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. 2026 Oct 20. Online ahead of print. doi: 10.1039/d6sc06531g

Single-component dual emission and full-spectrum color tuning from through-space charge transfer in asymmetric [2.2]paracyclophanes

Karla D Hernandez Gomora a,b, Bhargava R Jogi b,c, Philip Hadjipetkov a,b, Jack Shumway a, Samreen Ahmad a, Hayden Moody a, Dundappa Mumbaraddi a,b, Ajay Ram Srimath Kandada b,c, Cedric Schaack a,b,✉
PMCID: PMC13699521  PMID: 42936031

Abstract

Organic fluorophores are typically limited to a single emission color, because rapid relaxation funnels the excited population to one minimum before it emits. We report asymmetric pseudo-para [2.2]paracyclophanes that break this limit by placing a diphenylaniline donor and a triazine or cyano acceptor on opposite rings of the cyclophane. The rigid transannular contact supports an intramolecular through-space charge-transfer state that is populated alongside the locally excited state, so a single molecule emits from two states at once. Both emissive states here are intramolecular: their combined emission is invariant on dilution to 10−8 M and persists in a rigid polymer matrix. By utilizing solvent polarity to alter the thermodynamic equilibrium between these states, we demonstrate continuous color tuning across the visible spectrum, including the generation of single-molecule white light with CIE coordinates of (0.33, 0.32). Furthermore, we report a negative thermal quenching effect, in which the emission intensity rises rather than falls on heating, achieving photoluminescence quantum yields as high as 88%, a response documented in inorganic phosphors and semiconductors but rare among discrete organic emitters in solution. Translating thermal and dielectric fluctuations into ratiometric spectral readouts positions cyclophanes as a general scaffold for stimuli-responsive optoelectronics.

Traditional fluorophores emit a single band, whereas the scaffold in this report emits two. Consequently, broadly tunable colors, single-molecule white light, and a thermal response are observed and characterized.graphic file with name d6sc06531g-ga.webp

Introduction

Standard photophysical relaxation strictly limits fluorophores to radiative decay from a single, thermally equilibrated excited-state geometry, a paradigm formalized by Kasha's rule.1 Here, we circumvent this constraint by engineering a molecular framework that stabilizes two distinct emissive minima on a single excited-state surface. Because the relative thermal population between these geometries is sensitive to microenvironmental changes, this platform enables dynamic, environment-tunable dual emission for self-calibrating optical readout. Crucially, this dual emission is an intrinsic property of the isolated unimolecular structure, operating via bright conformational equilibria rather than non-radiative structural quenching or multi-molecular assemblies.

Single molecules that emit more than one color are the basis of single-component white-light sources and of self-referencing ratiometric probes. White light is usually generated by combining separate red, green, and blue emitters, but these components photobleach at different rates, so the color balance drifts over the lifetime of the device.2–6 A single molecule that emits balanced white light removes this failure mode at the source, yet examples of such properties remain rare.7 The second is ratiometric sensing. A probe that reports through the intensity ratio of two emission bands is self-referencing, so its readout does not depend on how much probe is present or on fluctuations in excitation. Fluorophores whose two bands respond differently to temperature, polarity, or other stimuli are therefore valuable for sensing, bioimaging, and adaptive optoelectronics (Fig. 1C).8–12 Both applications require the same thing: one molecule emitting from two states at once.

Fig. 1. Overview of photophysical processes that lead to light emission. (A) Kasha's rule specifies that emission can only occur from the lowest energy excited state, leading to monochromatic emission. (B) Strategies that circumvent simple S1 emission, such as Kasha violating molecules (e.g. azulene), triplet harvesting in thermally activated delayed fluorescence or phosphorescence, and intermolecular effects such as aggregation induced emission. (C) Our strategy produces an intramolecular equilibrium between locally excited and charge-transfer states, leading to multicolor emission from one species in response to temperature or solvent polarity.

