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An electron spin resonance study of copper porphin
IOURNALOFMAGNETICRESONANCE
26,341-349(1977)
An Electron Spin Resonance Study of Copper Porphin* JOSEPH BOHANDY
AND BORIS F. KIM
The Johns Hopkins University, Applied Physics Laboratory, Johns Hopkins Road, Laurel, Maryland 20810 Received August 20,1976; revision received November 1,1976 ESR spectra of magnetically concentrated copper porphin and of copper porphin diluted into single-crystal and polycrystalline triphenylene have been obtained, including low-temperature spectra of the concentrated and polycrystalline samples. Cu hyperline, nitrogen superhyperfine, and Cu isotope splittings are observed in the diluted samples. The results are used to estimate the molecular orbital coefficients for copper porphin. The data are consistent with copper porphin replacing triphenylene molecules substitutionally. INTRODUCTION
Metalloporphyrins are a class of compounds having a tetrapyrrole structure in which the metal is bonded to four nitrogen atoms in an approximate square planar array, usually assumed to have Ddhsymmetry. Replacement of the central metal by two hydrogens yields the so-called free base porphyrin. The parent compound of all porphyrins is called “porphin” (also referred to as the “unsubstituted porphyrin”) and has no substituents on the ring extremities. Various types of porphyrins are obtained by adding different side chains to the ring. Figure la shows a metalloporphin molecule. There have been a large number of electron spin resonance (ESR) and optical studies of these materials, in large part because of their presence in hemoglobin and chlorophyll. Ingram et al. (2) first observed the ESR spectra of copper tetraphenylporphin (CuTPP) and observed metal hyperfine structure even in the magnetically concentrated material. Assour (2) made ESR studies of copper, vanadyl, and cobalt TPP complexes in solution and diluted into the diamagnetic free base H2TPP. CuTPP and AgTPP were thoroughly studied by Manoharan and Rogers (3), using solutions, magnetically diluted single crystals, and polycrystalline powders. ESR measurements at 9.3 and 35.2 GHz on the cup:ric protoporphyrin IX dimethyl ester (CuPPDME) in chloroform and toluene solutions, and in the diamagnetic free base protoporphyrin IX dimethyl ester, were made by Hsu (4). The only previous report of ESR spectra of an unsubstituted porphyrin is that of van.adyl porphin (5). We report here the results of an ESR study of copper porphin (CUP) in its magnetically concentrated form, diluted into a single-crystal host lattice of triphenylene, and in polycrystalline samples of CUP in triphenylene obtained by crushing single crystal specimens. Spectra were taken at X band and KU band and at temperatures from room temperature down to 6 K. * Work supported by Naval Sea Systems Command under Contract NOOO17-72-C-4401 and Public Health Services Grant GM21897, National Institute of General Medical Sciences. Copyright 0 1977 by Academic Press, Inc. 341
All rights of reproduction Prinkd in Great Britain
in any form reserved.
ISSN 0022-2364
BOHANDY
342
U.4
AND
KIM
3
b
a/ FIG. 1. (a) A metalloporphin molecule. Replacement of the metal by two hydrogens gives the free base. (b) A representation of the normals to the four triphenylene planes in a unit cell relative to the crystallographic axes. EXPERIMENTAL
The ESR spectra were taken on a standard spectrometer at both X band and KU band. An Air Products Heli-tran variable-temperature unit, compatible with a Varian rectangular cavity, was used to obtain low-temperature spectra. CUP was purchased from the Mad River Chemical Company (now defunct). Crystals of triphenylene containing CUP as a guest molecule (denoted by CuP/TP) were grown by slow evaporation of mixed solutions. Triphenylene is an orthorhombic crystal having the space group P212,21 and four molecules per unit cell (6). The long dimension of the needle-shaped crystals is the crystallographic c axis, and the normals to the four triphenylene planes make equal angles of 51” with the c axis (see Fig. lb). The projections of these normals onto the a-b plane bisect a and b. The dominant crystal faces are the (1 10~type faces. RESULTS
A. Magnetically Concentrated CUP Room-temperature ESR spectra of an undiluted polycrystalline sample of CUP were obtained at 9.5 and 17.6 GHz. The spectra were characteristic of an axially symmetric system but, even at 17.6 GHz, the parallel and perpendicular portions were only
ESR OF COPPER PORPHIN
343
