Correlation between Crystal Habit and Luminescence Properties of 4

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Correlation between Crystal Habit and Luminescence Properties of 4,4-Difluoro-1,3-dimethyl-4-bora‑3a,4a‑diaza‑s‑indacene, An Asymmetric BODIPY Dye Christian Spies,† Anh-Minh Huynh,† Volker Huch,‡ and Gregor Jung*,† †

Biophysical Chemistry and ‡Inorganic and General Chemistry, Saarland University, Campus, Building B2 2, D-66123 Saarbrücken, Germany S Supporting Information *

ABSTRACT: The asymmetric substituted BODIPY dye 4,4-difluoro-1,3-dimethyl-4bora-3a,4a-diaza-s-indacene crystallizes in three different crystal habits, that is, as needles (I), leaves (II), or microcrystalline sublimed crystals (III). All crystals share the same crystal structure but exhibit varying solid-state fluorescence from yellow-orange to deep red. The crystal structure mainly consists of one-dimensional chains of J-aggregates which leads to excitonic luminescence at λ = 600 nm. The point-dipole approximation is used to calculate the excitonic splitting to a value of 3300 cm−1, which is in good agreement with the experimental observations. The influence of the macroscopic appearance on the luminescence properties is discussed in terms of reflectivity of the surface and reabsorption within the material. It turns out that the long-range order modulates the solid-state luminescence due to the small Stokes shift of the dye.

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INTRODUCTION Luminescent solids are nowadays of great interest in material science because of the possibility to create efficient light sources.1,2 Besides inorganic solids, namely, semiconductors or rare-earth metal ions as phosphors in an inorganic matrix,3,4 organic molecular solids are gaining more and more interest.5−8 Advantages of organic fluorophors are the huge variety of absorption and emission characteristics as well as usually high extinction coefficients and luminescence quantum yields. Moreover, they are compatible with polymer science9 and therefore crucial for the development of new organic light emitting diodes (OLEDs).10 One prominent challenge in the field of organic molecular solids is the transfer of excellent optical monomer properties to the solid state. Many promising molecules lack good luminescence properties in their crystalline form due to the high concentration of molecules and, consequently, concentration-dependent effective quenching processes.11 In addition, the strong electronic interaction of the molecules in a crystal might change the optical spectra; that is, blue- or redshifts compared to monomer absorption are frequently observed.11 Quite recently, some of these features were tuned by the molecular packing in the solid state.12−17 Understanding the structure−property relationship is essential to create new photonic material, for example, efficient light sources. Already in the middle of the 20th century, Davydov18,19 and Kasha20−23 successfully derived electronic properties in organic molecular crystals by the exciton model. In short, the strong electronic interaction of the molecules in a fixed geometry causes shifts as well as splitting of electronic levels and is thus the reason for the observed phenomena in optical spectra of molecular crystals. Their theory is still exhibiting a vivid interest nowadays.24,25 © XXXX American Chemical Society

One example for the above-mentioned mismatch between luminescent properties in solution and in the solid state are the dyes based on 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, BODIPY. The characteristics of molecular BODIPY dyes in solution were intensively studied since their first description by Treibs and Kreuzer26 and are now well-understood.27−30 They have found widespread applications such as laser dyes,27 light harvesters,31−33 and biomarkers.34 Dimers of BODIPY molecules were also intensively studied in the past33,35−38 and also appeared useful in biosciences, for example, for monitoring protein−protein contact.36 In the solid state most BODIPY dyes show hardly any fluorescence due to selfquenching but find application as constituents in rigid matrices39−41 or molecular dyads based on Förster energy transfer.42 There are some examples in the literature where luminescent BODIPY crystals were synthesized by introducing large spacer substituents at the core compound to avoid selfquenching.43−46 However, there is no conclusive description of solid state luminescence of BODIPY dyes without those spacers in the literature to our knowledge. During the extensive synthesis of BODIPY dyes in our laboratories,47,48 we observed that the asymmetric substituted derivative 4,4-difluoro-1,3-dimethyl-4-bora-3a,4a-diaza-s-indacene crystallizes in three different habits with differing luminescence properties (Figure 1). It turned out that all of these share the same crystal structure, that is, short-range order. The objective of this work is to explain the color and intensity change of the luminescence of the different crystal habits as a result of their macroscopic appearance. Received: February 28, 2013 Revised: August 9, 2013

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dx.doi.org/10.1021/jp404855s | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. (a) Photograph of the different crystal habits: The needles I, the leaves II in two different crystal sizes, and the sublimed crystals III. (b) The same crystals under UV irradiation (λ = 254 nm).

