J. Phys. Chem. B 2001, 105, 5341-5352
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Mechanisms of Excited-State Energy-Transfer Gating in Linear versus Branched Multiporphyrin Arrays Robin K. Lammi,† Richard W. Wagner,‡ Arounaguiry Ambroise,‡ James R. Diers,§ David F. Bocian,*,§ Dewey Holten,*,† and Jonathan S. Lindsey*,‡ Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130, Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695, and Department of Chemistry, UniVersity of California, RiVerside, California 92521 ReceiVed: March 6, 2001
We have investigated electrochemical switching of excited-state electronic energy migration in two optoelectronic gates with different architectures. Each gate consists of diarylethyne-linked subunits: a borondipyrrin (BDPY) input unit, a Zn-porphyrin transmission unit, a free-base-porphyrin (Fb-porphyrin) output unit, and a Mg-porphyrin redox-switched site connected either to the Fb porphyrin (linear gate) or to the Zn porphyrin (branched, T gate). Both the linear and branched architectures show Fb-porphyrin emission when the Mg porphyrin is neutral and nearly complete quenching when the Mg porphyrin is oxidized to the π-cation radical. To determine the mechanism of gating, we undertook a systematic photophysical study of the gates and their dyad and triad components in neutral and oxidized forms, using static and time-resolved optical spectroscopy. Two types of photoinduced energy-transfer (and/or charge-transfer) processes are involved in gate operation: transfer between adjacent subunits and transfer between nonadjacent subunits. All of the individual energy-transfer steps that funnel input light energy to the fluorescent output element in the neutral systems are highly efficient, occurring primarily by a through-bond mechanism. Similarly efficient energytransfer processes occur between the BDPY and the Zn and Fb porphyrins in the oxidized systems, but are followed by rapid and efficient energy/charge transfer to the redox-switched site and consequent nonradiative deactivation. Energy/charge transfer between nonadjacent porphyrins, which occurs principally by superexchange, is crucial to the operation of the T gate. Collectively, our studies elucidate the photophysics of gating and afford great flexibility and control in the design of more elaborate arrays for molecular photonics applications.
I. Introduction The development of molecular devices for information processing applications is one of the major objectives in materials chemistry. Such devices include sensors, wires, switches, logic elements, memory elements, and input/output components, which can be designed for use in photonic, electronic, or optoelectronic systems. Switches play an integral role in information processing; consequently, a great deal of effort has been devoted to the development of molecular-scale switches. Switches can be divided into two general categories based on whether conformational/diffusive motion is involved in converting the switch between two states. Switches that employ conformational/diffusive motion include the following: (1) Photochemical switches have been developed in which light causes interconversion of two conformers having different physical properties.1 Photochemical switches that yield altered electrochemical properties have enabled switching of electrontransfer in small arrays2 or of electron/hole migration along electronic wires.3-5 Other photoinduced structural transformations have been shown to alter electronic energy migration.6 (2) Redox switches have been developed in which electrochemistry causes conformational changes, including gross * Authors to whom correspondence should be addressed. † Washington University. ‡ North Carolina State University. § University of California.
mechanical motion.7-9 These switches have been employed in elegant molecular shuttles and other rudimentary molecular machines.10 Combinations of photochromic and electrochromic switches have also been developed.11-13 (3) Photoinduced electron transfer (PET) luminescent devices have been developed that employ a fluorophore attached to a receptor.14-18 Binding of a guest molecule or analyte to the receptor either suppresses or initiates an electron-transfer process with the neighboring fluorophore, enhancing or diminishing the fluorescence output. Swager has extended this basic concept to molecular wires for amplification of sensory signals upon binding of analytes.19 Two types of switches have been developed that do not involve conformational changes: (1) An extension of the PET idea employs a redox-active unit attached to a luminophore. In these systems, a change in oxidation state of the redox-active unit (e.g., quinone/hydroquinone) enables or precludes electron-transfer quenching of the luminophore.20 These systems are in many respects analogous to model systems of the photosynthetic reaction center, in which electron transfer occurs depending on the redox state of the electron acceptor. (2) A distinct design for an all-optical switch involves a molecule composed of two one-electron donors attached to one two-electron acceptor. Optical excitation of the donors (porphyrins) results in spectral changes upon transfer of one or two
10.1021/jp010857y CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001
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CHART 1: Structures of Linear and T-shaped Optoelectronic Gates
electrons to the acceptor. This system functions as an all-optical intensity-dependent photochromic switch.21,22 The use of redox processes as a gating mechanism enables the flow of excited-state energy to be controlled without conformational or diffusive motion. With this in mind, we developed the two optoelectronic gates shown in Chart 1.23 Each gate is composed of a molecular photonic wire that serves to transmit excited-state energy and a redox-switched site where the energy flow can be turned on and off.24 The wire consists of a boron-dipyrrin (BDPY) input unit, a Zn-porphyrin transmission unit, and a free base (Fb)-porphyrin output unit joined by diarylethyne linkers. A Mg porphyrin acts as the redoxswitched component in each array. In the linear gate, the Mg porphyrin is appended to the Fb porphyrin, while in the T gate, the Mg porphyrin is attached to the Zn porphyrin. In each neutral system, photoexcitation of the BDPY chromophore results in excited-state energy transfer followed by emission from the Fbporphyrin output unit. When the Mg porphyrin is oxidized, the fluorescence is almost completely quenched.25 The ON/OFF behavior of the linear gate can be generally explained by the relative excited-state energies of the component parts. The BDPY chromophore lies at highest energy, and the Zn-porphyrin and Fb-porphyrin excited states are each successively lower (Figure 1). The lowest excited state of the neutral Mg porphyrin lies above that of the Fb porphyrin (Figure 1A); therefore, energy transfer from the excited free base porphyrin to the neutral Mg porphyrin (denoted Fb* f Mg) is not possible and Fb emission occurs, with an excited-state lifetime of ∼13 ns (gate ON). In the oxidized form of the gate (Figure 1B), energy or charge transfer may occur from Fb* to the Mgporphyrin radical cation (Mg+). Nonradiative deactivation to the ground state then occurs by internal conversion, via the manifold of low-lying excited states of Mg+, or by reverse charge transfer (gate OFF).
