Synthesis, structure and physicochemical properties of a saddle-distorted porphyrin with a peripheral carboxyl group

Journal of Porphyrins and Phthalocyanines

J. Porphyrins Phthalocyanines 2011; 15: 421–432

DOI: 10.1142/S1088424611003379

Published at http://www.worldscinet.com/jpp/

Synthesis, structure and physicochemical properties of a

saddle-distorted porphyrin with a peripheral carboxyl group

Muniappan Sankar^a, Tomoya Ishizuka^a, Zeqing Wang^b, Tingli Ma^b, Motoo Shiro^cand

Takahiko Kojima*^a◊

^aDepartment of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennoudai 1-1-1,

Tsukuba, Ibaraki 305-8571, Japan

^bState Key Laboratory of Fine Chemicals, Dalian University of Technology, Gaoxinyuanqu, Dalian 116024, P. R.

China

^cX-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara, Akishima, Tokyo 196-8666, Japan

Dedicated to Professor John A. Shelnutt on the occasion of his 65^thbirthday

Received 21 April 2011

Accepted 11 May 2011

ABSTRACT: A saddle-distorted porphyrin bearing a carboxyl group as a hydrogen-bonding site on a

meso-phenyl group was synthesized and characterized. A supramolecular structure with intermolecular

hydrogen bonding was revealed by X-ray diffraction analysis. The effects of the peripheral carboxyl

group on the physicochemical properties of the porphyrin as well as on self-assembly were investigated

by spectroscopic measurements in solutions. The redox properties of the porphyrin and its Zn(II) complex

were also studied by electrochemical measurements and their application to dye-sensitized solar cells

was examined.

KEYWORDS: dodecaphenylporphyrin, hydrogen bonding, supramolecules, photovoltaics.

have reported on the formation of porphyrin nanotube

INTRODUCTION

structures by use of Mo^V-DPP complexes, where poly-

Supramolecular structures based on porphyrins and

oxomatalates (POMs) were included in the inner space

their derivatives have been intensively investigated for

of nano-sized tubular assemblies made of [Mo(DPP)

many decades due to the interest for the relevance to the

(O)(H₂O)]⁺with hydrogen bonds between an axial aqua

natural photosynthetic center [1], as well as the applicabi-

ligand at the central Mo^Vion in the porphyrin core and the

lity to functional materials such as photovoltaic cells [2].

terminal oxo ligands of the POMs [8]. In the nanotube,

In order to construct porphyrin supramolecules, various

non-covalent interactions such as hydrogen bonding [3],

the DPPs were integrated with intermolecular π–π inte-

ractions among the peripheral phenyl groups. The dipro-

π–π interaction [4], halogen bonding [5] and van der

Waals interaction [6] have been utilized so far. Among

so many porphyrin derivatives, we have focused on

non-planar porphyrins with saddle-type conformational

distortion and have reported various kinds of supramole-

cular structures by use of 2,3,5,7,8,10,12,13,15,17,18,20-

dodecaphenylporphyrin (H₂DPP) [7]. For example, we

tonated form of H₂DPP (H₄DPP²⁺) was also assembled

by intermolecular π–π interactions among the periphe-

ral phenyl groups to afford nanochannel structures [9],

which involve a relatively small inner space compared

to those of the nanotubes based on Mo-DPP complexes.

The inner space can be used for selective inclusion of

electron-donating guest molecules such as TTF or hydro-

quinones in the single crystals. In the host-guest system,

H₄DPP²⁺has been revealed to act as an electron acceptor

[9a, 9b, 10] in the photoinduced electron transfer from

included electron-donating guest molecules to exhibit

^SPP full member in good standing

*Correspondence to: Takahiko Kojima, email: kojima@chem.

tsukuba.ac.jp, tel/fax: +81 29-853-4323

422

M. Sankar et al.

photo-conducting properties [9a]. Furthermore, H₄DPP²⁺

behaves as a hydrogen donor for hydrogen bond for-

mation and takes two functional counter anions having

carboxyl groups such as a Zn(II)-phthalocyanine (ZnPc)

complex with 4-pyridine carboxylate as an axial ligand

[11] and ferrocene-carboxylate (Fc) [12], which strongly

interact with the H₄DPP²⁺core by hydrogen bonding. In

the supramolecular triads, H₄DPP²⁺also functions as an

electron acceptor and ZnPc or Fc as an electron donor to

allow us to observe photoinduced electron transfer reac-

tions to form charge-separated states.

In this study, we have introduced a carboxyl group as

a hydrogen-bonding site at the periphery of H₂DPP to

develop supramolecular structures by virtue of the inter-

molecular hydrogen bonding. This hydrogen bonding is

expected to be useful for emergence of novel structural

motifs and to control the spacial arrangement of porphy-

rin supramolecular structures such as nanochannels in the

crystal.

and ﬁltered by a silica gel column eluted with CH₂Cl₂

and a crude product was obtained, which was slightly

contaminated with corresponding chlorin derivatives. To

the obtained fraction containing the precursor compound

was added 2,3-dichloro-5,6-dicyano-p-benzoquinone

(DDQ) (1.85 g, 8.15 mmol) and the mixture was allowed

to reﬂux for 8 h until no green spot was observed on a

TLC plate. The reaction mixture was concentrated to the

minimum volume and puriﬁed with column chromato-

graphy on a silica gel column eluted with CH₂Cl₂, and

the target compound was obtained as the second fraction.

The solvent of the fraction was evaporated and the resi-

dual solid was recrystallized from CHCl₃/CH₃OH. Yield:

1.623 g (2.41 mmol, 10%). ¹H NMR (270 MHz; CDCl₃;

Me₄Si): δ_H, ppm 8.80 (d, 6H, J = 5 Hz, β-pyrrole-H), 8.73

(d, 2H, J = 6 Hz, β-pyrrole-H), 8.52 and 8.28 (ABq, J_AB

=

8 Hz, 4H, o- and m-H of ester-Ph), 8.22 (dd, 6H, J = 7.0,

2 Hz, o-H of meso-Ph), 7.82–7.71 (m, 9H, m and p-H of

meso-Ph), 4.09 (s, 3H, -COOCH₃), -2.80 (s, 2H, NH).