In the following, we will provide experimental data of the crystals. The differences in their luminescence behavior are described and interpreted. Finally, exciton theory is applied on the basis of the crystal structure to explain the experimental observations. In this context, the influence of the refractive index of the material and the interaction of the exciton with crystal phonons is also discussed.

Figure 2. (a) Crystal structure of the BODIPY under investigation as viewed along [100]. (b) Schematic illustration of the structure on the (001) plane. Electronic nearest neighbor interactions in the crystal according to the point dipole approximation are depicted in wavenumbers; the corresponding values using the model of Kuhn et al. are given in brackets.

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RESULTS Our approach is to derive the crystal luminescence from the photophysical properties of the molecular unit according to the exciton model. The relevant photophysical properties of the investigated BODIPY unit are collected in Table S1. In solution, it exhibits green fluorescence with an emission maximum at λem = 508 nm. The observed small Stokes shift of Δv ̅ ∼ 450 cm−1 and high extinction coefficient, which are both typical for this dye class, lead to a large calculated value of the Förster radius of R0 = 50 Å for energy transfer between identical molecules (Homo-FRET). This value indicates strong electronic interactions with nearby dye molecules. It also implies a large transition dipole moment, which we computed as |μba| = 6.8 D. This result is in excellent agreement with the value given for a similar compound by Bergström et al.36 According to the latter reference, |μba| is orientated along the long axis of the molecule, which is also generally accepted for all BODIPY dyes.27 The dye molecules are packed by crystallization to distances much smaller than the Förster radius, leading to even stronger electronic interactions. Crystallization was established by slow solvent evaporation from concentrated solutions (see experimental section in the Supporting Information). By varying the rate of the process and using sublimation techniques, three different habits of crystals were obtained (Figure 1a). They can be described as needles (I), leaves (II), and microcrystalline, sublimed crystals (III). While the leaves II can have lateral dimensions of some centimeters and a thickness of approximately some hundred micrometers, the needles I have the dimensions of about 0.1 × 1 cm. X-ray analysis of all crystal revealed a common crystal structure for all three habits (Table S6 and Figure 2). The dye crystallizes in the monoclinic space group Ia with the b-axis as the unique axis and four molecules per unit cell. The molecules are arranged in a herringbone fashion along the normal to (100). This type of arrangement is often observed in organic molecular crystals and has a great impact on the optical properties.13,16,49 The molecules have coplanarly orientated πsystems resulting in a high degree of interaction (Figure 2b). The measured angles will be employed for the calculation of the

orientation factor κ in the discussion of the electronic interactions. These angles yield a relatively high value of the orientation factor (κ ≈ 1) for translational equivalent molecules and thus hint at large interactions along the crystallographic aaxis. Smaller orientation factors are observed for other nearest neighbor interactions in the other directions (Table S2). The center-to-center distance of two molecular planes along the aaxis is measured as 6.373 Å, which is in between the distances reported for fluorescent BODIPY crystals (10 Å by Ozdemir45) and nonfluorescent crystals (3.8 Å by Bandichhor50). This shortest distance approximately exhibits the same length as that of the chromophore, which is determined as 6.765 Å between the two β-carbon atoms forming the long molecular axis. The absorption spectra of the different crystals are depicted in Figure 3. Therein, the vertical line indicates the lowest common electronic transition in the crystals I and II at λabs = 567 nm. As will be shown later, the spectral properties of III are in between I and II and, hence, are omitted here. Spectra in

Figure 3. Absorbance spectra of the BODIPY dye in its different habits. The vertical line indicates a common transition at 567 nm in the crystals I and II. In the reflectance mode, the maximum is shifted toward 583 nm. B



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Figure 4. Contour plots of excitation−emission fluorescence intensity of different crystal forms. (a) I at room temperature, (b) II at room temperature, (c) III at room temperature, (d) III at 77 K. The intensity at 77 K increases by a factor of 10 compared to the room temperature spectrum in c. The molecular structure of the dye molecule is shown in a as an inset.