In the T gate, the mechanism of gating is more complex (Figure 2). In the neutral gate (Figure 2A), energy is transferred from the photoexcited BDPY to the Zn porphyrin, from which it can flow downhill to both the Mg and Fb porphyrins. Based on previous studies on neutral dyads, Mg* will be formed in 73% of the arrays and Fb* in the remaining 27%.28 Because monomeric Mg porphyrins have multi-nanosecond excited-state lifetimes (∼10 ns), significant Mg-porphyrin emission might have been expected from the neutral T gate. The fact that fluorescence occurs almost exclusively from the Fb porphyrin (Figure 3)23 suggests that energy is shuttled efficiently from the Mg porphyrin to the nonadjacent Fb porphyrin (Figure 2A). In the oxidized T gate (Figure 2B), branching of energy (and possibly charge) flow occurs at the Zn porphyrin. Both energy transfer to the Fb porphyrin and energy (or charge) transfer to the oxidized Mg porphyrin can occur, with a 23/77 branching ratio based on results for the related dyads. Therefore, it is surprising that the Fb-porphyrin fluorescence seen in the neutral T gate is almost completely quenched in the oxidized array. This suggests that efficient communication occurs between the Fb porphyrin and the nonadjacent Mg-porphyrin π-cation radical. Given these provocative results, we undertook a detailed spectroscopic investigation of the linear and T gates and their component parts with two goals in mind: (1) to determine the mechanism(s) and time scale(s) of gating, and (2) to explain why the T gate functions as efficiently as the linear architecture, displaying predominantly Fb-porphyrin emission in the neutral form and nearly quantitative quenching when oxidized. Toward these ends, we examined the photophysics of the linear and T gates in both the neutral and oxidized forms. Parallel studies were performed on the oxidized dyad and triad components (Charts 2 and 3), which complement our previous work on the neutral forms of these and related arrays and monomer reference
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Figure 3. Emission spectra of the T gate in neutral (solid) and oxidized (dashed) forms. Obtained by excitation of the BDPY chromophore at 485 nm. Figure 1. Schematic energy-level diagram and processes for the lowest excited states of the components in the (A) neutral and (B) oxidized linear gate. The solid arrows represent energy-transfer processes except for the quenching of the excited Fb porphyrin by the adjacent Mgporphyrin radical cation in the oxidized array (B), which may also occur by charge transfer.
Figure 2. Schematic energy-level diagram and processes for the lowest excited states of the components in the (A) neutral and (B) oxidized T gate. The solid arrows represent energy transfer processes between adjacent porphyrins. The curved dashed arrows reflect energy transfer (and perhaps charge transfer for the oxidized gate) between the nonadjacent Fb and Mg porphyrins that is assisted by the intervening Zn porphyrin.
compounds (Chart 4).26-38 Collectively, these studies have achieved our goals by affording a comprehensive description of gating in these systems as well as mechanistic insights that more broadly impact our understanding of excited-state energy transfer in multiporphyrin arrays. II. Results Synthesis. Neutral Arrays. The synthesis and purification of the neutral arrays have been described elsewhere: BDPY-
ZnU;26 ZnFbU;36,39 ZnFbD;36 MgFbU;30 MgZnU;27 MgZnFbU;40 linear and T gates.41 Oxidized Arrays. The Mg-porphyrin components of the triad, dyads, and gates were oxidized electrochemically, by bulk electrolysis in CH2Cl2, CHCl3, or mixtures of CH2Cl2:CHCl3 (50:50 and 80:20), depending on solubility. (Chemical oxidations were also performed for some compounds, in CH2Cl2/ethanol (9:1) using tris(4-bromophenyl)aminium hexachloroantiimonate.) For each compound, the Mg-porphyrin half-wave potential was measured by cyclic voltammetry; bulk electrolysis was carried out at a potential 100-125 mV more positive than the E1/2 value. An electrolysis was deemed complete when the sample (specifically, the Mg-porphyrin component) was ∼99% oxidized. Spectroscopy. Static and time-resolved techniques were used in the examination of the gates and gate components. Below, we briefly describe the results for two oxidized dyad and triad components (Table 1). These studies aid in the subsequent detailed presentation of the results for the linear and T-shaped gates. Mg+ZnU. Figure 4 shows the Q-region electronic absorption and emission spectra of the neutral and oxidized T-gate components MgZnU and Mg+ZnU. The absorption spectrum of the neutral dyad is approximately the sum of the spectra of the individual Mg- and Zn-porphyrin monomers, as expected for these weakly coupled systems.27,29,42 The Zn-porphyrin Q(1,0) and Q(0,0) bands appear at 551 and 590 nm, respectively; the Mg-porphyrin analogues occur at 563 and 605 nm. The ground-state spectrum of the oxidized array shows the Znporphyrin Q-bands superimposed on the broad features characteristic of the Mg-porphyrin π-cation radical.38,43 These spectra demonstrate that the Mg porphyrin can be selectively oxidized without significant perturbation of the adjacent Zn porphyrin component. The fluorescence spectrum of neutral MgZnU, obtained by exciting the Zn porphyrin at 542 nm, shows the characteristic Mg-porphyrin emission bands at 611 and 664 nm (Figure 4 inset). No Zn-porphyrin emission is evident, indicating that Zn* f Mg energy transfer is nearly quantitative. Excitation of the Zn porphyrin in the oxidized array gives essentially no emission from either component, demonstrating efficient Zn* f Mg+ quenching followed by nonradiative deactivation. This overall quenching process was probed by time-resolved absorption
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CHART 2: Structures of Dyad Components of Gates. (Note that “U” indicates an unhindered linker and “D” indicates a distally hindered linker.)
studies on Mg+ZnU. The data (not shown) and analysis are analogous to those that we have described extensively for other porphyrin arrays26-34 and for the gates themselves (vide infra). In particular, a Zn* lifetime of 6 ( 2 ps was determined from the decay of bleaching in the Zn-porphyrin ground-state Q-bands following excitation with a 130-fs 542-nm flash. The decay
largely, if not completely, results in return to the ground state.44 Similar results were obtained upon chemical oxidation of the magnesium porphyrin. As described below, comparison of the 6 ps Zn* lifetime in the oxidized dyad with the ∼2 ns Zn* lifetime in reference monomers indicates that the rate constant for quenching of Zn* by the oxidized Mg porphyrin is (6 ps)-1.
