Molecular Electronics Wasielewski Research Group Updated by Dr. Lukas Kobr August 2012 Designing molecular systems for molecular electronics or for solar energy conversion that are capable of moving charge efficiently over long distances through molecular bridges requires a fundamental understanding of electron transport in donor-bridge-acceptor (D-B-A) systems. Charge transport has been studied in covalently linked D-B-A systems with various bridge molecules, including proteins, DNA, porphyrins, saturated and unsaturated hydrocarbons. Nevertheless, in many of these systems multiple charge transport mechanisms and pathways exist and the factors that favor particular mechanisms remain poorly understood. Continuing efforts toward understanding how the electronic structure and composition of the bridge governs the charge transport mechanism, and thus the lifetimes of photogenerated radical ion pairs (RPs), are important for the rational design of “molecular wires.” We are currently trying to answer three specific questions with our research: • How do the molecular structure and electronic properties of the bridge determine the

transition from the superexchange (tunneling) regime to the charge hopping regime? • What are the structural and electronic requirements for efficient photodriven electron or hole

transport by the charge hopping mechanism? How does the degree of hole or electron localization on discrete, energetically degenerate bridge sites affect this process?

• How can new theoretical insights into charge transport in cross-conjugated molecules impact the design of optimized molecular systems for photodriven charge transport?

We use inspiration from nature to design molecular systems that can shed light on the structural and electronic requirements for efficient, long distance, “wire-like” charge transport. A molecular wire is often described as a molecular bridge that is able to move charge efficiently over many chemical bond lengths. For example, in photosynthetic reaction center proteins, nature transfers charge over long distances using stepwise charge hopping reactions between redox cofactors. However, charge transfer in most synthetic systems occurs by a coherent tunneling process in which the energy barrier is determined by the electronic structure of the bridge molecule(s). This mechanism is known as superexchange. D-B-A systems allow us to study the structural and electronic requirements for transitioning between superexchange and charge hopping.

Figure 1. Schematic of superexchange vs. hopping mechanisms of electron transfer.

Bridge-mediated charge transport by the superexchange mechanism involves mixing of bridge states with those of the donor and acceptor and requires that the bridge states are energetically higher than and well-separated from those of the donor, resulting in an exponential dependence of the charge transport rate constant on distance, eq 1, )0(

0rr

ekk−−

=β (1)

where k0 is the rate constant at the van der Waals contact distance r0 (3.5 Å), and β is the damping coefficient for the decay. For systems undergoing superexchange charge transport, the overall electronic coupling matrix element, VDA, for charge transport is inversely dependent on the energy gap between the donor and bridge states as described by McConnell[1] (eq 2),

1−

∆∆

=N

DBBB

DBBADB

DA EV

EVVV (2)

where VDB and VBA are the matrix elements that couple the donor to the bridge and the bridge to the acceptor, respectively, VBB is the electronic coupling between bridge sites, N is the number of identical bridge sites, and ∆EDB is the energy gap for charge injection from the donor to the virtual bridge state.[1-2] In the superexchange mechanism charge does not reside on the bridge. If the bridge states energies are comparable to those of the donor, direct oxidation or reduction of the bridge may result in a charge hopping mechanism for transport, which typically has a much weaker distance dependence than superexchange (1/rn distance dependence, where 1 ≤ n ≤ 2). Achieving this weak charge transfer distance dependence in a D-B-A system is important for its potential use as a molecular wire. The use of rigid donors and acceptors with varying bridge lengths allows for the measurement of distance dependent charge transfer rates to elucidate how the superexchange and hopping charge transfer regimes within molecular systems depend on structure.[3-7] However, even in molecules having well-defined D-B-A distances, internal degrees of freedom, such as single-bond torsions, often modulate VDA. Moreover, work on oligomeric π-conjugated bridges, such as p-phenylenevinylene,[8] p-phenylene,[9] and fluorine[10] has demonstrated that a switch in mechanism from superexchange to hopping can occur at longer bridge lengths. Examples of Long Distance Charge Transport in Well-Defined D-B-A Molecule. A series of donor-bridge-acceptor (D-B-A) triads with 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) as the donor, the naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor, and with p-oligophenylene (Phn), 2,7-fluorenone (FNn) and p-phenylethynylene (PEnP) bridging units (Figure 2) was synthesized and studied using transient absorption spectroscopy in presence of magnetic fields and time-resolved EPR.[11-13] Nanosecond transient absorption spectroscopy shows that

