Ethylene-Bridged Oligo-BODIPYs: Access to ... - ACS Publications

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Cite This: J. Am. Chem. Soc. 2017, 139, 15104-15113

Ethylene-Bridged Oligo-BODIPYs: Access to Intramolecular J‑Aggregates and Superfluorophores Lukas J. Patalag,† Luong Phong Ho,‡ Peter G. Jones,‡ and Daniel B. Werz*,† †

Institut für Organische Chemie and ‡Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany S Supporting Information *

ABSTRACT: A versatile and rapid access to various chain lengths of ethylene-bridged BODIPY motifs was discovered. Corresponding oligomers comprising up to eight monomeric units were studied with respect to their microstructures by photophysical, X-ray crystallographic, and computational means. The investigation of three different dipyrrin cores revealed a crucial dependence on the substitution pattern of the core, whereas the nature of the meso-periphery is less critical. The impact of substituent effects on the conformational space was investigated by Monte Carlo simulations and a set of DFT methods (B3LYP, PBEh-3c, TPSS/PWPB95), including dispersion effects. Cryptopyrrole-derived oligo-BODIPYs are characterized by a tight intramolecular arrangement triggering a dominant J-type excitonic coupling with red-shifts up to 45 nm, exceptionally small line widths of the absorption and emission event (up to 286 cm−1), outstandingly high attenuation coefficients (up to 1 042 000 M−1 cm−1), and quantum yields of up to unity.

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INTRODUCTION The rich chemistry of the BODIPY motif,1 together with its beneficial photophysical properties, has markedly boosted the popularity of this user-friendly fluorophore over the last few decades.2 The diversity of easily incorporated fluorescence modulation modes has set the stage for a variety of sensorically active species3 and applications as an amenable, tailor-made fluorescent label,4 but the motif is also a work-horse for fundamental studies concerning the character and fate of electronically excited states.5 Dimerizations6 and oligomerizations7,8 have disclosed efficient exciton coupling phenomena and even heavy-atom-free intersystem crossing (ISC) processes, enabling novel designs for photosensitizing agents.9 Even though it represents a milestone in dye chemistry Jaggregation10 has only rarely been combined with the BODIPY motif so far,11 still being dominated by cyanine fluorophores.12 Independently discovered by Scheibe and Jelly in the 1930s,13 the phenomenon describes the microcrystalline-like selfassembly of dye monomers in solution leading to intense red-shifted absorptions with reduced vibronic progression, impressive sharpness, and an increased emissivity arising from intermolecular exciton coupling.14 The signature of the highly defined absorption energies is counterintuitive and depends on environmental as well as specific parameters of the engaged monomer. For typical cyanine-derived intermolecular Jaggregates fwhm-values of about 200 cm−1 were found;10a,15 intermolecular J-aggregates with BODIPY motifs recently revealed a narrowing down to 385 cm−1,11b values depending on temperature, purity, and the nature of the packing order. The process of J-aggregation is both highly concentration- and © 2017 American Chemical Society

structure-dependent, features that can be rewardingly utilized but also thwart a reproducible and individual exploitation of the induced photophysical properties. However, red-shifted absorptions, an outstanding brightness and photostability, and small line widths are now highly sought-after qualities in optoelectronic applications and multicolor super-resolution microscopic methods such as STED.16 Furthermore, quantum yields of unity are mandatory for an effective transport of trapped radiant energy, as realized for instance in luminescent solar concentrators, whose efficiency suffers extensively from interchromophoric disorder and re-absorption events within the chosen waveguide.17 Against the background of these considerations we were intrigued by the question whether Jaggregation with unconjugated subchromophores might also be realized in an intramolecular fashion employing a BODIPY motif as the central monomeric unit (Figure 1). A handful of synthetic approaches toward BODIPY-oligomers are known but are disadvantaged by conformational restrictions that inhibit the sensitive and sophisticated packing required for Jaggregation. One of the most prominent procedures is oxidative, tether-free β−β-coupling which can be realized by FeCl37a or hypervalent iodine oxidants7b such as [bis(trifluoroacetoxy)iodo]benzene (PIFA). Unless blocked, the α and β′ positions can also be addressed by the coupling protocol.6a,d More flexibility is allowed by C(sp3)-linkers6b or sulfur-based tethers6f which were recently introduced by Bröring et al. via acidic condensation with aldehydes and Received: August 2, 2017 Published: September 26, 2017 15104

