Distinct Crystalline Aromatic Structural Motifs: Identification

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Distinct Crystalline Aromatic Structural Motifs: Identification, Classification, and Implications Remya Ramakrishnan,‡ M. A. Niyas,‡ M. P. Lijina, and Mahesh Hariharan*

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School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Vithura, Thiruvananthapuram, Kerala 695551, India CONSPECTUS: Spatial noncovalent helical organization of nucleobases in DNA and radial organization of chromophores in natural light-harvesting systems are fascinating yet enigmatic. Understanding the numerous weak interactions that drive the formation of elegant supramolecular architectures in native natural systems and developing bioinspired design strategies have seen a surge of interest in recent decades. Self-assembly of functional chromophores in the crystalline phase is a definitive strategy to identify novel molecule−molecule interactions, in particular, atom−atom interactions, and to understand the synergistic nature of noncovalent interactions that stabilizes the supramolecular organization. This Account narrates our recent efforts in developing desirable supramolecular motifs employing weak interaction-based strategies and our observation of deviations from the common motifs chartered in aromatic systems. Modulation of long-range aromatic interactions through chemical modifications (acylation, benzoylation, haloacylation, and alkylation of chromophores) to attain a preferred stacking (herringbone, lamellar, or columnar) is presented. Particular attention has been given to attaining lamellar or columnar packing possessing potential interchromophoric electronic coupling mediated high charge mobility. Supramolecular arrangements of noncovalently or covalently associated donor−acceptor systems that open up additional possibilities of packing modes (segregated, mixed etc.) are explored. Our persistent efforts yielded distinct twisted-segregated and alternate distichous stacks for the nonparallel covalently linked donor−acceptor systems that favor a long-lived photoinduced charge-separated state. We further move on to discuss the unconventional packing motifs that were identified recently. The highly sought-after Greek cross (+) stacking of chromophores in crystalline phase and an elegant crystalline radial arrangement of chromophores are examined. The Greek cross (+) stacked architecture exhibits monomer-like emission characteristics owing to the absence of exciton coupling across the orthogonally stacked chromophores. Crystalline helical chromophore assembly is yet another emerging motif with far-reaching applications in domains ranging from asymmetric catalysis to chiral smart materials and has been accounted here by citing certain phenomenal examples from literature. Thus, this Account demonstrates that identifying and classifying new structural motifs based on topological aspects, such as interchromophoric orientation (cross) and extended chromophore arrangement in the crystal lattice (radial, helical, etc.), are crucial since such fundamental characteristics dictate the properties emerging out of the corresponding motifs. Encouraged from ours and others’ works, we propose the addition of new aromatic supramolecular structural motifs, namely, cross-stacked, helical, and radial arrangements, in order to expand the classification. We believe that identifying new emergent property-based supramolecular motifs and investigating the methods to achieve the desired motif will eventually have implications in fundamental crystal engineering, supramolecular chemistry, and biomimetic design of functional materials.

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INTRODUCTION Biological systems exhibit an ingenious choice of chromophores combined with a plethora of molecular organizations that harmoniously produce desired optical and electronic properties. For instance, nature adopted radially arranged porphyrin rings in light harvesting complexes 1 and 2 (LH1 and LH2) for light harvesting during photosynthesis, while for information storage and transfer, adenine, guanine, thymine, and cytosine were assembled helically. The radial chromophoric array in crystalline LH2 of Rhodoblastus acidophilus,1 horseshoe-shaped assembly of chromophores in crystalline LH1 of Rhodopseudomonas palustris,2 and S-shaped arrange© XXXX American Chemical Society

ment of chromophores in crystalline LH1 of Rhodobacter sphaeroides3 are a few other exquisite natural molecular scaffolds formed by synergistic noncovalent interactions (Figure 1).4 The complex supramolecular architectures have evolved to enable essential electronic properties such as energy and exciton migration. Nature uses subtle modifications in the arrangement of bacteriochlorophylls for both energy harvesting and photoinduced electron transfer in the photosynthetic apparatus of purple bacteria. Slip stacked (J-type coupled) Received: June 17, 2019

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DOI: 10.1021/acs.accounts.9b00320 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Figure 1. Crystal structures of (a) radial LH2 complex in Rhodoblastus acidophilus’, (b) horseshoe shaped RC-LH1 complex in Rhodopseudomonas palustris, and (c) S-shaped RC-LH1 complex in Rhodobacter sphaeroides. Adapted with permission from ref 4. Copyright 2017 Portland Press Limited. (d) Schematic representation of molecular orientation in LH2 and LH1.