UV-vis (CH₂Cl₂): λ_max, nm (log ε) 418 (5.70), 515 (4.33),

550 (3.97), 590 (3.82), 647 (3.77). MS (MALDI-TOF):

m/z 673.63 (calcd. for C₄₆H₃₃N₄O₂[M + H]⁺: 673.79).

Preparation of CuTPP(CO₂Me) [14]. H₂TPP(CO₂Me)

(1.25 g, 1.85 mmol) was dissolved in CHCl₃(200 mL) and

tothesolutionwasaddedthesuspensionofCu(OAc)₂·H₂O

(3.712 g, 18.6 mmol) in CH₃OH (50 mL). The reaction

mixture was reﬂuxed for 2 h, evaporated to dryness, and

the residue was dissolved in CHCl₃(150 mL). The CHCl₃

solution was washed with water, dried over Na₂SO₄, and

concentrated to the minimum volume. The solution was

loaded on a silica gel column, which was eluted with

CH₂Cl₂. The obtained crude product was recrystallized

from CH₂Cl₂/CH₃OH.Yield: 1.275 g (1.738 mmol, 94%).

UV-vis (CH₂Cl₂): λ_max, nm (log ε) 415 (5.73), 538 (4.34),

570 (sh). MS (MALDI-TOF): m/z 735.59 (calcd. for

C₄₆H₃₁N₄O₂Cu [M + H]⁺: 735.32).

Preparation of CuTPP(CO₂Me)-Br₈[14, 15].

CuTPP(CO₂Me) (1.267g, 1.72 mmol) was dissolved

in CHCl₃(200 mL) and the solution of Br₂(2.8 mL,

55 mmol) in CHCl₃(100 mL) was added dropwise for

30 min and the mixture was stirred for 4 h at rt. A solution

of pyridine (6 mL) in CHCl₃(100 mL) was added very

slowly and the reaction mixture was stirred overnight at

rt. Then, excess bromine was quenched with aqueous

solution of Na₂S₂O₅(20%, 200 mL), washed with water

(200 mL × 2), and concentrated to the minimum volume.

The solution was loaded on a silica gel column, which

was eluted with CH₂Cl₂. The crude product obtained

was recrystallized from CHCl₃/MeOH. Yield: 1.5 g of

EXPERIMENTAL

General

All commercially available chemicals were purchased

from appropriate sources and used as received unless

otherwise mentioned. Methanolic solution of (Me)₄NOH

(25 wt% solution, Aldrich) was received and appropri-

ate concentrations were prepared in spectrocopic-grade

1

methanol and used for UV-vis and H NMR titrations.

All NMR measurements were performed on a JEOL

EX270 spectrometer. UV-vis absorption spectra were

measured in spectroscopic-grade solvents on a Shimadzu

UV-3600 spectrophotometer at rt. Fluorescence spectra

were recorded on a Hamamatsu Photonics C9920-02

spectrometer at rt. Cyclic voltammograms were obtained

at rt under Ar on an ALS 710D electrochemical analyser

using a Pt wire as a counter electrode, a Pt working elec-

trode, and Ag/AgNO₃in CH₃CN as a reference electrode.

MALDI-TOF-MS spectra were measured on a Bruker

UltraﬂeXtreme-TN MALDI-TOF/TOF spectrometer

using dithranol as a matrix.

Synthesis

Preparation of mono-methyl ester of TPP

(5,10,15,20-tetraphenylporphyrin) (H₂TPP(CO₂Me))

[13]. Propionic acid (400 mL) was heated to 140 °C in a

1 L round-bottomed ﬂask, and to this solvent, were added

4-(methoxycarbonyl)benzaldehyde (3.97 g, 0.0242 mol),

benzaldehyde (7.6 mL, 0.074 mol) and pyrrole (6.7 mL,

0.097 mol) and the mixture was allowed to reﬂux for

90 min. The solution was cooled to rt and the solvent was

removed by evaporation under reduced pressure to give

a pink-gel residue. Hot water was added to the residue

to remove excess propionic acid and pink powder was

obtained. The obtained powder was dissolved in CH₂Cl₂

green powder (1.1 mmol, 64%). UV-vis (CH₂Cl₂): λ_max

,

nm (log ε) 365 (4.40), 453 (sh), 466 (5.11), 582 (4.21),

623 (3.78). MS (MALDI-TOF): m/z 1365.22 (calcd.

for C₄₆H₂₂N₄O₂Br₈Cu [M]⁺: 1365.47). Anal. calcd. for

C₄₆H₂₂Br₈N₄O₂Cu·H₂O: C; 39.93, H; 1.75, N; 4.05, found:

C; 39.70, H; 1.61, N; 3.81.

Preparation of H₂TPP(CO₂Me)-Br₈[14, 15].

CuTPP(CO₂Me)-Br₈(1.5 g, 1.1 mmol) was dissolved in

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SyntheSIS, StruCture anD PhySICOCheMICal PrOPertIeS Of a SaDDle-DIStOrteD POrPhyrIn

423

CHCl₃(100 mL), and the solution was cooled to 0 °C.

To the cooled solution, was slowly added conc. H₂SO₄

(10 mL) at 0 °C and the reaction mixture was stirred for

20 min until the CHCl₃layer became colorless. It was

transferred to a separating funnel, washed with water

(200 mL × 2) and then neutralized with NH₃aq (18%,

200 mL). The organic layer was washed with water

(150 mL), dried over Na₂SO₄and then the solvent was

evaporated. The residual solid was puriﬁed with column

chromatography on silica gel using CHCl₃as an eluent.