DOI: 10.1021/jacs.7b08176 J. Am. Chem. Soc. 2017, 139, 15104−15113

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Journal of the American Chemical Society Scheme 1. Plausible Mechanism of the Oxidative Oligomerization Process, Nomenclature, and Yields

Figure 1. Various realized modes for the oligomerization of the BODIPY motif.

SNAr reactions, respectively. A parallel orientation of the longitudinal axes of the subunits (as depicted in Figure 1), as a prerequisite for efficient J-type aggregation is sometimes geometrically feasible but has only been observed in isolated cases based on corresponding crystal structures or DFT calculations. Adding another methylene unit to the tether results in an ethylene bridge with three rotational degrees of freedom per dimeric subunit. Such a linkage theoretically allows a plane parallelism of the dipyrrin π systems in addition to a defined parallel alignment of the monomeric transition dipoles. This connectivity should provide a basis to study the morphology of oligo-BODIPYs as a function of substitution pattern and environmental influences.

a

First value is the isolated yield; recovered starting material is given in parentheses.

substitution pattern and lie in the range of 14% (tetramer), 5% (hexamer), and 2% (octamer). Usually significant amounts (20−40%) of the dimeric starting material were recovered despite the use of excess LDA. Scheme 2a) provides an overview of the oligomers that were synthesized. Even though the molecular polarity increases with a higher degree of oligomerization, gel permeation chromatography proved to be the method of choice to purify the reaction outcome. The higher oligomers were unambiguously characterized by HRMS and NMR spectroscopy. Whereas the signals for the pyrollic protons (R1 = H) in the inner sphere of the chain structure coalesce into one single peak and undergo a downfield shift by adjacent ring currents in 1H NMR spectra, the corresponding two protons at the termini of the chain are sufficiently shifted to higher field and can thus be used as a discrete internal standard for integration (see Supporting Information). Similarly, 19F NMR signals of the inner monomeric units are downfield shifted and can be distinguished from the signals of the two capping units. HRMS spectra of all oligomers were obtained indicating no impurities of different homologues. Spectroscopy. A relationship between the crystalline intermolecular arrangement of J-aggregates and the corresponding arrangement in concentrated dye solutions was established by Marchetti et al. by comparison of UV and fluorescence spectra.18 In order to gain insight into the microstructures of oligo-BODIPYs in solution we were able to obtain X-ray crystal structures of three dimeric species containing all three dipyrrin cores which were investigated (M-dim-Ar1, DM-dim-Ar1, EDM-dim-Ar1). All three molecules display crystallographic inversion symmetry, and the structures thus exhibit a face-to-

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RESULTS AND DISCUSSION Synthesis of Oligo-BODIPYs. To simplify access, we envisioned to exploit the acidity of α-methyl substituted BODIPY motifs to establish a versatile one-pot, oxidative coupling that would engage a broad range of diversely derivatized monomers (Scheme 1). In contrast to LiHMDS, LDA proved to be a sufficiently strong base to allow quantitative deprotonations of the α-methyl group within several minutes at −78 °C. The rapid addition of iodine monochloride (0.5 equiv) leads to a fast iodination of the respective α-position. The remaining lithiated species (0.5 equiv) can attack either via nucleophilic substitution or via addition if iodide is previously expelled by the nucleophilic capacity of the dipyrrin core (see Supporting Information for synthetic details). While the dimerization step (R0 = H) proceeds smoothly with yields up to 68%, a further formal tetramerization (with R0 = −(CH2CH2)− monomer) furnishes the respective tetramer together with higher homologues (hexamers, octamers, etc.) as a result of intermolecular anion scrambling between various lithiated species. A typical distribution of the oligomers thus formed is shown in Scheme 2b. Corresponding reaction yields are not influenced by the 15105


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Journal of the American Chemical Society Scheme 2. Overview of the Prepared Dimers and Respective Oligomers

a

Reaction scheme of the oligomerization with suitable dimers as starting material and reaction yields. bTypical size exclusion chromatogram after the oligomerization step demonstrating the product distribution.

