Full Text
UDC 535.333
PHYSICS
Academician of the Academy of Sciences of the BSSR A. N. SEVCHENKO, K. N. SOLOV'EV,
A. T. GRADYUSHKO, S. F. SHKIRMAN
QUASILINEAR ELECTRONIC SPECTRA OF METAL DERIVATIVES OF TETRABENZOPORPHIN AND PHTHALOCYANINE
Metalloporphyrins, i.e., complex compounds of porphyrin derivatives with metals, deserve careful study as analogues of compounds of major importance for biology—chlorophyll, heme, cytochromes, and others. A spectroscopic study of chlorophyll-like molecules should provide valuable information on the electronic structure of the chlorophyll molecule, which is essential for understanding the process of photosynthesis.
The introduction of a metal atom into the center of the porphyrin ring and related structures is accompanied by sharp changes in the electronic absorption spectra and by less significant changes in the fluorescence spectra, which at room temperature remain two-banded and approximately mirror-symmetric to the two longest-wavelength absorption bands (see the review (^1)). These phenomena are due, in general terms, to an increase in molecular symmetry from \(D_{2h}\) to \(D_{4h}\) upon introduction of the metal and to the influence of the nature of the metal on the intensity of the \(0\)—\(0\) band.
Until recently, the fine structure of the electron-vibrational spectra of porphyrins had not been studied. The use of E. V. Shpol'skii’s method of quasilinear spectra made it possible to solve this problem for a number of metal-free porphyrin derivatives (^2–^6). However, for the only metalloporphyrin investigated—Mg-phthalocyanine—the quasilinear spectrum obtained was considerably less sharp than that of metal-free phthalocyanine (^2). This circumstance, together with the complex character of the main “multiplet,” hindered interpretation of the spectrum.
For the magnesium and zinc complexes of tetrabenzoporphin (TBP) and tetrabenzo-tetraazaporphin (phthalocyanine, Pc), we have succeeded in finding conditions under which the quasilinear spectra are sufficiently sharp and the main multiplet sufficiently simple to permit a vibrational analysis of the fluorescence and absorption spectra. The work of selecting conditions for obtaining quasilinear spectra of metalloporphyrins is extremely laborious and, unfortunately, largely empirical. As in the case of the previously studied metal-free compounds (^4–^6), we used the method of dissolving additives to introduce the substance into a hydrocarbon matrix (\(n\)-octane). It turned out that the best additional solvent for the magnesium complexes studied is absolute ethanol, and for the zinc complexes, a pyridine–acetone mixture. A small addition of solution was introduced into octane immediately before freezing (the experimental temperature was \(77^\circ\) K). The spectra were recorded on an ISP-51 spectrograph with a UF-84 camera (\(F = 800\) mm) using Infra-720 photographic plates (for TBP) and Infra-760 (for Pc). The compounds studied were prepared and purified by T. F. Kachura: Mg-TBP according to (^7, ^8) with subsequent chromatography on \(\mathrm{Al_2O_3}\) using an acetone–trichloroethylene mixture as developer; Mg-Pc according to (^9); and the zinc complexes from spectrally pure TBP (^5) and commercial phthalocyanine by reaction with zinc acetate in boiling pyridine.
As already mentioned, the fluorescence spectra of metalloporphyrins at room temperature consist, as a rule, of two bands with frequency-
with an interval of about 1500 cm\(^{-1}\). Between them there is sometimes observed a less intense band, which is especially pronounced in metallophthalocyanines. The spectra of the compounds studied (Fig. 1) differ by an increased intensity of the 0—0 band, due to the removal of the quasi-forbiddenness of the long-wavelength electronic transition as a result of benzo- and aza-substitution. The absorption spectra are approximately mirror-symmetric to the fluorescence spectra, in accordance with Levshin’s law of mirror symmetry of absorption and emission spectra, but the frequency interval between the absorption bands in the TBP complexes is smaller than in the fluorescence spectra, while in the PC complexes it is somewhat larger.
