Emergence of Chiroptical Properties in Molecular Assemblies of

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Spectroscopy and Photochemistry; General Theory

Emergence of Chiroptical Properties in Molecular Assemblies of Phenyleneethynylenes: The role of Quasi-Degenerate Excitations Sabnam Kar, Swathi Konath, Cristina Sissa, Anna Painelli, and K George Thomas J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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The Journal of Physical Chemistry Letters

Emergence of Chiroptical Properties in Molecular Assemblies of Phenyleneethynylenes: The role of Quasi-degenerate Excitations Sabnam Kar,† K. Swathi,†, ‡ Cristina Sissa,*, ‡ Anna Painelli,‡ and K. George Thomas*,† †

School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM), Trivandrum-695551, India ‡

Dip. di Scienze Chimiche, della Vita e della Sostenibilità Ambientale, University of Parma, 43124 Parma, Italy

E-mail: *[email protected] , *[email protected]

ABSTRACT

Chiroptical properties of supramolecular assemblies originate through the asymmetric coupling of molecular transition dipole moments. Herein, we report a joint experimental and theoretical investigation to understand the influence of intermolecular interactions on chiroptical properties, particularly during the early stages of self-assembly. In this regard, phenyleneethynylene-based molecular systems appended with D- and L-isomers of phenylalanine have been synthesized with one as well as two electronic transitions in the spectral region of interest. When self-assembled,

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these molecules show distinctly different chiroptical properties with the right- and left-handed organizations, guided by the chirality of phenylalanines. The standard exciton approach explains the observation of a bisignated electronic circular dichroism signal in systems with a single transition, but fails when applied to systems with two nearby transitions. Here, we present a generalized exciton approach which addresses the unusual chiroptical properties of systems with multiple transitions.

TOC GRAPHICS

KEYWORDS: chiral aggregate, β-sheet, exciton coupling, phenyleneethynylene, phenylalanine The unique optical properties of molecular aggregates are primarily dictated by the mutual orientation of the molecular transition dipole moments.1 These aspects can be quantitatively explained using the exciton model, based on a perturbative treatment of intermolecular interactions, described in the dipolar approximation.2-4 The emergence of chiroptical properties in supramolecular and nanoscopic systems can also be explained within this model accounting for the asymmetric organization of transition dipole moments.1-22 To investigate these aspects, we have designed four chiral molecular systems having D- and L-isomers of phenylalanine,


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functionalized onto the meta-position of the terminal phenyl groups of two sets of phenyleneethynylenes (Chart 1): (a) 1,4-bis(phenylethynyl)-2,5-bis(dodecyloxy)benzene (D-PEOR and L-PE-OR) and (b) 1,4-bis(phenylethynyl)-2,5-bis(dodecyl)benzene (D-PE-R and L-PER), representing model systems. The dodecyloxy groups in the central phenyl ring of D- and LPE-OR are replaced by dodecyl groups in D- and L-PE-R to explore the effect of dialkoxy group on the electronic circular dichroism (ECD) spectra. The design strategy is based on the rationale that the molecular systems possess a hydrophobic chromophoric unit which can self-assemble in polar solvents. The functionalized amino acid groups provide helicity through various interactions, such as hydrogen bonding, leading to the formation of β-sheets.23-24

By adopting various

spectroscopic methods, we explore the asymmetric organization of these molecular systems and the role of interactions between the transition dipoles on the origin of chiroptical properties. These aspects are rationalized in terms of a simple and computationally cost-effective model that generalizes the exciton model to systems showing several nearby excitations. Phenyleneethynylene derivatives (Chart 1) are synthesized via palladium catalyzed HeckCassar-Sonagashira-Hagihara cross-coupling reactions.25 Details of the synthetic procedure adopted and the characterization of the intermediates and final products are presented as Schemes S1 and S2 in the Supporting Information. The optical spectra of D- and L-PE-OR in the region of 280-350 nm are governed by the same chromophoric unit, marginally perturbed by the phenylalanine substituents. Hence, both isomers show similar absorption spectral features in their molecularly dissolved forms in methanol (25 x 10-6 M) (trace a in Figures 1A and S1A). The shortwavelength band at 305 nm, with partially resolved vibronic features, is designated as band X. The long-wavelength band with a maximum around 365 nm is designated as band Y. Upon replacing the dodecyloxy groups in the central phenyl ring of D- and L-PE-OR with dodecyl groups, the