The obtained fraction was evaporated to dryness and

dried under vacuum. Yield: 1.42 g (1.09 mmol, 99%). ¹H

NMR (270 MHz; CDCl₃): δ_H, ppm 8.50 and 8.25 (ABq,

J_AB= 8 Hz, 4H, o- and m-H of ester-Ph), 8.17 (d, 6H,

J = 6 Hz, o-H of meso-Ph), 7.84 (m, 9H, m- and p-H of

(9:1 v/v). After adding a few drops of NEt₃, the solvent of

the collected fraction was evaporated to give a crude pro-

duct of (HNEt₃)[H₂DPP(COO)] (0.125 g). The resulting

residual solid (22 mg) of the crude porphyrin, (HNEt₃)

[H₂DPP(COO)], was dissolved in THF (5 mL), and to

the solution, HClO₄aq (60%, 0.1 mL) was added and

stirred for a few minutes. The solvent was evaporated

to dryness to yield green gel, and the resulting gel was

dissolved in CHCl₃(15 mL) and washed with water and

dried under vacuum. The green residue was recrystalli-

zed from CHCl₃/ether (1:6, v/v). Yield: 23 mg of green

powder (15.6 µmol, 91%). ¹H NMR (270 MHz; CDCl₃):

δ_H, ppm 8.05–7.84 (m, 8H, o-H of carboxy-Ph and o-H

of meso-Ph), 7.78 (d, 2H, J = 8 Hz, m-H of carboxy-Ph),

7.40–7.00 (m, 9H, m-, p-H of meso-Ph), 7.00–6.40 (m,

40H, β-pyrrole-Ph). UV-vis (CH₂Cl₂): λ_max, nm (log ε)

430 (sh), 487 (5.38), 586 (3.62), 651 (4.06), 704 (4.69).

FL (CH₂Cl₂): λ_max, nm 759. MS (MALDI-TOF): m/z

1269.49 (calcd. for C₉₃H₆₄N₄O₂⁺[M]⁺: 1269.53). Anal.

calcd. for C₉₃H₆₄N₄Cl₂O₁₀·H₂O: C; 75.15, H; 4.48, N;

3.77, found: C; 75.23, H; 4.57, N; 3.50.

meso-Ph), 4.1 (s, 3H, -COOCH₃). UV-vis (CH₂Cl₂): λ_max

,

nm (log ε) 370 (4.40), 470 (5.27), 570 (3.93), 628 (4.12),

740 (3.85). MS (MALDI-TOF): m/z 1305.28 (calcd.

for C₄₆H₂₅N₄O₂Br₈[M + H]⁺: 1304.96). Anal. calcd. for

C₄₆H₂₄N₄Br₈O₂·CHCl₃: C; 39.66, H; 1.77, N; 3.94, found:

C; 40.08, H; 1.91, N; 3.92.

Preparation of H₂DPP(CO₂Me). H₂TPP(CO₂Me)-

Br₈(0.35 g, 0.27 mmol), PhB(OH)₂(0.785 g, 6.44 mmol),

Pd(PPh₃)₄(59mg, 54µmol)andK₂CO₃(1.669g, 12.1mmol)

were loaded in a 200 mL three-necked ﬂask. This mixture

was dried under vacuum for 30 min and then the inside

of the ﬂask was ﬁlled with Ar. Toluene (100 mL) was

added and the mixture was heated to 90–100 °C under

Ar atmosphere and stirred for 36 h. After cooling to rt,

the volatile was removed by evaporation. The residue

was dissolved in CHCl₃(100 mL) and then washed with

water (100 mL), 12.5% NH₃aq (100 mL) and again with

water (150 mL) and ﬁnally brine (150 mL). The orga-

nic phase was dried over Na₂SO₄and puriﬁed on a silica

gel column eluted with 3–8% EtOAc in CHCl₃. Yield:

Preparation of ZnDPP(CO₂H). The crude sample

of (HNEt₃)[H₂DPP(COO)] (100 mg) right after column

chromatography was dissolved in CHCl₃(50 mL) contai-

ning a few drops of NEt₃. To the solution, was added

Zn(OAc)₂·2H₂O (130 mg, 0.592 mmol) in CH₃OH (5 mL)

and the mixture was heated to reﬂux for 2 h. After cooling

to rt, the solution was evaporated to dryness. The green

residual solid was dissolved in CHCl₃(40 mL) to wash

with water and the organic layer was dried over Na₂SO₄.

The solvent was removed under reduced pressure and

the residual solid was recrystallized from CHCl₃/Et₂O

(1:5 v/v). Yield: 100 mg of green powder (0.075 mmol,

1

95%). H NMR (270 MHz; CDCl₃): δ_H, ppm 8.05–7.25

(m, 8H, o-H of meso-Ph), 6.75–6.50 (m, 51H, m-, p-H

of meso-Ph and β-pyrrole-H). UV-vis (CH₂Cl₂): λ_max, nm

(log ε) 375 (4.37), 466 (5.17), 593 (4.01), 646 (3.82). FL

(CH₂Cl₂): λ_max, nm (φ) 735 (0.002). MS (MALDI-TOF):

m/z 1331.01 (calcd. for C₉₃H₆₀N₄O₂Zn [M]⁺: 1330.91).

Anal. calcd. for C₉₃H₆₀N₄O₂Zn·H₂O: C; 82.81, H; 4.63,

N; 4.15, found: C; 82.65, H; 4.48, N; 4.32.

1

0.254 g of green powder (0.198 mmol, 74%). H NMR

(270 MHz; CDCl₃): δ_H, ppm 7.57 (d, J = 5 Hz, 6H, o-H

of meso-Ph), 7.59 and 7.36 (ABq, J_AB= 8 Hz, 4H, o- and

m-H of ester-Ph), 6.50–6.90 (m, 49H, β-pyrrole-Ph

and m, p-H of meso-Ph), 3.95 (s, 3H, -COOCH₃). UV-

vis (CH₂Cl₂with one drop Et₃N): λ_max, nm (log ε) 376

(4.48), 469 (5.21), 566 (4.01), 618 (4.02), 725 (3.74). MS

(MALDI-TOF): m/z 1282.23 (calcd. for C₉₄H₆₅N₄O₂

[M + H]⁺: 1282.58).

