J. Phys. Chem. 1994,98, 7402-7410
7402
Energy Gap and Temperature Dependence of Photoinduced Electron Transfer in Porphyrin-Quinone Cyclophanes H. Heitele,' F. PtiLlinger, T. Hiiberle, and M. E. Michel-Beyerle' Institut j%r Physikalische und Theoretische Chemie, Technische Universititt Miinchen, Lichtenbergstrasse 4, 85747 Garching, FRG
H. A. Staab' Abteilung Organische Chemie, Max- Planck- Institut f i r medizinische Forschung, Jahnstrasse 29, 69028 Heidelberg, FRG Received: April 7 , 1994; In Final Form: May 13, 1994'
We have investigated intramolecular photoinduced charge separation and recombination in a series of cyclophanebridged porphyrinquinone systems by means of time-resolved fluorescence decay measurements. Rates of charge separation have been determined as a function of the free energy change of the reaction, of the polarity of the solvent, and of the temperature. In some systems a long-lived fluorescence is observed which is attributed to a thermal repopulation of the initially excited state from the charge transfer state. This delayed fluorescence allows the calculation of the rate of recombination in these cases. The observation of delayed fluorescence for a particular donor-acceptor compound in some solvent serves as a reference for the reaction free energy of the respective charge separation (AG, N 0 eV). The free energy change in other systems is estimated by correcting for differences in the redox potentials of the respective porphyrins and quinones. Electronic couplings and reorganization energies are determined by globally fitting standard rate expressions as a function of the free energy change to the experimental rate data. Three different kinds of fits are performed by (a) using both charge separation and recombination within the nonadiabatic approximation, (b) allowing for Landau-Zener adiabaticity corrections, and (c) fitting rates of charge separation (in the normal region) only. A particular focus lies in the specific effects imposed by the compact structure of the porphyrin-uinone cyclophanes. It is shown that electron transfer in these systems is nonadiabatic and dominated by intramolecular reorganization whereas the influence of the surrounding solvent is minimized by the close packing of electron donor and acceptor.
Introduction Investigations of photoinduced electron transfer in organic donor-acceptor molecules containing porphyrins and quinones have attracted enormous interest, partly motivated by the desire to mimick the primary charge separation in photosynthesis1-5 and partly by the variability of these systems which makes them particularly useful for mechanistic studies of electron transfer in general.6'2 With these goals in mind, the Heidelberg group in our joint program designed and synthesized13during the last decade a new family of porphyrin-quinone systems in which porphyrin and quinone units are linked in a cyclophane skeleton with vertical and cofacial stacking of porphyrins and quinones (Figure 1). Due to the twofold bridging, thestructures of theseporphyrin-quinone cyclophanesare relatively rigid and well-defined. X-ray structure analyses and low-temperature 'H-NMR studies have been published. 1 3 ~ ~ One characteristic feature of the cyclophanes is the direct contact between the porphyrins and quinones which minimizes the interaction between electron donor and acceptor and the solvent. The energetics of the electron transfer in these systems is systematicallyvaried by a judicious choice of the substituents at the quinones, by comparing diphenyl- and tetraphenylporphyrins, either as the free bases or as Zn-metalated porphyrins, and by changing the polarity of the solvent. Analogous compounds which contain electrochemically and spectroscopically inactive dimethoxybenzene derivatives instead of the quinones serve as reference substances for spectroscopic measurements. The results15 of a first series of time-resolved fluorescencedecay measurements of the five cyclophanes 1-5 in a series of solvents
* Abstract published in Advance ACS Abstracts, June 15, 1994. 0022-3654f 9412098-7402$04.50/0
