J. Phys. Chem. 1985,89, 1637-1643
1637
Time-Resolved EPR Spectroscopy of Electron Spin Polarized ZnTPP Triplets Oriented in a Liquid Crystal O d d Gonen' and Haim Levanon** Department of Physical Chemistry and the Fritz Haber Research Center for Molecular Dynamics,' The University, Jerusalem 91 904, Israel, and the Radiation L a b ~ r a t o r yUniversity ,~ of Notre Dame, Notre Dame, Indiana 46556 (Received: October 8, 1984)
A study of laser excitationdiode detection of the transient EPR triplet state of ZnTPP, in a uniaxial liquid crystal and a glass matrix at 100 K, is reported. This time-resolved technique enables us to monitor electron spin polarization in ZnTPP, which has thus far only been observed in spin thermalized states. Analysis of the transient kinetics gives values of -3 X lo6 s-l for the spin-lattice relaxation rates. Triplet EPR line shape analysis shows that (a) this metalloporphyrin, embedded in both matrices, deviates from axial symmetry and D = +0.0298, [El = 0.0098 cm-'; (b) the order parameters in the liquid crystal are evaluated to be 0.89 for the out-of-plane and 0.56 for the in-plane directions, respectively; (c) the out-of-plane direction is the active mode for the S T transition and the relative population rates of the triplet sublevels, A,, are 1:O:O for Az:Ax:Ay,respectively; (d) the sign of D can be determined.
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Introduction Light modulation (LM) EPR spectroscopy applied to porphyrins and biologically related systems provides substantial information on the spectra and spin dynamics of the triplet state.4 Consequently, valuable information can be extracted in these systems, on the structure, configuration, and photochemical and photophysical processes. Nevertheless, broad application of this method to primary photochemical events is limited by instrumental parameters to an overall time constant above 100 ps for EPR detection. The recently introduced time-resolved triplet EPR spectroscopy by pulsed laser excitation followed by transient magnetization detectionSShas been established and extended from singlet crystals to glass matrices.'-1° This technique significantly reduces the detection time constant to the submicrosecond region and enables one to directly monitor the development and fate of the magnetization formed by the light pulse excitation. It is evident that the major application of these novel methods are to those systems in which the transient behavior, as reflected by electron spin polarization (ESP), is below the time scale of the L M method. The porphyrin moiety to a large extent, both in vivo and in vitro, exhibit polarized photoexcited triplet spectra resulting from seT) paths between the correlective intersystem crossing (S sponding manifolds: There are, however, several systems whose triplet spectra reveal no ESP under conventional L M detection. The most conspicuous are the metalloporphyrins ZnTPP and MgTPP which in all previous reported EPR studies reveal complete spin thermalization." Moreover, the triplet spectra, in glass
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(1) In partial fulfillment of the requirements for a Ph.D. Degree at the Hebrew University of Jerusalem. (2) Permanent address: The Hebrew University of Jerusalem. (3) The research described herein was conducted at the Hebrew University and supported by the Israel Academy of Sciences and Humanities and the US.-Israel BSF, Grant No. 3443. The Fritz Haber Research Center is supported by the Minerva Gesellschaft fiir die Wrschung, GmbH, Miinchen, BRD. The Radiation Laboratory is supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2667. (4) For reviews on magnetic resonance in photosynthesis see: (a) Thurnauer, M.C. Reo. Chem. Intermed. 1979.3, 197. (b) Levanon, H.; Norris, J. R. 'Molecular Biology and Biophysics", Fong, Francis K., Ed.; SpringerVerlag: Berlin, 1982; Vol. 35, pp 153-195. (5) (a) Kim, S. S.; Weissman, S . I. Reu. Chem. Intermed. 1979, 3, 107. (b) For a review on transient methods in EPR spectroscopy see: Weissman, S. I. Annu. Rev. Chem. 1982, 33, 301. (6) Furrer, R.; Fujara, F.; Lange, C.; Stehlik, D.; Viethe, H. M.; Wollmann, W. Chem. Phys. Lett. 1980, 75, 332. (7) Mural, H.; Imamura T.; Obi, K. J . Phys. Chem. 1982, 86, 3279. (8) Mural, H.; Imamura, T.; Obi, K. Chem. Phys. Lett. 1982, 87, 295. (9) Terazima, M.; Yamauchi, S.; Hirota, N. Chem. Phys. Lett. 1983, 98. (10) Murai, H.; Hayashi, T.; THara, Y. J. Chem. Phys. Lett. 1984, 106, 139. (1 1) Scherz, A.; Levanon, H. J. Phys. Chem. 1980,84,324, and references therein.