Energy-Transfer Gating in Multiporphyrin Arrays
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5345
CHART 3: Structure of MgZnFbU Triad
CHART 4: Structures of MU′ Monomers
The two most likely quenching mechanisms, both of which are consistent with the fluorescence and transient absorption data, are as follows: (1) Energy transfer from Zn* to the oxidized Mg porphyrin producing (Mg+)*, followed by rapid internal conversion to the ground state. The first step (energy transfer) could reasonably have a rate of ∼(6 ps)-1 given that the analogous Zn* f Mg energy-transfer in the neutral array has a rate constant of (9 ps)-1 (Table 1).27 The second step (internal conversion) is expected to have a rate of g(10 ps)-1 based on studies of oxidized porphyrin monomers.45 (2) Electron transfer from Zn* to Mg+ producing Zn+Mg, followed by rapid return of the electron to the Zn porphyrin to give the ground state. Both of these steps could also conceivably have rate constants of >(10 ps)-1. The redox properties of the two components are such that both the photoinduced electron transfer the reverse charge shift are exothermic (by >1 eV and ∼0.2 eV, respectively). Additionally, the ultrafast through-bond energy transfer in MgZnU27 indicates that the electronic coupling is sufficient to support equally rapid charge transfer in the oxidized dyad. Thus, quenching of Zn* in the oxidized dyad Mg+ZnU could occur by either energy or charge transfer in ∼6 ps or less followed by equally rapid nonradiative deactivation. Mg+ZnFbU. Measurements were also made on the oxidized triad Mg+ZnFbU (Chart 3). The fluorescence that occurs exclusively from the Fb porphyrin in the neutral triad28 is almost completely absent in the oxidized form of the array. This result indicates that Fb* is efficiently quenched by the nonadjacent oxidized Mg porphyrin, followed by nonradiative decay to the ground state. It is evident from these and previous emission studies that three excited-state quenching processes occur in the oxidized Mg+ZnFbU triad. The first two represent the branched decay of Zn* involving the adjacent porphyrins: (1) Zn* f Mg+, which can occur by energy/charge transfer with a rate constant of ∼(6 ps)-1 (as inferred from studies on the Mg+ZnU dyad, described above), and (2) Zn* f Fb energy transfer which is inferred to have a rate constant of (24 ps)-1 based on studies on ZnFbU.29,30 The third process reflects the subsequent fate
of Fb*, namely quenching by the nonadjacent oxidized Mg porphyrin. We find that this Fb* f Mg+ quenching in the oxidized triad occurs with a time constant of ∼10 ps from transient absorption measurements in which the Fb porphyrin is excited (data not shown).46 The nonadjacent Fb* f Mg+ quenching process must use the intervening Zn porphyrin as an intermediate in a two-step energy/charge-transfer hopping mechanism or as an energy/ charge conduit in a superexchange mechanism. Analysis of the energetics and states involved indicates that the energy- and charge-transfer hopping mechanisms are implausible.47 Additionally, although superexchange-assisted Fb* f Mg+ electron transfer (forming MgFb+) is viable, it must be followed by corresponding nonadjacent Fb+ f Mg hole transfer. It may be difficult for these two sequential superexchange processes to occur within the ∼10 ps time observed. Although we cannot rule out this possibility, it seems more likely that nonadjacent quenching in Mg+ZnFbU occurs by superexchange-assisted Fb* f Mg+ energy-transfer mediated by the Zn porphyrin, followed by rapid internal conversion within the manifold of low-lying excited states of (Mg+)*. We have shown previously that similar Mg* f Fb nonadjacent energy transfer in the neutral triad occurs predominantly (if not exclusively) by superexchange.28 Linear Gate. Absorption and emission spectra of the neutral and oxidized linear gate (not shown) are very similar to those of the T gate (see Figures 3 and 7 and discussion below). Transient absorption experiments were performed on the gate in both its neutral and oxidized forms, exciting the BDPY at 490 nm. Representative absorption difference spectra for the neutral linear array are shown in Figure 5A. In this case, we expect successive energy transfers to occur, from the photoexcited BDPY to the Zn porphyrin and then to the Fb porphyrin. Shortly (3 ps) after excitation, the spectrum is dominated by features characteristic of BDPY*: bleaching of the prominent groundstate absorption band at 519 nm and stimulated emission at 547 nm.26 (In this spectrum (and the others) there is a small contribution of Fb* and perhaps Zn* due to direct porphyrin excitation in a fraction of the arrays.) At 23 ps, bleaching of the Zn porphyrin contributes prominently to the feature at 550 nm (as evidenced by the ratio of the features at 516 and 550 nm48), indicating that energy has reached the Zn porphyrin. Spectra at relatively long times after excitation (1.1 ns) show characteristic features of Fb*: bleaching of the Qy(1,0), Qy(0,0), and Qx(1,0) ground-state-absorption bands at 516, 550, and 592 nm, respectively, and a feature at 651 nm arising from the Qx(0,0) ground-state bleaching and stimulated emission. Kinetic analysis of data at a number of wavelengths between 500 and 700 nm resulted in dual-exponential fits; a representative plot of data in the 510-515 nm region is shown in Figure 6A. Following the instrument response (not shown), the BDPY* bleaching decays in 16 ( 2 ps and the growth of the Fb*
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TABLE 1: Gates and Gate Componentsa Linear Gate
T Gate
τ (ps)
ktrans-1 (ps)c
neutral gate (BDPY)*fZnfFb
70 ( 15
62
0.95
oxidized gate (BDPY)*fZnfFbfMg+
70 ( 15
69
0.94
1.8 ( 0.4e 16 ( 1e 47 ( 5f 31 ( 3g
2 (30%)e 17 (70%)e 45f 31g
0.98e
compoundb
gate components (BDPY)-ZnU ZnFbD MgFbU
Mg+FbU Fb* f Mg+
7(1
7
Φtransd
0.98f > 0.99g
> 0.99
compoundb neutral gate (BDPY)*fZnfFb (BDPY)*fZnfMgfFb oxidized gate (BDPY)*fZnfMg+ (BDPY)*fZnfFbfMg+ gate components (BDPY)-ZnU ZnFbU MgZnU Zn*fMg MgZnFbUh Mg*fFb Mg+ZnU Zn* f Mg+ Mg+ZnFbUh Fb*fMg+
τ (ps)
ktrans-1 (ps)c
250 ( 30
41 199
0.97 0.95
20 ( 5 55 ( 10
24 51
0.97 0.96
1.8 ( 0.4e 16 ( 1e 24 ( 2g
2 (30%)e 17 (70%)e 24g
0.98e
9 ( 1g
9g
170 ( 30i
173i
6(2
6
> 0.99
∼10
∼10
> 0.99
Φtransd
0.99g > 0.99g 0.98i
a
Measurements on neutral compounds made in toluene at room temperature. Cations generated electrochemically in CH2Cl2, CHCl3, or 1:1 CH2Cl2:CHCl3, depending on solubility (0.1 M TBAH, supporting electrolyte). b The donor in each donor-acceptor pair is listed first unless otherwise specified; where necessary, the specific energy-transfer process is given. c Inverse of energy- (or charge-) transfer rate, calculated from eq 3 and lifetimes in Table 2. d Energy/charge-transfer efficiency, calculated from eq 4. e From ref 26. f From ref 29. g From ref 27. h The triad shows some aggregation (see ref 46). i From ref 28.
