~ m ~

ELSEVIER Solar Energy Materials and Solar Cells 38 (1995) 139-155

Solar Energy Materials and Solar Cells

An artificial photosynthesis porphyrin tetrad molecule for photoelectric conversion

Yi Cao " Bao Wen Zhang, Wen Yuan Qian, Xiang Dong Wang, Jian Wei Bai, Xu Rui Xiao, Jian Guang Jia, Jin Wei Xu

Lab. of Photochemistry, Institute of Photographic Chemistry, Beijing 100101, China

Abstract

A new artificial photosynthesis porphyrin tetrad compound of porphyrin-terephthalic acid ester-viologen-carbarzole (1) was synthesized and compared with two triad compounds (2a, 2b) and two diad compounds (3a, 3b) for photoelectric conversion. The absorption and fluorescence spectra in solution and in binary solvent were investigated; especially, the effects of aggregation on the photophysical properties were studied. A mechanism including photoinduced electron transfer and multistep charge separation for tetrad compound was suggested. By using the LB technique, the synthetic molecular devices were prepared with molecules 1, 2a, 2b, 3a, 3b and the model compound of porphyrin (4b), respectively. With only one monolayer of tetrad LB film on the surface of SnOz conductive glass, rather high photodriven voltage and current were obtained.

1. Introduction

Natural photosynthesis is an efficient system for photoconversion. In biosys­terns, the molecules may organize themselves into complex functional entities - a reaction center with cooperating components of molecular dimensions. Photosyn­thesis takes place within the reaction center which really functions as a "solar cell", converting light to electricity with a photovoltage of 1.0 V and a solar power-conversion efficiency (at the level of primary step) of 15% [1]. Recently, there has been great interest in the biomimetric chemists of photosynthesis [2-12J.

Gust et al. [5b] synthesized a pentad molecule which consists of two covalently linked porphyrin moieties. The mimicry of the photosynthetic electron transfer of the pentad was carried out and the optoelectronic properties of the bilayer lipid

• Corresponding author.

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140 Y. Cao et al.j Solar Energy Materials and Solar Cells 38 (J995) 139-155

membrane of the pentad were determined. Schick et aI. [13] proposed a model for the organizations of porphyrin molecules in the monolayer assemblies. Kampas et aI. [14] prepared both sublimed and spin-coated thin films of a large number of porphyrins for testing in photoelectrochemical cells. Fujihira [1.5] has made efforts

n=3, 2a 6, 2b

:CH11n-o-Gt-C16HJ)

26r-0=3, 3&

6 3b

Scheme 1.

5

Y. Cao et aL / Solar Energy Materials and Solar Cells 38 (J995) 139-155 141

to modify Langmuir-Blodgett (LB) films with a monolayer of amphiphilic pyrene­viologen-ferrocene triad molecules on the surface of an electrode, which was fabricated into a molecular device and functioned as a photodiode. The formed quantum efficiency is 4.5 x 10-4 at stepped illumination with UV light of wave­length 350 nm. A number of works demonstrated that the optical properties of porphyrins in monolayers resemble those of dimerized or aggregated species in solutions and in micelles [16-18].

Hydrophobic interactions which play an important role in biochemical processes [19-23] are the primary force to drive aggregation. Micelles are composed of detergent molecules and generally viewed as the simplest models of mimetic biologic systems. However, recent studies reveal that in aquiorgano binary solvent neutral organic molecules tend to associate and the structures of the aggregates are even simpler than micelles [21-23].

ICO.COC.OIt • • o 0

IhC"3~ llr , IN!', ..

6

4a + 6 KH

THl". A

8

9

10

7 + 10 DI4F, b .. 1

Scheme 2.

142 Y. Cao et aL / Solar Energy Materials and Solar Cells 38 (J995) 139-155

In this paper, a series of covalently linked porphyrin with electron donor and electron acceptor amphiphilic compounds (Scheme 1) - a tetrad (1), two triads (2a, 2b), two diads (3a, 3b), porphyrin (4b) and carbazole (5) - were synthesized. The spectroscopic and aggregation properties of these molecules in solution and in dioxane-water binary solvent were studied and compared with each other. The photovoltaic behaviours of these amphiphilic porphyrin compounds in LB film were also measured and investigated.