X-ray crystallography

Preparation of [H₄DPP(CO₂H)](ClO₄)₂. H₂DPP-

(CO₂Me) (0.15 g, 0.12 mmol) was dissolved in a THF/

EtOH mixed solvent (3:1 v/v, 40 mL). To the solution

was added the solution of KOH (1.6 g, 0.029 mol) in

water (4 mL) and reﬂuxed under Ar overnight. After coo-

ling to rt, the solvent of the reaction mixture was remo-

ved under reduced pressure and acidiﬁed with 2 M HCl

(40 mL) yielded green precipitate. The precipitate was

washed with water several times, ﬁltered and dried under

vacuum. The diprotonated porphyrin was neutralized by

addition of Et₃N and the volatile was evaporated to obtain

the crude product. The crude porphyrin was puriﬁed on

a silica gel column eluted with CHCl₃/CH₃OH mixture

A dark-green single crystal of [H₄DPP(CO₂H)]

(OH)₂(for a vacuum-dried sample: Anal. calcd. for

C₉₃H₆₄N₄O₄·5H₂O: C; 80.27, H; 5.36, N; 4.03, found: C;

80.30, H; 5.43, N; 3.84) was obtained by recrystalliza-

tion of the crude sample of (HNEt₃)[H₂DPP(COO)] from

CHCl₃/MeOH with vapor diffusion of i-PrOH. The dif-

fraction data were measured on a Rigaku Mercury CCD

system at Rigaku Corporation (Akishima, Tokyo, Japan).

The data were integrated, scaled and corrected for absorp-

tion with the CrystalClear software [16]. Crystallographic

data: C₉₃H₆₂N₄O₂·2OH·6C₃H₈O, FW = 1662.10, monocli-

nic, space group P2₁/c, a = 30.628(2) Å, b = 30.1805(5)

Å, c = 40.738(3) Å, β = 113.212(8)°, V = 34,609(4) Å³,

J. Porphyrins Phthalocyanines 2011; 15: 423–432

424

M. Sankar et al.

T=123K,Z=16,D_c=1.10g.cm^-3,λ(CuKα)=1.54187Å,

Photovoltaic characterization. The current-voltage

curves of the DSSCs were obtained by applying an exter-

nal bias to the cell and by measuring the generated pho-

tocurrent under white light irradiation with a Keithley

digital source meter (Keithley 2601, USA). The intensity

of the incident light was 100 mW/cm², and the instrument

was equipped with a 300 W solar simulator (Solar Light

Co., INC., USA) that served as the light source. The pho-

ton ﬂux was determined by a power meter (Nova, Ophir

optronics Ltd.) and a calibration cell (BS-520, s/n 019,

Bunkoh-Keiki Co., Ltd.).

38,1208 reﬂections measured, 62,920 unique (R_int

=

0.0471) which were used in all calculations. All cal-

culations were performed using the Yadokari XG

crystallographic software package [17]. The structure

was solved by direct method (SHELXL-97) [18] and

reﬁned by full-matrix least-squares methods on F²with

3146 parameters: R1 = 0.1296 (I > 2σ(I)) and wR2 =

0.3552, GOF = 1.143, max/min residual density 1.870/

-0.397 eÅ^-3. In the course of the structure reﬁnements,

we could not determine the positions of the solvent mole-

cules of crystallization including water and 2-propanol

molecules, which were clearly identiﬁed in difference

Fourier maps, because of their severe disorder. Their

contribution was thus subtracted from the diffraction

pattern by the “Squeeze” program [19]. Crystallogra-

phic details are described in the cif ﬁle as Supporting

information.

RESULTS AND DISCUSSION

Synthesis

The target porphyrin was synthesized by the pro-

cedure described in Scheme 1, including abbrevia-

tions of synthetic intermediates and precursors together

with schematic descriptions of their structures. The

monoester-substituted TPP (5,10,15,20-tetraphenylpor-

phyrin), H₂TPP(CO₂Me) [13, 14], was synthesized by

Adler method with condensation of 4-(methoxycarbonyl)

benzaldehyde and benzaldehyde with pyrrole in the ratio

of 1:3:4 in propionic acid and puriﬁcation by column

chromatography gave the precursor in 10% yield. It was

further metalated with Cu(OAc)₂·H₂O, followed by bro-

mination using Br₂/pyridine in 60% yield for two steps

[14, 15]. Then, the central Cu ion in the porphyrin was

removed by acid demetalation followed by neutralization

with NH₃(aq) solution yielded freebase H₂TPP(CO₂Me)-

Br₈in quantitative yield. Suzuki-Miyaura coupling reac-

tion of H₂TPP(CO₂Me)-Br₈with phenylboronic acid

afforded H₂DPP(CO₂Me) in 74% yield. H₂DPP(CO₂Me)

was subjected to alkaline hydrolysis with KOH to afford

K[H₂DPP(COO)]. The potassium salt was neutralized

Device fabrication

Preparation of the working electrodes. The screen-

printable TiO₂and SnO₂colloidal pastes and working

electrodes were prepared according to the procedure

developed by Ma and coworkers [20]. The working

electrodes made of TiO₂and SnO₂were immersed

into a CH₂Cl₂/EtOH (9:1 v/v) solution of (HNEt₃)

[H₂DPP(COO)] or ZnDPP(CO₂H) (with and without

0.2 mM chenodeoxycholic acid (CDCA)) for 18 h. The

dye-sensitized TiO₂and SnO₂electrodes (thickness:

12–14 µm, area: 0.16 cm²) and a platinized counter

electrode were assembled to fabricate solar cells by

sandwiching a redox (I^-/I₃^-) electrolyte solution. The

electrolyte was composed of 0.03 M I₂, 0.06 M LiI, 0.6 M

1-butyl-3-methylimidazolium iodide (BMII), 0.1 M

guanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine

(4TBP) in acetonitrile and 3-methoxypropionitrile.