were interpreted in terms of a very fast photoinduced intramolecular charge separation within 1-10 ps with driving forces of AG- N -(0.05-0.7) eV and a charge recombination time on the order of 10 ps to several hundred picoseconds. Interestingly, the relevant electrontransfer parameters in some of these systems in weakly polar solvents were close to the values characteristic for the primary charge separationin photosynthetic reaction centers (RCs) of bacteria.l6 This analogy in the twocomponent porphyrin-quinone systems could be further extended" in measurements on a donor-acceptor triad (PQ1Qz) containing a porphyrin and two quinones in a sandwichlike configuration. Measurements on the compounds 16 and 19**were devoted to an investigation of the influence of the dynamics of dielectric relaxationof the surroundingsolvent on the rate of intramolecular electron transfer. Contrary to theoretical predictions, no effect of the dielectric relaxation properties of the solvent on the rate of electron transfer was observed. In the present study these earlier investigations are significantly extended in several ways. In particular, we now present sufficient experimental data to allow a detailed investigation of the rate of electron transfer as a function of the driving force of the reaction. Further measurements on the temperaturedependenceofthe rateof charge separation serve as an additional test for the consistency of the interpretation. Aquestion of special interest is the influenceof thevery compact structure of the cyclophanes on the general electron transfer kinetics. In a series of recent papers,19 Mataga et al. suggested that electron transfer, e.g. the rate of charge recombination in contact ion pairs, may vary exponentially with the driving force. On the other hand, charge recombination in solvent-separated Q 1994 American Chemical Society
Photoinduced ET in Porphyrin-Quinone Cyclophanes ion pairs of the same donor-acceptor combinationsZOobeys the usual bell-shaped curve which goes through a maximum.68a1-a With respect to the structure, the cyclophanescan be considered intermediate between these extreme cases. Furthermore, extensive investigations of linearly bridged porphyrinquinone systems6 revealed a strong effect of the polarity of the solvent on the energetics and the rates of charge separation and recombination. As will be shown below, thecompact structure of the cyclophanes dramatically reduces the influences of the solvent. Contrary to other donor-acceptor systems, the dominant reorganization in these compounds is most likely intramolecular in all solvents. Thus, the cyclophanes approach what has been called the "molecular" limit of electron transferz7which, in analogy to an intramolecular radiationless decay, interprets the electron transfer as a direct transition from the initial state to the dense manifold of intramolecular vibronic states of the products. This mechanism was invokedI8 to explain the absence of dielectric relaxation effects in these compounds.
The Journal of Physical Chemistry, Vol. 98, No. 30, 1994 7403 R' Me
4
OMe H H, CI
5
Cl
(I
Me OM8 H
7 (I
R'
9 10
R'
Me Me Me Me Me Me Me
Me
CI CF,
Me
Me Me Me Me Et Et Et Et Et
-
11: x = k 12: X Br
Materials and Methods The synthesis, the electrochemical properties, and the descrip tion of several X-ray structures of the porphyrinquinone cyclophanes have been published elsewhere.13J4 The fluorescence decay measurements in this study were performed by means of a single-photon timing apparatus and a synchroscan streak-camera (Hamamatsu (C1587) which were described b e f ~ r e . ~ The ~ J ~temporal J~ resolution was limited by the full width at half-maximum of the apparatus response function, which is about 30 ps for single-photon counting detection and 6-7 ps with the streak-camera, respectively. Using standard deconvolution procedures, we consider lifetimes down to 2 f 1 ps reliable. The porphyrin moieties of the donor-acceptor compounds were excited at the lowest energy transition which corresponds to a wavelength of 630 nm in the octaalkyldiphenylporphyrins,to 580 nm in the Zn-octaalkyldiphenylporphyrins,and to 650 nm in the tetraphenylporphryins. The fluorescence was detected at 710, 650, and 720 nm in the same series. Solvents were of spectroscopic purity and were used as purchased. The concentration of the samples was 10-5-10-4 M.
0
15
'OM.