Kasha’s limited point-dipole point-dipole approximation for description of excitonic couplings, we calculated energies of respective Davydov splittings VAB for general comparison (see also Supporting Information). Even though the X-ray crystal structures seem promising in terms of inducing the desired J-aggregation in solution, local fluctuation or environmental influences are likely to gain the upper hand and perturb the sensitive symmetrical arrangement. Despite their advantageous slip angles and symmetries in X-ray crystal structure dimers, M-dim-Ar1, DM-dim-Ar1, and EDMdim-Ar1 differ extensively in their photophysical properties. Compared to their monomeric precursors the absorptions and emissions of all three dimers are red-shifted by approximately 20 nm. Similarly, the degree of oligomerization accounts for a general stepwise decrease in fluorescence lifetimes (see Table 1). Stokes’s shifts as markers for intramolecular relaxation and/ or environmental reorganization processes however are highly inconsistent for M-dim-Ar1 and DM-dim-Ar1 regarding solvent interactions (DCM and THF), which indicates that their putative symmetrical conformations are rather sensitive in solution and might not remain unperturbed in the excited state. The absorption spectra of M-dim-Ar1 and DM-dim-Ar1 are described by a weakly pronounced exciton coupling of the S1 level contributing to two hypsochromically shifted twin peaks of low intensity (Figure 3). These signals might be rationalized as an overlay of the vibronic progression and a range of H-type

face and strictly parallel orientation (θAB = 0°) of the ethylenebridged dipyrrin cores and their transition dipole moments (Figure 2). Whereas the planes of the dipyrrin cores are nearly coplanar in dimer DM-dim-Ar1 (interplanar distance only 12 pm), M-dim-Ar1 and EDM-dim-Ar1 adopt significant interplanar offsets of 71 and 146 pm, respectively. This results in a staggered microstructure that could be regarded as a section of the brickwork or staircase arrangement anticipated for typical intermolecular J-aggregates.19 The mode of excitonic coupling (H- or J-type), however, is controlled by the so-called slip angle θslip. This specific angle stems from the offset of the assembled monomeric units in x-direction if parallelism of the single transition dipole moments is ensured (compare with Figure 1). According to the excitonic coupling theory of Kasha a value of θslip = 54.7° is the threshold below which bathochromically shifted J-type absorption bands can appear.20 Calculation of the slip angle was performed in consideration of all three space coordinates placing virtual transitions dipoles through the respective two β-carbon atoms of each BODIPY subunit (the complete set of interchromophoric angles and orientation factors is provided in the Supporting Information). For all three dimeric species the slip angle is smaller than the threshold according to X-ray data. Whereas M-dim-Ar1 and DM-dim-Ar1 exhibit similar angles of 36.5° and 36.3°, a larger slip angle of 44.5° was determined for EDM-dim-Ar1 as a result of the more pronounced staggered arrangement. Albeit in the knowledge of 15106


