Perylene diimide-based supramolecular polymer with temperature-sensitive ratiometric fluorescence responsiveness in solution and gels †

A novel amphiphilic fluorescence building block with perylene diimide (PDI) as the core and quadruple H-bonding groups (UPy) as wings (UPy–PDI–UPy) has been synthesized. It shows interesting thermo-responsive ratiometric dual-emission properties in both aqueous solution and the hydrogel state. Further contrast experiments with two other analogous derivatives, namely UPy–TPDI–UPy and TEG–PDI–TEG, indicated that the presence of supramolecular aggregation induced an emission enhancement effect between the UPy–PDI–UPy molecules, attributed to the synergetic effect of intermolecular intrinsic p – p stacking, the hydrophobic effect and highly directional quadruple H-bonding. In DMF/H 2 O (v:v = 1:1), UPy–PDI–UPy self-assembles into nanoparticles with obvious ratiometric fluorescence responsiveness towards temperature in the range of 20–80 1 C. Moreover, UPy–PDI–UPy can form a thermo-responsive hydrogel by dispersing in PEG-containing aqueous solution. The hydrogels show a temperature-dependent ratiometric dual-emission with a narrow responsive range of 20–39 1 C and an excellent renewable property. This innovative research helps to fabricate novel responsive luminescent materials by using the supramolecular self-assembly behavior.


Introduction
][12][13][14][15] Fabricating ratiometric fluorescence thermo-sensing materials with single-component dyes could be a good solution to overcome these disadvantages.However, the design of such dyes is very challenging due to Kasha's rule.
7][18][19][20][21][22][23] When the temperature lowers to allow the formation of directional H-bonding to order the fluorophores, the materials normally show weak fluorescence due to the quenching effect. 24It is difficult to obtain ratiometric fluorescence, especially for the normal fluorophores that show obvious aggregation-caused quenching (ACQ) in the solid-state or high concentration of solution.
Perylene diimide (PDI) is one of the most well-known fluorescent building blocks to fabricate supramolecular fluorescence materials. 257][38][39] Thus, the optimization of the supramolecular structure to modulate the p-p stacking of PDI is essential to achieve bright emission in the aggregated state, which can be realized by the introduction of various aromatic rings as isolation groups from the sides. 37,38However, this strategy is often hindered by the complicated synthesis routes, and the obtained dyes generally exhibit a stationary red or deep red emission nature. 40,41erein we report our strategy to prepare PDI-based ratiometric fluorescence supramolecular materials that are thermosensitive.In our strategy, the highly directional quadruple hydrogen bonding units of ureidopyrimidinone (UPy) are designed to conjugate with PDI to control the intermolecular p-p stacking for endowing the dye with bright red emission in the aggregated state, together with a temperature-dependent ratiometric fluorescence property.We demonstrate this concept by comparing three rationally designed amphiphilic PDI derivatives, UPy-PDI-UPy, UPy-TPDI-UPy and TEG-PDI-TEG (Scheme 1), which are composed of a hydrophobic PDI core and two hydrophilic wings.Thanks to the unsubstituted core and linked quadruple H-bonding group (UPy) as wings, only the supramolecular polymer UPy-PDI-UPy exhibited an intense fluorescence emission in the solid-state and a temperaturedependent ratiometric dual-emission (green and red) property in the nanoparticle dispersion when temperature increased from 20 to 80 1C.It is noteworthy that the subsequently prepared fluorescent hydrogel displayed a temperatureultrasensitive ratiometric fluorescence responsiveness and a visual sol-gel transition ranging from 20 to 39 1C.To the best of our knowledge, this is the first example of the fabrication of PDI-based supramolecular polymers as ratiometric fluorescence thermometers and can imply more inspiration to prepare novel responsive luminescent materials (Scheme 1).

Materials and methods
Unless otherwise stated, all the chemicals were purchased from commercial sources and directly used in the reaction.The detailed synthesis routes of relevant intermediates such as UPy-SH, OH-PDI-OH (2a) and OH-TPDI-OH (2b), as well as the final products including UPy-TPDI-UPy and TEG-PDI-TEG (Schemes S1-S3, ESI †), and the relevant apparatus and characterization including NMR spectra, mass spectra (MS), UV-vis absorption and fluorescence spectra, fluorescence lifetime, atomic force microscopy (AFM) and control experiments are displayed in the ESI.†