bacteriochlorophylls in a radial array are used for efficient funneling of energy to the reaction center,5 while a “special pair” of π−π stacked bacteriochlorophyll dimers6 facilitate quantitative electron transfer in the reaction center. High charge transport is predicted for a parallel-stacked dimer via Marcus−Hush formalism,7 while the energy transfer rate is governed by coherent resonance energy transfer8 and multichromophoric Förster resonance energy transfer theory9 and can be exclusively manipulated by organization of chromophores. Remarkable performance of natural photosynthetic systems owing to the precise assembly of chromophores in a molded environment has inspired several self-assembly strategies, aiming to manipulate the photoexcited state properties of chromophores, for the design of artificial photosynthetic systems exhibiting efficient light harvesting and photoinduced charge separation.10 Numerous biotemplated and bioinspired strategies have been adopted to understand the complexity and utilize the viability of biological supramolecular architectures. Directed chromophoric assembly through cyanine dye intercalation and minor groove binding in DNA,11,12 replacing nucleobases with fluorophores in DNA,13 and DNA templated multichromophoric helical assembly12,14 are few of the biotemplated synthetic strategies implemented to organize chromophores helically. Bioinspired crystalline aromatic foldamer based single helical assembly15 and β-sheets were developed recently16 by Huc and co-workers. Construction of radial molecular scaffolds mediated by hexaarylbenzene17 was employed as a covalent synthetic route to emulate the assembly of chromophores in natural photosynthetic systems. Lately, following the advent of crystal engineering, organization of chromophores via noncovalent interactions has become a promising and convenient strategy. Rational modulation of noncovalent interactions in assembling molecules to construct predesigned supramolecular

architectures requires a comprehensive understanding of the synergistic effects of weak intermolecular interactions. Small molecule organic crystallography possibly provides better insights into the three-dimensional (3D) supramolecular structures than the intuitive representations from the aggregate characterizations to delineate the packing motifs organized by the self-assembled chromophores. Numerous weak noncovalent interactions, including hydrogen bonding,18 halogen bonding,19 chalcogen bonding,20 pnictogen bonding,21 tetrel bonding,22 aerogen bonding,23 π−π interaction,24,25 and dipole−dipole interaction,26 were identified and examined through X-ray crystallography and computational tools.20,27−29 Recognition of novel interatomic interactions and 3D structural motifs is important to decipher the quaternary structure of proteins and leverage the cooperative effects and flexibility of noncovalent interactions to design efficient supramolecular extended architectures that may mimic natural systems. This Account deals with polycyclic aromatic hydrocarbon (PAH)-based chromophores and their 3D crystalline supramolecular assemblies that produce exciting emergent properties. We are interested in understanding the profitability and ultrafast photoexcited state processes of certain 3D supramolecular motifs and in developing strategies to crystallize desired supramolecular structures. This Account begins by introducing the current classification of PAHs and some of our strategies that yielded intended packing motifs. As the narration goes on, there is emphasis on the need for new additions to the classification of 3D supramolecular structural motifs for evolving a comprehensive sophisticated system to understand the rise of unique crystalline chromophoric assemblies and the corresponding emergent properties. Toward the conclusion, we propose the inclusion of crossB


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Figure 2. Flowchart of the current classification of crystalline motifs in organic systems.

Figure 3. Progressive acylation of anthracene, manifesting the modulation of π−π stacking by large perturbation of face-to-face π−π interaction. Adapted with permission from ref 36. Copyright 2018 Wiley-VCH.

interactions dominate in herringbone followed by sandwich herringbone, while face-to-face interactions govern β-motif followed by γ-motif. Controlling π-orbital overlap, and thereby the excitonic interactions, by modulating the 3D packing of chromophores was our principal motivation to work on crystalline assembly of chromophores.

stacked, helical, and radial supramolecular systems as new separate classes of crystalline structural motifs.

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CONVENTIONAL PACKING MODES IN POLYCYCLIC AROMATIC HYDROCARBONS Molecular assembly of PAHs is the best point to kick off a discussion on our motivation to diverge from conventional to hitherto unprecedented chromophoric arrangements. The seminal work by Desiraju and Gavezzotti, three decades ago,30 characterized the then available crystal structures of PAHs (Figure 2) into herringbone, sandwich herringbone, γ, and β structures and predicted the packing modes of a few undetermined crystal structures of PAHs. The competition of face-to-face (π−π) and edge-to-face (C−H···π) noncovalent interactions dictate the assembly of molecules into various packing modes in all-carbon PAHs.31 Edge-to-face aromatic

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MODULATING π−π STACKING Aromatic−aromatic interactions that include edge-to-face, edge-to-edge (parallel-displaced), and face-to-face (cofacial) π−π stacking are extremely important in protein folding32 and stabilization as well as functional material design. While proteins prefer edge-to-face or parallel-displaced π−π interactions over cofacial interaction, higher charge transport in semiconducting materials requires cofacial π−π interaction. The majority of the basic PAHs exhibit either herringbone C


