Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via

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Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via C-H Arylation for See-Through Photovoltaic G. Hanumantha Rao, Prem Jyoti Singh Rana, Ashraful Islam, and Surya Prakash Singh ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00862 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Energy Materials

Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via C-H Arylation for See-Through Photovoltaic G Hanumantha Rao,†,‡ Prem Jyoti Singh Rana,†Ashraful Islam,*§ Surya Prakash Singh*,†,‡ †

Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal road, Tarnaka, Hyderabad-500007, India ‡

Academy of Scientific and Innovative Research (AcSIR), CSIR-Human Resource Development Centre, (CSIR-HRDC) Campus, Kamla Nehru Nagar, Ghaziabad,Uttar Pradesh201002, India.

§

Photovoltaic Materials Unit, National Institute for Materials Science, Tsukuba, 305-0047, Japan E-mail: [email protected]

ABSTRACT:Developing organic molecules as light harvesters is the key for transparent solar cells technologies. In this paper, we have designed and synthesized a novel metal-free near-infrared (NIR) panchromatic asymmetrical squaraine sensitizers (SQ-SPS) with thethieno[3,2-b]thiophene (TT) unit as π-spacer, dicyanovinyl and cyanoacrylic acid unit as acceptor and successfully used this sensitizer for dye-sensitized solar cells (DSSCs) application. We adopted C-H arylation protocol to achieve the final target molecule which allows to avoid conventional and multi-step coupling reactions at low-cost. The time dependent density functional theory (TDDFT) gives an overview of transitions involved and is in good agreement with the experimental UV-Vis absorption data of SQ-SPS.The average life time of SQ-SPS is 2.45 nswhich is decent value for squaraine dye. SQ-SPS dye alone shows the power conversion efficiency (PCE) of 5.86 %, Voc 0.564 mV, Jsc15.00 mA cm-2 and after co-sensitization with N3 dye there is a tremendous increase in PCE to 8.84%, Voc 0.650 mV, Jsc 15.00 mA cm-2. The remarkable increase in efficiency is mainly due to entire spectral coverage of IPCE spectra after co-sensitization. The dye casted on TiO2 film shows the

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transparent nature which revealed that SQ-SPS dye has potential for transparent solar cell applications. KEYWORDS: Squaraine sensitizer, IR dye, C-H arylation, transparent solar cell, high efficiency INTRODUCTION Among organic photovoltaics, dye-sensitized solar cells (DSSCs)1 is a powerful technology which can be exploited to generate the electricity directly from sun at very low-cost.2 In DSSCs device, sensitizers play very vital role in converting photo-to-current. In this context, a series of sensitizers have been developed by researchers across the globe. These sensitizers are mainly categorized into two sections 1) metal-complexes and 2) metal-free dyes. However, metal-free sensitizers are more preferred over metal-complexes due to its easy synthesis and purification process, low-cost, high molar extinction coefficient and scope for further modification. Among various photovoltaic architecture, flexible and transparent photovoltaics have grabbed great attention.3 By see-through photovoltaic windows electricity can be generated by integration of transparent solar cells.4,5 Currently, transparent and semitransparent solar cells are commercially available with an energy conversion efficiency of 4-7% which is basically composed of amorphous silicon.6 Considering the drawback associated with silicon there is urgent need to develop low-cost, highly transparent and feasible for photovoltaic windows. Recently few research groups have explored their research program towards development of transparent solar cells7,8 which also includes perovskite9 and dye-sensitized solar cells.10 The transparency of a transparent DSSCs is also dependent on sensitizers use in DSSCs device. In DSSCs, ruthenium based metal-complexes have been used as sensitizers, which display the absorption in the high-eye-sensitivity range (500-600 nm)11 which is difficult to control the transparency as well as overall efficiency. Recently, Liyuan Han research group reported a mixed dye system containing a UV-absorbing dye and 2


