J . Phys. Chem. 1990, 94, 6647-6652
6647
Transient Raman Difference Spectroscopy of Nickel( I I ) Uroporphyrin 1r-1r Complexes B. A. Crawford, M. R. Ondrias, Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 871 31
and J. A. Shelnutt* Fuel Science Division 621 I , Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received: January 1 1 , 1990)
A transient Raman difference spectrometer has been constructed and used to investigate the excited-state dynamics of nickel
uroporphyrin I in a--Xcomplexes with either 5-chloro-l ,IO-phenanthroline or 2,2’-dimethyl-4,4’-bipyridinium dichloride (methylviologen). The nature and lifetime of the nickel uroporphyrin excited state observed within the IO-ns laser pulses are like those exhibited by uncomplexed nickel porphyrins. However, we find that structure-sensitive Raman lines of the porphyrin are affected differently by complex formation in the excited state than in the ground state for the methylviologen complex. This result suggests that conformational changes, accompanying the creation of the metal-centered d-d excited state, modulate the geometry of the a- complex. In particular, a conformational change from a ruffled, nonplanar geometry to a planar configuration appears to disrupt the unusual stacking interaction observed for the ground-state methylviologen complex.
Introduction Complex formation between metalloporphyrins and various aromatic molecules, such as caffeine, bipyridyls, phenanthroline derivatives, and viologens, has been known for years.’” These metalloporphyrin T--X complexes involve van der Waals and charge-transfer interactions but do not involve axial ligation at the metal ion bound to the porphyrin core. Investigation of these complexes is important because of the role similar species may play in photoredox reactions in artificial photosynthesis systems,’.s as models of electrophotographic processes9 and electron-transfer processes in proteins.I0 The a- complexes can also be considered as models of the interaction of the iron porphyrin with its protein surroundings in heme-containing proteins like hemoglobin and cytochrome oxidase. Finally, the a-x complexes mimic the stacking interaction of metalloporphyrins intercalated between DNA base pairs. l l , i In many of the above contexts, photogenerated metalloporphyrin excited states are involved. Consequently, questions arise concerning the effects of the porphyrin excited states upon formation of the a-a complex. The problem should be amenable to timeresolved optical spectroscopic techniques. However, because of the relatively small changes in the electronic absorption and vibrational spectra that accompany complexation in the ground state, it is expected that only small spectral changes will be observed in the excited state. For example, the shifts in the absorption bands of the porphyrin induced by complex formation are typically several nanometers, and the vibrational shifts are only a few wavenumbers. Detection of such small spectral differences in the excited state requires that the transient Raman instrument have high scanning accuracy and reproducibility. Alternatively, difference spectroscopic techniques can be used. Simultaneous collection of the spectra of a sample and a reference compound removes the requirement of high scanning accuracy, because the data point at a given frequency is obtained at the same spectrometer grating position for both samples. Simultaneous collection of spectra using Raman difference spectroscopy (RDS) has been used successfully with continuous wave laser sources to investigate ground-state phenomena of this The RDS technique has been a valuable tool in the comparison of nonbonding environmental factors influencing the ground state of metalloporphyrins in p r o t e i n ~ , ~ ~ -in~ ~a-a J*-~~ c ~ m p l e x e sand , ~ ~in~other ~ ~ environment^.^^-^^ I n this study, we have developed a novel transient Raman difference spectrometer. A conventional continuous wave (CW) Raman difference spectrometer4 was modified for use with a 10-11s
-
* To whom correspondence should be addressed. 0022-3654/90/2094-6647$02.50/0
