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HomeChemistryFluorescence-based monitoring of the pressure-induced aggregation microenvironment evolution for an AIEgen beneath...

Fluorescence-based monitoring of the pressure-induced aggregation microenvironment evolution for an AIEgen beneath a number of excitation channels


Pattern characterization beneath ambient and excessive strain

FTPE is a typical natural fluorophore with a monoclinic C2 symmetry; its atomic construction is proven in Fig. 1a. In crystals, the FTPE molecules are organized in a layered type alongside the b-axis. The molecular stacking of FTPE crystal is proven in Supplementary Fig. 1. In every layer, the molecules are linked by way of weak intermolecular interactions involving C–H···π interactions, however lack ππ interactions; that’s, the fluorene and benzene rings of neighboring molecules usually are not parallel. Earlier research have proven that the molecular conformation of FTPE performs a vital function in its luminescence habits: the upper the coplanarity of the fluorene and benzene rings is, the upper the diploma of intermolecular π-conjugation is. Moreover, the stacking mode and intermolecular interactions additionally contribute to the luminescence properties. Accordingly, the absorption edge within the ultraviolet-visible (UV–vis) spectrum and emission most within the luminescence spectrum exhibit a pink shift61. Determine 1b reveals the pictures of an FTPE single crystal captured throughout compression and decompression processes. The crystal was pale yellow with good transparency at atmospheric strain beneath daylight. In the course of the compression course of, the colour modified to orange pink after which darkish pink at 20.4 GPa. Upon releasing the strain to atmospheric strain, the FTPE single crystal pattern didn’t get better its preliminary coloration, indicating that the color-changing phenomenon was not reversible. Earlier research have demonstrated that the colour evolution throughout the compression course of is accompanied by a pink shift within the absorption edge38,49,52. Determine 1c reveals the UV–vis absorption spectrum recorded beneath completely different pressures. In ambient circumstances, the absorption fringe of FTPE was discovered be at 395 nm, equivalent to a band hole of two.88 eV, per earlier studies61. Particulars of the UV–vis absorption spectra beneath completely different pressures are proven in Supplementary Fig. 2. When the strain was elevated, the UV–vis absorption spectrum confirmed a transparent pink shift, as anticipated. Curiously, a brand new absorption edge appeared between 450 and 550 nm at 4.2 GPa. On the identical time, the form of the absorption spectrum modified considerably, suggesting essentially the most possible enhancement of the planarity of FTPE molecular conformation with larger conjugated diploma and the adjustments of stacking mode and intermolecular interplay by rising strain63. When the strain exceeded 12.4 GPa, the excellence between the unique and new absorption edges started to blur. With additional pressurization, new absorption peaks appeared at 474 nm and 531 nm. Determine 1d plots the change within the band hole (Eg) with strain, which was decided in Tauc coordinates methodology (Supplementary Fig. 3)64. Under 4.2 GPa, the band hole decreased solely barely from 2.88 to 2.82 eV. Between 4.2 and 10.8 GPa, the band hole decreased extra quickly from 2.82 to 2.50 eV. Additional, when the strain exceeded 10.8 GPa, the speed at which the band hole decreased turned even quicker, with the band hole lowering from 2.50 to 1.45 eV. Earlier research have demonstrated that the pink shift of the UV–vis spectrum might probably be because of a rise within the diploma of molecular conjugation, and that strain probably induced stacking structural transformation50,63,65.

Fig. 1: Absorption characterization of FTPE crystal beneath strain.
figure 1

a Atomic construction of FTPE molecule. b Pictures of FTPE single crystal throughout compression and decompression processes (0 GPa signifies ambient strain). In situ (c) UV–vis absorption spectra and (d) band gaps of FTPE at completely different pressures (P). The error bars are estimated in accordance with the intercept and slope of the match curve.

