first cellulose langmuir-blodgett films towards photocurrent generation systems

Communication

1270

First Cellulose Langmuir-Blodgett Filmstowards Photocurrent Generation Systemsa

Keita Sakakibara, Yasuhiro Ogawa, Fumiaki Nakatsubo*

Langmuir-Blodgett films of a cellulose derivative containing porphyrins, porphyrin-cellulose,were fabricated in order to construct a cellulose-based molecular photocurrent generationsystem. On visible light illumination of the LB monolayer film deposited on an ITO electrode,anodic photocurrents were observed with aquantum yield of 1.6% at an applied potentialof 0 V versus SCE, and 3.8–4.6% at 0.2–0.3 Vversus SCE. These values indicate that theself-quenching of the photoexcited porphyrinsin the cellulose LB film was suppressed, whileporphyrin moieties in the LB film had adensely packed structure. This is because theporphyrins are located at a distance of approxi-mately 1.0 nm along the cellulose backbone.

Introduction

The fabrication of photo-energy conversion systems is one

of the most important issues from the viewpoint of energy

and environmental problems. In biological photosyn-

thesis, sunlight is adsorbed and converted to electronic

excitation energy, leading to the generation of an efficient

charge separation state. Photoinduced electron transfer

K. Sakakibara, Y. Ogawa, F. NakatsuboDivision of Forest and Biomaterials Science, Graduate School ofAgriculture, Kyoto University, Kitashirakawa Oiwake-cho,Sakyo-ku, Kyoto 606–8502, JapanFax: þ81–75–753–6300; E-mail: [email protected]

a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mrc-journal.de, or from theauthor.

Macromol. Rapid Commun. 2007, 28, 1270–1275

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

with a high quantum efficiency of nearly 100% takes place

in highly organized pigment assemblies with the help of

the membrane proteins. In other words, protein scaffolds

hold pigments at intermolecular distances that optimize

electronic coupling, photon capture and energy transfer.[1–3]

Therefore, a precise arrangement of photofunctional mole-

cules on a molecular scale is essential for artificial photo-

energy conversion systems.

Much research on molecular organized layers has been

reported to date, such as Langmuir-Blodgett (LB) films,

self-assembled monolayers (SAMs) and layer-by-layer

assemblies, incorporating photofunctional molecules.[4–8]

In particular, assemblies of porphyrins, similar in structure

to the naturally occurring chlorophyll pigment, have been

widely studied because of their optical and electronic

properties: strong absorption and emission in the visible

region, semi-conductivity and strong photoelectrochemi-

cal activity.[9] For the achievement of highly efficient

DOI: 10.1002/marc.200700130

First Cellulose Langmuir-Blodgett Films towards Photocurrent Generation Systems

energy conversion, the stability and the highly ordered

structure of the porphyrin assemblies are of considerable

importance.[10] However, LB films have disadvantages

in terms of the stability and in-plane orientation, that is,

molecular ordering in the plane of monolayer films or

substrates, although they are known to form excellent

films having periodic molecular arrays along the film

deposition direction.[9,10]

LB films consisting of cellulose derivatives can overcome

these problems, because of the inherent rigidity of the

cellulose backbone, as well as the strictly defined molecular

structure with its three reactive hydroxyl groups per

anhydro glucose unit.[11–14] In addition, the LB films of

highly regioselectively substituted cellulose derivatives

with a controlled pattern of functionalization and with

well defined supramolecular nanoscale architecture, based

on the basic concept of the role-sharing functionalization

of cellulose, have bright prospects in the development of

molecular assemblies with highly efficient photo-energy

conversion.[15,16]

We report herein the fabrication of LB films consisting

of 2,3-di-O-stearoyl-6-O-[p-(10,15,20-triphenyl-5-porphyri-

nyl)-benzoyl]cellulose (1) (Figure 1).