pulsed laser source, which both creates (pumps) the excited state and excites the Raman spectrum of (probes) the transient state. The difference technique allows an accurate comparison of sample (complexed) and reference (uncomplexed) species because the two Raman spectra are obtained simultaneously. If the Raman spectra ( I ) Mauzerall, D. Biochemisfry 1965, 4 , 1801. (2) Blumberg, W. E.; Peisach, J. J . Bioi. Chem. 1965, 240, 870. (3) Shelnutt, J. A. J . Am. Chem. SOC.1981, 103,4275. (4) Shelnutt, J. A. J . Phys. Chem. 1983, 87, 605. (5) Shelnutt, J. A. Inorg. Chem. 1983, 22, 2535. (6) Shelnutt, J. A. J . Phys. Chem. 1984,88, 6121. (7) (a) Shelnutt, J. A. J . Am. Chem. SOC.1983,105, 7179. (b) Shelnutt,
J. A. US. Patent No. 4,568,435, Feb 4, 1986. (c) Kalyanasundaram, K.; Shelnutt, J. A.; Gratzel, M. Inorg. Chem. 1988, 27, 2820. (8) (a) Okura, 1.; Kusunoki, S.; Aono, S. Bull. Chem. SOC.Jpn. 1984,57, 1184. (b) Aono, S.; Okura, I.; Yamada, A. J. Phys. Chem. 1985,89, 1593. (c) Kinumi, Y.; Okura, I. Inorg. Chim. Acta 1988, 153, 77. (9) (a) Proceedings of the 12fh International Conference on Amorphous and Liquid Semiconductors; North-Holland: Amsterdam, 1987; pp 855-990. (b) Pai, D. M. J . Non-Crysr. Solids 1983,59/60, 1255. (c) Kanemitsu, Y.; Imamura, S. Appl. Phys. Lett. 1989, 54, 872. (IO) (a) Nocera, D. G.; Winkler, J. R.; Yocum, K. M.; Bordignon, E.; Gray, H. B. J . Am. Chem. Soc. 1984,106, 5145. (b) Kostic, N. M.; Margalit, R.; Che, C.-M.; Gray, H. B. J . A m . Chem. SOC. 1983, 105, 7765. (c) McLendon, G.; Miller, J. R.; Simolo, K.; Taylor, K.; Mauk, A. G.; English, A. M. In Excited States and Reactive Intermediates; Lever, A. B. P., Ed.; ACS Symposium Series 307; American Chemical Society: Washington, DC, 1986; Chapter 1 I . (d) Aono, S.; Kaji, N.; Okura, I. J . Mol. Caral. 1988, 45,
.
_.
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(1 1) Pasternack, R. F.; Gibbs, E. H.; Villafranca, J. J. Biochemisrry 1983, 22, 2406. (12) Birdsall, W. J.; Anderson, Jr., W. R.; Foster, N. Biochim. Biophys. Acra 1989, 1007, 176. ( 1 3) Kiefer, W. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H.; Hester, R. E., Eds.; Heyden: London, 1977, Vol. 3. p 1. (14) Shelnutt, J. A.; Rousseau, D. L.; Dethmers, J. K.; Margoliash, E. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 3865. (15) Shelnutt, J. A.; Rousseau, D. L.; Dethmers, J. K.; Margoliash, E. Biochemistry 1981, 20, 6485. (16) Rousseau, D. L. J. Raman Spectrosc. 1981, 10, 94. (17) Laane, J.; Kiefer, W. J. Chem. Phys. 1980, 722, 5305. (18) Shelnutt, J. A.; Rousseau, D. L.; Friedman, J. M.; Simon, S. R. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 4409. (19) Shiemke, A. K.;Scott, R. A.;Shelnutt, J. A . J . Am. Chem. Soc. 1988, 110, 1645. (20) Friedman, J. M.; Ondrias, M. R.; Rousseau, D. L. Annu. Reo. Phys. Chem. 1982, 33, 471. (21) Shelnutt, J. A.; Alston, K.; Ho, J.-Y.; Yu, N.-T.; Yamamoto, T.; Rifkind, J. M. Biochemisrry 1986, 25, 620. (22) Shelnutt, J. A.; Straub, K. D.; Rentzepis, P. M.; Gouterman, M.; Davidson, E. R. Biochemistry 1984, 23, 3946. (23) Shelnutt, J. A,; Dobry, M. M. J. Phys. Chem. 1983, 87, 3021. (24) Shelnutt, J. A,; Dobry, M. M.; Satterlee, J. D. J . Phys. Chem. 1984, 88, 4980. (25) Shelnutt, J. A,; Ginley, D. S. J. Polym. Sci., Parr A: Chem. 1986, 24, 1717. (26) Shelnutt, J. A,; Ginley, D. S. J . Phys. Chem. 1985, 89, 5473.
0 1990 American Chemical Society
6648
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
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of transient species in two samples are obtained concurrently, then differences in Raman line frequencies on the order of 0.1 cm-l can be reliably measured. Using the new transient RDS technique, we have measured the complex-induced shifts in the structure-sensitive Raman marker lines of a nickel porphyrin in its excited state. For the 5chlorophenanthroline complexes, the complex-induced shifts for the excited d-d state of nickel(I1) uroporphyrin I (NiUroP8-) are found to be similar to the complex-induced shifts found for the ground state. On the other hand, for the electrostatically bound T-T complex with methylviologen, the Raman marker lines of the excited state, remarkably, are found to shift in the opposite direction compared with the ground-state shifts. The latter result is explained on the basis of a transient conformational change from a nonplanar ruffled macrocycle to the planar one. The conformational transformation is a result of the nickel d-d27-35electronic transition, and this transformation disrupts the novel6 structure of the methylviologen ground-state complex.