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Figure 4. Haloacetylation yielded sandwich herringbone to lamellar or columnar motifs in pyrene. Adapted with permission from ref 39. Copyright 2016 American Chemical Society.

from herringbone to columnar or lamellar stacking.41 Synergistic cooperation of C−H···F interaction and noncovalent halogen bonds formed by iodine and nitrogen atoms assisted in composing long-range face-to-face π−π aromatic stacking. Side chain engineering of naphthalimide-perylenimide (NP) donor−acceptor (D-A) molecules demonstrated yet another systematic modulation of π−π interaction.42 Contrary to the “fastener effect” that was proposed for the fastening of π surfaces connected to long alkyl chains,43,44 we observed alkyl chains in NP to impede the π−π interaction, thereby causing a decline of exciton interaction. Encountering the complexities involved in modulation of packing in monochromophoric systems and confronting the challenges via directional noncovalent interactions emanating from functional group insertion or cocrystallization encouraged us to venture into the more formidable field of crystal engineering of donor−acceptor bichromophoric systems that has far-reaching consequences in artificial light-harvesting and optoelectronics.

(benzene, naphthalene, anthracene, phenanthrene, chrysene, picene, hexacene, etc.) or sandwich herringbone (pyrene, perylene, quaterrylene, etc.) packing modes. Addition of heteroatoms and substituent groups can impart new intermolecular interactions to alter the packing motif of chromophores and have significant influence on the charge transport properties of the system.33 Disk-shaped polyaromatic cores attached with flexible aliphatic chains are known to form discotic liquid crystals possessing elegant columnar architectures functioning as one-dimensional (1D) conduits for charge mobility. The intra- and intercolumnar arrangements in the discotic PAHs can be altered via noncovalent interactions such as hydrogen bonds and dipolar interactions induced by additional functional group substitutions (amido, ureido, carboxylic acid, monocyano, dimethoxy, etc.) on the aromatic core, which can thereby modulate the mechanical, optical, and electronic properties of the system.34 Halogenation, alkylation, or arylation in peri-positions of linear acenes are also reported to have changed the crystal packing from herringbone to πstacked motif.35 We adopted several functional group substitutions on chromophores to tune π−π interactions in organic small-molecule-based crystals. Systematic mono- to triacylation of anthracene36 (Figure 3) and mono- to tetraacylation of pyrene37 increased the face-to-face π−π interaction, thereby switching the packing from sandwich herringbone/herringbone to columnar/lamellar stacking, while mono- to tetrabenzoylation of pyrene38 reduced the face-to-face π−π interaction of chromophores. X-ray crystal structure characterized the systematic decrease/increase in π−π distance on acylation/benzoylation. Significant increase/ decrease in π−π interaction was also demonstrated in the red-/ blue-shifted emission spectra of the crystalline material. Haloacetylation yielded sandwich herringbone to lamellar or columnar stacking in pyrene39 (Figure 4). Electron density perturbation caused the formation of dihydrogen bonds, halogen bonds, and π−π interactions in haloacetylated pyrenes. Regioisomeric change in packing motif was clearly shown by the bisthiazolyl substitution on pyrenes where the 1,8substitution yielded columnar stacking while 1,6-substitution produced herringbone stacking.40 Other than the covalent substitution on the chromophore to deliver directional noncovalent interaction, we proposed the bicomponent cocrystallization of bipyridine connected PAHs with diiodotetrafluorobenzene as a strategy to tune chromophoric stacking

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DONOR−ACCEPTOR SYSTEMS Self-assembly of donor−acceptor (D-A) systems into stacked supramolecular architectures with emergent properties has received prodigious attention in recent years. Donor−acceptor systems with 1:1 stoichiometry (cocrystals or charge-transfer salts) generally form either slipped segregated-stack or interdigitated systems with the latter being dictated by charge transfer or quadrupolar interactions45 and more probable owing to the electronic complementarity of the electron donors and acceptors. The segregated stacks tend to exhibit ambipolar electrical conductivity, while the mixed stacking modes are inclined to display semiconductor or ferroelectric properties.46 The donor−π−acceptor dipolar dyes form a unique class of D−A chromophores that self-assemble into antiparallel stacks via electrostatic dipolar interactions and have consequently been confirmed to be useful as semiconductors in organic electronics and photovoltaics.26 Construction of desired supramolecular D−A edifices involves the judicious selection of chromophores with suitable symmetry and energy levels and the concerted interplay of various noncovalent intermolecular interactions such as π−π stacking, charge transfer (CT), hydrogen bonding, halogen bonding etc.46 We observed remarkable ordered assemblies of covalently linked nonparallel donor−acceptor systems, previously preD


