724
J. Phys. Chem. 1986, 90, 124-728
Rephasing of Collisionless Molecular Rotational Coherence in Large Molecules P. M. Felker,+ J. S. Baskin,' and A. H. Zewail* Arthur Amos Noyes Laboratory of Chemical Physics,$ California Institute of Technology, Pasadena, California 91 125 (Receiued: January 3, 1986)
We present experimental results on the rephasing of collisionless rotational coherence in jet-cooled trans-stilbene. The results represent the first observation of quantum coherence effects arising from the rotational level structure of molecules. We discuss the nature of the effect, its possible influence in time-resolved polarization experiments designed to probe intramolecular energy flow, and its possible uses as a new spectroscopic tool.
I. Introduction Time-resolved spectroscopy on isolated molecules, particularly on molecules cooled by free-jet expansion, has proved a rich source of information on molecular dynamics.' One aspect of dynamics of particular interest is intramolecular vibrational energy redistribution (IVR). Our time-resolved studiesz4 of this process have been aimed primarily toward the understanding of purely vibrational coherence-the quantum interference effects that arise from vibrational couplings that do not depend on rotational quantum numbers. Yet, one expects-and there is evidence to support this expectati~n~-'~-thatin some cases rovibrational couplings may markedly affect vibrational energy flow. A promising means by which to probe such intramolecular vibrational-rotational energy transfer (IVRET) experimentally is to measure polarization properties. For instance, Nathanson and McClelland" show that steady-state measurements of fluorescence polarization can give information pertaining to excited electronic state IVRET. One expects that even more information on dynamics might be available by adding an extra dimension to such experiments and making polarization measurements as a function of time. In fact, our groupI2 and Negus et al.I3 have reported polarization-dependent transients in isolated trans-stilbene which may reflect IVRET.14 At this juncture, however, the lack of theoretical results on what and how certain processes are reflected in time- and polarization-dependent observables inhibits interpretation of these results. One situation that can give rise to polarization-dependent transients has nothing to do with intramolecular energy flow. This situation is what we shall call a purely rotational coherence. It arises from the coherent preparation of rotational levels within a single vibronic state. This coherent superposition state is composed of rotational eigenstates that have different spatial properties and different energies. These two ingredients give rise to quantum interference effects which are detection polarization-dependent, that is, polarization-dependent transients. These transients, which do not necessarily vanish upon thermal averaging over initial states in the sample, are manifestations of a rephasing in the orientation of emission dipoles. It is important to realize that the effects can occur upon excitation to any vibronic state in the molecule. Clearly, an understanding of the manifestations of purely rotational coherence is necessary if one is to sort out dynamical contributions to polarization-dependent transients. In this Letter we report the first observation of transients that can be associated unambiguously with purely rotational coherence in isolated large molecules. We also present theoretical results to support this claim. The experimental results, obtained upon excitation of the vibrationless level of the first excited singlet state (Si)of jet-cooled trans-stilbene, demonstrate the prevalence of rotational coherence effects in picosecond experiments involving polarized observables. They also suggest that such effects can be used to advantage in a number of areas of molecular spectroscopy. 'Present address: Department of Chemistry, University of California, Los Angeles, CA 90024. * ARCS fellowship, Applied Physics Department, Caltech. §Contribution No. 7354.