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a NIR absorbing dye for high transparency of the devices can be achieved.10 They reported a transparent DSSCs using squaraine dye having a D-π-A molecular structures with a power conversion efficiency of 3.66%. Squaraine dyes belong to polymethine dyes family andwidely investigated in various applications such as photovoltaics,12-17 photoconductive materials,18 chemosensor,19 and photodynamic therapy20 due to its sharp, intense absorption and emission in the red and NIR region. Due to the arrangement of the intrinsic molecular dipole squaraine dye shows aggregation behaviour. The aggregated structure shows either hypsochromic shift (Haggregate) or bathochromic shift in absorption (J-aggregate) spectra with respect to their monomer.21,22 The synthesis of symmetrical squaraine dyes are simple than the unsymmetrical squaraine dyes. The condensation between electron-rich substrates and squaric acid yields symmetrical squaraine dye as a product.23 The co-sensitization approach shows potential in increasing photovoltaic performance of solar cells byusing visible region wavelength dye (e.g., ruthenium polypyridine)24 with organic dyes (squaraine)25 which covers the entire visible and NIR region of the solar spectrum. Before performing the co-sensitized experiment, the selection of efficient sensitizer is necessary, there are some important condition to be considered: 1) they should contain high molar extinction coefficients to reduce the thickness of semiconducting TiO2 film 2) the cosensitizers structure should be appropriate for competitive adsorption on the semiconducting TiO2 surface, whereas, it should successfully reduce dye aggregation; and 3) they must decrease the recombination of electrons in the semiconducting TiO2 film with I3¯. Herein, we report synthesis of new squaraine (SQ-SPS) dye via direct arylation reaction which allows to access the final product in step-economy and low cost manner. Molecular structure of novel SQ-SPS sensitizers is shown in Figure 1. Thieno[3,2-b]thiophene (TT) unit was selected as π-bridge between thedonor and the anchoring group as heterocyclic 3



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compounds with five-membered rings have been widely developed as efficient π-bridges for best charge transfer (CT).26 TT unit has good electron transfer capability, to attain planarity and intramolecularcharge transfer (ICT) for photovoltaic applications of organic dyes.27 The complete synthetic route of the SQ-SPS dye represented in Scheme 1. Figure 1.Molecular structures of novel SQ-SPS sensitizers. NC

CN C10H21 N

C4H9 N O

S S CN COOH

Scheme 1. Synthetic pathway of the novel SQ-SPS dye. NC N

I C10H21 N

I C10H21

Br

O

C4H9 N

reflux, overnight, 77%

O HN

Toluene/1-butanol

CN C10H21 N

C4H9 N O

reflux, 12h, 35%

Br

1

Br

2

3

SQ-Br

NC

CN

NC S

NC

CN

CN

O

H 4

S

via direct hetero-arylation

C4H9

C10H21 N

N

C4H9 N

NC COOH

O

C10H21 N O

CH3Cl/CH3CN piperidine, reflux, 12h, 44%

Pd(OAc)2, PivOH K2CO3, toluene, 47%

S

S S

S SQ-TT-CHO

CN

O SQ-SPS

4


COOH

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RESULTS AND DISCUSSION Currently, different types of organic dyes have been developed having various molecular architectures, which can be divided into acceptor-donor (A–D) type, donor-acceptor–spacer– donor (D–A–π–D) type, acceptor–donor-acceptor (A–D–A) etc. Here we have designed a novel SQ-SPS dye having donor–acceptor-donor-π-spacer-anchoring group (D-A-D-πanchoring group) type of architecture one of themost molecular design strategies to broaden absorption and obtain suitable energy levels.28 Absorption Spectroscopy

Figure 2. (a) Normalized absorption spectra of SQ-SPSdye in CHCl3 solution (green dotted lines), anchored on mesoporous TiO2 (red dotted lines) and fluorescence emission spectra in solution (blue solid lines) Inset: SQ-SPS dye in solution and adsorbed on mesoporous TiO2. (b) SQ-SPS dyes casted on TiO2 coated film. Figure 2 represents the UV−Vis absorption spectra of the SQ-SPS dye shows two broad absorption band ranging from 365-450 nm and 560-790 nm in CHCl3 solvent and the related data are summarized in Table 1. The longer wavelength absorption of SQ-SPS dye is centred at 720 nm with a vibronic progression band at 665 nm which is attributed to charge transfer ππ* transition from S0–S1 electronic excitation state. The new intense broad absorption band 5