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Figure 5. (a) Nonparallel segregated organization in NIN. Adapted with permission from ref 48. Copyright 2015 American Chemical Society. (b) Twisted-segregated stacked PA. Adapted with permission from ref 50. Copyright 2016 American Chemical Society. (c) Alternate distichous stack and (d) close packing arrangement in crystalline AdAn. Adapted with permission from ref 51. Copyright 2017 American Chemical Society. (e) Energy transfer in bichromophoric NP. Adapted with permission from ref 52. Copyright 2013 American Chemical Society.

mobility (μe) = 0.16−0.17 cm2 V−1 s−1) for the crystal suggesting PA as a potential candidate for photovoltaic and semiconducting device applications. We proceeded to construct a nucleobase−arene conjugate, 9-(adenin-9-yl)anthracene (AdAn, Figure 5c,d), hoping to capitalize on the propensity of nucleobases to self-assemble via hydrogen bonding and thereby explore photoexcited state dynamics of the aggregated nanostructures.51 AdAn self-assembled in a fascinating alternate distichous fashion with the adenines, held by C−H···N hydrogen bonding interactions, being adjacent to the anthracene-on-anthracene π-stacked columns. The aggregated AdAn formed long-lived photoinduced radical ion pair intermediates (τcr(aggregate) = 120 ms) owing to the segregated charge transport pathways, that is, delocalization of the electron through the columnar anthracene stacks and diffusion of the holes across the hydrogen-bonded adenine units. The self-assembled nanostructures of AdAn in solution underwent anthracene−anthracene photochemical dimerization and decomposition under high energy UV-irradiation, which reflected the crystal structure arrangement in the aggregated state. The alternate distichous arrangement exhibited by AdAn can be considered a novel assembly that can be mimicked for the design of robust charge transport devices. In another example,52 bichromophoric naphthalimide−peryleneimide (NP, Figure 5e) formed isolated dimers in the crystalline state engendering a highly conducive photoinduced energy transfer from excited singlet state of naphthalimide (1N*) to peryleneimide (P) followed by efficacious red solid-state photoluminescence (Φf = 0.5 ± 0.04) from the excited singlet state of peryleneimide (1P*). The interference from orthogonal naphthalimide and diisopropylphenyl units impeded the infamous strong H-type aggregation caused fluorescence quenching (ACQ) of peryleneimide (P) moieties. Consequently, a highly short-axis displaced weak H-type excitonic interaction in P moieties arose leading to high quantum yield of fluorescence. Parallel transition dipole−transition dipole

sumed to be difficult to attain,47 leading to an enhanced lifetime of the photoinduced charge separated state (CS). In 2015, 48 we reported a covalently linked D−A dyad (naphthalene−naphthalimide (NIN), Figure 5a) which exhibited >10000-fold enhanced CS lifetime in the aggregated state (CS lifetime, τcr > 1.2 ns) compared to that in the monomeric entity (τcr < 110 fs). NIN exhibited nonparallel segregated organization in the crystalline phase. The intramolecular hydrogen−hydrogen steric repulsion between the constituent units resulted in a nonparallel geometry for the dyad while the cooperative effect of C−H···π, π−π, and C− H···O interactions coupled with the electronic structure of D and A dictated a D-on-D and A-on-A segregated arrangement. The amplified lifetime for the CS state is attributed to the nonplanar geometry of the dyad and the delocalization of the photogenerated excitons over the antiparallel conduit across the stacks. The evolution of the charge resonance band in the near-IR region (λmax= 1020 nm) corresponding to naphthalene dimer radical cation in the femtosecond transient absorption spectrum of the aggregate state presented unambiguous evidence for the inheritance of the crystalline assembly (i.e., segregated stacking) to the aggregated state in solution. Qiu and co-workers49 theoretically predicted the appearance of interlayer electronic transition coupled with 2D nonlinear optical character for segregated stacked D−A systems. In yet another example, we succeeded in crystallizing an all-carbon D−A pyren-1-ylaceanthrylene (PA, Figure 5b) dyad into twisted-segregated stacks.50 The π−π and C−H···π interactions drove D-on-D and A-on-A bicontinuous stacks in the crystalline state. The augmented lifetime (τcr(aggregate) ≈ 1.28 ns vs τcr(monomer) ≤ 110 fs) of the photoinduced CS state in the self-assembled molecular structures in solution suggested a segregated morphology for the aggregates similar to that in the crystalline architecture. Further, Marcus theory of charge transfer rates proposed an ambipolar semiconducting property (hole mobility (μh) = 0.20−0.24 cm2 V−1 s−1 and electron E


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Figure 6. (a) Segregated stacking and (b) band structure of F6.Q. (c) Molecular structures of constituents of F6.Q/F6a.Q. (d) Mixed stacking and (e) band structure of F6a.Q. Adapted with permission from ref 54. Copyright 2018 Wiley-VCH.

