J. Phys. Chem. 1995, 99, 7514-7521
7514
Molecular Architecture and Environmental Effects in Intramolecular Electron Transfer. An Electron Paramagnetic Resonance Study Kobi Hasharoni and Haim Levanon* Department of Physical Chemistry and The Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Jdrg Gbtschmann, Heike Schubert, and Harry Kurreck Institute of Organic Chemistry, Free University of Berlin, 1419.5 Berlin, Germany
Klaus Mdbius Institute of Molecular Physics, Free University of Berlin, 14195 Berlin, Germany Received: August 3, 1994; In Final Form: November 9, 1994@
Intramolecular electron transfer (ET) in three photosynthetic model systems, oriented in liquid crystals (LCs), was monitored by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectroscopy: (1) zinc porphyrin (ZnTPP) linked via an amide spacer to a lumiflavin (PaF); (2) ZnTPP linked to a benzoquinone via a phenyl spacer in the para (p-PpQ); and (3) in the meta (m-PpQ) positions. The anisotropic liquid crystalline environment makes the ET products detectable over a wide range of temperatures, Le., 210 I T I 330 K. Under such experimental conditions the ET rates are reduced quite dramatically into the solvent controlled adiabatic regime. The spectral line shape differences reflect the effect of the molecular architecture, namely, the relative orientation of the donor-acceptor as well as the spacer moiety. These differences in molecular structures are manifested by the TREPR spectra through the magnitude of the spinspin coupling (J)and the dipolar interaction (D),thus leading to different electron spin polarization mechanisms.
I. Introduction
A
E'
The paramagnetic intermediates involved in the primary steps of charge separation in photosynthetic bacteria and green plants exhibit strong electron spin polarization (ESP).' One of the major goals in the mechanistic approach of model photosynthesis is to reproduce, in tailor made donor-acceptor systems, the electronic states associated with basic features of intramolecular electron transfer (ET) process and charge separation. A simplified model system consists of donor-spacer-acceptor (DsA) compounds combined with the matrix in which the molecule is embedded. The high sensitivity of time-resolved electron paramagnetic resonance (TREPR), with respect to the molecular architecture and the environment, makes this spectroscopy suitable to characterize ET reactions. Inspection of the processes shown in Figure 1 suggests that, upon photoexcitation, two types of transient species can be detected by TREPR: (i) radical anions and cations associated with several ESP mechanisms, such as radical pair, correlated radical pair, and triplet mechanisms (RPM, CRPM, and TM, respe~tively)*-~ and (ii) the triplet state of the radical pair, 3[D'+sA'-].5 In a recent publication> we have demonstrated that liquid crystals (LCs), due to their anisotropic properties, are ideal matrices for EPR detection of intramolecular ET reaction products. The advantage of using LCs is not only through the stabilization of the charge separated state via solvent dipole relaxation (A, in Figure 1) but mainly due to their intrinsic anisotropic properties. We have demonstrated that the range of TREPR detection of transient radical pairs could be spanned over a wide temperature range, of about 120 degrees, including ambient temperatures. At such elevated temperatures the LC
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, May 1, 1995. 0022-365419512099-7514$09.00/0
"DSA
is
Figure 1. Energy level diagram of the possible ET reactions: D, donor (porphyrin); s, spacer (amide or phenyl); A, acceptor (lumiflavin or
benzoquinone). The solvent reorganization energy is represented schematically by As. S-T mixing is valid only for small values of J (see text). exhibits fluid properties and the embedded chromophores are free to move with some orientational restrictions imposed by the molecular structure and phase diagram of the LC.' We report here on the TREPR detection of intramolecular ET in three covalently linked compounds (Figure 2) which are based on the structure: porphyrin donor-spacer-quinone acceptor, DsA, Le., zinc porphyrin (ZnTPP) linked to a lumiflavin via an amide spacer (PaF) and ZnTPP linked to a benzoquinone via a phenyl spacer in the para (p-PpQ) and in the meta (m-PpQ) positions. In PaF, the distance between the centers of the ZnTPP and the lumiflavin is -15 A, and for the respective p-PpQ and m-PpQ isomers, the distances are -11 and -9.3 A. While the amide spacer in PaF allows some flexibility of the flavin around the axis, this degree of freedom is reduced in p-PpQ and m-PpQ. Consequently, a variety of 0 1995 American Chemical Society
EPR Study of Intramolecular Electron Transfer
PaF
J. Phys. Chem., Vol. 99, No. 19, 1995 7515
P-PPQ I
0
0
Figure 2. Calculated minimum energy structures (Hyperchem, Autodesk Inc.) of PaF, p-PpQ, and m-PpQ. These structures do not take into account the LC environment used in the experiment, and thus, some differences are expected.
