effect of central metals in the porphyrin ring on photocurrent performance of cellulose...

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Effect of Central Metals in the Porphyrin Ringon Photocurrent Performance of CelluloseLangmuir-Blodgett Films

Keita Sakakibara, Fumiaki Nakatsubo*

Role-sharing functionalized cellulose derivatives havingMg, Zn, Cu, and Pd porphyrins are usedfor photocurrent generation system fabricated by the LB technique, mimicking the array ofnatural photosynthetic light-harvesting systems. These polymers exhibited different absorptionand redox properties in solution depending on thecentral metal ions. On visible light illumination,the LB film generated anodic photocurrent in theorder of Pd> free base> Zn>Cu>Mg. By insert-ing Pd ion, using myristoyl group, and adjustingthe DS, the photocurrent quantum yield was 6.9times as great as that of conventional porphyrincellulose. These findings may provide opportu-nities for the design of a new class of organic solarcells, based on cellulose molecular characteristics.

Introduction

The fabrication of photoelectric conversion systems is one

of the most important subjects not only for basic research

but also for practical use such as organic solar cells. In

particular, both dye-sensitized solar cells and photovoltaic

devices with bulk heterojunction structure are promising

K. Sakakibara, F. NakatsuboDivision of Forest and Biomaterials Science, Graduate School ofAgriculture, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, JapanFax: þ81 774 38 3658; E-mail: [email protected]. SakakibaraCurrent address: World Premier International (WPI) ResearchCenter for Materials Nanoarchitectonics (MANA), NationalInstitute for Materials Science (NIMS), Namiki 1-1, Tsukuba,Ibaraki 305-0044, JapanF. NakatsuboCurrent address: Research Institute for SustainableHumanosphere, Kyoto University, Gokasho, Uji, Kyoto 611-0011,Japan

Macromol. Chem. Phys. 2010, 211, 2425–2433

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlin

approaches toward inexpensive solar energy conversion.[1]

Needless to say, natural photosynthesis is regarded as the

most elaborate nanobiological device in theworld, because

the photoinduced electron transfer with a high quantum

efficiency of nearly 100% takes place in highly organized

pigment assemblies with the help of the membrane

proteins. This prominent functionality is based on the fact

that protein scaffolds hold pigments at intermolecular

distances that optimize electronic coupling, photon cap-

ture, and energy transfer.[2] There are several analogies

between natural photosynthesis and practical develop-

ment of organic solar cells. In fact, mimicking natural

photosynthesis is absolutely imperative to fabricate the

photoenergy conversionsystemin theartificiallyorganized

systems.[3]

Cellulose unique structure, a linear homopolymer

consisting of regio- and stereo-specific b-1,4-glycosidic

linked D-glucose units, attracts a great deal of interest for

creating advancedmaterials because of its stiffness, shape-

stable structure, hydrophilicity, stereoregularity, chirality,

multifunctionality, and the ability to form supramolecular

elibrary.com DOI: 10.1002/macp.201000257 2425

2426

K. Sakakibara, F. Nakatsubo

structures. So far, we have reported on some cellulose

Langmuir-Blodgett (LB) films with the function of photo-

current generation and revealed that introducing

porphyrin moiety to the cellulose scaffold has an impact

on the magnitude on the anodic photocurrent because of

the inhibition of the aggregation of the chromophores

(porphyrins).[4,5] Especially, a regioselectively substituted

cellulose derivative having both a porphyrin moiety and

stearoyl groups at the C-6 andC-2, 3 positions (H2PCS) could

be one of the promising candidates for well-defined

molecular assembly and highly efficient cellulosic solar

cells (Figure 1).[5] The structure of H2PCS was designed on

the basis of our fundamental concept, role-sharing

functionalization of cellulose; to impose precise function

for each hydroxyl group of repeating cellulose unit.[6]

The stearoyl groups at the C-2 and C-3 positions function in

self-organization, solubility and processability, and the

porphyrin group at the C-6 position in light-harvesting and

photoinduced electron transfer. The uniform and well-

defined structure affords precise control of the positioning

of porphyrins attached to cellulose scaffold, mimicking the

array of natural photosynthetic light-harvesting systems.

In addition, we have recently reported the fabrication of

mixedLBfilmsofH2PCSandfullereneC60andfoundthat the

photocurrent quantum yield was rather improved because

of the complexation of C60 with the porphyrin moieties

attached to rigid cellulose scaffold.[7]

R = stearoyl, M = 2H (H2PCS)M = Mg (MgPCS)M = Zn (ZnPCS)M = Cu (CuPCS)M = Pd (PdPCS)

O

ROOR

O

O

N

N

N

N

O

ROOR

OOH

O

M

R = myristoyl, M = Pd (PdPCM)

Figure 1. Structures and abbreviations of cellulose derivativesused in this study.


� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Several factors are known to affect photocurrent perfor-

mance in porphyrin LB films: side chain structure[8] and

length of peripheral alkyl chain.[9] Additionally, another

factor is central metal ion in porphyrin ring. For example,

Gervaldo et al. reported that photocurrent quantum yield

depends on the kind of metal ion in the porphyrin ring.[10]

They investigated the effect of central metal (zinc(II),

copper(II), palladium(II), nickel(II) and cobalt(II)) by using

low-molecular-weight porphyrins. However, polymeric

porphyrin derivatives have not been studied so far.