pa:rtiallyresolved.Cu hype&e structurewasnot observedasit wasin CuTPP (Z-3), in whichtherearefour phenylgroupson the porphinring and thedipolar interactions arereducedby the physicalstackingof the molecules.The absenceof Cu hfs indicates a smaller Cu-Cu distance in CUP. The values obtained for g,, andg, are 2.172 and 2.066, respectively. ‘Variable-temperature X-band ESR spectra of both CUP and CuTPP were recorded down to 17 K. The only significant change with temperature was the expected increase in intensity. B. Polycrystalline CuP/TP
Single-crystal specimens of triphenylene doped with CUP were crushed into a fine powder. ESR spectra obtained at X band and KU band are shown in Fig. 2. At X band, only the two lowest-field parallel components of the Cu hyperfine structure are separated from the much stronger perpendicular portion of the spectrum. At KU band, three parallel components are resolved, with the fourth just masked by the perpendicular spectrum. Each parallel hyperfine line is further split into lines separated by 14.2 G, because of the interaction of the unpaired electron with the four pyrrole 14N nuclei. Four equivalent nitrogen nuclei would give nine superhyperfine (shf) lines having an intensity ratio of 1:4: 10: 16: 19: 16: 10:4: 1. The observed spectra approximately adhere to this. In addition, between these shf lines there are lines which are approximately one-half as intense. These are either due to a magnetically inequivalent site or to the resolution of lines from the Wu isotope. The abundances of the two isotopes are 69.1 and 30.9 % for 63Cu and 6sCu, respectively. Thus, the expected intensity ratio would be approximately two to one. This suggests that these weaker lines are due to 6sCu. Further evidence for this is the fact that the weaker lines appear halfway between the stronger ones at both microwave frequencies with a splitting consistent with the ratio of the magnetic moments. This would not be the case if the weaker lines were due to alg-shift effect. Additional support for the isotope splitting will be given in the next section. The perpendicular portion of the powder spectrum is more complex, especially at X band. This is due not only to the shf structure but also to the fact that when there is a large anisotropy in the nuclear hyperfme interaction, as is the case for CuP/TP, angular anomalies result in significant contributions to the powder spectrum in the region between 0 = 70” and 0 = 80” (7). Their occurrence becomes less pronounced as the microwave frequency is increased. Such is seen to be the case at KU band, although one would like to have the spectrum at an even higher frequency. The data can be analyzed using the spin Hamiltonian of Chen, Abkowitz, and Sharp (0 cff = Bek,, Hz Sz + g,Wx Sx + H, &.)I + AS, 1, + B(& 1, + S, 1,)
Z3J+ b&, + Z4J1 + W-GI, + Z3A + 4L + Z4J + SzWl, + L + Z3,+ Z4A,
+ SML
+
PI
where Z1 and Z3 are the nuclear spins of the nitrogen ligands on the x axis, and Z, and Z4 are the nuclear spins of the nitrogen ligands on the y axis; g, and g,, A and B, and u and b are the two principal components of the g tensor, the copper nuclear hftensor,
344
BOHANDYiAND
KIM
9.246 GHz
14.2 G = b +I--
91 i
I 16.449 GHz FIG. 2. ESR spectra of polycrystalline samples of CUP in triphenylene crystal specimens.
obtained by crushing single-
and the 14N hf tensor, respectively. The resonance magnetic field in the (0, 4) direction of the principal coordinate system is HR = (l/g/I,) {hv - (M/g) (A’g: co2 8 + BZ g: sin2 O)1/2 - (m,,/g) [(a’ cos2 4 + b2 sin2 +)g: sin2 8 + b2gi cos2 0]1/2 PI - (m,,/g) [(b2 cos2 CJ+ a2 sin2 4) g: sir? 8 + b2 g: cos2 8]1’2}, where ml3 = ml + m3 and m24 = m, + m4 are the sums of the ligand nuclear spin components, M is the Cu nuclear spin component, and g2 = g: cos2 8 + glsin2 8.
ESR OF COPPER PORPHIN
345
Thereis confusionin theliteratureconcerningtheinterpretationof theexperimental valuesof a and b. Assour (2), for example, has taken the shf splitting on the parallel Cu hf lines to be a, the parallel shf tensor component, and b to be the shf splitting in the perpendicular powder spectrum. In the analysis of Guzy, Raynor, and Symons (9), the shf splitting on the parallel Cu hf lines is assigned to b, not a, since the principal direction of the 14N tensor is along the Cu-N bond perpendicular to the axis of molecular symmetry. Furthermore, the splitting on the Cu perpendicular features is 1/2(a + b) and not b. Our analysis follows this latter interpretion. Thus, g,,,A, and b can be obtained easily from the parallel portion of the powder spectrum and, with somewhat gre,ater difficulty, g,, B, and a from the perpendicular region. The spin-Hamiltonian parameters thus obtained are listed in Table 1. TABLE SPIN-HAMILTONIAN
1
PARAMETERS
OF
CUP”
g II
CUP Polycrystalline
CuP/TP
Sin@e-crystal CuP/TP
BI
A
B
-
-
2.172
2.066
2.190 +0.001
2.045 +0.002
196.3 kO.5
2.190 +0.001
2.043 f0.002
195.6 33 +0.5 +1.0 209.5 (“‘Cu)
34 +1.0
a
b
-
-
19.8 +0.2
14.2 +0.1
19.5 +0.2
14.1 +0.1
a :HF and SHF constants are in gauss.