200 K
250 K
300 K
4440 ns
3080 ns h
1720 ns
-
-
.
A
I
1040 ns
700 ns
360 ns
&
2880 3213 3541 3880
-
2880 3213 3547 3880
2880 3213 3547 3880 MAGNETIC FIELD (Gauss)
Figure 3. Diode-detection CW-EPR spectra of PuF in E-7 as a function of the time interval, ZD, following the laser pulse at three different temperatures. Note that the spectrum of 3PuF is detected over the whole temperature range between 100 and 320 K, while that of the Rp is limited to the 210-330 K range. Positive and negative signals are absorption (a) and emission (e) modes, respectively: microwave power, 105 mW; laser pulses (-12-ns pulse width) at 556 nm, 5 mJ/pulse at a repetition rate of 20 Hz.
orientations of the lumiflavin moiety with respect to the ZnTPP is expected, whereas with the aromatic spacer such a rotation is restricted. Finally, the effects of the ESP associated with the TREPR spectra are discussed in terms of the differences in molecular structure.
11. Experimental Section
The synthesis of the compounds will be described elsewhere.8 Two LCs (E-7 and ZLI-4389, Merck Ltd.)g were used as solvents and are characterized by the following phase transition
Hasharoni et al.
7516 J. Phys. Chem., Vol. 99, No. 19, 1995
TABLE 1: Spin Relaxation Times and ET Rates at 250 K in E-7
temperatures:
E-7 crystalline ZLI-4389: crystalline
210 K
250 K
soft-glass
soft-glass
263 K
253 K
nematic
nematic
333 K
isotropic
3T1-1 (lo6 s-l) k B F (106 s-I) RT1-1
335 K
isotropic
(106 s-1)
PaF
P-PPQ
m-PPQ
3.6 0.7 0.02
4.0 1.9 1.7
3.1 2.3 2.1
From best fit analysis. Uncertainty: 10%.
CW-TREPR measurements were carried out on a Bruker spectrometer (ESP-380, field modulation disconnected), interfaced to a pulsed laser (Continuum, TDL-60, 5 &/pulse, at a repetition rate of 20 Hz) pumped by the second harmonic of a Nd:YAG laser (Continuum, 661-20).6 Measurements were performed by dissolving the dimers in the LC in 4-mm 0.d. Pyrex tubes, degassed by several freeze-pump-thaw cycles on a vacuum line. The temperature was controlled by a variabletemperature N2 flow dewar in the EPR resonator. Samples were excited at selective wavelengths that correspond to the Q-band absorption of the porphyrin moiety, Le., 558 (PaF) and 556 nm @,m-PpQ). The time profile of the magnetization, My(t), is obtained by following the signal generated by the laser pulse at a specific magnetic field under constant microwave radiation. It should be noted that the rise and decay rates of the magnetization, in fluid LC environments, do not show any dependence on the microwave field strength, &.lo Thus, by using high microwave power (105 mW), the substantial improvement of the signal-to-noise ratio is achieved. 111. Results
The transient TREPR spectra of the three photoexcited molecules are attributed to the ET products which are the radical pairs (RPs) formed within the LC matrices. All attempts to observe the RPs in isotropic solutions, glass, soft glass, and liquid (ethanol or ethanol methylene chloride) under similar conditions, at low microwave power (1-10 mW to avoid saturation effects), failed, although the triplet of the porphyrins, (3PaF and 3PpQ) could be detected in the glass and soft glass. This rise and decay rates of the signals were extracted from analysis of the magnetization time profile, My(t), using a previously derived biexponential expression:
+
1
0
I
2
1 4
1
6
8
10
Time (10-6s) Figure 4. Magnetization, My(r),traces in E-7 at 250 K of (a) PaF, (b) p-PpQ, and (c) m-PpQ. The signal detected at early times (in absorption) is that of the precursor, 3ZnTPP. The slow signal buildup (absorption) in a and the two emissive signals in b and c are those of the RP. The continuous line is the result of best fit analysis, eqs 1 and 2 and ref 14.