Here, we study the effect of central metals in the

porphyrin rings on the photocurrent performance of

cellulose LB films. The central metal ionwas selected based

on the occurrence in porphyrin-based natural systems,

frequency of mention in scientific literature and the report

of Gervaldo et al.[10] Thereby, magnesium(II) (MgPCS;

similar in structure to chlorophyll), zinc(II) (ZnPCS),

copper(II) (CuPCS), and palladium(II) (PdPCS) were selected

(Figure 1). First, the derivatives in solution were character-

ized by UV-Vis absorption spectroscopy and cyclic voltam-

metry (CV). Next, the monolayer properties on the water

surface were studied by measuring the surface pressure

versus area (p-A) isotherms. The LB films transferred onto

solid substrates were characterized by UV-Vis absorption

spectroscopy. Then, thephotocurrentperformanceof the LB

films on an indium/tin oxide (ITO) electrode was deter-

mined. Finally, we demonstrate the improvement of

photocurrent performance by substituting stearoyl (C18)

to myristoyl (C14) group at the C-2 and C-3 positions and

adjusting the degree of substitution of the porphyrin

moiety (DSporphyrin) to be 0.36. This paper presents the

improved photocurrent generation systems by investigat-

ing the effect of central metals in the porphyrin ring,

thereby providing opportunities for the design of a new

class of organic solar cells, based on cellulose molecular

characteristics.

Experimental Part

Materials

2,3-Di-O-stearoyl-6-O-[p-(10,15,20-triphenyl-5-porphyrinyl)benzoyl]-

cellulose (H2PCS) was prepared regioselectively as described pre-

viously:[5] DSporphyrin¼0.64; degree of polymerization (DP)¼ 27.7.

2,3-Di-O-myristoyl-6-O-[p-(10,15,20-triphenyl-5-porphyrinyl)ben-

zoyl]cellulose (H2PCM) was newly synthesized in the same way:

DSporphyrin¼0.36. Other reagents were purchased from Nakarai

Tesque Inc. (Kyoto, Japan) or Wako Pure Chemical Industries, Ltd.

(Osaka, Japan).

Metalation of Porphyrin Groups

The completion of metalation except for MgPCSwas judged by the

change of the absorption spectrum and the disappearance of the

pyrrole N-H signal at d¼–2.7 from the 1H NMR analysis.

DOI: 10.1002/macp.201000257

Effect of Central Metals in the Porphyrin Ring on Photocurrent . . .

MgPCS: The insertion of a magnesium ion into the porphyrin

ringwas conducted bymodifying the general procedure of Lindsey

and Woodford for the magnesium(II) porphyrin synthesis[11] to fit

the cellulose derivative. To a solution of H2PCS (17.4mg, 11mmol

based on porphyrin moiety) in N,N-dimethylformamide (DMF)/

tetrahydrofuran (THF) (1:1v/v, 3mL),magnesium(II) bromideethyl

etherate (29.2mg, 110mmol) and triethylamine (Et3N) (30mL,

220mmol) were added. The reaction mixture was stirred at 100 8Cunder light shielding overnight, then extracted with dichloro-

methane (CH2Cl2), washed with saturated sodium hydrogen

carbonate (NaHCO3) aqueous solution, dried over sodium sulfate

(Na2SO4), and concentrated under reduced pressure. The polymer

was dissolved in CH2Cl2 and then poured into a large amount of

methanol (MeOH) to precipitate a polymer. The precipitated

polymer was collected to give MgPCS (8.8mg, 50%). Although the

absorption spectrum of MgPCS was different from that of H2PCS,

the pyrrole N-H signal at d¼–2.7 remained. Thus, the insertion of a

magnesium(II) ion into the porphyrin ring did not reach comple-

tion.

ZnPCS[7]: To a solution of H2PCS (9.0mg, 5.9mmol based on

porphyrin moiety) in CH2Cl2 (2mL), zinc(II) acetate (20mg, excess)

inMeOH (1mL)was added. The reactionmixturewas stirred under

light shielding overnight and then treated with the samework-up

procedure as MgPCS to give ZnPCS (6.0mg, 64%).

CuPCS: To a solution of H2PCS (8.1mg, 5.3mmol based on

porphyrinmoiety) inCH2Cl2 (3mL), copper(II) acetatemonohydrate

(10.5mg, 53mmol) and Et3N (15mL, 105mmol) were added. The

reaction mixture was stirred under light shielding overnight and

then treated with the same work-up procedure as MgPCS to give

CuPCS (7.8mg, 93%).

PdPCS: The insertion of a palladium ion into the porphyrin ring

was conducted according to the procedure by Scalise and

Durantini.[12] To a solution of H2PCS (14mg, 9.2mmol based on

porphyrin moiety) in DMF/THF (1:1 v/v, 5mL), palladium(II)

chloride (16mg, 92mmol) was added. The reaction mixture was

stirred at 100 8C for 2 days and then treatedwith the samework-up

procedure as MgPCS to give PdPCS (11.5mg, 76%).