Construction evolution of FTPE beneath excessive strain

To research the affect of strain on the construction of FTPE, we carried out in situ high-pressure ADXRD and IR spectral investigations in a diamond anvil cell (DAC) chamber at room temperature. Determine 2a reveals the ADXRD knowledge equivalent to discrete compression states (strain vary: 0.8 to 19.5 GPa), together with the information for the corresponding decompressed states at ambient strain. At pressures under 13.0 GPa, the diffraction peaks clearly indicated that the pattern was primarily within the crystal state, and all diffraction peaks shifted towards larger angles with rising strain due to the pressure-induced discount within the unit cell quantity and interplanar crystal spacing. Nonetheless, at pressures bigger than 13.0 GPa, the diffraction peaks turned wider owing to diffuse scattering, suggesting that the amorphous section was dominant. This consequence signifies that the sharp lower in band hole (Fig. 1d) was because of amorphization. Furthermore, upon releasing the strain, the diffraction sample didn’t correspond with the sample obtained for the unique pattern, indicating that the method was irreversible. To additional perceive the connection between the construction and strain, we carried out Le Bail refinement of the ADXRD patterns. Supplementary Fig. 4 and Supplementary Desk 1 present the ADXRD patterns obtained beneath completely different pressures and the corresponding refinement outcomes. The curves of the pressure-dependent lattice parameters and quantity could possibly be divided into three sections in accordance with the change fee, as proven in Fig. 2b, indicative of the isostructural section transition and amorphization of the pattern. The strain at which the isostructural section transition occurred coincided with the strain at which a brand new absorption edge emerged within the UV–vis absorption spectrum. Due to this fact, we deduced that, under 5.0 GPa, the lattice parameters and quantity decreased quickly owing to straightforward compression. Above 5.0 GPa, there was no extra accessible interspace for additional molecular packing. Nonetheless, the molecular conformation continued to vary in response to the strain enhance, resulting in the next elastic modulus and small pressure-dependent quantity change. When the strain was above 13 GPa, the amorphous part was dominant, per the sharp lower within the band hole. Concurrently, the refinement outcomes recommended that the compression of FTPE was anisotropic, with the b-axis being essentially the most compressible axis. Mixed with the molecular stacking of FTPE (see Supplementary Fig. 1), we inferred that the strain quickly shortened the intermolecular packing distance, on the identical time parallelized the molecular conformation, which can enhance the ππ overlapping diploma of adjoining molecules, and even to type excimers32. As well as, the MALDI-TOF of FTPE at ambient strain, launched from 6.6 GPa, 11.5 GPa, and 19.5 GPa indicated that there would most likely be no molecular polymerization manufacturing throughout the pressurization course of (see Supplementary Fig. 5).

Fig. 2: Structural evolution of FTPE crystal beneath excessive strain.
figure 2

In situ (a) ADXRD patterns of the FTPE powder and (b) unit cell quantity beneath strain. The pink dotted strains point out isomorphic and crystalline-to-amorphous section transitions. IR spectra of FTPE single crystal in (c) 600–1800 cm−1 and (d) 2800–3300 cm−1 ranges at completely different pressures within the vary of 0.2–19.1 GPa. Inset of (b) reveals the compression fee of the lattice constants (a/a0, b/b0, c/c0).

Additional, in situ high-pressure IR experiments had been carried out to acquire data on the chemical bonding and native construction. Determine 2c, d reveals the IR spectra of the FTPE single crystal within the 600 to 3300 cm−1 vary beneath completely different pressures. Supplies Studio was used to investigate the IR vibrational modes beneath atmospheric strain, and the outcomes are proven in Supplementary Desk 266. By elevating the strain, many of the IR peaks had been blue-shifted indicating that the interatomic distances had been decreased however in several extent50,54,55. Under 5.2 GPa, strain primarily affected the stacking mode, inflicting a dominating discount in cell quantity, which might prohibit the stretching vibration of molecules by intermolecular interactions, most likely contributing to the plain blue shift at high-wavenumber area above 2800 cm−1. The emergency of the brand new peak at 3165 cm−1 (3.5 GPa) could also be attributed to the formation of intermolecular C–H···π interactions. At pressures larger than 5.2 GPa, the strain primarily triggered a change within the molecular conformation, due to which the blue shift of the low-wavenumber peaks equivalent to molecular rotation was extra conspicuous as tight stacking would drastically hinder the fragrant rings to rotate freely63. The blue shift of the rotational modes turned clearer after the compression-induced conformational change, and the form of most IR peaks modified noticeably, suggesting that the utilized strain elevated the energy of intermolecular interactions. Above 11.6 GPa, most IR peaks turned broader and the stretching vibration alerts weakened, whereas the attribute peaks of the conformation-related rotational vibrations had been nonetheless noticed to some extent, indicating the emergence of an amorphous section, per the ADXRD knowledge. Supplementary Fig. 6a–d reveals the IR peaks of the pattern pressurized to 4.9 and 11.5 GPa. As proven in Supplementary Fig. 6e, after the discharge of strain, the IR peaks of the pattern pressurized to 4.9 and 11.5 GPa had been much like these beneath atmospheric strain, suggesting that the isostructural section transition was reversible. Nonetheless, following strain launch from 19.1 GPa the IR peaks of the pattern had been completely different from these of the unique pattern, per the ADXRD knowledge, indicating that the structural change was irreversible.

