Synthesis of Furan-Substituted N-Heteroacene-Based Liquid Material and Its Acid-Recognizing Behavior

In this study, we synthesized a novel N-heteroacene-based liquid material 6,7-bis(3,7,11-trimethyl-1-dodecyloxy)-2,3-difurylquinoxaline (RPNL 1), containing two furan rings. We revealed that RPNL 1 adopted a disordered liquid at 25 ◦C, determined by polarized optical microscopic observation, differential scanning calorimetry, and X-ray diffraction measurements. The fluorescent spectrum measurement revealed that RPNL 1 showed a blue emission at 25 ◦C. Dissolving benzene sulfonic acid (BSA) in RPNL 1 brought about dramatic changes in its physical properties, such as emission colors, as well as sample states. Upon recognizing BSA, photoluminescent color was changed into orange, as well as phase transition occurred from liquid to a liquid-crystalline phase. RPNL 1 can function as an acid-recognizing material, accompanied with the color changes in emission.


Introduction
Stimuli-responsive soft functional materials get much attention because these materials can respond to external stimulus to show dramatic changes in physical properties, such as emission color and shape, in fact, these materials can adopt more than two independent conditions, which are expected to be applied into optoelectronic devices, such as sensors, energy devices, and actuators [1][2][3][4][5].Recently, a lot of chemists have focused and developed a stimuli-responsive, solid-state luminescent materials response to various external stimuli, such as temperature, light, mechanical stress, and vapor, to give rise to dramatic changes in luminescent colors [6][7][8][9][10].These solid materials are facile to fabricate film states by a coating technology, however, films fabricated from solid-state materials have a possibility to form unexpected defects, such as grain boundary and contact resistance on electrode, which should remarkably lead to a decrease in their efficiency.If stimuli-responsive liquid materials are developed instead of solid-state materials, the disadvantages mentioned above are expected to be resolved, and the research field of stimuli-responsive materials will further expand.Herein, we report on a novel, room-temperature-photoluminescent, π-conjugated liquid (RPL) material, involving a stimuli-responsive behavior, itself.
Recently, RPL materials have been reported by some groups, which have been classified into novel, functional soft materials [11][12][13][14][15][16][17][18].Nakanishi's group has reported many liquid materials composed of polyaromatic hydrocarbons (PAH), such as naphthalene, anthracene, pyrene, and phenylenevinylene.Additionally, our group has reported a room-temperature-photoluminescent N-heteroacene-based liquid (6,7-bis (3,7,11-trimethyl-1-dodecyloxy)-2,3-difurylquinoxaline or (RPNL)) [13,18].N-heteroacene partially contains sp 2 imino-N atoms instead of sp 2 carbon atoms, [19][20][21][22][23][24][25] which can recognize a Lewis acid, such as a proton or metal ion, because imino-N atoms with lone-pair electrons serve as a Lewis base.The recognition of proton for imino-N atoms can give rise to an obvious change in the electronic property of the N-heteroacene framework [13,18,25].Accordingly, RPNL itself can function as an acid-recognizing material, in contrast to RPL composed of a PAH framework.In this study, we prepare novel RPNL 1, containing two furan rings.Although our group has reported thiophene-appended PRNL materials [18], it is important to investigate an influence on the electronic properties, as well as the acid-recognizing behavior, by substitution of various aromatic rings, leading to a sophisticated molecular design and tailor-made synthesis, for the development of novel PRNL materials with fine-tuned characters.Moreover, we investigated the theoretical calculations on electronic property, as well as a structure for furan-appended PRNL material, by using density functional theory (DFT) calculations.

