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Abstract
The stimulus-responsive smart switching of aggregation-induced emission (AIE) features has attracted considerable attention in 4D information encryption, optical sensors and biological imaging. Nevertheless, for some AIE-inactive triphenylamine (TPA) derivatives, activating the fluorescence channel of TPA remains a challenge based on their intrinsic molecular configuration. Here, we took a new design strategy for opening a new fluorescence channel and enhancing AIE efficiency for (E)-1-(((4-(diphenylamino)phenyl)imino)methyl)naphthalen-2-ol. The turn-on methodology employed is based on pressure induction. Combining ultrafast and Raman spectra with high-pressure in situ showed that activating the new fluorescence channel stemmed from restraining intramolecular twist rotation. Twisted intramolecular charge transfer (TICT) and intramolecular vibration were restricted, which induced an increase in AIE efficiency. This approach provides a new strategy for the development of stimulus-responsive smart-switch materials.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Stimulus-responsive smart luminescent materials [1–4] have been undoubtedly occupied position for their broad applications in light-emitting diodes [5,6], optical sensors [7,8], biological imaging [9,10], and 4D information encryption fields [11,12]. Particularly, these characteristic materials are applied to the turning-on mode of molecular switches owning to the extraordinary change of their emission intensity and/or color under external stimuli [13,14]. Fluorescence enhancement and color change exhibit non-negligible advantages in practical application [15,16].
Most traditional stimulus-responsive smart fluorescent materials exhibit an aggregation-caused quenching (ACQ) effect [17], and their fluorescence enhancement is severely limited in the switching field. Tuning the emission wavelength and increasing the fluorescence intensity are the key factors for turning on smart switches in stimuli-responsive application fields [18]. Aggregation-induced emission (AIE) molecules with an excited-state intramolecular proton transfer (ESIPT) skeleton have two kinds of light-emitting units, which can compensate for the defects of ACQ and insignificant fluorescence contrast [19–22]. Triphenylamine (TPA) is a typical member of the AIE groups [23,24]. Chemically linking TPA with an ESIPT structure is a plausible method toward novel fluorescent switches. Most AIE with ESIPT systems rely on regulating a known keto–enol fluorescence peak [25]. Nonetheless, the distorted mechanism results in some TPA derivatives having no AIE activity [26,27]. This defect severely prevents the regulation of AIE and ESIPT fluorescence peaks, which block the switch from being turned on. Traditionally, the invisible AIE channels could be visualized through altering the molecular constitution (e.g., introducing a benzothiadiazole group, benzaldehyde, benzene ring, or ester group) [28–30]. However, on the basis of the intrinsic molecular configuration, it is challenging to active a new AIE fluorescence channel.
Herein, (E)-1-(((4-(diphenylamino)phenyl)imino)methyl)naphthalen-2-ol (NNH) was selected, which consists of TPA and ESIPT groups. Such molecules play an important role in stimulating materials and light-emitting diodes fields [31,32]. First principles calculations indicate that the emission channels of K-state and E-state overlap together and the AIE property is inactive. Pressure-induced NNH has an enhancement and blue-shifted emission, exhibiting AIE-active behaviors. By harnessing this strategy, the new AIE fluorescence channel of TPA-based luminophores was activated to contrast the turning-on behavior. More importantly, as the pressure increased from 1.0 atm to 1.5 GPa, the system exhibited a 10-fold pressure-induced emission enhancement. Femtosecond transient absorption (fs-TA) in high-pressure experiments in situ revealed that pressure damages the ESIPT response and restricts twisted intramolecular charge transfer (TICT) and intramolecular vibration. Pressure effectively opens up a new AIE channel, realizing broad color aberration and high intensity. This is an innovative report of favorable opening of a new AIE channel and tuning responsive fluorescence switching by a pressure-inducing method.
2. Methods
2.1 Samples
NNH and CNN were purchased from Sigma (America) with 99% purity. All solvent was ultra-dry spectral grade and purchased from J&K (China).
