Electrical Properties of Some Porphyrins under High Pressure - The

360

D. W. WOOD,T. N. ANDERSEN,AND H. EYRING

Electrical Properties of Some Porphyrins under High Pressure

by D. W. Wood,’ T.N. Andersen, and H. Eying

Downloaded by UNIV OF MANITOBA on September 4, 2015 | http://pubs.acs.org Publication Date: February 1, 1966 | doi: 10.1021/j100874a007

Institute for the Study of Rate Processes, University of Utah, Salt Lake City, Utah (Received June 1‘7, 1966)

The electrical conductivities of protoporphyrin, hematoporphyrin free base, hematoporphyrin dihydrochloride, copper protoporphyrin, hemin, hematin, etioporphyrin, and vanadyl etioporphyrin have been measured a t the static pressures of 1-90 kbars and at temperatures of 25 to 100”. The studywas made in a Bridgman anvil press. A resistivity reduction of as great as five orders of magnitude was observed for the bridged compounds (hemin, hematin, and vanadyl etioporphyrin) over the pressure range, and a corresponding activation energy decrease by more than half was observed in the same pressure range. The resist,ivity us. pressure curves for these compounds showed a change in slope, and the activation energy us. pressure curves showed a discontinuity near 25 kbars. The resistivities and activation energies of the nonbridged compounds were essentially pressure independent. A hole-conduction mechanism independent of pressure for the nonbridged porphyrins accounts for the observed results. A conduction mechanism for the bridged compounds is proposed which involves passage of the electron from one chelated cation to the next through the bridging anion. Two rat,e-determining processes in series are required to explain the electron mobility.

Introduction The electrical conductivity of many organic molecules such as large polyaromatics and polycyclics lies in the semiconducting range,2s3 and several of these compounds show a decrease in resistivity of several powers of ten under increased p r e s ~ u r e . ~ - ~ Conduction through porphyrins, such as phthalocyanine and hemin, is often explained by ?r-electron overlap in the conjugated systems of neighboring molec ~ l e s ~ ~although ’ ~ ~ 1 ~ excitons have also been considered.6 I n the case of iron porphyrins containing imidazoles in the fifth and sixth coordinating positions, a grouptransfer mechanism through the iron-imidazole chain has been suggested.12 The porphyrins studied may be divided into three different classifications: those consisting of the parent organic skeleton, those containing a bivalent chelate, and those containing a polyvalent chelated ion with a bridging anion. The molecular structures of the planar or nearly planar mole~ules~3-’~ are schematically shown in E’igure 1. Structural determination of all porphyrins which have been studied show that molecules stack parallel to one another.’6-l7 Unfortunately, The Journal of Physical Chemistry

no structural determinations have been made of the bridged-type molecules to determine the shape of the anion-chelated cation-anion chain. (1) This paper is part of the work carried out in partial fulfillment of a Ph.D. degree. (2) C. G. B. Garret, ”Semiconductors,” N. B. Hannay, Ed., Reinhold Publishing Corp., New York, N. Y.,1959, p. 634. (3) H. Inokuchi and H. Akamato, “Solid State Physics,” Vol. 12, F. Seitz and D. Turnbull, Ed., Academic Press Inc., New York, N. Y., 1961, p. 93. (4) R. E. Harris, R. J. Vaisnys, H;,Stromberg, and G. Jura, “Progress in Very High Pressure Research, John Wiley and Sons, Inc., New Tork, N. Y., 1962, p. 165. (5) G. A. Samara and H. G. Drickamer, J . Chem. Phys., 17, 471, 474 (1962). (6) R. S. Bradley, J. D. Grace, and D. C. Munro, “The Physics and Chemistry of High Pressure,” London Symposium, 1962, Gordon and Breach, Science Publishers, Inc., New York, N. Y., 1963, p. 143. (7) M.A. Cook, R. T. Keyes, A. G. Funk, and F. A. Olson, Naval Ordnance Test Station Final Technical Report, Dee. 31, 1963, Contract N 123 (60530) 25906A. (8) H. A. Pohl, A. Rembaun, and A. Henry, J . A m . Chem. Soc.. 84, 2699 (1962). (9) R. B. Aust, W. H. Bentley, and H. G. Drickamer, J . Chem. Phys., 41, 1856 (1964). (10) D. D. Eley, G. D. Parfitt, &I. J. Perry, and D. H. Faysum, Trans. Faraday Soc., 49, 79 (1953). (11) M. H. Cardew and D. D. Eley, Discussions Faraday SOC.,27, 115 (1959).