Results and Discussion
For a molecular design of RPNL 1 to combine the inherently acid-recognizing property, it is indispensable that the two imino-N atoms are accessible from external stimuli, by other substituted moieties in RPNL 1, without any disturbance.Before synthesis of the RPNL 1, we performed DFT calculations to investigate the molecular structure of RPNL 1, by using the Spartan'18 software package (Wavefunction, Inc., Irvine, CA, USA).The optimized molecular geometry of the RPNL 1, calculated at the B3LYP/6-31G* level of theory, revealed that two long alkoxy chains did not interact with the imino-N atoms as acid-recognizing moieties in Figure 1a.Additionally, the interatomic distances between the O atoms on the furan rings and the imino-N atoms were 2.696 and 2.697 Å, which is shorter than the sum of van der Waals radius (O-N 3.07 Å), respectively [29].This result suggests that interatomic interactions occur between the O atoms on the furan rings and the imino-N atoms pyrazine framework, as a result, the acid-recognizing capability of the imino-N atoms might decrease.Therefore, we also investigated the acid-recognizing capability of RPNL 1 to benzene sulfonic acid (BSA), with lower pKa.DFT calculation of RPNL 1 with BSA revealed that the BSA molecule was capable of approaching one imino-N atom of RPNL 1, because the interatomic distance between the O atom of the BSA and the imino-N atom was accumulated to be 2.687 Å, in Figure 1b, despite a steric repulsion by the alkyl chains, as well as the furan rings.As a result of the hydrogen bonding, the distance between the O atom and the N atom were much shorter than the corresponding sum of the van der Waals radius.Moreover, the interatomic distance between the imino-N atom and the O atom for the furan ring, is almost the same as that before the recognition of the BSA.As a result, the non-bonding interaction between the imino-N atom and the O atom may be weak, which is expected, as the RPNL 1 can recognize an acid substance such as BSA.