with
where w1 is the microwave field and T1.2 are the relaxation times. The ET rate, ET, can be estimated from the rise time of the Rp signal, c-: ET = ET' dw, where dw is the inhomogeneous line width and ET' is the real ET rate. Thus, the rate obtained from these experiments is only a low limit (Table 1). PuF. TREPR spectra at three temperatures of photoexcited PuF, oriented in E-7, are shown in Figure 3. The spectra were taken at the parallel orientation, LI (B,where L is the LC director and B is the extemal magnetic field.'* The spectra exhibit two major features with respect to the time between the laser pulse and EPR detection. The first is the broad triplet spectrum of the ZnTPP moiety (3PaF) with a width of about 300 G, typical of ZnTPP triplets in the parallel orientation of the LC,697while the second feature is a narrow, -12 G (FWHM) line at g FZ 2.0 in complete absorption. This signal is long-lived (-50 ps
+
at 240 K) and does not change in time its shape in terms of width or phase. The kinetic traces in Figure 4a (Table 1) show the buildup of the porphyrin's triplet magnetization concurrently with the buildup of the narrow signal. The spectral and kinetic appearances were noticed in the temperature range of 220330 K.13 Similar spectra were detected when PaF was dissolved in ZLI-4389, a nematic LC whose parallel and perpendicular dielectric constants are more then twice as large as that of E-7. p-PpQ. Like that of PaF, photoexcitation of p-PpQ results in a superposition of two spectra that could be detected in the temperature range of 230-320 K. However, in this case the EPR spectra exhibit a completely different polarization pattem from that described above for PaF (Figure 5). The most noticeable change is the superposition of a four-line spectrum on the broad triplet of the porphyrin moiety, with an overall width of -90 G between the outer lines and -20 G between
-
J. Phys. Chem., Vol. 99, No. 19, 1995 7517
EPR Study of Intramolecular Electron Transfer 200 K
240 K
4440 ns
-
P
3080 ns
260 K
-
1720 ns
1040 ns
P
- IO0 ns
v
360 ns
2880 3213 3547 3880
2880 3213 3541 3880
MAGNETIC FIELD (Gauss) Figure 5. Diode-detection CW-EPR spectra of the triplet porphyrin, p3PpQ, and the triplet RP, p3[P+pQ'-],in E-7 as a function of to at three different temperatures. Experimental conditions are as in Figure 3.
-
the inner ones. This g 2.0 spectrum is clearly characterized by the emissiodabsorption (e/a) phase pattem of e,e,a,a, that changes above 270 K into a two-line spectrum, where the inner lines disappear, with an e,a phase pattem, and the spectral width remains unchanged. Above 280 K, the signal phase changes with respect to the time interval, ZD, between the laser pulse and the position on the magnetization curve, where the spectrum is accumulated; Le., the e,a pattem observed at low temperatures has changed into an a,e polarization retuming back to the e,a pattern at longer times. The same behavior was previously observed for cyclohexylene bridged porphyrin-quinone molecule (PcQ) and interpreted as the coexistence of singlet- and triplet-initiated ET routes,6 and will not be further discussed here. The rise time of the magnetization attributed to the narrow signal clearly correlates with the decay of P-~PPQ(Figure 4b and Table l), suggesting that at 250 K the narrow signal is a triplet-initiated RP. m-PpQ. TREPR spectra of photoexcited m-PpQ were found to be different from those of p-PpQ described above (Figure 6 ) . Over the entire temperature range of detection, only a twoline spectrum is superimposed on the broad triplet spectrum, m-3PpQ. It is -27-G wide and could be detected only in a narrow temperature range of 240-280 K. The signal phase change at high temperatures does not occur. The kinetic traces in Figure 4c exhibit the same temporal behavior as those obtained with the para isomer.