Br

Ph

Cu

Br

Ph

N

H

Br

Ph

Br

N

+

1) Cu(OAc)₂·H₂O

2) Br₂/pyridine

H

CO₂Me

Ph

PhCHO

+

H

EtCOOH

MeO₂C

CHO

Ph Br

Br

Ph

H₂TPP(CO₂Me)

CuTPP(CO₂Me)-Br₈

60%

10%

H₂SO₄

Ph

Br

Ph

M

Ph

Br

Ph

Br

Ph

Br

Ph

Br

PhB(OH)₂

Pd(PPh₃)₄/K₂CO₃

N

1) KOH

H

CO₂H

CO₂Me

H

2) HClO₄

toluene

Ph Ph

Ph Br

Ph

Br

H₂DPP(CO₂Me)

H₂TPP(CO₂Me)-Br₈

~100%

74%

M = H₄,[H₄DPP(COOH)](ClO₄)₂

84%

1) NEt₃

2) Zn(OAc)₂·2H₂O

M = Zn,

ZnDPP(CO₂H) 95%

Scheme 1. Synthesis of [H₄DPP(CO₂H)](ClO₄)₂and ZnDPP(COOH)

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SyntheSIS, StruCture anD PhySICOCheMICal PrOPertIeS Of a SaDDle-DIStOrteD POrPhyrIn

425

with HCl aq and the obtained crude chloride salt of the

diprotonated porphyrin, [H₄DPP(CO₂H)]Cl₂, was neutral-

ized again with excess triethylamine (Et₃N) and the crude

product was puriﬁed with column chromatography. The

perchloric acid salt of H₂DPP(COOH) ([H₄DPP(CO₂H)]

(ClO₄)₂) as the diprotonated species at the porphyrin core

was readily obtained by the treatment of crude (HNEt₃)

[H₂DPP(COO)], which was obtained from the column

chromatography, in CH₂Cl₂with aqueous HClO₄. In addi-

tion, after the neutralization of the porphyrin core, it was

metalated with Zn(OAc)₂·2H₂O to give ZnDPP(CO₂H) in

quantitative yield.

proton source during the recrystallization [21, 22]. The

[H₄DPP(CO₂H)]²⁺ion holds two OH^-ions as the coun-

ter anions, which formed hydrogen bonds with the pyr-

role NH of the diprotonated porphyrin core. The mean

O···N distance of the 16 hydrogen bonds was 3.19 Å in

the asymmetric unit containing four [H₄DPP(CO₂H)]

(OH)₂. The OH^-ion also forms hydrogen bonding with

i-PrOH, as shown in Fig. 1b, and this hydrogen bond-

ing may weaken the basicity of the OH^-ion to stabilize

it without accepting proton from [H₄DPP(CO₂H)]²⁺. In

the crystal packing, the [H₄DPP(CO₂H)]²⁺ions formed

a nanochannel structure similarly to the case of H₄DPP²⁺

[9] and the inner space of the nanochannel was occupied

with co-crystallized solvent molecules such as i-PrOH

and water. The pore size of the nanochannel was 0.97 ×

0.75 nm, which was almost the same as that of H₄DPP²⁺

nanochannnel (1.0 × 0.7 nm) [9]. The noteworthy dif-

ference between the nanochannel of [H₄DPP(CO₂H)]²⁺

and that of H₄DPP²⁺is relative spatial arrangement of

the channels: In the case of H₄DPP²⁺, nanochannels were

closely packed by π–π stacking among the adjacent

channels. On the other hand, nanochannels formed by

[H₄DPP(CO₂H)]²⁺were relatively separated due to the

Crystal structure of [H₄DPP(COOH)]²⁺

(HNEt₃)[H₂DPP(COO)] was recrystallized from a

CHCl₃/MeOH/2-propanol (i-PrOH) mixed solvent with

vapor diffusion method to afford dark-green single crys-

tals and the crystal structure was determined by X-ray

diffraction analysis (Fig. 1). The asymmetric unit con-

tained four independent porphyrin molecules and each

porphyrin core of the four molecules was found to

be diprotonated probably by the solvent or water as a

Fig. 1. Crystal structure of [H₄DPP(CO₂H)](OH)₂: (a) a top view; (b) a side view including hydrogen bonds between OH^-ions and

co-crystallized i-PrOH molecules; (c) a view of the crystal packing along the crystallographic a axis including solvent molecules

(i-PrOH and H₂O) of crystallization. Atoms in (a) and (b) are represented by thermal ellipsoids at the 50% probability level

J. Porphyrins Phthalocyanines 2011; 15: 425–432

426

M. Sankar et al.

formation of intermolecular hydrogen bonding between

the carboxyl groups at the peripheries of the porphyrins,

which belonged to neighboring channels.

727 nm (Q-band) in CH₂Cl₂. That of ZnDPP(CO₂H) in

CH₂Cl₂exhibited the same features as that of ZnDPP (red

line in Fig. 3a) and [H₄DPP(CO₂H)]²⁺shows a typical

spectrum for a diprotonated porphyrin with a red-shifted

Soret band (green line in Fig. 2a) compared to that of the

corresponding neutral porphyrin core, which is reminis-

cent of that of H₄DPP²⁺[9b, 10]. The electronic effect on

the UV-vis absorption of the DPP core by the introduc-

tion of the carboxyl group was very small: The shifts of

the Soret bands for (HNEt₃)[H₂DPP(COO)] (470 nm),

There were four independent carboxyl groups found

in the crystal and they formed hydrogen-bonding pairs

as shown in Fig. 2. Two of them formed a complemen-

tary hydrogen-bonding pair as shown in Fig. 2a. One of

[H₄DPP(CO₂H)]²⁺ions, which is depicted on the left-

hand side in Fig. 2b, exhibited positional disorder of the

carboxyl group into pseudo-orthogonal directions with

0.5 population for each. The half of them formed the

complementary hydrogen bonding. The other half exhib-

ited a partial hydrogen bonding, where one of the two

oxygen atoms of the carboxyl group formed hydrogen

bonding with two of the other carboxyl group and the

remaining oxygen atom, which did not participate in the

hydrogen bonding, interacted with severely disordered

i-PrOH molecules of crystallization (see Fig. 2b) [23].

As a result of this interchannel hydrogen bonding, the

distances between nanochannels increased (interchannel

distances of H₄MCDPP²⁺, 15.3 × 15.3 × 20.5 Å; those of

H₄DPP²⁺, 15.1 × 15.1 × 19.5Å [9]) and the space between

the channels was occupied with solvent molecules of

crystallization.

