3041
J. Phys. Chem. 1992, 96, 3041-3047
Photophysics and Photoinduced-Electron-Transfer Reactions of Zinc and Free-Base Octaethy lporphycenes Assia Berman,+Albert Michaeli; Jehuda Feitelson; Michael K. Bowman,*James R. Norris,*.g Haim Levanon,**+** Emanuel Vogel,ll and Peter Koch" Department of Physical Chemistry and The Farkas Center f o r Light- Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel, Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, and Institute of Organic Chemistry, The University of Koln, D-5000 Koln 41, FRG (Received: August 22, 1991; In Final Form: December 1 1 , 1991)
The photoexcited triplet states of free-base and zinc octaethylporphycenes (H20EPCand ZnOEPC, respectively) were investigated by laser photolysis in the liquid phase and by time-resolved CW-EPR spectroscopy in partially oriented liquid crystal matrices. The triplet and fluorescence yields of H20EPC are temperature dependent, and at room temperature they are lower by an order of magnitude than those of ZnOEPC. This temperature dependence is interpreted in terms of a thermally activated excited state in the singlet manifold, -2.5 kcal mol-' above SI.Frozen solutions of both porphycenes exhibit strong triplet EPR spectra, from which the magnetic and spin dynamics could be extracted. Electron transfer from the triplet state of H20EPCTand ZnOEPCTto duroquinone (DQ) was studied by the Fourier transform EPR method. The two triplet porphycenes transfer an electron to produce a spin-polarized anion radical DQ'- with a dominating radical-pair mechanism and a negligible contribution of triplet mechanism, due to mixing of in-plane and out-of-plane active spin states.
I. Introduction To date, many of the photosynthetic model systems have been based on porphyrins and metalloporphyrins as photodonors in electron-transfer reactions. This is because they are easier to make and more stable than the naturally occurring chlorophyll and related biological compounds. The intensive studies of primary photc~ynthesis,~-~ and the search for novel photodynamic agents?J have led in recent years to increased attention focused on the synthesis of a variety of porphyrinoid systems related to these fields of research.6-'0 Their vast potential for modeling studies and practical applications initiated the design and synthesis of novel porphyrinoid systems such as the sapphyrins of Woodward et a1.,6 the expanded porphyrins of Gosmann and Frank,' the porphycenes of Vogel et al.,8!9 and the texaphyrins of Sessler et a1.I0 these As stated in previous reports from our novel porphyrinoids exhibit different spectroscopic behavior as compared to that of conventional porphyrins, Le., long-wavelength absorption in the Q-band region, as well as unique dimerization properties in high-dielectric media. These properties are important in practical applications of photodynamic therapy as well as in the understanding of the interactions responsible for stabilizing the structure of the special pair in photosynthesis. In the present study we concentrate on the photophysics and photochemistry of two substituted porphycenes, namely, zinc and free-base 2,3,6,7,12,13,16,17-octaethylporphycene,ZnOEPC and H20EPC (Figure l).I4 The porphycenes, which are structural isomers of porphyrins, formally derived from the [ 18lannulene of C, symmetry or, alternatively, from the [ 16lannulene dianion of D2h.8As indicated previously, the differences in structure and symmetry between porphycenes and porphyrins are responsible for the changes of the photophysical and photochemical properties of the former porphyrinoids as compared to the latter o n e ~ . ~ J ' J ~ In order to evaluate the possible applications of porphycenes, in either photodynamic therapy or model photosynthesis, it is necessary first to establish quantitatively the photophysical properties, i.e., the processes these compounds undergo following their electronic excitation. Here we address ourselves to the primary photophysical and subsequent photochemical reactions of the excited state of ZnOEPC and HzOEPC in glassy matrices and in solution, which include the following processes 'The Hebrew University of Jerusalem. Argonne National Laboratory. 'The University of Chicago. II The University of Koln.
*
-+ + P
PT
Q
hu
P*
k,
ISC
PT
[PPI*+ Q,I'-]
[P,I'+ + Qpi'-I
PPI*+
km
'/TI
P,I'+ + Qpi*-
(1)
(2) (3)
Pq*+
(4a)
Qq*-
(4b)
1/TI
Q~I*-
+
+
