Pyrrole is an aromatic heterocycle and often used as a privileged building block for various dyes, such as carbazole[1-4], BODIPY[5-7], porphyrin[8, 9], and thiaporphyrin[10, 11]. Owing to their interesting spectral properties including tunable color and fluorescence changes upon external stimuli, pyrrole derivatives have been widely utilized in metal ions and anion sensing[12, 13], organic optoelectronic device[14], etc. Recently, we have developed aryl-substituted pyrrole derivatives as new types of organic dyeswith the aggregation-induced emission (AIE) or aggregation-enhanced emission (AEE) characteristic[15-19], and have achieved in situ detection of a trace amount of chemical compounds, such as aluminum ion, amine vapor, and carbon dioxide[20-23]. Moreover, these AIE/AEE dyes show various advantages in biological sensing or imaging because of their low background interference, a high signal to noise ratio, superior photostability, and so on[24, 25]. Therefore, the design of new AIE/AEE dyes has become a hot research topic in the field of optoelectric materials and biosensors/bioimaging[26-30].
The red-emitting organic dyes with AIE feature are ideal candidates for the development of new bioimaging reagents because the red emission of the fluorescence has a deeper tissue penetration than ultraviolet or visible emission, thus avoiding the interference of auto-fluorescence from the biological substrates[31, 32]. The introduction of strong electron donor (D) and acceptor (A) groups in organic dye can realize a red emission with AIE property[33]. This simple and convenient method has been used for designing various novel AIE/AEE dyes having longer-wavelength emissions and has been applied to optoelectric devices and bioprobes[34]. Such D-A systems typically exhibit solvatochromic effects attributed to the change in the local molecular microenvironment that originates from the variation of solvent polarity[35]. Owing to their sensitive responses to the solvent polarity, these dyes containing D-A structures can be used for monitoring microenvironmental polarity in biological systems. Recently, we developed the electron-accepting group methyl benzoate (—PhCOOCH3, MB) and the electron-donating group pyrrole as a D-A system and obtained the red fluorescent dye which specifically recognize the cell membrane of MCF-7 and 293T cell lines[15]. Generally, D-A systems are formed by the linear arrangement of these groups such as D-A[36], A-D-A[37], D-A-D[38], A-D-A-A[39], D-A-A-D[34], and D-A-D-A-D[40]. However, reports of T-shaped geometry molecules containing symmetrical or unsymmetrical D-A structures are still quite limited[41]. We speculated that the geometry of T-shaped D-A structure of pyrrole derivatives is related to their emissive behaviors. To confirm this assumed structure-property relationship, we designed other two pyrrole derivatives MB3PE2 and MB3 having π-T-shaped and T-shaped D-A structures, respectively, as shown in Scheme 1. MB3PE2 has the same D-A structure as MB3 except for the longer π-conjugation orginated from two extra phenylethynyl groups. Indeed, their emissive results reveal that the geometry of D-A groups has a considerable influence on the emissive properties of fluorophores, which can provide a guidance for designing new dyes with high performance and long emissive wavelength.
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Scheme1 Schematic drawing of geometries of D-A system and chemical structures of pyrrole derivatives |
Phenylacetylene and iodine monochloride were purchased from J&K Scientific Ltd. Other chemical reagents such as catalysts and solvents were purchased from Aladdin Industrial Inc. All chemicals were used without further purification. Compounds 1-methyl benzoate-2, 5- di(4-bromophenyl)-pyrrole (1)[15] and methyl 4-ethynylbenzoate[22] were synthesized according to our previous work. Triple-distilled water was used for all the experiments.
1.2 Instrumentations and MethodsThe UV-Vis spectra were recorded on a TU-1901 UV-Vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). Photoluminescence (PL) spectra were collected on a Hitachi F-7000 fluorescence spectrophotometer at room temperature. The nuclear magnetic resonance (NMR) spectra were recorded on a BrukerAMX-400 spectrometer using deuterated chloroform or dichloromethane as solvent. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was performed by using α-cyano-4-hydroxycinnamic acid (CCA) as the matrix under the reflector mode for data acquisition. PL quantum yields were measured by using an integrating sphere on Nanolog FL3-2iHR fluorescence spectrometer (Horiba Jobin Yvon Company). Fluorescence decays were measured by using a Lifespec Ⅱ Picosecond fluorescence lifetime spectrometer (Edinburgh Instruments Ltd.), while the lifetime values were obtained by reconvolution fit analysis of the decay profiles with the aid of software.
