A fluorescence molecular switch with high contrast multi-emissions and ON/OFF states

Abstract

By integrating an AIE active tetraphenylethylene (TPE) with a molecular switch of spiropyran (SP) through a single bond linkage, a single molecular switch with four high contrast states of red, cyan, blue and dark has been achieved by synergistic control of the molecular structures and their packing modes in the solid state.

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Multi-stimuli responsive emission switching materials have attracted much attention in the past few decades both for fundamental study and promising applications in sensors,¹ displays,² data recording,³ security inks,⁴ and bioimaging,⁵ etc. Stimuli such as chemicals, light, heat, electrons, as well as (hydraulic) force have been applied to trigger the color switching, and emission switching between two or three, even four colors has been achieved.⁶ Furthermore, ON/OFF emission switching systems which have advantages in high-contrast optical memory and sensing devices have also been developed.⁷ However, the current ON/OFF molecular switch is only realized between homochromous emission and dark. Molecular switches with both multi-emissions (>2) and ON/OFF switching properties are still a big challenge due to the difficulties in successive control the molecular structure/molecular packing/intermolecular energy transfer. Such molecular switches can largely improve the information storage density and realize multi-emission display simultaneously, which to the best of our knowledge have not been reported. In this work, two functional moieties of molecular switch and aggregation induced emission (AIE) molecule are integrated into one molecule to achieve the goal. The moiety of molecular switch can change its structures between ring-closed form (RCF) and ring-open form (ROF) under external stimuli and exhibit different photophysical property.⁸ While the AIE moiety plays the role in both fluorescence enhancement in the aggregation state to conquer aggregation-caused quenching effect and fluorescence switching by alternating its molecular packing modes.^9,10 Although some AIE molecules modified molecular switches have been synthesized by us and others,¹¹ emission switching both among the multi colors and between the multi-color and OFF-state haven't been achieved so far.

Here, a new molecular switch of SPTPE was achieved by modifying spiropyran (SP) with tetraphenylethylene (TPE) through a single bond linkage (Fig. 1). Two fashions of (i) molecular structure change and (ii) molecular packing mode change have been utilized to work synergistically, and four high contrast state of red, cyan, blue and dark have been achieved both in solution phase and in solid state. The reversible switch between any of the four states has been realized with chemical/mechanical/thermal stimuli. The existence of dark state together with three well distinguishable color states can not only provide higher data density, but also enable high contrast signal ratios, as well as cater to the reading habit. As an example, application of the fluorescent molecular switch in high contrast rewritable multi-emission paper is demonstrated.

Fig. 1 Molecular structure of SPTPE and its acid/base triggered isomerization.