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are significantly lowered regardless of solvent polarity (fwhmA = 516 cm−1, fwhmA (monomer) = 896 cm−1 (DCM)). As a consequence the attenuation coefficient ε of EDM-dim-Ar1 does not simply amount to the sum of two monomeric BODIPY-units as observed for M-dim-Ar1 and DM-dim-Ar1 but increases from ε = 71 000 M−1 cm−1 to ε = 219 000 M−1 cm−1 in DCM and from ε = 76 000 M−1 cm−1 to ε = 241 000 M−1 cm−1 in THF. The oscillator strength per monomeric unit ( f/u), however, does not change and stays at f/u = 0.45−0.50 for all EDM oligomers with respect to the main transition (see Table 1). These characteristics indicate a well-defined conformational microstructure, a parallel alignment of the stimulated transition dipoles, and thus an efficient J-type excitonic coupling. The sole inconsistency involves the fluorescence quantum yields, which are slightly lowered (ΦF = 0.68) compared to the monomer in DCM but undergo a boost to unity in THF. This tendency is strictly maintained for higher homologues with an EDM dipyrrin core. Figure 4a demonstrates how the excitation energies and line widths change for the EDM-Ar 2 series with the degree of oligomerization. While the extinction still increases disproportionately by stepwise addition of dimeric subunits the relative intensities of respective side peaks (see absorption shoulder at 490 nm of the monomer) decrease relative to the corresponding main absorption in the same order, also in the UV-domain below 400 nm. The origin of the consistently hypsochromically shifted oligomeric sidepeaks would be consistent with a weak contribution of H-type excitonic coupling as a result of a weakly perturbed parallelism between the oscillating transition dipoles within the J-aggregate. Since the existence of high energy arrangements in shape of discrete H-aggregates can be broadly excluded on the basis of our computational studies (see below), the sidepeaks should also incorporate the vibronic progression of the dominant J-bands.21 EDM-hex-Ar2 reaches an attenuation coefficient of ε = 1042 000 M−1 cm−1 in THF with a fluorescence quantum yield of unity. The latter applies generally to all J-aggregating oligomers in this solvent. The maximum absorption wavelength is redshifted from λmax = 522 nm (monomer) to λmax = 567 nm for EDM-hex-Ar2 (1520 cm−1), while the fwhmA-value decreases dramatically from 855 cm−1 (monomer) to only 286 cm−1 (EDM-hex-Ar2) in THF (similarly in DCM). The exchange narrowing of J-bands in dye aggregates is often rationalized by the theory of coherent exciton scattering (CES).22 Subsequent theoretical studies23 stated that the absorption line widths diminish with growing amounts of monomeric units (N) according to 1/√N, which is reasonably mirrored in the spectra of the EDM-Ar2 series (Figure 4b). This is in stark contrast to tether-free β−β-coupled BODIPYs, with broadened absorption and emission peaks,7a and respective α−α-coupled species, which exhibit a more symmetrical Davydov splitting6a with substantial amounts of H-type coupling. For a rough estimation of the average exciton delocalization,14a,23c,24 expressed as the effective coherence length Neff, eq 1 is often used.10g,11a,25 Δν̃ represents the line width at two-thirds of the maximum height of the corresponding main absorption band with the monomeric line width Δν̃monomer being 609 cm−1. 2/3

Figure 2. X-ray crystal structures involving all three dipyrrin cores (M, DM, EDM) studied. 4′-Isopropylidene groups are omitted for better visualization. θAB is the angle between the superimposed transition dipole moments of the subunits. κ is the orientation factor. Transition dipole moments μeg were calculated by integration of the respective main absorption bands in DCM according to eq S2 (see Supporting Information).10h,11d

excitonic coupling contributions of various high-energy conformers present in solution. Striking for DM-dim-Ar1, but especially for M-dim-Ar1, is the broadened emission peak (fwhmF ≈ 1564 cm−1, fwhmF(monomer) ≈ 1024 cm−1) implying significant rotational freedom on lowering the degree of dipyrrin substitution. Despite the structural flexibility of Mdim-Ar1 its fluorescence quantum yields (ΦF = 0.56 (DCM), ΦF = 0.68 (THF)) are doubled in DCM and even tripled in THF compared to the monomeric precursor. These are unusual values for BODIPY species with meso-unrestricted aryl residues and are possibly related to the decreasing fluorescence lifetimes. This holds true, albeit to a lesser extent, for DM-dim-Ar1 in which two methyls at the β′-positions ensure enough orthogonal fixation of the meso-aryl ring. Emission efficiencies compared to the monomeric precursor rise from ΦF = 0.58 to ΦF = 0.70 in DCM and from ΦF = 0.55 to ΦF = 0.90 in the more polar THF. A narrowing of the absorption and emission peaks, however, as a typical indication of J-type aggregation was observed neither for M-dim-Ar1 nor for DM-dim-Ar1. This situation changes drastically when the β-positions are derivatized by ethyl residues as present in EDM-dim-Ar1. Not only are the absorption and emission now red-shifted and the Stokes shift decreased compared to the monomeric precursor, but also the line widths for both absorption and emission peaks