When porphyrins are regioselectively attached at the

C-6 position, the self-quenching of the photo-excited

porphyrins along the rigid cellulose backbone must be

suppressed, so the photocurrent will be enhanced by

the efficient charge separated state. We investigated the

structural and photoelectrochemical properties of the LB

films in detail using surface pressure (p)-area (A) isotherm

measurements, UV-vis absorption and photoelectro-

chemical measurements. To the best of our knowledge,

this is the first report on a cellulose-based molecular

photocurrent generation system.

1

O

OO

O

O

C17H35

OC17H35

O

HN

N

N

NH

O

OO

O

OH

C17H35C17H35 O

O

O

Figure 1. Structure of the porphyrin-cellulose 1; the degree ofsubstitution (DS)¼ ca. 0.6.



Experimental Part

Synthesis of 2,3- Di-O-stearoyl- 6-O-[p-(10,15,20-

triphenyl-5-porphyrinyl)-benzoyl]cellulose (1)

The introduction of porphyrins at the C-6 position of 2,3-di-O-

stearoylcellulose, the preparation and analytical results for which

can be seen in the Supporting Information, was carried out using

the modified method of Redl et al.[13]. 2,3-Di-O-stearoylcellulose

(50 mg) was dissolved in 2 mL of dichloromethane. The solu-

tion was treated with 1,3-dicyclohexylcarbodiimide (DCC) (30 mg,

0.14mmol), 4-(dimethylamino)pyridine (DMAP) (18mg, 0.14mmol)

and 5-(40-carboxyphenyl)-10,15,20-triphenylporphin (95 mg,

0.14 mmol) at 0 8C with constant stirring. The mixture was

stirred at room temperature for 5 d under a nitrogen atmosphere.

The mixture was then poured into a large amount of methanol

(MeOH). The precipitates were filtered off and washed thoroughly

with MeOH under suction. The precipitates were dissolved in THF

and then precipitated with MeOH for purification. This procedure

was repeated several times. Finally, the polymer was purified by

gel permeation chromatography (GPC), (LH-20) eluted with

dichloromethane, to afford the porphyrin-cellulose 1 as a purple

solid in a yield of 77mg (90.0%) The DS (calculated from elemental

analysis) was 0.64 and the DS calculated from UV-vis absorbance

measurements was 0.61.Mn (GPC) was found to be 2.73�104,Mw

(GPC) was 5.56� 104, Mw=Mn (GPC) was 2.04 and DPn (GPC)

was 27.7.1H NMR (CDCl3): d¼�2.80 (NH), 0.81 (broad s, stearoyl –CH3),

1.19 (broad-s, stearoyl –CH2–), 1.61 (s, stearoyl –CO–CH2–CH2–),

2.22 (broad s, stearoyl –CO–CH2–), 3.08–5.46 (HAGU), 7.28–7.86 (10,

15, 20-aromatic-meta and para-H), 7.86–8.26 (10,15,20-aromatic-

ortho-H), 8.26–8.60 (5-aromatic-ortho and meta-H), 8.60–8.96

(b-H).13C NMR (CDCl3): d¼100.8 (C-1), 72.0–75.0 (C-2,3,4), 67.6 (C-5),

62.6 (C-6), 14.1 (stearoyl –CH3), 22.6, 24.7, 29.7, 31.8, 33.9 (stearoyl–CH2–), 118.2, 120.3, 126.5, 127.6, 130.9, 134.5, 142.0, 147.8 (por-

phyrin-C), 166.0 (porphyrin –C––O), 171.8, 172.2 (stearoyl –C––O).

IR (KBr): 3 462, 3 315, 3 055, 3 026, 2 922, 2 851, 2 706, 2 606,

2 532, 1 755, 1 724, 1 652, 1 607, 1 558, 1 491, 1 468, 1 441, 1 400,

1 350, 1 267, 1 222, 1 213, 1 177, 1 153, 1 115, 1 072, 1 032, 1 020,

1 001, 980, 966, 918, 878, 866, 847, 800, 754, 729, 700, 658, 642, 619,

484 cm�1.

