Continuously tunable emission color based on the molecular aggregation of (2Z,2′Z)-2,2′-(1,4-phenylenae)bis(3-(4-(dodecyloxy)phenyl)acrylonitrile)

Abstract

The development of organic solid-state luminescent materials with continuously tunable emission is a topic of interest for both fundamental research and practical applications. A luminescent liquid crystal, (2Z,2′Z)-2,2′-(1,4-phenylenae)bis(3-(4-(dodecyloxy)phenyl)acrylonitrile) (PDPA) is reported, which exhibits a continuously tunable emission color from green to orange at room temperature when cooled from an isotropic liquid at different cooling rates. PDPA self-assembles to a layer structure and/or a helical structure with different pitch length depending on the stacking rate, which influences the conformation coplanarity. The conjugation lengths shorten and the π–π interactions weaken, inducing a photoluminescence spectrum blue-shift from 600 nm to 506 nm; a considerable luminescent shift. Therefore, this material has potential uses in novel optical devices, as its emission profile can be tuned by controlling the molecular aggregation.

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1. Introduction

Organic solid-state luminescent materials are of great interest for optoelectronic devices because of their superior thermal stability, structural diversity, and unique photophysical properties.¹ They have been widely used in organic light emitting diodes (OLED),² lasers,³ sensors⁴ and hybrid materials for biological imaging.⁵ They are also low cost, easily processed, and flexible, which has led to potential uses in soft optical devices and large-scale flat panel displays.⁶ The ability to tune the luminescence properties of solid-state materials is highly desired in the optical materials community.⁷ For various applications—such as displays and dye lasers—precise control of the luminescence properties are essential to optimizing optical device performance.⁸ For low molecular weight organic solid-state materials, it has been recognized that the bulk luminescence properties differ from that of dilute solutions; emission in the solid state is dictated by the ensemble rather than individual molecules. During the last decade, a number of pure organic solid-state luminescent materials with controllable emission properties have been developed. The luminescence properties of these materials are generally tailored by altering the molecular packing mode within the crystal structure. For example, Liu et al. reported that two trifluoromethyl (CF₃)-substituted aromatic–amine compounds exhibit a photoluminescent color switch—from yellow to red and from green to yellow—by adjusting the π–π interactions in the two crystals.⁹ Tian et al. reported that a biindenyl derivative with two bodipy groups emits distinct orange and red light in response to torsion angles tuning between the indene moieties and the bodipy units, which influenced the coplanarity of the conformation.¹⁰ Such tunable polymorph-based emissive behavior not only reveals an effective way to tailor molecular material luminescence, but also provides an insight into the relationship between the molecular arrangement/packing and the bulk optical properties. However, only a few examples of luminescent molecules with continuously-tunable fluorescent emission in solid-state have been reported. One notable example is the anthracene derivative (9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene), which continuously changes photoluminescence color from green to red in response to external pressure ranging from 0 GPa to 8 GPa; this pressure leads to an enhanced intermolecular π–π interaction between adjacent anthracene planes in the crystal.¹¹ While this continuously tunable luminescent state is not stable, its photoluminescence color returns to its initial state upon pressure removal. The development of a continuously tunable emission system is still challenging.

The cyanostilbene unite is a π-conjugated platform with interesting optical properties including aggregation induced emission enhancement (AIEE) properties, multi-stimuli luminescence switching, two-photon absorption, lasing, and liquid crystal behavior.¹² π-conjugated cyanostilbene derivatives exhibit a unique “elastic twist”; large torsional conformational changes readily occur in response to intermolecular interactions.¹³ Previous studies have proposed that the molecular aggregation modes may bring about the adjustment of non-covalent intra- or intermolecular interactions such as π–π stacking, hydrogen bonding, or hydrophobic interactions, possibly resulting in the modulation of the luminescence properties.¹⁴ It should be noted that the cyano moiety has the ability to interact with both cyano groups or hydrogen atoms in aromatic rings.^15,16 Cyano interactions could cooperate with other intermolecular interactions to create supramolecular architectures. Thus, the photophysical properties of cyanostilbene-based luminophores can vary greatly.^10–12

