Distance-Dependent Rates of Photoinduced Charge Separation and

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11 Distance-Dependent Rates of Photoinduced Charge Separation and Dark Charge Recombination in Fixed-Distance Porphyrin-Quinone Molecules Michael R. Wasielewski and Mark P. Niemczyk Chemistry Division, Argonne National Laboratory, Argonne, IL 60439

Three zinc tetraphenylporphyrin-anthraquinone derivatives were prepared in which the edge-to-edge distances between the porphyrin and quinone π systems are fixed by a rigid hydrocarbon spacer molecule. Triptycene, trans-1.2diphenylcyclopentane, and adamantane were used to fix the porphyrin-anthraquinone distance at 2.5, 3.7, and 4.9 Å, respectively. These molecules possess 1,2, and 3 saturated carbon atoms, respectively, between the porphyrin donor and the quinone acceptor. Rate constants for photoinduced electron transfer from the lowest excited singlet state of the zinc tetraphenylporphyrin donor to the anthraquinone acceptor were measured. In addition, the corresponding radical ion pair recombination rate constants for each of these molecules were also determined. The rate constants for both photoinduced charge separation and subsequent radical ion pair recombination decrease by approximately a factor of 10 for each saturated carbon atom intervening between the porphyrin donor and the quinone acceptor. These results are consistent with a model in which the rate of electron transfer is determined by weak mixing of the σ orbitals of the saturated hydrocarbon spacer with the π orbitals of the donor and acceptor. In photosynthetic reaction centers the distances between the various electron donors and acceptors are restricted by the surrounding protein.(I) Studies of model systems possessing well-defined donor-acceptor distances and geometries are necessary to fully understand the critical role of these parameters in determining the efficiency of photoinduced charge separation. Porphyrins possessing covalent linkages to quinones have become increasingly important in the study of photoinduced electron transfer reactions.(2^4) Yet, few of these models possess linkages between the porphyrin electron donor and the quinone electron acceptor which restrict both the distance and the orientation between the donor and the acceotor.(2u-v.3.4) We recently prepared a series of restricted distance porphyrin-quinone donor-acceptor molecules designed to study the dependence of the rate of electron transfer proceeding from the lowest excited singlet state of the porphyrin on the free energy of reaction.(3-4) We now report measurement 0097-6156/ 86/ 0321 -0154506.00/ 0 © 1986 American Chemical Society

Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

11.

WASIELEWSKI A N D N I E M C Z Y K

Distance-Dependent Rates

of the rate constants for both radical ion pair formation and recombination in a series of three molecules in which a zinc tetraphenylporphyrin is positioned at three different fixed edge-to-edge distances relative to an anthraquinone. The structures of these molecules, 1-3 are depicted in Figure 1. Compounds 1, 2, and 2 possess 1, 2, and 3 saturated carbon atoms, respectively between the ττ system of the zinc tetraphenylporphyrin electron donor and that of the anthraquinone electron acceptor. This results in an edge-to-edge distance of 2.5, 3.7, and 4.9 À , for I , 2, and 2, respectively.©


