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. 2026 Sep 8. Online ahead of print. doi: 10.1039/d6sc04162k

π-Stacked supramolecular contact ion pairs prohibit quenching and provide control of symmetry in fluorescent dyes

Laura Kacenauskaite a, Andrew H Olsson b, Tumpa Sadhukhan b, Krishnan Raghavachari b, Amar H Flood b,✉, Bo W Laursen a,✉
PMCID: PMC13579366  PMID: 42751269

Abstract

Cationic chromophores like cyanines and xantheniums are prevalent fluorescent dyes used for imaging and as probes. In non-polar environments, however, electrostatic interactions drive formation of ion pairs with the accompanying anions that may assemble further into aggregates of ion-pairs, strongly affecting and deteriorating photophysical properties. Here, we investigate contact ion pairs and higher order aggregates of cationic fluorophores by leveraging a symmetry-sensitive amino-triangulenium dye (ATOTA+) and its combination with anion-binding macrocycle cyanostar (CS). Absorption, fluorescence, and NMR titrations in non-polar solvents show that cyanostar binds the dye's counter ions (X−) in a stable 2 : 1 sandwich-like anion complex (CS2X−) that prohibits dye aggregation. Fluorescence anisotropy and energy transfer measurements show that ATOTA+ and the CS2X− anion complex form well-defined and tightly π-stacked contact ion pairs resulting in highly increased fluorescence quantum yield and lifetime. These effects are found to be general. Addition of cyanostar to six cationic dyes (rhodamine, cyanine, oxazine, triangulenium and styryl type chromophores) in benzene solutions and polystyrene thin films improves photophysical properties assigned to the supramolecular contact ion-pairs formed between dyes and the cyanostar–anion complex.

In low polarity solvents cationic dyes form contact ion pairs and aggregates that are quenched, addition of anion receptor leads to supramolecular ion pairs with high quantum yields and controlled symmetry.graphic file with name d6sc04162k-ga.webp

Introduction

Cationic chromophores are ubiquitous in colorants, fluorescent dyes, and probes due to their unique electronic structure enabling strong and tunable redshifted electronic transitions in relatively small conjugated systems.1–3 Cyanines, triarylmethyliums, xantheniums, and acridiniums are all cationic π-systems with an odd number of sp2 centers. In practical applications they are prevalent among fluorescent dyes used in challenging bioimaging applications, as well as probes, photosensitizers and laser dyes.4–8 With an intrinsically cationic π-system, electrostatic interactions with the environment are consequential and always need to be considered. For example, electrostatic interactions are a key element in the assembly of cationic fluorescent DNA probes with anionic DNA.9–11 In low polarity conditions formation of contact ion pairs (CIPs) between the dyes and available anions is a fundamental consequence that, to a large extent, defines the optical and photophysical properties of the cationic dyes. In many cases, CIPs can cause complications from enhanced aggregation, quenching, and distorted spectral and photophysical properties. However, the strong electrostatic interactions between ions in low polarity conditions also opens up the possibility of actively using CIP formation to obtain specific properties. Selection of anions with different sizes has, for example, been used to modify the association of dyes, favoring either monomer, dimer, or excimer as the emitter.12 The size and charge density of the anion can be used to modulate the strength of the electric field imposed on the cationic dye in CIPs.13 This effect has been used to modulate charge transfer (CT) complexes with cationic acceptors in bistable rotaxanes.14 For NIR-emissive heptamethine dyes, CIP formation with small hard anions break the perfect cyanine symmetry (with no bond length alternations) and leads to asymmetric CIPs with a dipolar structure, increased bond length alternation, and broad CT transitions.15,16 However, introduction of large soft anions (like TRISPHAT and fluorinated tetraphenylborate) reinstates the symmetric cyanine structure in the CIP and turn on sharp and intense transitions, event in non-polar conditions where CIPs are favored.

These effects have been used for the optimization of NIR and SWIR cyanine dyes in non-polar perfluorocarbon nano- and micro-droplets for bioimaging,17 wherein use of large soft anions improves both molar absorption coefficients and fluorescence quantum yields.18 For threefold symmetric triarylmethylium dyes (like crystal violet), symmetry breaking by solvent dipoles and by anions has for a long time attracted a significant research interest and controversy.19–24 In low polarity conditions it now seems well established that the observed splitting of the absorption spectrum is caused by formation of low symmetry CIPs between the dye and its accompanying anion, breaking orbital degeneracy. The size of the anion has also been used to modulate the transient interconversion between ground state CIP and excited state CIP with different location of the anion and thus different fluorescence properties.25,26 CIPs, rather than isolated dyes, must also be considered when cationic dyes are embedded in low polarity polymers for thin-film applications or in dye-loaded polymer nanoparticles for bioimaging.27,28 In recent years, the importance of CIPs has been clearly demonstrated by the tremendous improvements in brightness and FRET efficiency for dye-loaded polymer nanoparticles obtained by Klymchenko and co-workers through introduction of large fluorinated counterions and engineering of polymer polarity.29–32