P*+ Q*P Q (5) where P and Q stand for the porphycene chromophore and quinone, respectively. The square brackets indicate the photochemical cage, where the electron spins are polarized (pol); l / T l is the inverse spin-lattice relaxation time; k,, and kb are the rate constants for the forward and back electron-transfer reactions, respectively; and kdiffis the rate for the escape from the cage. ( I ) (a) Wasielewski, M. R. Photochem. Photobiol. 1988, 47, 923. (b) Wasielewski, M. R. In Metal Ions In Biological Systems; Sigel, H., Ed.; Marcel Dekker: New York, Vol. 27, in press. (2) Gust, D.; Moore, T. A. Science 1989, 244, 35. (3) Hoff, A. J., Ed. Advanced EPR. Applications in Biology and Biochemistry; Elsevier: Amsterdam, 1989. (4) (a) Schennann, G.; Volcker, A.; Seikel, K.; Schmidt, R.; Brauer, H.-D.; Montforts, F.-P. Photochem. Photobiol. 1990, 51, 45. (b) Schermann, G.; Schmidt, R.; Volcker, A,; Brauer, H.-D.; Mertes, H.; Franck, B. Photochem. Photobiol. 1990. 52. 741. (5) Pandey, R.K.;Bellnier, D. A.; Smith, K. M.; Dougherty, T. J. Photochem. Photobiol. 1991, 53, 65. (6) Bauer, V. J.; Clive, D. L. J.; Dolphin, D.; Paine 111, J. B.; Harris, F. L.; Kina. M. M.; Loder, J.; Chien Wan& - S.-W.; Woodward, R. B. J . A m . Chem. soc. 1983, 105, 6429. (7) Gosmann, M.; Franck, B. Angew. Chem., Int. Ed. Engl. 1986,25, 1100. ( 8 ) (a) Vogel, E.; Kiicher, M.; Schmickler, H.; Lex, J. Angew. Chem., In!. Ed. Engl. 1986, 25, 257. (b) Vogel, E.; Kiicher, M.; Lex, J.; Ermer, 0. Isr. J. Chem. 1989, 29, 257. (9) (a) Vogel, E.; Haas, W.; Knipp, B.; Lex, J.; Schmickler, H. Angew. Chem., Int. Ed. Engl. 1988, 27,406. (b) Vogel, E.; Sicken, M.; Rohring, P.; Lex, J.; Schmickler, H.; Ermer, 0. Ibid. 1988, 27, 411. (10) (a) Sessler, J. L.; Murai, T.; Lynch, V.; Cyr, M. J . A m . Chem. Soc. 1988, 110, 5586. (b) Sessler, J. L.; Murai, T.; Lynch, V. Inorg. Chem. 1989, 28, 141. (11) Ofir, H.; Regev, A.; Levanon, H.; Vogel, E.; Kkher, M.; Balci, M. J. Phys. Chem. 1987, 91, 2686. (12) Levanon, H.; Toporowicz, M.; Ofir, H.; Fessenden, R. W.; Das, P. K. J. Phys. Chem. 1988, 92, 2429. (13) (a) Regev, A,; Berman, A,; Levanon, H.; Murai, T.; Sessler, J. L. Chem. Phys. Lett. 1989,160,401. (b) Regev, A,; Micheli, S.; Levanon, H.; Cyr, M.; Sessler, J. L. J. Phys. Chem., in press. (14) Koch, P. Dissertation, University of Koln, 1990; to be published.
0022-3654/92/2096-3041$03.00/00 1992 American Chemical Society
Berman et al.
3042 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 Y
\ /
I
1 I' \
L
,' n
0 m w
M
\N\
I
X
,N& Q
% M= Zn, 2H Figure 1. Schematic structures of ZnOEPC and H20EPC. The indicated X and Yaxes are the ZFS tensor components fixed in the molecular plane. The angle a0is the angle between the X axis and the liquid crystal director, L. It indicates the most probable angle for the in-plane molecular orientation about the Z axis.
The participation of paramagnetic species in the set of reactions 1-4 makes this system suitable for a combined study employing optical and EPR spectroscopies. As will be shown below, all four processes differ in their details from those found for the tetraphenylporphyrin (H,TPP) and its Zn complex (ZnTPP).I5-I7 11. Experimental Section Zinc and free-base octaethylporphycenes were prepared and purified as described by Vogel et al.;I4 duroquinone (DQ) and anthraquinone (AQ) (Aldrich Chemical Co.) were further purified by recrystallization from petroleum ether and acetic acid, respectively. Benzoquinone (BQ) (Fluka AG) was purified by sublimation. Samples were prepared under dark conditions, and all experiments were carried out immediately after sample preparation. The liquid crystal, E-7I8 (BDH), and spectroscopic grade ethanol (Merck Analyzed) were used without further purification. Steady-state absorption measurements were carried out on a HP Vectra ES diode array spectrophotometer. a. Laser Photolysis. The steady-state fluorescence of ZnOEPC and H,OEPC in ethanol (1 X 10" M) was measured on a SLM 4800 fluorimeter, and their lifetimes were determined on a home-built upgraded single-photon counting apparatus.Ig Triplet-triplet absorption was recorded by exciting M solutions with a N,-pumped dye laser (Molectron) a t 580 nm and measuring the light absorption via a Jarrell-Ash 25-cm monochromator using a Hamamatsu R-928 photomultiplier. The signal was then channeled into a digitizer (Tektronix 2430A) followed by on-line processing using a microcomputer (Olivetti 24M). Experimental data were analyzed in terms of their decay kinetics. All solutions (I-cm optical path) were deoxygenated by a freeze-pumpthaw procedure. Measurements of ZnOEPC were carried out at room temperature (298 K), while H,OEPC was studied overe a temperature range between 262 and 303 K. Temperature was controlled by circulating an ethylene glycolwater mixture into the sample holder, at the desired temperature. To check for any permanent changes in the sample absorption, optical spectra were taken before and after the photolysis experiment. b. Time-Resolved EPR. EPR experiments were carried out in two modes utilizing selective laser excitation, namely, CW direct detection and Fourier transform (FT) pulsed EPR. Samples of (15) (a) Gonen, 0.; Levanon, H. J . Phys. Chem. 1985, 89, 1637. (b) Gonen, 0.;Levanon, H. J . Chem. Phys. 1986,84, 4132. (16) (a) Prisner, T.;Dobbert, 0.; Dinse, K. P.; van Willigen, H. J . Am. Chem. SOC.1988, 110, 1622. (b) Pliischau, M.; Zahl, A,; Dinse, K. P.; van Willigen, H. J . Chem. Phys. 1989, 90, 3153. (17) (a) Angerhofer, A.; Toporowicz, M.; Bowman, M. K.; Norris, J . R.; Levanon, H. J. Phys. Chem. 1988,92,71. (b) Bowman, M. K.; Toporowicz, M.; Norris, J. R.; Michalski, T.J.; Angerhofer,A,; Levanon, H . Isr. J . Chem. 1988, 28, 215. (18) (a) Levanon, H. Reu. Chem. Intermed. 1987,8, 287. (b) Regev, A,; Levanon, H.; Murai, T.; Sessler, J. L. J. Chem. Phys. 1990, 92, 4718. (19) Barboy, N.; Feitelson, J . J . Phys. Chem. 1986, 90, 271.
