Peptide-induced chirality transfer and circularly polarized luminescence in achiral BODIPY emitters via halogen bonding

Abstract

This study explores peptide-mediated chiral induction and circularly polarized luminescence (CPL) in achiral BODIPY dyes, leading to a high g_lum value of up to −1.2 × 10⁻² through orthogonal halogen bonding and hydrogen bonding. It unravels the impact of these combined directional interactions on the formation of heterostructures and their thermal stability.

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Circularly polarized luminescence (CPL), which is the differential emission of right- and left-circularly polarized light,^1a,b has gained increased attention over the past decade for its ability to offer critical information on the chirality of electronic excited states and its possible uses in optoelectronic technologies,² 3D displays,³ chiral recognition, and sensing.⁴ Organic molecules featuring both chirality and fluorescence have been explored to generate CPL-active materials. However, typically tuning the CPL properties via chemical modulation of different molecules involves tedious synthesis, making it difficult to establish the structural basis for systematic analysis of the excited state chiroptical features. Supramolecular approaches for CPL induction in achiral fluorophores by chirality transfer from chiral templates broaden the scope of structure–property analysis by allowing independent adjustment of the emission properties of fluorescent dyes and the chirality of the templates.⁵ Such modular strategies have emerged in recent years, with the added benefit of constructing stimuli-responsive ON–OFF CPL switches.⁶ Halogens, with their unique electronic structure, can participate in both Lewis acidic and basic interactions. This enables the utilization of halogen (X)-bonding^7a–c between Lewis bases and electrophilic σ-holes, centered on the halogen atom, as a powerful directional noncovalent force in crystal engineering,⁸ materials science,⁹ anion transport,¹⁰ self-assembly,¹¹ and many other fields. In comparison to the extensively studied hydrogen (H)-bonding, X-bonding is more directional¹² and demonstrates comparable strength. Nevertheless, hierarchical supramolecular chirality mediated by halogen bonding is an understudied field,^13a,b and its relevance in CPL induction or amplification has rarely been investigated.^14a–c On the other hand, hydrogen-bonded peptide assemblies, exhibiting amplification of chirality, have been established as a promising system for designing chiroptical materials.^15a,b We envisaged that additional immobilization of a halogen bond-donating unit in peptide-based chiral templates could be useful for supramolecular chirality transfer to achiral fluorophores if equipped with complementary halogen bonding motifs. Such modular designs for engineering CPL in achiral molecules based on orthogonal operation of H-bonding and X-bonding remain nearly unexplored. To this end, herein, we designed a new peptide molecule (D₁) functionalized with a tetrafluoroiodophenyl (TFIP) moiety,¹⁶ as the halogen bond donor for two achiral boron-dipyrromethene (BODIPY) based fluorescent halogen bond acceptors A₁ and A₂ (Scheme 1), which were co-assembled by C–I⋯F–B X-bonding.¹⁷ This study exhibits new insights into the impact of X-bonding as a structure-directing tool in controlling supramolecular chirality in both ground and excited states.

Scheme 1 Chemical structures of peptides (D₁ and D₂) and achiral BODIPYs (A₁ and A₂) and CPL induction in their co-assemblies (D₁–A₁ and D₁–A₂) by halogen bonding.