3(D+-B-A-)

kCRTkCRT

kTT

kRPST

kCRS D-B-3*A

kCS1(D+-B-A-)

D-B-A

CT(D-B-A)

Ener

gy

3*D-B-A

b)

Figure 2. a) Donor-Bridge-Acceptor systems used in this study. b) Charge transfer scheme for FN1-3, PE1-3P, and Ph1-5.

a)

n

N N N

O

O

O

On

n = 1 - 51-3: R = n-C8H17 ; 4-5: R = 2,5-di-t-butylphenyl

PEnPn = 1 - 3 n = 1 - 3

DMJ An

FNn

NI

1,2: R = n-C8H17 ; 3: R = 2,5-di-t-butylphenyl

O

n n

Bridge R

Phn

1-3: R = 2,5-di-t-butylphenyl

Bridge =

photoexcitation of DMJ-An into its charge transfer band and subsequent electron transfer to NI results in a nearly quantitative yield of 1(DMJ+•-An-Bridge-NI-•), which undergo rapid radical pair intersystem crossing (RP-ISC) to produce the triplet RPs, 3(DMJ+•-An-Bridge-NI-•). The CR rate constants, kCR, in toluene were measured over a temperature range from 270 to 350 K, and a kinetic analysis of kCR in the presence of an applied static magnetic field was used to determine both the singlet and triplet charge recombination rate constants (kCRS and kCRT), as well as the intersystem crossing rate constant (kST). Plots of ln (kT1/2) versus 1/T for the PE1P-bridged compound show a crossover at 300 K from a temperature-independent singlet CR pathway to a triplet CR pathway that is positively activated with a barrier of 1047 ± 170 cm-1. The singlet CR pathway via the FN1 bridge displays a negative activation energy that results from donor-bridge and bridge-acceptor torsional motions about the single bonds connecting them. In contrast, the triplet CR pathway via the FN1-2 and PE1-2P bridges exhibits positive activation energies. The activation barriers to these torsional motions range from 1100 to 4500 cm-1 and are well reproduced by semiclassical electron transfer theory. While in the presence of a magnetic field the CR dynamics is modulated by the spin dynamics that arise from S-T mixing, the CR dynamics proceed by Marcus inverted region pathways in the absence of a magnetic field.

Femtosecond transient absorption spectroscopy performed on the FNn-bridged series showed FN-

• as an intermiediate, which indicates a stepwise electron transfer mechanism for the charge transport through the bridge. The overall rate exhibited exponential distance dependence. Kinetic analysis suggests that the overall kinetics is dominated by the initial injection of an electron to the first FN unit, which is the slowest step of the process due to strong electrostatic interactions between DMJ+• and An-•. As the charge separation becomes larger, the electrostatic attraction decreases, and the subsequent electron transfers increase in rate. The rich diversity in electron transfer dynamics observed in these systems, which have only relatively small structural variations, underscores the need for thorough understanding of charge transport in order to rationally design functional materials.

Figure 3. Plots of ln (kT1/2) versus 1/T for PE1P-bridged (left) and FN1-bridged derivative (right).

Cross-conjugation is defined as “a compound possessing three unsaturated groups, two of which although conjugated to a third unsaturated center are not conjugated to each other. The word conjugated is defined here in the classical sense of denoting a system of alternating single and double bonds.”[14] Although the term cross-conjugation is used infrequently, molecules that exhibit this particular type of conjugation are common in chemistry, e.g. quinones, radialenes, fulvalenes, and various fused aromatics.[15] The effects of cross-conjugation on charge transfer states in molecular systems have been examined with a view toward non-linear optical materials, magnetic materials, as well as donor-acceptor interactions. Recently, we have developed a quantum interference model,[16-18] which predicts that the rates of bridge-mediated charge transport through cross-conjugated bridges can exhibit large anti-resonances as a function of charge injection energy, i.e. the difference in energy between the initial donor state and the bridge state into which charge is injected (Figure 4). In principle, this means that the charge transfer rate through a molecular bridge can be changed over many orders of magnitude by the appropriate choice of injection energy. We are using new D-B-A systems (Figure 5) to study what effect these anti-resonances have on the rates of charge separation and recombination reactions. Our initial results show that photoinitiated CS from the DMJ-An excited singlet state through the cross-conjugated 1,1-diphenylethene bridge (1) to NI is about 30 times slower than through its linearly-conjugated trans-stilbene counterpart (2), and is comparable to that observed through the diphenylmethane bridge (3). This result implies that cross-conjugation strongly decreases the π; orbital contribution to the donor-acceptor electronic coupling, so that electron transfer most likely uses the bridge σ system as its primary CS pathway. In contrast, the CS rate through the cross-conjugated xanthone bridge is comparable to that observed through the linearly-conjugated trans-stilbene bridge. Molecular conductance calculations on these bridges show that cross-conjugation results in quantum interference effects that greatly alter the through-bridge donor-acceptor electronic coupling as a function of charge injection energy. These calculations display trends that agree well with the observed trends in the electron transfer rates.