In our previous studies, we prepared a cyanodistyrylbenzene-based luminescent liquid crystal ((2Z,2′Z)-2,2′-(1,4-phenylenae)bis(3-(4-(dodecyloxy)phenyl)acrylonitrile), PDPA), which exhibited different luminescent colors (green, yellow, and orange) dependent on the self-assembled structure.¹⁷ The PDPA luminescence can vary between those three different colors in response to mechanical shearing or thermal annealing. In this work, PDPA emission color is tuned continuously by controlling the self-assembled structure. To gain an insight into this phenomenon, we have comprehensively explored the thermodynamics, optical, and photophysical properties of this highly fluorescent solid-state material.

2. Experiment

All chemicals were purchased from Sigma Aldrich and used without further purification. ¹H NMR spectra and ¹³C NMR were measured using an Agilent VNMRS600 (600 MHz) spectrometer. UV-vis absorption spectra were measured from solid state samples using an ultraviolet spectrometer (Shimadzu UV2550). Photoluminescence spectra were measured with a HORIBA FluoroMax-4 spectrofluorometer using solid state samples. The absolute photoluminescence quantum yields of solid samples were measured using a HORIBA FluoroMax-4 spectrofluorometer with an integrating sphere. Differential scanning calorimetry (DSC) was accomplished with a METTLER 82le/400 instrument, and a high-precision thermal controller (INSTEC mK 1000) was used to control the heating/cooling rate. Polarizing optical microscopy (POM) was carried out with a DM2500 M polarizing optical microscope (Leica, Germany). XRD measurements were made using a thin film X-ray diffractometer (X'Pert PRO MPD, Netherlands). Infrared spectroscopy was carried out with a Nicolet 67 Fourier transform infrared spectrometer (Thermo Nicolet, America). Mass spectra were measured using an ACQUITY UPLC LCT Premier XE mass spectrometer (Waters, America).

(2Z,2′Z)-2,2′-(1,4-Phenylenae)bis(3-(4-(dodecyloxy)phenyl)acrylonitrile), PDPA was synthesized via Knoevenagel reaction of 4-(dodecyloxy)benzaldehyde and (4-cyanomethyl-phenyl)acetonitrile. The specific synthesis steps are described in the literature.¹⁷ The purified material was characterized by ¹H NMR spectra, ¹³C NMR spectra, and mass spectroscopy.

¹H NMR (CDCl₃, 600 MHz, ppm): δ = 7.89 (d, 2H, ArH, J = 8.4 Hz), 7.70 (s, 2H, CH₂), 7.64 (d, 1H, ArH, J = 8.4 Hz), 7.50 (s, 1H, ArH), 7.49 (d, 2H, ArH, J = 8.8 Hz), 7.48 (d, 2H, ArH, J = 8.0 Hz), 7.13 (d, 1H, ArH, J = 8.4 Hz), 6.97 (d, 2H, ArH, J = 8.4 Hz), 6.75 (d, 1H, ArH, J = 8.4 Hz), 4.02 (m, 4H, CH₂), 1.77 (m, 4H, 2 × CH₂), 1.27 (m, 36H, 18 × CH₂), 0.88 (t, 6H, CH₂, J = 6.0 Hz).

¹³C NMR (CDCl₃, 150 MHz, ppm): δ = 161.3, 142.2, 135.1, 131.4, 126.2, 126.1, 118.3, 114.9, 68.3, 31.9, 29.7, 29.3, 26.0, 22.7, 14.1.

m/z (EIMS) calculated for C₄₈H₆₄N₂O₂, 701.0; found, 701.1.

