Facile synthesis of β-functionalized “push-pull” Zn(II) porphyrins for DSSC applications

Article abstract of DOI:10.1016/j.dyepig.2017.07.053

Three new β-substituted “push-pull” Zn(II) porphyrin dyes with various electron donors at meso-positions and cyanoacetic acid as acceptor at β-position have been designed and synthesized. These porphyrins have been characterized by UV-Vis, Fluorescence, <

Full text of DOI:10.1016/j.dyepig.2017.07.053

Accepted Manuscript
Facile synthesis of β-functionalized “push-pull” Zn(II) porphyrins for DSSC
applications
Kamal Prakash, Shweta Manchanda, Vediappan Sudhakar, Nidhi Sharma,
Muniappan Sankar, Kothandam Krishnamoorthy
PII:
S0143-7208(17)31109-9
10.1016/j.dyepig.2017.07.053
DYPI 6142
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 12 May 2017
Revised Date: 19 July 2017
Accepted Date: 21 July 2017
Please cite this article as: Prakash K, Manchanda S, Sudhakar V, Sharma N, Sankar M, Krishnamoorthy
K, Facile synthesis of β-functionalized “push-pull” Zn(II) porphyrins for DSSC applications, Dyes and
Pigments (2017), doi: 10.1016/j.dyepig.2017.07.053.
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ACCEPTED MANUSCRIPT
Facile synthesis of
β-Substituted “Push-Pull” Zn(II) Porphyrins for DSSC Applications
Kamal Prakash, Shweta Manchanda, Vediappan Sudhakar, Nidhi Sharma, Muniappan Sankar and Kothandam
Krishnamoorthy
β
-Functionalized “push-pull porphyrin” dyes have been designed, synthesized and studied for DSSC applications. These
dyes displayed power conversion efficiency (PCE) of
η = 1.72-3.13% where co-sensitized ZnT(Mes)P(CN-COOH)(N719)
dye demonstrated maximum PCE efficiency up to 5.35%, with a Jsc of 11.8 mA cm-2, a Voc of 630 mV and a fill factor (FF)
of 0.72.
CN
COOH
N
N
N
N
Zn

ACCEPTED MANUSCRIPT
Facile Synthesis of
Applications
β-Functionalized “Push-Pull” Zn(II) Porphyrins for DSSC
Kamal Prakash,a Shweta Manchanda,aVediappan Sudhakar,bNidhi Sharma,aMuniappan Sankar*aand Kothandam
Krishnamoorthy*b
ABSTRACT
Three new β-substituted “push-pull” Zn(II) porphyrin dyes with various electron donors at meso-positions and cyanoacetic acid as acceptor at β-
position have been designed and synthesized. These porphyrins have been characterized by UV-Vis, Fluorescence,1H NMR and 13C
NMRspectroscopic techniques and cyclic voltammetric studies. The Soret and Q band of Zn(II) porphyrin dyes were found to be red-shifted (30-35
nm) as compared to ZnTPP.The fluorescence quenching andthe decrement in quantum yield and lifetime suggest intramolecularcharge transfer from
donor to acceptor. Zn porphyrinsexhibited anodic shift in their first redox potentials (0.03-0.11 V) as compared to ZnTPP. The HOMO-LUMO
-
energy levels of Zn porphyrin dyes were compared with the conduction band of TiO2 and the electrolyte I-/I3 . The HOMO levels of all the dyes are
sufficiently higher than the energy level of electrolyte I-/I3- and LUMO levels significantly lowerthan the conduction band of TiO2 which reflect the
feasibility of facile electron-transfer. ZnT(Mes)P(CN-COOH) has been co-sensitized with N719 dye to further improve the PCE efficiency. These
dyes displayed power conversion efficiency (PCE) of η = 1.72-3.13% where co-sensitized ZnT(Mes)P(CN-COOH) (N719) dye demonstrated
maximum PCE efficiency up to 5.35%, with a Jsc of 11.8 mA cm-2, a Voc of 630 mV and a fill factor (FF) of 72% due to better light harvesting
capacity.
1. Introduction
As the global energy demand increased from the last decades, there is an urgent requirement to look for new sustainable and renewable energy
sources to fulfil the advance energy demand for the next generation.1-3Silicon based solar cells have been developed for the alternate energy source
but its production cost is very high. Dye sensitized solar cells (DSSC) were aroused as favourable candidate for green and clean energy due to their
high efficiency and low cost of production and turn outa better alternate to the Si based solar cells.1-3 Traditionally, Ruthenium-based sensitizers have
been utilized for DSSC to achieve high power conversion efficiency up to 11% but these complexes areexpensive, rare, need skillful synthesis as well
as hazardous tothe environmentwhich restrict their commercial application.4,5From this point of view, tremendous efforts have been made to design
and synthesize new, cost-effective, high yield and efficient metal free sensitizers.6-9 Many organic dyes such as perylene,10BODIPY,11diimide,12
courmines,13 triarylamines14 and carbazole15 also reported as efficient sensitizers for DSSCwith power conversion efficiency in the range 5-9% due to
their comparable cost, high yield synthesis, stability and large molar absorptions coefficients. Among the various type of sensitizers, porphyrin and
phthalocyanine derivatives are gained much more importance as potential sensitizers forDSSC applications.16-20Porphyrinoids play crucial role to
sustain life in living organism such as chlorophyll in green leaves, heme protein in red blood cells etc.21,22 Chlorophyll in green leaves absorb light
from the sun for the process of photosynthesis to convert photon energy into chemical energy.In this regard, porphyrins and phthalocyanine have
attracted much interest for researchers as they can be mimicked of neutural photosynthetic architectures.23,24Porphyrins have intense absorptions in
the visible region due to their π-conjugated aromatic system.They are easy to synthesize and functionalize at meso- andβ-pyrrole positionswhich lead
to interesting electronic spectral andelectrochemical redox properties.25-28Porphyrins can be synthesized in very high yield, have facile synthetic
routes as well as cheaper and environmental friendly as compared to Ru-based dyes.
Porphyrins, first reported as sensitizers for DSSC by Grätzel and co-workers in 1991.29 Since then,porphyrins have emerged as a class of promising
molecules as DSSC materials and alot of research has been carried out on them to achieve high power conversion efficiency for solar cells.30-33D–π–
A type structures have electron-donor (D), π-linker (π), and an electron-acceptor anchor (A) which enhance their photophysical properties and
efficient electron injection due to intramolecular charge transfer (CT). From last few years, many meso-substituted push-pull porphyrinswere
reported with high PCE where different donor group and acceptor carboxylic acceptor group situated at opposite meso-positons.34-37The zinc
porphyrin sensitizer YD2-o-C8 and SM315exhibited an impressive PCE of 12-13% using a cobalt-based electrolyte.38,39The extension of
porphyrinmacrocyle can be achieved by functionalization of porphyrin at meso-position with promising chromophores such as aceneor pyrene by
increasing number of aromatic groups in the porphyrin core is a good strategy to extend the absorption spectra to greater wavelength. Fluorene-
substituted sensitizer LD2240 showed high PCE value of 8.1% and pyrene-substituted sensitizer LD441 exhibited PCE of 10.1%, suggesting that they
showed good light-harnessing capability in the near IR and visible regions.Imahori and co-workers studied the meso-naphthyl fused porphyrin dyes
1

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namely fused-ZnP1 with ƞ-value of 4.1% which is found to be much greater than non-fused porphyin dye ZnP1.42Porphyrin array has been
developed for DSSC solar cell due to their excellent light harvesting ability. Diau and co-workers synthesized YDD0and YDD1 porphyrin dimer
with ƞ-value of 4.07% and 5.23%, respectively.43However, porphyrin arrays enhance their light harvesting ability as compared to monomeric
porphyrins whereas the possibility of reduced electron ejection and dye aggregation lowers the η-value of these arrays. Co-sensitization is a key tool
to enhance the DSSC performance by combining two or more dyes with complementary spectral features sensitized on semiconductor surface
together which extend the light harvesting ability as well as increase photocurrent of device. Firstly, Gräzel and co-workers reported
porphyrin/organic dye co-sensitization in 2010 where YD-2 co-sensitized with a organic dye which showed an enhanced device performance of
ƞ=6.9% as compared to individual porphyrin dye, ƞ = 5.6%.44 Co-sensitization increase the Jsc as well as Vocbecause of enhancement of light
harvesting ability and suppression of charge recombination. Chen and co-workers reported co-sensitization of porphyrinHD18 with PT-C6 which
displayed ƞ value of 10.1%.45
Functionalization of porphyrin at β-position provide a new way to synthesize β-functionalized porphyrin dyes for DSSCs.46-53β-Substituent have
better electronic communication with porphyrin π-system which can lead superior light harvesting ability. β-Functionalizedporphyrin dyes (GD1,
GD2 and Zn-1) were reported for DSSC applications with ƞ-value of 5-7%.46,47 Kim and co-workers prepared twoβ-push-pullporphyrins namely tda-
1b-d-Zn and tda-2b-bd-Zn with a higher PCE of 7.47% and 5.91%, respectively.48With suitable β-functionalization of porphyrins having strong
electron donating and acceptor groups can modulate the optical and electronic properties which further improve the performance of porphyrin
sensitizers.49,50β-formylation or bromination of porphyrins are of special interest due to their easy transformation in several other functional groups.51-
53β-Bromosubstituted porphyrins can befunctionalized by various donor groups via coupling reactions whereas β-formyl group can also be converted
into different acceptor groups e.g. carboxylic acid or cyanoacetic acid groups.54-59
To improve the efficiency of β-substituted porphyrins for DSSC, herein, we have synthesized three β-substituted push-pull Zn(II) porphyrin dyes by
varying the substituents at meso- and β-positions as shown in figure 1.β-monoformylporphyrins were utilized as the precursors for the targeted
porphyrin dyes in which formyl group undergo Knoevenagelcondensation with cyanoacetic acid to introduce cyano-carboxylic acid as the acceptor
group. Bulky groups such as tert-butyl and mesityl groups were introduced at meso position to prevent the dye aggregation of porphyrin molecules at
TiO2 surface and also provide an effective shielding to electron recombination with the electrolyte. Zn mesitylporphyrinic dye was further co-
sensitized with N719 organic dye to improve the power conversion efficiency for DSSC. These porphyrin molecules were synthesized in good yields
and characterized by UV-Vis., Fluorescence,1H-NMR and MALDI-MS spectroscopic techniques and elemental analysis.