Figure 7. (a) Exciton splitting diagram for a molecular dimer at different orientations. Schematic representation of molecular structure and the close-packing in (b) PTE-Br2, (c) PTE-Br0, and (d) PTE-Br4. (e) Scheme for the charge-filtering (selective hole transfer (kh)) phenomenon in PTE-Br2 Greek cross (+) aggregate. Adapted with permission from ref 55. Copyright 2018 Wiley-VCH.

exhibited efficient Förster type energy transfer leading to high orange-red fluorescence quantum yield (Φf = 0.54 ± 0.02).53 The kinetically stable supramolecular gel assembly of (R)NP(OH)2 thus formed above the critical gelator concentration in the dichloromethane/hexane mixture was found to undergo transition to thermodynamically stable crystal via gliding of the

orientation of naphthalimide and peryleneimide units in the same molecule, nearest and non-nearest neighbor molecules in the crystal lattice, resulted in efficient intra- and intermolecular energy transfer. The NP derivative with the amphiphilic (R/S)α,β-dihydroxypropyl side-chain substituted (R/S-NP(OH)2) on the naphthalimide moiety formed a chiral vesicle gel and F


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phores is predicted to possess lower energy splitting between the lowest two excited states and therefore greater probability of transition from the second lowest excited state to the first lowest excited state. A substantial transition dipole moment develops for the lowest energy transition in the cross-dipole stacked aggregate and consequently ensures efficient solid state light emission properties including stimulated and amplified spontaneous emission characteristics making them suitable for organic laser diodes.63 Ma and co-workers provided the first experimental evidence for the cross-stacked architecture (α = 70°, X-aggregate) in crystalline trans-2,5-diphenyl-1,4-distyrylbenzene, which was found to exhibit strong blue fluorescence and considerable electron-charge transporting properties.57 The cross-stacked aggregates exhibit minimal excitonic coupling and display strong luminescence properties while simultaneously opening up π-ways for charge carrier mobility. Several cross-dipoles (0 < α < 90°) emerged in the following years.63−68 Orthogonally stacked aggregates have been established in certain columnar liquid crystals via solid-state NMR,69 two-dimensional wide-angle X-ray scattering (2DWAXS),70 and molecular modeling.71 Though thermodynamically favorable,72 an ideal cross-dipole (α = 90°) alignment in the crystalline phase that can provide graphic insights into the noncovalent interactions and packing arrangements remained chimerical. In 2018,55 we succeeded in providing the single crystal evidence for an explicitly orthogonally stacked arrangement of chromophores (α = 90°) in crystalline 1,7dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br2, Figure 7b), which was termed “Greek cross (+)” aggregate and can be considered to be obtainable on rotating the alternate chromophores in an ideal H-aggregate by 90°. The perpendicularly cross-stacked columns constituted a convex type dimer dictated by π−π interactions and concave type dimer governed by C···Br and Br···O interactions. Noncovalent C−H···O and C−H···π interactions directed the 2D arrangement in the crystalline PTE-Br2. Interestingly, nonbrominated PTE-Br0 (perylene-3,4,9,10-tetracarboxylic tetrabutylester) and tetrabrominated PTE-Br4 (1,6,7,12-tetrabromoperylene-3,4,9,10-tetracarboxylic tetrabutylester) were found to display X-aggregate (α = 70°) and J-aggregate (θ = 48.4°) type packing, respectively, in their single crystalline phase (Figure 7c,d). The insertion of Br atoms in the bay position twisted the perylene core in PTE-Br2 (dihedral angle, φ = 24.8°) and PTE-Br4 (φ = 38°) owing to the steric repulsion. The symmetry of the chromophore and the synergistic effect of various noncovalent interactions directed the assembly of chromophores in the PTE derivatives under investigation. The PTE-Br2 was found to retain the monomeric optical characteristics (fluorescence quantum yield, fluorescence lifetime, and radiative decay rates) in crystalline state as expected and theoretically exhibited exceptional selective hole mobility (μh/μe = 575.8, Figure 7e). It is interesting to note that despite both crystalline PTE-Br0 and PTE-Br2 having a similar percentage of π stacking interactions they exhibit dissimilar mobility properties (μh/μe = 3.86 [PTE-Br0]), which can be attributed to the difference in interchromophoric orientation (α = 70° and α = 90°). The amalgamation of high solid-state fluorescence quantum yield and charge transport in an organic semiconductor is indeed a challenging goal and necessitates the diligent tuning of interchromophoric alignment (i.e., in terms of intermolecular distance, molecular symmetry, and spatial orientation) in crystal packing.