2. Experimental details

2.1. Materials

THF, DMF, dioxane and CHCl 3 were reagent grade and were purified [24] before use. H 2S04 , steric acid and other reagents were reagent grade and were used as received. Water was deionized and distilled three times.

2.2. Instrumentation

The absorption spectra were recorded on a Hewlett-Packard 8451 spectropho­tometer. Fluorescence spectra were carried out on a Hitachi Model MPF-4 spectrophotometer. Lifetimes were measured by the technique of time-correlated single photon counting using a Photochemical Research Associates Horiba-NAES Model 1100 lifetime fluorometer. The LB films were performed by a Model JC-1 LB machine.

2.3. Synthesis

The synthesis of compound 1 was carried out as shown in Scheme 2. Compound 4a was prepared by condensation of pyrrole, tolualdehyde and 4-hydroxybe­nzaldehyde with a yield of 12% [25]. Potassium terephthalate reacted with 1,3-di­bromopropane to give 6. The substitution of 6 by 4a gave 7 in 55% yield. Starting material 8 was prepared from 1,10-decadiol and carbazole derivative 9 was pro­duced via substitution of 8 by carbazole. 10 was obtained from the addition of 4,4-dipyridyl to 9 in 70% yield. The coupling reaction of 7 and 9 gave the purpose compound 1 with a yield of 14%. Compounds 2a, 2b, 3a, 3b, 4b and 5 were synthesized according to reference [26]. All the products involved were purified and characterized by UV, IR and IH-NMR spectroscopies.

2.4. Preparation of LB film

The CHCl3 solutions of the amphiphilic compounds 1, 2a, 2b, 3a, 3b and 4b mixed with steric acid (1:3.12) were spread on a distilled water subphase, respec­tively. Well organized monomolecular layer assemblies of compound 1, 2a, 2b, 3a, 3b and 4b were constructed on Sn02 conductive glass by using the LB technique.

Y. Cao et aL / Solar Energy Materials and Solar Cells 38 (J 995) 139-155 143

The transfer of monolayer to Sn02 electrode under a surface pressure of 25 dyn/cm2 resulted in Y type LB film.

2.5. Measurement of photovoltaic behaviour

The photovoltaic behaviours were measured in 0.1 M H 2S04 solution under irradiation of focused white light from a 250 W tungsten halide lamp equipped with a 7 em water filter to separate heat. The intensity of incident light was 75 mW /cm2• A saturated calomel electrode (SeE) and a Pt foil were used as reference and counter electrode respectively for measuring the open-circuit volt­age (Voc ) and short-circuit current Usc). In the action spectrum measurement, monochromatic irradiation was performed with a 250 W tungsten halide lamp and a monochromator. The measurements were taken with digital multimeter (Model GDM-8045) and lock-in amplifier (Model FS-4) in conjunction with a light chopper (EG & G, Model 197).

3. Results and discussions

3.1. Photo physical properties in solution

The absorption spectra of compound 1, 2 and 3 are the same as those of porphyrin 4 in the region of 350-700 nm, Amax = 416 nm. This indicates that there is no appreciable interaction between porphyrin and the other moieties - tereph­thalic acid ester, viologen and carbazole in the ground state.

Fluorescence spectra of compounds 1, 2 and 3 in the region of 650-730 nm are similar to that of porphyrin. In addition, there is another emission band for compounds 1 and 2 ranging from 350-380 nm, which is ascribed to the emission of carbazole moiety. On the other hand, relative fluorescence intensities of 1, 2 and 3 in THF are lower than that of 4. This suggests that photoinduced electron transfer takes place. Fluorescence lifetimes (T,TO) were measured by single photon count­ing, and electron-transfer rate constants (k et ) were calculated from the following equation:

k et = l/T -1/To'

Table 1 Fluorescence lifetimes and electron transfer rate constants of porphyrin compounds in THF and in micelle a

