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Picosecond optical studies of nickel(II) protoporphyrin IX dimethyl ester
Volume f 64, number 1
CHEMICAL PHYSICS LETTERS
PICOSECOND OPTICAL STUDIES OF NICKEL(I1) PROTOPORPHYRIN
1 December 1989
IX DIMETHYL ESTER *
Scott H. COURTNEY, Tom M. JEDJU, Joel M. FRIEDMAN AT&T Bell Laboratories, Murray Hill, NJ 07974, USA
Rhett G. ALDEN and Mark R. ONDRIAS ’ Department of Chemistry, UniversityofNew Mexico, Albuquerque, NM 87131, USA Received 25 July 1989; in final form 18 September 1989
Time-resolved Raman and adsorption spectroscopies were used to characterize the vibrational and electronic structure of the *B,, excited state of nickel@) protoporphyrin IX dimethyl ester. Evidence for structural relaxation processes within the initial B,, excited state was observed on picosecond time scales. We conclude that interconversions between structural conformers are important in these processes. Such conformational interconversions in metalloporphyrins may have a major role in the complex mechanisms of many biological systems.
1. Introduction Metalloporphyrins are important in a variety of biological functions, such as oxygen transport, electron transfer, and energy conversion. In addition, metalloporphyrins have been used in photochemical cycles as catalytic charge transfer intermediates. To understand many of these functions, the excited state dynamics of these macrocycles must be elucidated. Consequently, these systems have been studied by a number of optical spectroscopic techniques (for a good overview, see ref. [ 1 ] ), including transient absorption [ 2-5 ] and resonance Raman spectroscopies [ 6-8 1. Systematic transient absorption studies of nickel porphyrins in both coordinating and noncoordinating solvents have led to significant progress [ 4,5 1. However, time-resolved resonance Raman spectroscopy (TRR) can be utilized to analyze the photodynamics of these systems in more detail by examining the effects ofthe electronic state on the vibrational structure of the porphyrin moiety. Find-
* This work was performed at AT&T Bell Laboratories and was supported by the NSF (DMB 8604435) and the NIH (GM 33330). ’ To whom correspondence should be addressed.
sen et al. [ 7 ] characterized several nickel porphyrins by transient Raman spectroscopy, and an initial characterization of Ni (OEP ) by picosecond time-resolved CARS was recently reported [ 61. Photoexcitation of nickel porphyrins in noncoordinating solvents leads to the formation of a relatively long-lived excited state (which we will call *B1,) that decays to the ‘Al, ground state in z 300 ps. Based on transient absorption results, Kim et al. [4,5] identified this long-lived state as the 3Blg,concluding that excitation into the Q-band region results in fast intersystem crossing primarily from the ‘B,, to the 3B,,. They estimated the lifetime of the ‘B,, to be < 15 ps. Alternatively, excitation into the B-band (Soret) region of the absorption spectrum may activate additional relaxation pathways from the singlet manifold to the 3B,, state. These additional decay pathways presumably arise from the high density of triplet states above the Q band. Theoretical calculations by Ake and Gouterman [9], using the odd electron perturbation method, placed the relative energy level spacing for the ‘B1, and 3B,, excited states to be = 6000 cm-‘, while the energetic spacing between the 3B,, and ‘Alp ground state is ~3000 cm-‘. The 3B,, state can be viewed
0009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland )
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Volume 164, number I
as a combination of the porphyrin ‘Ai, ground state and the nickel ‘B,, ‘( dZ~,dX~_-yZ) configurations. Thus, photoexcitation causes a net d-d transition on the metal. Nanosecond
transient
1 December 1989
CHEMICAL PHYSICS LETTERS
resonance
Raman
spectros-
tion spectra. This system is tunable and regularly produces 300 fs, 1 mJ pulses. The detector was a Princeton Instruments double diode array mounted on a single monochromator with a dual fiber optic bundle coupled to the input slit.
copy has been used to characterize what was taken to be the *B,, excited state [ 71. Here we present a study of nickel( II) protoporphyrin
IX dimethyl
es-
ter (NiPPDME) in a noncoordinating solvent using picosecond time-resolved Raman and absorption spectroscopies. Our results confirm that the transient Raman spectra correspond to the *Br, and clarify the relaxation kinetics of the initial excited state.
