Self‐Sorting Supramolecular Polymerization: Helical and Lamellar Aggregates of Tetra‐Bay‐Acyloxy Perylene Bisimide

Abstract A new perylene bisimide (PBI), with a fluorescence quantum yield up to unity, self‐assembles into two polymorphic supramolecular polymers. This PBI bears four solubilizing acyloxy substituents at the bay positions and is unsubstituted at the imide position, thereby allowing hydrogen‐bond‐directed self‐assembly in nonpolar solvents. The formation of the polymorphs is controlled by the cooling rate of hot monomer solutions. They show distinctive absorption profiles and morphologies and can be isolated in different polymorphic liquid‐crystalline states. The interchromophoric arrangement causing the spectral features was elucidated, revealing the formation of columnar and lamellar phases, which are formed by either homo‐ or heterochiral self‐assembly, respectively, of the atropoenantiomeric PBIs. Kinetic studies reveal a narcissistic self‐sorting process upon fast cooling, and that the transformation into the heterochiral (racemic) sheetlike self‐assemblies proceeds by dissociation via the monomeric state.


Introduction
Perylene bisimides (PBIs) are amongst the most studied colorants in supramolecular chemistry because of their unmatched combination of favorable optical and redox properties. [1][2][3][4] Thus,t hey can afford fluorescence quantum yields close to unity [5] and can be reduced at moderate potentials to give radical anions of high stability. [6] Further, their high tendency to self-assemble by p-p interactions [7][8][9] in solution as well as the solid state allows construction of functional (nano)materials with tailored properties and has led to the implementation of PBIs in field-effect transis-tors, [10][11][12][13] solar cells, [14] and photonic devices. [15][16][17] This use in different applications is supported by the ease of modification of the monomeric PBI building block, allowing fine-tuning of the inter-a nd intramolecular interactions,a sw ell as optical and redox properties. [18] Particularly,s ubstitution at the bay position with halogen, [19,20] cyano, [21] amino, [22] methoxy, [23] or phenoxy substituents [24,25] is widely used to adjust the chromophoreso ptoelectronic features.H owever,t he substitution at bay positions further causes ad istortion of the p system because of the repulsive interactions of substituents in close proximity,r esulting in ac onformational chirality of these dyes. [25] Usually,t he interconversion process between the P-a nd M-atropoenantiomers in solution is fast and separation of the two is only possible by using sufficiently large,f or example,b romo substituents, [26] by fixation of the chirality with tethers connecting the 1,7-and/or 6,12-positions, [27,28] or by introduction of 2,2'-biphenol units in 1,12-or 6,7-positions. [29] Therefore,i nm ost cases an equilibrium between the two atropoenantiomers exists,a nd strongly influences the self-assembly pathway by either homo-or heterochiral contacts of the chromophores. [29] Homochiral self-assembly leads to the formation of one-dimensional, helical fibers of either P-o rM-chirality.T his self-assembly was demonstrated for avariety of PBIs which form helices in the self-assembled state in solution [8,29,30] or the columnar liquid-crystalline phase. [31][32][33] In contrast, heterochiral selfassembly yields two-dimensional structures of alternating Pand M-atropoenantiomers,however, only observed in singlecrystals of tetra-and octachloro-substituted PBIs. [34,35] In the current study we introduce PBI1,anew wellsoluble PBI derivative that exhibits more similar optical properties to the parent core-unsubstituted PBI compared to the widely applied tetraphenoxy-substituted PBIs. PBI1 is functionalized with four acyloxy groups at the bay position and bears free imides facilitating hydrogen-bond (H-bond) directed self-assembly into supramolecular polymers (Figure 1a). Most interestingly, PBI1 is capable of forming both homochiral one-dimensional fibers and heterochiral twodimensional sheets,depending on the cooling rate applied to ah ot solution of monomers in methylcyclohexane (MCH; Figure 1b)a sd etermined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Thet wo supramolecular polymorphs, [36,37] Agg1 and Agg2,e xhibit distinctive absorption properties originating from the unique interchromophoric arrangement, which they retain in the solid state.P olarizing optical microscopy of the polymorphs revealed their liquid-crystalline (LC) behavior which enabled detailed studies of the supramolecular arrangement by acombination of wide-angle X-ray scattering (WAXS) as well as polarized UV/Vis and FT-IR spectroscopy.T he pathway complexity [38][39][40][41][42] of the supramolecular polymerization [43][44][45] that distinguishes between the two polymorphs was investigated by UV/Vis spectroscopy.