Synthesis of the two peptides (D₁ and D₂) is reported in the ESI† and that of BODIPYs (A₁ and A₂) is reported elsewhere.¹⁸ All of them showed solubility in methanol (MeOH), where they exhibited monomeric properties (Fig. S1 and S2, ESI†). A₁ and A₂ in 10% MeOH/water (c = 10 μM) showed an absorption maximum centered at ∼529 nm, which is assigned to the S₀–S₁ transition, and a low-intensity band around 400 nm for S₀–S₂.¹⁹ Upon co-assembly with the peptide D₁, A₁ exhibited a significant bathochromic shift of 34 nm, resulting in the emergence of a prominent band at λ_max = 563 nm (Fig. 1a) with a concomitant increase in the band intensity. In contrast, only ∼4 nm red shift²⁰ was observed for the co-assembly of A₂ with D₁. Thus, co-assembled D₁–A₁ and D₁–A₂ exhibit different molecular packing of the two BODIPY dyes. The emission intensity increased for both A₁ and A₂ in their respective co-assemblies (Fig. 1b), which is also reflected in the increased quantum yields of D₁–A₁ (0.36) and D₁–A₂ (0.45) compared to the homo-aggregates of A₁ (0.16) and A₂ (0.15) (see ESI†). In 10% MeOH/water, D₁ exhibited a bisignated circular dichroism (CD) spectrum with two negative signals at 221 nm and 278 nm, along with a positive peak at 254 nm, indicating the formation of a secondary helical structure (Fig. 1c and Fig. S2b, ESI†). As expected, both A₁ and A₂, lacking any chiral entity, did not show any CD signals (Fig. 1c and d). However, in the case of D₁–A₁ and D₁–A₂, a new band centered at ∼563 nm and ∼530 nm emerged (Fig. 1d), precisely matching with the newly shifted absorbance peaks for A₁ and A₂, in their respective co-assemblies. This suggests the chirality transfer from the self-assembled peptide to the achiral BODIPYs in their co-assembled states. Also, the CD signals corresponding to the D₁ absorbance showed a notable change in the co-assembly (Fig. 1c), indicating that the intrinsic helical arrangement of the peptide is influenced by the presence of the two BODIPYs. The existence of ground state chirality and enhanced emission both in D₁–A₁ and D₁–A₂ prompted us to further check their chiral emission in the excited state. While achiral BODIPYs exhibit no CPL signals, they showed CPL activity when co-assembled with the peptide at 25 °C (Fig. 2a and b). Pleasingly, CPL signals emerged at ∼580 nm for D₁–A₁ and ∼554 nm for D₁–A₂, with an appreciably high luminescence dissymmetry factor (g_lum) of −1.2 (±0.075) × 10⁻² and −0.91 (±0.049) × 10⁻², respectively (Fig. S3, ESI†). g_lum = 2(I_L − I_R)/(I_L + I_R), where I_L and I_R refer to the luminescence intensities of left- and right-circular polarized components, respectively. The CPL signals align well with the emission wavelengths of A₁ and A₂ in their co-assembled states. Furthermore, D₁–A₁ and D₁–A₂ showed ON and OFF CPL responses at 25 °C and 75 °C (Fig. 2a and b), respectively, which implies that induced supramolecular chirality in BODIPY acceptors by the peptide donor is lost around 75 °C. To gain insights into the thermal stability of the self-assembled D₁ and its co-assembly with A₁ and A₂, temperature-dependent CD and UV-Vis studies were performed. D₁ showed a notable reduction in the CD band intensity with increasing temperature from 25 °C to 95 °C (Fig. 2c). The CD melting curve monitored at 278 nm showed complete disassembly at ∼70 °C for the helical structure of D₁ (Fig. 2d). In concomitance, variable-temperature UV-Vis studies with D₁ revealed spectral broadening along with a hypochromic shift for the band centered at 263 nm (Fig. 2e), which is ascribed to the tetrafluoroiodophenyl (TFIP) moiety. Complete disassembly was noticed at ∼70 °C (Fig. 2h), corroborating with the CD results. The peptide-induced CD bands for D₁–A₁ and D₁–A₂, originating from the two achiral BODIPYs, remained largely intact up to 60 °C, above which the signal intensity began to decrease rapidly and showed complete loss above 80 °C (Fig. 2d and Fig. S4, ESI†). In contrast, the helical assembly of D₁ alone was significantly disrupted at 60 °C. Thus, compared to D₁ alone, the two co-assemblies demonstrated greater thermal stability (Fig. 2d). A similar observation was made from the variable-temperature UV-Vis studies. With increasing temperature from 25 °C to 95 °C, the co-assemblies showed a decrease in the absorption intensity of A₁ and A₂ (Fig. 2f and g) without any significant spectral shift, and the UV-Vis spectra at 95 °C did not resemble that of the monomeric spectra observed in MeOH (Fig. S1, ESI†). This suggests that A₁ or A₂ remained aggregated even after disassembly from D₁ possibly due to the hydrophobic dominance in water. α₅₀ values (temperature at which fraction of aggregation is 0.5) obtained from the UV-Vis spectral changes revealed greater thermal stability for co-assembled D₁–A₁ (66 °C) and D₁–A₂ (76 °C) than the homo-assembly of D₁ (57 °C) (Fig. 2h), in agreement with the CD data. Noteworthily, D₁–A₂ exhibits greater stability than D₁–A₁, which is attributed to the stronger hydrophobic effect of A₂ in 10% MeOH/water. This is owing to the absence of the polar amine group in A₂ unlike in A₁, which makes D₁–A₂ co-assembly more hydrophobic and less hydrated than the D₁–A₁ co-assembly. Comparing the UV-Vis melting curves of A₁ and A₂ with D₁–A₁ and D₁–A₂, respectively, we further ascertained augmentation of BODIPY stability by the peptide in the two co-assemblies (Fig. S5, ESI†). This is attributed to the synergistic effect of multiple directional non-covalent interactions operating within the two co-assemblies, in contrast to the formation of solvophobically-driven collapsed structures in the BODIPY homo-assemblies.

