Amino acid-appended pyromellitic diimide liquid materials, their photoluminescence, and the thermal response that turns the photoluminescence off

Abstract

We report a liquid material based on an L-valine-appended pyromellitic diimide framework. This liquid adopts a room-temperature liquid with T_g at −50 °C and can dissolve naphthalene derivatives to show various photoluminescent colors. Furthermore, the on/off photoluminescence of these solutions can be controlled by heating.

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Solvent-free π-conjugated liquid materials, classified differently from ionic liquids, are gaining attention as environmentally friendly soft materials that can be used by various conditions, without the use of harmful volatile organic solvents.^1–12 As an alternative to solid materials without organic solvents, they have the potential to achieve some of the sustainable development goals (SDGs) such as goal 9.^13–15 Also, π-conjugated liquid materials can dissolve or disperse various substances, such as inorganic/organic compounds, which can be coated to fabricate homogeneous composite materials onto various substrates at even high concentrations without grain boundaries.

Strategies for the molecular design of photoluminescent (PL) π-conjugated liquid materials at room temperature, or supercooled liquids, are to introduce branched alkyl chains, suppressing intermolecular interactions between π-conjugated frameworks^16,17 and to lower the melting point by using asymmetric structures.^18,19 One of problems with using a supercooled liquid is that its solution dissolving solutes tends to bring about phase separation between the solute and the crystallized liquid over time. Therefore, it is desirable for a molecular design that prevents adapting a supercooled-liquid state by effectively suppressing intermolecular interactions between the π-conjugated frameworks. In this study, we report on incorporating valine as an amino acid into the π-conjugated framework as a new molecular design for room-temperature liquid materials.

Amino acids are low-cost, commercially available, and essential components of proteins in our lives. They have a bulky chiral center, which can be used as a substituent to induce supramolecular chirality.^20–23 On the other hand, we focus on steric bulkiness for amino acids, which are expected to fine-tune the degree of suppression of intermolecular interactions between π-conjugated sites. The pyromellitic diimide (PMDI) framework was selected as a model compound to investigate the effect of the introduction of amino acids.

PMDI is the smallest arylene diimide and is the essential building block that can develop organic electronics as solid-state materials in both industry and academia.^24–27 In contrast, the luminescent properties of PMDI derivatives are limited because of the PL quenching due to dipole–dipole interactions in the solid state,²⁸ though there are many reports on CT such as crystalline,^29–33 liquid⁷ and liquid-crystalline materials.^34,35 Nevertheless, some researchers have recently achieved significant PL properties by employing molecular designs to restrict undesirable interactions using sterically bulky substituents,^28,36,37 and to substitute with halogen atoms showing phosphorescence.^37,38 These strategies inspired us to diversify material designs to explore material characteristics.

Herein, we report on the synthesis and PL property of amino acid-substituted PDMI liquid (BR-L-Val-PMDI (1)). The introduction of L-valine dramatically lowers the melting point or the glass transition point (T_g) compared to that of liquid materials reported in our previous study.³⁹ The obtained liquid was able to dissolve various naphthalene derivatives (NA-#s) in concentrations as high as equimolar. Furthermore, the PL color can be easily tuned by selecting NA-#s as a solute. Unlike the conventional methods of introducing branched alkyl chains, the present approach for introducing amino acids has never been reported and is a useful molecular design strategy to develop π-conjugated liquid materials, because bulky amino acids substituted near the π-conjugated framework can inhibit intermolecular interaction between π-conjugated frameworks due to their steric repulsion. Moreover, the PL on/off of this solution can be controlled by heating (Fig. 1).

Fig. 1 Molecular structure of BR-L-Val-PMDI (1), its photograph under room light, and photographs of 1 and NA-#s/1 solutions under black light (365 nm).

Initially, we elucidated the fluidic behaviors of 1 by using polarized optical microscopic (POM) observation, X-ray diffraction (XRD) analysis, and differential scanning calorimetry (DSC) (Fig. 2). The POM observation of 1 at 25 °C showed a dark-field image without the birefringence characteristic of a crystalline state (Fig. 2a). Also, the XRD measurement of 1 at 25 °C shows a halo peak at 25° which is derived from the molten alkyl chains (Fig. 2b). In addition, DSC measurements of 1 exhibit no transition peak from the liquid to the solid state under this experimental condition (10 and 1 °C min⁻¹, see ESI,† Fig. S3-11), although T_g was observed at −50 °C (Fig. 2c). These results indicate that 1 adopts a liquid state in a wide range including room temperature and is thermally stable without decomposition under experimental conditions.

