Letter pubs.acs.org/JPCL
Suppressing Excimers in H‑Aggregates of Perylene Bisimide FoldaDimer: Role of Dimer Conformation and Competing Assembly Pathways Samaresh Samanta and Debangshu Chaudhuri* Department of Chemical Sciences and Centre for Advanced Functional Materials (CAFM), Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India S Supporting Information *
ABSTRACT: Long-lived excitons in H-aggregates hold great promise for efficient transport of excitation energy, provided they are not scavenged by structurallly relaxed excimers. We report solution self-assembly of a perylene bisimide (PBI) folda-dimer that exhibits two distinct kinetic stages: an initial fast assembly leads to metastable aggregates with large excimer contribution that is followed by a slower growth of stable, extended H-aggregates free of excimers. Mechanistic investigations reveal an interplay of two competing aggregation pathways, where suppression of excimers is directly linked to the crossover from an isodesmic to cooperative aggregation. How the comeptition between two self-assembly pathways is influenced by the conformational flexibility of the folda-dimer is also discussed.
T
ment in extended aggregates is nontrivial, because the outcome of a self-assembly is often dictated by competing kinetics of different pathways,15−19 and not so much by the free-energy of the assembled state. Consequently, suppressing excimer formation in H-aggregates by a rational design has hitherto remained elusive. In this work, we demonstrate for the first time the feasibility of eliminating excimers in strongly excitoncoupled H-aggregates of a flexible bichromophoric molecule through a delicate interplay of competing aggregation pathways. The molecule under investigation is 2PBI (Figure 1), a covalently bridged perylene bisimide (PBI) folda-dimer that is prepared by condensation of a presynthesized perylene monoimide with m-xylenediamine (see Supporting Informa-
he greatest appeal of supramolecular dye assemblies lies with the possibility of realizing newer functionalities and emergent phenomena that are not accessible to its constituent molecules. In this context, the term “emergent” implies a useful or desirable outcome. However, in certain instances, selfassembly gives rise to antagonistic effects that can severely limit the intended function of the assembled structure. A case in point is that of cofacially organized H-aggregates, characterized by a symmetry-forbidden, long-lived excited state. Long-lived excitations are key to an efficient migration of photoexcitation energy,1,2 making H-aggregates good candidates for lightharvesting applications.3,4 In contrast to this, a particularly detrimental and yet common-place manifestation of chromophoric interactions in H-aggregates are excimers that form as a result of a structural relaxation of the long-lived photoexcited state. Formation of excimers contributes to a significant loss of excitation energy, and even a small fraction of these can act as low-energy traps impeding energy5,6 and charge7,8 migration. Thus, in the context of energy harvesting, a competition between the long-lived exciton-coupled aggregate state and excimers dictates material performance. To achieve an H-aggregated dye assembly that promotes strong exciton-coupling and yet dissuades excimer formation presents certain inherent challenges. Foremost is the lack of a molecular-level understanding of the factors that favor excimer formation in a multichromophoric assembly. Although excimers have been extensively studied in structurally well-defined model bi- and trichromphoric systems, 9−12 the influence of interchromphoric arrangement is significantly more complex in extended aggregates that can support multimeric excimers.13,14 Additionally, aiming for a desired molecular arrange© XXXX American Chemical Society
Figure 1. Structures of the folda-dimer 2PBI and its two conformers. Received: May 29, 2017 Accepted: July 11, 2017 Published: July 11, 2017 3427
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The Journal of Physical Chemistry Letters
Figure 2. 2PBI self-assembly. (a) Temporal evolution of absorption spectra of 2PBI in 99.7% MCH (1 μM), at 298 K. Inset: Time-dependent A0−0/ A0−2 reveals two kinetic stages of aggregation (before and after t = 1 h) with distinctly different rates. (b) Time-dependent aggregate size distribution from DLS. Inset: Increase in aggregate size (black circles) and the extent of exciton coupling (A0−0/A0−2, green circles) follow similar kinetics. (c) Excimer PL at 610 nm (λex = 540 nm) diminishes during the course of 2PBI self-assembly. Inset: Near-identical time dependence of excimer PL intensity (PL610, purple circles) and A0−0/A0−2 ratio (green circles). Solid lines are guides to the eye.