OCH3
C(CH3)3
CN
COOH
CN
COOH
CN
COOH
N
N
N
N
N
N
N
N
N
N
Zn
Zn
C(CH3)3
Zn
(H3C)3C
OCH3
H3CO
N
N
Ph
Ph
C(CH3)3
OCH3
(ZnT(4-t-Bu)P(Ph2)(CN-COOH)
ZnT(4-MeO)P(CN-COOH)
ZnT(Mes)P(CN-COOH)
Figure1 Molecularstructures of synthesized Zn(II) porphyrin dyes.
2. Experimental Section
2.1 Material
Pyrrole, propionic acid, piperidine, p-chloroanil, Cu(OAc)2óH2O andZn(OAc)2ó2H2O were taken from HiMedia, India and N-Bromosuccinimide,
cyanoacetic acid, sodium hydroxide, BF3óOEt2 were taken from SRL, India and used as received. PhB(OH)2, 4-tert-butylbenzaldehyde and
mesityldehyde were purchased from Alfa Aesar. 4-Methoxybenzaldehyde andPd(PPh3)4 were taken from Sigma Aldrich and used without further
purification. DMF and K2CO3 were received from Rankem. POCl3 was taken from Thomas Baker. All solvents used in this work were distilled and
then used. Column chromatography was carried on silica gel (Rankem laboratory, 100-200 mesh). TBAPF6 was recrystallized twice with ethanol.
Dry CH2Cl2 used in CV analysis was distilled twice with P2O5 and third time from CaH2.
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2.2 Instrumentation and methods
Optical absorption spectra were recorded in dry CH2Cl2 using an Agilent Cary 100 spectrophotometer using a pair of quartz cells of 3.5 mL volume
and 10 mm path length and fluorescence spectra were recorded using a Hitachi F-4600 spectrofluorometer using a quartz cell of 10 mm path length.
1H NMRand 13C NMR spectra were recorded using JEOL ECX 400 MHz spectrometer using DMSO-d6 and CDCl3 as solvents. Mass spectra were
measured using a BrukerUltraflextreme-TN MALDI-TOF-MS spectrometer using HABA (2-(4՛-hydroxybenzeneazo)benzoic acid) as matrix.
Electrochemical measurements were carried out using CH instrument (CH 620E). A three electrode assembly used was consist of a platinum working
electrode, Ag/AgCl as a reference electrode and a Pt-wire as a counter electrode. The concentration of all compounds was maintained at 1 mM. All
measurements were performed in triple distilled CH2Cl2 containing 0.1M TBAPF6 as supporting electrolyte, which was degassed by argon gas
purging.
2.3The fabrication of DSSCs
The dye-sensitized TiO2 film was used as a photoanode in the solar cell and Pt foil used as counter electrode. Fluorine-doped SnO2 glass (TEC-15,
2.2 mm thickness, Solaronix) was used for transparent conducting electrodes. Both the substrate was first cleaned in an ultrasonic bath using a
detergent solution, acetone and ethanol, respectively (each step was 20 min long). The FTO glass plates were immersed into a 40 mM aqueous TiCl4
solution at 70 °C for 30 min and washed with water and ethanol and then FTO plates were sintered at 500 °C for 30 min. A 9-10 µm thick layer of 20
nm sized TiO2 particles deposited on FTO and sintered and were gradually heated under air flow at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C
for 15 min, and 500 °C for 15 min. Then 4 µm thick layer of 400 nm scattering layer was deposited over the 400 nm sized TiO2 particles and
gradually heated under air flow condition at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and 500 °C for 15 min. The electrodes were
again immersed into a 40 mM TiCl4 solution at 70°C for 30 minutes and washed with water and ethanol and sintered at 500°C. After cooling to room
temperature, FTO plates are heated at 70°C and electrodes were then immersed in a 0.1 mM of porphyrin sensitizer in DMF/MeOH (volume ratio:
4:1) solutions and the dye solution contains 0.4mM CDCA (Solaronix, Switzerland) as an additive. Then the photoanodes underwent dipping
condition for 20 h under dark conditions at a room temperature. After dye loading, photoanodes were washed with EtOH and dried by air flow. The
co-sensitization was carried out by dipping porphyrin sensitized (ZnT(Mes)P(CN-COOH)TiO2 in 0.5 mM N719 dye in ethanol for 2 h.The same
experiment was repeated for0.5 mMN719 dyealone. Finally electrolyte (AN-50, Solarnix, Switzerland) solution was introduced into the space
between the photoanodes and Pt sheet counter electrode.
2.4 Synthesis of Zn(II) Porphyrin dyes
2.4.1 Synthesis of 2ʹ-Cyano-12,13-diphenyl-3ʹ-(2-(5,10,15,20-meso-tetra-(4-tertiarybutylphenyl)PorphyrinatoZinc(II)yl)acrylic acid (ZnT(II)(
4-t-Bu)P(Ph2)(CN-COOH)):
H2T(4-t-Bu)P:6018.0 mL of Propionic acid was taken in a 100 mL round-bottom (RB) flask and warmed to about 10 minutes. To this, 0.5 mL (3
mmol) of 4-t-butylbenzaldehyde and 0.2 mL (3 mmol) pyrrole were added and then refluxed at 90 ºC for 90 minutes. The reaction mixture was
cooled to room temperature and filtered using G-4 sintered crucible washed and thoroughly with MeOH to remove polypyrrolic impurities. The
compound was purified through silica gel column chromatography using chloroform as an eluent. The solvent was evaporated using rotary
evaporator and the compound was recrystallized using CHCl3/MeOH. The yield was found to be 16% (100 mg). UV-Vis. (CH2Cl2, λmax in nm): 420,
517, 554, 590, 648. 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.87 (s, 8H, β-pyrrole-Ph), 8.15(d, 8H, J = 8 Hz, ortho-Ph), 7.76(d, 8H, J = 8 Hz, meta-
Ph), 1.61(s, 36H, t-Bu-H), -2.74 (s, 2H, -NH proton).Elemental analysis calcd.for C60H62N4: C, 85.88; H, 7.45; N, 6.68% and found C, 85.15; H,
6.22; N, 7.75%.
Cu(II)T(4-t-Bu)P: 100 mg (0.12 mmol) of H2T(4-t-Bu)P was dissolved in 15 mL of CHCl3.Cu(OAc)2óH2O (0.240 mg, 10 eq.) in 3.0 mL of MeOH
was added to this porphyrin solution in CHCl3 and refluxed for 30 minutes. After completion, the reaction mixture was washed two times with water
to remove excess of Cu metal salt and the organic layer was separated with CHCl3. The crude porphyrin was purified by silica gel column
chromatography using CHCl3 as eluent and recrystallized using CHCl3/MeOH. The yield of the product was found to be 90 % (95 mg). UV-Vis.
(CH2Cl2, λmax in nm): 412, 540.Elemental analysis calcd.for C60H60N4Cu: C, 80.01; H, 6.71; N, 6.22% and found C, 80.86; H, 6.27; N, 6.31%.
H2T(4-t-Bu)P(CHO):A 250 mL round bottom flask equipped with condenser, drying tube and thermometer was charged with DMF(1.0 mL)and
POCl3(0.9 mL)at 0 ºC and stirred for 15 minutes at room temperature until a golden-yellowish liquid was formed which indicates the formation of
Vilsmeier-complex. 90 mg(0.10 mmol) CuT(4-t-Butyl)Pwas diluted with a 15.0 mL of 1,2-dichloroethane under argon atmosphere. This porphyrinic
solution was then added to the Vilsmeier-complex and refluxed for 7 h at 80 ºC. After that the reaction mixture was cooled at room temperature for
overnight. Then, 2.0 mL conc. H2SO4 was added to the reaction mixture and stirred for 10 minutesin ice bath. It was then neutralized with ice cold
aqueous solution of NaOH (5.0 g in 100 mL distilled water).Then the reaction mixture was transferred to the separating funnel and CHCl3 was added.