Figure 4. Q-region ground-state spectra of MgZnU (solid) and Mg+ZnU (dashed). The corresponding emission spectra, obtained by excitation at 542 nm, are shown in the inset.
bleaching occurs in 70 ( 15 ps (Table 1). This Fb* formation time incorporates both the BDPY* f Zn and Zn* f Fb energytransfer processes and is fully consistent with the time constants of the individual steps deduced from studies on the component dyads (vide infra). In the oxidized form of the gate, it was not possible to measure time constants for individual energy-transfer steps.49 Instead, we measured the overall decay of the spectrum resulting from the successive BDPY* f Zn and Zn* f Fb energy transfer steps, followed by energy/charge transfer from Fb* to the oxidized Mg porphyrin and nonradiative decay. Kinetic analysis of the data gives a time constant of 70 ( 15 ps (Table 1). T Gate. The Q-region ground-state absorption spectra for the neutral and oxidized forms of the T gate are shown in Figure 7. The spectrum of the neutral compound has a large BDPY feature with a maximum at 516 nm and a smaller shoulder at
Figure 5. Representative time-resolved absorption spectra for the (A) neutral linear gate and (B) the neutral T gate, elicited by excitation of BDPY at 490 nm.
485 nm in addition to the bands from the Zn, Fb, and Mg porphyrins: Zn Q(1,0), 551 nm; Zn Q(0,0), 592 nm; Fb Qy(1,0), 516 nm (obscured by the BDPY peak); Fb Qy(0,0), 551 nm; Fb Qx(1,0), 592 nm; Fb Qx(0,0), 648 nm; Mg Q(1,0), 563 nm; Mg Q(0,0), 603 nm. In the spectrum of the oxidized compound, the features of the BDPY and the Zn and Fb porphyrins are visible on top of the broad spectrum of the oxidized Mg porphyrin.43 Emission spectra of the neutral and oxidized forms of the T gate were obtained with BDPY excitation at 485 nm (Figure 3). In the neutral array, a small amount of Mg-porphyrin
Energy-Transfer Gating in Multiporphyrin Arrays
Figure 6. (A) Kinetic data (510-515 nm region) and dual-exponential fit for the neutral linear gate. (B) Kinetic data (510-515 nm region) and dual-exponential fit for the neutral T gate.
Figure 7. Ground-state absorption spectra of the T gate in neutral (solid) and oxidized (dashed) forms.
emission is evident at 612 nm (the Q(0,0) band); however, the Fb-porphyrin Q(0,0) and Q(0,1) bands (at 649 and 715 nm, respectively) are dominant. These findings indicate that energy transfer from the photoexcited BDPY input unit to the Fbporphyrin output unit is quite efficient. In the oxidized form of the T gate, the Fb-porphyrin emission is largely quenched, indicating that Fb* f Mg+ energy/charge transfer and subsequent (Mg+)* nonradiative decay (by internal conversion or reverse charge transfer) have occurred. The neutral and oxidized forms of the T gate were examined by transient absorption spectroscopy, exciting the BDPY chromophore at 490 nm. Spectra for the neutral array are shown in Figure 5B. In this case, we expect the following processes to occur: (1) energy transfer from BDPY* f Zn, (2) branched energy flow from Zn* to both the Mg and Fb porphyrins, and (3) energy transfer from Mg* to the nonadjacent Fb porphyrin.