hlost previous authors have reported very little, if any, temperature dependence of the ESR spectra of porphyrins. Figure 3 shows the perpendicular portion of the X band
FIG. 3. The perpendicular 175 and 55 K.
portion
of the X band ESR spectrum of polycrystalline
CuP/TP at
ESR spectrum of polycrystalline CuP/TP at low temperature and one sees that there is a definite temperature effect. At 175 K, the high-field lines develop shoulders. Resolved splittings are seen at 125 K. As the temperature is lowered below 100 K, microwave
346
BOHANDY
AND
KIM
power saturation effects occur and, aside from this, no significant changes take place in the spectra down to 6 K. The 55 K spectrum is typical of all spectra below 100 K. The magnitudes of the splittings and the intensities again suggest their assignment to a Cu isotope splitting. C. Single-Crystal CuPjTP Triphenylene has four molecules per unit cell. If the CUP goes in substitutionally, then the normals to the four porphin planes would make equal angles of 51” with the c axis and the single-crystal ESR spectrum would be expected to consist of 4 x 9 x 4 = 144 lines, in general. Instead of trying to do a complete rotational study, we decided to see if the symmetry directions and principal axis directions were the same as those found for vanadyl porphin in triphenylene (5). Such was found to be the case. With the
FIG. 4. The low-field “parallel” Cu hypertine component of a single-crystal sample of CuP/TP showing shf and isotope splittings. The stick diagram shows the expected spectrum.
magnetic field parallel to the crystallographic c axis, the spectrum coalesced into four Cu hyperfine lines with shf structure on each line. The “parallel” spectrum of one site was obtained with the crystal mounted on the (110) face and with the angle between the magnetic field and c equal to 51”. This orientation yielded g,, , A, and b. Rotating the magnet to 51’ on the opposite side of c gave another “parallel” spectrum. Figure 4 shows the low-field line of one site clearly isolated from the rest of the spectrum in this orientation. The A4 = 3 transition consists of nine shf lines having the approximate intensity ratio of 1: 4 : 10 : 16 : 19 : 16 : 10 : 4 : 1 with well-resolved smaller lines midway between. The ratio of the magnetic moments of the two copper isotopes is 1.07. A(63C~) = 195.6 G, so that @%u) is expected to be 209.3 G, and the M = + transition for 65Cu should be shifted 21 G to lower field, roughly 1.5 times the nitrogen splitting. A stick diagram showing the expected spectrum is shown and it agrees well with the experimental one. Therefore, these smaller lines are assigned to ‘Yu. Values of A(63Cu), @%u), b, and g,, are obtained from this spectrum and are given in Table 1. This
ERR
0~
C~PPERPORPI~IN
341
interpretation is consistent with the results of Hsu (4> on isotopically pure YuPPDME. In that case, there was no problem of superimposed spectra in the parallel portion of the spectrum and his value of 199 G for (k3Cu) is very close to our value of 196 G for the unsubstituted CUP. T:here are several orientations of the crystal for which the magnetic field is perpendicular to at least one of the normals to the porphin planes. However, it was not possible to isolate completely the perpendicular spectrum of one site from overlapping lines due to o’ther sites. Furthermore, when a # b, one can see from Eq. (2) that there is a I$ dependence of the shf splitting and more than nine lines for each Cu hf lines may be observed. When the magnetic field bisects the two crystallographic axes perpendicular to the c axis, it is perpendicular to two of the normals to the CUP planes. The best values of g,, B, and a were obtained in this orientation and were in good agreement with the powder data. The single-crystal data are consistent with CUP molecules in substitutional triphenylene sites. DISCUSSION
The antibonding molecular orbitals required to discuss the ESR data are, assuming 4 symmetry (2,3), W,,)
= 81&-y2
-
+B;(oI
-
02
t+b(alg)= ud,z - &‘(a, + ~72+ W,)
+
~73
(l/d%
GPI
-p3)
= E&z - (1 Ifi)
Gp2
- pa),
= &dxz -
03
-
cd,
+ Q),
[31
= P24, - !&(PI -p2 +p3 - ~4). parameters are related to the molecular orbitals by the equations
W,,)
The spin-Hamiltonian
141
’
t51
161 [71
a = WI)‘WeBNgNF [
+ f5 W3>2p]3
PI
b = (383' 28,BNgN ; ~J~(W - A
[91
[
42CW
In these equations, K = Fermi contact term, A = spin-orbit coupling constant of the free Cu ion, A ,, = E(B,,) - E(B,,), A, = E(B,,) - E(E,), and gN is the magnetic moment of nitrogen. Also, p = 28,~NgCu(dx2-y21r-31dx2-Y2), WI 12
348
BOHAfiDY
AND
KIM
where gcu = magnetic moment of Cu,
1111
s = 2&-&), and
T(n) = n _ 8R( 1 - n2P2 6% - 2,) (Zsz&“” , WI (Zs + ZJY 6 where n = (2/3)‘12 for the assumed sp2 trigonal hybridization of nitrogen, R is the metal-nitrogen internuclear distance, and Z, and Z,, are the effective nuclear charges on the nitrogen atom. /3; can be calculated from [8] and [9] using the experimental values of ccand b. Then, jI1 is obtained from the normalization condition on ti(bI,), (BJ2 + (P;)” - v, Pi s = 1.