2. Experimental NiPPDME was purchased from Porphyrin Products and further purified by column chromatography on alumina. HPLC grade chloroform was obtained from Aldrich Chemicals. The sample integrity and purity were monitored by absorption spectra on a Hewlett-Packard mode1 8452 spectrophotometer. A backscattering collection geometry ( =f 1.5) was used to obtain the Raman spectra using an Instruments SA UlOOOdouble monochromator with both 600 grooves/mm and 2400 grooves/mm gratings. The detector was an intensified optical multichannel analyzer (Princeton Instruments IRY700). Approximately 50% of the second harmonic (532 nm) from a Quantel active-passively mode-locked Nd: YAG laser ( z 250 mJ/pulse, 30 ps, 10 Hz) was focused in a high pressure hydrogen cell (300 psi). The first antiStokes shift (436 nm) was separated with a prism and focused at the sample with cylindrical optics. The spatial overlap for the two-color “pump-probe” experiments was achieved by recombining some of the remaining 532 nm pump pulse and the 436 nm “probe” with a dichroic mirror. The pump beam was temporally delayed using a Klinger GV88 optical delay stage, and the temporal overlap was verified using excited state absorption of rhodamine 6G. An amplified femtosecond dye laser system consisting of a mode-locked continuously pumped YAG laser with a fiber optic pulse compressor (Spectra Physics), a synchronously pumped dye laser (Coherent 702), and a dye laser amplifier operating at 10 Hz was used to obtain the time-resolved absorp40
3. Results and discussion Fig. 1 shows the ground state recovery kinetics of NiPPDME in chloroform. Previous absorption studies [2-41 have observed this transient and placed the lifetime at M280 ps. We obtained a slightly longer value (the fit to a single exponential in fig. 1 gave 330 ps). Kobayashi et al. [ 31, assigned this “bottleneck” state as the ‘Big, while others suggested that the 3B,, is populated via the ‘B,, and the lowest porphyrin ( ‘IC,x*) triplet [ 2,4]. In noncoordinating solvents, Kim et al. [ 5 ] observed spectral evolution on the red side of a strong transient absorption band (at -575 nm, which they assigned as the Q(O,O) band of 3B,,). The red shoulder was taken as evidence of the ‘Big, and they estimated that the intersystem crossing ‘Blg-+‘Big requires 10-l 5 ps. Figs. 2 and 3 summarize the results obtained from
: 0.02 ; 73 0 .r( f : -0.02 a:
o.31
-0.5
1
0
1
0.5
rime
1.5
Ins)
Fig. 1. Kinetics of the ground state recovery of the Q( 0,O) band of NiPPDME in chloroform at 56 1nm. The solid line is a tit to a single exponential with a time constant of 330 ps. The decay was generated using the 10 Hz Nd: YAG laser ( Y 30 ps), and the excitation wavelength was 532 nm.
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1500 Shift
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Fig. 2. Resonance Raman spectra of NiPPDME in chloroform. The center spectrum is that of the ground state obtained using only the 436 nm pulse, and the upper spectrum was obtained with a time delay between the pump (532 nm) and the probe (436 nm ) pulseof 20 ps. The lower spectrum is the excited state spectrum obtained by subtracting the residual ground state fraction (0.6 in this case) from the upper spectrum.
time-resolved resonance Raman studies of NiPPDME in the 30 ps to 3 ns time scale. We had initially hoped to see evidence in picosecond time-resolved Raman spectra for higher lying excited state species. However, our time-resolved absorption results (see below) make it quite unlikely that resonance Raman spectra obtained with 30 ps pulses contain significant scattering from the putative initial state(s). Fig. 2 clearly shows the formation of a transient d-d excited state when the sample is irradiated with an intense (= 10” W/cm’, 532 nm) pump pulse spatially and temporally overlapped with the relatively weak probe pulse ( z 1O8W/cm’, 436 nm ). At higher laser fluences, bands appear at 1370, 1495, 1576 and 1632 cm-’ corresponding to v;, Y;, u; and vTo,respectively, of the excited state species. The intensity of the probe beam was attenuated to a level at which these peaks were not observed, and the positions of v4, y3, v2, and vi0 in the center spectrum of fig. 2 are consistent with those of the
1300
1400
Raman
1500
1600
Shift
(cm-')
1700
Fig. 3. Time-resolved resonance Raman difference spectra of NiPPDME in chloroform. The difference spectra were obtained as in fig, 2, and the four spectra represent different time delays between the pump and probe pulses (- 10 ps, 20 ps, 220 ps, and 1ns from bottom to top). The fraction ofground state subtracted in each spectrum was 0.59,0.60, 0.76, and 0.82, respectively.