1.3 Synthesis of 1-Methyl benzoate-2, 5-di(4-bromophenyl)-3, 4-di(phenylethynyl)-pyrrole (3)In a three-necked, round-bottomed flask under argon were placed compound 2 (0.7630 g, 1 mmol), phenylacetylene (0.5107 g, 5 mmol), dichlorobis (triphenylphosphine) palladium(Ⅱ) (0.0140 g, 0.02 mmol), triphenylphosphine (0.0105 g, 0.04 mmol), and copper(Ⅰ) iodide (0.0076 g, 0.04 mmol). Freshly distilled triethylamine (50 mL) and toluene (50 mL) were then added. The resulting mixture was stirred at room temperature for 12 h. After solvent evaporation, the solid was dissolved in DCM and washed with aqueous solution of NH4Cl. The organic layer was dried over MgSO4 and then filtered. The solvent was removed under reduced pressure and the crude product was purified by silica gel column chromatography using n-hexane/DCM (2:1, V/V) as eluent. A white solid was obtained in 60% yield (0.4268 g). 1HNMR(400 MHz, CDCl3): 7.94(d, J=8.6 Hz, 2H), 7.47(dd, J=7.7, 1.8 Hz, 4H), 7.39(d, J= 8.5 Hz, 4H), 7.32(d, J=6.6 Hz, 6H), 7.12 (d, J=8.5 Hz, 4H), 7.02(d, J=8.5 Hz, 2H), 3.92(s, 3H). MS (MALDI-TOF, m/z): calcd for C40H25Br2NO2: 711.0; found 711.1.
1.4 Synthesis of 1-Methyl benzoate-2, 5-di{4-{2-[(4-methoxycarbonyl)phenyl] ethynyl}phenyl}-3, 4-di(phenylethynyl)-pyrrole (MB3PE2)In a three-necked, round-bottomed flask under argon were placed compound 3 (0.7114 g, 1 mmol), methyl 4-ethynylbenzoate (0.8009 g, 5 mmol), dichlorobis(triphenylphosphine) palladium(Ⅱ) (0.0140 g, 0.02 mmol), triphenylphosphine (0.0105 g, 0.04 mmol), and copper(Ⅰ) iodide (0.0076 g, 0.04 mmol). Freshly distilled triethylamine (50 mL) and toluene (50 mL) were then added. The resulting mixture was stirred at 65 ℃ for 24 h. After solvent evaporation, the solid was dissolved in DCM and washed with aqueous solution of NH4Cl. The organic layer was dried over MgSO4 and then filtered. The solvent was removed under reduced pressure and the crude product was purified by silica gel column chromatography using n-hexane/DCM/ethyl acetate (14:5:1, V/V/V) as eluent. A yellow solid was obtained in 24% yield (0.2131 g). 1HNMR(400 MHz, CD2Cl2): 8.04(d, J=8.3 Hz, 4H), 7.98(d, J=8.5 Hz, 2H), 7.63(d, J=8.3 Hz, 4H), 7.53(dd, J=12.8, 5.2 Hz, 8H), 7.45-7.31 (m, 10H), 7.15 (d, J=8.5 Hz, 2H), 3.93(d, J=6.1 Hz, 9H). HR-MS (APCI, m/z): calcd for [M+H]+ 870.2851; found 870.2847; mass error -0.5 ppm.
1.5 Synthesis of 1-Methyl benzoate-2, 5-di{4-{2-[(4-methoxycarbonyl)phenyl] ethynyl}phenyl}-pyrrole (MB3)In a three-necked, round-bottomed flask under argon were placed compound 1 (0.5112 g, 1 mmol), methyl 4-ethynylbenzoate (0.8009 g, 5 mmol), dichlorobis(triphenylphosphine) palladium(Ⅱ) (0.0140 g, 0.02 mmol), triphenylphosphine (0.0105 g, 0.04 mmol), and copper(Ⅰ) iodide (0.0076 g, 0.04 mmol). Freshly distilled triethylamine (50 mL) and toluene (80 mL) were then added. The resulting mixture was stirred at 65 ℃ for 24 h. After solvent evaporation, the solid was dissolved in DCM and washed with aqueous solution of NH4Cl. The organic layer was dried over MgSO4 and then filtered. The solvent was removed under reduced pressure and the crude product was purified by silica gel column chromatography using n-hexane/DCM/ethyl acetate (14:5:1, V/V/V) as eluent. A yellow-green solid was obtained in 30% yield (0.2016 g). 1HNMR(400 MHz, CDCl3): 7.99(dd, J=18.3, 8.0 Hz, 6H), 7.55(d, J=7.5 Hz, 4H), 7.36(d, J=7.1 Hz, 4H), 7.10(d, J=6.9 Hz, 2H), 7.02(d, J=7.7 Hz, 4H), 6.56(s, 2H), 3.92(s, 9H). HR-MS (APCI, m/z): calcd for [M+H]+ 670.2225; found 670.2225; mass error 0 ppm.