IV. Discussion The TREPR line-shape variations of photoexcited PaF, m-PpQ, and p-PpQ, as a function of temperature, are the result of solvent-mediated intramolecular ET. l4 Moreover, since an identical environment is assumed for all three dimers, the spectral differences must be due to skctural variations in terms of (1) donor-acceptor separation and/or relative orientation, (2)
acceptor characteristics in terms of redox potentials, and (3) the chemical nature of the bridging moiety. Such differences in molecular structure should result in different electron spinspin coupling (4and dipolar terms (D), thus routing the electron spin transitions into different mechanisms, which can be probed by the TREPR experiment. It is easy to distinguish between PaF as compared to m-PpQ and p-PpQ with respect to J. In the former, we expect J to be rather small because of the relatively large donor-acceptor separation and the exponential dependency of J on the separation. Moreover, this interaction is averaged out even to a smaller value due to the free rotation of the lumiflavin part. In the case of m-PpQ and p-PpQ, J is much larger for the same reasons, Le., because of a smaller separation and restricted rotation about the donor-acceptor axis. In the forthcoming discussion, two cases will be discussed. A. Weak Spin-Spin Coupling: PuF Case. Unlike the PcQ studied earliefi and the PpQ isomers described below, the narrow spectra shown in Figure 3 cannot be assigned to a polarized triplet of the RP, 3[P+aF'-],formed via the triplet precursor, 3PaF (Figure 1). This is because a narrow single-line spectrum that results in from a triplet RP would require fast molecular rotation to average out the zero-field splitting (ZFS) terms. This is an unlikely process to occur with the bulky PaF in the LC.15 Since a single absorptive EPR line is detected, the analysis of the signal origin is somewhat ambiguous. Two possible cases will be considered; the first involves a thermal EPR spectrum, while the other involves a spin-polarized one. The trivial interpretation of the TREPR spectra, shown in Figure 3, is that the single line spectrum is due to a superposition of two uncoupled free radicals on a single molecule, Le., P+aF'-, which are the final products in a thermal spin equilibrium. Using the similar g-factors and line widths of P+ and F, Le., 2.0025 and 7 G16 and 2.0032 and 11 G,17 respectively, the superimposed two signals result in a single
-
7518 J. Phys. Chem., Vol. 99, No. 19, I995
200 K
Hasharoni et al. 250 K
270 K
4440 ns
3080 ns
v
1720 ns
700-
3602883 3216 35503883
-
2883 3216 3550 3883
-
2883 3216 3550 3883
MAGNETIC FIELD (Gauss) Figure 6. Diode-detection CW-EPR spectra of photoexcited m-PpQ (3Pand 3RP) in E-7 as a function of Experimental conditions are as in Figure 3. undistorted Gaussian line shape (the separation of the resonant fields at the X-band is -1.2 G). Thus, within the present experimental setup, nothing can be said about any ESP mechanism. However, two experimental observations do not comply with such a thermal spin assignment. The first, is the very high signal-to-noise ratio of the TREPR single-line spectrum (Figure 3), and the second is the nonexisting correlation between the triplet magnetization decay rate and the buildup rate of the narrow signal (such a correlation is expected for the one-step ET: 3PaF P+aF'-). The