when excited at λexc = 410 nm and λexc = 510 nm. We attribute this second band either to excitation of monomeric BODIPY, as can be inferred from the faint green emission (λem = 520 nm), or a higher photonic band of the crystal. An additional emission in the red at λem = 700 nm is found with an excitation maximum at λexc = 650 nm and a pronounced shoulder at λexc = 583 nm. The latter value is in accordance with the bathochromic shift of the maximum absorption wavelength of II compared to the needles I (Figure 3). It is interesting to note that no distinct red emission can be excited with wavelengths shorter than λexc = 565−570 nm, whereas excitation below λexc = 550 nm hardly produces any luminescence above λem = 570− 575 nm. It should also be mentioned that the overall luminescence intensity of II compared to I is lower by a factor of about 20. However, despite its bright appearance to the naked eye, the luminescence yield of all habits turned out to be below 1% (data not shown) as observed by direct measurement as described elsewhere.51 The sublimed crystals III show the most complex spectrum and can be considered as a combination of the other two types (Figure 4c). Beside the orange luminescence (λem ≈ 600 nm) which corresponds to the emission of the needles I, red emission (λem ≈ 700 nm), already known from the leaves II, is found in these crystals as well. In contrast to the leaves II, excitation can be achieved throughout the whole spectral range. Even more, this red emission completely vanishes in favor of the orange emission if the temperature is lowered to T = 77 K (Figure 4d). In fact, this spectrum becomes similar to the spectrum of I except for a small temperature shift of the

transmission could only be obtained with the larger leaf-like crystals II. Nevertheless, reflectance spectra provide the same information about the electronic states of the crystals. Despite some small differences in absolute transition wavelengths, similar structured spectra of the different crystal habits are obtained. A distinct long wavelength absorption maximum at λabs = 567 nm is seen with a distinct long tail into the red range up to λ = 650 nm in all spectra. The maximum absorption of II is shifted to longer wavelengths (λmax = 580 nm) and shows an inflection point at λ = 567 nm. At higher energies all spectra share a broad, unstructured band between 450 and 510 nm less pronounced in Iwith faint maxima at λabs = 470 and λabs = 506 nm as well as a further absorption maximum at λabs = 410 nm. We will focus the discussion on the longest wavelength absorption which is red-shifted by about Δλ ≈ 70 nm (Δv ̅ ≈ 2440 cm−1) from the monomer absorbance at λabs = 498 nm. Excitation−emission spectra of all crystal habits are displayed in Figure 4a−c. We start in the description of the luminescence characteristics of the needles I as they exhibit only one emission band at around λem = 600 nm (Figure 4a). This orange emission (see also Figure 1b) is observed by exciting all bands which were already found in absorption, that is, the distinct bands at λabs = 410 nm and λabs = 567 nm. We state here explicitly that this emission is surprisingly bright concerning the compact structure of the BODIPY dye and the often observed self-quenching. Some green luminescence (λexc ≈ 500 nm, λem = 510 nm) is detected and presumably arises from monomer. In the spectrum of the leaves II (Figure 4b) the maximum emission wavelength is blue-shifted to about λem = 570 nm C



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nm in the crystals (Figure 2), most likely due to excitonic interactions. The green emission at λem ≈ 520 nm is seen in all crystals as a minor emission peak and might stem from monomers. The reason for observing monomer emission is from the fact that the material is very fragile. However, excitation of the monomeric dye at λexc ≈ 500 nm might also be superimposed by a higher absorption band of the crystals. The maximum emission wavelength of the crystals is seen at λem ≈ 570−600 nm, depending on the crystal habit. This orange emission can be found in all crystals investigated and may hence be assigned to an intrinsic emission behavior of aggregated BODIPY dyes in J-type conformation.36,37 This luminescence stems mainly from the crystal edges in the case of I if it is excited in the depth of the crystals but comes from the surface if excited by one-photon absorption. An additional red emission band with λem ≈ 700 nm is observed if the crystal size is large and has extended planar surfaces. This red emission vanishes at T = 77 K. As it is only visible at the crystals II and III, it cannot be assigned to an intrinsic transition due to short-range effects but must have its origin in the macroscopic appearance of the crystals. This is corroborated by powdering the samples, which results in a somewhat similar emission of all habits (see Figure S2). Furthermore, the green reflecting leaves II show a red-shifted absorption and blue-shifted emission behavior compared to the other crystals. The excitation−emission spectrum (Figure 4b) indicates by its kind of compressed form that some “forbidden” process at the transition wavelength λabs ≈ 567 nm might be responsible for the observed shifting of the bands. Together with the high reflectivity of the leaves II, we assign this energetic shift to a modulation of the reflectivity so that photons are unable to enter or leave the crystals close to the transition wavelength (see below).