centred at 397 nm corresponds to the extended conjugation arising from the dicyanovinyl unit. The fluorescence spectrum of SQ-SPS dye shows the emission maximum at 758 nm. We observe very low 38 nm Stokes shifts in SQ dye solution due to the conformational rigidity, i.e., a similar configuration of the dye in theground as well as the excited states.29 Figure 2a shows the normalized optical absorption spectra of SQ-SPS dyes in solution and the SQ-SPS dye anchored on mesoporous semiconducting TiO2 film. The absorption spectrum of SQ-SPS-TiO2 shows spectral broadening, increase in full width half maximum (FWHM) and shifted 15 nm hypsochromically compared to the SQ-SPS dye in solution. The broadening in absorption spectra is attributed to the interaction of anchoring group COOH in SQ-SPS dye with TiO2 surface and aggregation of molecules in a condensed state.30,31 The vibronic shoulder from 550-650 nm for SQ-SPS-TiO2 become more pronounced compared SQ-SPS in solution due to aggregation of dye. The unsymmetrical squaraine dyes show prominent absorption with abroad peak in visible region from 400-500 nm which is due to H-aggregate formation by intermolecular interaction. Previous reports also reveal that there is a blue-shift in squaraine-based dye due to H-aggregation when adsorbed on SnO2 surface.32,33 The life time of SQ-SPS is 2.45 ns and the life decay curve is shown in Figure S12. To test the realistic transparent solar cells application we have casted our sensitizer SQ-SPS on TiO2 coated film and displayed in Figure 2b. From Figure 2b one can conclude SQ-SPS has potential to use in transparent solar cells. Electrochemical Properties The cyclic voltammetry (CV) method was used to determine the redox behaviour of the SQSPS dye in chloroform solution (Figure 3) and the related data are summarized in Table 1. The experimental details are presented in supporting information. SQ-SPSdye shows two quasi-reversible oxidation waves at 0.74V and 1.04 V in the anodic region which corresponds to the removal of an electron from indole and indolium unit. The redox potential was also 6


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measured via differential pulse voltammetry showed three distinct peaks centred at 0.56 V, 0.732 V, 1.032 V for SQ-SPS dye. The peak at 0.56 V is assigned for [Fc]0/[Fc]+. The Eoxd onset of SQ-SPSdye is 0.74 V. In the same way, the HOMO and LUMO energy levels are calculated to be −4.98 eV and −3.32 eV respectively. CV of SQE DPV of SQE

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.1

1.0 0.9 0.8 0.7 0.6 0.5 Potential vs Ag/AgCl (V)

0.4

Figure 3. Cyclic voltammograms and differential pulse voltammetry of SQ-SPS dye (1 mM solution in CHCl3 using 0.1 M (Bu)4NPF6 supporting electrolyte, Ag/AgCl as a reference electrode and [Fc]+/[Fc]0 as the internal standard with a scan rate of 100 mV s-1).

Figure 4. Schematic representation energy level conduction band of TiO2 and redox electrolyte along with HOMO and LUMO of the SQ-SPS dye. 7



The SQ-SPS dye displays the more negative LUMO of −0.92 V compared to the CB of TiO2 (−0.5 V) which make the electron injection thermodynamically more feasible from the excited state of the dye to the conduction band of TiO2. Whereas, the HOMO of SQ-SPS dye 0.74 V is more positive than the redox potential of the electrolyte (I¯/I3¯= 0.4 V) which is favourable for dye regeneration from the redox electrolyte. The lowering in the LUMO of SQ-SPS dye is due to the presence ofelectron withdrawing dicyanovinyl acceptor unit. Figure 4 shows the schematic representation energy level conduction band of TiO2 and redox electrolyte along with HOMO and LUMO of the SQ-SPS dye. 0.6 0.5 Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.3 0.2 0.1 0.0 680

690

700

710 720 730 Wavelength (nm)

740

750

Figure 5. UV-visible spectroelectrochemical changes of the SQ-SPS dye recorded in CHCl3 at applied potentials of 0.74 V. In DSSCs, the sensitizer absorbs the photon and transfer the electron from ground state to the excited state of dye. Further, these excited electrons are transferred from excited state of dye to conduction band of semiconductor (TiO2) in femtosecond time scale. We performed the spectrochemical study to better understand the electronic properties of the oxidized species of SQ-SPS dye under applying potential shown in Figure 5. We observe that the absorption peak at 720 nm decrease gradually when controlled oxidation potential of 0.74 V is applied, this change is due to formation of squaraine cation radical.34We also applied 1.04 V Vs SCE to the

