Conductance Control of Porphyrin Solids by Molecular Design and

solids. The type of conductance for porphyrin films is hardly controlled by the doping technique. ... the doping technique is widely used to control t...
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J . Phys. Chem. 1989, 93, 5311-5315

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Conductance Control of Porphyrin Solids by Molecular Design and Doping Kazuo Yamashita,* Yutaka Harima, and Tatsuro Matsubayashi Division of Material and Life Sciences, Faculty of Integrated Arts and Sciences, Hiroshima University, Hiroshima 730, Japan (Received: October 31, 1988; I n Final Form: January 17, 1989)

Photovoltaic effects observed with a thin sublimed film of 5,10,15,20-tetra(4-pyridyl)porphyrin (T(4-Py)P), T(3-Py)P, or T(2-Py)P sandwiched between AI and indium-tin oxide (ITO) electrodes are explained in terms of the n-type semiconducting behavior of the pyridylporphyrin film and its formation of a blocking contact (Schottky barrier) with ITO. The doping effects of electron donors and acceptors into the porphyrin solids upon the photocurrents are also consistent with the n-type conductance of the pyridylporphyrins. Similar n-type semiconducting behaviors are observed with other porphyrins having heterocyclic substituents such as 6-quinolyl and 6-quinoxalyl groups. A series of phenyl/4-pyridyl meso-substituted porphyrins were investigated to find a relationship between the chemical structure of the porphyrin molecule and the conductance type of the porphyrin solid. In consequence, it was found that at least three pyridyl groups are necessary for the porphyrin film to exhibit the n-type conductance. However, this is not a sufficient condition, because introduction of a Zn(I1) ion into the center of the porphyrin ring of T(4-Py)P leads to the conductance change from the n-type to the p-type as the first ring reduction potential varies from -0.93 V to -1.16 V vs SCE. This indicates that the ease of reduction of the porphyrin molecule is also necessary for manifestation of the n-type conductance in addition to the presence of the heterocyclic groups. The first reduction potential of the porphyrin molecule is adopted as an index relating with the conductance type of porphyrin solids. The type of conductance for porphyrin films is hardly controlled by the doping technique. Finally, the origin of the n-type conductance observed is discussed.

Introduction Recent studies of organic photoconductors indicate a great number of possibilities of their applications to reprography, vidicon television tube, photomemory, photovoltaic cell etc. As for solar cells, at present organic devices that utilize their photovoltaic or photoelectrochemical properties stand still far from practical use because of low energy conversion efficiencies as well as poor long-term stability compared with traditional inorganic solar cells. The superiority of inorganic materials induces some pessimism for organic solar cells. However, it is unjustified because organic materials have some unique advantages over the inorganic ones as to diversity, flexibility, ease of fabrication, low cost, etc. Establishment of a technology for controlling electric and photoelectric properties of organic materials is urgent to build more efficient organic devices. In the case of inorganic semiconductors the doping technique is widely used to control the type of conductance. However, it is difficult to apply the technique to organic semiconductors. Of course the dark conductivity or photoconductivity of the organic materials is increased or decreased by But, it hardly results in doping electron acceptors or the change of conductance from a ptype to an n-type or vice versa. Several years ago we reported first that substitution of four phenyl groups in a 5,10,15,20-tetraphenylporphyrin (TPP) molecule by four 3-pyridyl groups led to the conductance change of the porphyrin film from the p-type to the n-type and that a blocking contact was formed at the interface between TPP (p-type) and T(3-Py)P ( n - t ~ p e ) . ~ This suggests the feasibility that the type of conductance of organic molecular solids is controlled by the molecular design. In this paper, at first we describe the details of the n-type semiconducting properties of porphyrins such as T(4-Py)P, T(3Py)P, T(Z-Py)P, and the like. Second, the effects of central metal ions on the first ring reduction potentials and on the type of conductance are presented for metallo derivatives of T(4-Py)P. Finally we show the replacement effects of 4-pyridyl groups in a T(4-Py)P molecule by phenyl groups upon the type of conductance of the porphyrin films. This study contributes to a better understanding of the relationship between the type of conductance and the molecular structure for porphyrins, which leads to the

establishment of the method to control the type of conductance of organic semiconducting materials by the molecular design.