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Chart 1. D- and L-isomers of phenylalanine functionalized on phenyleneethynylenes. The shaded regions indicate two main functional moieties which play important role in the self-assembly. The molecular systems are labeled with alphabetic extensions as D, L, PE, OR and R representing Disomer, L-isomer, phenyleneethynylene unit, alkoxy and alkyl substitutions, respectively. absorption spectrum appears as a single band in the short-wavelength region (trace c in Figures 1B and S1B). Absorption and emission of two sets of phenyleneethynylene derivatives in methanol are presented as Figure S2 and their photophysical properties are summarized in Table 1. These results

are

analogous

with

the

previous

observation

that

the

unsubstituted

1,4-

bis(phenylethynyl)benzene shows a single absorption band, while two well-resolved bands appear upon substituting the central benzene ring with dialkoxy groups.26 Since the two distinct absorption bands in D- and L-PE-OR originate from two electronic transitions, it is indeed important for the subsequent analysis to establish the relative orientation of their transition dipole moments. Therefore, we measured fluorescence anisotropy spectra of D- and L-PE-OR in viscous and frozen solvents (Figure S3). Based on this analysis and with the support from TD-DFT calculations (Section 18 of Supporting Information), we estimate the angle between the two transition dipole moments as ≈ 10° (Scheme S3).


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Figure 1. (A) Absorption spectra (250-450 nm) of D-PE-OR in methanol (trace a; black) and 75% (v/v) H2O/MeOH mixture (trace b; red), (B) absorption spectra (250-450 nm) of D-PE-R in methanol (trace c; black) and 70% (v/v) H2O/MeOH mixture (trace d; red) (C) ECD spectra in methanol: D- and L-PE-OR (both are CD silent; black traces) and 75% (v/v) H2O/MeOH mixture: D-PE-OR (e, red trace) and L-PE-OR (f, blue trace), (D) ECD spectra in methanol: D-PE-R and L-PE-R (both are CD silent; black traces) and 70% (v/v) H2O/MeOH mixture: D-PE-R (g, red trace) and L-PE-R (h, blue trace), (E) schematic representation of D-PE-OR with two transition dipoles showing standard exciton (SE) and generalized exciton (GE) approaches, and (F) schematic representation of D-PE-R with one transition dipole (notes: the absorption spectra of L-PE-OR and L-PE-R are provided as Figure S1; all the compounds are CD silent in methanol, in the spectral region of 250-450 nm). Solvent-dependent absorption spectra indicate that phenyleneethynylene derivatives selfassemble upon increasing the water content in methanol (Figures 1A,B and S1A,B). The


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Table 1. Photophysical characterization of phenyleneethynylene derivatives Molecules

Experimental a

Absorbance

-1

-1

max, nm (, M cm )

Emissionb max, nm (𝐟 )

4

405 (0.81  0.03)

4

405 (0.84  0.03)

D-PE-OR

305, 365 (3.2 x 10 )

L-PE-OR

305, 365 (3.3 x 10 )

D-PE-R

328 (1.07 ×10 )

L-PE-R

328 (1.06 ×10 )

4

352 (0.39 ± 0.02)

4

352 (0.40 ± 0.02)

max,  and f denote wavelength of the absorption/emission maximum, the extinction coefficient and the quantum yield, respectively.  of D-/L-PE-OR are measured at 365 nm. f of D-/L-PEOR are determined using quinine hemisulphate in 0.5 M H2SO4 (f = 0.57) as reference (excitation wavelength 345 nm). f of D-/L-PE-R are determined using naphthalene in cyclohexane (f = 0.23) as reference (excitation wavelength 290 nm). a,b

aggregates are stable when the percentage composition of water in methanol is high, which is evident from the temperature-dependent absorption, emission and ECD studies. For example, Dand L-PE-OR aggregates formed in 75% (v/v) water in methanol are stable up to 60 °C (Figure S4). The D- and L-PE-R aggregates are stable in 70% (v/v) water in methanol, however, tend to precipitate on further increasing the water content. The D-/L-phenylalanine groups are introduced on to the phenyleneethynylene core to guide these molecular systems to a chiral supramolecular arrangement. ECD spectra of all the derivatives in methanol are fully silent in the 250-450 nm spectral region (Figures 1C,D), confirming that the chiral end groups hardly interact with the central phenyleneethynylene chromophoric unit. However, in the same spectral region, supramolecular assemblies of D- and L-PE-OR (Figure 1C) in 75% (v/v) water in methanol show distinct ECD signals with opposite sign. The ECD signals measured in this spectral region are therefore ascribed to the exciton-coupling of transition dipole moments on different molecules in