matrim and in uniaxial liquid crystals (LC), suggest that the axial symmetry of these porphyrins has been removed. This is reflected by the nonvanishing IEI term in the spin Hamiltonian with the apparent relation ID1 = 31EI. The utilization of LC as host matrices for large molecules facilitates the triplet EPR spectral analysis due to the partial orientation of the guest solute in the host matrix. This has been demonstrated on several porphyrins and chlorophyll systems partially oriented in uniaxial and discotic L C employing L M EPR.l2-I4 In uniaxial LC this partial orientation imposes a cylindrical distribution of the porphyrin about the director L having the following _effFts on the EPR spectrum14 (cf. also Figure 1): (a) When &llB, since the ZnTPP planes are distributed uniaxially about L, $ere is a vanishing probability for our plane axis to be parallel to B, leading to the total exclusion of the Z lines from the s p e c p ? . (b) When L I B (obtained by-rotating the frozem sample by 1712 in an axis perpendicular to B), there is a finite nonvanishing probability ,of finding any one of the three canonical orientations parallel to B. The overall effect is that the triplet EPR spectra benefit from such organization in improved S / N ratio and suppression or enhancement of various lines in the spectrum due to alterations of the probability of certain molecular axis to be parallel to B. To get a closer insight into the points mentioned, we have studied the triplet EPR line shape and spin dynamics of ZnTPP at a high temperature (100 K) in an isotropic (glass) and oriented (liquid crystal) media. This was accomplished by laser excitation and diode detection time-resolved EPR spectroscopy. As expected, a highly polarized transient triplet EPR spectrum was detected. The time domain in which the spectra are observed is within the spin relaxation time and the highly anisotropic line shape in the liquid-crystalline phase confirms the nonaxial symmetry of ZnTPP in the media studied here. Line shape simulation enables extraction of the spatial distribution of ZnTPP in the LC as well as evaluation of the sublevel spin dynamics.
Experimental Section ZnTPP (chlorin free) from Midcentury Chemical Co., liquid from BDH, and toluene (AR) and ethanol (AR) crystal (E-7)I5 from Baker Analyzed were all used without further purification. Glass samples of ZnTPP ( M) dissolved in a 50% mixture of toluene-ethanol were prepared in Pyrex tubes (4 mm o.d.), degassed by freeze-vacuum-thaw cycles, and sealed under vacN
(12) Grebel, V.; Levanon, H. Chem. Phys. Lett. 1980, 71, 218. (13) Gonen, 0.;Levanon, H. J . Chem. Phys. 1983, 78, 2214. (14) Gonen, 0.;Levanon, H. J. Phys. Chem. 1984, 88, 4223. (15) E-7 (BDH) is an eutectic mixture of RrPh-Ph-CN; R I = C5H,, (51%); R2 = C7H15 (25%); R, = CaH170 (16%); R4 CSHllPh (8%).
0022-3654/85/2089-1637$01.50/00 1985 American Chemical Society
1638 The Journal of Physical Chemistry, Vol. 89, No, 9, 1985
a)
Gonen and Levanon
b)
t’
A
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Figure 1. (a) Schematic representation of cylindrical distribution of porphyrin dimers about the director (parallel configuration); structural organization is face to face with perpendicular direction of one subunit relative to the other. X,Y,Z and x,y,z are molecular (on one subunit) and laboratory coordinate systems, respectivzly. (b) Same as in (a) except that the sample is rotated (in the frozen _state) by x = r / 2 , here the cylindrical distribution is perpendicular to B (perpendicular configuration). In this case, any axis may be parallel to B. (c) Distribution of the porphyrin plane about L due to rotational fluctuations about the Z axis (prior to sample freezing). This distribution is characterized by eq 11. (d) Orientation of the external magnetic field Bin the molecular (ZES) frame ( X , Y , Z ) . (e) Result of sample rotation by an Sngle x (see (b) above) and fluctuations of the porphyrin plane about L by an angle 8’ (prior to freezing) on the minimal angle B,Z can make with b. This distribution is characterized by eq 12.
