Chain-dependent emission color codes of extended tetraphenylethylene derivatives: discrimination between water and methanol

Abstract

In this paper, we report the emission properties of two luminogens, which possess an identical extended tetraphenylethylene (TPE) aromatic core, but different hydrophobic dodecyl (1) and hydrophilic di(ethylene oxide) (2) peripheral chains. In comparison to the dilute chloroform solutions (0.1 μM), the emission of the chloroform solutions (10 μM) was red-shifted, which indicates an intermolecular interaction with increasing concentration. To investigate the chain-dependent emission behavior of 1 and 2, polar methanol and nonpolar n-hexane were employed as two poor solvents. As a consequence, upon adding each poor solvent, the emission properties were predominantly dependent upon the solubility between the peripheral chain and the mixture solvent (chloroform/poor solvent). Despite the identical aromatic chromophore, the emission could be guided by the nature of the attached peripheral chains. Upon increasing the fraction of the poor solvent for a peripheral chain, the crystallization of the TPE-based aromatic cores occurred, which produced a bright bluish emission. Scanning electron microscopy observation could confirm the ordered morphologies of the bluish aggregates. By comparing the integral values of the aromatic segments in the ¹H-NMR spectra with increasing the poor solvents, it can be said that the crystallization is initiated by the retardation of the conformational motions of the TPE aromatic ring, not the whole aromatic segment. In addition, by replacing chloroform with water-mixable tetrahydrofuran, two polar solvents, i.e., water and methanol, could be distinguished using the two emission color codes of 1 and 2. The molecular approach (the simple variation of the peripheral chain) in this study proved an alternative way to tune the solution emission of the identical chromophores.

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Introduction

Luminogens that are the core component in photovoltaic and light emitting diodes, are still an interesting subject worth studying. To date, tremendous effort has been made to enhance emitting efficiencies which are necessary to optimize material functions.¹ In particular, luminogens are necessary to show better emitting efficiency in the solid state for practical applications. In comparison to the solution state, however, most of the luminogens decrease the emitting intensity in the solid state, which is known as aggregation-caused quenching (ACQ).² On the other hand, some organic luminogens with non-planar structures display more efficient emission in the solid state than in the solution state.³ Such an emission enhancement is known as an aggregation-induced emission (AIE), which is mostly due to the restricted intramolecular rotation in the solid state.⁴

Tetraphenylethylene (TPE) consisting of four phenyl rings conjugated to central ethylene group is an iconic luminogen showing AIE effect.⁵ In addition to the emission enhancement, TPE-based luminogens are also interesting in regard to fluorescence color, i.e., emission wavelength. In contrast to conventional luminogens, their fluorescence shifts to shorter wavelength when they crystallize. To differentiate it from the AIE effect, this can be referred to as a crystallization-induced emission (CIE) effect.⁶ The blue-shifted emission of TPE derivatives is attributed to the crystallization of twisted conformers that are stabilized by a hydrogen bonding between the C–H and π electron.⁷ Based on the CIE effect, the fluorescence color can be switched in response to external stimuli such as fuming, thermal, and mechanical force.⁸

Solvatochromism is the ability of a chromophore to alter color depending on the type of solvent.⁹ In general, the solvatochromic effect is due to the change in solvent polarity. By the electrostatic interactions between chromophore and solvent, either excited or ground state can be stabilized or destabilized, which leads to the change in solution fluorescence color.

On the other hand, we envisioned that the CIE effect of TPE-based chromophores could be employed as an alternative way to tune the emission color. In this case, we thought that the solubility of TPE derivatives was an important factor at a given solvent system. In contrast to classical solvatochromism, the change in the fluorescence color could be operated by the morphology in the solution. In this regard, we intended to demonstrate that the morphology engineering (i.e., crystalline vs. amorphous) could tune the fluorescence color of TPE-based compounds. To understand the mechanistic aspect of the color-tuning and expand the versatility, the molecular approach in this study was to alter the type of peripheral chains that are attached to an identical TPE-based chromophore. To this end, we designed two TPE derivatives 1 and 2 which consist of hydrophobic dodecyl and hydrophilic di(ethylene oxide) (DEO) chains on the periphery, respectively (Fig. 1). The aromatic structure could be accomplished by employing click chemistry (see ESI†), by which the central TPE core is extended by the triazolyl groups.¹⁰ Since the compounds are composed of the distinct peripheral chains, the aggregation process may behave differently depending on the solubility of the peripheral chains in a given solvent. The compatibility of the peripheral chains with the solvent must significantly influence the TPE crystallization, finally producing different fluorescence colors. In a word, the nature of the peripheral chain is able to guide the fluorescence color of the aromatic chromophore.