Fluorescence measurements of FTPE beneath excessive strain

To look at the affect of the stacking mode and molecular conformation on the emission properties, three excitation wavelengths had been chosen primarily based on the absorption knowledge to discover the emission properties beneath excessive strain. Determine 3 reveals the pressure-dependent 3D and 2D fluorescence spectra of FTPE single crystal excited with 355, 532, and 633 nm lasers (the separated fluorescence spectra had been displayed in Supplementary Figs. 79). Mixed with the structural knowledge, it could possibly be inferred that the molecular motion was extra restricted beneath strain than that within the preliminary crystal state owing to the decreased distance between the molecules. Due to this fact, the power was launched within the type of radiative transition, leading to a rise within the depth of the emission obtained beneath 355 nm excitation under 0.3 GPa (Fig. 3a). At low pressures, a blue shift was noticed within the high-wavenumber IR peaks associated to cell quantity, indicating that the strain primarily shortened the intermolecular distance of adjoining molecules. Accordingly, with rising strain, the emission obtained beneath 355 nm excitation decreased in depth, and there was a pink shift of the emission most. Additional enhance in strain led to a planar conformation, leading to a rise in intramolecular conjugation65. On the identical time, the lower of intermolecular distance could favor the polarization impact on adjoining molecules, enhance the intermolecular interplay, or drive the change from crystalline state to amorphous state, which would scale back the power of the bottom excited state, resulting in a pink shift within the fluorescent wavelength and a lower in depth67,68. Consequently, the emission confirmed an obvious pink shift of the emission most, and the emission depth decreased additional because of elevated conjugation and the transformation of the crystal state from its preliminary state to a state with planar conformation (Fig. 3b). When the pattern turned amorphous, the emission most didn’t shift with the strain. Ultimately, the emission band virtually disappeared at 20.4 GPa because of the fairly low quantum effectivity for the amorphous state and emission-detriment intermolecular interactions akin to ππ interactions. Following the strain launch, the emission depth didn’t return to the preliminary worth, indicating that the conformational change was irreversible (Fig. 3c). Though the molecular conformation tended to get better to the preliminary one, the stacking mode had modified, and the molecules had rearranged. In consequence, the transformation was irreversible. The pressure-dependent coloration change beneath 355 nm laser excitation is proven in Supplementary Fig. 10, together with the corresponding emission spectra.

Fig. 3: Fluorescence monitoring of FTPE crystal structural evolution beneath excessive strain.
figure 3

Stress-dependent 3D and 2D fluorescence spectra of FTPE single crystal throughout the compression course of beneath laser excitation at (a, b) 355 nm, (d, e) 532 nm, and (g, h) 633 nm. Adjustments within the fluorescence depth with strain throughout compression and decompression processes beneath laser excitation at (c) 355 nm, (f) 532 nm, and (i) 633 nm. The colour in (a, d, g) signifies the magnitude of fluorescence depth. P+ and P- in (c, f, i) point out the fluorescence throughout the compression and decompression, respectively.