P31
Using the valuesg, = 0.403,4*(2.s) = 32.4 x 10e4 cmm3(IO), (r-3)2, = 20.9 x 1O24cmm3 (ZO), and the experimental value of b, one computes j?L2= 0.326 and pi = 0.571. The crystal structure of CUP has not been determined but the distance between the opposite nitrogen atoms in free base porphin is 4.1 A (II) so that the Cu-N distance in CUP can be taken to be approximately the same as CuTPP. The quantities S and T(n) have been evaluated for CuTPP by Assour (2) as 0.092 and 0.33, respectively. Therefore, [13] gives a value of /I: = 0.766 and /I1 = 0.875 for CUP. These values for /I1 and /?; agree with those reported by Hsu (4) for CuPPDME, although his method involved calculating Bi from the average of the spin-Hamiltonian parameters found in measurements on room-temperature solutions. Unless A ,, and A, are known from optical data, the other bonding coefficients cannot be computed. This is the case for porphyrins and phthalocyanines in which the d-d transitions are masked by the much stronger X-rc* transitions of the porphyrin ring. There have been no positive assignments of various weak transitions observed in porphyrins to d-d transitions. However, one can compute values for the so-called reduced energies, /3i/A,, and ~~14,. With il = -830 cm-’ and neglecting the term in T(n), one obtains from [4] and [5] pi/A,, = 3.928 x 1O-5,
E~/A~= 3.407 x 1O-5.
For the case of zero in-plane and out-of-plane n-bonding, i.e., p: = 1 = s2, one computes A ,, = 25,445 cm-l and A, = 29,351 cm-l. The spectral position of the intense Soret band of CUP has been measured in this laboratory to be 25,239 cm-l. This would most certainly obscure the transition corresponding to A ,,. These calculated values of A ,, and A, would place the b,, level above the e, level. The extended Htickel calculations of Zerner and Gouterman (12) predict that e, is above b,, by approximately 1000 cm-‘. The assumption of p$ = 1 is probably a good one since, as pointed out by Guzy et al. (9), the 2p, and 2p, orbitals necessary for in-plane 71bonding are fully used in maintaining the rigid porphyrin crskeleton and are not available for 71bonding. If /If = 1, a small amount of out-of-plane K bonding must be introduced to raise the e, level above b,,. A value of E = 0.91 will place e, 1000 cm-l above b,, if Bi = 1. These coefficients generally agree with those reported for CuTPP (3). However, Guzy et al. (9) report considerable out-of-plane n bonding for Cu phthalocyanine with E’ = 0.60 or 0.51. In summary, the suggested bonding scheme for CUP in triphenylene is considerable in-plane 0 bonding with j31 = 0.875 and /I; = 0.571, zero in-plane a bonding with b2 = 1,
ESR OF COPPER
PORPHIN
349
and a small amount of out-of-plane 71bonding with E = 0.91; A ,, = 25,445 cm-’ and A, g 24,445 cm-‘. Until the optical transitions corresponding to A ,, and A, can be experimentally determined, it is felt that the values for E, as well as &, must be viewed wit:h a large degree of uncertainty. REFERENCES 1. D. J. E. INGRAM,
J. E. BENNETT,
P. GEORGE,
AND J. M. GOLDSTEIN,
J. Amer. Chem. See. 78, 3545
(1956). 2. J. M. ASSOUR, J. Chem. Phys. 43,2477 (1965). 3. P. T. MANOHARAN AND M. T. ROGERS, “Electron Spin Resonance of Metal Complexes” Ed.), p. 143, Plenum, New York, 1969. 4. Y. Hsu, Mol. Phyx 21,1087 (1971). 5. J. BOHANDY, B. F. KIM, AND C. K. JEN, J. Mugn. Resonance 15,420 (1974). 6. A. KLUG, Acta CrystalIogr. 3, 165 (1950). 7. L. D. ROLLMANN AND S. I. CHAN, J. Chem. Phys. 50,3416 (1969). 8. I. CHEN, M. ABKOWITZ, AND J. H. SHARP, J. Chem. Phys. 50,2237 (1969). 9. C. M. GUZY, J. B. RAYNOR, AND M. C. R. SYMONS, J. Chem. Sot. A, 2299 (1969), 10. E. CLEMENTI, C. C. ROOTHAAN, AND M. YOSHIMINE, Phys. Rev. 127, 1618 (1962). If. L. E. WEBB AND E. B. FLEISCHER, J. Chem. Phys. 43,310O (1965). 12. M. ZERNER AND M. GOUTERMAN, Theoret. Chim. Acta 4,44 (1966).