ELECTRONIC PROPERTIES OF PORPHYRINS UNDER HIGHPRESSURE

361

OH CH:CHz


hemin

*CH:CHz

CHzCOzH

hematoporphyrin dihydrochloride

CH,

CH=CHz

CHzCOzH

CH~CO~H CH~CO~H

hematin

CH3

copper protoporphyrin

CHzCH2

I

etioporphyrin

vanadyl etioporphyrin

I

TT

CHzCOzH

CH3

I

I

CHzCOzH

protoporphyrin Figure 1. Structural formulas of compounds studied.

Since these materials decompose rather than melt or sublime at elevated temperatures, purification is effected by chromatography and recrystallization. The source and grade of each material studied is given in Table I. From the Table I it can be seen that minor amounts of impurities are probably present in all samples. The possible effect of these will be discussed later.

(12) D. Urry and H. Eyring, J . Theoret. BWZ., 8 , 198 (1965). (13) A. E. Martell and M. Calvin, ,6Chemistry of the Metal Chelate Compounds,” Prentice-Hall, Inc., New York, N. Y., 1959, pp. 26, 176. (14) D. F. Koenig, Acta Cryst., 18, 663 (1965). (15) J. M. Robertson, J . Chem. SOC.,615 (1935); 1195 (1936). (16) L. E. Webb and E. B. Fleischer, J . Am. Chem. Soe., 8 7 , 667 (1965). (17) M. B. Crute, Acta crust., i z , 2 4 (1959).

Volume 70, Number 92 February 1966

362

D. W. WOOD, T. N. ANDERSEN,AND H. EYRING

Table I : Source and Grade of Compounds Studied


7. Etioporphyrin 8. VanadyI etio-

porphyrin

Sigma Chemical Co. Mann Research Laboratories

Recrystallized three times 98% (Spectrograde)

Mann Research Laboratories

Unspecified

Calbiochem

C grade

Calbiochem Prepared and donated by Dr. G. Cartwright, Department of Medicine, University of Utah Prepared and donated by Mr. A. Sonntag, Department of Chemistry, University of Utah Prepared and donated by Mr. R. Garvey, Department of Chemistry, University of Utah

C grade Multiple recrystallization

Although all of these porphyrins show approximately the same room pressure resistivity after compaction, there is a marked difference observed at high pressures which would indicate that the conduction mechanism in these different molecules is not the same.

Experimental Procedure A Bridgman opposed-anvil high-pressure apparatus has been used t,o subject the organic compounds to pressures in excess of 75 kbars. The samples consisted of thin wafers of 0.64-cm. diameter and a thickness after compact,ion of about 0.25 cm. These samples were kept in a desiccator for several days before being subjected to pressure. The samples were first subjected to the maximum pressure of the experiment in order that compaction be obtained. The pressure was then released, and all electrical measurements were made as the pressure was again increasing. Constant current values were obtained within 10 min. after the potential was applied. After the initial compaction, repetitive experiments on a sample yielded essentially identical results. The sample resistivity, p , was calculated by using the equation p = RA/d where R is the measured resistance, A is the cross-sectional area, and d is the final thickness of the sample.l8 Although it is known that pressure gradients exist across flat anvil face^,^^'^^^ it has been shown for anvils of about the present diameter that the gradient is small if samples of diameter to thickness of approximately 25:l are used.lg This was further substantiated experimentally by measuring the resistances of small strips of bismuth embedded in some of the The Journal of Physical Chemistry