Optical Properties of the Monomeric Dye
They ellow colored solution of monomeric PBI1 in dichloromethane exhibits its absorption maximum at 512 nm (43 000 cm À1 m À1 ), the first vibronic progression at 479 nm (30 000 cm À1 m À1 ), and the S 0 -S 2 transition at 398 nm (7500 cm À1 m À1 ;F igure 2a). In comparison to other common PBIs,that is, PBI2 (l max = 527 nm), PBI3 (l max = 576 nm), and PBI4 (l max = 610 nm;F igure 2b), PBI1 absorbs at lower wavelength, that is,e ven 15 nm hypsochromically compared to the parent PBI2.T his blue-shift can be explained by the electron-withdrawing effect (ÀI-effect) and the negligible mesomeric effect (+ M effect) of the acyloxy bay substituents compared to the + M effect provided by phenoxy and methoxy in PBI3 and PBI4.S imilar to PBI3 and PBI4, PBI1 shows ab roadened lineshape and al ess pronounced vibronic fine structure.T hese features are characteristic for bay-substituted PBIs and are attributed to the core-twist induced by the steric congestion in the PBI bay-area (see Figure S1 in the Supporting Information). [23,47] Thee mission properties of PBI1 were determined in chloroform where the dye exhibits its emission maximum at 544 nm, corresponding to as mall Stokes shift of 1100 cm À1 . Thefluorescence lifetime was determined to 4.8 ns which is in Figure 1. a) Chemical structure of PBI1.b )Schematic illustration of the cooling-rate dependent homo-or heterochiral self-assembly into fibers or sheets, respectively,and their subsequent organization into columnar or lamellar liquid crystals. the expected range for monomeric PBIs (see Figure S2). [1] PBI1 exhibits af luorescence quantum yield of F Fl = 1.0 similar to PBI2 and PBI3,w hile substitution by methoxy groups leads to aq uenching of the fluorescence to F Fl = 0.68. [46]

Supramolecular Polymorphism
In chlorinated and aromatic solvents like dichloromethane,c hloroform, tetrachloromethane,o rt oluene,w hich solubilize PBI1 well, the dye retains its monomeric state even at higher concentrations (see Figure S3). In contrast, when dissolving PBI1 in nonpolar aliphatic solvents like MCH (c 0 = 40 mm), in which H-bonds and p-p interactions can thrive, [7] the dye self-assembles into two different aggregates (Agg1 and Agg2)d epending on the cooling rate applied to ah ot solution of monomers ( Figure 3c;f or FT-IR analyses,s ee Figure S4). Rapid cooling (10 Kmin À1 )leads to the formation of Agg1,w hile slow cooling (0.6 Kmin À1 )l eads to the formation of Agg2.T he polymorphs differ distinctively in their absorption profile and accordingly in color. Agg1 shows ab athochromically shifted absorption maximum at l max = 600 nm that increases in extinction and exhibits an arrowing of the 0,0 vibronic band with af ull-width-at-half-maximum (FWHM) of 300 cm À1 compared to the monomer with 570 cm À1 (Figure 3a). These optical features are characteristic for PBI J-aggregates and have been observed in structurally related tetra-bay-phenoxy substituted PBIs. [8,31,32,48,49] In contrast, Agg2 exhibits its main transition at 541 nm with an arrowed FWHM of 370 cm À1 compared to the monomer, av ibronic progression at 498 nm and ar ed-shifted weaker band at 588 nm (Figure 3b).
At room temperature, Agg1 transforms into apurple gellike phase,w hile Agg2 forms ar ed precipitate in MCH ( Figure 4). Both polymorphs can be isolated in the solid state and retain their distinctive UV/Vis absorption properties (see Figure S6). Thea bsorption maximum of Agg1 in the solid state can be observed at 601 nm. TheF WHM of the main absorption signal increases only slightly compared to Agg1 in solution from 300 cm À1 to 350 cm À1 .The absorption maximum of Agg2 can be observed at 538 nm and the FWHM also only slightly increases compared to the solution from 370 cm À1 to 470 cm À1 .T his minor increase in FWHM indicates that both polymorphs retain ah ighly defined intermolecular order in the solid state,e nabling the elucidation of their structure by means of microscopy and scattering techniques.

