J. Phys. Chem. 1988, 92, 307-314
307
Photodynamics of Nickel Porphyrins in Noncoordinating Solvents: Characterization of d-d Excited States Using Transient Raman Spectroscopyt E. W. Findsen,** J. A. Shelnutt,l and M. R. Ondriad Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, and Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received: April 24, 1987)
The photodynamic behavior of several nickel porphyrins in noncoordinating solvents was investigated with absorption and transient Raman spectroscopies. A recent transient absorption study of nickel porphyrins, in noncoordinating solvents, by Kim et al. (Chem. Phys. 1983, 75, 305) proposed that, upon photoexcitation from the ground 4-coordinate state (lAI8),the excited-state decay path traverses a state in which the metal is in an electronic state (designated here as *BlJ) similar to that observed for the metal in coordinating solvent systems (3B1,). This excited state involves the net promotion of one of the two nickel electrons in the d s orbital into the empty d+,,z orbital. While the lifetime of the *B1,state has been estimated to be -250 ps, it can be observed by using 10-ns-widelaser pulses under appropriate experimental conditions. Simple calculations demonstrate the effect of state lifetimes upon the population of the excited-state species within the nanosecond laser pulse. The *B1, state is reversibly formed for several nickel porphyrins in all noncoordinating solvents used. This state is identified in transient Raman spectra by shifts in several lines. Three of these lines are sensitive to changes in the core size of the porphyrin and indicate expansion of the porphyrin core to accommodate the increased in-plane electron density caused by the electron promotion on the metal. Detailed examination of one nickel porphyrin, NiPPDME, indicates a dependence of the *BIKstate's vibrational characteristics upon the solvent environment and implies that the porphyrin a2, orbital is very sensitive to solvent environment. Absorption studies indicate that while both the a,, and orbitals can be influenced by the solvent environment, the a2" orbital is the most affected.
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Introduction Metalloporphyrins serve nature in a variety of ways. In the biochemical world, they are the active sites of proteins whose functions range from oxygen transport and storage (hemoglobin, myoglobin) to electron transfer (cytochrome c, cytochrome oxidase) to energy conversion (chlorophyll). Recently, a nickelcontaining tetrapyrrole was found to play a catalytic role in the biosynthesis of methane by methanogenic bacteria.' Additionally, porphyrins have proven to be efficient sensitizers and catalysts in a number of inorganic chemical and photochemical systems. The mechanisms by which metalloporphyrins perform such diverse functions continue to be the focus of extensive spectroscopic investigation. It is particularly important that the excited-state properties of these complex molecules be understood in order to make full use of the unique photochemical abilities of metalloporphyrins. The study of the photodynamics of metalloporphyrins has, in the past, been the domain of workers using transient absorption techniques. The formation of an excited-state species in a nickel porphyrin in a noncoordinating solvent was first proposed by Cheung et al.,2 but no attempt was made to characterize this species. Later studies using transient absorption spectroscopy characterized the decay mechanism of nickel porphyrins in both coordinating and noncoordinating The most detailed studies in this area are those of Kim et al.3*4Their reports present a complete picture of the decay paths and the lifetime of the states involved. Building on this extensive background, we can, using transient Raman spectroscopy, observe the effects of the excited electronic state of the metal on the vibrational properties of the porphyrin macrocycle. Recently, transient Raman spectroscopy has been used to characterize the vibrational response of several nickel porphyrins to photoinduced d e l i g a t i ~ n . ~These * ~ studies clearly demonstrated the applicability of transient Raman spectroscopy to this problem as well as the wealth of structural information this technique can provide. While previous studies concentrated upon the photodynamics of nickel porphyrins in coordinating solvents, some preliminary results of experiments with nickel protoporphyrin in two noncoordinating solvent systems were also presented. This report expands upon those earlier studies +Thiswork performed at the University of New Mexico and Sandia National Laboratories, Albuquerque, New Mexico. *University of New Mexico. Sandia National Laboratories.
0022-3654/88/2092-0307$01.50/0
providing a detailed analysis of the structural characteristics of the excited states of nickel porphyrins in noncoordinating solvents.