Grade

Source

Porphyrin

1. Hemin 2. Hematoporphyrin dihydrochloride 3. Hematoporphyrin (free base) 4. Protoporphyrin (Lipostain) 5. Hematin 6. Copper protoporphyrin

Recrystallized three times and chromatographed Recrystallized and chromatographically homogeneous

samples. Quite sharp resistance changes occurred at the expected transition pressures. Although some error undoubtedly exists because of the pressure gradients, it is relatively unimportant for the semiquantitative purposes of the present experimental data. Therefore, pressures on the sample were calculated assuming uniform distribution of pressure across the anvil faces. The d.c. resistances of the samples were measured by applying a constant voltage and recording the current through the anvils and sample on a Keithley Model 610A electrometer. Voltage us. resistance calibration showed that the value of the resistance was independent of the applied voltage in the range 1-90 v. Substituting various metal contacts between the sample and anvils gave identical results. These experiments indicate that space charge is negligible and that any contact potential junction must be ohmic. Since the resistances of the samples were much larger than that of the anvils, the resistance of the latter could be neglected. Considering the large resistance changes with pressure, the change in the sample thickness with pressure could also be neglected. It appears certain, also, that the resistance measured is that of (18) We have chosen to report the results as resistivity, rather than resistance since the difference only involves geometrical differences, and resistivity is generally more useful. The results are, of course, subject to small corrections due to pressure gradients across the anvils and to changing thickness of sample with pressure change. Our results are for a compacted powder and may be somewhat different than for a single crystal. (19) F. Dachille and R. Roy, “The Physics and Chemistry of High Pressure,” London Symposium, 1962. Gordon and Breach, Science Publishers, Inc., New York, N. Y., 1963, p. 77. (20) E. R. Lippincott and H. C. Duecker, Science, 344, 1119 (1964).



the substance and not that of loose crystallite contacts. The material is soft, and these contact resistances would be eliminated before the first few kilobars, as was shown for several organic materials by Eley and Parfitt.21 The sample was heated by means of a small-split Variac controlled furnace with a thermocouple placed as close to the sample as possible. Equilibrium resistance values were obtained after holding the Variac constant for at least 1 hr. beyond the time required to reach a definite temperature. The compressibility us. pressure curve for hemin was obtained by means of a piston-cylinder T.E.C. kiloton hydropress. Details of this high-pressure apparatus have been published e l ~ e r n h e r e . ~ ~ ~ ~ ~

363

o n . .

13.0-

0

h \

0

0

a

e

0

0

\v

b

:::- 9\-v\ -. :- --- -- - .

11.0- A

A A A A A

- .. .. .. .

---.*a

A - A - -

Results The plot of the resistivity us. pressure for the porphyrins is shown in Figure 2. The listed experimental results are the average of from two to as many as ten runs (in the case of hemin), and the deviation in p was less than half an order of magnitude a t a given pressure. Log R us. 1/T plots were obtained from 15 to 100" at various pressures and resulted in single straight lines, the slopes of which gave the activation energies. was calculated on the basis This activation energy, EaCt, of the equation cr = uo exp(-E,,$kT) where cr is the conductivity. Therefore P = Poe

Eact/kT

.8

where Eaetis shown as a function of pressure in Figure 3. Repetitive measurements a t given pressures produced a precision of activation energies to within *0.05 e.v. Although the anvils were open to the atmosphere, the effect of oxygen on the resultant curves is not significant as shown by Cook and co-workers,' who compared p us. P curves at room temperature for several of these materials with and without vacuum applied to the press. Likewise, adsorption of water vapor from the atmosphere during a measurement was shown not to affect the results (although the previously cited authors' obtained an effect at low pressures for protoporphyrin). Free volumes of activation, AV*, were calculated in the case of hemin and hematin from the equation AV*

=

RT b In p/bP

-

(1)

0

Protoporphyrin

X

Copper protoporphyrin

0

Hemotin

Hemin

-6-

\

2

A Vonodyl Etioporphyrin

.c

\

P\ ' \ I

\'\.ci

\

P (kb)-

Figure 3. Experimental energy of activation, Eactus. pressure.