Determine 3d, e reveals the pressure-dependent fluorescence spectra obtained beneath 532 nm laser excitation over the strain vary of 0 to twenty.5 GPa. In response to the UV–vis spectra (Fig. 1c), the absorbance at ~532 nm elevated marginally with strain, and accordingly, the depth of the emission beneath 532 nm laser excitation elevated slowly throughout low strain vary. After the isostructural section transition above 4–5 GPa, the absorbance elevated abruptly with the looks of a brand new absorption edge between 450 and 550 nm, leading to a major enhancement of the emission depth (Fig. 3f). Due to this fact, these fascinating emission behaviors beneath 532 nm laser excitation could possibly be moderately employed to analyze the isostructural section transition inside crystal beneath strain. Notably, there was no apparent pink or blue shift of the fluorescence band, completely different from the outcomes obtained for many luminescent supplies beneath excessive pressures49,55. As talked about above, the fluorescence band underwent a pink shift because of the pressure-induced conformational planarization, stacking mode change, and intermolecular interplay enhancement, as noticed within the fluorescence spectra obtained beneath 355 nm laser excitation. Mixed with the connection of the emission depth, it is perhaps inferred that the molecules that had been progressively turning planar might need been excited by the 532 nm laser. Due to this fact, there was a pointy enhance within the emission depth after the isostructural section transition of FTPE, however the excited state power could loosen up to the potential effectively to type an equilibrium state69 or the molecular conjugation might maybe be unchanged, leading to an virtually unchanged emission most with rising strain. At pressures larger than 10.8 GPa, the fluorescence depth decreased quickly till the weakly luminescent state was reached at a strain of 20.5 GPa because of the transformation of the extremely emissive crystal to a weakly emissive amorphous section61. Due to this fact, regardless of the gradual enhance within the molecular absorbance with strain, the emission depth decreased. When the amorphous state dominated, the emission depth decreased extra slowly. Following the strain launch, the fluorescence traits didn’t revert to the preliminary ones, per the outcomes obtained beneath 355 nm laser excitation, indicating that the method was irreversible. Furthermore, the pressure-dependent normalized lifetime spectra excited by the 532 nm laser and the calculated decay instances of FTPE additionally offered the three distinct processes of 0.6–4.8 GPa, 4.8–10.1 GPa, and 10.1–19.0 GPa, as proven in Supplementary Fig. 11, that are effectively equivalent to the PL and X-ray diffraction outcomes.

Equally, when excited with the 633 nm laser beneath strain, along with the absorbance habits, the emission habits was additionally fully completely different from these noticed beneath different excitation wavelengths. In response to the UV–vis outcomes, the absorbance at ~633 nm was virtually zero at low strain, and there was no emission or weak emission with a relentless pressure-dependent depth under 5.3 GPa (Fig. 3g, h). After the isostructural section transition, the depth of the emission elevated progressively. Additional, the emission most didn’t shift with rising strain, as noticed beneath 532 nm laser excitation. Determine 3i reveals an abrupt enhance within the emission depth with strain and the next fee of the emission depth enhance after 12.9 GPa, which is the strain equivalent to the emergence of the amorphous state in accordance with the ADXRD leads to Fig. 2a. As well as, the pattern exhibited weak luminescence when the strain was launched. Mixed with the strain–construction knowledge, it could possibly be inferred that the fluorescence noticed beneath the 633 nm laser excitation is perhaps because of the amorphous section generated beneath the compression pressure. The crystal exhibited no luminescence between 0 and 5.3 GPa beneath 633 nm laser excitation as a result of the molecular construction was not within the amorphous section on this strain vary. Because the nonhydrostatic strain was elevated additional, part of the crystal section turned amorphous, leading to a gradual enhancement of the emission. Above 13.0 GPa, the construction progressively modified from a crystalline to amorphous section, accompanied by an considerable enhancement of the emission. These outcomes evidently supported that 633 nm laser excitation channel could possibly be used to observe the pressure-dependent emission habits of the amorphous state. Following the strain launch, the pattern confirmed a faint glow, implying that the molecules couldn’t revert to their preliminary state fully from the amorphous state after being pressurized to 19.5 GPa, per the outcomes obtained beneath 355 and 532 nm laser excitation. Such phenomenon motivated us to additional monitor the fluorescence spectra of FTPE in amorphous state beneath completely different pressures by the pattern ready in situ after releasing from the strain of about 20 GPa. As proven in Supplementary Fig. 12c, d, the emission depth elevated linearly beneath the strain starting from environment strain to about 20.0 GPa beneath 633 nm laser excitation. This fluorescence noticed beneath the 633 nm laser excitation was assigned to the amorphous section generated beneath compression, which was much like the emission habits beneath the strain starting from 12.5 GPa to twenty.0 GPa in Fig. 3g–i and Supplementary Fig. 12a, b, additional implying the transformation between the crystalline and amorphous states for FTPE. As well as, Supplementary Fig. 13 reveals the pressure-dependent emission traits beneath 355, 532, and 633 nm laser excitations when the pattern was pressurized as much as ~10 GPa. In the course of the compression and decompression processes, the emissive states excited by the 355 and 532 nm lasers had been per the preliminary state, whereas the emissive state excited by the 633 nm laser was not (Supplementary Fig. 13g–i), suggesting the doable transformation strain between crystalline and amorphous states is about 10 GPa.

