J. Phys. Chem. B 2002, 106, 8657-8666
8657
Radical Induced Impeding of Charge Recombination Ivan Vlassiouk,† Sergei Smirnov,*† Olaf Kutzki,‡,§ Michael Wedel,‡,| and Franz-Peter Montforts‡ 1Department 2UniVersita ¨t
of Chemistry and Biochemistry, New Mexico State UniVersity, Las Cruces, New Mexico, Bremen, Institut fu¨ r Organische Chemie, Leobener Str. NW2, 28359 Bremen, Germany
ReceiVed: April 7, 2002; In Final Form: July 1, 2002
We demonstrate that stable radicals, such as molecular oxygen and TEMPO, can inhibit back electron transfer by enhancing intersystem crossing of a singlet radical ion pair into its triplet state. The phenomenon is demonstrated on a series of dyads between porphyrin/chlorin and fullerene C60. The effect is observed only when the energy of the charge separated state is lower than that of the locally excited triplet states. Because of the spin statistics, the reverse intersystem crossing is less efficient, allowing use of oxygen and other paramagnetic species for impeding charge recombination in various electron-transfer systems. The interpretation is also confirmed by measurements of singlet oxygen yield and (lack of) magnetic field effects.
Introduction Mimicking photosynthesis has been a long-standing goal of research in the field of photoinduced electron transfer. Optimization of that process requires that the light induced charge separation effectively competes with undesired relaxation via energy transfer and back electron transfer. Numerous studies have demonstrated the importance of the redox properties of the donor and acceptor moieties, the separation distance in the donor-acceptor pair, and the reorganization energy of species involved.1-6 Increasing the distance between separated charges usually impedes their recombination and often improves the yield of charge separation. Nevertheless, mere distancing of the donor and acceptor moieties participating in photoinduced charge separation is often not sufficient. Novel approaches to slowing recombination of the charge transfer states include the use of electrolytes7 and magnetic fields.8 The latter takes advantage of intersystem crossing within the ion radical pair driven by magnetic nuclei on the ion radicals. Because the triplet pair recombination to the ground state is a spin forbidden process, the lifetime of such ion pair state is substantially longer.9 The intersystem crossing time is usually on the order of 10-7 s due to small magnetic fields from the nuclei and thus might happen within the lifetime of the singlet ion pair state but with not very high yield. High magnetic fields from stable radicals can more rapidly induced the intersystem crossing and thus improve the yield and lifetime of charge separation. We will show that stable radicals, such as oxygen and TEMPO, can be employed either as catalysts or inhibitors10 of the back electron-transfer reaction from a fullerene radical anion to a porphyrin-like cation in the photoinduced charge separated state of four dyads. This is the first example in which O2, the most important biological oxidant, acts as an inhibitor rather than an active participant (oxidant)11 * To whom correspondence should be addressed. E-mail:
[email protected]. † New Mexico State University. ‡ Universita ¨ t Bremen, Institut fu¨r Organische Chemie. § Present address: Yale University, Department of Chemistry, P.O. Box 208107, New Haven, Connecticut 06520-8107. | Present address: Duke University, Department of Chemistry, LSRC, Suite A005, Box 90317, Durham, North Carolina 27708.
SCHEME 1: Molecular Structures for the Four Molecules under Study
or a catalyst12 of back electron transfer. The effect of radicals was investigated using four types of porphyrin-C60 linked molecules shown in Scheme 1. The important role of intersystem crossing induced by paramagnetic species will be demonstrated and the mechanism of its utilization will be suggested. The mechanisms of interaction of the electronically excited states of organic molecules with molecular oxygen have been a classic problem in photochemistry and a field of intensive research for decades.13,14 Unique involvement of molecular oxygen in electron transfer phenomena arises from the combination of different roles it plays: it is a fairly good electron acceptor and thus can oxidize excited state of some molecules,9 but besides that, as a paramagnetic species in the ground state (3Σg-), it can also induce intersystem crossing between the singlet and triplet states of molecules.13 Such a combination results in different mechanisms for quenching of singlet and triplet excited states. Equations 1-7 below demonstrate a
10.1021/jp025908l CCC: $22.00 © 2002 American Chemical Society Published on Web 08/01/2002
8658 J. Phys. Chem. B, Vol. 106, No. 34, 2002
Vlassiouk et al.