We recently reported a universal and highly reproducible method to arrange cationic fluorophores into well-defined charge-by-charge structures using the cyanostar (CS) macrocycles as an anion receptor to bind the anion (X−) in a large disc-shaped 2 : 1 anion complex (CS2X−).33 This supramolecular anion complex then acts as a structure directing unit, forming an isolating lattice around the cationic dyes in materials called small-molecule ionic-isolation lattices (SMILES, Fig. 1G). In SMILES, dyes are electronically decoupled and their attractive spectral properties found in dilute polar solution are reinstated in solids.33–36 It has also been found that addition of cyanostar to cationic dyes enables high loading into non-polar polymers without the observation of aggregation-caused quenching (ACQ). For SMILES, as well as for all other materials based on cationic dyes (crystals, thin films, nanoparticles and blends in low polarity polymers), we propose that the fundamental building blocks determining the packing and optical properties is a CIP between dye and anion (or anion complex; CS2X−) as illustrated in Fig. 1B and F. The large CS2X− anion complex, however, is structurally different from all the other large anions previously applied to modify dye CIPs, where the variation has been the anion size, from small hard chloride to large soft fluorinated tetraphenylborates. The CS2X− anion complex has the negative charge placed in the center of a large flat π-surface able to form close π–π contacts with the cationic π-systems of the dyes, while at the same time allowing strong coulombic interactions due to short distance between positive and negative regions in the dye and anion complex.

Fig. 1. Schematic representation of cationic fluorophores and their accompanying anions (X−) in various environments without (A–C) and after (E–G) addition of two molar equivalents of anion binding receptor cyanostar, CS (D). Arrows indicate dominant species in non-polar solvents with and without CS. (G) Charge-by-charge packing of cationic dye and anion complex characteristic for SMILES (Small Molecule Ionic Isolation Lattices) crystal structures.

Fig. 1

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Here we investigate the formation, structure, and optical properties of supramolecular CIPs formed between cationic dyes and the cyanostar anion complex (CS2X−) in low polarity solvents and in polymer matrixes. We find that the simple addition of cyanostar to solutions of cationic dyes leads to efficient formation of well-defined π–π stacked supramolecular CIPs. This special CIP structure allows, for the first time, symmetry conservation of triarylmethylium type dyes. The CIPs also become a means to control dye solubility in low polarity solutions, avoid ACQ, reinstate high quantum yield and, raise the predictability of photophysical properties in non-polar solvents and polymers that are otherwise incompatible with these dyes.

Results and discussion

Selection of symmetry-sensitive fluorophores

To study the structure and photophysical properties of the fundamental ion pairs between cationic dyes and cyanostar anion complexes we turn to the D3h symmetric and highly fluorescent amino-trioxatriangulenium chromophore (ATOTA+, Fig. 2).37,38 To ensure solubility in even very non-polar conditions, we selected a derivative with two dodecyl chains.39 Even though the formal symmetry is broken by this substitution pattern we find that it has a minor impact on the symmetry of the chromophore itself (Fig. S1). ATOTA+ was previously shown, both theoretically and experimentally, to be extremely sensitive to the placement of the anion with respect to the cationic core (Fig. 2).13,37 In solvents of medium polarity like dichloromethane (DCM, εr = 9) and acetonitrile (εr = 37), which help ensure independent solvation of both anion and cation at µM to mM concentrations (Fig. 1A), the positive charge on the ATOTA+ core is delocalized symmetrically and results in a degenerate absorption transition, represented by a single absorption peak around 470 nm (Fig. 2A). However, as the polarity of the solvent decreases, anion and cation form tight ion pairs or even higher order aggregates rather than individually solvated ions (Fig. 1B and C). In benzene solutions, ion pair formation is favored by the low polarity (εr = 2.3),40 but higher order π-aggregates are disfavored by the relative high polarizability of the aromatic solvent. As the anions in the ion pairs tend to locate in an asymmetric position relative to the dye, the high local electric field in the ion pair breaks the symmetry of the orbitals and electronic transitions in ATOTA+ (local Stark effect).41 This breaking of degeneracy splits the absorption band into two characteristic peaks (around 450 nm and 480 nm. Fig. 2B and S2). In this way the shape of the absorption spectrum becomes diagnostic for formation of asymmetric ion pairs. In this study we selected DCM and benzene as the general solvents since we found that DCM in all cases of dilute solutions provides free (non ionpaired) dyes, benzene on the other hand favors the isolated CIPs, while less polar solvents such as methylcyclohexane leads to aggregation of CIPs.

Fig. 2. Schematic representation of photophysical properties and structure of ATOTA·PF6 in different polarity solvents. (A) – in dichloromethane as free ions, (B) – in benzene as asymmetric ion-pairs. R1 = n-dodecyl.