Fig. 1 (a) UV-Vis, (b) photoluminescence (PL) and (c,d) CD spectra of D₁, A₁, A₂, D₁–A₁ and D₁–A₂; conc. = 0.1 mM in 10% MeOH/H₂O.

Fig. 2 (a) CPL spectra of D₁–A₁ and (b) D₁–A₂ at 25 °C and 75 °C; (c) variable-temperature CD spectra of D₁; variable-temperature UV-Vis spectra of (e) D₁, (f) D₁–A₁ and (g) D₁–A₂; (d) normalized CD vs. temperature plot and (h) α_aggvs. temperature plot for D₁, D₁–A₁, and D₁–A₂. Individual conc. = 0.1 mM in 10% MeOH/water.

While D₁ exhibited an entangled fibrillar morphology (Fig. 3a), A₁ and A₂ produced ill-defined spherical aggregates (Fig. S6a and b, ESI†), as visualized from the transmission electron microscopy (TEM) images. In contrast, co-assembled D₁–A₁ formed heterogeneous structures with short nanorods attached to small fibers (Fig. 3b). Upon heating the solution to 90 °C followed by cooling, elongated fibers and nanorods were regenerated (Fig. 3c), but surprisingly the nanorods were found to be detached from the fibers and could be isolated (Fig. 3d). This is due to the precipitation of the peptide fibers, likely caused by the slow escape of the MeOH during the heating process. Non-reversibility of the UV-Vis and CD spectra (Fig. S7a and b, ESI†) after re-cooling the solution and repeating the heat–cool cycle further establishes that chirality transfer does not occur once the nanorods are dissociated from the helical fibers. In contrast, co-assembled D₁–A₂ exhibited fibrillar morphology (Fig. S6c, ESI†) similar to D₁, and no segregated nanorods were seen attached to the fibers, unlike observed in D₁–A₁ co-assembly. Possibly, the greater hydrophobicity of A₂ owing to the absence of the polar amine groups enables its better intercalation within the fibrillar network of D₁. This results in improved colloidal stability through increased hydrophobic shielding from the bulk water during the co-assembly. In contrast, A₁ formed separate nanorods on the peptide fibers by possibly facing its hydrated amine group to bulk water. This accounts for the greater thermal stability of D₁–A₂ than D₁–A₁.

Fig. 3 TEM images of (a) self-assembled D₁, and (b) co-assembled D₁–A₁ showing short fibers attached to small nanorods; (c) D₁–A₁ morphology after a heat–cool treatment showing the co-existence of elongated fibers and detached nanorods, and (d) isolated nanorods by selective precipitation of the peptide fibers. Individual conc. = 0.1 mM in 10% MeOH/H₂O.