2 Results and Discussion 2.1 SynthesisThe target compounds MB3PE2 and MB3 were synthesized via optimized routes as described in Scheme 2. The Sonogashira cross-coupling reaction of 1 with methyl 4-ethynylbenzoate gave the compound MB3 in 30% yield. Iodination of 1 using ICl afforded compound 2, 1-methyl benzate-2, 5-di(4-bromophenyl)-3, 4-diiodo-pyrrole, in good yield (83%). Two successive Sonogashira cross-coupling reactions of 2 with phenylacetylene and methyl 4-ethynylbenzoate gave the compound 3 (60% yield) and the target compound MB3PE2 (25% yield), respectively. All compounds were fully characterized by spectroscopic methods, and the detailed synthetic procedures are provided in the above experimental part.
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Scheme2 Synthetic routes to MB3 and MB3PE2 (ⅰ and ⅳ) Methyl 4-ethynylbenzoate/PdCl2(PPh3)2/CuI/ PPh3, 65 ℃; (ⅱ) ICl/NaHCO3, r.t.; (ⅲ) phenylacetylene/PdCl2(PPh3)2/CuI/PPh3, r.t. |
The optical properties of two compounds were studied by UV-Vis absorption and photoluminescence (PL) spectroscopy in solution or aggregation state in water or hexane mixtures. The absorption profiles of diluted solutions of MB3PE2 and MB3 are quite different due to their distinct chemical structures. In the case of THF solutions (Figure 1a), the absorption bands of MB3PE2 exhibits a prominent peak at 300 nm with a shoulder peak at about 360 nm, and the absorption spectrum of MB3 has two clear peaks at 275 and 350 nm. The peaks at lower wavelength correspond to the π-π* transition of phenyl rings, and those at higher wavelength originate from the conjugated aromatic backbone. No obvious ICT characteristic was observed inthe absorption bands of MB3PE2 and MB3 despite their typical D-A structural feature, probably due to the weak donor (pyrrole) and the weak acceptor (—PhCOOCH3). Moreover, altering the solvents such as ethyl acetate (EA), dichloromethane (DCM), chloroform, tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile (AN), dimethyl sulfoxide (DMSO) hardly influenced their UV-Vis absorption spectra (Figures 1b-1c). The two compounds were excited by the excitation wavelengths of 360 nm and exhibited single emission peak, as shown in Figure 1d. It is noteworthy to mention that the maximum emission wavelength of MB3 in THF is longer than that of MB3PE2, probably due to the better coplanarity of conjugated backbones of MB3. This result is in agreement with that the UV spectrum of MB3 in longer wavelength showed intensified absorption. Moreover, their PL spectra in different solvents showed more distinct differences (Figure 1e-1f). Both MB3PE2 and MB3 emitted blue emissions in most of the investigated solvents and showed a remarkable solvatochromism feature. In the case of MB3PE2 excited at 360 nm, the solvent change from hexane to DMSO resulted in a 65 nm red-shift while MB3 showed an 82 nm red-shift, which MB3 had more obvious solvent effect. Based on these findings, we can conclude that the UV-Vis absorption and PL properties of these aryl-substituted pyrrole derivatives are significantly affected by their geometric conjugated structures (as shown in Scheme 1) even though they have very similar D-A structure.