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5347 The time-dependent absorption changes parallel those described above for the neutral linear gate, with the addition of the contributions due to the formation and decay of Mg*. At 4 ps after excitation, the spectrum is dominated by the BDPY ground-state-absorption bleach at ∼520 nm and stimulated emission at 547 nm.26 (The spectrum also contains contributions of Fb*, best seen in isolation in the 2.2-ns spectrum, due to excitation of the Fb porphyrin in a fraction of the gates.) At 48 ps, the BDPY bleaches have disappeared as energy has flowed to the Zn porphyrin and then begun to pass to the Mg and Fb porphyrins. Thus, the Fb-porphyrin features are joined by bleachings of the Zn-porphyrin ground-state absorption bands at 551 and 592 nm,50 together with Mg-porphyrin features at 563 and 603 nm. At long times (2.2-ns spectrum), the Zn- and Mg-porphyrin bands have disappeared and only features associated with formation of Fb* remain. These features have the appropriate intensity ratios based on the static optical spectra and include Qy(1,0), Qy(0,0), and Qx(1,0) ground-state-absorption bleachings at 516, 551, and 592 nm, respectively, and the combination of Qx(0,0) bleaching and stimulated emission at 649 nm. Kinetic analysis was performed on data at several wavelengths between 500 and 700 nm. A representative kinetic trace, generated for the 510-515 nm region, is shown in Figure 6B. These data were fit to a dual-exponential function with time constants of 13 ( 1 ps and 250 ( 30 ps (Table 1). Similar results were found in other spectral regions. The 13-ps component is attributed to the decay of the (BDPY)* excited state and the 250-ps component is ascribed to the arrival of energy at the Fb porphyrin (via the Zn and Mg porphyrins). These results will be discussed in detail below. In the oxidized form of the T gate, we again measured the overall spectral decay expected to result from the following processes in combination:48 (1) BDPY* f Zn energy transfer; (2) branched decay of Zn* by energy transfer to the Fb porphyrin and energy/charge transfer to the oxidized Mg porphyrin; (3) energy (or charge) transfer from Fb* to the nonadjacent oxidized Mg porphyrin; (4) nonradiative deactivation of (Mg+)* (or MgFb+) by internal conversion (or charge shift) to give the ground state. Kinetic analysis of the data at several wavelengths resulted in a dual-exponential fit with time constants of 20 ( 5 ps and 55 ( 10 ps (Table 1). As described below, these two times are consistent with the expected composite processes, which include two routes for quenching: one that involves only steps between adjacent porphyrins (steps 1, 2, and 4 above) and another that additionally involves nonadjacent porphyrins (step 3). Excited-State Energy-Transfer Rates and Yields. The microscopic rate constants and yields of individual energytransfer steps in the dyads and triad (e.g., Zn* f Mg+ in Mg+ZnU) were calculated using eqs 1-4
1/τD ) krad + kisc + kic
(1)
1/τDA ) krad + kisc + kic + ktrans
(2)
ktrans ) 1/τDA - 1/τD
(3)
Φtrans ) ktransτDA ) 1 - τDA/τD
(4)
and the lifetime data in Table 2. In these equations, τDA is theexcited-state lifetime of the donor in the presence of the acceptor (e.g., Zn* in Mg+ZnU, τD is the excited-state lifetime of the (donor) porphyrin monomer (e.g., Zn* in ZnU′; see Chart 4), ktrans is the energy-transfer rate constant, and Φtrans is the energy-transfer efficiency. These equations assume that, other
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TABLE 2: Excited-State Lifetimes of Porphyrin Monomersa τ (ns) monomersb
Zn*c
MgU′ ZnU′ ZnU′-βf FbU′
2.4 ( 0.2e 2.4 ( 0.2g
a
Mg*c
Fb*c
9.7 ( 0.4d 13.3 ( 0.5e b
Data in toluene at room temperature. See structure of U′ monomers in Chart 4. c Zn*, Mg*, and Fb* lifetimes were measured by time-resolved emission spectroscopy. d From ref 27. e From ref 35. f Zn(II)-5,10,15-trimesityl-2-(4-ethynylphenyl)porphyrin. g From ref 56.
than energy transfer, there are no pathways for depopulating the excited state of the donor porphyrin in the multiporphyrin arrays which do not also exist in the porphyrin monomers (radiative decay (rad), intersystem crossing (isc) and internal conversion (ic)). The transient absorption and static emission data support this assumption. The only potential deviation from this scheme is in the case of quenching of a photoexcited porphyrin by an oxidized Mg porphyrin component in an array. In these cases, charge transfer is also possilbe (vide supra); for simplicity, ktrans will designate the excited-state quenching pathway by either (or both) energy or charge transfer in the oxidized arrays. We expect that the microscopic rate constants obtained from eqs 1-4 for the component dyads and triad are applicable to the gates. As noted above, it is difficult to extract this information directly from the kinetic data on the gates due to the presence of multiple processes with roughly similar rate constants operating in series or parallel (see Figures 8 and 9). Thus, to see whether the observations on the gates are consistent with those for the component units, we estimated the overall end-to-end time constants for the multistep processes in the gates (e.g., (BDPY)* f Zn f Fb in the neutral linear gate) by two means. (1) We inserted the individual microscopic rate constants (obtained from the dyad and triad component units) into the analytical expressions for a two-step sequential process (A f B f C). This sequence represents the serial processes that occur in both gates; a similar analysis was performed for branching at Zn* that occurs in the T gate. As an example, for the neutral linear gate, the calculated time constant for arrival of energy at the Fb porphyrin (BDPY* f Zn* f Fb) is 72 ps. (2) Given the relative rates and the kinetic schemes involved, a crude approximation of the end-to-end time constants in the gates involves summing the time constants of the individual steps (e.g., [BDPY* f Zn] + [Zn* f Fb] ≈ 17 ps + 45 ps ≈ 62 ps for the neutral linear gate). Both methods agree reasonably well with the measured value of 70 ( 15 ps. Thus, for simplicity we used the summing method to obtain values to compare with the measured end-to-end time constants. Anticipated overall efficiencies were determined by multiplying the calculated efficiencies for the constituent steps (e.g., {[(BDPY)*fZn][Zn*fFb]} ≈ (0.98)(0.98) ≈ 0.95). The results of all these calculations are given in Table 1. III. Discussion A main purpose of this study was to understand how the two types of gates function in their neutral state (ON ≡ light emitted by Fb-porphyrin output element) and oxidized state (OFF ≡ no light output). Based on simple distance and kinetic arguments, one might expect the linear architecture to be preferred over the T gate. In particular, two effects resulting from the branched architecture of the T gate could have compromised its ON/OFF