Preparation of LB Mono- and Multilayer Films

The cellulose derivatives were dissolved in toluene at a concentra-

tion of �0.1�10�3M with respect to the porphyrin unit. The

ultrapurewater at a normal resistance of 18.2MV � cm (Simpli Lab,

Millipore) was used for the subphase. The subphase temperature

was kept constant at 20 8C by circulating thermostatted water

system. The solutionwas spreadonto awater subphase in a Teflon-

coatedLangmuir trough(100�250�5mm3,FSD-300,USI-system).

The surfacepressurewasmeasuredby theWilhelmymethod.After

spreading the sample, the solvent was allowed to evaporate for

30min. The p-A isothermsweremeasured at a compression rate of

6mm �min�1.

For the preparation of the LB mono- and multilayer films, the

horizontal lifting method (the Langmuir-Schafer technique) was

used to deposit the surfacemonolayer onto several substrates such

as quartz (for UV) and an ITO (Geomatec) electrode (for

photocurrent measurement). The upward and downward stroke

rate was 1mm �min�1 together. The surface pressure was fixed at

10mN �m�1 during deposition. All substrateswere cleaned in a 5%



aqueous detergent (Scat-20X-N, Nakarai Tesque Inc.) for 24h and

thenplaced inwater, acetone, and chloroform in anultrasonic bath

for 15min each.

Measurements

UV-Vis spectra were obtained on a Jasco V-560 UV-Vis absorption

spectrophotometer (Jasco, Japan). Toobserveabsorptiondichroism,

GPH-506 polarizer and RSH-680 revolving sample holder (Jasco,

Japan) were attached to the spectrometer and the absorbance was

measured for vertically or horizontally polarized light.

Electrochemical measurements were carried out in a micro cell

(MCA-microcell,BAS, Japan)havingathree-electrodeconfiguration

using an electrochemical analyzer (ALS650B, BAS, Japan). The

working and the counter electrode for CV measurements were a

platinum electrode (1.6mm diameter) and a platinum wire,

respectively, and the reference electrode was Ag/AgCl (sat. KCl).

Ferrocene (Fc) was added as an internal reference and all the

potentials were referenced relative to the ferrocenium/ferrocene

(Fcþ/Fc) couple. Tetrabutylammonium hexafluorophosphate

(Bu4NPF6) in CH2Cl2 was used as the supporting electrolyte. The

sample solutionswere deoxygenated by a stream of nitrogen prior

to the CV measurements. The scan rate was 100mV � s�1 and all

experiments were performed at room temperature (�20 8C).Photocurrent measurements were carried out at a constant

appliedpotentialusinganelectrochemical analyzer (ALS650B,BAS)

at room temperature (�20 8C) on light irradiation. A 500W xenon

arc short lamp (UXL-500SX,Ushio, Japan)wasusedasa light source,

equipped with a UV and IR cut-off filter (SCF, Ushio, Japan). Action

spectra were obtained with monochromatic light from a xenon

lamp through MIF-W-type metal interference filters [400–550nm

wavelength, 10 nm full width at half maximum (fwhm), Vacuum

Optics Corporation of Japan]. Light intensity at the irradiation

substrate surface was measured with a thermopile (MIR-100C,

TAZMO, Japan). LB film on an ITO electrode as a working electrode

was contacted with aqueous solution containing 0.1M sodium

sulfate (Na2SO4) as an electrolyte and 5�10�2M hydroquinone

(H2Q) as a sacrifice electron donor. A saturated calomel electrode

(SCE)as the referenceelectrodeandaplatinumwireelectrodeas the

counter electrode were used. The electrolyte solution was initially

purgedwithnitrogen for 15minand thenmaintainedunder aflow

of nitrogen. The configuration of the electrochemical cell was

referred to the literature of Aoki et al.[13] An electrode area of

1.54 cm2 was exposed to the electrolyte solution. The single-sided

photoactive layerwas irradiatedthroughthebackof the ITO-coated

glass, which caused unwanted loss of the light intensity. However,

the obtainedphotocurrent values and themeasured light intensity

were used without any correction.

To evaluate the efficiency of photocurrent generation, the

quantum yield (F) based on the number of photons adsorbed by

porphyrin moiety in the LB films was calculated according to the

following equation:

F ¼ 100ði=eÞIð1�10�AÞ

; I ¼ Wl

hc(1)

where i is the observed photocurrent density, e is the charge of the

electron, I is the number of photons per unit area and unit time, lis the wavelength of light illumination, W is light power

www.mcp-journal.de 2427

2428


irradiation at l nm, h is the Planck constant, c is the light velocity,

and A is the absorbance of the monolayer at l nm. At the same

time, the incident photon-to-current conversion efficiency (IPCE)

was determined as follows:[14]

Tab

Sam

H2P

Mg

ZnP

CuP

PdP

a)Half-

Macrom

� 2010

IPCEð%Þ ¼ 100� 1 240i

Wl(2)

Figure 2. UV-Vis absorption spectra of cellulose derivatives inCHCl3: (a) H2PCS, (b) MgPCS, (c) ZnPCS, (d) CuPCS, and (e) PdPCS.Inset shows magnification of Q-band region of each spectrum.

