27 V with respect to NHE.[49] The LUMO of the dyes are more

EP

negative when compared to CB of TiO2 which helps to inject the electrons efficiently. All the dyes have more positive oxidation potentials than the redox electrolyte, which facilitates the

AC C

regeneration of the dye after injection of electron. [50] Thus, the dyes (T1-T3) possess favorable energy levels for electron injection and dye regeneration process. The incorporation of fluorene help to raise the LUMO when compared to phenyl-substituted triazine based dyes and favor electron injection into the conduction band of TiO2. T1

T2

22

T3

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LUMO+3

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LUMO+2

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SC

LUMO+1

LUMO

AC C

EP

HOMO-1

TE D

HOMO

HOMO-2

Fig. 9. Electronic distributions in the frontier molecular orbitals of the dyes T1-T3.

23

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3.4. Theoretical calculations

To gain more insight into the electronic transitions of the dyes, we have performed the

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density functional (DFT) theoretical calculations. The electronic distributions in the frontier molecular orbitals of the selected dyes are displayed in Fig. 9 and the computed parameters are compiled in Table 2. The HOMOs of the dyes are mainly located on the diphenylfluorenyl amine

SC

and LUMOs of the dyes are located on thiophene cyanoacrylic acid and little contribution from fluorene unit. The observed differences in optical properties of these dyes mainly originate from

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the number of diphenylamine and cyanoacrylic acid units. In order to understand these contributions for each electronic transition, we have performed orbital contributions calculations to individual units such as diphenylamine donor, cyanoacrylic acid acceptor and triazine auxiliary acceptor. For the dye T1, longer wavelength absorption originates mainly from HOMO to LUMO+2 electronic excitation. These orbitals are majorly composed by triarylamine (57%)

TE D

and triazine (40%) units, respectively. For the dye T2, the longer wavelength absorption arises from HOMO-1 to LUMO+1 electronic excitation. HOMO-1 is localized on the triarylamine units and the LUMO+1 is distributed over triazine, bithiophene and cyanoacrylic acid units.

EP

Therefore, this absorption may be termed as charge transfer from triarylamine to cyanoacrylic

AC C

acid unit. For the dye T3, longer wavelength absorption originates from HOMO to LUMO+3 electronic transition. Since HOMO is on triarylamine unit (65%) and LUMO+3 spreads over triazine (36%) and cyanoacrylic acid (26%) units, this absorption also causes charge to migrate from triarylamine to cyanoacrylic acid. [47] Table 2. Computed vertical excitation energies and their oscillator strengths, assignments, dipole moments and band gaps for the dyes at B3LYP/6-31G level.

24

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assignments

T1

460.0

1.27

454.7

1.40

501.4 462.8

1.85 0.82

453.7

0.40

467.9 452.0

1.18 2.12

434.1

0.19

HOMO→LUMO+2 (67%), HOMO→LUMO+1 (13%) HOMO-2→LUMO (54%), HOMO-1→LUMO+2 (33%) HOMO-2→LUMO (95%) HOMO-1→LUMO+1 (70%), HOMO→LUMO+2 (27%) HOMO→LUMO+2 (69%), HOMO-1→LUMO+1 (27%) HOMO→LUMO+3 (79%) HOMO-2→LUMO (43%), HOMO-1→LUMO+1 (25%), HOMO-1→LUMO (21%) HOMO-1→LUMO (71%), HOMO-2→LUMO (18%)

T2

T3

µ g, D 7.96

HOMO, eV 4.94

LUMO, eV 2.72

Eg, eV

2.22

7.71

4.94

2.80

2.14

6.03

5.00

2.80

2.20

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f

SC

λ, nm

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Dye

3.5. DSSC characteristics

The promising absorption properties of the dyes motivated us to construct DSSC devices employing these dyes as sensitizer. Fig. 10 displays the I-V curve and incident photon to current

TE D

conversion efficiency (IPCE) action spectra of the dyes. The photovoltaic parameters of the dyes

EP

10

8

6

4

2

100

T1 T2 T3

T1 T2 T3

75

AC C

Photocurrent density, mA cm

-2

12

IPCE (%)

are compiled in Table 3.

50

25

(a)

(b) 0

0 0

100

200

300

400

500

600

700

400

800

450

500

550

600

650

Wavelength, nm

Voltage, mV

Fig. 10. (a) I-V characteristics and (b) IPCE spectra of the DSSCs fabricated using the dyes in solvent bath ACN/tert-BuOH/DMSO (3.5/3.5/3 v/v).

