J. Phys. Chem. 1996, 100, 8791-8795
8791
Effect of Peripheral Substitution on Electron and Energy Transfer in Electrostatically Bound Zinc-Magnesium Porphyrin Dimers Alexander Berg, Mazal Rachamim, Tamar Galili, and Haim Levanon* Department of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: January 31, 1996; In Final Form: March 19, 1996X
We report on intramolecular electron and/or energy transfer (IET and IEnT), in two photoexcited electrostatically bound dimers: (1) zinc(II) tetrakis(4-sulfonatophenyl)porphyrin and magnesium(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin, [ZnTSPP-MgTTAP]; and (2) zinc(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin and magnesium(II) tetrakis(4-sulfonatophenyl)porphyrin, [ZnTTAP-MgTSPP]. The photochemical properties of both dimers, dissolved in dimethylsulfoxide-glycerol (DMSO-Glyc, 140 K), were investigated by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectroscopy. It is demonstrated that by changing the peripheral substitution of the particular metalloporphyrin in the dimer, an IET route, parallel to the singlet IEnT, can be triggered. Thus, singlet IEnT from the Zn to the Mg constituent is the only process in [ZnTSPP-MgTTAP], while IET to produce a charge-separated state, [ZnTTAP•+-MgTSPP•-] accompanies the IEnT in [ZnTTAP-MgTSPP].
Introduction The primary steps in photosynthesis occur between a number of different pigments, which are organized in a so-called photosynthetic reaction center (RC). Despite the fact that the structure of some RCs is known, their functioning in the mechanism of electronsand energystransfer (ET and EnT) is still under intensive study.1,2 In an effort to understand the structure-function relationship, the implementation of the mechanistic approach is very useful. Thus, by utilizing different model systems, attempts are carried out to mimic states and intermediates that are related to those participating in primary photosynthesis. It is clear that porphyrins and porphyrin-like chromophores are leading structures in many model systems being investigated by modern time-resolved EPR spectroscopy.1 Most ET studies are still concentrated on covalently linked donor-acceptor systems.1,3-5 Different approaches of employing base-paired donor-acceptor systems6-9 and electrostatically bound dimers,10-13 in studying intramolecular ET, have also been described recently. The last two approaches are important since the chromophores of the in ViVo RC are held in space by the ordering and charge distribution of the protein environment,14 both playing a crucial role in the unidirectional ET route in the in ViVo apparatus.2 We concentrate here on studying the photophysics and photochemistry of electrostatically bound dimers, by timeresolved EPR (TREPR) spectroscopy. These dimers may be suitable as model systems because of the following reasons: (1) a simple synthesis from their monomer constituents; (2) high efficiency of triplet state production upon photoexcitation; (3) the relatively long lifetime of the photoexcited triplet state of these chromophores, which allows their participation in ET and EnT reactions. The high sensitivity of TREPR, with respect to spectral and time resolution, makes this spectroscopy suitable for the investigation of the paramagnetic species, which are involved in the light-induced reactions in this type of donoracceptor system. We report on intramolecular ET and EnT in two electrostatically bound metalloporphyrin dimers (structure I), consisting of zinc(II) tetrakis(4-sulfonatophenyl)porphyrin X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00329-2 CCC: $12.00
and magnesium(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin, i.e., [ZnTSPP-MgTTAP], and zinc(II) tetrakis(4-N,N,Ntrimethylanilinium)porphyrin and magnesium(II) tetrakis(4sulfonatophenyl)porphyrin, i.e., [ZnTTAP-MgTSPP]. As will be shown below, the metal inversion in the dimer strongly affects its photochemical behavior. Experimental Section The metalloporphyrin monomers (sodium and chloride salts) were obtained from Mid-century Chemical Co. The other chemicals used were glycerol (Glyc) (BDH, analytical reagent), dimethylsulfoxide (DMSO) (Aldrich, ACS spectrophotometric grade), and triple-distilled water (TDW). All the chemicals were used without further purification. The synthesis of the electrostatically bound [ZnTSPPMgTTAP] and [ZnTTAP-MgTSPP] dimers was carried out according to the procedure described elsewhere.11 It was shown by elementary analysis that mixing of cationic and anionic watersoluble metalloporphyrin monomers in polar solvents leads to the formation of stable 1:1 dimer complexes.10,11 Optical measurements were carried out on a Hewlett-Packard UV-vis spectrophotometer in 2-mm-wide quartz cells. Due to the low solubility of the dimers, we did not perform a precise quantitative analysis of their optical spectra, which are presented in relative absorption units. CW-TREPR measurements were carried out on a Varian E-12 spectrometer (X-band) in direct detection (DD) mode, interfaced to a pulsed dye laser (Quanta Ray, PDL-1, 5 mJ/pulse, at a repetition rate 10 Hz) pumped by the second harmonic of a Nd:YAG laser (Quanta Ray, DCR© 1996 American Chemical Society
8792 J. Phys. Chem., Vol. 100, No. 21, 1996
Berg et al.