(2)

pressure, showing a change in the slope a t 20-30 kbars for hemin and hematin. A break in the activaThe average calculated AV* for hemin is -5.6 ~ m . ~ / le P > mole for P < 25 kbars and -1.66 ~ m . ~ / m o for (21) D. D. Eley and G. E. Parfitt, Trans. Faraday Soc., 51, 1529 30 kbars. For hematin AV* = -2.70 ~ m . ~ / m o for le (1959). P < 25 kbars and - 1.59 ~ m . ~ / m ofor l e P > 30 kbars. (22) F. R. Boyd and J. L. England, J . Geophys. Res., 6 5 , 741 (1960). Figure 2 shows that the bridged porphyrins give a (23) G. C. Kennedy and R. C . Newton, "Solids under Pressure," marked decrease in resistivity, p , with an increase in McGraw-Hill Book Go., Inc., New York, N. P., 1962, p. 163. ~~

~

~~

Volume 70, Number 8 February 1966

364

tion energy plots also occurs in this region (see Figure 3). For lack of quantity of sample in the case of vanadyl etioporphyrin, the complete curves could not be studied. As a further study of the break in the activation energy in the 20-30-kbar range, a compression study of hemin was made a t room temperature with an observed change in slope of the P vs. AV/Vo curve a t 25 kbars as shown in Figure 4. The nonbridged porphyrins show no change in resistivity or activation energy with pressure.

D. W. WOOD,T. N. ANDERSEN, AND H. EYRINQ

Or

-05-

.I0 -

: P

.IS-

.20-


Discussion The mechanism of conduction in the nonbridged porphyrins is insensitive to pressure and also appears to involve the mechanism a t least partially operative This conclusion is based on the similarities of zero in the bridged compounds a t atmospheric pressure.

~ .305

2 IS

pressure (extrapolated) resistivities of bridged porphyrins and their respective nonbridged porphyrins, along with the pronounced difference in resistivities of etioporphyrin and the carbonyl-bearing porphyrins. Considering that many polyaromatics and phthalocyanines are p-type semiconductors6 and also that hemin is a p-type semiconductor at atmospheric pressure,24with an activation energy similar to the above compounds, this semiconduction can be understood if it is due to electron trapping by the crystals (principally selftrapping rather than impurity-trapping) followed by hole conduction.6,26s26The fact that the resistances of phthalocyanine, copper phthalocyanine, pentacene, tetracene, pyrolytic graphite, and anthracene decrease with increasing pressure indicates that a bridge is not necessary for semiconduction under pressure if there is enough ?r-orbital overlapping. One must remember, however, that substituents on the pyrrole ring could prevent appreciable overlap in the case of the porphyrins studied. The difference between the conductivities of etioporphyrin and the protoporphyrins or hematoporphyrin can be understood as due to the presence of carbonyl groups on the latter compounds (etioporphyrin has only aliphatic groups). The carbonyl groups are in a better geometric position to overlap with neighboring molecules than are the more protected pyrrole rings. Impurity donors are, of course, a source of charge carriers. Evidence against a major contribution by such extrinsic conduction is the fact that the conductivities of the bridged porphyrins approach that of the respective nonchelated porphyrins at low pressure. Also, if impurities account for semiconduction in both bridged and nonbridged compounds, it is difficult to understand why pressure does not affect the conductivity in both. The Journal of Physical Chemistry

20

25

30

3!i

40

45

P (kbl-

Figure 4. Compression us. pressure for hemin at 25”.