The reaction mixture was washed with water until green color disappeared followed by washing with 20% aqueous solution of NaHCO3(2 × 100
mL). The organic layer was separated and dried over anhydrous sodium sulphate. The solvent was evaporated using rotary evaporator and the crude
porphyrin was purified by column chromatography using CHCl3as eluent. The yield of the product was found to be 80% (70 mg). UV-Vis. (CH2Cl2,
λmax in nm): 434, 539, 581, 606, 681. 1H NMR (CDCl3, 400 MHz): δ (ppm) 9.45 (s, 1H, -CHO proton), 9.25 (s, 1H, β-pyrrole-H) 8.93-8.88 (m, 4H,
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β-pyrrole-H), 8.80 (t, 2H, J = 4.8 Hz, β-pyrrole-H), 8.18-8.12 (m, 8H, ortho-PhH), 7.80-7.76 (m, 8H, , meta-PhH), 1.61-1.60 (m, 36H, t-Bu-H), -2.50
(s, 2H, -NH proton).Elementalanalysis calcd.for C61H62N4O: C, 84.49; H, 7.21; N, 6.46% and found C, 84.41; H, 6.51; N, 6.47%.
H2T(4-t-Bu)P(Br2)(CHO):In a 100 mL RB flask, H2T(4-t-Bu)P(CHO) (150 mg, 0.172 mmol) was taken in 40 mL of CHCl3.To this solution, NBS
(76 mg, 2.5 eq.) was added. The flask was covered with aluminum foil to protect the mixture from light and allowed to reflux for 24 h at 75 ºC. The
solvent was removed by rotary evaporation and the crude product was purified with column chromatography using 1:1 CHCl3/Hexane as an eluent.
The yield of the product was found to be 50% (90 mg). UV-Vis. (CH2Cl2, λmax in nm):441(5.16), 538(3.92), 687(3.75). 1H NMR (CDCl3, 400 MHz):
δ (ppm) 9.34 (s, 1H, -CHO proton), 9.16 (s, 1H, β-pyrrole-H), 8.94-8.85 (m, 4H, β-pyrrole-H), 8.20-8.09 (m, 8H, , ortho-PhH), 7.83-7.79 (m, 8H, ,
meta-PhH), 1.60-1.61 (m, 36H, t-Bu-H), -2.62 (s, 2H, -NH proton). MALDI-TOF-MS: m/z 1022.65 for [M]+ ( calcd. 1022.31).Elemental analysis
calcd.for C61H60Br2N4O: C, 71.48; H, 5.90; N, 5.47% and found C, 74.12; H, 5.80; N, 5.67%.13CNMR (100 MHz, CDCl3) : δ (ppm) 188.8, 186.6,
152.8, 152.5, 152.2, 152.1, 151.9, 151.8, 151.3, 149.2, 148.2, 141.1, 140.8, 139.5, 139.2, 139.0, 138.1, 138.1, 138.0, 137.7, 137.6, 137.3, 136.7,
135.8, 135.6, 135.3, 135.2, 130.0, 129.7, 129.2, 129.1, 125.1, 124.8, 124.7, 124.6, 124.5, 124.2, 123.2, 122.9, 122.1, 120.7, 120.6, 120.5, 120.3,
120.2, 35.0, 31.7, 22.7, 14.7.
H2T(4-t-Bu)P(Ph2)(CHO):The two-neck round-bottom flask (100 mL) containing distilled toluene (20 mL) was charged with (H2T(4-t-
Bu)(CHO)Br2) (50 mg, 0.048 mmol) and purged with argon gas. Then phenyl boronic acid (46 mg, 0.384 mmol, 8 eq.), K2CO3 (105 mg, 0.768
mmol, 16 eq.), Pd(PPh3)4 (12 mg, 20 mol %) were added and purged again with argon for 15 minutes. The reaction mixture was refluxed for 6 h at
100ºC under inert atmosphere. After completion of the reaction, the solvent was evaporated under reduced pressure using rotary evaporator. The
resulting residue was dissolved in minimum amount of CHCl3 and washed with saturated aqueous NaHCO3solution followed by 30% aqueous NaCl
solution. The organic layer was separated and dried over anhydrous Na2SO4. Solvent was evaporated using rotary evaporator. The crude porphyrin
1
was purified by column chromatography using CHCl3. The yield was found to be 50 % (26 mg). UV-Vis. (CH2Cl2, λmax in nm):443, 536, 682. H
NMR (CDCl3, 400 MHz): δ (ppm) 9.39 (s, 1H, -CHO proton), 9.27 (s, 1H, β-pyrrole-H), 8.79-8.75 (m, 2H, β-pyrrole-H), 8.61 (d, 2H, J = 4 Hz, β-
pyrrole-H), 8.20 (d, 2H, J = 8 Hz, ortho-PhH), 8.16 (d, 2H, J = 8.4 Hz, ortho-PhH), 7.81-7.72 (m, 8H, ortho and meta-PhH), 7.26-7.21 (m, 4H,
meta-PhH), 6.91-6.88 (m, 4H, β-pyrrole-PhH), 6.85-6.80 (m, 6H, β-pyrrole-PhH), 1.62-1.59 (m, 36H, t-Bu-H), -2.21 (s, 2H, -NH proton). MALDI-
TOF-MS: m/z 1019.365 for [M + H]+ ( calcd. 1018.55).Elemental analysis calcd.for C73H70N4O: C, 86.01; H, 6.92; N, 5.50% and found C, 86.15; H,
6.91; N, 5.06%.13CNMR (100 MHz, CDCl3) : δ (ppm) 189.2, 149.6, 149.0, 139.9, 139.4, 139.2, 138.6, 138.4, 137.9, 137.2, 137.0, 136.0, 135.4,
135.2, 135.0, 132.5, 132.3, 131.6, 131.5, 130.3, 130.1, 128.6, 126.5, 125.2, 124.9, 124.5, 124.0, 123.2, 122.9, 122.6, 122.0, 114.6, 35.0, 34.6, 33.9,
32.0, 31.7, 31.5, 29.8, 29.6, 29.5, 29.3, 29.0.
Zn(II)T(4-t-Bu)P(Ph2)(CHO):H2T(4-t-Bu)P(Ph2)(CHO) (20 mg, 0.019 mmol) was dissolved in 10mL of CHCl3. To this solution, Zn(OAc)2ó2H2O
(43 mg, 10 eq.) in 1 mL of MeOH was added and reflux for 30 minutes. The reaction mixture was then washed twice with water to remove excess of
zinc metal salt and then the organic layer was separated with CHCl3 and dried over anhydrous sodium sulphate. The crude porphyrin was purified by
column chromatography using CHCl3 as eluent and the yield of the product was found to be 85% (18 mg). UV-Vis (CH2Cl2, λmax in nm):440, 567,
608. Elemental analysis calcd.for C73H68N4OZn: C, 80.98; H, 6.33; N, 5.17% and found C, 80.27; H, 6.73; N, 5.01%.
Zn(II)T(4-t-Bu)P(Ph2)(CN-COOH): Zn(II)T(4-t-Bu)P(Ph2)(CHO) (15 mg, 0.0138 mmol) was dissolved in 10 mL of distilled CHCl3 in 100 mL RB
flask. Cyanoacetic acid (12.5 mg, 10 eq.) in 1 mL of CH3CN was added to this solution followed by drop wise addition of piperidine (0.01 mL)and
the resultant reaction mixture was refluxed overnight at 80 ºC. After that, the reaction mixture was washed with H3PO4 (2.00 M solution). The
organic layer was separated with CHCl3 and dried over sodium sulphate. The solvent was removed under vacuum and the crude porphyrin was
purified by silica gel column chromatography using CHCl3-MeOH (9: 1) as eluent. The yield of the product was found to be 87% (13 mg). UV-Vis
1
(CH2Cl2, λmax in nm):447, 574, 628. H NMR (DMSO-d6, 400 MHz): δ (ppm) 9.97 (s, 1H, -COOH), 9.38 (s, 1H, β-pyrrole), 8.69-8.57 (m, 2H, β-
pyrrole-H), 8.42-8.36 (m, 2H, , β-pyrrole-H), 8.08-7.92 (m, 11H, ortho, meta-PhH and 1-ethenyl-H), 7.76-7.70 (m, 4H, meta-PhH), 7.51(d, 2H, J=7.2
Hz, meta-PhH), 7.02(d, 4H, J=6.4 Hz, β-pyrrole-ortho- PhH), 6.81-6.68 (m. 6H, β-pyrrole-metaandp-PhH), 1.38-1.19 (m, 36H, t-Bu-H). MALDI-
TOF-MS: m/z 1147.249 for [M]+ ( calcd. 1147.59).Elemental analysis calcd.for C73H70N4O: C, 79.39; H, 6.05; N, 6.09% and found C, 79.87; H,
6.42; N, 5.12%.
2.4.2 Synthesis of 2ʹ-Cyano-3ʹ-(2-5,10,15,20-meso-tetra(4-methoxyphenyl)PorphyrinatoZinc(II))acrylic acid(Zn(II)T(4-MeO)P(CN-COOH)):
H2T(4-MeO)P: Synthetic procedure is similar to above reported Adler-Longo method. 16% yield. UV-Vis. (CH2Cl2, λmax in nm): 421(5.52),
518(4.09), 554(4.18), 596(3.60), 651(3.72).Elemental analysis calcd.for C48H38N4O4: C, 78.45; H, 5.21; N, 7.62% and found C, 78.26; H, 5.63; N,
5.10%.
Cu metallation of H2T(4-MeO)P and the formylation of CuT(4-MeO)P were carried out using the above mentioned procedures.
Cu(II)T(4-MeO)P: 87% yield. UV-Vis. (CH2Cl2, λmax in nm): 417, 541, 577.Elemental analysis calcd.for C48H36N4O4Cu: C, 72.39; H, 4.56; N,
7.04% and found C, 72.36; H, 4.10; N, 5.41%.