Materials and Methods Nickel( 11) uroporphyrin I was obtained from Porphyrin Products (Logan, UT) and either was used without further purification or was purified by chromatography on a Sephadex G-75 gel filtration column. Methylviologen (Aldrich) and 5-chloro1,IO-phenanthroline (Aldrich) were used as obtained. Samples of NiUroP8- (- 5 X 10-5 M) in 0.1 M KOH were added to one side of a partitioned cell, while NiUroP complexed with methylviologen or 5-chlorophenanthroline was added to the chamber on the other side of the cell. Raman spectra were obtained in a backscattering geometry from a partitioned cell rotating at 50 Hz. As the cell rotates, the (27) Chirvonyi, V. S.; Dzhagarov, B. M.; Timinskii, Y. V.; Gurinovich, G. P. Chem. Phys. Lett. 1980, 70, 79. (28) Kim, D.-H.; Kirmaier, C.; Holten, D. Chem. Phys. Lett. 1983, 98, 584. (29) Kim, D.-H.; Holten, D. Chem. Phys. 1983, 75, 305. (30) Holten, D.; Gouterman, M. In Proceedings of the Symposium on Optical Properties and Struct ures of Tetrapyrroles; Springer-Verlag: New York. 1984. .. ( 3 i ) Findsen, E. W.; Shelnutt, J. A,; Friedman, J. M.; Ondrias, M. R. Chem. Phys. Lett. 1986, 126, 465. (32) Findsen, E. W.; Shelnutt, J. A.; Ondrias, M. R. J . Am. Chem. SOC. Complexes in Fossil Fuels: Geochemistry, Characterization, and Processing Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; Chapter 24. (34) Shelnutt, J. A.; Alston, K.; Findsen, E. W.; Ondrias, M. R.; Rifkind, J. M. In Porphyrins: Excited States and dynamics; Gouterman, M., Rentzepis, P. M., Straub. K. D., Eds.; ACS Symposium Series 321; American Chemical Society: Washington, DC, 1986; Chapter 16. (35) Findsen, E. W.; Shelnutt, J. A.; Ondrias, M. R. J. Phys. Chem. 1988, 92. 307.
Crawford et al. sample and reference species were probed alternately. The average pulse energy of the 406- and 402-nm radiation used was maintained at -0.4 mJ/pulse at the sample. The repetition rate of the laser was 25 Hz. The laser radiation was focused onto the rotating cell using spherical optics. No sample decomposition was noted in the absorption spectrum or successive scans of the transient Raman spectra. The instrumental resolution was 9 cm-'. A diagram of the transient Raman difference instrument is provided in Figure 1. The most crucial aspect of the transient RDS technique is the timing involved in probing the spinning sample cell at times coincident with one chamber of the cell or the other. A electronic timing pulse from the optical chopper, used as a rotator for the sample cell, is sent to the divide-by-N circuit and delay generator which triggers the N2 laser. The pulse from the rotator comes at a time when a simultaneous pulse of light from the laser probes the reference chamber of the cell. For the 5 0 - H ~rotation rate, the chopper pulses have a 20-ms period. The pulse output from the divide-by-N circuit, which actually triggers the laser, generates a light pulse that probes the reference chamber every N revolutions of the cell. After a preset number of laser pulses probe the reference species, the computer switches on the digital delay generator. As a result of the delay, the output pulses of the divide-by-N circuit, and consequently the laser pulses, are delayed by half the rotation time (10 ms). The sample chamber of the cell is then probed by the laser for the same preset number of pulses. Scattered light from the samples contained in the cell compartments is collected by a lens and imaged onto the entrance slit of a SPEX 1401 double monochromator for analysis and detection. The scattered light is detected by a thermoelectrically cooled (-20 "C) Hamamatsu P928 photomultiplier tube (PMT). The PMT pulses are amplified and sent to an EG&G PAR gated integrator (Model 4422) and boxcar averager (Model 4420). After the user-selected number of light pulses, the intensities measured during 5-11s intervals centered on the peak of the PMT pulses are averaged and the average value is stored in the computer. The timing of the triggering of the boxcar averager must be accurate with respect to the pulse from the PMT; therefore, a fast photodiode located in the dye laser is used to trigger the gated integrator and boxcar averager. After both sample and reference species are probed by the preset number of light pulses, the monochromator is then stepped by the computer to the next frequency position and the sample and reference species are then probed again. By repeating the procedure, the spectrum of the two samples is generated simultaneously with respect to the grating position so that an accurate comparison of the two spectra can be made. Because a high-power pulsed laser is used to excite the spectra, each spectrum obtained from this process may contain Raman lines from both the excited-state (or transient intermediate) species and the ground-state species.