Synthesis and characterization of PDI derivatives
The chemical structures of the designed PDI derivatives are shown in Scheme 1. TEG-PDI-TEG consists of a PDI core and two hydrophilic 2-[2-[2-(2-aminoethoxy)ethoxy]ethoxy]ethanol (TEG) wings.UPy-PDI-UPy is made from a TEG-PDI-TEG structure and two ureido-4-pyrimidinone (UPy) terminal groups.Such a structure allowed UPy-PDI-UPy to form supramolecular polymers via the self-complementary quadruple H-bonding between the UPy moieties. 18,19Compared to UPy-PDI-UPy, UPy-TPDI-UPy has a tetrachloro-substituted PDI core.The substituted groups in TPDI were designed to weaken the p-p stacking interaction of the PDI core. 42,43These PDI derivatives were synthesized by coupling TEG/UPy with bis-N,N 0 -( 2 Fig. 1 shows the fluorescent properties of the three PDI derivatives in the solid-state.Interestingly, under 365 nm light irradiation, UPy-PDI-UPy emits a bright red light with a high fluorescence quantum yield of 7.19% while both PEG-PDI-PEG and UPy-TPDI-UPy show weak fluorescence with low fluorescence quantum yields of 0.19% and 0.26%, respectively (Fig. 1a).Such a unique property is supported by their fluorescence spectra (Fig. 1b) in which the maximum fluorescence emission intensity of UPy-PDI-UPy in the solid state at 694 nm is 53 times or 41 times higher than those of TEG-PDI-TEG at 693 nm or UPy-TPDI-UPy at 636 nm.The absence of emission in both PEG-PDI-PEG and UPy-TPDI-UPy was attributed to the ''aggregation-caused quenching'' (ACQ) effect.It is well documented that the fluorescence of PDIs is generally quenched in the solid-state due to consecutive p-p interactions and/or dipole-dipole interactions between the neighboring planar PDI fluorophores. 39Wu ¨rthner and co-workers have demonstrated that bulky substituents on the PDI fluorophore favor the formation of discrete PDI-PDI p-dimers and undistorted planar PDI fluorophores, which inhibit long-range PDI aggregation in the solid state. 44,45As displayed in the 1 H NMR spectrum in Fig. S9 (ESI †), with increasing the concentration of UPy-PDI-UPy, the peaks at 8.2 and 8.4 ppm that belong to the PDI-PDI p-dimers gradually rise, indicating that the bulky UPy side groups in UPy-PDI-UPy can lead to the formation of discrete PDI-PDI p-dimers. 46,47Also, no chemical shifts related to trimers or other long-range PDI aggregation can be observed in Fig. S9 (ESI †).We thus attribute the bright deep-red emission efficiency of UPy-PDI-UPy in the solid state to the appropriate p-p stacking of PDI induced by UPy dimerization.
Because of their amphiphilic structures, the PDI derivatives could self-assemble in aqueous solutions.UV-vis absorption and fluorescence spectroscopies were used to study their selfassemblies (Fig. 2).A DMF/H 2 O mixture solvent was employed by increasing the fraction of water in DMF.At a concentration of 50 mM, UPy-PDI-UPy completely dissolved in pure DMF, showing typical absorption and emission images of welldispersed PDI derivatives (Fig. 2a and b). 48,49When the content of water was increased to 30 vol%, dramatic changes in both the absorption and emission spectra were observed.The S 0-1 transition (l = 490 nm) is more intense than the S 0-0 transition (l = 525 nm) while the fluorescent intensity at around 545 nm is significantly reduced, indicating a p-p stacking aggregation state.With further increase of the water fraction, a slight bathochromic shift of l max occurred and the well-structured absorption curve turned into a less structured peak.The absorbance ratio of A 525 /A 490 was declined from 1.6 to 0.4 (Fig. S10a, ESI †), indicating enhanced p-p stacking interaction. 48Interestingly, with  a further increase of water fraction, UPy-PDI-UPy shows a significant enhancement of fluorescence intensity at 646 nm, accompanied by a change of emission color from green to red.This unusual enhancement in emission is different from most previous reports of the PDI derivatives, even UPy-linked ones. 48,49e attributed this to the formation of appropriate intermolecular p-p stacking through multiple H-bonding interactions.Such a synergetic effect was supported by the results from UPy-TPDI-UPy in which the tetrachloro substituents prevent the intermolecular p-p stacking interaction and TEG-PDI-TEG, a PDI derivative without UPy moieties.UPy-TPDI-UPy only exhibits a slight change in absorption along with the absorbance ratio of A 518 /A 490 (Fig. S10b, ESI †), and obvious fluorescence quenching at a higher water fraction (Fig. 2c and 3d).On the other hand, TEG-PDI-TEG shows an obvious decrease in the absorption peak at S 0-0 transition (Fig. 2e) but a stepwise promotion in that of the S 0-1 transition.As a result, the absorbance ratio of A 525 /A 490 declines from 1.45 to 0.7 (Fig. S10c, ESI †).In addition to this change, clear quenching in fluorescence intensity was observed, due to the formation of strong p-p interaction mediated aggregates, as reported in the literature.
To further investigate the aggregation behavior of UPy-PDI-UPy in DMF/H 2 O mixed solution, AFM, TEM and DLS were employed to study the size of its aggregatesunder different water fractions.As exhibited in Fig. 3 and Fig. S11 (ESI †), when the water fraction was increased from 20% to 80%, the particle size data from DLS gradually decrease and the size distribution was also reduced.The results were also confirmed by AFM and TEM measurements.Fig. 3a and c display the AFM images of the supramolecular aggregates of UPy-PDI-UPy in DMF/H 2 O mixed solution with 30% or 50% fraction.In the case of 30% water fraction, the aggregates show an average size of 396.5 nm (DLS).Increasing water fraction reduces both the average particle size and size distribution.The TEM images as shown in Fig. S12 (ESI †) further confirm the aggregation behavior of UPy-PDI-UPy in the mixture of DMF/H 2 O.This indicated that solvent polarity can be used to regulate the supramolecular selfassembly behavior of UPy-PDI-UPy in the current system.