(T. F. Yen,
26,341-349(1977)
An Electron Spin Resonance Study of Copper Porphin* JOSEPH BOHANDY
AND BORIS F. KIM
The Johns Hopkins University, Applied Physics Laboratory, Johns Hopkins Road, Laurel, Maryland 20810 Received August 20,1976; revision received November 1,1976 ESR spectra of magnetically concentrated copper porphin and of copper porphin diluted into single-crystal and polycrystalline triphenylene have been obtained, including low-temperature spectra of the concentrated and polycrystalline samples. Cu hyperline, nitrogen superhyperfine, and Cu isotope splittings are observed in the diluted samples. The results are used to estimate the molecular orbital coefficients for copper porphin. The data are consistent with copper porphin replacing triphenylene molecules substitutionally. INTRODUCTION
Metalloporphyrins are a class of compounds having a tetrapyrrole structure in which the metal is bonded to four nitrogen atoms in an approximate square planar array, usually assumed to have Ddhsymmetry. Replacement of the central metal by two hydrogens yields the so-called free base porphyrin. The parent compound of all porphyrins is called “porphin” (also referred to as the “unsubstituted porphyrin”) and has no substituents on the ring extremities. Various types of porphyrins are obtained by adding different side chains to the ring. Figure la shows a metalloporphin molecule. There have been a large number of electron spin resonance (ESR) and optical studies of these materials, in large part because of their presence in hemoglobin and chlorophyll. Ingram et al. (2) first observed the ESR spectra of copper tetraphenylporphin (CuTPP) and observed metal hyperfine structure even in the magnetically concentrated material. Assour (2) made ESR studies of copper, vanadyl, and cobalt TPP complexes in solution and diluted into the diamagnetic free base H2TPP. CuTPP and AgTPP were thoroughly studied by Manoharan and Rogers (3), using solutions, magnetically diluted single crystals, and polycrystalline powders. ESR measurements at 9.3 and 35.2 GHz on the cup:ric protoporphyrin IX dimethyl ester (CuPPDME) in chloroform and toluene solutions, and in the diamagnetic free base protoporphyrin IX dimethyl ester, were made by Hsu (4). The only previous report of ESR spectra of an unsubstituted porphyrin is that of van.adyl porphin (5). We report here the results of an ESR study of copper porphin (CUP) in its magnetically concentrated form, diluted into a single-crystal host lattice of triphenylene, and in polycrystalline samples of CUP in triphenylene obtained by crushing single crystal specimens. Spectra were taken at X band and KU band and at temperatures from room temperature down to 6 K. * Work supported by Naval Sea Systems Command under Contract NOOO17-72-C-4401 and Public Health Services Grant GM21897, National Institute of General Medical Sciences. Copyright 0 1977 by Academic Press, Inc. 341
All rights of reproduction Prinkd in Great Britain
in any form reserved.
ISSN 0022-2364
BOHANDY
342
U.4
AND
KIM
3
b
a/ FIG. 1. (a) A metalloporphin molecule. Replacement of the metal by two hydrogens gives the free base. (b) A representation of the normals to the four triphenylene planes in a unit cell relative to the crystallographic axes. EXPERIMENTAL
The ESR spectra were taken on a standard spectrometer at both X band and KU band. An Air Products Heli-tran variable-temperature unit, compatible with a Varian rectangular cavity, was used to obtain low-temperature spectra. CUP was purchased from the Mad River Chemical Company (now defunct). Crystals of triphenylene containing CUP as a guest molecule (denoted by CuP/TP) were grown by slow evaporation of mixed solutions. Triphenylene is an orthorhombic crystal having the space group P212,21 and four molecules per unit cell (6). The long dimension of the needle-shaped crystals is the crystallographic c axis, and the normals to the four triphenylene planes make equal angles of 51” with the c axis (see Fig. lb). The projections of these normals onto the a-b plane bisect a and b. The dominant crystal faces are the (1 10~type faces. RESULTS
A. Magnetically Concentrated CUP Room-temperature ESR spectra of an undiluted polycrystalline sample of CUP were obtained at 9.5 and 17.6 GHz. The spectra were characteristic of an axially symmetric system but, even at 17.6 GHz, the parallel and perpendicular portions were only
ESR OF COPPER PORPHIN
343
pa:rtiallyresolved.Cu hype&e structurewasnot observedasit wasin CuTPP (Z-3), in whichtherearefour phenylgroupson the porphinring and thedipolar interactions arereducedby the physicalstackingof the molecules.The absenceof Cu hfs indicates a smaller Cu-Cu distance in CUP. The values obtained for g,, andg, are 2.172 and 2.066, respectively. ‘Variable-temperature X-band ESR spectra of both CUP and CuTPP were recorded down to 17 K. The only significant change with temperature was the expected increase in intensity. B. Polycrystalline CuP/TP