Results We applied the new transient RDS technique to measure the small (- 1 cm-I) shifts in the Raman lines of the d-d excited state of nickel uroporphyrin that result from T-T complex formation. The transient Raman difference spectra (Figures 2-5) show that the net d - d excited state, previously observed in uncomplexed nickel porphyrin^,*^-^^ is also apparent within the IO-ns excitation pulses for the T-T complexes. Previous CW RDS ~ t u d i e s ~have -~ shown that shifts of 1-5 cm-I for the o ~ i d a t i o n - s t a t e ~and ~*~' c o r e - s i ~ emarker ~ ~ lines of ground-state metallouroporphyrins occur upon complex formation with methylviologen and many other aromatic molecules. Methylviologen Complexes. Figures 2 and 3 show the transient RDS data for the comparison of the transient resonance Raman spectra of the methylviologen complex and uncomplexed NiUrop-. (36) Yamamoto, T.; Palmer, G.;Gill, D.; Salmeen, 1. T.; Rimai, L. J. Biol. Chem. 1973, 248, 5211. (37) Spiro, T. G.;Strekas, T. C. J . A m . Chem. S o t . 1974, 96, 338. (38) (a) Spaulding, L. D.; Chang, C. C.; Yu, N.-T.; Felton, R. H. J . Am. Chem. SOC.1975, 97, 2517. (b) Spiro, T. G.In Iron Porphyrins; Lever, A, B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1982; Part 11, p 89.
Spectroscopy of Nickel(I1) Uroporphyrin Complexes
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6649 - - ~ _ _ _ _ _
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Figure 2. Transient Raman difference spectrum in the region of the oxidation-state marker line u4, comparing (middle) uncomplexed nickel uroporphyrin and (bottom) the nickel uroporphyrin-methylviologen complex. The difference spectrum (top) shows a derivative shape characteristic of a shift to higher frequency for the methylviologen complex. Laser excitation is at 406 nm.
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Figure 3. Transient Raman spectra of nickel uroporphyrin (lower) and its methylviologen complex (upper) in the region of the core-size marker lines obtained simultaneously by using the dual-channel transient Raman difference spectrometer. The complex-induced shifts are determined by fitting Lorentzian lines to the Raman data (see text). Laser excitation is at 406 nm. Notation: vi* represents the excited-state Raman lines and v, represents the ground-state lines.
Figure 2 shows the spectra of the oxidation-state marker line u4. The line shapes are complicated because the Raman lines from the excited-state ( u 4 * ) and the residual ground-state ( u 4 ) species are not well separated. Nevertheless, the line shape is dominated by the u4* contribution and the changes in line shape are consistent with an increase in frequency of u4* of the complex by 1.4 f 0.4 cm-I relative to uncomplexed N i U r o p . The shift Av is calculated from the expression Au = 0.38I'Id/IO, where r is the line width, Io is the intensity at the peak of the Raman line of the complex, and 1, is the peak-to-valley intensity in the difference spectrum.l3I8 The value of Au4* agrees with the shift inferred from the difference
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Figure 4. Transient Raman difference spectrum in the region of the oxidation-state marker line comparing (bottom) uncomplexed nickel uroporphyrin and (middle) the nickel uroporphyrin-5-chlorophenanthroline complex. The difference spectrum (top) shows a derivative shape characteristic of a shift to a higher frequency for the 5chlorophenanthroline complex. Laser excitation is at 402 nm.