Spectral and electrochemical properties

The absorption spectrum of (HNEt₃)[H₂DPP(COO)],

which was obtained by treatment of [H₄DPP(CO₂H)]

(ClO₄)₂(7.6 × 10^-6M) with excess amount of NEt₃, exhib-

ited the absorption maxima at 470 (Soret), 567, 620, and

Fig. 2. Hydrogen bonding for the carboxyl group: (a) a comple-

mentary hydrogen bonding pair; (b) another hydrogen bonding

pattern for the carboxyl groups

Fig. 3. UV-vis (a) and ﬂuorescence spectra (b) of (HNEt₃)

[H₂DPP(COO)] (black), [H₄DPP(CO₂H)](ClO₄)₂(green) and

ZnDPP(CO₂H) (red) in CH₂Cl₂

J. Porphyrins Phthalocyanines 2011; 15: 426–432

SyntheSIS, StruCture anD PhySICOCheMICal PrOPertIeS Of a SaDDle-DIStOrteD POrPhyrIn

427

a

Table 1. Redox potentials (E_1/2) of DPP(CO₂H) in CH₂Cl₂

DPP as the protonated sites direct out of the porphyrin

plane by the distortion of the porphyrin skeleton and are

easily accessible for protons. In addition, the diproto-

nated species of DPPs are stabilized by hydrogen bond-

ing between counter anions, where the two counter anions

are placed at the space above and below the porphyrin

plane and form hydrogen bonds with trans-located two

pyrrolic N-Hs of the saddle-distorted porphyrin, which

are diagonally positioned each other, as can be seen in

Fig. 1.

We examined the spectroscopic titration of

[H₄DPP(CO₂H)](ClO₄)₂in CHCl₃with [Me₄N]OH at

rt to elucidate the order of acidity of protons included.

Upon the addition of the ﬁrst equivalent of OH^-, the

absorption spectrum of [H₄DPP(CO₂H)](ClO₄)₂showed

only small change without showing isosbestic points, as

depicted in Fig. 4a. In sharp contrast, the addition of

the second and the third equivalents of the base allowed

us to observe signiﬁcant spectral change without isos-

bestic points, as shown in Fig. 4b. Further addition of

the base did not afford any spectral change. The absor-

bance change at 467 nm was monitored relative to the

equivalency of OH^-to the diprotonated porphyrin. The

resultant titration curve is displayed in Fig. 4c, indicating

that the ﬁrst step is mono-deprotonation followed by two

successive deprotonation processes. Thus, we concluded

that the ﬁrst deprotonation occurs at the carboxyl group

and then the deprotonation of the protons attached to the

pyrroles in a successive manner without formation of the

corresponding mono-protonated form of the porphyrin

core [26], as summarized in Scheme 2.

Oxidation, V

Reduction, V

I

II

(HNEt₃)[H₂DPP(COO)] ^b

[H₄DPP(CO₂H)](ClO₄)₂

ZnDPP(CO₂H)

H₂DPP

+0.26 ^c

+1.18

+0.31

+0.32 ^c

+1.11

+0.29

+0.75

+0.60

—

-1.55 ^c

-0.55

-1.61 ^c

-1.57 ^c

-0.59

—

+0.42

—

[H₄DPP](ClO₄)₂

ZnDPP

+1.40

+0.40

+1.08

+0.91

-1.65

H₂TPP

-1.50 ^c

ZnTPP

-1.61

^aSupporting electrolyte: TBAPF₆(0.1 M), potentials vs. Ag/

AgNO₃in CH₃CN as a reference electrode, a Pt working elec-

trode, a Pt wire as a counter electrode. In order to avoid the

protonation at the porphyrin core, a few drops of triethylamine

was added to the sample solution. ^cFor irreversible redox proc-

esses, potentials of the DPV peaks were given.

b

[H₄DPP(CO₂H)](ClO₄)₂(487 nm) and ZnDPP(CO₂H)

(466 nm) were 1∼2 nm toward lower energy in each

case in comparison with those of the corresponding

DPP counterparts [24]. The ﬂuorescence spectra of

(HNEt₃)[H₂DPP(COO)], [H₄DPP(CO₂H)](ClO₄)₂, and

ZnDPP(CO₂H) were measured in CH₂Cl₂at rt to observe

the emission maxima at 793, 759, and 735 nm, respec-

tively, as depicted in Fig. 3b. These emission maxima are

also marginally shifted relative to those of MDPP (M =

H₂, H₄, Zn) [25].

In the previous reports, we utilized the hydrogen

bonding between diprotonated DPP and counter anions

having a carboxylate group for formation of supramo-

lecular structures [11, 12]. The carboxylate-appended

[H₄DPP(COO)]⁺, which should be derived from the ﬁrst

deprotonation at the carboxyl group of H₄MCDPP²⁺(see

Scheme 2), is expected to form self-assembled struc-

tures through intermolecular hydrogen bonding between

the diprotonated porphyrin core and the peripheral car-

boxylate moiety. Thus, we examined the concentration

dependence on the absorption spectra of the carboxylate-

appended porphyrin dication, [H₄DPP(COO)]⁺, in CHCl₃

(Fig. 3d) to observe that the Soret band gradually low-

ered its molar absorption coefﬁcient with increasing the

concentration of the porphyrin. This kind of behaviors

can be ascribed to the self-aggregation of chromophores

as observed in that of water-soluble porphyrins [27]. In

addition, the observation that the spectral change in the

course of the deprotonation processes does not give isos-

bestic points (vide supra) also suggests the emergence of

supramolecular structures in accordance with the forma-

tion of the [H₄DPP(COO)]⁺species.

Redox potentials of (HNEt₃)[H₂DPP(COO)],

[H₄DPP(CO₂H)](ClO₄)₂and ZnDPP(CO₂H) in CH₂Cl₂

are summarized in Table 1 together with the data of the

corresponding DPP and TPP species. The redox waves

due to the oxidation and reduction processes are revers-

ible for the diprotonated species and Zn complexes,

whereas those for (HNEt₃)[H₂DPP(COO)] and H₂DPP are

irreversible. The ﬁrst oxidation and reduction potentials

of (HNEt₃)[H₂DPP(COO)], [H₄DPP(CO₂H)](ClO₄)₂and

ZnDPP(CO₂H) were almost similar to those of the corre-

sponding DPP counterpart despite of the introduction of

a carboxyl group. Comparison of the redox potentials of

DPP(CO₂H)s to those of TPPs revealed a clear tendency

of the narrower HOMO–LUMO gaps of highly distorted

porphyrins relative to those of planar ones. In addition,

the DPP(CO₂H)s showed more negative LUMO levels

than the conduction-band edge of TiO₂, but the HOMO

levels are only a little positive compared to the oxida-

tion potential for the I^-/I₃^-mediator. The latter situation

may be disadvantage for the dye regeneration in a DSSC

system (vide infra).

