Evolution of HJ Coupling in Nanoscale Molecular Self-Assemblies

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C: Physical Processes in Nanomaterials and Nanostructures

Evolution of HJ Coupling in Nanoscale Molecular Self-Assemblies Sarah R Marques, Joelle A. Labastide, and Michael D. Barnes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03200 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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The Journal of Physical Chemistry

Evolution of HJ Coupling in Nanoscale Molecular Self-Assemblies Sarah R. Marquesa, Joelle A. Labastide,a,b and Michael D. Barnesa,b* a

Department of Chemistry,bDepartment of Physics

University of Massachusetts Amherst, Amherst, MA 01003 (Corresponding author email: [email protected])

Abstract: We investigated the evolution of combined Frenkel exciton and charge-transfer chromophore coupling in nanoscale self-assembled clusters of 7,8,15,16-tetraazaterrylene (TAT). Using spatially- and wavelength-resolved fluorescence imaging, we observed significant changes in the relative intensities of vibronic transitions (J-like to H-like) in isolated clusters with increasing size (along the crystal growth axis), suggestive of a change in the interference of the combined short and long-range interactions upon assembly. Large clusters and nanowires showed spectral signatures that appear superficially like H-aggregates with a diminished origin (0-0) intensity relative to the higher vibronic sidebands, while small nanoclusters – appearing as diffraction limited spots – had distinct J-aggregate spectral signatures (enhancement of the origin intensity) suggestive of different packing structures in small clusters. The isolation of the different nanoscale assemblies allows for the opportunity to understand changes in molecular registration upon solution phase crystallization.

Introduction Understanding the correlation between molecular architecture and chromophore coupling as well as their impact on directional energy and/or charge transport is a long-sought

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goal in organic optoelectronics.1,

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Organic semiconductor materials such as perylene

derivatives have demonstrated crystallochromic effects induced by sub-angstrom slips in the cofacial molecular alignment.3-6

These subtle changes in the molecular architecture affect

packing geometry (and therefore inter-molecular coupling) and are signaled by distinct changes in absorption or emission spectra. Recent theoretical work suggests that these different packing motifs can selectively influence exciton mobility or charge transport within crystalline organic networks.7, 8 To date, very little experimental information is available on the effects of variations in molecular packing motif on the sign and magnitude of intermolecular interactions in solid state aggregates.

9-14

In this work, we isolated different self-assembled

clusters of 7,8,15,16 tetraazaterrylene (TAT) at various stages of growth in order to interrogate the evolution of excitonic coupling from the isolated molecule limit to extended crystal. From spatially resolved spectral signatures, we show that large TAT clusters have three distinct spectral signatures corresponding to different dominant coupling mechanisms, suggesting different packing motifs which signal a change in the balance between long-range Coulombic exciton coupling and short range charge-transfer coupling upon self-assembly. As shown extensively by Spano and coworkers, the relative intensities of vibronic transitions encode information on the sign and magnitude of the intermolecular coupling. In organic semiconductors, where a C-C double bond stretch (ω ≈ 150 meV) is usually strongly coupled to the electronic excitation, H-type or J-type aggregation is signaled by suppression or enhancement respectively of the origin (0-0) electronic transition relative to the higher vibronic sidebands.15,

16,17

In a coupled exciton picture, this effect derives from a

destructive/constructive interference between neighboring transition dipoles. Recent work by Spano and co-workers pointed to the importance of the combined long range, dipole-dipole or Frenkel exciton coupling (FE), and short range, charge transfer (CT) interactions, in determining the overall spectral signature.15, 16 Due to each coupling mode having both a sign (negative J; positive, H), and magnitude, the mixing of CT and FE coupling determines the curvature of the exciton band (sum of FE and CT couplings) and the observed spectral signature.

18, 19

The sign and magnitude of the combined electron and hole CT interaction

depends sensitively on the wavefunction overlap and precise registration between neighboring


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molecules.17 Computational studies indicate that sub-angstrom lateral displacements (slips) results in significant change in both sign and magnitude of the overall CT coupling.7, 15 This implies a route toward wavefunction engineering of the exciton band curvature that potentially impacts charge and energy transport in organic semiconductors.7, 15 Interestingly, recent computational findings show that a CT interaction can dominate over the FE coupling (i.e. charge screening of the dipole moment by neighboring molecules) causing the formation of an hJ or jH aggregate in which the spectral signatures are that of a J or H aggregate respectively.7, 18, 19 With a multitude of different possible coupling combinations that may or may not be dependent on packing geometry of a molecular assembly, it is difficult to predict how the spectral signatures of an HJ aggregate like TAT manifest experimentally. Recent experimental work aimed at the realization of tunable HJ aggregate systems focused on manipulation of molecular packing through the addition of side chains and/or immersion in different dielectric environments.10-12,

14, 20, 21

Maity et al. used the length of

poly(ε-caprolactone) (PCL) grafted on graphene quantum dots to direct molecular packing, forming J and H aggregates.14 Egawa and coworkers immersed films of 5,10,15,20-tetrakis (4sulfonatiphenyl)porphorhyrin (TPPS) and poly(allylamine)(PAA) in acidic and neutral solutions to induce a reversible J-H transition in the TPPS assemblies.