units in the extended aggregates.22 Such a departure from the perfect H-aggregated structure renders the lowest energy transition partially allowed. Using absorption spectroscopy, we elucidated the evolution of exciton coupling during the course of 2PBI self-assembly. In Figure 2c, we present how self-assembly affects the photoluminescence (PL) of 2PBI. Direct excitation at 540 nm gives rise to a PL at 610 nm (PL610) that is typical of an excimer: broad, structureless, and strongly Stokes shifted (∼70 nm).23 Corresponding PL excitation (PLE) spectrum (Figure S5) confirms that the species emitting PL610 is indeed H-aggregated 2PBI. It is, however, intriguing that this aggregate state is shortlived; PL610 diminishes completely during the first hour of selfassembly.24 Evidently, self-assembly of 2PBI in MCH leads to two distinct aggregate states at different time scales. The excimer state is accessible only in the initial stages of aggregation. With time, a presumably more stable H-aggregate is formed, which upon photoexcitation does not relax into excimers. Quite remarkably, a plot of integrated PL610 intensity versus time, when compared to that of A0−0/A0−2 (inset, Figure 2c) shows a spectacular match, implying a common underlying cause behind the two phenomena. Figure 3 presents the effect of 2PBI concentration on the kinetics of self-assembly. In MCH, the folda-dimer is expected to undergo a near-instantaneous folding, at all concentrations. This is evident even at 0.1 μM, the tinitial spectrum exhibits a clear sign of H-type exciton coupling, with A0−0/A0−2 = 2.9, which is well below the characteristic open conformer value of 4.1. Geometry optimization of 2PBI confirms that the folded
tion). In an appropriate solvent, free rotations about the benzylic carbons of the m-xylylene bridge allow 2PBI to switch between the noninteracting open and the intramolecularly pistacked folded conformers that can be characterized using optical absorption and 1H NMR spectroscopy (unpublished results). Since both conformers can conceivably act as selfassembly building blocks, we anticipate the course of 2PBI aggregation to depend on the dynamic nature of its folding equilibrium. Self-assembly of 2PBI was carried out in methylcyclohexane (MCH), under kinetic control. A solution of 2PBI in CHCl3 (10 μL, 0.3 mM) was mixed with 3 mL MCH under constant stirring, such that the final 2PBI concentration was 1 μM, and the solution composition was predominantly (99.7%) MCH. Figure 2a presents the temporal evolution of the 2PBI absorption spectrum. When compared to the spectrum in CHCl3, the first (tinitial) spectrum of the time series, measured within 2 min of dispersing 2PBI in MCH, exhibits a clear signature of H-type aggregation (Figure S4): an overall hypochromism along with a decrease in the absorbance ratio of 0−0 to 0−1 vibronic peaks (A0−0/A0−1). Subsequent changes in absorption spectrum mirror the progress of 2PBI selfassembly: A0−0 declines faster than A0−1, while the 0−2 band grows steadily to become the most dominant feature. Temporal variation (inset, Figure 2a) in the relative intensities of the 0−0 and 0−2 bands (A0−0/A0−2) summarizes the observed changes. An extensive theoretical work by Spano has shown that exciton coupling in H-aggregates results in a transfer of oscillator strength from 0 to 0 to higher vibronic bands,20 and the extent of this spectral distortion is related to the number of coherently coupled molecules within the H-aggregate. Thus, the observed changes indicate an increase in the number of exciton coupled 2PBI molecules in the H-aggregate. Dynamic light scattering (DLS) confirms the growth of aggregate size (Figure 2b), during the first 10 h. That the extent of electronic coupling continues to increase with the aggregate size indicates a relatively defect-free self-assembly. From the plot of A0−0/A0−2 vs time, one can identify two distinct kinetic stages of aggregation: during the first hour, A0−0/A0−2 decreases sharply from 4.1 (open form)21 to 0.75, and is followed by a much slower decline to the steady-state value of 0.62. Such a drastic change in the rate indicates a significantly complex selfassembly process. Further, lack of any isosbestic point implies the coexistence of multiple aggregate species. A gradual development of the red-shifted absorption feature (540 and 565 nm) suggests a small rotational offset between the 2PBI
Figure 3. Time dependence of A0−0/A0−2 at different 2PBI concentrations in 99.7% MCH. Inset: Height-normalized tfinal absorption spectrum at 0.5, 1, and 8 μM 2PBI concentrations, respectively. 3428