X-Ray evidence on analogous platelike compounds leads to the expectation that the porphyrins will assume a preferred orientation under pressure with the plane of the porphyrin ring parallel to the anvil faces.2’ Since the measurements for Figures 2 and 3 are perpendicular to the anvil faces, the conductivity data reflect changes perpendicular to the molecular planes. This is further substantiated by the fact that the resistance of hemin measured parallel to the anvil faces does not decrease with increased pressure as does the resistance measured perpendicular to the anvils (cJ Figure 2). The following model is accordingly proposed for the n-type conduction in hemin at elevated pressures as well as for the other bridged compounds studied. Some Fe3+ ions acquire an electron from the organic conjugated system followed by transport of the electron along the Fe-C1-Fe chain (the holes left in the organic part of the molecule may also give p-type conduction by migration to neighboring porphyrins). Pressure aids the n-type conduction in two ways: (1) the neighboring Fe-CI groups are brought more closely together so that the mobility of electrons is increased, and (2) the number of n-type carriers is increased in accord with the general results that semiconductors become more metallike with pressure. (24) A. Terenin, Proc. Chem. Sac., 321 (1961). (25) N. Riehl, “Symposium on Electrical Conductivity in Organic Solids,” H. Kallmann and M. Silver, Ed., Interscience Publishers, Inc., New York, N. Y.,1961, p. 61. (26) A. Terenin, ref. 25, p. 29. (27) R. Sehr, M. M. Labes, M.Bose, H. Ur, and F. Wilhelrn, ref. 25, p. 309.



In this particular case the iron, which a t low pressures is out of the porphyrin plane, is forced closer to the conjugated system by pressure thus lowering its energy by delocalization or^ the extra electrons into the ring system. Supporting this mechanism are the facts that (a) electron transfer between Fe2+ and Fe3+ ions, bridged by a third anion such as C1-, occurs easily in the case of aqueous solutions2* and (b) the conductivity of solids with a nonintegral number of electrons per atom, such as Fe304, is many orders of magnitude higher than the conductivity of solids with an integral number of electrons per atom (cf. Fe203).29 The criterion for rapid conduction in these systems is that two adjacent metal ions of different valence states are in nearly equivalent potential energy positions. Therefore, when an electron is transferred from one to the other, the final state is at essentially the same energy as the initial state.3O We observe a change in the charge-transport mechanism for hemin and hematin at approximately 25 kbars as evidenced by (a) the change in slope of log p vs. P , (b) the discontinuity in the plot of E,,, us. P, and (c) the change in slope of AV/V us. P. These phenomena show that the resistance to migration is now arising principally from steps which a t larger pressures than 25 kbars were relatively inconsequential. The conductivity, u, is the product of the number of carriers per milliliter, N, their charge /el, and their mobility p.

Here n, is the number of barriers of kind, i, that a charge must surmount in moving 1 cm. while (rJV=1 is the time required to surmount each barrier when the applied potential is 1 v./cm. We will designate the jump between neighboring iron atoms as mechanism 1. If X1 is the distance between two adjacent iron atoms along the line Fe-C1-Fe-C1, then nl = 1/X1. The distance spanned by the sum of all other types of Xi in the electron migration is assumed negligible by comparison. The second type of barrier t o migration which experiment reveals as important at pressures above 25 kbars will be described as mechanism 2. There may well be other steps, but they are not significant in our experiments. We can also write32

- -1 (Ti)

v-1

- ki = KIcTh e - AG,*/RT

(4)

where, assuming the volume of activation to be pressure independent

365

+

AH*p=l - TAS*~=.~ ( P - l)AP* (5) Thus for our case we have for the resistivity AG*p

=

Well below 25 kbars the berm n 2 ( ~ 2 ) ~is, ~negligibly small, whereas well above this pressure the term n1(71)~~1 is negligible. Equation 6 is for two resistances in series and corresponds to the observed behavior as indicated by the two full lines in Figure 5. Had the two resistances been in parallel, the equation for the resistance would have been

(7) and would have followed the dotted lines of Figure 5 in disagreement with experiment. n-Type and ptype conduction are for example two parallel mechanisms. Other possible parallel conductors would be impurity acceptors or donors. Such complications are not observed in our experiments. Barrier one can be considered to be the C1- bridge between a Fe2+ and Fe3+ ion. The second barrier which the mobility exhibits could be understood if there is a misfit in the lattice causing a misalignment in the Fe-C1-Fe chain at certain places; such a barrier is a modified bridge. Our experiments show that this