Fig. 1. Absorption and fluorescence spectra of Mg-phthalocyanine (solid curves) and Zn-tetrabenzoporphin (dashed curves) at room temperature
In frozen octane solutions, the spectra described split into a series of quasilines; moreover, the first band corresponds to an intense principal multiplet and a group of weak vibrational satellites with small frequencies (which justifies the name 0—0 band), while the second and intermediate bands correspond to groups of lines, indicating the complex character of these bands (see Fig. 2). Under the conditions we selected, only two components of the principal multiplet possess appreciable intensity, one of them being much more intense than the other, so that the quasilinear spectrum appears as consisting of singlets (except for Mg-TBP). The absorption spectra begin with a principal “multiplet,” resonantly coinciding with the principal “multiplet” of fluorescence, and likewise consist practically of singlets, with the exception of Zn-TBP, where the absorption spectrum has a “doublet” structure. The reason for the latter is unclear; it may be connected with the presence of two forms differently solvated by the solvent additives—pyridine and acetone.
On the basis of the data obtained, a vibrational analysis was carried out and the frequencies of the normal vibrations active in the electronic spectra of the compounds studied were determined. The results of the analysis of the fluorescence spectra, i.e., the frequencies of the ground state, are summarized in Table 1, where data are also presented for metal-free TBP \((^5)\) and PC \((^3)\).
Comparison of the spectra of the metal derivatives with the spectra of the free bases makes it possible to draw a number of conclusions. First, the general character of the vibrational structure changes little upon introduction of the metal: the frequencies change only slightly, by no more than 50 cm\(^{-1}\); the same normal vibrations are the most active in the spectrum. However, for some vibrations the activity changes appreciably upon introduction of the metal. Secondly, it is possible to correlate the majority of frequencies in both types of compounds—TBP and PC—with the exception of the most active frequencies in the interval 1500–1600 cm\(^{-1}\). Thirdly, the changes of the corresponding vibrations in the series free base—magnesium—zinc are analogous for TBP and PC, which confirms the kinship of the indicated vibrations and points to a similar influence of the metal atom on the dynamics of the molecule in both cases. Fourthly, most vibrations can be assigned to two types: the first type is characterized by an increase in frequency upon introduction of magnesium, the second by a decrease in frequency upon introduction of magnesium; in both cases, on going from magnesium to zinc the frequencies increase. The first type includes, mainly, low frequencies, and the second includes frequencies above 1000 cm\(^{-1}\). It may be supposed
Fig. 2. Quasi-line fluorescence spectra at 77° K in n-octane:
a — Mg-phthalocyanine, b — Zn-phthalocyanine, c — Mg-tetrabenzoporphin, d — Zn-tetrabenzoporphin.
Table 1*
| TBP \((^5)\), \(\nu,\ \mathrm{cm}^{-1}\) | TBP, int. | Mg-TBP, \(\nu,\ \mathrm{cm}^{-1}\) | Mg-TBP, int. | Zn-TBP, \(\nu,\ \mathrm{cm}^{-1}\) | Zn-TBP, int. | Mg-PC, \(\nu,\ \mathrm{cm}^{-1}\) | Mg-PC, int. | Zn-PC, \(\nu,\ \mathrm{cm}^{-1}\) | Zn-PC, int. | PC \((^8)\), \(\nu,\ \mathrm{cm}^{-1}\) | PC, int. |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 118 | med. | 130 | very weak | 137 | very weak | 138 | very weak | 148 | very weak | 139 | med. |
| 127 | med. | 160 | extr. weak | 153 | very weak | 182 | weak | 173 | extr. weak | 184 | weak |
| 218 | int. | 238 | very weak | 244 | very weak | 244 | very weak | 270 | very weak | 235 | weak |
| 351 | extr. weak | 366 | extr. weak | — | 354 | extr. weak | 344 | extr. weak | — | ||
| 480 | extr. weak | 485 | weak | 486 | weak | 485 | med. | 484 | med. | 488 | weak |
| 510 | extr. weak | 519 | extr. weak | 512 | extr. weak | 561 | extr. weak | — | 546 | weak | |
| 566 | extr. weak | 586 | extr. weak | — | 588 | weak | 590 | med. | 572 | med. | |