function: (1) Zn* f Mg energy transfer in the neutral T gate could have produced long-lived Mg* emission rather than quantitative energy transfer to the Fb-porphyrin output element, and (2) significant Zn* f Fb energy transfer in the oxidized gate could have resulted in sub-quantitative quenching of Fbporphyrin emission. Neither of these possibilities occurs to any great extent. Thus, it is clear that there is efficient communication between the distant Fb and Mg porphyrins (in addition to that between adjacent units) in both the neutral and oxidized forms of the T gate. Given the fast energy-transfer rates relative to the calculated Fo¨rster rates51 and the dependence on linker motif in both the oxidized and neutral dyads and triad,28,52 it is clear that energy transfer between adjacent units in the linear and T gates occurs via a linker-mediated through-bond process. Based on our previous work on MgZnFbU and related triads,28 it is also clear that energy transfer from Mg* to the nonadjacent Fb porphyrin in the neutral T gate occurs by a superexchange mechanism using the intervening Zn porphyrin as an energy conduit. The results obtained here on Mg+ZnU indicate that Zn* is quenched by the adjacent Mg porphyrin radical cation in the oxidized T gate, and that this occurs by energy (or charge) transfer followed by rapid nonradiative deactivation of (Mg+)* by internal conversion (or of MgZn+ by reverse charge transfer). The same is true of quenching of Fb* by the adjacent oxidized Mg porphyrin in the oxidized linear gate. The results obtained here on Mg+ZnFbU indicate that Fb* is quenched by the nonadjacent Mg porphyrin radical cation in the oxidized T gate, and that this most likely occurs by superexchange-assisted energy transfer using the intervening Zn porphyrin, followed by nonradiative decay. [Nonadjacent charge-transfer quenching is less probable (vide supra)]. All of these processes are rapid and highly efficient, and underlie effective operation of the gates: only adjacent processes occur in the linear gate and both adjacent and nonadjacent processes are utilized in the T gate. The observed overall transfer rates for the gates themselves are fully in keeping with the rates and mechanisms deduced for the dyad and triad components. These issues will be discussed below. Linear Gate. In the neutral linear gate, photoexcitation of the BDPY input element results in significant Fb-porphyrin emission. This “gate-ON” operation is facilitated by two highly efficient (primarily through-bond) energy-transfer processes (Figure 8A): BDPY* f Zn and Zn* f Fb, both of which have been examined in the requisite (BDPY)-ZnU53 and ZnFbD54 dyads (see Table 1).26,29 Accordingly, the time constant (inverse rate constant) of energy transfer from the photoexcited BDPY to the Fb porphyrin (via the Zn porphyrin) in the linear gate is calculated to be ∼62 ps. This value compares quite well with the time constant of 70 ( 15 ps determined experimentally. When the Mg porphyrin is oxidized to the radical cation, the light output of the linear gate is negligible (Figure 8B). Energy is transferred from BDPY* f Zn and Zn* f Fb as in the neutral array; however, the adjacent oxidized Mg porphyrin can quench Fb*. This occurs either by energy transfer from Fb* to Mg+ followed by rapid nonradiative decay to the ground state (via the manifold of low-lying excited states of Mg+) or by forward and reverse charge transfer between these two units. Thus, fast (∼7 ps) energy/charge transfer between the excited Fb porphyrin and the adjacent Mg-porphyrin cation radical competes very favorably with the ∼13-ns inherent excited-state lifetime of the Fb porphyrin. The result is that more than 99% of the energy reaching the Fb porphyrin is dissipated via quenching and nonradiative deactivation involving the oxidized Mg porphyrin.
Energy-Transfer Gating in Multiporphyrin Arrays
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5349
Figure 8. Representation of photophysical processes occurring in the (A) neutral and (B) oxidized forms of the linear gate. Times listed are inverse rate constants for energy transfer, as determined for the dyad and triad components of the gates (in ps) and the Fb-porphyrin excited-state lifetime, determined for the porphyrin monomer (in ns).
The time constant for this overall multistep “gate-OFF” operation is experimentally measured to be 70 ( 15 ps, which is consistent with the ∼69 ps time constant (inverse rate constant) calculated from the individual steps (17 ps + 45 ps + 7 ps; Table 1). All of the processes involved in the operation of the neutral and oxidized forms of the linear gate are very efficient (all have yields greater than or equal to 98%): 95% of the excitation energy arrives at the Fb porphyrin in the neutral gate and 94% arrives at the Mg-porphyrin π-cation radical in the oxidized compound. Thus, the ON/OFF behavior of the linear gate is the result of a series of very fast, efficient (primarily throughbond) energy/charge-transfer steps between adjacent components. T Gate. Initially, we characterized three energy-transfer processes relevant to those in the neutral T gate: BDPY* f Zn, Zn* f Mg, and Zn* f Fb, all of which have been investigated in the requisite (BDPY)-ZnU, ZnFbU, and MgZnU dyads (Table 1 and Figure 9A).26,27 The first BDPY* f Zn step is the same as that which occurs in the linear gate. Branching then occurs: 73% of the Zn* decay occurs by energy transfer to the Mg porphyrin with a rate constant of (9 ps)-1; the remaining 27% occurs by energy transfer to the Fb porphyrin (via the unhindered linker)54 with a rate constant of (24 ps)-1 (from measurements on MgZnU and ZnFbU; see Table 1).28 All of the energy ultimately arrives at the Fb porphyrin as the result of energy transfer from the excited Mg porphyrin, which occurs via a superexchange mechanism using the orbitals/states
of the intervening Zn porphyrin.28 The superexchange-mediated energy transfer is rapid and efficient ((173 ps)-1, 98% efficiency; Table 1). Measurements on the neutral T gate give a time constant of 250 ( 30 ps for the overall transfer of energy from the BDPY to the Fb porphyrin. This value is relatively consistent with the expected time constant calculated from the composite of individual steps (from measurements on dyads and triads) for the longer, superexchange-aided pathway, BDPY* f Zn f Mg f Fb (∼199 ps; 95% efficiency). Based on the branching ratio for the decay of the Zn* state (mentioned above), approximately 73% of the arrays should utilize this route for energy-transfer to the Fb porphyrin; the other 27% should proceed from the photoexcited BDPY to the Zn porphyrin and then to the Fb porphyrin. The overall time constant of energy transfer for the less-traveled (BDPY)* f Zn f Fb pathway is expected to be on the order of 41 ps. This component is not evident in the kinetic analysis because the amplitude of the transient absorption signal is small (the majority of the signal is attributed to the decay of BDPY*). Oxidation of the Mg porphyrin in the T gate again introduces a pathway for partial quenching of the adjacent Zn porphyrin and nearly quantitative quenching of the distant Fb porphyrin (Figure 9B). As in the neutral gate, Zn* is formed by quantitative energy transfer from BDPY*. Upon Mg-porphyrin oxidation, 77% of the Zn* decay occurs by energy/charge transfer to the Mg-porphyrin cation radical with a rate constant of (6 ps)-1; the remaining 23% occurs by energy transfer to the Fb porphyrin
5350 J. Phys. Chem. B, Vol. 105, No. 22, 2001
Lammi et al. Fb porphyrin (which would emit) and not be quenched by the Mg+. The absence of these two superexchange processes would completely compromise the efficient ON/OFF behavior of the T gate. IV. Conclusions Our examination of the linear and T gate architectures and related dyad and triad components has revealed the following: (1) Gating involves a series of fast, efficient through-bond energy-transfer processes in the neutral state (GATE ON ≡ light output) state and energy- or charge-transfer steps in the oxidized state (GATE OFF ≡ no light output). Energy/charge transfer occurs between adjacent subunits in the linear gate and between both adjacent and nonadjacent units in the T gate. (2) Energy (and perhaps charge) transfer between the distant Mg and Fb porphyrins in the T gate occurs by superexchange (utilizing the orbitals/states on the intervening Zn porphyrin) and is the key to the operation of the T gate. Without these Mg* f Fb and Fb* f Mg+ steps, the clear ON/OFF gating response observed for the linear gate would be completely compromised in the branched architecture. Collectively, the studies of all of these arrays has elucidated the mechanisms operative in prototypical redox-switched optical gates that do not depend on conformational or diffusive motion, and guidance for the rational design of more elaborate architectures for application in molecular photonics. V. Experimental Methods
Figure 9. Representation of photophysical processes occurring in the neutral (A) and oxidized (B) forms of the T gate. Other details follow those given in Figure 8.