other alternative involves ESP. Spin-polarized spectra are commonly typified by emission and/or absorption line-shape pattems. These spectral features are useful in identifying and analyzing the ESP mechanism. Evidently, a single EPR line in absorption makes the analysis more complicated. The results can be accounted for in terms of a fixed multicomponent molecular assembly. In such a case, spin polarization develops in one or more stages that correspond to consecutive ET processes.18J9 The first ET step generates a RP with nonzero magnetic interactions (exchange and/or dipolar), which is accompanied by S-TO mixing. A consecutive ET will form a new RP whose spin population depends on the interactions present during the lifetime of the f i s t RP. If the magnetic interactions are small during the lifetime of the second RP (i.e., J and D 0), spin polarization due to RPM will develop, characterized by opposite line phases of the two radicals. These requirements are probably fulfilled in PaF (-15 %, interradical separation), where both J and D are very small. From a point dipole approximation, D zz -8 G, and because of the exponential dependence of J with distance, J < D. From the spectra, it is evident that CRPM, typified by its characteristic symmetric derivative-like s p e ~ t r u mis, ~not ~ ~valid ~ here. The narrow (RPM-polarized) single-line EPR spectrum of photoexcited PaF can be attributed to either the cation radical
-
-
t~ at
three different temperatures.
of the porphyrin, P+, or to the anion radical of the lumiflavin, F-.We can differentiate between them on the basis of the signal width. Overlap of the two radical spectra, this time with opposite signs, results in a distorted derivative-like line shape, thus disputing the experimental observation, where only one radical is apparently being detected. On the other hand, the spectrum could be fitted with a single Gaussian line shape function with an 11-G width, in accordance with that reported for lumiflavin anion r a d i ~ a l . ' ~This . ~ ~ anion radical is known to exhibit a rich hyperfine EPR structure,L7which is lost when the flavin is attached to a protein to form flavoprotein, where a single line, very similar to that reported here, is observed.21The loss of hyperfine structure occurs when the thermal rotation rate of the radical is much smaller than the anisotropic interaction; i.e., TR > l/dw, where dw is the anisotropic hyperfine interaction and t~ is the rotational correlation time. The largest anisotropic interaction in lumiflavin is on the order of 20-25 M H Z ,from ~~ which ZR can be estimated to be at least IO-* s. While the anisotropic broadening in the flavoprotein was attributed to a tight binding of the flavin to the protein, it seems that in our case the reason for this behavior is the LC environment rather than binding of the flavin to the relatively small porphyrin. Indeed, LC media are known to induce a long t ~ supported , by the experimental results, where triplet EPR detection in fluid LCs is f e a ~ i b l e . ~ $ * ~ Two schemes can account for the slow buildup of the RP magnetization; the first involves the amide spacer as the intermediate site, Le.,
while the second involves the solvent
'PaF hv 3*PaF-,P'+aF
+ LC'-
-
P'+ar-
(4)
J. Phys. Chem., Vol. 99, No. 19, 1995 7519
EPR Study of Intramolecular Electron Transfer
i a.
-
X'
b.