ground state species. As the time delay between the pump and probe pulses increases, the intensity of these new lines decreases. The difference spectra in fig. 3 show clearly that, within 1 ns after the excitation pulse, the Raman spectrum is identical to that of the ground state. This relaxation time ( 1 ns ~3 lifetimes) confirms the association of these new lines to the “bottleneck” excited state [7]). It therefore seems quite certain that the transient Raman peaks are associated with a *B,, excited state, yet the multiplicity of this state is uncertain. The enhanced time resolution of these spectra allow for a cleaner resolution of the structural properties of the *B,, state. Two of the vibrational modes shown to decrease with increasing core size of the macrocycle, v, and vlo, shift from 1522 and 1658 cm-’ to 1495 and 1632 cm-’ (see table l), respectively [ lo]. In addition, v.,, a totally symmetric C,N stretching mode that is highly sensitive to the x electron density of the porphyrin, broadens and shifts from its ‘Alg ground state value of 1381 cm-’ to a 41
Volume 164, number 1 Table 1 Raman modes of NiPPDME a)
*e r? V2 VI0
Ground state (‘A,,)
Excited state
1381 1522 1595 1658
1370 1495 ~1576~’ 1632
(*RI,)
*) Raman frequencies are given in cm-‘. b, The excited state value of uz may be distorted by underlying ground state modes, such as Y , ,
peak position at approximately 1370 cm-‘. It is tempting to assign another peak at z 1576 cm-’ (see the lower spectrum in fig. 3) to u; (at 1595 cm-’ in the ground state). In a previous transient resonance Raman study, no correlation to the solvent E,(30) value#’ was found for this peak; whereas, of and ~7, showed a clear correlation [ 71. Hence, it seems likely that this peak is highly distorted by another ground state mode, possibly u11 (the shoulder in the ground state spectra). v; is slightly distorted by a chloroform solvent line at z 1500 cm-‘, but is not significant based on the strong correlation of &(30). The time-resolved Raman difference spectra presented in this study show no evidence of spectral evolution of the vibrational modes on a 30 ps and longer time scale. The linewidths of ~3, v;, and VT0are relatively narrow ( IO- I4 cm - ’). In contrast, the linewidths of these modes in equilibrium nickel porphyrins in noncoordinating solvents are 16-25 cm-‘. The broader linewidths of the equilibrium species result from multiple conformations of the macrocycle in solution. Nickel porphyrins are known to crystallize in three distinct conformations: two triclinic planar forms and a tetragonal form [ 12,131. The tetragonal form exhibits a contracted metal core and highly distorted nonplanar macrocycle relative to the planar forms. Recent studies by Alden et al. [ 14,15 ] using resonance Raman spectroscopy, molecular mechanics, and molecular orbital (MO) cal#’The E( 30) parameter is based on the wavelength of a chargetransfer band of apyridinium 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 on the dye frontier molecular orbit&. See ref. [ 111.
42
1 December 1989
CHEMICAL PHYSICS LETTERS
culations have shown that multiple conformers of nickel porphyrins also exist in solution. The narrower lines of the *B,, excited state are consistent with those of a planar species and indicate that there is no conformational heterogeneity of the macrocycle in the excited state. Thus, the nuclear dynamics accompanying the Alg+*Blg transition produce a planar macrocycle within 30 ps of excitation. To further investigate the spectral evolution observed by Kin et al. [ 3,4], we undertook a subpicosecond transient absorption measurement of NiPPDME in chloroform (the three prominent absorption bands are B(0,0)=393, Q( 1,0) =524 and Q(O,O) =562 nm). Fig. 4 illustrates the results. We observed an instrument-limited rise (pulse duration w 300 fs) for the bleaching signal at 560 nm, and the nonradiative relaxation from ‘Q appears to require
B a
l”“l”“l”“l”“I”“l 525
550
575 600 Wavelength lnm)
625
650
Fig. 4. Transient absorption spectra as a function of time for NiPPDME in chloroform. There is a small distortion at the laser excitation wavelength (570 nm) due to scatter. The spectra correspond to -2, 0, + 1, + 2, +4, and + IO ps (from bottom IO top). The maximum absorption change was 0.05 at 576 nm.
