Subhrajyoti
Bhandary
a,
Rik
Van Deun
b,
Anna M.
Kaczmarek
c and
Kristof
Van Hecke
*a
aXStruct, Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium. E-mail: kristof.vanhecke@ugent.be
bL3 – Luminescent Lanthanide Lab, Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium
cNanoSensing Group, Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium
First published on 15th August 2022
Mechanically responsive organic luminescent crystals are one of the promising choices of materials for flexible photonic devices. However, the change in phosphorescence emission as a function of the flexibility of a crystal has never been reported. Our current findings demonstrate two-dimensional (2D) and one-dimensional (1D) macroscopic elastic deformability, under mechanical stress, in elastically flexible single crystals of dibenzothiophene, and its brominated derivative, respectively. Unlike the presence of dual fluorescence (FL) and room temperature phosphorescence (RTP) in dibenzothiophene single crystals, the derivative was found to show only RTP. Interestingly, upon elastic deformation, single crystals of the dual emissive dibenzothiophene show a noticeable blue shift (∼20 nm) of RTP emission when compared to their pristine crystals (straight and naturally bent). However, their FL peaks remain nearly unchanged irrespective of the crystal deformation. A hierarchy of structure-elastic functionality to RTP modulation has been quantitatively mapped by rationalizing the role of chalcogen-involved weak interactions.
Here, we present 2D elastic performance in single crystals of dibenzothiophene (DBS) and 1D elastic flexibility of 2-bromodibenzothiophene (DBS–Br) crystals (Fig. 1) under applied mechanical stress. DBS is one of the earliest known crystallization-induced dual FL and RTP metal-free materials.12,13a The inclusion of thiophene moieties and chalcogen atoms have emerged as a desirable class of structural core for the design of new persistent RTP metal-free crystalline materials.13 Moreover, the findings on dual-functional elasticity and fluorescence properties in single crystals of thiophene core-containing extended π-conjugated molecules include a new dimension to the advancement of luminescent flexible crystals.5a,10 In the current case, flexible single crystals of DBS–Br were also found to show RTP properties. Importantly, the elastically bent single crystals of DBS show an evident switching of their RTP emission peak (blue shifted) when compared to their pristine undeformed or naturally bent crystals.
The straight shaped single crystals (∼2–3 mm long, acicular) of both DBS and DBS–Br have been observed to be mechanically flexible (Fig. 3). At ambient temperature (18–25 °C), DBS single crystals could be macroscopically bent into a semi-circular arc, down the major face (i.e. the (101)/(−10−1) plane) using a three-point support of a pair of tweezers and needle (Fig. 3a and Movies S1 and S2†). The original straight shape could also be successfully restored upon relaxation of force, which is indicative of elastic deformation (see ESI† for bending experiments). The process of bending and subsequent shape restoration can be performed in a loop without breaking the single crystals. Remarkably, the DBS crystals also display elastic flexibility from the lateral faces (001)/(00−1), however, to a lesser extent in comparison to the major (101)/(−10−1) faces (Fig. 3c and Movies S3 and S4†). Hence, DBS crystals can be categorized as 2D elastically flexible organic crystals. Similarly, DBS–Br single crystals could only be deformed into an arc down the major faces (001)/(00-1), which is followed by shape recovery upon removal of stress (Fig. 3b and Movie S5†). The DBS–Br crystals are bendable from 1D and are brittle from lateral (011)/(0−1−1) faces (Movie S6†). However, beyond a certain onset of deformation, the crystals are fragmented for both compounds. We did not observe any stress-induced permanent (plastic) deformation for single crystals of both compounds.
The naturally bent (from their major faces) as grown crystals of both molecules also display behavior when subjected to similar mechanical stress. Furthermore, the crystal structures of such naturally deformed DBS and DBS–Br crystals have been determined by SCXRD analysis, which shows no change in structural parameters when compared to their respective straight shape crystals (Table S1†). It has also been realized that the single crystallinity of naturally grown bent crystals (at the bent region) is slightly compromised in comparison with their straight counterparts, i.e. the deformed crystals clearly result in relatively less precise X-ray diffraction data without having any structural change in comparison to their straight crystals (Table S1†). Moreover, the intrinsic integrity of DBS single crystals upon elastic bending has also been confirmed by mounting an elastically deformed (from major face) single crystal on a home source single-crystal X-ray diffractometer. The observed diffraction spots at low angles (weak in intensity and broad) at the bent region of the crystal clearly indicate the preservation of single crystallinity upon elastic bending (Fig. S4†).
The elastic strain of single crystals of both compounds at their maximum deformation limit was calculated by the Euler–Bernoulli equation14 (see ESI† for details, Fig. S5–S7†). The elastic strain on the major faces of DBS (101)/(−10−1) and DBS–Br (001)/(00−1) were measured to be around 3.3% and 2.5%, respectively. This suggests that DBS crystals from major faces can accommodate more stress in comparison to DBS–Br. However, the elastic bending ability from the lateral faces (001)/(00−1) of DBS crystals are calculated to be around 0.8% only.