Results and Discussion
For a molecular design of RPNL 1 to combine the inherently acid-recognizing property, it is indispensable that the two imino-N atoms are accessible from external stimuli, by other substituted moieties in RPNL 1, without any disturbance.Before synthesis of the RPNL 1, we performed DFT calculations to investigate the molecular structure of RPNL 1, by using the Spartan'18 software package (Wavefunction, Inc., Irvine, CA, USA).The optimized molecular geometry of the RPNL 1, calculated at the B3LYP/6-31G* level of theory, revealed that two long alkoxy chains did not interact with the imino-N atoms as acid-recognizing moieties in Figure 1a.Additionally, the interatomic distances between the O atoms on the furan rings and the imino-N atoms were 2.696 and 2.697 Å, which is shorter than the sum of van der Waals radius (O-N 3.07 Å), respectively [29].This result suggests that interatomic interactions occur between the O atoms on the furan rings and the imino-N atoms pyrazine framework, as a result, the acid-recognizing capability of the imino-N atoms might decrease.Therefore, we also investigated the acid-recognizing capability of RPNL 1 to benzene sulfonic acid (BSA), with lower pKa.DFT calculation of RPNL 1 with BSA revealed that the BSA molecule was capable of approaching one imino-N atom of RPNL 1, because the interatomic distance between the O atom of the BSA and the imino-N atom was accumulated to be 2.687 Å, in Figure 1b, despite a steric repulsion by the alkyl chains, as well as the furan rings.As a result of the hydrogen bonding, the distance between the O atom and the N atom were much shorter than the corresponding sum of the van der Waals radius.Moreover, the interatomic distance between the imino-N atom and the O atom for the furan ring, is almost the same as that before the recognition of the BSA.As a result, the non-bonding interaction between the imino-N atom and the O atom may be weak, which is expected, as the RPNL 1 can recognize an acid substance such as BSA.We prepared a novel RPNL 1, containing two furan rings, in Scheme 1. 4,5-Bis(3,7,11-trimethyl-1-dodecyloxy)-1,2-dinitrobenzene was reduced with hydrazine monohydrate, using by Pd/C in dry EtOH, to obtain a diaminobenezene derivative unstable to air, then followed by thermal condensation with 1,2-di(2-furyl)ethane-1,2-dione, in dry EtOH, to afford RPNL 1.To study a fluidic liquid property of RPNL 1, polarized optical microscopic (POM) observation, differential scanning calorimetry (DSC), and X-ray diffraction (XRD) measurements were operated, as shown in Figures 2 and 3. POM observation of the RPNL 1 at 25 • C, clarified that no birefringence was observed in Figure 2. Additionally, the XRD pattern of RPNL 1 at 25 • C showed no peak, due to the self-assembled structures and a halo peak organized from the molten-branched alkoxy chains in Figure 3.Both results strongly indicate that RPNL 1 is a fluidic liquid at 25 • C. DSC trace of RPNL 1 showed that no phase transition peak appeared from an isotropic liquid to a crystal, on first cooling and then heating.Additionally, glass transition temperatures (T g ) were observed to be at −35.5 • C on first cooling, and at −32.8 • C on heating, indicating that RPNL 1 adopted a liquid state in a wide temperature range, including up to 25 • C.
We added BSA (equimolar to RPNL 1) into RPNL 1 to investigate the dissolving behavior of RPNL 1. Upon heating at 100 • C, the BSA was dissolved in RPNL 1, whereas the POM observation of the RPNL 1•BSA (in which RPNL 1 dissolves BSA), showed birefringence, under a cross-Nicols condition, at 25 • C, in Figure 3; of which observation, the result was clearly different from that for RPNL 1.This result implies that RPNL 1 can recognize BSA to form a complex, via hydrogen bonding [18].Moreover, an appearance of birefringence indicates that RPNL 1•BSA self-assemblies into the formation of an ordered structure.To confirm that a phase transition of RPNL 1 occurs by the addition of the BSA, we carried out the DSC measurements for the RPNL 1•BSA.
DSC measurements revealed that the T g at 7.8 • C for RPNL 1•BSA was much higher than that at −32.8 • C for RPNL 1, and the melting point (T m ) clearly appeared at 44.3 • C on heating, as shown in Figure 3b.In particular, the appearance of the T m strongly suggested that the phase transition of RPNL 1 would happen from a disordered liquid-to-liquid-crystalline (LC) state, by a complexation between RPNL 1 and BSA.The phase transition of RPNL 1 from a liquid state to an LC phase, by recognition of the BSA, was derived from the induction of polarity.Before recognition, RPNL 1 comprises aromatic moiety and aliphatic alkyl chains, which is a nonpolar molecule.On the other hand, the complexation of the RPNL 1 with the BSA generates a polarity because the RPNL 1•BSA becomes an amphiphilic molecule composed of ionic N-heteroacene framework, as well as hydrophobic alkyl chains.This dramatic change in a polarity of the aromatic moiety works as a driving force to self-assemble into an LC phase.
It is noted that the LC structure of the RPNL 1•BSA could be aligned by a mechanical shearing in the sandwiched glass substrates, in Figure 2, suggesting that the RPNL 1•BSA self-assembled to form the columnar LC phase.The direction of the long axis of the columns for the RPNL 1•BSA corresponds to the shearing direction, of which birefringence is alternately bright and dark, upon rotation by 45 • , under the cross-Nicols condition.It is known that these phenomena are consistent with the RPNL 1•BSA forming the columnar LC phases, reported by other groups [30].To investigate the self-assembled structure of the RPNL 1•BSA, the XRD pattern of the RPNL 1•BSA at 25 • C, was operated.The XRD pattern of the RPNL 1•BSA, showed three peaks at 42.1 (11), 38.4 (20), and 21.1 (31), in the small-angle region at 25 • C, as shown in Figure 3d, indicating that the RPNL 1•BSA self-assembled into the rectangular columnar (Col r ) LC structure, with the space group C2/m [31].We anticipated that each adjacent π-conjugated framework in the mixture of RPNL 1•BSA would adopt alternating inversion arrangements because of the acid recognition made by RPNL 1-an amphiphilic molecule-to induce the electrostatic interactions [18].Moreover, an amphiphilic property of the RPNL 1•BSA induced nanosegregation to stabilize the Col r LC phase.