Fig. 2

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Addition of anion receptor to dye solutions

Fig. 3 shows changes in the absorption, emission, and excitation spectra of ATOTA·PF6 as cyanostar is added to benzene and dichloromethane solutions. As expected from previous results,37,41 two absorption peaks are clearly observed for ATOTA·PF6 in benzene solution (black spectrum, Fig. 3A), showing that ATOTA+ is present as an asymmetric ion pair with its PF6− counterion (Fig. 2B). As cyanostar is added, changes in the absorption spectra can be seen both in regions where cyanostar (300–400 nm) and ATOTA+ (400–500 nm) absorb. As the relative concentration of cyanostar increases, the two absorption peaks of the ATOTA ion pair merge into one absorption peak at around 480 nm, indicating the ATOTA+ cation is no longer in a symmetry-breaking environment. Once the amount of cyanostar reaches ∼2–2.5 molar equivalents (with respect to ATOTA·PF6), no further significant changes in 400–500 nm region can be seen. The equivalence point is in good agreement with all previous studies of cyanostar binding, where two molar equivalents form a very stable 2 : 1 sandwich-like complex with the PF6− anion (CS2PF6−).33,42–44 These changes in the absorption spectra are largely reproduced in the excitation spectra (Fig. 3A, inset). Importantly, the presence of a cyanostar peak (at 320 nm) in the excitation spectra shows that excitation of the cyanostar-anion complexes (CS2PF6−) leads to emission from the ATOTA+ dye. This observation proves energy transfer from the anion complexes to ATOTA+,35,36 which must occur within a ∼50 Å proximity consistent with ion pair formation. Thus, while the formation of the CS2PF6− anion complex has restored the 3-fold symmetry of the ATOTA+ dye, this restoration does not originate from dissociation of the ion pair.

Fig. 3. ATOTA+ as a probe for ion pair formation and symmetry in apolar solvents. (A) Changes in the ATOTA+ absorption spectra in benzene as a function of increasing CS concentration. Insert shows excitation spectra (em. 540 nm) of the same solutions. (B) Fluorescence spectra (ex. 355 nm) of ATOTA+ in benzene at various CS : ATOTA+ ratios, showing energy transfer between cyanostar and ATOTA+. Insert show changes of the emission intensity of the ATOTA+ with various concentrations of CS in benzene after direct excitation of ATOTA+ (450 nm) and indirect excitation via cyanostar (355 nm). (C) Changes in the ATOTA+ absorption spectra in dichloromethane as a function of increasing CS concentration. Insert shows excitation spectra (em. 540 nm) of the same solutions. (D) Fluorescence spectra (ex. 355 nm) of ATOTA+ in dichloromethane at various CS : ATOTA+ ratios. Insert show changes of the emission intensity of the ATOTA+ with various concentrations of CS in dichloromethane after direct excitation (450 nm) and excitation of cyanostar (355 nm). Emission intensities are corrected for the changes in absorption after addition of cyanostar when exciting at 450 nm.

Fig. 3

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The behavior seen in benzene is contrary to that observed in DCM at these µM-level concentrations (Fig. 3C). In the more polar solvent (DCM), the only change in absorption spectra is the increasing cyanostar peak at 320 nm, while the excitation spectra (Fig. 3C, inset) are unchanged upon addition of cyanostar. These observations confirm the absence of energy transfer, and thus of any ion pairs, solvent separated or otherwise, even though CS2PF6− anion complexes are formed under these conditions (Fig. S3). The presence of freely solvated cations and anion complexes in DCM solutions (Fig. 1A and E) is also confirmed by the invariant fluorescence emission spectra (Fig. 3D).

For benzene solutions, the fluorescence intensity from ATOTA+ upon excitation into the cyanostar band (355 nm) increases as function of added cyanostar until the expected 2 molar equivalents (Fig. 3B, inset) are reached, in agreement with formation of 2 : 1 CS2PF6− anion complexes and their association with ATOTA+ as ion pairs. Excess cyanostar does not contribute significantly to the fluorescence from the ATOTA+ species in benzene, since these excess neutral cyanostar molecules do not associate with the dyes. Furthermore, small fractions of quenched (ATOTA·PF6)n ion pair dimers (n = 2) and higher order aggregates (n > 2, Fig. 1C)13 are also suppressed by the addition of cyanostar, pushing the equilibria towards the ion pair with the anion complex (Fig. 1F). This effect is most clearly seen for extremely non-polar solvents, where aggregates (Fig. 1C) are the dominating species in the absence of cyanostar. For example, addition of cyanostar to methylcyclohexane solutions (εr = 2.0), breaks up these aggregates and yields spectra practically identical to those in benzene (see Fig. S4 and S5).