I

25

P (kb) Figure 5. Resistivities p1 and p2 as functions of pressure: solid line, over-all resistivity for p1 and pz in series; dotted line, over-all resistivity for p1 and pz in parallel. -

(28) H. Taube, Chemistru, 38, 18 (1965). (29) E. J. Verwey, P. W. Haayman, and F. C. Romeijn, J . Chem. Phys., 15, 181 (1947). (30) E. J. Verwey and J. H. deBoer, Rec. Traa. Chim., 5 5 , 531 (1936). (31) R.B.Parlin and H. Eyring, “Ion Transport across Membranes,” Academic Press Inc., New York, N. Y., 1954,p. 103. (32) S. Glasstone, K. J. Laidler, and H. Eyring, “The Theory of Rate Processes,” McGraw-Hill Book Co., Inc., New York, N. Y., 1941,Chapter IV.

Volume 70, Number 2

February 1966

366

D. FENNELL EVANS AND ROBERT L. KAY


barrier decreases with pressure more slowly than the first. I n fact, since experiment only yields the sum of the effects of pressure on ionization and on mobility, we cannot, rule out the possibility that the mobility of carriers according to mechanism ( 2 ) actually decreases with pressure. This will be evident if one examines eq. 5 and notes how the pressure effect comes into AG,the free energy of ionization in carrier production, and into AG,*, the activation free energy for mobility. The coinpression data (Figure 4) indicate that a t low pressures the normal lattice undergoes compression resulting in the C1- ions being moved into more sym-

metrical positions, thus improving charge mobility. Above 25 kbars the important effect on mobility according to our model is the effect of pressure on misalignment.

Aclcnourledgment. The authors gratefully wish to acknowledge the Army Research Office (Durham) for financial support of the work under Grant No. DAARO-D-31-124-G-618. We also wish to thank contributors of compounds as listed in Table I. The valuable advice of Professors Ivan Cutler and Owen Johnson is gratefully acknowledged.

The Conductance Behavior of the Symmetrical Tetraalkylammonium Halides in Aqueous Solution at 25 and 10"

by D. Fennel1 Evans and Robert L. Kay Mellon Institute, Pittsburgh, Pennsylvania 16813 (Received June $8, 1866)

Precise conductance measurements are reported for Me4NC1, RIe4NBr, RIe4NI, EttNBr, Pr4NC1, PrtNBr, Pr4N1, Bu4NCI, Bu4NBr, and Bu4NI in aqueous solutions at 25 and 10" for the concentration range 5 X 10-4 to low2M . Salt purity was verified by the agreement obtained in the limiting ionic conductances. A Fuoss-Onsager analysis gave low ion-size parameters that increased with cation size for the chlorides, remained constant for the bromides, and decreased with increasing cation size for the iodides, indicating increasingly abnormal behavior as the anion size increased. The opposite order and much higher values have been reported for the alkali halides. This effect is attributed to increasing association as both the anion and cation size increases. Only the data for Pr4NI and Bu4NI at 25" and Bu4XI a t 10" analyzed directly for any significant amount of association.

Introduction The symmetrical tetraalkylammonium ions have long been used as good examples of spherical ions having a large variation in size. A number of recent systematic investigations of the properties of these electrolytes in aqueous solution have indicated that the interaction of water with the hydrocarbon portion of the electrolyte is of considerable importance. LindenThe Journal of Physical Chemistry

baum and Boyd2 have shown that the activity coefficients for the tetraalkylammonium chlorides increase with increasing cation size, whereas the opposite behavior is observed in the case of the bromides and (1) Presented in part a t the 147th National Meeting of the American Chemical societv. Chicago. - . 111.. SeDt 1964. (2) s. Lindenbaum and G. E. Boyd, J. P h w . C h m . , 68,911 (1964). "

I

Electrical Properties of Some Porphyrins under High Pressure - The

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