The Journal of
Physical Chemistry VOLUME 97, NUMBER 19, MAY 13,1993
0 Copyright 1993 by the American Chemical Society
LETTERS Extreme Viscosity Dependence for the Magnetic-Field-Sensitive Recombination Rates of Chain-Linked Triplet Radical Ion Pairs Vladimir Ya. Shafirovich’ and Elena E. Batova Institute of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia
Peter P. Levin Institute of Chemical Physics, Russian Academy of Sciences, 11 7334 Moscow, Russia Received: August 21, 1992; In Final Form: December 23, 1992
Twochain-linked triplet radical ion pairs (RIPs), 3[P+-(CH2)n-Vi’+] (P+ = Zn porphyrin and Vi’+ = viologen radical cations; n = 6 and 10) have been studied by transient optical absorption and external magnetic field ( B I0.21 T ) effects on the RIP recombination rates, k,(B) in methanol-glycerol mixtures of widely varying viscosities (0.55-123 cP). Extreme (with a maximum in k,(B = 0)) and monotonous (with a decrease of k,(B = 0)) viscosity dependences have been found for [P+-(CH&-Vi’+] and [P0+-(CH2),~Vi*+] ,respectively. The increase of k,(B = 0) correlates with the narrowing of the corresponding magnetic-field-dependent recombination rate spectra, whereas the decrease of k,(B = 0) correlates with the broadening of those.
Introduction Recombination of chain-linkedtriplet radical ion pairs (RIPs) is governed by the interplay of spin and chain dynamics.’ To probe chain dynamics in flexible triplet biradicals, Doubleday, Turro, and their collaborators2used the effects of solvent viscosity (7) on the recombination rate constant (k,) and found two kinetic limits. Biradicals show a pronounced viscosity dependence (monotonous decrease of k,) in the limit of “slow chain motions” and no dependence3 in that of “fast chain motions”. To obtain information on spin dynamics, Weller, Staerk, and co-workers4 studied the viscosity perturbations in magnetic-field-dependent (MFD)reaction yield spectra of polymethylene-linked RIPs and found the narrowing of MFD spectra with an increase of viscosity. In terms of static ensemble approximation (SEA) theory of Shulten and Bitt1,s this effect means the enhancement of the intersystem crossing (ISC) in RIPs. In this paper we wish to report that in a regime of “fast chain motions” the viscosity perturbations of the ISC in chain-linked triplet RIPs appear in two kinetic regularities: (i) an unprecedentedin classical kinetics extreme viscosity dependence with a maximum in the recombination rute constant; (ii) “classical” monotonous viscosity dependencewith a decrease of the recombination rate constant. These regularities are related to chain length. The extreme dependence is observed in RIPs with a “middle” polymethylene
chain ( n = 6)and the monotonous one is observed in RIPs with a “long” chain ( n = 10). Two zinc porphyrin-viologen dyads, P-(CH2)n-Vi2+ ( n = 6 and 10) were chosen from a series of 14 dyads6 with systematic variations inchainlength (n = 2-16). Triplet RIPs weregenerated by photoinduced intramolecular electron transfer (ET) from 3P to Vi2+. The intramolecular triplet quenching being controlled by a restricted diffusion was used to probe chain motions. The changesof the ISC were assessed from the viscosity perturbations of the MFD recombination rate constant spectra.
d Experimental Section
The absorption spectra and decay kinetics of the intermediates were recorded by nanosecond laser flash photolysis using a PRA LN-102dye laser (420 nm) pumped by an LN-1000 nitrogen laser as an excitation source.’ Kinetic curves were averaged over
0022-3654/93/2091-4811~04.00/0 0 1993 American Chemical Society
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4878 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
128 laser pulses by a Biomation 6500 waveform recorder coupled to an Apple IIe microcomputer. In the magnetic field experiments the quartz cell (8 X 8 mm cross-section) was placed between two pole pieces of a permanent magnet. The distance between the pole pieces can be varied and magnetic field strength B I0.21 T. The dyads containing PF6- as a counteranion were prepared and purified by methods described previously.8 Commercially available “chemically pure” grade methanol and glycerol were used without purification. Viscosities of methanol-glycerol mixtures were measured by an Ubbelohde viscometer. Sample concentrationswere (2-5) X 10-5M. Before the laser flashstudies the solutions were carefully flushed with argon.