Fig. 1

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While excited-state intramolecular proton transfer (ESIPT),13–15 twisted intramolecular charge-transfer (TICT),16,17 and intermolecular excimer formation have been employed to engineer dual-emissive pathways,3,18–20 each carries fundamental limitations. ESIPT requires specialized proton-transfer-capable functional groups, which restricts structural versatility and environmental suitability.21–24 TICT states are frequently dominated by non-radiative torsional decay, limiting achievable luminescence intensities. Excimer and aggregation-based emission generate a second band through intermolecular interactions, resulting in an emission that depends on concentration and on solid-state packing rather than being an intrinsic property of the isolated molecule.25–28

The [2.2]paracyclophane (PCP) core provides the geometry to make the exciplex-type state intramolecular.29–31 Two benzene rings are held cofacially below the van der Waals distance by two ethane bridges, which fix the inter-ring geometry and allow electronic communication across the transannular gap.31–34 The rigid bridges also suppress the large-amplitude torsions that drive non-radiative decay in flexible systems, while still permitting the controlled transannular relaxation characteristic of the scaffold. Consequently, the PCP architecture has been utilized across diverse optoelectronic applications, including chiral OLEDs,35–43 two-photon absorbers,44 layered π-electron-system polymers,45–48 and thermally activated delayed fluorescence emitters.39,49,50 Miyazaki et al. demonstrated that pseudo-para donor–donor–acceptor (D–D–A) cyclophanes support charge-transfer (CT) states whose oscillator strength depends sensitively on substitution pattern,51 while Tanaka et al. showed that PCP-organoboron complexes exhibit large excited-state dipole moments and pronounced solvatofluorochromism driven by intramolecular CT.52 These studies highlight the widespread interest in PCP as a luminescence platform, including capabilities for thermally activated delayed fluorescence that result in high photoluminescence quantum yields (PLQY). To our knowledge, no paracyclophane has been developed as a tunable dual emitter, in which both the locally excited and charge-transfer states are deliberately populated and steered across the spectrum.

Here, we report the synthesis and photophysical characterization of pseudo-para-disubstituted [2.2]paracyclophanes featuring an N,N-diphenylaniline (DPA) electron donor paired with either a cyano (CN) or 1,3,5-triphenyltriazine (TRZ) acceptor, alongside their symmetric analogs (Fig. 2B). The transannular donor–acceptor dipole gives rise to two emissive states within a single molecule: a localized excited (LE) state and a transannular CT state, whose relative population is set by the environment. We find this behavior is not explained by the established routes to bimodal emission, and the origin appears to be entirely intramolecular and independent of large conformational change. We further observe that the scaffold displays negative thermal quenching (NTQ), with emission rising rather than falling on heating, a property largely restricted to inorganic phosphors and semiconductors.53,54 Together, dielectric and thermal control turn a single rigid molecule into a self-referencing reporter of its environment, reading out both polarity and temperature as color.

Fig. 2. (A) Chemical structures of the pseudo-para PCP compounds reported here, accessed via a one-pot Sonogashira cross-coupling. X-ray crystallographic structures shown as ORTEP representations for (B) asymmetric DPA–CN 5 and (C) symmetric DPA–DPA 2. (D) Normalized UV-vis absorption spectra of the compound library in toluene. (E) Normalized steady-state photoluminescence spectra of the 1–5 in non-polar toluene, establishing the high-energy localized excited state emission baseline.

Fig. 2

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Results and discussion

Synthesis and molecular design

We have previously established that breaking the structural symmetry of the PCP scaffold with modest donor–acceptor pairs, including methoxy and cyano substituents, is sufficient to establish a transannular dipole, unlock CT emission, and induce directional charge transport in the solid state.32 Seminal studies by Bazan and coworkers further demonstrated that pseudo-para D–A cyclophanes sustain highly polarizable through-space charge-transfer states whose energy and oscillator strength depend sensitively on substitution pattern.55,56 Kaiser et al. showed that cyclophane-bridged dyads can support long-lived singlet and triplet charge-separated states.57 We hypothesized that tuning the transannular dipole would afford an equilibrium between this CT state and a locally excited state, resulting in two distinct emissive populations.

