Distinct bulk emission properties of the hexacatenar molecules by varying the polarity of peripheral chains†

Seongwon Park‡ ^a, Jinyoung Jun‡^a, Jaeduk Byun^b, Ho-Joong Kim*^c and Byoung-Ki Cho*^a
aDepartment of Chemistry, Dankook University, 119, Dandae-ro, Chungnam, 448-701, Korea. E-mail: chobk@dankook.ac.kr
^bDepartment of Physics, Dankook University, 119, Dandae-ro, Chungnam, 448-701, Korea
^cDepartment of Chemistry, Chosun University, Gwangju, 501-759, Korea

First published on 15th August 2024

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Introduction

Intramolecular charge transfer (ICT) which is a phenomenon where charge moves from an electron-rich donor (D) to an electron-deficient acceptor (A) within a molecule, is a widely studied topic in molecular spectroscopy. Applications utilizing ICT include organic light-emitting diodes (OLEDs),^1,2 solar cells,³ fluorescence detectors,⁴ chemical sensors,⁵ and biological imaging.^6,7

The structure of molecules with ICT characteristics consists of a π-conjugated linker between the donor (D) and acceptor (A).^8,9 The ICT process occurs in the excited state of the molecule, which has absorbed an appropriate wavelength, promoting charge transfer in this state. The charge transfer involves the movement of electrons through the π-conjugated linker within the D–A system. This results in a different electronic distribution with varying degrees of charge separation within the molecule, leading to changes in the emission wavelength.^10,11 The wavelength shift due to charge transfer is significantly influenced by the polarity of the solvent; as solvent polarity increases, the charge-separated electronic distribution becomes stabilized. Consequently, a red-shift in the emission wavelength is observed, along with changes in quantum yield and fluorescence lifetime characteristics.^12,13 However, the application potential of the solution is fundamentally limited due to low mechanical properties.^14–16 Thus, controlling the emission color change of ICT chromophores in the bulk state is a highly intriguing research topic.

Most of the research on the engineering of ICT emission properties in the bulk state has primarily focused on structural variations within the chromophore itself. For example, in the push–pull type systems, the strength of ICT has been manipulated by changing the donor or acceptor substituents while keeping the main aromatic backbone unaltered.^17–19 Additionally, the introduction of bulky groups linked to chromophores has altered their stable conformational states, leading to variations in the energy levels of the ICT state.^20,21

Alternatively, we propose that the bulk emission properties of an ICT chromophore could be engineered by varying the non-chromophore part rather than the chromophore backbone (Fig. 1), although this molecular approach has not been attempted to date. Our research aims to investigate differences in bulk ICT emission by introducing flexible chains with different polarities to the periphery of a chromophore capable of ICT phenomena. With this in mind, we designed two hexacatenar molecules consisting of an identical D–A–D chromophore, but with nonpolar decyl chains and polar tri(ethylene oxide) (TEO) chains for 1 and 2, respectively. The chromophore includes an electron acceptor (A) group composed of naphthalene-conjugated 1,3,4-oxadiazoles, and an electron donor (D) group comprised of 3,4,5-trialkoxy benzene. 1,3,4-Oxadiazole was chosen due to its excellent brightness, strong electron-accepting ability, and thermal stability.^22–28 The synthesized compounds 1 and 2 were examined in bulk for their assembled structures and emission characteristics. Remarkably, they displayed different emissions, specifically sky-blue and green colors at room temperature (RT) for 1 and 2, respectively (Fig. 1). Moreover, their bulk emission colors changed with increasing temperature. We hypothesize that 1 and 2 have different degrees of charge separation in the ICT state, due to the influence of the polarity of the peripheral chains. The emission properties in both the bulk and solution states were characterized to support our findings related to ICT characteristics. This paper will address the synthesis, thermotropic, photophysical, and dynamics properties of the hexacatenar compounds.

Results and discussion

Synthesis of 1 and 2

The synthetic procedure is outlined in Scheme S1 (ESI†). The introduction of aliphatic peripheral chains was conducted through the alkylation reaction of decyl bromide and tosylated tri(ethylene oxide) (TEO) with propyl gallate. Subsequently, the hydrazidation reaction produced aliphatic chains-terminated hydrazide compounds (b-1 and b-2). The synthesis of 1 and 2 involved amide coupling with 1,4-naphthalene dicarboxylic acid, followed by cyclization in the presence of thionyl chloride.