22

Balakrishnan et al. used the

dielectric environment and conformational changes in packing through solvent vapor annealing and hydrophilic and hydrophobic side chains to form H and J aggregates from assemblies of perylene diimide chromophores.23 Work in our laboratory investigated the H- to J-aggregate transition in poly-3-hexyl thiophene (P3HT) crystalline nanowires’ dependence on molecular weight, regioregularity, and solvent processing.24 Interestingly, larger molecular weight P3HT undergoes chain folding associated with a decrease in torsional disorder (increased planarity of the single P3HT chains), which results in a slightly smaller inter-lamellar spacing;24 however, perhaps counterintuitively, the larger molecular weight P3HT forms Jaggregate nanofibers with very weak interchain coupling as revealed by time- and polarization resolved photoluminescence and Kelvin-probe imaging.25 In this work, we used 7,8,15,16-tetraazaterrylene (TAT) as a material platform to study the evolution of intermolecular coupling from isolated molecules to extended crystalline


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nanowires.26 TAT, an n-type organic semiconductor, has a number of interesting properties that make for ready spectroscopic access and interpretation. First and foremost, in contrast with perylene diimide (PDI) aggregates whose photoluminescence is dominated by excimer emission,27-29 vibronic structure in TAT nanocluster and nanowire photoluminescence is preserved, allowing for direct spectroscopic interrogation of HJ aggregate behavior. Second, TAT crystallizes in a monoclinic unit cell, with no known polymorphism.16 Finally, detailed theoretical information on the intermolecular couplings within TAT aggregates (and associated photophysics) has been developed by Spano and coworkers allowing for a quantitative comparison between theory and experiment.7, 16, 18, 19 Yamagata and Spano were able to quantitatively model the absorption spectrum of TAT single crystals using a positive (H) Frenkel exciton coupling, and negative (J-) CT interaction.16 In this work, spatially resolved spectral signatures of isolated large clusters show that TAT undergoes superficially J-to-H transition upon assembly signifying a competition between CT and FE components during crystal growth. This is supported by films cast from dilute TAT in chloroform and poly(methylmethacrylate) (PMMA). Samples yielded diffraction limited species that have J- type spectral signatures and extended crystals that have H type spectral signatures indicating a significant difference in the electronic properties between the different self-assemblies.

Experimental: Stock solutions of 400 µM TAT in chloroform were made and vigorously sonicated at 3 minute intervals for a total of 6 or 9 minutes. From the stock solution, dilutions were prepared at 40 µM, 4.0 µM, and 0.4 µM. The dilutions were drop-cast onto plasma-cleaned glass coverslips. Photoluminescence spectra were acquired using a 488 nm argon air-cooled laser (Spectra-Physics 360C) configured for epi-illumination. An inverted microscope (Nikon TE 300) and oil immersion objective (1.4 N.A./100×) were used to collect fluorescence through a dichroic mirror and long pass filter combination and transmitted to an Acton 2150i spectrometer (300 grooves/in grating, blazed at 550 nm) aligned to the side port of the microscope. The single molecule/pre-nucleation species were done by diluting the stock


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solution with a 1 mg/mL solution of PMMA (Sigma Aldrich 350 kDa) in chloroform (Omnisolv). Studies of these samples were done under vacuum. Aliquots (50 mL) were spin-coated at 2200 rpm, with an acceleration of 400 rpm/sec, and spin-time of 120 seconds under nitrogen. The nucleation sites and extended crystals were drop-cast from the 400 µM stock solution directly onto glass substrates and did not require a vacuum for extended study.

