Magnetic circular dichroism studies—63. Sign variation in the magnetic circular dichroism spectra of some perimeter symmetric metallo porphyrins

Vol.4OA, No.3,

Specrrochimica Acta, Printed in Great Britain.

pp.

281-291,

0584-8539184

1984

Q 1984

S3.00

+

0.00

Pergamon PressLtd.

Magnetic circular dichroism studies-63.* Sign variation in the magnetic circular dichroism spectra of some perimeter symmetric metallo porphyrins JOSEPHD. KEEGAN,EDWARD BUNNENBERG and CARLDJERASSI~ Department of Chemistry, Stanford University, Stanford, CA 94305, U.S.A. (Received 8 March 1983) Abstract-It is known from previous work that porphine dication, porphine dianion and the ethanol adduct of magnesium porphine are exceptions to the “golden rule” of MCD spectroscopy that positive A terms are associated with the Q and B (Soret) electronic transitions of perimeter symmetric centrally substituted porphyrins since for them a negative A term is found for the Q,, transition, giving rise to an overall electronic MCD band sign pattern of + - - +. The present work uncovers additional examples in a series of zinc tetrakis(ortho-halophenyl)porphyrins. Michl’s perimeter model is used to provide insight into structural effects which might lead to the condition of AHOMO 5 ALUM0 required to explain the sign inversions on a purely electronic basis. The possibility that the sign variations have a more general vibronic origin is also considered.

INTRODUCTION

The first measurement of the magnetic rotatory dispersion of cytochrome c by SHASHOUA in 1964 inaugurated an era of intense interest in the magnetic optical activity of porphyrins especially as measured by the technique of magnetic circular dichroism (MCD) and much of this work has been summarized in several recent reviews [l-3]. In the course of these studies, it has been repeatedly observed[4+] that perimeter symmetric centrally substituted diamagnetic porphyrins exhibit positive A terms for the two lowest energy singlet electronic transitions, the Q. and B (Soret) transitions and, further, that the MCD B term bands associated with the corresponding electronic transitions (Qi, Qg, B”, and BY,) of the free-base derivatives of perimeter symmetric porphyrins appear in the same sign sequence, i.e. - + - + with increasing energy. Because of the weight of the number of observations and the theory developed [4,7,8] for the general case of the unsubstituted porphyrin ring system, the appearance of the normal, - + - +, sign sequence for the electronic MCD bands of symmetrically substituted free-base and metallo porphyrins has effectively attained the status of a “golden rule”. There are, however, exceptions to these rules. In particular, we have recently demonstrated [9] that the rule for DZhfree-base porphyrins is largely a matter of the circumstances of the peripheral substituents since substituents, e.g. meso-pentafluorophenyl groups, can be chosen which result in a distribution of the energy levels of the highest occupied (HOMOs) and lowest unoccupied (LUMOs) molecular orbitals such that the signs of the Qz and Qi MCD bands, but not those of the Soret bands, are inverted, giving rise thereby to the

*For Part 62, see ref. [9]. tTo whom all correspondence should be addressed.

overall sign pattern sequence of + - - +. There are also several exceptions to the rule for symmetric “metallo” porphyrins in that the MCD associated with the Q. transitions of porphine dication (PH:+)[6], porphine dianion (P’- ) [lo] and the ethanol adduct of magnesium porphine (MgP.EtOH) [ll, 121 is also inverted, giving rise again to the overall sign pattern sequence of + - - +. Since the existence of these exceptions to the rule for porphyrins ordinarily considered to have effective D4,, symmetry seems to constitute an enigma in the theoretical framework for understanding the MCD of porphyrins and since the very existence of these exceptions may point to additional information about the electronic structure of porphyrins, we felt that it was important to undertake a search for additional examples of perimeter symmetric centrally substituted porphyrins which might show sign inversion in the MCD bands associated with the Q. transition. In the present work we summarize the results and successes of our search for a series of zinc tetraarylporphyrins. In a previous publication [ll], we discussed the occurrence of sign inversion in the low temperature spectrum of MgP.EtOH in terms of the reorientation of the electric moments due to the symmetry lowering ethanol ligand(s). In the present discussion and interpretation of our results we will utilize MICHL’Sperimeter model [S, 13 ] since we have found in previous investigations of the MCD of porphyrins[9] and reduced porphyrins [l&16] that this model, especially when used with a protocol based on experimental absorption spectral data, provides a sensitive probe of the relative energies of the four-orbital [17] MOs of porphyrins. We do so, however, with some caution since, as will be discussed, it is quite possible that the MCD effects we observe are of vibronic origin and thus outside the present scope of MICHL’Selectronic perimeter model. Nevertheless, the perimeter model does provide a useful framework against which to view 287

288

JOSEPH D. KEEGAN et al.

some of the factors which may be important consider in a more extended treatment.

to

I \

::J’ jN . :”/ ’