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Figure 1 The normalized UV-Vis (a) and PL (d) spectra of MB3PE2 and MB3 in THF; the UV-Vis spectra of (b) MB3PE2 and (c) MB3 and the PL spectra of (e) MB3PE2 and (f) MB3 in hexane, toluene, EA, DCM, THF, DMF, AN, DMSO; insert: the maximum emission wavelength (λem) in different solvents. [MB3PE2] = [MB3] = 10 μmol·L-1; excitation wavelength (λex): 360 nm |
As discussed above, the PL behaviors of both compounds exhibit a different red-shift response in various solvents with varying polarities. Given that these compounds have good solubility in THF and THF is miscible with most of other solvents, we investigated the PL properties of their aggregates by adding nonsolvent into their THF solutions. Water and hexane were selected for this study because both of them are non-solvents for this two compounds. In addition, water has the highest polarity while hexane can be considered as a nonpolar solvent. The aggregation behavior was investigated by monitoring the change in PL intensity with the addition of increasing amounts of water or hexane in their solvent mixtures, as shown in Figure 2. Adding water to MB3PE2 THF solution decreased its emission slowly from the water fraction (fW) 10% to 40% and rapidly decreased when fW reached 50% (Figures 2a and 2b), mainly due to the aggregation formation. Meanwhile, its maximum emission wavelength showed a 16 nm red-shift from 443 to 459 nm when the fW reached 40% (Figure 2c). Subsequently, a slight blue-shift of 13 nm from 459 to 446 nm was observed when fW reached 60%. Then the maximum emission wavelength stabilized around 446 nm. The former red-shift before the fW reached 50% was possibly attributed to a change of solvent polarity, and the latter blue-shift was likely caused by the alignment change when the molecules of MB3PE2 rapidly aggregated. There is a totally different PL behaviour for MB3 when the fW reached 50%, which increased its emission intensity until the fW exceeded 70%. Additionally, the PL intensity of MB3PE2 and MB3 slowly increased when hexane was used as non-solvent, as shown in Figures 2d-2e. Moreover, their maximum emission wavelengths exhibited an apparent blue-shift when fH reached 99%. As shown in Figure 2f, the blue-shifts of MB3PE2 and MB3 were 22 nm and 32 nm, respectively. The track of the transmittance of different fW showed that obvious aggregation were formed when fW reached 50%, but there were no aggregate in any fH, as shown in Figure 3. Since the molecules of MB3PE2 and MB3 have the same D-A structure but the different conjugation geometry, their PL behaviors showed different responses upon aggregation. Taken together, these results indicate that the conjugation and/or D-A geometry of pyrrole derivatives can significantly influence PL performance, especially in aggregation state.
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Figure 2 PL spectra of (a) MB3PE2 and (b) MB3 in the THF-water mixtures with different water fraction (fW) and (c) their correlation between net increase in PL intensity [(I-I0)/I0] vs. fW and λem vs. fW; PL spectra of (d) MB3PE2 and (e) MB3 in the THF-hexane mixtures with different hexane fraction (fH) and (f) their [(I-I0)/I0] vs. fH and λem vs. fH. [MB3PE2] = [MB3] = 10 μmol·L-1; λex =360 nm |
The fluorescence lifetime (τ) and quantum yield (Φ) are the most critical parameters of an organic fluorophore. To quantitatively evaluate the compounds′ emission, we investigated the fluorescence lifetime of MB3PE2 and MB3 indifferent states, including THF solutions, aggregates by water or hexane, and powders. Moreover, their absolute PL quantum yields were measured using an integrating sphere. The resulting data are summarized in Table 1. Compounds MB3PE2 and MB3 had similar lifetimes in the same state. However, their PL quantum yields exhibited significant difference. For example, the quantum yield of MB3PE2 in THF solution was 67.05% while MB3 had 26.02%, which was 2.6 times because MB3PE2 had a larger conjugated system. The lowest quantum yields of MB3PE2 and MB3 were observed when they aggregated in the presence of water, which clearly exhibited a typical solvatochromic effect, the higher the polarity of solvent the lower the quantum yield[42]. On the other hand, both of them had nearly identical values of Φ in powder, indicating they have the similar PL mechanism in the solid state. The Φ values of MB3PE2 and MB3 in aggregation by hexane were less than those of in THF solution, which is attributed to the reduced full width at half-maximum (FWHM) through their slightly enhanced maximum intensity when fH reaches 99% (Figures 2d and 2e).