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11. Theory A. Purely Rotational Coherence. We shall consider the specific situation depicted in Figure 1. It corresponds closely to the case of the S,-So 0; transition of trans-stilbene. A prolate symmetric top molecule in vibronic state ISovo)and rotational level IJ&&f0) is excited with a short pulse of linearly polarized light to the vibronic level JS,u,). (We take the excitation polarization direction, Z1, to be along the laboratory Z axis.) This excitation process creates a coherent superposition of rotational levels in the excited vibronic manifold. Since we have assumed that the absorption transition dipole fil lies along the symmetry axis of the symmetric top, the excited rotational levels composing this superposition state i@([))are those allowed by the selection rules A J = 0, f l , AK = 0, and AM = O:I5 I@(t)) = (clJJo- 1KoMo)e-i2"(Jo-1)Josr
+
c,lJoKoMo)e-'z"Jo(Jo+1)BI + CdJo -t1KoMo)e-i2?r(Jo+ I )(J0+2)Bf)e-i2r(u,+(a-B)Ko*)re-~1/2
ISlVl)
(1)
where A and B are the usual rotational constants of a prolate symmetric topi5(in GHz), r is the decay rate of the ISlvl)vibronic state, v,, is the energy (in GHz) of ISlvl),and cl, c2, and c3 are constants which depend on Jo, KO,and Mo. Now consider the time development of the polarized fluorescence from this rotational superposition state to all possible rotational levels of a vibronic level JSovf)in the ground electronic (1) P. M . Felker and A. H. Zewail in Applications of PicosecondSpectroscopy to Chemistry, K. B. Eisenthal, Ed., Reidel, Dortrecht, 1984, pp 273-291. (2) W. R. Lambert, P. M. Felker, and A. H. Zewail, J . Chem. Phys., 75, 5958 (1981); P. M. Felker and A. H. Zewail, Chem. Phys. Lett., 102, 113 (1984). (3) P. M . Felker and A. H. Zewail, Phys. Reu. Lett., 53, 501 (1984); P. M. Felker and A. H. Zewail, Chem. Phys. Lett., 108, 303 (1984). (4) P. M. Felker and A. H. Zewail, J . Chem. Phys., 82,2961, 2975, 2994, 3003 (1985). (5)'E. K: C. Lee and G . L. Loper in Radiationless Transitions, S. H. Lin, Ed., Academic, New York, 1980, p 81. (6) D. B. McDonald, G.R. Fleming, and S. A. Rice, Chem. Phys., 60, 335 (1981). (7) E. Reidle, H. J. Neusser, and E. W. Schlag, J . Phys. Chem., 86, 4847 (1982); E. Riedle and H. J. Neusser, J . Chem. Phys., 80, 4686 (1984). (8) D. A. Dolson, C. S. Parmenter, and B. M. Stone, Chem. Phys. Lett., 81, 360 (1981); C. S. Parmenter, J . Phys. Chem., 86, 1735 (1982). (9) W. R. Lambert, P. M. Felker, and A. H . Zewail, J. Chem. Phys., 81, 2217 (1984). (10) B. E. Forch, K. T.Chen, H. Saigusa, and E. C. Lim, J . Phys. Chem., 87, 2280 (1983). ( 1 1 ) G. M. Nathanson and G. M. McClelland, J . Chem. Phys., 81, 629 ( 1 985). (12) J. W. Perry, N. F. Scherer, and A. H. Zewail, Chem. Phys. Lett., 103, 1 (1983); N. F. Scherer, J. F. Shepanski, and A. H. Zewail, J . Chem. Phys., 81, 2181 (1984). (13) D. K. Negus, D. S. Green, and R. M. Hochstrasser, Chem. Phys. Lett., 117, 409 (1985). (14) Y. Matsumoto, L. H. Spangler, and D. W. Pratt, Chem. Phys. Lett., 95, 343 (1983); 98, 333 (1983), have observed some fluorescence polarization anisotropy which may reflect rotational involvement in singlet-triplet coupling in pyrazine. Here, we do no concern ourselves with such effects nor with quantum beats arising from singlet-triplet coupling. (15) G. Herzberg, Molecular Spectra and Molecular Structure. Vol. 111. Electronic Spectra and Electronic Structure of Polyatomic Molecules, Van Nostrand-Reinhold, New York, 1966.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 725
Letters
TABLE I: Functions Pertinent to the Rotational Coherence Decay Represented by Eq 2 of the Text"
Figure 1. Schematic representation of the preparation and detection of rotational coherence in a molecule. The case depicted corresponds to the linearly polarized excitation (polarization vector 2,) of a symmetric top molecule in ground state-rovibronic level ~S,uo;JoKoMo) to those rotational levels of the excited vibronic state [ S l u l )allowed by the rotational selection rules germane to a parallel-type transition moment. The excitation process creates a superposition state of three rotational levels, the coherence properties of which can be probed by time-resolving the polarized fluorescence (polarization Pf) to the manifold of ground-state or by probing with a second, variably rovibronic levels ISOuf;JIKfMI), time-delayed laser pulse (polarization 2;).