1
H2T(4-MeO)P(CHO): The yield was found to be 70%. UV-Vis. (CH2Cl2, λmax in nm): 437(5.64), 531(4.34), 572(4.09), 606(3.89), 675(4.08). H
NMR (CDCl3, 400 MHz): δ (ppm) 9.41 (s, 1H, -CHO proton), 9.34 (s, 1H, β-pyrrole-H) 8.90 (m, 4H, β-pyrrole-H), 8.80 (t, 2H, J = 5.6 Hz, β-pyrrole-
H), 8.14-8.10 (m, 8H, ortho-PhH), 7.30-7.27 (m, 8H, , meta-PhH), 4.10-4.08 (m, 12H, -OCH3 proton), -2.50 (s, 2H, -NH proton). MALDI-TOF-MS:
4

ACCEPTED MANUSCRIPT
m/z 762.93 for [M + H]+ ( calcd. 762.28). Elemental analysis calcd.for C49H38N4O5: C, 77.15; H, 5.02; N, 7.34% and found C, 77.26; H, 5.19; N,
7.93%.13CNMR (100 MHz, CDCl3): δ (ppm) 189.6, 160.4, 159.9, 159.7, 136.3, 136.0, 135.9, 135.8, 135.0, 134.4, 134.2, 122.5, 120.3, 120.0, 1119.8,
113.0, 112.5,
Zn(II)T(4-MeO)P(CHO): 80% (45 mg). UV-Vis. (CH2Cl2, λmax in nm):437(5.30), 560(3.97), 605(3.86). 1H NMR (CDCl3, 400 MHz): δ (ppm) 9.56
(s, 1H, -CHO proton), 9.41 (s, 1H, β-pyrrole-H), 8.97-8.93 (m, 6H, β-pyrrole-H), 8.16-8.07 (m, 8H, ortho-PhH), 7.33-7.21 (m, 8H,meta-PhH), 4.10-
4.08 (m,12H, MeO-proton). MALDI-TOF-MS: m/z 825.02 for [M + H]+ ( calcd. 825.20).Elemental analysis calcd.for C49H36N4O5Zn : C, 71.23; H,
4.39; N, 6.78% and found C, 71.56; H, 4.19; N, 6.06%.13C NMR (100 MHz, CDCl3): δ (ppm) 190.3, 160.1, 159.0, 159.4, 152.2, 152.1, 152.0, 151.7,
151.1, 150.5, 147.7, 146.7, 141.6, 136.0, 135.8, 135.7, 135.7, 135.6, 135.5, 134.9, 134.7, 133.3, 133.0, 132.9, 132.7, 132.4, 123.4, 121.3, 121.0,
120.6, 121.7, 112.4, 55.7.
Zn(II)T(4-MeO)P(CN-COOH):The Knoevenagel condensation was carried out as mentioned above. The yield of the product was found to be 88%
(38 mg). UV-Vis. (CH2Cl2, λmax in nm): 452(4.96), 568(3.92), 619(3.91). 1H NMR (DMSO-d6, 400 MHz): δ (ppm) 9.44 (s, 1H, β-pyrrole-H) 8.82-
8.74 (m, 6H, β-pyrrole-H), 8.10-7.93 (m, 9H,ortho-PhH and 1-ethenyl-H), 7.35-7.28 (m, 8H, meta-PhH), 4.03-4.01 (m, 12H, -OCH3-proton).
MALDI-TOF-MS: m/z 891.63 for [M]+ ( calcd. 891.20).Elemental analysis calcd.for C52H37N5O6Zn : C, 69.92; H, 4.17; N, 7.84% and found C,
69.86; H, 4.59; N, 7.90%.13CNMR (100 MHz, DMSO-d6): δ (ppm) 160.2, 159.4, 159.3, 151.0, 150.8, 150.1, 142.1, 136.1, 135.7, 135.3, 135.2,
135.0, 134.8, 133.1, 132.7, 132.6, 132.5, 132.4, 132.3, 132.2, 120.8, 120.5, 113.4, 114.8, 79.7, 55.9, 34.0, 34.3, 33.7, 33.1, 32.0, 31.8, 31.2, 29.2.
2.4.3 Synthesis of 2ʹ-Cyano-3ʹ-(2-(5,10,15,20-meso-tetramesityl) PorphyrinatoZinc(II))acrylic acid (Zn(II)T(Mes)P(CN-COOH)):
H2T(Mes)P:Tetramesitylporphyrin was synthesized according to reported Lindsey method.62Yield 29%. UV-Vis. (CH2Cl2, λmax in nm): 418(5.63),
1
514(4.20), 548(3.57), 594(3.70), 648(3.48). H NMR (CDCl3, 400 MHz): δ (ppm) 8.62 (s, 8H, β-pyrrole-H), 1.85 (s, 24H, ortho-ArCH3), 1.62 (s,
12H, para-ArCH3), -2.51 (s, 2H, -NH proton).Elemental analysis calcd.for C56H54N4: C, 85.89; H, 6.95; N, 7.15% and found C, 85.61; H, 6.38; N,
7.91%.
CHO
NH
N
N
NH
N
N
(i) Cu(OAc)2.H2O
(ii) DMF/POCl3,
CH2Cl2, H2SO4
HN
HN
70%
H2T(Mes)P(CHO)
H2T(Mes)P
(i) Zn(OAc)2.2H2O
CN
COOH
(ii) Cyanoacetic acid,
piperidine, H3PO4
N
N
N
N
Zn
77%
ZnT(Mes)P(CN-COOH)
Scheme 1: Synthetic Route of ZnT(Mes)P(CN-COOH) dye
Cu(II)T(Mes)P: Yield (80 mg). UV-Vis. (CH2Cl2, λmax in nm): 415, 539.Elemental analysis calcd.for C56H52N4Cu: C, 79.64; H, 6.21; N, 6.63% and
found C, 79.50; H, 6.13; N, 6.82%.
H2T(Mes)P(CHO): Theformylation was performed as mentioned above. The yield was found to be 70% (60 mg). UV-Vis. (CH2Cl2, λmax in nm):
437, 531, 572, 606, 675. 1H NMR (CDCl3, 400 MHz): δ (ppm) 9.41 (s, 1H, -CHO proton), 9.30 (s, 1H, β-pyrrole-H) 8.68-8.57 (m, 6H, β-pyrrole-H),
7.29-7.27 (m, 6H, , meta-PhH), 7.26-7.25 (m, 2H, , meta-PhH), 2.63 (d, 12H, J = 3.6Hz, para-ArCH3 proton), 1.89 (d, 18H, J = 7.6Hz, meta-ArCH3
proton), 1.83 (s, 6H, meta-ArCH3 proton), -2.21 (s, 2H, -NH proton). MALDI-TOF-MS: m/z 811.23 for [M + H]+ ( calcd. 811.43).Elemental analysis
calcd.for C57H54N4O: C, 84.41; H, 6.71; N, 6.91% and found C, 84.07; H, 6.21; N, 6.32%.13CNMR (100 MHz, CDCl3): δ (ppm) 190.4, 139.8, 139.5,
139.4, 139.3, 138.0, 137.9, 137.8, 137.6, 129.1, 128.56, 128.48, 128.3, 128.0, 127.9, 125.4, 120.6, 118.2, 117.9, 117.8, 32.0, 31.7, 30.43, 30.3, 29.8,
29.6, 29.5, 29.4, 29.2.
5

ACCEPTED MANUSCRIPT
1
Zn(II)T(Mes)P(CHO): 88%. UV-Vis. (CH2Cl2, λmax in nm):436(5.53), 562(4.23), 604(4.08). H NMR (CDCl3, 400 MHz): δ (ppm) 9.43 (s, 1H, -
CHO proton), 9.37 (s, 1H, β-pyrrole-H), 8.69-8.62 (m, 6H, β-pyrrole-H), 7.26-7.23 (m, 8H, meta-PhH), 2.61 (d, 12H, J = 4.8 Hz, para-ArCH3
proton), 1.85 (d, 18H, J = 7.2 Hz, meta-ArCH3 proton), 1.81 (s, 6H, meta-ArCH3 proton). MALDI-TOF-MS: m/z 873.27 for [M + H]+ ( calcd.
873.34).Elemental analysis calcd.for C57H52N4OZn: C, 78.29; H, 5.99; N, 6.41% and found C, 78.48; H, 6.09; N, 6.26%.13CNMR (100 MHz, CDCl3)
: δ (ppm) 191.3, 151.5, 151.1, 150.4, 155.6, 139.7, 139.5, 139.4, 139.3, 139.2, 138.9, 137.8, 137.7, 135.0, 132.3, 132.2, 131.4, 128.4, 127.9, 118.5,
116.1, 114.3, 45.0, 34.9, 34.0, 32.1, 31.8, 31.2.