Preparation of LB Mono- and Multilayer Films

A diluted solution of the porphyrin-cellulose 1 (0.05 wt.-%) in

chloroform was spread onto a water sub-phase in a Teflon coated

trough (100�250�5 mm, FSD-300, USI System). Ultrapure water

at a normal resistance of 18.2 V � cm�1 (Simpli Lab, Millipore) was

used for the sub-phase. The sub-phase temperature was kept at

20 8C. The surface pressure was measured using a Wilhelmy-type

film balance. After 30 min were allowed for the solvent to

evaporate off, the p-A isotherms were measured at a constant

compression rate of 6 mm �min�1.

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

horizontal lifting method (Langmuir-Schafer technique) was used

to deposit a surface monolayer onto several substrates, such as

quartz plates (for UV) and indium tin oxide (ITO, GEOMATEC)

www.mrc-journal.de 1271

K. Sakakibara, Y. Ogawa, F. Nakatsubo

0

5

10

15

20

25

30

35

40

45

1.21.00.80.60.40.2

Area / nm2 per AGU

Surf

ace

pres

sure

/ m

N m

-1

Figure 2. Surface pressure (p)-area (A) isotherm of 1 at the air-water interface at 20 8C.

Figure 3. AFM image of a monolayer of 1 deposited on freshlycleaved mica at a pressure of 10 mN �m�1 at 20 8C.

1272

electrodes (for photocurrent measurements). The upward and

downward stroke rate was 1 mm �min�1 together. During the

deposition, the surface pressure was fixed at 10 mN �m�1 to

prepare the LB films. All substrates were cleaned in a 5% aqueous

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

placed in acetone, chloroform, acetone, and then water in an

ultrasonic bath for 15 min each.

Photocurrent Measurements

The photocurrent measurements were carried out at a constant

applied potential using an electrochemical analyzer (ALS650B,

BAS) at room temperature (approximately 25 8C) on light

illumination. A 500 W xenon arc short lamp (UXL-500SX, Ushio,

Japan) was used as a light source. Monochromatic light was

obtained from a xenon lamp filtered with metal interference

filters of MIF-W type (400–550 nm wavelength, 10 nm fwhm,

Vacuum Optics Corporation of Japan). The light intensity at the

irradiation substrate surface was measured with a thermopile

(MIR-100C, TAZMO, Japan). The LB films on an ITO electrode as a

working electrode were contacted onto an aqueous solution

containing 0.1 M Na2SO4 as an electrolyte and 50�10�3M

hydroquinone (H2Q) as a sacrifice electron donor. The configura-

tion of the electrochemical cell was referred to in the literature

article of Aoki et al.[17]. An electrode area of 1.54 cm2 was exposed

to the electrolyte solution. A saturated calomel electrode (SCE) as a

reference electrode and a platinum wire electrode as a counter

electrode were used. The electrolyte solution was initially purged

with nitrogen for 15 min and then maintained under a flow of

nitrogen.

Results and Discussion

Structural Properties of LB Films

Porphyrin-cellulose 1was prepared according to the modi-

fied procedures of Redl et al.[13].The degree of substitution

(DS) values of porphyrin were calculated from both

elemental analysis (DS¼ 0.64) and UV-vis absorption

measurements (DS¼ 0.61). These values imply the alter-

nate arrangement of porphyrin moieties at the C-6

position along the cellulose backbone. That is, the mean

interval of porphyrin moieties is approximately equal to

the pitch per turn of the helical strand of cellulose, 1.03 nm.

Therefore, porphyrin-cellulose 1 has the potential to act as

a scaffold to control the distance between porphyrin

moieties in a confined space in the LB film.

The existence of a stable monolayer of porphyrin-

cellulose 1 on the water surface was confirmed by surface

pressure (p)-area (A) isotherm measurements (Figure 2).

The limiting molecular area was almost 0.73 nm2 per

anhydro glucose unit, obtained by the extrapolation of

the steepest part of the isotherm to zero surface pressure.

The cross-sectional areas of a glucopyranose ring and a

porphyrin moiety are known to be ca. 0.55–0.6 nm2 and

2–2.5 nm2, respectively.[9,16] Therefore, the limiting area



of 1 suggests that the porphyrin rings are oriented almost

perpendicular to the water surface.