Results and Discussion: Figure 1 shows the solution-phase emission spectra of TAT in chloroform in increasing concentrations: 0.4 µM (green), 4 µM (blue), 40 µM (red), and 400 µM (purple). The solution phase PL spectra is a representation of all the emissive species in solution including monomers, dimers, small aggregates and large aggregates in various contributions. The crystal photoluminescence spectrum (shaded orange) is superimposed and scaled to fit the trailing red edge of each of the individual spectra to illustrate TAT’s aggregation is seeded in solution; most likely due to the molecules being weakly soluble in most organic solvents. The crystal

spectra

subtracted

from

cannot the

simply

solution

be

phase

spectra as there are multiple aggregates present in solution that have different spectral origins and intensity envelopes. The arrows (green and purple) indicate the Figure 1: Solution phase PL spectra of TAT in chloroform change in spectral intensity and crystal from various concentrations: 0.4 µM (green), µM (blue), 40 µM (red), and 400 µM (purple). Super-imposed in orange is contribution to the overall spectral the crystal spectrum. The green arrow indicates the change in I00 peak intensity, and the blue arrow indicates the change in the crystal PL contribution as a function of concentration. 5 ACS Paragon Plus Environment


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intensity as a function of monomer concentration. We believe the presence of small and large aggregates in solution results in the varying solution phase spectral signatures as seen through the diminished contribution of the crystal spectra in dilute concentrations. In the most concentrated solutions (40.0 µM and 400.0 µM), the intensity profile superficially resembles that of an H- Aggregate with diminished origin intensity, while the most dilute solution (0.4 µM) appears as the uncoupled monomer (0-0/0-1 intensity ratio of ≈ 1.4).17 With the knowledge of the monomer peak intensity ratio, nominal H- or J- aggregation can be defined for species that have a 0-0/0-1 intensity ratio < or >1.4.18, 19 The intermediate solution (4.0 µM) appears neither as a distinct dominant coupling H or J rather the 0-0/0-1 intensity ratio ≈ 1, indicating a gradual change in the coupling of the emissive species present. Spectra from the concentrated solutions agree with Spano’s computational models for the overall positive sign of the combined CT and FE coupling in crystalline TAT, indicating that the long-range interactions are dominated by H-type Coulombic coupling.7 The increasingly large contribution of TAT crystal luminescence on the red edge of the spectral envelope17 suggests an increased presence of large aggregates in solution. In addition, the spectra associated with the most dilute solution shows a slight red shift relative to the higher concentration and shows a 0-0/0-1 intensity ratio (≈1.4) consistent with uncoupled monomer emission. Spectra from the most concentrated solution (400 µM) shows diminished 0-0 PL intensity, consistent with a Kasha H-aggregate model.17 The solution with intermediate concentration (4.0 µM) shows a 0-0/0-1 peak intensity ratio ≈ 1, indicating the presence of fewer emissive species in solution as seen by the small contribution of the crystal spectra and blue shift from the dilute solution spectra. Figure 2 shows representative spectra and statistics of nanoscale clusters assayed from ~1 µM, ~700 nM, and ~600 nM chloroform solutions of TAT, in a PMMA-supported film. Solution samples were spin-coated on glass coverslips producing PMMA-supported films of mostly very small clusters appearing in photoluminescence as diffraction limited images. Figure 2A shows histograms of peak intensity ratios obtained from a total of 220 different sampled spectra. The individual histograms were comprised of spectra 70 different diffraction limited spots assayed


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from a ~600 nM solution (gold), with an average 0-0/01 intensity ratio of 1.4. The single molecule (inset) was seen to have a peak intensity ratio of 1.5. Species assayed from a 700 nM solution (red, 99 spectra) gave an average 0-0/0-1 intensity ratio of 1.55, while spectra from clusters (blue, 51 spectra) assayed from ~ 1 µM solution showed an average peak ratio of 1.9. Figure 2B shows sampled spectra from the fringes of the distribution J aggregates (pink and green) and H aggregates (teal and blue) along with their 0-0/0-1 intensity ratios.17 The histograms show that as concentration increases, the distribution of intensity ratios broadens and the presence of outliers (species with intensity ratios far away from the mean) increases indicating the formation of J aggregates and H aggregates seen in panel 2B. We hypothesize these small aggregates are prenucleation sites that seed solution phase crystallization.30 While the presence of the H-aggregate species is explained by the formation of assemblies with dominant long-range interactions,16,

17

the presence of the J-aggregate species with a

peak intensity ratio of 2 is not as easily explained. In the HJ Aggregate model calculated for TAT, the charge transfer interaction is modeled as J due to the sign and magnitude of electron and hole transfer integrals, (in the absence of the FE coupling) even though the molecular registration of two neighboring cofacial molecules is slightly slipped (~1Å in x and y).7,

16

In the absence of CT coupling,

locally within a π-stack, two cofacial TAT molecules would have an FE coupling strength of ~1100 cm-1;16 however, the FE coupling is screened by the π-stack in the extended crystal resulting in coupling strength of 300-400 cm magnitude comparable

of

the

with

-1

FE that

such that the coupling of

the

is CT

component, but opposite in sign.7

Figure 2: Panel A shows the 0-0/0-1peak intensity ratio histogram and single molecule spectrum (inset) from samples cast from different concentrations. Panel B shows representative spectra and the 0-0/0-1peak intensity ratio for outliers from histograms in Panel A.