Experimental Section Nickel protoporphyrin IX (NiPP), nickel protoporphyrin IX dimethyl ester (NiPPDME), and nickel octaethylporphyrin (NiOEP) were obtained from Porphyrin Products (Logan UT) and were used without further purification. Organic solvents were of spectral or AR grade (Aldrich, Fisher) and were used without further purification. Cetyltrimethylammonium bromide (CTAB; Sigma Chemical), and sodium dodecyl sulfate (SDS; United States Biochemical Co.) were also used as received. Samples were contained in either a quartz EPR tube (Wilmad) or a standard quartz cuvette. All spectra were obtained at ambient temperature (20-25 "C). Since no sample degradation was observed in aerobic vs anaerobic samples, deoxygenation of the samples was not routinely performed. Sample integrity was monitored with UVvisible spectroscopy. Transient Raman data were obtained by using an instrument described in detail elsewhere.1° Soret excitation was provided by a Molectron N,-pumped dye laser. Visible excitation was provided by a Quanta-Ray DCR-I1 Nd:YAG laser using the second harmonic (532 nm) of the fundamental. A backscattering geometry was utilized, and the scattered radiation was focused into a SPEX Industries 1403 scanning double monochromator. The Raman scattered radiation was detected by a photomultiplier in a cooled housing. The photomultiplier output was fed into an EG&G boxcar (162/165), whose output was then directed into a SPEX Industries Datamate controller, which controlled the (1) Wolfe, R. S. Trends Biochem. Sci. 1985, 10, 396-399. (2) Cheung, L. D.; Yu, N.-T.; Felton, R. H. Chem. Phys. Lett. 1978,55, 527-530. (3) Kim, D.-H.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305-322. (4) Kim, D.-H.; Holten, D. Chem. Phys. Lett. 1983, 98, 584. (5) Holten, D.; Gouterman, M. In Proceedings of the Symposium on Optical Properties and Structures of Tetrapyrrole; Konstanz, SpringerVerlag: New York, 1984. ( 6 ) Kobayashi, T.; Straub, K.; Rentzpeis, P. M. Photochem. Photobiol. 1979, 29, 925. (7) Chirvonyi, V. S.; Dzhagarov, B. M.; Timinskii, Y . V.; Gurinovich, G. P. Chem. Phys. Lett. 1980, 70, 79. (8) Kim, D.-H.; Spiro, T. G. J. Am. Chem. SOC.1986, 108, 2099. (9) (a) Findsen, E. W.; Shelnutt, J. A,; Friedman, J. M.; Ondrias, M. R. Chem. Phys. Lett. 1986, 126, 465-471. (b) Findsen, E. W.; Shelnutt, J. A.; Ondrias, M. R. J . Am. Chem. SOC.1986, 108, 4009-4017. (IO) Findsen. E. W. Doctoral Disseration, University of New Mexico, 1986
0 1988 American Chemical Society
Findsen et al.
308 The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
N
z
1300
1400
Ramon
1500
Shift
1600
1700
(cm-')
Figure 2. Transient resonance Raman spectra of NiOEP in toluene using 406 nm excitation. Spectrum a was obtained at low photon densities with a very defocused cylindrical lens (see Experimental Section). Spectrum b was obtained at high photon densities with a sharply focused spherical lens. A solvent vibrational mode may contribute to the intensity of the region between ISSO and 1660 cm-I. The influence of this interference upon porphyrin line positions is minimal due to the relatively high porphyrin concentration (-50 wM).
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Control of photon densities at the sample was necessary to obtain spectra of the ground- and excited-state species. Laser power densities were controlled in one of three ways or in combination: (1) use of either a spherical or cylindrical lens to image the laser beam onto the sample; (2) the actual focus of the imaging optic; (3) use of neutral density filters. We estimate the power densities at the sample, for Soret excitation, to be in the range of 10 mJ/cm2 (for low-power studies) to 400 mJ/cm2 (for high-power studies).
monochromator and stored the data during acquisition. The data were then transferred to an Apple computer for storage and graphic manipulation." All spectra are the unsmoothed sum of several scans.