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Discussion To obtain information on the relationship between spin and chain dynamics, rates of chain motions should be estimated unambiguously. These rates can be assessed from the observed rates of the intramolecular triplet quenching. The monotonous retardation of the quenching process in viscous media (Figure 1a) and independence of the k, value on solvent dielectric constant (e, = 2-110)699e lend support that chain motions are the ratelimiting step and the subsequent intramolecular electron transfer (ET) from 3P to Vi2+is fast. Thus, the intramolecular triplet quenching occurs in a regime of “slow chain motions”,las2and k, can be taken as an effective rate constant of those. Figure 1 shows that the values of k, are greater than those of k, at all viscosities. Such a ratio between k, and k, lends an unequivocal evidence that the triplet RIP recombination occurs in a regime of ‘fast chain motions”,lav2 and the rate-limiting step
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Results Laser photoexcitation of P-(CH2),-Vi2+ in methanol-glycerol mixtures result in theappearance of the triplet excited, 3P-(CH2)nViz+and RIP, P*+-(CH2),-Vi*+ states with characteristic maxima in transient absorption ~ p e c t r a . ~The , ~ traces of the transient absorption contain the fast component, which corresponds to the intramolecular quenching of 3Pby Vi2+(absorption decay at 470 nm) accompanied by the triplet RIP formation (absorption rise at 4 10 and 620 nm) and slow component reflecting the RIP decay (absorption decay at 410, 470, and 620 nm). The observed rate constant of the intramolecular quenching of 3P by Vi2+ (ke)decreases with the increase of solvent viscosity for all dyads (Figure la). The lifetimes of RIPS are stronglyperturbed by solvent viscosity and by application of an external magnetic field. The extreme dependence (with a maximum) of the RIP recombination rate constant on solvent viscosity in zero magnetic field is observed in the case of P‘+-(CH&,-Vi*+ (Figure lb). P’+-(CH2),rVi0+ with the longer spacer exhibits the monotonous decrease of the k,(B = 0) value with the increase of solvent viscosity (Figure IC). Large magnetic field effect, k,(B = O)/k,(B = 0.21 T) lends strong support that RIPs are born in the triplet alignment.’ In high magnetic field (B = 0.21 T) the recombination rates for both triplet RIPs are practically independent of solvent viscosity (Figure lb,c). The MFD rate spectra judging by their shapes can be described by the following parameters: B,,,, magnetic field strength at which k,(B) has a maximum; B 1 p ,magnetic field strength at which magnetic field effect attains half of its saturation value. The viscosity perturbations of these parameters are strongly dependent on the spacer length. In the case of 3[P’+-(CH2)6Vi*+]the increase of solvent viscosity results in the narrowing (shifts of B,,, and Bl12in low fields) followed by the broadening (shifts of B,,, and B112in high fields) of the MFD rate spectra (Figure 2a). In the caseof ~[P’+-(CH~)IO-VP+]the pronounced broadening of the MFD rate spectra is observed only (Figure 2b).
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Viscosity (cp) Figure 1. Solvent viscosity dependence of the rate constants of the
intramolecular electron transfer from the porphyrin triplet excited state to viologen (a) and the rate constants of the triplet RIPs recombination (b and c) in methanol-glycerol mixtures at 18 “C:(a) P-(CH&-Vi2+ ( 0 ) ;P-(CHz)lrVi2+(A);(b) 3[P’t-(CH2)6-Vi*t], B = 0 ( O ) ,B = 0.21 T (B); (e) 3[P’+-(CH2)lrVi*+],B = 0 (O), B = 0.21 T (B). is the backward intramolecular ET from Vi*+to P+, Dielectric properties of media might be important in determining the recombination rate. However, we f o ~ n that d ~the ~ k,(B) ~ ~ values in all fields (B = 0-0.21 T) within an experimental accuracy were the same in ethyl acetate, acetone, and acetonitrile (es = 6.0, 20.7, and 37.5, respectively). Weller, Staerk, and co-workers4 found that dielectric constant did not affect the shape of the MFD triplet yield spectra. The independenceof k,(B) on esmeans that the recombination is controlled by the ISC, which to a first approximation is independent of dielectric properties of solvent. In nonviscous media anisotropic interactions are averaged to zero by the molecular tumbling motion, and the ISC is governed by the relationship of the electron-nuclear hyperfine coupling (hfc), the Zeeman splitting, the exchange interaction (4,and the spin-orbit coupling.’ The exchange interaction and the SOC in the RIPs are determined by distances between the individual
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The Journal of Physical Chemistry, Vol. 97, NO. 19, 1993 4879
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22 0 B ImT) Figure 2. Magnetic-field-dependent recombination rate spectra in methanol-glycerol mixturesat 18 O C : (a) 3[P'+-(CH2)6-Vi'C]; (b)I[P.+(CH2) l,~-Vi'+].