Results and Discussion Analysis of the Fluorescence Kinetics. The fluorescencedecay kinetics of the systems studied here (Table 1) follows the qualitative picture developed in ref 15. Briefly, we distinguish three different regimes with (a) monoexponential decay with a characteristic time constant comparable to the unquenched porphyrin moiety ( 7 = 11-13 ns in the free-base porphyrins, 1.3-1.6 ns in the Zn-porphyrins), (b) strongly quenched biexponential fluorescencedecay, and (c) very short-livedfluorescence on the order of 1 ps. In addition to these dominant terms in the decay kinetics, in many cases additional weak fluorescence components with a total relative amplitude of 55%are observed. Such componentsare attributed to impurities and photoproducts and are ignored in the analysis. Representative examples of the fluorescence decay are shown in Figure 2: 1 in hexane (a); 1 in acetonitrile (b); 3 in methylene chloride (c). The occurrence of an essentiallybiexponential fluorescencedecay for 1in acetonitrile is evidenced by comparing the quality of biexponentialand tripleexponential fits to the decay curve (Figure 2b,c). The inclusion of a weak longer-lived component accounting for impurity fluorescence is always necessary. Yet, inspection of the residuals and their autocorrelation shows that the dominant fluorescence in the biexponential fit (17 ps, 91%) is in fact composed of two clearly distinct components ( 5 ps, 67%; 30 ps, 30%). The three regimes can be rationalized in terms of the photoinduced electron transfer cycle in Figure 3 if proper account is taken of the influence of the acceptor strength and the solvent
14: X
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OM8 15: X = H 16: x CI
-
18 19 20 21
Me
Me Me Me
OMe
Me
H
CI
22 H
23 CI
Me Me
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Et Et Et Et
Figure 1. Structures of the porphyrinquinone cyclophanesin this study.
Heitele et al.
7404 The Journul of Physical Chemistry, Vol. 98, No. 30, 1994
TABLE 1: Fluorescence Decay Times ri and W u Respective Amplitudes (in a), as Well as Rates of Cbarge Separation k,and Recombination & from Eq 1. compd
1 2 7 8 9 10 12 16 18 19 21
1 2 3 4 5 6 8 9 10 12 16 17
18 19 20 21 1 2 3 4 5 6
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 1
2 3 4 5 11 13 14 15 16
AGa [eV]
ka [1O1O AG, s-l] [eV]
9200 19 (18), 432 (82) 25 (17), 750 (83) 20 (20), 350 (80) 1 1 1 2 (93), 28 ( 5 ) 1 1 1
In Hexane 0.19 0.03' 0.03' 0.02' -0.24 -0.36 -0.22 -0.08" -0.7 -0.88 -0.58
10.05 1.1 0.8 1.2 -100 -100 -100 24b -100 -100 -100
120 (13), 1360 (82) 8 (73), 32 (25) 3 (89), 31 (10) 1 1 140 (16), 1700 (84) 4 (82), 27 (14) 1 1 1 1 2 1 1 3 1
In Toluene 0.02' 0.17 -1.98 -0.05" 10 -1.91 -0.06' 30 -1.9 -0.2 =loo -0.34 =loo 0.02' 0.16 -1.98 22 -1.9 -0.06' 100 -0.34 100 -0.42 =100 -0.24 >5os -0.22 50 -0.5 100 -0.76 -100 -0.94 33 -0.5 =100 -0.64
3 (71), 20 (28) 2 ( 9 9 , 18 ( 5 ) 1 1 1 3 (70), 17 (25) 2 (95),17 (14) 1 1 1 3 (70), 9 (30) 1 3 (75), 10 (23) 8 (75), 18 (25) 3 (95), 14 (5) 1 2 1 1 2 1 1 1
In CHzC12 25 -0.04" -0.08' 48 N 100 -0.14 100 -0.28 N 100 -0.42 26 -0.04" 48 -0.09" 100 -0.15 100 -0.41 100 -0.53 27 -0.05' 100 -0.35 28 -0.05" 5.46 -0.05" 166 -0.07" -0.34 >5@ -0.69 50 100 -0.95 -100 -1.13 -0.69 50 100 -0.83 N 100 -0.95 N 100 -1.13
71 (ai)
[p]
5 (67), 30 (30) 4 (89), 30 (7) 2 1 2 3 (67), 10 (33) 4 (75), 12 (23) 8 (70), 10 (30) 3 (95), 15 (5) 1
ka [lO'O d]
-1.99 -1.99 -1.98
1.0 0.6 1.1
-1.83
3.7
--
0.33 3.9 3.5 0.14 4.2
---
In MeCN -0.03' -0.07" -0.13 -0.27 -0.41 -0.04' -0.06" -0.05' -0.07' -0.34 . _
-
15 23 50 100
-
k, = { ( X + Y)+ d [ X - YJ2+ 4k,k,)/2 k2={(X+Y)-d[X-YJ2+4k,k,)/2 X-k2
k1- kz
-1.92 -1.88
6.6 5.8
-1.92 -1 -87
7.5 6.2
-1.86
14
-1.86 -1.86 -1.84
12 6.5 7.4
-1.93 -1.89
4.5 3.6
-1 -92 -1.85 -1.86 -1.84
13 9.1 7.4 6.9
50
26 21 5.36 166
-
a1 =-.