Figure 4. The computed electron transmission through the bridges bound between metallic electrodes. Over a large range the transmission through bridges 1 and 3 is significantly lower than that through bridges 2 and 4.

The rational design of functional materials capable of efficient electron transfer requires a fundamental understanding charge separation (CS), and charge transport processes. Although various covalent donor-acceptor systems have been developed to address each of these challenges, it remains unclear how nuclear motions affect electron transfer. Time-resolved vibrational spectroscopy allows for the interrogation of structural changes that occur during these processes, which are difficult to resolve using transient absorption techniques. The recent development of femtosecond stimulated Raman spectroscopy (FSRS) overcomes several limitations of other vibrational spectroscopy techniques because it is insensitive to fluorescence or scattering, has high spectral (~10 cm-1) and temporal (~60 fs) resolution, and is self-phase matched. The ultrafast vibrational dynamics of the photoinduced charge transfer reaction between perylene (Per) and perylene-3,4:9,10-bis(dicarboximide) (PDI) in Per-Xy-PDI (Figure 5) were investigated using femtosecond stimulated Raman spectroscopy (FSRS).[19] Probing the structural dynamics of PDI following its selective photoexcitation in a covalently-linked dyad reveals vibrational modes uniquely characteristic to the PDI lowest excited singlet state and radical anion between 1000 cm-1 and 1700 cm-1. A comparison of these vibrations to those of the ground state (Figure 5) reveals the appearance of new 1*PDI and PDI-• stretching modes in the dyad at 1593 cm-1 and 1588 cm-1, respectively. DFT calculations show that these vibrations are parallel to the long axis of PDI, and thus then may be integral to the charge separation reaction. The ability to differentiate excited state from radical anion vibrational modes allows the evaluation of the influence of specific modes on the charge transfer dynamics in donor-bridge-acceptor systems based on PDI molecular constructs.

Figure 5. Chemical structures of the molecules used to study quantum interference effects in molecular bridges.

N N N

O

O

O

O

1

DMJ An

2

NI

Bridge

3

Bridge =O

O

4

Figure 5. Time-resolved FSRS and resonance-enhanced ground state Raman spectra of (A) PDI-Xy, and (B) PDI-Xy-Per in DCM. Solid lines: evolution of S1 PDI modes; dotted lines: evolution of PDI-radical-anion modes. References: (1) McConnell, H. M. Intramolecular Charge Transfer in Aromatic Free Radicals, J. Chem. Phys. 1961, 35, 508-515. (2) Ratner, M. A. Bridge-assisted electron transfer: Effective electronic coupling, J. Phys. Chem. 1990, 94, 4877–4883. (3) Lambert, C.; Noll, G.; Schelter, J. Bridge-mediated hopping or superexchange electron-transfer processes in bis(triarylamine) systems, Nat. Mater. 2002, 1, 69-73. (4) Davis, W. B.; Wasielewski, M. R.; Ratner, M. A.; Mujica, V.; Nitzan, A. Electron Transfer Rates in Bridged Molecular Systems: A Phenomenological Approach to Relaxation, J. Phys. Chem. 1997, 101, 6158-6164.