2a 2b 3a 3b 4b

7", ns 7.54 b 11.1 11.1 11.7 ket in THF, S-1 4.72 x 107 4.62 X 106 4.62x 106

7", ns 9.11 11.1 9.15 11.1 11.7 ket in micelle, S-1 2.43 X 107 4.62x 106 2.38 x 107 4.52x 106

• x 2 < 1.32. bIn CHCI 3•

144 y. Coo et aL / So/arEnergy Materials and Solar Cells 38 (J995) 139-155

8

~ 0

~

0.6

0.5

0.4

0.3

0.2

0.1

0

0.6

0.5

0.4

0.3

0.2

0.1

o -0.1

300

300

400 500 600 700 Wavelength/run

400 500 600 700 Wavelength/run

-30115 --6Ons

800 900

-o-3Ons --6Ons -o-90ns ---12Ons --25Ons -sOns

triad

800 900

Fig. 1. Laser flash photolysis.

2a

where T is the lifetime of compound 1, 2 or 3 and TO is the lifetime of 4. The values of T, TO and ket are listed in Table 1. The higher ket value in 2a and 3a may be due to the relatively shorter distance between porphyrin and viologen moieties than that in 2b and 3b. After all, the value of ket in 1 is the highest one, which might be attributed to more step of electron transfer in 1 than that in 2 and 3.

The transient absorption spectra of tetrad 1 and triad 2a were detected by laser flash photolysis (Fig. 1). The absorbance of carbazole cation at 800 nm for tetrad is higher than that of triad at the same time interval. This strongly proved that photoinduced charge separation actually occurs and indicated that charge separa­tion of tetrad is better than that of triad.

3.2. Photo physical properties in aquiorgano binary solvent

The absorption spectra and fluorescence spectra of 2b at room temperature in dioxane-water binary solvent are shown in Fig. 2; <P is defined as the volume

Y. Cao et al.j Solar Energy Materials and Solar Cells 38 (1995) 139-155

-:, ..s

310 LlO

.;

510 610 k(rvn)

III

- q,;.o 0.3

0.' 0.8

145

Fig. 2. The absorption spectra and fluorescence spectra of 2b at room temperature in dioxane-water binary solvent.

fraction of water in binary mixture. When (/> < 0.5, only the monomer absorption band (A max = 420 nm) and monomer fluorescence emission (excitation wavelength 410 nm, Emax = 655 nm) of the porphyrin moiety can be detected. An increase in (/>

value «(/> = 0.6) leads to a decrease in the absorbance of the monomer Soret band, and a new broad absorption band with "-max = 444 nm appears, which is in good agreement with reported aggregates [6]. As (/> = 0.7, only the porphyrin aggregate absorbtion band (A max = 444 nm) can be observed and there no fluorescence emission can be detected. Aggregate formation requires a high enough concentra­tion of substrate. Obviously, aggregate formation of 2b is due to the local concentration enhancement induced by intermolecular aggregation driven by hy­drophobic force.

The ratio of the absorbance of aggregate absorption band (A a, "-max = 444 nm) to the sum of the absorbance of the aggregate and monomer absorption band ("-max = 410 nm) for 2b against (/> value in dioxane-water is shown in Fig. 3. It exhibits a clearly discernible break in the plot. The value corresponding the break in the plot in Fig. 3 is defined as critical solvent composition (C</» for aggregate formation. When (/> > C</>, aggregate formation is dependent on substrate concen-

146 Y. Cao et al. / Solar Energy Materials and Solar Cells 38 (]995) 139-155

E 1.0

r 4: ... III -< 0.8 , .. 4:

0.6

0.4 i o :

0.2 /

/1 0 -.

0 0.2 o.~ 0.6 0.8 1.0 .p

Fig. 3. The ratio of the absorbance of aggregate absorption band (A •• Am.,. = 444 nm) to the sum of the absorbance of the aggregate and monomer absorption band (Am.,. = 410 nm) for 2b against <P value in dioxane-water.

tration. Fig. 4 shows the plot of Aa/(Am + A) against substrate concentration. The break in this plot indicates the occurrence of critical aggregation concentra­tion (CAC).

