Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
pubs.acs.org/JACS
Singlet Fission in Covalent Terrylenediimide Dimers: Probing the Nature of the Multiexciton State Using Femtosecond Mid-Infrared Spectroscopy Michelle Chen, Youn Jue Bae, Catherine M. Mauck, Aritra Mandal, Ryan M. Young,* and Michael R. Wasielewski*
Downloaded via TUFTS UNIV on July 12, 2018 at 23:22:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Chemistry and Institute for Sustainability and Energy at Northwestern, Northwestern University, Evanston, Illinois 60208-3113, United States S Supporting Information *
ABSTRACT: Singlet fission (SF) is a spin-allowed process that involves absorption of a photon by two electronically interacting chromophores to produce a singlet exciton state, 1(S1S0), followed by rapid formation of two triplet excitons if the singlet exciton energy is about twice that of the triplet exciton. The initial formation of the multiexciton correlated triplet pair state, 1(T1T1), is thought to involve the agency of charge transfer (CT) states. The dynamics of these electronic states were studied in a covalent slip-stacked terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer in which the conformation of two TDI molecules is determined by a xanthene spacer (XanTDI2). Femtosecond mid-infrared (fsIR) spectroscopy shows that the multiexciton 1(T1T1) state has absorptions characteristic of the T1 state in the carbonyl stretch region of the IR spectrum, in addition to IR absorptions specific to the CT state in the CC stretch region. The simultaneous presence of CT and triplet state features in both high dielectric constant CH2Cl2 and low dielectric constant 1,4-dioxane throughout the multiexciton state lifetime suggests that this state has both CT and triplet character.
■
as pentacene,4−12 tetracene,13−16 diphenylisobenzofuran,17,18 diketopyrrolopyrrole (DPP),19−22 carotenoids,23,24 zethrenes,25 and rylenes.26−30 Recent efforts have focused on elucidating the SF mechanism and determining the nature of the intermediate electronic states involved. Two general SF mechanisms have been proposed: direct coupling and mediation by a charge transfer (CT) state.1,2 In the direct coupling mechanism, the initial 1(S1S0) state involving a neighboring pair of molecules is directly coupled to the 1(T1T1) multiexciton state via a concerted two-electron process. This direct mechanism has been observed in crystalline tetracene31 and bipentacene.32 The mediated mechanism occurs through two consecutive one-electron processes and involves a CT state that couples the 1 (S1S0) state to the 1(T1T1) state. Studies have indicated that the interactions of the initial 1(S1S0) state and the 1(T1T1) product state of SF are far more complex and involve additional states.7,33−36 Information about the 1(T1T1) state has proven elusive because of the difficulty of observing it directly.5,33,37 Recent studies on intermolecular SF in zethrenes25 and pentacenes8,11 have resulted in the observation of the 1(T1T1) state. Although the 1(T1T1) state can be
INTRODUCTION In certain organic molecular solids, a singlet exciton formed by the absorption of a single photon can spontaneously downconvert to give two triplet excitons, each on an adjacent molecule through a phenomenon known as singlet fission (SF). For SF to occur, a pair of adjacent chromophores must have the correct electronic coupling and energetics, which are defined by the electronic structure of the organic chromophore and its interactions with its neighboring molecules. There are two energetic requirements for rapid, efficient SF: (1) the energy of the singlet exciton (S1) must be greater than or equal to twice the energy of the lowest energy triplet exciton (T1) (E(S1) ≥ 2E(T1)) and (2) the energy of the second lowest energy triplet exciton (T2) must also obey the relationship E(T2) ≥ 2E(T1).1,2 Despite being known since the 1960s, SF remained largely unexplored until it was shown theoretically in 2006 that SF could increase the efficiency limit of singlejunction solar cells beyond the Shockley-Queisser limit of 33% to nearly 45%.2,3 Given this advantage, research has focused on understanding how the electronic coupling between adjacent SF chromophores determines SF efficiency. To understand the role of electronic coupling in SF, intermolecular SF has been studied extensively in amorphous and crystalline films as well as in aggregates such as nanoparticles; these intermolecular SF studies have involved a variety of organic chromophores such © XXXX American Chemical Society
Received: May 8, 2018 Published: June 27, 2018 A
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society observed in films and aggregates, covalent dimers provide significant advantages for studying the 1(T1T1) state. For example, the system energetics of the dimer are defined by the electronic structure of the organic chromophore, while the electronic coupling between the chromophores comprising the dimer can be tailored and probed by placing them on a rigid organic scaffold. Moreover, the 1(T1T1) state cannot dissociate into two independent triplet states making it easier to study this multiexciton state. High-yield intramolecular SF has been observed in polythiophenes,38 covalent dimers of bisalkynylated pentacene39−42 and tetracene derivatives,43 and terrylene-3,4:11,12-bis(dicarboximide) (TDI).44 More importantly, these intramolecular SF studies in covalent dimers have resulted in the observation of the 1(T1T1) state, which can provide insights on the involvement of the CT states in the SF mechanism.42−46 The participation of CT states in the SF mechanism has been suggested by the solvent dielectric constant-dependent SF rates observed in diphenylisobenzofuran47 and pentacene dimers.40,41,48 In our previous study of TDI dimers, SF occurred in low dielectric constant solvents, while symmetrybreaking charge separation was observed in high dielectric constant solvents.44 The higher dielectric constant solvent stabilizes the CT state, lowering its energy and allowing it to act as a trap state, whereas the lack of CT state stabilization in lower dielectric constant solvents allows the CT state to act as a virtual state in a superexchange interaction that promotes SF. Despite evidence of CT participation in SF, the nature of the relationships between the CT state and the 1(S1S0) and 1 (T1T1) states is not well understood. The interaction of the CT state with the 1(S1S0) and 1(T1T1) states can be interpreted either classically or quantum mechanically. If the energy of the CT state is similar to that of the 1(S1S0) and 1 (T1T1) states, then a classical equilibrium among the three states may occur. This would be an extension of the equilibrium between the 1(S1S0) and 1(T1T1) states proposed previously for TDI dimers.44 From a quantum mechanical standpoint, the 1(S1S0), CT, and 1(T1T1) states could exist either as a coherent superposition or an incoherent mixed state. Previously, the 1(T1T1) state has been suggested to be part of a coherent superposition of the 1(S1S0), 1(T1T1), and a virtual CT state.49 Furthermore, a 1(S1S0)-CT mixed state has been observed in TDI films,29 whereas a study on DPP films that undergo SF has suggested CT-1(T1T1) state mixing.20 Time-resolved electronic spectroscopies often lack structural insight due to significant spectral congestion from broad line widths and overlapping peaks, making it difficult to ascertain whether SF involves the initial creation of mixed states that evolve in time as SF occurs. However, time-resolved vibrational spectroscopy provides the naturally narrow line widths and distinct structural specificity that can deliver better insights into the structure of electronically excited states. Previously, pump−probe femtosecond transient infrared (fsIR) spectroscopy has been used to study electron transfer reactions,50−54 charge transport in bulk heterojunction films,55,56 excimer dynamics in organic chromophores,57,58 and exciton delocalization.59,60 FsIR spectroscopy has also been used to probe the excited-state dynamics of the rylene family member, perylene3,4:9,10-bis(dicarboximide) (PDI), by studying its strongly IR active CC and CO modes.57,60,61 More recently, fsIR spectroscopy has been used to study SF in TIPS-pentacene10,11 and cyano-substituted tetracene derivatives.16
Here, we investigate the nature of the multiexciton state involved in the SF mechanism in a covalently linked TDI dimer (XanTDI2) in which a xanthene bridge holds the two TDI molecules in a π-stacked geometry and a biphenyl spacer introduces a slip-stacked geometry (Figure 1). TDI has been a
Figure 1. Chemical structures of the xanthene TDI dimer (XanTDI2), xanthene TDI monomer (XanTDI), and C15 TDI monomer (C15TDI).