Neff =

Δν2/3 ̃ monomer Δν2/3 ̃ oligomer

(1)

The values for the EDM-Ar2 series are consistently higher than theoretically predicted (Figure 4c), which may be a direct 15107


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Journal of the American Chemical Society Table 1. Excerpt of Prepared Compounds and Corresponding Spectroscopic Data at Room Temperaturea

a J-aggregating species are highlighted in gray. First value is that in DCM; second value (parentheses) is that in THF. bOscillator strength according to eq S1; for values in THF see Supporting Information. cExperimental fluorescence lifetime. dFluorescence rate constant according to ΦF/τF. e Transition dipole moment according to eq S2;10h,11d for values in THF see Supporting Information. fValues are obscured by the overlap of a side peak and estimated. gValues are obscured by the overlap of a side peak. hBroad peaks. iCould not be determined. jValue in THF might be inaccurate because of solubility.

Figure 4. (a) Normalized absorptions of the EDM-Ar2 series in THF showing the bathochromic shift and narrowing of corresponding absorption bands. (b) Absorption line widths correlate according to 1/ √N. (c) Effective coherence lengths according to eq 1.

Figure 3. Absorption and emission profile of M-dim-Ar1 in DCM. Monomer data are shown for comparison.

result of the approximative quality of eq 1, but also indicates exciton delocalization over the entire structural entity. Ethylene-bridged oligo-BODIPYs of the EDM-Ar2 series also reveal a specific and consistent tendency in their fluorescence spectra (Figure 5). However, the extent of the red-shift is smaller, λmax = 534 nm (monomer) to λmax = 569 nm for EDM-hex-Ar2 (1152 cm−1) which results in a Stokes’ shift of only Δν̃ = 62 cm−1. Here again, side emission bands decrease in relative intensity compared to the main emission peak, rendering the superfluorophore exceptionally dark beyond the main emission domain.

The narrowing of emission line widths appears to be a unique feature of ethylene-bridged J-aggregating oligo-BODIPYs and has not yet been reported for respective tether-free or C(sp3)-coupled BODIPY motifs.6,7 Thus, EDM-oligo-Ar2 species develop into highly efficient monochromatic emitters with increasing amounts of incorporated subunits. An EDMequipped congener with a meso-hexyl residue (EDM-dim-Hex) shows the same tendencies in all spectroscopic data, revealing the meso-periphery to participate less in the critical Jaggregating molecular sphere (Table 1). To rule out a potential 15108


