The photoinduced charge separation at the water-porphyrin interface

Volume 50, numhcr 1

THE PHOTOINDUCED

CHbMICAL

CHARGE

PHYSICS LtTTERS

SEPARATION

AT THE WATER-PORPHYRIN

1.5 August 1977

INTERFACE

Mitsuyuki SOMA

Received 9 August 1976 Revised manuscript rceivcd

17 May 1977

The photomcluced charge \cp.lration nntl chqqc transfer at the I~ountl:~ry bctwccn ‘3 tctraphcnylporphmc crystal and an aqut3xb clcctrode were observed wlth the porphyrin Iaycrs prepared on d goh.kodtcd ,tlurnuiiurn fad arid on a tr:in\parcn t msulating polynwr film.

1. Introduction Efforts in the direction of understanding the primary processes in photosynthesis have stimulated studies of the photoelectric cffccts in, and at the surface of, aggregates of porphyrin molecules and related compounds [ 1,2]_ The possible utilization of porphyrin pigments as active elements for solar energy conversion has recently amplified interest in such work [3,4]. Studies relevant to the photoelectronic processes in the photosynthesis apparatus arc the photoelectric phenomena at the interface between sohd-state porphyrin and an aqueous clectrocle. Recent developments in investigation using bilayer lipid membranes containing chlorophyll have shown that one can construct model systems which at least partly demonstrate photoelectric behaviour similar to actual photosynthesis systems [5]. On the other hand cfficient charge transfer and/or charge injection across the surface of single crystals of some aromatic hydrocarbons in contact with aqueous or other liquid elcctrodcs have been shown to occur [G-8] _ Unfortunatcly, the techniques used in such work are not always applicable to porphyrin crystals because of the difficulties in obtaining single crystals of the desired size and geometry. However, some pioneering work, as well as the recent studies have revealed the possibility of studying the photoelectric behaviour of porphyrin layers deposited on appropriate substrates in contact with an aqueous electrode [4,9- 111. The aim of the prcscnt work is to demonstrate the

feasibility of relatively simple systems of an evaporalcd porphyrm film in contact with an aqueous electrode in the study of photoinduccd charge generation at the interface. Using a subhmcd film of tctraphcnylporphine as an example, it will be demonstrated that photoinduced charge separation at the water-porphyrin intcrface glvcs rise to a photovoltagc and a photovoltaic current in the porphyrin film on a gold-coated metal electrode and that, from additional measurements of the photoinduced potential changes and the transient displacement current observed in the film deposited on a transparent insulating polymer membrane, the photoelectric effect at the water-porphyrin boundary can be separated from that at the other boundary in the porphyrin 1iQWr.

2. Experimental Commercial tetraphenylporphine free base (TPP, Strcm “chlorin free” grsdc) was used without further purification. The photoelement was made by the vacuum sublimation of TPP onto a substrate. The substrate was either an i~lunliniurTl foil onto which was evaporated a gold film coating or an insulating membrane of a copolymer of tetrafluoroethylene and hexafluoropropylenc (FEP polymer). The FE1 film is transparent within the wavelength range of interest, the lo.ss of light intensity across the film being less than 5%. The photoelement was fixed by vacuum grease as 93

Volume SO, number 1

CitEhIICAL

PHYSICS LIXl-ERS

Fig. I. A schematic cross sectional view of the pbotnccll.

1 Pyrex glals window, 2 nqucous clectrodc, 3 Wmplc mcmbmne, 4 polyethylene plate, 5 platinum wire clcctrode, G fac;tener.

shown m Fig. 1 in the photoelectric cell (made of glass or polyethylene) which is similar to those described previously [S] . TIE two comparti~ents which were separated by .I membrane (3) were filled with distilled water (2). Platinum wire elcctrodcs (5) in contact with the aqueous electrode were connected to an clectrometer. The area of the TPP film in contact with the water was ca. 1.2 cm2. The excitation was effected using a 300-W tungsten lamp combined with a Toshiba glass filter (a high-pass filter with a cut-off wavelength at 390 ~1) and a JarreII-Ash 0.25-m monochromator. Only an exit slit of 100 p was used in order that the light intensity bc sufficient to obtain an easily detectable photoelectric signal. The photovobge and current were measured using a Takeda model TR--84M ~brating-reed electrometer. In the current measurement mode the input resistance was 10” or 10” ohm. The measured current was independent of the input resistance, thereby indicating that the main potential drop occurs across the sample membrane. After the measurement, the TPP layer was dissoivcd in benzene and the amount of TPP deposited on the substrate was determined from the absorption spectrum in the visible range. The amount was typically of the order of low7 mole.