.0 t
40
: iim
1
00 320
00 416
512
608
704
800
320
416
512
608
704
800
h (nm)
A (m)
Figure 2. (a) Ground-state absorptions (full lines) and fluorescence (dotted lines) spectra of ZnOEPC and H,OEPC in ethanol at room temperature. The fluorescence intensities are given in arbitrary units. (b) Differential triplet-triplet absorption curves of ZnOEPC and HzOEPC (&rcitarion = 580 nm) in ethanol at room temperature. (c) Calculated triplet-triplet absorption spectra of ZnOEPC and H,OEPC. The extinction coefficients (M-' cm-') were determined as described in the text.
ZnOEPC and H,OEPC (-3 X lo4 M) dissolved in ethanol were prepared in Pyrex tubes (4." o.d.), degassed by freezepumpthaw cycles, and sealed under vacuum. Samples in the liquid crystal (LC) with the same chromophore concentrations were similarly prepared by first evaporating the solvent (leaving the solute powder on the tube walls) and then introducing the LC and were degassed and sealed as described above. LC sample alignment was camed out as described previously.'* Temperature was maintained by a nitrogen flow system at 100 K for triplet-state detection and -240 K for the electron-transfer measurements."J8 Only DQ (-2 X 10-*-2.5 X M) was used as an electron acceptor, because of its relatively high redox potential and stability over the long time period of the experiment, in the electron-transfer measurements. In the direct detection mode, samples in the microwave cavity were photoexcited at 640 nm by a dye laser (Quanta Ray, PDL-1) pumped by the second harmonic of a Nd:YAG (Quanta Ray, DCR-1A). An Exciton DCM-640 dye was used, providing pulse widths of 10 ns (18 mJ/pulse) at a repetition rate of 10 Hz. This excitation corresponds to the Q-band optical transition of the porphycenes. C W spectra, at different specified times, were obtained from the transverse magnetization, My(?),decay curves. A detailed description of light excitation, signal detection, and the real time data acquisition is given elsewhere.I3J5 Triplet line shape analysis was carried out using the density matrix formalism presented in previously published work.I3 Further data analysis was carried out on a CCI Power 6/32 minicomputer. FT-EPR measurements were carried out on a home-built spectrometer,20using the same sample conditions (except for liquid ethanol solutions and Suprasil quartz tubes). In the pulsed FT experiments, light excitation pulses (640 nm) of 10-ns width (1 mJ/pulse) a t a repetition rate of 100 Hz were provided by an excimer-pumped dye laser (Lambda Physik EMG-5OE using XeC1).17*20All FT-EPR spectra were normalized to the same number of averaged FID's, with a typical number of 2000-4000 averages.
-
111. Results and Discussion
a. Basic Photophysical Processes: Ground-State Absorption, Fluorescence, and Triplet-Triplet Absorption. The absorption (20) (a) Massoth, R. J. Ph.D. Thesis, University of Kansas, 1987. (b) Angerhofer, A.; Massoth, R. J.; Bowman, M. K. Isr. J . Chem. 1988, 28, 227.
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3043
Zinc and Free-Base Octaethylporphycenes
TABLE I: Photophysical and Photochemical Parameters of Porphycenes (Laser Photolysis Experiments)" TP qJf 7; h c ket(BQId k,,(DQ)d 22.5 12.0 0.92 3.2 0.05 85 ZnOEPC 140 -0.04 H2OEPC 5.0 0.02 0.4 9.8 0.38 200 H2TPrPC 270 0.3 H2PC 10.2 0.44 0.88 2.7 0.04 1250 ZnTPP 0.82 13.6 0.13 1350 HZTPP "Estimated errors (this work): f 5 % for
T~
and
and *lo% for ke,.
T ~ ,
spectra of ZnOEPC and H20EPC in ethanol (Figure 2a) are similar to those of the porphyrins but reflect differences in symmetry between porphyrins and porphycenes.* For example, the Q-band absorption of the porphycenes appears at longer wavelengths (642 nm for ZnOEPC and 664 nm for H20EPC) than in the porphyrins. Both ZnOEPC and H20EPC show strong long-wavelength absorption with maxima at X = 642 and 664 nm, respectively, with molar extinction coefficients of e = 8.8 X lo4 and 2.25 X lo4 M-' cm-' at the respective wavelengths. The Soret band maxima lie at A = 390 nm with e = 1.49 X lo5 M-I cm-I and A = 382 nm with e = 1.02 X lo5 M-I cm-I for ZnOEPC and H20EPC, respectively. The fluorescence spectra (Figure 2a) of ZnOPEC and H20EPC exhibit long-wavelength emission peaks at 660 and 685 nm, indicating Stokes shifts. The fluorescence lifetimes, Tf, of both chromophores are on the order of 5 ns. Quantum yields of fluorescence, 4f, were determined by comparison with the known value of tetraphenylporphyrin2I (Table 1). Triplet-triplet (T-T) absorption spectra were determined by laser photolysis experiments. The differential T-T absorption spectra of ZnOEPC and H20EPC are presented in Figure 2b. It is immediately noticed that the relative optical density (OD) changes of the metalloporphycene exceed those of the free-base porphycene by over an order of magnitude. To quantify the differential T-T absorption spectra, we have calculated the absolute T-T spectra (Figure 2c) via eq 6 (after correcting for the wavelength dependence of the photomultiplier response)
ODT ICT
eT=-=
O D G ~ I S+CAOD,, ICS4ISC
(6)
where OD, and ODG are the triplet- and ground-state absorptions, POD,, is the experimental differential T-T absorption, I is the optical path, Cs22and CT are the excited singlet and triplet concentrations, and 4rscis the quantum yield of intersystem crossing (determined by the heavy-atom method,23using ethyl iodide as a quencher). For ZnOEPCT, the maximum T-T absorption is at X = 390 nm with an extinctioin coefficient, e, of 8 X lo4 M-' cm-I, while the T-T absorption maximum for H20EPC is at X = 380 nm with t = 5.52 X lo4 M-I cm-l. Finally, the triplet lifetimes, T T , were determined from the first-order decay kinetics of the triplet-triplet absorption. The calculated photophysical parameters (using ethanol as a solvent) are summarized in Table I. For comparison, we also present relevant data of related porphycenes and tetraphenylporphyrin. The sum of the quantum yields, qbf 41sc,of ZnOEPC is close to unity, indicating that the fluorescence and intersystem crossing are the only significant deexcitation pathways for the singlet excited state of this metalloporphycene. However, in the case of H,OEPC, r$f q51sc is very low (cf. Table I) and seems to be solvent independent. Such behavior is very similar to that of 2,7,12,17-tetra-n-propylporphycene,H,TPrPC,24 where the
+
+
(21) (a) Harriman, A. J . Chem. SOC.,Faraday Trans. 1 1980, 76, 1978. (b) Brookfield, R. L.; Ellul, H.; Harriman, A,; Porter, G . J . Chem. SOC., Faraday Trans. 2 1986, 82, 219. (22) Under sufficiently high laser intensities, T-T absorption approaches saturation; Le., all porphycene molecules in the laser path are excited. Under these experimental conditions, we assume that C, C, (ground-state concentration). (23) Medinger, T.; Wilkinson, F. Trans. Faraday SOC.1965, 61, 620.