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the supramolecular assembly. An exciton coupled ECD signal is observed in the region of the short-wavelength transition (X) for both the compounds. Specifically, the bisignated ECD signals of D-PE-OR showed a negative Cotton effect (310-340 nm) followed by a positive Cotton effect in the spectral region of 250-310 nm (termed as negative chirality or negative couplet). As expected, in the same spectral region, L-PE-OR showed exactly opposite ECD spectra (positive chirality or positive couplet). Interestingly, the zero crossover in the ECD spectrum of both systems matches with the maxima of the X absorption band (Figure 1A). The shape of the ECD signal observed in the long-wavelength region (Y band) is instead somewhat unexpected with a non-bisignated negative signal for D-PE-OR and a positive signal for L-PE-OR (Figure 1C). Quite interestingly, the ECD spectrum of D- and L-PE-R (model systems) in 70% water in methanol appears as a single bisignated band in the short-wavelength region (Figure 1D). The observed negative and positive couplets for D- and L-PE-R are very similar to those observed in the X-region for D- and L-PE-OR (Figure 1C). Before discussing the theoretical model to rationalize ECD spectra, it is important to explore how functional groups linked on phenyleneethynylenes drive the organization of molecules with opposite helicity by taking D- and L-PE-OR as representative examples. Structural information about the supramolecular organization of D- and L-PE-OR are obtained by monitoring the 1H-NMR spectra (Figures 2B and S5B) by varying the composition of water in methanol. All protons in the aromatic region of D- and L-PE-OR are assigned with the help of 1

H-1H COSY spectra, recorded in CD3OD (Figures 2A, S5A). The singlet D proton at 7.89 ppm is

coupled with two doublets at 7.65 (A proton) and 7.72 ppm (B proton), respectively. The triplet observed at 7.45 ppm (C proton) is mutually coupled with A and B protons, whereas E proton at 7.12 ppm remains uncoupled. Stable aggregates obtained in 75% (v/v) water in methanol- d4 have


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Figure 2. (A) 1H-1H COSY spectrum in CD3OD (7-8 ppm), (B) solvent-dependent 1H-NMR spectra in mixtures of H2O/CD3OD (500 MHz, 298 K, 25 x 10-6 M) from 7-8 ppm and (C) schematic illustration of stacking of D-PE-OR and the influence of aromatic π-cloud on the Dproton of adjacent molecule. (D,E) The ECD spectra in the spectral range of 200-240 nm (D) in methanol: L-PE-OR (trace a, blue) and D-PE-OR (trace b, red) and (E) in 75% (v/v) H2O/MeOH mixture: L-PE-OR (trace c, blue) and D-PE-OR (trace d, red). (F) Schematic illustration on molecular level interactions in 1-D helix in 75% (v/v) H2O/MeOH mixture. (Notes: (i) At high water to methanol ratio, a distinct peak is observed at 7.95 ppm originating from the water exchanged amide protons, (ii) broadening of aromatic proton peaks on increasing water in methanol-d4 beyond 45% (v/v), is attributed to the formation of higher-order aggregates, (iii) NMR studies of L-PE-OR is presented as Figure S5 and the ECD spectrum of D-/L-PE-R in 70% (v/v) H2O/MeOH mixture as Figure S7B.