EXPERIMENTAL Tetrakis(pentafluorophenyl)porphyrin (TF,PP) was prepared according to the procedure described by LONGO et al.[18], converted to the zinc complex by the refluxing dimethylformamide method of ADLER et at. [19], and freed from the chlorin impurity by chromatography on a column of basic alumina (activity grade 1) with benzene as the eluant as suggested by SPELLANE rf al. [20]. Tetraphenylporphyrin (TPP), tetrakis(orrho-fluorophenyl)porphyrin (T-o-FPP) and tetrakis(orrho-methyIphenyl)porphyrin (T-o-CH,PP) were prepared according to the general procedure of ADLER et al. [21], converted to the zinc derivatives, and freed from the chlorin impurities directly by treatment[22] with 2,3dichloro-5,6-dicyanobenzoquinone. Zinc porphine (ZnP) was prepared from porphine (Sigma) which had been purified by chromatography. Toluene, the solvent used for most ambient temperature measurements, was of spectroscopic quality (Baker) and was distilled from freshly pressed sodium under a stream of dry nitrogen just prior to use. Solutions containing 25 7, pyridine (v:v) were used to generate the pyridinate complexes. The cyano complexes of ZnTF,PP and ZnP were prepared by dissolving the sample in a benzene solution 1 M in KCN and 0.04 M in dicyclohexano-I 8-crown6(Aldrich). Measurements at 77°K were made in EPA (diethylether-isopentaneethanol, 5:5:2 v:v) and in EPA. Pyr (diethylether12: 10:6:0.5v:v). A volume isopentaneethanol-pyridine, contraction of 30 ;, was taken into account in normalizing the spectra measured at 77°K. MCD measurements were carried out in a 15 kG magnetic field using a JASCO J-40 circular dichrometer. MCD spectra are plotted as molar magnetic ellipticity, [O],W, which has the units of degcm’ dmol-’ G-l. Absorption spectra were measured using a Cary 14M spectrophotometer. Both instruments are interfaced to a NOVA 840 computer for normalization, storage, and curve fitting of the spectral data as described elsewhere [6, 111.

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RESULTS

In addition to their high symmetry, a spectral feature which the symmetric porphyrins (PHj+, P2and MgP.EtOH) previously known to exhibit MCD band sign inversion have in common is the very low intensity of their Q. absorption bands. For this reason, our search for further examples which might provide additional clues as to the cause of the phenomenon turned to tetraarylporphyrins where it is known [17a, l&20,23-29] that variations in the central substituent or in the substituents on the phenyl rings modulate the intensity of the Q. transition. The full set of zinc tetraarylporphyrins investigated in our work [30] is shown in Fig. 1. In the present report, however, we select the data for a few representative compounds and summarize how our results for this limited series related to our findings for the entire set. Spectra for both the visible and Soret regions of ZnTPP are shown in Fig. 2. However, since sign variation does not occur in the Soret MCD bands, only the absorption and MCD spectra (at 293°K in toluene) in the visible region are given for ZnTF,PP (Fig. 3), ZnT-o-FPP (Fig. 4) and ZnT-o-CH,PP (Fig. 5). The spectra of a

80

WAVELENGTH

(nm)

Fig. 2. MCD and absorption spectra of zinc tetraphenylpor) and its pyridinate complex (- - -) in toluene phyrin (-and in toluene + 25 4” pyridine (v : v).

289

Magnetic circular dichroism studies

20

ZnTFSPP

-

t ZnTFSPP.Pyr

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----

-

,. .!

25

225

m

m 50

150

x

x s

O?!l

1 7

WAVELENGTH

(nm)

5

Fig. 3. Visible region MCD and absorption spectra of zinc tetrakis(pentatIuorophenyl)porphyrin (-) and its pyridinate (- - -) and cyano ( . . .)complexes. The solvent for the cyano complex was benzene 1 M in KCN and 0.04 M in dicyclohexano-1%crown-6.

20

t

ZnTpFPP

r!l

r5

WAVELENGTH

(nm)

Fig. 5. Visible region MCD and absorption spectra of zinc and its pyritetrakis(ortho-methylphenyhporphyrin (-) dinate complex (- - -). new example of sign inversion in the unsubstituted porphine series, the cyano complex of zinc porphine (ZnPCN-) is presented in Fig. 6. The low temperature spectra of ZnT-o-FPP in EPA and ZnTFsPP in EPA.Pyr are shown in Figs 8 and 9, respectively.

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25

m 00 -1 50

m x

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4 0

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WAVELENGTH

(nm)

Fig. 4. Visible region MCD and absorption spectra of zinc and its pyritetralcis(ortho-fluorophenyl)porphyrin (-) dinate complex (- - -).

WAVELENGTH

(nm)

Fig. 6. Visible region MCD and absorption spectra of the pyridinate (---) and cyano (. ’ .) complexes of zinc porphine.

JOSEPHD. KEEGANef al.

290

Pertinent spectral data and parameters obtained by curve fitting are summarized in Table 1. In the discussion of our results we will turn first to a consideration of the intimate but not decisive relation between the intensity of the Q, transition and the occurrence of sign variation; next to a brief summary of the perimeter model [S, 131 and to some comments on its possible limitations with regard to the present series; then to a consideration of symmetry lowering structural effects required for the perimeter model to be directly applicable; and finally to the vibronic effects which may also be important.