recent target of SF studies due to its high stability, easy synthetic modification, and strong visible absorption (e.g., C15TDI: ε651 = 127,100 M−1 cm−1, E(S1) = 1.87 eV).62 Its lowenergy T1 state (E(T1) ≤ 0.77 eV)44 allows it to fulfill the energetic requirement E(S1) ≥ 2E(T1) for exoergic SF and therefore enables SF in the absence of favorable entropic effects in covalent dimer systems.2 The use of the xanthene bridge with tert-butyl groups greatly increases the dimer solubility compared to the previously studied analogue using a triptycene scaffold.44 In high dielectric constant CH2Cl2 (ε = 8.93), XanTDI2 primarily undergoes symmetry-breaking charge separation, while SF dominates in low dielectric constant 1,4-dioxane (ε = 2.25). However, in both cases fsIR spectroscopy shows that vibrational features characteristic of the CT and 1(T1T1) states are present at all times observed (300 fs to 8 ns). We also present results on a monomeric TDI control molecule (XanTDI) with the xanthene bridge and biphenyl spacer to confirm the absence of redox chemistry between the xanthene bridge and TDI as well as a symmetric TDI monomer (C15TDI) to obtain the IR spectra of the TDI triplet state and radical anion.
■
EXPERIMENTAL SECTION
Synthesis. The xanthene TDI monomer (XanTDI) and dimer (XanTDI2) were synthesized as described in the Supporting Information, SI, by modifications of literature procedures (Figure S1). 29,44,63,64 The symmetric TDI monomer (C15TDI) was synthesized according to literature procedures.65 Steady-State Characterization. Absorption spectra were collected on a Shimadzu 1800 spectrophotometer. Emission spectra were collected on a Horiba Nanolog fluorimeter with a perpendicular arrangement of the excitation source and detector. All emission spectra were corrected for monochromator wavelength dependence and CCD-detector spectral response functions. Long integration B
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society times of 10 s were needed to record spectra of the TDI dimer solutions. Quantum yields were determined using cresyl violet perchlorate in methanol as a standard. FTIR spectra were collected on a Shimadzu IRAffinity-1 spectrophotometer in a demountable liquid cell (Harrick scientific) with a 150 or 500 μm Teflon spacer and 2 mm thick CaF2 windows. Transient Spectroscopy. The experimental femtosecond transient absorption (fsTA) and nanosecond transient absorption (nsTA) setup has been previously described.44,66 Pump pulses at 655 or 660 nm generated by using the output of a laboratory-built collinear optical parametric amplifier were used to excite 3.5 μM solution samples in 1−2 mm cuvettes. The experimental femtosecond transient IR (fsIR) setup has been previously described.54,57 Excitation pulses were tuned to 655 or 660 nm (3 μJ/pulse) and samples were prepared in the same demountable liquid cell but with either a 500 or 630 μm Teflon spacer. Time-resolved fluorescence (TRF) data were collected using a Hamamatsu C4334 Streakscope streak camera system. Data were collected in a 5 ns window with a fwhm instrument response function of 66 ps. Excitation pulses were generated with a commercial directdiode-pumped 100 kHz amplified femtosecond laser (Spirit 1040, Spectra-Physics), which produced the fundamental beam of 1040 nm; the fundamental beam pumped a noncollinear optical parametric amplifier (Spirit-NOPA, Spectra-Physics), which delivered tunable, high-repetition-rate pulses at 600 nm (1 nJ/pulse). Computational Details. Using QChem (version 4.0), optimized ground- and excited-state geometry and normal mode calculations were performed using density functional theory (DFT) and timedependent density functional theory (TD-DFT) at the B3LYP/631G* level. To model the FTIR spectra, a scaling factor of 0.9667 and Lorentzian line shapes (fwhm 5 cm−1) were applied to the normal mode calculations. These computations were done on monomeric TDIs with methyl groups replacing the xanthene bridge and C15 swallowtail groups at the imide positions. Computations, with a d3dispersion correction, were also completed on a simplified version of XanTDI in which the tert-butyl groups on the xanthene bridge and the C15 swallowtail at the imide position were all replaced with methyl groups. The optimized ground state geometry calculation of XanTDI2 was obtained by implementing DFT in TeraChem 1.5K at the B3LYP/6-31G** level with a d3-dispersion correction. For this computation, the C15 swallowtail groups at the imide positions were replaced with methyl groups. Full computed structures and selected mode assignments are given in the SI.