Fig. 1 Molecular structures of 1 and 2 that consist of the identical clicked TPE-based aromatic core, but hydrophobic and hydrophilic peripheral chains, respectively.

Herein, we report the fluorescence properties and the role of the peripheral chains in the solutions of 1 and 2. In this study, we demonstrated which part of the extended aromatic structure is mainly involved in the crystallization. Additionally, the observed morphology-dependent fluorescent color codes of 1 and 2 could be finally applied to the differentiation of two polar solvents such as water and methanol.

Results and discussion

Fluorescence properties of the chloroform solutions of 1 and 2

Before examining the emission change induced by the addition of poor solvents, the emission properties of the chloroform solutions with two different concentrations (0.1 μM and 10 μM) were investigated. According to the UV absorption spectra, the steady-state photoluminescence (PL) spectra were detected with the excitation at 350 nm (Fig. S2†). Both dilute chloroform solutions (0.1 μM) of 1 and 2 showed almost identical PL spectra (Fig. 2(a)). Their λ_max values appeared near 390 nm.¹¹ Since chloroform is a good solvent for the aromatic core and peripheral chains, the fluorescence information of the dilute solutions is considered to be derived from single molecular state.

Fig. 2 (a) Emission spectra of the chloroform solutions (0.1 μM) of 1 and 2, and (b) the emission spectral change of the chloroform solution of 1 with increasing the concentration from 0.1 μM to 10 μM. In (b), for the concentration compensation, the spectrum at 0.1 μM is magnified hundredfold. Excitation wavelength: 350 nm.

In contrast to the dilute solutions, the λ_max of the concentrated solution (10 μM) moved to a longer wavelength of 518 nm (Fig. 2(b)). Taking into account the hundredfold increase in concentration, the PL intensity of the concentrated solution is lower than that of the dilute solution. As consistent with the spectroscopic data, the concentrated solution displayed a weak greenish emission that could be detected with the naked eye (see the emission color of the solution at 0% in Fig. 3(a)). From the significant red-shift in the λ_max, a kind of intermolecular interaction may be involved in the concentrated solution, which is probably due to more frequent molecular collisions with increasing the concentration.

Fig. 3 (a and c) Emission photos and (b and d) spectra of the chloroform/methanol solutions of 1 and 2 as a function of f_(MeOH). The solutions in (a and c) were taken under UV illumination at 365 nm. The excitation wavelength of the emission spectra is 350 nm.

Fluorescence properties of the chloroform/methanol solutions of 1 and 2

By addition of poor solvents to the chloroform solution, the fluorescence color change was investigated. In this study, we chose two solvent extremes, i.e., polar methanol and nonpolar n-hexane, as the poor solvent, and investigated the emissive behavior with increasing the volume fraction (f) of the poor solvent. The concentration of the solutions was set to be 10 μM.

First, the emission properties of 1 and 2 as a function of f_(MeOH) were examined with the naked eye under the UV illumination at 365 nm (Fig. 3(a) and (c)). For 1, the initial greenish emission became dimmer with increasing f_(MeOH), and almost no emission was observed in the solution with f_(MeOH) = 60%. Upon further increasing f_(MeOH), the solutions with f_(MeOH) = 70% and 80% suddenly emitted a bright bluish color. By adding more methanol, the bluish emission changed into a greenish blue emission at f_(MeOH) = 90%. This emission color variation in the chloroform/methanol solutions of 1 could also be identified by the PL spectra (Fig. 3(b)). The initial emission with the λ_max of 518 nm became weaker, as f_(MeOH) increased up to f_(MeOH) = 60%. At f_(MeOH) = 70%, the λ_max blue-shifted to 454 nm and the intensity considerably increased. Above f_(MeOH) = 70%, the PL spectra moved back to the longer wavelength and became weaker.^6b,12 These PL spectra were consistent with the observation with the naked eye.