8


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sensitizer but did not observe any significant change in absorption spectra. When the applied potential was removed the absorption, spectra was retained tooriginal absorption. The detail related to spectroelectrochemical study is provided in supporting information. Theoretical Calculation To better understand the optical transition and geometrical structures of SQ-SPSdye, We performed density functional theory (DFT) using Gaussian 09 software35 and the ground state geometry was optimized via B3LYP/6-31G* level of theory.36 The self-consistent reaction field (SCRF) calculation was performed using conductor-like polarizable continuum model (CPCM) to incorporate solvation effect from CHCl3. Using the optimizedgeometry the theoretical absorption spectra of SQ-SPS dye was calculated by time-dependent density functional theory (TDDFT) in CHCl3.37 The alkyl chains were replaced with methyl groups to reduce computational time. The alkyl chains approximation to methyl groups will not affect in the present study, as the effect will be minor, not considerable. The optimised geometry of SQ-SPS is shown in Figure 6. The experimental UV-Vis absorption spectra and TDDFT calculated energy transition with oscillator strength of SQ-SPS dye is shown in Figure 7. Figure 8 shows the electronic distribution of the frontier molecular orbitals (FMO) for the SQ-SPS dyes and the computed data is summarized in Table 2. The performance of SQ-SPS dye is dependent on stereoisomerism, as it influences electronic charge distributions on HOMO and LUMO of dye. Previous reports reveal that CN-functionalized SQ dyes are locked in cis-configuration owing to steric hindrance and our results are also consistent with them.38 The theoretical spectra consist the excitations from 300 nm to 750 nm because first 20 excitations gives the information related to entire UV, Visible and NIR region. The simulated electronic absorption spectra of SQ-SPS dye showed good alignment with the experimental absorption spectra. We observed the intense peak at 705 nm for HOMO to LUMO transition in theoretical absorption spectra. At the ground state (HOMO) for SQ dye, the electrons 9



density is mainly localized on squaraine core, electron withdrawing dicyanovinyl unit extending upto the fused phenyl rings and on the illumination of light, the electron starts to move from HOMO to LUMO by intramolecular charge transfer transition which involves ππ* electronic excitation. In LUMO the electronic cloud is mainly located onsquarine core centre and finally reached to the anchoring groups via the presence of π-bridge. Due to such electron distribution in HOMO and LUMO recommend the efficient electron transfer from donor moiety to anchoring acceptor unit. Dihedral angles for the optimized ground state geometry of SQ-SPS dyes were calculated for the planes topresent between indolenine and cyanosquaric unit (A-B and B-C). The plane (C-D) were present between thienothiophene (TT) and SQ units (C-D) (Figure 6). For ∠A-B and ∠B-C, SQ unit is nearly planar whereas ∠C-D of 28.50º envisage that TT is somewhat out of theplane which results in minor restriction in the conjugation. The new peak covers the region from 350 nm to 500 nm with some transition such as S0→S3, S0→S4, S0→S5. In S0-S3, S0-S4 the major contribution is from H–2 → L, H–1 → L. Whereas in HOMO-1 the electron density is spreadover CN group of squaraine, indoline (C ring), TT, and acceptor part (-COOH), while in HOMO-2 the electron cloud is localized at indoline A ring and CN group of sqiuirine unit. In S0-S5 involve the major contribution from H – 1 → L + 1, whereas in L+1 the electron density is localized at squaraine, TT (D) and acceptor part (COOH).

Figure 6. The optimized geometry of SQ-SPS dye. 10


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Absorbance (a.u.)

1.0

0.8

SQ-SPS (Exp.) SQ-SPS (Theor.)

1.6

1.2 0.6 0.8

0.4

0.4

0.2

Oscillator strength (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 400

500

600 700 Wavelength (nm)

800

Figure 7. Experimental UV-Vis absorption spectra (Green) and TDDFT calculated energy transition with oscillator strength shown as avertical black line of SQ-SPS dye.