In the ¹H-NMR spectrum of 1, the hydrogen peaks of naphthalene are observed at 9.41, 8.36, and 7.83 ppm, and the hydrogen peak of pyrogallol is found at 7.39 ppm (Fig. S1, ESI†). The hydrogen peaks of the nonpolar dodecyl chains are observed from 1.87 to 0.88 ppm (Fig. S1, ESI†). For compound 2, the hydrogen peaks of naphthalene are observed at 9.40, 8.38, and 7.83 ppm, and the hydrogen peak of pyrogallol is found at 7.39 ppm (Fig. S2, ESI†). The hydrogen peaks of the polar TEO chains are seen from 3.93 to 3.36 ppm (Fig. S2, ESI†). The ¹³C-NMR spectra of 1 and 2 reveal ten chemically distinct aromatic carbon signals, including two for 1,3,4-oxadiazole, five for naphthalene, and four for pyrogallol, respectively (Fig. S1 and S2, ESI†). The NMR data are in good agreement with the designed molecular structures. Additionally, the elemental analysis results of 1 and 2 match the calculated values within a margin of error of 0.5%, confirming that the designed molecules were successfully synthesized.

Thermotropic properties of 1 and 2

DSC measurements were performed to investigate the temperature-dependent phase behavior of 1 and 2 (Fig. S3, ESI†). In the first heating of 1, two endothermic transitions are observed at 51.2 °C (65.0 kJ mol⁻¹) and 90.4 °C (2.0 kJ mol⁻¹). A single-phase transition is observed at 86.7 °C (2.0 kJ mol⁻¹) during the cooling process. Unlike the first heating data, the thermogram obtained from the second heating exhibits only the phase transition at 90.4 °C (2.0 kJ mol⁻¹). The phase transition at 51.2 °C observed in the first heating shows a large enthalpy change of 65.0 kJ mol⁻¹, which indicates the melting temperature (T_m). On the other hand, the phase transition at 90.4 °C corresponds to the isotropization from the liquid crystalline (LC) phase to the isotropic liquid (Liq) phase, due to its relatively small enthalpy change of 2.0 kJ mol⁻¹.

During the observation of 1 under a polarized optical microscope (POM), a crystallized texture is observed at temperatures below 50 °C (Fig. 2a). Upon melting, the LC phase exhibits a birefringent fan-like texture which is inferred as a hexagonal columnar LC phase (Col_hex) (Fig. 2a).^29,30 Upon cooling from the isotropization temperature (T_i), the fan-like texture in the LC phase remains visible down to RT. Crystallization could be confirmed in POM after annealing for 12 hours at RT. The results from DSC and the thermal analysis using POM are found to be consistent. In contrast to 1, compound 2 exists in a Liq state at RT. No phase transition is observed in DSC upon heating, and a dark texture indicative of the Liq state is observed in POM (Fig. S4, ESI†).

Morphological analysis of 1

The morphological analysis of compound 1 was conducted using XRD. In the XRD data, multiple sharp peak reflections are observed at RT (Fig. 2b). These numerous peaks represent a crystalline diffraction. In the LC phase at 60 °C, only a primary reflection is observed at q = 0.22 Å⁻¹ (Fig. 2b). Assuming a hexagonal columnar structure as inferred from POM, the intercolumnar distance (a) is calculated to be 33 Å using the primary reflection (100). The broad peak at q = 0.39 Å⁻¹ corresponds to the reflection from the polyimide (PI) cover used in the XRD cell.³¹ In the wide-angle region, a broad peak at q = 1.83 Å⁻¹ is observed (Fig. 2b). The d-spacing of this peak is 3.4 Å, which corresponds to the columnar stratum distance involved in π–π stacking (Fig. 2d).

For a more precise structural analysis, grazing incidence X-ray diffraction (GIXRD) was utilized. In the GIXRD data measured for the LC phase of 1, a primary peak reflection is observed at the equatorial position (Fig. 2c). This indicates that columns are oriented perpendicular to the substrate.^32,33 Additionally, a meridional reflection with a d-spacing of 3.4 Å is observed at q = 1.83 Å⁻¹ (Fig. 2c), corresponding to the typical π–π columnar stacking. Consequently, the stacking direction of aromatic segments within a column is parallel to the columnar axis (Fig. 2d).