Analysis of contact ion pair formation by NMR and UV-vis titration

The assembly of the ion pairs between CS2PF6− and ATOTA+ was also studied by NMR titration in benzene with 5% chloroform added to enhance solubility at the higher concentrations (0.25 mM). In a titration (Fig. 4A) monitoring the addition of ATOTA+ as the PF6− salt to the cyanostar we observe direct formation of the ATOTA·CS2PF6 ion pairs with saturation in peak shifts and intensities at the 0.5 equivalents expected of the 2 : 1 stoichiometry between cyanostar and PF6−. The slow-exchange cyanostar-based peaks decrease intensity from 0 to 0.42 equivalents while the peaks for the anion complex CS2PF6− grow in over this range. Diastereomeric peaks are observed in key protons (e.g., Hb and Hc) and its origin is well documented to stem from the π-stacking of the two cyanostar macrocycles in the anion complex. Consistently, protons for the ATOTA+ cation grow in and display a single set of positions until ∼0.5 equivalents. Thereafter, the peaks shift towards the positions seen for a solution of the ATOTA·PF6 alone. In these low-polarity solutions, ATOTA+ is present as ion pairs either with CS2PF6− or PF6−. The inverse titration shows consistent results (Fig. 4B). Upon addition of cyanostar, the ATOTA+ peaks all shift to the positions seen for the ATOTA·CS2PF6 ion pair. For instance, HA’ resonates at 5.86 in the ion pairs formed by titration in either direction. The cyanostar-based peaks also saturate in intensity beyond 2 equivalents and are seen at the same positions as in the forward titration.

Fig. 4. NMR titration data showing formation of the ATOTA·CS2PF6 ion pair species. (A) Forward titration showing CS (0.25 mM) upon addition of ATOTA·PF6. (B) Reverse titration showing ATOTA·PF6 (0.25 mM) upon addition of CS. (600 MHz, 5 : 95 CDCl3:benzene-d6, dilution corrected).

Fig. 4

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The peak positions provide insight into the chemical environments that change during titrations. Characteristic of the π-stacked cyanostar dimer in the anion complex CS2PF6−, we see the diastereomer signals of the meso (MP) and chiral (MM/PP) combinations of the M or P cyanostar macrocycles. The inner CH-hydrogen bonding protons (Ha, Hd) are both shifted downfield upon anion complexation while the upfield shifts in the outer protons signify the dominance of the π-stacking environment. Similarly, all aromatic signals (e.g., HA) for ATOTA+ are shifted upfield by ∼0.6 ppm when the PF6− anions are converted into CS2PF6− indicative of forming the π-stacked contacts with cyanostar. The methylene signals on the decyl chains and all the N-methyl signals (HB) are also observed to respond to the ring current of the cyanostar π-system with shifts of ∼0.5 ppm. The methylene protons located further away from the cation's core, HC and HD, show smaller shifts but HE does not shift at all, consistent with the effect of proximity.

To quantify the ion-pairing behavior of ATOTA·CS2PF6 in benzene, a UV-vis titration was performed by adding cyanostar to ATOTA·PF6 in benzene (Fig. S7). The UV-vis titration was subject to an equilibrium-restricted factor analysis45 (Fig. S8) to determine the equilibrium constants and most prominent equilibria present in solution. Variable concentration studies confirm the ATOTA·PF6 in benzene is ion-paired under the conditions of the titration. We found that the best model involved direct formation of the ATOTA·CS2PF6 ion pair upon addition of cyanostar following the equilibrium below:2CS + ATOTA·PF6 ⇌ CS2PF6·ATOTA

The extinction coefficient spectra and speciation curves obtained from the factor analysis of the titration data agrees with the three main species (Fig. S8). The analysis returns a stability constant for forming the ATOTA·CS2PF6 ion pair of log β = 14 and corresponds to ΔG211 = −80 kJ mol−1. This value surpasses the binding affinity of cyanostar for large ions in more polar solvents (ΔG2:1 = −70 kJ mol−1, 40% methanol v/v in dichloromethane) and is thus consistent with enhanced electrostatic stabilization in low polarity conditions.46

Energy transfer in supramolecular contact ion pairs

Cyanostar itself is weakly fluorescent and this feature provides additional means to investigate the energy transfer between cyanostar and ATOTA+ in the supramolecular ion pair ATOTA·CS2PF6. Fig. 5A shows changes of cyanostar fluorescence intensity (ex. 355 nm, em. 420 nm, spectra in Fig. S9) in benzene and dichloromethane as a function of adding ATOTA·PF6 to the respective solutions. In dichloromethane solution, fluorescence intensity increases by a factor of two as the anion-binding cyanostar macrocycles form the more rigid CS2PF6− anion complexes47 and thereby limits non-radiative relaxation pathways in cyanostar. In benzene solutions, cyanostar emission is nearly completely quenched when the molar ratio between ATOTA·PF6 and cyanostar reaches 0.5 (=1 : 2 stoichiometry matching ATOTA·CS2PF6). However, it seems that only around 45% of the excitations into the CS2PF6− complex are transferred to ATOTA+. This excitation energy transfer efficiency (EET%) can be derived from the difference in ratio between cyanostar and ATOTA+ peaks in the absorption (Fig. 3A) and excitation spectra (Fig. 3A insert), and is shown in Fig. 5B. By correcting for absorption of non-binding cyanostar macrocycles present in excess beyond 2 equivalents, constant excitation energy transfer efficiency throughout different CS : ATOTA+ ratios are found. Furthermore, identical EET efficiency can be observed throughout a broad range of concentrations (Fig. 5C and S10), verifying that the ATOTA·CS2PF6 ion pairs are stable in this concentration range (20 nM to 20 µM) in benzene and rules out dissociation of ion pairs as the reason for the partial energy transfer. In dichloromethane, energy transfer is not observed at any concentration (Fig. 5C), confirming that ion pairs are not formed and that individual ions are well solvated and do not interact at the investigated concentrations.