Figure 2. (a) Absorption spectra of XanTDI (red), XanTDI2 (blue), and C15TDI (green) in CH2Cl2. (b) The ground state structure of XanTDI2 calculated in TeraChem (B3LYP/6-31G**).
two TDI molecules within XanTDI2 is modest. This is consistent with observations of larger decreases in extinction coefficients in more strongly coupled pentacene dimers,40 and reported increases in extinction coefficient for many dimers with very weak electronic coupling.41,47,70,71 We note that the fluorescence from XanTDI2 is highly quenched in CH2Cl2 (ϕF = 0.022 ± 0.003) and 1,4 dioxane (ϕF = 0.039 ± 0.005) compared to monomeric TDI (ϕF = 0.84 ± 0.03)44 due to photophysical processes that are highly competitive with radiative decay in TDI (Figure S6a). The FTIR spectra of XanTDI, XanTDI2, and C15TDI are given in Figure 3a, alongside the computed ground state IR spectra for the singlet, triplet, anion, and cation states and the lowest excited singlet state of monomeric N,N-dimethyl TDI in Figure 3b, and the FTIR spectrum of C15TDI−• in Figure 3c. The main contributions to the spectra come from the imide carbonyl and terrylene core stretches between 1300 and 1700 cm−1. Of the states calculated, TDI−• has the strongest vibrational modes (Figure S2a). The calculated IR spectra suggest that TDI−• has an intense CC mode at 1564 cm−1, whereas TDI+• exhibits weaker features in the CO region at 1674 and 1711 cm−1. This is borne out by the intense CC stretches observed at 1572 and 1532 cm−1 and the 1644 cm−1 CO stretch in the experimental spectrum of C15TDI−• (Figure 3c). The triplet spectrum is dominated by CO stretches at 1651 and 1683 cm−1, which shows that the modes of TDI−• and 3*TDI are distinguishable from those of 1*TDI and the TDI ground state, and therefore, the CC stretch region of TDI−• will be an important signature of CT state involvement in SF. We note that the xanthene bridge has no strong IR contributions in the region of interest.61 This is supported by comparison of the experimental FTIR spectrum of XanTDI and the computed IR spectrum that includes the xanthene bridge (Figure S2b). We do not expect any xanthene excited-state modes to contribute to the region of interest in the fsIR spectra since the xanthene bridge does not absorb the excitation wavelength and does not participate in any photochemical processes.
■
RESULTS AND DISCUSSION Steady-State Characterization. Steady-state absorption spectra of XanTDI, XanTDI2, and C15TDI in CH2Cl2 are shown in Figure 2a. The absorption spectrum of C15TDI has 0−0 and 0−1 vibronic bands centered at 649 and 596 nm, respectively, whereas those of XanTDI occur at 656 and 603 nm, respectively. In the absorption spectrum of the dimer XanTDI2, the relative intensity of the 0−0 and 0−1 vibronic bands at 660 and 606 nm, respectively, change slightly in comparison to XanTDI due to the modest electronic coupling between the two TDIs from slip-stacking.68 Slip-stacking is important as it destabilizes the TDI excimer state, which hinders SF in TDI dimers.44 According to DFT calculations, the dimer has a slip angle of θ = 20° and is slipped by 8.6 Å with an average π−π distance of 3.5 Å (Figure 2b), consistent with its analogue that uses a triptycene spacer.44 The peak molar extinction coefficient of XanTDI2 (ε660 = 56,300 M−1 cm−1) is smaller than that of XanTDI (ε656 = 67,900 M−1 cm−1) and C15TDI (ε649 = 137,700 M−1 cm−1). The extinction coefficients of N-alkylated TDI molecules62 are generally about 30% higher than those of the N-arylated derivatives.69 The small decrease in extinction coefficient of XanTDI2 relative to that of XanTDI suggests that the electronic coupling of the C
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
Figure 3. (a) Ground state FTIR spectra of XanTDI (red), XanTDI2 (blue), and C15TDI (green). *Denotes artifacts from solvent subtraction. (b) Calculated IR spectra of the indicated states of monomeric N,N-dimethyl TDI. (c) FTIR spectrum of C15TDI−• in CD2Cl2 generated using CoCp2.
are assigned to TDI−•,44 suggesting that symmetry-breaking charge separation occurs between the TDIs within the dimer. In this high dielectric constant medium, charge separation occurs in τ = 5.5 ± 0.3 ps, which explains the quenched fluorescence, while charge recombination occurs in τ = 437 ± 3 ps. Global analysis of the XanTDI2 fsTA data in CH2Cl2 using an A → B → ground (GS) model clearly shows the evolution of the 1(S1S0) state (A) into the CT state (B) (Figure 4b, see also Figure S4). However, state A also shows a significant tail that extends beyond 1300 nm, which suggests potential mixing of the CT state with 1(S1S0) at the earliest times observed. Moreover, there is a small positive absorption feature characteristic of the Tn ← T1 transition at 618 nm.44 A pure GSB should have a larger negative feature at this wavelength. These data hint at the possibility of CT and 1 (T1T1) state involvement in the excited state dynamics at very early times following photoexcitation. FsIR spectroscopy will be used to examine this possibility in more detail as described below. In contrast, photoexcitation of XanTDI2 at 660 nm in low dielectric constant 1,4-dioxane results in the appearance of the Sn ← S1 NIR absorption bands at 867, 1062, and 1223 nm, which decay rapidly with the concomitant growth of a feature with a maximum at 618 nm (Figure 4c). This feature is the Tn ← T1 excited-state absorption (ESA) as indicated by its similarity to the anthracene-sensitized TDI triplet state absorption spectrum.44 The 618 nm maximum is observed because the GSB overlaps strongly with the Tn ← T1 ESA. As the triplet feature appears, the Sn ← S1 band at 1062 nm blueshifts toward 1005 nm suggesting some contribution from TDI−• to the SF process. It is also notable that little or no SE is observed. In addition, the weak feature at 760 nm that appears with the triplet feature can be attributed to TDI+•.44 The appearance of both TDI−• and TDI+• character as SF occurs strongly supports the involvement of the CT state in the SF mechanism. The fact that the fsTA spectra resemble those of the TDI triplet and CT states with only modest shifts and bandwidth changes suggests that that the electronic coupling of the CT and 1(T1T1) states is also modest. Global analysis of the XanTDI2 fsTA data in 1,4-dioxane allows us to uncover the role of the CT state as XanTDI2 undergoes SF. An A → B → C → ground (GS) model was chosen as it allows us to monitor the contribution of the 1 (S1S0), CT, and 1(T1T1) states over time by observing their relative contributions to the evolution-associated spectra. An equilibrium model was not used because the number of required rate constants is larger than the number of observed