emission to the blue and the typical sharpening of the bands at lower temperature. Again, some faint monomer emission at λem = 520 nm is detected, but monomer excitation predominantly produces orange luminescence. Alternatively, the excitation band λexc = 506 nm might represent another electronic state. We conclude from the spectra in Figure 4 that all transitions found in absorption are also visible in the excitation spectra, however, with different intensity in the three habits. It is also worth mentioning that the luminescence properties of all crystals become conform after being powdered (see Figure S2). To learn more about the nature of the emission, we recorded fluorescence images using a confocal laser scanning microscope with variable excitation wavelengths. The luminescence of the leaves was hardly found and, if ever, only at some scratches (images not shown). More information was obtained from the images of the needles I using one- and two-photon excitation (Figure 5).

Figure 5. Confocal fluorescence image of a needle I (λdet = 560−615 nm). (a) λexc = 488 nm, (b) λexc = 916 nm.

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Both ways of excitation cause different fluorescence behaviors: Regular one-photon excitation leads to a crystal luminescence that is homogenously distributed over the surface (Figure 5a). By using two-photon excitation, which is supposed to penetrate deeper into the crystal, light is mainly emitted at the edges of the crystal (Figure 5b). From the images, it can also be seen that the crystals I are built up from many very small “leaves” which are grown unsteadily together. This observation gives reason for the common crystal structure of all three habits. Summing up all experimental results, we conclude that the lowest energy band is shifted from the monomer to λabs = 567

DISCUSSION

The optical properties of the crystals can be derived from the properties of the molecular unit. Excitonic interactions are the dominant interactions as the molecules can no longer be described as monomers but by the exciton model. This model is frequently applied to other organic molecular crystals, molecular dimers, or polymers.17,49,52,53 The level of theory in this work is based on the pointdipole−dipole approximation as introduced in exciton theory by Kasha et al.23 and the strong coupling case of Simpson and Peterson.54 We also compared the results with the extended

Figure 6. Excitonic splitting of the optically allowed energy levels in the crystal according to the two employed models. The interaction with the translationally equivalent molecules shifts the excited state to a hypothetical level (dashed); the interaction with nonequivalent molecules creates a splitting of the energy level. Experimental values are shown in gray and brackets for comparison. D



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dipole approximation by Kuhn and co-workers55 (see SI for a detailed description). The computed values for the nearest neighbor interactions are shown in Figure 2b. Negative values of interaction energies result from κ < 0 and lead to a bathochromic energy shift of the allowed transitions. According to our computation the S0 → S1 transition in the monomer at λexc = 498 nm is shifted by −1350 cm−1 and split by 1650 cm−1 in two optically allowed excitonic states (Figure 6).56 Based on this, the visible transitions are expected at λ = 491 and λ = 585 nm. These values are assigned to the experimentally measured peaks at λabs = 470 and λabs = 567 nm in the electronic spectra, which is a fairly good agreement concerning the accuracy of the level of theory used. A similar good agreement is obtained with the model of Kuhn. Whereas the point-dipole approximation slightly overestimates the experimental wavelengths, Kuhn’s extended model underestimates them (Table S4). We conclude from this comparison that the point dipole approximation serves as a suitable model here, even though the intermolecular distances are very short compared to the molecular length. The further emission peaks at about 700 nm in the spectra of II and III cannot be explained by the exciton model. They are much lower in energy than the first transition seen in absorption and not visible in the spectrum of I. Furthermore, the red emission vanishes at low temperature in the spectrum of II (Figure 4d) as well as in the spectrum of III (data not shown). Lifetime measurements do not support phosphorescence from a triplet state (Table S5). In contrast to the orange luminescence, this red emission may not be pure crystal emission as defined by Hochstrasser but rather emission from imperfections in the crystal structure.56 The soft material, which is only held together by weak π−π-interactions, easily undergoes translational or rotational movements of the monomer thus modulating the transition energies.57 A closer inspection of Figures 3 and 4b reveals that absorption and emission wavelengths are shifted in the case of the crystals II compared to I and III. We explain this finding with the large flat surface of the crystals II: a high reflectivity of incident light has its origin in the high refractive index of the material.58 The latter varies largely in the proximity of an electronic transition, causing a modulation of the reflectivity at these wavelengths.59 As can be seen in Figure 1 this is indeed the case as the leaves II show a green reflectivity, but the other crystal habits do lack this iridescence. The inflection point observed in the absorption spectrum at λabs = 567 nm (Figure 3) is in agreement with this interpretation since the modulation of the refractive index has also its inflection point at the transition wavelength.59 The reflectivity is also the reason why red emission in the leaves II cannot be excited with light λexc < 570 nm. Even the part of electromagnetic radiation which is not reflected cannot excite excitons deeply in the crystal. A penetration depth of roughly 23 nm for one-photon excitation is estimated. The small path length explains why luminescence, excited with one-photon absorption, can only originate from the excitons near the surface as the excitation light cannot enter deeper. This is even observable in the needles I despite the much smaller crystal dimensions compared to the more extended crystals II. In the leaves II, the blue shift of the emission (λem = 570 nm) compared to the habits I and III (λem = 600 nm) again results from the high reflectivity close to the transition wavelength

