Circularly polarized luminescence from a common alkoxy pillar[5]arene and its co‐aggregates with π‐conjugated rods

Cylinder‐shaped macrocycles composed of π‐panels have attracted special attention as one of the best platforms for the development of organic molecule‐based chiroptical materials. Pillar[n]arenes are a class of macrocycles with the advantage of easy preparation but have not been extensively investigated from the perspective of luminescent molecules. However, common alkoxy pillar[n]arenes are fluorescent in non‐haloalkane solvents, showing potential to be used for molecule‐based chiroptical materials. In this work, circularly polarized luminescence (CPL) spectra are reported for a pillar[5]arene with stable planar chirality using tetrahydrofuran (THF) and cyclohexane as solvents, which has been missing for many years. The pillar[5]arene also forms co‐aggregates with 1,4‐bis(phenylethynyl)benzene and 1,4‐bis[(pentafluorophenyl)ethynyl]benzene in THF/H2O mixtures, owing to a hydrophobic effect. The co‐aggregates with the fluorinated π‐rod display a new low‐energy absorption peak and broad emission band as well as intense circular dichroism and CPL signals. Chiral information from the enantiopure pillar[5]arene core is efficiently transmitted to the co‐aggregates with the π‐conjugated rod, leading to the highest dissymmetry factor for CPL (2.9 × 10−2 at 472 nm) among pillar[n]arene‐based CPL materials.


INTRODUCTION
Exploitation of chirality is a key area of interest in contemporary science as it can give additional degree of freedom for advanced information technologies and outputs independent of achiral background signals.[3][4] Moleculebased chiroptical materials have advantages due to their lightweight, flexible, and solution-processable characteristics as well as their unique use in solution-state applications.However, chiroptical organic compounds with superior photophysical properties and dissymmetry factors (g abs and g lum ) [7][8] Among many kinds of π-conjugated platforms, macrocyclic oligomers have drawn particular attention [9][10][11][12][13][14][15][16] as their intrinsically high This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.© 2024 The Authors.Aggregate published by South China University of Technology; AIE Institute and John Wiley & Sons Australia, Ltd. symmetry can prevent undesirable bias of electronic distribution from deteriorating the dissymmetry factors; a high g lum value has indeed been reported for a cyclic chrysene tetramer (1 in Figure 1). [13]Although chiral carbon nanotubes and their discrete fragment molecules should be an ideal solution, difficulties with their selective synthesis and separation from many isomers gave rise to a demand for promising alternatives with easy preparation.
[22][23][24][25] The π-panels exhibit smooth rotation when the substituents are small enough to pass through the internal cavity, while bulky cyclohexylmethoxy groups inhibit interconversion to give chirality-aligned enantiomers exclusively.The enantiomers of P5OCH 2 Cy were stable at 40 • C for 18 h [26] and showed a mirror-imaged pair of sharp circular dichroism (CD) signals with an attractive g abs value in the order of 10 −2 .Conversely, fluorescence intensity was very weak in chloroform, in addition to short response wavelengths (ca.320 nm).29] Since 2020, several circularly polarized luminescence (CPL)-active pillar[n]arenes have been developed by functionalizing the rim substituents.[32] The resulting series of derivatives (2a-d) improved photo-responsive wavelengths and luminescence quantum yields, although they did not exhibit fluorescence in polar solutions.Unfortunately, unsymmetric electronic distributions largely decreased dissymmetry factors from that of an alkoxy pillar [5]arene, except for the compound substituted with boron-based acceptor units on both sides (2e). [33]Kato, Ogoshi et al. took a C 5 -symmetric approach to change the photophysical properties of alkoxy pillar[n]arenes (3a-c), [34,35] succeeding at retaining high dissymmetry in a one-rim arylated derivative 3a.However, steric hindrance around the biaryl C-C bonds led to large dihedral angles and a short fluorescence wavelength.Apart from these molecules with the trade-offs, hybrid systems with other photo-responsive units enabled easy alteration of their basic optical properties.They attained comparable or higher dissymmetry as single molecules [36][37][38] (4) and supramolecular aggregates [39,40] (5) due to the effective transfer of the planar chirality.This research prompted us to combine a simple alkoxy pillar [5]arene with a π-conjugated guest in expectation of a chiroptical response based on the highest occupied molecular orbital (HOMO) of the former and the lowest unoccupied molecular orbital (LUMO) of the latter.
Characteristically, pillar [5]arenes form strong host-guest complexes with electron-poor alkyl chains (G1, G2), [41,42] but form weak complexes with aromatic compounds due to the size mismatch.][45] In this work, neutral π-conjugated molecules with good luminescence properties but weak complexing abilities are investigated (6 F and 6 H ). Instead of selecting good host-guest pairs, the environment was altered by adding water, so that a hydrophobic effect induced complexation against the weak binding trends.Indeed, mixtures of tetrahydrofuran (THF) and H 2 O gave co-aggregates and intense CD and CPL bands were observed with maximum dissymmetry factors of g abs = 1.3 × 10 −2 at 351 nm and g lum = 2.9 × 10 −2 at 472 nm, respectively.