state. From the form of eq 1, it is clear that this fluorescence decay
(tf being the detection polarization and ji the dipole moment operator) will be modulated at the three beat frequencies ul 2 BJo, v2 = 2B(Jo l ) , and vi v2. The quantum beats, whose phases and modulation depths depend on the orientation of Ef with respect to P,I6 and on the orientation of the emission transition dipole jif s (S,viIjiISovf) with respect to ji,, are the manifestations of purely rotational coherence. It can be showni7 for an isotropic sample and for jifllji, (remember jiI is assumed to lie along the molecule's symmetry axis) that the fluorescence decay as a function of initial level I J , & o ~ o ) , summed over Mo to account for the isotropy of the sample, is given by
+
I(Jo,Ko,Pf,t) = ( a
+
+ p COS ( 2 ~ ~ +l t ) COS ( 2 ~ ~ 2+t ) 6 cos (2s(vi
+ v2)t))e+
(2)
where a , 8, y, and 6 are functions of Jo, KO,and Zf, and are presented in Table I for Ptllti and t f l t i . Now in any real sample of large molecules, an observed decay will not correspond to eq 2 but to some thermal average of eq 2 over Jo and KO.That is, for a prolate top I(gf,t) =
I(Jo,Ko,Cf,t)e(J~(J~+l)B+(A-B)Koz))lksT (3)
7 JOKO
(16) These characteristics of polarization-sensitive coherence are similar in many ways to quantum beats arising from fine or hyperfine structure, or Zeeman level splittings in atoms. For a review see S . Haroche in High Resolution Loser Spectroscopy, K. Shimoda, Ed., Springer, New York, 1976, p 254. (17) P. M. Felker and A. H. Zewail, J . Chem. Phys., to be submitted.
where 7 is a constant. It turns out, contrary to what one might think, that this thermal averaging does not necessarily preclude the observation of rotational coherence effects, even for large molecules which tend to have many rotational levels thermally populated. The reasons for this are twofold. Firstly, all possible v,, v2, and vi u2 values, although they depend on Jo, are integer multiples of 2B since Jo is an integer--i.e., they are commensurable. Secondly, from Table I, the signs of the coefficients for all of the many cosine terms entering into eq 3 are identical if Efile^, or if t f l e l - t h e coefficients of the cosines are all positive for parallel polarization and all negative for perpendicular polarization. These two facts ensure that at times n/2B, n = l , 2, ,.. there will be transients in the thermally averaged decay since at these times all cosine terms are in phase. For parallel detection these transients are positive, for perpendicular they are negative. Similarly, at times m / 4 B , m = 1, 3, 5 ...,cos ~ T ( u , u2)t = -1 and either cos 2 m l t = -1 or cos 2 r v 2 t = -1. Therefore at such times one expects transients of opposite polarity to those at times n/2B. Two of the properties associated with the manifestations of rotational coherence in fluorescence decays are analogous to properties associated with the manifestations of solvated molecule-rotational diffusion in fluorescence decays.'* Firstly, detection through a polarizer set such that Cfis at 5 4 . 7 O with respect to E , (such decays are proportional to I,, 21,) completely eliminates transients arising from molecular rotation. This is well known in the case of rotational diffu~ion.'~From the results of Table I, it can be seen to apply also to the case of purely rotational coherence that we considered herein. Secondly, the quantity
+
+
+
r(0)
W) - I,(O) 1ll(O) + 21,(0)
can be calculated from the results of Table I. It is found to be 2!5, independent of Jo and KO.The same value applies to rotational diffusion studiesi8J9when absorption and emission dipoles are parallel. Essentially, the similarities between purely rotational coherence effects in isolated molecules and rotational diffusion in solution, when each is probed by time-resolved fluorescence, exist because both types of measurements probe the correlation function ( (jil(0)-jif(t))2).( T h i s will be di.;cussed f u r t h e r elsewhere.I7) The differences in the types of transients expected in the two cases arise due to the different types of rotational motions involved. B. The Rephasing Effect. Physically, the transients that arise ~~~~~~~~
~
(18) For example, see T. J. Chuang and K B. Eisenthal, J . Chem. Phys., 57, 5094 (1972) (19) See, for example, A. J. Cross and G . R. Fleming, Biophys. J., 46, 45 (1984).
726 The Journal of Physical Chemistry. Vol. 90, No. 5, 1986
Letters
Rephosing of Rotational Coherence 0 ) Two
initial J, K levels
8 b)
Thermally averaged initio1 J, K levels
Time: 0.0
0.177
0.404
0.5
0.707
0.056
I .o
I
- - I, -, h
el
Figure 2. The angular distribution of parallel-type emission dipoles at various times subsequent to the excitation of a symmetric top molecule via a parallel-type absorption transition. In both a and b the plots correspond to the instantaneous probability per unit angle 0 that the emission dipole lies between 0 and 0 + de. The time (in units 1/(28) after excitation) corresponding to each plot is given at the bottom of b. In the plots of a the distributions corresponding to excitation from two different ground state rotational levels are shown. In b, ensemble angular distributions of the dipoles (obtained by thermally averaging over initial states) are plotted. One notes that ensemble averaging does not affect the rephasing at 1/(28). At the bottom of the figure we indicate schematically a crude picture of the temporally changing alignment of dipoles (arrows) in the sample.