1.0

0.8

E 4: + CIS 0.6 <C

"-CII «

0.4

0.2

0

0 -0/

0 1 2 3 4

concentration

Fig. 4. Plot of A. /(A m + A.) against substrate concentration.

Y. Cao et al. / Solar Energy Materials and Solar Cells 38 (1995) 139-155

, . '" 2' " .a (; B «

!

510 .\(nmJ

Q

-q,=o 0.7

610

b -¢-o

0·7

Fig. 5. The absorption and fluorescence spectra of 3b in dioxine-water binary solvent.

147

The absorption and fluorescence spectra of porphyrin moiety in aggregate of 2b 3b and 4b resemble each other very well (Figs. 2, 5 and 6). Using the same method as for 2b, we also found the C", and CAC of 3b, 4b and 5, which are listed in Table 2.

For compound 5, aggregation makes the absorption bands redshifted (Fig. 7a) and new structured fluorescence bands (Emax = 409,431 nm) appeared (Fig. 7b) in dioxane-water binary solvent. The study of temperature (Fig. 8) and concentration (Fig. 9) effects indicates that the new fluorescence bands come from the carbazole dimer which is formed due to the intermolecular aggregation driven by hydropho­bic force. Under the same condition, no any carbazole dimer fluorescence emission of 1, 2a and 2b could be detected. This implies that in the aggregate of 1, 2a and 2b, the carbazole moieties separated by a large porphyrin moiety which inhibited carbazole dimer formation. For carbazole moiety of 2b, the absorption and fluorescence spectra in dioxane-water binary solvent are similar to that in dioxane, but the fluorescence intensity is lowered (Fig. 2). The favourite conformation of 1 and 2 in organo binary solvent might be in folded type which is schemed in Scheme 3. Therefore, the decrease in fluorescence intensity of the carbazole moiety might

148 Y. Coo et aL / Solar Energy Materials and Solar Cells 38 (J995) 139-155

. " .. u C o

.l> (; III

.l> <I.

A. (nm)

b

- 4'==0 --- C;=O·5

a

- <1>-0 ---4>-0·5

Fig. 6. The absorption and fluorescence spectra of 4b in dioxine-water binary solvent.

be attributed to electron transfer fluorescence quenching by an adjacent porphyrin cation radical (Scheme 4, Step 5) and the final charge separation, P-T - V+ -Cz + was obtained, in which V+ and Cz+ is separated by -(CH Z)lO-' 9-Methyl car­bazole (Cz-CH 3) and methyl, lO-N-Carbazoleyldecyl viologen dibromide were

Table 2 C'" and CAC of 2b, 3b, 4b and 5 in dioxane-water

Compound C'" •

2b 3b 4b 5

a Concentration: 2 X 10-5 M.

0.70 0.60 0.40 0.55

b", = 0.8 for 2b; 0.65, 3b; 0.5, 4b and 0.6, 5.

CACbM

2x 10 5

1 X 10-5

1.5 X 10-6

5xlO- 6

Y. Cao et al. / Solar Energy Materials and Solar Cells 38 (]995) 139-155 149

synthesized and their fluorescence behaviours were studied. We found that the fluorescence life times of CZ-CH3 and Cz-(CH 2)IO-y2 + are 14.8 ns and 14.9 ns in THF, respectively. It implies that there is no significant electron transfer interaction between Cz and y2+ units in Cz-(CH3)1O-V2+.

41

g :": td

..::l ' L. " o •• ' D'I .0 «

ZiO 310 350

(a)

--q,=o

········<P=O.6

370 wavelength (nlll)

(bl

:: .' -- <P= 0 c:

: . ........... <P= O.S >. ...; : II

tJ . ' -' . ::

c 0

D'I r-

'. e , tal ,

~- ' . . '"

320 380 HO 520

wavelength (n.) Fig. 7. For compound 5, aggregation makes the absorption bands redshifted (a) and new structured fluorescence bands (Em"" = 409, 431 nm) appeared (7b) in dioxane-water binary solvent.