X
L) Z
that this ET path differs from ordinary intramolecular ET encountered in PpQ and PcQ (see below and ref 6). B. Strong Spin Coupling: PpQ Case. The observed ESP associated with m-PpQ and p-PpQ (Figures 5 and 6) are quite different from that of PuF (Figure 3). The spectral line shape excludes a doublet state signal; i.e., they do not originate from RPM or CRPM. Moreover, in either case, the line separation should reflect the g-value difference between the porphyrin cation and the quinone anion, which is -3 G compared to the -20 G found in the spectra.24 Also, the width of the four lines are much broader then those of the corresponding radicals. The close similarity of the p-PpQ spectra to those of PcQ6 allows us to utilize the same interpretation in assignment of the spectrum; i.e., it is the triplet state spectrum of the RP (3RP, i.e., 3[P+pQ'-]). In order for such a mechanism to be operative, all EPR transitions must be within the three triplet sublevels, Le., the probability for IS) IT) mixing (S-TO and S-T-) ought to be negligible so that IS) will not acquire the necessary triplet character required for transitions between these four sublevels. The mixing probabilities are25
Y'
Figure 7. (a) Coordinate-axis system used to describe the orientation of the chromophores, relative to each other and to the LC director; (b) possible orientation of the molecules in the LC. The porphyrin and FW frames of reference are X,YJ and x',y',z', respectively. The amide path is unlikely due to its high redox potential with respect to ZnTPP. Thus, the multistep ET probably involves the LC solvent to form the first RP, [P+uF* *Lc'-],which transfers the electron to the lumiflavin at a slow rate (as expected for a bimolecular diffusion process in a viscous solvent). It is an open question why the spectrum of P+escapes detection, where, in mixtures of porphyrins and quinones in solutions16*24 or in the bacterial reaction center,l both radicals are observed. A possible explanation might be due to the inhomogeneous broadening imposed by the LC. Although the proposed ET process in PuF is not fully understood, it is evident
where D is the dipolar interaction strength of the 3RP, Q' and Q" are the mixing coefficients, which for S-TO mixing is a function of the g-value difference and hyperfine interaction between the two radicals and for S-T-1 mixing depends only on the hyperfine interaction. Evidently, large J values should reduce the singlet-triplet mixing, and transitions within the three triplet sublevels of 3[P+pQ'-]will dominate the EPR spectrum. Unlike the PuF case, at a separation of -10 8, between of P+ and Q-, J cannot be neglected. Indeed, a value of - 1370 G was calculated for J in the PcQ case.25 Using the point-dipole approximation, D is estimated to be -27 G, which is in line with the 45 G value, found from the spectra (an uncertainty of about 1.5 8, in the center-to-center separation is reasonable). The ESP of the 3RP evolves in a single ET step from the precursor state. This can be seen from the kinetic traces (Figure
a. X Y
z
Po)
Z' X'
Y'
lA-/
IY
'
b. T(P+*pQ-*), D < 0 , E#O
P-)
Figure 8. Energy level diagrams of the triplet sublevels and EPR transitions: (a) 3P; (b) 3Rp. Transitions are due to a triplet precursor.
7520 J. Phys. Chem., Vol. 99, No. 19, 1995
Hasharoni et al.
4b,c), which show that the decay of the magnetization of, e.g., p 3 P p Q (kd = 2.7 x lo6 s-l) correlates well with the rise of the 3RP magnetization (kr = 1.7 x lo6 s-’). Moreover, since the 3RP exhibits a polarized spectrum, it is reasonable to assume that the polarization transfer competes with the spin-lattice relaxation of the triplet, 3PpQ; i.e., 3T1-1 5 ET. This ESP mechanism is conceptually similar to CRPM since, in both cases, two electron spins are interacting via the exchange and dipolar terms, where the only distinction is in the value of J and the resulting EPR transition pattern. The triplet EPR spectrum of a chromophore embedded in a LC is a function of the molecular alignment with respect to the director, L, and magnetic field, In the experiments reported here, LllB and, because of the cylindrical distribution of the chromophores, the fluctuations of L about the direction of B can be neglected, making L and B collinear. The signal amplitude is a product of the magnetization along a molecular