Photoluminescence (PL) characterization of one straight single crystal (acicular) of each compound suggests that they both show weak RTP in the solid-state (see ESI† for details). The RTP emission peaks, for DBS and DBS–Br single crystals, were detected approximately at 563 nm (lifetime of 11.6 μs) and 441 nm (lifetime of 8.7 μs), respectively (Fig. S8–S11 and Table S3†). Moreover, the pristine straight single crystal (∼4 mm long) of DBS showed dual FL (peak at ca. 356 nm) and RTP (peak at ca. 563 nm) emission phenomena (Fig. 4), which corroborates with previous reports.12,13a In order to investigate the effect of stress on FL and RTP emissions, for differently manipulated DBS single crystals, we recorded PL spectra for a pristine straight, an elastically bent and a naturally bent single crystal (Fig. S12†).15 All three DBS single crystals display a FL peak approximately at 356 nm, which means there is no prominent effect of mechanical bending on the FL spectra of DBS crystals (Fig. 4a), although a broadening of the PL spectrum of the elastically bent DBS crystal is noticed, which could possibly be attributed to an increase of local defects within the DBS single crystal lattice (inhomogeneous long-range order), due to the employed elastic deformation. The straight, naturally deformed, and elastically bent single crystals are blue emissive under a 365 nm UV lamp (Fig. 4b). Interestingly, a closer look of their RTP spectra clearly indicates a blue shifting (maximum shift from 563 nm to 543 nm) of the emission peak for the elastically deformed crystal in comparison to the pristine straight and naturally bent crystals (Fig. 4c). The shifted RTP peak was observed to return to the initial position after removing stress from the elastically deformed crystal, however, performing switching cycle (fatigue) experiments were not possible for the same crystal, as the DBS single crystals were not found to be sustainable (breaking) after multiple elastic bending experiments, depending on their size and morphology. Furthermore, the plot of their CIE color coordinates also reveals the modification of the RTP emission from the yellow (inset) to green region on the account of elastic deformation in DBS crystals (Fig. 4d).
The origin of the observed impressive 2D/1D elastic flexibilities of both compounds and observed RTP change in the deformed DBS single crystals can be rationalized from the quantification of their supramolecular features and interaction topology. Therefore, we first computed molecular electrostatic potential (ESP) maps and energy frameworks (based on pairwise interaction energy) for both π-surfaced molecules using CrystalExplorer17.5,16,17 at the CE-B3LYP/DGDZVP level of theory (Fig. 5). The ESP plot of DBS (Fig. 5a) indicates two highly electron rich phenyl rings (red, ESP ranges from −49.0 to −52.5 kJ mol−1) and a relatively less electron dense thiophene core (ESP of −26.0 kJ mol−1). In the case of DBS–Br (Fig. 5b), the addition of a heavy halogen atom significantly modifies the ESP distributions in the molecule. Two phenyl rings become less rich and anisotropic (−21 kJ mol−1 and −31.5 kJ mol−1) compared to the former. Moreover, the formation of an electron deficient π-hole18a on the thiophene surface of DBS–Br is notable (see blue positive surface, ESP of 13.4 kJ mol−1). It is now clear that the DBS–Br molecules are more prone to form stacking interactions between aromatic rings for better electrostatic complementarity between phenyl and thiophene units compared to DBS. For this reason, weak slip stacking of molecules has been favored in DBS crystals (through S⋯π contacts to minimize repulsion among π-rings) having an interaction energy (IE) of −16.1 kJ mol−1 (side face in Fig. 2c). In the case of DBS–Br, a stronger stacking (IE −36.7 kJ mol−1) is associated with S⋯π (hole) and π⋯π interactions (see side face packing in Fig. 2f). Another intriguing fact about the ESP plots here is the presence of two small electron deficient σ-holes18 (black arrows, blue regions) on light chalcogen atoms (S) for both DBS and DBS–Br crystals (ESP ranges from 36.6 to 39.0 kJ mol−1). A weak σ-hole on the Br atom is also observed (ESP of 33.8 kJ mol−1). However, such σ-holes on chalcogen mostly remain unutilized by molecules in both crystals except for the formation of C–H⋯π supported relatively directional S(σ-hole)⋯π (IE −13.4 kJ mol−1) interactions in DBS (shaded ribbons in Fig. 2b). Furthermore, the computed energy framework17 of DBS (Fig. 5c) depicts a nearly hexagonal interaction topology (represented by cylindrical tubes) down the ac-crystallographic plane, constructed mostly through C–H⋯π (IE −26.0 kJ mol−1), π⋯π stacking (−20.6 kJ mol−1), and C–H⋯π supported S(σ-hole)⋯π interactions (−13.4 kJ mol−1). Furthermore, the S⋯π guided displaced stacking along the needle axis of the crystal [010], having an IE of −16.1 kJ mol−1, makes the energetic topology nearly isotropic in terms of its comparable strength. Within this unique hexagonal isotropic topology of DBS, the cooperative interplay of C–H⋯π (dimeric motifs along the major face (101)) and different S-involved (S⋯π and C–H⋯S) interactions along [001] of the crystal (see shaded ribbons in Fig. 2b) could be the origin for exhibiting 2D elastic macroscopic flexibilities from both faces of the crystal. Unlike DBS, an anisotropic energetic topology has been evident in DBS–Br crystals, which is primarily stabilized by relatively strong (S⋯π(hole) stacking, −36.7 kJ mol−1, thicker tubes) vs. weak (C–H⋯S, −15.4/−14.7 kJ mol−1, thin tubes) interactions in nearly orthogonal fashion (Fig. 5d). Despite this anisotropic interaction topology, the manifestation of structure-guiding S⋯π-hole associated stacking and C–H⋯S dimers is capable to promote elastic bending in DBS–Br single crystals on major faces (001/00−1) only. We hypothesize that such chalcogen-involved (σ/π-holes) interactions in both DBS and DBS–Br crystals can act as a restorative force for the reversible bending through accommodating the applied stress within their stacking columns by the expansion-contraction fashion (Fig. 6 and S13†).