Results and Discussion

Spectroscopic Studies and Electrochemical Propertiesof the Cellulose Derivatives

Figure 2 shows UV-Vis absorption spectra of the cellulose

derivatives in CHCl3. The typical Soret- and Q-bands were

observed due to p-p� transitions of the conjugated

macrocycles. The absorbance peaks of the derivatives are

summarized inTable 1. The absorption spectrawere similar

to those of the corresponding low-molecular-weight

porphyrins.[12] In every case, the distinct Soret absorption

bands were much more intense than the Q-bands. The

metalatedderivatives exhibited thedecrease in thenumber

of Q-bands relative to H2PCS since they enhanced

symmetry which makes the energy levels involved in the

Q-band absorption degenerate.[15] The spectrum of MgPCS,

however, did not show such degeneration due to incom-

plete insertion of a magnesium(II) ion into the porphyrin

ring. The molar extinction coefficients at the Soret

band varied with changes in the central metal ions from

1.1 to 4.2� 105 L �mol�1 � cm�1: ZnPCS>H2PCS>CuPCS>

MgPCS> PdPCS (Table 1). The half bandwidths of the Soret

band were almost similar to those of the corresponding

low-molecular-weight porphyrin, indicating no aggrega-

tion insolutionbetweentheporphyrinmoietiesattachedto

cellulose backbone.

Figure 3 shows cyclic voltammograms of the cellulose

derivatives measured in CH2Cl2 with Bu4NPF6 to examine

le 1. Spectroscopic and electrochemical data for cellulose derivat

ple Absorption

lmax Sore

nm e

10�5 L�mol�1�cm�1

CS 420, 516, 551, 591, 648 3.1

PCS 426, 516, 564, 604 2.0

CS 424, 553, 595 4.2

CS 416, 540 2.8

CS 418, 524 1.1

width potentials determined by means of CV (all potentials vs. F

ol. Chem. Phys. 2010, 211, 2425–2433

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

their redox properties. The data of the half-wave potentials

(E1/2) of the oxidation and reduction at a scan rate of

100mV � s�1 are summarized in Table 1. Reversible oxida-

tion and reduction waves were detected in Figure 3A,

corresponding to the formation of radical p-cation and p-anion, respectively. The second oxidation and reduction

waves were not obscured by solvent limit. The oxidation

and reduction values of H2PCS were almost analogous to

those of methylcellulose derivative containing porphyrin

moieties reported by Redl et al.[16]

It is said that reduction and oxidation of porphyrin

molecules are governed by the activation or deactivation of

the p electron conjugated system through electrostatic

interaction of the central metal ions.[17] The oxidation

potential in this studybecamemore cathodic in the order of

PdPCS (þ0.71V)>H2PCS (þ0.59V)>CuPCS (þ0.57V)> ZnPCS

(þ0.37V)>MgPCS (þ0.25V). On the other hand, this

order was inapplicable to the reduction potential: H2PCS

(–1.65V)>MgPCS (–1.66V)> PdPCS (–1.71V)>CuPCS

(–1.77V)> ZnPCS (–1.82V). In the case of MgPCS, ZnPCS

ives in solution.

Electrochemical potentialsa)

t band V

Half bandwidth 1st Ox. 2nd Ox. 1st Red.

nm

16 0.59 – –1.65

19 0.25 0.56 –1.66

19 0.37 0.68 –1.82

14 0.57 – –1.77

20 0.71 – –1.71

cþ/Fc, potential sweep rate:100mV � s�1; 0.1 M Bu4NPF6 in CH2Cl2).

DOI: 10.1002/macp.201000257

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Potential (V vs Fc+ / Fc)

(A)

(C)

(D)

(B)

(E)

H2P+/0H2P0/-

ZnP+/0ZnP0/-

ZnP2+/+

PdP+/0

PdP0/-

MgP+/0MgP0/-MgP2+/+

CuP+/0

CuP0/-

Figure 3. Cyclic voltammograms of (A) H2PCS, (B) MgPCS,(C) ZnPCS, (D) CuPCS, and (E) PdPCS in CH2Cl2 containing 0.1 MBu4NPF6 as a supporting electrolyte. Scan rate: 100mV � s�1.

0

10

20

30

40

50

0.2 0.4 0.6 0.8 1.0 1.2 1.4

Sur

face

pre

ssur

e (m

N·m

-1)

Area (nm2 per AGU)

a e

db

c

Figure 4. Surface pressure versus area (p-A) isotherms of(a) H2PCS, (b) MgPCS, (c) ZnPCS, (d) CuPCS, and (e) PdPCS atthe air-water interface at 20 8C.


and CuPCS, the redox processes were observed at more

negative potentials than H2PCS because of increased

electron density on porphyrin rings provided by the metal

ions. Notably, the second oxidation processes for MgPCS

and ZnPCS were more clearly defined as a result of the

negative shifts in potentials (Figure 3b and c). On the

contrary, the redox process of PdPCS was observed at more

positive potentials than that of H2PCS. It should be

mentioned that Fuhrhop has found that oxidation poten-

tials of porphyrins with stable divalent metal ions, such as

Mg(II), Zn(II), Cu(II), and Pd(II), are proportional to electro-

negativity of the central metals: electronegativities of

Mg¼ 1.2, Zn¼ 1.5,Cu¼ 1.8, Pd¼ 2.0.[17] Consequently, Pd(II)

ion in the porphyrin ring brought heightened oxidation

potentials, so that the oxidation of PdPCS is more difficult

than that of H2PCS. This difficulty in oxidationwill exert an

influence on the generation of anodic photocurrent, as

described later.