2.2 High-pressure generation
Pressure was made by using diamond anvil cell (DAC) [33,34]. Ruby and sample were put into a DAC chamber with 800 µm diameter, constructed from a T301 steel gasket, which was preindented to 400 µm. Afterwards, a hole with 500 µm diameter was drilled in the center of indentation through the laser that drilled machine. The pressure values were calibrated by the standard ruby fluorescent technique.
2.3 Spectroscopic measurements
The steady-state emission spectra were obtained for the NNH and CNN on the RF5301 fluorescence spectrophotometer. The femtosecond TA measurements were performed adopt a power of 4 W with a pulse width of 50 fs under 1 kHz repetition rate, the wavelength is 800 nm with 50 fs for full width at half-maximum. The 400 nm pump pulses were delivered by BBO crystal with Coherent Legend (50 fs, 1 kHz, 800 nm). The excitation energy of the sample attenuates the pump pulse to 4 µ J, Surface Xplorer 2.2 software was used for fitting the kinetic traces.
2.4 Theoretic methods
The stabilized geometry structures were optimized with TDDFT methods and DFT in first excited-state and ground. In all situations, the computations were carried out by Gaussian 09 package [35]. The THF model implicitly selects integral equation formalist variables (IEFPCM) [36–38].
3. Results and discussion
The structure of NNH molecule is shown in Fig. 1(a). In tetrahydrofuran (THF) solution, the spectral information of NNH was consistent with calculations by density functional theory (DFT)/B3LYP with the TZVP basis set. According to the report of Keli Han et al. [39–41], this means that the functional and basis set are suitable for this system. The E-state and K-state peak positions were 573 nm and 585 nm by calculation, indicating the emission channels of K-state and E-state overlap. We measure the absolute fluorescence quantum yields for solids of NNH is only 1.68%. To understand of their AIE behaviors, a mixture of THF/water solution was used to determine photoluminescence (PL) activity (Fig. 1(b)). The emission of NNH was enhanced, with a maximum at 565 nm. We found that NNH exhibited very feeble nano-aggregation, with a αAIE (50% water fraction) value of 2.05 [Fig. 1(c)], and the fluorescence peak barely moved. Therefore, it can be inferred that NNH was AIE-inactive.
Fig. 1. (a) Structure of NNH. (b) PL spectra of NNH in THF/water solution. (c) Diagram of αAIE (PL intensity I/I0) versus the component of THF/water mixtures for NNH. (d) The calculated charge values on C2, C3, and C4. Blue represents N atoms, red represents O atoms, yellow represents C atoms. (e) PL spectra of NNH in MeOH/glycerol mixed solution. (f) PL spectra of NNH at different temperatures. (g) SEM image of aggregate formation about NNH.
Furthermore, on the basis of quantum mechanics, assist further predicting the AIE-activity. The asymmetric charge distribution of the TPA luminescent group indicates that it has AIE activity. NNH is further characterized, including the charges on C2, C3, and C4, which are calculated by utilizing the natural bond orbital analysis method [42], those values are 0.1523 a.u., 0.1488 a.u. and 0.1498 a.u., respectively (Fig. 1(d)). Parameter ‘‘D’’ is calculated with atomic charge in the following equation:
It has been previously proposed that AIE is greatly influenced by TICT [43]. Figs. S1(a) and S1(b) displayed the absorption spectrum and fluorescence spectra of NNH in nonpolar n-Hexane, and polar ACN solvents. The fluorescence intensity decreased with the increase of solvent polarity, which indicates polarity accelerate transition from ICT state to TICT state. The calculated molecular orbital results indicate that the charge is transferred among different parts of NNH from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) [Fig. S1(c)]. Therefore, ICT occurs in the excited-NNH. Optimized geometric structure displayed that TPA group has non-planar structure in the excited state [Fig. S2 and Table. S1], indicating NH has TICT property. Accordingly, intriguing (PL) properties might be expected under varying pressure. For our specimens in diamond