In order to shed some lights on the self-assembly of

the carboxylate-appended porphyrin dication derived

from the carboxylate-appended [H₄DPP(COO)]⁺, we con-

ducted titration experiments using ¹H NMR spectroscopy

Spectral changes with acid-base titration

The saddle-distorted DPP tends to be easily diproto-

nated because the lone pairs of the pyrrolic nitrogens of

J. Porphyrins Phthalocyanines 2011; 15: 427–432

428

M. Sankar et al.

Fig. 4. Spectroscopic titration of [H₄DPP(CO₂H)](ClO₄)₂(1.2 × 10^-5M) with [Me₄N]OH in CHCl₃at rt: (a) spectral change in the

course of the addition of 1 eq of the base; (b) spectral change for the addition of 2–3 eq of the base; (c) change of absorbance at

467 nm relative to the equivalency of the base; (d) effect of concentration on the absorption spectra at different porphyrin concen-

trations (3 µM, 6 µM, 12 µM) with corresponding 1 eq addition of [Me₄N]OH in CHCl₃

2+

+

–

Ph

NH

N

HN

N

1 eq OH^–

2 eq OH^–

–

CO₂H

CO₂

HN

Ph Ph

Ph

[H₄DPP(CO₂H)]²⁺

[H₄DPP(COO)]⁺

[H₂DPP(COO)]^–

Scheme 2. A plausible deprotonation sequence of [H₄DPP(CO₂H)]²⁺

on the 2.0 mM solution of [H₄DPP(CO₂H)](ClO₄)₂in

CDCl₃(Fig. 5). In the initial spectrum, a doublet due to

the m-H of the meso-carboxyphenyl group was observed

at 7.76 ppm (J = 8 Hz) and multiplets appeared at

7.85–8.0 and 6.55–7.00 ppm were assigned to the o-H

of meso-carboxyphenyl and meso-phenyl groups and the

β-phenyl groups, respectively (Fig. 5a). Addition of 1 eq

of Me₄NOH made the spectrum broadened (Fig. 5b) and

further addition of 2 more eq of Me₄NOH gave the upﬁeld-

shifted and sharp signals at 7.50–7.74, 7.15–7.40 and

6.43–6.98 ppm for the meso-carboxyphenyl, meso-phenyl

and the β-phenyl groups, respectively (Fig. 5c). Based

on the UV-vis titration experiment, the spectra obtained

by the addition of 1 eq of Me₄NOH was assigned to that

1

of [H₄DPP(COO)]⁺and signiﬁcant up-ﬁeld shifts of H

NMR signals were observed for the carboxylate-phenyl

J. Porphyrins Phthalocyanines 2011; 15: 428–432

SyntheSIS, StruCture anD PhySICOCheMICal PrOPertIeS Of a SaDDle-DIStOrteD POrPhyrIn

429

protons. The chemical shifts were 6.94 and 5.56 ppm for

o- and m-phenyl protons, which were originally observed

at 7.97 and 7.76 ppm, respectively (see the dotted arrows

in Fig. 5). This up-ﬁeld shifts were probably due that they

were positioned above the porphyrin ring of other mol-

ecules and were affected by the shielding effect of the

ring current [28].

As described above, 1 eq addition of Me₄NOH to the

solution of [H₄DPP(CO₂H)]²⁺afforded the carboxylate-

appended porphyrin dication and the formation of supra-

molecular structures was assumed on the basis of the

1

results of the UV-vis and H NMR spectroscopic mea-

surements. At this point, although we have not obtained

any deﬁnitive evidence to support yet, we assume the

supramolecular structure formed by the self-assembly

of [H₄DPP(COO)]⁺as depicted in Scheme 3, based on

our previous observations on formation of hydrogen-

bonded supramolecular assemblies made of H₄DPP²⁺

and carboxylate-appended molecular components [11,

12, 14, 26].

Cell fabrication and photovoltaic characteristics

The porphyrins were adsorbed as photosensitizers onto

TiO₂and SnO₂nanocrystalline ﬁlms to serve as working

electrodes in dye-sensitized Solar cells (DSSCs) [29].

The working electrodes were prepared as mentioned in

the Experimental section and then they were immersed

into a solution of porphyrin ((HNEt₃)[H₂DPP(COO)] or

ZnDPP(CO₂H)) in CH₂Cl₂/EtOH (9:1 v/v). Pt electrodes

were used as counter electrodes. The DSSCs were fabri-

cated by sandwiching a redox (I^-/I₃^-) electrolyte solution.

Through measurements of I-V curves, we assessed the

performance of the DSSC devices and the open-circuit

photovoltage (V_oc), the short-circuit photocurrent density

(J_SC), ﬁll factor (FF) and power conversion efﬁciency

(PCE, η) were summarized in Table 2. The I-V curves of

the solar cells made of the porphyrin/TiO₂sensitized ﬁlms

are shown in Fig. S1 in the Supporting information. The

PCE (η) value for (HNEt₃)[H₂DPP(COO)] is similar to

1

Fig. 5. H NMR spectral change of [H₄DPP(CO₂H)](ClO₄)₂

upon addition of (Me)₄NOH in CDCl₃. (a) [H₄DPP(CO₂H)]