Energy calculations for the molecular conformation were conducted using the density functional theory (DFT) method to elucidate the predominant conformer in the LC phase. The conformational energy of the molecule was analyzed as a function of the dihedral angle (α) between the naphthalene and 1,3,4-oxadiazole units (Fig. 3a). Given that two oxadiazole groups are linked to the central naphthalene unit, the syn- and anti-models should be considered individually.³⁴ In both cases, the zero dihedral angle is defined as the planar conformation where the H⁶ atom of the naphthalene is closest to the nitrogen atom of the oxadiazole (Fig. 3b). For both models, the most unstable conformations are found at α = 90° due to the lack of electronic coupling stabilization.³⁵ In the syn-model, the minimum energy state is a planar conformer at α = 0°, characterized by a 2.34 Å distance between the H⁶ atom of the naphthalene and the nitrogen atom of the oxadiazole (Fig. 3b and c). Conversely, in the anti-model, the minimum energy state is found at a tilt conformer at α = 30° (or 150°) (Fig. 3c). In the planar conformer at α = 0° in the anti-model, the oxygen atom of one of the oxadiazoles approaches the H⁶ of the naphthalene to the closest distance of 1.83 Å (Fig. 3b), causing the steric repulsion. Consequently, the simulation data suggest that the syn-conformer at α = 0° is the most energetically favored, being more stable than the anti-conformer at α = 30° by approximately 17.4 kJ mol⁻¹.

Photophysical properties in the solutions

In the UV-vis absorption spectra, both 1 and 2 exhibit an absorption peak around 365 nm, regardless of the solvent used (Fig. S5, ESI†). Despite being different chains, they show the same absorption wavelength. Compared to other samples, the cyclohexane (CycloHX) solution of 2 exhibits a lower absorption signal. This is attributed to the poor solubility of the hydrophilic TEO groups in the hydrophobic CycloHX, resulting in only partial dissolution of 2.

To confirm that the transition to the excited state involves the same electronic distribution change in both compounds, DFT simulations were performed. The DFT results reveal that the electronic distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for 1 and 2 are similar (Fig. S6, ESI†). The orbital geometries of the HOMO and LUMO strongly suggest the ICT from the electron-donating 3,4,5-trialkoxy benzene to the electron-accepting central group composed of naphthalene-conjugated 1,3,4-oxadiazoles during the electronic excitation.

Like Beer's law in UV-vis absorption spectroscopy, fluorescence spectroscopy also exhibits a linear relationship between emission intensity and sample concentration. However, at higher concentrations where molecules are close enough to collide, this linear relationship breaks down.^36,37 Therefore, the photoluminescence (PL) measurements of the THF solutions were performed at different concentrations. An emission peak appears around 500 nm (Fig. S7, ESI†). The emission spectra as a function of concentration reveal that the highest intensity signal is observed at a concentration of 10⁻⁵ M. This indicates that the emission signals in solutions lower than the 10⁻⁵ M result from individual molecules, with no interaction between them. Subsequently, the fluorescence studies on monomeric emission were conducted at a fixed concentration of 10⁻⁵ M. To investigate the ICT characteristics, the emission properties were analyzed by altering the solvent polarity (Fig. 4a and b).³⁸ Under UV irradiation at 365 nm, the solutions of 1 and 2 in non-polar solvents like CycloHX and toluene emit blue light. With an increase in solvent polarity, the emission shifts from blue to green, and then to yellow. As a result, this solvatochromism demonstrates that both compounds display emission characteristics in solution attributable to ICT.

Chromophores with ICT properties typically show a red-shift in emission wavelength and a decrease in emission intensity as the solvent polarity increases. Therefore, PL measurements were conducted to precisely analyze the changes in emission wavelength and intensity (Fig. 4c and d). In solutions with non-polar CycloHX and toluene, the emission spectra are sharp and exhibit vibronic structures, indicating locally excited (LE) state emissions. In contrast, only a broad peak is observed in solutions with polar methylene chloride (MC) and THF. The emission data show a significant bathochromic shift from 434 nm to 560 nm and become deintensified with increasing polarity. These emission results are attributed to the formation of a stable charge-transfer state in the polar environment.³⁹ The low emission intensity of 2 in CycloHX is due to the poor solubility of the polar TEO chains (Fig. 4d).