ZnT(Mes)P(CN-COOH):The synthetic procedure of this compound is same as previously described.77% (25 mg). UV-Vis. (CH2Cl2, λmax in nm):
1
453(5.53), 562(4.23), 612(3.90). H NMR (DMSO-d6, 400 MHz): δ (ppm) 8.99 (s, 1H, β-pyrrole-H) 8.44-8.40 (m, 6H, β-pyrrole-H), 7.90 (s, 1H,
1-ethenyl-H), 7.23-7.21 (m, 8H, meta-PhH), 2.45 (d, 12H, J = 6.4 Hz, para-ArCH3-proton), 1.74 (d, 18H, J = 4 Hz, meta-ArCH3-proton), 1.68 (s,
12H, meta-ArCH3-proton). MALDI-TOF-MS: m/z 940.28 for [M + H]+ ( calcd. 940.35).Elemental analysis calcd.for C60H53N5O2Zn: C, 76.54; H,
5.67; N, 7.44% and found C, 76.98; H, 5.17; N, 7.31%.13CNMR (100 MHz, DMSO-d6): δ (ppm) 163.7, 150.6, 150.48, 150.44, 150.36, 150.0, 159.4,
139.3, 139.2, 139.0, 138.9, 138.88, 137.5, 132.0, 131.7, 131.6, 131.59, 131.3, 128.7, 128.2, 128.16, 128.0, 118.2, 117.6, 79.7, 34.7, 29.6.
C(CH3)3
C(CH3)3
CHO
NH
N
N
NH
N
(i) Cu(OAc)2.H2O
C(CH3)3
C(CH3)3
(H3C)3C
(H3C)3C
(ii) DMF/POCl3,
CH2Cl2, H2SO4
HN
N
HN
2.5 eq. NBS.,
Dist. CHCl3
(H3C)3C
(H3C)3C
H2T(4-t-Bu)P
C(CH3)3
80%
H2T(4-t-Bu)P(CHO)
CHO
NH
N
N
C(CH3)3
(H3C)3C
HN
C(CH3)3
Br
CN
COOH
Br
50%
(H3C)3C
N
N
N
Zn
N
H2T(4-t-Bu)P(Br2)(CHO)
C(CH3)3
(H3C)3C
Ph
C(CH3)3
CHO
Ph
PhB(OH)2, K2CO3,
Pd(PPh3)4
(i) Zn(OAc)2.2H2O
(H3C)3C
NH
N
(ii) Cyanoacetic acid,
piperidine, H3PO4
C(CH3)3
(H3C)3C
HN
N
87%
Ph
Zn(II)T(4-t-Bu)P(Ph2)(CN-COOH)
Ph
50%
H2T(4-t-Bu)P(Ph2)(CHO)
(H3C)3C
Scheme 2: Synthetic Route of Zn(II)T(4-t-Bu)P(Ph2)(CN-COOH) dye
3. Result and discussion
3.1 Synthesis
We designed and synthesized three new β-functionalized porphyrins by varying the meso-position with bulky groups (t-butyl and mesityl) and
incorporating electron-donating Ph groups at β-position of porphyrin. The targetedZn(II) Porphyrinswere synthesized by facile routes having high
yields. The synthetic routesforZn(II)T(Mes)P(CN-COOH) and Zn(II)T(4-t-Bu)P(Ph2)(CN-COOH) dyes were shown in scheme 1 and 2. The
synthetic route forZn(II)T(4-MeO)P(CN-COOH) was shown in ESI (scheme S1). Firstly, meso-tetraarylporphyrins were synthesized by Adler-Longo
and Lindsey method which further underwent formylation to synthesize free-base monoformyltetrarylporphyrins.60,62 The formylation and Cu/Zn
6

ACCEPTED MANUSCRIPT
metallation have been carried out according to reported procedure in good yields (70-90%).27,61,55To obtain the target porphyrinic dyes, Zn(II)
monoformyl porphyrins were directly converted into targeted dyes by condensation with cyanoaetic acid.46 But, in case of t-Bu-Ph porphyrinic dyes,
regioselective dibromination and Suzuki coupling reaction with phenyl boronic acid were carried out in order to append an electron-donating
phenylsubstituent on β-pyrrolic position of porphyrin core52and then it further proceeded for Zn metallation and Knoevenagel condensation to obtain
desired porphyrin dye.46All dyes were synthesized in very good yields and characterized by UV-Vis, Fluorescence,1H-NMR, 13C-NMRand MALDI-
TOF-MS spectroscopic techniques. The photophysical and electrochemical measurement have also been performed for all theseZn(II) porphyrin
dyes.
3.2 Electronic spectral andNMR studies
The UV-Vis absorption spectra of all porphyrin dyes and their precursors were recorded in CH2Cl2 at 298 K. The typical strong Soret and Q bands
wereobserved due to ꢀ −π* transitions of conjugated porphyrin systems. The spectral data of all porphyrins are listed in Table 1. The monoformyl
porphyrins H2T(4-MeO)P(CHO), H2T(Mes)P(CHO) and H2T(4-t-Bu)P(CHO) exhibited 10-12 nm shift in the absorption band from their parent
unsubstituted porphyrin.27
Table 1. UV-Visible and Emission data of all synthesized porphyrin molecules in CH2Cl2 at 298 K.
Porphyrins
B band, λmax, nm
Emission,
Quantum Yield, Life Time,τ (ns)
λem, nm
ϕf
H2T(4-MeO)P
421(5.52), 518(4.09), 554(4.18), (3.60), 651(3.72)
437(5.64), 531(4.34), 572(4.09), 606(3.89), 675(4.08)
437(5.30),560(3.97), 605(3.86)
660
693
0.1956
0.0675
0.0082
0.0055
0.1362
0.0919
0.0162
0.0114
0.2384
0.0786
0.0008
8.23
4.17
0.84
1.73
9.65
7.49
1.58
1.16
7.67
5.72
0.87
H2T(4-MeO)P(CHO)
ZnT(4-MeO)P(CHO)
ZnT(4-MeO)P(CN-COOH)
H2T(Mes)P
637
452(4.96), 568(3.92), 619(3.91)
682
418(5.63)(4.20), 548(3.57), 594(3.70), 648(3.48)
433, 310, 348, 526, 569, 606, 663
658
H2T(Mes)P(CHO)
678
ZnT(Mes)P(CHO)
436(5.53), (4.23), 604(4.08)
622
ZnT(Mes)P(CN-COOH)
H2T(4-t-Bu)P
399(sh)(4.51), 453(4.89), 562(3.89), 612(3.90)
420, 517, 554, 590, 648
666
688
H2T(4-t-Bu)P(CHO)
H2T(4-t-Bu)P(CHO)(Br2)
434(5.57),(4.33), 573(4.07), 606(3.96), 665(4.07)
441(5.16), 538(3.92), 687(3.75)
688
622,731
H2T(4-t-Bu)P(CHO)(Ph2)
443, 682
447, 628
723
651
0.0372
0.0032
*
ZnT(4-t-Bu)P(Ph2)(CN-COOH)
0.74
*lifetime was too low to detect. aThe values in parentheses refer to logɛ values, ɛ in dm3 mol-1 cm-1; sh = shoulder; error in quantum yield values 1–2%.
Figure2a represents absorption spectra of target Zn(II) porphyrin dyes. All synthesized porphyrin dyes exhibited broad Soret band in the range of
445-455 nm and moderate Q bands in the range of 560-630 nm. Porphyrin dyes having mesityl and methoxy groups at meso-position showed
marginal red-shift (7-8 nm) in Soret band as compared to dyes having phenyl group at β-pyrrole position.The synthesized porphyrins were also
characterized by fluorescence spectroscopy to investigate the effect of different groups on porphyrin moiety.
(a)
(b)
(c)
Figure 2: (a) UV-Vis absorption spectra of Zn(II) Porphyrin dyes. (b) Fluorescence Spectra of compound all Zn(II) Porphyrin dyes. (c) Comparison of Fluorescence
Spectra of Zn(4-MeO)P(CHO), Zn(Mes)P(CHO) to their corresponding dyes ZnT(4-MeO)P(CN-COOH) and ZnT(Mes)P(CN-COOH) in CH2Cl2 at 298K.
7

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The emission spectra of all three Zn porphyrin dyes in CH2Cl2 at 298 K were presented in fig. 2b and their emission data is illustrated in table 1.
ZnT(4-MeO)P(CN-COOH), ZnT(4-t-Bu)P(Ph2)(CN-COOH) and ZnT(Mes)P(CN-COOH) dyes exhibited broad emission spectral features.
Fluorescence quenching was observed for all Zn(II) porphyrin dyes compared to their parent Zn(II) monoformyl porphyrins as shown in fig 2c,
which indicatesan efficient charge transfer from donor to acceptor. Figures S1-S6 in the ESI represent the UV-Vis and fluorescence spectral features
of all synthesized porphyrins. The Quantum yield decreases in this order: ZnT(Mes)P(CN-COOH) >ZnT(4-MeO)P(CN-COOH) >ZnT(4-t-
Bu)P(Ph2)(CN-COOH) which suggest an efficient charge-transfer in ZnT(4-t-Bu)P(Ph2)(CN-COOH) as compared to other two dyes. The ZnT(4-t-
Bu)P(Br2)(CHO) exhibited fluorescence quenching and led to the decrement in the quantum yield due to the heavy atom effect of the bromo
substituent at β-position.