Optical properties of alkoxy pillar[5]arene in various solvents
First, we investigated solvent effects on the optical properties of a common alkoxy pillar [5]arene P5OCH 2 Cy (Figure S3-1 and Table S3-1).In pillar[n]arene chemistry, chloroform is the most widely used solvent for diverse solution-state measurements, as it can dissolve many kinds of neutral guests and pillar[n]arenes but it does not form host-guest complexes with pillar [5]arenes, in addition to its high availability in the deuterated form.However, alkoxy pillar [5]arenes are typically non-fluorescent in haloalkane solvents and cause gradual decomposition into unidentified compounds.Therefore, the fluorescence of P5OCH 2 Cy was not observed in chloroform and only very weakly in CH 2 Cl 2 , while it was evident in THF, cyclohexane, and acetonitrile with quantum Abbreviations: CD, circular dichroism; CPL, circularly polarized luminescence; FL, fluorescence; THF, tetrahydrofuran; UV, ultraviolet.a Excitation wavelengths were set at 290 and 240 nm for FL and CPL measurements, respectively.b B CPL values were calculated as 0.5 × ε × |g lum | × Φ lum using extinction coefficients (ε) at UV/vis absorption peak tops.c Not acquired due to low solubility.

F I G U R E 2
Circularly polarized luminescence (CPL) spectra of P5OCH 2 Cy in cyclohexane and tetrahydrofuran (THF).The excitation wavelength was set at 240 nm to avoid the influence of excitation light.
yields of 13%-19% (Table 1).Meanwhile, the shapes of the ultraviolet/visible (UV/vis) absorption, CD, and fluorescence spectra were almost identical to each other within peak shifts of 5 nm, confirming small solvent effects.The CD signals in CH 2 Cl 2 and THF showed higher dissymmetry factors at the CD peak tops [(1.3-1.4) × 10 −2 ] than those in other solvents [(0.91-0.94)× 10 −2 ], which suggested that host-guest complexation with these solvent molecules suppressed the swing motion of the π-units to give sharp CD signals.In cyclohexane and THF, CPL spectra were also recorded, giving peak tops at almost the same wavelengths as those in the fluorescence spectra (Figure 2).The dissymmetry factor was higher in THF (0.66 × 10 −2 ) than cyclohexane (0.47 × 10 −2 ) in accordance with the trend in CD response.Each value for CPL was smaller than that for CD, which may be due to structural and/or electronic deviation at the excited state from the ground-state D 5 -symmetric form.