Structure Elucidation
We could investigate the morphological properties of both polymorphs by SEM of samples (MCH, c 0 = 40 mm)d ropcasted on silicon wafer. Agg1 shows an etwork of entangled fibers on the scale of several micrometers.T he height of the individual fibers could be determined as 3.1 AE 0.2 nm by AFM (see Figure S7). In contrast, Agg2 exhibits two-dimensional sheetlike structures with alength of several micrometers and awidth of up to 1micrometer.The height of these sheets was determined as 1.7 AE 0.2 nm by AFM (see Figure S8). These results are in accordance with the observation of ag el-like phase for Agg1 and the precipitation of Agg2.
Both polymorphs show birefringence when investigated by polarizing optical microscopy.T hey can be aligned by mechanical shearing,i ndicating the fluid nature of the material originating in the nanosegregation of the rigid pcore and the flexible alkyl chains (see Figure S9). Accordingly,w ew ere able to investigate the supramolecular assemblies by WAXS experiments of aligned fibers to elucidate the respective intermolecular arrangement. Both polymorphs were prepared on a1 0mgs cale by applying the required cooling rates to hot solutions of PBI1 in MCH (c 0 = 40 mm), and were isolated by centrifugation and dried under reduced pressure.T he resulting LC materials were subsequently aligned by fiber extrusion at ambient temperature from their LC states.
TheW AXS pattern of al ying fiber of Agg1 shows equatorial reflections which can be indexed according to ac olumnar rhombohedral lattice (Col rhomb )w ith a = 28.8 and g = 78.88 8 (Figure 5a). Thed iffuse halo at 4.4 corre-sponds to the liquid-like alkyl chains.T he X-ray diffraction pattern of astanding fiber of Agg1 shows meridional and offmeridional reflections that indicate aperiodic organization of the chromophores along the column (Figure 5b). Thef irst meridional signal corresponds to the length of the axial translation subunit and can be observed at 13.8 ,which is the size of the PBI chromophore along the long axis. [31][32][33] This data indicates that the PBIs are oriented parallel to the columnar long axis forming H-bonded strands,a nd is supported by polarized FT-IR (see Figure S10 a) and UV/ Viss pectroscopy (see Figure S11 a) of shear-aligned thin films,w hich reveal that NH stretching vibrations in H-bonds and the S 0 -S 1 transition of the PBIs are aligned parallel to the shearing and column direction. This result is further confirmed by ad iffuse signal at 3.6 on the equator, which is characteristic for such PBI assemblies and originates in the pp stacking of the chromophores perpendicular to the columnar long-axis. [31][32][33] Them eridional signal at 13.8 can be indexed as layer line L = 16 of ah elical arrangement and consequently all other diffuse meridional and off-meridional signals are positioned at layer lines L = 24, 31, 38, 48, 51 and L = 62. Accordingly,1 6m olecules form the helical repeat of 16 13.8 = 220.8 .T he correlation length of the equatorial signal at 3.6 amounts to six molecules (see Supporting Information). This value implies that about six strands of Hbonded PBIs form ac olumn and is supported by the reasonable density of 1.05 gcm À1 when the columnar stratum is filled by the integer number of six molecules (for details see the Supporting Information). Therefore, Agg1 is composed of asextuple-stranded 16 1 helix with a22.58 8 twist per molecule. Such ahelical arrangement can only be formed by homochiral self-assembly leading to the formation of either P-o rMhelices.A sPBI1 is achiral, both M-a nd P-helical selfassemblies coexist in the columnar liquid crystal. The individual H-bonded strands are longitudinally displaced by about 7 ,leading to aslipped-stack arrangement causing the absorption spectrum which is typical for PBI J-aggregates. [8,31,32,34] Thes elf-assembled structure was modelled with the program Accelrys Materials Studio 2017 R2. Accordingly, core-twisted PBIs with the same axial chirality were arranged to form H-bonded strands in as lipped-stack in the sextuplestranded helix (Figure 5c,f or details see the Supporting Information). Theh elix was optimized in the rhombohedral unit cell with the force field COMPASS II, applying the Ewald summation method until the nonbonding energy was strongly negative.W ith the optimized structure,w ec ould simulate the fiber diffraction pattern with the program CLEARER. [50] Thes imulated pattern is in good agreement with the experiment showing the essential signals corresponding to the Col rhomb lattice as well as the meridional signal at 13.8 (see Figure S13 a).