3. Results and discussion

Motivated by the extraordinary AIEE effect, a variety of cyanostilbene derivatives have been developed. We have designed and synthesized a new cyanostilbene-based luminescent liquid crystal (PDPA, Fig. 1A) by incorporating two terminal dodecyloxy fragments into the dicyanodistyrylbenzene core. This structure shows thermochromic and piezochromic luminescence switching due to the changes in the molecular packing structure.¹⁷ The PDPA mesomorphic properties were investigated using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The phase-transition temperatures and related enthalpy values are depicted in the DSC traces, as shown in Fig. 1C. Clearly, the highest transition temperature during the cooling and the heating processes is rather close, which is usually associated with a liquid crystal transition. The transition at the lowest temperature during cooling is 10 °C lower than that observed during heating, which means that the transition is related to the crystal structure formation. PDPA transitions to the SmC* phase at about 66 °C, to the smectic C phase at about 118 °C, and to the isotropic state at about 198 °C during the heating process. Note that the SmC* phase emits green fluorescence and the SmC phase emits yellow light, as shown in Fig. 1B; this indicates that PDPA is a luminescent liquid crystal. During the cooling process, a new unidentified liquid crystal texture is observed between 108 °C and 93 °C. This unidentified phase is unstable and transforms to SmC* after annealing at 105 °C for 20 minutes, as shown in Fig. 2. This temporal mesomorphic property is unique and offers an alternative method of controlling PDPA luminescent properties.

Fig. 1 (A) PDPA chemical structure; (B) PDPA POM images taken at 150 °C (left) and 70 °C (right) during the cooling process. Insets are images of the PDPA with yellow fluorescence at 150 °C (left) and with green fluorescence at 70 °C (right); (C) PDPA DSC curves at a heating/cooling rate of 5 °C per minute.

Fig. 2 PDPA POM images at 105 °C after cooling from the isotropic phase with different annealing times: (A) 10 seconds (SmC); (B) 1 minute; (C) 2 minutes (unidentified phase); (D) 5 minutes; (E) 8 minutes; and (F) 20 minutes (SmC*).

To gain insights into the transitions and clarify the nature of the temporal mesophase, we obtained DSC thermograms at various cooling rate, as shown in Fig. 3. The two transition temperatures (isotropic-SmC and SmC*-crystal) are independent of the cooling rate, which indicates such transitions approach to thermodynamic equilibrium.¹⁸ With an increased cooling rate, the second- and third-order exothermic transition peaks (SmC-unidentified and unidentified-SmC*) become slightly broader and shift to lower temperatures; particularly for the unidentified-SmC* transition. These transition temperatures depend on the cooling rate, meaning that these transitions are kinetic equilibrium processes.¹⁹ Apparently, the PDPA structure self-assembly can be controlled by varying the cooling rate.

Fig. 3 PDPA DSC thermograms during cooling process at different scan rates.

As a result, the PDPA emission color can be continuously tuned by varying the cooling rate. As the cooling rate increases, the PDPA film fluorescence emission in the quartz cell exhibits a gradual red shift (Fig. 4A). By varying the cooling rate from 0.5 °C min⁻¹ to 50 °C min⁻¹, a more noticeable luminescence color change is observed (Fig. 4B)—which is larger than that obtained through grinding.¹⁷ The emission changes from green (506 nm) to orange (600 nm), which is a considerably large luminescent shift. The photoluminescence (PL) spectrum produced with an applied cooling rate of 0.5 °C min⁻¹ is quite similar to the frozen film from the SmC*, with λ_max = 506 nm.¹⁷ As the cooling rate increases to 50 °C min⁻¹, the luminescence spectra shifts to red, with λ_max = 600 nm; this is redder emission than that obtained from a film ground with a pestle. Apparently, the grinding method is not powerful enough to cause a strong luminescence shift. We believed that the dramatic change in the luminescence color is the result of changes to the molecular aggregation state.

Fig. 4 (A) PDPA film normalized photoluminescence spectra produced at the following cooling rates: 0.5 °C min⁻¹ (a); 5 °C min⁻¹ (b); 10 °C min⁻¹ (c); 20 °C min⁻¹ (d); 30 °C min⁻¹ (e); 40 °C min⁻¹ (f); and 50 °C min⁻¹ (g). (B) PDPA film images in a quartz cell under UV irradiation (365 nm).