DISCUSSION

The correlation between the spkttiny of the HOMOs and the occurrence of sign variation The electronic transitions of porphyrins having effective Ddh symmetry can often be usefully interpreted on the basis of GOUTERMAN’Sfour-orbital model [17, 251 as arising from single electron excitations from a pair of nearly degenerate HOMOs, the a,, and alu orbitals, to a pair of LUMOs, the e: and e,’ orbitals (Fig. 7) which, because of the high symmetry usually assumed for the porphyrin ring system, are taken to be degenerate. The resulting nearly degenerate singlet configurations, ’ (a, u es) and ’ (a,, eg), interact to give the Soret and Q states. For the Soret state, the electronic transition dipoles add but the magnetic dipoles subtract. Consequently, electronic excitations into the Soret state carry large dipole strength as is shown for ZnTPP in Fig. 2, but the Soret state has a relatively small magnetic moment (5 0.03 8,) associated with it. Conversely, the electronic transition dipoles subtract but the magnetic

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dipoles add in the Q state. The result is that although the Q state typically has a relatively large magnetic moment (- 3%5p,), there is much less dipole strength associated with transitions into it (Fig. 2). In the event that the Q and Soret electronic states are formed from an equal mixture of the singlet excited configurations, the dipole strength of the Q. transition vanishes; thus, the intensity of the Q. absorption band of symmetric porphyrins serves as a measure of the splitting between the a,, and al, orbitals. However, since the vibronic Q, transition becomes allowed by borrowing from the Soret stateand since the intensity of the Qi transition is relatively constant for a number of porphyrins, the ratio E,~~Q~/E,,,,, Qi, suggested by SPELLANEet al. [20], provides a convenient normalized measure of the splitting in the HOMOs. The spectra shown in Figs 3-6 are limited to the visible region and so we will center our discussion around the values of this ratio. In Table 1, we also list values for D(Q,)/D(B) which is a more accurate normalized measure of AHOMO as well as the actual extinction coefficients of the Q. transition. The structural features which engender weak Q. transition intensities in the present series can be treated by simple notions of perturbation theory which parallel those given in more detail in our treatment of substituent effects and sign variation in the MCD of reduced porphyrins [14, 161. Porphine dianion is again taken (Fig. 7) as the reference porphyrin for which the HOMOs and LUMOs are assumed to be degenerate as judged by the low value (0.06) of the ratio of its visible absorption band intensities. This is a particularly appropriate choice in the context of this work since the MCD associated with the Q,, transition of Pz - is inverted [lo]. Because of the nodal properties of the porphin HOMOs (see Fig. 5, ref. [17a]), symmetric substitution at the center of the ring or at the

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---Zn

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Fig. 7. Energy level diagrams for the porphyrins discussed in the text scaled according to the ratio amax QOI%,, Qr The solid lines are the relative energy levels adduced for the dianions. Changes in the energy of the azU orbital subsequent to variation in the central substituent are indicated by the arrows and the dashed lines. Possible splittings in the es orbitals are not shown.

Magnetic circular dichroism studies

291

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JOSEPHD. KEEGANrt al

meso positions has little effect on the energy of the a,, orbital; consequently, in Fig. 7 energy level shifts proportional to the ratio of the visible absorption band intensities are indicated only for the u2” orbital. The small splittings in the LUMOs that may exist for some of the compounds are also not shown in Fig. 7 but will be commented on in a later section. When zinc is inserted into the core of Pz- to form ZnP, overlap of its R orbital with those of the porphyrin ring lowers the energy of the u2” orbital as is evidenced by the increase in the absorption band ratio from 0.06 to 0.44. However, when the electron donating ligand pyridine is added to the solution to form the pyridinate complex, the energy level of the azu orbital rises and a measure of this change is that the ratio decreases to 0.13. For both ZnP and ZnP.Pyr, the MCD associated with the Q. transition exhibits the normal - + sign pattern (ref. [l l] and Fig. 6). The energy of the a,. orbital is raised even further when CN- is the axial ligand since the ratio decreases to 0.05 and sign inversion is observed for ZnP.CN(Fig. 6) as it was for P2- [lo]. A detailed understanding of the specific modes of interaction between the substituted phenyl groups and the porphyrin ring which result in the modulation of the intensities of the Q0 transitions of tetraarylporphyrins is complicated by a number of factors. In the para substituted series resonance effects appear to dominate inductive effects [27] whereas in the orrho substituted series the situation is more complex and it is generally necessary to consider’ a combination of resonance, inductive, and steric effects as has been discussed in some detail by LONGOei ai. [ 18, 26, 311 in their investigations of tetraphenylporphyrins having halogen substituents in the orrho-phenyl positions. One of their conclusions is that there is also a through-space interaction between the orbitals of the halogen substituents and the porphyrin ring orbitals. This interaction is believed by them to result in a shift in the energy level of the a,, orbital which is in addition to the resonance and inductive effects which operate via the a2” orbital. We will return to a consideration of this through-space interaction and its potential for effecting small splittings in the e, orbitals in a later section. Here we are concerned with the effective splittings in the HOMOs and for reasons of simplicity and clarity (Fig. 7) have followed SPELLANEet al. [20] and adopted the operational viewpoint that the modulation in the HOMO splitting is effected primarily through changes in the energy level of the ulu orbital. Substitution of four +E [32] unsubstituted phenyl groups at the meso positions of P2- raises the energy of the a2” orbital strongly (Fig. 7) as judged by the increase in the ratio from 0.06 for P2 to 1 for TPP’ -. Insertion of zinc lowers the energy of the a,, orbital (the ratio changes to 0.15 for ZnTPP) as it did for P* and again pyridine raises the energy of this orbital since the ratio increases to 0.47 for ZnTPP.Pyr. In each of these derivatives of TPP the splitting in the HOMOs is relatively large and the MCD associated