Fluorescence spectral changes in THF/H 2 O mixtures
To extend the response wavelengths to the visible region, we tested host-guest complexation of P5OCH 2 Cy and a πconjugated rod with pentafluorophenyl terminals, which gave electron-poor central paraphenylene units (6 F in Figures 1D  and S10-2).Cyclohexane was used as the solvent with an expectation of improved association constants owing to low solvation ability.Despite these conditions, no UV/vis absorption and fluorescence spectral changes were observed within the conditions suitable for optical measurement (Figure S3-2).
To circumvent the insufficient assembly formation, we planned to exploit the hydrophobic effect. [28,46]igure 3A represents fluorescence spectra of equimolar mixture of P5OCH 2 Cy and 6 F (hereafter, represented as P5OCH 2 Cy⋅6 F ) in mixtures of THF and H 2 O with variable water fractions in percent (f w ).In pure THF (f w = 0), the fluorescence spectrum of P5OCH 2 Cy⋅6 F was the simple sum of the bands for P5OCH 2 Cy (324 nm) and 6 F (347 and 365 nm).When the water fractions increased above f w = 40, the intensities of the bands decreased to become zero at f w > 70 (Figure 3B).Furthermore, a new broad band at around 475 nm appeared above f w = 40, which could be ascribed to some assemblies induced by the water-rich solvents.Indeed, dynamic light scattering (DLS) measurements showed clear evidence of aggregation by increasing the water fractions; at f w = 0, no meaningful scattering was observed, but at f w = 40, the particle size was estimated to be around 2.7 nm and further grew up to over 50 nm above f w = 60 (Figure 4A).Scanning electron microscope (SEM) images also showed apparent changes between f w = 40 and 60 (Figure 4B,C).
Although rod compound 6 F itself also showed a longwavelength emission at around 475 nm and a similar DLS profile above f w = 50, more drastic changes in the band intensities were observed in f w = 60-70 (Figures S3-4c and S7-1).Upon aggregation, P5OCH 2 Cy⋅6 F indicated a decrease of fluorescence quantum yield from 0.50 at f w = 40 to 0.27 at f w = 60, while the yields for 6 F did not show a clear trend (0.57-0.66 at f w = 40-60) (Figure 3C).Fluorescence lifetimes at 465 nm also included a longer component (10-13 ns) in P5OCH 2 Cy⋅6 F than 6 F (5-7 ns) above f w = 60 (Table S6-2), although detailed interpretation for the fitting was difficult, probably due to the various aggregated species.These results suggest that the presence of P5OCH 2 Cy had a considerable influence on the low-energy-emitting species in the aggregated states.
In the current system with THF and H 2 O, P5OCH 2 Cy did not cause aggregation-induced emission but showed a gradual decrease in fluorescence intensities with increasing water fractions (Figure S3-4a).This is in sharp contrast to the behaviors of pillar [5]arenes with linear alkoxy chains in CH 2 Cl 2 /EtOH. [28]The non-zero fluorescence intensities of P5OCH 2 Cy alone in aggregated states are different from the complete quenching of pillar [5]arene fluorescence in P5OCH 2 Cy⋅6 F , which again supports that the co-aggregated states of P5OCH 2 Cy⋅6 F are different from independent aggregates of 6 F and P5OCH 2 Cy.

CD spectral changes in THF/H 2 O mixtures
Next, the chiroptical properties of P5OCH 2 Cy⋅6 F were investigated in mixtures of THF and H 2 O with variable water fractions (f w ), by using enantiopure P5OCH 2 Cy (Figures S2-2 and S2-3).While UV/vis absorption spectra showed non-trivial baseline shifts, probably because of light scattering by the co-aggregates, a low-energy shoulder peak at 358 nm gradually appeared above f w = 50 (Figure S3-6).In the CD spectra of (pR)-P5OCH 2 Cy⋅6 F , negative Cotton effects were observed at 347-430 nm and positive ones at 310-347 nm above f w = 40 (Figure 5A,B).The spectral shape was the sharpest with g abs = 1.3 × 10 −2 at f w = 60, then the spectra became broad in the water-rich media.For the DLS measurement, the largest size of aggregates with ca.400 nm was observed at f w = 60 and decreased to ca. 90 nm at f w = 90 (Figure 4A).The CD signals were obviously red shifted from those of (pR)-P5OCH 2 Cy at around 315 nm, and a mirrorimage relationship with (pS)-P5OCH 2 Cy counterpart was confirmed at f w = 60 (Figure S3-3).Therefore, the planar chirality of (pR)-P5OCH 2 Cy was effectively transferred to the co-aggregates with closely interacted 6 F .

