Chinese Chemical Letters  2015, Vol.26 Issue (07):834-838   PDF    
Synthesis and application of Vis/NIR dialkylaminophenylbuta-1, 3-dienyl borondipyrromethene dyes
Peng-Cheng Shia, Xin-Dong Jiangb, Rui-Na Gaoa, Yuan-Yuan Doua,, Wei-Li Zhaoa     
a The Key Laboratory for Special Functional Materials, Ministry of Education, Henan University, Kaifeng 475004, China;
b College of Applied Chemistry, Shenyang University of Chemical Technology, Shenyang 110142, China
Abstract: Mono- and bis-dialkylaminophenylbuta-1,3-dienyl boron-dipyrromethenes (BODIPYs) 1-12 were synthesized in 36%-42% yields by a Knoevenagel-type condensation. The absorption and emission maxima (λlabs = 614-739 nm; (λlem = 655-776 nm in CHCl3) of 1-12 covered from the visuble to the nearinfrared region. Probe 1 was ratiometric Vis pH probes. Such probe was almost non-fluorescent. Upon the protonation of the tertiary amine function of 1, the strong fluorescence (φf = 0.97) was released and the florescence intensity was dramatically increased by one thousand folds. The sharp isosbestic points were discovered at 590 nm, which was a ratiometric pH probe.
Key words: BODIPY     Near-infrared     Fluorescent dyes     Spectroscopic properties    
1. Introduction

The measurement of pH is one of the most important analytical methods in chemical laboratories and industry [1, 2, 3, 4, 5, 6, 7, 8, 9]. Fluorescent measurements offer many advantages over conventional electrochemical approaches including high sensitivity,ease of miniaturization, and non-invasive read-out etc. [10, 11, 12, 13]. Ratiometric probes are particularly preferable because of the measurement of the ratio of the fluorescence intensities at two wavelengths,which is insensitive to orientation,probe concentration and background fluorescence [14, 15, 16, 17].

The most popular strategy for pH-responsive fluorescent sensors takes advantage of intramolecular charge transfer (ICT) or the photoinduced electron transfer (PET) [18, 19, 20, 21, 22, 23]. Dimethylamino group [24, 25] is one of the fragments frequently used for the purpose of ICT or PET [26, 27, 28, 29, 30, 31]. The BODIPY is a family of very widely used fluorescent dye for potential applications and often designed for pH probes [32, 33] wherein the excited state of the fluorophore can be quenched by the electron transfer from electron donating amine to the fluorophore. Upon recognition of a proton,the electron transfer is "switched off" and in turn the emission of fluorescence is "switched on". It is well-known that when a donor group (e.g. aniline or phenol) is linked in the meso position of BODIPY chromophore,the maxima of absorption and emission wavelengths are little affected,however the fluorescent intensity is dramatically changed during protonation or deprotonation (Fig. 1a-d) [34, 35, 36, 37]. When the donor group e.g. dimethylaminostyryl substituent is attached to the 3-position of BODIPY (Fig. 1e),both wavelengths and intensities of absorption and emission are affected significantly during the sensing process [38, 39, 40]. Significant buleshifts (about 100- 200 nm) are frequently observed for the pH response. As a result,the emission maximum of pH probe is often located at the visible region upon protonation [34, 35, 36, 37, 38, 39, 40]. Near-infrared (NIR, 650-900 nm) fluorescent dyes have been the fast developing dyes because of the various advantages over visible dyes: significant reduction of the background absorption,fluorescence, low light scattering,high sensitivity,and deep penetration [41, 42]. Very recently,modifications on the BODIPY core to tune fluorescence emission to red-NIR region became a hot topic [43- 46]. One of the very effective methods is an extension of the conjugated system in BODIPYs through Knoevenagel reaction [47, 48, 49, 50]. The long standing interest of our group lies in design and synthesis of novel near-infrared fluorescent materials and their applications [51, 52, 53]. Literature searching show that only limited BODIPY dyes with 4-phenylbuta-1,3-dienyl group or pentadienoate moiety were documented [54, 55, 56]. For the dialkylaminophenylbuta-1,3-dienyl BOIDIPYs,only one example containing a single dialkylaminophenylbuta-1,3-dienyl group has been documented [54]. However,the spectroscopic properties were not reported. Herein,we report synthesis of BODIPYs containing mono- and bis-dialkylaminophenylbuta-1,3-dienyl moieties (Fig. 1).