Fig. 5. Photophysical properties of ATOTA+ in solvents with different polarity as a function of CS concentration. (A) Cyanostar emission intensity (ex. 355 nm, em. 420 nm) at different ATOTA+ : CS ratios in benzene and dichloromethane, (B) EET efficiency between cyanostar and ATOTA+, (C) EET efficiency between cyanostar and ATOTA+ at CS : ATOTA+ = 2 as a function of ATOTA+ concentration, (D) rotational correlation time of ATOTA+ in benzene solution (µM range).

Fig. 5

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Further insight into the ion pair association is obtained by measurement of the rotational correlation time (θ) using time-resolved fluorescence anisotropy (see Section S12 for details). Fig. 5D show the increasing average rotational correlation time θ of the ATOTA+ emitters as the equivalents of cyanostar are increased in benzene. The initial asymmetric ion pair with PF6− (Fig. 1B) display a rotational correlation time θ = 0.3 ns. As the amount of cyanostar reaches two molar equivalents (with respect to ATOTA·PF6), the rotational correlation time plateaus at θ = 0.8 ns, indicating that the size of the emitter does not increase any further, and that excess cyanostar macrocycles mostly stay in solution independently from the formed ATOTA·CS2PF6 ion pairs, in agreement with the energy transfer measurements (Fig. 5B). The fluorescence anisotropy decay probes the rotational motions of the ATOTA+ fluorophore, and the observed 2.4 times increase clearly show that, in benzene solution, the ATOTA+ dye does not rotate independently of the anion complex (CS2PF6−), suggesting that the dominating species is indeed a contact ion pair (CIP). The increase of rotational correlation time also agrees well with the increase in overall mass/size upon formation of an ATOTA·CS2PF6 CIP, as it should increase around 3 times, based on the molar mass of the individual components (MATOTA·PF6 = 811 g mol−1, MCS2 = 1832 g mol−1). It might be unreasonable to expect proportional increase in rotational correlation time for the CIP, as that does not account for non-spherical shape of the molecules and would imply completely stationary arrangement of the individual components in ATOTA·CS2PF6 CIP, which is most likely not the case in liquid solution. However, the Stokes–Einstein's equation predicts that the 0.8 ns rotational correlation time corresponds to a volume of around 3.2 nm3 (see Section S12 for more details), in good agreement with the typical volume of one cationic dye and one cyanostar anion complex (CS2X−) found in single crystal SMILES lattice 3.5–4.0 nm3.33 We also find it in a good agreement with the diffusion coefficient measured by NMR methods, which establishes an average size of 4.8 nm3 from the Stokes–Einstein equation (see Section S13 and Fig. S12).

Structure of the contact ion pairs

Based on the spectroscopic evidence it can be concluded that 2 : 1 anion complexes (CS2PF6−) and their contact ion pairs with ATOTA+ (ATOTA·CS2PF6) are strongly favored in apolar solvents (like benzene, ε = 2.3, and methylcyclohexane, ε = 2.0) in a broad concentration range. With this in mind, the characteristic spectrum of symmetric ATOTA+ in the CIP with CS2PF6− (Fig. 3A) can only be rationalized with a face-to-face geometry where the anion complex and negative charge is placed above the cation center of ATOTA+ (see Fig. 1F), thus reinstating 3-fold symmetry and degenerate S0→S1 transition.