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The Journal of Physical Chemistry Letters conformer is ∼0.65 eV more stable than the corresponding open form (Figure S7). Moreover, the ratio at 0.1 μM does not decrease with time, suggesting that the critical aggregation concentration of 2PBI in MCH at 298 K is above 0.1 μM. At 0.5 μM and higher concentrations, folded 2PBI undergoes further self-assembly, and the rate of decrease of A0−0/A0−2 ratio exhibits a strong concentration dependence. The A0−0/ A0−2 values for the tinitial spectra, which indicate the extent to which self-assembly progresses in the first 2 min, are 1.8, 1.45 and 0.75, in 0.5, 1, and 8 μM solutions, respectively. Likewise, the effect of concentration in the slow stage of aggregation is apparent from the lowest value of A0−0/A0−2 that could be eventually achieved in each case, and the corresponding tfinal spectra (inset, Figure 3). Finally, a perfect agreement between the kinetics of PL610 darkening and that of decreasing A0−0/ A0−2 was confirmed for all three concentrations (Figure S8). While discussing the concentration dependence of 2PBI selfassembly, it is important to note that the monomeric Ref-PBI remains unaggregated in MCH, even at 60 μM concentration (Figure S9), thus highlighting that the aggregation constant of the folda-dimer 2PBI is significantly higher than that of RefPBI in MCH, despite the fact that the nature of noncovalent interactions leading to aggregation are very similar in two cases: pi-stacking and dispersion interactions between the hydrophobic alkyl chains. The effect of solvent viscosity on the kinetics of aggregation (both exciton coupling and excimer PL) is qualitatively the same (Figure S10, S11), with self-assembly being significantly slower in high viscosity media. Having demonstrated a spontaneous suppression of excimer formation in strongly exciton-coupled H-aggregates of 2PBI, we now elucidate the underlying self-assembly mechanism, and identify different aggregate states that can account for the observed time-dependence. Figure 4a presents the temperaturedependent optical absorption spectra of excimer-free 2PBI aggregates in 99.7% MCH. Prior to the experiment, 2PBI solution (1 μM) was incubated for 48 h, at 298 K, allowing the self-assembly to reach the steady-state. Heating under thermodynamic control (1 K/min) causes disassembly and a corresponding increase in A0−0/A0−2. Temperature-dependent degree of aggregation (αAgg) (Figure 4b and S12) derived from the absorption spectra exhibits a nonsigmoidal behavior that can be reliably fitted to a cooperative model (see Supporting Information for details).25,26 Clearly, formation of excimer-free final aggregate state follows a cooperative mechanism, which is characterized by an unfavorable nucleation step followed by a spontaneous elongation process. Molecular enthalpy release during elongation (ΔHe) and the characteristic elongation temperature (Te), estimated from the fitting, are −76.7 kJ/mol and 353 K, respectively. We note that the dissociated 2PBI molecules (at T > Te) remain in the folded state, presumably due to the aggregating nature of MCH. Upon cooling the solution (1 K/min), a contrastingly different aggregation pathway is revealed. The onset of aggregation occurs at a lower temperature (325 K), and αAgg increases in a sigmoidal fashion that is typical of an isodesmic growth.26 It is important to note that the self-assembly in the cooling run progresses under kinetic control. The picture that emerges points at two distinct aggregation pathways for 2PBI in MCH: its rapid folding marks the onset of a fast, kinetically controlled OFFpathway aggregation that leads to an isodesmic growth of small H-aggregates. These metastable aggregates gradually reorganize into more stable, excimer-free, extended H-aggregates following a cooperative ON pathway. An excellent agreement between
Figure 4. Self-assembly mechanism. (a) Temperature-dependent optical absorption spectra of 2PBI in 99.7% MCH show aggregate dissociation upon heating (1 K/min) (b) Degree of aggregation derived from absorption spectra for the heating (pink circles), cooling (blue circles), and reheating runs (yellow circles) reveal contrasting aggregation mechanisms. The solid black lines are least-squared fits using cooperative (heating), isodesmic (cooling) and a linear combination of the two (reheating) growth models. Red diamonds representing the growth of excimer PL (PL610) during the cooling run closely follows the corresponding αAgg curve.