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CHEMICAL PHYSICS LETTERS
< I ps. The spectral evolution of the red shoulder is complete in < 4 ps, as evidenced by the top two spectra in fig. 4 ( + 10 and + 4 ps). The spectral evolution of the transient absorption signal in the SO-650 nm region exhibits nonexponential kinetic behavior. This phenomenon has also been observed in the Soret band region by Rodriguez and Holten [ 161. They found evidence for the creation of a metastable d-d excited state within 350 fs of excitation. However, the absorption band of this excited state at x 425 nm continued to evolve, reaching its steady state form after ~20 ps. In light of these results, the previous assignment of the red spectral shoulder to the ‘B,, by Kim et al. [ 31 seems less secure due to the nonexponential evolution of the band. The intersystem crossing ‘Blg+ 3B,g would nominally display single exponential decay kinetics, and the relaxation time observed is very short for this process. Furthermore, there is no evidence in the 30 ps time-resolved Raman spectra for an additional electronic excited state. The lower spectrum of fig. 3 (on the rising edge of the excitation pulse) is qualitatively the same as the other time-resolved difference spectra. The most straightforward explanation for the nonexponential behavior of the < 10 ps transient absorption spectra is structural dynamics in the *BI, excited state. There is precedence for excited state structural reorganization in large molecular systems such as 1,I ‘-binaphthyl [ 171. The conformational heterogeneity exhibited by ‘A,, ground state nickel porphyrins make them likely candidates for such effects, The rapid ( < 1 ps) creation of the *B1, excited state should result in a distribution of macrocycle conformations reflective of the distribution of the original lA,, ground state in solution. The nonplanar, or “ruffled”, conformers have red-shifted ground state absorption spectra relative to the “planar” conformer, as evidenced by MO calculations and Raman excitation profiles of nickel porphyrins in noncoordinating solvents [ 15 1. The core of the macrocycle expands upon photoexcitation due to population of the d+_,,Z orbital in the *B,, state. This favors a planar conformation of the porphyrin in the excited state. The nonexponential spectral evolution can be accounted for by the conversion of the ruffled conformers to a single planar conformation in the excited state. The ruffled
1 December 1989
conformers would convert to the favored planar form in the excited state at different rates and produce the observed blue-shift at short time scales. Clearly, other processes may contribute to the complicated subnanosecond photodynamics of NiPPDME, including a “vibrationally hot” species or dielectric relaxation of the solvent. However, based upon the demonstrated conformational heterogeneity of nickel porphyrins, we conclude that conformational. interconversion of NiPPDME in the excited state plays a dominant role in the observed behavior. Recent structural data on (bacterio)chlorophylls in proteins [ 18,191 and model compounds [ 20,2 1 ] have demonstrated that protein constraints and crystal packing forces can significantly alter the conformation of the macrocycle. This has been suggested as a possible mechanism for altering the optical and redox properties of chromophores in a protein matrix [ 2 11. Hemoglobin reconstituted with nickel (II) protoporphyrin IX, for example, has been shown to display a predominantly planar configuration in contrast to a solution environment [ 151. The short time scale for conformational interconversion suggested by this study may play an important role in the short-lived electron transfer processes that occur in reaction center proteins. Future Raman experiments using 1 ps time resolution are currently in preparation to determine more precisely the origins of the early time dynamics of nickel porphyrins.
Acknowledgement We thank Dr. J.A. Shelnutt for helpful discussions.
References [ 1] M. Gouterman,
P. Rentzepis and K. Straub, eds., Porphyrins: excited state and dynamics, ACS Symp. Ser. 32 1 (Am. Chem. Sot., Washington, 1987). [2] V.S. Chirvonyi, B.M. Dzhagarov, Yu.V. Timinskii and G.P. Gurinovich, Chem. Phys. Letters 70 (1980) 79. [3] T. Kobayashi, K.D. Straub and P.M. Rentzepis, Photochem. Photobiol. 29 (1979) 925. [4] D.H. Kim and D. Holten, Chem. Phys. Letters 98 ( 1983) 584. [ 51 D.H. Kim, C. Kinnaier and D. Holten, Chem. Phys. 75 (1983) 305.
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[6] A.Yu. Chikishev, V.F. Kamalov, N.I. Koroteev, V.V. Kvach, A.P. Shkurinov and B.N. Toluetaev, Chem. Phys. Letters 144 (1988) 90. [7] E.W. Findsen, J.A. Shelnutt and M.R. Ondrias, J. Phys. Chem. 92 (1988) 307. [8] W.A. Oertling and G.T. Babcock, J. Am. Chem. Sot. 107 (1985) 6406. [9] R.L. Ake and M, Gouterman, Theoret, Chim. Acta 17 (1970) 408. [JO] S. Choi, T.G. Spiro, KC Langry, K.N. Smith, D.L. Budd and G.N. La Mar, J. Am. Chem. Sot. 104 (1982) 4345. I 1] C. Reichardt, in: Solvent effects in organic chemistry (Verlag Chemie, Weinheim, 1978). 121T.D. Brennan, W.R. Scheidt and J.A. Shelnutt, J. Am. Chem. Sot. 110 (1988) 3919.
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[ 13 ] D.L. Cullen and E.F. Meyer Jr., J. Am. Chem. Sot. 96 (1974) 2095. [ 141R.G. Alden, B.A. Crawford, R. Doolen, M.R. Ondrias and J.A. Shelnutt, J. Am. Chem. Sot. 1I I (1989) 2070. [ 15 ] R.G. Alden, M.R. Ondrias and J.A. Shelnutt, J. Am. Chem. Sot., in press. 161J. Rodriguez and D. Holten, J. Chem. Phys., in press. 171 D.P. Millar and K.B. Eisenthal, J. Chem. Phys. 83 (1985) 5076. 181H. Micheland J. Deisenhofer, EMBO J. 5 (1985) 2445. [ 191 D.E. Tranrud, M.F. Schmid and B.W. Matthews, J. Mol. Biol. 188 ( 1986) 443. [20] K.M. Barkigia, J. Fajer, C.K. Chang and R. Young, J. Am. Chem. Sot. 106 (1984) 6457. [ 2 1 ] K.M. Barkigia, L. Chantranupong, KM. Smith and J. Fajer, J. Am. Chem. Sot. 110 (1988) 7566.