25

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Table 3 Photovoltaic parameters of the devices using the dyes (T1-T3) JSC (mA cm-2)

ff

η (%)

7.04

VOC (mV) 702

0.58

2.88

3.54 × 10‒7

10.67

679

0.59

4.29

9.92 × 10‒8

6.36

665

0.66

2.80

T1

Dye amount (mol cm‒2) 2.85 × 10‒7

T2 T3

Rrec Rct2 (ohm) (ohm) 63.89 20.50

τe (ms)

12.50

RI PT

Dye

59.79

15.90

8.41

53.57

24.68

1.41

SC

The dyes T1 and T3 showed IPCE spectra covering from 400 to 550 nm while T2 showed

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broad spectrum extended up to 600 nm. The dye T3 showed IPCE spectral value of 84% at 420 nm. The device based on T1 displayed a PCE of 2.88% (JSC = 7.04 mA cm-2, VOC = 702 mV and ff = 0.58) similar to the dye T3 (PCE = 2.80%, JSC = 6.36 mA cm-2, VOC = 665 mV and ff =

0.66). Dye T1 with mono anchoring unit shows slight high PCE than dye T3 containing dianchoring units due to higher JSC of the former dye resulted from higher molar extinction

TE D

coefficient of former when compared to the later dye. Over all dye, T2 possessing bithiophene in conjugation showed higher PCE of 4.29% with JSC of 10.67 mA cm-2, VOC of 679 mV and ff of 0.59. The broad light harvesting of the dye T2 is responsible for higher PCE when compared to

AC C

[51]

EP

other dyes, which have similar LUMO level and may inherit similar electron injection efficiency.

The interfacial electron transport of the devices were studied by electrochemical impedance spectroscopy and Nyqusit plots of the dyes under dark conditions displayed in Fig. 11. In Nyqusist plot, three semicircles were observed and each one represents the charge transport resistance at the corresponding interface. The recombination of electrons from the conduction band of TiO2 with the electrolyte or oxidized state of the dyes leads to decrease of VOC of the device. The frequency of the middle semicircle represents the charge recombination resistance 26

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(Rrec) and charge transfer resistance (Rct2) under dark and illuminated conditions, respectively. The Rrec of the dyes follows the increasing order, T3 < T2 < T1. The higher recombination resistance of T1 results in higher VOC of 702 mV when compared to other dyes (T2 and T3). It is

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noteworthy to mention that the di-anchoring dyes lowers the conduction band of TiO2, which release more protons when it adsorbed on TiO2. [21] The lowering of conduction band of TiO2 decreases the VOC of the dyes and it follows the trend for T1 and T3 dyes. Fig. 12(a) displays the

SC

Nyquist plot under illumination of the devices and the deduced Rct values follow the order T2 < T1 < T3. Smaller the Rct2 higher is the charge collection efficiency. The dye T2 possessed

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smaller Rct2 in the series and produces higher JSC when compared to other dyes. 30

25

15

10

EP

5

TE D

-Z'', ohm

20

T1 T2 T3

0

AC C

20

40

60

80

100

Z', ohm

Fig. 11. Nyquist plots of the dyes under dark.

27

120

ACCEPTED MANUSCRIPT

14

20

16

-Theta, deg

-Z'', ohm

T1 T2 T3

8

5

12

8

2 4

30

40

50

60

-1

1x10

0

SC

20

RI PT

T1 T2 T3

11

Z', ohm

1x10

Frequency, Hz

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Fig. 12. (a) Nyquist and (b) Bode phase plots of the dyes under illumination. The middle frequency peak of the Bode-phase plot (Fig. 12(b)) of the devices constructed by the dyes related to Rrec and life time can be extracted by the formula τe = 1/ωmin.[52] The order of lifetime follows the increasing order of T3 (1.41 ms) < T2 (8.41 ms) < T1 (12.50 ms). The

TE D

highest electron lifetime of T1 indicates effective suppression of the back electron transfer with the oxidized dye and redox electrolyte. Consequently, it allows more number of electrons

VOC.

4. Conclusions

EP

available in conduction band of TiO2, shifts the conduction band upwardly, and results in high

AC C

We have successfully synthesized a set of triazine-based dyes containing star-shaped bis(donor)-acceptor and donor-bis(acceptor) molecular configurations. The synthesis of dyes was accomplished by selective C-N coupling and Stille coupling reactions. All the dyes exhibited absorption in the visible region with high molar extinction coefficients when compared to the dyes, which lack triazine. In addition, the dyes showed suitable HOMO and LUMO levels for better electron injection and regeneration of the oxidized dye. The mono-anchoring dyes

28

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achieved broad light harvesting properties when compared to bi-anchoring dye. In dye bath solution of ACN/tert-butanol/DMSO solvent mixture, the device based on dye T2 exhibited highest PCE of 4.29% attributed to higher JSC, minimized recombination of electrons and high

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electron lifetime. Acknowledgements

SC

KRJT is thankful to DST, New Delhi for generous financial support (Ref. No. DST/TSG/PT/2013/09). AV acknowledges a research fellowship from UGC, New Delhi.

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We are also thankful to DST for the purchase of ESI mass spectrometer via the FIST grant to the Chemistry Department, IIT Roorkee. References

[1]

O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-sensitized

(a) Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye sensitized solar cells. Chem Rev 2010;110:6595-663. (b) Mishra A, Fischer MKR, Bäuerle P. Metal-free

EP

organic dyes for dye-sensitized solar cells: from structure: property relationships to design rules. Angew Chem Int Ed 2009;48:2474-99. (c) Ooyama Y, Harima Y. Photophysical and electrochemical properties, and molecular structures of organic dyes

AC C

[2]

TE D

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