J. Phys. Chem. 1994, 98, 1758-1761
1758
Free Energy Dependence of Photoinduced Charge Separation Rates in Porphyrin Dyads Janice M. Wraziano, Paul A. Liddell, Lana Leggett, Ana L. Moore,' Thomas A. Moore,' and Devens Gust* Center for the Study of Early Events in Photosynthesis, Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287 Received: November 18, 1993'
A series of covalently linked porphyrin dyads in which the thermodynamic driving force for interporphyrin photoinduced charge separation spans a range of 1.13 eV has been prepared. Time-resolved fluorescence studies have yielded 22 rate constants for photoinduced electron transfer in dichloromethane solution ranging from 4.1 X 107 to 5.0 X 10'' s-l. The data are consistent with the theoretical treatments of Marcus and Levich, although there is no evidence for inverted behavior. In the normal region, electron transfer between free base porphyrin moieties is about 4 times faster than transfer involving a zinc porphyrin and having the same thermodynamic driving force, based on electrochemical measurements. Photoinduced electron transfer to an excited singlet state and electron transfer from an excited singlet state have the same dependence upon free energy change. Introduction The first step in the conversionof excitation energy to chemical potential in photosynthetic reaction centers involves photoinduced electron transfer from a chlorophyll excited singlet state to generate a charge-separated species. The majority of synthetic reaction center model studies mimic this process through photoinduced electron transfer from the first excited singlet state of a porphyrin taa covalentlylinked quinoneor other smallorganic acceptor.1-5 Investigations of photoinduced charge separation as a function of free energy change in these system^^^^^^^ have shown that the data are in general accord with electron-transfer theory,1&12with reorganization energies of about 1 eV in organic solvents and usually a strong temperature dependence. The initial electron-transfer event of photosynthesis,however, occurs between two large cyclic tetrapyrrole molecules. This process can be characterized by a relatively small reorganization energy and a small inverse temperature dependence. Although these differences from the behavior of the model systems are undoubtedly duein large part to the protein matrix within which photosynthetic charge separation occurs, the cyclic tetrapyrrole donor and acceptor moieties must also play a role. In order to begin to investigate their contribution, a number of research groups have undertaken studies of interporphyrin photoinduced electron transfer." Herein, we report a study of the dependence of the rate of photoinduced charge separation upon free energy change for a series of covalently linked porphyrin dyad species.
CHART 1
g I
k.
-
P-PF:
R, = CH3. R2 = CH3CONH. R, = R, = F. & H, M = H2
CP-PF:
R,
CH,,
R,
p&pF
R, = CH,,
R,
CPZ-PF
R1 CH,,
R, = R,
-
-
I
-
R,
I
F. R, = H, M = Hp
& = H.
CH,CONH, R3 iR, IF. I
M -Zn
F. R5 = H, M = Zn
-
,
P-XI:
R1 = CH,,
Results
PZ-PCI:
R1=CH3, R2=CH3CONH. R 3 = H , R 4 = k = C I . M=Zn
A total of 11 porphyrin (P) dyads and carotenoid (C) bearing triads of the form PA-PB and C-PA-PB were prepared using techniquessimilar to those we have described p r e v i ~ u s l y . ~(See ~J~ Chart 1.) The tetraarylporphyrin moieties in these molecules are all joined through amide linkages. These linkages, with their potential double bond character, constrain interchromophore separation to 19 A ~enter-to-center.'~The angular relationship between the planes of the porphyrin rings is less well constrained, but it is reasonable to assume that the conformations are fairly similar for all of the molecules under investigation. These molecules all undergo photoinduced electron transfer from the porphyrin first excited singlet states to yield an interporphyrin charge-separated state. Estimation of the thermodynamic driving force for this process (Table 1) requires a knowledge of the energies of the first excited singlet states and the PA.'+-PB'- charge-separated species. The excited-state en-
PD-PCI:
R1 =Re = (CH&N, R, = H. R,
I
R, =CI. M = H,
PmXI:
R1 I R2 = (CH3),N, R3 = H. R,
I
R,
0022-365419412098- 1758304.50/0
N
e
R1 = R2
Abstract published in Advance ACS Absrracrs, February 1, 1994.
k.