traditional description of bimolecular processes for the excited singlet (1-4) and triplet (5-7) state relaxation caused by molecular oxygen. kd[O2]
k1
M* + 3O2y\ z 3(1M* ... 3O2) 98 3M* + 1O2*(1∆g’1Σg+) k-d (1)
1
k2
98 3M* + 1O2
(2)
k3
98 1M + 1O2*(1∆g,1Σg+) k4
98 1M + 3O2
(3) (4)
A molecule in its excited singlet state, 1M*, can relax upon collision with the ground-state molecular oxygen 3O2 either to the (excited) triplet state, 3M*, or directly to the ground state, 1M. Intersystem crossing (reaction 2) is thought to be the most efficient route for the deactivation of 1M*.13 Indeed, reaction 1 in many cases does not proceed because of a small S-T energy gap, and the other energy transfer reaction (4) is usually inefficient because of a small Franck-Condon factor. Reaction 3 is believed to be slow because the corresponding process is spin-forbidden. Reaction 2, is believed to proceed via transient charge transfer state, M+...O2-, where oxygen serves as an electron accepting species.13 The energy of such a transient CT state often is higher than the triplet state energy making the corresponding rate constant often less than diffusion controlled. The excited triplet molecule, 3M*, reacts with oxygen via more options (reactions 5-7): 5/9kd[O2]
k-d
3/9kd[O2]
kic
98 5(3M* ... 3O2)98 3M* + 3O2
(5) k-d
M* + 3O2 y\ z 3(3M* ... 3O2) 98 3(1M ... 3O2) 98 k
3
-d
1 1/9kd[O2]
ken
M + 3O2 (6)
k-d
z (3M* ... 3O2) 98 1(1M ... 1Oe*) 98 y\ k -d
M + 1O2* (1∆g, 1Σg+) (7)
1
Because of the spin statistics, the total spin state for the collided pair of the triplet molecule, 3M*, and the ground-state oxygen can be either zero, one, or two. Assuming that the reactions proceed without change of the total spin in the pair, as shown in reactions 5-7, simple spin-statistical assessments of these processes can be made. For example, only 3/9th of the collisions with oxygen happen in a triplet spin state and thus can lead to the triplet state relaxation via reaction 6. The remaining 5/9th of the diffusional encounters (5) happen in a quintet spin state and thus are spin forbidden toward any relaxation involving change of the spin states of the molecule and oxygen. A total of 1/9 of the collisions happen in a singlet state (7) and can result in what is formally called the energy transfer process, yielding the ground-state singlet of the molecule, 1M, and an excited singlet state of oxygen. Depending on the triplet state energy, there are two possible singlet states of oxygen, 1Σg+ (1.62 eV) and 1∆g (0.98 eV), that can be produced in the quenching. The collisions encountered in the total triplet state (reaction 6) have an option to produce the ground-state molecule via intersystem crossing in a spin allowed fashion. The last two reactions are also believed to proceed via transient charge transfer state.13
Transient charge transfer state, M+...O2- is required for decoupling electron spins in M. Thus, intersystem crossing efficiency drops when energy of M+...O2- noticeably exceeds that of M*. We suggest that in cases where the excited state of a dyad A-D already has charge-transfer character, A--D+, i.e., electron spins already are decoupled, the role of the paramagnetic species can be reduced to pure magnetic dephasing of the spin states in A--D+. In the latter case, there is no need for charge transfer onto paramagnetic species. Experimental Section Synthesis of the dyads used in this study was described elsewhere.15-19 Solvents, toluene and THF (both HPLC grade), and TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl) were purchased from Aldrich and used without further purification. Light induced dipole moment changes were measured using standard time-resolved transient displacement current setup.20 Solution of a dyad with a typical concentration 10-4 M was circulated through a dipole cell. The cell has two parallel flat stainless steel electrodes and quartz windows for laser excitation. In this geometry, the incident light is perpendicular to the electric field, but the light polarization can be varied from parallel to perpendicular with respect to the electric field. All experiments were done at room temperature, and an external voltage of 600 V was applied across 0.70 ( 0.05 mm gap between the electrodes. The third harmonic of a Nd:YAG laser (“Orion SBR” from MPB) shifted on CH4 (to make 396 nm) was used for excitation. The laser pulse duration was ca. 20 ps, and the incident energy was kept below 100 µJ. The charge displacement (dipole) signal was measured across the 1 MΩ input resistance of a P6243 active probe (Tektronix) and digitized by a 1 GHz digital oscilloscope, TDS 684A (Tektronix). In the experiments with a polar solvent (THF), because of a high dark current, the 1 MΩ probe could not be used, and the signal was measured across 50 Ω, i.e., in the displacement current mode.20 The oxygen concentration was varied by changing its partial pressures above the solution (up to 3.7 atm), and the amount was calculated using Henry coefficient 120 atm/mole.21 Zero oxygen concentration was achieved by bubbling solutions with pure nitrogen for 5 min. TEMPO concentration in the solution was monitored by optical absorbance. Absorption spectra were recorded using a Perkin-Elmer Lambda 40 UV/vis spectrometer. Locally excited triplet state energies of chlorins and phorphyrins were estimated from the phosphorescence spectra of their toluene solutions frozen to 77 K. For that purpose, glassified droplets of a solution in a quartz dewar filled