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similar spectral properties as observed for 75% (v/v) water in methanol (Figure S6). Solventdependent 1H-NMR studies are carried out in mixtures of water and methanol-d4 (Figures2B, S5B; the water peak is suppressed in the NMR experiments). The concentration of both the compounds used in 1H-NMR are kept same as in optical spectroscopic studies (25 x10-6 M) and the temperature is maintained at 298 K. Hydrophobic phenyleneethynylene units minimize the nonpolar surface of the molecules upon increasing the water content in methanol, and these interactions invariably are reflected in the proton signals in the aromatic region. An appreciable shielding for D proton of Dand L-PE-OR is observed upon increasing the fraction of water in methanol-d4 from 0 to 45% (v/v). However, in the above solvent compositions, the position of other aromatic protons of both the isomers remain practically unaffected, even though the coupling features are perturbed. As a result of stacking of chromophores, upon increasing the water content in methanol-d4, the D proton undergoes shielding due to the influence of the aromatic π-cloud of phenyl ring on the adjacent phenyleneethynylene molecule. This observation further points towards the possibility of a twist between the chromophoric units during their organization (Figure 2C). However, information on the directionality of twist of the chromophoric units, cannot be obtained through NMR studies. The ECD spectrum in the far-UV region can provide valuable information on this aspect. The ECD spectra of D- and L-PE-OR in the far-UV region (200-240 nm), recorded in methanol and 75% (v/v) water in methanol are presented in Figures 2D,E. The ECD spectrum of the molecularly dissolved form of D- and L-PE-OR in methanol showed negative and positive signals, respectively (at 219 nm), originating from the chirality of phenylalanine groups (Figure 2D). More interestingly, the characteristic ECD signals observed for D- and L-PE-OR in 75% (v/v) water in methanol, with distinct negative and positive couplets, signify the formation of β-sheet structures (Figure 2E; D-/L-PE-R in Figure S7B). The spectral profile of various secondary structures23-24,


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are presented as Figure S7A for comparison. The β-sheet structures in the aggregated states of

D-/L-PE-OR and D-/L-PE-R are formed due to the H-bonding interaction between –C=O and – NH groups of adjacent molecules in the supramolecular assembly (Figure 2F). The directionality in the twist of the chiral assembly is guided by the chirality of the D- and L- isomers of phenylalanine groups. It is reported that amide groups play a significant role in inducing twist in the supramolecular structures and in amplifying the chiral response.28-30 Thus, solvent-dependent NMR and far-UV ECD spectra provide unequivocal evidence on the role of amide groups in the formation of -sheets. The formation of chiral supramolecular systems is often explained based on a helical supramolecular geometry,1, 5, 30-32 and the sign of ECD spectrum is used as a thumbrule to assign their handedness.33-35 Accordingly, we ascribe a left-handed (M-type) helical arrangement to D-PE-OR and D-PE-R aggregates and right-handed (P-type) arrangement to LPE-OR and L-PE-R aggregates (Figures 3A,B). The behaviour of a dimer of equivalent molecules, each bearing a single excitation, is known since date and sets the basis of our understanding of supramolecular chirality.3-4, 33-37 In the exciton model for a chiral dimer, the two degenerate states of the excited monomers are mixed by intermolecular interactions resulting in symmetric and antisymmetric states whose relative energies depend on the sign of the interaction term (Section 10 of Supporting Information).2, 5, 3841

The two states in chiral assemblies respond in opposite way to circularly polarized light and a

characteristic bisignated signal, also called as exciton couplet, appears in ECD spectra.4, 33-36 This concept works well for supramolecular assemblies of D- and L-PE-R molecules wherein a single transition dominates the monomer spectra in the region of interest (Figures 1B and S1B). However, when this concept is applied to systems such as D- and L-PE-OR with two nearby excitations per molecule, the standard exciton (SE) model, only accounting for the interaction between degenerate


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excitations (X-X and Y-Y), predicts the presence of two bisignated signals (one for each monomer transition, Figures S12 and S13) in the ECD spectrum. This contrasts strikingly with the experimental observation in Figure 1C. To rationalize the anomalous features in the ECD signals of D- and L-PE-OR, we introduce a generalized exciton (GE) approach that accounts for degenerate (X-X and Y-Y) as well as non-degenerate (X-Y) excitonic interactions. Table 2. Molecular parameters from the absorption spectrum of D- and L-PE-OR in methanol Transitionsa