αAgg and the growth of PL610 intensity during the cooling run further validates that excimer formation is endemic to the OFFpathway aggregates. Reorganization of OFF- to ON-pathway aggregates is best revealed during the second heating run (5 K/ min): αAgg exhibits an unconventional temperature dependence that closely follows the isodesmic curve below 300 K, but crosses over to resemble the cooperative model at higher T. Assuming that a fraction of OFF-pathway aggregates continuously reorganizes into the ON-pathway aggregates at T < Te (353 K), it is possible to simulate their combined disassembly process in the reheating run, using a linear combination of the two models (see Supporting Information). We now focus on the mechanism of OFF- to ON-pathway aggregate reorganization that represents the key step toward inhibiting excimer formation. Since each of the elementary aggregation steps leading up to the OFF aggregate, including 2PBI folding, is reversible, we argue that a minority of 2PBI remains in solution as the open conformer. In order to investigate the role of open conformer in self-assembly, we monitored (Figure 5a) the evolution of PL intensity at 520 nm (PL520, λex = 470 nm). Since PL520 is characteristic of an isolated PBI chromophore (Figure S13a), it can be used to exclusively monitor the solution concentration of the open 2PBI conformer. Interestingly, PL520 intensity changes nonmonotonically with time (Figure 5b). Its increase in the initial stage implies a rise in the fraction of open 2PBI, and the eventual decline indicates that the regenerated open form gets subsequently used up. A similar trend was confirmed for different 2PBI concentrations (Figure S13). The fact that the 3429
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Figure 5. Aggregate reorganization. (a) Temporal evolution of PL spectrum (λex = 490 nm) shows a nonmonotonic change in 520 nm PL (PL520) intensity (shaded green), in contrast to the steady decline of PL610. (b) Integrated PL520 intensity versus time for different 2PBI concentrations (0.1, 0.5, 1, and 8 μM). Inset: An initial increase in PL520 intensity signifies a rise in the fraction of open 2PBI conformer. (d) A schematic representation of the role of 2PBI folding equilibrium in the competition between the OFF and ON aggregation pathways, along with speculated aggregate structures.
reversal of PL520 intensity is coincident with the loss of PL610 implies a common origin behind the two phenomena, namely the crossover from OFF to ON pathway. A direct reorganization of OFF to ON-pathway aggregate through unfolding of multiple 2PBI units is likely to involve a very high energetic cost,27 which is why we propose reorganization through a disassembly reassembly process.15 Cooperative ONpathway aggregation drives the disassembly of kinetically trapped, OFF-pathway aggregates in solution (Figure 5c), resulting in the loss of excimer PL. The dissociated 2PBI molecules undergo a disfavored unfolding step in MCH, thereby releasing the open conformer that subsequently participates in the ON-pathway aggregation. The proposed model is also consistent with the contrasting formation mechanisms (isodesmic vs cooperative) of the two aggregates. One-dimensional OFF-pathway aggregation of the folded 2PBI requires only one noncovalent interaction along the growth axis (Figure 5c)π-stacking between the adjacent folded 2PBI unitsand is therefore likely to follow an isodesmic growth.28,29 Cooperativity on the other hand, results from the participation of multiple noncovalent interactions along the stacking axis.26,30,31 Each open conformer can employ a pair of π-stacking interactions with multiple neighboring units to form the ON-pathway aggregate.32 Such interaction also explains the significantly higher aggregation constant of 2PBI over monomeric Ref-PBI discussed earlier (Figure S9). The fact that the formation of the excimer-free H-aggregates is driven by cooperative self-assembly with the OFF pathway providing an additional kinetic control makes it possible to translate this success to larger and practically relevant
aggregates. Field emission scanning electron micrograph (FESEM) of 2PBI aggregates grown from MCH (5 μM) reveals a nanofibrillar morphology (Figure 6a), that are several
Figure 6. (a) FESEM of 2PBI nanowires, self-assembled from 5 μM solution in MCH, at 298 K. Scale bar: 5 μm. (b) Absorbance normalized PL spectra (λex = 540 nm) of nanowires and that of a freshly prepared solution in MCH (5 μM). Inset: absorption spectrum of the nanowires.