Thus, by deciding on three completely different laser excitation wavelengths, the adjustments within the molecular stacking and molecular conformation could possibly be monitored, as proven in Fig. 4. Regardless of the slight modulation of molecular conformation, the nearer stacking mode, equivalent to the change of cell quantity, performed a significant function under ~5 GPa. That’s, the blue shift of the low-wavenumber IR peaks related to the molecular conformation was not apparent, whereas the sensitivity of the high-wavenumber peaks to quantity change was obvious. In the meantime, the lower within the distance between the molecules within the crystal would possibly result in a rise within the intermolecular interplay, polarization impact on adjoining molecules or the amorphous evolution, and the power was dissipated by nonradiative pathways. Thus, the emission depth decreased progressively, accompanied by a pink shift of the emission most beneath 355 nm laser excitation. In the intervening time, the continual pink shift in PL emission and the lowering band gaps calculated by the UV–vis absorbance mirrored the planarity tendency of molecules and conjugation-benefit intermolecular interactions that enhanced the intramolecular and intermolecular conjugation. When the isostructural section transition occurred, the molecules flattened progressively beneath strain, leading to a blue shift of the low-wavenumber IR peaks related to the conformation and a readily observable change within the peak shapes. The planarization of the molecular conformation causes a discount within the variety of molecules within the preliminary conformation and a rise within the variety of molecules within the crystal state with planarized molecules. Such a numeric correlation offers rise to a rise within the fluorescence beneath 532 nm excitation till the looks of amorphous. What’s exceptional within the pressure-dependent fluorescence beneath 532 nm excitation is that the emission depth all of a sudden elevated at an increasing fee above 5 GPa, pointing to a different issue that led to the enhancement of the depth. The impact of restriction of intramolecular motions was thought of because of the compression of molecular bonds and the enhancement of the intermolecular interactions these had been noticed in emergency of doable C–H···π at 3.5 GPa within the infrared spectra. Upon rising the strain, the molecular state progressively modified from a extremely emissive crystal state right into a weakly emissive amorphous state61. Due to this fact, regardless of the rise in absorption with the strain, the emission beneath 532 nm excitation decreased quickly above 10.8 GPa. Nonetheless, the 633 nm laser could possibly be used to observe the emission of the amorphous state. Beneath excitation by 633 nm laser, the emission of amorphous state could possibly be noticed to extend with the strain above 5 GPa, whereas there may be virtually no emission earlier than ~5 GPa is perhaps ascribed to the issue of transformation between crystalline and amorphous states beneath such low strain. It has been proved that the amorphous state was unable to return to the crystalline state by merely eradicating the strain, with no reversibility of fluorescence depth and wavelength, in addition to the IR absorbance, UV–vis absorbance, and ADXRD.

Fig. 4: Schematic of FTPE crystal structural evolution beneath excessive strain by way of a number of excitation channels.
figure 4

All the compression course of will be divided into three levels: (1) the strain reduces the gap between the molecules, notably alongside the b-axis, with out apparent molecular conformation change, (2) owing to the restricted area, the molecular conformation would alter to be planar for adapting to the smaller area beneath strain, and (3) an extra compression results in the amorphization of the construction.

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