𝐸 (eV)b 𝜇 (D)c

𝛾 (eV)d

X

3.95

7

0.22

Y

3.32

6

0.22

a

X and Y represent short- and long-wavelength bands, respectively. b-dE, 𝜇 and 𝛾 denote transition energy, transition dipole moment and half-width at half-maximum of the transition band, respectively. To start with, we consider a dimer, where each one of the two equivalent molecules, A and B, has two excited states, X and Y. The two molecular excitons are characterized by their energy (𝐸𝑋/𝑌 ) and transition dipole moment (𝜇𝑋/𝑌 ), that are estimated from the analysis of the absorption spectra of molecularly dissolved D- and L-PE-OR in methanol. Since the absorption spectra of D- and LPE-OR are equivalent, the same model parameters apply to both the molecules (Table 2). As discussed earlier, the angle (α) between the X and Y transition dipole moments is estimated from fluorescence anisotropy spectra as α ≈ 10° (Scheme S3 and Figure S3). The exciton basis for a dimer of D-/L-PE-OR then has two pairs of degenerate states: one pair of states corresponds to the two states where either molecule A or B is in the X excited state, the second pair has either molecule A or B in the Y excited state. Electrostatic intermolecular interactions are introduced in the point-dipole approximation that applies as long as the intermolecular distances are large with respect to the dipole length, a reasonable approximation in our case where the strongest dipole


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(7 D) corresponds to a dipole length of about 1.3 Å. In the dipolar approximation, the interaction energy between transition dipole moments located in molecules A and B is described by equation (1), ⃗ 𝐴 ∙𝜇 ⃗𝐵 𝜇

1

𝑉 = 4𝜋𝜀

0

𝑛2

(

𝑟3

−3

(𝜇 ⃗ 𝐴 ∙𝑟 )(𝜇 ⃗ 𝐵 ∙𝑟 )

) ,

𝑟5

(1)

where 𝜀0 and 𝑛 denote the vacuum permittivity and refractive index of the solvent, respectively. 𝜇𝐴/𝐵 represents the (X and/or Y) transition dipole moment of A/B molecules, and 𝑟 is the vector that connects the two dipoles. In either approaches (SE or GE), the excitonic Hamiltonian is diagonalized and the four eigenstates are used to calculate the optical spectra of aggregates. Assigning a Lorentzian spectral profile to each state, absorption spectra are calculated using equation (2),37 𝐸𝛾

𝐴(𝐸) ∝ ∑𝑖 |𝜇𝑖 |2 (𝐸 −𝐸)2 +𝛾2 ,

(2)

𝑖

where 𝑖 runs on the dimer eigenstates, characterized by the transition dipole moment 𝜇𝑖 and energy 𝐸𝑖 . Since electrons are localized on each molecular unit, the dipole moment operator is the vectorial sum of the dipole moment operators of each molecule. Accordingly, the transition dipole moment associated to each excited state can be written as the sum of the contributions from each molecule, 𝜇𝑖 = ∑𝑛 𝜇𝑖,𝑛 , where 𝑛 runs on the molecular sites. The ECD spectrum is finally calculated using equation (3),3-4, 37

Δ𝐴(𝐸) ∝ ∑𝑖

𝐸𝛾 𝐸𝑖 𝐺 (𝐸𝑖 −𝐸)2 +𝛾2 2ℏ 𝑖

,

(3)

where 𝑖 runs on the system eigenstates, corresponding to the excited states of the dimer, and 𝐺𝑖 , defined by equation (4), only depends on the geometrical factors:


12

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𝐺𝑖 = ∑𝑛,𝑚 𝑟𝑛,𝑛+𝑚 ∙ (𝜇𝑖,𝑛 × 𝜇𝑖,𝑛+𝑚 ) ,