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higher oligomers we tried to elucidate the oligomeric microstructures and the responsible interactions for Jaggregation by means of computational modeling on the basis of density functional theory. The common B3LYP functional with a standard double-ζ basis set (6-31G**) was chosen as a starting point for geometrical optimizations and compared with a corresponding dispersion-corrected26 recalculation (B3LYP-D3). The latter approach turned out to be reliable in the rationalization of unusual azobenzene switching properties and was able to highlight the significance of London dispersion forces.27 Considering the molecular size of our oligomers and the mutual spatial accessibility of the chromophoric units we concluded that dispersion interactions might be mandatory for our systems as well. Finally we contrasted these results with the novel reparametrized PBE0 method, termed PBEh-3c which employs a modified double-ζ basis set (def2-mSVP) and includes a dispersion as well as a geometrical counterpoise correction (gCP).28 PBEh-3c performed excellently in various benchmarks, often reaching the quality of a triple-ζ AO basis set second-order perturbation approach (MP2/TZ) at considerable lower computational costs. A scan of possible energy minimum geometries was carried out with preceding Monte Carlo simulations utilizing the OPLS_2005 force field. To save computational time we replaced the meso-Ar1 group by a hydrogen atom H since experimental data revealed that J-aggregation operates independently of the meso substituent involved. The individual conformational spaces were initially scanned with dimeric structures (M-dim-H, DM-dim-H, and EDM-dim-H), essentially revealing four basic conformers with ordered arrangements that might play a role with respect to defined excitonic couplings (see Supporting Information for detailed discussion). In addition, a range of disordered microstructures, predominantly reflecting corresponding gauche conformers, was found. On applying further geometrical optimizations with the set of DFT methods only EDM-dim-H, with the J-aggregating dipyrrin core, revealed a clear energetic preference for two distinct microstructures that outperform competing arrangements by up to 10 kcal. As expected, one of these was derived from the X-ray crystallographic structure, with a staggered geometry, albeit with a modified interplanar offset. The second arrangement resembles a saddle-like microstructure, in which the transition dipoles are fixed in a small curvature of ca. 27°. The derivation of a resulting H-type excitonic coupling contribution according to eq S5 in the Supporting Information, however, proves that the ratio (J-type/H-type = 1.89/0.11) is not sufficient to explain fully the intensity of the side peak of EDM-dim-Ar2 in Figure 4, underscoring the influence of potential vibronic progression on the high-energy side of the main band. Independently of the DFT method chosen, the staggered and the saddle conformations of EDM-dim-H lie very close in energy (ΔE ≈ 2 kcal) which makes it hard to identify a single dominant microstructure. The experimentally detected small absorption line widths, however, necessitate a clear decision and rule out a balanced coexistence of two disparate geometries with highly differing excitonic coupling energies (staggered: VAB ≈ 180 cm−1, saddle: VAB ≈ 480 cm−1) that would obscure the appearance of a consistently red-shifted, narrowed absorption band. Insight into the kinetic stability of dimeric species with saddle and staggered geometries is provided in the Supporting Information. In order to approach a more realistic scenario and forecast the morphology of Jaggregating higher homologues of the EDM series, we

Figure 5. Normalized emissions of the EDM-Ar2 series in THF demonstrating the narrowing of corresponding emission bands.

intermolecular origin of these characterics the spectroscopic studies were repeated at different concentrations without any differences in red-shifts or line widths. The thermodynamic stability of the J-aggregated subunits within the oligomeric microstructure was estimated by temperature-dependent absorption spectra (Figure 6) of EDM-hex-Ar2, recorded in

Figure 6. Temperature-dependent absorption spectra of EDM-hex-Ar2 in toluene.

steps of 10 K in several solvents (toluene, anisole, 2methylpyridine, trichloroethene). Within the instrumental limitations no indications for a “melting” of the microstructure were detected. The line width increased only from 284 cm−1 at room temperature to 343 cm−1 at 80 °C, accompanied by a negligible red shift of about 1 nm (30 cm−1). On recooling the original absorption spectrum was perfectly restored. Measurements with a high-intensity metal−halide lamp revealed an overall increased photostability especially for the J-aggregating EDM-Ar2 series. While the monomer is bleached within 50 min in this setup, about 50% of EDM-dim-Ar2, EDM-tet-Ar2, and EDM-hex-Ar2 still remain intact (see Supporting Information). Computations. Since the bulky Ar2 residues defeated any attempt to obtain appropriate crystals for X-ray diffraction of 15109