Experimental Section Molecular structures of metal-free porphyrins used are represented in Figure 1. The free bases (5,10,15,20-tetra(4pyridyl) porphyrinato)copper(I I) (CUT(4-Py)P) and ZnT( 4-Py) P were prepared and purified according to the methods in the lite r a t ~ r e . ~Phenothiazine -~ (Pz) and o-chloranil (Chl) used as an electron donor and acceptor were purified by recrystailization from toluene and acetone, respectively. Iodine used as an acceptor was purified by sublimation prior to use. Ammonia gas was dried over calcium hydride and sodium metal just before use. Other chemicals used were of reagent grade. An indium-tin oxide (ITO) film was produced on a piece of cleaned slide glass by means of Tokuda CFS-8EP closed-field sputtering apparatus.I0 The film thickness was about 100 nm, the electric resistance was about a few hundred ohms cm-2 and the transparence was greater than 80% in the visible region of the solar spectrum. Films of porphyrins and metals were prepared by sublimation and evaporation at a pressure of Pa, respectively. Porphyrin films doped with Chl, Pz, and I, were prepared by using a Kyowa-Riken K359SW spinner. Fabrication of the sandwich-type cell of M/porphyrin/M’, where M or M’ denotes an I T 0 or a metal electrode, were carried out as described previously.I0 Dark electric and photoelectric properties of the photovoltaic cells were measured by the apparatus controlled by an NEC PC-8801MK2 microcomputer (Figure 2). The measurements were carried out after standing the cells in a dark place and in air for a week after preparation unless otherwise specified. Photocurrents were normalized in such a way that the intensity of monochromatic light incident on the surface of the dye film through the illuminated metal was constant at about 2.70 X lOI3 photons cm-2 s-l with aid of the microcomputer. Inset 1 depicts the top and side views of the sandwich-type photovoltaic cells prepared on a piece of slide glass, where eight units of the cell are fabricated. Inset 2 illustrates the vessel used for measurements of effects of gases (O,, H2, and NH3) on the photo(6) Shamim, A.; Worthington, P.; Hambright, P. J . Chem. Sot. Puk. 1981,

3, 1 .

(1) Tollin, G . ; Kearns, D. R.; Calvin, M. J . Chem. Phys. 1960, 32, 1013. (2) Kearns, D. R.; Tollin, C.; Calvin, M. J . Chem. Phys. 1960, 32, 1020. (3) Loutfy, R . 0.;Menzel, E. R. J . A m . Chem. SOC.1980, 102, 4967. (4) Leempoel, P.; Fan, F-R.; Bard, A. J. J . Phys. Chem. 1983, 87, 2948. (5) Yamashita, K.; Matsumura, Y.; Harima, Y.; Miura, S.; Suzuki, H. Chem. Lett. 1984, 489.

0022-3654/89/2093-531 1$01.50/0

( 7 ) Williams, G. N.; Williams, R. F. X.; Lewis, A,; Hambright, P. J . Inorg. Nucl. Chem. 1979, 41, 41. (8) Fleischer, E. B. Inorg. Chem. 1962, I , 493. (9) Bull, R. A.; Bulkowski, J. E. J . Colloid Interface Sci. 1983, 92, I . (IO) Yamashita, K.; Harima, Y.; Iwashima, H. J . Phys. Chem. 1988, 91, 3055.

0 1989 American Chemical Society

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Yamashita et al.

The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5

I

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1

1

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1 substituents at positions 5 IO 15 20 Ph Ph Ph Ph la 2-Py 2-Py 2-Py Ib 2-Py 3-Py 3-Py 3-Py IC 3-Py 4-Py 4-Py 4-Py 4-Py Id 4-Py Ph Ph 4-Py le 4-Py Ph 4-Py Ph If 6-4 6-Q 6-Q 6-Q Ig 6-QxI 6-QxI 6-QxI 6-QxI Ih Figure 1. Chemical structures of the meso-substituted porphyrins used. l a , 5,10,15,20-tetraphenylporphyrin(TPP); I b , 5,10,15,20-tetra(2pyridy1)porphyrin [T(2-Py)P]; IC, T(3-Py)P; Id, T(4-Py)P; le, 5,15di(phenyl)-l0,20-di(4-pyridyl)porphyrin (?rans-Ph2Py2P);If, 5.1 O-di(phenyl)-]5,20-di(4-pyridyl)porphyrin (cis-Ph,Py2P); lg, 5,10,15,20tetra(6-quinoly1)porphyrin [T(6-Q)P]; Ih, 5,10.15,20-tetra(6quinoxaly1)porphyrin [T(6-Qxl)P]. Abbreviations used: Ph, phenyl group; Py, pyridyl group; Q, quinolyl group; Qxl, quinoxalyl group. compd

