Significant effect of alkyl chain length on fluorescent thermochromism of 9,10-bis(p-alkoxystyryl)anthracenes

Abstract

9,10-Bis(p-alkoxystyryl)anthracenes (DSA-pn_n) with different length of alkyl chains (n = 4, 8, 16) were synthesized. It was found that they exhibit aggregation-induced emission (AIE) properties and significant fluorescent thermochromism. Besides, the thermochromic behaviours of DSA-pn_n show great dependence on the length of alkyl chains.

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The luminescent materials showing fluorescent colour, intensity or lifetime changes at different temperatures are classified as thermochromic fluorescent (TCF) materials.¹ Recently, this class of “smart” materials attracts increasing interest due to their promising applications as rewritable paper,² memories,³ luminescent temperature sensors⁴ etc. Generally, they are ideally utilized as solid films, however, tuning fluorescent properties or colours by chemical reactions in solid state has often suffered insufficient conversions or loss of fluorescence properties. Alternatively, altering solid-state molecular packing mode was found to be a promising way to dynamically regulate fluorescence properties.⁵ Since Tang et al. reported the aggregation-induced emission (AIE) phenomena in 2001,⁶ rapid progress has been achieved in the solid-state TCF field.

Continuously, a series of metal-doped complexes,^4,7 phasmidic molecules,⁸ derivatives of diphenyldibenzofulvene,⁹ cyanostilbene¹⁰ etc. were reported to show thermochromic aggregation-induced emission (TAIE) properties. Up to date, the TAIE compounds with controllable molecular packing modes were still limited. Recent years, piezofluorochromism of 9,10-bis(arylvinyl)anthracene derivatives has been intensively investigated.^11–14 It was found that not only the structures of peripheral aryl units connected to 9,10-anthylene core but also the alkyl chains at aryl units lay a significant impact on solid-state fluorescence and piezofluorochromic property.^15–19 Recently, Chi and Yang et al. reported that 9,10-bis(alkoxystyryl)anthracenes present tuneable fluorescent response upon external stimuli, including force, solvent vapour or heat.^20,21 However, their thermochromism was rarely touched.

Herein, a series of 9,10-bis(p-alkoxystyryl)anthracenes (DSA-pn_n) with different length of alkyl chains (n = 4, 8, 16, Fig. 1) were synthesized and their fluorescence properties were fully investigated. Along with verified AIE characteristics, dramatic fluorescent thermochromism were observed for all of them upon a thermal process with quantitatively different thermo-induced spectral shifts (Δλ_TCF = 47–72 nm). Moreover, the measured colour transition temperatures of the three DSA-pn_n are different and show dependence on the chain length. These findings revealing the structure–property relationship of TAIE compounds would benefit the development of luminescent thermochromic solids.

Fig. 1 Chemical structures of DSA-pn_n.

DSA-pn_n were facilely synthesized by Wittig–Horner reactions of alkyoxy-substituted (n-butyl, n-octyl and n-hexadecyl) benzaldehydes and 9,10-bis(diethoxyphosphorylmethyl)anthracene. The target compounds were characterized by ¹H NMR, ¹³C NMR and HRMS.

Firstly, their photoluminescence (PL) emission spectra in different concentrations of THF aqueous solutions were recorded. As shown in Fig. 2, they all exhibit faint fluorescence in pure THF but strong emissions in dilute THF aqueous solutions. Since DSA-pn_n are soluble in THF but insoluble in water, so the molecules of DSA-pn_n would aggregate when their THF solutions were diluted with water. The fluorescence triggered by the formation of aggregates in dilute THF aqueous solution is the typical AIE phenomenon. Thus, DSA-pn4, DSA-pn8 and DSA-pn16 were demonstrated to be AIE-active. Moreover, enhanced fluorescence isn't observed until the THF concentration is less than 40% for DSA-pn4, 40% for DSA-pn8 and only 70% for DSA-pn16 correspondingly. Therefore, their AIE properties show somewhat chain length-dependent. This phenomenon can be ascribed to the poorer solubility of DSA-pn_n in water with the longer chain length,²² which makes the longer-chained DSA-pn_n more easily aggregate in THF aqueous solutions.