Experimental The synthesis of compound I has already been outlined.Q) Compound 2 was prepared as follows: trans.-1,2-diphenylcyclopentane, 4 was prepared in 65% yield by bis-alkylation of trans-stilbene with l,3-dichloropropane.(£) Compound 4 was alkylated with C H C 1 0 C H in C H C 1 using T i C l catalyst.(7) Hydrolysis of the resulting chloroether during work-up yielded trans-l-(4-formylphenyl)-2-phenylcyclopentane, £ , 51%. Aldehyde £ was acylated with phthalic anhydride in 1,2-dichloroethane using an A1C1 catalyst to yield keto-acid 6, 83%. Keto-acid 6 was treated with polyphosphoric acid at 100° for 1 hr to give trans-1 -(2-anthraauinonvl)-2phenylcyclopentane, 2 in 10% yield. Treatment of 1 molar equivalent of 7 with 4 equivalents of pyrrole and 2 equivalents of benzaldehyde in refluxing propionic acid(S) for 30 min gave an 8% yield of the free base of 2 after chromatography. Treatment of the free base with excess Z n O A c in refluxing C H C 1 for 10 min gave 2 in quantitative yield: mass spectrum, m / z calcd 951.4, found 951.3. Compound 2 was prepared as follows: 1,3-diphenyladamantane, 8 was prepared from 1-bromoadamantane in 47% yield by a one step literature route.(9) Compound & was alkylated with C H C l O C H as described above to give after work-up and chromatography l-(4-formylphenyl)-3phenyladamantane, 2 in 63% yield. Aldehyde 2 was acylated as described above with phthalic anhydride to give keto-acid 10 in 79% yield. Ring closure of keto-acid 1£ in polyphosphoric acid at 100° for 1 hr gave l - ( 2 anthraquinonyl)-3-phenyladamantane, ϋ in 40% yield. One step Adler porphyrin synthesis(8) using ϋ , benzaldehyde, and pyrrole gave the free base of 3 in 4% yield after chromatography. Treatment of free base with Z n O A c as above gave 2 in quantitative yield: mass spectrum, m / z calcd 1017.4, found 1017.5. Redox potentials for 1-2 were determined in butyronitrile containing 0.1 M tetra-n-butylammonium perchlorate using a Pt disc electrode at 21°. These potentials were measured relative to a saturated calomel electrode using ac voltammetry.(lQ) Both the one electron oxidations and reductions of 1-3 exhibited good reversibility. The half-wave potentials for the oneelectron oxidation and reduction of 1-2, Z n T P P , and two model quinones are given in Table I. Fluorescence quantum yields for 1-2 and Z n T P P in butyronitrile were determined using 1 0 ' M solutions in 1 cm cuvettes at the 90° geometry. The samples were excited at 549 nm and the resultant fluorescence emission spectra were digitized. The emission spectra of 1-2 differed only in relative intensity. A direct comparison between the integrated and normalized fluorescence spectra of 1-2 was made with that of Z n T P P . The data normalized to the known fluorescence quantum yield of ZnTPPQJL) are listed in Table II. Fluorescence lifetimes were determined on Ι Ο " M solutions of 2

3

2

2

4

3

2

3

2

3

2

7

7


Figure 1.

Structures of compounds

1-3.




11.



Table I. Energetics

Compound

E\/

ZnTPP

-1.30

triptyceneanthraquinone

-0.82

2-methylanthraquinone

-0.82

E

2

î/2

-^G

-AG

ca

{

0.82

I

-0.82

0.82

0.43

1.64

2

-0.82

0.82

0.43

1.64

3

-0.82

0.82

0.43

1.64

Table II. Fluorescence Quantum Yields and Lifetimes

Compound

ZnTPP

0.030

2.0 ns

I

0.001

0.048

2

0.003

0.21

3

0.020

1.3

1-3 and Z n T P P by time-correlated photon counting(12) and are also listed in Table II. Picosecond time-resolved transient absorption measurements were obtained as follows: Solutions of 2 χ Ι Ο " M 1-3 were each prepared in butyronitrile contained in 2 mm pathlength cuvettes. The samples were degassed by three freeze-pump-thaw cycles. A 2 mm diameter spot on the sample cell was illuminated with the pump and probe beams of the transient absorption apparatus. The 600 nm, 0.4 ps, 0.5 nJ output of a mode-locked A r - synchronously pumped R 6 G / D Q O C I dye laser was amplified to 1.5 mJ using a 4-stage R640 dye amplifier pumped by a frequency-doubled N d - Y A G laser operating at 10 H z . The amplified laser pulse was split with a dichroic beam splitter. A 600 nm, 0.5 ps, 1.2 mJ pulse was used to generate a 0.5 ps white light continuum probe pulse. The remaining 0.3 mJ, 600 n m , 0.5 ps pulse was used to excite the sample. Absorbance measurements were made with a double beam probe configuration which employed optical multichannel 4

+


158

P O R P H Y R I N S : E X C I T E D STATES A N D D Y N A M I C S

detection. Time delays between pump and probe pulses were accomplished with an optical delay line. Analyses of the kinetics were carried out using the method of Provencher.Ql) Results and Discussion The one electron redox potentials of 1-1 along with those of the appropriate reference compounds are presented in Table I. The redox potentials of both the porphyrins and the quinones are not altered by linking the two molecules. These potentials were used to obtain the exothermicity of the charge separation, -AG and that of the charge recombination, - A G from Equations 1 and 2, respectively:


CB

-AG

c r

C 8

= E(Si) - E°è

-AG

c r

= E°g

-E

+E r

e

i

r e

+ el/er

i

-e2/€r

where E(Si) is the energy of the lowest excited singlet state of the molecule, in this case E(Si) = 2.07 eV for zinc tetraphenylporphyrin(H), E°g and E ^ are the measured values for the one electron oxidation and reduction of the porphyrin donor and the quinone acceptor, respectively, e is the solvent dielectric constant, r is the average center-to-center distance between the ions within the radical ion pair, and e is the charge of the electron. The term in equations 1 and 2 that depends on e is the solvent dependent coulomb energy change upon ion pair formation or recombination. For compounds 1-3 r is about 11.5 - 15 À , thus, in butyronitrile (c= 20) the term el/er varies from 0.05 - 0.06 e V . Thus, for compounds 1-2 the exothermicities of both the charge separation and radical ion recombination reactions are approximately constant. The degree of electronic interaction between the electron donor and acceptor in both the ground and lowest excited singlet states of 1-3 can be determined by an examination of their ground state optical absorption spectra and their fluorescence emission spectra. The ground state optical absorption spectra and fluorescence emission spectra of the porphyrins in 1-3 are not perturbed by the presence of the appended quinones and are. typical of Z n T P P . This is illustrated for 1 in Figure 2. However, the fluorescence quantum yields of these compounds are a strong function of the distance between the porphyrin and the quinone, Table II. We have shown previouslyQ) that the fluorescence quenching mechanism for 1 in butyronitrile is electron transfer from the lowest excited singlet state of the Z n T P P to the anthraquinone. The fluorescence data in Table II suggest that electron transfer competes effectively with the intrinsic decay pathways of the *ZnTPP* state in 2, but competes much less effectively with these pathways in 3. We have obtained additional evidence supporting the electron transfer mechanism of fluorescence quenching in 2 and 2. from picosecond transient absorption and fluorescence measurements. The fluorescence lifetimes of 1-3 in butyronitrile are reported in Table II. These lifetimes are proportional to the observed fluorescence quantum yields of these compounds and therefore indicate that the observed fluorescence quenching is not due simply to a change in the radiative rate for emission. The transient absorption spectrum observed 100 psec after excitation of 1 in butyronitrile with a 0.5 psec, 600 nm laser pulse is shown in Figure 3. We assign this spectrum to the Z n T P P A Q " radical ion pair, since it is very r e

Q

+


(1) (2)




0.0 H 360

1

400

1

460

\

1

600

WAVELENGTH

1

660

1

600

660

nm

Figure 2. Optical absorption and emission (inset) spectra of I in butyronitrile.

0.20

Ί

-0.06 i 400

1

460

1

600

1

660

WAVELENGTH

1

600

1

Γ

660

700

nm

Figure 3. Transient absorption spectrum of a 2 χ ΙΟ" M solution \ in butyronitrile at 100 ps following a 0.3 mJ, 0.5 ps, 600 nm laser flash. Filters that reject stray excitation light cut out the 580-620 nm wavelength region, while the sharp cutoff at 440 nm is due to the intense absorption of the porphyrin Soret band at 419 nm. 4


160


similar to the published spectrum of Z n T P P . ( l 5 ) Specifically, the molar extinction coefficient for Z n T P P is 3.3 χ 10 at 460 nm and 10 at 640 nm.(15) The distinct band in the 600 - 700 nm region of the spectrum is highly characteristic of Z n T P P formation. Both ZnTPP*(16) and Z n T P P * Q 7 ) possess extinction coefficients that are about 10 times smaller than that of Z n T P P in the 600 - 700 nm region. Thus, the Z n T P P spectrum strongly dominates the spectrum in Figure 3. Similar transient spectra develop and decay for 2 and 3 with kinetics that are shown in Figure 4. The decay of the transient absorption of I at 640 nm is shown in Figure 4A. It is clear that the radical ion pair produced following excitation of I decays cleanly to ground state. In Figure 4B the rise of the absorbance at 640 nm due to formation of the radical ion pair of 2 as well as its decay to ground state are both much slower than those of I in Figure 4A. Since the rate constant for formation of the Z n T P P band at 640 nm as shown in Figure 4C is smaller than that of I and 2, direct intersystem crossing of Z n T P P * to Z n T P P * is very competitive with the formation of the radical ion pair. Nanosecond flash photolysis measurements show that the absorbance at 640 nm due to ZnTPP* " decays in 10 ns. The dependence of electron transfer rate constants on distance has been described by Equation 3: +