Results The reversible formation of an excited-state species at high power is clearly demonstrated in Figures 1-3. A spectrum of the 4-coordinate ground-state species taken at low power is shown in Figure la. This spectrum is typical of those observed for nickel porphyrins in noncoordinating solvents. Nickel protoporphyrin in H20/acetone exhibits values of 1652, 1592, 1522, and 1382 cm-' for vibrations that are assigned to 4 ~ 1 0 4, ~ 2 ,4v3, and 4v4,25 respectively. These values are characteristic of 4-coordinate specie^.*^^ The excited-state species has modes at 1628, 1577, 1494, and 1374 cm-I which are assigned to *q0,*v2, *v3, and *v4, respectively. These frequencies are assigned by analogy to modes observed for NiPP in coordinating solvents and NiPP incorporated in hemoglobin and myoglobin'2 but are distinct from them. Table I presents the results of power-dependence studies on NiPP, NiPPDME, and NiOEP, in a number of noncoordinating solvent systems. No significant frequency dependence on the porphyrin peripheral substituents was observed (compare, for instance, the spectral data of NiPP and NiPPDME in the same solvents, as well as the results of studies of NiOEP). Figure 1 presents spectra of NiPPDME and NiPP in 10% H20/acetone. Figure 2 presents spectra of NiOEP in toluene obtained with low and high photon densities. Spectra of NiOEP in dichloromethane were similar to those obtained in toluene but are not presented. Figure 3 presents spectra of NiPP in CTAB micelles obtained with 532-nm excitation at several power levels. As the power increases, the intensities of several new modes, which
( I I ) Findsen, E. W.; Ondrias, M . R. Photochem. Photobiol. 1986, 43, 229-232.
(12) Shelnutt, J. A,; Alston, K.; Ho, J.-Y.; Yu, N.-T.; Yamamoto, T.; Rifkind, J . M. Biochemistry 1986, 25, 620-626.
I 1300
1400
I
1 1500
I
I 1600
1
1
Roman S h i f t (crn'l ) Figure 1. Raman spectra of the 4-coordinate equilibrium ground-state species (a and d) of NiPP and NiPPDME in 10%H20/acetone. Spectra
b and e are representative of those obtained using high photon density. The new peaks appearing in the spectra are due to the *Big state formed under these conditions. The excitation wavelength used, 406 nm, was chosen to correspond to the absorption maximum of the 4-coordinate equilibrium species, thus maximizing the production of the excited-state species. The experimental parameters, such as the laser beam focus and sample concentration, were identical for all spectra (in a given series). The incident laser power was controlled by the use of neutral density filters.
Nickel Porphyrins in Noncoordinating Solvents
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 309
TABLE I: Raman Modes Observed in Noncoordinating Solvents’ mode solvent
4v4
4v3
4v2
H20/acetone SDS CTAB NaOH/MeOH DMSO
1382 1384 1383 1386 1385
1522 1523 1524 1524 1524
1592 1597 1596 1598 1594
H20/acetone toluene benzene Br-benzene CHI3 CH2C12 acetone DMSO
1381 1383 1382 1382 1382 1381 1382 1382
1521 1522 1521 1521 1522 1522 1520 1522
1593 1598 1592 1596 1594 1601 1593 1593
toluene CH2CI2
1385 1387
1522 1524
1582e 1578e
4vI~ Ni(PP) 1656 1662 1660 1657 1654
*v2
*VI0
1494 1495 1492
1577 1577 1572
1490
1575
1628 1632 1627 1626 1624
1372 1377 1375 1374 1373 1372 1376 1373
1490 1497 1495 1493 1494 1493e 1490 1490
1575 1574 NR 1578 1583 1583 1573 1573
1624 1632 1629 1626 1626 1624 1628 1626
1380 1381
1496 1498e
NR 1580e
1629 1631e
*v4
*Y3
1374 1376 1374 1378 1374
Ni(PPDME) 1656 1658 1657 1658 1657 1657 1655 1657 Ni(0EP) 1657 1658
“Asterisk indicates excited-state species. Accuracies are assumed to be &1 cm-’. The lowercase e beside some values indicates that this position is estimated because of the presence of interfering bands. N R indicates not resolved.