Single-crystal specimens of triphenylene doped with CUP were crushed into a fine powder. ESR spectra obtained at X band and KU band are shown in Fig. 2. At X band, only the two lowest-field parallel components of the Cu hyperfine structure are separated from the much stronger perpendicular portion of the spectrum. At KU band, three parallel components are resolved, with the fourth just masked by the perpendicular spectrum. Each parallel hyperfine line is further split into lines separated by 14.2 G, because of the interaction of the unpaired electron with the four pyrrole 14N nuclei. Four equivalent nitrogen nuclei would give nine superhyperfine (shf) lines having an intensity ratio of 1:4: 10: 16: 19: 16: 10:4: 1. The observed spectra approximately adhere to this. In addition, between these shf lines there are lines which are approximately one-half as intense. These are either due to a magnetically inequivalent site or to the resolution of lines from the Wu isotope. The abundances of the two isotopes are 69.1 and 30.9 % for 63Cu and 6sCu, respectively. Thus, the expected intensity ratio would be approximately two to one. This suggests that these weaker lines are due to 6sCu. Further evidence for this is the fact that the weaker lines appear halfway between the stronger ones at both microwave frequencies with a splitting consistent with the ratio of the magnetic moments. This would not be the case if the weaker lines were due to alg-shift effect. Additional support for the isotope splitting will be given in the next section. The perpendicular portion of the powder spectrum is more complex, especially at X band. This is due not only to the shf structure but also to the fact that when there is a large anisotropy in the nuclear hyperfme interaction, as is the case for CuP/TP, angular anomalies result in significant contributions to the powder spectrum in the region between 0 = 70” and 0 = 80” (7). Their occurrence becomes less pronounced as the microwave frequency is increased. Such is seen to be the case at KU band, although one would like to have the spectrum at an even higher frequency. The data can be analyzed using the spin Hamiltonian of Chen, Abkowitz, and Sharp (0 cff = Bek,, Hz Sz + g,Wx Sx + H, &.)I + AS, 1, + B(& 1, + S, 1,)
Z3J+ b&, + Z4J1 + W-GI, + Z3A + 4L + Z4J + SzWl, + L + Z3,+ Z4A,
+ SML
+
PI
where Z1 and Z3 are the nuclear spins of the nitrogen ligands on the x axis, and Z, and Z4 are the nuclear spins of the nitrogen ligands on the y axis; g, and g,, A and B, and u and b are the two principal components of the g tensor, the copper nuclear hftensor,
344
BOHANDYiAND
KIM
9.246 GHz
14.2 G = b +I--
91 i
I 16.449 GHz FIG. 2. ESR spectra of polycrystalline samples of CUP in triphenylene crystal specimens.
obtained by crushing single-
and the 14N hf tensor, respectively. The resonance magnetic field in the (0, 4) direction of the principal coordinate system is HR = (l/g/I,) {hv - (M/g) (A’g: co2 8 + BZ g: sin2 O)1/2 - (m,,/g) [(a’ cos2 4 + b2 sin2 +)g: sin2 8 + b2gi cos2 0]1/2 PI - (m,,/g) [(b2 cos2 CJ+ a2 sin2 4) g: sir? 8 + b2 g: cos2 8]1’2}, where ml3 = ml + m3 and m24 = m, + m4 are the sums of the ligand nuclear spin components, M is the Cu nuclear spin component, and g2 = g: cos2 8 + glsin2 8.
ESR OF COPPER PORPHIN
345
Thereis confusionin theliteratureconcerningtheinterpretationof theexperimental valuesof a and b. Assour (2), for example, has taken the shf splitting on the parallel Cu hf lines to be a, the parallel shf tensor component, and b to be the shf splitting in the perpendicular powder spectrum. In the analysis of Guzy, Raynor, and Symons (9), the shf splitting on the parallel Cu hf lines is assigned to b, not a, since the principal direction of the 14N tensor is along the Cu-N bond perpendicular to the axis of molecular symmetry. Furthermore, the splitting on the Cu perpendicular features is 1/2(a + b) and not b. Our analysis follows this latter interpretion. Thus, g,,,A, and b can be obtained easily from the parallel portion of the powder spectrum and, with somewhat gre,ater difficulty, g,, B, and a from the perpendicular region. The spin-Hamiltonian parameters thus obtained are listed in Table 1. TABLE SPIN-HAMILTONIAN
1
PARAMETERS
OF
CUP”
g II
CUP Polycrystalline
CuP/TP
Sin@e-crystal CuP/TP
BI
A
B
-
-
2.172
2.066
2.190 +0.001
2.045 +0.002
196.3 kO.5
2.190 +0.001
2.043 f0.002
195.6 33 +0.5 +1.0 209.5 (“‘Cu)
34 +1.0
a
b
-
-
19.8 +0.2
14.2 +0.1
19.5 +0.2
14.1 +0.1
a :HF and SHF constants are in gauss.