Single bond linkage of SP with TPE in the para position generates a new partially conjugated system. Compared with SP (abs peak: ∼298 nm) and TPE (abs peak: ∼300 nm), the absorption of SPTPE has a red shift (abs peaks: 298 nm and 322 nm) and is largely enhanced, indicating SP and TPE have partially conjugated (Fig. S1, ESI†). Although UV light can be used to trigger the RCF to ROF isomerisation of SP, stable ROF can not be obtained with UV light irradiation in solvents, such as THF, acetonitrile, DMF, EtOAc etc., because the phenolic oxygen anion of ROF need an electron withdrawing group to stabilize its negative charge in ROF, while TPE moiety at its para-position is an electron donating group, which facilitate the ring-closing process.¹² Nevertheless, SPTPE still has acid–base triggered tautomerization between RCF and ROF originating from SP moiety (Fig. 1 and 2A), however, both RCF and ROF isomers show more complicated AIE properties than TPE (Fig. 2B and C). There is no emission for both RCF and ROF isomers in solution due to the non-irradiative energy decay. When water was added to RCF to induce the aggregation, it shows water fraction dependent blue and cyan two emissions, which are proved to be corresponding to the crystalline and amorphous phase of the aggregates respectively (Fig. 2B and D). The UV-vis absorption spectrum of the amorphous aggregates has a significant red shift with the crystalline one, which is in accordance with the red-shift of the emission, indicating the more planar conformation in the amorphous phase (Fig. S2A, ESI†). Since TPE can only exist in crystalline state,¹³ the presence of SP enable SPTPE with both crystalline and amorphous phases, which further indicates SPTPE will have mechanochromic properties. Because the aggregation rate is tuned with water fraction, when aggregation rate is faster than crystallization rate, amorphous phase is obtained, otherwise, crystalline phase is obtained. While, when water was added to the solution of ROF, it shows more complicated phenomena. When water fraction reaches 70%, amorphous RCF aggregates are obtained, whose emission at <500 nm was absorbed by the soluble ROF in solution, giving out a green emission (Fig. S3 ESI†). Although the emission of amorphous aggregated RCF is observed at 70% water fraction, its percentage is very little, because no significant change of the absorption spectra is observed (Fig. S2B, ESI†). Then, at higher water fractions >80%, the ionic ROF SPTPE precipitate out as crystals without ring-closing process, exhibiting a red emission (Fig. 2C and D), and UV-vis spectra have a significant red shift, indicating the molecules take more planar conformation and/or there are π–π interactions between the ROF isomers in its crystal (Fig. S2B, ESI†). The water fraction dependent different existing states are because water plays both roles as a poor solvent and a proton capture agent. In detail, because of the poor solubility of both RCF and ROF SPTPE in water, they tend to precipitate with the increasing of water fraction; in the meanwhile, the proton capture effect of water will greatly decrease the acidity of HCl in organic solvents and cause ROF SPTPE to switch to RCF SPTPE. Therefore, the competition between the precipitation effect and the ring-closing effect of water leads to the different existing states at different water fractions. So, here SPTPE not only preserves the functions of SP and TPE, but also shows synergistic interactions between the two moieties to realize water fraction sensitivity for both isomers, leading to more emission states in AIE process.

Fig. 2 (A) UV-vis spectra of SPTPE in acetonitrile solution (1 × 10⁻⁵ M) with the addition of HCl (acidized for 12 h), then adequate NaOH. (B and C) PL spectra of RCF SPTPE (1 × 10⁻⁴ M) and ROF SPTPE (obtained from RCF SPTPE by treating with 50 equiv. of HCl for 12 h, 1 × 10⁻⁴ M) in acetonitrile/water mixtures with different water volume fractions. (D) XRD pattern of the above aggregates with different emissions. Inset: corresponding fluorescent images under 365 nm.

Since the SPTPE isomers exhibit multiple emissions in aggregated states from solution (Fig. S4A and B, ESI†), we turn our attention to their stimuli-responsive properties in their solid states. Excitingly, four states of SPTPE were obtained in solid, they are crystalline ring-closed form (CRCF), amorphous ring-closed form (ARCF), crystalline ring-open form (CROF) and amorphous ring-open form (AROF), corresponding to four emission colors of blue, cyan, red and dark, respectively (Fig. S4C and D, ESI†). Furthermore, any two of the four states can be reversibly converted to each other by appropriate external stimuli (Fig. 3). Take the CRCF SPTPE re-crystallized from dichloromethane (DCM)/hexane as initial state, it gave out a blue emission centered at 455 nm [Φ_F = 62.10%, τ = 2.5 ns]. Theoretical calculation by Gaussian 09 (ref. 14) shows the electron density of HOMO and LUMO are distributed on indole and TPE moiety respectively in the RCF SPTPE, and HOMO−1 and LUMO both distributed on TPE (Fig. S5A, ESI†). Considering the similar emission wavelength to TPE (Fig. S6, ESI†) and the short life time, the blue emission should be assigned as the local excited emission of TPE moiety between HOMO−1 and LUMO.

Fig. 3 (A) Fluorescence images of SPTPE powders in different states, as well as the reversible switching between each other triggered by various stimuli, and their corresponding (B) PL spectra and (C) XRD patterns.

