Rong Rong
Cui
a,
Yuan Chao
Lv
b,
Yong Sheng
Zhao
b,
Na
Zhao
*a and
Nan
Li
*a
aKey Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, 710119, China. E-mail: nzhao@snnu.edu.cn; nli@snnu.edu.cn
bKey Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
First published on 6th February 2018
Solid-state fluorescent materials have attracted a surge of interest in recent years due to their wide applications in the fields of photoelectric devices, memory storage and fluorescent probes. Compared to the synthesis of new molecules, exploring new properties in known molecules is a facile approach to obtain functionalized fluorescent materials. In this report, we systematically explored the solid-state photoluminescence properties and applications of 7-(diethylamino)coumarin-3-aldehyde (DCA) and 7-(diethylamino)coumarin-3-carboxylic acid (DCCA). Both fluorophores exhibited a special concentration-dependent emission effect. They displayed polymorphism dependent solid-state emission and single crystal analysis revealed that enhanced overlap between neighbouring molecules resulted in a red-shifted emission. Crystal-to-crystal transformation has also been achieved for both DCA and DCCA by employing an external thermal treatment. In addition, the solid powder of DCA and DCCA displayed fluorescence response to HCl and NH3 gas with high sensitivity. Furthermore, 1D micromaterials were assembled for both fluorophores and DCA exhibited outstanding optical waveguide behavior.
Synthesis of fluorescent molecules with structure diversity is the most creative activity in the field of fluorescent materials. However, creativity in functionalized fluorescent materials has often arisen from novel conceptual interpretations of well-established molecules.7 Compared to the development of new molecules that may suffer from synthesis complexity and property uncertainty, investigating known fluorophores to explore their superior properties and versatile applications is undoubtedly a shortcut strategy to obtain functionalized fluorescent materials. Indeed, many outstanding properties of star fluorescent molecules were dug out after the first report.8
Coumarin, one of the most noted organic fluorophores, has been widely and extensively used in both material and biological sciences due to its inherent physicochemical and photophysical characteristics, such as reasonable stability and relative ease of synthesis.9 However, in comparison with abundant reports about the photoluminescence research of coumarins in solution, investigations of fluorescence properties as well as potential utilization in the solid state are still rare.10
Herein, we report on the exploration of the solid-state optical properties and applications of two fluorophores based on an aminocoumarin scaffold, 7-(diethylamino)coumarin-3-aldehyde (DCA) and 7-(diethylamino)coumarin-3-carboxylic acid (DCCA) (Scheme 1).11 Both fluorophores displayed special concentration-dependent emission behaviour. Importantly, two types of crystal with varied emission color were obtained for DCA and DCCA. Subsequent single crystal analysis revealed the origin of this polymorphism phenomenon. Upon thermal treatment, crystal-to-crystal transformations were achieved for both fluorophores. Meanwhile, the solid state of DCA and DCCA exhibited sensitive fluorescence response to HCl and NH3 gas with different modes. Moreover, DCA and DCCA were able to assemble into 1D micromaterials, while DCA possessed remarkable optical waveguide behaviour.
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Fig. 1 Normalized PL spectra of DCA (A) and DCCA (B) in THF at different concentrations. Inset: Photograph of DCA (A) and DCCA (B) in THF with varied concentrations under 365 nm UV irradiation. |
Compd | λ em [nm] | Φ F (%) | τ avg [ns] | k f [109 s−1] | k nr [109 s−1] |
---|---|---|---|---|---|
a Absolute fluorescence quantum yield was measured by using the calibrated integrating sphere system. b Average fluorescence lifetime. | |||||
DCA-Y | 564 | 7.5 | 8.25 | 0.009 | 0.112 |
DCA-R | 601 | 7.2 | 16.77 | 0.004 | 0.055 |
DCA-P | 562 | 7.9 | 5.73 | 0.014 | 0.160 |
DCCA-G | 518 | 9.6 | 3.77 | 0.025 | 0.240 |
DCCA-R | 584 | 16.5 | 45.13 | 0.004 | 0.018 |
DCCA-P | 573 | 8.5 | 19.53 | 0.004 | 0.047 |
Interestingly, two forms of crystals with a nod and plate shape (called DCCA-G and DCCA-R) were obtained simultaneously by slowly evaporating the dichloromethane/methanol solution of DCCA (Fig. 2E–H). A green emission at 518 nm was observed for DCCA-G with ΦF of 9.6% and τavg of 3.77 ns. In contrast, DCCA-R emitted red light at 584 nm (ΦF = 16.5%, τavg = 45.13 ns). Different from DCCA-G and DCCA-R, the as-prepared powder sample of DCCA (named DCCA-P) displayed the orange emission color (λem = 573 nm, ΦF = 8.5%, τavg = 19.53 ns). It's obvious that both DCA and DCCA presented significantly polymorphism-dependent emission.14 Their solid-state emission could be easily tuned by altering the preparation precedures.