Rl
PDPF:
.
I
(CH,)pN,
R3 = R4 = F, R5 = H,
R2 = CH3CONH. R, = H, R,
-
I
MI
Zn
- R,
CI, M = H,
CI, M = Zn
PD-PFCI:
R1 R, = (CH3),N, R3 I R, = F, R, = CI. M = H2
PmPFCI:
R, = R,
= (CH,),N.
R,
I
N
R, = F. R, = CI, M = Zn
ergies were determined from the average of the energies of the longest-wavelength porphyrin absorption maximum and the shortest-wavelength porphyrin fluorescence maximum in dichloromethane. The energies of the PA'+-PB'- states were estimated from cyclic voltammetric measurements on the dyads themselves or appropriate model porphyrin amides or esters. Some of the redox potentials were reported previously.14J5 The others were measured at a platinum button electrode in benzonitrile solution containing 0.1 M tetra-n-butylammonium hexafluorophosphate with ferrocene as an internal reference redox system. The photochemistryof the dyads and triads will be exemplified by the results for dyad P-PCl. A model for porphyrin P in dichloromethane solution has absorption maxima at 420 (Soret), 0 1994 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 98, No. 7, I994 1759
80 a U a +
.-
E
.9 +
40
-ma a
4
0
I 6
680
720
760
Wavelength (nm) Figure 1. Decay-associatedfluorescenceemission spectra for dyad P - E 1 in air-saturateddichloromethaneat ambient temperaturesfollowing 590nm, 9 - p laser pulse excitation. Global analysis yielded four exponential components with lifetimes of 0.038 (V), 227 (O), 0.900 ( O ) , and 6.00 ns (A). The x 2 goodness-of-fit value was 1.24.
518,554,594, and 650 nm. A model for PCl has maxima at 448, 546, 586, 634, and 702 nm. The absorption spectrum of dyad P-PCl is nearly identical with a linear combination of those of the two porphyrin models with maxima at 420, 448, 518, 552, 592,650, and 702 nm. The emission spectrum of the P model has maxima at 657 and 720 nm, whereas that of the PCl model features maxima at 735 and 790 nm. The emission spectrum of the dyad has features of the emission of both models, but the intensity is strongly quenched. The emissionand absorption data locate the energies of the IP-PC1 and P-lPC1 states at 1.90 and 1.73 eV, respectively. The fluorescencequenching noted in the steady-state spectrum of P-PCl was also observed in time-resolved studies. The dyad in dichloromethane solution (- 1 X 10-5 M) was excited at 590 nm, where both porphyrins absorb, with 9-ps laser pulses. The fluorescencedecays at nine wavelengthsin the 645-780-nm region were determined by the single photon timing method.l6 Global analysisI7 yielded decay-associated spectra (Figure 1) with a satisfactory fit to four exponentials (x2 = 1.24). The two major components had lifetimes of 38 and 227 ps. The other two contributed less than 1%to thedecay amplitude at all wavelengths and will be ignored. The spectrum of the 38-ps component of the decay has the general shape of the P emission in the 645-700-nm region, where that moiety has its fluorescence maximum. It has a negative amplitude beyond 730 nm, where most of the emission is due to PCl. This spectral shape is indicative of singlet-singlet energy transfer from 1P to PCl. Thus, the lifetime of IP-PC1 is 38 ps. The decay pathways for 'P-PCl are shown in Figure 2. Step 3 represents decay by internal conversion, intersystem crossing, and fluorescence, and k3 is estimated as 1.3 X lo8 s-l based on the 7.6-11s fluorescence lifetime of the P model. In principle, 1P-PCl can also decay by singlet-singlet energy transfer to PCl as mentioned above and by electron transfer to PC1, yielding p'+-PCP-. Energy transfer was further investigated using steady-state fluorescence excitation spectroscopy. The emission intensity as a function of excitation wavelength was monitored at 800 nm, where essentially all of the emission comes from PCl. The corrected excitation spectrum was normalized to the dyad absorption spectrum in the 448-452-nm region, where most of the absorption is due to PC1. The excitation spectrum had features characteristic of both PCl and P, signifying energy transfer. Quantitatively, the singlet-singlet energy-transfer quantum yield (+I) was 0.47. Singlet energy-transfer phenomena in similar molecules have been reported.14 The absorption and emission spectra discussed above show that the thermodynamic driving force for step 1 is -0.17 eV. If energy transfer between the two
Figure 2. Transient states and their interconversion pathways for porphyrin dyad P-PCI.