CHEMICAL PHYSICS LETTERS
PICOSECOND OPTICAL STUDIES OF NICKEL(I1) PROTOPORPHYRIN
1 December 1989
IX DIMETHYL ESTER *
Scott H. COURTNEY, Tom M. JEDJU, Joel M. FRIEDMAN AT&T Bell Laboratories, Murray Hill, NJ 07974, USA
Rhett G. ALDEN and Mark R. ONDRIAS ’ Department of Chemistry, UniversityofNew Mexico, Albuquerque, NM 87131, USA Received 25 July 1989; in final form 18 September 1989
Time-resolved Raman and adsorption spectroscopies were used to characterize the vibrational and electronic structure of the *B,, excited state of nickel@) protoporphyrin IX dimethyl ester. Evidence for structural relaxation processes within the initial B,, excited state was observed on picosecond time scales. We conclude that interconversions between structural conformers are important in these processes. Such conformational interconversions in metalloporphyrins may have a major role in the complex mechanisms of many biological systems.
1. Introduction Metalloporphyrins are important in a variety of biological functions, such as oxygen transport, electron transfer, and energy conversion. In addition, metalloporphyrins have been used in photochemical cycles as catalytic charge transfer intermediates. To understand many of these functions, the excited state dynamics of these macrocycles must be elucidated. Consequently, these systems have been studied by a number of optical spectroscopic techniques (for a good overview, see ref. [ 1 ] ), including transient absorption [ 2-5 ] and resonance Raman spectroscopies [ 6-8 1. Systematic transient absorption studies of nickel porphyrins in both coordinating and noncoordinating solvents have led to significant progress [ 4,5 1. However, time-resolved resonance Raman spectroscopy (TRR) can be utilized to analyze the photodynamics of these systems in more detail by examining the effects ofthe electronic state on the vibrational structure of the porphyrin moiety. Find-
* This work was performed at AT&T Bell Laboratories and was supported by the NSF (DMB 8604435) and the NIH (GM 33330). ’ To whom correspondence should be addressed.
sen et al. [ 7 ] characterized several nickel porphyrins by transient Raman spectroscopy, and an initial characterization of Ni (OEP ) by picosecond time-resolved CARS was recently reported [ 61. Photoexcitation of nickel porphyrins in noncoordinating solvents leads to the formation of a relatively long-lived excited state (which we will call *B1,) that decays to the ‘Al, ground state in z 300 ps. Based on transient absorption results, Kim et al. [4,5] identified this long-lived state as the 3Blg,concluding that excitation into the Q-band region results in fast intersystem crossing primarily from the ‘B,, to the 3B,,. They estimated the lifetime of the ‘B,, to be < 15 ps. Alternatively, excitation into the B-band (Soret) region of the absorption spectrum may activate additional relaxation pathways from the singlet manifold to the 3B,, state. These additional decay pathways presumably arise from the high density of triplet states above the Q band. Theoretical calculations by Ake and Gouterman [9], using the odd electron perturbation method, placed the relative energy level spacing for the ‘B1, and 3B,, excited states to be = 6000 cm-‘, while the energetic spacing between the 3B,, and ‘Alp ground state is ~3000 cm-‘. The 3B,, state can be viewed
0009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland )
39
Volume 164, number I
as a combination of the porphyrin ‘Ai, ground state and the nickel ‘B,, ‘( dZ~,dX~_-yZ) configurations. Thus, photoexcitation causes a net d-d transition on the metal. Nanosecond
transient
1 December 1989
CHEMICAL PHYSICS LETTERS
resonance
Raman
spectros-
tion spectra. This system is tunable and regularly produces 300 fs, 1 mJ pulses. The detector was a Princeton Instruments double diode array mounted on a single monochromator with a dual fiber optic bundle coupled to the input slit.
copy has been used to characterize what was taken to be the *B,, excited state [ 71. Here we present a study of nickel( II) protoporphyrin
IX dimethyl
es-
ter (NiPPDME) in a noncoordinating solvent using picosecond time-resolved Raman and absorption spectroscopies. Our results confirm that the transient Raman spectra correspond to the *Br, and clarify the relaxation kinetics of the initial excited state.