To gain insight into the specific PDPA self-assembly structures that influence luminescence change, we characterized the films with X-ray diffraction spectroscopy and POM. Fig. 5A shows the XRD patterns for PDPA films in different states. Three sharp peaks in the small-angle region and crowded peaks in the wide-angle region are observed for the PDPA film with the lowest cooling rate (0.5 °C min⁻¹, Fig. 5A). The X-ray diffraction shows peaks at 14.6 Å, 11.0 Å, and 8.8 Å with a reciprocal d-spacing ratio of 3 : 4 : 5 corresponding to (300), (400), and (500) reflections, respectively. According to the sharp peaks in the small-angle region, the layer spacing, d, is about 44.0 Å. This is shorter than the estimated molecular length (48.0 Å, Fig. S1†), suggesting that PDPA molecules form a tilting structure (phase I). Meanwhile, striations are observed from the POM images. The POM and XRD results imply that PDPA molecules self-assemble to form an ordered macroscopic helical structure—like the ferroelectric phase with the molecules titled at angle to the normal (23.5°, Fig. S2†)—when cooling from isotropic state with a lower cooling rate (0.5 °C min⁻¹). The pitch length—corresponding to the distance the director rotates through an angle of 2π—is about 5.0 μm. Furthermore, sharp peaks are observed in the wide angle region, with an obvious diffraction peak with a d-spacing of 3.6 Å. This is characteristic of an effective π–π stacking distance and consistent with the distance between adjacent molecules, indicating strong intermolecular interactions (include π–π intermolecular effect) between PDPA molecules in phase I.²⁰ When the cooling rate is lower than 20 °C min⁻¹, a similar helical structure with a different pitch length is observed with X-ray diffraction spectroscopy and POM, as shown in Fig. 5B. As the cooling rate increases, the pitch length decreases and the intermolecular interactions are enhanced. For cooling rates above 30 °C min⁻¹, X-ray diffraction spectroscopy reveals that PDPA forms a new self-assembly layer structure (phase II) with a longer d-spacing (47.2 Å). Meanwhile, phase I is still present and the pitch length is so short that it cannot be observed clearly using POM (Fig. S3†). Simultaneously, the phase I content gradually decreased as the stacking speed increases.

Fig. 5 (A) PDPA X-ray diffraction patterns at room temperature with different cooling rates from the isotropic state: 0.5 °C min⁻¹ (a); 5 °C min⁻¹ (b); 10 °C min⁻¹ (c); 20 °C min⁻¹ (d); 30 °C min⁻¹ (e); 40 °C min⁻¹ (f); and 50 °C min⁻¹ (g). (B) PDPA POM images for cooling at 0.5 °C min⁻¹ (a); at 5 °C min⁻¹ (b); at 10 °C min⁻¹ (c); and at 20 °C min⁻¹ (d).

The PDPA film self-assembled structures were also studied using IR spectroscopy, as shown in Fig. 6. The C≡N stretching band observed in phase I shows that the full width at half maximum (FWHM) of the peak gradually broadens as the cooling rate increases. It can be concluded that the self-assembled structures have become disordered. Meanwhile, the phenyl ring stretching band at 3032 cm⁻¹ is weaker, suggesting that the PDPA molecular motion has weakened. Thus, the fluorescence quantum yield increases from 26.37% to 55.95% (see Table S1 in ESI†). As the cooling rate increases above 30 °C min⁻¹, phase II and phase I are both present. The peak corresponding to the C≡N stretching band shifts from 2220 cm⁻¹ to 2215 cm⁻¹ (Fig. 7), and a new peak is observed at 3342 cm⁻¹ (Fig. S4†). These peaks indicate that there are strong H-bond intermolecular forces in phase II. These PDPA film UV absorption spectral bands (Fig. S5†) in phase I exhibit an apparent redshift due to an increase to the conjugation length as the cooling rate increased. This result corresponds to a fluorescence emission wavelength change from 506 nm to 540 nm (Table S1†). When forming phase II, the observed absorption spectra is broader and exhibits more redshift than phase I, which is attributed to excimer formation of the cyanostilbene moieties in phase II.^20,21 The result accords with an orange emission color and a weakening of the fluorescence quantum yield (Table S1†). On the other hand, the emission lifetimes of PDPA film as the cooling rate 50 °C min⁻¹ (0.15 ns) are almost 2.5 times longer than that at 0.5 °C min⁻¹ (0.06 ns) (Fig. S6†).