-
polarityon thefreeenergyof thecharge-separatedstate. Absence of fluorescence quenching can be attributed to a positive free energy change for the charge separation from the excited porphyrin (P) to the quinone (Q):P*Q PQ-. More polar solvents or a stronger acceptor shifts the free energy of the charge transfer state PQto free energies closer to that of the excited state P*Q. Biexponential decay is interpreted in terms of a fast charge separation and a thermally activated back-reaction from the charge-separated to the locally excited state P+Q- P*Q, leading to a prompt and a delayed fluorescence component. The rates of charge separation k, and of the back-reaction &+as well as the rate of recombination to the ground state k,and the free energy change AG, of the charge separation can be calculated from the lifetimes 71, 7 2 and relative amplitudes u1, (1 - al) of the two components of the biexponential fluorescence decay, Z(t) = 01 exp(-klt) + (1 - ul)exp(-kzt), and the fluorescence lifetime 70 of the unquenched donor with the following set of equations (kl = 1/71, k2 = 1/72, k" = 1/70):
>506 ~.
a Free energies of charge separation AG, and charge recombination A ~ are , from 1, otherwise from qs and with the daw in Table 3. Charge separation rates in thedonor-acceptor compoundscontaining two auinones were divided bv 2. For the exwrimental accuracv of the dataiee the text and Figurd4.
,x= k, + kg;Y = k, + ,k
(1)
For large driving forces the back-reactionis negligible with k, = 1 / -~1/70 H 1 / (~7 >> TO),where is the (monoexponential) fluorescence lifetime of the donor-acceptor system. The corresponding rates are summarized in Table 1. In the compounds 14-16 the presence of two identical quinones offers two equivalent pathways for theinitial charge separation. For better comparison with the systems with only one acceptor, the rates kg for the symmetric compounds were divided by 2. As we have noted before," the characterization of the ET propertieson the basis of time-resolved fluorescence measurements alone suffers from the lack of a direct proof of the formation of a charge transfer state and its subsequent decay. The lifetime of the charge transfer is only indirectly inferred from the observation of a long-lived fluorescence component. The assignment of this component to delayed fluorescence is not unique. The presence of several conformers with different rate constants constitutes an alternative explanation for an effectively biphasic or multiphasictime dependence. Since complementarytransient absorbance measurementsare only availableZBfor a very limited number of these systems, the interpretation of the decay kinetics in terms of a thermally activated back-recombination to the initially excited state has to rely on the internal consistency of the data and on a careful comparison with related systems: The main arguments supportingthis interpretation are the following: (1) The change in behavior from unquenched to biexponential tostronglyquenched monoexponential decay is strongly correlated with the free energy change of the charge separation.15 The reaction cycle in Figure 3 provides a satisfactory interpretation of these trends. (2) Transient absorption measurements with a temporal resolution better than 100 fs for some selected systems28are in satisfactoryagreement with picosecond time-resolved fluorescence kinetics. Comparabledata exist for four compounds in methylene chloride solution; the respective data are compiled in Table 2. The characteristic decay constants for the short-time behavior are identical within experimental accuracy. In particular, there is no evidence for a timedependent process on a time scale much shorter than 1 ps, which would be beyond our present resolution. Furthermore, the long-time decay of the transient absorbance is in good agreement with the long-lived flU0rWcence component. This euualitv - .is exmted. if the long-lived absorbanceis assigned to the charge-separated state t h e decay of which is the-rate
Photoinduced ET in Porphyrin-Quinone Cyclophanes
The Journal of Physical Chemistry, Vol. 98, No. 30, 1994 7405
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