anvil cell (DAC), the pressure-dependent PL spectra were taken up to 2.0 GPa (Fig. 2). One can clearly notice that the NNH spectrum found at 1.0 atm displayed a single emission-peak centered at 556 nm (Fig. 2(a)). Interestingly, once the pressure was imposed, an abnormal phenomenon was the appearance of a new emission peak at 468 nm. In order to determine the pressure-induced fluorescence peaks, we used molecule of (E)-N1-(naphthalen-1-ylmethylene)-N4,N4-diphenylbenzene-1,4-diamine (CNN) without hydroxyl group for comparison. Compared with NNH, the CNN molecule only had one luminescence state through the TPA group. In the Fig. 2(b), the position of the PL peak of CNN was 468 nm, which is consistent with NNH under pressure. Therefore, the pressure-induced fluorescence herein was attributed to the AIE state of TPA. The compression process significantly enhanced fluorescence intensity. The emission reached its maximum intensity at 1.5 GPa, which was approximately 10-fold higher than at 1.0 atm. During compression from 1.0 atm to 1.5 GPa, the quantum yield of NNH increased 24.06-fold (Fig. 2(c)). The fluorescence intensity decreased at 2.0 GPa because the decreased intermolecular distance induced increased π−π interaction. High-pressures can significantly enhance AIE efficiency for NNH compared with mixing in the THF/water [44,45]. The NNH/THF solution formed a transparent and clear solution from 1.0 atm to 2.0 GPa. It is worth noting that the sample exhibited needle-like crystals [46] when the pressure rose to 2.5 GPa (Fig. (2d)). Laser cannot pass through the needle-like crystal, so hardly obtained values of femtosecond transient absorption (fs-TA). Based on the CIE chromaticity diagram, the emission of NNH at 1.0 atm lies in yellow-green (0.41, 0.56) (Fig. 2(e)). As pressure was increased from 1.0 atm to 2.0 GPa, the NNH emission showed a wide color evolution from yellow-green to blue (0.12, 0.40). Furthermore, fluorescent images of NNH were consistent with the CIE chromaticity diagram. Figure 2(f) shows how the fluorescence intensity rose gradually with increasing pressure, in which the luminescence color changed from green to blue. The blue luminous peak is derived from the TPA luminous group, which pressure successfully opening the AIE channel. When there was pressure stimulation, the blue switch from AIE fluorescence was turned on. NNH was efficiently switched “on” by the compression.
Fig. 2. Characterization of NNH under different pressures. (a) Fluorescence spectra of NNH under high pressure from 1.0 atm to 2.0 GPa. (b) Normalized PL spectra of NNH, NNH under 0.5 GPa and CNN measured with 400 nm excitation. (c) Fluorescence quantum yield ratio for different pressures to 1.0 atm. (d) Conformation of NNH in THF solvent liquid state (left) and crystalline state (right). (e) Pressure-dependent chromaticity coordinates of NNH emission. (f) Fluorescent images of NNH at 1.0 atm and 2.0 GPa.
To describe the AIE channel unlocking and significantly improved efficiency for NNH during compression, as derived from the intramolecular stretching vibration and rotation configuration of NNH, we put forward an ideal mechanism (Fig. 3). Primitively, NNH locates at the ESIPT state after photoexcitation. Particles then rapidly transfer from ICT to the dark TICT state at 1.0 atm, which leads to depressed emission efficiency. At present, the AIE state is inactive due to intramolecular vibration. Only the ESIPT channel is open, emitting green fluorescence. Intramolecular rotation and vibration are restricted with pressure, which improves AIE efficiency and induces particles efficiently transferred to the AIE state. A new AIE-active channel is then opened. The blue AIE switch of TPA part is turned on, and the ESIPT process is turned off. Hence, the TICT process and intramolecular vibration are both restricted effectively, which improves AIE efficiency. In addition, intramolecular proton transfer is limited.
Fig. 3. Schematic diagram of relaxation dynamics of the excited-state NNH molecule at 1.0 atm and compression process. Design concept of TPA via pressure-induced luminescence.