150 Y. Cao et al.; Solar Energy Materials and Solar Cells 38 (J995) 139-155

3.3. LB film behaviour

By LB technique, the chloroform solution of amphiphilic porphyrin compounds tetrad 1, triads 2a, 2b, diads 3a, 3b and porphyrin 4b were spread onto the tri-stilled water subphase to form the film with unidirectionally oriented monomolecular layer, respectively. Only the hydrophillic moiety viologen of these molecules was reasonably located in LB film surface, therefore the conformation of 1 and 2 in LB film should be arranged spatially in folded type like that in aquiorgano binary solvent (Scheme 3).

The surface pressure-area isotherms of amphiphilic porphyrin compounds 1, 2a, 3a and 4b are shown in Fig. 10. For isotherms reveal the behaviours of steep increase of the surface pressure on reduction of the area per molecule and exhibited the formation of solid densely packed monomolecular layer in the air-water interface with the cross sectional area of about 192.5 A2, 174.8 A2, 153.6 A2 and 22.5 A2 per molecule in the case of compounds 1, 2a, 3a and 4b respectively, A smaller monomolecular area for compound 4b suggested that the ring plane of porphyrin molecules was squeezed from the water surface and laid on the top of steric acid molecules, In addition, the surface pressure of LB film has an

.' .A. /'

45 cC " '\ " J .

I 30°C : !: , -13°C :: . I : .. :

:l 1 ., I

>- I '-\ ..., \ ....

UI 1\ " c i\;\ .. ' , ... c I '. I

\ . ,

c , 0 \

\ ,

1/1 III

, :

E .... :

til

320 360 400 4~Cl 480 520

wavelength (nm)

Fig. 8, Study of temperature (Fig. 8).

Y. Cao et aL / Solar Energy Materials and Solar Cells 38 (J995) 139-155

c c III III .... E

W

.. ..

: (': .. \. :/ .. :' r ·1·' ;. \',,-:

:1 ,.

- lxlO-=1.t

_._. hlO-=1.t

. •••.. 5xlO-=1.t

320 360 400 440 480 520 560

wavelength (nm) Fig, 9, Study of concentration.

151

effect on photocurrent. The photoinduced short-circuit current of tetrad mono­layer LB film increased from 1.85 J..LA to 2.79 J..LA during the surface pressure changing from 20 dyn/cm2 to dyn/cm 2

.

3.4, Photovoltaic behaviour

To investigate the photovoltaic behaviours of these artificial photosynthesis molecules and to confirm the contribution of porphyrin, vioiogen, terephthalic acid

p

v r-------iez

Scheme 3.

152 Y. Cao et al. / Solar Energy Materials and Solar Cells 38 (J995) 139-155

Ip_T_y 2+ -Cz

1 iJ1

P-T-y2+-CZ

Scheme 4.

ester and carbazole, which act as sensitizer, electron acceptor and electron donor in the unimolecule, well-organized monomolecular layer assemblies of compound 1, 2, 3 and 4 were constructed on Sn02 conductive glass by using the LB technique. The absorption spectra of tetrad 1 in LB film (Amax = 440 nm), which resemble closely with those of compounds 2a, 2b, 3a, 3b and 4b are shown in Fig. 11. The maximum absorption of these amphiphilic compounds 1, 2, 3 and 4b in LB film (440 nm) which is red shifted about 20 nm compared with that in solution (416 nm) is very much closed to that of the aggregate formed due to the hydrophobic force in aquiorgano binary solvent (444 nm).

The open-circuit voltage and short-circuit current observed with one monolayer of 1, 2a, 2b, 3a, 3b and 4b which were modified on Sn02 electrode surface are

0 20 40 60

0 lQ

\ ""' \IJ ... I 8 0 \\ ~"" s::: \\ >. \ ."

....... 0

\. (l)C')

'"' ;:l \\ '" '" \' (1)0

1:l.C'I \' (I) \\ CJ \\ alo \\ '):: .....

"'~ ..... ;:l r:n " . ................ -----

0 80 160 240 320 400 Aera (A2)

Fig. 10. The surface pressure-area isotherms of amphiphilic porphyrin compounds 1 (_. -' -), 2a ( ...... ), 3a (--) and 4b with stearic acid (- - - - - -).