axis with the projection of that axis on L,637 and the sign of each transition is determined by the population difference between the two participating levels. Because the spectra at 130 K are practically the same as those of 3ZnTPP,69z7we deduce that the molecular alignment of 3PpQis dictated by the porphyrin moiety (X,Y,Z frame of reference in Figure 7a). It implies that L passes through the longest axis of the molecule. From previous line-shape analysisz7 the angle (4’ ) between L and one of the porphyrin in-plane axes is -45”; i.e., L passes through the phenyl group linked to the quinone. Due to the axial symmetry of the charge separated state, both x’ and y’ (x’,y’,z’ are the RP’s frame of reference) have equal probabilities to gain polarization from the out-of-plane axis, Z, which is selectively populated via triplet singlet SO-ISC, as shown schematically in Figure 7a,b. When BllY (or X), both x’ (or y ’ ) and z’ will assume polarization from the Z-axis, because of the deviation of the dipolar axis, z’, by the angle 0 from the porphyrin plane. This spin polarization is distributed between the high-field triplet energy levels through the combination of the zero-field wave functions.z8 In Figure 8 we show the populations of the different spin levels, the shaded circles represent the amount of polarization found in a spin level and their relative diameters are in the correct proportions. Although all of the triplet-EPR transitions can be deduced from this diagram, the relative intensities deviate from the observed ones. This is because the projection of the magnetization on L should also be considered. The procedure to calculate these projections is outlined in the Appendix. Thus, the observed 3RP transitions are the product of the 3PpQ 3[P+pQ‘-]polarization transfer (Figure 8b) and the projection of the magnetization on L (eq A3). The two limiting cases of the para and meta isomers will be discussed. From the separation of the inner (-20 G) and outer (-90G) lines in p-PpQ (Figure 5 ) , it is evident that the spectra do not comply with an axial symmetry as a priori expected, namely, D and E # 0. Assuming a nonaxial triplet spectrum, we find an E value of 8.3 G, and the separation between the two additional transitions is calculated to be -70 G, located on the inner side of the outer lines (easily observed on an extended field scale that is not shown). This is confirmed by the larger line width of the outer lines (-22 G) compared to the inner ones (-8 G). Using eq A3 for a = 0, the expressions for the components of the director are L,=O
-
-
L, = Pz sin 20 (7) Since the larger population difference is expected for the Bllz’
case (Figure 8b), we expect that the z’ lines will be more intense than the y’ (or x ’ ) lines, if 0 is sufficiently small. This is confirmed by the molecular modeling of p-PpQ (Figure 2). Since the x’ transition is easily detected, we can assume that a is slightly larger than zero and that the quinone deviates from the director, probably due to a stress induced by the LC. The small amplitude of this line can also be realized from Figure 8b. The two-line spectrum (-27 G separation) of the RP of m-PpQ, shown in Figure 6, cannot be assigned to the same origin as that of p-PpQ. If these two lines are due to the z’ transitions, it would imply a ZFS value of D 13.5 G. Such a value is not feasible due to the small distance between the electrons in 3RP, as compared to p-PpQ (9.3 vs 11.0 A between the centers of the P and Q, respectively). A plausible explanation is that the detected lines are those of the .t! transitions. This assignment is in line with the larger line separation expected for this isomer as compared with the p-PpQ case. Equation A3 in the other limit, i.e., a = 90”, results in the following set of equations:
-
L, = Pz sin 8 L, = Pz cos 8 sin 0