in purely rotational coherence represent rephasings in the alignment of the emission dipoles of molecules in the sample. For instance, for the particular case we have treated herein, one can show” that at t = 0 the probability per unit angle ( p ( 8 , t ) ) that ,Zr is at an angle between 8 and 6’ d8 with respect to tl is proportional to cos2 6 sin 8. (Note that the probability per unit solid angle, that ilf is oriented in the infinitesimal solid angle element centered about (e,$), is proportional to cos’ 8 and independent of 4.) Because of the rotational motion of the molecules, this initial alignment rapidly dephases. However, due to the quantized nature of free molecular rotation, rephasing in the alignment of dipoles can occur. In particular, at times n/2B there are full recurrences in the cos2 8 sin 0 distribution. (In a sense, this behavior is analogous to that which occurs in a photon echo experiment, except that, in the rotational coherence case, the rephasing is microscopic and arises due to the quantized nature of free molecular rotation.) The rephasing effect is depicted in Figure 2. The plots in the figure correspond to p(8,t) vs. 8 for seven values o f t . In Figure 2a we have plotted the probability distributions corresponding to absorption from two different ground-state rotational levels. One sees that, although the distributions are generally different, at times 0 and 1/(2B) they are identical. (At time 1/(4B)the distributions are nearly identical, but not exactly so.) This recurrence in the alignment of emission dipoles occurs at 1/ ( 2 B ) independent of initial rotational level. It is the rephasing of which we speak. The point is further illustrated by the plots of Figure 2b. These correspond to p(8,t) averaged over the thermal distribution of ground-state rotational
+
levels and represent instantaneous dipole distributions of the sample as a whole. The rephasing is evident in the fact that thermal averaging has no effect on p(0,t) at t = 0 and t = 1/(28) (Le., compare with Figure 2a). The bottom of Figure 2 schematically depicts the alignment of emission dipoles in the sample at times 0, 1/(4B), and 1/(2B). It can be seen that there is a bias toward alignment along e^l at t = 0 and 1 /(2B), and toward ,Zr being at 90’ to tl at t = 1/(4B). C. Simulations. In Figure 3 we show calculated decays pertaining to rotational coherence in jet-cooled trans-stilbene. All decays were calculated by assuming trans-stilbene to be a symmetric top with rotational constants A = 2.678, B = 0.256 GHZ,~O assuming A-axis directions for the absorption and emission dipoles of this near prolate rotor, and using a temperature of 5 K. The top two decays, in order to display the decay behavior of the rotational coherence, were calculated by assuming essentially infinite response of detection and infinite fluorescence lifetime. The bottom two decays correspond to what one would observe experimentally given a finite detection response (45 ps fwhm) and given the fluorescence lifetime of the SI - Oolevel of trans-stilbene (2.6 ns).” In the simulated experimental decays one can readily (20) In fact, trans-stilbene is only an approximate symmetric top. We discuss in section IV the effect of molecular asymmetry on the transients arising from rotational coherence. The rotational constants used were taken from B. W. Keelan and A. H. Zewail, J . Phys. Chem., 89,4939 (1985). Our B rotational constant is taken as the average of the constants B and C (adjusted upward -2% to match the experimental results of section IV) from this reference.
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 727
Letters Simulations without convolution
Perpendicula
Simulations with convolution
,
)
\-
z
I
i
I
I
I
2
3
4
Time ( n s ) Figure 3. Simulated fluorescence decays showing the effects of purely rotational coherence. Decays were calculated by using the results of Table I and parameters given in the text. The top two decays correspond to infinite temporal resolution and infinite fluorescence lifetime. "Parallel" and "perpendicular" refer to the orientation of zf with respect to 1,. The bottom two decays are just convolutions of the top two decays with a finite detection response function, assuming, in addition, a 2.6-ns fluorescence lifetime.
