Crystallization-Induced Red Phosphorescence and Grinding-Induced Blue-Shifted Emission of a Benzobis(1,2,5-thiadiazole)-Thiophene Conjugate.

Mechanochromic luminogens are of significant importance in both academic and technical aspects. Thus far, most mechanochromic compounds exhibit bathochromically shifted emission upon grinding; the examples of those that exhibit blue-shifted emission still remain limited. Herein, a donor-acceptor-donor (D-A-D)-structured conjugate, namely 4,7-di(2-thienyl)-2,1,3-benzothiadiazole (DTBT), comprising benzobis(1,2,5-thiadiazole) and thiophene units, has been carefully synthesized and investigated. DTBT exhibits typical intramolecular charge transfer (ICT) characteristics, crystallization-induced phosphorescence (CIP), and remarkable mechanochromism. Although it merely emits fluorescence in solutions with distinct ICT features, its crystals demonstrate bright-red room-temperature phosphorescence (616 nm) with efficiency up to 25.0% and generate yellow excimer fluorescence (578 nm) upon mechanical grinding, accompanying decreased lifetimes from 10.9 μs to 3.5 ns and a blue-shifted emission of 38 nm. These results highly indicate the feasibility to fabricate novel CIP luminogens with blue-shifted mechanochromism.

Introduction

Mechanochromic luminogens, whose emission can be finely modulated upon mechanical stimulation through the changing of molecular arrangement, have attracted intense interest on account of their fundamental importance and promising technical applications in memory devices, strain sensors, security inks, and camouflage.[1−10] Currently, most reported organic mechanochromic luminogens exhibit red-shifted emissions in response to mechanical stimuli;[11−15] those with blue-shifted emissions are rarely reported,[16−18] despite their importance for the comprehensive understanding of mechanisms and for summarizing the universal design principle. On the one hand, most reported conventional emitters normally demonstrate aggregation-caused quenching (ACQ) in the aggregation state,[19] making it difficult to receive intense solid emission and consequently hard to observe mechanochromism. On the other hand, in comparison with other stimuli (i.e., pH, temperature) responsive materials, mechanochromic luminogens still remain in their infant stage. Although increasing examples have been reported, general design principles remain scarce.[1−3,14,16] Further exploration of novel mechanochromic materials with high contrast is thus strikingly desirable.

In 2001, Tang and co-workers discovered an aggregation-induced emission (AIE) phenomenon, which is exactly the opposite of ACQ, from twisted siloles.[20,21] Many AIE luminogens (AIEgens) can exhibit mechanochromic behaviors. Among different AIEgens, those with crystallization-induced emission (CIE) have attracted great attention.[22−27] CIE compounds weakly emit in amorphous solids but become highly emissive in crystalline states,[22,24] which makes them highly promising as mechanochromic luminogens.[2,24] The CIE phenomenon is ascribed to the highly rigidified conformations in crystals owing to the presence of effective intermolecular interactions, which significantly frustrate the intramolecular molecular motions, thus depressing nonradiative deactivations and lighting up the emissions.[21−23] Apart from fluorescence, crystallization-induced phosphorescence (CIP) was also noticed in 2010,[25,28] which offers an effective method toward efficient room-temperature phosphorescence (RTP) from pure organics. Owing to the extraordinary susceptibility of triplet states to environmental quenchers (i.e., oxygen and moisture), CIP luminogens are highly potential mechanochromic emitters.[29−31] Meanwhile, compared with fluorescence, phosphorescence owns much larger Stokes shift, whose emissive maximum is even redder than the excimer fluorescence. If the regular molecular packing in crystals is destroyed and converted into disordered amorphous states by mechanical stimulation, triplet excitons are readily quenched by vibrational dissipations and external quenchers.[28−30] Consequently, no remarkable triplet emission can be expected. Therefore, destruction of CIP crystals may serve as a general strategy toward mechanochromic compounds with high contrast and mechanically induced blue-shifted emission.

To construct such mechanochromic CIP luminogens, in this contribution, a donor–acceptor–donor (D–A–D)-structured luminogen, namely, 4,7-di(2-thienyl)-2,1,3-benzothiadiazole (DTBT, Figure A), consisting of two thiophenes and benzo[c][1,2,5]thiadiazole units, was synthesized. DTBT is highly planar with small torsion angles less than 6° between D and A planes. It emits fluorescence in solutions and disordered amorphous solids, but bright-red RTP in crystals, exhibiting CIP characteristics with high efficiency (Φ) up to 25.0%. DTBT also exhibits different emission maxima in various states with obvious mechanochromism (Figure B). Specifically, upon grinding, the emission color, maximum, and lifetime of the crystals changed from red, 616 nm, and 10.9 μs to orange, 578 nm, and 3.5 ns, respectively, displaying a blue shift in response to mechanical stimuli.