A monolayer could be successfully deposited onto

several substrates at a surface pressure of 10 mN �m�1

by the horizontal lifting method. Atomic force microscopy

(AFM) images of the monolayer showed that the surface

was smooth and homogeneous (Figure 3).

DOI: 10.1002/marc.200700130


0.0

0.5

1.0

1.5

2.0

2.5a)

Time / s

Phot

ocur

rent

den

sity

/ µA

cm

-2

10 s

OnOn OffOff OnOn OffOff

b)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6Ph

otoc

urre

nt d

ensi

ty / µ

A c

m-2

0.00

0.05

0.10

0.15

0.20

0.25

650550450350

Wavelength / nm

Abs

orba

nce

4 layers

3 layers

2 layers

1 layer

Figure 4. UV-vis absorption spectra of the LB films of 1 (solid line)as a function of deposited layer and 1 in CHCl3 (dashed line). Thespectrum of 1 in CHCl3 is normalized at the Soret band of the 4layered LB films of 1 for comparison.

Figure 4 shows UV-vis absorption spectra of the LB

mono- and multilayer films and 1 in CHCl3. A linear

increase of the band of LB multilayer films demonstrates

the successive deposition of the monolayer. The porphy-

rin-cellulose in solution exhibits an intense Soret band

at 420 nm and four Q-bands at 516, 551, 590 and 649 nm.

For the LB films, the Soret band was broader and red-

shifted at 429 nm, whereas no appreciable change was

seen in the Q-bands. Similar phenomena were reported for

SAMs and LB films of porphyrins.[9,18] A stacked side-by-

side porphyrin p-aggregation (J-aggregate) leads to a red

shift, whereas face-to-face porphyrin p-aggregation (H-

aggregate) leads to a blue shift. Therefore, the J-aggregate

stacked structure of porphyrins was formed along the

cellulose scaffold in the LB monolayer films.[19]
550500450400
Wavelength / nm

Figure 5. (a) Photoelectrochemical response of the LB monolayerfilm of 1 on an ITO electrode with illumination at 420 nm:electrolyte solution N2 saturated 0.1 M Na2SO4 aqueous solutioncontaining 50� 10�3 M hydroquinone (H2Q) as a sacrifice donor;input power 2.0 mW � cm�2; applied potential 0 V versus satu-rated calomel electrode (SCE). (b) Action spectrum of the LBmonolayer (circles). The solid curve represents its absorptionspectrum (same as in Figure 4).

Photoelectrochemical Properties of LB Films

Photocurrent measurements were performed for the LB

monolayer film on an ITO electrode in a nitrogen saturated

0.1 M Na2SO4 aqueous solution containing 50� 10�3M of

hydroquinone (H2Q) as a sacrifice electron donor using the

modified ITO electrode as a working electrode, a platinum

counter electrode and a SCE reference electrode. Figure 5(a)

shows the photocurrent generation by on-and-off illumi-

nation of the LB monolayer film excited at 420 nm with a

power density of 2.0 mW � cm�2. A steady state anodic

photocurrent appeared during light illumination.

The plot of photocurrent as a function of the illumina-

tion wavelength, that is, the action spectrum [Figure 5(b)],

closely resembles the absorption spectrum of the LB

films of 1 in the range 400–550 nm, indicating that the

porphyrins are the photoactive species responsible for the

photocurrent generation. Therefore, the anodic photocur-

rent is certainly produced by the electron transfer reaction

from the photoexcited porphyrins to the ITO electrode.