Quantum yields (ϕ) were calculated by using the equation given below
ϕsample = (ϕref × Asample × Iref)/(Aref × Isample
)
1H-NMR spectra of Zn porphyrin dyes and their precursor molecules were recorded in DMSO-d6 and CDCl3, respectively at 298 K and shown in the
ESI(figure S7-14).The key feature of proton NMR of all mono-formyl porphyrin is -CHO, β-pyrrole, meso-phenyl and imino protons. All porphyrins
exhibited their characteristic signals for mesityl CH3--OCH3and tert.-butyl protons. The characteristic signal of formyl group and adjacentβ-pyrrole-
H appeared in the range of 9.15-9.50 ppm whereas other β-pyrrole-H signals were found in the range of 8.57-8.95 ppm. The signal of formyl proton
for H2T(4-t-Bu)P(CHO) showed downfield shift by 0.04-0.06 ppm relative to other mono-formyl porphyrins. The β-pyrrole proton ofH2T(4-t-
Bu)P(Ph2)(CHO) and H2T(Mes)P(CHO) exhibited upfield shift than other porphyrins.The ortho-, meta-PhH and imino- proton signals of H2T(4-t-
Bu)P(Br2)(CHO) and H2T(4-t-Bu)P(Ph2)(CHO) exihibited downfielded and upfielded shift, respectively as compared to other porphyrins due to
different electronic nature of bromo and phenyl group. The β-pyrrole PhH of H2T(4-t-Bu)P(Ph2)(CHO) were found in the range of 6.80-6.90 ppm.
All Zn porphyrin dyes show one proton signals for β-pyrrole-H adjacent to cyanoacetic acid group in the range of 8.91-9.38 ppm whereas other β-
pyrrole-H signals were found in the range of 8.35-8.85 ppm. The ethenyl protons of Zn porphyrin dyes were generally featured with ortho-phenyl
protons (PhH)whereas the ethenyl proton of ZnT(Mes)P(CN-COOH) was found as a singlet at 7.90 ppm due to the absence of ortho protons. The
-COOH proton of cyanoacetic acid group of ZnT(4-t-Bu)P(Ph2)(CN-COOH) was appeared as a singlet at 9.97 ppm. The13C-NMR spectra of newly
synthesised porphyrins were recorded in DMSO-d6 and CDCl3 at 298 K (figures S17-24 in the ESI). However, the solubility ofZnT(4-t-
Bu)P(Ph2)(CN-COOH) was too poor in deuterated solvents (DMSO-d6 and CDCl3) to record the 13C NMR spectrum even at higher number of scans
(upto 10000). The number and position of carbon signals in the spectra are in accordance with proposed structure of the corresponding porphyrins.
The formyl carbon appeared around 190 ppm in parent monoformyl porphyrins and ethenyl carbon signal was found at 79 ppm in target Zn
porphyrin dyes. The methoxy carbon signals were found at 55 ppm in methoxy porphyrins series. The porphyrin macrocycle signals appeared in the
range of 110-160 ppm whereas alkyl carbon signals appeared in the range of 30-60 ppm in all synthesised porphyrins. The MALDI-TOF mass data
were also recorded and shown in the ESI (figure S25-S33).
3.4 Electrochemical Redox properties
The electrochemical studies were performed for all porphyrins to examine the effect of substitutions on the porphyrin core. The electrochemical
redox data of porphyrins were measured in CH2Cl2 containing 0.1M TBAPF6 at 298 K and listed in table 2. The redox potentials of porphyrins can be
affected by the variation of meso- and β-substituent and with different metal ions.
(a)
(b)
Fig. 3 (a) CV of Zn(II) porphyrin Dyes in CH2Cl2 at 298 K and (b) Energy level diagram of dyes compared with the conduction band of TiO2 and I-/I3- electrolyte.
Figure 3a represents the cyclic voltammograms (CVs) of all β-substituted Zn porphyrin dyes in CH2Cl2 at 298 K. The CV diagrams of all other
porphyrins were shown in ESI (fig. S34). The first oxidation potential of all porphyrin molecules were found the range of 0.84 to 1.16 V whereas the
8

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first reduction potential varied from -0.90 to -1.34 V. Free base tetraarylporphyrins show anodic shift in their first ring oxidation (0.02-0.16 V) and
reduction potential as compared to H2TPP. In case of H2T(Mes)P(CHO), a dramatic cathodic shift in their first oxidation potential (0.07 V) and
reduction potential (0.012 V) as compared to H2T(Mes)P was observed whereas an opposite trend was observed for H2T(4-t-Bu)P(CHO) as
compared to H2T(4-t-Bu)P. The first ring oxidation potential of H2T(4-MeO)P(CHO) was unaltered as compared to H2T(4-MeO)P whereas cathodic
shift was observed for the first ring reduction potential.
Table 2. Cyclic Voltammetry Studies depicting Oxidation & Reduction Potentials of all the Synthesized Porphyrins in CH2Cl2 at 298 K.
Porphyrins
Oxidation (V)
Reduction (V)
∆
E1/2 (V)
I
II
III
-
1.56a
I
II
-1.31a
H2T(4-MeO)P
0.99
1.02
1.15
1.40
-1.05 i
-1.17 a
2.04
2.19
H2T(4-tbu)P
-
H2T(Mes)P
1.16
0.99
1.63
1.16
-
-1.14 a
-
2.30
2.07
H2(4-MeO)P(CHO)
-
-1.08
-1.30a
H2T(4-tbu)P(CHO)
H2T(Mes)P(CHO)
1.05
1.09
1.06
0.87
0.95
0.84
0.87
0.95
1.27
1.63a
1.21
1.16
1.31
1.09
1.08
1.30
1.51a
-1.07
-1.26a
-0.90
-1.20
-1.34
-0.98a
-1.18a
-1.23a
-1.29a
2.11
2.35
1.97
2.06
2.29
1.82
2.05
2.18
-
-
-
-
-
-
-
-
H2T(4-tbu)P(CHO)(Br2)
ZnT(4-MeO)P(CHO)
-
-
ZnT(Mes)P(CHO)
-
ZnT(4-MeO)P(CN-COOH)
ZnT(4-tbu)P(Ph2)(CN-COOH)
ZnT(Mes)P(CN-COOH)
-1.54a
-
-
Scan rate = 0.1 V/s. aData taken from DPV. b∆E1/2 = Ioxd. - Ired. Pt working and Pt wire counter electrodes were used.
These Zn porphyrin dyes exhibited anodic shift in their first reduction and oxidation potential (0.03-0.11 V) as compared to ZnTPP whereas the first
oxidation potential of ZnT(4-MeO)P(CN-COOH) has shown only marginal changes. Figure 3b depicted the HOMO-LUMO energy levels of Zn
-
porphyrin dyes and compared with the conduction band of TiO2 and the electrolyte I-/I3 . The result showed that the HOMO levels of all the dyes are
-
sufficiently higher than the energy level of electrolyte (I-/I3 ) which indicates that the oxidized sensitizers could be easily regenerated by the
electrolyte. The LUMO levels of these dyes are significantly lower than the conduction band energy levels of TiO2 which provides an efficient
electron injection into the TiO2 conduction band from the excited sensitizers. This shows the feasibility of electron transfer in DSSC.
3.5 Photovoltaic Studies
Dye-sensitized solar cells (DSSCs) were fabricated with β-substituted push pullZn(II)-porphyrin-sensitized 12 µm thick TiO2 photoanodes (0.16 cm2)
assembled into standard sandwiched cells having Pt sheet as counter electrode. The space between the photoanodes is filled with iodide/triiodide
-
electrolyte I-/I3 . The ZnT(Mes)P(CN-COOH) dye was co-sensitized with N719 organic dye and also studied for DSSC application. The photocurrent
density-voltage (I-V characteristics) of synthesized Zn(II) porphyrin dyes are shown in figure 4a and the corresponding photovoltaic data under AM
1.5G solar light illumination (power 100 mW cm-2) with an active area of 0.16 cm2, is listed in table 3. The data for commercial N719dye under
similar conditions is also added for comparison.
Table 3. Photovoltaic parameters of Zn(II) porphyrins under AM 1.5G sun illumination (power 100 mW cm2) with an active area of 0.16 cm2
Zn-Porphyrin dyes
ZnT(Mes)P(CN-COOH)
ZnT(4-MeO)P(CN-COOH)
ZnT(4-t-Bu)P(Ph2)(CN-COOH)
ZnT(Mes)P(CN-COOH)(N719 dye)
STD-N719 (ref)
Voc (V)
0.575
0.570
0.58
0.63
0.61
Jsc (mA/cm2 )
FF (%)
70
70
69
72
η (%)
7.8
5.7
4.3
11.8
6.7
3.13
2.27
1.72
5.35
2.77
68
The overall power conversion efficiencies (PCE) lie in the range of 1.72-5.35%, follows the order as ZnT(4-t-Bu)P(CN-COOH) (η = 1.72%) <
ZnT(4-MeO)P(CN-COOH) (η = 2.27 %) < ZnT(Mes)P(CN-COOH) (η = 3.13 %) < ZnT(4-Mes)P(CN-COOH)(N719 dye) (η = 5.35%).
