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 GI 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
-
Ru(bpy),,+
Ru(bpy),+
where PQ2+ is paraquat 0 1986 American Chemical Society
(1)
+ PQ' + DMA'
(2)
(3)
The Journal of Physical Chemistry, Vol. 90, No. 5, 1986 729
Letters
with Sassoon, it was suggested that back electron transfer between oxidative and reductive equivalents stored separately on redox sites in different soluble polymers could be greatly inhibited compared to reactions between comparable monomers. We recently reported the preparation and characterization of soluble redox polymers containing acceptor PQ2+sites and/or donor PTZ sites (PTZ is phenothiazine) based on chloromethylated poly~tyrene,~ e.g. ~~CH-CH~-HCH-CH~~CH-CH~*CH-CHZ~-,
I
I
I
(PS
I
-C PQ2+l/CCH,C
I
go CH2CI
-
5 00 m=/d
IY
-~cH-cH~-+~cH-cH,~CH-CH,-~~CH-CH-CH~~+
time
I
Figure 1. Intensity of transmitted light vs. time curves before and after visible flash photolysis of a CH2CI2/0.1M [(Bu,N)(PF,)] solution conM in taining 5 X 10” M [(5-H2N-phen)Ru(bpy)2](PF,)2, 3.2 X M P T Z as polymer-bound sites in PS-[PTZ],I[CH2CI],9,and 1 X in PQ2+as sites in PS-[PQ2C]s[CH2CI]22: (A) an absorbance increase observed at 456 nm and 50 ps/division, (B) an absorbance decrease observed at 753 nm and 100 ms/division, and (C) the same but at 5 5 5 nm and 500 ms/division.
CH2
I
I
CH,CI
I
+@+-eM
(PS-CPTZI,CCH,CIln,_,)
and DMA is dimethylaniline (PhNMe2). At sufficiently high concentrations of added quenchers, a redox yield “doubling” occurs R ~ ( b p y ) ~ ,++ DMA-
+
R ~ ( b p y ) , + PQ2+
+ DMA’ R ~ ( b p y ) 3 ~++ PQ+
Ru(bpy),’+
-
(4) (5)
before back electron transfer can occur between Ru(bpy)3+ and DMA’, R ~ ( b p y ) , ~and + PQ+, or Ru(bpy),+ and Ru(bpy),,+. However, charge storage in the redox products PQ+ + DMA’ is a transient event because of rapid back electron transfer.
PQ+
I
+ DMA+
k(CH3CN) = (2.2
-
PQ2+ + DMA
(6)
* 0.1) X lo9 M-I s-I
In a recent report by Rabani and Sassoon2 and in a conversation (2) (a) Rabani, J.; Sassoon, R. E . J. Photochem. 1985, 29, 7. (b) Sassoon, R. E. J . Am. Chem. SOC.1985, 107,6133.
We report here an application of these polymeric materials to the prolongation of photochemically produced charge storage times in solution. It is known from earlier work that polyviologens quench the excited state(s) of R ~ ( b p y ) , ~ + .We ~ have verified that PSCH2-[PQ2+Ij and PS-CH2-[PTZIk also quench the MLCT excited state(s) of [(5-H2N-phen)Ru(bpy)2]2+5 (5-H2N-phen is 5-amino-1,lO-phenanthroline;T~ = 1151 ns in CH2C12at room temperat~re)~~ (3) Margerum, L. D.; Murray, R. W.; Meyer, T. J. J. Phys. Chem., in
press. (4) (a) Lee, P. C.; Matheson, M. S.;Meisel, D. Isr. J . Chem. 1982, 22, 133. (b) Sassoon, R. E.; Gershuni, S.; Rabani, J. J . Phys. Chem. 1985, 89, 1937. (c) Nishijima, T.; Nagamura, T.; Matsuo, T. J . Polym.Sci., Polym. Lett. Ed. 1981, 19, 65. (5) (a) Ellis, C. D.; Meyer, T. J. Inorg. Chem. 1984,23, 1748. (b) Hupp, J. T.; Otruba, J. P.; Parus, S. J.; Meyer, T.J. J . Electrwnal. Chem. Interfacial Electrochem. 1985, 190, 287. (c) Margerum, L. D. Ph.D. Dissertation, University of North Carolina at Chapel Hill, 1985.