Monolayer Behavior at the Air/Water Interface

Solution of cellulose derivatives was spread onto a water

surface to prepare a floating monolayer. Because of high

aggregation tendency of the porphyrinmoiety, an uniform

monolayer filmon the surface couldnot be formed fromthe

chloroform stock solutions. Finally, a monolayer film was

successfully fabricated when the derivatives were dis-

solved in toluene at a low concentration as a spreading

stock solution to minimize that adverse effect. Figure 4



shows surface pressure versus area (p-A) isotherms of the

cellulose derivatives at the air-water interface at

20 8C. H2PCS and PdPCS formed stable and well packed

monolayers with steeper increase and higher collapse

pressure than MgPCS, ZnPCS and CuPCS. Characteristics of

thep-A isothermsmightbeassociatedwith thepreferential

molecular orientation of porphyrin moieties, since the

hydrophilic parts in thederivativeshave the samestructure

in common. Nagatani et al. studied the orientation of Zn

porphyrins at the air-water interface anddescribed that the

axial hydration to the Zn center in the porphyrin ring

disturb the formation of condensedmonolayer.[18] Liu et al.

described the Cu(II) ion in the porphyrin ring coordinates

with water molecules in the subphase.[19] Hence, the

gradual increases of the surface pressure in the p-A

isotherms for MgPCS, ZnPCS, and CuPCS might be caused

by the coordination of water molecules to the axial site of

central metals. This is supported by many reports

concerning about the axial coordination with oxygen or

nitrogen containing molecules from the environment.[20]

On the contrary, the low affinity of water molecules

toH2PCS and PdPCSwould be attributed to the formation of

the stable monolayer.

Spectroscopic Studies of LB Films

The monolayer of the cellulose derivatives could not be

depositedwell to substratesby thevertical dippingmethod,

but successfully by the horizontal lifting (the Langmuir-

Schafer) method at a surface pressure of 10 mN �m�1.

Figure 5 shows UV-Vis absorption spectra of the LB mono-

and multilayer films of the cellulose derivatives. Linear

increases of the Soret band in the multilayer films


0.00

0.04

0.08

0.12

0.16

350 400 450 500 550 600 650 700Wavelength (nm)

Abs

orba

nce

0.000

0.006

0.012

480 520 560 600 640 680

A)

424516

551

590 651

0.00

0.04

0.08

0.12

0.16

350 400 450 500 550 600 650 700Wavelength (nm)

Abs

orba

nce

B)

0.000

0.002

0.004

480 520 560 600 640 680428

518

553

0.00

0.04

0.08

0.12

0.16

350 400 450 500 550 600 650 700Wavelength (nm)

Abso

rban

ce

C)

0.000

0.005

0.010

0.015

480 520 560 600 640 680

433 557

0.00

0.04

0.08

0.12

0.16

350 400 450 500 550 600 650 700Wavelength (nm)

Abs

orba

nce

D) E) F)

0.000

0.010

0.020

480 520 560 600 640 680

423 541

0.00

0.04

0.08

0.12

0.16

350 400 450 500 550 600 650 700Wavelength (nm)

Abs

orba

nce

0.000

0.010

0.020

480 520 560 600 640 680

424

527

0.00

0.04

0.08

0.12

0.16

0 1 2 3 4 5

Abs

orba

nce

Number of layers

Figure 5. (A-E) UV-Vis absorption spectra of LB films of (A) H2PCS, (B) MgPCS, (C) ZnPCS, (D) CuPCS, and (E) PdPCS on a quartz substrate as afunction of deposited layers from bottom (mono-layer) to top (five-layer). Inset showsmagnification of Q-band region of each spectrum. (F)Plots of absorbance at Soret band against number of layers: H2PCS (solid circles), MgPCS (open circles), ZnPCS (solid triangles), CuPCS (opentriangles), and PdPCS (solid squares).

2430


demonstrates that the monolayers were uniformly depos-

ited onto a quart substrate, indicating X-typemultilayer LB

films were fabricated successfully (Figure 5F).

UV-Vis spectra are generally used to investigate the

aggregation and orientation of porphyrin molecules.[21]

The Soret bands in the LB filmswere slight red-shifted to be

2–9nm in comparison with those in solution (Table 1,

Figure5). Thehalfbandwidthsof theSoretbandwere25nm

(H2PCS LB film), 24nm (MgPCS LB film), 25nm (ZnPCS LB

film), 27nm (CuPCS), and 27nm (PdPCS LB film), whose

values were close to those in solution (14–20nm) in

comparison with those of the LB films consisting of

low-molecular-weight porphyrins which exhibit notably

spectral changes such as a large red-shift and peak

broadening.[22] This difference can lead to an appropriate

conclusion that the cellulose backbone effectively pre-

vented the aggregation of the attached porphyrinmoieties,

yielding a uniform distribution of metalated porphyrins in

a two-dimensional plane.