FsTA Spectroscopy. FsTA spectra of XanTDI were acquired following excitation at 660 nm in CH2Cl2 (Figure S3a). The excited-state spectra and dynamics of XanTDI are similar to other monomeric TDI derivatives.29,44 Following photoexcitation, the transient spectrum of XanTDI exhibits ground state bleach (GSB) minima at 600 and 660 nm, a stimulated emission (SE) feature at 736 nm, and three sharp Sn ← S1 features in the near-infrared (NIR) region at 862, 1064, and 1227 nm. These features decay biexponentially (τ1 = 186 ± 8 ps and τ2 = 2.73 ± 0.02 ns) due to structural relaxation (Figure S3b−d), which has been observed previously in TDI monomers.44 Similar dynamics were observed in 1,4-dioxane. The fsTA spectra confirm the lack of triplet state formation and redox chemistry between the xanthene bridge and TDI in XanTDI. Upon photoexcitation of XanTDI2 in CH2Cl2 at 660 nm, we observe strong GSB features at 600 and 658 nm, a very weak SE feature at 734 nm, and Sn ← S1 NIR absorptions at 874, 1055, and 1230 nm (Figure 4a). At later times, the S1 features decay, while absorptions at 757, 900, 1028, 1233, and 1311 nm appear concomitantly. The feature at 757 nm is assigned to TDI+•, whereas the features at 900, 1028, 1233, and 1311 nm
Figure 4. Transient visible/NIR absorption spectra of XanTDI2: (a) in CH2Cl2 and (b) its evolution-associated spectra; (c) in 1,4-dioxane and (d) its evolution-associated spectra. Data near 660 and 820 nm are omitted due to pump scatter and residual light used to generate the white light probe pulse, respectively. D
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
occurring between the xanthene bridge and TDI and assist in the assignment of TDI singlet excited state IR absorptions. To identify the vibrational modes of the triplet (T1) state, C15TDI was excited at 655 nm in iodoethane, which induces intersystem crossing (ISC) via the heavy atom effect observable within the 8 ns pump−probe delay window of our fsIR experiment (Figure 5b). Formation of 3*C15TDI within 8 ns was confirmed initially by fsTA and nsTA data (Figure S8). Consistent with XanTDI, the fsIR spectrum of 1 *C15TDI at early times in iodoethane exhibits GSB features at 1589 and 1666 cm−1 corresponding to the CC ring and C O stretches, respectively (Figure 5b), which as expected, is similar to that of 1*XanTDI. 1*C15TDI has two excited-state CC stretches at 1537 and 1553 cm−1 and a strong CO stretch at 1640 cm−1 in addition to three weaker modes at 1609, 1617, and 1623 cm−1. These modes decay in τ = 1.9 ± 0.2 ns as ISC occurs to form the triplet state. The 1*C15TDI and 3*C15TDI transient spectra were generated by global analysis of the fsIR data and are shown in Figure 6a (see also
rates leading to an underdetermined mathematical model. The evolution-associated spectra for states A, B, and C are shown in Figure 4d (see also Figure S5). State A has primarily singlet character with strong NIR absorptions at 867, 1062, and 1223 nm and SE at 730 nm and is therefore assigned to 1(S1S0), which decays in τA = 2.7 ± 0.3 ps. States B (τB = 161 ± 5 ps) and C (τC = 1.34 ± 0.04 ns) are assigned to mixed 1(T1T1) and CT populations that result from either a classical equilibrium or quantum mechanical state mixing. States B and C may have different relative contributions from the CT and 1(T1T1) states, which will be probed using fsIR spectroscopy. Time-resolved fluorescence decay measurements give complementary lifetimes of τB = 159 ± 33 ps and τC = 1.58 ± 0.07 ns (Figure S6b). We note that the fluorescence yield of XanTDI2 in 1,4-dioxane is higher than that in CH2Cl2. This is due to the presence of singlet character in all the states involved in SF as shown in the NIR region of the evolutionassociated spectra. Assigning the IR Absorptions of TDI Singlet, Triplet, and Anion States. Assigning the IR absorptions of potential SF intermediate states will provide us with structural information and allow for their identification in the SF mechanism. FsIR spectra of XanTDI at selected time delays are shown in Figure 5a. The excited-state IR spectrum of TDI
Figure 6. Species-associated spectra of (a) 3*C15TDI in iodoethane, and the evolution-associated spectra of XanTDI2 in (b) CD2Cl2 and (c) 1,4-dioxane-d8. The dashed vertical lines in the spectra are located at prominent T1 and the TDI−• features.
Figure S9). We note that 3*C15TDI has three prominent modes at 1628, 1641, and 1653 cm−1 that live past the 8 ns pump−probe time delay window of the fsIR experiment. As expected from the calculations, 3*C15TDI has stronger carbonyl stretches than CC ring stretches. Upon photoexcitation of XanTDI2 at 660 nm in CD2Cl2 strong GSB features at 1589 (CC), 1666 (CO), and 1675 (CO) cm−1 along with ESA at 1549, 1557, 1573, 1626, 1638, and 1653 cm−1 appear (Figure 5c). The appearance of a strong CC stretch at 1573 cm−1 is assigned to TDI−• based on the experimental and computed IR spectra (Figure 3 and Table S3), and previous work on PDI, in which a strong PDI−• feature was also observed in the CC stretch region around 1579 cm−1.61 The evolution-associated spectra (Figure 6b, see also Figure S10) show that this dominant feature appears immediately, then shifts slightly to 1576 cm−1 in τ = 4.2 ± 0.5 ps followed by decay of all spectral changes in τ = 490 ± 10 ps. These time constants agree very well with those observed by fsTA spectroscopy for symmetry-breaking charge separation followed by charge recombination in XanTDI2 (Figures 4a and b). Interestingly, the immediate presence of the strong 1573
Figure 5. FsIR spectra at selected time delays of (a) XanTDI in 1,4dioxane-d8, (b) C15TDI in iodoethane, (c) XanTDI2 in CD2Cl2, and (d) XanTDI2 in 1,4-dioxane-d8.