with the Q. transition exhibits the normal - + sign pattern (Fig. 2; the MCD spectrum of TPP* - is given in ref. [9]). Similar substituent effect energy level shifts occur for TF,PP except that the five fluorine substituents on each of the phenyl rings cause the phenyl groups to be less electron donating as is reflected by the lower ratio (0.19) observed for TFsPP2as compared to TPP2-. As a result of the initially lower level of the a,, orbital, the absorption band ratio increases to 0.23 on conversion to the zinc complex since now, in contrast to the situation for ZnTPP, the u2,, orbital of ZnTF,PP lies below that of the a,, orbital (Fig. 7). The subsequent decrease in the ratio to 0.09 on ligation with pyridine is consistent with this orbital energy level picture. Ligation with CN- should, based on the changes observed for ZnP, raise the level of the a2” orbital even further and in Fig. 7 we place it above the a,, orbital since the same ratio (0.09) was measured for both ZnTF,PP.Pyr and ZnTF,PP.CN-. In the TF,PP series sign inversion is observed only for the zinc pyridinate and cyano complexes (Fig. 3). For the dianion [9] and the bare zinc derivative (Fig. 3) where the ratios are larger, the normal sign pattern is found. When only a single fluorine substituent is present in the ogres-phenyl position as is the case for T-o-FPP’-, the energy of the aZ” orbital should not be as strongly depressed as when five fluorines are present. However, attempts to generate T-o-FPP’.. by the usual method led to a mixture of ionic species. Consequently, an experimental estimate of its relative position is not available. In any case, this supposition (indicated in parentheses in Fig. 7) is supported by the observation that the ratio for ZnT-o-FPP is 0.07 but increases to 0.19 when pyridine is the axial ligand. The positions of the nZu orbitals for ZnT-o-FPP and ZnT-o-FPP.Pyr shown in Fig. 7 are based on a correlation between the ratio E,,, QO/s,,, Qi and the energy of the Q. transition [30]. Sign inversion is again found in the MCD associated with the Q. transition of ZnT-o-FPP for which the ratio is small but not for ZnT-o-FPP’Pyr which has a larger value of the absorption band ratio (Fig. 4). In the preceding discussion of the relation between the intensity of the Q. absorption band and the occurrence of sign variation in the limited series of porphine and fluorine substituted tetraphenylporphyrin derivatives it has developed that sign inversion occurs in the MCD associated with the Q. transition when the ratio E,,,_ QO/smaxQ 1 is less than 0.1. A similar finding has been noted throughout the full set of zinc tetraarylporphyrins investigated [30]. Thus, sign inversion occurs within the Qc transition of ZnT-o,o’CI,PP.Pyr (0.06) ZnTCl,PP.Pyr (0.07) ZnT-o-CIPP (0.07) and ZnT-o-BrPP (0.08) but not for ZnT-o,o’Cl,PP.CN(O.l8), ZnTCI,PP-CN(0.16), ZnT-oFPP.Pyr (0.19), ZnT-o-ClPP.Pyr (0.14) or ZnT-oBrPP.Pyr (0.14). Furthermore, sign inversion is not observed for any other urrho or para substituted zinc tetraarylporphyrin in the series, e.g. ZnT-o-NO,PP

293

Magnetic circular dichroism studies (0.2) and ZnT-p-NO,PP (0.31), where the value of the ratio is greater than 0.1. This direct correlation between the intensity of the Q. transition and the occurrence of MCD band sign inversion breaks down, however, in the cases of ZnT-oCHsPP (Fig. 5) and ZnT-o-OCH3PP [30] since, for both, the ratio is 0.1 or less (0.08 and 0.1, respectively), yet both show the normal - + sign pattern for the MCD associated with the Q. transition rather than the inverted + - pattern expected on the basis of their ratios. Since there is some lack of precision in the use of the ratio E_ Qe/s,,, Qr, the ratio between the dipole strengths of the Q. transition and the Soret transition, D(Q,)/D(B), is also given in Table 1. This ratio for ZnT-o-CHJPP is the same (0.0025) as that for ZnT-oFPP and less than the values found for ZnTF,PP+Pyr and ZnTF,PP.CN-, each of which exhibit Q. MCD band sign inversion. Consequently, while ZnT-o0CH3PP might be at the transition point, the lack of sign inversion in the case of ZnT-o-CHJPP may have important implications with regard to the structural and/or vibronic effects that may be responsible for sign inversion (see below). In constructing the energy level diagrams for the tetraphenylporphyrin and fluorophenylporphyrin derivatives in Fig. 7, we have assumed that the shifts in the energies of the a,, orbitals are governed primarily by changes in the electron donating power of the phenyl groups and less by differences in the steric interaction between the orrho-phenyl substituents and the porphyrin ring. Steric effects are, however, generally important in the ortho substituted series and it is probably this factor which accounts for the difference in the position estimated for the azu orbital of T-o-CHJPP2(the band ratio is 0.74) as compared to TPP’- in Fig. 7. The ordering of the al, and al, orbitals for ZnT-o-CHsPP as compared to ZnT-o-FPP is based on a consideration of this steric factor and also on the correlation [30] between the visible absorption band ratio and the energy of the Q. transition. In summary, steric and electronic considerations can account for the variation in the intensities of the Q. transition of tetraphenylporphyrins. They do not, however, directly explicate the factors which may be responsible for MCD band sign variation. For this, we turn first to a consideration of MICHL’Sperimeter model. The perimeter model and possible structural sources of sign variation in the MCD of perimeter symmetric porphyrins