On the other hand, the N,N'-dimethylformamide (DMF) solution with the highest polarity among the used solvents exhibits negligible emission signal which is barely distinguishable from the X-axis in the PL spectra (Fig. 4c and d). The quantum yields (QY) of the DMF solutions of 1 and 2 are determined to be 4.0% and 6.1%, respectively, which are significantly lower than the QYs of other solutions (Fig. 4a and b). This significant suppression in emission intensity is presumed to be due to the twisted intramolecular charge transfer (TICT) phenomenon,⁴⁰ which has been observed in highly polar solutions. During the excited state, TICT-mediated decay involves the rapid 90° rotation of a single bond connecting the electron donor and acceptor groups in highly polar solvents, leading to complete charge separation.⁴¹ Consequently, the TICT in the excited state results in non-radiative decay and reduces fluorescence lifetime.^42,43 Therefore, the sharp decline in PL intensity observed in the DMF solutions is attributed to the TICT.

By analyzing the relaxation mechanisms from the excited states, it could distinguish between ICT and TICT. Fluorescence lifetime experiments were conducted on three solutions with a concentration of 10⁻⁵ M: CycloHX, THF, and DMF (Fig. 4e and f). The solutions were irradiated with 372 nm light to induce excitation, followed by the measurement of changes in fluorescence intensity over time. For 1 and 2, the lifetimes in the non-polar CycloHX are measured to be approximately 0.77 ns. In the polar THF, the lifetime increases to 3.16 and 3.46 ns for 1 and 2. As mentioned previously, the increase in solvent polarity stabilizes the ICT state, resulting in a delayed emission and thus, an increase in fluorescence lifetime. However, a rapid decrease in fluorescence lifetimes to 0.19 and 0.24 ns for 1 and 2 is observed in the highly polar DMF, confirming the transition to the TICT state in the excited state.

Even in solvents of high polarity, the transition to TICT can be inhibited if rotational motions are slowed down.^44,45 To test this, as polar and viscous glycerol was added to DMF solutions, the PL properties of 1 and 2 were investigated (Fig. S8, ESI†). For 1, the addition of glycerol results in a blue-shift of the wavelength from 561 nm to 493 nm, and the PL intensity increases approximately 26-fold. Similarly, for 2, the wavelength blue-shifts from 549 nm to 514 nm, and the PL intensity increases about 6 times. Additionally, the fluorescence lifetime significantly increases, reaching up to 8.72 ns and 7.96 ns in the 80% glycerol solutions (Gly80) of 1 and 2, respectively (Fig. 4e and f). The solution samples of both 1 and 2 exhibit similar emission trends with increasing glycerol fraction, although there are some differences in emission intensity and wavelength. The increased intensity and the blue-shifted wavelength are due to restricted rotational motions caused by the higher viscosity of glycerol. In more viscous media, the rotational motions of the C–C bond connecting the electron-accepting oxadiazole and the electron-donating 3,4,5-trialkoxy benzene become slower, inhibiting non-radiative decay pathways. In such media, the C–C bond does not reach the 90° rotation required for the formation of the TICT state with complete charge separation. Consequently, the addition of viscous glycerol leads to ICT states with reduced charge separation, resulting in blue-shifted emissions.

Photophysical properties in the THF/water mixture solutions

Before analyzing the emission properties in the bulk state, the emission properties were analyzed as a function of water fraction (f_w) in the THF/water mixture solutions. Given that the alkyl chains of 1 are hydrophobic, it was expected that aggregation would occur at high f_w. On the other hand, the TEO chains of 2 are hydrophilic, making them soluble in THF/water mixture solutions. The emission images of 1 and 2 under UV irradiation at 365 nm were examined (Fig. 5a and d). The emission differences between 1 and 2 at high f_w might result from aggregation and changes in polarity due to the addition of water.

For accurate analysis, the PL data of 1 and 2 in THF/water mixture solutions were measured (Fig. S9, ESI†). In the case of 1, a red-shift of the emission wavelength and a decrease in emission intensity are observed with increasing to f_w = 30% (Fig. 5b). However, when f_w increases beyond 40%, the emission wavelength blue-shifts to 495 nm, and the emission intensity increases (Fig. 5b). This observation resembles aggregation-induced emission (AIE). However, it would be more reasonable to explain it as a result of a lower polar environment within the molecular aggregates,^46,47 because the PL intensity is weaker than in the THF-only solution. The occurrence of aggregation is identified by the broadening of the UV-vis absorption spectrum at f_w = 40% (Fig. S10, ESI†).⁴⁸ The interior of the aggregates is less polar than the external polar environment, leading to an increase in PL intensity and a blue-shift in wavelength. This explanation can be corroborated by fluorescence lifetime experiments (Fig. 5c). The lifetime increases with the increase in f_w. The fluorescence lifetimes at f_w = 30, 60, and 90% are measured to be 1.02 ns 4.19 ns, and 6.43 ns, respectively. Compared to the emission wavelength observed in CycloHX, as detailed in the previously mentioned solvent-dependent PL data, the emissions from the aggregates appear at longer wavelengths and lack vibronic structures. Additionally, these emissions exhibit extended lifetimes. Such emission characteristics suggest that ICT emissions occur in the aggregated state.