1A). Laser excitation at 570 nm corresponds to the Q-band absorption of the porphyrin moiety. All metalloporphyrin monomers in this study exhibit similar extinction coefficients at this wavelength. EPR measurements were performed on samples dissolved in the DMSO-Glyc (1:1) mixture in 4-mm-o.d. Pyrex tubes, degassed by several freeze-pump-thaw cycles on a vacuum line. The concentration of the samples was ∼10-4 M. The temperature was controlled by a variable-temperature nitrogenflow dewar inside the EPR resonator. The time profile of the magnetization, My(t), was obtained by following the signal generated by the laser pulse at a specific magnetic field, under constant microwave irradiation. It should be noted that the observed EPR signals were completely reversible and did not show any saturation effect; thus, by using high microwave power (100 mW), a substantial improvement of the signal-to-noise ratio was achieved. A detailed description of light excitation, signal detection, data acquisition, and triplet line shape analysis is given elsewhere.15-18 The EPR signal decay rates were extracted by analysis of the magnetization time profile, My(t), using a previously derived biexponential expression:19,20
My(t) ) P0ω1 with
[e-c+t - e-c-t] Peq t [e-c+t′ - e-c-t′] + ∫ ω c- - c+ dt′ c- - c+ T1 0 1 (1)
[
]
1/2 T2-1 + T1-1 (T2-1 - T1-1)2 2 c( ) - ω1 2 4 where ω1 is the microwave field, T1 and T2 are the relaxation times, and P0 and Peq are the initial and equilibrium triplet state populations, respectively.
Figure 1. Ground state optical absorption spectra (Q-band) of (a) ZnTTAP (dotted line) and MgTSPP (solid line); (b) ZnTSPP (dotted line) and MgTTAP (solid line), dissolved in DMSO-Glyc (1:1). Porphyrin concentrations are ∼10-4 M. The arrow indicates the wavelength of laser excitation.
Results Figure 1 shows the absorption spectra (Q-band) of the metalloporphyrin monomers ZnTTAP and MgTSPP (a) and ZnTSPP and MgTTAP (b), dissolved in DMSO-Glyc (1:1) at room temperature. ZnTTAP and ZnTSPP solutions exhibit identical optical spectra, both in the Soret-band (not shown) and Q-band regions. MgTTAP and MgTSPP monomer solutions have identical Soret bands, but their Q-band absorptions are slightly different: MgTSPP exhibits a shoulder at 484 nm and four distinctive peaks at 516, 546, 602, and 642 nm, with relative intensities of 4.5:5.6:3.2:1, respectively; MgTTAP exhibits peaks at 516, 564, 604, and 654 nm with relative intensities of 3.15:6.9:3.4:1, respectively. Light excitation was carried out at 570 nm, where all monomers have the same optical density (Figure 1). We found it convenient to compare the optical spectra of the dimers (Figure 2, traces a) with the 1:1 mixtures of the relevant monomers in control experiments (Figure 2, traces b). For a better comparison, spectra of the dimers and the mixtures, in the Q-band, were normalized to the same height. The ratio of the Soret- and Q-band intensities is one of the characteristic features of the optical spectra of porphyrins. While for the monomeric 1:1 mixture, this ratio is 15-16, it is reduced significantly for the corresponding dimers; that is, it is 5 for [ZnTTAP-MgTSPP] and 7 for [ZnTSPP-MgTTAP]. The spectral width of the Soret band (measured at half-height) is unchanged for [ZnTSPP-MgTTAP], but is broadened approximately by 290 cm-1 for [ZnTTAP-MgTSPP], as compared to that of the corresponding control mixtures. For both dimers the Soret bands are not shifted. On the other hand, while the Q-band of [ZnTSPP-MgTTAP] does not show any shift,
Figure 2. Ground state optical absorption spectra of [ZnTTAPMgTSPP] (top) and [ZnTSPP-MgTAPP] (bottom) in the Soret band and Q-band. Superimposed (dotted lines) are the optical spectra of the 1:1 mixtures of the relevant Zn- and Mg-porphyrin monomers. Solvent and concentration are as in Figure 1.
that of [ZnTTAP-MgTSPP] is slightly red shifted by 4 nm, and some band broadening is noticeable in the red region of the spectrum. This behavior suggests that some ground state interaction between the subunits occurs in both dimers. Photoexcitation of frozen solutions of MTSPP and MTTAP (M ) Mg, Zn) monomers in DMSO-Glyc gives rise to polarized triplet spectra, i.e., 3*MTSPP and 3*MTTAP. In Figure 3, we present their EPR triplet spectra with superimposed computer simulations. It should be noted that the triplet EPR spectra of ZnTSPP and MgTTAP are identical with those of ZnTTAP and MgTSPP, respectively, exhibiting aaaeee and
Effect of Peripheral Substitution
J. Phys. Chem., Vol. 100, No. 21, 1996 8793
Figure 4. Direct detection CW-EPR spectrum of a photoexcited 1:1 mixture of ZnTTAP and MgTSPP monomers (solid line) in DMSOGlyc (1:1) at 140 K. The computer fit was made by adding together the separate spectra of the relevant monomers as described in the text (dotted line). Experimental conditions are as in Figure 3.