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uum. The liquid crystal sample ( M) was prepared in a similar tube by evaporating the toluene solvent (leaving the ZnTPP powder on the tube walls) and then introducing the E-7, degassing, and sealing as described above. Measurements were carried out on a Varian E- 12 EPR spectrometer with field modulation disconnected. The LC sample is aligned in the cavity by warming in a nitrogen flow system to above the clearing point (338 K) and cooling in a strong magnetic field (12 kG) into the nematic range (338-263 K). After -10 min a t room temperature the sample is cooled fast (- 10 deg/min) to the required temperature (100 K). The sample in the microwave cavity was photoexciied by the second harmonic of a Nd:YAG laser (Quanta Ray, DCR-1 A) producing 18-25-mJ pulses at 532 nm (of which 50% passed through the cavity grid), 10 ns width, at a repetition rate of 10 Hz. This excitation corresponds to the ZnTPP Q-band optical transition. The EPR signal is taken from the preamplifier connected directly to the X-band microwave diode detector with care taken to avoid any intereference from the 100-kHz filters. This signal is then fed into an automatic back-off amplifier (50-MHz bandwidth, 12 dB gain) triggered prior to the laser pulse, ensuring the transient signal is on a dc level. The output of the back-off system is connected to a Nicolet Explorer 111-A digital oscilloscope interfaced to a Data General Nova- 1200 microcomputer on line with the experiment. To avert long lines and pickup, the detection system is triggered by a fast photodiode (rise time < 50 ns). The individual digital kinetic trace (1024 points at 50 ns/point) obtained after each laser pulse is transferred to the computer and processes in real time, within the interval between two successive laser pulses. Spectral and kinetics measurements are performed. ( a ) Spectrum. As was pointed out in earlier s t ~ d i e d , ~the *~J~ triplet EPR line shape can be determined from the amplitudes of the transient intensities. The time-resolved spectra (Figures 2 and 3) are taken in real time at 0.5-ps intervals after the laser pulse. The spectral amplitudes a t a given magnetic field are obtained in the following manner:
(1) To improve the S / N ratio the computer accumulates time profiles from 40 laser pulses to represent a field point. Since the magnetic field is scanned at 1 G/s and the laser repetition rate is 10 H z then, for the 1000-G field scanned at 960 s, the 4 s of averaging per point implies a field resolution of -4 G. (2) After the 40 time profiles were summed the computer averages the intensity at four adjacent time points at the beginning ~ along that kinetic trace. Each such average of every 0 5 interval represents one amplitude in the time-dependent spectra. This results in improvement of the S / N ratio and the time resolution in the spectra is reduced to flOO ns. (3) To further reduce dc noise (mainly laser pickup), the computer subtracts the amplitudes at the first (off-resonance) field from the corresponding ones at every successive field location. ( b ) Kinetics. After a spectrum is acquired, the 240 points are plotted by the comptuer on the EPR instrument console for field reference. The field is set manually to one of the canonical fields and the kinetic profile there is accumulated by the computer until a reasonable S/N is achieved. The final result is correctly normalized to show relative amplitudes. To reject the laser pickup the same number of accumulations are performed at an off-resonance field and subtracted from the on-resonance signal. The spectral and kinetic profiles are stored on a floppy disk during the experiment and transferred off line to a VAX 11/750 for any further processing and graphics.