Therelated analysis of the WAXS pattern of alying fiber of Agg2 shows four equidistant reflections that could be indexed as 001, 002, 003, and 004 signals of alamellar lattice (c = 27.1 ;F igure 5d). However,t hese signals are very broad, indicating aw eak correlation between the lamellae. Accordingly,the correlation length could be calculated to be only two lamellae (for details see Supporting Information). In contrast, the meridional signals related to the intralamellar arrangement of the PBIs are well defined (Figure 5e). They can be apparently indexed according to ar ectangular centered lattice (a = 14.1 , b = 18.2 ). Thecorresponding unit cell comprises four PBI molecules assuming ar easonable density of 1.11 gcm À3 .T he PBIs are oriented in parallel with the layer direction, that is,p erpendicular to the c-axis as determined with polarized FT-IR (see Figure S10 b) and UV/ Visspectroscopy (see Figure S11 b).
To rationalize the absorption spectra of Agg2 and thus gain further information on the arrangement of the dyes within the unit cell, an interplay between short-and longrange coupling has to be taken into account. [51,52] Thel atter arises from the interaction of the transition dipole moments as described within the conventional Kasha exciton theory, [53] whereas the short-range coupling is caused by the HOMO-HOMO and LUMO-LUMO overlap of the p-stacked chromophores,a nd is very sensitive to structural arrangements. [52,54] Therefore,s mall changes of the longitudinal shift of the PBI chromophores can lead to distinctly different absorption spectra, enabling one to derive information on the chromophore arrangements. [52,55] Theabsorption spectrum of Agg2 is in very good agreement with the calculated spectrum for p-stacked perylene dyes exhibiting al ongitudinal shift of about 5 as reported by Hestand and Spano (see Figure S15). [52] Since the HOMO and LUMO distribution of perylene [53] and PBI [1] chromophores are almost identical, we can use the perylene spectra as reference.A ccordingly,t he spectral signature of Agg2 results from the interference of long-range and short-range coupling in the so-called resonant regime (i.e.t he Frenkel and charge-transfer state are of similar energy) [51] and shows one intense absorption peak at 539 nm and less intense absorption bands at higher and lower energies (for ad etailed discussion see the Supporting Information).
With the help of the exciton-vibrational spectral pattern analysis,t he unit cell of LC Agg2 was generated with PBI chromophores that are longitudinally shifted by 5 (see Figure S12). This special in-plane shift, however,b reaks the first assigned centered symmetry of the four strands and the planar unit cell must be primitive.T hus the absence of reflection with h + k = 2n + 1i sa ccidental, and the cell is apparently only pseudocentered (see the Supporting Information). Figures 5f and gs how as upercell of this arrangement that highlights the alternating arrangement of P-and Matropoenantiomers.U sing the constructed unit cell, the diffraction pattern was simulated using CLEARER, which confirms the absence of the most prominent reflections for the primitive unit cell (see Figure S13 b). [50] Pathway Complexity of the Supramolecular Polymerization Lastly,wewere interested in amore detailed investigation of the supramolecular polymerization and the underlying pathway complexity that leads to the formation of the respective homochiral one-dimensional and heterochiral two-dimensional polymorphs.T emperature-dependent UV/ Visspectroscopy of PBI1 (c 0 = 40 mm,MCH) showed that the self-assembly of both, Agg1 (cooling rate 10 Kmin À1 )a nd Agg2 (cooling rate 0.6 Kmin À1 ), follow ac ooperative nucleation-elongation mechanism (see Figure S17). This mechanism is reasonable as more than one intermolecular force, namely p-p interactions and H-bonding,c ontribute to the formation of the supramolecular polymers.W henm ixing Agg1 and Agg2 in a1:1 ratio (c 0 = 40 mm,MCH), the UV/Vis absorption spectrum shows as uperposition of the individual spectra (blue line,F igure 6a). Time-dependent experiments at 35 8 8Cr evealed an interconversion of Agg1 into Agg2 as indicated by the decreasing absorption at 600 nm and the concomitant shift to 588 nm as well as the increasing absorption at 541 nm ( Figure 6a). This interconversion