see the recurrences and the changing phase behavior with polarization that are expected based on our discussion above. These results indicate that in a picosecond experiment one should be able to observe the manifestations of purely rotational coherence. We shall compare these calculated decays to ones which we observe experimentally. 111. Experiment Experiments were performed with an apparatus described in detail e l ~ e w h e r e . ~Briefly, ,~ the sample was a jet-cooled gaseous mixture consisting of a small fraction of trans-stilbene in an inert carrier gas (He or Ne). Expansion parameters are given with the experimental data. The trans-stilbene was excited to the vibrationless level of its S, state (P = 32 234 cm-1)21with a pulse of frequency doubled, linearly polarized laser light (Av = 5 cm-], 20 ps) from a synchronously pumped, cavity dumped (4 At MHz) dye laser. Fluorescence was collected at right angles, passed through a polarizer and monochromator (detection of single bands in the fluorescence spectrum can be very important to the observation of rotational coherence effects"), and detected with a multichannel plate. Fluorescence decays were measured by time-correlated single-photon counting. The total detection response of the system (with an aperture in the monochromator to reduce the transit-time spread through it) was 45 ps fwhm.
-
IV. Results and Discussion Figure 4 shows experimental decays of the 0; fluorescence band of trans-stilbene for parallel and perpendicular detection polar(21) J. A. Syage, W. R. Lambert, P. M. Felker, A. H. Zewail, and R. M. Hochstrasser, Chem. Phys. Lett., 88, 266 (1982); J. A. Syage, P. M. Felker, and A. H. Zewail, J . Chem. Phys., 81, 4685 (1984).
J
I
1
I
1
0
I
2
3
4
Time(ns) Figure 4. (Top) Experimental fluorescence decays corresponding to the exitation and detection of the SI-Ox band of jet-cooled trans-stilbene: expansion orifice 70 pm, 75 psig N e backing pressure, nozzle T N 150 "C, laser-to-nozzle distance 3 mm. (Bottom) Fluorescence anisotropies r ( t ) . The experimental trace was obtained directly from the parallel and perpendicular decays at the top of the figure by using the expression for r ( t ) . The upper theoretical trace was obtained from decays calculated for an asymmetric top (rotational constants 2.678,0.262,a d 0.250 GHz) at 5 K with convolution of the experimental response function accounted for. The bottom trace was calculated from the bottom two decays (symmetric top) of Figure 3.
ization. One can see by comparison of Figure 4 with Figure 3 that the experimental and theoretical decays match very well. Particularly noteworthy is the observed phase change upon changing detection polarization. Also, the recurrence behavior that occurs in the experimental decays of Figure 4 vanishes when fluorescence is detected through a polarizer at 54.7'. Finally, it is evident that the measured r(t) agrees very well with the theoretical simulations. Given the match between theory and experiment, it seems clear that we have observed the recurrence behavior associated with rotational coherence in the trans-stilbene molecule. Nevertheless, we must be concerned with two points. Firstly, trans-stilbene is only an approximate symmetric top (calculated So rotational constants20are A = 2.678, B = 0.259, and C ='0.246 GHz). Elsewhere," we show that while asymmetries are expected to reduce the recurrences associated with rotational coherence, they generally do not eliminate them. This is demonstrated in Figure 4 by the theoretial asymmetric top r(t) trace. However, one does expect that the recurrences will occur at a frequency B + C rather than t h e 2 8 frequency expected for a s y m m e t r i c top. I t is encouraging that our measured excited-state value of 0.5 13 GHz is so close to the calculated ground-state value. Secondly, we must be concerned with the directions of the absorption and emission dipole moments. Other work20,22has established that the Sl-So, 0; transition in trans-stilbene is at least approximately along the long ( A ) axis of the molecule. Hence, the fact that our results match the theoretical predictions that were based on the as(22) R. H. Dyck and D. S. McClure, J . Chem. Phys., 36, 2326 (1962).