U Y

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Figure 2. Block diagram of the apparatus used for measurements of current and voltage characteristics of the porphyrin photovoltaic cells. LS, light source; SL, slit; SH, shutter; OC, switch for shutter; MO, monochromator; WS, sweeper of wavelength; L, lens; C, cell; B, power supply; R, variable resistance; SW, circuit-changing switch; E, electrometer; D, display; COM,microcomputer; REC, recorder; SPM, spectrophotometer. inset 1 illustrates the top and side views of the sandwichtype photovoltaic cells. Inset 2 shows the vessel used for the study of doping effects of gases.

400

500

600

Wavelength / nm Figure 3. Action spectra of photocurrents for the ITO/T(4-Py)P/AI cell with the optical absorption spectrum of the T(4-Py)P film used. Solid and dashed curves represent the spectra obtained oh illumination of the IT0 and the AI electrode, respectively. Voltages are applied to the IT0 electrode with respect to the AI electrode

I T 0 electrode. Inset shows the optical absorption spectrum of the T(4-Py)P film used. Current passing from AI to I T 0 through the cell is taken to be positive. The action spectra obtained on illumination of the I T 0 side closely follow the absorption spectrum of the porphyrin film, though sometimes slight shifts of peaks to longer wavelength are observed. The red shift of the peak in the action spectra may be due to the surface recombination of charge carriers in the porphyrin fi1ms.l' Broadening and weakening observed for the blue (Soret) band in the optical absorption spectrum are similar to those for other porphyrin films and attributable to the strong dipole-dipole interactions in solid. These facts indicate that the singlet or triplet state of the porphyrin molecule is the plausible precursor of the charge carrier generation.12 On the other hand, the action spectra obtained on illumination of the AI electrode are quite different in shape from the spectra obtained on illumination of the I T 0 electrode and the absorption spectrum, though the photocurrents observed flow from AI to I T 0 through the cell regardless of the direction of illumination. The photocurrent at the Soret band decreases with increasing thickness of the porphyrin layer, when illumination is made from the AI side. This phenomenon is ascribed to the optical filtering effect due to the porphyrin phase.5 Only the light absorbed near the I T 0 blocking contact is efficient in producing charge carriers (holes and electrons), resulting in the photocurrents, while an Ohmic contact is formed at the AI/T(CPy)P junction. Similar results are obtained for homologues such as T(3-Py)P and T(2-Py j P and other porphyrins having heterocyclic substituents, for example, 5,10,15,20-tetra(6-quinolyl)porphyrin(T(6-Q)P) and 5,10,15.20-tetra(6-quinoxalyl)porphyrin (T(6-Qx1)P). The contacts of these porphyrins with noble metals such as Pt, Au, and Ag, of which work functions are relatively large, are found to be blocking, too. Figure 4 illustrates the voltage dependence of the dark current and photocurrents for the ITO/T(CPy)P/AI cell with the dependence of illumination direction on the photocurrents. No obvious rectifying effect is observed on the dark-current-voltage curve for the cell in air. This behavior is similar to that of lightly doped or nearly intrinsic semiconductors. However, at the reduced pressure (ca. Pa) a rectifying phenomenon appears slightly on the I-V curve. On the other hand, the photocurrents are

response. All measurements were carried out at room temperature.

Results and Discussion Figure 3 displays the action spectra of photocurrents for the ITO/T(4-Py)P( 100 nm)/Al cell at 0 and -1.5 V applied to the

(11) Yamashita, K.; Kihara, N.; Shimidzu. H.; Suzuki, H. Photochem. Photobiol. 1982, 35, 1. (12) Kampas, F. J.; Yamashita, K . ; Fajer, J. Nature (London) 1980, 284,

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The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5313

Conductance Control of Porphyrin Solids

a

*2-

2

b

?