The most intense and blue-shifted emission happened at f_(MeOH) = 70%. As hinted by TPE-based luminogens reported previously, this can be interpreted as the formation of crystalline aggregates.¹³ Since crystalline morphologies minimize free volume and do not allow rotational motions, the crystalline aggregation can restrict the rotation of the phenyl segments of the TPE-based aromatic core. TPE-based luminogens are known to crystallize via C–H and π interaction between adjacent aromatic cores. These interactions stabilize a fixed twisted conformer with a shorter π-conjugation length, which results in a blue-shifted emission. The aggregate morphology of the brightest sample could be identified by scanning electron microscopy (SEM). In the SEM image, needle-like aggregates were revealed (Fig. 4(a)), which indicates a crystalline morphology.^6b,9e And, the SEM sample exhibited the identical bluish emission under the radiation at 365 nm (see the photo in the inset of Fig. 4(a)).

Fig. 4 SEM images of (a) the aggregates of 1 at f_(MeOH) = 70%, and (b) the aggregates of 1 at f_(MeOH) = 90%. The inset images are the fluorescence photos of the SEM samples under UV illumination at 365 nm.

On the other hand, the red-shifted fluorescence of the solution with f_(MeOH) = 90% may be explained in terms of the molecular dynamics when exposed to more polar solvent environment. The conformation of the extended TPE core can be kinetically frozen in more polar environment. In this situation, TPE-based cores may not have enough time to form thermodynamically stable crystalline aggregates consisting of the twisted conformers. Thus, the solution with f_(MeOH) = 90% possibly resulted in amorphous aggregates. The amorphous morphology could be confirmed by the SEM sample of 1 with f_(MeOH) = 90%. In contrast to the crystalline morphology at f_(MeOH) = 70%, the SEM image displayed structureless aggregates, indicative of an amorphous morphology (Fig. 4(b)).^6b,9e In contrast to the crystalline morphology, the amorphous morphology must contain less-twisted conformers. Therefore, the amorphous aggregates displayed the green emission (see the inset of Fig. 4(b)). Besides the red-shift in the emission wavelength, the fluorescence intensity at f_(MeOH) = 90% was weaker than that at f_(MeOH) = 70%. We conjecture that interstitial vacant spaces in kinetically frozen amorphous aggregates at f_(MeOH) = 90% allows limited segmental motions of the aromatic segments. If this dynamic consideration is true, the initial amorphous aggregates should return to the thermodynamically stable crystalline morphology in a sufficient time.^12a Indeed, the solution with f_(MeOH) = 90% displayed an almost identical bluish emission to that of the solution with f_(MeOH) = 70% after being left for 6 hours (Fig. S3†).

As above-described, 1 showed the emission enhancement by forming the crystalline aggregates above f_(MeOH) > 60%. Like the reported prototype TPE molecule, the addition of the poor solvent may hinder the rotational motion of aromatic segments.¹⁴ Nevertheless, the conformational dynamics of the “extended” TPE core during the crystallization was not clear. For example, we wanted to understand which part (central TPE or peripheral phenyl groups) of the extended aromatic core would be mainly involved in the conformational restriction. This could be identified by a ¹H-NMR method. In the NMR technique, the signals of conformationally restricted parts experience line-broadening, which causes the signal loss. As a result, the relative integration of these signals decreases with respect to other resonances. In this study, we checked the variation in the integral value of each aromatic hydrogen with increasing f_(MeOH). By doing so, we could figure out the above argument. The solution concentration of 10 μM employed in the fluorescence experiments was too dilute to be proper for the ¹H-NMR study. Thus, the concentration of the NMR samples was set to be 1 mM. For 1, the ¹H-NMR spectral data could be obtained to f_(MeOH) = 30%, above which the NMR sample was not transparent due to the precipitation.

Fig. 5(a) represents the ¹H-NMR spectra of 1. The chemical shift of the aromatic resonances changed, because the polarities of the solutions became different as a function of f_(MeOH). As f_(MeOH) increased, the integral values of H_a and H_b of the internal TPE ring decreased exclusively, although H_a was overlapped with H_d. On the contrary, the integral values of other aromatic hydrogens kept constant. Accordingly, the addition of methanol suppresses the rotational motions of the C–C bond connecting between ethylenic group and the phenyl rings, while the conformations of the triazolyl and terminal phenyl groups are not significantly affected. It is probably due to the star-shaped molecular shape. The sparse peripheral aromatic parts may rotate more freely than the denser center. Consequently, the ¹H-NMR result suggests that the TPE segments are predominantly involved at the onset of the crystallization via the reduction in the conformational mobility upon addition of methanol.