lo9 s. 'In IO6 s.
ket(AQ)d 1.15
ref this work this work 24 24 21 21
lo-* M-I s-l.
8
I 3
B
,
,
,
so+.
,
,
,
,
3.5
3.2
,
,
3.8
Figure 3. Semilog plots describing changes in relative triplet-triplet (left scale) absorbance of free-base prphycene in ethanol at 420 nm (0) = 580 nm ( 0 )(right scale) and fluorescence intensity, of the 685 nm, LLXc vs 1/T. Conditions of the laser photolysis experiments are as described in the text. The solid lines are best-fit least-mean-squares plots. A schematic representation of the relevant transition energies and the proposed state, A2, is shown in the inset. The position of the energy levels in the triplet manifold is arbitrary.
fluorescence and triplet yields were found to be strongly dependent on the temperature and peripheral substitution.I2 Indeed, temperature-dependent experiments carried out on the HzOEPC clearly demonstrate that the triplet and the fluorescent singlet-state populations strongly depend on other primary processes of the system. The temperature-dependent data are summarized in Figure 3, where the measured differential T-T absorption and fluorescence intensities vs 1 / T are presented. The mechanisms used to explain the strong temperature-dependent relaxation traditionally fall into two types.25 The first mechanism is based on the thermal population of vibrational levels of the initial electronic state, whereas the second involves thermal population of a second electronic excited state. These mechanisms were applied in the treatment of temperature dependence of the lifetimes of excited radicals26 and in previous work on other free-base porphycenes.I2 Similarly, a second singlet level is invoked here following the scheme shown in Figure 3. The energy differences between A2and SI were calculated by eq 7. The calculated values
[A21 = exp([SI1
g)
(7)
of AE from the T-T absorption and fluorescence data are 2.4 and (24) (a) Aramendia, P. F.; Redmond, R. W.; Nonell, S.; Schuster, W.; Braslavsky, S.E.; Schaffner, K.; Vogel, E. Photochem. Photobiol. 1986, 44, 5 5 5 . (b) Nonell, S.;Aramendia, P. F.; Heihoff, K.; Negri, R. M.; Braslavsky, S.E. J . Phys. Chem. 1990, 94, 5819. (25) (a) Heller, D.; Freed, K. F.; Gelbart, W. M. J . Chem. Phys. 1972, 56, 2309. (b) Freed, K. F. In Topics in Applied Physics; Fong, F. K., Ed.; Springer-Verlag: New York, 1976; Vol. 15, Chapter 2. (26) Meisel, D.; Das, P. K.; Hug, G . L.; Bhattacharyya,K.; Fessenden, R. W. J . A m . Chem. SOC.1986, 108, 4706.
3044 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
Berman et al.
z h
-1
r"
-1
c v
[MIXIO
04
I
320
I
l
116
,
l
,
512
,
608
l
,
l
704
I
SO0
4
Figure 5. Stern-Volmer plots of triplet state (ZnOEPCT)quenching by BQ (e),D Q (0),and A Q (H)in ethanol at room temperature. The T and T~ are the triplet half-life times with and without the quinone, respectively.
(nm)
ZnOEPC
H,OFPC
Figure 4. Differential triplet-triplet absorption spectra of ZnOEPC dissolved in ethanol in the presence of DQ ( 5 X lo4 M) at room temperature. The spectrum, solid line, was taken 300 ns after the laser pulse. Absorption curve, dotted line, was obtained 12 ws after the laser pulse. Normalized kinetic profiles of the triplet-triplet absorption decay and radical anion rise are shown in the inset. The arrows a and b indicate the wavelength at which the decay and rise signals were monitored, a (350 nm) and b (415 nm).