Irradiation time/ min

Figure 6. Effects of O2in the air on the photocurrents for the sandwich-type cells of (a) ZnTPP and (b) T(3-Py)P. Illumination is carried out through the AI electrode for the ZnTPP cell and through the IT0 electrode for the T(3-Py)P cell at the wavelength of the respective Soret peak. 1.0

0 -1

-1.0

E'V N

-

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Figure 4. Dark-current-voltage and photocurrent-voltage characteristics for the ITO/T(4-Py)P/AI cell. Curves 1 and 2 are obtained on illumination of the IT0 and the AI electrode, respectively. z5L

'

"

"

*SLl

400 500 Wavelength/

600

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Figure 7. Doping effects of I2 and phenothiazine on the action spectra for the Au/T(4-Py)P/AI cell. Spectrum 1, no doping; 2, doped with I1; 3, doped with phenothiazine.

1

L

A

from these results that T(4-Py)P is characterized as an n-type semiconducting material rather than p-type. No clear rectifying effect observed with the ITO/T(4-Py)P/Al cell in air is attributable to the presence of 02,which acts as acceptor and thus hampers the n-type conductance of the T(4-Py)P film. In principle, the type of conductance of organic solids can be determined by measurements of the Hall effect, the Seebeck effect, etc. in the same manner as inorganic semiconducting materials. Actually, however, the result obtained by one method is not always consistent with another. Especially, for organic thin films it is very hard to obtain reliable data for measurements of the Hall effect because of their high resistivities. Hereafter, therefore, we determine the type of conductance of the porphyrin film from the optical filtering effect and the dopants effect. Figure 6a illustrates the effect of O2 in the air on the shortcircuit photocurrent obtained on illumination of the AI electrode in the ITO/ZnTPP/AI cell. The pressure in the vessel containing Pa, and the photovoltaic cell was reduced down to about 3 X then the air was rapidly introduced by opening the stopcock. At first the photocurrent increased promptly, then decreased gradually with the lapse of time, and reached a certain limiting value, being fairly large in comparison with the initial stationary photocurrent obtained at the reduced pressure. This O2effect coincides well with the p-type semiconducting behavior of ZnTPP itself, since O2molecules act as good electron acceptors and as a matter of course they expedite the light-induced charge separation in the ZnTPP film.I0 On the other hand, the photocurrent obtained on illumination of the I T 0 electrode for the ITO/T(3-Py)P/AI cell decreased by introducing the air into the vessel (Figure 6b). At present we cannot understand the initial rapid increase of the photocurrents observed for both cells, though it might reflect the capacitance change in the cells.

--/ 0

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a Figure 5. Dark-current-voltage and photocurrent-voltage characteristics for the ITO/ZnTPP/AI cell. Curves 1 and 2 are obtained on illumination of the IT0 and the AI electrode, respectively.

strikingly enhanced at negative voltages applied to the I T 0 electrode. This indicates that the forward bias corresponds to the IT0 electrode being positive and the blocking contact or Schottky barrier is formed at the ITOIT(4-Py)P junction, where the conduction- and valence-band edges bend upward at 0 V or short-circuit conditions. The results are contrary to those observed with p-type porphyrins such as ZnTPP and MgTPP.12 Surely, in both cells of T(4-Py)P and ZnTPP the photocurrents flow in the same direction from A1 to I T 0 through the porphyrin phases (Figures 4 and 5 ) . However, it should be noted here that there is a substantial difference between them, which is that the strong rectifying and photovoltaic effects observed for the ITO/ ZnTPP/AI cell are attributable to the blocking contact formed at the ZnTPP/AI junction and not to the I T 0 contact. In this case, the band edges bend downwards at 0 V or under short-circuit conditions. Hence, photogenerated electrons in the space charge layer move along the electric field toward the AI electrode and holes move in the opposite direction. We know that a blocking contact is formed at the junction of ZnTPP with ITO, but it is very weak in comparison with the A1 ~ 0 n t a c t . l ~It is deduced

(13) Yamashita, K.; Harima, Y.; Matsumura, Y. Bull. Chem. SOC.Jpn. 1985, 58, 1761.