The perimeter model was developed by MICHL [S, 131 as a general first order method for relating the absolute signs of the MCD associated with the four lowest energy purely electronic transitions of n systems which can be derived from a (4N +2)-electron [nlannulene perimeter to their molecular structures. Algebraic expressions are derived for the A and B terms of n systems with a three-fold or higher axis of symmetry as well as for the B terms of systems of lower

symmetry. These expressions incorporate the magnetic moments p- and /1+ which are associated with sensepreserving and sense-reversing excitations to the Band L (the latter is labeled Q in the usual porphyrin nomenclature) states of cyclic polyenes, respectively, as well as other parameters which relate to the conversion of the parent annulene to the molecule of interest. The practical utility of the perimeter model stems from MICHL’Sproposition that, in most cases, the signs of the A- and B-term MCD bands can be directly related to the relative magnitudes of AHOMO and ALUM0 and that frequently these splittings can be estimated without recourse to specific numerical calculations. A summary of the predictions of the perimeter model is given in Table 2. The initial orbital energy distribution that is of particular interest in most substituent effect studies is exemplified by the soft MCD chromophore case which is characterized by the condition AHOMO = ALUM0 # 0. If this condition is exact, then the Bterm MCD associated with the four lowest energy purely electronic transitions (for porphyrins, Qt, Q& B”,and BY,)is governed only by the small p- moment of the Soret state. Since interstate mixing is involved in this case, the MCD associated with the Q state will be weak, however, to date there are very few clear cut examples of the - - - + sign pattern for porphyrin derivatives [16]. As this condition of equality between AHOMO and ALUM0 breaks down due to the effects of substituents, the larger contributions of the p+ moment of the Q state begin to play a more effective role in the determination of the overall sign pattern. Consequently, as the condition of AHOMO c ALUM0 but with IAHOMO - ALUM01 small is reached, sign inversion occurs initially only for the MCD of the Q transitions and the overall sign pattern becomes + - - +. A particularly pertinent example of this pattern has been found recently in the MCD spectrum of the free-base derivative of TFsPP[9]. When the value of IAHOMO - ALUM01 becomes larger, sign inversion may also occur in the Soret MCD bands and there are a number of examples of the resulting + - + - pattern in the MCD spectra of reduced porphyrins [14-161. On the other hand, when the substituents force the condition of AHOMO > ALUM0 the pc+contribution gives rise to the - + sign pattern for the MCD associated with the Q transitions but gradual departures from the initial condition of equality between AHOMO and ALUM0 Table 2. Perimeter model predictions for porphyrins IF

AHOMO > ALUM0 AHOMO < ALUM0 AHOMO 5 ALUM0 AHOMO=ALUMO#O AHOMO = ALUM0 = 0

Predicted band sign pattern for MCD A and/or B terms B Q

-+ ++- 0 0

-+ +-+ -+ -+

294

JOSEPHD. KEEGAN

in this direction are difficult to discern directly since both the p(- and p+ contributions give rise to the - + pattern for the Soret MCD bands. This overall - + + sign pattern is found for most free-base porphyrins[9]. This is also the sign pattern found for most centrally substituted perimeter symmetric porphyrins such as ZnTPP (Fig. 2) since normally the splitting in the HOMOs ensured by the substituents is far greater than any zero field splitting (ZFS) that may exist in the otherwise degenerate, on symmetry grounds, LUMOs. In contrast to the more common soft MCI) chromophore case just outlined, the initial condition that is most applicable to the symmetric porphyrins which exhibit sign inversion only in the MCD associated with the Q. transition is that of the double-soft MCD chromophore which is characterized by the condition AHOMO = ALUM0 = 0. This is the case since a number of these porphyrins would ordinarily be considered to have a four-fold rotational axis and thus, on symmetry grounds, degenerate LUMOs. Furthermore, we have demonstrated in the preceding section that AHOMO 2 0 is a necessary but not suficient condition for the occurrence of sign inversion. However, if the condition AHOMO = ALUM0 = 0 obtains exactly, then, as indicated in Table 2, no purely electronic MCD should be observed for the Q transition since the dipole strength of transitions into the Q state is zero under this condition. Consequently, any MCD or absorption intensity present would be of vibronic origin. Such vibronic effects may offer a viable explanation for the sign variation we observe and we will return to a further consideration of them in the next section. In the subsequent portion of this section, however, we examine the possibility that the sign variation observed in the zinc tetraarylporphyrin series is of electronic origin. In other words, we consider the possibility that for these porphyrins the substituents force the splitting in the HOMOs to be small and that the occurrence of sign inversion indicates that AHOMO s ALUMO; and thus that the occurrence of sign inversion provides direct evidence of small static or structurally induced excited state ZFSs. On this basis, the difference between the present seemingly double-soft perimeter symmetric centrally substituted porphyrins and porphyrins which belong to the more common soft or almost-soft classes becomes a matter of degree rather than kind. In a number of cases then, the MCD associated with the Q. transitions of the zinc tetraarylporphyrins should be regarded formally not as positive or negative A terms but as closely spaced B terms of opposite sign arising from the ZFS Q$ and Qg components. However, since the B term values for the separate components cannot be uniqueiy evaluated, we tabulate A-term and quasi-A-term values, B-term values (which provide a measure of the asymmetry of the positive and negative lobes of the A term), and values for the magnetic moment (p = 2A/D) in Table 1. Low symmetry axial ligands are well known struc-