Figure 1

(A) Synthetic route of DTBT synthesis and (B) schematic illustration of its emission properties at varying states.

Results and Discussion

DTBT was obtained by the well-known Stille C–C coupling reaction between 4,7-dibromo-2,1,3-benzothidiazole and (2-thienyl)tributylstannane with palladium catalyst (Figure A) in a high yield of 75%. DTBT owns good solubility in common organic solvents such as toluene, acetonitrile, dichloromethane (DCM), chloroform (TCM), tetrahydrofuran (THF), dimethylformamide (DMF), and dimethylacetamide. The chemical structure of DTBT was characterized by high-resolution mass spectrum (HRMS) and NMR spectra (Figures S1–S3), with satisfactory results obtained. For example, the reaction product gave a [M + H]+ peak at m/z 299.9854 (calcd 299.9850) in the HRMS spectrum, thus confirming the formation of the target adduct. Furthermore, single-crystal analysis directly suggests the successful preparation of DTBT (Figure S4, Table S1, and CCDC 1872296).

DTBT exhibits two absorption bands at around 308 and 446 nm in toluene, which can be assigned to the π–π* and intramolecular charge transfer (ICT) transitions (Figure A), respectively. This absorption profile slightly varies in diverse solvents, which indicates that its ground-state electronic structures are almost independent of the solvent polarity. Photoluminescence (PL), however, is generally bathochromically shifted, peaking at 550, 565, 559, 568, 576, and 580 nm in toluene, TCM, THF, DCM, acetonitrile, and DMF (Figure B), with Φ values of 52.8, 44.5, 49.1, 45.1, 35.4, and 32.8%, respectively. These results are highly indicative of the ICT feature of DTBT.[32,33] The relatively small variations in emission maximum and Φ values of D–A–D-structured DTBT in different solvents suggest its rigid conformation and/or small change in dipole moment at excited states. The electron densities of DTBT calculated by Gaussian 16 at the B3LYP/6-31G(d) level are shown in Figure . Although the electron cloud of the highest occupied molecular orbital (HOMO) level is predominantly distributed on the phenyl ring and the D unit of thiophene, that of the lowest unoccupied molecular orbital (LUMO) level is almost localized on the A moiety of thiadiazole, thus testifying the ICT nature of DTBT.[34,35]

Figure 2

(A) UV–vis absorption and (B) PL spectra of diverse DTBT solutions (20 μM). Excitation wavelength (λex) in (B) is 450 nm.

Figure 3

HOMO and LUMO energy levels of DTBT.

PL spectra of DTBT in solvent and solvent/nonsolvent mixtures were also recorded. As can be seen from Figure , the PL intensity of DTBT in pure THF is so high that a prominent peak at 558 nm is recorded. With increasing water fraction (fw), the emission intensity continuously decreases to a negligible level when fw = 90%, which suggests ACQ nature (fluorescence) of DTBT. It is also noted that the PL peak also changes with fw. With boosted fw, the maximum is bathochromically shifted until fw = 70%, then dramatically blue-shifted, changing from 558 to 584 nm (fw = 70%), and then to 567 nm (fw = 90%). When fw is ≤70%, evident signals are recorded owing to the molecular dissolution of DTBT in mixtures. Increasing fw promotes the mixture polarity, subsequently leading to red-shifted emission along with lowered PL intensity owing to the ICT effect. However, when fw is further increased to 80%, on account of the strikingly decreased solvating power of the mixture, DTBT molecules begin to aggregate. Consequently, a much less polar microenvironment is formed because of the molecular self-wrapping, thus resulting in blue-shifted emission. Further addition of water affords more serious aggregation and even a less-polar microenvironment, thus resulting in weakened emission with a blue-shifted maximum.

Figure 4

(A) Emission spectra and (B) peak intensity of DTBT in THF and THF/water mixtures with varying fws (20 μM, λex = 390 nm). The inset picture in (B) is the photograph of DTBT in THF and in 10:90 THF/water mixture taken under 365 nm UV irradiation.

Despite DTBT showing typical ACQ characteristics in THF/water mixtures, astonishingly, its single crystals emit a bright-red light peaking at 616 nm (Figure A,B) with a Φ value as high as 25.0%. Even more interestingly, DTBT solids show distinct mechanochromic luminescence. After being ground with a pestle, the red emissive crystals are converted into fine powders, which generate an orange light with a maximum at 578 nm (Figure A,B), demonstrating a distinct change in emission color with a wavelength blue shift of 38 nm. On further DCM fuming for 3 min, the bright-red emission is reverted. To attain further insights into the mechanism of mechanochromism, the powder X-ray diffraction (PXRD) measurement was conducted. As can be seen from Figure C, for DTBT crystals, the sharp and intense diffractions suggest their highly ordered crystalline attributes; for their ground counterparts, the extremely weak diffractions reveal their predominantly disordered amorphous features. The fumed powders, however, exhibit recovered sharp diffractions, thus implying the restoration of regular crystalline packing. Therefore, the switchable conversion from crystalline to amorphous states of DTBT should be responsible for its reversible mechanochromism.