The J-aggregate spectral signatures for


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these clusters suggest a different molecular packing than the bulk crystal during the initial crystallization process. Multi-component solution phase crystallization processes have been observed previously. For example, perylene bisimide derivative (N,N-di[N-(2-aminoethyl)3,4,5-tris(dodecyloxy)benzamide]-

1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-

tetracarboxylic acid bisimide) has been shown to undergo competing crystal growth pathways that form two different prenucleation sites (small clusters) depending on solvent and temperature.31 Figure 3 A, B, and C show histograms of the 0-0 transition energies taken from the spectra described in Figure 2. The aggregate spectra cast from the ~600 nM solution (gold) has an average origin energy of 2.22 eV, while the ~700 nM solution (red) is bimodal with maxima at 2.22 eV and 2.34 eV. The distribution of origin energies for clusters assayed from ~1 µM solution (blue) is peaked at an origin energy of 2.34. Figure 3 D, E, F, and G show the 2D Gaussian fits of the 00/01 peak intensity ratio vs. origin energy, as well as count rate vs. origin energy collected from spectra of aggregates cast from ~600 nM (gold), ~700 nM (red) and~1

Figure 3: A,B,&C Histograms of transition energies taken from the spectra described in Figure 2. The aggregate spectra cast from the ~600 nM solution (gold) A, the ~700 nM solution (red) B ,~1 mM solution (blue)C. Figure 3 D,E,F, and G shows the 2D Gaussian fits of the 00/01 peak intensity ratio vs. transition energy ,D and E, and count rate vs. transition energy, F and G, collected from spectra of aggregates cast from 600 nM (gold), 700 nM (red) and~1 mM (blue) solutions TAT in chloroform and PMMA. Figure 3H shows the count rate vs 00/01 peak intensity ratio of spectra from aggregates assembled from cast ~600 nM (gold), ~700 nN solution (red), and 1 mM (blue) solutions TAT in chloroform and PMMA.


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µM (blue) solutions TAT in chloroform and PMMA. Figure 3 B, E, and G show the J-aggregate spectra are blue-shifted from the monomer, and in the sample cast from the intermediate solution both monomer and J-aggregate species are present and result in a bimodal distribution in the transition energy. The count rate vs. 00/01 peak intensity ratio shows an increase in 00/01 peak intensity ratio increases the count rate, which is consistent with the emissive properties of a J-aggregate.17 The blue shift of the J-aggregate spectra, however, is somewhat surprising in that one usually expects a red shift in the origin transition relative to the monomer.17 It seems likely that the blue shift is attributable to a slightly different dielectric environment within the PMMA matrix.

Wei and coworkers have

shown, for example, the effect a polymer matrix can have on emission spectra through the submersion of quantum dots into a PMMA matrix causing a change in stokes shift on the order of 0.9 eV.32 As a subject of future study, understanding the early stages of solution phase

crystallization,

and

the

packing

structure of small cluster or prenucleation sites consisting of only a few molecules are important for tuning photophysical properties and directional charge separation exhibited by the extended crystals. 30, 33 A final bit of structural insight was afforded by examining the axially-resolved PL spectra from elongated aggregates slightly larger than the diffraction limit (250 nm). Figure 4: Photoluminescence image (PL) with resolution of 250 nm and spatially resolved PL with and without image with color bars corresponding to spectral position. A) Null spectra with 00/01 peak intensity ratio of 1.4 B) B spectra from the competition of the FE and CT coupling that has 00/01 peak intensity ratio of 1.4 C&D) H aggregate spectra that shows FE interaction dominant with 00/01 peak intensity ratio 9 ACS Paragon Plus Environment of 0.5.