2. Experimental NiPPDME was purchased from Porphyrin Products and further purified by column chromatography on alumina. HPLC grade chloroform was obtained from Aldrich Chemicals. The sample integrity and purity were monitored by absorption spectra on a Hewlett-Packard mode1 8452 spectrophotometer. A backscattering collection geometry ( =f 1.5) was used to obtain the Raman spectra using an Instruments SA UlOOOdouble monochromator with both 600 grooves/mm and 2400 grooves/mm gratings. The detector was an intensified optical multichannel analyzer (Princeton Instruments IRY700). Approximately 50% of the second harmonic (532 nm) from a Quantel active-passively mode-locked Nd: YAG laser ( z 250 mJ/pulse, 30 ps, 10 Hz) was focused in a high pressure hydrogen cell (300 psi). The first antiStokes shift (436 nm) was separated with a prism and focused at the sample with cylindrical optics. The spatial overlap for the two-color “pump-probe” experiments was achieved by recombining some of the remaining 532 nm pump pulse and the 436 nm “probe” with a dichroic mirror. The pump beam was temporally delayed using a Klinger GV88 optical delay stage, and the temporal overlap was verified using excited state absorption of rhodamine 6G. An amplified femtosecond dye laser system consisting of a mode-locked continuously pumped YAG laser with a fiber optic pulse compressor (Spectra Physics), a synchronously pumped dye laser (Coherent 702), and a dye laser amplifier operating at 10 Hz was used to obtain the time-resolved absorp40
3. Results and discussion Fig. 1 shows the ground state recovery kinetics of NiPPDME in chloroform. Previous absorption studies [2-41 have observed this transient and placed the lifetime at M280 ps. We obtained a slightly longer value (the fit to a single exponential in fig. 1 gave 330 ps). Kobayashi et al. [ 31, assigned this “bottleneck” state as the ‘Big, while others suggested that the 3B,, is populated via the ‘B,, and the lowest porphyrin ( ‘IC,x*) triplet [ 2,4]. In noncoordinating solvents, Kim et al. [ 5 ] observed spectral evolution on the red side of a strong transient absorption band (at -575 nm, which they assigned as the Q(O,O) band of 3B,,). The red shoulder was taken as evidence of the ‘Big, and they estimated that the intersystem crossing ‘Blg-+‘Big requires 10-l 5 ps. Figs. 2 and 3 summarize the results obtained from
: 0.02 ; 73 0 .r( f : -0.02 a:
o.31
-0.5
1
0
1
0.5
rime
1.5
Ins)
Fig. 1. Kinetics of the ground state recovery of the Q( 0,O) band of NiPPDME in chloroform at 56 1nm. The solid line is a tit to a single exponential with a time constant of 330 ps. The decay was generated using the 10 Hz Nd: YAG laser ( Y 30 ps), and the excitation wavelength was 532 nm.
Volume 164, number I
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1500 Shift
1600
1700
(cn1-~1
Fig. 2. Resonance Raman spectra of NiPPDME in chloroform. The center spectrum is that of the ground state obtained using only the 436 nm pulse, and the upper spectrum was obtained with a time delay between the pump (532 nm) and the probe (436 nm ) pulseof 20 ps. The lower spectrum is the excited state spectrum obtained by subtracting the residual ground state fraction (0.6 in this case) from the upper spectrum.
time-resolved resonance Raman studies of NiPPDME in the 30 ps to 3 ns time scale. We had initially hoped to see evidence in picosecond time-resolved Raman spectra for higher lying excited state species. However, our time-resolved absorption results (see below) make it quite unlikely that resonance Raman spectra obtained with 30 ps pulses contain significant scattering from the putative initial state(s). Fig. 2 clearly shows the formation of a transient d-d excited state when the sample is irradiated with an intense (= 10” W/cm’, 532 nm) pump pulse spatially and temporally overlapped with the relatively weak probe pulse ( z 1O8W/cm’, 436 nm ). At higher laser fluences, bands appear at 1370, 1495, 1576 and 1632 cm-’ corresponding to v;, Y;, u; and vTo,respectively, of the excited state species. The intensity of the probe beam was attenuated to a level at which these peaks were not observed, and the positions of v4, y3, v2, and vi0 in the center spectrum of fig. 2 are consistent with those of the
1300
1400
Raman
1500
1600
Shift
(cm-')
1700
Fig. 3. Time-resolved resonance Raman difference spectra of NiPPDME in chloroform. The difference spectra were obtained as in fig, 2, and the four spectra represent different time delays between the pump and probe pulses (- 10 ps, 20 ps, 220 ps, and 1ns from bottom to top). The fraction ofground state subtracted in each spectrum was 0.59,0.60, 0.76, and 0.82, respectively.