TABLE II: Calculated Core Sizes (A) of Equilibrium and Excited-State Nickel Porphyrins” solvent
ET(30)
v3
v2
VI0
*v2
*v3
*VI0
2.027 1.954 1.954 1.95 1.948 1.963 1.958
1.994 2.007 1.994 NR 1.999
2.017 2.021 2.015 NR 2.021
2.01 1 2.014 2.004 2.017 2.02
pyrrolidine(6) pyrrolidine(4) H20/acetone SDS CTAB NaOH/MeOH DMSO
NA NA NA NA NA NA 45
2.015 1.951 1.951 1.953 1.94 1.948 1.956
Ni(PP) 2.039 1.957 1.954 1.957 1.949 1.952 1.958
piperidine(6) piperidine pyrrolidine(6) pyrrolidone H20/acetone toluene benzene Br-benzene CHCl3 CH2CI2 acetone DMSO
35.5 35.5 NA NA 33.9 34.5 37.5 39.1 41.1 42.2 45
2.017 1.948 2.016 1.951 1.955 1.94 1.956 1.946 1.951 1.933 1.953 1.953
Ni(PPDME) 2.024 1.954 2.03 1.957 1.959 1.954 1.957 1.957 1.954 1.954 1.959 1.954
2.027 1.958 2.029 1.958 1.954 1.954 1.956 1.954 1.956 1.956 1.96 1.956
1.999 2.002 NR 1.992 1.979 1.979 2.004 2.004
2.027 2.01 2.015 2.019 2.017 2.019 2.026 2.025
2.02 2.004 2.01 2.016 2.016 2.02 2.012 2.016
toluene CHzCl2
33.9 41.1
NR NR
Ni(0EP) 1.952 1.95
1.958 1.954
NR NR
2.012 2.008
2.01 2.006
a Core sizes calculated using the observed empirical correlations between given mode frequencies and core s i ~ e . ~ ~ All. ’ ground-state ~ species are 4-coordinate except those denoted as 6-coordinate by 6. In coordinating solvents, an equilibrium between 4- and 6-coordinate species exists and values for both ground-state species are given. In these cases, the 4-coordinate species is a minor component, making observation of its transient species impossible. Abbreviations: NR, not resolved; NA, not available.
we ascribe to the excited-state species, increase. The dependence of the excited state upon solvent environment was explored by examining the behavior of the excited state of NiPPDME in a number of noncoordinating solvents. A systematic dependence upon solvent ETf30)(defined below) was observed for the positions of *v3, *vIo,and *v4. The range of solvents used in the Raman study was limited by the solubility of NiPPDME in these solvents. These dependencies (or lack thereof in the case of *v2) are demonstrated in t h e plots of Figure 4 as well as in Tables I and 11. The absorption spectra of NiPPDME in the same series of noncoordinating solvents were also recorded. Spectral parameters were plotted against several solvent parameters, such as dipole moment, dielectric, and ET(30).’3The ET(30)parameter is based
upon the wavelength of a charge-transfer band of a pyridinium N-phenoxide betaine dye as a function of the solvent and thus reflects the combined effects of the dipolar and inductive properties of the solvent upon the dye’s frontier molecular 0rbita1s.l~ The dye has a nitrogen conjugated with a a electron system. This parameter yielded better fit to the data the than other solvent parameters. Moreover, Figure 5 clearly depicts a trend that indicates a specific electronic response by the porphyrin to the solvent environment. A correlated response to solvent ET(30) was found for the average energy of the B and Q (Soret and a) bands, the difference between the energy of these two bands, and the intensity ratio of the CY-@ bands. The average energy of the B and Q bands corresponds to the parameter Allgof the four-orbital model of G 0 ~ t e r m a n . l ~The energy difference between the B
(13) Reichardt, C. In Solvent Effects in Organic Chemistry; Verlag Chemie: Weinheim, 1978.