dyad over a time period of 48 h. The metastable (R)-NP(OH)2 gel formed a robust co-gel in the presence of indole. The dyad (R)-NP(OH)2/indole (1:10) co-gel underwent photoinduced electron transfer from indole to 1N*/1P* and produced enhanced lifetime for the CS state (τcr(gel) ≈ 1.4 ns vs τcr(solution) ≈ 6 ps,). The emergence of a broad band at 640 nm corresponding to the spectroscopic signature of π−π stacked P radical anion evinced the existence of extended charge delocalization through the π-stacked assemblies of P in the gel state. The various packing modes emerging out of the synthesized D−A systems intrigued and prompted us to explore the structure−packing−property correlation in a series of TTF− TCNQ (tetrathiafulvalene−tetracyanoquinodimethane) based cocrystals.54 TTF-TCNQ-based derivatives serve as a model system for understanding the factors influencing a certain type of crystal packing through functional group substituents. Exploiting the computational tools (Hirshfeld surface analysis, PIXEL energy calculations, and quantum theory of atoms in molecules (QTAIM)) available at our disposal, we observed that the presence of hydrogen bond inducing functional groups, or heavy elements or a modification in the length of side chain deterred the segregated stacking for TTF−TCNQbased cocrystals. Energy decomposition analysis (PIXEL) elucidated the higher contribution of the dispersion component toward total lattice stability irrespective of the type of packing. Band-structure and DOS calculations revealed metallic and semiconductive characteristics for the segregated (F6.Q) and mixed stacking (F6a.Q) polymorphs (Figure 6), respectively, for a TTF−TCNQ-based cocrystal providing distinct evidence for the inherent higher charge transport potential of D-on-D and A-on-A segregated architectures.

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CROSS-STACKED SYSTEMS Alteration of excitonic and electronic coupling via variation in the slip angle (θ), rotational angle (rotational angle between long axes of stacked chromophores, α), or interchromophoric distance (Figure 7a) within the stacked chromophores can have tremendous impact on the emergent charge carrier mobility and optical properties.55−58 Cofacial stacking (Haggregate) of chromophores extends the “supramolecular conjugation” imparting high charge mobility.59 However, the H-aggregates (θ > 54.7°) exhibit lowered fluorescence quantum efficiency (aggregation induced quenching (AIQ)) owing to effective exciton splitting followed by forbidding of electronic transition from ground state to the lower excited state. Excitonic splitting in slip-stacked transition dipoles (Jaggregates) favor ground state to lower excited state transition resulting in high fluorescence quantum efficiency coupled with weak to considerable charge transport character.5,60 Ma and co-workers56 observed efficient near-infrared emission and electron mobility for the single crystal of a PBI derivative (N,N′-(bis(4-methoxybenzyl)-perylene-3,4,9,10-bis(dicarboximide)), obtained by physical vapor transfer method, possessing magic angle stacking (θ = 54.7°). Theoretical investigations by Kasha and co-workers and later by Brédas and co-workers predicted cross-stacking mode preferably with the transition dipoles being exactly perpendicular (α = 90°) to one another as the most efficient alignment possessing null excitonic interactions and therefore retaining monomeric fluorescence quantum efficiency in the condensed media.61,62 The cross-stacked arrangement with a definite angle between the long molecular axes of stacked chromoG


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Figure 8. Schematic representation of different types of dihalogen interactions: (a) type I trans, (b) type I cis, (c) type II dihalogen bonding, (d) X3 synthon formed by type II dihalogen interactions, and (e) X6 synthon formed by type I dihalogen interactions. (f) Trigonal and (g) radial arrangement of NIBr2. (h) Bond critical point (green dots), ring critical point (red dots), and CCP (blue dot) obtained from QTAIM analysis of NIBr2. Adapted with permission from ref 73. Copyright 2019 American Chemical Society.

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RADIAL SYSTEMS Small organic molecular crystallography aids in identifying novel molecule−molecule interactions and adds unforeseen dimensions to the current understanding of existing noncovalent interactions. For example,73 we recently reported a stable noncovalent type-I hexabromine (Br6) synthon (Figure 8) that assists the radial assembly of NIBr 2 (1,8dibromonaphthalene(2,6-diisopropylphenyl)imide) in the crystal state possessing a rare R3̅c symmetry. The au courant notion of the stability of interhalogen interactions was based on the electrostatic model that relies on the amphoteric nature and σ-hole theory. Accordingly, type-II halogen−halogen interaction was classified as the only true halogen−halogen interaction, while type-I was considered as destabilizing and thereby a consequence of close packing. However, the role of quantum covalency in the origin of halogen−halogen interaction had been proposed in recent years. In our work, six bromine atoms were found to interact inter- and intramolecularly to form a rare noncovalent cage as evidenced from the cage critical point (CCP) identified in QTAIM analysis. Penda’s interacting quantum atom method (IQA) revealed the stabilizing nature of the hexahalogen synthon with exchange-correlation as the major contributor toward the stability (Table 1). The significant role of exchange-