Then, polarized UV-Vismeasurementwas performed for

probing structural anisotropy within the films.[23] The

absorption intensity at the Soret band forp- and s- polarized

lightwasmeasured as a function of the tilt angle (u) against

the incident light. Absorbance forp- and s- polarized light at

u are defined as Ap(u) and As(u), and that at 08 as Ap(0) and

As(0), respectively. Absorbance ratios ofAs(u)/As(0) to Ap(u)/



Ap(0) as a function of the tilt anglewere analyzed according

to Ogi et al (Figure 6).[24] In every case, absorbance ratios

were in agreement with the theoretical curve of the

parallel-type orientation. These results were almost analo-

gous to those obtained by flexible polymer having

porphyrin moieties.[24] In the present study, we found

littledifference in theorientationofporphyrin rings inspite

of the central metal ions. As a consequence, environment

around the porphyrin moieties in the LB filmwas common

among metalated derivatives.

Photocurrent Properties of LB Films

Photocurrentmeasurementswereperformed for theLBfilm

deposited on an ITO electrode in a nitrogen-saturated 0.1M

Na2SO4 aqueous solution containing 5� 10�2M H2Q as a

sacrifice electrondonor.A steady-stateanodicphotocurrent

appeared during light irradiation.[5] Figure 7 shows plots of

photocurrent density as a function of irradiation wave-

length, that is, action spectra. Each spectrum closely

resembled its corresponding absorption spectra in the

range of 400–550nm (Figure 5). This result indicates that

the porphyrin moieties were exactly photoactive species

responsible for the photocurrent generation: the anodic

photocurrent was certainly produced by electron transfer

DOI: 10.1002/macp.201000257

Figure 6. Absorbance ratio of As(u)/As(0) to Ap(u)/Ap(0) measuredat Soret bands of porphyrin moieties as a function of tilt angles u:five-layer LB films of H2PCS (solid circles), MgPCS (open circles),ZnPCS (solid triangles), CuPCS (open triangles), and PdPCS (solidsquares). Black-solid, gray-solid, and dotted lines represent theor-etical curves of the parallel, random, and perpendicular orien-tations. The angle u is corrected by the refractive index ofcellulose triacetate (n¼ 1.48).


reaction fromthephotoexcitedporphyrin in thecelluloseLB

film to the ITO electrode.

The PdPCS LB monolayer film exhibited larger photo-

current densities compared to the other LB films. Table 2

shows photocurrent quantum yield (F) and IPCE value

of the LB monolayer film calculated by the Equation (1)

and (2). F decreased in the order PdPCS (7.2%)>H2PCS

(1.4%)> ZnPCS (0.92%)>CuPCS (0.68%)>MgPCS (0.21%)

excited at the Soret band under no bias potential. The F

value of the PdPCS LB filmwas 5.1 and 34.2 times as high as

0

400

800

1200

1600

400 450 500 550

Pho

tocu

rren

t den

sity

(nA

·cm

-2)

Wavelength (nm)

Figure 7. Action spectra of LB monolayer films of H2PCS (solidcircles), MgPCS (open circles), ZnPCS (solid triangles), CuPCS (opentriangles), and PdPCS (solid squares) on an ITO electrode; electro-lyte solution: a N2-saturated 0.1 M Na2SO4 aqueous solutioncontaining 5� 10�2 M H2Q as a sacrifice donor at a bias potentialof 0mV versus SCE.



that of H2PCS and MgPCS, respectively. Furthermore, an

increase of the anodic photocurrent was observed with

increase in bias potential (from 0 to þ100mV). Finally, F

reached 14% for the PdPCS LB monolayer film at a bias

potential ofþ100mV. This valuewas comparable to that of

11% obtained for self-assembled monolayers (SAMs) of

ferrocene-zinc porphyrin-fullerene linked triad on an ITO

electrode[14] or 15% obtained for special-pair mimic

porphyrin dimer assemblies on a titanium-modified ITO

electrode.[25] As a consequent of the high quantum yield,

the PdPCS LB film can serve as a good photosensitizer to

construct photocurrent generation systems. However, the

IPCE value of the PdPCS film (0.56%)was still small because

of thepoor light-harvestingproperty inherent tomonolayer

system.[14] Remarkable differences between the quantum

yield of methalated porphyrin-cellulose should be depen-

dent on the effect of central metal ions in the porphyrin

rings. As has been pointed out by Gervaldo et al., the

photocurrent quantum yield increases as the oxidation

potential of the derivatives becomes more anodic.[10]

Indeed, the quantum yields of the cellulose LB films were

shown with some tendency to be elevated in the order

corresponding to their oxidation potentials (Table 1 and 2).

This could be explained by their observation that back

electron transfer kinetics decreased with increase in

oxidation potential.[10] It can also be presumed that the

electron transfer from H2Q in bulk to the porphyrins after

excitation might be one of the dominant factors for the

enhancement of performance.