et al

tural sources of excited state ZFS for porphyrins. For example, DVOKNIKOVet al. [33] report that the Qe and Q$ transitions of the water, ethanol, and pyridinate complexes (but not the ligand free forms) of MgP and MgTPP are split by 10&l 50 cm- ’ in glassy solvents at 77°K and in the case of the five-coordinate complexes of ZnP in EPA, the splitting has been estimated to be 60 cm-’ [12] or as much as 108 cm-’ [I l]. Splittings of this size can often be observed directly in an ordinary absorption spectrum provided, in contrast to the present series, that the bands are sufficiently strong and sharp as is the case for magnesium octaethylporphyrin in EPA and zinc octaethylporphyrin in EPA,Pyr glasses at 77°K [34] and for ZnTPP in polypropylene foil at 4S’K [35]. On the other hand, for most metallo porphyrins an MCD spectrum, whether obtained at low temperature [34] or at ambient temperature as shown for ZnTPP.Pyr in Fig. 2, provides much less direct evidence for splittings on the order of 100 cm-‘. For example, in Table 1,2A/I) for ZnTPP.Pyr is only marginally lower than the value for ZnTPP and this difference may relate as much to the changes in the skeletal structure of the porphyrin from square-planar for the unligated species [36] to squarepyrimidal[37] for the ligated derivative as to ZFS per se. In addition, the inequality in the [@],,,s of the lobes of the A term which often accompanies ligand induced ZFS in the porphine and ~taalkylporphyrin series [I 1, 34, 381 is not a reliable general guide in the tetraarylporphyrin series since as shown for ZnTPP and ZnTPP.Pyr in Fig. 2, the shape of the A term is asymmetric regardless of whether a ligand is present or not. In the special cases of ZnTF,PP.Pyr (Fig. 3) ZnT-o, o’-Cl,PP.Pyr and ZnTCl,PP.Pyr [30], it is not unreasonable to suppose that a ligand induced ZFS of about 100 cm- ’ is also present and that, since for each of them AHOMO is small in contrast to ZnTPP.Pyr (Fig. 7) the sign inversions and the attendant changes in the value of 2A/D (Table 1 ref. 130)) provides direct evidence of the ligand induced excited state ZFS. However, the presence of a fifth ligand of low symmetry may not be the only source of the static ZFS needed for sign inversion under the tenets of MICHL’S electronic perimeter model since, given that the cylindrically symmetric CN- ligand is normal to the plane of the porphyrin ring, then ZnTF,PP-CNshould have a domed, idealized fits,, geometry (the core of ZnTPP.Pyr has a quasi-l),, rullhng in the crystal [37]) under which the degeneracy in the e, orbitals should not be lifted. Thus, if an overt symmetry lowering structural feature is to be invoked as the source of sign inversion for ZnTF,PPCN (Fig. 3) it appears that it must arise through an unsymmetrical orientation of the perfluorophenyl groups. It should be noted, however, that the splitting which may occur for the 6 orbitals in this manner is not generally sufficient to cause sign inversion since, because of larger values of AHOMO, ZnT-o,o’-Cl,PP.CNand ZnTCL, PP.CNboth exhibit the normal sign pattern for the

Magnetic circular dichroism studies MCD within the Q. transition [30]. In the mono-ortho-halo series (Fig. 1) sign inversion occurs only in the absence of an axial ligand (CN-, pyridine, amines and ethanol) as is illustrated for ZnTo-FPP and ZnT-o-FPP.Pyr in Fig. 4. Porphyrins with a single substituent in the ortho-phenyl position exist in solution as a mixture of atropisomers [39] in the statistical ratio of 1:4: 2: 1 for the “~-UP”; “~-UP, ldown”; cis-“2-up, 2-down”; and trans-“2-up, 2-down” isomers, respectively. Since only the “~-UP” isomer has the possibility of a four-fold rotational axis, an asymmetrical interaction between the orbitals of the phenyl groups and those of the porphyrin ring for the more abundant isomers could give rise directly to the splitting in the LUMOs which, given the small value of AHOMO, is required for sign inversion. This kind of direct interaction via the meso position could well be augmented by a specific unsymmetrical through-space interaction between the fluorine orbitals and the es orbitals as was previously proposed by LONGOet al. [18, 26, 311 for the a,, orbital. Some support for the reality of a specific electronic interaction of this kind comes from the fact that sign inversion is not observed for ZnT-o-CHsPP (Fig. 5) even though its HOMOs are judged (Fig. 7) to be split not more than those of ZnT-o-FPP. A similar mechanism for splitting the LUMOs may also exist for other ortho-phenyl substituted porphyrins, e.g. ZnT-o-N02PP, but the splitting is not directly observed via sign inversion since for them AHOMO is relatively large. In summary, a case can be made that in the zinc tetra(ortho-halophenyl)porphyrins the occurrence of MCD band sign variation provides direct evidence of small structurally induced splittings in the electronic components of the Q state. This viewpoint could be directly extended to the sign inversion which occurs in the MCD spectrum of MgP in EPA at 77°K [ll, 121 since ligand induced splittings of the Qt and Q$ transitions of 100-150 [33] and 195 cm-’ [40] have been reported. It is, however, more difficult to make the further extension to the sign inversions which occur for PHi+ [6], P2- [lo] and ZnP.CN- (Fig. 6) since for these porphyrins overt symmetry lowering structural features which would lead to the loss of a four-fold rotational axis are not evident. In the next section, we comment on the possibility of a more general vibronic origin for the sign inversions observed. Possible vibronic sources of sign variation Sign variation within the vibronic manifold of the Q transition of metallo porphyrins is well documented [5, 6, 11, 34, 41-431 and it has been shown by GOUTERMAN et al. [7,42,43] that the negative vibronic A terms arise by virtue of a symmetry imposed reversal in the phase of the electric moments when the symmetries of the vibrations responsible for the intensity borrowed from the strongly allowed Soret transition are big or bzs whereas A terms of positive sign appear for vibrational modes of alg and azg symmetry.