L, = p Zcosz 8 Due to sterical constraints, 0 becomes larger then 45” (supported by molecular modeling without, however, taking into account the strains imposed by the LC on this molecule). Such a molecular configuration leads to the conclusion that the x’ transition will dominate the spectrum. The y’ and z’ lines are expected to be very weak and probably escape EPR detection. The large value of a required for an observable spectrum is the result of the meta orientation imposed on the quinone. It implies that, similar to the p-PpQ case, the m-PpQ aligns itself in the LC exactly as ZnTPP. It is noteworthy that the ET in m-PpQ is a much less favorable process as compared to the p-PpQ presently studied and PcQ reported elsewhere.6 This is reflected by the very narrow temperature range of detection (-40 degrees) and the much smaller signal-to-noise ratio of the EPR spectra. Finally, it is gratifying to show how different DsA systems embedded in LCs exhibit conspicuous differences between their spectral features, thus demonstrating how ET is affected by modifying the spacer as well as by the donor-acceptor mutual orientation. Acknowledgment. We are grateful to Dr. J. R. Miller for his valuable comments. This work was supported by a USIsrael BSF grant (H.L.) and by the Deutsche Forschungsgemeinschaft (Grant SFB 337) and Fonds der Chemischen Industrie (H.K.). A special grant of the Erna and Victor Hasselblad Foundation (H.L.) is gratefully acknowledged. The Farkas Research Center is supported by the Minerva Gesellschaft f i r die Forschung, GmbH, Miinchen, FRG. This work is in partial fulfillment of the requirements for a Ph.D. degree (K.H.) at the Hebrew University of Jerusalem. Appendix: Expressions Derived for the Polarization Transfer To describe triplet polarization transfer during ET in a LC, we have to consider that such a polarization transfer to the RP is not the only criterion governing the EPR spectra. Triplet spectra are detected when the magnetic field is parallel to one of the molecular principal axes, while there is a projection of the RP axis system on L. In order to quantify these requirements, two successive transformations are carried out. The first
EPR Study of Intramolecular Electron Transfer
J. Phys. Chem., Vol. 99, No. 19, 1995 7521
transformation, produces the polarization transferred from the 3ZnTPP frame of reference (P: X,Y,Z) to the Rp frame of reference (p’: x’,y’,z‘ ). This is carried out by a rotation of the coordinate system: p‘ = AP, where A is an Euler rotation matrix with = 0: sin 4
8 is the angle that the dipolar axis (z’ ) makes with the porphyrin X Y plane, and 4 is the angle of its X Y projection with one of the molecular in-plane axes (Figure 7a). Because of the highly selective, out-of-plane spin polarization in 3ZnTPP,26only P , should be considered giving rise to the following expressions for p’:
p;=o
p i = Pz sin 8 p; = pZcos e
(‘42)
As explained in the text, the x’ and y‘ components of p’ are interchangeable. The second transformation required is the one from the RF’ frame of reference to that of L, Le., back to the porphyrin molecular axis system (because this is the system where the director is defined). From Figure 7a it is seen that the relevant angles in this case are a, which is the angle between X Y projection of z’ and L, and 8, which is the same as that discussed above. The transformation is L = A‘p‘ where A‘ is another Euler matrix, where 4 has been replaced by a. The final expressions for the polarization component on L are
Lx = Pz sin 8 sin a L, = P,[sin 8 cos 8 cos a
L, = P,[COS’
+ cos 8 sin 81
e - sin’ e cos a]
(‘43)
References and Notes (1) Snyder, S. W.; Thumauer, M. C. In The Photosynthetic Reaction Center; Noms, J. R., Deisenhofer, J., Eds.; Academic Press: New York, 1993; Vol. II, pp 285-330.
(2) Salikhov, K. M.; Molin, Y. N.; Segdeev, R. 2.;Buchachenko, A. L. Spin Polarization and Magnetic Effects in Radical Reactions; Elsevier: Amsterdam, 1984. (3) Closs, G. L.; Forbes, M. D. E.; Noms, J. R. J. Phys. Chem. 1987, 91, 3592. (4) Atkins, R. W.; Evans, G. T. Mol. Phys. 1974, 27, 1633. (5) Lendzian, F.; von Maltzan, B. Chem. Phys. Lett. 1991, 180, 191. (6) Hasharoni, K.; Levanon, H.; von Gersdorff, J.; Kurreck, H.; Mobius, K. J. Chem. Phys. 1993, 98, 2916. (7) Regev, A.; Galili, T.; Levanon, H. J . Chem. Phys. 1991,95,7907. (8) Schubert, H.; Kurreck, H. Manuscript in preparation. R1: = C5Hll (9) E-7 is an eutectic mixture of R ~ - C ~ H ~ - C ~ S - C N (51%); R2 = C7His (25%); R3 = C8Hi70 (16%); R4 = C ~ H I ~ C (8%). ~HS The chemical composition of ZLI-4389 is not available. (10) The decay rates of the TREPR transient kinetics, in fluid LC matrices, do not show any noticeable dependence on the microwave power. The nonoscillatory behavior of the magnetization is indicative of an overdamping condition: wIzT1T2