is similar to those of other rylene derivatives.57,60 Photoexcitation of XanTDI at 660 nm results in GSB at the 1587 cm−1 CC stretching band and at the 1666 and 1675 cm−1 C = O bands. The ESA of the singlet state has two sharp CC ring stretches at 1534 and 1558 cm−1 and two sharp CO stretches at 1642 and 1657 cm−1, which is consistent with the computed IR spectra (Table S4). These excited-state modes decay concomitantly in τ = 3.29 ± 0.06 ns (Figure S7). These fsIR data again confirm that there is no redox chemistry E
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
Figure 7. Time evolution of the mixed state population (red vector), which includes |1(S1S0)⟩, |CT⟩, and |1(T1T1)⟩ with (A) a small contribution from |CT⟩, (B) a significant contribution from |CT⟩ but a small contribution from |1(T1T1)⟩, and (C) a significant contribution from both |CT⟩ and |1(T1T1)⟩.
cm−1 band indicates that the 1(S1S0) state of XanTDI2 has significant CT character (see below). This band is notably absent in the Sn ← S1 and Tn ← T1 spectra of monomeric TDI (Figure 5a and b). In addition, the bands at 1626, 1638, and 1653 cm−1 correspond closely to those of 3*C15TDI as seen by comparing Figure 6a and b, thus corroborating the fsTA data that some triplet character is also present during this process. When XanTDI2 is photoexcited in 1,4-dioxane-d8 strong GSB features appear at 1589 (CC) and 1678 (CO) cm−1 along with ESA at 1551, 1572, 1628, 1640, and 1656 cm−1 (Figure 5d). Once again, the evolution-associated spectra obtained by applying an A → B → C → GS model exhibit an intense mode at 1572 cm−1, which is characteristic of TDI−• (Figure 6c, see also Figure S11) and absent in both the Sn ← S1 and Tn ← T1 spectra of TDI (Figure 5a and b). In addition, comparing Figure 6a and c, the 1628, 1640, and 1656 cm−1 IR CO modes are consistent with those of 3*C15TDI. This initial spectrum relaxes in τA = 5 ± 2 ps to a similar spectrum in which there are small frequency shifts in the CO region and a shift of the 1572 cm−1 mode to 1575 cm−1. Once again, the time constant for this frequency shift is consistent with the
formation of a mixed CT-1(T1T1) state as indicated by the fsTA experiment, where states B (τB = 110 ± 1 ps) and C (τC = 1.21 ± 0.02 ns) are again assigned to the mixed CT-1(T1T1) state with the triplet character larger in state C. Nature of the Multiexciton State. It is clear from the fsIR data that both the 1(S1S0) and 1(T1T1) states have significant contributions from the CT state that evolve in time. The three-state vector model of the mixed excited-state population illustrated in Figure 7 will be used to discuss the spectral data. The length of the vector represents the total mixed state population, while its direction displays the state composition. When CT state mixing with either 1(S1S0) or 1 (T1T1) is weak, the 1(S1S0) population evolves to 1(T1T1) with little or no contribution to the transient spectra from features characteristic of the CT state (Figure 7a). This is clearly not the case for XanTDI2 in either CD2Cl2 or 1,4dioxane-d8 because the CT state, as indicated by the appearance of TDI−• spectral features, appears in the fsIR spectra within the instrument response. The data strongly suggest that 1(S1S0)-CT mixing occurs very early, which may be a consequence of the two states being close in energy. If F
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society mixing with 1(T1T1) remains weak or negligible, for example, if the CT state is energetically below both 1(S1S0) and 1(T1T1), then the population of the initial photoexcited state as indicated by the population vector in Figure 7b will start at an angle 0 < θ < 90° within the 1(S1S0)-CT plane and rotate in that plane until it lies primarily along the CT state direction. The most interesting case is the one in which there is CT state mixing with both 1(S1S0) and 1(T1T1), as illustrated in Figure 7c, where the population vector initially points at an angle 0 < θ < 90° within the 1(S1S0)-CT plane with a small 1(T1T1) contribution. The time evolution of the population vector then reflects contributions from all three states before it settles into the CT-1(T1T1) plane indicating that there is CT-1(T1T1) mixing in the multiexciton product state. Depending on how the diabatic surfaces of each state interact, the population vector can take many different paths to arrive the product state. The situation illustrated in Figure 7c is exemplified by the XanTDI2 data in both CD2Cl2 and 1,4-dioxane-d8. Figure 8
Although we and others have predicted and observed a role for the CT state in other SF chromophores such as pentacene72,73 and DPP;20 here we are able to observe mixing of the 1(S1S0), CT, and 1(T1T1) states directly by fsIR spectroscopy.49 The observed mixing of the CT state with 1 (S1S0) within the fsIR instrument response and the apparent mixing of the CT state with 1(T1T1) at all times during its decay to ground state make it difficult to accurately determine or even define the SF yield. In the strictest sense, SF can be defined as requiring the formation of two independent triplet states from the 1(T1T1) state, which in this case does not happen. Nevertheless, the ultrafast spectra and kinetics of the overall state evolution show clearly that the formation of the multiexciton statethe primary singlet fission eventoccurs with significant state mixing in XanTDI2. The analysis of the multiexciton state presented here may also allow us to better understand the excimer-like intermediates observed in previous SF studies. In tetracene and DPP, the transient spectra of the observed excimer-like intermediates bear spectral similarities to both the 1(S1S0) and 1 (T1T1) states.20,37,43 Excimer-like intermediates, which frequently have partial CT character, may mix with 1(S1S0) and 1(T1T1) if the excimer state energy is close to that of these states, and thus, the excimer state becomes part of the state description.
■
CONCLUSIONS Covalent dimers are important model systems for mechanistic SF studies because the energetics of the system are defined by the electronic structure of the organic chromophore, whereas the interchromophore electronic coupling can be tailored and probed by placing the chromophores on a rigid scaffold. In this study, a slip-stacked TDI dimer (XanTDI2), covalently linked by a xanthene spacer, was synthesized to study SF dynamics in solution, which were characterized by fsTA and fsIR spectroscopies. FsTA spectroscopy shows some indications of the presence of both the CT and 1(T1T1) states as reflected in change of absorption maxima and bandshapes; however, fsIR spectroscopy shows clearly that the multiexciton state has vibrational modes characteristic of the T1 state in the carbonyl stretch region of the IR spectrum, in addition to IR absorptions specific to the CT state in the CC stretch region. The simultaneous presence of CT and triplet state features in both high and low dielectric constant media throughout the multiexciton state lifetime suggests that this state has both CT and triplet character. These observations support the idea that the multiexciton state is a mixed state including contributions from the 1(S1S0), CT, and 1(T1T1) states. This study demonstrates that fsIR spectroscopy is a valuable tool for understanding the various electronic states involved in SF. In the future, we will investigate the multiexciton state using twodimensional electronic spectroscopy to further understand the relationships among the 1(S1S0), CT, and 1(T1T1) states, since in principle, two-dimensional electronic spectroscopy can ascertain whether photoexcitation produces a coherent superposition of these states,7,74,75 while one-dimensional transient spectroscopies in general cannot.