hlost previous authors have reported very little, if any, temperature dependence of the ESR spectra of porphyrins. Figure 3 shows the perpendicular portion of the X band
FIG. 3. The perpendicular 175 and 55 K.
portion
of the X band ESR spectrum of polycrystalline
CuP/TP at
ESR spectrum of polycrystalline CuP/TP at low temperature and one sees that there is a definite temperature effect. At 175 K, the high-field lines develop shoulders. Resolved splittings are seen at 125 K. As the temperature is lowered below 100 K, microwave
346
BOHANDY
AND
KIM
power saturation effects occur and, aside from this, no significant changes take place in the spectra down to 6 K. The 55 K spectrum is typical of all spectra below 100 K. The magnitudes of the splittings and the intensities again suggest their assignment to a Cu isotope splitting. C. Single-Crystal CuPjTP Triphenylene has four molecules per unit cell. If the CUP goes in substitutionally, then the normals to the four porphin planes would make equal angles of 51” with the c axis and the single-crystal ESR spectrum would be expected to consist of 4 x 9 x 4 = 144 lines, in general. Instead of trying to do a complete rotational study, we decided to see if the symmetry directions and principal axis directions were the same as those found for vanadyl porphin in triphenylene (5). Such was found to be the case. With the
FIG. 4. The low-field “parallel” Cu hypertine component of a single-crystal sample of CuP/TP showing shf and isotope splittings. The stick diagram shows the expected spectrum.
magnetic field parallel to the crystallographic c axis, the spectrum coalesced into four Cu hyperfine lines with shf structure on each line. The “parallel” spectrum of one site was obtained with the crystal mounted on the (110) face and with the angle between the magnetic field and c equal to 51”. This orientation yielded g,, , A, and b. Rotating the magnet to 51’ on the opposite side of c gave another “parallel” spectrum. Figure 4 shows the low-field line of one site clearly isolated from the rest of the spectrum in this orientation. The A4 = 3 transition consists of nine shf lines having the approximate intensity ratio of 1: 4 : 10 : 16 : 19 : 16 : 10 : 4 : 1 with well-resolved smaller lines midway between. The ratio of the magnetic moments of the two copper isotopes is 1.07. A(63C~) = 195.6 G, so that @%u) is expected to be 209.3 G, and the M = + transition for 65Cu should be shifted 21 G to lower field, roughly 1.5 times the nitrogen splitting. A stick diagram showing the expected spectrum is shown and it agrees well with the experimental one. Therefore, these smaller lines are assigned to ‘Yu. Values of A(63Cu), @%u), b, and g,, are obtained from this spectrum and are given in Table 1. This
ERR
0~
C~PPERPORPI~IN
341
interpretation is consistent with the results of Hsu (4> on isotopically pure YuPPDME. In that case, there was no problem of superimposed spectra in the parallel portion of the spectrum and his value of 199 G for (k3Cu) is very close to our value of 196 G for the unsubstituted CUP. T:here are several orientations of the crystal for which the magnetic field is perpendicular to at least one of the normals to the porphin planes. However, it was not possible to isolate completely the perpendicular spectrum of one site from overlapping lines due to o’ther sites. Furthermore, when a # b, one can see from Eq. (2) that there is a I$ dependence of the shf splitting and more than nine lines for each Cu hf lines may be observed. When the magnetic field bisects the two crystallographic axes perpendicular to the c axis, it is perpendicular to two of the normals to the CUP planes. The best values of g,, B, and a were obtained in this orientation and were in good agreement with the powder data. The single-crystal data are consistent with CUP molecules in substitutional triphenylene sites. DISCUSSION
The antibonding molecular orbitals required to discuss the ESR data are, assuming 4 symmetry (2,3), W,,)
= 81&-y2
-
+B;(oI
-
02
t+b(alg)= ud,z - &‘(a, + ~72+ W,)
+
~73
(l/d%
GPI
-p3)
= E&z - (1 Ifi)
Gp2
- pa),
= &dxz -
03
-
cd,
+ Q),
[31
= P24, - !&(PI -p2 +p3 - ~4). parameters are related to the molecular orbitals by the equations
W,,)
The spin-Hamiltonian
141
’
t51
161 [71
a = WI)‘WeBNgNF [
+ f5 W3>2p]3
PI
b = (383' 28,BNgN ; ~J~(W - A
[91
[
42CW
In these equations, K = Fermi contact term, A = spin-orbit coupling constant of the free Cu ion, A ,, = E(B,,) - E(B,,), A, = E(B,,) - E(E,), and gN is the magnetic moment of nitrogen. Also, p = 28,~NgCu(dx2-y21r-31dx2-Y2), WI 12
348
BOHAfiDY
AND
KIM
where gcu = magnetic moment of Cu,
1111
s = 2&-&), and
T(n) = n _ 8R( 1 - n2P2 6% - 2,) (Zsz&“” , WI (Zs + ZJY 6 where n = (2/3)‘12 for the assumed sp2 trigonal hybridization of nitrogen, R is the metal-nitrogen internuclear distance, and Z, and Z,, are the effective nuclear charges on the nitrogen atom. /3; can be calculated from [8] and [9] using the experimental values of ccand b. Then, jI1 is obtained from the normalization condition on ti(bI,), (BJ2 + (P;)” - v, Pi s = 1.