2.9 kcal/mol, respectively. Although the exact mechanism relevant to the present result is not yet determined, we analyze the results of Figure 3 by invoking a second level (A2)12in the singlet manifold, lying between S1 and Sa. If the A2 state is thermally populated and its deactivation to the ground singlet is radiationless, it is expected that the triplet and fluorescence yields should be temperature dependent. In addition, the low quantum yields of fluorescence and triplet formation in the case of H20EPCare in line with the enhanced nonradiative decay in the singlet manifold caused by the specific site of the peripheral substitution.I2 In comparison, changing the number of peripheral substituents and their site, e.g., freebase tetrapropylporphycene (H2TPrPC)," there is a noticeable increase in the fluorescence and triplet yield values (cf. Table I). b. Electron Transfer: Laser Photolysis Experiments. In the presence of quinones, which are good electron acceptors, the triplet-state lifetime of ZnOEPC decreases appreciably. Figure 4 shows that the triplet decay is accompanied by birth of a new absorbing species at 350 nm. This new absorption appears in the presence of duroquinone ( 5 X M) with a rise time similar to that of the triplet decay, Le., 77 = T~ = 1.8 ps. Time evolution of the transient absorption (Figure 4) indicates that the spectrum, 12 ps after the laser pulse, where the porphycene triplet has practically disappeared, is identical with the absorption spectrum of the duroquinone anion radical, DQ.-,27which disappears slowly with a decay time of -80 ps. The determinatioin of the electron-transfer rates was carried out by employing a Stern-Volmer analysis. The corresponding plots are shown in Figure 5 , from which the quenching rate constants, k,,, for ZnOEPCTand benzoquinone, duroquinone, and anthraquinone in ethanol, were determined (cf. Table I). The electron-transfer rate constants follow the order k,,(BQ) > k,,(DQ) >> ket(AQ). The quantum yield of the duroquinone anion radical was determined from the triplet yield and the extinction coefficient of the duroquinone anion radical at X = 440 nm, e(DQ)'- = 7600 M-1 cm-l ,28 resulting in a value of +(DQ)*- 20%. It should be noted that attempts to observe the cation radical of ZnOEPC'+ (reaction 2) failed. Presently, we have no satisfactory explanation for that. Also, because of the low triplet yield in the case of H20EPC, analogous electron-transfer reaction could not be observed by the laser photolysis method. This photochemical reaction
-
(27) Meisel, D.; Czapski, G. J . Phys. Chem. 1975, 79, 1503. ( 2 8 ) Patel, K. B.; Willson, R. L. J . Chem. Soc., Faraday Trans. I 1973, 69, 814.
I
2750
3250
MAGNETIC
FNJ)
3750
(GAUSS)
2750
3250
3750
MAGNETIC FELD (GAUSS)
Figure 6. Diode detection transient EPR triplet spectra of the two chromophores in ethanol and E-7 liquid crystal at the parallel and perpendicular configurations, at 110 K. Spectra at microwave power of 10 mW were taken 500 ns after the laser pulse. The smooth curves superimposed on the spectra are computer simulations (for details, see refs 13 and 15).
can be monitored by time-resolved EPR. c. Time-ResolvedEPR Measurements. The clear involvement of the photoexcited triplet state in the photophysics and photochemistry of the porphycenes prompted us to investigate these processes also by time-resolved EPR methods. The main advantage of this spectroscopic method is the unique identification of the transient paramagnetic species formed upon light excitation, namely, the photoexcited triplet of the chromophore (S = 1) and the doublet of the acceptor (S= I/*) species. Two modes of EPR detection were employed, i.e., CW and pulsed FT direct detection with 200- and IO-ns time resolution, respectively. Both time resolutions are with respect to the delay time between laser excitation and microwave detection. We start first with the characterization of the triplet state of both chromophores (magnetic parameters and spin dynamics), and in the next stage we present
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3045
Zinc and Free-Base Octaethylporphycenes TABLE 11: Magnetic Parameters and Electron-Transfer Rates" Ea.b 0.0080 0.0050 0.0030 0.0098 0.0080 0.0090
P*b
ZnOEPC HZOEPC H2TPrPC ZnTPP HZTPP HZTPP
0.0266 0.0280 0.0310 0.0298 0.0385 0.0371
A,:A,:A: 1.0:0.8:0.2 1.0:0.9:0.5
TI b*C 8.5 7.0
T,b,C 0.035 0.050
0.o:o.o: 1 .o 1.0:0.6:0.3 1.0:0.6:0.3
0.4U 8.3 22
0.020
k.,d,' 1.7 1.6
T , (DO")'" 9.0 9.5
11.4
7.0
ref
~
this work this work 11 17 18 15b
" In cm-]. bTriplet-state parameters in E-7 LC calculated from CW-EPR (100 K). 'In lo6 s. M-' s-'. 'Calculated from FT-EPR (240 K). /Triplet SLR calculated from FT-EPR (240 K). gEstimated errors (this work): *5% for D and E ; *20% for TI,*of triplet state (from triplet line shape analysis), T , of DQ'-, and ket.
- 0.1 ps
!
-50
.
(
.
*
0
.
(
.
+so
I
-50
0
+so
MHZ MHZ Figure 7. FT-EPR spectra of DQ'- in ethanol at 243 K, employing the two porphycenes as electron donors, at different delay times (indicated) between excitation and detection. Magnetic field, 3246.0 G, microwave pulse width, 12 ns. In the inset is shown the diode detection transient EPR spectrum of DQ'- (ZnOEPC was used as donor) in ethanol at 243 K, at 10 mW microwave power, and 1.2 ps between excitation and EPR detection.