In the case of 2, on the other hand, aggregation does not occur because of the water-soluble TEO chains. As f_w increases, a red-shift occurs due to the solvatochromic effect, and the PL intensity gradually decreases (Fig. 5d and e). The lifetimes at f_w = 30, 60, and 90% are determined to be 0.24 ns, 0.52 ns, and 0.94 ns, respectively (Fig. 5f). The lifetimes of less than 1.0 ns confirm that the emission characteristics are attributed to the TICT state.

Photophysical properties in the bulk

As mentioned previously, the bulk samples of 1 and 2 exhibit distinct emission colors at RT (Fig. 1). The emission spectra of 1 and 2 at RT show λ_max values at 476 and 514 nm, respectively (Fig. 6b and f). This noticeable difference in emission wavelength indicates that the emission of 2 originates from a relatively more stable ICT state with greater charge separation, which is formed by the polar TEO chains, similar to how a polar solvent stabilizes the ICT state.

In addition, they exhibit emission color change with temperature. Remarkably, their emissions blue-shift upon heating. The emission of 1 changes from greenish-blue to sky-blue, while the emission of 2 changes from green to greenish-blue (Fig. 6a and e, and Fig. S11, ESI†). To analyze their temperature-dependent emission properties in the bulk state, the emission wavelength and intensity are plotted as a function of temperature (Fig. 6c and g). For both samples, their emission wavelengths exhibit a blue-shift as temperature increases from RT to 150 °C. This blue-shift can be explained by considering the shift in the thermal equilibrium between LE state and ICT state.^44,49–53 Indeed, the LE emission signals in the CycloHX and toluene solutions are observed as a shoulder in the bulk emission spectra at higher temperatures. Therefore, the lower-energy ICT state is predominantly occupied at lower temperatures. However, as the temperature increases, thermal activations overcome the energy barrier between the excited states, increasing the population of the higher energy LE state. As a result, the rise in temperature leads to a blue-shift emission.

Notably, the emission intensity of 1 changes discontinuously with each phase transition (Fig. 6c). During the transition from the Cry to the Col_hex LC phase, the emission intensity suddenly decreases due to the enhanced molecular motions.^54,55 Conversely, during the transition from the Col_hex LC to the Liq phase, an increase in emission intensity is observed (Fig. 6c). This negative thermal quenching of the emission is likely attributed to the increased average distance between aromatic chromophores. This can be supported by the XRD data (Fig. 2b), where the reflection corresponding to π–π stacking disappears upon transitioning to the Liq phase. It is assumed that less emissive LC aggregates dissociate in the Liq phase, and thereby reducing the intermolecular interactions and consequently decreasing non-radiative decay pathways.^56,57

In the temperature-dependent emission spectra of 2 (Fig. 6f), the emission intensity gradually decreases up to 80 °C (Fig. 6g), due to increased thermal motions. Unexpectedly, 2 displays a sudden blue-shift and an increase in emission intensity between 80 °C and 90 °C (Fig. 6g). This suggests a sudden change in the intramolecular dynamic motion, leading to a negative thermal quenching effect.

Time-resolved emission data for both samples show a decrease in lifetime as temperature increases (Fig. 6d and h). This trend is linked to a gradual reduction in ICT character at elevated temperatures. Notably, the lifetime of 1 decreases sharply from 7.75 ns to 5.13 ns at the phase transition from the Cry to the Col_hex LC phase (Fig. S12, ESI†). This reduction is attributed to the abrupt activation of rotational motions upon entering the LC phase.

Temperature-dependent dielectric relaxation spectroscopy studies

To provide evidence for the temperature-dependent molecular motions of 1 and 2, dielectric relaxation spectroscopy (DRS) measurements were conducted with a heating rate of 2 °C min⁻¹ and a frequency range of 0.1–10⁵ Hz (voltage: 0.2 V μm⁻¹) (Fig. 7).