ZnT(Mes)P(CN-COOH) exhibited higher PCE efficiency as compared to ZnT(4-t-Bu)P(CN-COOH) and ZnT(Mes)P(CN-COOH) as dye aggregation
has been effectively suppressed by mesityl group. Notably, the co-sensitization of ZnT(Mes)P(CN-COOH) with N719 dye enhanced the PCE
efficiency upto 5.35%, with a Jsc of 11.8 mA cm-2, a Voc of 630 mV and a fill factor (FF) of 72%. N719 increase the device voltage which leads to
high photo conversion efficiency for DSSCs. The incident photon-to-current conversion efficiency (IPCE) spectra were recorded as a function of
incident wavelength using a Newport IPCE system as shown in figure4b and 4c. These spectra slightly broader but follow the absorption spectra of
the corresponding dyes where red shift seen in Soret and Q band absorption of Zn(II) porphyrin dyes in IPCE spectra.The broadness of IPCE spectra
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demonstrate the better light harvesting ability of Zn porphyrin dyes. As shown in figure 4b, ZnT(Mes)P(CN-COOH) exhibited broader IPCE spectra
as compared to other porphyrin dyes. Co-sensitized ZnT(Mes)P(CN-COOH) with N719 dye shows much broader IPCE spectrum in the range 400-
700 nm as similar to Ru-dyes which suggests that co-sensitization of ZnT(Mes)P(CN-COOH) dye with N719 dye improve their light harvesting
ability as well as their Power conversation efficiency (PCE) upto 5.35%. The broader the IPCE spectra, the greater the light harvesting ability of Zn
porphyrin dyes and higher the PCE value as depicted in table 3. Bulky groups such as tert-butyl and mesityl groups present in ZnT(4-t-
Bu)P(Ph)2(CN-COOH) and ZnT(Mes)P(CN-COOH) supressed the dye aggregation of these dyes at TiO2 surface and also reduce the possibilities of
charge recombination with the electrolyte that enhance the efficiency of DSSCs.
The IPCE spectra of co-sensitized ZnT(Mes)P(CN-COOH)(N719) porphyrin showed that the IPCE value of co-sensitized porphyrin dye is higher
than ZnT(Mes)P(CN-COOH) and N719 dye with higher Jsc,, Voc, and FF of72%, corresponding to overall conversion efficiency of 5.35%. The IPCE
values for the dyes follow the order: ZnT(4-t-Bu)P(Ph)2(CN-COOH) <ZnT(4-MeO)P(CN-COOH) <ZnT(Mes)P(CN-COOH) <co-sensitized
ZnT(Mes)P(CN-COOH)(N719) which is in agreement with Jsc values obtained and the corresponding PCE values as shown in table 3. Further, the J-
V curves under standard AM 1.5G illumination arein qualitative agreement with the photo-action spectra of these Zn(II) porphyrin dyes. Hence, the
mesityl group prevents the dye aggregation and charge recombination. Futher, the co-sensitization of N719 dye with Zn(II) porphyrin enhances the
light harvesting capacity in visible region as shown figure 4c.
(a)
(b)
(c)
Fig. 4 (a) The typical I-V characteristics of the DSSCs using Zn(II) porphyrin dyes. IPCE action spectra of (b) Zn(II) porphyrin dyes and(c) ZnT(4-Mes)P(CN-
COOH)(N719 dye) and N719 dye only
3.6 Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy (EIS) is a powerful tool to analyze interfacial and charge-transfer processes in the DSSC.63Figure5 shows
the Nyquist plots for solar cells based on synthesized Zn(II) porphyrin sensitizers under a bias voltage of -0.6 V in dark condition. Three semicircles
are observed in Nyquist plots. The first semicircle is ascribed to the charge transfer resistance at the counter electrode-electrolyte interface in the high
frequency region.The larger semicircle is associated with interfacial charge transfer resistances (Rct) at the TiO2-dye-electrolyte interface in the
middle frequency region and low frequency region is assigned to impedance related to ion diffusion resistance in the electrolyte of the solar cell.
Fig. 5 Nyquist plots for synthesized Zn(II) porphyrin sensitizers under forward bias voltage of -0.6 V in dark condition.
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Table 4. Electrochemical Impedence spectroscopy parameters of Zn(II) porphyrins under under a bias voltage of -0.6 V in dark condition.
Zn-Porphyrin dyes
Rct(Ω)
94
Cµ (µF/cm2)
τr (ms)
7.8
IPCE
27
ZnT(Mes)P(CN-COOH)
ZnT(4-MeO)P(CN-COOH)
8.3
6
48
2.88
19
ZnT(4-
t
-Bu)P(Ph2)(CN-COOH)
65
4.4
2.73
12
13
50
ZnT(Mes)P(CN-COOH)(N719 dye)
108
10.2
-
Rct defines the charge recombination rate between injected electrons and electron acceptors (I3 ) in the electrolyte and a higher value Rct slow down
the electron transfer between the electrolyte and the TiO2 with increase in the Voc values.64,65Rct is related to the charge recombination rate between
-
injected electrons and electron acceptors (I3 ) in the electrolyte and a large Rct implies slow electron transfer between the electrolyte and the TiO2,
resulting in an increase in the Voc values.66The fitted Rct increased in the order of ZnT(4-MeO)P(CN-COOH) <ZnT(4-t-Bu)P(CN-COOH)
<ZnT(Mes)P(CN-COOH) <ZnT(4-Mes)P(CN-COOH)-Co-sensitized N719.
This trend is consistent with the order of Voc. From the Rctvalue, electron life time (τ) was obtained by implying the following equation, τ = Rct
×
Cchem (Cchem = Chemical capacitance).66The electron lifetime (τ) reflects the response time constant for recombination and which is positively
correlated withVoc. ZnT(4-Mes)P(CN-COOH)(N719) exhibited highest chemical capacitance as shown in table 4 and this is correlated with the
charge recombination rate. A longer electron lifetime indicates the effective reduction of the reverse reaction of the injected electron (slower
recombination rate) with the triiodide in the electrolyte and therefore a higher photovoltage.67 For these dye molecules, the calculated electron
lifetime values increased in the order of ZnT(4-t-Bu)P(CN-COOH) ( 2.73 ms) <ZnT(4-MeO)P(CN-COOH) (2.88 ms) <ZnT(Mes)P(CN-COOH) (7.8
ms) <ZnT(Mes)P(CN-COOH)(N719) (12 ms).ZnT(Mes)P(CN-COOH)co-sensitized N719 dye exhibited the high recombination resistance (108 Ω),
high capacitance (10.2×10-3 F/cm2 ) and impressive longest electron lifetime (12 ms) which confirms the effective suppression of the electron
recombination of the injected electron with the I3- in the electrolyte and TiO2 electrode. This results in higherJsc (11.8 mA/cm2), Voc (0.630 V), FF (72
%) and a highest PCE value of 5.35% for ZnT(Mes)P(CN-COOH)(N719)dye. On the basis of above studies,ZnT(Mes)P(CN-COOH) showed better
photovoltaic performance over other Zn(II) porphyrin dyes but its co-sensitization with N719 dye improved the efficiency of the cells due to its
relatively increased photocurrent voltage and effective retardation of dye aggregation. The EIS results of Zn(II) porphyrin dyes are in good
agreement with the photovoltaic performance.
4. Conclusions
Three new β-functionalized push-pull Zn(II) porphyrin dyes have been successfully designed, synthesized by facile synthetic route in good yields and
characterized by various spectroscopic techniques. These dyes exhibited remarkable red-shift as compared to ZnTPP due to the electron-withdrawing
nature of cyanoacetic acid group and charge-transfer interactions. The decrement in the quantum yield and lifetime as well as the comparison of
HOMO-LUMO energy levels of Zn(II) porphyrin dyes from cyclic voltammetry studies clearly suggest the feasibility of electron transfer for DSSC
appliation. Photovoltaic studies of Zn(II) porphyrin dyes exhibited the power conversion efficiency from 1.72 to 3.13% but co-sensitization of
Zn(II)T(Mes)P(CN-COOH) with N719 dye improved the PCE upto 5.35%. The overall power conversion efficiencies (PCE) and the IPCE values for
the dyes follow the order as ZnT(4-t-Bu)P(CN-COOH) (η = 1.72%) <ZnT(4-MeO)P(CN-COOH) (η = 2.27 %) <ZnT(Mes)P(CN-COOH) (η = 3.13
%) < ZnT(Mes)P(CN-COOH)(N719) (η = 5.35%). This work demonstrates the effect of meso- and β-substituents on electronic spectral features and
photovoltaic properties.
Acknowledgements
We are grateful for the support provided by Board of Research in Nuclear Science (2012/37C/61/BRNS/2776), Council of Scientific and Industrial
Research (01(2694)/12/EMR-II) and Science and Engineering Research Board (SB/FT/CS-015/2012). KP and NS thank Ministry of Human
Resource Development (MHRD), India for the senior research fellowship(SRF) and national postdoctoral fellowship (N-PDF), respectively.
aDeparment of Chemistry, Indian Institute of Technology Roorkee, Roorkee - 247667, India. E-Mail: sankafcy@iitr.ac.in. Tel: +91-1332-284753;
Fax: +91-1332-273560.
bPolymers and Advanced Materials Laboratory, National Chemical Laboratory, Pune-411008, India. E-mail: k.krishnamoorthy@ncl.res.in; Tel:
+91-20-2590-3075; Fax: +91-20-2590-2615.
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Electronic Supplementary Information
Facile Synthesis of β-Substituted “Push-Pull” Zn(II) Porphyrins for
DSSC Applications
Kamal Prakash,a Shweta Manchanda,aVediappan Sudhakar,b Nidhi Sharma,a Muniappan Sankar*aand
Kothandam Krishnamoorthy*b
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee – 247667, India
bPolymers and Advanced Materials Laboratory, National Chemical Laboratory, Pune-411008, India.
E-mail: k.krishnamoorthy@ncl.res.in; Tel: +91-20-2590-3075; Fax: +91-20-2590-2615.
Table of content
Page No.