CPL spectra in THF/H 2 O mixtures
Encouraged by the strong CD induction, CPL spectra of (pS)-P5OCH 2 Cy⋅6 F were recorded (Figure 5C).When P5OCH 2 Cy alone in THF (f w = 0) was converted into an aggregate at f w = 60, only a slight decrease in CPL intensities at around 320 nm was observed.In contrast,

Effect of molar ratios on the optical properties of the co-aggregate
To gain insights into the co-aggregates of P5OCH 2 Cy and 6 F , a set of optical measurements was repeated at f w = 60, using different amounts of P5OCH 2 Cy.The UV/vis absorption spectra showed large baseline shifts with increasing ratio of P5OCH 2 Cy (Figure S4-1) because of severe light scattering associated with large hydrodynamic diameter (Figure S7-1).The most intense CD signal at 351 nm was observed with a P5OCH 2 Cy/6 F molar ratio of 1.0/1.0; a further increase of P5OCH 2 Cy led to peak broadening, similar to what was observed for P5OCH 2 Cy⋅6 F in water-rich conditions (Figure 5A,B).
In the fluorescence spectra, an increase in the P5OCH 2 Cy/6 F ratio showed complex trends of peak intensities at 347, 365, and 475 nm (Figure 6A,B).The pillar [5]arene-based fluorescence band at 324 nm showed no signal below a ratio of 1.0/1.0 and began to increase in intensity at a ratio of 1.5/1.0 and above.A plot of fluorescence quantum yields also indicated a sharp drop from 0.61 to 0.27 below a ratio of 1.0/1.0 and was roughly constant above that ratio (Figure 6C).These behaviors suggested that the consumption of free 6 F without interactions with P5OCH 2 Cy occurred smoothly to form the 1:1 co-aggregate predominantly.Over a ratio of 1.0/1.0,some P5OCH 2 Cy molecules were not closely complexed with 6 F , which implied the weak association including further P5OCH 2 Cy molecules.
The CPL signal at around 500 nm was also enhanced by the increasing P5OCH 2 Cy/6 F ratio, similar to the CD peak at around 350 nm, but the maximum intensity was observed at a ratio of 2.0/1.0 due to additional long-range influence by weakly associated P5OCH 2 Cy molecules among other factors (Figures 7 and S4-4).Importantly, the observed g lum value of 2.9 × 10 −2 at 472 nm is much higher than those of pillar[n]arene derivatives which can be used in solution, [30,31,[33][34][35][36][37][38] and even superior to any system exploiting the pillar[n]arene planar chirality, including cast film materials. [32,39,40]The B CPL value [47] was calculated as a hypothetical 1:2 complexed species with ε = 7.5 × 10 4 M −1 cm −1 (λ abs = 312 nm) to be ca.3.0 × 10 2 M −1 cm −1 (λ lum = 472 nm), which is also the highest among pillar[n]arene-based CPL systems. [25]3.2 Studies by 1 H nuclear magnetic resonance spectroscopy and theoretical calculations Finally, 1 H nuclear magnetic resonance (NMR) spectra were recorded for the 1:1 co-aggregate of P5OCH 2 Cy and 6 F and its components in mixtures of THF-d 8 and D 2 O (Figures S9-1 to S9-4).Although the optical measurement was performed at a concentration of 1.64 × 10 −5 M, the 1 H NMR spectroscopy was conducted at (0.60-1.0) × 10 −3 M to gain enough