This interpretation of the spectral data was further supported by DFT calculations. Geometry optimizations were conducted using a functional to help account for dispersion effects, B3LYP-D3BJ/6-31G(d), and with an implicit solvation model to represent the behaviors in benzene and dichloromethane. The ATOTA+ cation was truncated to N-methyl substitutions and in cyanostar, the tert-butyl groups were replaced with hydrogen atoms. The calculated geometry of the ATOTA·PF6 ion pair, in benzene suggests that the anion is positionally delocalized over 38 sites (Fig. S13), only two of which are symmetrically located. This finding is consistent with the analysis of the solvent-dependent spectroscopy (vide supra) and refines prior theoretical studies showing how the asymmetric CIP display distinct localization of HOMO and HOMO−1 orbitals relative to the anion position13,37 by accounting for the configurational entropy (Table S1) in the anion's possible positions. In the case of the ATOTA·CS2PF6 CIP in benzene, the optimized geometry shows (Fig. S14) that the ATOTA+ is centrally located over the 2 : 1 CS2PF6− anion complex. No detailed conformational search was performed for the ATOTA·CS2PF6 CIP, however considering the size and structure of the anion complex it is hard to envision stable conformations with a very different relative placement of the negative charge, as is the case of the ATOTA·PF6 CIP. TD-DFT studies of the electronic transitions (Table S2) are also consistent with the spectroscopic signatures of the various ion pairs: upon formation of the ATOTA·CS2PF6 CIP in benzene, the two transitions become quasi-degenerate and are only 0.03 eV apart. This degeneracy is also seen in the spectra and TD-DFT results for the bare ATOTA+ cation, as present in dichloromethane solution.

The face-to-face stacking of ATOTA+ and CS2PF6− in the CIPs resembles the structural motifs found in the previously reported single crystal structures of SMILES materials, where the cationic dyes in all cases were found in similar positions (though with various angles and offsets that most likely results from crystal packing effects).33,48 Interestingly, in the ATOTA·CS2PF6 CIP the ATOTA+ chromophore obtains even more perfect electronic symmetry than as a free cation in DCM. This outcome is witnessed by the narrowing of the absorption band from 2150 cm−1 FWHM in DCM to 1570 cm−1 in benzene where it becomes a near-perfect mirror image of the fluorescence spectrum (Fig. S15). This observation supports the much debated hypothesis about symmetry breaking in crystal/ethyl violet and similar 3-fold symmetric dyes by anions or solvent dipoles.19–24 The contact ion pairs with CS2X− thus represents a unique case where 3-fold symmetry can be fully established in solution for the first time. This high symmetry is only possible due to the special co-facial center-to-center structure favored in the CIP with CS2X− and the absence of solvent dipoles.

One attractive feature of the aforementioned SMILES materials is their ability to retain solution-like spectral properties and high fluorescence quantum yields of cationic dyes in apolar polymers.33 We hypothesize that this feature exactly stems from the CIP formation between dyes and cyanostar anion complexes, increasing solubility, partially restricting rotational freedom in flexible fluorophores, prohibiting dye aggregates with dye–dye contact and thus eliminating concentration quenching (ACQ). By studying ATOTA·PF6 with and without cyanostar in polystyrene thin films we are now able elucidate this mechanism of suppressed quenching and gain more insight on the detailed structure of the ion pairs between dyes and cyanostar anion complexes.

In the polymer matrix molecular tumbling is prohibited and we can use fluorescence anisotropy to further study the relative orientation of electronic transitions and thus the molecular components in the CIPs. Fig. 6A and B show the steady-state fluorescence anisotropy (r) of ATOTA·PF6 and ATOTA·CS2PF6 in polystyrene thin films, superimposed on the corresponding excitation and emission spectra. In polystyrene films, like in benzene solution, ATOTA·PF6 display the split excitation spectrum characteristic of the asymmetric ion pairs. The model of symmetry breaking by the anion (Fig. 2)37,41 is also confirmed by the steady state fluorescence anisotropy measured at different excitation wavelengths (Fig. 6A), showing that the two peaks (450 and 480 nm) originate from two separate, partially overlapping but perpendicular transitions. Theoretical values for fully separated and perpendicular transitions would be r = 0.4 and r = −0.2, but it is hardly achievable in practice as the two transitions partially overlap and we find values of r450 ≈ 0.0 and r480 ≈ 0.32.41,49

Fig. 6. (A–D) – Steady-state fluorescence, excitation and anisotropy spectra in polystyrene thin films (dye concentration ∼6 µM) of different fluorophores with and without two molar equivalents of CS. (E) − structure of DIOC2. (F) – Steady state anisotropy changes as a function of concentration of DIOC2·CS2PF6.

Fig. 6

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By contrast, the spectra acquired for ATOTA·CS2PF6 in polystyrene (Fig. 6B) closely resemble the single peak spectra seen in benzene solution. Most interestingly, now the fluorescence anisotropy is low, r ≈ 0.1, no matter the excitation wavelength (Fig. 6B). The value of r = 0.1 is exactly the value expected for a fluorophore where absorption and emission transitions are degenerate in the same plane (e.g. D3h symmetry49,50), confirming the placement of the CS2PF6− anion complex on the center of the ATOTA+ dye in the CIP. Furthermore, we also find r ≈ 0.1 when the CS2PF6− anion complex is excited (at 355 nm, Fig. 6B) and fluorescence is emitted from ATOTA+, after energy transfer. First, this confirms EET and thus CIP formation, and second, it shows that the CS2PF6− complex must be parallel with the ATOTA+ dye in the CIP (as sketched in Fig. 1F).