Figure 4 shows two representative PL images of a single larger cluster one


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the PL dispersed along a wavelength dimension in the z direction. Figure 4A-D shows the extracted spectral slices from the spectrally resolved PL image. These spectra show the large clusters spectroscopically seem disordered. At the top edge of the large cluster, the spectrum appears as blue shifted compared to the other lateral spectral slices with a 0-0/0-1 of 1.4 (Figure 4A). Shown in successive z-slices Figure 4B, C, and D show the spectra evolve from an intensity ratio of 1.2, to 0.5, which is indicative of the typical H-type spectral signature of the TAT crystal. The blue shifted spectral features with 00/01 peak intensity ratio of 1.4 is similar to the uncoupled monomer suggesting that the coupling type within this specific area is a null aggregate. Due to the size of these aggregates being smaller than the 0-0 transition wavelength, we believe self-absorption is minimal. The null aggregate spectrum describes coupling between multiple molecules, possibly disordered, where the CT and FE coupling have canceled yielding monomer like spectral signatures, which is innately different than the monomer spectrum which is from one isolated molecule.18, 19 In the HJ aggregate model proposed by Spano, a “null” aggregate is formed in the situation where the CT and FE interactions are equal in magnitude and opposite in sign resulting in an exciton band with nearly zero curvature producing spectral signatures of the uncoupled monomer.18, 19 The red shift of the null aggregate from the uncoupled monomer spectrum also implies crystalline order within the aggregate.18,

19

In Figure 4B, the spectral

signatures infer an exciton band curvature that has become slightly more negative due to the cumulative sign of the FE and CT coupling resulting in a spectrum with an I00/I01 < monomer ratio, 1.4, and is almost unity. The lack of a strong H or J type spectral signature leads us to believe it is this part of the crystal growth process would be most sensitive to tuning of the molecular architecture and thereby exciton band curvature.

As shown in In Figure 4D, the

spectrum appears superficially as an H aggregate suggestive of dominant FE coupling; however, there is no information on whether the sign of the CT coupling has changed producing an HH aggregate as opposed to an Hj aggregate. The fact that only one edge of the crystal has null aggregate spectral signatures suggest a change in the molecular packing upon crystallization as well as the J aggregate spectral signatures of the aggregates from dilute solutions. These aggregates provide a new platform to study the impact of molecular packing


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on combined CT and FE interactions, and a molecular understanding of how these interactions evolve upon solution phase crystal growth.

Discussion/Conclusion: Figure 5 shows a summary of the spectral signatures observed from the isolatable species from TAT’s self-assembly and the inferred dominant coupling from the I00/I01 intensity ratio. The competition between the CT and FE interactions at early stages of growth are easily discernible. The uncoupled monomers have no exciton band present, and small clusters where the CT component seems to be dominant over the FE component has Jaggregate spectral features. Large clusters have spectral signatures of the null aggregate, uncoupled monomer, on one edge where the FE and CT has canceled as well as an equivalency point where the FE and CT are slightly imbalanced, and the rest of the cluster and extended crystal exhibited dominant FE coupling. We have shown how the overall sign and magnitude of combined FE and CT intermolecular coupling evolves from single molecule to extended nanowire crystal as

Figure 5: Steady state emission spectra of TAT monomer (red), J-Aggregate (blue), Null Aggregate (red), HJ- Aggregate (green), and extended crystal (gold).


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signaled by distinct changes in the PL spectra. Drop-cast films of TAT in chloroform formed large clusters that have spectral signatures of the uncoupled monomer, HJ aggregate, and an H aggregate indicating a change in the chromophore coupling upon growth. Spin-coated films of TAT in chloroform and PMMA produced PL spectra characteristic of uncoupled monomers, and small aggregates where the CT component was dominant over the FE component yielding J- aggregate spectral features. Extended crystals had completely H aggregate spectral signatures. This transition showed that naturally upon growth TAT spectral signature transitions from a J - aggregate in small clusters to Null –HJ- H in large aggregates indicating an exciton band inversion. The reason for the change possibly, molecular packing, is undetermined. The observed change in the exciton band curvature gives support to the idea of tuning of chromophore coupling through molecular architecture and, in turn, unit cell geometry. The exciton band curvature of these small molecule organics is directly correlated to long exciton diffusion lengths and production of long lived free charge carriers due to the constructive interference delaying radiative combination.34-36 Finding simpler ways to manipulate coupling for charge transfer and stimulated emission properties are key for developing optoelectronic devices. The next step in realizing tunable small organic molecule architecture is understanding how these different couplings play a part in exciton diffusion and polaron pair formation, ultimately, leading to active layer structures with efficient directional charge and energy transport.

ACKNOWLEDGEMENT This work was supported in part by the Northeast Alliance for Graduate Education and the Professoriate and STEM Faculty-Student Research Grant from the Graduate School at the University of Massachusetts Amherst.


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Evolution of HJ Coupling in Nanoscale Molecular Self-Assemblies

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