data on the electron-transfer reactions using both modes of detection. As will be shown, both the CW and FT data complement each other and are essential for a coherent analysis of the two complementary experimental techniques. i. Triplet State. Triplet spectra of ZnOEPC and H,OEPC distributed isotropically in glass and partially oriented LC matrices are shown in Figure 6 . The nonderivative line shapes reflect spin-polarized spectra that exhibit an e,e,e,a,a,a pattern (from low to high field), where e stands for emission and a for absorption. In the case of H,OEPCT, line shape analysis results in the relative singlet-triplet ISC selective population rates governed by spin-orbit coupling, i.e., A , > A? >> A, (Table 11), where the principal axes of the zero-field splitting (ZFS) tensor axes X and Yare confined to the molecular plane, and Z is the out-of-plane axis. With these assignments, the population rates indicate a preferential in-plane spin polarization as expected for the free-base porphycene triplet." The same population rates pattern was found for the ZnOEPC triplet. These values contrast with the population rates of ZnTPP,15 in which the active spin state is associated with the outof-plane ZFS tensor axis, Z.This result, for ZnTPP, is consistent with the large spin-orbit coupling and higher order terms in the spin-orbit H a m i l t ~ n i a n . ~For ~ ZnOEPCT,the observed in-plane active spin states may be due to a reduction of the spin-orbit interaction, which arises from the following reason: The small effective cavity size defined by the four nitrogens of the porphycene (7.46 A2)8compared to that of porphyrin (8.29 A)3omay force (29) Metz, F.; Friedrich, S.;Hohlneicher, G . Chem. Phys. Leu. 1972, 16, 353. (30) Scheidt, W. R.; Mondal, J. U.; Eigenbrot, C. W.; Adler, A,; Radonovich, L. J. Inorg. Chem. 1986, 25, 195.
the ZnZ+out of the molecular plane, thus decreasing the contribution of the d orbitals to the spin-orbit interaction. In that respect it is noteworthy the difference between ZnOEPC and ZnTPP regarding the location of their in-plane dipolar ZFS axes (X, Y)in the molecular frame (Figure 1). Unlike with ZnTPP,15 the triplet line shape analysis in the nematic liquid crystal assigns the direction of the X dipolar axis to be rotated 45O from the director, L (Figure 1). In the case of ZnTPP, on the other hand, the in-plane principal axes are found to coincide with the lines connecting the two diagonal nitrogens in the cavity.15 Such a relative change in the ZFS dipolar axes may also affect the ratio of the singlet-triplet population rates in ZnOEPC as compared to that in ZnTPP. Furthermore, while the ZFS parameters D calculated for ZnOEPC and H,OEPC are similar (0.0266 and 0.0280 cm-', respectively), the corresponding values for ZnTPP and HzTPP triplets are different and significantly larger (0.0298 and 0.0385 cm-I, respectively; cf. Table II).15*18The small D values of these porphycenes are in line with the effective distance of the triplet electrons, which is larger along one axis in porphycenes than in porphyrins.]] The spin relaxation times, TI and T2,were calculated from the kinetic profiles at different microwave powers and the corresponding decay The values are close to those obtained with porphyrin triplets, where the T , >> T2,Le., indicating an overdamped response.31 ii. Electron Transfer. As stated above, DQ (2.5 X M) was chosen as an acceptor for the electron-transfer reaction study in the EPR mode. Time-resolved FT-EPR spectra of the anion (31) Hore, P. J.; McLauchlan, K. A. Mol. Phys. 1981, 42, 533.
3046 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
Berman et al.
radical, DQ'-, produced under photoexcitation of the two porZIlOEPC H, OEPC phycenes, are shown in Figure 7. With both triplet precursors the line shape, low to high on the frequency scale, exhibits an a/e pattern (or e/a, on the field scale, inset Figure 7). These observations clearly suggest that the CIDEP effect originates from . .., I triplet porphycene precursors. It is immediately noticed that the resulting spin-polarized spectra exhibit unique features, not observed previously with other metallotetraphenylp~rphyrins:~~~-~~ (a) The spectra mainly governed by the radical-pair mechanism .. (RPM) with negligible contribution of the triplet mechanism 01 I IO 100 01 1 IO 100 (TM), thus causing the polarized spectra to appear at relatively Time ( P s) Time ( P s) long delay times (->200 ns) after light excitation. (b) Once the Figure 8. Time dependence of R P M contribution to the CIDEP effect doublet state of DQ'- is detected, it remains spin polarized for of DQ'- produced by ZnOEPCT and HzOEPC'. The RPM polarization a long time, most noticeably when ZnOEPCT is the precursor was determined as described in the text, the lines were calculated em(-50 ps). The long-lived polarized DQ'- signal may suggest an ploying eq 10, and the best-fit parameters are given in Table 11. operative F-pair mechanism due to the reaction DQ'- DQ DQ + DQ*-,20bS32 chemical reactivity toward DQ. A rapid rise with a maximum The absence of TM is quantitatively justified by the expression at 3 ps after the laser pulse is followed by a slow decay of the for the TM polarization, PfIl2of the doublet DQ'- radical formed polarized signal (Figure 8 ) . The smooth line superimposed on under TM33J4 the experimental results was obtained by a best-fit analysis taking into account the rise and decay of the intensity difference, APill2 = (ml=fl), and fitting into eq lOI7 4 1 - ( A , + A, A,) f - [ D ( A , A, - 2A,) 3E(A, - A,)J p = A[exp(-k,t) - exp(-t/T,)] (10) 15B 3 (8) where p denotes the polarization in terms of the intensity difference, A(m,=*l), k, is the rise time of the signal intensity where A,, A,, and A , are the selective ISC population rates, D difference and is equal to the electron-transfer rate k,,, A is the and E are the ZFS parameters, and B is the external magnetic relative amplitude, and TI is the spin-lattice relaxation time of field (-3200 G). From the triplet EPR line shape analysis all DQ'-. It is apparent that eq 10 fails to account for the experithe magnetic and spin dynamic parameters were calculated; thus mental results in short delay times. In addition to the scrambled the enhancement factor, J can be calculated via eq 9:34 