signal-to-noise ratio.Because of the higher concentrations, aggregation occurred with lower fractions of D 2 O in the NMR studies.In pure THF-d 8 , aromatic peaks for the co-aggregate appeared at the same chemical shifts as those for independent P5OCH 2 Cy and 6 F , indicating no interactions between the two components.When the ratio of D 2 O increased above f w = 10, the peaks decreased in intensity but afforded no peak shifts or new shielded signals due to the paraphenylene protons of 6 F .Although the aromatic peak of P5OCH 2 Cy decreased in intensity at lower f w than that of 6 F , the absence of other information hampered the structural supposition of the co-aggregates.We also attempted to obtain the single crystal structure of a complex of P5OCH 2 Cy and 6 F but failed in several conditions.
To deal with this difficulty, we optimized 6 F , 6 H , P5OCH 3 , π-stacked dimers of 6 F and 6 H , a few conformations of the 1:1 complex of P5OCH 3 and 6 F , and a 1:1 complex of P5OCH 3 and 6 H at the ωB97X-D/6-31G(d) level (Figure S10-1).In these calculations, the dimer of 6 F and complexes of P5OCH 3 and 6 F showed lower total energies than their components by 23 and 20-43 kcal mol −1 , respectively.A host-guest inclusion configuration was more stable by 23 kcal mol −1 than a hetero π-stacking configuration with guest exclusion.Although real aggregates were formed under the influence of many interactions that were not included in the calculations above, the host-guest inclusion state was favored at least in the isolated 1:1 complex.
Next, time-dependent density-functional theory calculations were conducted for the optimized structures (Figures S10-3 to S10-18 and Tables S10-1 to S10-7).In line with experimental spectra, free 6 F provided a large oscillator strength due to a π-π* transition at 312 nm, which is lower in energy than P5OCH 3 (254-264 nm).It is noted that HOMO and LUMO levels of 6 F and the dimer of 6 F were lower in energy than corresponding levels of P5OCH 3 , which could lead to charge-transfer transitions in the 1:1 complexes of P5OCH 3 and 6 F .However, every assembly model had additional excited state(s) with small oscillator strength(s) in the low-energy shoulder region of the π-π* transition of 6 F , which could account for the new low-energy absorption peak in aggregated conditions (Figure S3-6).For the assembly models of P5OCH 3 and 6 F , rotary strengths due to the lowenergy excitation showed non-zero values but were much smaller than those expected from the experimental spectra.Therefore, other complexing conformations or higher assembly structures may be the origin of the intense CD bands at around 350 nm.In general, it is very difficult to distinguish host-guest inclusion states and CH/π-or π/π-complexed ones outside of the macrocycles unless the assembly gives crystal structures or single discrete species in a dilute solution.Therefore, detailed structures of the co-aggregates were unable to be deduced rationally from the experiments and theoretical calculations in the present research.