(λabs = 567 nm), but now in the opposite direction: luminescence light can only leave the crystal distinctly away from the exciton transition. Thus, the high reflectivity also explains the low-luminescence intensity of the leaves II. Whenever the reflectivity is absent, for example, in the crystals III or by powdering the leaves, also both effects, the missing excitability of the red emission below λexc = 565−570 nm as well as the blue-shifted exciton emission disappear in agreement with the experimental finding. There is, however, another impact of the high reflectivity in combination with the small Stokes shift of the luminescence. Due to the considerable spectral overlap between emission and absorption reabsorption likely occurs. The red tail in the absorption spectrum is caused by thermal population of vibrations and, especially, phonons. Even luminescence light that is red-shifted by Δv ̅ = 1200 cm−1 (≈ 6kbT) maximally travels 10 μm before reabsorption. At lower temperature (77 K), however, vibrations and phonons in the electronic ground state are much less populated, and the absorption coefficient in the red tail of the excitonic band is reduced to a large extent: the bright excitonic luminescence becomes visible. In the leaves II, the effect is even more pronounced as fluorescence photons are internally reflected thus prolonging the interaction length of the photons with the material; that is, coupling to phonons becomes more important. Only distinctly red-shifted luminescence can therefore leave the macroscopic crystal at room temperature. Based on the observed temperature-dependent luminescence behavior we assume that the red band originates from polariton−exciton emission.60 However, a detailed description within the polariton−exciton emission model is beyond the scope of this work.

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CONCLUSIONS In summary, we report brightly fluorescent crystals of a small BODIPY dye without spacer groups. The missing quenching, often observed for aggregated dyes, arises from the molecular packaging as in J-aggregates. The orange luminescence is of excitonic type and can be calculated by simple exciton theory based on the point-dipole−dipole interaction term. Despite the common short-range order of the molecules in terms of the crystal structure, a very long-range order, that is, the crystal habit, is crucial for the optical properties of the investigated BODIPY crystal. Increasing the size of the crystals leads to a lowered emission intensity and an additional red emission. BODIPY dyes are especially appropriate for observing such strong photonic effects as, first, their transition dipole moment is large, leading to a large energy splitting and high absorbance. Second, the intrinsic Stokes shift is small resulting in a strong dependence on temperature effects and macroscopic crystal appearance, that is, optical path length effects. To the best of our knowledge this is the first observation of a modulated emission color by varying crystal size in the field of molecular organic crystals.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, details on the calculation of the excitonic splitting, fluorescence lifetime values, photophysical parameters of the dye molecule, fluorescence spectra after powdering the samples, and crystal structure of the dye. This material is available free of charge via the Internet at http://pubs.acs.org. E



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 (0)681 302 64846. Phone: +49 (0)681 302 64848. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Michel Orrit and Holger Kohlmann for fruitful discussions, Michael Schmitt and Leon Muijs for help with some measurements, and Klaus Meerholz for putting his quantum yield measurement setup to our disposal. This work was supported by the German Science Foundation (DFG, JU650/3-1).

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