In contrast to 1, the chloroform/methanol solutions of 2 with the hydrophilic DEO peripheries displayed no considerable emission over the entire range of f_(MeOH) (Fig. 3(c)). From the PL spectra of 2, the greenish emission with the λ_max of 518 nm became weaker, as similar to the chloroform/methanol solutions of 1 with the smaller f_(MeOH) range (Fig. 3(d)). In addition, the λ_max of the emission did not change, regardless of the f_(MeOH). No emission enhancement of 2 is attributed to the compatibility between the DEO periphery and methanol, although the aromatic core disfavors methanol. In comparison to 1, the DEO peripheral chains of 2 solvated by methanol are conformationally flexible, which affects the dynamics of the aromatic core segments. Inevitably, the aromatic cores cannot crystallize by the disturbance of the mobile DEO peripheries. This interpretation could be evidenced by the ¹H-NMR data. Over the entire f_(MeOH) range to 50%,¹⁵ the integration ratios of the aromatic hydrogens did not change (Fig. 5(b)). This means that even the central TPE aromatic groups can rotate with the aid of the mobile DEO peripheries.

Fig. 5 ¹H-NMR spectra of 1 and 2 as a function of f_(MeOH).

Fluorescence properties of the chloroform/n-hexane solutions of 1 and 2

In addition to polar methanol, n-hexane was employed as the hydrophobic poor solvent. Like the chloroform/methanol solutions, the emission behavior of 1 and 2 was examined upon adding n-hexane to the chloroform solutions. In these cases, both 1 and 2 showed the CIE behavior toward n-hexane. In particular, the CIE effect of 1 appeared at f_(hexane) = 90% (Fig. 6(a) and (b)), although the dodecyl peripheries of 1 are hydrophobic. This indicates that the rigid aromatic core does not favor n-hexane. At this point, it is interesting to compare the chloroform/methanol solutions of 2 with hydrophilic DEO peripheries to the chloroform/n-hexane solutions of 1 with hydrophobic dodecyl peripheries. In terms of the solvent polarity, the aromatic core seemed to dislike methanol more than n-hexane. However, the CIE effect happened solely in the chloroform/n-hexane solution of 1 (Fig. 3(c) and 6(a)). From this unexpected phenomenon, it can be speculated that the triazolyl groups of the aromatic core is polar enough to be immiscible with n-hexane, but compatible with methanol. As another possible factor, more conformationally flexible DEO chains may hinder the crystalline packing more effectively than less-flexible dodecyl chains.

Fig. 6 (a and c) Emission photos and (b and d) spectra of the chloroform/n-hexane solutions of 1 and 2 as a function of f_(hexane). The solutions in (a and c) were taken under UV illumination at 365 nm. The excitation wavelength of the emission spectra is 350 nm.

In comparison to 1, 2 with the hydrophilic DEO peripheries began to show the CIE effect at a relatively lower f_(hexane) of 70% (Fig. 6(c) and (d)). And, the solution at f_(hexane) = 80% displayed the brightest blue emission which turned into a greenish blue emission at f_(hexane) = 90%. Like the chloroform/methanol solutions of 1, this emission change can be interpreted by the formation of kinetically frozen amorphous aggregates. The solution at f_(hexane) = 90% went back to the blue emitted solution after being left for 1 hour (Fig. S4†). This solution of 2 returned to the bluish solution more quickly than the chloroform/methanol solution of 1 at f_(MeOH) = 90%. This might be due to the faster dynamics of the DEO chains as compared to the dodecyl chains of 1. The morphologies of 2 at f_(hexane) = 80% and 90% were checked by SEM. The aggregates at f_(hexane) = 80% looked like a rice seed, while the aggregates at f_(hexane) = 90% were structureless (Fig. 7). These morphological features were consistent with the emission colors of the SEM samples under UV illumination at 365 nm (see the insets of Fig. 7).

Fig. 7 SEM images of (a) the aggregates of 2 at f_(hexane) = 80%, and (b) the aggregates of 2 at f_(hexane) = 90%. The inset images are the fluorescence photos of the SEM samples under UV illumination at 365 nm.

For these chloroform/n-hexane solutions, the ¹H-NMR integration change was examined as a function of f_(hexane). For both 1 and 2, the integration values of the TPE hydrogens (H_a and H_b) solely decreased with increasing f_(hexane) (Fig. 8). This NMR result agrees well with that of the chloroform/methanol solutions of 1. Therefore, it can be said that the crystallization producing the CIE effect is mainly associated with the conformational restriction of the TPE segment.