P31
Using the valuesg, = 0.403,4*(2.s) = 32.4 x 10e4 cmm3(IO), (r-3)2, = 20.9 x 1O24cmm3 (ZO), and the experimental value of b, one computes j?L2= 0.326 and pi = 0.571. The crystal structure of CUP has not been determined but the distance between the opposite nitrogen atoms in free base porphin is 4.1 A (II) so that the Cu-N distance in CUP can be taken to be approximately the same as CuTPP. The quantities S and T(n) have been evaluated for CuTPP by Assour (2) as 0.092 and 0.33, respectively. Therefore, [13] gives a value of /I: = 0.766 and /I1 = 0.875 for CUP. These values for /I1 and /?; agree with those reported by Hsu (4) for CuPPDME, although his method involved calculating Bi from the average of the spin-Hamiltonian parameters found in measurements on room-temperature solutions. Unless A ,, and A, are known from optical data, the other bonding coefficients cannot be computed. This is the case for porphyrins and phthalocyanines in which the d-d transitions are masked by the much stronger X-rc* transitions of the porphyrin ring. There have been no positive assignments of various weak transitions observed in porphyrins to d-d transitions. However, one can compute values for the so-called reduced energies, /3i/A,, and ~~14,. With il = -830 cm-’ and neglecting the term in T(n), one obtains from [4] and [5] pi/A,, = 3.928 x 1O-5,
E~/A~= 3.407 x 1O-5.
For the case of zero in-plane and out-of-plane n-bonding, i.e., p: = 1 = s2, one computes A ,, = 25,445 cm-l and A, = 29,351 cm-l. The spectral position of the intense Soret band of CUP has been measured in this laboratory to be 25,239 cm-l. This would most certainly obscure the transition corresponding to A ,,. These calculated values of A ,, and A, would place the b,, level above the e, level. The extended Htickel calculations of Zerner and Gouterman (12) predict that e, is above b,, by approximately 1000 cm-‘. The assumption of p$ = 1 is probably a good one since, as pointed out by Guzy et al. (9), the 2p, and 2p, orbitals necessary for in-plane 71bonding are fully used in maintaining the rigid porphyrin crskeleton and are not available for 71bonding. If /If = 1, a small amount of out-of-plane K bonding must be introduced to raise the e, level above b,,. A value of E = 0.91 will place e, 1000 cm-l above b,, if Bi = 1. These coefficients generally agree with those reported for CuTPP (3). However, Guzy et al. (9) report considerable out-of-plane n bonding for Cu phthalocyanine with E’ = 0.60 or 0.51. In summary, the suggested bonding scheme for CUP in triphenylene is considerable in-plane 0 bonding with j31 = 0.875 and /I; = 0.571, zero in-plane a bonding with b2 = 1,
ESR OF COPPER
PORPHIN
349
and a small amount of out-of-plane 71bonding with E = 0.91; A ,, = 25,445 cm-’ and A, g 24,445 cm-‘. Until the optical transitions corresponding to A ,, and A, can be experimentally determined, it is felt that the values for E, as well as &, must be viewed wit:h a large degree of uncertainty. REFERENCES 1. D. J. E. INGRAM,
J. E. BENNETT,
P. GEORGE,
AND J. M. GOLDSTEIN,
J. Amer. Chem. See. 78, 3545
(1956). 2. J. M. ASSOUR, J. Chem. Phys. 43,2477 (1965). 3. P. T. MANOHARAN AND M. T. ROGERS, “Electron Spin Resonance of Metal Complexes” Ed.), p. 143, Plenum, New York, 1969. 4. Y. Hsu, Mol. Phyx 21,1087 (1971). 5. J. BOHANDY, B. F. KIM, AND C. K. JEN, J. Mugn. Resonance 15,420 (1974). 6. A. KLUG, Acta CrystalIogr. 3, 165 (1950). 7. L. D. ROLLMANN AND S. I. CHAN, J. Chem. Phys. 50,3416 (1969). 8. I. CHEN, M. ABKOWITZ, AND J. H. SHARP, J. Chem. Phys. 50,2237 (1969). 9. C. M. GUZY, J. B. RAYNOR, AND M. C. R. SYMONS, J. Chem. Sot. A, 2299 (1969), 10. E. CLEMENTI, C. C. ROOTHAAN, AND M. YOSHIMINE, Phys. Rev. 127, 1618 (1962). If. L. E. WEBB AND E. B. FLEISCHER, J. Chem. Phys. 43,310O (1965). 12. M. ZERNER AND M. GOUTERMAN, Theoret. Chim. Acta 4,44 (1966).
(T. F. Yen,