(5) Johnson, M. D.; Miller, J. R.; Green, N. S.; Closs, G. L. Distance dependence of intramolecular hole and electron transfer in organic radical ions, J. Phys. Chem. 1989, 93, 1173-1176. (6) Bixon, M.; Jortner, J. Electron Transfer: From Isolated Molecules to Biomolecules, Part 1, Adv. Chem. Phys. 1999, 106, 3. (7) Jortner, J.; Bixon, M.; Langenbacher, T.; Michel-Beyerle, M. E. Charge transfer and transport in DNA, Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 12759-12765. (8) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Molecular wire behaviour in p-phenylenevinylene oligomers, Nature 1998, 396, 60–63. (9) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R. Making a Molecular Wire: Charge and Spin Transport through para-Phenylene Oligomers, J. Am. Chem. Soc. 2004, 126, 5577-5584. (10) Goldsmith, R. H.; Sinks, L. E.; Kelley, R. F.; Betzen, L. J.; Liu, W. H.; Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. Wire-like charge transport at near constant bridge energy through fluorene oligomers, Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 3540-3545. (11) Ricks, A. B.; Brown, K. E.; Wenninger, M.; Karlen, S. D.; Berlin, Y. A.; Co, D. T.; Wasielewski, M. R. Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor−Bridge−Acceptor Molecules: Implications for Wirelike Behavior, J. Am. Chem. Soc. 2012, 134, 4581-4588. (12) Colvin, M. T; Ricks, A. B.; Scott, A. M.; Co, D. T.; Wasielewski, M. R. Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor−bridge−Acceptor Molecules, J. Phys. Chem. A 2012, 116, 1923-1930. (13) Colvin, M. T.; Ricks, A. B.; Wasielewski, M. R. Role of Bridge Energetics on the Preference for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor Molecules, J. Phys. Chem. A 2012, 116, 2184-2191. (14) Phelan, N. F.; Orchin, M. Cross conjugation, J. Chem. Educ. 1968, 45, 633-637. (15) Gholami, M.; Tykwinski, R. R. Oligomeric and Polymeric Systems with a Cross-conjugated pi-Framework, Chem. Rev. 2006, 106, 4997-5027. (16) Solomon, G. C.; Andrews, D. Q.; Hansen, T.; Goldsmith, R. H.; Wasielewski, M. R.; Van Duyne, R. P.; Ratner, M. A. Understanding quantum interference in coherent molecular conduction, J. Chem. Phys. 2008, 129, 054701/054701-054701/054708. (17) Andrews, D. Q.; Solomon, G. C.; Goldsmith, R. H.; Hansen, T.; Wasielewski, M. R.; Van Duyne, R. P.; Ratner, M. A. Quantum Interference: The Structural Dependence of Electron Transmission through Model Systems and Cross-Conjugated Molecules, J. Phys. Chem. C 2008, 112, 16991-16998. (18) Solomon, G. C.; Andrews, D. Q.; Goldsmith, R. H.; Hansen, T.; Wasielewski, M. R.; Van Duyne, R. P.; Ratner, M. A. Quantum Interference in Acyclic Systems: Conductance of Cross-Conjugated Molecules, J. Am. Chem. Soc. 2008, 130, 17301-17308. (19) Brown, K. E.; Veldkamp, B. S.; Co, D. T.; Wasielewski, M. R. Vibrational Dynamics of a Perylene-Perylenediimide Donor-Acceptor Dyad Probed with Femtosecond Stimulated Raman Spectroscopy, J. Phys. Chem. Lett. 2012, article ASAP, DOI: 10.1021/jz301107c.