To systematically test this hypothesis, we designed and synthesized five pseudo-para-disubstituted [2.2]paracyclophanes via a one-pot Sonogashira cross-coupling (Fig. 2B).32,57 The asymmetric targets feature a strong N,N-diphenylaniline electron donor paired with either a strong 1,3,5-triphenyltriazine acceptor (4) or a weaker cyano acceptor (5). Symmetric TRZ–TRZ (1) and DPA–DPA (2) analogs were synthesized as controls, while the synthesis and photophysical characterization of CN–CN 3 was reported previously.32 Single-crystal X-ray diffraction of 2 and 5 (Fig. 2B and C) confirms a transannular interarene distance of 3.05 Å, indicating no ground-state donor–acceptor association irrespective of the substituents (Fig. S1). Interestingly, DPA–CN 5 packs in a head-to-head manner, with parallel nitrile groups with short intermolecular contact distances of ∼3.5 Å (Fig. S2). Ground-state density functional theory calculations illustrate the spatial separation of the frontier molecular orbitals, with the HOMO localized on the donor unit and the LUMO localized on the acceptor for asymmetric compounds 4 and 5, establishing a pronounced transannular dipole across the PCP core (Fig. S3 and S4).

The photophysical baselines of the compound library were established in non-polar toluene. The UV-vis absorption spectra display characteristic broad π–π* transitions (Fig. 2D). Upon photoexcitation, all five compounds emit a single, high-energy band, and under these non-polar conditions the asymmetric compounds 4 and 5 are indistinguishable from the symmetric controls 1–3, establishing a common LE emission baseline (Fig. 2E; see Section S5 for additional spectroscopic data). The symmetric scaffolds exhibit robust photoluminescence quantum yields (PLQYs) that remain high across a broad dielectric range. In non-polar toluene, compounds 1 and 2 demonstrate efficient emission with PLQYs of 54% and 50%, respectively, increasing to 79% for 1 and 81% for 2 in highly polar monoglyme. The largely polarity-independent PLQYs of the symmetric controls suggest the excited states retain an apolar character typical of a LE π–π* transition, and are largely immune to solvatochromic shifts. Compounds 4 and 5 were measured side by side in the same set of solvents (Fig. 3C and D), and compared to the symmetric analogs (Fig. 3A and B). As the dielectric constant increases, a broad, lower-energy band emerges alongside the fixed high-energy band, and the two respond to solvent in opposite ways: the high-energy band does not shift substantially, while the low-energy band is strongly solvatochromic, red-shifting as polarity increases. For compound 4, this redshift is entangled with an early onset of non-radiative quenching (Fig. 3C and Table S6), so the band's apparent position and intensity confound solvatochromism and quenching within this polarity window. This is in contrast to 5 (Fig. 3D and Table S7), which remains brightly emissive. In other words, a single molecule now carries one emission that ignores its environment and one that reports on it.

Fig. 3. (A–D) Normalized photoluminescence spectra of symmetric controls (A) TRZ–TRZ 1 and (B) DPA–DPA 2, alongside asymmetric donor–acceptor cyclophanes (C) DPA–TRZ 4 and (D) DPA–CN 5 in five representative solvents. 1 and 2 exhibit monochromatic emission, whereas 4 and 5 display bimodal solvatochromism in solvents of intermediate polarity. (E–H) Corresponding Lippert–Mataga plots correlating the Stokes shift (Δν) to the solvent orientation polarizability (Δf) across the complete 16-solvent series. While 1 (E) and 2 (F) demonstrate a single emissive state with minor solvent dependence the asymmetric compounds 4 (G) and 5 (H) resolves into two distinct linear correlations: a LE state (blue) whose shallow slope mirrors the symmetric controls, and a highly polar CT state (red) exhibiting a steep positive slope.

Fig. 3

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The two bands separate cleanly in a Lippert–Mataga analysis. Across the 16-solvent series, the symmetric controls 1 and 2 give a single shallow-sloped trendline in response to solvent polarity (Fig. 3E and F). The asymmetric compounds instead resolve into two lines (see Section S5.5 for deconvolution): a shallow one that tracks the symmetric controls, and a steep one whose large slope marks a substantial transannular excited-state dipole (Fig. 3G and H). The two slopes quantify what the spectra show: a solvent-independent LE state and a solvent-driven CT state coexisting in one molecule.