ground state species. As the time delay between the pump and probe pulses increases, the intensity of these new lines decreases. The difference spectra in fig. 3 show clearly that, within 1 ns after the excitation pulse, the Raman spectrum is identical to that of the ground state. This relaxation time ( 1 ns ~3 lifetimes) confirms the association of these new lines to the “bottleneck” excited state [7]). It therefore seems quite certain that the transient Raman peaks are associated with a *B,, excited state, yet the multiplicity of this state is uncertain. The enhanced time resolution of these spectra allow for a cleaner resolution of the structural properties of the *B,, state. Two of the vibrational modes shown to decrease with increasing core size of the macrocycle, v, and vlo, shift from 1522 and 1658 cm-’ to 1495 and 1632 cm-’ (see table l), respectively [ lo]. In addition, v.,, a totally symmetric C,N stretching mode that is highly sensitive to the x electron density of the porphyrin, broadens and shifts from its ‘Alg ground state value of 1381 cm-’ to a 41
Volume 164, number 1 Table 1 Raman modes of NiPPDME a)
*e r? V2 VI0
Ground state (‘A,,)
Excited state
1381 1522 1595 1658
1370 1495 ~1576~’ 1632
(*RI,)
*) Raman frequencies are given in cm-‘. b, The excited state value of uz may be distorted by underlying ground state modes, such as Y , ,
peak position at approximately 1370 cm-‘. It is tempting to assign another peak at z 1576 cm-’ (see the lower spectrum in fig. 3) to u; (at 1595 cm-’ in the ground state). In a previous transient resonance Raman study, no correlation to the solvent E,(30) value#’ was found for this peak; whereas, of and ~7, showed a clear correlation [ 71. Hence, it seems likely that this peak is highly distorted by another ground state mode, possibly u11 (the shoulder in the ground state spectra). v; is slightly distorted by a chloroform solvent line at z 1500 cm-‘, but is not significant based on the strong correlation of &(30). The time-resolved Raman difference spectra presented in this study show no evidence of spectral evolution of the vibrational modes on a 30 ps and longer time scale. The linewidths of ~3, v;, and VT0are relatively narrow ( IO- I4 cm - ’). In contrast, the linewidths of these modes in equilibrium nickel porphyrins in noncoordinating solvents are 16-25 cm-‘. The broader linewidths of the equilibrium species result from multiple conformations of the macrocycle in solution. Nickel porphyrins are known to crystallize in three distinct conformations: two triclinic planar forms and a tetragonal form [ 12,131. The tetragonal form exhibits a contracted metal core and highly distorted nonplanar macrocycle relative to the planar forms. Recent studies by Alden et al. [ 14,15 ] using resonance Raman spectroscopy, molecular mechanics, and molecular orbital (MO) cal#’The E( 30) parameter is based on the wavelength of a chargetransfer band of apyridinium 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 on the dye frontier molecular orbit&. See ref. [ 111.
42
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culations have shown that multiple conformers of nickel porphyrins also exist in solution. The narrower lines of the *B,, excited state are consistent with those of a planar species and indicate that there is no conformational heterogeneity of the macrocycle in the excited state. Thus, the nuclear dynamics accompanying the Alg+*Blg transition produce a planar macrocycle within 30 ps of excitation. To further investigate the spectral evolution observed by Kin et al. [ 3,4], we undertook a subpicosecond transient absorption measurement of NiPPDME in chloroform (the three prominent absorption bands are B(0,0)=393, Q( 1,0) =524 and Q(O,O) =562 nm). Fig. 4 illustrates the results. We observed an instrument-limited rise (pulse duration w 300 fs) for the bleaching signal at 560 nm, and the nonradiative relaxation from ‘Q appears to require
B a
l”“l”“l”“l”“I”“l 525
550
575 600 Wavelength lnm)
625
650
Fig. 4. Transient absorption spectra as a function of time for NiPPDME in chloroform. There is a small distortion at the laser excitation wavelength (570 nm) due to scatter. The spectra correspond to -2, 0, + 1, + 2, +4, and + IO ps (from bottom IO top). The maximum absorption change was 0.05 at 576 nm.