Figure 3. Direct detection CW-EPR spectra (and their computer simulations) of photoexcited MgTSPP (top) and ZnTTAP (bottom) monomers in DMSO-Glyc (1:1) at 140 K. Microwave power is 100 mW; laser excitation is at 570 nm, with 5 mJ/pulse.
TABLE 1: Magnetic and Kinetic Parameters of the Monomers and Dimers at 140 K compound a
ZnP1 MgP2a [ZnP1-MgP2 ]a [ZnP2-MgP1]a
D (G) 330 320
E (G)
T1 (µs)
T2 (µs)
Ax:Ay:Az
110 100
0.33b
0.11b
0.33b 0.33 0.34
0.11b 0.09 0.12
0:0:1 0.72:0.8:1
a P and P stand for TSPP and TTAP substituents, respectively. 1 2 Obtained from line shape analysis and by using eq 1. Uncertainty (10%.
b
eeeaaa patterns from low to high field. All magnetic parameters extracted from these spectra are typical to the well-known EPR data of metalloporphyrins.10,21 The line shape analysis (Figure 3) provides us with the following information: (1) the zerofield-splitting (ZFS) parameters of the photoexcited triplet states; (2) the relative population rates (Ax:Ay:Az) of triplet r singlet spin-orbit intersystem crossing (SO-ISC); and (3) triplet spin relaxation times T1 and T2. The obtained parameters are summarized in Table 1. In the control experiments performed on the porphyrin mixtures, photoexcitation gives rise to a triplet spectrum which is a one-to-one sum, within experimental error, of spectra of the monomers, as may be expected under photoexcitation at 570 nm, where both chromophores in their ground states have the same optical density. For example, Figure 4 shows the spectrum of the 1:1 mixture of the 3*ZnTTAP and 3*MgTSPP monomers (solid line) as compared to a simulation curve (dotted line) and will be further discussed below. Photoexcitation of a 1:1 mixture of ZnTSPP and MgTTAP results in the same spectrum as that in Figure 4. The EPR spectra of both dimers, however, are significantly different. In Figure 5 we present the spectrum of [ZnTSPPMgTTAP] (a) and that of [ZnTTAP-MgTSPP] (b) with the fitted superpositions of the relevant monomers’ spectra. It is evident that the triplet spectrum of [ZnTSPP-MgTTAP] exhibits practically the same line shape as that of 3*MgTTAP monomer, with a very small contribution of 3*ZnTSPP monomer (compare Figure 3 with Figure 5a). The triplet spectrum of the second dimer, [ZnTTAP-MgTSPP], is also dominated by that of 3*MgTSPP monomer, but with a much larger contribution
Figure 5. Direct detection CW-EPR spectra of (a) photoexcited [MgTTAP-ZnTSPP] and (b) [ZnTTAP-MgTSPP] in DMSO-Glyc (1:1) at 140 K (solid line). The computer fit was made by superposition of the relevant monomer spectra (dotted line) as described in the text. Experimental conditions are as in Figure 3.
of 3*ZnTTAP monomer (compare Figure 3 with Figure 5b). The simulations of the triplet spectra of both dimers were carried out by computer summation of the experimental triplet spectra of the relevant monomers, taken with different weighing factors, with the following resulting ratios: 3*ZnTSPP/3*MgTTAP ≈ 1:9 for [ZnTSPP-MgTTAP] and 3*ZnTTAP/3*MgTSPP ≈ 3:7 for [ZnTTAP-MgTSPP]. Both ratios were found to be unchanged with respect to the time evolution, i.e., at different times after the laser pulse. Thus, these results clearly indicate that in [ZnTSPP-MgTTAP] an efficient EnT from the photoexcited ZnTSPP to MgTTAP subunit takes place (see the Discussion section below). The same arguments lead to the conclusion that EnT occurs by the same route in [ZnTTAPMgTSPP], but with much less efficiency. Moreover, in the case of [ZnTTAP-MgTSPP] (Figure 5b), an additional spectral feature could be observed, as reflected by a narrow signal at 3244 G (g ) 2.0026). The time evolution of this complex spectrum is shown in Figure 6 and will be discussed below. In Figure 7, we show the kinetic traces of the transient magnetization My(t) upon photoexcitation of [ZnTTAPMgTSPP], taken at 3148 G (broad triplet signal) and at 3244 G (the superimposed narrow signal).
8794 J. Phys. Chem., Vol. 100, No. 21, 1996
Berg et al. porphyrin subunits in their singlet or triplet excited states.33 Indeed, the EPR spectrum of [ZnTSPP-MgTTAP] resembles that of 3*MgTTAP monomer, but with a small contribution (