Next, we investigated the supramolecular interactions governing the self-assembly of the peptide D₁ and its co-assembly with A₁ and A₂. D₁ exhibited a H-bonded amide I band at 1649 cm⁻¹ (Fig. S8, ESI†), with no observed shift in the two co-assemblies, suggesting the retention of the peptide H-bonding in the co-assembled states. Noteworthily, the amine moiety in A₁ is not a decisive interacting group for the co-assembly with D₁, as A₂ in its absence also interacts with D₁ and exhibits induced chirality in both the ground and excited states. To gain insight into the presence of X-bonding, ¹⁹F NMR spectroscopy was performed using D₂O as a locking solvent in a sealed capillary tube. D₁ displayed two peaks at −121.27 ppm and −150.31 ppm, respectively, for the ortho- and meta-fluorine atoms (with respect to the iodine atom) associated with the TFIP moiety in D₁, which showed significant upfield shift upon co-assembly with A₁ (Δ 1.47, Δ 0.22) and A₂ (Δ 2.23, Δ 0.12), typically observed during halogen bonding (Fig. 4a and Fig. S9, ESI†). Contrarily, A₁ and A₂ exhibited a quartet resonance at −156.88 and −156.92 ppm, respectively, for the two fluorine atoms attached to the boron centers (Fig. 4a and Fig. S9, ESI†). Upon co-assembly with D₁, a notable downfield shift to −155.44 and −154.80 ppm was observed in D₁–A₁ and D₁–A₂, respectively (Fig. 4a and Fig. S9, ESI†). The spectral shift was more prominent for A₂ (Δ 2.12) as compared to A₁ (Δ 1.44) in the presence of D₁, which indicates stronger binding of D₁ with A₂, as previously observed from the temperature-dependent UV-Vis spectroscopy studies. For further confirmation of the involvement of X-bonding, ¹¹B NMR spectroscopy was performed, as the boron center is highly sensitive to the ligand around its vicinity. A₁ and A₂ exhibited a triplet peak at −4.84 and −4.90 ppm, respectively, for the B–F coupling, which significantly shifted to −2.63 ppm and −2.32 ppm when co-assembled with D₁ (Fig. 4b and Fig. S10, ESI†). Comparing both co-assemblies revealed a stronger hydrophobically-assisted X-bonding interaction in D₁–A₂ than D₁–A₁.²¹ Validation of B–F⋯I–C halogen bonding was further pursued by studying the co-assembly of A₁ and A₂ with a control peptide (D₂), where the proline is attached to a pentafluorophenyl group (Scheme 1), which has no X-bonding donating ability due to the absence of the iodine atom. D₂ also self-assembled into a similar fibrillar structure as D₁ (Fig. S11, ESI†). UV-Vis and CD analysis of the co-assemblies showed no spectral change or chiral induction in the BODIPY acceptors (Fig. S12, ESI†), ruling out any possibility of their nonspecific interactions with the peptide D₂. This confirms that directional X-bonding between chiral peptide donor D₁ and the achiral BODIPY emitters drives chirality transfer and CPL induction, enabling symmetry breaking and chiral amplification in π-stacked A₁ and A₂ co-assemblies via orthogonal H-bonding. This accounts for the shape transition from hydrophobically-collapsed disordered spherical homo-aggregates to more ordered helical heterostructures.

Fig. 4 (a) ¹⁹F NMR spectra and (b) ¹¹B NMR spectra in 10% MeOH/water locked with D₂O. (*) Reference peak for phenylboronic acid.

This work reveals a versatile halogen bonding-mediated strategy for CPL generation (g_lum value reaching up to −1.2 × 10⁻² in the solution state) in achiral amine substituted/unsubstituted BODIPY acceptors, A₁ and A₂, respectively, by chirality transfer from a chiral peptide donor (D₁). The halogen-bonded co-assemblies demonstrate temperature-dependent ON/OFF CPL switching, co-existing fibrillar and rod-like morphology attached to each other (originated from D₁–A₁ co-assembly), and their physical separation by a simple heat–cool treatment, which was not observed for the D₁ and A₂ pair. D₁–A₂ co-assembly showed superior thermal stability compared to D₁–A₁ due to the stronger hydrophobic interaction-driven halogen bonding in the D₁–A₂ co-assembly. This work reveals novel perspectives on the capability of directional halogen bonding in regulating supramolecular chirality in both ground and excited states, leading to strong CPL signals from achiral BODIPY fluorophores. This study opens up new opportunities in designing tailorable CPL active responsive materials, exploring orthogonal halogen bonding in hydrogen-bonded peptide assemblies.

S. S. & A. J. thank CSIR, New Delhi, and S. G. thanks BRNS for a fellowship. A. D. & S. M. thank DAE-BRNS (No. 58/20/10/2022-BRNS/37054) for funding and IACS for generous support to create the CPL facility.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc05814c

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