Figure 8. Frontier molecular orbitals of SQ-SPS involved in various electronic transitions. Photovoltaic Properties By the addition of co-sensitized dyes, the wide range of thesolar spectrum is covered from 350 to 750 nm and the squaraine peak at 720 nm play a vital role in the increase of PCE than standard N3 Dye. Figure 9 represents the current-voltage (J-V) plots and IPCE of the DSSC devices sensitized by SQ-SPS (TBP 0.1), N3, SQ-SPS+N3 using semiconducting mesoscopic TiO2 photoanodes under AM 1.5 illumination solar condition and the relevant data is summarized in Table 3. The device fabricated without co-sensitization SQ-SPS (TBP 0.1 11



mM) shows PCE of 5.86%, and Jsc of 15.0 mAcm-2 and low Voc of 0.56 V. The lower PCE is mainly due to less coverage of solar spectrum from 300-550 nm with IPCE of 68% and 85% of IPCE in the range from 550-780 nm. On the other hand, the co-sensitization SQ-SPS+N3 dye display increase in Voc of 0.69 V and Jsc of 15.11 mA cm-2. The energy level of the dye plays an important role in photocurrent generation, the high LUMO ofSQ-SPSdye and N3 (3.77 eV)39 thermodynamically facilitate the electron injection from both SQ-SPSdye and cosensitized N3 dye and enhance the Voc value.40,41 The considerable increase in Voc was in good harmony with the IPCE values at 350–700 nm and hence produce the high PCE of 8.94% for SQ-SPS+N3 and was higher than the isolated SQ-SPS dyes. Co-sensitization increases the light harvester ability due to adsorption of dyes on semiconducting TiO2 layer via different anchoring sites such as carboxylic acid and cyanoacrylic acid.42 SQ-SPS and N3 single-dye having Voc values of 0.564, 0.632 respectively. SQ-SPS dye when co-sensitized with N3 dye shows the Voc value (0.650) which higher than that of N3. 20

100

(a)

(b)

15

75

IPCE (%)

Photocurrent density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

5

50

25

SQ-SPS TBP 0.1 N3 SQ-SPS+N3

SQ-SPS TBP 0.1 N3 SQ-SPS +N3 0

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400

Voltage (V)

500

600

700

800

900

Wavelength (nm)

Figure 9. (a) I-V characteristics and (b) IPCE spectra of DSC device fabricated using SQSPS, N3 and SQ-SPS+N3.

12


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Table 1. Spectroscopic and photophysical, electrochemical data of unsymmetrical SQ-SPS dye in chloroform solution at room temperature (RT). Dye

SQSPS

ߣ௘௠ ௠௔௫

ߣ௔௕௦ ௠௔௫

(dm3

ɛ -1

-

(nm)

(nm

mol

a

)b

1

720

758

2.46 × 105

(705)

cm

)

Eo-

Stoke

FWH

o (eV

s shift

M(nm)

)c

(nm)d

e

1.66

38

98.78

(ns)

Eox*

Eox

Ʈ f

(V)

g

(V)

h

Eo-o (eV)

i

HOM

LUMO

O

(eV)k

(eV)j 2.45

0.74

-0.92

1.66

-4.95

-3.32

4

5.25 × 10

392 (419)

a

Optical absorption maxima of dye in the chloroform solution. Values in parentheses

represent the absorption spectra calculated by using TD-DFT. bEmission maxima of dyes in the chloroform solution. cBandgap (E0-0) was estimated from the intersection of absorption and emission spectrum of dye in chloroform solution. dStokes shift was calculated from absorption maximum and emission maximum separation. eThe FWHM was estimated from the electronic absorption of dyes anchored on TiO2 surface. ffluorescence lifetime decay. gEoxd is the ground state oxidation potential calculated from CV. hThe excited state potential (Eoxd*) calculated by using ground state potential. IE0-0 is evaluated from the intersection point of normalized absorption and emission spectrum.J Highest Occupied Molecular Orbital (HOMO) k

Lowest Unoccupied Molecular Orbital (LUMO) energy levels.

Table 2. Theoretical absorption wavelength, oscillator strength (f) orbital transition of SQSPS dye in CHCl3. Dye

Electronic transition

Absorption wavelength ߣ௔௕௦ ௠௔௫

oscillator strength (f in a.u.)