with liquid nitrogen were excited by laser (at 396 nm), and luminescence spectra were measured using a SD2000 CCD spectrometer from Ocean Optics. Fluorescence was eliminated from the detection by setting a ca.1 ms delay on the CCD shutter. The singlet state energies were measured analogously in room-temperature solutions, and the results are published elsewhere.19 Phosphorescence of singlet oxygen was measured under irradiation with the same laser using InGaAs photodiode (G8370, Hamamatsu, with 1.5 V of reverse bias) terminated by a high load resistance. The oxygen luminescence at 1270 nm was filtered by a Si filter made from a Si wafer. Such an inexpensive filter efficiently filters out luminescence shorter than 950 nm and transmits ca. 8% in the range of oxygen phosphorescence. A small amount of residual fluorescence (and phosphorescence) of porphyrins/chlorins and C60 was nevertheless detected. The oxygen luminescence portion was obtained by subtracting luminescence without oxygen from that one with
Radical Induced Impeding of Charge Recombination
J. Phys. Chem. B, Vol. 106, No. 34, 2002 8659
TABLE 1: Dipole Moments, µ, and the Energies of the Local Triplet (ET) and the Charge Transfer States (ECT) for the Four Dyads along with the Singlet Charge Recombination Rates in Toluene (kCRS) ZnChl-C60 Chl-C60 ZnPor-C60 Por-C60 C60(COOEt)2 Da
µ, ET, eVc ECT, eVe kCRS, 109s-1
39 1.47 1.32 0.71
41 1.77 1.52 0.25
37 1.78 1.52 0.19
15b 1.82 1.72 0.16
0 1.50d
a Apparent dipole moment in Debye, also reflects the yield of the CT state; see also ref 23. b Smaller because of the much less than unity yield of the CT state in toluene. c Local triplet energies of appropriate porphyrin derivatives obtained from the maxima of phosphorescence spectra except for the last one, where the triplet energy is that of C60. d Reference 24. e Calculated using eq 9, see also ref 25.
oxygen. The resulting time resolution (enforced by a large load resistor) did not allow accurate measure of the oxygen formation time, but the characteristic decay time, τ ) 29 µs, of oxygen phosphorescence is clearly recognized and agrees well with published values.22 Results I. Dipole Measurements with O2. In our previous studies, we have characterized the two series of dyads between the fullerene C60 and either porphyrin15 or chlorin16,17 linked molecules (ZnP-C60, P-C60, ZnChl-C60, and Chl-C60, see Scheme 1) using the transient displacement current technique.18,19 In this technique, the signal is formed by reorientation of newly formed dipoles (CT states) in the applied electric field and results in appearance of the transient displacement current in the dipole cell and the external circuit. The dipole signal is measured across a load resistor (high impedance probe in these experiments) and is proportional to the concentration of dipolar species. The details of the setup can be found elsewhere.19,20 Table 1 summarizes the photoinduced dipole measurements data in toluene and shows that the dipole moments after excitation are similar among the dyads except for Por-C60, in which the yield of charge separation in toluene is noticeably less than unity. The latter yield increases in more polar solvents, and the dipole moment becomes close to that of other dyads.19 All in all, the four dyads represent four energetically different realizations of charge separated states with approximately equal separations distance between donor (porhyrin or chlorin) and acceptor (C60) moieties.25 Figure 1 demonstrates what is usually expected of oxygen in photoinduced electron transfer: the lifetime of the charge separated state in ZnPor-C60, created upon laser excitation, shortens with oxygen concentration. The same effect is observed for two other dyads, Por-C60 and Chl-C60. The rate constants of quenching are almost identical for all three dyads (see Table 2), which lets us conclude that they are equal to the diffusion control rate constant in toluene, kd ) 2.2 × 1010 M-1s-1. The situation is quite different for ZnChl-C60. Figure 2 displays that addition of oxygen in this solution produced a longer-lived component instead. Upon further oxygen concentration increase, the enhancement of the long-lived component intensity is accompanied by shortening of its lifetime. All of these phenomena are consistent with oxygen induced intersystem crossing depicted in Scheme 2 and by reactions 8 and 10: Being a paramagnetic species in the ground state, oxygen can induce intersystem crossing without participation in actual electron transfer, opposite to what is thought of the oxygen effect in neutral molecules. The process can take
Figure 1. A. Transient displacement charge signals for ZnPor-C60 dyad in toluene with varying oxygen concentrations from 0 M to 20.3 mM, achieved by different oxygen pressure above the solution. All signals are normalized to the same amount of absorbed energy, 50 µJ, from excitation at 396 nm and 600 V of external voltage applied across the 0.70 mm electrode gap. B. Rates of back electron-transfer induced by oxygen, kS[O2], for the three dyads: 0, ZnPor-C60; ), Por-C60, and O, Chl-C60 in toluene obtained from fitting the signals in 2A. Solid line is a guide to an eye. Rate constants obtained from linear fits to each set individually23 are given in Table 1.
TABLE 2: Rate Constants of “Oxygen Quenching” for the Four Dyads and Fullerene in Toluene
kS,a 1010 M-1s-1 kisc,c 1010 M-1s-1 k-isc,d 1010 M-1s-1 φ(1O2)e a
ZnChlC60
ChlC60
ZnPorC60
PorC60
C60(COOEt)2
b 1.15 0.4