295

In detailed studies using spectra obtained in glassy solution at 77°K [ll, 411, we noted for a series of metallo porphines and octaethylporphyrins for which a positive A term is associated with the Q. transition that the band at v0 + 620-725 cm- ’ was frequently the first vibrational mode to exhibit a negative A term. In the room temperature MCD spectrum of ZnP.Pyr (Fig. 6), for example, this negative A term is found at v,, + -6OOcn-‘. In contrast, for ZnP.CNwhere a negative A term is observed for the Q. transition, a positive A term is found for the first evident vibrational mode at about v,, + 830 cm-‘. The same pattern of sign alteration is also observed for PH: + [6] and Pz [lo] but not for MgP.EtOH [ll, 121 since for it negative A terms are associated with both the Q,, transition and the first evident vibrational mode. These differences may relate to the specific mechanism which leads to sign inversion for MgP.EtOH on the one hand and for PH:+, Pz- and ZnP.CN- on the other. In the spectra of the zinc tetraarylporphyrins in toluene solution at 293”K, the MCD associated with the poorly resolved vibrational band at about v0 + 780 cm- ‘, indicated for ZnTPP in Fig. 2, is always best fitted as a positive A-term regardless of whether sign inversion (Fig. 3 and 4) occurs for the MCD within the Q. transition or not. In order to further examine the question of the sign pattern of the A terms within the Qi manifold of the zinc tetra(ortho-halophenyl) porphyrins, we have also measured several of them in glassy solution at 77°K. ZnT-o-FPP in EPA at 293°K and at 77°K (Fig. 8) shows the normal sign pattern for the MCD associated with the Q. transition in contrast to the inverted pattern found in the toluene spectrum (Fig. 4) and this change can be attributed to the formation of a ligated complex. At 77°K a positive vibronic A-term becomes clearly resolved at about v0 + 840 cm - 1 and a much less well resolved, but clearly negative, A term can be discerned at about v. +440cm-‘. Thus, the sign pattern sequence previously noted in the metallo porphine and octaethylporphyrin series which have a positive A term for the Q. transition is replicated in the spectra of ZnT-oFPP.EtOH (Fig. 8), but with the difference that the inverted vibronic A term is found at a lower frequency. On the other hand, the results obtained for ZnTF,PP.Pyr in EPA (Fig. 9) were unexpected. At 293°K the MCD associated with the Q. transition of ZnTF,PP.Pyr in EPA exhibits the inverted sign pattern just as was observed in toluene (Fig. 3); however, at 77°K the bands blue-shift and, except for a very weak and questionable positive band at about 585 nm, the normal sign pattern appears for the MCD associated with the Q. transition. At somewhat higher energy the negative lobe of what may be a negative A term at about v. + 270 cm-’ appears, and this is followed by the ubiquitous positive vibronic A term at The blue-shift and the nearly about v,+7OOcm-‘. complete sign inversion observed for ZnTF,PP.Pyr in EPA (Fig. 9) at 77°K could be interpreted as indicating that the zinc-pyridine bond becomes stronger at the

JOSEPHD. KEEGANet al

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lower temperature. This would cause the energy of the azu orbital to increase to the point that AHOMO > ALUM0 which would lead (see above) to the normal sign pattern. Alternatively, it may be that the temperature effects are indicative of the vibronic