Fig. 8 ¹H-NMR spectra of 1 and 2 as a function of f_(hexane).

Fluorescence properties prior to the CIE effect

In this paper, it is worth noting the emission behavior of the solutions prior to the CIE effect. As shown in Fig. 2(b), an emission quenching happened more frequently in the concentrated chloroform solution (10 μM) than in the dilute chloroform solution (0.1 μM). In addition, the solutions of 1 and 2 showed almost similar fluorescence behavior prior to the CIE effect regardless of the identity of the poor solvent. Before the CIE effect happened, the initial greenish emission became weaker by keeping the λ_max constant (518 nm) as the f increased. This emission behavior is somewhat distinct from examples reported previously.

In most AIE-active compounds, the emission gradually intensified upon adding poor solvent.^1a,16 Although some exceptional TPE compounds displayed a gradual decrease in fluorescence intensity, the λ_max values moved to longer wavelengths. The red-shift was explained by the intramolecular charge transfer (ICT).¹⁷ As solvent polarity increased upon addition of more water, the ICT state could be more stabilized, which decreased the fluorescence intensity. In contrast to the previous examples, the λ_max of 518 nm in this study did not change with increasing f_(MeOH). Furthermore, the solutions using hydrophobic n-hexane displayed the identical λ_max. Therefore, it can be concluded that the gradual reduction in the fluorescence intensity observed in this study is not due to the ICT mechanism.

To obtain a clue, we measured the time-resolved fluorescence of the chloroform and chloroform/methanol solutions of 1. All solutions displayed biexponential decay curves (Fig. 9). The average lifetimes (τ_av) are shown in Table 1. The τ_av of the dilute chloroform solution (0.1 μM) was measured to be 1.05 ns, while the τ_av decreased to 0.25 ns in the concentrated solution (10 μM). Taking into consideration the concentration factor, more frequent intermolecular collisions possibly contribute to the reduction in the τ_av,¹⁸ which agrees with the decrease in the normalized emission intensity of the concentrated solution.

Fig. 9 Time-resolved fluorescence spectra of the chloroform and chloroform/methanol solutions of 1.

Table 1 Lifetime data of the solution samples of 1

Sample	τ1 (ns)	τ2 (ns)	τav (ns)	ΦF^a	kr^b (ns^−1)	knr^b (ns^−1)
a Calculated using quinine sulfate as the reference.b Calculated using the following equations: kr = ΦF/τav, knr = (1 − ΦF)/τav.
0.1 μM (in CHCl3)	3.17 (20.1%)	0.52 (79.9%)	1.05
10 μM (in CHCl3)	0.25 (99.8%)	1.62 (0.2%)	0.25	0.019	0.08	3.92
f(MeOH) = 20%	0.24 (99.8%)	1.61 (0.2%)	0.24	0.015	0.06	4.10
f(MeOH) = 40%	0.24 (99.7%)	1.86 (0.3%)	0.24	0.011	0.04	4.12
f(MeOH) = 40%	3.35 (43.5%)	1.37 (56.5%)	2.23

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The chloroform/methanol solutions with the concentration of 10 μM prior to the CIE effect exhibited almost identical lifetime value (0.24 ns) to that (0.25 ns) of the concentrated chloroform solution. This may also support the collision argument, because their concentrations are identical. The excited molecules can live until they collide with each other. On the other hand, for the gradual reduction in the fluorescence intensity with increasing f_(MeOH), the reason is still ambiguous. At the present moment, based on the observed PL data, it is sure that no ICT mechanism is applied in this case. However, considering the immiscibility of the aromatic core toward both hydrophilic methanol and hydrophobic n-hexane, a subtle variation in the degree of interaction between the aromatic cores upon the collision may occur depending on the volume ratio of the poor solvents. This may affect the radiative (k_r) and non-radiative (k_nr) rate constants when excited molecules relax. Thus, we measured the fluorescence quantum yields (Φ_F) of the solutions with f_(MeOH) = 0%, 20%, and 40% using quinine sulfate as a standard reference (Table 1). According to the following equations, k_r = Φ_F/τ_av and k_nr = (1-Φ_F)/τ_av, the k_r and k_nr can be calculated. The k_nr increases, as f_(MeOH) increases. The interaction between chromophores upon collision would be closer in the solutions with a greater f_(MeOH), which may facilitate non-radiative decay channels.^19,20 As a consequence, this may result in the change in the fluorescence intensity.