Results and Discussion
For a molecular design of RPNL 1 to combine the inherently acid-recognizing property, it is indispensable that the two imino-N atoms are accessible from external stimuli, by other substituted moieties in RPNL 1, without any disturbance.Before synthesis of the RPNL 1, we performed DFT calculations to investigate the molecular structure of RPNL 1, by using the Spartan'18 software package (Wavefunction, Inc., Irvine, CA, USA).The optimized molecular geometry of the RPNL 1, calculated at the B3LYP/6-31G* level of theory, revealed that two long alkoxy chains did not interact with the imino-N atoms as acid-recognizing moieties in Figure 1a.Additionally, the interatomic distances between the O atoms on the furan rings and the imino-N atoms were 2.696 and 2.697 Å, which is shorter than the sum of van der Waals radius (O-N 3.07 Å), respectively [29].This result suggests that interatomic interactions occur between the O atoms on the furan rings and the imino-N atoms pyrazine framework, as a result, the acid-recognizing capability of the imino-N atoms might decrease.Therefore, we also investigated the acid-recognizing capability of RPNL 1 to benzene sulfonic acid (BSA), with lower pKa.DFT calculation of RPNL 1 with BSA revealed that the BSA molecule was capable of approaching one imino-N atom of RPNL 1, because the interatomic distance between the O atom of the BSA and the imino-N atom was accumulated to be 2.687 Å, in Figure 1b, despite a steric repulsion by the alkyl chains, as well as the furan rings.As a result of the hydrogen bonding, the distance between the O atom and the N atom were much shorter than the corresponding sum of the van der Waals radius.Moreover, the interatomic distance between the imino-N atom and the O atom for the furan ring, is almost the same as that before the recognition of the BSA.As a result, the non-bonding interaction between the imino-N atom and the O atom may be weak, which is expected, as the RPNL 1 can recognize an acid substance such as BSA.It is noted that the LC structure of the RPNL 1•BSA could be aligned by a mechanical shearing in the sandwiched glass substrates, in Figure 2, suggesting that the RPNL 1•BSA self-assembled to form the columnar LC phase.The direction of the long axis of the columns for the RPNL 1•BSA corresponds to the shearing direction, of which birefringence is alternately bright and dark, upon rotation by 45°, under the cross-Nicols condition.It is known that these phenomena are consistent with the RPNL 1•BSA forming the columnar LC phases, reported by other groups [30].To investigate the self-assembled structure of the RPNL 1•BSA, the XRD pattern of the RPNL 1•BSA at 25 °C, was operated.The XRD pattern of the RPNL 1•BSA, showed three peaks at 42.1 (11), 38.4 (20), and 21.1 (31), in the small-angle region at 25 °C, as shown in Figure 3d, indicating that the RPNL 1•BSA self-assembled into the rectangular columnar (Colr) LC structure, with the space group C2/m [31].We anticipated that each adjacent π-conjugated framework in the mixture of RPNL 1•BSA would adopt alternating inversion arrangements because of the acid recognition made by RPNL 1-an amphiphilic molecule-to induce the electrostatic interactions [18].Moreover, an amphiphilic property of the RPNL 1•BSA induced nanosegregation to stabilize the Colr LC phase.Then, to evaluate the electronic properties of the RPNL 1 and RPNL 1•BSA, we carried out UV-vis absorption and FL spectra measurements, due to the ground and excited states, respectively.In a liquid state, the UV-vis absorption spectrum of RPNL 1 showed two peaks at 277 and 392 nm, in Figure 4a, which was almost same as that of RPNL 1, in a CHCl3 solution in Figure 4c.The FL spectrum of RPNL 1 showed a broad peak at 456 nm in a liquid state, in which the photoluminescent color was blue, Figure 4b.Compared to the peak, around at 450 nm, in the FL spectrum of RPNL 1 in a CHCl3 solution (10 −3 -10 −6 M), that of RPNL 1 in a liquid state was bathochromically shifted.These Then, to evaluate the electronic properties of the RPNL 1 and RPNL 1•BSA, we carried out UV-vis absorption and FL spectra measurements, due to the ground and excited states, respectively.In a liquid state, the UV-vis absorption spectrum of RPNL 1 showed two peaks at 277 and 392 nm, in Figure 4a, which was almost same as that of RPNL 1, in a CHCl 3 solution in Figure 4c.The FL spectrum of RPNL 1 showed a broad peak at 456 nm in a liquid state, in which the photoluminescent color was blue, Figure 4b.Compared to the peak, around at 450 nm, in the FL spectrum of RPNL 1 in a CHCl 3 solution (10 −3 -10 −6 M), that of RPNL 1 in a liquid state was bathochromically shifted.These results suggest that molecules at an excited state in a condensed liquid should bring about a weak electronic coupling, via an intermolecular interaction with each other.Additionally, an electronic coupling between the molecules at ground state, was weak.
Dissolving BSA in RPNL 1 gave rise to remarkable changes in the UV-vis absorption and the FL spectra.In a condensed state, the UV-vis absorption spectrum of the RPNL 1•BSA showed two peaks at 297 and 465 nm.In particular, the peak of longer wavelength for RPNL 1•BSA was red-shifted by protonation, compared to that of RPNL 1.Additionally, the color of RPNL 1 clearly changed from light yellow to orange.The UV-vis absorption spectrum of RPNL 1 with HCl in the CHCl 3 solution (10 −3 -10 −6 M) was 468 nm, which was almost the as same that of RPNL 1•BSA in a condensed state, in which the behavior was consistent with that before complexation between RPNL 1 and BSA.It should be noted that the peak FL spectrum for RPNL 1•BSA was much red-shifted by ca.140 nm.The blue emission for RPNL 1 clearly turned into an orange emission by protonation.On the other hand, the FL spectrum of RPNL 1 with HCl in the CHCl3 solution (10 −3 -10 −6 M) showed a peak at around 530 nm, with a light green emission, which clearly differed from that of RPNL 1•BSA in the condensed state.The remarkable red-shift for the RPNL 1•BSA, in a condensed state, contributed to the strong electronic coupling between molecules, because the induction of polarity should enhance the intermolecular interaction, leading to a tightly molecular packing for RPNL 1•BSA, in the condensed Colr LC state.These pronounced changes by acid recognition were derived from RPNL 1, as a liquid material, in a condensed state, and not observed as a solution state.The dramatic changes in the fluorescent colors were advantages characteristic of a condensed liquid material.It should be noted that the peak FL spectrum for RPNL 1•BSA was much red-shifted by ca.140 nm.The blue emission for RPNL 1 clearly turned into an orange emission by protonation.On the other hand, the FL spectrum of RPNL 1 with HCl in the CHCl 3 solution (10 −3 -10 −6 M) showed a peak at around 530 nm, with a light green emission, which clearly differed from that of RPNL 1•BSA in the condensed state.The remarkable red-shift for the RPNL 1•BSA, in a condensed state, contributed to the strong electronic coupling between molecules, because the induction of polarity should enhance the intermolecular interaction, leading to a tightly molecular packing for RPNL 1•BSA, in the condensed Col r LC state.These pronounced changes by acid recognition were derived from RPNL 1, as a liquid material, in a condensed state, and not observed as a solution state.The dramatic changes in the fluorescent colors were advantages characteristic of a condensed liquid material.