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nature of the sources of sign variation in porphyrins for which AHOMO is small. In our vibronic intensity studies [ll, 411, we did not explicitly consider JahnTeller coupling [4447] as a specific mechanism of intensity borrowing which might give rise to negative A terms within the vibronic manifold of the Q transition although it could have been used to explain our observations since under this mechanism the excited square or square-pyrimidal [47] porphyrin skeleton is unstable and is subject to symmetry lowering distortions. Those of b,, symmetry lead to vibronically degenerate rectangular distortions whereas those of b,, symmetry lead to vibronically degenerate diamond-shaped distortions. JahnTeller instabilities have, however, been focused upon by the Leiden Group [ 12,40,48, 491 in their high resolution Zeeman studies of MgP, ZnP and PdP in n-alkane single crystals at h 4°K. In the n-alkane host the low symmetry of the site gives rise to a crystal field splitting (6) that removes the vibronic degeneracy of the Jahn-Teller components. For example, in the case of ZnP in n-decane the ZFS x, y components of the Q,, transition are clearly evident in the absorption spectrum with 6 = 109 cm- ’ along with a vibration having a frequency of 180 cm-’ which can be identified as a Jahn-Teller active mode. There may, of course, be other JahnTeller active modes at higher frequencies such as the v0 + 620-709 cm- ’ band of b,, symmetry which we have identified in earlier studies [ 11,411, or the one at v,, + 440 cm-’ in the spectrum of ZnT-oFPP.EtOH in EPA (Fig. 8). The Leiden Group has also extended their consideration of Jahn-Teller coupling to the MCD of ZnP in EPA at 77°K [12] where now the solvent ligands play the role of the crystal field in lifting the degeneracy of the Q state and they have been able to fit their spectrum with the requisite negative A term at v0 + 180 cm- ‘. However, as noted above, relatively low frequency vibrational modes exhibiting negative A terms are not evident in the MCD spectra of tetraarylporphyrin or porphine derivatives (with the exception of MgP’EtOH) which have negative A terms within their Q. transitions. Thus, whether an explanation for the sign variations we observe can ultimately be derived on the basis of Jahn-Teller coupling remains a moot question which is further tempered by the finding of PERRINet al. [42] that Jahn-Teller distortions vanish in the special case of porphyrins for which AHOMO = 0. Finally, we note that throughout this work we have taken the extinction coefficient or the dipole strength as a measure of the electronic oscillator strength of the Q. transition and thus as a direct index of the size of AHOMO. We have found, however, that regardless of the porphyrin substrate or the ligand that some intensity is inevitably found in the Q. absorption band. This finding led to the proposition that, at least in particular instances, the sign variations we observe could be entirely of vibronic rather than electronic origin. In this connection, MCD may provide an interesting test for the conclusions reached by

Magnetic circular dichroism studies

297

[16] J. D. KEEGAN,A. M. STOLZENBERG, Y. C. Lu, R. E. LINDER, G. BARTH, A. MOSCOWITZ,E. BUNNENBERG and C. DJERASSI,J. Am. t-hem. Sot. 104, 317 (1982). [17a] M. GOUTERMAN,J. molec. Spectrosc. 6, 138 (1961). [17b] M. GOUTERMAN,G. H. WAGNIEREand L.C. SNYDER, CONCLUSION J. moiec. Spectrosc. 11, 108 (1963). [17c] C. WEISS,JR., H. KOBAYASHI and M. GOUTERMAN,J. We believe that the results and analyses presented molec. Spectrosc. 16, 415 (1965). [18] F. R. L~NCO, M. G. FINARELLIand J. B. KIM, J. here again demonstrate the utility of an MCD specHeterocvcl. Chem. 6. 927 (1969). trum in combination with the perimeter model as a [19] A. D. ADLER, F. R. LONG& F. KAMPASand J. KIM, J. probe of porphyrin four-orbital electronic structure. inorg. nucl. Chem. 32, 2443 (1970). In particular, our present results probe the intriguing r201 _ - P. J. SPELLANE,M. GOUTERMAN,A. ANTIPAS,S. KIM and Y. C. LIU, Inorg. Chem. 19, 386 (1980). case wherein the condition AHOMO = ALUM0 = 0 I211 A. D. ADLER. F. R. LONGO, J. D. FINARELLI,J. is closely approximated and demonstrate that the GOLDMACHER,J. ASSOURand L. KORSAKOFT,J. org. previously known cases of sign inversion in the MCD Chem. 32,476 (1967). associated with the Q,, transition are not unique to the r221 _ _ G. H. BARNETT, M. F. HUDSONand K. M. SMITH, peripherally unsubstituted porphine chromophore. Tetrahedron Letf. 2887 (1973). r231 G. D. DOROUGH.J. R. MILLERand F. M. HUENNEKENS. Two general explanations for sign variation are conJ. Am. them. So;. 73, 4315 (1951). sidered, but the exact weighting of vibronic and [24] J. R. MILLERand G. D. DOROUGH,J.Am. them. Sot. 74, symmetry lowering structural effects is not clear. It is 3977 (1952). clear, however, that our uncovering of the wider r25]_ M. GOUTERMAN,The Porphyrins, IIIA, p. 1 (edited by _ generality of the phenomenon suggests that continuing D. DOLPHIN).Academic Press, New York (1978). f261 D. J. OUIMBYand F. R. LONGO. J. Am. them. Sot. 97, efforts to understand the causes of the effects are 5111 (1975). worthwhile. [27] M. MOET-NER and A. D. ADLER, J. Am. them. Sot. 97, 5107 (1975). [28] J. DALTON, L. R. MILGROMand S. M. PEMBERTON,J. them. Sot. Perkin Trans. II, 370 (1980). Acknowledgements-We wish to thank RUTHRECORDSfor her [29] M. NAPPAand J. S. VALENTINE,J. Am. them. Sot. 100, expert assistance with the exuerimental measurements. 5075 (1978). Financial support was provided by grants from the National [30] J. D. KEEGAN,Ph.D. thesis, Stanford University (1981). Science Foundation (CHE-80-25733) and the National Institutes of Health (GM-20276). [31] J. B. KIM, J. J. LEONARDand F. R. LINGO, J. Am. them. Sot. 94, 3986 (1972). [32] M. J. S. DEWAR and R. C. DOUGHERTY,The PM0 Theory oforganic Chemistry. Plenum Press, New York REFERENCES (1975). [33] S. S. DVORNIKOV,K. N. SOLOV’EVand M. P. TSVIRKO, [l] J. C. SUTHERLAND, The Porphyrins, IIIA, p. 225 (edited Biophysics, 24, 811 (1980). by D. 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