dominated type-I halogen−halogen interaction in driving an elegant radial chromophoric assembly and thereby advocate for a reevaluation of the currently accepted notion of electrostatics as the exclusive basis of halogen−halogen interaction. The evolution of the impressive radial assembly with higher order symmetry (R3̅c) in the crystalline phase from a low symmetric monomer (C1) manifests the dynamic nature of various interand intramolecular atom−atom interactions. The radial chromophoric pattern reminds one of the circular organization of bacteriochlorophylls in the light-harvesting systems (LH) of purple bacteria. Nature utilizes the J-type coupled chromophores in a radial array for ultrafast exciton passage within the LH2/LH1 complex and efficient channeling of energy to the reaction center.5 Thus, identifying and assessing the origin of radial chromophoric architectures, wherein appreciable excitonic coupling between adjacent chromophores is additionally observed, can prove invaluable for the design of artificial light-harvesting systems.

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HELICAL SYSTEMS Over the years, myriad exquisite supramolecular scaffolds have evolved out of the chemists’ desire to mimic natural systems and functions. The discovery of α-helical proteins and the right-handed double helical strands of DNA triggered several endeavors in the development of synthetic chiral helical assemblies.74 Self-assembly of chiral and achiral molecules into crystalline helical architectures is a particularly demanding avenue and exploits synergistic combinations of noncovalent interactions arising from functional groups within the chromophores. Champness and co-workers75 reported an unanticipated triple helical arrangement of di-4-pyridylsubstituted 3,4,9,10-perylenetetracarboxylic diimide (1) in the crystalline phase assembled by a cooperative interplay of weak C−H···O interactions (constituting ortho pyridyl hydrogen and one of the carbonyl group oxygen) and intrahelical π−π stacking interactions among the core-twisted PDI backbones (Figure 9a−c). The pairs of P atropisomers and M atropisomers are found to alternately self-assemble along the main axis of the helix. However, highly ordered single, double, and quadruple helical arrangements have been rationally designed in a bottom-up approach via coassembly

Table 1. IQA Interaction Energy of Br···Br Interactions interactions

Eta

Ecla

Exca

Br···Br (intermolecular) Br···Br (intramolecular)

−4.95 −17.32

0.63 0.50

−5.58 −17.82

a

Energies are given in kcal/mol. Et = total interatomic energy, Exc = exchange correlation energy, and Ecl = classical component.

correlation, an expression of covalency, in stabilizing the type-I halogen−halogen interaction highlighted the precedence of a covalent component in the nature of halogen−halogen interactions. The radial arrangement is further reinforced by the through space charge transfer interaction between the diisopropylphenyl donor and naphthalimide acceptor in the adjacent NIBr2 units in the radial assembly. Herein, we provide the photographic proof for the ability of a covalent component H


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Figure 9. (a) Molecular structure of 1, (b) three helices within a tube of 1 viewed parallel to the c-axis, and (c) trimer of 1 formed by intermolecular C−H···O interactions viewed along the crystallographic c-axis. Adapted with permission from ref 75. Copyright 2018 American Chemical Society. (d) 2-(Iodoethynyl)quinoline. (e) Two crystallographically independent halogen-bonded supramolecular triangles in the crystal structure. (f) Left- and right-handed supramolecular double helices formed by C−I···N halogen bonds. Adapted with permission from ref 79. Copyright 2018 Wiley-VCH.

Figure 10. Schematic representations of the newly proposed aromatic packing motifs: (a) cross stacking, (b) helical assembly, and (c) radial assembly.

of intermolecular ethynyl C−I···N halogen bonding and π−π stacking interactions (Figure 9d−f). The unique natural chiral resolution of the enantiomeric pairs in the crystalline phase highlights the significant role of molecular structure and crystallization conditions in induction and amplification of chirality from 1D helical strand to the 3D crystalline architectures. Understanding the factors governing origin and progressive transfer of chirality from the primitive nucleation phase to the macroscopic level80 is a nascent area of research and may eventually lead to control over supramolecular chirality and helical inversion, which has the potential to revolutionize the field of biomimetic and smart materials.

or self-assembly of intramolecular noncovalent interaction driven foldamers functionalized with identical groups (in the presence of a coformer) or electronically complementary groups at the helical ends. Hydrogen bonding,76 halogen bonding,77 or metal coordination78 is employed to align the crescent-shaped monomers in a seamless 1D helix, which is further stabilized by intrahelical π−π stacking interactions. The hollow tubular cavities formed within the helical array have potential applications as solvent channels,76 as supramolecular catalysts,78 in guest entrapment and transport,77 and in asymmetric catalysis.78 These helical frameworks either are racemic conglomerates possessing helices of both handednesses or exhibit high fidelity to helical sense leading to formation of chiral crystals. In an exceptional example, Mak and co-workers 79 realized the crystallization of 2(iodoethynyl)quinolone, an achiral planar heterocycle, into a supramolecular triangular motif from certain solvents (chloroform, benzene, and p-xylene) and a pair of enantiomeric double helical motifs from acetonitrile via concerted interplay