+

4

4

+

1

3


+

+

+

1

3

1

k

e t

=

i/e-«

(3)

r

where the pre-exponential factor, υ includes the dependence of the electron transfer reaction on the electron exchange matrix element, the FranckCondon factors, and the free energy of reaction, α is a constant that depends on the overlap of the wavefunctions for the donor and acceptor, and r is the donor-acceptor distance. Thus, a plot of In k vs r should have a slope equal to - a and an intercept of In v. The rate constants determined for both photoinduced charge separation and subsequent radical ion pair recombination in 1-3 are plotted as a function of the edge-to-edge distance between the π systems of the donor and acceptor in Figure 5. The slope of the least squares fits to the data yield a = 2.3 and ν - 8 χ 10 sec" for the charge separation reaction and a - 2.1 and ν = 4 χ 10 sec for charge recombination reaction. Since the values of ν depend on several factors noted above, in the absence of additional data such as the temperature dependence of the electron transfer rate constants for i - 3 it is difficult to analyze the apparent difference between ν for the charge separation reaction and that of the radical ion pair recombination reaction. However, the difference between these two values of ν is not unreasonable given that the charge separation involves oxidation of an excited state of the donor, while radical ion pair recombination involves two ground state radicals. Small changes in the nuclear coordinates of the donor and acceptor for these two reactions should be sufficient to produce the observed difference in v. The electronic coupling factor between Z n T P P * and A Q should be different than that between Z n T P P and A Q " . The data clearly show that the rates of both of these electron transfer reactions diminish by about a factor of 10 for each saturated carbon atom between the edge of the π system of the donor and that of the acceptor. This decrease is more pronounced than that observed in small aromatic donor-acceptor molecules.(18.) In these molecules the energy of the lowest excited singlet state of the electron donor is about 3.5 eV and the energy of e t

12

11

1

- 1

1

+



Figure 4. Transient absorption changes at 640 nm for 1-3 in butyronitrile following a 0.3 mJ, 0.5 ps, 600 nm laser flash: A ) I , B) 2, C) 3.




DONOR-ACCEPTOR DISTANCE angstroms Figure 5. Plot of log k for charge separation, φ , and log k for radical ion pair recombination, , vs edge-to-edge distance between the π systems of the Z n T P P donor and anthraquinone acceptor in compounds 1-3 in butyronitrile at 21°. e t

e t


II.



the radical ion pair state is about 3 e V . These energies are substantially higher than the 2.07 e V S energy and the 1.64 e V radical pair energy for 1-3. Thus, mixing of the high energy states involved in the electron transfer reactions of small aromatic molecules with the high energy states of the spacer should be stronger than mixing of the corresponding low energy states of 1-2 with those of the spacer. This mixing results in participation of the intervening spacer bonds in the electron transfer reaction. The weak participation of the spacer bonds of 1-2 in the electron transfer reaction results in α values for the data presented in Figure £ that are somewhat higher than most a values previously reported experimentally.(19) The mechanism of electron transfer may involve either direct overlap of the π orbitals of the donor with those of the acceptor or the admixture of spacer orbitals with these orbitals.(2fi) The fact that there appears to exist an absolute dependence of the electron transfer rate on the donor-acceptor energy levels relative to those of the intervening spacer suggests that the spacer orbitals participate in the electron transfer reaction. In most rigid spacer molecules thus far studied the σ orbitals of the spacer are in the correct geometry to promote electronic interaction between atoms that are several bonds removed from one another.(2JL) This interaction is closely related to the phenomenon of spin density propagation along a w-plan hydrocarbon backbone which results in hyperfine coupling of the unpaired electron spin at a radical center to a nuclear spin several bonds away from the radical center.(21) Both electron-nuclear spin coupling and electron transfer have similar dependencies on the orientation of the spacer orbitals.(22)


x

Conclusion Our data show that a molecular spacer consisting of a rigid saturated hydrocarbon attenuates the rate of both photoinduced charge separation and subsequent radical ion pair recombination in porphyrin-quinonesdonoracceptor molecules by about an order of magnitude for each intervening saturated carbon atom between the donor and the acceptor. This strong distance dependence is most likely due to the fact that the radical ion pair states in our porphyrin-quinone model systems are sufficiently low in energy that they mix weakly with the σ orbitals of the saturated hydrocarbon spacer. Since the energies of the radical ion pair states in photosynthetic electron transfer proteins are similar to those of the model systems studied here, our results suggest that saturated bonds of the protein backbone that lie between electron donors and acceptors in these proteins may attenuate the rate of electron transfer reactions between these donors and acceptors to a degree similar to that observed in our model systems. Acknowledgment The authors wish to thank M r . W. A . Svec for his assistance in the preparation of the compounds described in this work. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U . S . Department of Energy under Contract W-31-109-Eng-38.

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2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.


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Distance-Dependent Rates of Photoinduced Charge Separation and

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