with a rate constant of (24 ps)-1 (based on the results for Mg+ZnU and ZnFbU; see Table 1). Energy transfer (and perhaps charge transfer) occurs from the Fb porphyrin to the nonadjacent Mg-porphyrin cation radical via superexchange mediated by the Zn porphyrin (with a rate constant of ∼(10 ps-1); see Table 1). Using the individual steps, the expected overall time constant of BDPY* f Zn f Mg+ transfer is 23 ps (97% efficiency), which is similar to the time constant of the shorter of the two components measured in the gate (20 ( 5 ps). Again, based on the branching ratio mentioned above for the decay of Zn*, approximately 77% of the arrays should utilize this transfer route to Mg+; the other 23% should use the superexchange-aided route from BDPY* f Zn f Fb f Mg+. This latter pathway is expected to occur with an overall time constant of 51 ps (17 ps + 24 ps + 10 ps; 95% efficiency). The slower of the two components measured for the oxidized form of the T gate (55 ( 10 ps; see Table 1) compares well with this value. The above considerations show that the successful operation of both the linear and T gates is the result of a series of very fast (picosecond time scale), efficient energy-transfer processes in the neutral forms of the arrays and energy or charge transfer processes in the oxidized systems (depicted in Figures 8 and 9). The key features that allow the T gate to operate as efficiently as the linear architecture are the two nonadjacent processes that occur via superexchange mediated by the intervening Zn porphyrin (Mg* f Fb in the neutral gate; Fb* f Mg+ in the oxidized form of the gate). Without these steps, 73% of the energy in the neutral compound would pass to the Mg porphyrin (which would emit) and not reach the Fb porphyrin; 23% of the energy in the oxidized form of the array would pass to the
General. SolVents. CH2Cl2 (99.9%, Aldrich) was dried over molecular sieves prior to use. CHCl3 (99.99%, Omnisolv) was chromatographed on basic alumina immediately prior to use. Electrochemistry. General. All electrochemistry was performed in CH2Cl2, CHCl3, or mixtures of CH2Cl2:CHCl3 (50: 50, 80:20), with 0.1 M TBAH as the supporting electrolyte. An Ag/Ag+ reference electrode, a Pt wire (CV) or gauze (BE) working electrode, and a Pt gauze auxiliary electrode were employed in a three-compartment cell containing ground glass frits. (Chemical oxidation was performed on some compunds in CH2Cl2/ethanol (9:1), using tris(4-bromophenyl)aminium hexachloroantiimonate as an oxidant.) Cyclic Voltammetry. Cyclic voltammetry (BAS CV-50W) was performed on the neutral arrays. In each case, the lowest (least positive) anodic peak potential was identified as deriving from the Mg porphyrin and the half-wave potential corresponding to this porphyrin was determined to be 320-400 mV, depending on compound (dyad, triad, gate) and solvent. Bulk Electrolysis. For each compound, bulk electrolysis (BAS CV-50W, ESC potentiostat, coulometer) was performed, with stirring, at a potential 100-125 mV more positive than the Mgporphyrin half-wave potential. Electrolysis was halted when the measured current dropped to approximately 1% of its initial value (signifying that oxidation was ∼99% complete). Spectroscopic Methods. General. Samples for time-resolved fluorescence measurements were degassed by several freezepump-thaw cycles on a high-vacuum line; samples for static fluorescence and static and time-resolved absorption experiments were not degassed. Static Absorption and Fluorescence Spectroscopy. Static absorption (Cary 100) and fluorescence (Spex Fluorolog Tau2, Spex Fluoromax) measurements were performed as described previously.42,55 For the absorption measurements, a 0.5-nm data interval, a scan speed of 600 nm/min, and a spectral bandwidth of 0.5 nm were used. Fluorescence measurements utilized ∼2 µM samples, 5 nm band-pass in both excitation and detection
Energy-Transfer Gating in Multiporphyrin Arrays paths, and a right-angle geometry. Emission spectra were corrected for the sensitivity of the detection system. Time-ResolVed Fluorescence Spectroscopy. Fluorescence lifetimes were measured using fluorescence modulation (phase shift) methods on a Spex Fluorolog Tau2 spectrofluorimeter as described previously.42,55 Modulation frequencies from 10 to 250 MHz were utilized and both the fluorescence phase shift and amplitude modulation were used in modeling the data. Samples (∼5 µM) were excited in an appropriate Q-band and the emission over the entire profile was isolated with appropriate colored glass filters. Time-ResolVed Absorption Spectroscopy. Transient absorption data were obtained at room temperature as discussed elsewhere.42 Samples (∼0.1 to 0.2 mM) were placed in a 5-mm path length cuvette and stirred with a magnetic stir bar. The samples were excited at 10 Hz with 130 fs, 25-30 µJ pump pulses and probed with white light pulses of comparable duration. Each spectrum was acquired at a specific pump-probe delay time using 2D-detection and represents the average of data from 300 laser pulses. Kinetic data were generated by averaging the ∆A values in specific wavelength ranges for the entire set of pump-probe delay times and plotting these values as a function of time. A nonlinear least-squares algorithm was used to fit the kinetic traces to functions consisting of either a single or double exponential plus a constant. Acknowledgment. This research was supported by NSF Grant CHE 9988142. References and Notes (1) Feringa, B. L.; Jager, W. F.; de Lange, B. Tetrahedron 1993, 49, 8267-8310. (2) Doron, A.; Portnoy, M.; Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937-8944. (3) Saika, T.; Irie, M.; Shimidzu, T. J. Chem. Soc. Chem. Commun. 