Fig. 2 (a) POM observation and (b) XRD pattern of 1 at room temperature. Arrows indicate the analyzer (A) and polarizer (P) axes. (c) DSC traces of 1 at a scanning velocity of 10 °C min⁻¹. Dashed lines are drawn to clarify T_g. (d) Single-crystal X-ray diffraction analysis of L-Val-PMDI.

It should be noted that 1 does not show freezing and melting points (f.p. and m.p.), and T_g dramatically decreases compared to those of previously reported compounds without amino-acid moieties.³⁹ This result suggests that the molecular strategy for the direct incorporation of L-valine to the core framework should suppress the intermolecular interactions between π-conjugated frameworks. To verify the influence on the incorporation of L-valine, we carried out a single-crystal X-ray diffraction analysis of the model compound L-Val-PMDI without alkyl chains (Fig. 2d and ESI,† Fig. S2). Isopropyl and carboxylic acid moieties can protrude in the vertical direction with large torsion angles (−49.92° and 77.82°) to the planar π-conjugated PMDI framework, restricting the π–π interaction between the cores. Moreover, bulky branched-alkyl chains can promote making the 1 molecule have a fluidic property, effectively suppressing the crystallization of 1 to afford 1 in a liquid state at r.t.

Furthermore, to verify whether the core of 1 with bulky substituents can interact with other molecules, we prepared the 1 : 1 co-crystal of L-Val-PMDI and naphthalene (NA-H) from ethyl acetate solution (Fig. 3a). The single crystal analysis revealed that L-Val-PMDI and NA-H formed a 1 : 1 complex with an alternately stacking structure with each other, leading to a one-dimensional (1D) structure along the b-axis. NA-H and L-Val-PMDI frameworks partially overlap with each other, with the distance between both cores estimated as 3.472 Å. This result indicates that weak intermolecular interaction would occur because this distance is longer than the corresponding van der Waals radii (3.40 Å for the distance between two p-orbitals of carbon). These results suggest that 1 and NA-#s can interact with each other even in a solution state.

Fig. 3 (a) Single-crystal X-ray diffraction analysis of NA-H/L-Val-PMDI along the a-axis (left) and the b axis (right). (b) TG analysis of NA-H/1 over the temperature range from 40 to 600 °C under a N₂ atmosphere. (c) UV-vis absorption spectra of NA-H/1 in a neat state. (d) and (e) UV-vis absorption spectra and the HOMO and LUMO of NA-H/Me-L-Val-PMDI produced with TD-CAM-B3LYP-GD3(BJ)/6-31+G(d,p). (f) PL spectra of NA-H/1 in a neat state.

Then, we prepared a series of solutions, including NA-#: 1 = 1 : 1 (mol/mol) (referred to as NA-#/1). It was found that 1 can easily dissolve equimolar amounts of solid NA-#s without using organic solvent. For all NA-#/1s, there are no solid substances by optical microscopic (OM) and POM observations, XRD patterns, and DSC measurements (Fig. S3-1 to S3-9, ESI†), resulting in all NA-#/1s adopting solution states at r.t. (see Fig. S3-1 to S3-9, ESI†). Moreover, we carried out thermogravimetric (TG) analysis to elucidate the miscible/dispersed states of NA-#/1.³⁹

1 can vaporize up to 400 °C without decomposition (Fig. S3-11, ESI†). Also, every TG trace of NA-#/1 shows clearly two weight loss steps up to 400 °C (Fig. 3b). The former declining step indicates the weight loss of volatile NA-#s, and the latter one corresponds to that of 1. It should be noted that the weight-loss rates of NA-#/1 are different from those of solid NA-# (Tables S3-1, ESI†). Thus, we estimate these activation energies ΔE due to the vaporization of NA-# in both solid and solution states in NA-#/1 (Fig. S3-11, ESI†). Every ΔE of NA-# in NA-#/1 tends to be smaller than those of solid NA-#. It is known that the vaporization mechanisms of NA-#s are dependent upon the surrounding environment.⁴⁰ Solid NA-#s interact with one another via π–π and C–H-π interactions, whereas NA-# dissolved in 1 should weaken the intermolecular interaction with one another, leading to a smaller ΔE of NA-# in NA-#/1 than those of solid NA-#.³⁹ These results indicate that NA-#/1 has fluidic properties that solvate NA-# at the molecular level.