Fig. 2 PL spectra of DSA-pn_n in different concentrations of THF aqueous solutions.

Their solid-state photoluminescence spectra upon heating at every 10 K from 298 K to 498 K were described in Fig. 3 and Table 1 (detailed data in ESI Table S1†). As the pristine compounds at room temperature (r.t.), DSA-pn4 and DSA-pn8 emit similar emissions at 520 nm and 521 nm respectively, while the emission of DSA-pn16 is more blue-shifted at 505 nm. With the increase of temperature, their PL spectra curves display slight changes until large spectral wavelength shifts are witnessed at certain temperatures (colour-transition temperature, T_c-t). The distinguishable peaks occur at 567 nm (DSA-pn4), 569 nm (DSA-pn8) and 577 nm (DSA-pn16). This phenomenon exhibiting significant wavelength changes (Δλ_TCF) upon heating clearly demonstrate that all three DSA-pn_n are sensitive to thermal stimuli, namely they are excellent TAIE materials. Furthermore, their thermochromism shows great dependence on alkyl chain length as the longest-chained DSA-pn16 shows the largest Δλ_TCF of 72 nm, compared to 47 nm and 48 nm for DSA-pn4 and DSA-pn8, respectively. Also, their colour-transition temperature (T_c-t) differs greatly from each other as the order of DSA-pn4 (488 K) > DSA-pn8 (438 K) > DSA-pn16 (408 K). These results together demonstrate the significant effect of alkyl chain length on fluorescent thermochromism of DSA-pn_n.

Fig. 3 Peak emission wavelengths of DSA-pn_n at different temperatures.

Table 1 Peak emission wavelengths (λ/nm) of DSA-pn_n at room temperature and colour-transition temperature (T_c-t)^a

Samples	λr.t./nm	λmelt/nm	ΔλTCF/nm	λc-t/K
a Thermo-induced spectral shift, ΔλTCF = λmelt − λr.t.
DSA-pn4	520	567	47	488
DSA-pn8	521	569	48	438
DSA-pn16	505	577	72	408

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Fig. 4 presents the maximum fluorescence intensity of DSA-pn16 at different temperatures upon heating. Normally, fluorescence intensity will reduce at high temperatures as a consequence of thermal decay pathway.¹ Although DSA-pn16 displayed a tendency of intensity decrease upon heating, a significant decline wasn't happened until 398 K, at this temperature it exhibits 87.5 (a.u.) of fluorescence intensity compared to 281 (a.u.) at room temperature. The sharp intensity drop point approximately matches the colour-transition temperature of DSA-pn16, which indicated this phenomenon shouldn't be simply attributed to the thermal decay pathway. As larger π–π overlapping in molecular packing would induce a bathochromic shift, accompanying with the reduction of fluorescent intensity, so variations of packing mode were expected during the thermal process. Similar fluorescence intensity curves were also observed for DSA-pn4 and DSA-pn8. (ESI Fig. S1†).

Fig. 4 The maximum fluorescence intensity curve of DSA-pn16 at different temperatures upon heating.

Correspondingly, fluorescence images of three DSA-pn_n samples during heating and cooling processes (from r.t. to T_c-t and reverse) were recorded under a 365 nm UV lamp to observe the fluorescent colour changes directly (Fig. 5). Well consistent with the above results, DSA-pn4 and DSA-pn8 show similar green-yellow fluorescence while DSA-pn16 presents much greener emission at room temperature. After heating to T_c-t, rapid colour changes from the original colours to orange were witnessed for DSA-pn4, DSA-pn8 and DSA-pn16. When repeating the heating and cooling process for several times, their fluorescent colours undergo a circle of yellow/green → orange → yellow/green → orange, which demonstrates that their thermochromic processes are reversible and repeatable.