In order to further understand the origin of this bimodal emission, we systematically performed control experiments to probe excimer formation, aggregation, ground-state conformational heterogeneity, or ESIPT.7,11,58,59 Normalized emission spectra in chloroform are invariant down to 10−8m concentration range, confirming the lower-energy band is an intrinsic intramolecular state (Fig. S35 and S36). Additionally, incremental addition of 1,3,5-triphenyltriazine into symmetric compound 2 yielded no intermolecular charge-transfer band (Fig. S37), further disproving aggregation effects. Photoluminescence excitation spectra monitored at the two emission bands for 4 and 5 were near-identical, suggesting that both bands originate from a common ground-state species and thermalized S1 manifold (Fig. S17 and S18). Having excluded intermolecular origins, we investigated intramolecular photophysical alternatives. The absence of protonation sites likewise precludes ESIPT. Comparing photoluminescence of 4 in solvents of identical polarity but different viscosity affords qualitatively equivalent bimodal emission profiles (Fig. S39). Moreover, Zysman-Colman and co-workers and Fang et al. have reported through-bond analogues in which the DPA donor and the TRZ or CN acceptor are a part of conjugated para-phenyl chromophores, and found no evidence of TICT character.60,61 Finally, solid-state PMMA films of 4 and 5 display dual emission that persists across 1, 5, and 10 wt% concentrations, confirming the phenomenon remains robust in the solid state (Fig. S55 and S57), consistent with the absence of a TICT pathway. Triplet-mediated pathways are excluded on two independent grounds. Room-temperature phosphorescence emits through the forbidden triplet-to-ground transition, while thermally activated delayed fluorescence (TADF) produces a characteristic two-component decay (and only one emission band): prompt fluorescence followed by a delayed tail as the singlet is repopulated by reverse intersystem crossing. Both leave a long-lived signature in the emission decay. We observe neither: the emission of 4 and 5 decays as a single prompt component within τ = 5–15 ns, with no delayed tail (Fig. S53). Time-resolved photoluminescence maps across multiple solvents further confirm the prompt decay dynamics of both emissive states (Fig. S54), with higher polarity conditions showing shorter lifetimes. Triplet-mediated emission is also typically quenched by molecular oxygen, whereas the emission here is oxygen-insensitive (Fig. S27). Taken together, these observations indicate that the bimodal emission arises from the rigid through-space architecture of the PCP scaffold. The mechanistic origin of this dual emission is distinct from the aforementioned examples, and we aim to elucidate and harness this property in future work.

Thermodynamic steering and the energy gap law

The interplay of these two states gives 4 and 5 access to an unusually wide emission range. As solvent polarity increases, the transannular CT state is progressively stabilized, and its emission red-shifts continuously across a wide window while the LE band stays fixed. This continuous tuning is the basis of the color control developed below. At the highest polarities the trend reaches a limit: as the CT state is stabilized to lower energy, the narrowing S1–S0 gap accelerates non-radiative decay through the Energy Gap Law, and the CT emission gradually weakens. In strongly polar solvents such as benzonitrile (ε ∼25, Fig. 3C) and DMF (ε ∼36.7), the low-energy band of DPA–TRZ 4 is no longer observed, and the spectrum reverts to the single high-energy LE band (λ ∼420–450 nm, Fig. S20), an effect governed by the Energy Gap Law discussed below.

The weaker acceptor analog DPA–CN 5 reaches this non-radiative threshold at a higher polarity (Fig. S23). This difference is quantified by PLQY (Tables S6 and S7): 4 is already deep into Energy-Gap-Law quenching across the entire polarity range surveyed, falling from 55% in hexane to just 2% in CH2Cl2, whereas 5 retains a substantially higher PLQY over the same range (52% in hexane, 41% in THF, 22% in CH2Cl2) and only reaches a comparably low value (10%) in benzonitrile. Because the cyano group induces a smaller transannular dipole than TRZ, the CT state of 5 is less deeply stabilized at a given dielectric constant. This allows the intermediate state to remain radiatively competitive further into the polar regime; even in pure benzonitrile (ε ∼25), 5 retains a bimodal profile spanning 410 nm to 600 nm (Fig. 3D). This comparison establishes that acceptor strength controls the polarity threshold at which the Energy Gap Law quenches the lower-energy emission. The complete mapping of 4 and 5 against our 16-solvent series can be found in Section S5.2 in the SI, including CIE 1931 plots that establish colorimetry.