(14) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1977; Val. 111, pp 1-165.
Findsen et al.
310 The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
I
1573
I498
I496 1494
1492
1480
Sol vent
I 1290
I
I 1390 Ramon
I
I 1490 Sh 1 f t
I
E, (30)
I
1590
1690
(cni-1 )
Figure 3. Transient Raman spectra of NiPP in CTAB micelles (0.1 M NaOH) using 532 nm excitation. All spectra were obtained under identical experimental conditions with the exception of the incident laser power. The incident laser power was controlled by the use of calibrated neutral density filters. Spectra were obtained in an alternating sequence of higher and lower laser powers in order to ascertain that all spectral changes were reversible. Some chlorin formation was observed (by absorption spectroscopy) after the highest power spectra were obtained, but the amount formed is estimated to be less than 10% and was not resonantly enhanced. Laser energies used to obtain the spectra: (a) -0.5, (b) -1.3, (c) -2.5, and (d) -4.8 mJ/pulse.
and Q bands is related to A,, and Alllg parameters of the same model. The application of the four-orbital model to these data will be discussed below. In the course of these spectroscopic determinations, three solvents were determined to induce NiPPDME spectral behavior that did not follow one or more of the correlations described above. One solvent in particular, benzyl alcohol, failed all correlations. In the case of benzyl alcohol, if the ET(30)value of the parent molecule (toluene) is used in all of the plots presented above, the NiPPDME spectral values fall very close to the best fit line through the data points. This suggests that the aromatic properties of benzyl alcohol dominate the properties of the OH group for interaction with the porphyrin .n system. Methanol and ethanol appear to fit all the correlations except for the intensity of the a-P ratio vs ET(30). This behavior appears to be common to solvents with ET(30)values greater than -47. In this study, the anomalous solvents are all alcohols. They appear to form a separate correlation for the plot of a-(3 ratio vs ET(30)(see Figure
5). Discussion The characterization of the excited-state species serving as both the transition state for ligand binding and the "bottleneck" state for deactivation of electronically excited Ni-porphyrins is vital for a complete understanding of the complex photodynamic behavior of these molecules. Transient absorption spectroscopy has proven to be a very useful technique in the pursuit of this knowledge. However, transient absorption spectroscopy directly probes the electronic changes in the system, yielding only indirect information on the geometric response of the molecule to the
1576
I T
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E, (30)
I
I622
a
'
1
35
.
1
40
Solvent
.
I
43
.
Y)
E, (30)
Figure 4. Relative response of several core size sensitive Raman lines to solvent ET(30) (see text). As can be seen, u3 and u,,, (graphs A and C) appear to correlate with this solvent parameter while u2 (graph B) does not. Error bars denote estimated accuracy of Raman line position (3tl cm-'). The lines are a linear least-squares fit to the data in each panel.
electronic changes. Transient Raman spectroscopy, however, provides very detailed structural information about the properties of the molecular excited state via the vibrational response of the molecule. Nickel Porphyrin Photodynamics. Nickel(I1) porphyrins have eight metal d electrons, six of which fill the d,, d,, and dyzorbitals. The other two electrons change their orbital occupation depending on the ligation state of the system. For 4-coordinate nickel porphyrins, both electrons reside in the dZ2 orbital, which is
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 311
Nickel Porphyrins in Noncoordinating Solvents
7500
-
n
I
m
7400
E
u "
7300
8 0 I
7200
4
PI L
for 4- and 5-coordinate nickel porphyrin systems. The lifetimes of excited states in the decay path of several nickel porphyrin systems have been determined with transient absorption spectro~copy.~-' The studies by Kim et aL3v4present the most complete picture of the decay paths of nickel porphyrins in both coordinating and noncoordinating solvents and indicate that the porphyrin T-T* excited states decay within a few picoseconds of excitation to a metal-centered, metastable excited state. For nickel porphyrins that have the IA,, state as their ground state, the metal-centered excited states were found to have the following lifetimes and a decay path
0
In
7100
7000
30
35 .
40
45
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60
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