As shearing (hydraulic) force is a common way to induce the packing mode change of organic solids, grinding was performed on CRCF SPTPE. The emission switched from blue to cyan at 488 nm [Φ_F = 57.23%, τ₁ = 2.2 ns A₁ = 47.14; τ₂ = 4.4 ns A₂ = 52.86], and CRCF changed to ARCF spontaneously (Fig. 3). The bathochromic-shift of the PL spectrum was assigned as the planarization of SPTPE as described by our early work.¹⁵ We think hand grinding cannot lead to homogenous molecular packing, and the ground sample is composed of the crashed amorphous nanocluster.¹⁶ The double lifetime is resultant from the different molecular environments in the nonnegligible surfaces and cores in the nanoclusters. Furthermore, this process is reversible. When the powder of ARCF SPTPE is subjected to solvent fuming or thermal annealing, CRCF SPTPE is obtained by re-crystallization process (DSC analysis indicates that the amorphous to crystalline transition temperature is about 150 °C, Fig. S7, ESI†). The reversible mechanochromic luminescence can be repeated many cycles with good emission stability and constant quantum yields (Fig. S8A, ESI†).

Strong acid such as hydrochloric acid (HCl) was used to trigger the ring-opening of ARCF SPTPE in solid state. When a good solvent of SPTPE, such as ethyl acetate, was used together with HCl, ARCF SPTPE changed to CROF by successive processes of dissolution, acid-triggered ring-opening and recrystallization together. CROF of SPTPE gave out a red emission centered at 641 nm [Φ_F = 9.64%, τ₁ = 1.3 ns A₁ = 37.54; τ₂ = 2.55 ns A₂ = 62.46]. This red emission is assigned to intramolecular charge transfer as HOMO and LUMO are distributed on TPE and indolium moiety respectively from theoretical calculation (Fig. S5B, ESI†). The double lifetimes indicate the re-crystallized crystals are very small due to the low mobility of molecules in the fuming condition, and the surface states can still not be neglected. The inverse process from CROF to ARCF could be achieved by Et₃N fuming, Et₃N acts as a base to make the ROF deprotanate and convert to RCF, meanwhile, as a poor solvent of RCF SPTPE, Et₃N do not allow SPTPE to re-crystallize. Therefore, reversible switching between cyan and red was achieved by acid/base stimuli, and this process can be repeated for several times without changing the emission colors (Fig. S8B, ESI†).

Mechanical force was performed to change the packing mode of CROF SPTPE further. CROF changed to AROF after grinding (Fig. 3). Surprisingly, the corresponding emission of AROF SPTPE quenched dramatically [Φ_F < 0.01%]. This is a typical fluorescence ON/OFF process.¹⁷ The reasons of emission quenching after ground may be explained as follow. Firstly, grinding changed the molecular packing from crystal to amorphous, which cause large areas of loosely bonded surface and facilitate the non-radiative transitions. Secondly, molecular packing becomes tighter in the centre of the amorphous clusters because grinding can cause more planar configurations,^15,16 which makes intermolecular charge transfer more convenient, and quenches the emission eventually. These deductions are in consistent with the phenomenon that the single molecular level dispersed ROF SPTPE in PEG polymer (wt% = 0.1%) gave out a red emission (Fig. S9, ESI†). This emission switching process is reversible too as most mechanochromic systems show.¹⁸ When treated with solvent vapor, such as ethyl acetate/DCM, AROF convert to CROF accompanied with the reappearance of red emission, and this process can be repeated for several times without obvious change on the fluorescent color as well as quantum yield (Fig. S8C, ESI†).