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The Journal of Physical Chemistry, Vol. 93, No. 13, 1989

Yamashita et al. N

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Figure 8. Schematic representation of energy bands at the (a) AI/ ZnTPP/Au and (b) AI/T(4-Py)P/Au junctions.

The O2effect observed with T(3-Py)P or T(4-Py)P is similar to the effect of l 2 or o-chloranil, but contrary to that of phenothiazine (Pz), as shown in Figure 7. Namely, the photocurrents for the Au/T(4-Py)P/AI cell, where a Au film is used as an electrode in place of I T 0 to form a blocking contact with T(4Py)P, are decreased by doping 12, but increased by Pz. The concentrations of 1, and Pz in the T(4-Py)P films were about 8 mol %. Of course, the photocurrents for the ZnTPP cell are enhanced by doping l 2 and c-chloranil, but decreased by Pz. These facts indicate that the films of T(4-Py)P and those of T(3-Py)P can, undoubtedly, be characterized as n-type semiconductors. It is worth noting here that the photoactive region in the cell of Au/T(4-Py)P/AI (or Au/ZnTPP/AI) does not shift from the Au contact to the AI contact (or from the A1 contact to the Au contact) by doping electron acceptors (or donors). The energy diagram for the AI/ZnTPP/Au junctions in contradistinction to that for the AI/T(4-Py)P/Au junctions is represented in Figure 8, where the T(4-Py)P solid is assumed to be an extrinsic n-type semiconductor and the energy levels of T(4Py)P are estimated from the results described previously and those reported in ref 13. Besides the pyridylporphyrins, as described previously, porphyrins such as T(6-Qxl)P and T(6-Q)P exhibit the n-type semiconducting behavior. In all cases four heterocyclic groups are present commonly as the peripheral substituents in the porphyrin molecules. Such heterocyclic groups seem to have an important role for the porphyrin to show the n-type conductance. Figure 9 shows the photocurrent action spectra for the Al/ PhPy3P(80 nm)/Au cell, where PhPy3P denotes 5-(phenyl)10,15,20-tri(4-pyridyl)porphyrin. The observed large difference in shape between the spectra 1 and 1' is ascribed to the optical filtering effect due to the PhPy3P phase and explained in the same manner as T(4-Py)P. When the PhPy3P cell was exposed to NH, vapor, the photocurrents were increased distinctly (spectrum 2). On the contrary, doping I2 into the porphyrin film resulted in the reduction of the photocurrent. These doping effects are the same with those observed with T(4-Py)P, indicating the n-type conductance of the PhPy,P film. In fact, the replacement of one 4-pyridyl group in a T(4-Py)P molecule by one phenyl group does not lead to any change of the type of conductance. Substitution effects of two pyridyl groups by two phenyl groups upon the photovoltaic features are illustrated in Figure IO. The dependence of illumination direction on the photocurrent for the Au/trans-Ph2Py2P/Al cell, where trans-Ph2Py2Pdenotes 5,15diphenyl- 10,2O-di(4-pyridyl)porphyrin, is the same as that of cis-Ph2Py2P,which is 5,IO-diphenyl- 15,20-di(4-pyridyl)porphyrin, indicating that the photoactive region in the cell is the porphyrin/AI interface and not the Au interface. The p-type semiconducting behavior observed with trans- and cis-Ph2Py,P agrees well with the doping effect of NH3 on the photocurrent (spectrum 2). The photocurrents were drastically decreased by N H 3 gas. Thus

Wavelength/ nm Figure 9. Photocurrent action spectra and effects of N H 3 on the photocurrents for the AI/PhPy,P/AI cell. Spectra 1 and 1' are obtained for the cell in air on illumination of the Au and AI electrodes, respectively. Action spectrum 2 is obtained for the cell in NH3 gas on illumination of the Au electrode.