Figure 5

(A) Luminescent photographs (365 nm UV irradiation), (B) emission spectra, and (C) PXRD patterns of DTBT crystals, ground solids, and fumed powders. Lifetimes of (D) DTBT crystals and (E) their ground solids.

Generally, most reported mechanochromic luminogens exhibit prominent red-shifted emission; a dramatically hypsochromic shift is rarely observed, particularly for those under ambient conditions. Considering the rigid planar structure and incorporation of heteroatoms, which tend to form excimers and promote spin–orbital coupling, respectively, there are several possibilities for the blue-shifted emission: (1) excimer emission in crystals and monomer emission in ground solids, (2) RTP emission in crystals and fluorescence in ground powders. To obtain more information, time-resolved emission measurement was conducted. As shown in Figure D,E, although the pristine crystals own the ⟨τ⟩ value as large as 10.9 μs, that of the ground solids is merely 3.5 ns, which are suggestive of their RTP and fluorescence nature, respectively, thus favoring the second case. Considering the absence of RTP in solutions and ground solids, the bright RTP emission of crystals suggests the CIP characteristic of DTBT. Moreover, such efficient red RTP emission (Φ = 25.0%) is also rare, which renders DTBT highly promising for organic light-emitting diode and bioimaging applications. Therefore, the mechanochromism of DTBT should be ascribed to the conversion from phosphorescence to fluorescence, accompanied with transformation from ordered crystalline states to disordered amorphous states.

To gain more information about the solid emission, particularly, to clarify whether excimer emission is present in the solid state, DTBT-doped poly(methyl methacrylate) (PMMA) films with different doping concentrations of 0.1, 1, and 10 wt % were fabricated. With increasing DTBT concentration, the emission gradually red shifted from 541 to 553 nm and then to 573 nm, with the emission color changing from yellow to yellowish-orange and then to orange for the 0.1, 1, and 10 wt % DTBT/PMMA films (Figure A,B), respectively. Such results clearly illustrate how aggregation impacts the emission of doped films. It is assumed that in the 0.1 and 10 wt % DTBT/PMMA films, monomer and excimer emissions are observed, respectively. Generally, compared with monomer fluorescence, excimer emission owns much longer lifetime and decreased PL efficiency.[19] To testify the above conjecture, ⟨τ⟩ and Φ values of different films were thus measured, which are 34.2/28.6/15.3% and 4.2/8.9/12.5 ns for the 0.1, 1, and 10 wt % DTBT/PMMA films (Figures D, S5, and 6E), respectively. The red-shifted emission, decreased efficiency, and prolonged lifetime highly suggest the excimer emission in the 10 wt % DTBT/PMMA film. On account of the similar highly aggregated states and approaching emission maxima (∼575 nm) of the concentrated doped film and ground solids, these results strongly indicate the excimer fluorescence in the ground solids. Meanwhile, these films also show no phosphorescent emissions. The absence of phosphorescence in PMMA films together with bright-red RTP in the crystalline states duly confirms the CIP feature of DTBT at ambient conditions.[25,28] Furthermore, these results imply that the above mechanochromism should be ascribed to the switchable transformation from RTP to excimer fluorescence between crystalline and amorphous states.

Figure 6

(A) Emission spectra, (B) photographs taken under 365 nm UV irradiation, and (C, D) lifetimes of DTBT/PAAM films with different weight fractions of DTBT as indicated.

Single-crystal structure can provide exact conformation and molecular packing of the compounds in crystals, thus beneficial to the understanding of the solid emission. Single crystals of DTBT were thus cultured through slow evaporation from its CHCl3–methanol solution. As depicted in Figure , DTBT molecules adopt nearly coplanar conformations, with small dihedral angles of 5.18 and 1.71° for the planes of P1 and P3 as well as P2 and P3 (Figure B), respectively. Despite their almost planar conformation, there are no strong π–π stackings between adjacent DTBT molecules because of their perpendicular or slipped parallel packings. Specifically, noncovalent intermolecular electronic communications of C–H···π (2.840 Å) among adjacent molecules and intramolecular short contacts of S···N (2.879 Å) are present in crystals (Figure C). On the one hand, these contacts further rigidify the conformations; on the other hand, the presence of heteroatoms and S···N contacts could promote the n−π* transition, orbital coupling, and consequently intersystem crossing (ISC) process, thus favorable for the generation of RTP emission.[36,37] Promoted ISC process, rigid planar structure, and absence of intense π···π contacts render DTBT in crystals highly RTP-emissive. Upon manual grinding, intermolecular short contacts are destroyed and triplet excitons tend to be quenched owing to the activated vibrational deactivations and exposure to external quenchers, thus resulting in the disappearance of RTP emission. Meanwhile, mechanical grinding may force the molecules to stack much closer to form remarkable π···π stackings, which afford excimer fluorescence at a much bluer wavelength when compared to the RTP emission, thus yielding the blue-shifted emission.