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Scheme S1.Synthetic Route of Zn(II)T(4-MeOPh)P(CN-COOH)
Fig. S1 UV-Vis Spectrum of (a) Free Base (4-tbu) Porphyrins( the intensity of Q
band is enhanced by a factor of 5 for clarity.) (b) Zn(II) (4-tbu) Porphyrins in
CHCl3 at 298K.
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Fig. S2 UV-Vis Spectrum of (a) Free Base (4-MeO) Porphyrins( the intensity of Q
band is enhanced by a factor of 5 for clarity.) (b) Zn(II) (4-MeO) Porphyrins in
CHCl3 at 298 K.
Fig. S3 UV-Vis Spectrum of (a) Free Base (Mes) Porphyrins( the intensity of Q
band is enhanced by a factor of 5 for clarity.) (b) Zn(II) (Mes) Porphyrins in CHCl3
at 298 K.
Fig. S4 UV-Vis Spectrum of Zn(II) mono-formylPorphyrins ( the intensity of Q
band is enhanced by a factor of 5 for clarity.) in CHCl3 at 298 K.
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Fig. S5 Emission Spectrum of (a) Free Base tetraarylPorphyrins (b) mono-
formylPorphyrins in CHCl3 at 298 K.
Fig. S6 Emission Spectrum of (a) Free Base (4-tbu) Porphyrins (b) Zn(II)
Porphyrins in CHCl3 at 298 K.
Fig. S71H-NMR spectrum of H2T(4-t-Bu)P(CHO) in CDCl3 at 298 K
Fig. S81H-NMR spectrum of H2T(4-t-Bu)P(Br2)(CHO) in CDCl3 at 298 K
Fig. S91H-NMR spectrum of H2T(4-t-Bu)P(Ph2)(CHO) in CDCl3 at 298 K
Fig. S101H-NMR spectrum of ZnT(4-t-Bu)P(Ph2)(CN-COOH) in DMSO at 298 K
Fig. S111H-NMR spectrum of H2T(4-MeO)P(CHO) in CDCl3 at 298 K
Fig. S121H-NMR spectrum of ZnT(4-MeO)P(CHO) in CDCl3 at 298 K
Fig. S131H-NMR spectrum of ZnT(4-MeO)P(CN-COOH) in DMSO-d6 at 298 K.
Fig. S14 1H-NMR spectrum of H2T(Mes)P(CHO) in CDCl3 at 298 K
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Fig. S151H-NMR spectrum of ZnT(Mes)P(CHO) in CDCl3 at 298 K
Fig. S16 1H-NMR spectrum of ZnT(Mes)P(CN-COOH) in DMSO-d6 at 298 K
Fig. S17 13C-NMR spectrum of H2T(4-t-Bu)P(Br2)(CHO) in CDCl3 at 298 K
Fig. S1813C-NMR spectrum of H2T(4-t-Bu)P(Ph2)(CHO) in CDCl3 at 298 K
Fig. S1913C-NMR spectrum of H2T(4-MeO)P(CHO) in CDCl3 at 298 K
Fig. S2013C-NMR spectrum of ZnT(4-MeO)P(CHO) in CDCl3 at 298 K
Fig. S2113C-NMR spectrum of ZnT(4-MeO)P(CN-COOH) in DMSO-d6 at 298 K
Fig. S2213C-NMR spectrum of H2T(Mes)P(CHO) in CDCl3 at 298 K
Fig. S2313C-NMR spectrum of ZnT(Mes)P(CHO) in CDCl3 at 298 K
Fig. S2413C-NMR spectrum of ZnT(Mes)P(CN-COOH) in DMSO-d6 at 298 K
Fig. S25 MALDI-TOF-MS of H2T(4-tbu)P(Br2)(CHO)
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Fig. S26 MALDI-TOF-MS of H2T(4-t-Bu)P(Ph2)(CHO)
Fig. S27 MALDI-TOF-MS of ZnT(4-tbu)P(Ph2)(CN-COOH)
Fig. S28 MALDI-TOF-MS of H2T(4-MeO)P(CHO)
Fig. S29 MALDI-TOF-MS of ZnT(4-MeO)P(CHO)
Fig. S30 MALDI-TOF-MS of ZnT(4-MeO)P(CN-COOH)
Fig. S31 MALDI-TOF-MS of H2T(Mes)P(CHO)
Fig. S32 MALDI-TOF-MS of ZnT(Mes)P(CHO)
Fig. S33 MALDI-TOF-MS of ZnT(Mes)P(CN-COOH)
Fig. S34 Cyclic Voltagrammes of (a) free base tetraarylporphyrins (b) free base
mono-formylporphyrins in CH2Cl2 at 298 K
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OCH3
OCH3
CHO
NH
N
NH
N
(i) Cu(OAc)2.H2O
OCH3
OCH3
H3CO
H3CO
(ii) DMF/POCl3,
CH2Cl2, H2SO4
N
HN
N
HN
70%
H3CO
H3CO
(i) Zn(OAc)2.2H2O
H2T(4-MeO)P(CHO)
H2T(4-MeO)P
(ii) Cyanoacetic acid,
piperidine, H3PO4
OCH3
CN
COOH
N
N
N
N
Zn
OCH3
H3CO
88%
Zn(II)T(4-MeO)P(CN-COOH)
H3CO
Scheme S1. Synthetic Route of Zn(II)T(4-MeOPh)P(CN-COOH)
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Fig. S1 UV-Vis Spectrum of (a) Free Base (4-t-Bu) Porphyrins ( the intensity of Q band is
enhanced by a factor of 5 for clarity.) (b) Zn(II) (4-tbu) Porphyrins in CHCl3 at 298 K.
Fig. S2UV-Vis Spectrum of (a) Free Base (4-MeO) Porphyrins ( the intensity of Q band is
enhanced by a factor of 5 for clarity.) (b) Zn(II) (4-MeO) Porphyrins in CHCl3 at 298 K.
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Fig. S3UV-Vis Spectrum of (a) Free Base (Mes) Porphyrins( the intensity of Q band is enhanced by a
factor of 5 for clarity.) (b) Zn(II) (Mes) Porphyrins in CHCl3 at 298 K.
Fig. S4UV-Vis Spectrum of Zn(II)mono-formylPorphyrins ( the intensity of Q band is enhanced by a
of 5 for clarity.) in CHCl3 at 298 K.
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Fig. S5 Emission Spectrum of (a) Free Base tetraarylPorphyrins (b) mono-formylPorphyrins in
CHCl3 at 298 K.
Fig. S6 Emission Spectrum of (a) Free Base (4-t-Bu)Porphyrins (b) Zn(II) Porphyrins in CHCl3 at
298 K.
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Fig. S71H-NMR spectrum of H2T(4-t-Bu)P(CHO) in CDCl3 at 298 K
Fig. S81H-NMR spectrum of H2T(4-t-Bu)P(Br2)(CHO) in CDCl3 at 298 K
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Fig. S91H-NMR spectrum of H2T(4-t-Bu)P(Ph2)(CHO) in CDCl3 at 298 K
Fig. S101H-NMR spectrum of ZnT(4-t-Bu)P(Ph2)(CN-COOH) in DMSO-d6 at 298 K
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Fig. S111H-NMR spectrum of H2T(4-MeO)P(CHO) in CDCl3 at 298 K
Fig. S121H-NMR spectrum of ZnT(4-MeO)P(CHO) in CDCl3 at 298 K
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m-PhH
o-PhH &
-OCH3 H
β-pyrroleH
Ethynyl H
Fig. S131H-NMR spectrum of ZnT(4-MeO)P(CN-COOH) in DMSO-d6 at 298 K.
Fig. S141H-NMR spectrum of H2T(Mes)P(CHO) in CDCl3 at 298 K
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m-Ph H
m-ArCH3 H
m-ArCH3 H
p-ArCH3 H
-CHO H
β-pyrrole H
Fig. S151H-NMR spectrum of ZnT(Mes)P(CHO) in CDCl3 at 298 K
m-ArCH3
H
p-ArCH3
H
m-ArCH3
H
Fig. S161H-NMR spectrum of ZnT(Mes)P(CN-COOH) in DMSO-d6 at 298 K
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C(CH3)3
CHO
NH
N
C(CH3)3
(H3C)3C
N
HN
Br
Br
(H3C)3C
Fig. S1713C-NMR spectrum of H2T(4-t-Bu)P(Br2)(CHO) in CDCl3 at 298 K
C(CH3)3
CHO
NH
N
N
C(CH3)3
(H3C)3C
HN
Ph
Ph
(H3C)3C
Fig. S1813C-NMR spectrum of H2T(4-t-Bu)P(Ph2)(CHO) in CDCl3 at 298 K
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OCH3
CHO
NH
N
N
OCH3
H3CO
HN
H3CO
Fig. S1913C-NMR spectrum of H2T(4-MeO)P(CHO) in CDCl3 at 298 K
OCH3
CHO
N
N
N
N
Zn
OCH3
H3CO
H3CO
Fig. S2013C-NMR spectrum of ZnT(4-MeO)P(CHO) in CDCl3 at 298 K
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OCH3
CN
COOH
N
N
N
N
Zn
OCH3
H3CO
H3CO
Fig. S2113C-NMR spectrum of ZnT(4-MeO)P(CN-COOH) in DMSO-
d6 at 298 K
Fig. S22 13C-NMR spectrum of H2T(Mes)P(CHO) in CDCl3 at 298 K
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