The dielectric loss (e′′) spectrum for 1 indicates the presence of an absorption mode in the range of 2 × 10²–10⁴ Hz (Fig. 7a). Given its low intensity, this mode is associated with a small displacement, indicative of subtle movements of the terminal 3,4,5-trialkoxy benzene groups linked to decyl chains. With increasing temperature, this absorption mode shifts towards higher frequencies, reflecting a decrease in emission intensity. Upon transitioning into the LC phase, the absorption frequency becomes saturated. At 90 °C, a slower absorption mode in a lower frequency range of 0.1–1.0 Hz becomes more prominent (Fig. 7a). The pronounced loss values for this mode suggest that it involves the rotational motion of the C–C bond connecting the polar oxadiazole and naphthalene groups.³⁵ Such rotational motion within the aromatic core increases the average distance between chromophores, leading to weaker intermolecular interactions. This dynamic motion contributes to the negative thermal quenching above T_i.

For 2, the absorption mode observed between 2 × 10²–10⁴ Hz is attributed to the motion of the 3,4,5-trialkoxy benzene group linked to TEO chains (Fig. 7b). The TEO chain is more flexible compared to the decyl chain,^58,59 resulting in larger displacement fluctuations of the terminal aromatic groups. As seen in the temperature-dependent emission results, there is a sudden decrease in the emission wavelength near 85 °C. At this point, a new absorption mode of the C–C bond rotation between naphthalene and oxadiazole in the range of 0.1–10² Hz is observed (Fig. 7b). This implies that the rotational motion within the acceptor group is activated at temperatures above 85 °C. To confirm this behavior, the variation in the energy gap (ΔE) between HOMO and LUMO was simulated as functions of the dihedral angles α and β (Fig. 7c). Compared to the β, ΔE increases significantly with increasing the α, suggesting that the rotational motion within the acceptor group leads to an increase in the energy levels of the ICT state to an unstable state. Consequently, a blue-shift in the emission wavelength occurs.

Conclusions

In summary, we reported the distinct emission properties of an identical D–A–D chromophore in the bulk state depending on the type of peripheral chain. The chromophore consisted of an electron acceptor made of naphthalene-conjugated 1,3,4-oxadiazoles, and an electron donor composed of 3,4,5-trialkoxy benzene. The peripheral chains of 1 and 2 consisted of nonpolar decyl chains and polar TEO chains, respectively. 1 transitioned from a Cry to an LC, and then to a Liq state as the temperature increased. In contrast, 2 existed in a Liq state at RT and remained in the Liq state across a wide temperature range without undergoing a phase transition. Both 1 and 2 exhibited a red-shift in the emission wavelength and a decrease in emission intensity as the solvent polarity increased. These emission properties in the solution state indicated the ICT characteristics, which were supported by DFT simulations. In the highly polar DMF solution, they exhibited a TICT phenomenon. More notably, the bulk samples of 1 and 2 exhibited different emission colors at RT, which blue-shifted with increasing temperature. The emission properties including wavelength and intensity were associated with molecular dynamics characterized by DRS studies. The PL difference between the bulk samples of 1 and 2 could be elucidated by the degree of stabilization of ICT state depending on the type of the non-emissive peripheral chains. The findings presented in this study demonstrated that the emission properties of the same chromophore could be engineered by the polarity of the peripheral chains in the bulk state.

Author contributions

Seongwon Park: formal analysis, investigation, methodology, writing – original draft, visualization. Jinyoung Jun: formal analysis, investigation, data curation, visualization. Jaeduk Byun: validation, data curation. Ho-Joong Kim: writing – review & editing, resources, formal analysis. Byoung-Ki Cho: conceptualization, writing – review & editing, formal analysis, visualization, funding acquisition, supervision.

Data availability

Data for this article are either included in the manuscript and the ESI† or are available upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by grants from the National Research Foundation (NRF) of Korea (Grant No. 2021R1A2C1006891). Experiments at PLS-II 6D UNIST-PAL Beamline were supported in part by MSIT, POSTECH, and UNIST.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details and characterization data; 1H- and 13C-NMR spectra, DSC thermogram of 1; POM texture of 2 at RT; UV-vis absorption spectra of the solutions; DFT simulation of the HOMO and LUMO; emission spectra of the THF solutions; emission images and spectra with increasing glycerol fraction; emission spectra of the THF/water solutions; UV-vis absorption spectra of the THF/water solutions of 1; 1931 CIE chromaticity upon heating. See DOI: https://doi.org/10.1039/d4tc02637c

‡ These authors contributed equally to this work.

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