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Theory The EPR signal is a measure of the transverse magnetization M,(t) induced by the microwave radition, and its time dependence for the case where the inhomogeneous broadening exceeds y B , = w , is given byss6 M,(t) = j =-_ g ( w o ) w , [ w 1 2+
(0
- wo)2]-1/2 x
e-rlTsin {[wlz + ( w - w 0 ) 2 ] 1 / 2duo t ] (1) where g(wo) is the shape function, T i s the phase memory time
The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1639
EPR of ZnTPP in Liquid Crystals
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Magnetic Field (Gauss) Figure 2. Diode-detected transient EPR triplet absorption spectra of ZnTPP in a liquid crystalline (E-7) matrix at parallel (left) and perpendicular (right) configurations at 100 K and microwave power of 50 mW at different times after the laser pulse (indicated on each trace). The traces in each frame are in their true relative amplitude. The smooth curve suDerimwsed on the spectrum at 0.1 ps is a simulation of eq 14 with the parameters given in the text and 1/ T2of 12 6. -
1
which is equal in this case to On resonance, for a weak microwave field (wI
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Magnetic Field (Gauss) Figure 4. Computer derivatives of the diode-detected absorption spectra in glass and LC matrices parallel and perpendicular orientations (left). Also presented (right) are LM first harmonic representation (dx"/dB) of ZnTPP in E-7; the light modulation frequency is 7 Hz and the microwave power
is 5 mW. TABLE I: First-Order Decay Rates of M J t ) at Different Power Levels ( B , ) and l/Tl at the Canonical Fields ( X 1 0 4 d)" 50 mW 10mW 2 mW 0.5 mW 0.12 G 0.06 G 0.03 G 0.015 G 2.2 1.4 0.9 0.7 Z,(glass) 1.8 1.1 0.6 0.5 21 (LC) 2.1 1.4 0.8 yz (LC) Xl (LC) 2.4 1.4 XI (LC) 2.2 1.5 0.6 y, (LC) 2 2
(LC)
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This is the advantage of the L C matrix; as in ZnTPP ID1 31EJ the intense Z line overlaps any trace of Y. In the perpendicular orientation different spectra are obtained, where the Z components are substantially enhanced as compared to the parallel configurations or that of the glass sample (compare S/Nratios in Figures 2 and 3); the polarization patterns are the same as in the glass. The first spectrum shown in Figure 2 (LC) was treated quantitatively via eq 14 by line shape analysis?' Due to long-lived triplet vs. fast spin relaxation time, (9) can be substituted into (3) and in a cylindrical distribution M(Q) = dB d4. The parameters required to obtain the line shape are the zfs ID1 and 14, the line width 1/T2, the three population parameters 9, and the distribution parameters uv,u4,and C#@ The molecular parameters obtained from the fit of eq 14 to the experimental results are 0.0298 and 0.0098 & 0.0005 cm-' for ID1 and IEI, respectively, in agreement with results obtained previously" for ZnTPP employing L M EPR. The population rates, by substituting the obtained apinto (8), yield the ratio 1:-0-0 for AZ=Ax:AZ,in accordance with the results obtained for zinc porphinez4 and Zn-substituted c h l o r ~ p h y l l where , ~ ~ the active spin state is the out-of-plane Z component. The distribution parameters obtained by the simulation are 12' and 25' for uv and u4, respectively, and 45O for 40. Substituting these values into eq 13 yields the (23) The analysis is carried out on the earlicst spectrum only as in later times the only change is an exponential decrease in signal intensity (eq 9). (24) Chan, I. Y.;van Dorp, G.; Schaafsma, T. J.; van der Waals, J. H. Mol. Phys. 1971, 22, 741. (25) Clarke, R. H.; Connors, R. E.; Schaafsma, T. J.; Kleibeuker, J. F.; Plantenkamp, R. J. J . Am. Chem. SOC.1976, 98, 3674.
order parameters S, = 0.89, SI,= 0.56. The trace resulting from the simulation is given in Figure 2 for both the parallel and perpendicular configurations where all parameters are common to both orientation^.'^ So that the results obtained in the time-resolved method could be compared with the slower LM technique, the parallel and perpendicular derivative spectra, via the latter method, were recorded and are presented in Figure 4. Also presented in this figure are the computer derivatives of the absorption spectra in Figures 2 and 3. There is a striking difference &tween the line shapes obtained by the different methods. The line between the X and Y canonical orientations in the L M spectrum is apparently missing in the time-resolved experiment. The occurrence of this component has been previously" accounted for in terms of aggregation and intermolecular energy transfer of the type PTP ~t PPT, where P is a monomeric species having ID1 31EI. This treatment assumes a dimeric structure as shown schematically in Figure 1 consisting of two monomers organized face to face in perpendicular directions. Upon exchange, the X and Y axes of the interacting monomers are interchanged by a solid line ?r/2 jump.26 The absence of this line in the time-resolved spectrum can be explained in terms of the exchange model combined with the results of the population rates obtained above. Since the Z component is overpopulated, the X and Y states at the triplet birth have very little population to exchange. This will result in a very intense line in the Z canonical orientation as indeed observed in both glass and LC matrices. In the X and Y orientations, however, the high-field approximation (gj3B >> D) causes an admixture of the Z spin state into the upper and iower levels and consequently the appropriate transitions (X and Y) are also observed but are of lower intensity (Figures 2 and 3). Moreover, spin relaxation and energy transfer equalize the X and Y populations, thus 'washing out" the transient spectrum. In the L M experiment, however, the spin system is thermalized and steady-state conditions are approached, allowing for observation of the exchange process (Figure 4). The distribution parameters are also in line with the interacting dimer model as the value of 45O for dois conceivable considering the structure of such a pair (Figure IC) where the most probable alignment is the intersecting line between the two subunits. Moreover, the relative high value of ug vs. ug.reflects a higher degree of rotational freedom compared to the longitudinal
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(26) Baram, A.; Luz, 2.;Alexander, S. J. Chem. Phys. 1916, 64, 4321.