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Journal of the American Chemical Society transferred the staggered and saddle arrangements to trimeric counterparts and considered two plausible superstructures termed SC (staircase-like) and UD (up−down) to discriminate between the related spatial orientations of the two ethylene tethers in a s-cis-s-trans related isomerism. (Figure 7). The plethora of twisted microstructures found by a supporting Monte Carlo sampling is summed up in the last row including an energy range. In order to circumvent erroneous contributions owing to the intramolecular basis set superposition error (IBSSE)29 we equipped the B3LYP functional with a 6-311G** basis set. On applying our three computational methods, B3LYP-D3 gave rather inconsistent results with regard to the spectroscopic data, implying a set of highly distorted arrangements as alleged global conformational energy minima. Since this is in stark contrast to the narrowed line widths and the well-defined absorption and emission profiles of the EDM series, we conclude that the 6-311G** basis set might be illsuited due to a higher intrinsic tendency for a BSSE than respective Ahlrichs basis sets.30 Remarkably, the outdated and modest B3LYP approach without D3 correction provided consistent energies, but in favor of the staggered arrangement with a SC superstructure (staggered-SC). As observed for the dimeric case, PBEh-3c switches to saddle-like microstructures as global conformational minima, probably in consideration of the higher London dispersion contributions at this geometry. The favored up−down superstructure (saddle-UD), however, is still very close in energy to a SC congener, saddle-SC (ca. 0.2 kcal) but also to staggered arrangements independent of the superstructure (ca. 2 kcal). In order to benefit from a higher computational predictive power, we finally submitted the selected microstructures to a geometrical optimization at the triple-ζ level employing the D3-corrected TPSS functional.31 For a subsequent single-point refinement the double-hybrid functional PWPB95-D3 was used, which proved to deliver highly precise energies with a relatively low basis set dependency.32 Since we now abandoned the objective of maintaining comparability to B3LYP energies we replaced the CPCM solvation model by Truhlar’s SMD model (Solvation Model based on Density) which tends to be a more suitable choice for structures of low polarity making use of its cavitydispersion-solvent-structure term.33 Indeed, the PBEh-3c derived tendency for saddle-like microstructures increased significantly. Staggered geometries (staggered-SC and staggered-UD) are revealed to be clearly disfavored by ca. 4 kcal, being also less dominated by attractive London dispersion interactions (ca. 5−7 kcal less compared to saddle microstructures (averaged, TPSS-D3, and B3LYP-D3).34 The energy of saddle-SC shows a high dependency on the orientation of inner β-ethyl residues (ΔE = up to 3 kcal) strongly favoring their outward orientation as a consequence of the dense arrangement. Steric effects seem to gain the upper hand with regard to the related saddle-UD superstructure, which is disfavored overall by 1.3 kcal despite a more favorable dispersion energy on the basis of greater compactness. This small energetic discrimination, however, is not contradictory to the experimental data, since saddle-SC and saddle-UD exhibit very similar intrachromophoric distances and angles, which results in essentially identical excitonic coupling conditions. A coexistence of these superstructures, also as partial arrangements within higher homologues, is thus not only allowed but quite likely, especially when entropy contributions become greater in larger oligomeric systems. Figure 8 illustrates the modeled, helically shaped hyperstructure of an EDM-hex-H

Figure 7. Pictorial representation of various minimum energy microstructures of EDM-tri-H and their corresponding relative energies. Geometrically optimized by B3LYP, B3LYP-D3, PBEh-3c (CPCM: dichloromethane) and TPSS-D3 (SMD: dichloromethane). Single point refinement of TPSS-D3 geometry with PWPB95-D3/ def2-TZVPP (SMD: dichloromethane). Energy range for saddle and staggered reflect the orientation of respective β-ethyl groups. Angles, distances, and excitonic coupling energies are averaged. θAB is the angle between the superimposed transition dipole moments of two adjacent subunits. a Slip angle (θslip) is not applicable for this geometry. 15110