radical ions and their mutual orientations. Kaptein and DeKanterIO and Turro, Doubleday, and co-workers1a.2showed the importance of the SOC in the recombination of triplet biradicals. In the triplet RIPs under consideration the SOC induces the spin-forbidden transition (intersystem ET with the rate constant ksoc)to the ground The value of ksoc does not depend on B, and its contribution to k,(B) value can be estimated from the limit value of k,at high B, k,(B = 0.21 T), when intersystem ET is a limiting step of the RIP recombination. Parts b and c of Figure 1 show that the k,(B = 0.21 T) values do not depend on a spacer length, and their viscosity perturbations are weak. It is not surprising because the recombination occurs in a regime of "fast chain motions", and the RIP conformations in which the SOC is highly efficient are formed in a nonlimiting step. In zero magnetic field the relative contribution of the intersystem ET decreases with the increase of a spacer length; this implies the appearance of an alternative route of the ISC induced by the hf~,8.9d,e
The exchange interaction, that energetically separates singlet and triplet states of RIPs, decreases the efficiency of the ISC induced by the hfc.] In nonviscous media the "effective", i.e. the most probable triplet-singlet splitting, 2J,rr corresponds to B,,, in the MFDrateconstant spectra.'q4s5 Themagnetic field changes the Zeeman splitting between the triplet sublevels T+, TO,and T-, but leaves 2Jeff unchanged. Level crossing between S and T-for negative JandT+ for positive Jgive maxima in k,at B comparable to 2Jeff.In nonviscous media thedecreaseof 2Jeffwiththe increase of chain length" appears in the enhancement of k,(B = 0) and the narrowing of the MFD rate constant spectra (B,,, and B1p shifts in low fields). These perturbations of the recombination rates and parameters of the MFD spectra induced by the changes of chain length are well established in chemistry of chain-linked RIPs and flexible biradicals and were observed in many research groups.l a - c ~
Figure 2 shows that the viscosity perturbations of the k,(B = 0) values correlate with those of the MFD rate constant spectra: (i) an increase of k,(B = 0) is related to the narrowing of the MFD rate constant spectra; (ii) a decrease of k,(B = 0) is related to the broadening of the MFD rate constant spectra. The qualitative interpretation of this correlation can be made in terms of the SEA theory of Shulten and Bittl.5 Within this theory, Jew is a function of both the interradical distance distribution p ( r ) and the time scale of the distance variation r(t). Using the SEA, Weller, Staerk, and co-workers4 showed that by assuming a constantp(r) distribution, a decrease of chain motion rates results in a decrease of 2Jeff,which appears in the narrowing of the MFD spectrum. Indeed, for 3[P'+-(CH2)6-Vi'+] the decrease of k, (Figure la) correlates with the increaseof k,(B = 0) (Figure lb) and the narrowing of the MFD rate spectra (Figure 2a) at middle viscosities (28 cP). Since the triplet RIPs are born in closed conformations (a tight contact between 3P and Vi2+is required for ET), in highly viscous media the maximum of p(r) function can move to small r values in the ISC time scale. The distortion of the distance distribution p(r) can result in the increase of Jeff. Thus, at high viscosities the'decrease of k,(B = 0) values (Figures lb,c), associated with the broadening of the MFD rate spectra (Figure 2), can be explained by the increaseof Jeff. An alternative interpretation of the MFD rate spectra broadening can include the effects of the dipolar interaction between unpaired electrons. At high viscosities an incomplete averaging of the dipolar interaction1 can make a substantial contribution in the T d splitting and change the ISC induced by the hfc, so a difference between B,, and We,may exist. Another effect of an incompletely averaged anisotropic (hfc, dipolar) interaction is the possibility of efficient spin relaxation transitions.14 These transitions might be responsible for the delayed MFD ratespectra (Figure 2) in the intermediate fields (10 < B < 200 mT). We understand clearly that further research is necessary to better elucidate the contribution of the different couplingsin theviscosity perturbations of the ISC, and this work is in progress now.