Recent Group Publications on Molecular Electronics: Direct Observation of the Preference of Hole Transfer Over Electron Transfer for Radical Ion Pair Recombination in Donor-Bridge-Acceptor Molecules, Z. E. X. Dance, M. J. Ahrens, A. M. Vega, A. Butler-Ricks, D. W. McCamant, M. A. Ratner, and M. R. Wasielewski, J. Am. Chem. Soc. 130, 830-832 (2008). Challenges in Distinguishing Superexchange and Hopping Mechanisms of Intramolecular Charge Transfer Through Fluorene Oligomers, R. H. Goldsmith, O. DeLeon, T. M. Wilson, D. Finkelstein-Shapiro, M. A. Ratner, and M. R. Wasielewski, J. Phys. Chem. A 112, 4410-4414 (2008). Unexpectedly Similar Charge Transfer Rates Through Benzo-annulated Bicyclo[2.2.2]octanes, R. H. Goldsmith, J. Vura-Weis, A. M. Vega, S. Borkar, A. Sen, M. A. Ratner, and M. R. Wasielewski, J. Am. Chem. Soc. 130, 7659-7669 (2008). Understanding quantum interference in coherent molecular conduction, G. C. Solomon, D. Q. Andrews, T. Hansen, R. H. Goldsmith, M. R. Wasielewski, R. P. Van Duyne, and M. A. Ratner, J. Chem. Phys. 129, 054701/1-054701/8 (2008). Quantum Interference: The Structural Dependence of Electron Transmission through Model Systems and Cross-conjugated Molecules, D. Q. Andrews, G. C. Solomon, R. H. Goldsmith, T. Hansen, M. R. Wasielewski, R. P. Van Duyne, and M. A. Ratner, J. Phys. Chem. C 112, 16991-16998 (2008). Quantum Interference in Acyclic Systems: Conductance of Cross-conjugated Molecules, G. C. Solomon, D. Q. Andrews, R. H. Goldsmith, T. Hansen, M. R. Wasielewski, R. P. van Duyne, and M. A. Ratner, J. Am. Chem. Soc. 130, 17301-17308 (2008). Towards a n-Type Molecular Wire: Electron Hopping within Linearly-linked Perylenediimide Oligomers, T. M. Wilson, M. J. Tauber, and M. R. Wasielewski, J. Am. Chem. Soc. 131, 8952-8957 (2009). Direct Measurement of Photoinduced Charge Separation Distances in Donor-Acceptor Systems for Artificial Photosynthesis using OOP-ESEEM, R. Carmieli, Q. Mi, A. Butler Ricks, E. M. Giacobbe, S. M. Mickley, and M. R. Wasielewski, J. Am. Chem. Soc. 131, 8372-8373 (2009). Excitation Energy Transfer Pathways within Asymmetric Covalent Chlorophyll a Tetramers, V. L. Gunderson, T. M. Wilson, M. R. Wasielewski, J. Phys. Chem. C 113, 11936-11942 (2009). Photoinitiated Charge Transport through π-Stacked Electron Conduits in Supramolecular Ordered Assemblies of Donor-Acceptor Triads, J. E. Bullock, R. Carmieli, S. M. Mickley, J. Vura-Weis, and M. R. Wasielewski, J. Am. Chem. Soc. 131, 11919-11929 (2009).

Spin-Selective Charge Transport Pathways through p-Oligophenylene-Linked Donor-Bridge-Acceptor Molecules, A. M. Scott, T. Miura, A. Butler Ricks, Z. E. X. Dance, E. M. Giacobbe, M. T. Colvin, and M. R. Wasielewski, J. Am. Chem. Soc. 131, 17655-17666 (2009). Interrogating the Intramolecular Charge-Transfer State of a Julolidine-Anthracene Donor-Acceptor Molecule with Femtosecond Stimulated Raman Spectroscopy, J. V. Lockard, A. Butler Ricks, D. T. Co, and M. R. Wasielewski, J. Phys. Chem. Lett. 1, 215-218 (2010). Rapid Intramolecular Hole Hopping in meso-meso and meta-Phenylene-Linked Linear and Cyclic Multi-Porphyrin Arrays, T. M. Wilson, T. Hori, M.-C. Yoon, N. Aratani, A. Osuka, D. Kim, and M. R. Wasielewski, J. Am. Chem. Soc. 132, 1383-1388 (2010).

Geometry and Electronic Coupling in Perylenediimide Stacks: Mapping Structure - Charge Transport Relationships, J. Vura-Weis, M. A. Ratner, M. R. Wasielewski, J. Am. Chem. Soc. 132, 1738-1739 (2010). Excimer Formation Dynamics of Intramolecular π-Stacked Perylenediimides Probed by Single Molecule Fluorescence Spectroscopy, H. Yoo, J. Yang, A. Yousef, M. R. Wasielewski, and D. Kim, J. Am. Chem. Soc. 132, 3939-3944 (2010). Electron Hopping among Cofacially Stacked Perylenediimides Assembled using DNA Hairpins, T. M. Wilson, T. A. Zeidan, M. Hariharan, F. D. Lewis, and M. R. Wasielewski, Angew. Chem. Int. Ed. 49, 2385-2388 (2010). Comparing Spin-Selective Charge Transport through Oligomeric Aromatic Bridges in Donor-Bridge-Acceptor Molecules, A. M. Scott, A. Butler Ricks, M. T. Colvin, and M. R. Wasielewski, Angew. Chem. Int. Ed. 49, 2904-2908 (2010). Time-resolved EPR Studies of Charge Recombination and Triplet State Formation within Donor-Bridge-Acceptor Molecules having Wire-like Oligofluorene Bridges, T. Miura, R. Carmieli, and M. R. Wasielewski, J. Phys. Chem. A 114, 5769-5778 (2010). Crossover from Single-Step Tunneling to Wire-like Multi-Step Hopping for Molecular Triplet Energy Transfer, J. Vura-Weis, S. H. Abdelwahed, R. Shukla, R. Rathore, M. A. Ratner, and M. R. Wasielewski, Science 328 1547-1550 (2010). The Chameleonic Nature of Electron Transport through π-Stacked Systems, G. C. Solomon, C. Herrmann, J. Vura-Weis, M. R. Wasielewski, and M. A. Ratner, J. Am. Chem. Soc. 132, 7887-7889 (2010).