Together, solvent dielectric and donor–acceptor character provide two orthogonal handles on the emission. Continuous solvent titrations map this control across the visible spectrum. As dichloromethane is added to toluene, the emission of 4 moves continuously from deep blue, through yellow–white, to red (Fig. 4A). Compound 5, featuring a cyano-group as acceptor, follows a parallel but less red-shifted path from blue to green (Fig. 4B), reaching warm-white only upon addition of benzonitrile.

Fig. 4. 3D emission spectra and corresponding macroscopic photographs demonstrating the polarity-induced red-shift for (A) DPA–TRZ 4 and (B) DPA–CN 5 upon titration of CH2Cl2 into toluene (PhMe); normalized emission spectra for quantitative solvent titration series of (C) DPA–TRZ 4 titration of CH2Cl2 into toluene, (D) DPA–TRZ 4 titration of THF into toluene PhMe : THF, (E) DPA–CN 5 titration of CH2Cl2 into toluene, and (F) DPA–CN 5 titration of benzonitrile into hexane. (G–J) Corresponding plots of the ratiometric intensity of the CT versus LE states (ICT/ILE) (K–N) Corresponding CIE 1931 chromaticity trajectories. Together, these trajectories highlight the precise colorimetric control and partitioning of the S1 manifold to achieve broadband, single-molecule white-light emission.

Fig. 4

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Three-dimensional spectral overlays and 2D titration profiles reveal the static nature of the high-energy LE band alongside the dynamic emergence and continuous red-shift of the lower-energy band (Fig. 4C–F). The thermodynamic partitioning of excitons between these states is tracked by the monotonically increasing intensity ratios (ICT/ILE) as a function of polar solvent fraction (Fig. 4G–J). By targeting specific intermediate dielectric environments, excitons can be partitioned to achieve specific colorimetric targets, including single-molecule white light (Fig. 4K–N). For 4, fine-tuning the local polarity by titrating THF into toluene (Fig. 4D) similarly modulates the state populations (Fig. 4H). The ICT/ILE ratio increases at first, reaching a maximum at 60% THF, before decreasing at higher polarity due to the energy gap law. A 100% THF broadband mixing of the emissive bands produces chromaticity coordinates of (0.33, 0.32), which coincides nearly perfectly with the pure white-light equal energy point (Fig. 4L). Normalized emission spectra recorded across a tenfold concentration range in this solvent mixture are invariant, confirming this color balance is an intrinsic single-molecule property. Finally, the colorimetric coverage is further expanded by the weaker acceptor 5; modulating hexanes (ε ∼2) with benzonitrile (ε ∼25), shown in Fig. 4F, systematically tunes the excited-state partitioning (Fig. 4J) to sweep the emission coordinates across the white-to-green color spaces (Fig. 4N). In pure benzonitrile, 5 also exhibits almost white-light emission with CIE coordinates (0.33, 0.35). The energy gap law also limits compound 5, though the dielectric range is significantly enhanced, showing how 4 and 5 are complementary ratiometric probes for environmental dielectric sensing.

Negative thermal quenching

Most fluorophores dim when heated, as thermal energy opens non-radiative decay channels. Compounds 4 and 5 do the reverse: they brighten. Across every solvent tested, heating increases the luminescence intensity (Fig. 5A and D), and results in a continuous hypsochromic shift (Fig. S44–S50). The enhancement is oxygen-insensitive, and the fluorescence shows no delayed component, suggesting a mechanism different from TADF. For DPA–CN 5 in triglyme, heating from 25 to 110 °C increases the intensity 2.6-fold; because heating has a negligible effect on the absorption (Fig. S43), this corresponds to a photoluminescence quantum yield of ∼88% at 110 °C (Fig. 5F) compared to 34% at 25 °C (Section S5.9). Additionally, both compounds are thermally stable, showing no decomposition by 1H-NMR after heating in toluene at 100 °C for two hours (Section S4.4).