Ratiometric fluorescence responsiveness of temperature in solution and gels
Temperature-dependent fluorescence emission spectra of the UPy-PDI-UPy nanoparticle dispersion in DMF/H 2 O mixed solution (v/v = 50/50) were recorded from 20 1C to 80 1C (Fig. 4).With an increase in temperature, the fluorescence peak at 646 nm decreases and the emission around 545 nm becomes dominant (Fig. 4a).The average lifetime of the dispersion at 646 nm also declines from 15.9 ns to 8.9 ns (Fig. S13, ESI †).The change in emission was attributed to the disassembly of PDI cores associated with both p-p stacking and H-bonding interactions.The absorption spectra of the dispersion as shown in Fig. S14 (ESI †) supported the conclusion that the p-p stacking interaction was reduced with temperature promotion.In this process, particle size was reduced (Fig. S15, ESI †), due to the segmental disaggregation of the formed supramolecular aggregates.Notably, a prominent linear fitting curve, with a correlation coefficient of R 2 = 0.98, was also achieved by plotting the logarithm of I 545 /I 646 versus temperature (20.0-80.01C).This perfect fitting implied that the nanoparticle dispersion can serve as a favorable ratiometric fluorescent sensor for temperature.In addition, when the temperature was alternated between 20 and 80 1C, the nanoparticle dispersion exhibited reversibly switchable fluorescence nature (Fig. 4c).A slight ''fatigue'' effect was observed in reversibility (Fig. 4c) and fluorescence brightness (Fig. 5d).As shown in Fig. S16 (ESI †), when the  temperature was decreased from 80 1C to 20 1C, some big UPy-PDI-UPy nanoparticles form, which increases the distribution of nanoparticles.This result is significantly different from that of the nanoparticles formed in the mixture of DMF/H 2 O solvent (Fig. 3c and d).The different assembly strategies make the color and the fluorescence emission spectra of the nanoparticles (Fig. 4d and Fig. S17, ESI †) different from that of the initial sample at 20 1C.In addition, the ''fatigue'' emission still exists, even when the incubation time of cooling was prolonged to 46 h (Fig. S17, ESI †).However, marked fluorescent switching can still be visualized after five test cycles.In addition, the photostability of UPy-PDI-UPy nanoparticles in the mixture of DMF/H 2 O is provided in Fig. S18 (ESI †).The nanoparticles displayed good photostability under the continuous excitation of 490 nm for 60 min.Thus, the nanoparticle dispersion exhibited attractive properties for renewable temperature sensing in various applications.
A UPy-PDI-UPy hydrogel could be prepared by adding polyethylene glycol (PEG, M w = 10 000) into the solution.This hydrogel (Gel1) was also thermo-sensitive.Elevating the temperature from 20 to 39 1C induced a sol-gel transition (Fig. 5a).Interestingly, Gel1 displays an obvious fluorescence color change from pink to greenyellow, accompanied by the sol-gel transition.The emission was further studied using fluorescence spectroscopy.As displayed in Fig. 5b, UPy-PDI-UPy in a hydrogel (Gel1) exhibits a high thermo-sensitivity in fluorescence, even more sensitive than that in the solution state.The ratiometric fluorescent intensity I 542 /I 646 was further used to evaluate the thermo-sensitivity (Fig. 5c).It was found that a sharp change in I 542 /I 646 (1.1 to 5.3) occurred in the range of 35-41 1C (critical temperature range).We attribute this to the phase-transition temperature of PEG in the as-prepared hydrogel intensifying the disaggregation of the formed supramolecular aggregates during the phase-transition process.Moreover, a good linear fitting curve with a correlation coefficient of 0.98 (R 2 ) was achieved by plotting the I 542 /I 646 versus temperature (35.0-41.01C).Note that this change perfectly matched the human physiological temperature range, implying a potential application in body-monitor devices.A temperature-sensitive color-changing bracelet by the encapsulation of Gel1 in a plastic tube was thus fabricated to show this potential (Fig. 5a).In addition, Gel1 also exhibits excellent reversibility in thermoinduced fluorescence change between 20 and 39 1C (Fig. 5d).In comparison with the corresponding nanoparticle dispersion, no detectable ''fatigue'' effects were observed in the reversibility even after five cycles.Furthermore, the critical temperature range of the gel can be easily varied by changing the ratio and molecular weight of PEG, as exhibited in Table S1 and Fig. S19-S21 (ESI †).In the current system, PEG is the major component (58 and 75 wt% for Gel1 and Gel2, respectively) and water is absorbed in the PEG as a plasticizer in the gel state.PEG with M w of 10 000 shows a melting point of ca.60 1C, which is expected to decrease after the addition of water.The gel-sol transition is the melting of the mixture of PEG and water.This hypothesis is also supported by the system of the PEG with lower M w (2000).Decreasing the M w can reduce the melting point of PEG itself and also decrease the transition temperature of the system (system of M w 10 000: 41 1C; system of M w 2000: 36 1C).Based on this mechanism, the concentration of PEG should make an important contribution to the phase transition, as the results show in Table S1, Fig. 5a and Fig. S19 (ESI †) (39 1C for Gel1 and 41 1C for Gel2).In this system, the UPy-PDI-UPy molecules disperse in the media and would thus change their aggregated state with the media.Moreover, similar to UPy-PDI-UPy, TEG-PDI-TEG can also form a hydrogel.However, in contrast with the obvious temperaturedependence of UPy-PDI-UPy hydrogel, the corresponding TEG-PDI-TEG in gel (Gel4, Table S1, Fig. S19 and S22, ESI †) did not show any fluorescence color change except for the presence of fluorescence quenching and gel-sol transition with the increase of temperature.