AROF SPTPE could convert to CRCF under the treatment of base and appropriate solvent together, such as triethylamine (TEA) along with acetic ether or sodium hydroxide (NaOH) solution together with ethanol, which go through another process of dissolution, deprotonation and recrystallization. Mono-treatment of heating can induce the conversion too. In this process, firstly, HCl volatilized under high temperature and ring-close occurred simultaneously, then thermal annealing induced re-crystallization (Fig. S10, ESI†). In order to verify this process, thermal gravimetric analysis (TGA) was used to characterize the thermal stability of AROF SPTPE. The weight loss at the platform of about 200 °C corresponds to the lost HCl calculated by molecular weight (Fig. S11, ESI†). Furthermore, when SPTPE is protonated with nonvolatile acid, such as HPF₆, the corresponding AROF SPTPE can hardly change to RCF upon heat treatment. If the temperature reaches too high, the molecule will be damaged and lose their fluorescence (Fig. S12, ESI†). Meanwhile, CRCF to AROF could also be achieved by acid and appropriate solvent treatment, such as HCl/DCM fuming (Fig. S8D, ESI†). In this process, selection of solvent is essential. DCM facilitate the reaction of acid and SPTPE and volatilize quickly, which results in ARCF switch to AROF and re-crystallization will not occurred. As for strong acid treatment only, such as HCl, ring-opening reaction will occur just at the surface of the crystal, its color changed from white to red under visible light, however, PL and XRD indicate that it is still RCF inside the crystal (Fig. S13, ESI†).

Besides, switching between two crystal forms (CRCF and CROF) can be realized under the stimuli of acid/base with the assistant of good solvent (Fig. S8E, ESI†). Meanwhile, switching between two amorphous forms (ARCF and AROF) can be achieved with acid/base treatment without solvent (Fig. S8F, ESI†). Although the fully structure transition takes hours, the clear emission quenching/appearing happen within minutes (Fig. S14, ESI†). Thus, reversible conversion between any two states was achieved by modulating the structure and packing mode change under appropriate external stimuli.

These multi-stimuli responsive properties of SPTPE encouraged us to explore its application on multi-fluorescence display and rewritable multi-emission paper. First, AROF of SPTPE was deposited on filter paper by soaking in its DCM solution then dried up, as initial state, it gave out no emission. When using TEA/DCM, NaOH/ethanol and acetic ether as inks to write respectively, characters of “JLU” with emissions of cyan, blue and red were obtained simultaneously on the same strip (Fig. 4A). In order to obtain clear image of red emission because of its low quantum yield, sufficient exposure time of camera was required, which results in a bright cyan emission. Further, the same piece of the SPTPE-incorporated paper can display different emission colors depending on the stimuli. As a demonstration, a paper-cut cartoon image could reproducibly change its emission between any color of red/cyan/blue through a dark state every time (Fig. 4B).

Fig. 4 Photographic images of SPTPE deposited filter paper: (A) characters of “JLU” written in turn on one piece of paper using TEA/DCM, NaOH/ethanol and acetic ether as inks respectively under visible light (up) and UV light (down). (B) The same piece of paper cut of a girl kicking shuttlecock treated with EtOAc (red); DCM (dark); triethylamine/DCM then heated (cyan); HCl/DCM (dark); sodium hydroxide/ethanol (blue) in sequence (all dark states were omitted).

In summary, we have synthesized a novel fluorescent molecular switch SPTPE by combining spiropyran with TPE through a single bond. By elaborate control of the multiple fluorescence decisive parameters, such as molecular structure, molecular packing, intermolecular energy transfer with chemicals, heat and grinding, four states (CRCF, ARCF, CROF, AROF) corresponding to three well distinguishable emission colors (blue, cyan, red) and a dark state have been achieved. The reversible switching between the three color states and the OFF state in such a small molecule makes it very promising in fabricating functional materials. As an example, multi-emission display and rewritable paper based on SPTPE were realized utilizing the multi-stimuli responsive properties and high ON/OFF ratio between emission and dark state. The success of our work will encourage the development of new multi-stimuli responsive molecular switches by combining different functional moieties and controlling different photo-chemical and photo-physical processes.

Acknowledgements

This work was financially supported by the National Science Foundation of China (51303063, 21574058).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of synthesis and characterization of SPTPE. ¹H NMR, ¹³C NMR spectra, UV-vis, PL spectra, PXRD for SPTPE, spiropyran and TPE. See DOI: 10.1039/c6ra21639k

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