730 The Journal of Physical Chemistry, Vol. 90, No. 5, 1986
+ k + PS--CH2-[PTZ+] [PTZ]k-l
( 5-H2N-phen)Ru(bpy)22+* PS-CH,-[PTZ]
(5-H,N-phen)Ru(bpy)2+
k,(CH,C12) = (1.2 f 0.2)
X
SCHEME I
--*
lo8 M-'
(7)
X
lo8 M-' s
-
2( 5-NH2-phen)Ru(bpy)22+*
hu
s-I
~
~
~
The point of the experiments described here was to show that when solutions containing a Ru-bpy chromophore, PS[PQ"],[CH2C];i,g and PS-[PTZ]k[CH2C1],-Aare photolyzed, PS-[PQ'] [PQ ],_l[CH2CI],-, and PS-[PTZ+] [PTZ],-,[CH2Cl],k appear in solution. Further, because of the slowness of back electron transfers between redox sites localized on separate polymer strands, the separated oxidative and reductive redox equivalents exhibit substantially enhanced lifetimes. In a typical experiment, [(5-H2N-phen)Ru(bpy),](PF6), and the quencher polymers PS-[PTZ]k[CH2C1],-k and PS-[PQ(PF6),],[CH2CI], were dissolved in dry methylene chloride with 0.1 M tetra-n-butylammonium hexafluorophosphate [(Bu4N)(PF,)] added as electrolyte. The quencher polymers were prepared from a sample of chloromethylated polystyrene of average molecular weight 7600 g/mol (average n = 29.7). The solution was transferred to a 10-cm flash photolysis cell, bubble-deaerated with dry N 2 for 35 min, and sealed with an airtight septum cap. Conventional (microsecond) flash photolysis with a previously6 described apparatus was used to monitor the sequence of events induced by the flash. UV photolyzing light was filtered out with Corning 3-73 filters. Shown in Figure 1 are oscilloscopic traces obtained following visible flash photolysis of a solution of 0.1 M [(Bu4N)PF6]/CH,C1 M [(5-HzN-phen)Ru(bpy)2](PF6)2, 3.2 X containing 5 X M PTZ as polymer-bound sites in PS-[PTZ]l,[CH2C1]19, and 1.0 X M PQ2+ as polymer-bound sites in PS-[PQ(PF6)2]8[CH,C1]22.At 456 nm, the major absorbing species is [(5-H2N-phen)Ru(bpy),]*+ ( E ~ ~ , ( C H ~ C=N )1.59 X lo4 M-' cm-I). The rapid reappearance of Ru" (Figure 1A) on the 50 ys/division time scale confirms that a rapid quenching reaction or reactions did occur during the flash. At 456 nm, PS[PTZ+]k[CH2Cl]n_k also absorbs relatively strongly ( t 3300 M-' cm-I for 10-methylphenothiazinium cation7) which explains the short time scale loss in transmitted intensity past the preflash base line in Figure I A . In parts B and C of Figure 1 are shown the relatively longer lived transient decays which arise from disappearance of PS-[PTZ'] [PTZ]lo[CH2C1]19 (753 nm, E = 1300 M-l ~ m - l and ) ~ PS-[PQ+][PQ2+]7[CH,C1]2z (555 nm, t = 12000
-
M-I
2( 5-NH2-phen)Ru(bpy)?+
io-'
(5-H,N-phen)R~(bpy),~+* + PS-CH2-[PQ2+lj ----* (5-H,N-phen)R~(bpy),~' + PS-CH,-[PQ+] [PQ2'l,-I (8) k,'(H,O) = 2.7
Letters
,-1),4b
The transient absorption changes observed are consistent with the proposed series of reactions in Scheme I. In the scheme, oxidative and reductive quenching (reactions 7 and 8) occur within the pulse time of the flash lamp (10-1 5 p).The concentrations of the transients [ (5-NH2-phen)Ru(bpy),]+, [ (S-NH,-phen)( 6 ) (a) Keene, F. R.; Young, R. C.; Meyer, T. J. J . Am. Chem. SOC.1977. 99, 2468. (b) Nagel, J. K. Ph.D. Dissertation, University of North Carolina at Chapel Hill, 1978. (7) Litt, M. H.; Radovic, J. J . Phys. Chem. 1974, 78. 1750.
(5-NH,-phen)R~(bpy),~+*+ PS-[PTZ], 1[CH2C1119
-
(6)
k,
(5-NH2-phen)Ru(bpy),+ + PS[PTZl+[PTZlIO[CH,CII19 (7)
-
+ +
k,'
( 5-NH,-phen)R~(bpy),~+* PS-[PQ2+] 8 [CH,C1] 2 2 ( 5-NH2-phen)Ru( b ~ y ) , ~ +PS- [PQ+] [PQ"] [ CH2Cl]22 (8)
(S-NH,-phen)Ru(bpy),+ + PS-[PQ2fl,[CH2C1122 (5-NH2-phen)Ru(bpy),'+ + PS-[PQ+l [PQ2+17[CH2C1122 (9)
2hv
2PS-[PTZ]l1[CH+21]19 + 2PS-[PQ2+]8[CH2CI]22 2PS-[PTZ+] [PTZ]lo[CH,Cl] 2PS-[PQ+] [PQ2+]7[CH2Cl]22(AG = +2.2 eV) (1 1)
+
Ru(bpy),I3+, PS-[PTZ+] [PTZ] Io[CH2C1]19,and PS-[PQ+][PQ2+],[CH,C12],, after the flash are low (