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event (up to 324 cm−1), quantum yields of unity in THF, and an overall improved photostability. These features should contribute to broadening the spectral variety of the BODIPY motif, ensuring exceptionally distinct fluorescence events of highest emissivity for multicolor super-resolution microscopy, organic photonics (OLEDs), and solar energy concentrators. The general synthetic strategy promises to allow direct application to suitable aza-BODIPY, BOIMPY, BOPHY di(iso)indomethene, or even tailored rhodamine-derived motifs, whose acidity at suitable methyl positions should be sufficiently high to enter the described reaction pathway. Products hereof might enrich the current toolbox of organic emitters with finely tunable and red-shifted candidates, especially in the far-red and near-infrared domain. The specific influence of β-ethyl groups on the thermodynamics and kinetics of the J-aggregating process renders this position appealing to steer the system toward novel micro- and superstructures or amplify the aggregation up to a “locked in” state. Finally, we believe that this work will provide valuable insights also into the understanding of the archetypical intermolecular J-aggregation phenomenon and might contribute to the final steps of its clarification.

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Figure 8. Geometrically extended, helically shaped hyperstructures of EDM-hex-H and EDM-dodeca-H on the basis of computationally predicted most stable saddle-SC geometry.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08176. Synthetic procedures; experimental and computational details; spectroscopic and crystallographic data (PDF) X-ray crystallographic data for M-dim-Ar1 (CIF) X-ray crystallographic data for DM-dim-Ar1 (CIF) X-ray crystallographic data for EDM-dim-Ar1 (CIF)

species derived from the computationally predicted, most stable all-saddle-all-SC arrangement. A geometrical homologation to an EDM-dodeca-H congener visualizes the pitch of the helix which amounts to ca. 3.0 nm, requiring about 8 repeating monomeric units. The outer and inner diameters of the helix are estimated at 2.2 and 1.0 nm, respectively.

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CONCLUSION AND OUTLOOK Resisting the plausible urge to force monomeric units into close proximity within the oligomerization strategy this study shows that BODIPY scaffolds tethered by an α−α-ethano unit are able to shift excitonic coupling effects efficiently toward the J-type mode, independent of dye concentration and only slightly influenced by solvent polarity. Potential H-type contributions are weak which should allow to term the structural origin of the coherently coupled oscillators as unprecedented, intramolecular J-aggregates with BODIPY motifs. The synthetic entry relies on a straightforward and easily transferable mechanism and allows to engage a wide range of BODIPY monomers varying in their dipyrrin core and their meso substitution. Computations at the PBEh-3c and TPSS-D3 level suggest that β-ethyl residues at the dipyrrin cores give rise to a slightly curved (27°), saddle-like microstructure, aligning the oscillating transition dipole moments in an essentially parallel fashion. This results in a helically shaped hyperstructure adopted by corresponding oligomeric homologues. Kinetic energy barriers were found to be small enough to allow interconformational exchange at room temperature. All oligo-BODIPYs, J-aggregating and non-Jaggregating, benefit from a tunable red-shift and a greatly increased attenuation coefficient, and exhibit decreased fluorescence lifetimes. Unusually high fluorescence efficiencies were found for BODIPY species with unrestricted meso aryl residues. Five cryptopyrrole-derived counterparts with strong intramolecular J-type excitonic coupling were prepared and are additionally characterized by extremely high attenuation coefficients of up to 1 042 000 M−1 cm−1, small absorption line widths (up to 286 cm−1), notably also of the emission

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Daniel B. Werz: 0000-0002-3973-2212 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.J.P. thanks Dr. Alexey Butkevich, Dr. Vladimir Belov, and Prof. Dr. Stefan Hell (MPI for Biophysical Chemistry, Göttingen) for the possibility to measure fluorescence quantum yields, and Prof. Dr. Stefan Grimme, Christoph Bannwarth (University of Bonn), Prof. Dr. Jörg Grunenberg, Dr. Kai Brandhorst, Oscar Arias, and Katharina Bolsewig (TU Braunschweig) for kind support. D.B.W. thanks Prof. Dr. Philip Tinnefeld (Ludwig Maximilian University of Munich) for initial discussions.

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REFERENCES

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