3. Results and discussion Fig. 2 shows the spectral dependence of the photovoftaic current obtained with the TPP film on the gold-coated aluminium foil, the current flowing with the Pt electrode facing the illuminated TPP surface being always negative, Tlirou~iout this paper the photovoltagc and current are defined as positive when

01 400

500

WAVELENGTH

600

(nm)

1

1’~. 2. The yxxtd dcpcndence uf the photovoltaic current m the tctr~phunylporpflirle film on B gold cudted alumini~m foci. Tlte upper curves are tllc absorption spectra for thin and tt ;ck TPP tilms on the FEP memhranl?.

the Pt electrode facing the TPP layer is positive, that is, when the potential drops across the water-TPPsubstrate direction. Also shown in the figure are the absorption spectra of the TPP films on the FEP membrane. The absorption spectra are almost identical to the solution spectra [ 121, except for the small red-shift of each absorption band, the partly resolved structures of the Q-bands which are recognizable as shoufders and the considerable broadening of the Soret band. AIthough the action spectrum is of a very qualitative nature without any correction for the spectral intettsity distribution of the lamp and the characteristics of the monochromator, one can clearly see the close correspondence between the absorption spectrum and the photoelectric action spectrum. The observation indicates that only excited molecules near the water/ TpP interface contribute to the charge generation which is responsible for the photoelectric effect, as the thickness of the TW filrlii was sucii that one could not obtain such sharp structures in the action spectrum if all the excited molecules in the volume of the TPP film participated equally in the charge generation. The photovoltage showed essentially the same wavelength dependence. Its polarity is always negative in accordance with the direction of the photocurrent.

Volume

50, number

CHEMICAL

1

I’IIYSICS

LETI-1:KS

The value of the maximum at 560 nm is 35 mV (with 100 p slit). The polarity of the photovoltage and the direction of photocurrent require the accumulation of positive charge carriers in the TPP film with respect to the Pt electrode facing it and accordingly indicate electron transfer from TPP to the aqueous electrode. An applied external voltage up to 1 V simply increased the dark current but did not affect the photocurrent irrespective of its polarity, suggesting that a porous structure of the porphyrin layer allows a leak dark current. In order to detelminc more clcally where the charge scpalation occurs, the light induced polari/.ation developed in the TPP covered FEP membrane was measured. When the membrane was illumin~~tcd, the potcntial bctwcen the two Pt electrodes changed and reached a Stati0Ililly value. When the light was turned off, the poteutial decayed to the dark level. The charge generation in the TPP layer induces polarization in

the insulating FEP membrane, the polarity of ~11~11 depends on the direction of charge separation in and/or on the surface of the TPP layer. Thus, with the current mode of the measurement a transient displacement current was observed corresponding to the charging and discharging

of the membrane

systcrn

at the

mo-

of the initiation and termination of the illuminnCon, respectively. The rise time of the current was about 0.1 s which was limited by the recorder time constant. The magnitude of the transient photocurrent for the second shot of the excitation depended on the length of the preceding dark period. A dark period of about ten minutes was required for the complete recovery of the transient signal, which can bc attributed to the slow decay of the trapped charges. By keeping the excitation and dark period constant (15 s each) we could obtain a reproducible transient current, which was about 2/3 of its maximum value, i.e. the transient for excitation after il prolonged dark period. Fig. 3a shows the spectral depcndcncc of the peak transient current thus obtained with the illumination through the water/TPP interface, the Pt electrode of the dluminated side being ncgative. The general features of the spectrum arc in good agreement with the results shown in fig. 2. The photoinduced potential change exhibited a similar spectral dependence, the illuminated electrode being always negative. Accordingly, the charge separation nient

15 August

1377

a

-

H201TPP/FEP

b Ii20 1TPWFEP

4iio

-

500 WAVELENGTH

600 (ram)

700

1.1g. 3. llie spcctrnl depuntlencc of the tr:mwnt d:qAaccrnent photocurrcnt in the tctrd~~hcnylt~orptlirie fllrri on tlic I’CP membrane for (a) illunim.ltwn through the watcr/‘l‘l~IB interface, nnct (b) ~llurmn~tion through the I LI’/Tl’I’ intcrfxe.

The ordmatcs

for (a) drrd (1,) are shown

in the sdine units.

in the S~IC direction ;I$ in the TPP layer on the gold film, clectlons transferring from TPP to WiItCr across the interface. On the other hand, when tlic membrane was excited through the FEP/TPP interface, the sign of the transient current changed with the wavelength of cxcited light (fig. 31)). The positive current obtained in the wavelength region where the optical density of the TPP film is large can be attributed to the charge separation in the vicinity of the FtP/TPP interface. The resulting polarization 1s in the TPP-FEP direction. In the longer wavelength region where the optical density of the film decreases (see fig. 2), the sign was inverscd to negative, the same sign as when the sample was illuminated through the watcr/TPP occtxs

interface.