T M discussed above, the delay may also be due to some consecutive reaction resulting in the polarized DQ'-.36 The F-pair (9) mechanism is associated with the recombination process and probably responsible for the long-lived polarized DQ'The best-fit analysis results are presented in Table 11, where the For the present case, where A , > A, >> A , at 240 K, the Boltzcalculated values are based upon RPM excluding the other promann population difference is about 0.090%, so that the relevant cesses mentioned a b ~ v e . The ~ ~ EPR . ~ ~ experiments give a lower values (Table 11) for A i(i = x , y, z ) result in f values on the order electron-transfer rate than the laser photolysis experiment. This of -46 and -28 for DQ'- formed via the triplet state of ZnOis expected from the temperature difference between the two EPCT and H20EPCT,respectively. These values are smaller when experiments and is attributed to the viscosity changes of ethanol. compared to cases where selective ISC rates are governed by either The calculated spin-lattice relaxation times of DQ'- are in good the out-of-plane dipolar spin axis or in-plane axis/axes. The latter agreement with values of 1/T, of DQ'- obtained with different cases are found with triplet metalloporphyrins, Le., ZnTPPT cf triplet p r e c u r s ~ r s . l ~ $ ~ ~ = 173) and MgTPPT cf = 8 5 ) , respectively, undergoing elecWe have shown in this paper the compatibility between timetron-transfer reactions with DQ." In other words, when both resolved CW- and FT-EPR methods in sorting out the spin-poin-plane and out-of-plane axes are active and mixed together, larization mechanism that governs electron transfer between a triplet memory transfer is scrambled, thus giving rise to a small triplet precursor and an electron acceptor. As to the specific TM contribution to the CIDEP effect. In addition, differences porphycene precursors, we found that the photophysics and in electron-transfer and spin-lattice relaxation rates can modify photochemistry of H20EPC and ZnOEPC differ from those of the amplitude of the observed TM polarization. (Unfortunately, the respective porphyrins, H2TPP and ZnTPP. Two parameters no triplet spin-lattice relaxation rates are available for triplet were found to affect the spectroscopicproperties of the porphycenes porphycenes in liquid ethanol.) Therefore, spectral analysis fostudied in this work, namely (a) symmetry and structure and (b) cused on the RPM (including Ag) provides the electron-transfer peripheral substitution. The former are intrinsic to the porrates. Having triplet precursors in the electron-transfer reaction phycenes, affecting the optical and magnetic properties. Regarding is further supported by inspection of the S/N ratio of the polarized the latter, which is more general, we are confident from the present spectra of DQ'- (Figure 7). As discussed above, the singlet-triplet and previousI2studies that peripheral substitution plays a dominant ISC yield is much higher for ZnOEPC (Table I). These results role in affecting the photophysical and photochemical properties should affect the S/N ratio of the radical's FT spectra shown in of porphycenes in the liquid phase. The extent to which these Figure 7, which indeed are compatible with the quantum yields chemical modifications are unique to porphycenes is under current results. investigation in our laboratories. The analysis of the FT-EPR spectra, using linear prediction method,3s was similar to that previously described (Figure 8).17 Acknowledgment. This research was supported by a grant from The time evolution of the A(mI=fl) hyperfine components of the GIF, the German-Israeli Foundation for Scientific Research DQ'- is shown in Figure 8 for each triplet precursor. As expected, both precursors (H20EPCTand ZnOEPC') are similar in their > .
(e)
+
+
+
-
-
+
(32) McLauchlan, K. A. In ref 3, Chapter 10. (33) (a) Atkins, P. W.; McLauchlan, K. A. In Chemically Induced Magnetic Polarization; Lepley, A. R., Closs, G . L., Eds.; Interscience: New York, 1973. (b) Atkins, P. W.; Evans, G. T. A d a Chem. Phys. 1975, 35. (34) Wong, S. K.; Hutchinson, D. A.; Wan, J. K. S . J. Chem. Phys. 1973, 58, 985. (35) Tang, J.; Norris, J. R. J. Magn. Reson. 1988, 79, 190.
(36) For example, a possible mechanism in a consecutive reaction is pair dissociation. However, it does not appear to be the limiting rate at high DQ concentration (2 X lo-* M). Because of poor S/N in this time regime, we do not treat quantitatively this part of the fit. In any event, the majority of the rise gives a single time constant, which is pseudo-first-order in DQ, implying that the calculated rate is for the electron-transfer rate. (37) The polarized signal due to F-pair mechanism is weak and should show up at long times; thus, it will have a small influence on the calculated rate constants.
J. Phys. Chem. 1992, 96, 3047-3050 and Development (H.L. and E.V.), by the U S . Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract W-31-109-Eng-38 (J.R.N., M.K.B., and H.L.), by the Israel Council for Research and Development, and by DFG (H.L.). The Farkas Research Center is supported by the Bundesministerium fur die Forschung iind Technologie and
3047
the Minerva Gesellschaft fur die Forschung GmbH, FRG. A.B. (HUJ) is supported by a L. Eshkol stipend for conducting her Ph.D. Thesis. Registry No. DQ, 527-17-3; DQ'-, 3572-98-3; ZnOEPC, 13035128-9; H,OEPC, 13035 1-26-7.
Rate Constants for Reactions of Hydrazine Fuels with O(3P) Valerie I. Lang Space and Environment Technology Center, The Aerospace Corporation, P.O. Box 92957, M2/251, Los Angeles, California 90009-2957 (Received: August 28, 1991; In Final Form: December 16, 1991)
Room-temperature rate constants for the reactions of three hydrazine fuels with O(3P) were measured. A laser photolysis-resonance fluorescence technique was used in the experiments. The reactions were monitored by the decay of 131 nm, O('S3P) resonance fluorescenceunder both high-pressure (-50 Torr) and low-pressure (1-3 Torr) conditions. The measured rate constants were 0.99 (f0.12) X lo-'] cm3moleculed s-] for N2H4, 1.6 (10.34) X 10-l' cm3molecule-] s-I for CH3HN2H2, cm3 molecule-] s-l for (CH3),N2H,, all at 296 K. The rate constants increase with the number and 2.3 (f0.34) X of hydrogen atoms per hydrazine molecule.