Further, the effect of number of layers on the photo-

current density and the quantum yield was studied for the

PdPCS LB films (Figure 8). Though the improvement in the

photocurrent performancewas expected, the photocurrent

densities stayed constant, so that the quantum yield

decreased. As discussed by Taniguchi et al.,[26] the photo-

current can be generated as a result of electron transport

from the excited porphyrins to the ITO electrode via an

electron hopping mechanism. Thus, the constant photo-

current density resulted from the electron hopping

mechanism. At a bias potential of þ100mV, the photo-

current density of the PdPCS LB two-layer LB film increased

up to 3.2mA � cm�1, giving rise to the highest IPCE value of

0.64%.

Based on the study on structure-property relationship of

porphyrin-cellulose LB films, the optimal side chains and

DSporphyrin have been found to bemyristoyl (C14) group and

about 0.4, respectively.[27] To further improve the photo-

current performance for the LBmultilayer film, the stearoyl

group at the C-2 and C-3 positions was substituted by a

shorter myristoyl group to reduce interlayer distance and

the DSporphyrin was adjusted to be 0.36. The photocurrent

density and quantum yield obviously increased by a factor

of two in the two-layer system (Figure 8). At a bias potential

ofþ100mV, thephotocurrent density of the PdPCMLB two-


Table 2. Photocurrent quantum yield (F) and IPCE for anodic photocurrent of LB monolayer film.

Sample At 0 mV At 50 mV At 100 mV

F IPCE F IPCE F IPCE

% % %

H2PCSa) 1.4 0.063 2.2 0.095 2.9 0.13

MgPCSa) 0.21 0.006 0.31 0.008 0.41 0.011

ZnPCSb) 0.92 0.045 0.98 0.048 1.25 0.061

CuPCSa) 0.68 0.034 0.82 0.041 1.06 0.053

PdPCSa) 7.2 0.28 9.9 0.38 14 0.56

a)Excited at 420nm; b)Excited at 430nm.

Figure 8. Photocurrent density and quantum yield excited at420nm as a function of number of layers for PdPCS and PdPCMon an ITO at a bias potential of 0mV versus SCE: photocurrentdensity of PdPCS (solid squares) and PdPCM (solid circles); quan-tum yield of PdPCS (open squares) and PdPCM (open circles).

2432


layer filmwas 5.3mA � cm�1, giving rise to the highest IPCE

value of 1.04%. Finally, the photocurrent quantum yield

was 6.9 times as great as that of conventional H2PCS by

inserting Pd ion, using myristoyl group, and adjusting the

DSporphyrin to be 0.36.

Overall, the photocurrent performance of porphyrin-

cellulose LB films was greatly improved by investigating

the effect of central metal ions in the porphyrin ring. We

emphasize that this cellulosic photocurrent generation

system studied here are a convenient model system.

Cellulose derivatives with other organic semiconductors

in application for polymer-based solar cells are currently

underway and will be reported.

Conclusion

Metalated porphyrin-cellulose derivatives, MgPCS, ZnPCS,

CuPCS, and PdPCS, were prepared and used as a building



block for the fabrication of the LB mono- and multilayer

film. CV showed reversible oxidation and reduction waves

corresponding to the radical p-cation and p-anion, respec-tively. The oxidationpotential becamemore cathodic in the

order PdPCS>H2PCS>CuPCS> ZnPCS>MgPCS, which

was relative to the electronegativity of the central

metals. H2PCS and PdPCS formed stable and well-packed

monolayers with steeper increase and higher collapse

pressure than MgPCS, ZnPCS, and CuPCS because of the

coordination of water molecules to the axial site of central

metals. These monolayers were transferred successively

onto quartz and an ITO electrode by the horizontal lifting

method, yielding X-type LB films. Absorption spectra of the

LB films indicated that cellulose backbone prevented the

porphyrin aggregation. UV-Vis dichroism measurements

revealed that the porphyrin rings in the LB film tend to lie

parallel to the substrate, indicating that environment

around the porphyrin moieties in the LB filmwas common

among the cellulose derivatives. Anodic photocurrents

were observedby light irradiation to the LBmonolayer film.

The generated photocurrents increased in the order of

PdPCS>H2PCS> ZnPCS>CuPCS>MgPCS, which was

almost correlated with that of their oxidation potentials.

The insertion of Pd ions into the porphyrin rings yielded

high photocurrent quantum yield (7.2% under no bias

potential) and IPCE value (0.28%). Finally, the photocurrent

quantum yield was 6.9 times as great as that of the

conventional H2PCS by inserting Pd ion, using myristoyl

group, and adjusting the DSporphyrin to be 0.36. From these

results, we propose that cellulose is expected to be useful

scaffold for the fabrication of photovoltaic thin film in

application for polymer-based solar cells.

Acknowledgements: We are grateful to Prof. Tokuji Miyashita andDr. Jun Matsui (Tohoku University) for useful suggestions forphotocurrent measurements. This investigation was supportedby a Grant-in-Aid for Scientific Research from the Ministryof Education, Science, and Culture of Japan (nos.17380107).