1994, 2123-2124. (4) Stellacci, F.; Bertarelli, C.; Toscano, F.; Gallazzi, M. C.; Zotti, G.; Zerbi, G. AdV. Mater. 1999, 11, 292-295. (5) Laine, P.; Marvaud, V.; Gourdon, A.; Launay, J.-P.; Argazzi, R.; Bignozzi, C.-A. Inorg. Chem. 1996, 35, 711-714. (6) Walz, J.; Ulrich, K.; Port, H.; Wolf, H. C.; Wonner, J.; Effenberger, F. Chem. Phys. Lett. 1993, 213, 321-324. (7) Ashton, P. R.; Balzani, V.; Becher, J.; Credi, A.; Fyfe, M. C. T.; Mattersteig, G.; Menzer, S.; Nielsen, M. B.; Raymo, F. M.; Stoddart, J. F.; Venturi, M.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 3951-3957. (8) Deans, R.; Niemz, A.; Breinlinger, E. C.; Rotello, V. M. J. Am. Chem. Soc. 1997, 119, 10863-10864. (9) Canevet, C.; Libman, J.; Shanzer, A. Angew. Chem., Int. Ed. Engl. 1996, 35, 2657-2660. (10) Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369, 133-137. (11) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 275284. (12) Kawai, S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 285-293. (13) Goulle, V.; Harriman, A.; Lehn, J.-M. J. Chem. Soc. Chem. Commun. 1993, 1034-1035. (14) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. AdVances in Supramolecular Chemistry; JAI Press: Stamford, CT, 1997; Vol. 4, pp 1-53. (15) Fabbrizzi, L.; Poggi, A. Chem. Soc. ReV. 1995, 197-202. (16) Bergonzi, R.; Fabbrizzi, L.; Licchelli, M.; Mangano, C. Coord. Chem. ReV. 1998, 170, 31-46. (17) Bissell, R. A.; de Silva, A. P.; Gunaratne, H. Q. N.; Lynch, P. L. M.; Maguire, G. E. M.; McCoy, C. P.; Sandanayake, K. R. A. S. Top. Curr. Chem.; Springer-Verlag: Berlin Hiedelberg, 1993; Vol. 168. (18) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515-1566. (19) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207. (20) Daub, J.; Beck, M.; Knorr, A.; Spreitzer, H. Pure Appl. Chem. 1996, 68, 1399-1404.
J. Phys. Chem. B, Vol. 105, No. 22, 2001 5351 (21) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L., III Wasielewski, M. R. Science 1992, 257, 63-65. (22) Debreczeny, M. P.; Svec, W. A.; Wasielewski, M. R. Science 1996, 274, 584-587. (23) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 3996-3997. (24) We used the term “gate” to describe these devices, in which electrochemical modulation turns the light output of the Fb porphyrin on and off. (25) Determination of the extent of quenching is complicated by residual fluorescence from molecules that have undergone demetalation of magnesium during the oxidation process (yielding a neutral Fb porphyrin in place of the oxidized Mg porphyrin). Such demetalation is estimated to occur in ∼5% of molecules. We have previously reported extents of quenching as high as 97% (ref 23); thus, correcting for the residual fluorescence resulting from demetalation, quenching is nearly quantitative. (26) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H., III; Singh, D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1998, 120, 10001-10017. (27) Hascoat, P.; Yang, S. I.; Lammi, R. K.; Alley, J.; Bocian, D. F.; Lindsey, J. S.; Holten, D. Inorg. Chem. 1999, 38, 4849-4853. (28) Lammi, R. K.; Ambroise, A.; Balasubramanian, T.; Wagner, R. W.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 2000, 122, 7579-7591. (29) Hsiao, J.-S.; Krueger, B. P.; Wagner, R. W.; Johnson, T. E.; Delaney, J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.; Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181-11193. (30) Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J.; Lindsey, J. S.; Holten, D.; Bocian, D. F. J. Mater. Chem. 1997, 7, 1245-1262. (31) Strachan, J. P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey, J. S.; Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 1119111201. (32) Strachan, J.-P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey, J. S.; Holten, D.; Bocian, D. F. Inorg. Chem. 1998, 37, 1191-1201. (33) Yang, S. I.; Lammi, R. K.; Seth, J.; Riggs, J. A.; Arai, T.; Kim, D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 1998, 102, 9426-9436. (34) Yang, S. I.; Seth, J.; Balasubramanian, T.; Kim, D.; Lindsey, J. S.; Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1999, 121, 4008-4018. (35) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann, S.; Kim, D. H.; Holten, D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines 1999, 3, 117-147. (36) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1996, 118, 11166-11180. (37) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1994, 116, 10578-10592. (38) Seth, J.; Palaniappan, V.; Wagner, R. W.; Johnson, T. E.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 11194-11207. (39) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W. Tetrahedron 1994, 50, 8941-8968. (40) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J. S. Chem. Mater. 1999, 11, 2974-2983. (41) Ambroise, A.; Wagner, R. W.; Rao, P. D.; Riggs, J. A.; Hascoat, P.; Lindsey, J. S. J. Org. Chem., to be submitted. (42) Li, J.; Diers, J. R.; Seth, J.; Yang, S. I.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 9090-9100. (43) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 5, pp 53-125. (44) It is difficult to ascertain whether a longer-lived state forms in low yield due to a small amount of photodecomposition that often occurs in the oxidized arrays during the course of the experiments. (45) (a) The photoexcited cation radicals of monomeric porphyrins decay very rapidly to the ground electronic state, presumably via the manifold of low-lying excited states present in these species (as is evident from the weak, broad features extending past 1000 nm in the absorption spectra (ref 43)). The decays appear to occur in at least two phases, one