Figure 8. Normalized species/evolution-associated spectra comparing the fsIR spectra of the multiexciton state generated by global analysis of the XanTDI2 data in 1,4-dioxane-d8 (black) and in CD2Cl2 (blue) compared to the 3* C15TDI spectrum (red). The dashed vertical lines in the spectra are located at prominent T1 and TDI−• features.
provides an overlay of the evolution-associated fsIR spectra of 3 *C15TDI with those of the longest-lived species observed for XanTDI2 in CD2Cl2 and 1,4-dioxane-d8. The carbonyl region of the spectrum in both solvents shows bands at 1628, 1641, and 1653 cm−1 that are characteristic of 3*TDI, which suggests that the multiexciton state is structurally similar to the T1 state. Comparing the spectra of XanTDI2 in CD2Cl2 and 1,4-dioxane shown in Figure 8, the CC stretch mode assigned to TDI−• appears in both spectra, only sharpening and slightly blueshifting from 1572 to 1574 cm−1 in the higher dielectric constant solvent; yet, the basic spectral features remain essentially the same. As noted above, there are significant differences between the 3*C15TDI spectrum and the spectrum of the multiexciton state in the CC stretch region. The 3 *C15TDI spectrum does not have any positive features between 1570 and 1582 cm−1 and instead exhibits GSB features in that region. Therefore, the presence of the intense TDI−• mode at 1574 cm−1 in the multiexciton state spectrum strongly supports CT-1(T1T1) state mixing.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b04830. G
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
■
(16) Margulies, E. A.; Kerisit, N.; Gawel, P.; Mauck, C. M.; Ma, L.; Miller, C. E.; Young, R. M.; Trapp, N.; Wu, Y.-L.; Diederich, F.; Wasielewski, M. R. J. Phys. Chem. C 2017, 121, 21262−21271. (17) Ryerson, J. L.; Schrauben, J. N.; Ferguson, A. J.; Sahoo, S. C.; Naumov, P.; Havlas, Z.; Michl, J.; Nozik, A. J.; Johnson, J. C. J. Phys. Chem. C 2014, 118, 12121−12132. (18) Schrauben, J. N.; Ryerson, J. L.; Michl, J.; Johnson, J. C. J. Am. Chem. Soc. 2014, 136, 7363−7373. (19) Hartnett, P. E.; Margulies, E. A.; Mauck, C. M.; Miller, S. A.; Wu, Y.; Wu, Y.-L.; Marks, T. J.; Wasielewski, M. R. J. Phys. Chem. B 2016, 120, 1357−1366. (20) Mauck, C. M.; Hartnett, P. E.; Margulies, E. A.; Ma, L.; Miller, C. E.; Schatz, G. C.; Marks, T. J.; Wasielewski, M. R. J. Am. Chem. Soc. 2016, 138, 11749−11761. (21) Miller, C. E.; Wasielewski, M. R.; Schatz, G. C. J. Phys. Chem. C 2017, 121, 10345−10350. (22) Mauck, C. M.; Hartnett, P. E.; Wu, Y.-L.; Miller, C. E.; Marks, T. J.; Wasielewski, M. R. Chem. Mater. 2017, 29, 6810−6817. (23) Wang, C.; Angelella, M.; Kuo, C.-H.; Tauber, M. J. Proc. SPIE 2012, 8459, 845905. (24) Wang, C.; Tauber, M. J. J. Am. Chem. Soc. 2010, 132, 13988− 13991. (25) Lukman, S.; Richter, J. M.; Yang, L.; Hu, P.; Wu, J.; Greenham, N. C.; Musser, A. J. J. Am. Chem. Soc. 2017, 139, 18376−18385. (26) Eaton, S. W.; Shoer, L. E.; Karlen, S. D.; Dyar, S. M.; Margulies, E. A.; Veldkamp, B. S.; Ramanan, C.; Hartzler, D. A.; Savikhin, S.; Marks, T. J.; Wasielewski, M. R. J. Am. Chem. Soc. 2013, 135, 14701− 14712. (27) Eaton, S. W.; Miller, S. A.; Margulies, E. A.; Shoer, L. E.; Schaller, R. D.; Wasielewski, M. R. J. Phys. Chem. A 2015, 119, 4151− 4161. (28) Le, A. K.; Bender, J. A.; Roberts, S. T. J. Phys. Chem. Lett. 2016, 7, 4922−4928. (29) Margulies, E. A.; Logsdon, J. L.; Miller, C. E.; Ma, L.; Simonoff, E.; Young, R. M.; Schatz, G. C.; Wasielewski, M. R. J. Am. Chem. Soc. 2017, 139, 663−671. (30) Le, A. K.; Bender, J. A.; Arias, D. H.; Cotton, D. E.; Johnson, J. C.; Roberts, S. T. J. Am. Chem. Soc. 2018, 140, 814−826. (31) Burdett, J. J.; Bardeen, C. J. J. Am. Chem. Soc. 2012, 134, 8597− 8607. (32) Fuemmeler, E. G.; Sanders, S. N.; Pun, A. B.; Kumarasamy, E.; Zeng, T.; Miyata, K.; Steigerwald, M. L.; Zhu, X. Y.; Sfeir, M. Y.; Campos, L. M.; Ananth, N. ACS Cent. Sci. 2016, 2, 316−324. (33) Chan, W.-L.; Ligges, M.; Jailaubekov, A.; Kaake, L.; MiajaAvila, L.; Zhu, X. Y. Science 2011, 334, 1541−1545. (34) Greyson, E. C.; Vura-Weis, J.; Michl, J.; Ratner, M. A. J. Phys. Chem. B 2010, 114, 14168−14177. (35) Berkelbach, T. C.; Hybertsen, M. S.; Reichman, D. R. J. Chem. Phys. 2013, 138, 114103. (36) Monahan, N.; Zhu, X. Y. Annu. Rev. Phys. Chem. 2015, 66, 601−618. (37) Stern, H. L.; Musser, A. J.; Gelinas, S.; Parkinson, P.; Herz, L. M.; Bruzek, M. J.; Anthony, J.; Friend, R. H.; Walker, B. J. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 7656−7661. (38) Varnavski, O.; Abeyasinghe, N.; Arago, J.; Serrano-Perez, J. J.; Orti, E.; Lopez Navarrete, J. T.; Takimiya, K.; Casanova, D.; Casado, J.; Goodson, T. J. Phys. Chem. Lett. 2015, 6, 1375−1384. (39) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M. T.; Choi, B.; Xia, J.; Taffet, E. J.; Low, J. Z.; Miller, J. R.; Roy, X.; Zhu, X. Y.; Steigerwald, M. L.; Sfeir, M. Y.; Campos, L. M. J. Am. Chem. Soc. 2015, 137, 8965−8972. (40) Lukman, S.; Musser, A. J.; Chen, K.; Athanasopoulos, S.; Yong, C. K.; Zeng, Z.; Ye, Q.; Chi, C.; Hodgkiss, J. M.; Wu, J.; Friend, R. H.; Greenham, N. C. Adv. Funct. Mater. 2015, 25, 5452−5461. (41) Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E. T.; Casillas, R.; Basel, B. S.; Thoss, M.; Tykwinski, R. R.; Guldi, D. M. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5325−5330. (42) Basel, B. S.; Zirzlmeier, J.; Hetzer, C.; Phelan, B. T.; Krzyaniak, M. D.; Reddy, S. R.; Coto, P. B.; Horwitz, N. E.; Young, R. M.; White,
Experimental details and characterization (DOCX)
AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *
[email protected] ORCID
Michelle Chen: 0000-0002-4336-2971 Catherine M. Mauck: 0000-0002-6432-9724 Aritra Mandal: 0000-0002-8680-3730 Ryan M. Young: 0000-0002-5108-0261 Michael R. Wasielewski: 0000-0003-2920-5440 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, DOE under Grant DE-FG02-99ER14999 (M.R.W.). M.C. gratefully acknowledges support from the Ryan Fellowship and the Northwestern University International Institute for Nanotechnology. C.M.M. was supported by an NSF Graduate Research Fellowship under Grant No. DGE1324585. We thank Ms. Claire E. Miller for preliminary fsTA measurements, Dr. Nathan T. La Porte for assistance with the FTIR of C15TDI−•, and Mr. Chenjian Lin for preparing C15TDI.
■
REFERENCES
(1) Smith, M. B.; Michl, J. Annu. Rev. Phys. Chem. 2013, 64, 361− 386. (2) Smith, M. B.; Michl, J. Chem. Rev. 2010, 110, 6891−6936. (3) Hanna, M. C.; Nozik, A. J. J. Appl. Phys. 2006, 100, 074510. (4) Ramanan, C.; Smeigh, A. L.; Anthony, J. E.; Marks, T. J.; Wasielewski, M. R. J. Am. Chem. Soc. 2012, 134, 386−397. (5) Walker, B. J.; Musser, A. J.; Beljonne, D.; Friend, R. H. Nat. Chem. 2013, 5, 1019−1024. (6) Musser, A. J.; Liebel, M.; Schnedermann, C.; Wende, T.; Kehoe, T. B.; Rao, A.; Kukura, P. Nat. Phys. 2015, 11, 352−357. (7) Bakulin, A. A.; Morgan, S. E.; Kehoe, T. B.; Wilson, M. W. B.; Chin, A. W.; Zigmantas, D.; Egorova, D.; Rao, A. Nat. Chem. 2016, 8, 16−23. (8) Pensack, R. D.; Ostroumov, E. E.; Tilley, A. J.; Mazza, S.; Grieco, C.; Thorley, K. J.; Asbury, J. B.; Seferos, D. S.; Anthony, J. E.; Scholes, G. D. J. Phys. Chem. Lett. 2016, 7, 2370−2375. (9) Pensack, R. D.; Grieco, C.; Purdum, G. E.; Mazza, S. M.; Tilley, A. J.; Ostroumov, E. E.; Seferos, D. S.; Loo, Y.-L.; Asbury, J. B.; Anthony, J. E.; Scholes, G. D. Mater. Horiz. 2017, 4, 915−923. (10) Grieco, C.; Kennehan, E. R.; Rimshaw, A.; Payne, M. M.; Anthony, J. E.; Asbury, J. B. J. Phys. Chem. Lett. 2017, 8, 5700−5706. (11) Grieco, C.; Kennehan, E. R.; Kim, H.; Pensack, R. D.; Brigeman, A. N.; Rimshaw, A.; Payne, M. M.; Anthony, J. E.; Giebink, N. C.; Scholes, G. D.; Asbury, J. B. J. Phys. Chem. C 2018, 122, 2012− 2022. (12) Hart, S. M.; Silva, W. R.; Frontiera, R. R. Chem. Sci. 2018, 9, 1242−1250. (13) Burdett, J. J.; Bardeen, C. J. Acc. Chem. Res. 2013, 46, 1312− 1320. (14) Piland, G. B.; Bardeen, C. J. J. Phys. Chem. Lett. 2015, 6, 1841− 1846. (15) Margulies, E. A.; Wu, Y.-L.; Gawel, P.; Miller, S. A.; Shoer, L. E.; Schaller, R. D.; Diederich, F.; Wasielewski, M. R. Angew. Chem., Int. Ed. 2015, 54, 8679−8683. H
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society
(68) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371−392. (69) Holtrup, F. O.; Müller, G. R. J.; Quante, H.; De Feyter, S.; De Schryver, F. C.; Müllen, K. Chem. - Eur. J. 1997, 3, 219−225. (70) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Steigerwald, M. L.; Sfeir, M. Y.; Campos, L. M. Angew. Chem., Int. Ed. 2016, 55, 3373− 3377. (71) Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J. J. Am. Chem. Soc. 2007, 129, 14240−14250. (72) Beljonne, D.; Yamagata, H.; Brédas, J. L.; Spano, F. C.; Olivier, Y. Phys. Rev. Lett. 2013, 110, 226402. (73) Zeng, T.; Hoffmann, R.; Ananth, N. J. Am. Chem. Soc. 2014, 136, 5755−5764. (74) Ferretti, M.; Hendrikx, R.; Romero, E.; Southall, J.; Cogdell, R. J.; Novoderezhkin, V. I.; Scholes, G. D.; van Grondelle, R. Sci. Rep. 2016, 6, 20834. (75) Sun, K.-W.; Yao, Y. J. Chem. Phys. 2017, 147, 224905.