Conclusions
We prepared novel RPNL 1 comprising of two furan rings.Dissolving BSA in RPNL 1 gave rise to the remarkable change in physical properties of RPNL 1, such as electronic properties, as well as sample condition.Upon complexation with BSA, the photoluminescent color of blue emission for RPNL 1 turned into an orange emission.Additionally, the phase transition happened from a disordered liquid state to a self-organizing columnar LC phase.As a result, RPNL 1 can function as an acid-responsive material.Further investigations of the various PRNLs showing functionality and stimuli-responsive capability are in progress, in our laboratory.
Author Contributions: A.I. synthesized RPNL 1 and carried out all experiments.K.I. designed and supervised this project, and wrote the manuscript.

Figure 2 .
Figure 2. Photomicrographs of the optical microscopic (OM) and the polarized optical microscopic (POM) observations, the RPNL 1, and the RPNL 1•BSA.A, P, and S indicate the analyzer, polarizer, and shearing direction, respectively.

Figure 3 .
Figure 3. Differential Scanning Calorimetry (DSC) traces of the (a) RPNL 1 and (b) RPNL 1•BSA, on a scanning rate of 10 K/min; (c) XRD patterns of RPNL 1 and RPNL 1•BSA at 25 • C; (d) XRD pattern of the magnified view for RPNL 1•BSA; and (e) Predicted schematic illustration of the molecular packing for RPNL 1•BSA in a disc-like stratum of a column.

Figure 4 .
Figure 4. (a) UV-vis absorption spectra and (b) fluorescent spectra of RPNL 1 and RPNL 1•BSA.(c) UV-vis absorption spectra and (d) fluorescent spectra of RPNL 1 and RPNL 1 with HCl in CHCl3 solutions.Photographs of (a) and (c) under room light and (b) and (d) under black light (365 nm).

Figure 4 .
Figure 4. (a) UV-vis absorption spectra and (b) fluorescent spectra of RPNL 1 and RPNL 1•BSA.(c) UV-vis absorption spectra and (d) fluorescent spectra of RPNL 1 and RPNL 1 with HCl in CHCl 3 solutions.Photographs of (a) and (c) under room light and (b) and (d) under black light (365 nm).