Raman spectra of NNH were obtained to further confirm the vibration confinement mechanism. The intensity of the Raman band was dominated by the change in the molecular polarization [47,48]. The peaks of NNH exhibited a gradual blue-shift under high pressure, with a gradual weakening of peak intensity; this reveals that the intermolecular distance decreased with shortened bond lengths (Fig. 4(a) and 4(b)). NNH showed two significantly Raman peaks at 910 cm-1 and 2958 cm-1, which were attributed to C-N from TPA and the O–H stretching vibration, respectively. The Raman peaks of C-N from 910 to 930 cm−1 in DAC showed an obvious blue-shift (Fig. 4(c)), illustrating that the stretching vibration of the C−N bond was restricted. Of particular interest, the O–H stretching vibration Raman band showed a gradual blue-shift from 2958 to 2984 cm−1 (Fig. 4(d)), indicating that the distance of O–H decrease, which is unfavorable to the conversion of NNH from E* to K* in the excited state.
Fig. 4. Representative high-pressure Raman patterns of NNH: (a) 600-1300 cm-1 and (b) 1340-4500 cm-1. Raman frequency of C−O bond vibration (c) and C−N bond vibration (d) as a function of pressure.
To better understand the TICT process of the NNH molecule under an increasing pressure, in situ high pressure TA spectroscopy was employed [49–51]. Global fitting was used to analyze the relaxation dynamics of the excited-state NNH molecule with sets of kinetic curves simultaneously. We obtained femtosecond transient absorption spectra of NNH in THF solvent under different pressures. Figures 5(a) and 5(b) showed that when the NNH molecule was excited by light, an excited state adiabatic absorption signal appeared at 510 nm, and an emission signal appeared at 600 nm. As the pressure increased, the excited state absorption signal and emission signal were significantly weakened. The reason for the acceleration of the ICT process is that the increased pressure shortened the molecular bond length, and the intramolecular interaction became stronger. It can be seen that the non-radiative TICT process was effectively restrained under the action of pressure, which resulted in fluorescence intensity enhancement.
Fig. 5. (a)–(b) TA spectra of NNH in THF solution at 1.0 atm, and 0.5 GPa. (c)–(d) kinetics of the TA spectra of NNH at corresponding pressures. (e) Graphs of ICT time (τ2) and (f) TICT time (τ3) as a function of pressure.
The fs-TA spectra of NNH in the THF solvent are shown in Fig. 5. Under normal pressure, the related process could be conformed to three exponential-decay lifetimes: τ1, τ2, and τ3. τ1 can be derived from the ESIPT process, τ2 should be attributed to the ICT process, and τ3 is related to the TICT state [52,53]. The frontier molecular orbital of NNH is calculated to detect the dynamics process in the excited state (Fig. S3). The charge is transferred among different parts of NNH from the HOMO to the LUMO. Therefore, ICT occurs in the excited-NNH. Figures 5(c) and 5(d) showed the assessment for the best-fit. Curves use a global analysis. One can safely infer that τ1 vanished under pressure. In other words, under contact pressure, the ESIPT reaction was halted. However, the τ2 lifetime decreased from 1.43 ps at 1.0 atm to 0.42 ps at 2.0 GPa (Fig. 5(e)), indicating that the rate of the excited ICT process of the NNH molecule was accelerated under pressure. Nevertheless, the values of τ3 increased remarkably from 44.97 ps at 1.0 atm to 644.12 ps at 2.0 GPa (Fig. 5(f)). This means that pressure accelerated the ICT process, but pressure inhibited the TICT process.
4. Conclusion
In summary, NNH, was an AIE stimulus-responsive molecule during compression, NNH consists of two functional moieties: TPA and ESIPT groups. Theoretical calculations indicate that at 1.0 atm only the ESIPT process emits light, and AIE is inactive. We put forth a new design strategy for responsive fluorescence switching by a pressure-induced AIE-active channel turning on the switch. Under pressure, the intramolecular vibrations and TICT process of NNH are restricted, resulting in AIE intensity enhancement. The compression strategy successfully realized broadening the fluorescence color-change range and enhancing the fluorescence intensity at the same time. Thus, this approach is an important step for expanding the scope of the stimulus-responsive switch family of molecules.
Funding
National Natural Science Foundation of China (No. 11874180); Young and Middle-aged Scientific and Technological Innovation leaders and Team Projects in Jilin Province (20200301020RQ); National Basic Research Program of China (Grant No. 2019YFA0307701).
Disclosures
The authors declare no competing financial interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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