Introduction Unburned liquid rocket propellants are often ejected into the atmosphere at altitudes where ground-state atomic oxygen is a predominant species. In addition to anhydrous hydrazine (N2H4), two substituted derivatives, monomethylhydrazine (CH3HN2H2) and unsymmetrical dimethylhydrazine ((CH3),N2H2), are commonly used as propellants. Chemiluminescent emission from the reaction of N2H4 with oxygen atoms was first observed in 1956.l~~ The emitting species were identified as excited NH, NH2, OH, and NO. In the early work, it was also noted that the reaction between N2H4and atomic oxygen was very fast.]-' A complete understanding of the mechanisms and kinetics of the primary atmospheric reactions of each of the hydrazine species is needed to fully describe their distinct chemiluminescent signatures. The reaction chemistry of N2H4 has been studied in more detail than that of the other hydrazine-type fuels. Two kinetic studies of the N2H4 and 0 atom reaction have been done by Gehring et al.49S They reported the rate coefficient of the reaction 0 + N2H4 N2H2 HzO (1)
-
(
+
in the Arrhenius form as
k l = 8.5 X l o i 3exp -
1200;~mol
1
cm3 mol-' s-I (2)
and later modified the preexponential coefficient to 6.3 X The latter value corresponds to a rate constant at 296 K of 1.4 X lo-" cm3 molecule-' s-I. The experiments were done in a fast-flow reactor with electron spin resonance and mass spectrometry detection. Shane and Brenned estimated the reaction rate constant to be considerably slower, 3.0 (f1.5) X 10-l2 cm3 molecule-] s-I at 295 K. There have been no rate constants previously reported for the reactions of the methylated hydrazines with atomic oxygen. The primary reaction step may be hydrogen abstraction, similar to reaction or attack at the methyl groups.* 1,497
The purpose of this work was to compare the room-temperature reactivity of N2H4, CH3HN2H2,and (CH3)2N2Hztoward O(3P). The rate constants for each of the three primary reactions are reported, and in the case of N2H4 our result is compared to the previous results of Gehring et al.43Sand to that of Shane and Brennena6 Our experimental technique of laser photolysis-resonance fluorescencewas specific to monitoring ground-state atomic oxygen.
Experimental Section One of the greatest difficulties associated with performing kinetic studies on hydrazine-type compounds is their rapid decomposition on the surfaces of many common laboratory materials. However lists of hydrazine compatible substances have been published?JO With these guidelines, our experimental apparatus was designed to prevent loss of hydrazine in any part of the system prior to reaction with the 0 atoms. Second, kinetic experiments in which only the hydrazine concentration is monitored are prone to result in reaction rates which are too fast if any portion of the hydrazine reacts on the walls or with contaminants such as water vapor. This bias was eliminated in our experiments by observing the decay of the other reactant, O(3P). Using our technique, any loss of hydrazine reactant would result in an apparent 0 atom decay rate which is slower than the actual reaction rate with hydrazine. Under the current experimental conditions, there were no loss processes for the O(3P) atoms which were kinetically comparable to the reaction with hydrazine. Anhydrous N2H4, CH3HN2H2,and (CH3),N2H2liquids were obtained from Aldrich with stated purities of 98%, 99%, and 98%, respectively, and from SIGMA (N2H4) with a stated purity of 98.8%. Commercial N 2 (Air Products, UHP, 99.999%) and 0, (MG Industries, 99.999%) were used without further purification. One percent hydrazine vapor (N2H4, CH3HN2H2, or (CH3),N2H2)mixtures in N, diluent gas were prepared in a 30-L pyrex manifold. The liquid hydrazine was handled under a N 2 purge before it entered the manifold and then purified by several freezelthaw cycles at dry icelacetone temperatures. The hy-
(1) Moore, G. E.; Shuler, K. E.; Silverman, S.; Herman, R. J . Phys. Chem. 1956, 60. 813.
(2) Hall, A. R.; Wolfhard, H. G. Trans. Faraday Soc. 1956, 52, 1520. (3) Becker, K. H.; Bayes, K. D. J . Phys. Chem. 1967, 71, 371. (4) Gehring, M.; Hoyermann, K.; Wagner, H. G.; Wolfrum, J. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 956. ( 5 ) Gehring, M.; Hoyermann, K.; Schacke, H.; Wolfrum, J. Fourteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1972; pp 99-105. (6) Shane, E. C.; Brennen, W. J . Chem. Phys. 1971, 55, 1479.
(7) Foner, S. N.; Hudson, R. L. J . Chem. Phys. 1970, 53,4377. (8) Dimpfl, W. L.; Bernstein, L. S.;Alder-Golden, S. M.; Cox, J. W.;
Cunningham, K. W.; Pritt, A. T. Proceedings of the 1991 JANNAF (Joint ArmyNauy-NASA-Air Force) Exhaust Plumes Technology Subcommittee Meeting, Laurel, MD, May 1991. (9) Cadwallader, E.; Piper, L. B. Hydrazine Compatibility Suruey; Chemical Propulsion Information Agency: Silver Springs, MD, 1973. (10) Schmidt, E. W. Hydrazine and Its Deroatives: Preparation, Properties, Applications; John Wiley and Sons: New York, 1984; pp 454-483.
0022-365419212096-3047$03.00/00 1992 American Chemical Society