microns long. Comparing the absorbance normalized PL spectra of these fibers with that of a freshly prepared (tinitial) solution of 2PBI in MCH (Figure 6b) shows over 95% reduction in the intensity of PL610, confirming an effective suppression of excimers in the extended aggregates. To conclude, we demonstrate an effiicent suppression of excimers in extended H-aggregates of the folda-dimer 2PBI. The significance of this study may be seen in light of the fact that the photophysics of an overwhelming majority of PBI based H-aggregates is dominated by excimers.23 The key lies in the conformational flexibility of the dimer that allows its open 3430
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and Conductivity of Self-assembled pi-pi Stacks of Perylene Bisimide Dyes. Chem. - Eur. J. 2007, 13, 436−449. (8) Howard, I. A.; Laquai, F.; Keivanidis, P. E.; Friend, R. H.; Greenham, N. C. Perylene Tetracarboxydiimide as an Electron Acceptor in Organic Solar Cells: a Study of Charge Generation and Recombination. J. Phys. Chem. C 2009, 113, 21225−21232. (9) De Schryver, F. C.; Collart, P.; Vandendriessche, J.; Goedeweeck, R.; Swinnen, A. M.; Van der Auweraer, M. Intramolecular Excimer Formation in Bichromophoric Molecules linked by a Short Flexible Chain. Acc. Chem. Res. 1987, 20, 159−166. (10) Brown, K. E.; Salamant, W. A.; Shoer, L. E.; Young, R. M.; Wasielewski, M. R. Direct Observation of Ultrafast Excimer Formation in Covalent Perylenediimide Dimers using Near-Infrared Transient Absorption Spectroscopy. J. Phys. Chem. Lett. 2014, 5, 2588−2593. (11) Lindquist, R. J.; Lefler, K. M.; Brown, K. E.; Dyar, S. M.; Margulies, E. A.; Young, R. M.; Wasielewski, M. R. Energy Flow Dynamics Within Cofacial and Slip-Stacked Perylene-3,4-dicarboximide Dimer Models of π-Aggregates. J. Am. Chem. Soc. 2014, 136, 14912−14923. (12) Stangl, T.; Wilhelm, P.; Schmitz, D.; Remmerssen, K.; Henzel, S.; Jester, S. S.; Höger, S.; Vogelsang, J.; Lupton, J. M. Temporal Fluctuations in Excimer-like Interactions between π-Conjugated Chromophores. J. Phys. Chem. Lett. 2015, 6, 1321−1326. (13) Son, M.; Park, K. H.; Shao, C.; Würthner, F.; Kim, D. Spectroscopic Demonstration of Exciton Dynamics and Excimer Formation in a Sterically Controlled Perylene Bisimide Dimer Aggregate. J. Phys. Chem. Lett. 2014, 5, 3601−3607. (14) Sung, J.; Kim, P.; Fimmel, B.; Würthner, F.; Kim, D. Direct Observation of Ultrafast Coherent Exciton Dynamics in Helical πStacks of Self-Assembled Perylene Bisimides. Nat. Commun. 2015, 6, 8646−8652. (15) Korevaar, P. A.; George, S. J.; Markvoort, A. J.; Smulders, M. M. J.; Hilbers, P. A. J.; Schenning, A. P. H. J.; De Greef, T. F. A.; Meijer, E. W. Pathway Complexity in Supramolecular Polymerization. Nature 2012, 481, 492−496. (16) Fennel, F.; Wolter, S.; Xie, Z.; Plötz, P.