References and Notes (1) For recent reviews see: (a) Doubleday, C., Jr.; Turro, N. J.; Wang, J.-F. Acc. Chem. Res. 1989, 22, 199. (b) Steiner, U. E.; Ulrich, T. Chem. Rev.1989, 89, 51. (c) Closs, G. L.; Forbes, M. D. E.; Piotrowiak, P. J. Am. Chem. SOC.1992, 114, 3285, and references cited therein. (2) (a) Zimmt, M.; Doubleday, C., Jr.; Turro, N. J. J . Am. Chem. SOC. 1985, 107, 6724. (b) Zimmt, M.; Doubleday, C., Jr.; Turro, N . J. J . Am. Chem. SOC.1986, 108, 3618. (3) Nonmonotonouschangesofbiradicallifetimesonviscosityordielectric constant were observed in the solvents covering a range of properties.',' An intrinsic molecular effect (chain length) was found, rather than a medium effect. (41 . , (a) ~,Staerk. H.: Busmann.H.-D.:Kuhnle. W.: Wel1er.A. Chem. Phvs. Lett. 1989,155, 603. (b) Busmann, H.:D.; Staerk, H.; Weller, A. J . Ch& Phys. 1989, 91, 4098. (5) (a) Shulten, K.; Bittl, R. J . Chem. Phys. 1986,84, 5155. (b) Bittl, R.; Shulten, K. Chem. Phys. Le??. 1988, 146, 58; 1990, 173, 387; (c) Bittl, R.; Shulten, K. J . Chem. Phys. 1989, 90, 1794. ( 6 ) Shafirovich, V. Ya.; Batova, E. E.; Levin, P. P. Manuscript in preparation. (7) Levin, P. P.; Pluzhnikov, P. F.; Kuzmin, V. A. Chem. Phys. Lett. 1988, 147, 283. (8) Shafirovich,V. Ya.; Batova, E. E.; Levin, P. P.J. Chem. SOC.,Faraday Trans. 1992, 88, 935. (9) (a) Nakamura, H.; Uehata, A.; Motonaga, N.; Ogata, T.; Matsuo, T. Chem. Lett. 1987,543. (b) Uehata, A.; Nakamura, M.; Usui, S.;Matsuo, T. J . Phys. Chem. 1990,93,8197; (c) Sait0.T.; Hirata, Y.; Sato, H.; Yoshida, T.; Mataga, N. Bull. Chem.SOC. Jpn. 1988,61,1925; (d) Levin, P. P.;Batova, E. E.; Shafirovich, V. Ya. Chem. Phys. 1990, 142,279. (e) Shafirovich, V. Ya.; Batova, E. E.; Levin, P. P. Chem. Phys. 1992, 162, 155. (10) Kaptein, R.; DeKanter, F. J. J. J. Am. Chem. SOC.1982,104,4759. (1 1) The both through-space" and through-bondI3models assume the pronounced decrease of J with increase of polymethylene chain length. (12) DeKanter, F. J . J.; den Hollander, J. A.; Huizer, A. H.; Kaptein, R. Mol. Phys. 1977, 34, 857. ( 1 3) McConnel1.H. M. J . Chem.Phys. 1%1,35,508. Closs,G. L.; Miller, J. R. Science 1988, 240, 440. (14) Hayashi, H.; Nagakura, S. Bull. Chem. SOC.Jpn. 1984, 57, 322.