Belt-Shaped π-Systems: Relating Geometry to Electronic Structure in a Six-Porphyrin Nanoring Sprafke, J. K.; Kondratuk, D. V.; Wykes, M.; Thompson, A. L.; Hoffmann, M; Drevinskas, R.; Chen, W.-H.; Yong, C. K.; Kaernbratt, J.; Bullock, J. E.; Malfois, M.; Wasielewski, M. R.; Albinsson, B.; Herz, L. M.; Zigmantas, D.; Beljonne, D.; Anderson, H. L. J. Am. Chem. Soc. 2011, 17262-17273.

Low-Temperature Frequency Domain Study of Excitation Energy Transfer in Ethynyl-Linked Chlorophyll Trefoils and Aggregates, Neupane, B.; Dang, N. C.; Kelley, R. F.; Wasielewski, M. R.; Jankowiak, R. J. Phys. Chem. B 2011, 115, 10391-10399. Mechanically Stabilized Tetrathiafulvalene Radical Dimers Coskun, A.; Spruell, J. M.; Barin, G.; Fahrenbach, A. C.; Forgan, R. S.; Colvin, M. T.; Carmieli, R.; Benítez, D.; Tkatchouk, E.; Friedman, D. C. Sarjeant, A. A.; Wasielewski, M. R.; Goddard, W. A; Stoddart, J. F. J. Am. Chem. Soc. 2011, 133, 4538-4547. Ultrafast Intersystem Crossing and Spin Dynamics of Zinc meso-Tetraphenylporphyrin Covalently Bound to Stable Radicals, Colvin, M. T.; Smeigh, A. L.; Giacobbe, E. M.; Conron, S. M. M.; Ricks, A. B.; Wasielewski, M. R. J. Phys. Chem. A 2011, 115, 7538-7549. Temperature Dependence of Spin-Selective Charge Transfer Pathways in Donor-Bridge-Acceptor Molecules with Oligomeric Fluorenone and p-Phenylethynylene Bridges, Scott, A. M.; Wasielewski, M. R. J. Am. Chem. Soc. 2011, 133, 3005-3013. Extraordinary Nonlinear Absorption in 3D Bowtie Nanoantennas, Suh, J. Y.; Huntington, M. D.; Kim, C. H.; Zhou, W.; Wasielewski, M. R., Odom, T. W. Nano Lett. 2012, 12, 269-274. Controlling Switching in Bistable [2]Catenanes by Combining Donor−Acceptor and Radical−Radical Interactions, Zhu, Z.; Fahrenbach, A. C.; Li, H.; Barnes, J. C.; Liu, Z.; Dyar, S. M.; Zhang, H.; Lei, J.; Carmieli, R.; Sarjeant, A. A.; Stern, C. L.; Wasielewski, M. R.; Stoddart, J. F. J. Am. Chem. Soc. 2012, 134, 11709-11720. Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor−Bridge−Acceptor Molecules: Implications for Wirelike Behavior, Ricks, A. B.; Brown, K. E.; Wenninger, M.; Karlen, S. D.; Berlin, Y. A.; Co, D. T.; Wasielewski, M. R. J. Am. Chem. Soc. 2012, 134, 4581-4588. Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor−bridge−Acceptor Molecules, Colvin, M. T; Ricks, A. B.; Scott, A. M.; Co, D. T.; Wasielewski, M. R. J. Phys. Chem. A 2012, 116, 1923-1930. Role of Bridge Energetics on the Preference for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor Molecules, Colvin, M. T.; Ricks, A. B.; Wasielewski, M. R. J. Phys. Chem. A 2012, 116, 2184-2191. Vibrational Dynamics of a Perylene-Perylenediimide Donor-Acceptor Dyad Probed with Femtosecond Stimulated Raman Spectroscopy,Brown, K. E.; Veldkamp, B. S.; Co, D. T.; Wasielewski, M. R. J. Phys. Chem. Lett. 2012, article ASAP, DOI: 10.1021/jz301107c.