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by about 2 cm-I for the complex but decreases by about 2 cm-' for the ground-state complex. The complex-induced shifts found for the ground-state lines in the present transient RDS work using a Lorentzian curve-fitting procedure described earlier39are in good agreement with the shifts of -5 f 1 and -1.4 f 0.5 cm-I for u3 and u I 0 , respectively, observed in the C W RDS studies.6 The complex-induced shifts determined using transient RDS are listed in Table I . To summarize the results for the methylviologen complex, the shifts in the Raman marker lines for the excited-state complex are all to higher frequency relative to the uncomplexed porphyrin, while for the ground-state species, the complexation induces shifts to lower frequency for these modes.6 The magnitudes of the excited-state and ground-state shifts are comparable. Chlorophenanthroline Complexes. The transient RDS results for the methylviologen excited-state complexes are in sharp contrast with those obtained for the 5-chlorophenanthroline complex (Figures 4 and 5). Figure 4 shows the Raman data for the oxidation-state marker line. The peak position of u4* increases by 0.7 cm-' for the complex; the exact shift is somewhat uncertain because of the same complications noted for u4 for the methylviologen complex. Figure 5 gives the transient RDS data for the region of the core-size marker lines u3, u2, and uI0. The shift in u3 of 1.4 f 0.8 cm-I is in agreement with C W measurements of the ground-state complex-induced shifts (2.5 cm-I determined for the 2,9-dimethyl-I ,10-phenanthroline-NiUroP8- complex or the 2.1-cm-I shift measured for the 5-chlorophenanthroline-copper( 11) uroporphyrin ~ o m p l e x ) For . ~ the excited state, u3* shifts to higher frequency by 2.5 f 0.9 cm-l. Similarly, complex-induced shifts for vi, and ul0* are both to higher frequency and of comparable magnitude. Thus, the transient RDS results for the phenanthroline complex are qualitatively different from those obtained for the methylviologen complex. Specifically, the complex-induced shifts in the excited state of the phenanthroline complex are similar to those observed in the ground state, while the excited-state shifts of the methylviologen complex are in the opposite direction from shifts observed in the ground state. Discussion Complex-Induced Shifts in Raman Lines of the Ground State of NiUrop-. The ground-state complexes of methylviologen and phenanthroline derivatives with metallouroporphyrins have been investigated extensively with both UV-visible absorption and C W RDS However, for the nickel uroporphyrin derivatives, additional factors complicate r-r complexation. Nonplanar conformations of the macrocycle are favored in the ground state of uncomplexed nickel porphyrins because of the short optimum Ni-N bond distancem3 of the low-spin (d,2*) nickel ion. Ruffling allows the porphyrin core to contract for more favorable Ni-N bonding. On the other hand, the conjugated bonding of (39) Stump, R. F : Deanin, G . G.; Oliver, J . M.; Shelnutt. J . A . Biophys. J 1987. 51. 605
Crawford et al. the macrocycle favors a planar macrocycle. These competing energetic factors lead to an equilibrium between planar and ruffled low-energy conformers of the uncomplexed nickel porphyrin. Formation of the methylviologen complex results in ruffling of the planar conformer, and thus, the outcome is (at least) two ruffled species for the complex.42 Formation of the phenanthroline complex, on the other hand, is expected to maintain (or perhaps increase) the amount o f t h e planar species, in analogy with the effect of x - x dimerization of nickel u r ~ p o r p h y r i n . ~ ~ , ~ ~ Furthermore, the determination of the complex-induced shifts in the core-size lines is more complicated for NiUroP8- than for the other metallouroporphyrin complexes because of the conformational heterogeneity brought about by macrocycle rufA bimodal distribution of ruffled and planar conformations is observed for nickel porphyrins in solution. As a result of ruffling, the line shapes of core-size marker lines are complicated, and further, the detailed line shapes are influenced by complex formation.42 The effect of conformational heterogeneity is most pronounced for u l o , because it shows the largest decrease in frequency upon ruffling. In most previous C W RDS studies, the ground-state shifts were determined from the difference spectra, 13-1 5 and that analysis ignores the presence of multiple conformations of the macrocycle. Likewise, in the present work, no effort was made to resolve the lines into ruffled and planar components in either the ground or excited state. The more detailed analysis of the line shapes42 was not warranted given resolution and signal-to-noise ratio of the transient RDS data. Also, the difference m e t h ~ d ' ~gives - ' ~ slightly different shifts than the Lorentzian curve-fitting technique used in the present work. Considering the difference in methods, the values are in good agreement. The phenanthroline complexes are generally representative of most x - r complexes. For these complexes, the core-size and oxidation-state marker lines all increase in frequency by several wavenumbers upon complex Many other x-x complexing agents such as bipyridine and quinoline cause shifts to higher f r e q u e n ~ y . ~Similarly, -~ the stacking interaction in x-x dimers of metallouroporphyrins result in small positive frequency shifts in these lines.24 The Raman studies show that the nature of the metal incorporated into the uroporphyrin core has little effect on the frequency shifts that result from complex formation.3d Phenanthroline even forms a T-T complex with metal-free uroporphyrin. Small positive complex-induced shifts are observed for the Cu(II), Ni(II), Fe(III), Co( I I ) , and other metal uroporphyrin derivatives. For example, the shifts for u4, vj, and ulo for the 5-chlorophenanthroline-Fe(111)UroP8- complex are 0.7, 2.1, and 1.9 cm-I, re~pectively.~ In general. the complex-induced shifts in the core-size markers (1-5 cm-I) are larger than the shift for the oxidation-state marker u4 (