CONCLUSION
In this work, we revealed the CPL properties of a representative alkoxy pillar [5]arene with stable planar chirality.Although the response wavelength was rather short at around 320 nm, g lum values were as high [(0.47-0.66)× 10

F
I G U R E 1 (A) A chiral macrocyclic oligomer 1. (B) Chemical structures of common alkoxy pillar[n]arenes PnOR and typical guests G1-G3, and their interconversion between two conformations with aligned planar chirality via π-unit rotation.(C) Representative examples of pillar[n]arene-based circularly polarized luminescence (CPL)-active derivatives and supramolecular systems, 2-5.(D) Combination of alkoxy pillar[5]arene P5OCH 2 Cy and rod-shaped π-conjugated molecules 6 F and 6 H in this work.

F
I G U R E 3 (A) Fluorescence spectra and (B) plots of fluorescence intensities at peak tops of (pR)-P5OCH 2 Cy⋅6 F in tetrahydrofuran (THF)/H 2 O mixtures with different water fractions (f w ).Fluorescence intensities at 324 and 475 nm are plotted with three times the values.(pS)-P5OCH 2 Cy was used instead of (pR)-P5OCH 2 Cy at f w = 40.(C) Fluorescence quantum yields of 6 F , (pR)-P5OCH 2 Cy, and (pR)-P5OCH 2 Cy⋅6 F in THF/H 2 O mixtures with different water fractions (f w ).The concentration and excitation wavelength were set at 1.64 × 10 −5 M and 290 nm, respectively.F I G U R E 4 (A) The hydrodynamic diameter of (pR)-P5OCH 2 Cy⋅6 F in tetrahydrofuran (THF)/H 2 O mixtures with different water fractions (f w ).Scanning electron microscopy (SEM) images of (pR)-P5OCH 2 Cy⋅6 F in THF/H 2 O mixtures at (B) f w = 40 and (C) f w = 60.F I G U R E 5 (A) Circular dichroism (CD) spectra and (B) plots of CD intensities at peak tops of (pR)-P5OCH 2 Cy⋅6 F in tetrahydrofuran (THF)/H 2 O mixtures with different water fractions (f w ).(pS)-P5OCH 2 Cy⋅6 F was used instead of (pR)-P5OCH 2 Cy⋅6 F and the CD and gabs signs were reversed at f w = 40.(C) Circularly polarized luminescence (CPL) spectra of (pS)-P5OCH 2 Cy and (pS)-P5OCH 2 Cy⋅6 F in THF/H 2 O mixtures at f w = 0, 60 (λ ex = 250 nm).The sign-reversed values are plotted for consistence with CD studies.The concentration and excitation wavelength were set at 1.64 × 10 −5 M and 250 nm, respectively.

F I G U R E 6
(A) Fluorescence spectra, (B) plots of fluorescence intensities at peak tops, and (C) fluorescence quantum yields of 6 F (1.64 × 10 −5 M) with varying equivalents of (pR)-P5OCH 2 Cy in a tetrahydrofuran (THF)/H 2 O mixture (f w = 60).The excitation wavelength was set at 290 nm.(pS)-P5OCH 2 Cy⋅6 F gave the same sign of CPL signals at around 350 nm and new sign-reversed signals at around 500 nm at f w = 60 (Figure S3-9), which corresponded to the bands in the fluorescence spectra.The g lum value of the latter signal reached 0.80 × 10 −2 at f w = 60, indicating again the effective transfer of the planar chirality to this low-energy emission in the co-aggregate.These spectral measurements were also conducted for a combination of P5OCH 2 Cy and another π-conjugated rod with simple phenyl groups (6 H ) in THF/H 2 O mixtures but CD and CPL induction was negligible (Figures S5-1 to S5-4).

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I G U R E 7 (A) Circularly polarized luminescence (CPL) spectra and (B) plots of maximum glum values of 6 F (1.64 × 10 −5 M) with varying equivalents of (pS)-P5OCH 2 Cy in a tetrahydrofuran (THF)/H 2 O mixture (f w = 60).The sign-reversed values are plotted for consistence with circular dichroism (CD) studies.The excitation wavelength was set at 250 nm.

− 2 ]
as a simple organic molecule, similar to its CD response at around 310 nm [(0.91-1.4)× 10 −2 ].Apart from synthetic modification of pillar[n]arenes and hybrid systems including other photo-responsive π-units with small HOMO-LUMO gaps, this work tested the simple combination of an alkoxy pillar[5]arene with stable chirality and other rod-shaped π-units with comparable HOMO-LUMO gaps, using co-aggregating conditions in THF/H 2 O mixtures.With the assistance of electron-withdrawing pentafluorophenyl groups, P5OCH 2 Cy and 6 F formed co-aggregates around f w = 60, affording sharp UV/vis absorption and CD peaks and a broad fluorescence band in the low-energy region.While the structural and electronic details of the co-aggregates were not clear, the obtained chiroptical properties were outstanding in pillar[n]arene-based CPL systems, indicating g lum = 2.9 × 10 −2 and B CPL = ca.3.0 × 10 2 at 472 nm for the optimum conditions.This work presents the prospect of pillar[n]arene-based (pseudo)rotaxanes with π-conjugated axles as a molecular platform with excellent chiroptical properties, which is being actively pursued in our laboratory.
Summary of optical properties of P5OCH 2 Cy at spectral peak tops (λ max[nm]).a TA B L E 1 B CPL (M −1 cm −1 ) b