F. J.; Hampel, F.; Clark, T.; Thoss, M.; Tykwinski, R. R.; Wasielewski, M. R.; Guldi, D. M. Nat. Commun. 2017, 8, 15171. (43) Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.; Haiges, R.; Krylov, A. I.; Bradforth, S. E.; Thompson, M. E. J. Am. Chem. Soc. 2016, 138, 617−627. (44) Margulies, E. A.; Miller, C. E.; Wu, Y.; Ma, L.; Schatz, G. C.; Young, R. M.; Wasielewski, M. R. Nat. Chem. 2016, 8, 1120−1225. (45) Tayebjee, M. J. Y.; Sanders, S. N.; Kumarasamy, E.; Campos, L. M.; Sfeir, M. Y.; McCamey, D. R. Nat. Phys. 2017, 13, 182−188. (46) Basel, B. S.; Zirzlmeier, J.; Hetzer, C.; Reddy, S. R.; Phelan, B. T.; Krzyaniak, M. D.; Volland, M. K.; Coto, P. B.; Young, R. M.; Clark, T.; Wasielewski Michael, R.; Guldi Dirk, M. Chem. 2018, 4, 1092−1111. (47) Johnson, J. C.; Akdag, A.; Zamadar, M.; Chen, X.; Schwerin, A. F.; Paci, I.; Smith, M. B.; Havlas, Z.; Miller, J. R.; Ratner, M. A.; Nozik, A. J.; Michl, J. J. Phys. Chem. B 2013, 117, 4680−4695. (48) Zirzlmeier, J.; Casillas, R.; Reddy, S. R.; Coto, P. B.; Lehnherr, D.; Chernick, E. T.; Papadopoulos, I.; Thoss, M.; Tykwinski, R. R.; Guldi, D. M. Nanoscale 2016, 8, 10113−10123. (49) Chan, W.-L.; Ligges, M.; Zhu, X. Y. Nat. Chem. 2012, 4, 840− 845. (50) Rubtsov, I. V.; Kang, Y. K.; Redmore, N. P.; Allen, R. M.; Zheng, J.; Beratan, D. N.; Therien, M. J. J. Am. Chem. Soc. 2004, 126, 5022−5023. (51) Rubtsov, I. V.; Redmore, N. P.; Hochstrasser, R. M.; Therien, M. J. J. Am. Chem. Soc. 2004, 126, 2684−2685. (52) Dattelbaum, D. M.; Omberg, K. M.; Hay, P. J.; Gebhart, N. L.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J. Phys. Chem. A 2004, 108, 3527−3536. (53) Sazanovich, I. V.; Alamiry, M. A. H.; Best, J.; Bennett, R. D.; Bouganov, O. V.; Davies, E. S.; Grivin, V. P.; Meijer, A. J. H. M.; Plyusnin, V. F.; Ronayne, K. L.; Shelton, A. H.; Tikhomirov, S. A.; Towrie, M.; Weinstein, J. A. Inorg. Chem. 2008, 47, 10432−10445. (54) La Porte, N. T.; Martinez, J. F.; Hedstrom, S.; Rudshteyn, B.; Phelan, B. T.; Mauck, C. M.; Young, R. M.; Batista, V. S.; Wasielewski, M. R. Chem. Sci. 2017, 8, 3821−3831. (55) Pensack, R. D.; Guo, C.; Vakhshouri, K.; Gomez, E. D.; Asbury, J. B. J. Phys. Chem. C 2012, 116, 4824−4831. (56) Jeong, K. S.; Pensack, R. D.; Asbury, J. B. Acc. Chem. Res. 2013, 46, 1538−1547. (57) Mauck, C. M.; Young, R. M.; Wasielewski, M. R. J. Phys. Chem. A 2017, 121, 784−792. (58) Wu, Y.; Zhou, J.; Phelan, B. T.; Mauck, C. M.; Stoddart, J. F.; Young, R. M.; Wasielewski, M. R. J. Am. Chem. Soc. 2017, 139, 14265−14276. (59) Mani, T.; Grills, D. C.; Newton, M. D.; Miller, J. R. J. Am. Chem. Soc. 2015, 137, 10979−10991. (60) Kennehan, E. R.; Grieco, C.; Brigeman, A. N.; Doucette, G. S.; Rimshaw, A.; Bisgaier, K.; Giebink, N. C.; Asbury, J. B. Phys. Chem. Chem. Phys. 2017, 19, 24829−24839. (61) Hartnett, P. E.; Mauck, C. M.; Harris, M. A.; Young, R. M.; Wu, Y.-L.; Marks, T. J.; Wasielewski, M. R. J. Am. Chem. Soc. 2017, 139, 749−756. (62) Nolde, F.; Pisula, W.; Müller, S.; Kohl, C.; Müllen, K. Chem. Mater. 2006, 18, 3715−3725. (63) Lefler, K. M.; Brown, K. E.; Salamant, W. A.; Dyar, S. M.; Knowles, K. E.; Wasielewski, M. R. J. Phys. Chem. A 2013, 117, 10333−10345. (64) Nolde, F.; Qu, J.; Kohl, C.; Pschirer, N. G.; Reuther, E.; Müllen, K. Chem. - Eur. J. 2005, 11, 3959−3967. (65) Pisula, W.; Kastler, M.; Wasserfallen, D.; Robertson, J. W. F.; Nolde, F.; Kohl, C.; Muellen, K. Angew. Chem., Int. Ed. 2006, 45, 819−823. (66) Young, R. M.; Dyar, S. M.; Barnes, J. C.; Juricek, M.; Stoddart, J. F.; Co, D. T.; Wasielewski, M. R. J. Phys. Chem. A 2013, 117, 12438−12448. (67) Irikura, K. K.; Johnson, R. D.; Kacker, R. N. J. Phys. Chem. A 2005, 109, 8430−8437. I
DOI: 10.1021/jacs.8b04830 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