DOI: 10.1002/macp.201000257


K. S. acknowledges the Research Fellowships of the Japan Societyfor the Promotion of Science (JSPS) for Young Scientists.

Received: May 12, 2010; Revised: July 27, 2010; Published online:October 15, 2010; DOI: 10.1002/macp.201000257

Keywords: cellulose; electrochemistry; LB films; structure-prop-erty relations; supramolecular structures

[1] [1a] M. Gratzel, Nature 2001, 414, 338; [1b] C. J. Brabec, N. S.Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001, 11, 15.

[2] [2a] W. Saenger, P. Jordan, N. Krauß, Curr. Opin. Struct. 2002,12, 244; [2b] J. R. Dunetz, C. Sandstrom, E. R. Young, P. Baker,S. A. V. Name, T. Cathopolous, R. Fairman, J. Cd. Paula, K. S.Akerfeldt, Org. Lett. 2005, 7, 2559; [2c] O. Y. Kas, M. B. Charati,K. L. Kiick, M. E. Galvin, Chem. Mater. 2006, 18, 4238.

[3] [3a] H. Imahori, S. Fukuzumi, Adv. Funct. Mater. 2004, 14, 525;[3b] T. Konishi, A. Ikeda, S. Shinkai, Tetrahedron 2005, 61,4881.

[4] K. Sakakibara, F. Nakatsubo, Cellulose 2008, 15, 825.[5] K. Sakakibara, Y. Ogawa, F. Nakatsubo, Macromol. Rapid

Commun. 2007, 28, 1270.[6] S. Ifuku, Y. Tsujii, H. Kamitakahara, T. Takano, F. Nakatsubo,

J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5023.[7] K. Sakakibara, F. Nakatsubo, Macromol. Chem. Phys. 2008,

209, 1274.[8] B. Choudhury, A. C. Weedon, J. R. Bolton, Langmuir 1998, 14,

6199.[9] J.-H. Yu, F.-Y. Li, X.-S. Wang, Y. Huang, B.-W. Zhang, C.-H.

Huang, Y. Cao, Opt. Mater. 2002, 21, 467.[10] M. Gervaldo, F. Fungo, E. N. Durantini, J. J. Silber, L. Sereno,

L. Otero, J. Phys. Chem. B 2005, 109, 20953.



[11] J. S. Lindsey, J. N. Woodford, Inorg. Chem. 1995, 34, 1063.[12] I. Scalise, E. N. Durantini, J. Photochem. Photobiol., A. 2004,

162, 105.[13] A. Aoki, Y. Abe, T. Miyashita, Langmuir 1999, 15, 1463.[14] H. Imahori, M. Kimura, K. Hosomizu, T. Sato, T. K. Ahn, S. K.

Kim, D. Kim, Y. Nishimura, I. Yamazaki, Y. Araki, O. Ito,S. Fukuzumi, Chem. Eur. J. 2004, 10, 5111.

[15] A. D. F. Dunbar, T. H. Richardson, A. J. McNaughton,J. Hutchinson, C. A. Hunter, J. Phys. Chem. B 2006, 110, 16646.

[16] F. X. Redl, M. Lutz, J. Daub, Chem. Eur. J. 2001, 7, 5350.[17] J.-H. Fuhrhop, K.M. Kadish, D. G. Davis, J. Am. Chem. Soc. 1973,

95, 5140.[18] H. Nagatani, H. Tanida, T. Ozeki, I. Watanabe, Langmuir 2006,

22, 209.[19] H.-G. Liu, X.-S. Feng, Q.-B. Xue, L. Wang, K.-Z. Yang, Thin Solid

Films 1999, 340, 265.[20] M. P. Byrn, C. J. Curtis, S. I. Khan, P. A. Sawin, R. Tsurumi, C. E.

Strouse, J. Am. Chem. Soc. 1990, 112, 1865.[21] F. T. Buoninsegni, L. Becucci, M. R. Moncelli, R. Guidelli,

A. Agostiano, P. Cosma, J. Electroanal. Chem. 2003, 550/551, 229.[22] [22a] D. Gust, T. A. Moore, A. L. Moore, D. K. Luttrull, J. M.

DeGraziano, N. J. Blodt, Langmuir 1991, 7, 1483; [22b] K. E.Splan, J. T. Hupp, Langmuir 2004, 20, 10560.

[23] S. Ito, K. Kanno, S. Ohmori, Y. Onogi, M. Yamamoto, Macro-molecules 1991, 24, 659.

[24] T. Ogi, H. Ohkita, S. Ito, M. Yamamoto, Thin Solid Films 2002,415, 228.

[25] M.Morisue, N. Haruta, D. Kalita, Y. Kobuke, Chem. Eur. J. 2006,12, 8123.

[26] T. Taniguchi, Y. Fukasawa, T. Miyashita, J. Phys. Chem. B 1999,103, 1920.

[27] Y. Ogawa, Structure-property relationship of cellulosic thinfilm on photocurrent generation, Master Thesis, KyotoUniversity, Kyoto 2008.


effect of central metals in the porphyrin ring on photocurrent performance of cellulose...

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