-A.; Kühn, O.; Würthner, F.; Lochbrunner, S. Biphasic Self-Assembly Pathways and SizeDependent Photophysical Properties of Perylene Bisimide Dye Aggregates. J. Am. Chem. Soc. 2013, 135, 18722−18725. (17) Shao, C.; Stolte, M.; Würthner, F. Quadruple π Stack of Two Perylene Bisimide Tweezers: a Bimolecular Complex with Kinetic Stability. Angew. Chem., Int. Ed. 2013, 52, 7482−7486. (18) Baram, J.; Weissman, H.; Rybtchinski, B. Supramolecular Polymer Transformation: a Kinetic Study. J. Phys. Chem. B 2014, 118, 12068−12073. (19) Mattia, E.; Otto, S. Supramolecular Systems Chemistry. Nat. Nanotechnol. 2015, 10, 111−119. (20) Spano, F. C. The Spectral Signatures of Frenkel polarons in Hand J-Aggregates. Acc. Chem. Res. 2010, 43, 429−439. (21) At time t = 0 of self-assembly, 2PBI exists in the open form (in CHCl3) characterized by A0−0/A0−2 = 4.1. (22) Che, Y.; Yang, X. M.; Balakrishnan, K.; Zuo, J. M.; Zang, L. Highly Polarized and Self-Waveguided Emission from SingleCrystalline Organic Nanobelts. Chem. Mater. 2009, 21, 2930−2934. (23) Würthner, F.; Saha-Möller, C. R.; Fimmel, B.; Ogi, S.; Leowanawat, P.; Schmidt, D. Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials. Chem. Rev. 2016, 116, 962−1052. (24) Turbidity assay of 2PBI in 99.7% MCH (Figure S6) confirms the absence of any precipitation related PL quenching. (25) Smulders, M. M. J.; Schenning, A. P. H. J.; Meijer, E. W. Insight into the Mechanisms of Cooperative Self-Assembly: the “Sergeantsand-Soldiers” Principle of Chiral and Achiral C3-Symmetrical Discotic Triamides. J. Am. Chem. Soc. 2008, 130, 606−611. (26) de Greef, T. F. A.; Smulders, M. M. J.; Wolffs, M.; Schenning, A. P. H. J.; Sijbesma, R. P.; Meijer, E. W. Supramolecular Polymerization. Chem. Rev. 2009, 109, 5687−5754. (27) Lohr, A.; Lysetska, M.; Würthner, F. Supramolecular Stereomutation in Kinetic and Thermodynamic Self-Assembly of Helical
and folded conformers to coexist in a dynamic equilibrium. In solution, both conformers act as self-assembly building blocks, albeit in two contrastingly different aggregation pathways. The folded 2PBI triggers a fast, OFF-pathway isodesmic aggregation that yields small aggregates with pronounced excimeric contribution, while the open conformer participates in a slower, ON-pathway cooperative self-assembly that drives the formation of excimer-free H-aggregates. The fact that a competing OFF-pathway aggregation functions as a kinetic trap by sequestering the open conformer from solution makes it possible to grow large one-dimensional nanowires with negligible excimer contribution. Efforts to further improve sample quality through the use of living supramolecular polymerization strategies,33−35 such as seeding, are currently underway.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01338. Experimental methods, synthesis and characterization, and additional figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Debangshu Chaudhuri: 0000-0002-8941-4327 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge IISER Kolkata and Department of Science and Technology (DST), India (Project: EMR/2014/000223 and SB/FT/CS-164/2013), for financial support. S.S. acknowledges UGC for scholarship. We also thank Mr. Sumit Naskar and Dr. Mousumi Das, IISER Kolkata for DFT calculations.
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REFERENCES
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