(4)

where n and m run on molecular sites and 𝑟𝑛,𝑛+𝑚 = 𝑟𝑛 − 𝑟𝑛+𝑚 is the vector joining the n, n+m molecular centers. We notice that the geometrical factor 𝐺𝑖 defined in equation 4, is related to the rotational strength, 𝑅𝑖 , as usually defined by 𝑅𝑖 ∝ 𝐸𝑖 ∗ 𝐺𝑖 . To highlight the difference between the SE and GE approaches, Table 3 lists the four geometrical factors, 𝐺𝑖 , calculated for a dimer in helicoidal geometry in the two approaches, and additional results are shown as 𝐺𝑖 plots in Figure S8. In the SE approach, the X and Y excitons do not mutually interact. Hence, the ECD spectrum calculated in this approach is simply the sum of the spectra calculated separately for each exciton pair. In the SE approach,

we then expect (and calculate) equal and

opposite

𝐺1 -𝐺2 values, as relevant to the X-exciton couplet, and equal and opposite 𝐺3 -𝐺4 values for the Yexciton couplet (Table 3 and Figure S8A). The results are completely different in the GE approach, where the X-Y interaction are also accounted for, leading to a mixing of the four excitonic states with a redistribution of the overall 𝐺𝑖 as presented in Table 3 and Figure S8B. Thus, in GE approach, the concept of exciton couplet is modified by involving non-degenerate interactions. We are now in a position to qualitatively understand the different chiroptical properties of supramolecular assemblies of D-/L-PE-R and D-/L-PE-OR. We consider the simplest chiral aggregate structure wherein N molecules are organized as a helix (Figure 3). In an aggregate formed by D-/L-PE-R molecules, since each molecule bears a single excitation in the spectral region of interest, N degenerate states are present. These degenerate states split by intermolecular interactions forming the exciton band. Absorption and ECD spectra are again calculated using equations 2 and 3, allowing the sum run on all the N excitonic states (Figures S9 and S10). These calculations are carried out by imposing the molecular transition energy and dipole moment as 3.78 eV and 6 D, respectively and setting γ and the refractive index as 0.22 eV and 1.33,


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respectively. As expected, the calculated ECD spectrum in Figure 3E shows a well-defined exciton couplet which matches with experimental results (Figure 1D). Table 3. 𝐺𝑖 values and transition energies (𝐸𝑖 ) calculated for dimersa forming a right-handed helix. Tilt Approachc angle τb

Y exciton

X exciton

𝐺1 [𝐸1 eV)]

𝐺2 [𝐸2 eV)]

𝐺3 [𝐸3 eV)]

𝐺4 [𝐸4 eV)]

SE

12.50 [3.12]

-12.50 [3.52]

17.02 [3.68]

-17.02 [4.22]

GE

21.71 [3.05]

-5.36 [3.45]

7.82 [3.76]

-24.17 [4.28]

SE

24.63 [3.13]

-24.63 [3.51]

33.52 [3.70]

-33.52 [4.20]

GE

42.14 [3.06]

-10.96 [3.45]

16.03 [3.77]

-47.21 [4.26]

SE

36.00 [3.15]

-36.00 [3.49]

49.00 [3.72]

-49.00 [4.18]

GE

60.02 [3.09]

-17.04 [3.44]

25.01 [3.78]

-67.98 [4.24]





 a

the two molecules of phenyleneethynylenes bearing dodecyloxy groups are superimposed with intermolecular distance set to 4 Å; bresults are shown for different tilt angles; cSE and GE denote standard and generalized exciton approaches, respectively. More interesting are the results obtained for a similar helix made of D-/L-PE-OR molecules. The presence of two excitations in the relevant spectral region leads to 2N exciton states that are all mixed up in the GE approach. An ECD spectrum compatible with the experiment (Figure 1C), showing a non-bisignated signal in the low-frequency region and a bisignated signal in the high frequency region, is obtained for a helical geometry as shown in Figure 3F. This helical geometry is compatible with the formation of -sheets, but does not rationalize the weak red-shift observed in the absorption spectra of aggregates with respect to monomers (Figure 1A). This can be ascribed to various intermolecular interactions resulting from the assembly of helices as bundles as observed in AFM images (Figure S11). Results based on GE approach for different geometrical


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parameters of the helix demonstrate that, irrespective of the specific structure adopted for the aggregate, significant deviations are observed when compared with SE approach (Figures S12S15).

Figure 3. Illustrations of right- and left-handed helical geometries of molecules containing (A) one and (B) two transition dipoles. The theoretical absorption spectra of helical assemblies of molecules with (C) one transition dipole moment using SE approach, (D) two transition dipole moments using GE approach in their monomeric and aggregated states. The calculated ECD spectra of right- and left handed assemblies of molecules having (E) one transition dipole moment using SE approach, (F) two transition dipole moments using GE approach. Results are shown for helices formed by ten molecules, intermolecular distance 7 Å and tilt angle 10°. Results for various other geometries are presented as Figures S9 and S10 (one transition dipole) and S12-S15 (two transition dipoles).