Fig. 5 Fluorescence images of DSA-pn_n upon heating and cooling under a 365 nm UV lamp.

To understand the thermochromic fluorescent behaviours of DSA-pn_n, they were characterized by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). As shown in DSC diagrams (ESI Fig. S2†), the high-temperature endothermic peaks, recognized as the isotropic melt transitions (T_H), are very close to their colour-transition temperatures (492 K for DSA-pn4, 420 K for DSA-pn8 and 397 K for DSA-pn16), denoting phase-transitions induced thermochromic mechanism of DSA-pn_n. Then, their WAXD patterns at room and T_c-t temperatures were recorded respectively (Fig. S3†). The sharp and intense peaks observed at room temperature indicated an ordered crystalline structure of the pristine samples. On contrast, lower-intensity reflections along with some peaks missing in WAXD curves of DSA-pn4 and DSA-pn8 at T_c-t indicated the changes from crystal to somewhat amorphous state. Meanwhile, complete transition to amorphous phase occurred with the disappearance of all diffraction peaks for DSA-pn16. This result clearly indicated their phase-transition really happened during thermochromic process.

Further investigations on thermochromic mechanism of DSA-pn_n have been carried out by examining single crystal diffraction. Single crystals with highly ordered molecular packing structures of DSA-pn4, DSA-pn8 and DSA-pn16 were obtained by slowly vaporizing petroleum ether into their saturated THF solution at room temperature. The resolved structures are shown in Fig. 6.

Fig. 6 Molecular stacking modes of DSA-pn4, DSA-pn8 and DSA-pn16 in crystals.

Twisted molecular patterns between anthracene nucleus and peripheral aryl rings were observed for three crystals, which embodied with different torsion angles as 68.25° for DSA-pn4, 70.29° for DSA-pn8 and 72.81° for DSA-pn16. The decrease of molecular conjugation degree by twisted backbones consequently led the most blue-shifted emission of DSA-pn16 at room temperature. On the other hand, all of them present different packing structures as the vertical distances between two neighbouring anthracene planes are 3.476 Å, 3.606 Å, and 3.481 Å for DSA-pn4, DSA-pn8 and DSA-pn16, respectively. And the inclination angels along the long axis of two neighbouring molecules are in the order of 39.54° (DSA-pn4) < 45.86° (DSA-pn8) < 87.37° (DSA-pn16), indicating a succession of weaker intermolecular interactions and looser stacking modes with the extension of alkyl length. Thus, easily-destructible structures and great tendency to form a more planar conformation can be expected for long-chained DSA-pn_n, which agree well with their TCF behaviours, increase in spectral shifts and decrease in colour transition temperatures. Overall, alkyl chain length can significantly affect molecular backbone conformations and solid-state packing structure, so further influence the fluorescent thermochromism of 9,10-bis(p-alkoxystyryl)anthracenes.

In summary, a series of 9,10-bis(p-alkoxystyryl)anthracenes with different length of alkyl chains (DSA-pn4, DSA-pn8 and DSA-pn16) were synthesized and their TCF behaviours were intensively evaluated. All of them show remarkable fluorescent thermochromism and obvious AIE properties, indicating 9,10-bis(p-alkoxystyryl)anthracene derivatives are possibly ideal TAIE materials. And it was found that alkyl chain length has significant effect on their TCF properties, showing a pattern of increase in spectral shifts while decrease in colour transition temperatures with the extension of alkyl length. DSC and WAXD measurements have signified crystal to amorphous phase transitions upon heating should be responsible for the thermochromic behaviours. Furthermore, X-ray crystallographic analyses reveal that twisted molecular patterns of DSA-pn_n make them sensitive to thermal stimuli and alkyl chain length could endow them with unique molecular backbone conformations and solid-state packing structures, inducing different optical properties. The present findings, as a prior trial in exploring the structure–property relationship of TAIE compounds, would benefit the development of luminescent thermochromic solids.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1057818. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra08809g

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