The photocurrent quantum efficiency (F), based on the

number of photons adsorbed by the porphyrin moiety in

the LB film, was calculated according to Equation (1)

F ¼ ði=eÞ=½Ið1� 10�AÞ�; I ¼ ðWlÞ=ðhcÞ (1)

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

the electron, I is the number of photons per unit area and

unit time, l is the wavelength of light illumination, W is

the light power irradiation at l nm, h is Planck’s constant, c

is the light velocity and A is the absorbance of the


K. Sakakibara, Y. Ogawa, F. Nakatsubo

1274

monolayer at l nm.[20] We observed the quantum effici-

ency (F) of the LB monolayer film to be 1.6% under the

experimental conditions (applied potential¼ 0 V versus

SCE, light intensity¼ 2.0 mW � cm�2, l¼ 420 nm). This

value is higher than that of F� 0.4% obtained for LB films

of low molecular weight porphyrin molecules,[21] sug-

gesting that the self-quenching of the photoexcited

porphyrins in the cellulose LB film was suppressed well

because of the rigid cellulose scaffold. In addition, the

present value is comparable to those of 0.8–1.1% obtained

for polymer LB monolayer films containing a tris(2,20-

bipyridine)ruthenium complex on ITO electrodes (applied

potential¼ 0 V, light intensity¼ 2.2 mW � cm�2, l¼460 nm).[22] Taking into account the content of porphyrins

in the porphyrin-cellulose (DS� 0.6, that is, rough estimate¼60 mol-%) and that of ruthenium complexes in the polymer

(6.8–9.8 mol-%),[22] the present LB films are effective for

obtaining higher optical density per unit area; a high

external quantum yield, IPCE (incident photon- to-current

efficiency), is possible, by depositing LB multilayer films.

The photocurrent quantum yield (F) of the LB mono-

layer film increased on applying more positive potentials,

and increased at around 3.8–4.6% at 0.2–0.3 V versus

SCE (Figure 6). TheseF values are comparable to the values

of 3.4� 0.6% obtained for a porphyrin SAM on an

ITO electrode (applied potential¼ 0.4V, light intensity¼0.83 mW � cm�2, l¼ 423� 5 nm) reported by Imahori

et al.[20]. They pointed out that the self-quenching of the

porphyrin excited state cannot be reduced in conventional

molecular assemblies such as LB films.[20] However, we

achieved the same quantum yields of photocurrent

generation as those of SAMs. In other words, the

porphyrins in the LB film have a densely packed structure,

analogous to SAMs. This is because the porphyrins are

located at a distance of approximately 1.0 nm along the

cellulose backbone. While the F values are not so large at

0.0

1.0

2.0

3.0

4.0

5.0

0.30.20.10-0.1

Potential / V vs SCE

Qua

ntum

Yie

ld a

t 420

nm

/ %

Figure 6. Quantum yield at 420 nm versus applied potential of theLB monolayer film of 1 on an ITO electrode. The electrolytesolution was a N2 saturated 0.1 M Na2SO4 aqueous solutioncontaining 50� 10�3 M hydroquinone (H2Q) as a sacrifice donor.



the present stage, they are reasonable considering that the

film is composed only of the porphyrin-cellulose 1without

incorporation of either electron donor or acceptor layers.

Hetero-structure LB films formed by successive deposition

of electron donor or acceptor monolayers will be required

in the next step.

Conclusion

In conclusion, we have successfully fabricated LB films of a

role-sharing functionalized cellulose containing porphy-

rins on an ITO electrode. The porphyrins in the LB films

were very densely packed, and the self-quenching of

porphyrins was suppressed, analogous to porphyrin SAMs.

These results demonstrate that the utilization of cellulose

as a scaffold for porphyrins or other chromophores on ITO

electrodes is highly promising for the construction of

photocurrent generation systems.

Acknowledgements: We are grateful to Prof. Tokuji Miyashita andDr. Jun Matsui (Tohoku University) for useful suggestions for thephotocurrent measurements. This investigation was supported inpart by a Grant-in-Aid for Scientific Research from the Ministry ofEducation, Science and Culture of Japan (no.17380107).

Received: February 20, 2007; Revised: April 1, 2007; Accepted:April 4, 2007; DOI: 10.1002/marc.200700130

Keywords: cellulose; functionalization of polymers; LB films;photocurrent generation system; porphyrins

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first cellulose langmuir-blodgett films towards photocurrent generation systems

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