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< I ps. The spectral evolution of the red shoulder is complete in < 4 ps, as evidenced by the top two spectra in fig. 4 ( + 10 and + 4 ps). The spectral evolution of the transient absorption signal in the SO-650 nm region exhibits nonexponential kinetic behavior. This phenomenon has also been observed in the Soret band region by Rodriguez and Holten [ 161. They found evidence for the creation of a metastable d-d excited state within 350 fs of excitation. However, the absorption band of this excited state at x 425 nm continued to evolve, reaching its steady state form after ~20 ps. In light of these results, the previous assignment of the red spectral shoulder to the ‘B,, by Kim et al. [ 31 seems less secure due to the nonexponential evolution of the band. The intersystem crossing ‘Blg+ 3B,g would nominally display single exponential decay kinetics, and the relaxation time observed is very short for this process. Furthermore, there is no evidence in the 30 ps time-resolved Raman spectra for an additional electronic excited state. The lower spectrum of fig. 3 (on the rising edge of the excitation pulse) is qualitatively the same as the other time-resolved difference spectra. The most straightforward explanation for the nonexponential behavior of the < 10 ps transient absorption spectra is structural dynamics in the *BI, excited state. There is precedence for excited state structural reorganization in large molecular systems such as 1,I ‘-binaphthyl [ 171. The conformational heterogeneity exhibited by ‘A,, ground state nickel porphyrins make them likely candidates for such effects, The rapid ( < 1 ps) creation of the *B1, excited state should result in a distribution of macrocycle conformations reflective of the distribution of the original lA,, ground state in solution. The nonplanar, or “ruffled”, conformers have red-shifted ground state absorption spectra relative to the “planar” conformer, as evidenced by MO calculations and Raman excitation profiles of nickel porphyrins in noncoordinating solvents [ 15 1. The core of the macrocycle expands upon photoexcitation due to population of the d+_,,Z orbital in the *B,, state. This favors a planar conformation of the porphyrin in the excited state. The nonexponential spectral evolution can be accounted for by the conversion of the ruffled conformers to a single planar conformation in the excited state. The ruffled
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conformers would convert to the favored planar form in the excited state at different rates and produce the observed blue-shift at short time scales. Clearly, other processes may contribute to the complicated subnanosecond photodynamics of NiPPDME, including a “vibrationally hot” species or dielectric relaxation of the solvent. However, based upon the demonstrated conformational heterogeneity of nickel porphyrins, we conclude that conformational. interconversion of NiPPDME in the excited state plays a dominant role in the observed behavior. Recent structural data on (bacterio)chlorophylls in proteins [ 18,191 and model compounds [ 20,2 1 ] have demonstrated that protein constraints and crystal packing forces can significantly alter the conformation of the macrocycle. This has been suggested as a possible mechanism for altering the optical and redox properties of chromophores in a protein matrix [ 2 11. Hemoglobin reconstituted with nickel (II) protoporphyrin IX, for example, has been shown to display a predominantly planar configuration in contrast to a solution environment [ 151. The short time scale for conformational interconversion suggested by this study may play an important role in the short-lived electron transfer processes that occur in reaction center proteins. Future Raman experiments using 1 ps time resolution are currently in preparation to determine more precisely the origins of the early time dynamics of nickel porphyrins.
Acknowledgement We thank Dr. J.A. Shelnutt for helpful discussions.
References [ 1] M. Gouterman,
P. Rentzepis and K. Straub, eds., Porphyrins: excited state and dynamics, ACS Symp. Ser. 32 1 (Am. Chem. Sot., Washington, 1987). [2] V.S. Chirvonyi, B.M. Dzhagarov, Yu.V. Timinskii and G.P. Gurinovich, Chem. Phys. Letters 70 (1980) 79. [3] T. Kobayashi, K.D. Straub and P.M. Rentzepis, Photochem. Photobiol. 29 (1979) 925. [4] D.H. Kim and D. Holten, Chem. Phys. Letters 98 ( 1983) 584. [ 51 D.H. Kim, C. Kinnaier and D. Holten, Chem. Phys. 75 (1983) 305.
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[6] A.Yu. Chikishev, V.F. Kamalov, N.I. Koroteev, V.V. Kvach, A.P. Shkurinov and B.N. Toluetaev, Chem. Phys. Letters 144 (1988) 90. [7] E.W. Findsen, J.A. Shelnutt and M.R. Ondrias, J. Phys. Chem. 92 (1988) 307. [8] W.A. Oertling and G.T. Babcock, J. Am. Chem. Sot. 107 (1985) 6406. [9] R.L. Ake and M, Gouterman, Theoret, Chim. Acta 17 (1970) 408. [JO] S. Choi, T.G. Spiro, KC Langry, K.N. Smith, D.L. Budd and G.N. La Mar, J. Am. Chem. Sot. 104 (1982) 4345. I 1] C. Reichardt, in: Solvent effects in organic chemistry (Verlag Chemie, Weinheim, 1978). 121T.D. Brennan, W.R. Scheidt and J.A. Shelnutt, J. Am. Chem. Sot. 110 (1988) 3919.
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[ 13 ] D.L. Cullen and E.F. Meyer Jr., J. Am. Chem. Sot. 96 (1974) 2095. [ 141R.G. Alden, B.A. Crawford, R. Doolen, M.R. Ondrias and J.A. Shelnutt, J. Am. Chem. Sot. 1I I (1989) 2070. [ 15 ] R.G. Alden, M.R. Ondrias and J.A. Shelnutt, J. Am. Chem. Sot., in press. 161J. Rodriguez and D. Holten, J. Chem. Phys., in press. 171 D.P. Millar and K.B. Eisenthal, J. Chem. Phys. 83 (1985) 5076. 181H. Micheland J. Deisenhofer, EMBO J. 5 (1985) 2445. [ 191 D.E. Tranrud, M.F. Schmid and B.W. Matthews, J. Mol. Biol. 188 ( 1986) 443. [20] K.M. Barkigia, J. Fajer, C.K. Chang and R. Young, J. Am. Chem. Sot. 106 (1984) 6457. [ 2 1 ] K.M. Barkigia, L. Chantranupong, KM. Smith and J. Fajer, J. Am. Chem. Sot. 110 (1988) 7566.