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SUMMARY AND OUTLOOK The emergent properties from the self-assembly of chromophores are governed by the chemical structure of the chromophore and its arrangement within the assembly. Apart from identifying the stabilizing interatomic interactions and understanding the nature of cooperativity of multiple nonI


Article

Accounts of Chemical Research Author Contributions

covalent interactions, classification of emergent property-based supramolecular structural motifs is necessary to design crystalline functional materials of interest. Conventional classification of crystalline supramolecular motifs based on π−π stacking interactions needs a revival with an extensive classification owing to the identification of hitherto unprecedented motifs exhibiting exceptional emergent properties. Here, based on ours and others’ works, we recommend the addition of cross-stacked (variation in rotational angle in the stacked chromophores), radial, and helical packing motifs (Figure 10). Crystallography of aromatic chromophores can give a better picture regarding (a) the nature and role of various novel or existing noncovalent interactions in materializing the unique supramolecular motifs and (b) the emergent properties aided by the corresponding motifs. The former has been exemplified by the hexabromine synthon mediated radial assembly of chromophores evincing the contribution of a covalent component to halogen−halogen interactions. The latter is embodied in examples of covalently linked donor−acceptor systems wherein the emergent longlived photoinduced charge-separated state in the aggregate is attributed to delocalization of the excitons through the segregated stacks as revealed in the crystalline phase. Monomer-like optical behavior caused by the null excitonic splitting in Greek cross (+) stacking coupled with the intrinsic selective hole mobility exhibited by the Greek cross (+) stacked motif is yet another example of an emergent property from a supramolecular motif. Cross-stacked architectures with significant π−π stacking interactions tend to exhibit rotational angle dependent emergent properties. Decoding the origin and evolution of chiral helical supramolecular motifs can have radical implications in multiple avenues of research ranging from asymmetric catalysis, chiral recognition and amplification to guest entrapment and transport and design of memory storage and smart materials. A relook into the existing crystalline assemblies and correlation with their emergent properties is essential for the design of tailor-made supramolecular architectures for specific applications. However, several factors need to be taken into account for the construction of supramolecular assemblies with the desired architecture. The solvents used and the conditions employed for crystallization have the potential to dictate the emerging motif and could be chromophore- and functional-group specific. There is still a long way to go for the impeccable design of cross-stacked, radial, helical, and many other unique assemblies mirroring natural supramolecular scaffolds and prediction of the type of packing mode, mixed or segregated, adopted by D−A (complex or covalently linked) systems. One way to move forward is to cautiously scrutinize the transpiring unprecedented structural motifs, thereby diagnosing and examining every aspect of any novel or known noncovalent intermolecular interactions existing within and attempting to decipher the origin of the supramolecular motif.

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‡

R.R. and M.A.N. contributed equally. The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Remya Ramakrishnan is currently pursuing her Ph.D. under the guidance of Dr. Mahesh Hariharan at IISER-TVM. Her research interests are in exploring the various noncovalent interactions that guide the packing and solid state photophysical properties in photonic crystals. M. A. Niyas is currently doing his integrated B.S.−M.S. course and working with Dr. Mahesh Hariharan at IISER-TVM. His research interests are in understanding the nature and role of noncovalent interactions and the origin of emergent properties in the crystalline state. M. P. Lijina is currently pursuing her integrated Ph.D. course with Dr. Mahesh Hariharan at IISER-TVM. Her research interests are in understanding the strength of weak interactions in crystalline organic molecules and their interaction with light. Mahesh Hariharan is a faculty member in School of Chemistry, IISER Thiruvananthapuram. After completing doctoral research with Dr. D. Ramaiah from CSIR-NIIST India, Dr. Hariharan carried out postdoctoral research (2007−2009) with Prof. Frederick D. Lewis at Northwestern University. Dr. Hariharan’s research efforts focus on understanding the ultrafast excited state dynamics of biomolecules and crystalline and twisted organic materials.

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ACKNOWLEDGMENTS The authors thank present and past Hariharan lab members for their invaluable contributions to the projects highlighted. M.H. acknowledges the Department of Science and Technology (DST) Nanomission [DST-SR/NM/NS-23/2016(C)], Govt. of India, for the support of this work. M.A.N. is thankful for a DST-INSPIRE Fellowship. R.R. and M.P.L. acknowledge UGC and IISER-TVM for financial assistance.

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