Conclusions
In summary, we have synthesized three amphiphilic PDI derivatives, PEG-PDI-PEG, UPy-PDI-UPy, and UPy-TPDI-UPy.They have a PDI core and PEG/PEG-UPy wings.It was found that UPy-PDI-UPy exhibited an intense deep-red fluorescence emission not only in the solid-state, but also in the nanoparticle dispersion, implying an aggregation-induced-enhancement in fluorescence.We attributed such enhancement in fluorescence to the formation of appropriate p-p stacking.Interestingly, UPy-PDI-UPy in a DMF/H 2 O mixture solution (the volume ratio of H 2 O is 50%) displayed obviously ratiometric fluorescence responsiveness against temperature in the range of 20-80 1C, together with a good reversible cycling property.Moreover, the as-prepared fluorescent gels consisting of UPy-PDI-UPy and PEG not only exhibited visual sol-gel transition, but also showed temperature-sensitive ratiometric dual-emission signals at a narrow range of 20-39 1C with excellent renewable nature, which perfectly matched the human physiological temperature range.Those temperature-dependent ratiometric emission properties suggested potential application in various visual luminescent devices such as anti-counterfeiting labels and ratiometric fluorescent thermometers.

Fig. 3
Fig. 3 AFM images and DLS distribution graph of the nanoparticles via the self-assembly of UPy-PDI-UPy (50 mM) in DMF/H 2 O mixed solution with a water fraction of 30 vol% (a and b) and 50 vol% (c and d), respectively.

Fig. 4
Fig. 4 (a) Temperature dependent fluorescence emission of the UPy-PDI-UPy (50 mM) nanoparticle dispersion in DMF/H 2 O: 50/50 (v/v).l ex = 490 nm.(b) The ratiometric fluorescence intensity (I 545 /I 646 ) of the nanoparticle dispersion versus temperature, as well as the corresponding linear fitting curve.(c) Renewable cycles of the corresponding fluorescence intensity at 546 nm and 649 nm change with cooling at 20 1C and heating at 80 1C.(d) Fluorescence photographs of the nanoparticle dispersion at the first cooling at 20 1C and the switchable fluorescence states between 20 1C and 80 1C.

Fig. 5
Fig. 5 (a) Fluorescence photographs of Gel1 at various temperatures, and the temperature-sensitive bracelet made of Gel1.(b) Temperaturedependent fluorescence emission of Gel1.l ex = 490 nm.(c) The ratiometric fluorescence intensity (I 542 /I 646 ) of Gel1 versus temperature, and the corresponding linear fitting curve (inset).(d) Switching cycles of the I 542 /I 646 of Gel1 between 20 1C and 39 1C.