PXRD of DCA in different states was performed and the results showed that the diffraction profile of DCA-P was almost the same as that of DCA-Y (Fig. S3A, ESI†), suggesting their similar packing arrangement and almost identical emission. Additionally, the different diffraction pattern of DCCA in three states indicated its distinct packing assignment (Fig. S3B, ESI†), which resulted in the distinguishing emission.
To gain further insight into the mechanism of the above pronounced polymorphism properties, single crystal X-ray structural analysis was carried out and the corresponding crystal data are summarized in Table S2 (ESI†). X-ray diffraction analysis revealed that the molecules in DCA-Y adopted a coplanar conformation (Fig. 3A and B) and exhibited a head-to-tail antiparallel packing mode with two types of overlap arrangement (Fig. 3C and D), which resulted in the formation of polymer columns along the b-axis direction (Fig. 3E). Within each column, the distances between adjacent molecules were 3.668 Å and 3.515 Å, respectively, suggestive of the strong intermolecular π–π stacking (Fig. 3F). A modest molecular overlap led to a negligible slip angle along the long molecular axis, whereas the slip angle along the short molecular axis was 76.3° (Fig. 3G). Meanwhile, multiple C–H⋯O (2.524 Å, 2.613 Å and 2.633 Å) interactions existed within the crystal (Fig. S4A, ESI†), which linked columns to form a 3D structure in the crystal.
Although the conformation of the DCA-R crystal was similar to that of DCA-Y (Fig. 3H), the packing mode was entirely different. In the DCA-R, molecules also adopted a head-to-tail antiparallel packing mode (Fig. 3I and J) with two kinds of overlap arrangement, which led to the formation of molecular columns along the b-axis (Fig. 3K). The neighbouring molecules in the column also displayed intermolecular π–π stacking (3.396 Å and 3.285 Å) (Fig. 3L). The different molecular overlap in DCA-R resulted in a slip angle (59.7°) along the long molecular axis (Fig. 3L), while a very small slip was observed along the short axis (Fig. 3M). Multiple intermolecular C–H⋯O interactions (2.461 Å, 2.505 Å and 2.520 Å) have also been found throughout the crystal of DCA-R (Fig. S4B, ESI†), which hold the columns together in a 3D structure. Compared to the packing of DCA-Y, the degree of molecular overlap in DCA-R is relatively larger. In addition, DCA-R exhibited stronger π–π stacking due to the short distance between neighbouring molecules, which indicated larger electronic excited-state delocalization of DCA-R and resulted in its red-shifted emission.
X-ray diffraction analysis revealed that molecules in DCCA-G exhibited a coplanar conformation (Fig. 4A and B). In the DCCA-G, molecules were in a head-to-head parallel packing mode (Fig. 4C), which resulted in polymer columns along the a-axis direction (Fig. 4D). The distance between adjacent molecules in the column was 3.404 Å (Fig. 4E). Meanwhile, the small molecular overlap gave a slip angle of 43.7° along the long molecular axis (Fig. 4E), and the slip angle was 70.0° along the short molecular axis (Fig. 4F). Multiple intermolecular C–H⋯O interactions (2.659 Å, 2.675 Å and 2.680 Å) have also been found throughout the crystal of DCCA-G (Fig. S5A, ESI†), which helped the columns to form a 3D structure.
The conformation of DCCA-R was also coplanar (Fig. 4G). Nevertheless, the packing mode in the crystal was very different from that of DCCA-G. When viewed along the a-axis, the molecules in DCCA-R packed into a 1D chain along the c-axis direction (Fig. 4J), which adopted a head-to-tail parallel packing pattern (Fig. 4H and I) with two types of overlap arrangement. The distances between adjacent molecules were 3.438 Å and 3.420 Å (Fig. 4K). The higher degree of molecular overlap led to a larger slip angle (61.1°) along the long molecular axis (Fig. 4K), while there was a very small slip along the short molecular axis (Fig. 4L). Different from DCCA-G, multiple C–H⋯O interactions (2.460 Å, 2.515 Å and 2.630 Å) existed throughout the crystal (Fig. S5B, ESI†), which held the chains in a 3D structure and helped to rigidify the molecular conformation. Upon carefully checking the crystal packing it was revealed that the distance of π–π stacking interactions in DCCA-G and DCCA-R was similar, but the molecular overlap degree in DCCA-R was higher than that in DCCA-G. This feature led to the relatively larger electronic delocalization in DCCA-R and thus it achieved a bathochromic-shift emission.