Y. Cao et aL / Solar Energy Materials and Solar Cells 38 (J995) 139-155

1.6 r-----------'---.<J.25

0.20

c o

0.15 ~ P., s..

0.10 ~ .D <:

0.05

0.0 0.00 400 480 560 640 720 800

Wavelength (nrn)

153

Fig. 11. The photocurrent action spectrum (--) and absorption spectrum (- - - - - -) of tetrad 1.

shown in Table 3_ Comparison of the photovoltage (Voc ) and the photocurrent Usc) results among them clearly indicated that the highest values of Voc and Isc were obtained for tetrad 1 (Voc 758 mY, Isc 2.56 JLA/cm2

). The order of the photo­voltaic values of both the voltage and current is tetrad 1 > triads 2a, 2b > diads 3a, 3b > porphyrin 4b. No photoresponse was observed for bare Sn02 electrode. These results reveal an important contribution of the electron donor and electron acceptor for increasing the charge separation efficiency.

A mechanism including photoinduced charge transfer and multistep charge separation for tetrad was suggested (Scheme 4): Mter excitation of 1 (step 1), an intermediate charge separation state P+ - T-_y2+ -Cz was immediately obtained by fast electron transfer from excited porphyrin to terephthalic acid ester (step 2). Especially, because of the viologen and terephalic ester moieties as an electron acceptors and carbazole moiety as an electron donor, the back electron transfer from intermediate charge separation state to its ground state (step 3) was retarded more efficiently than that of triads [26]_ The further steps of charge separation (step 4 and step 6) subsequently occur from P+ - T--y2+ -Cz to the another two intermediates P+-T-Y+-Cz and P-T--y2+-Cz+ respectively. The final charge separation state P-T-Y+-Cz+ with a long lifetime was formed (step 5 and step 7) and detected by laser flash photolysis (Fig. 1).

Table 3 Photovoltage (Voc ) and photocurrent (Ise) of monolayer LB films modified on Sn02 electrode

Compound Photovoltage Photocurrent (mV) (IJ.A/cm2)

758 2.55 h W6 1~

2b 743 0.81 3a 464 0.14 4b 421 0.11

154 Y. Cao et al. / Solar Energy Materials and Solar Cells 38 (J995) 139-155

Fig. 11 illustrates photocurrent action spectrum and absorption spectrum of tetrad 1. The photocurrent action spectrum in a monolayer resembles its absorp­tion spectrum obtained with 25 monolayers. It reveals that the observed photocur­rent mainly arose from the excitation of the photosensitizer, i.e., the porphyrin moiety. The quantum efficiency which is defined as the ratio of the number of collected electrons over the number of photons incident on the monolayer of tetrad at the excitation wavelength of 440 nm was 1.17 X 10-3•

4. Conclusions

A new tetrad compound of covalently linked porphyrin-terephthalic acid ester­viologen-carbazole (1) was synthesized. Compared with the triads (2a and 2b), diads (3a and 3b) and the porphyrin with the long carbon chain (4b), the tetrad (1) has the highest efficiency for photoinduced electron transfer and charge separa­tion.

A systematic study on photophysical properties in aquiorgano binary solvent was carried out. In dioxane-water, due to the intermolecular aggregation driven by hydrophobic force, aggregates of porphyrin and/or carbazole were formed, which resemble LB films.

Using LB techniques to assemble the tetrad (1) on Sn02' a rather high photovoltage of 758 mV and photocurrent of 2.56 ,...A/cm2 with only one mono­layer were obtained. It is further confirmed that well organized molecules of porphyrin tetrad with a electron donor and two electron acceptors make a multistep charge separation which leads to more inhibitation of back charge transfer and additional increase of efficiency for photoelectric conversion as compared with porphyrin triads.

Acknowledgements

This work was supported by National Advance Material Committee of China and National Science Foundation of China. The transient absorption spectra were detected by laser flash photolysis in the Lab of Prof. K. Tokumaru, Tsukuba University.

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