Transition Assignment

% Contribution

(eV) 1.7579

1.4728

H→L

99 %

S0 →S2 S0 →S3

(nm) 705.30 (720)* 599.49 472.10

2.0682 2.6262

0.1910 0.8014

S0 →S4

455.50

2.7219

0.1790

S0 →S5

419.50

2.9555

0.3788

H→L+1 H–2→L H–1→L H–2→L H–1→L H–1→L+1

98 % 31 % 59 % 53 % 37 % 92 %

S0 →S1 SQ-SPS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


*Shows experimental absorption maximum of SQ-SPS dye. 13



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Table 3. Photovoltaic parameters of DSSCs based on SQ-SPS, N3 and SQ-SPS+N3. Dye

Jsc(mA cm -2)

Voc (V)

FF

Efficiency (%)

SQ-SPS TBP 0.1

15.00

0.56

0.69

5.86

N3

15.11

0.63

0.72

6.91

SQ-SPS+N3

17.78

0.65

0.76

8.84

CONCLUSION We have successfully designed and synthesized a NIR panchromatic asymmetrical squaraine sensitizers (SQ-SPS) with thieno[3,2-b]thiophene as π-spacer,dicyanovinyl and cyanoacrylic acid unit as an acceptor. A cost effective and atom economy C-H arylation reactions were adopted to synthesize the SQ-SPS sensitizer. The experimental optical and electrochemical studies of SQ-SPS are in well alignment with theoretical calculations. Under AM 1.5 G solar conditions, the isolated SQ-SPS shows the power conversion efficiency of 5.86%, Voc 0.564 mV, and Jsc 15.00 mA cm-2 after co-sensitization with N3 dye (SQ-SPS+N3) there is a remarkable enhancement in PCE to 8.84%, Voc 0.650 mV, and Jsc 15.00 mA cm-2. The increase in efficiency is mainly due to entire spectral coverage of IPCE spectra after cosensitization. The sensitizers have shown the potential for transparent photovoltaic. Further, work are underway to explore its see-through photovoltaic applications. Experimental Section Synthesis: All starting chemicals and reagents were purchased from commercial sources and used as without any further purification. 5-bromo-2,3,3-trimethyl-3H-indole (1),43 trimethylammonium (E)-2-((3-butyl-1,1-dimethyl-1H-benzo-[e]indol-2(3H)-ylidene)methyl)3-(dicyanomethylene)-4-oxocyclobut-1-enolate

(3)44andthieno[3,2-b]thiophene-2-

carbaldehyde45 were prepared according to reported procedures.43-45 5-Bromo-1-decyl-2,3,3-trimethyl-3H-indol-1-ium iodide (2):5-bromo-2,3,3-trimethyl-3Hindole (1) (1.5 g, 0.006 mmol) taken in round bottom flask, to this iododecane (3.37 g, 0.012 14