Figure 7

(A) Molecular packing in a unit cell; (B) dihedral angles; and (C) fragmental molecular packing with denoted intra- and intermolecular interactions of TDBT single crystals.

Conclusions

In summary, D–A–D-structured DTBT with highly planar conformations was synthesized and investigated in terms of photophysical properties. It exhibits typical ICT characteristics with good emission efficiency and relatively small variations in emission maxima in different solvents, owing to its highly rigid conformations. DTBT merely generates fluorescent emissions in solution and doped polymeric films, but generates efficient red RTP with an impressively high Φ up to 25.0%, demonstrating CIP characteristics. Furthermore, upon grinding, with the conversion from the ordered crystalline state to the disordered amorphous state, red RTP (616 nm) of DTBT disappeared, accompanying the emergence of orange excimer emission (578 nm), thus demonstrating remarkably blue-shifted emission. Such efficient red RTP along with mechanical-stimuli-induced blue-shifted emission is scarcely reported. The CIP feature, outstanding red RTP emission, and remarkable mechanochromism render DTBT highly promising for versatile optoelectronic applications.

Experimental Section

Materials and Instruments

4,7-Dibromo-2,1,3-benzothidiazole, tetrakis(triphenylphosphine) palladium(0) [Pd(PPh3)4], (2-thienyl)tributylstannane, and PMMA were obtained from Aladdin. Toluene/acetonitrile/DCM and THF were, respectively, distilled from CaH2 and sodium/benzophenone under nitrogen before use. DMF as well as other commercially available reagents were directly used. 1H (500 MHz) and 13C NMR (126 MHz) measurements were conducted on a Bruker AVANCE III HD 500 MHz spectrometer using deuterated dimethyl sulfoxide (DMSO) as a solvent and tetramethylsilane (δ = 0 ppm) as an internal standard. High-resolution mass spectrum (HRMS) and absorption were recorded on a ThermoFisher Q-Exactive mass spectrometer with an EI ion source and a UV-3600 UV–vis photospectrometer, respectively. Emission spectra were recorded on a Cary Eclipse fluorescence spectrophotometer under ambient conditions. PL quantum yields of DTBT in solvents were evaluated by the comparison method using quinine sulfate (Φ = 54% in 0.1 N H2SO4) as a standard, whereas those of the single crystals and doped films were determined by the absolute method using an integrating sphere. Single-crystal XRD data were collected on a Bruker–Nonices Smart Apex CCD diffractometer, and refinement was conducted using the SHELTL suite of X-ray programs (version 6.10). The PXRD measurement was performed on an X’Pert PRO X-ray diffractometer. The Gaussian 16, Revision B.01 package[38] was used for geometry optimization at the B3LYP/6-31G(d) level. Visualization of the molecular orbitals were conducted with Multiwfn software package.[39]

Synthesis of DTBT

4,7-Dibromo-2,1,3-benzothidiazole (500 mg, 1.7 mmol), (2-thienyl)tributylstannane (1.522 g, 4.1 mmol), and anhydrous toluene (30 mL) were placed into a dried 100 mL two-neck round-bottomed flask. Pd(PPh3)4 (90 mg, 0.078 mmol) was then added under nitrogen; the resulting mixture was stirred and refluxed for 24 h. Upon cooling to room temperature, the mixture was diluted with 30 mL of toluene and then poured into 150 mL of distilled water. After being extracted with 200 mL of toluene three times and then washed by brine twice, the collected organic phase was dried over MgSO4 overnight. After being filtrated, toluene was evaporated. The red crude product was further purified with a silica column using DCM/petroleum ether (1:5 by volume) as an eluent. Red crystals were obtained in 75% yield. 1H NMR (500 MHz, DMSO-d6) δ (ppm): 8.14 (d, J = 3.7 Hz, 2H), 8.05 (s, 2H), 7.79–7.72 (m, 2H), 7.27 (dd, J = 5.0, 3.8 Hz, 2H). 13C NMR (126 MHz, DMSO-d6) δ (ppm): 152.14, 138.87, 128.69, 128.53, 127.87, 126.25, 125.46. HRMS (EI): m/z calcd for C14H8N2S3 [M + H]+ 299.9850, found 299.9854.