| 629 | extr. weak | 643 | very weak | — | 611 | extr. weak | 611 | extr. weak | — | ||
| 698 | int. | 701 | med. | 703 | med. | 683 | int. | 682 | int. | 684 | int. |
| 723 | med. | 738 | med. | 740 | med. | 748 | int. | 749 | int. | 726 | int. |
| 801 | med. | 829 | med. | 827 | weak | 830 | med. | 834 | weak | 801 | int. |
| — | — | — | 950 | weak | 949 | weak | 1009 | weak | |||
| 1018 | extr. weak | 1011 | extr. weak | 1025 | extr. weak | 1011 | extr. weak | 1018 | extr. weak | 1028 | very weak |
| 1045 | extr. weak | 1068 | extr. weak | 1066 | extr. weak | 1113 | very weak | 1107 | very weak | 1085 | weak |
| 1125 | extr. weak | 1124 | extr. weak | 1123 | very weak | 1144 | med. | 1144 | med. | 1143 | int. |
| 1156 | extr. weak | 1154 | extr. weak | 1159 | weak | — | — | — | |||
| 1223 | med. | — | 1205 | extr. weak | 1188 | weak | 1181 | very weak | 1188 | med. | |
| 1250 | med. | 1245 | med. | 1253 | med. | 1222 | weak | 1225 | weak | 1233 | very weak |
| — | — | — | 1308 | very weak | 1308 | very weak | 1318 | weak | |||
| 1331 | med. | 1331 | med. | 1335 | int. | 1347 | med. | 1346 | int. | 1348 | int. |
| 1418 | extr. weak | — | — | 1424 | extr. weak | 1433 | extr. weak | 1404 | weak | ||
| 1451 | extr. weak | 1444 | very weak | 1456 | weak | 1444 | extr. weak | 1455 | extr. weak | 1455 | med. |
| — | — | — | 1513 | int. | 1514 | int. | 1517 | med. | |||
| 1526 | extr. weak | 1530 | extr. weak | 1531 | very weak | 1562 | very weak | — | — | ||
| 1596 | int. | 1556 | int. | 1570 | med. | 1582 | int. | 1540 | med. | 1555 | int. |
| 1624 | int. | 1612 | int. | 1624 | int. | — | — | — | |||
| \(1651=1513+138\) | weak |
* Conventional designations: TBP—tetrabenzoporphin, PC—phthalocyanine; int.—intense, med.—medium intensity, weak—weak, very weak—very weak, extr. weak—extremely weak (estimate of the intensities of the corresponding quasilines).
It may be assumed that the first type is due to stabilization of the macrocycle upon introduction of a metal as a result of the conjugation effect, which increases in connection with the rise in symmetry from \(D_{2h}\) to \(D_{4h}\), while the second type is due to the superposition on this effect of a stronger effect associated with the disappearance of the repulsion of the two central hydrogen atoms upon introduction of the me-
Table 2
| Mg-TBP, \(\nu,\ \mathrm{cm}^{-1}\) | Mg-TBP, int. | Zn-TBP, \(\nu,\ \mathrm{cm}^{-1}\) | Zn-TBP, int. | Mg-PC, \(\nu,\ \mathrm{cm}^{-1}\) | Mg-PC, int. | Zn-PC, \(\nu,\ \mathrm{cm}^{-1}\) | Zn-PC, int. |
|---|---|---|---|---|---|---|---|
| — | 133 | very weak | 138 | extr. weak | 123 | very weak | |
| 160 | extr. weak | — | 177 | med. | 170 | ||
| 220 | very weak | 240 | very weak | 243 | very weak | 268 | very weak |
| 362 | extr. weak | — | — | — | |||
| 483 | very weak | 479 | weak | 481 | med. | 483 | weak |
| — | — | 540 | extr. weak | — | |||
| 584 | extr. weak | — | 584 | weak | 580 | med. | |
| 643 | very weak | — | 607 | weak | — | ||
| 697 | med. | 694 | int. | 679 | int. | 676 | int. |
| 740 | very weak | 723 | weak | 747 | int. | 740 | med. |
| 817 | med. | 820 | med. | 805 | med. | 840 | med. |
| — | — | 939 | med. | 942 | weak | ||
| — | — | 1006 | very weak | 1011 | extr. weak | ||
| 1020 | extr. weak | 1025 | weak | 1088 | extr. weak | — | |
| 1094 | very weak | 1078 | med. | 1132 | med. | 1130 | med. |
| 1116 | very weak | 1090 | med. | — | — | ||
| 1174 | very weak | — | 1180 | weak | — | ||
| 1222 | very weak | 1251 | int. | 1225 | med. | 1246 | weak |
| — | — | 1295 | extr. weak | 1290 | weak | ||
| 1327 | int. | 1335 | int. | 1346 | int. | 1336 | med. |
| — | — | 1407 | weak | — | |||
| 1437 | very weak | 1422 | weak | — | 1443 | very weak | |
| — | — | 1497 | very weak | 1495 | very weak | ||
| 1514 | med. | 1505 | int. | — | — | ||
| — | 1550 | weak | 1565 | med. | — | ||
| 1605 | weak | 1595 | weak | — | — | ||
| \(1628=1497+138\) | int. | \(1610=1495+123\) | med. | ||||
| \(1672=1497+177\) | med. | \(1667=1495+170\) | int. | ||||
| \(1710=1495+225\) | med. | ||||||
| \(1763=1495+268\) | weak |
...of the metal. The increase in frequencies on going from magnesium to zinc indicates the stabilizing influence of the stronger complex-forming agent.