Fig. 6 A possible molecular level change of stacking columns of DBS molecules upon elastic bending of a pristine DBS crystal on its major face, showing the weakening (stretching) of S⋯π interactions (face-to-edge S⋯C distance between adjacent molecules) because of reversible rotation of molecules (black arrows) to result in a contracted inner (red in color) to expanded outer (green) arc of the bent region. The stretching of S⋯π distance in the outer side is greater than the inner side of the bend (d0 < d1 < d2) due to a larger molecule-to-molecule separation in the former region. Note that stacking columns extend along the needle axis (b-axis) of crystals as shown in Fig. 2c. |
It is worth stating that the chalcogen-involved intermolecular interactions (various σ/π-hole types S⋯π, and C–H⋯S) and associated molecular stacking also plays a central role in the generation of RTP properties in both DBS and DBS–Br crystals. This could be rationalized in terms of efficient intersystem crossing (via n–π* type transition) guided by hetero-chalcogen atoms13c and various S-associated intermolecular interactions that restrict the molecular motion in both crystals. Moreover, the electrostatic nature of different σ/π hole driven S⋯π interactions may contribute to a similar effect (intermolecular electron coupling and restricted molecular motion) to RTP as halogen bonding.18b,19a It is also recognized that the face-to-face aromatic π⋯π stacking and introduction of hetero S-atoms may endorse the mechanophosphorescence process in crystals.6b,c,19 In DBS crystals, the displaced stacking columns are primarily stabilized by hetero-atom mediated S⋯π interactions, avoiding pure (face-to-face) π⋯π stacking along the needle axis of the crystal (blue arrows in Fig. 2c). During elastic bending of a crystal having a planar molecular core11 like DBS, there must be a local variation of packing patterns within the stacking columns across the bend of the acicular crystal (Fig. 6 and S13†). Such reorganization of stacking patterns upon bending could be feasible via redistribution of the weak S⋯π (energetically less demanding, IE −16.1 kJ mol−1) interactions via reversible rotation of DBS molecules across the bend (black arrows in Fig. 6). In effect, the stacking column of DBS molecules in the outer arc of the bend would be expanded and subsequent contraction of molecule-to-molecule stacking distance is expected in the inner part to accommodate the stress (Fig. 6). Despite the expansion and contraction of intermolecular stacking columns from outer to inner columns upon bending, the rotation of DBS molecules across the bend (black arrows) causes a larger face-to-edge S⋯C separation of molecules (d0 < d1 < d2, Fig. 6) which subsequently weakens S⋯π interactions in crystal. Hence, on bending, the stretching of S⋯π distances in both outer and inner arc of stacked DBS columns, could be the primary reason for the observed hypsochromic shift of RTP emission peak. Furthermore, it has previously been realized that the adjustment of pure π⋯π stacking distances upon bending causes macroscopic changes in FL emission for flexible crystals.5c,e In the current case, S⋯π associated columns (displaced stacking) have minor contributions from pure π⋯π interactions of DBS rings due to electrostatic reasons (reduce repulsion) and hence, it is expected to influence less likely the FL peak of DBS upon mechanical bending.
To get further insight into the importance of such chalcogen-cantered interactions in mechanical-emission properties, we analyzed the crystal structures of well-known elastically bendable luminescent π-conjugated crystals having thiophene moieties.5a,10 Such investigation also suggested that in addition to the displaced stacking, the S-centered weak interactions (e.g., S⋯F, S⋯π, S⋯S, and C–H⋯S) have also been noticed to play a crucial role in their interaction topologies (Fig. S14†).
Footnote |
† Electronic supplementary information (ESI) available: Materials, crystallographic and photoluminescence studies. CCDC 2120853, 2120854, 2166317 and 2166318. For ESI and crystallographic data in CIF or other electronic format see https://doi.org/10.1039/d2sc03729g |
This journal is © The Royal Society of Chemistry 2022 |