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Table 1 The lifetime and quantum yields of MB3PE2 and MB3 in different states |
Fluorescence dynamics can reveal more information about the photophysical processes of organic dyes when they are photo-excited, which is very useful for understanding the emission mechanism[43, 44]. By using the τ and Φ values, the excited-state decay rates were estimated according to the following equations, τ = 1/(kr+knr), Φ = kr/(kr+knr), where kr and knr are the radiative and non-radiative decay rates, respectively. The kr values of MB3PE2 and MB3 in THF solution are very different (Table 2). The calculated kr value of MB3PE2 in THF solution is 5.451×108 s-1, which is about two-fold higher than that of MB3, whereas their kr values of aggregates caused by water or its solid exhibit similar results. In addition, MB3PE2 and MB3 aggregates that caused by hexane exhibit greater kr values than those of aggregates that caused by water. This phenomenon is reasonable because MB3PE2 and MB3 show obvious solvatochromic effect. The knr values of MB3PE2 and MB3 aggregates that caused by water are obviously greater than those of other states, which could be the likely cause of poor emissive performances when they aggregate by water. The knr value of MB3PE2 in powder is greater than that in THF solution, which could be caused by the π-π stacking interactions in the solid state. This finding could be explained by the knr values of MB3 in THF solution and powder are almost identical. Since compound MB3 has strong intermolecular π-π stacking interactions, the non-radiative decay rates of MB3 are almost identical whether in THF solution (6.490×108 s-1) or powders (6.168×108 s-1). These results unambiguously demonstrate that the geometry of emissive dyes has a significant influence on the radiative decay rates, whether in solution or solid state.
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Table 2 The radiative decay rates (kr) and non-radiative decay rates (knr) of MB3PE2 and MB3 in different states |
To further understand the optical properties of these luminogens at the molecular level, density functional theory (DFT) calculations were carried out using a suite of Gaussian 09 program. The nonlocal density functional of B3LYP with 6-31G (d) basis sets was used for the computation. The calculated frontier molecular orbitals are shown in Figure 4. The orbital distributions of the highest occupied molecular orbital (HOMO) of MB3PE2 and MB3 are similar, which mainly originate from the pyrrole rings as well as the phenylacetylene groups at the 2, 5-position of pyrrole in both compounds and 3, 4-position phenylacetylene groups of pyrrole in MB3PE2. There is almost no electron distribution in HOMO of 1-position methyl benzoate in the two compounds. The lowest unoccupied molecular orbital (LUMO) exhibits similar electron densities of MB3PE2 and MB3, which are mainly delocalized over the 2, 5-position 4-{2-[(4-methoxycarbonyl)phenyl] ethynyl}phenyl groups, an obvious electron density separation is observed. This spatially directed separation can be considered as a preferable condition for ICT between donor and acceptor units[38], which contributes to the fluorescence solvatochromic properties upon photo-excitation. But MB3PE2 has a bigger dihedral angles between pyrrole ring and benzene ring linked on the 2, 5-position than that of MB3, which is in good agreement with their maximum emissive wavelength in PL spectra.
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Figure 3 Transmittance (T%) of (a) MB3PE2 and (b) MB3 in the THF-water mixtures with different water fraction and (c) MB3PE2 and (d) MB3 in the THF-hexane mixtures with different hexane fraction. [MB5] = [MB3PE2] = [MB3] = 10 μmol·L-1 |
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Figure 4 The calculated molecular orbital amplitude plots of the HOMOs and the LUMOs of MB3PE2 and MB3 and their dihedral angles between pyrrole ring and benzene rings on 2, 5-position using B3LYP/6-31G basis set with G09 program |
In summary, two pyrrole derivatives with different geometry structures were synthesized, and their optical property was investigated. MB3PE2 and MB3 emit blue light with apparent solvatochromic effects in different solvents. MB3PE2 exhibits strong fluorescence in the solution state, but it can be decreased in the aggregation state. The fluorescence decrease of MB3PE2 in the aggregation state intrinsically originates from the increasing knr and decreasing kr simultaneously, where the values of kr and knr were obtained by using the experimental τ and Φ values. The obtained data and mechanistic explanations of the fluorescent properties of pyrrole derivatives unambiguously demonstrates that the geometry of D-A and/or conjugated groups in the organic dyes has a profound impact on their PL behaviors. The present work not only provides useful insights into designing new fluorophores with good performance but also demonstrates that the PL property in aggregation state could be regulated by optimizing their geometries of conjugation and/or D-A groups.
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