In the first excited state, as the analysis of the absorption spectra shows, the frequencies of the normal vibrations have values close to the frequencies of the ground state (see Table 2), with some frequencies being retained to an accuracy of \(2\text{–}3\ \mathrm{cm}^{-1}\), i.e., within the experimental error, while others change (as a rule, decrease) more noticeably upon electronic excitation. The intensities of the transitions change more sharply.
Our data make it possible to determine the nature of the deviations from mirror symmetry in the spectra of the compounds studied. Although at room temperature the symmetry of the frequencies is violated while the symmetry of the band intensities is satisfactory \((^1)\), the low-temperature quasilinear spectra show that deviations from mirror symmetry are caused not by changes in the vibrational frequencies upon electronic excitation, but by differences in the probabilities of the corresponding vibronic transitions. In the absorption of TBP metal derivatives, transitions with vibrations in the range \(1000\text{–}1400\ \mathrm{cm}^{-1}\) are relatively more intense, whereas in the fluorescence spectrum the most active vibrations are those near \(1600\ \mathrm{cm}^{-1}\), which leads to a decrease in the frequency interval between the bands of the absorption spectrum at room temperature. In the absorption spectra of PC metal derivatives, the combination frequencies corresponding to excitation together with the most active vibration near \(1500\ \mathrm{cm}^{-1}\) of small vibrational quanta \((100\text{–}300\ \mathrm{cm}^{-1})\) are more intense than in the fluorescence spectra; as a result, the frequency interval between the absorption bands at room temperature is larger than between the fluorescence bands.
Since the lower excited state of metalloporphyrin molecules is doubly degenerate \((^1)\), in the low-temperature spectra one might have expected the manifestation of the Jahn–Teller effect, i.e., splitting of the degenerate level as a consequence of the interaction of electronic and vibrational motions. We believe, however, that the doublet structure observed in the quasilinear spectra of the compounds studied is not due to the Jahn–Teller effect, since an analogous doublet structure is observed for the less symmetric free bases, where there is no degeneracy. Therefore, the increased width of the quasilines, as compared with metal-free porphyrins, should be associated with this effect; i.e., in the present case the Jahn–Teller splitting does not exceed several reciprocal centimeters.
In conclusion, the authors express their gratitude to T. F. Kachura for preparing the compounds studied.
Institute of Physics
Academy of Sciences of the BSSR
Received
24 II 1966
CITED LITERATURE
- G. P. Gurinovich, A. N. Sevchenko, K. N. Solov’ev, UFN, 79, 173 (1963).
- F. F. Litvin, R. I. Personov, DAN, 136, 798 (1961).
- R. I. Personov, Optics and Spectroscopy, 15, 61 (1963).
- A. N. Sevchenko, K. N. Solov’ev et al., DAN, 153, 1391 (1963).
- A. N. Sevchenko, K. N. Solov’ev et al., DAN, 161, 1313 (1965).
- S. F. Shkirman, K. N. Solov’ev, Izv. AN SSSR, ser. fiz., 29, 1378 (1965).
- P. A. Barrett, R. P. Linstead et al., J. Chem. Soc., 1940, 1079.
- K. N. Solov’ev, S. F. Shkirman, T. F. Kachura, Izv. AN SSSR, ser. fiz., 27, 767 (1963).
- G. T. Byrne, R. P. Linstead, A. R. Lowe, J. Chem. Soc., 1934, 1017.