In comparison to the above solutions, the CIE-active solution with f_(MeOH) = 70% has a long τ_av (2.23 ns). It is because the excited state is stabilized by the formation of the immobile crystalline aggregate, which increases the lifetime of the excited state and suppresses quenching processes.²¹

Discrimination of methanol and water using the tetrahydrofuran solutions of 1 and 2

The above-results demonstrated that the solution fluorescence color can vary depending on the nature of the peripheral chain, although the constituting aromatic core is identical. From the lesson of the fluorescence properties of 1 and 2, we attempted to distinguish two polar solvents, i.e., water and methanol. Methanol is a representative polar organic solvent, and sometimes it is used instead of water in laboratory. However, in contrast to biologically non-toxic water, methanol is a highly toxic solvent and it is difficult to differentiate from water without performing chemical analyses. Thus, it is worth distinguishing the two polar solvents with the naked eye.

Since chloroform is not miscible with water, we replaced chloroform with water-mixable tetrahydrofuran (THF) as the good solvent. In terms of polarity, the TPE-based aromatic core (in 1 and 2) and dodecyl peripheries (in 1) were considered to be more incompatible with water than methanol. From their subtle solubility differences toward the two polar solvents, a change in fluorescence color was expected, which could give a clue to distinguish between water and methanol.

As similar to the chloroform solutions, the THF solutions of 1 and 2 showed a weak greenish fluorescence (Fig. 10). The emission properties of their THF/water and THF/methanol solutions were examined as a function of f_{(poor solvent)} (Fig. S5†). Fig. 10 represents the emission photos of the solutions with f_{(poor solvent)} = 90%, because their emission colors can distinguish between water and methanol. For 1 with the hydrophobic dodecyl chains, the THF/water solution displayed a blue emission, while a greenish blue emission was observed in the THF/methanol solution (Fig. 10(a)). These two emission colors could be clearly distinguished with the naked eye. Furthermore, the differentiation between water and methanol could be confirmed by a different color code in the solutions of 2 with the hydrophilic DEO chains (Fig. 10(b)). The THF/water solution of 2 showed a strong green emission. In contrast, no emission was observed in the THF/methanol solution of 2, which is similar to the chloroform/methanol solution of 2. This contrast in the emission of 2 is attributed to the degree of the immiscibility between the aromatic core and poor solvents. Considering the green emission of the THF/water solution of 2, the crystallization did not occur completely, and thereby all aromatic segments wouldn't form regularly twisted conformation of the CIE-active samples. In comparison to the non-emissive THF/methanol solution, however, tighter aggregation of the aromatic segments might occur due to the greater immiscibility toward water, which resulted in the green emission with the enhanced intensity. Consequently, the cross-checking of the two emission color codes of 1 and 2 could distinguish between water and methanol.

Fig. 10 Emission photos of THF, THF/water, and THF/MeOH solutions of (a) 1 and (b) 2 under UV illumination at 365 nm.

Conclusions

We prepared two luminogens consisting of the identical TPE-based aromatic core, but different hydrophobic and hydrophilic peripheral chains. Upon adding poor solvents such as hydrophilic methanol and hydrophobic n-hexane, the fluorescence behavior of their chloroform/poor solvent solutions was investigated. In this study, we observed that the emission color and intensity of the solutions are strongly dependent upon the compatibility between the peripheral chain and the added solvent. In contrast to classical solvatochromism, the molecular approach (i.e., tailoring the nature of peripheral chains connected to the identical chromophore) offers an alternative way to tune the solution emission color. The peripheral chain can be considered as a guiding group to tune the fluorescence behavior of the TPE-based chromophore. Additionally, the obtained fluorescence results of the two compounds could be extended to the solvent discrimination. Consequently, water and methanol were able to be distinguished by cross-checking the emission color codes of the two compounds.

Acknowledgements

This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea (no. 2012R1A2A2A01045017). We are grateful to Jinhee Kim for the support of the synthesis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, synthesis of 2,¹H- and ¹³C-NMR spectra of 2, UV absorption spectra, emission color changes of 1 and 2 as a function of temperature. See DOI: 10.1039/c4ra14233k

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