TABLE 1: Rate Constants for Photoinduced Charge Separation ( k )and Free Energy Change Values (-AGO) for Porphyrin Dyads and Triads compound P-IPF' C-P-'PF' 'P-PF' C-'P-PFa PZ-'PF' C-PZ'PF' lPZ-PFa C-'PZPFa PZD-'PFb IPZD-PFb P-'PCI
kt (s-I) -AGO (eV) 4.1 x 107 0.10 9.7 x 107 0.10 4.1 X lo1 0.13 0.13 9.7 x 101 0.32 2.5 X 10' 0.34 2.3 X 10' 0.48 2.2 x 109 3.5 x 109 0.50 4.5 x 109 0.59 1.4 X 10'0 0.68 3.0 x 109 0.26
compound 'P-PCI p z w i 'Pz-PCI PD-'PCI 'PD-PCI PZD-1PCl lPZD-PCI PD-1PFCI lPD-PFCI PZD-IPFCI IPZD-PFCI
ka (s-I) 1.4 X 6.2 x 7.9 x 3.4 x 4.9 x 5.1 X 3.0 X 2.5 X 3.3 X 2.0 X 5.0 X
10'0 109 10'0 10'0 10'0
10'0 10'1
10" 1011 10'1 10'1
-AGO (eV)
0.43 0.48 0.83 0.60 0.70 0.73 1.02 0.8 1 0.91 0.94 1.23
Results from ref 14. Results from ref 15. porphyrin moieties were an equilibrium process, then at 298 K, k2 would be substantially slower than the other processes depopulating the P-IPC1 state. Ignoring k2 for this reason, kl is given by divided by the lifetime of 'P-PCl and equals 1.2 x 1010 s-1. As the singlet energy-transfer quantum yield is only 0.47 and k3 is small relative to the reciprocal of the fluorescence lifetime of 1P-PCl, another decay pathway must be operative. This is ascribed to electron transfer to yield P+-PCl*- (step 6 in Figure 2). This assignment is supported by transient absorption studies of PZD-PF, in which the ionic state has been observed directly,lS and of C-PZ-PF, where charge separation to form C-Pz'+PF'- is followed by a second electron transfer to yield a long-lived (240 ns) C'+-PZ-PF'-state.l4 The 38-ps lifetimeof 'P-PCl and the values for kl and k3 given above allow calculation of k6 as 1.4 X 1010 s-1. The 227-ps component of the fluorescence decay associated spectrum has the general shape of the emission of lPC1 and is assigned to the decay of P-1PCl. The P-lPC1 excited singlet state can decay by the usual photophysical pathways (step 4 in Figure 2) or by accepting an electron from P to form the P.+PCP- state discussed above (step 5). The value of k4 is estimated as 1.4 X IO9 s-1 from the 0.69-nsfluorescencelifetime of the PCl model. This value and the 227-ps lifetime of P-'PCl allow calculation of the rate constant for electron transfer kS as 3.0 X 109 S-1.
The other dyads and triads show steady-state and time-resolved fluorescence behavior analogous to that described for P-PC1. The rate constants for photoinduced charge separation originating from 'PA-PB and PA-'PB, determined in a similar fashion, are listed in Table 1. In the case of P-'PF and IP-PF, the first excited singlet states are nearly isoenergetic, and singlet-singlet
Letters
1760 The Journal of Physical Chemistry, Vol. 98, No. 7, 1994 energy transfer between them is very rapid relative to electron transfer. The two rate constants could not be extracted independently but were assumed, to a first approximation, to be equal. (Note that the values of the thermodynamic driving force for electron transfer are also nearly identi~al.1~) The same situation was encounted for C-P-PF. In all of the fluorescence decay measurements, suitable goodness-of-fit criteria were satisfied. However, the fluorescence lifetimes of 'PZ-PCl, 'PZD-PC1, lPD-PFCl, and 'PZD-PFCl were