As a control for these anisotropy results we measured similar data for the non-degenerate linear cyanine chromophore DIOC2 (Fig. 6E). We obtained r = 0.4 independent of whether the dye was present as an ion pair with either PF6− (Fig. 6C) or with CS2PF6− (Fig. 6D). This observation is in agreement with the transition dipole direction not being susceptible to the location of the anion (PF6−) or anion complex (CS2PF6−). However, excitation at 355 nm into the CS2PF6− complex drops anisotropy of the DIOC2 fluorescence to r ≈ 0.1 exactly as expected for the CIP (the linear DOIC2 chromophore sensitized by excitation of the 2D degenerated CS2PF6− complex in a co-facial parallel orientation).

To ensure that the anisotropy measurements are not influenced by long range energy transfer between ion pairs, we investigated the anisotropy of DIOC2·CS2PF6 in polystyrene films as function of increasing dye concentration (Fig. 6F). As the concentration of DIOC2·CS2PF6 increases above ∼10 µM (average distance between fluorophores ∼12 nm), anisotropy value drops. At this point, distances between molecules are approaching the range where homo-FRET between the dyes becomes non-negligible leading to anisotropy loss. As the concentration reaches ∼100 mM, anisotropy becomes completely scrambled and approaches r = 0, as energy migration between the dyes with different orientations becomes very efficient.

Cyanostar CIPs as a general solution to ACQ of cationic dyes

Based on the studies of ATOTA·PF6 and DIOC2·PF6 with cyanostar we can now conclude that well-defined CIPs of cationic dyes and cyanostar-anion complexes form into π-stacked parallel co-facial arrangements and are highly stable in low polarity solvents and polystyrene matrixes in the µM to mM range. These properties of the supramolecular cyanostar CIPs suggest that the persistent packing of various cationic dyes with the cyanostar-anion complexes in solid state SMILES structures is secured by the high stability of the CIP making this the actual building block of SMILES materials. To investigate if cyanostar CIPs in general provides these enhanced photophysical properties to cationic dyes in non-polar solvents and polymers, we screened the photophysical properties of six cationic dyes representative of the major classes of cationic dyes (Fig. S16) in dilute (below 15 µM) dichloromethane and benzene solutions, and in polystyrene thin films with and without two molar equivalents of cyanostar. Their quantum yields and fluorescence lifetimes are compiled in Table 1. Extended table including radiative and non-radiative decay rates can be found in SI (Tables S3 and S4).

Table 1. Fluorescence lifetimes and quantum yields of various cationic fluorophores in dichloromethane, benzene and polystyrene, with and without addition of two molar equivalents of CS. Green fluorescence lifetime entries indicate monoexponetial decay fits. Yellow fluorescence lifetime entries indicate two-exponential fluorescence decay with second component of < 15% of counts. Red fluorescence lifetime entries indicate multiexponential fluorescence lifetime decays where second and third component comprises more than 20% of total counts. General structure of the fluorophores (yellow), their anions (red, X−) and cyanostar (black) are shown.

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For dyes without cyanostar, measurements in dichloromethane solution all resulted in mono-exponential fluorescence lifetimes (indicated by green numbers) in accordance with homogeneous non-aggregated solutions of separated ions (column 1, Table 1). Lifetimes and quantum yields agreed with literature values. However, in benzene and polystyrene solutions without cyanostar we find clear signs of aggregation and quenching for all the cationic dyes (Table 1, column 2 and 3). This interpretation is most evident from the fluorescence lifetimes that are non-mono-exponential (indicated by red numbers) testifying to the presence of multiple emissive species (aggregates). Significant reductions in quantum yields and fluorescence lifetimes are observed for all dyes except for the flexible cyanine and styryl dyes (DIOC2 and LDS751). Therein, quantum yields and fluorescence lifetimes are higher than in dichloromethane due to rigidization and reduced rates of non-radiative deactivation in the aggregates and in solid polystyrene matrix.