suggests that Agg2 is the thermodynamically favored product, while Agg1 is formed under kinetic control. Thep athwaycomplexity can be probed by time-dependent UV/Vis experiments at different concentrations. [38,39] Accordingly,t he conversion of Agg1 into Agg2 was followed at ar ange of concentrations from c 0 = 20 mm to c 0 = 60 mm in MCH at 35 8 8C (see Figure S18). At c 0 = 60 mm,nosignificant spectral change can be observed over ap eriod of 10 minutes.H owever,a t lower concentrations the absorption signal at 600 nm corresponding to Agg1 decreases in intensity whilst the new absorption band at 541 nm corresponding to Agg2 arises.This change can be followed by the change in absorption at 541 nm over time,t hereby demonstrating an increasing rate with decreasing concentration (Figure 6b,s ee Figure S18). These results indicate that Agg1 is an off-pathway kinetic product and that the interconversion of Agg1 into Agg2 takes place by the fully dissociated monomeric state and is therefore favored at lower concentrations where the concentration of available monomer is higher. [38][39] Putting these results into perspective, it is reasonable to assume that the initial dimer pair formation, that is,h omochiral (M/M or P/P)o rh eterochiral (P/M or M/P)d etermines the outcome of the self-assembly pathway and thereby the final helical fiber or sheet-type morphology. Agg1 is formed by ah omochiral assembly of either only P-oronly M-atropisomers,while the formation of Agg2 requires an alternating,h eterochiral self-assembly. Apparently,o nce P-o rM-chirality is established within the supramolecular structure of the kinetic product Agg1,t he fibers cannot come unwound to form the heterochiral, sheetlike structure Agg2 even though it is the thermodynamically favored product. Thepathway via the monomeric state, however, enables the structural transformation because of the fast interconversion between the two atropoenantiomers in solution. It is noteworthy that the herein elucidated narcissistic versus social self-sorting [56] in one-and two-dimensional supramolecular polymerization into Agg1 and Agg2 under either kinetic or thermodynamic control directly relates to the formation of conglomerate and racemate crystals in threedimensional self-assembly.U nfortunately,t he conversion of the kinetic polymorph Agg1 into the thermodynamic polymorph Agg2 by aseeded [37,57] supramolecular polymerization approach could not be achieved in as atisfactory manner as even at high seed ratios of Agg2 only aminor transformation into the thermodynamically favored polymorph was observed (see Figure S19).

Conclusion
In summary,w ep resented an ew PBI1 dye bearing four acyloxy substituents at bay positions.Inthis study,weshowed that PBI1 forms two polymorphs upon cooling ahot solution in methylcyclohexane,depending on the applied cooling rate. Thet wo polymorphs show distinctive absorption profiles in solution and in the liquid-crystalline state,w hich could be related to ad ifference in the longitudinal shift of the Hbonded strands in the self-assembled structures.T he interchromophoric arrangement in the respective one-and twodimensional polymorph could be elucidated by polarized spectroscopy and X-ray scattering revealing the formation of acolumnar (Agg1)and alamellar structure (Agg2). Based on these results we were able to propose packing models for both polymorphs.While the helical structure of Agg1 is formed by ah omochiral arrangement of the respective P-a nd Matropoenantiomers,t he sheets of Agg2 are formed by an alternating heterochiral arrangement of the two.Accordingly, formation of Agg1 and Agg2 are particularly illustrative examples of conglomerate versus racemic self-sorting phenomena. Time-dependent UV/Vis spectroscopy in solution indicated that Agg2 whose formation is initiated from heterochiral dimer pairs is the thermodynamically favored product while Agg1 is formed in ak inetic process from homochiral dimer pairs in an off-pathway mechanism. Accordingly, PBI1 provided unprecedented insights into self-assembly pathways from the monomer via aggregates in solution up to the bulk liquid-crystalline state.G iven their interesting (opto)electronic properties, PBI1 or its derivatives might also be interesting candidates for future applications in photovoltaic and photonic devices.