Fig. 6 PDPA film IR spectra produced with different cooling rates: 0.5 °C min⁻¹ (a); 5 °C min⁻¹ (b); 10 °C min⁻¹ (c); 20 °C min⁻¹ (d); 30 °C min⁻¹ (e); 40 °C min⁻¹ (f); and 50 °C min⁻¹ (g).

Fig. 7 Relationship between emission wavelength and pitch length.

The PDPA self-assembled structure in phase I is a special helical structure. The helical structure pitch length can be estimated by directly measuring the distances between striations visible in the POM images, as shown in Fig. S7.† The pitch length gradually decreases as the cooling rate increases (Fig. 5B). Fig. 7 shows the emission wavelength as a function of pitch length. As shown in Fig. 6, the pitch length declines from 5.0 μm to 1.4 μm, thus the emission wavelength redshifts from 506 nm to 540 nm. In order to achieve continuously tunable emission color, the helical structure pitch length should be proactively controlled in phase I. This can be accomplished by controlling the cooling rate.

PDPA molecules exhibit a unique “elastic twist” feature, which plays an important role in self-assembled structures.¹³ In an isotropic liquid, the molecules adopt a twisting conformation and are virtually non-emissive because of intramolecular rotation. As the isotropic liquid is gradually cooled, the molecular conformations first start an intramolecular planarization process. This process is induced by the intramolecular torsional motion deactivation caused by decreasing thermal energy and increasing medium viscosity. Two highly planarized molecules easily associate with each other and form an excimer with a long conjugation length and strong π–π interactions; the layer spacing is 47.2 Å. Proposed structures for the self-assembled PDPA are shown in Fig. 8. The excimer is responsible for the significant red-shift, but also the weakened and unstructured emission.^13,22 The terminal flexible alkyl chains do not favor alignment due to both the steric effect and the incompatibility with the aromatic cores. As a result, stacking modes transform to helical structures in the next self-assembled stage. If the stacking speed is slow enough, the molecular aggregation state has enough time to form a long pitch helical structure. When the system comes to room temperature, the whole stacking process is terminated. Meanwhile, the smaller degrees of co-planar conformation lead to a shortening of the conjugation length and the π–π interactions weaken as the pitch is increased. Therefore, the film PL spectrum changes from a yellow emission to a green emission. PDPA molecular self-assembly is a kinetic process, so the final molecular aggregation state is dependent on the current stacking stage—which is suspended at room temperature. In this self-assembly process, the PDPA molecular aggregation state experiences a layer structure and a helical structure with pitch that varies from short to long. Therefore, the PDPA PL emission in solid state can be changed by controlling its molecular stacking stage.

Fig. 8 Stacking modes and corresponding emission colors for various molecular aggregation structures.

4. Conclusion

In summary, we have produced a luminescent solid-state material (PDPA) with continuously tunable fluorescence color based on the molecular aggregation state. PDPA can form two self-assembled structures with different conjugation lengths dependent on the cooling rate. On the basis of structural, optical, photo-physical, and computational studies, we identified the two packing structures: a helical structure with different pitch length (phase I) and a layered structure (phase II). In the self-assembly process, the PDPA molecular aggregation first experiences a layer structure with a long conjugation length due to efficient excimer formation. Afterwards, the motion of flexible alkyl chains induces the formation of a helical structure with a tuned pitch length. Here the excimer is diminished, while the helical structure luminescent color can shift from green to yellow by proactively controlling the pitch length—as a result of conjugation length shortening—and weakening π–π interactions. Therefore, by controlling the self-assembled evolution, the PDPA PL emission color in solid state can be continuously changed. Continuous tuned emission color leads to new applications of solid luminescent materials that make optoelectronic devices more flexible, sophisticated, and functional.

Acknowledgements

This work was mainly supported by the National Natural Science Foundation of China (grant No. 61107014, 51573036), and Program for New Century Excellent Talents in University (grant No. NCET-12-0839) for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22024j

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