728
J. Phys. Chem. 1986, 90, 728-730
sumption of A-axis polarization is quite reasonable. In light of the fact that purely rotational coherence effects in trans-stilbene can be observed with 45-ps resolution, one can certainly expect such effects to be a major influence in time-resolved polarization experiments with several picoseconds resolution. From the top two calculated decays of Figure 3, one can see that with high temporal resolution one should observe large transients near t = 0 (such transients are washed out by convolution effects at lower temporal resolution-see the bottom decays of Figure 3). It is not inconceivable, therefore, that the polarization-dependent transients near t = 0 observed at higher energies in the SI level structure of trans-stilbene by our groupI2 and by Negus et may have contributions from purely rotational coherence. At this time, it is unwise to speculate further as to the magnitude of this contribution until theoretical simulations that precisely account for the experimental features are performed. For instance, the experiments from our groupI2 are pump-probe experiments using photoionization detection. The manifestations of purely rotational coherence in such experiments can differ in significant ways from the fluorescence case (especially if the probe ionization step proceeds through a resonant intermediate). The experiments of Negus et al.I3 were done on trans-stilbene at high temperature (398 K) with detection of total (wavelength unresolved) fluorescence. Therefore, the effect of vibrational hot bands and the fact that different fluorescence bands may be characterized by different emission dipole directions must be considered. Nevertheless, given our experimental results, it is important to stress that the possible contribution of purely rotational coherence effects to polarization-dependent transients on a picosecond time scale must be carefully assessed before any conclusion can be made concerning ro-vibrational energy flow. Recently, we have made measurements of r(t) on trans-stilbene at excitation energies where the molecule is known to undergo dissipative IVR.4 Our experimental and theoretical results show that purely rotational coherence effects influence the apparent time scale and magnitude of transients arising from IVR. We shall treat this subject in more detail in another paper.23
Given our success in observing the effects of purely rotational coherence, it is not difficult to imagine a number of uses to which the phenomenon might be very fruitfully applied. Firstly, it is clear that the time-resolved method can be used to obtain the rotational constants of single vibronic levels of large molecules.24 Secondly, beat frequencies and beat phase behavior depend on the relative orientation of 3, and itf and on their orientation with respect to the inertial axis system of the m o l e c ~ l e . 'Thus, ~ one might use rotational coherence effects to assign the symmetries of vibronic levels. Thirdly, one might expect the recurrences associated with rotational coherence to be quite sensitive to MJ-changing collisions. Thus, the effect may be useful in studying intermolecular forces in the gas phase. Finally, since the recurrence behavior depends on the regular spacings between rotational levels in a vibronic manifold, one expects intramolecular perturbations that disturb that spacing (e.g., Coriolis coupling) to affect the rotational coherence. Hence, in principle, one has a new probe of intramolecular dynamics. The situation we have considered herein, that involving linear , i i l in turn is parallel to the symmetry polarizations, and ~ f ~ ~which, axis of a prolate symmetric top, is clearly not the most general situation imaginable as regards purely rotational coherence. Elsewhere,I7 we consider the theory of much more general cases. In particular, we allow for circular polarizations, different directions of and Ph and deviations from symmetric top molecules. In addition, further work aimed toward understallding rotational coherence effects in cases involving nontrivial dynamics is in progress. Acknowledgment. We thank the National Science Foundation for support of this work through Grant No. DMR-8521191.
(23) J. S.Baskin, P. M. Felker, and A. H. Zewail, J . Chem. Phys., to be submitted. (24) J. S. Baskin, P. M. Felker, and A. H. Zewail, J. Chew. Phys., in press.
Transient Storage of Photochemically Produced Oxidative and Reductive Equivalents in Soluble Redox Polymers Lawrence D. Margerum, Royce W. Murray,* and Thomas J. Meyer* Kenan Laboratories, Department of Chemistry, University of North Carolina a t Chapel Hill, Chapel Hill, North Carolina 2751 4 (Received: December 2, 1985)
Optical excitation and electron-transfer quenching of the metal to ligand charge-transfer (MLCT) excited state(s) of [(5-NH2-phen)Ru(bpy)2]2+occur in solutions containing polystyrene polymers in which either pendant paraquat (PQ2') or phenothiazine (PTZ) sites are attached. The quenching and subsequent electron-transfer steps lead to the appearance of -PQ+ and -PTZ+ on separate polymers. Since electron transfer between -PQ+ and -PTZ+ sites on separate polymers is slow, the lifetime of the photochemically produced oxidative and reductive equivalents is enhanced by io3 compared to related monomeric quenchers under similar conditions.
-
In the presence of both oxidative and reductive quenchers the metal to ligand charge-transfer (MLCT) excited state(s) of Ru( b p ~ ) , ~can ' undergo parallel oxidative and reductive quenching, e.g. (1) Nagel, J.
K.;Young, R. C.; Meyer, T. J. Inorg. Chem. 1977, 16, 3366.
0022-3654/86/2090-0728$01.50/0
-
R ~ ( b p y ) , ~ + Ru(bpy)3'+* Ru(bpy),*+* + PQ2+ R ~ ( b p y ) , ~ ++* DMA
-
where PQ2+ is paraquat 0 1986 American Chemical Society
+ PQ' Ru(bpy),+ + DMA' Ru(bpy),,+
(1)
(2)
(3)