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mmol) and acetonitrile (10 ml) were added. Reaction mixture was stirred overnight. Solvent was removed under reduced pressure and that mixture was washed several times with diethyl ether and n-pentane to get (2) as a solid. Yield: 2.6 g (77 %). m.p 160-162 °C. 1H NMR (500 MHz, CDCl3, TMS): δ (ppm) 7.71-7.73 (m, 2H), 7.63 (d, J = 10 Hz, 1H), 4.65 (t, J = 10 Hz, 2H), 3.09 (s, 3H), 1.89-1.95 (m, 2H), 1.68 (s, 6H), 1.41-1.47 (m, 2H), 1.32-1.37 (m, 2H), 1.23-1.28 (m, 10H), 0.87 (t, J = 5 Hz, 3H);13C NMR (100 MHz, CDCl3): δ (ppm) 143.3, 139.8, 132.7, 126.7, 124.5, 117.0, 54.7, 50.3, 31.6, 29.1, 27.8, 26.7, 23.1, 22.5, 17.0, 13.9. (Z)-4-((5-bromo-1-decyl-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(3-butyl1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)methyl)-3-(dicyanomethylene)cyclobut-1enolate (SQ-Br): A mixture of 3 (0.30 g, 0.73 mmol) and 2 (0.432 g, 1.46 mmol) were taken in round bottom flask, to this 1:1 ratio 8 ml 1-butanol/toluene was added and refluxed overnight. After completion of reaction, solvents were removed under vacuum and dichloromethane was added, washed with water and dried over sodium sulphate. Crude product was purified by column chromatography on silica gel using dichloromethane/hexane (80:20, v/v) as an eluent to obtain SQ-Br asgreen solid. Yield: 0.198 g (35%). m.p. 208210 °C, IR (KBr) ῦ = 3451, 2922(νC-H), 2185(νC-N), 1720(νC=O) cm−1.1H-NMR (500 MHz, CDCl3): δ (ppm)8.20 (d, J = 10 Hz, 1H), 7.91-7.94 (m, 2H), 7.58-7.61 (m, 1H), 7.46-7.49 (m, 1H), 7.42-7.44 (m, 2H), 7.36 (d, J = 5 Hz, 1H), 6.87 (d, J = 10 Hz, 1H), 6.63 (s, 1H), 6.47 (s, 1H), 4.18 (t, J = 10 Hz, 2H), 3.95 (t, J = 10 Hz, 2H), 2.05 (s, 6H), 1.84-1.91 (m, 2H), 1.78 (s, 6H), 1.60 (moisture, 3H), 1.50-1.56 (m, 2H), 1.40-1.46 (m, 2H), 1.31-1.35 (m, 2H), 1.23-1.27 (m, 12H), 1.00 (t, J= 10 Hz, 3H), 0.87 (t, J = 10 Hz, 3H); 13C-NMR (100 MHz, CDCl3): δ (ppm)174.4, 173.1, 169.6, 167.7, 166.9, 164.9, 144.1, 141.3, 138.8, 135.5, 131.7, 130.7, 130.0, 129.7, 128.3, 127.5, 125.5, 124.9, 122.5, 118.9, 118.8, 116.7, 110.9, 110.4, 89.4, 88.9, 51.4, 49.0, 44.6, 44.2, 40.8, 31.8, 29.8, 29.6, 29.4, 29.3, 29.2, 27.0, 26.7, 26.6, 26.1, 22.6, 20.0, 14.0, 13.8;HRMS (ESI, m/z) calcd for C47H53BrN4O:769.8539(M–H-), found:m/z769.3504. 15



(Z)-2-((E)-(3-butyl-1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)methyl)-4-((1-decyl-5(5-formylthieno[3,2-b]thiophen-2-yl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-3(dicyanomethylene)cyclobut-1-enolate(SQ-TT-CHO): SQ-TT-CHO was synthesized by using the reported procedure.46 In a Schlenk tube SQ-Br (0.10 g, 0.13 mmol) and thieno[3,2b]thiophene-2-carbaldehyde (0.087g, 0.54 mmol) were taken. The Shclenk tube is evacuated and refilled with nitrogen four times. Pd(OAc)2 (5 mol %), P(Cy)3(10 mol %), PivOH (30 mol %) and K2CO3 (2.5 eq.) were added to it followed by 4 mL of anhydrous toluene. The mixture was stirred at 110 oC for 24 h. After completion of the reaction, the mixture was poured into water and extracted with dichloromethane. The organic layer was then washed with brine, dried over sodium sulphate and concentrated under vacuum. Crude product was purified by column chromatography to give pure compound of SQ-TT-CHO.Yield: 0.053 g (47%). m.p. 232-240°C. IR (KBr) ῦ = 3450(νC-H), 2921(νC-H), 2189(νC-N), 1719 (νC=O) cm−1. 1

H-NMR (400 MHz, CDCl3): δ (ppm)9.95 (s, 1H), 8.21 (d, J = 8 Hz, 1H), 7.92-7.95 (m, 2H),

7.73 (d, J = 4 Hz, 1H), 7.58-7.63 (m, 3H), 7.46-7.50 (m, 1H), 7.37-7.40 (m, 2H), 7.15 (d, J = 8 Hz, 1H), 6.68 (s, 1H), 6.55 (s, 1H), 4.20 (t, J = 8 Hz, 2H), 4.03 (t, J = 8 Hz, 2H), 2.07 (s, 6H), 1.86 (s, 6H), 1.48-1.58 (m, 6H), 1.37-1.42 (m, 2H), 1.24-1.28 (m, 12H), 1.01 (t, J = 8 Hz, 3H), 0.85-0.88 (m, 3H);