An inversion

of the sign of the photopoten-

teal took place in the same way. In the intcrmediatc wavelength region the initial positive spike of the current was followed by a nep,ative spike (qee below). In fig. 3b, the value of the major peak has been rccorded. With the following additional observations for the photoeffcct of the membrane illuminated through the FEP/TPP boundary (especially items 2 and 3), we attribute the negative photoeffect not to the d&rapping of the charge in the bulk of TPP

Votumc 50, number 1

CHEhfiCAL PHYSICS LETl-ERS

film but to the charge separation at the water/TPP boundary excited by the light passing through the TPP layer. (1) The change of the photoinduccd potential with the elapsed time after the initiation of the illumination was obsekyed. Broad-band excitation without slits in the monochromator was applied in order to obtain sufficiently Iargc signals. For 405 nm light only positive changes in the potential appeared. On the other hand, for 580 nm light, the initial rise of the potential was followed by a sharp decrease within 0.1 s and the potential changed sign and attained a stationary value. A similar bchaviour of the photovoltaic effect in chlorophyll with an asymmetric contact was reported by Tang and AIbrecht [ 131. The initial phase of the effect corresponds to the charge separation at the FEP/TPP interface and the latter to that at the water/TPP interface. For light of 670 run, only the negative phase was observed. (2) If the TPP lltycr was extremely thin, only the negative photoeffect was observed irrespective of the wavelength of the exciting light. Apparently the negative photoeffect exceeds the positive effect of the FEP/TPP interface. When the thickness of the TPP Idyer is increased, the wavelength at which the inversion of titc pI~otc~volta~~ effect occurs 1s sftrfteti to that corresponding to lower extinction. (3) The addition of an appropriate electron acccp tor, e.g. ~-ben~oqtl~rloI~e, to the aqueous electrode in contact with the TW film enhances the charge separation at the water/TPP interface with a concomltant increase rn the range of the invcrscd regions. Firlally, let us consider the possible electron acccptor in the charge separation at the water/TPP interface. The effect of removing oxygen present in t11c aqueous phase was examined in two ways. In one case, the water phiist: was charged thoroug~l~y by b~Ib~l~rig dry nitrogen and then the cell was sealed just before the photoexcitation. In another case, the transfer of the sublixncd TPP film from a glass container, in wbieh a sublimed film of TPP was prepared, to the measuring cell, the filling of the cell with water deaerated beforehand and the scaling of the ceil were clone in a nitrogen dry box. In both casts, no significant changes in the photoeffcct from that obtained

15 August 1977

under air were observed. Accordingly it is suggested that water is directly involved in the p~lotoinduc~d charge transfer process, although the possibility that a very small amount of residual oxygen in the aqueous phrase plays an important role cannot be cxchlded under the present experimental conditions. Tn conclusion, charge separation at the water/TPP boundary was demonstrated to occur with higher efficiency than that at the boundary between TPP and an inert polymer membrane. It is believed that the &tailed mechanism of the charge separation merits furrhcr investigation.

Acknowledgement The author wishes to tflank Professor Kenzi Tamaru for his encouragement. This work was partly supported by a grant from the Ministry of Education.

References [ 11 hf. Calvin, J. Tttcor. 13iol. 1 (1961) 258 [ 2J G. Tollin, J. Tl~cor. Biol. 2 (1962) 105. [31 AK. Gbosh, D.L. hforcl, ?- Bztg, R.S. Sltnw dnd C.A. Rowe Jr., 3. Appi. Phys. 45 (1974) 230. (4 1 F K. Fang and N. Wmograd, J. Attt. Chem. Sot. 98 (I 976) 2287. IS] H.T. Ticn, Pliotochcm. Pitotobiol. IG (1372) 271. [G] Ii. Kallmtlnn and hf. Pope. J. Citem. Pity\. 30 (19.59) 58.5; 32 (1960) 300; Nature 185 (1960) 753; 186 (1960) f7]’ ~-h*~b~ and T-hi. I+.&, Di~cttssion~ rnraday Sot. 45 (1968) 31. [S] hf. Soma, J. Am. Clam. Soe. 92 (1970) 3289; Bull. Chcm. Sot. Japan 43 (1970) 2247; A. Yarwgishi and hf. Soma, 1~~11.C&n.

Sot. Japan 43 (2970) 3741. 19 ] V.R. Evsttgncev and A.N. Tercnitt, Dokl. Aknd- Auk SSSR 81 (1951) 223. [ 101 GA- Alfcrov, V.I. Scva\t’yanov, V.A. Ilatovskii, Yu.S. Stturnav and C.G. Kontissarov, Dokl. Akad. Nattk SSSR 207 (1972) 628. J.H. Wang, Pruc. NatI. Acad. Sci. US 62 (1969) 653. K. Rousseau and II. Dolphin, Tctr&cdron Letters (1974) 4251. C.W. Tang nnd AC. Albrccltt, J. Chem. Ploys. 63 (1975) 953.