Gonen and Levanon
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The resulting simulated curve due to the isotropically distributed triplets is shown in Figure 3 on the first trace.23 The imperfect fit can be accounted for in terms of anisotropic spin relaxation rates, a factor not taken quantitatively in the present study. A good fit of the glass spectrum could also be obtained with different dynamic parameters, namely, AY > AZ >> Ax,where the zero field eigenvalues are ordered Z > X > Y,implying a negative D value. Nevertheless, this possibility is ruled out because (a) such parameters do not fit either one of the LC spectra and (b) it does not seem likely that the population rates will be affected to such an extent on changing the host matrix. The fact that spectra in both matrices could be simulated with the same dynamic parameters, namely, AZ = 1, Ax = AY = 0, implies that the zero field eigenvalues are ordered Y > X > Z for which D > 0. As to the sign of the E term it cannot be, at this stage, determined unambiguously from the time-resolved spectra as the order of X and Y is unspecified. Several conclusions can be drawn from the spectral behavior and the discussions above. (1) The directions of polarization from low to high field are a, a, a, e, e, e; where a and e are enhanced absorption and emission lines, respectively. (2) The relative intensities of the line, particularly in the LC, associated with the canonical orientations provide a measure of the relative population rates from the singlet into the triple sublevels and subsequently determination of the sign of D. (3) The absence of an exchange line in the time-resolved spectra, and the relatively intense Z line, supports the X-Y interaction model against one involving Z . Kinetics. Figure 5 displays the time dependence of the EPR signal at the Z canonical orientation of the glass sample as a function of the microwave power. Due to the line shape it is not possible to monitor this dependence at other canonical orientations. In the LC sample (Figure 6 ) this problem is overcome as the Y orientations can be unveiled by the total suppression of the Z component and the X lines are intensified due to the general improvement of the S/N ratio. The time dependence of the magnetization at the canonical fields is treated quantitatively in
Time ( b s ) Figure 5. Kinetic traces of M,(t) for the out-of-planeZ orientations (cf. Figure 3) of ZnTPP in a glass matrix at different microwave powers. The power is indicated on each trace and all traces are shown in their true relative intensity. Each trace is the result of 1020 computer accumulations. The smooth traces superimposed on the experimental curves are simulation of eq 15 with the parameters given in Table I.
'wobble" (Figure 1) as to be expected from the nematic structure of the medium. The molecular parameters obtained from the L C spectral analysis were substituted into eq 3 where M(8,b) = sin 8 d8 db.
h
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Time ( ~ s ) Figure 6. Kinetic traces (M,(t)) at the three canonical orientations (cf. Figure 2) of ZnTPP in E-7 at different microwave powers. The power is indicated on each trace and all traces are shown in their true relative intensity. Each trace is the result of 1020 computer accumulations. The smooth traces superimposed on the experimental curves are simulations of eq 15 with the parameters given in Table I .
J. Phys. Chem. 1985,89, 1643-1646 terms of eq 1 On resonance, for lines where the inhomogeneous broadening is larger than wl, eq 1 is simplified and becomes5p6 .5*6327
M,(t) = -rMz(0)e-f/Tisin w l t (1 5) Three possible cases arise from (15): (a) at high microwave power
(wl >> l/Tl), the magnetization will display damped oscillations; (b) at low power (wl