For investigating the photophysical properties of NA-#/1, we carried out UV-vis absorption and PL measurements (Fig. 3c). 1 is colorless, although NA-#/1 shows a new peak at 392 nm different from those of both 1 and NA-H. To investigate the characteristics of the peak observed at a longer wavelength, time-dependent density functional theory (TD-DFT) calculations were carried out by using Gaussian 16 at the CAM-B3LYP-D3(BJ)/6-31+G(d,p) level of theory by using a Me-L-Val-PMDI : NA-H = 1 : 1 complex (Fig. 3d and Fig. S4, ESI†). The functional is selected to be suitable for CT systems due to the long-range correction.^33,41 TD-DFT calculations show the longer-wavelength peak, which is almost consistent with that experimentally obtained in NA-H/1. Also, this peak at 376 nm produced by the TD-DFT calculation is contributed by the LUMO located on Me-L-Val-PMDI as an electron acceptor and the HOMO located on NA-H as an electron donor (Fig. 3e). This result indicates that this peak should be the CT band derived from the electronic transition from NA-H to Me-L-Val-PMDI. It noted that the CT band was red-shift dependent on higher HOMO levels of NA-#, of which values are correlated with the gap between HOMO level for NA-# and LUMO level for 1 (Fig. 3e and Tables S3-1, ESI†). For electron-donating NA-N(CH₃)₂ with the highest HOMO level, the CT band in NA-N(CH₃)₂/1 was observed at the longest wavelength (532 nm). On the other hand, electron-accepting NA-Cl, NA-COCH₃, NA-CO₂CH₃, NA-COH, and NA-CN have almost no CT peaks.

For the PL spectra, both 1 and NA show less emission by themselves, while NA-#s/1 shows various emission colors different from those of both 1 and NA-#s (Fig. 3f), of which emission might be derived from CT and exciplex emission. The PL spectrum of NA-H/1 shows a peak at 522 nm with a yellow-green emission. Also, the PL peaks at 581 of NA-CH₃/1 and 598 nm of NA-OCH₃/1 are bathochromically shifted compared to that of NA-H/1 as they are higher electron donors than NA-H (Table S3-1 and Fig. S3-12, ESI†). Conversely, for NA-Cl/1, NA-HCO/1, and NA-CN/1 substituted with electron-withdrawing groups, those PL peaks are shifted to shorter wavelengths than those of NA-H/1. The emission wavelength was also correlated with the difference between the LUMO of 1 and the HOMO of NA-#s calculated by DFT calculation (Fig. S3-12, ESI†).

Finally, we investigated the thermal-responsive property of NA-#s/1 solutions in response to heating (Fig. 4). NA-H/1 painted onto filter paper shows a similar emission color to that of the NA-H/1 solution, whereas the painted filter paper shows no emission upon heating at 100 °C for 5 minutes (Fig. 4). This is because heating can vaporize NA-H from the NA-H/1 solution, leaving 1 with less emission on the filter paper. Similarly, it is observed that heating at 150 °C or 200 °C can vaporize NA-#s from NA-#s/1 (Fig. S6-1, ESI†). These results indicate that NA-#s/1 can function as temperature-responsive liquid materials.

Fig. 4 Thermal-responsive behavior of NA-H/1.

In conclusion, we synthesized an amino-acid appended PMDI liquid material. The introduction of the bulky L-valine and branched alkyl chains into a PDMI framework can dramatically lower T_g to adapt a room temperature liquid. Liquid 1 can dissolve NA-#s in concentrations as high as equimolar without organic solvents. PL colors of NA-#s/1 can be tuned by selecting NA-#s, which are dependent upon HOMO levels of NA-#s. Furthermore, tuning PL of NA-#s/1 on/off can be controlled by heating. As a result, NA-#s/1 functions as a thermal-responsive liquid material. Currently, we are investigating influences on rheological and chiroptical properties by incorporating with different amino acids.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2373409 and 2353953. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc02229g

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