Such crystal-to-crystal conversion was also investigated using differential scanning calorimetry (DSC). As seen in Fig. 5C, two types of crystal (DCA-Y and DCA-R) gave consistent melting points at 167 °C. During the process of heating, DCA-Y displayed a distinct exothermic peak at approximately 87 °C, whereas no peak was detected for DCA-R. Combining the emission change, this exothermic peak could be attributed to the thermal phase transition from DCA-Y to the thermodynamically stable state of DCA-R. The same melting point at 230 °C was observed for crystals of DCCA-G and DCCA-R (Fig. 5F). However, DCCA-G exhibited an exothermic peak at about 105 °C but not DCCA-R, suggesting that DCCA-G experienced a phase transition to the relatively stable state of DCCA-R.
The solid-state emission of DCA and DCCA could also be tuned by fuming with HCl and NH3 gas. As shown in Fig. 6A and B, the powder of DCA emitted an intense orange emission at 560 nm. However, upon exposing to HCl gas for ∼2 minutes, the fluorescence was completely quenched. After being fumed with NH3 gas for a few minutes, the sample gave a red emission color and the emission peak appeared at 600 nm, which red-shifted by about 50 nm compared with the initial sample of DCA. This “off–on” emission switch could be repeated by fuming HCl–NH3 gas. In sharp contrast, a different phenomenon was found for DCCA under the same conditions (Fig. 6C and D). When DCCA powder was fumed with HCl gas, the emission color was converted from orange to blue, and the emission peak shifted from 570 nm to 420 nm. Subsequently, treating the sample with NH3 gas brought about a green emission peaked at 500 nm. The emission color between blue and green could be repeated by using HCl–NH3 gas. The above results confirmed that DCA and DCCA can be employed as a fluorescent sensor for HCl and NH3 gas with high sensitivity.
In order to understand the mechanism of the above fluorescence response, 1H NMR analysis of DCA and DCCA at different states was performed. As shown in Fig. S7 (ESI†), DCA possessed a typical aldehyde proton signal at 10 ppm. After the treatment with HCl gas, however, the aldehyde proton disappeared and the signal of aromatic proton shifted significantly. This suggested that both amino and aldehyde groups of DCA could undergo a protonation process in an acidic environment. The protonated structure destroyed the ICT process and led to the quenching of fluorescence. The aldehyde proton signal was not recovered after the treatment with NH3 gas, which could be ascribed to the reaction between NH3 and the aldehyde group under acidic conditions to give the imine species. The resulting product exhibited stronger ICT character and gave a red-shifted emission compared to its initial state. For DCCA, fuming with HCl gas shifted all proton signals down field except for those of the methyl group, suggesting that the protonation of the amino group occurred (Fig. S8, ESI†). As a result, the ICT process was decreased and remarkable blue-shifted emission was observed. After being treated with NH3 gas, the carboxylic acid group might be ionized due to the neutralization which reduced the ICT effect. Consequently, the green emission was observed.
The above results confirmed that the solid-state emission of DCA and DCCA could be efficiently tuned by various external stimuli including heating and fuming with acidic or basic gas. In particular for DCCA, a considerably wide range of emission was achieved (from 420 nm to 584 nm), which is extremely rare in previous reports about tunable organic solid-state emitters.
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Fig. 7 Fluorescence microscopy and SEM images of DCA (A and C) and DCCA (B and D) micromaterials. Scale bar in A and B: 20 μm. |
According to the fluorescence microscopy images, bright emission spots at both ends of the micromaterials, while a relatively weak emission from the body, implied that the micromaterials might exhibit optical waveguide behaviour.17 In order to prove such properties, a distance-dependent fluorescence image of a single microrod (DCA) was taken using a near-field scanning optical microscope. As shown in Fig. 8A, the chosen microrod was excited using a focused laser at six different positions along its length. Obviously, a yellow emission was observed at the end of the rod except for the excited sites. This phenomenon indicated that the microrod propagated the light to the end of the rod, indicative of the optical waveguide behaviour. The emission spectra were collected in the fixed emitting ends with varied excitation positions (labelled with 1–6) (Fig. 8B). The results showed that the emission intensity was decreased with increasing distance between the excited site and the emitted end, which originated from the optical loss during the propagation process. The emission intensities (550 nm) at the fixed end (Iend) and the excited site (Ibody) were recorded, and the optical loss coefficient (a) was calculated by using the equation log(Iend/Ibody) = −αx, where x is the distance between the excited site and emitting end (Fig. 8C). The α value of DCA was determined to be 58 dB cm−1, which was quite low compared with previous reports.6c,15b,17a,18 The well-ordered arrangement of microrods as well as their smooth surface should be responsible for such excellent optical waveguide behaviour.
Footnote |
† Electronic supplementary information (ESI) available: Synthesis and characterization, optical spectra, and crystal data. CCDC 1557654. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qm00617a |
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