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In conclusion, we have investigated the chiroptical properties of two sets of helical assemblies, one with molecules having single excitation (D-/L-PE-R; Figure 1F) and the other with two close lying excitations (D-/L-PE-OR; Figure 1E). The standard exciton model fails to rationalize the supramolecular chirality of molecules having multiple nearby optical transitions. We, therefore, present a generalized exciton model that properly accounts for the contributions of multiple molecular excitations to ECD spectrum, rationalizing the puzzling chiroptical responses of supramolecular assemblies of molecular systems like D-/L-PE-OR having multiple transitions in close proximity. Furthermore, the insights gained are useful for the design of supramolecular and nanoscopic materials with tailored chiroptical properties having potential applications in chiral sensing, synthesis, and catalysis. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website: synthesis and characterization of D-/L-PE-OR and D-/L-PE-R, spectroscopic, microscopic and theoretical investigations of D-/L-PE-OR and D-/L-PE-R. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENTS KGT thanks the Department of Science and Technology (DST Nanomission Project (SR/NM/NS23/2016) and J. C. Bose National Fellowship of SERB-DST, Government of India for financial support. SK acknowledges the University Grants Commission, Government of India for senior research fellowship. The Indo-Italian Executive Program 2017-2019 (KGT, CS, AP) of Cooperation in Scientific & Technological Cooperation (No. INT/Italy/P-9/2016(ER)) and the Joint PhD degree program (Swathi) between IISER-TVM and University of Parma are acknowledged for support of exchange of personals. CS thanks the University of Parma for financial support (Project Cybamat, FIL 2016 “Quota incentivante”). Authors thank Dr. Vinesh Vijayan, IISER-TVM, for helpful discussion on NMR studies and Mr. P. P. Rafeeque for graphical support. REFERENCES 1. Thomas, R.; Kumar, J.; George, J.; Shanthil, M.; Naidu, G. N.; Swathi, R. S.; Thomas, K. G. Coupling of Elementary Electronic Excitations: Drawing Parallels Between Excitons and Plasmons. J. Phys. Chem. Lett. 2018, 9, 919-932. 2. Davydov, A. S. Theory of Molecular Excitons. 1st ed.; Springer New York, 1971; p 313. 3. Condon, E. U. Theories of Optical Rotatory Power. Rev. Mod. Phys. 1937, 9, 432-457. 4. Craig, D. P.; Thirunamachandran, T. Molecular Quantum Electrodynamics : An Introduction to Radiation-Molecule Interactions. Academic Press: London, 1984; p 324. 5. van Dijk, L.; Bobbert, P. A.; Spano, F. C. Extreme Sensitivity of Circular Dichroism to Long-Range Excitonic Couplings in Helical Supramolecular Assemblies. J. Phys. Chem. B 2010, 114, 817-825. 6. Kistler, K. A.; Pochas, C. M.; Yamagata, H.; Matsika, S.; Spano, F. C. Absorption, Circular Dichroism, and Photoluminescence in Perylene Diimide Bichromophores: PolarizationDependent H- and J-Aggregate Behavior. J. Phys. Chem. B 2012, 116, 77-86. 7. Kumar, J.; Nakashima, T.; Kawai, T. Circularly Polarized Luminescence in Chiral Molecules and Supramolecular Assemblies. J. Phys. Chem. Lett. 2015, 6, 3445-3452. 8. Korevaar, P. A.; George, S. J.; Markvoort, A. J.; Smulders, M. M. J.; Hilbers, P. A. J.; Schenning, A. P. H. J.; De Greef, T. F. A.; Meijer, E. W. Pathway Complexity in Supramolecular Polymerization. Nature 2012, 481, 492-496. 9. Kumar, J.; Thomas, K. G.; Liz-Marzan, L. M. Nanoscale Chirality in Metal and Semiconductor Nanoparticles. Chem. Commun. 2016, 52, 12555-12569.


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Emergence of Chiroptical Properties in Molecular Assemblies of

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