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Figure 10. Photocurrent action spectra and effects of NH, on the photocurrents for the Alltrans-Ph,Py,P/Au cell. Spectra 1 and 1' are obtained for the cell in the air on illumination of the AI and Au electrodes, respectively. Action spectrum 2 is obtained for the cell in NH, gas on illumination of the AI electrode.

the replacement of two pyridyl groups by two phenyl groups brings about the change of the type of conductance from the n-type to the p-type. It is, therefore, inferred that at least three pyridyl groups as the peripheral substituents in the porphyrin molecule are necessary for the porphyrin solid to exhibit the n-type conductance. The type of conductance for a porphyrin solid can also be changed by insertion of a suitable metal ion into the center of the porphyrin. For example, films of metallo complexes of T(4-Py)P such as (5,10,15,20-tetra(4-pyridyl)porphyrinato)magnesium(II) (MgT(4-Py)P) and ZnT(4-Py)P exhibit clear p-type semiconducting behaviors. On the other hand, a CuT(4-Py)P film shows the same n-type photovoltaic features as T(4-Py)P. Generally, introducing a divalent metal, whose electronegativity is smaller than that of hydrogen atom, into the T(4-Py)P molecule tends to lead to the change of conductance from the n-type to the p-type, while insertion of a metal having larger electronegativity produces no influence on the conductance. Previously, we have demonstrated that the more easier-to-oxidize compound exhibits the higher quantum yield of photocurrent.I2 The exponential correlation between the quantum yield and the first ring oxidation potential for ptype porphyrins indicates that the photocurrent or quantum yield is in direct proportion to the rate constant of charge carrier formation process. The reason is that the rate constant correlates exponentially with the oxidation potential of the porphyrin molecule at the excited or ground state. In the case of p-type porphyrins, therefore, the first ring oxidation potential of the porphyrin molecule becomes a very useful criterion

The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5315

Conductance Control of Porphyrin Solids TABLE I: Summary of First Ring Reduction Potentials, Type of Conductance. and Doping Effects E', /2/V "S acceptors donors porphyrins type I2 Chl O2 NH3 H2 Pz T(4-Py)P -0.93 -0.91" n - - + + + PhPy3P -0.97 -0.95' n - + cis-Ph2PyzP -0.99' p + trans-Ph2Py2P -0.99" p + Ph3PyP -1.04" p + TPP -1.08 -1.09' p + + + CuT(4-Py)P ZnT(4-Py)P ZnTPP

-1.04 -1.16 -1.35

n P p

f + +

+

+

+

-

-

-

+,

"After Shamin et aI.l4 Symbols -, and A denote the increase, decrease, and no change of photocurrents when the porphyrin films are doped.

to evaluate the quantum yield and is very instructive in the preparation of a more efficient porphyrin. In other words, introducing an ion of metal having lower electronegativity into the center of the porphyrin ring or placing electron-releasing groups as peripheral substituents near the porphryin ring is very effective to increase the photocurrent quantum yield. For n-type porphyrins, on the other hand, the reduction potential of the porphyrin molecule seems to be one of important parameters relating the photocurrent quantum yield with the type of conductance of the porphyrin solid, although the data listed in Table I are not enough to conclude it. One can see that the n-type semiconducting porphyrins such as T(4-Py)P and T(6-Qxl)P are more easily reduced than p-type porphyrins such as ZnTPP.I4 It is difficult to draw a definite line regarding reduction potential between the p-type and the n-type porphyrins. However, the boundary is supposed to be located at around -1.0 V vs S C E within the metal-free Ph/Py meso-substituted porphyrins and other free bases. It is worth noting here that the reduction potential of a cis- or trans-Ph2Py2molecule is -0.99 V and very close to that of the n-type porphyrin PhPy3. Nevertheless, the conductance type of the film of cis- or trans-Ph2Py2Pdoes not change from the p-type to the n-type by doping donors such as Pz and NH3. The reverse applies to PhPy3P and CuT(4-Py)P. Namely, their conductance types are hardly changed from the n-type to the p-type or vice versa by doping acceptors or donors. The conductance type of organic solids often is well understood on the basis of the energy band model in the same manner as inorganic semiconductors. For example, the p-type conductance observed with solid films of most porphyrins and phthalocyanines studied so far is attributable to the presence of oxygen molecules in the organic solids. Oxygen molecules act as good electron acceptors or provide sites for electron traps,I0J5 and inject holes into the valence band, and thus give rise to the ptype conductance. O2molecules, of course, exist in the T(4-Py)P film sandwiched between AI and Au or ITO. However, the T(4-Py)P solid shows n-type conductance, which is hard to understand from the presence of O2acceptors in the solid film. Furthermore, it is very interesting and worth noting that the n-type conductance of T(4-Py)P is independent of doping additional acceptors such as I2 and Chl into the solid. If T(4-Py)P solids are extrinsic n-type semiconductors, there must be donors rather than acceptors, but it seems to be improbable. (14) Shamim, A.; Hambright, P.; Williams, R. F. X.J. Znorg. Nucl. Chem. Lett. 1979, 15, 243. ( I 5 ) Stanbery, B. J.; Gouterman, M.; Burgess, R. M. J . Phys. Chem. 1985, 89, 4950.