Fig. 5. Temperature-dependent photoluminescence spectra of the asymmetric cyclophanes (A) DPA–TRZ 4 in triglyme and (D) DPA–CN 5 in benzonitrile, demonstrating a continuous bathochromic shift as the heated solutions (110 °C) cool to 25 °C. (B and E) Corresponding CIE 1931 trajectories tracking the color transitions. Insets: photographs of the emission at heated (left) and room-temperature (right) state for DPA–TRZ 4, and room-temperature (left) and heated (right) for DPA–CN 5 (C and F) PLQY at room temperature versus 110 °C (65 °C for THF and 80 °C for monoglyme) across various solvent environments. The PLQY is calculated from the PLQY at 25 °C, by calculating the ratio of luminescence intensity after heating, as we observe no changes in absorbance in response to temperature.

Fig. 5

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For 4, this enhancement follows the expected thermodielectric response: thermal expansion lowers the solvent dielectric constant, raising the CT-state energy and slowing non-radiative decay through the Energy Gap Law. In triglyme, heating 4 results in a hypsochromic shift from ∼594 to ∼530 nm (Fig. 5A), and a corresponding thermochromic shift from purple at room temperature to yellow–green upon heating. Interestingly, the heated triglyme quantum yield of 4 (∼7.5%) remains below the room-temperature value in chlorobenzene (23%). Compound 5 behaves differently. Its heated triglyme quantum yield of ∼88% exceeds its room-temperature value in any less-polar solvent (66% in anisole), while resulting in color change from white to white–green. We believe this cannot be purely explained through thermodielectric effects, and studies are currently underway to explain the photophysical behaviors of 4 and 5. Practically, the temperature-dependent color shift turns both compounds into self-referencing thermometers, most vividly 4, whose purple-to-green swing encodes temperature directly as a spectral coordinate (Fig. 5B and E) for calibration-free readout.

Conclusion

In summary, a covalently enforced through-space donor–acceptor contact across a [2.2]paracyclophane produces two coexisting emissive states, a locally excited state and a transannular charge-transfer state, from a single molecule. Dual emission of this LE/CT type normally requires a rotatable bond between the donor and acceptor, through which a twisted charge-transfer state forms. Here the second state arises across the rigid transannular gap instead, without bond rotation, labile functional group, or intermolecular contact, making the emission intramolecular, concentration-independent, and free of the torsional non-radiative channel that limits twisted systems. This design yields a broad optical response. Solvent polarity tunes the balance between the two states, sweeping a single compound across the entire visible spectrum and through balanced white light at (0.33, 0.32), with varying acceptors acting as complementary probes across a wide dielectric range. The emission rises on heating, reaching a quantum yield of 88%; this negative thermal quenching, common in inorganic phosphors but rare in discrete organic molecules in solution, turns the accompanying color shift into a ratiometric temperature readout. Building the second emissive state into the architecture, rather than into a reactive group or a conformational coordinate, provides a robust route to stimuli-responsive multicolor emission. The scaffold's planar chirality further opens a path to circularly polarized dual emission, while the vapor response points toward solid-state sensing. The excited-state dynamics governing the temperature response are under investigation.

Author contributions

C. S. and K. D. H. G. conceived the idea. K. D. H. G., P. H., H. M., and J. S. synthesized the compounds. K. D. H. G. and P. H. characterized the steady state. A. R. S. K. and B. R. J. performed all time-resolved measurements and helped inform photophysical understanding. C. S., A. R. S. K., K. D. H. G., and B. R. J. jointly wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

SC-OLF-D6SC06531G-s001

Acknowledgments

C. S. gratefully acknowledges financial support from the American Chemical Society Petroleum Research Fund (PRF #70454-DNI4).

Data availability

The data supporting this article, including additional spectroscopic titrations, crystallographic information, synthetic protocols and characterization, as well as DFT calculations have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6sc06531g.

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Associated Data

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Supplementary Materials

SC-OLF-D6SC06531G-s001
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Data Availability Statement

The data supporting this article, including additional spectroscopic titrations, crystallographic information, synthetic protocols and characterization, as well as DFT calculations have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6sc06531g.

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