For dyes with 2 molar equivalents of cyanostar added, photophysical data observed in DCM (Table 1, column 4) were identical to those without cyanostar, in agreement with freely dissolved dyes not interacting with anion complexes. In benzene solution however, dramatic effects of cyanostar are observed, as all dyes now display non-aggregated properties with mono-exponential fluorescence lifetimes. Quantum yields and lifetimes are now similar or higher than those in dichloromethane. In solid polystyrene the same trend is observed, though with some dyes showing minor second components in the fluorescence decays (yellow lifetimes), indicating some degree of inhomogeneity in the solid matrix. Since lifetimes and quantum yields in all cases are higher in the polymer matrix than in both DCM and benzene we rule out dye aggregation as the main course and suggest local variations in the rigidity of the polymer matrix impact non-radiative decay rates. A clear indication of CIPs formation is the large increased quantum yields and lifetimes of the flexible dyes DIOC2 and LDS751 in benzene solution (relative to solutions in DCM), proving that conformational changes in these dyes is strongly inhibited by the close contact to the anion complex. Further rigidification in polystyrene only adds moderate additional improvements, highlighting how large this effect is in the CIP already in solution. Comparison of all the spectral and photophysical data leads us to conclude that CS2·X− anion complexes efficiently prohibits ACQ of the cationic dyes and has no negative impact whatsoever on lifetimes or quantum yields. The special co-facial π–π stacked CIP formed between the cyanostar anion complex (CS2X−) and cationic dyes consistently across different dyes classes constitutes a unique entity for studying and using cationic dyes in low polarity solvents and polymers and as a building block for optical materials, such as SMILES. We expect the anti ACQ effect to be functional in all environments where the CS2·X− anion complexes are favored.42 This include also quite polar solvents such as MeOH/DCM mixtures with effective dielectric constants in the range of εr ≈ 20, in such system aggregation is however only expected at much higher concentrations. We expect that large cationic dyes at high concentrations will be less protected from ACQ when the dyes become larger than CS. Recent studies of SMILES nanoparticles of Cy7 and Cy9 NIR dyes seems to support this.51 More complex supramolecular CIPs made from dyes with 2+ charge and/or anion complexes with 2− charge can be envisioned, both as building blocks for advanced optical materials and as discrete entities in the solution state.52

Conclusion

Addition of cyanostar to cationic dyes in non-polar solutions leads to the formation of contact ion pairs, CIPs, with unique properties. The large π-surface and central negative charge of the supramolecular anion complex CS2·X− favors tight face-to-face contact with the cationic dyes. These well-defined CIPs improve solubility and prohibit aggregation to reinstate intrinsic spectral and photophysical properties of the dyes. In this way, the addition of cyanostar leads to highly improved fluorescence quantum yields and lifetimes in low polarity solutions and polymers. In the π–π stacked supramolecular CIP, the CS2X− anion complex is parallel to the dye and acts via excitation energy transfer (EET) as an antenna for enhanced brightness under UV excitation. The π–π stacking of dye and anion complex in the CIP is so tight that it provides a steric barrier for structural relaxation in flexible dyes, like DIOC2, leading to reduced rates of internal conversion and highly improved emission quantum yields and lifetimes, not otherwise possible in liquid solution. The unique structure of the cyanostar CIPs, for the first time, allows formation and study of cationic dyes with threefold symmetry, like crystal violet (D3) and ATOTA+ (D3h), by conserving the electronic symmetry and degeneracy of these dyes. Finally, this study provides a fundamental insight into the structural and electronic properties of the dye – cyanostar anion complex (CS2X−) CIP that is the fundamental building block of SMILES materials.

Author contributions

L. Kacenauskaite – investigation (sample preparation, UV-vis and fluorescence spectroscopy), writing original manuscript draft. A. H. Olsson – investigation (UV-vis and NMR titrations), T. Sadhukhan – investigation (quantum chemical calculations), K. Raghavachari – supervision (quantum chemical calculations), A. H. Flood – supervision, and funding acquisition, B. W. Laursen – conceptualization, supervision, funding acquisition, project administration. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Abbreviations

ACQ

Aggregation-caused quenching

ATOTA+

Amino-trioxatriangulenium

CIP

Contact ion pair

CS

Cyanostar

DCM

Dichloromethane

DFT

Density functional theory

DIOC2

3,3′-Diethyloxacarbocyanine

EET

Excitation energy transfer

FWHM

Full width at half maximum

QY

Quantum yield

SMILES

Small molecule ionic isolation lattices

TD-DFT

Time dependent density functional theory

Supplementary Material

SC-OLF-D6SC04162K-s001

Acknowledgments

The work was supported by the Novo Nordisk Foundation (NNF20OC0062176) and National Science Foundation (DMR-2118423).

Data availability

Experimental data are available from corresponding author upon reasonable request.

Supplementary information (SI): Materials and methods section, spectra of different ATOTA+ derivatives, spectroscopic investigation of CS binding anions, NMR titrations, characterization of ATOTA+ solubility, factor analysis, calculations of rotational correlation times, DFT calculations, structure of cationic fluorophores, spectroscopic properties of ion pairs in different environments. See DOI: https://doi.org/10.1039/d6sc04162k.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

SC-OLF-D6SC04162K-s001
SC-OLF-D6SC04162K-s001.pdf (1.6MB, pdf)

Data Availability Statement

Experimental data are available from corresponding author upon reasonable request.

Supplementary information (SI): Materials and methods section, spectra of different ATOTA+ derivatives, spectroscopic investigation of CS binding anions, NMR titrations, characterization of ATOTA+ solubility, factor analysis, calculations of rotational correlation times, DFT calculations, structure of cationic fluorophores, spectroscopic properties of ion pairs in different environments. See DOI: https://doi.org/10.1039/d6sc04162k.

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