In the meantime, we know a case where a substitution of the CH bridge in a molecule of acridine yellow or pyronine G by a nitrogen atom leads to a change from p-type to n-type conductance.I6 The introduction of nitrogen is connected with a red shift in absorption spectrum by about 100 nm. This shift in absorption has been attributed to a change in electron density in the excited state at the position of the nitrogen atom. The increase of the electron affinity might be one of the factors responsible for the n-type conductance. A similar consideration may be applicable to the present case, though there is a great difference in chemical structure between acridine yellow or pyronine G and the porphyrin. In fact, T(4-Py)P is reduced more easily than TPP, indicating the increase of the electron affinity by the introduction of pyridyl groups in place of phenyl groups, though no distinct red shift in the absorption spectrum is observed. The n-type or p-type conductance may be observed with porphyrin solids, even though the porphyrin solids are intrinsic semiconductors. This is attributed to ( 1 ) a difference between the , n-type conductance hole and electron mobility (&, and K ~ ) viz. if M~ > w h and p-type conductance if &, > pe, (2) a difference between drift mobilities of holes and electrons, caused by different trap concentrations and trap depths, or (3) asymmetric trapping processes of holes and electrons.17 At this stage, we cannot explain thoroughly the reason why the presence of heterocyclic substituents such as pyridyl groups is effective for the manifestation of the n-type conductance. However, it is evident that the pyridyl groups are electron-withdrawing substituents and so introduction of the pyridyl groups into the porphyrin molecule results in the ease of the reduction of the porphyrin ring. As a matter of fact, the ease of reduction is surely one of the most important and necessary conditions for manifestation of the n-type conductance, but it is not regarded as a sufficient condition. For instance, a 5,10,15,20-tetrakis(p-nitropheny1)porphyrin molecule, being reduced somewhat easily compared with the T(4-Py)P molecule,'* shows a p-type conductance. Measurements of work functions or Fermi levels of the mesosubstituted porphyrin solids by means of the ultraviolet photoelectron spectroscopic technique are in progress to establish the mechanism of the n-type conductance. In conclusion, the films of porphyrins having heterocyclic substituents such as pyridyl groups, quinoxalyl group, and the like are characterized as n-type semiconducting materials. For a series of phenyl/4-pyridyl meso-substituted porphyrins, the films of T(4-Py)P and PhPy3P show n-type conductance, but other derivatives, i.e., cis- and trans-Ph2Py2Pand Ph3PyP, exhibit p-type conductance. This indicates that no less than three pyridyl groups are necessary for the porphyrins to manifest t h e n-type conductance. Thus we can control the conductance type of porphyrin solids by the molecular design. In addition, it is confirmed that the type of conductance of the porphyrins is not changed by doping electron donors or acceptors, though the dark conductance and photoconductance can be varied. Registry No. la, 917-23-7; l b , 40904-90-3; IC, 40882-83-5; Id, 16834-13-2; le, 71410-72-5; If, 71410-73-6; lg, 120926-75-2; lh, 120926-76-3; CuT(4-Py)P. 14518-23-1; ZnT(4-Py)P, 31 183-1 1-6; ZnTPP, 14074-80-7; ITO, 50926-1 1-9; AI, 7429-90-5; 0 2 , 7782-44-7; 12, 7553-56-2; Au, 7440-57-5; PhPy,P, 71 188-40-4; NH3, 7664-41-7; phenothiazine, 92-84-2. (16) Meier, H. In Monographs in Modern Chemistry; Ebel, H. F., Ed.; Verlag Chemie: Weinheim, 1974; Vol. 2, pp 30 and 155-157. (17) Reference 16, pp 413-418. (18) Callot, H. J.; Giraudeau, A,; Gross, M. J . Chem. SOC.,Perkin Trans. 2 1975, 1321.