(ClO₄)₂(b) [H₄DPP(CO₂H)](ClO₄)₂+ 1 eq of (Me)₄NOH

(c) [H₄DPP(CO₂H)] (ClO₄)₂+ 3 eq of (Me)₄NOH

O

Ph

N

Ph

H

O

N

H

N

H

N

+

Ph

O

NH

HN

self-assembly

CO₂

O

Ph

N

O

N

H

O

H

N

Ph

H

N

H

O

N

Ph

O

Scheme 3. A possible self-assembled structure of [H₄DPP(COO)]⁺

J. Porphyrins Phthalocyanines 2011; 15: 429–432

430

M. Sankar et al.

that for ZnDPP(CO₂H) (entry 1 and 3). When chenode-

oxycholic acid (CDCA) was added as a co-adsorbate

for dye-loading to avoid aggregation of the dye on

the TiO₂surface (entry 2 and 4) [29b], the improvement

of J_SCand V_OCwas not observed both for the cases of

(HNEt₃)[H₂DPP(COO)] and ZnDPP(CO₂H). This means

that the aggregation of the porphyrins on the TiO₂and

SnO₂surface seems not severe. We also fabricated DSSCs

based on SnO₂ﬁlm electrodes adsorbed the porphyrin sen-

sitizers. Although the conduction band of SnO₂is lower

than that of TiO₂to show further enhancement in the efﬁ-

ciency, the results were not much improved. Regarding

the reasons for the low efﬁciency for both of the DSSCs

based on the TiO₂and SnO₂working electrodes, one is

desorption of porphyrins from the electrode surfaces dur-

ing the measurements and another would be the proxim-

ity of the oxidation potentials between DPP(CO₂H)s and

I^-to give less efﬁciency of the dye regeneration by the

reduction of radical cations of DPP(CO₂H)s.

CONCLUSION

Supramolecular structures of a saddle-distorted por-

phyrin with a carboxyl group as a hydrogen-bonding

site were conﬁrmed by the X-ray diffraction analysis.

[H₄DPP(CO₂H)]²⁺, having a carboxyphenyl group at a

meso-position, formed a nanochannel structure by self-

assembly and the nanochannel structure was similar to

that consisted of H₄DPP²⁺. In the nanochannel structure

of [H₄DPP(CO₂H)]²⁺, however, intermolecular comple-

mentary hydrogen bonding of the carboxyl groups was

formed to regulate the mutual distances between the nano-

channels. As well as the inner space of the nanochannel

[9], the resulting interchannel spaces can be also used to

include functional guest molecules. The deprotonation of

the carboxyl group in [H₄DPP(CO₂H)]²⁺afforded another

supramolecular structures derived from its self-assembly

in solution by virtue of intermolecular hydrogen bonding

between the appended carboxylate group and the dipro-

tonated porphyrin core as observed in the combination

of H₄DPP²⁺with molecules having a carboxylate group

[11, 12, 26]. We have also examined the DSSC perfor-

mances of (HNEt₃)[H₂DPP(COO)]^-and ZnDPP(CO₂H)

and the efﬁciencies were moderate but the further inves-

tigation for the improvement of the efﬁciency by avoid-

ing desorption and the alteration of sacriﬁcial reductants

to sufﬁciently regenerate sensitizing dyes is currently

underway.

Acknowledgements

This work was supported by Grants-in-Aid for Scien-

tiﬁc Research (Nos. 21350035 and 20108010) from JSPS

and MEXT. M. S. appreciates support from a JSPS pos-

tdoctoral fellowship (No. P09049). This work was also

supported by the National High Technology Research

and Development Program for Advanced Materials of

China (Grant 2009AA03Z220). We appreciate Prof. Tat-

suya Nabeshima (University of Tsukuba) for his kind

accommodation for the ﬂuorescence measurements.

Supporting information

Fig. 6. Images of dye-sensitized ﬁlms (a) of (HNEt₃)

[H₂DPP(COO)] on TiO₂and (b) on SnO₂ﬁlms and (c) of

ZnDPP(CO₂H) on TiO₂and (d) on SnO₂ﬁlms

I-VcurvesfortheDSSCsusing(HNEt₃)[H₂DPP(COO)]

and ZnDPP(CO₂H) are given in the supplementary

Table 2. Photovoltaic parameters of MCDPP-sensitized solar cells

Entry

Sensitizer

V_oc, mV

J_SC, mA.cm^-2

η, %

FF

1^a

2^a

3^a

4^a

5^b

6^b

(HNEt₃)[H₂DPP(COO)]

(HNEt₃)[H₂DPP(COO)] + CDCA

ZnDPP(CO₂H)

430

390

530

560

300

390

0.21

0.19

0.25

0.11

0.13

0.52

0.07

0.15

0.10

0.14

0.03

0.13

0.76

0.67

0.70

0.72

0.62

ZnDPP(CO₂H) + CDCA

(HNEt₃)[H₂DPP(COO)]

ZnDPP(CO₂H)

^aUse of TiO₂electrode. ^bUse of SnO₂electrode.

J. Porphyrins Phthalocyanines 2011; 15: 430–432

SyntheSIS, StruCture anD PhySICOCheMICal PrOPertIeS Of a SaDDle-DIStOrteD POrPhyrIn

431

material. This material is available free of charge via the

Internet at http://www.worldscinet.com/jpp/jpp.shtml.

Crystallographic data for [H₄DPP(CO₂H)](OH)₂

have been deposited at the Cambridge Crystallographic

Data Center (CCDC) under number 825053. Copies can

be obtained on request, free of charge, via www.ccdc.

cam.ac.uk/conts/retrieving.html or from the Cambridge

Crystallographic Data Centre, 12 Union Road, Cam-

bridge CB2 1EZ, UK (fax: +44 1223-336-033 or email:

deposit@ccdc.cam.ac.uk).

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[H₂DPP(COO)] was protonated during the recrys-

tallization. The spectra exhbited intense absorption

around 715 nm and the Soret band (λ_max= 492 nm)

showed red shift by 24 nm compared to that of

[H₂DPP(COO)]^-formed by adding excess amount

J. Porphyrins Phthalocyanines 2011; 15: 431–432

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found to form hydrogen bonding with a severely

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which was eliminated by the Squeeze method.

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468 nm for H₂DPP, 491 nm for H₄DPPCl₂, and

465 nm ZnDPP.

25. Emission maxima in CH₂Cl₂: 786 nm for H₂DPP,

761 nm for H₄DPPCl₂, and 684 nm ZnDPP.

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J. Porphyrins Phthalocyanines 2011; 15: 432–432

Article Doi

Synthesis, structure and physicochemical properties of a saddle-distorted porphyrin with a peripheral carboxyl group

DOI: 10.1142/S1088424611003379

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Full text of DOI:10.1142/S1088424611003379

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