13

C-NMR (100 MHz, CDCl3):δ (ppm) 184.5, 174.9, 173.1, 169.5,

167.8, 167.6, 164.7, 143.9, 143.5, 143.3, 143.2, 140.8, 139.4, 138.8, 135.8, 133.6, 131.8, 130.0, 129.8, 129.6, 128.3, 128.0, 127.6, 125.1, 123.1, 122.6, 120.7, 118.9, 118.8, 110.5, 109.9, 89.7, 89.4, 51.6, 48.8, 44.7, 44.3, 41.0, 31.8, 29.8, 29.6, 29.5, 29.4, 29.2, 27.2, 26.9, 26.7, 26.1, 22.6, 20.0, 14.0, 13.9;HRMS (ESI, m/z) calcd for C54H56N4O2S2: 857.1780 (M+), found: m/z 857.3969. (Z)-2-((E)-(3-butyl-1,1-dimethyl-1H-benzo[e]indol-2(3H)-ylidene)methyl)-4-((5-(5-((E)-2carboxy-2-cyanovinyl)thieno[3,2-b]thiophen-2-yl)-1-decyl-3,3-dimethyl-3H-indol-1-ium2-yl)methylene)-3-(dicyanomethylene)cyclobut-1-enolate (SQ-SPS): SQ-TT-CHO (0.08 g, 16


Page 16 of 23

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0.09 mmol) was dissolved in 3 mL of chloroform and 3 mL of acetonitrile in round bottom flask. To this 5 eq. cyanoacetic acid was added and followed by one drop of piperidine. The reaction mixture was stirred at 80 oC for 15 h. After completion of reaction, solvents were removed and dissolved in 30 mL of dichloromethane. The organic layer was washed with water, brine solution and dried over sodium sulphate. The solvent was removed under reduced pressure and purified by column chromatography by silica gel using MeOH/CHCl3 as an eluent. Pure compound SQ-SPS was obtained as dark green solid. Yield: 0.038 g (44%). m.p.250-252 °C. IR (KBr) ῦ = 3449(νC-H), 2922(νC-H), 2190(νC-N), 1721 (νC=O) cm−1. 1H-NMR (500 MHz, CDCl3): δ(ppm) 8.48 (s, 1H), 8.19 (d, J = 10 Hz), 7.93 (t, J = 10 Hz, 2H), 7.75 (d, J = 5 Hz, 1H), 7.59 (t, J = 10 Hz, 1H), 7.46-7.60 (m, 3H), 7.37-7.39 (m, 2H), 7.16 (d, J = 10 Hz, 1H), 6.68 (s, 1H), 6.55 (s, 1H), 4.20 (t, J = 5Hz, 2H), 4.02 (t, J = 10 Hz, 2H), 2.05 (s, 6H), 1.86 (s, 12H), 1.49-1.54 (m, 6H), 1.37-1.41 (m, 3H), 1.04-1.10 (m, 3H), 1.00 (t, J = 5Hz, 3H), 0.86 (t, J = 5 Hz, 3H);13C-NMR (100 MHz, CDCl3): δ(ppm) 174.9, 173.1, 169.6, 167.8, 164.6, 144.3, 143.5, 143.3, 140.7, 138.8, 135.8, 132.3, 131.8, 130.0, 129.9, 129.7, 129.4, 128.5, 128.3, 127.6, 127.1, 125.2, 125.1, 123.3, 127.7, 122.6, 120.5, 118.9, 110.5, 110.2, 89.7, 89.7, 67.6, 52.2, 51.6, 50.6, 48.8, 48.2, 44.7, 44.3, 41.0, 31.8, 29.8, 29.5, 29.2, 27.2, 26.8, 26.7, 26.1, 22.6,

20.0,

14.1,

13.9;

MALDI-TOF

MS

calcd

for

C57H57N5O3S2:

923.39

(M+),found:m/z932.46. Supporting Information Supporting

information

include

detail

Spectroelectrochemical studied, 1H-NMR,

13

description

regarding

Cyclicvoltametric,

C-NMR, HRMS, MALDI-TOF MS, Life time

decay curve. Acknowledgements We gratefully acknowledge financial support received from the SUNRISE project. GHR thanks AcSIR for PhD enrolment and CSIR for SRF. 17



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(46) Bisht, R.; Fairoos, M. K. M.; Singh, A. K.; Nithyanandhan, J. Panchromatic Sensitizer for

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The table of contents

23


Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via

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