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Fig. 1.Comparison of chemical structures of BODIPY pH probes reported and used in the present study.
2. Experimental

1H NMR spectra were recorded on a VARIAN Mercury 400 MHz spectrometer. 1H NMR chemical shifts (δ) are given in ppm downfield from Me4Si,determined by residual chloroform (δ 7.26). 13C NMR spectra were recorded on a VARIAN Mercury 100 MHz spectrometer in CDCl3,all signals are reported in ppm with the internal chloroform signal at δ 77.0 as standard. Mass spectrometric measurements were performed by the mass spectrometry service of the ETHZ on a Bruker Reflex MALDI as matrix (20 kV). The refractive index of the medium was measured by 2W Abbe’s refractometer. All reactions were carried out under N2. Tetrahydrofuran (THF) was freshly distilled from Na-benzophenone,and other solvents were distilled over CaH2. Merck silica gel 60 was used for the column chromatography.

Fluorescence spectra were recorded on FluoroSENS spectrophotometer and are reported as cm-1. UV-vis spectra were recorded on Perkin-Elmer Lambda 35 UV/vis spectrophotometer at room temperature. The fluorescence quantum yields (Фf) of the BODIPY systems were calculated using the following relationship (Eq. (1)):

where F denotes the integral of the corrected fluorescence spectrum,A is the absorbance at the excitation wavelength,and n is the refractive index of the medium (n = 1.4455 in CHCl3; n = 1.4550 in DMSO/H2O (4:1,v/v) at 20℃),ref and sampl denote parameters from the reference and unknown experimental samples,respectively. The reference systems used were nile blue as standard (Фf = 0.27,λex = 625 nm,0.5% (v/v) 0.1 mol/L HCl in ethanol) [57] for the mono-substituted BODIPYs,and boronazadipyrromethene compound aza-BODIPY as standard (Фf = 0.36 in chloroform) [46] for di-substituted BODIPYs,respectively. 2.1. Synthesis of 3-(4-morpholinophenyl)acrylaldehyde (13)

4-Morpholinobenzaldehyde (1.91 g,10 mmol) was dropwise added to 98% H2SO4 (5 mL) at 0℃. Then,acetaldehyde (1.7 mL, 30 mmol) was added the mixture and stirred for 3 h at room temperature. The mixture was neutralized with 10% NaOH aq. and extracted with CH2Cl2 (2× 80 mL). The organic layer was washed with brine (2× 60 mL),and dried over anhydrous Na2SO4. After removing the solvent by evaporation,the residue was separated by column chromatography (n-hexane:CH2Cl2 = 2:1) to afford 13 (1.52 g,70%) as a yellow solid. 1H NMR (CDCl3,400 MHz): δ 9.62 (d, 1H,J = 7.6 Hz),7.59 (d,2H,J = 8.8 Hz),7.39 (d,1H,J = 16 Hz),6.89 (d,2H,J = 8.8 Hz),6.59 (dd,1H,J = 16,7.6 Hz),3.87-3.85 (m,4H), 3.36-3.27 (m,4H). 2.2. General procedure for the synthesis of BODIPYs (1-12)

4-(Dialkylaminophenyl)acrylaldehyde (2 mmol),TM-BDP or 8-phenyl-TM-BDP (0.5 mmol),AcOH (0.5 mL),and piperidine (0.5 mL) were stirred for 24 h at 95℃ in dry toluene (6 mL) in the presence of a small amount of activated 4Å molecular sieves. The mixture was cooled to room temperature,quenched with water,extracted with CH2Cl2,washed with brine,dried over Na2SO4,evaporated and purified by TLC to afford mono- and bisdialkylaminophenylbuta- 1,3-dienyl BODIPYs. (Synthesis of 3-12, please see the Supporting information.) 2.3. Synthesis of 1 and 2

4-(Dimethylaminophenyl)acrylaldehyde,TM-BDP were used as the starting materials,and BODIPYs 1 (76 mg,0.18 mmol,36%) and 2 (101 mg,0.18 mmol,36%) were obtained. 1: 1H NMR (CDCl3, 400 MHz): δ 7.50-7.45 (m,1H),7.35 (d,2H,J = 8.8 Hz),7.14-7.04 (m,2H),6.94 (s,1H),6.91-6.88 (m,1H),6.85 (d,2H,J = 8.8 Hz),6.60 (s,1H),6.04 (s,1H),3.00 (s,6H),2.55 (s,3H),2.27 (s,3H),2.24 (s, 3H). 13C NMR (CDCl3,100 MHz): δ 154.9,150.5,140.5,139.3,138.7, 137.5,135.4,129.2,128.3,126.5,125.0,122.6,121.2,120.3,118.5, 117.3,115.7,112.2,40.2,14.6,11.3. HRMS-MALDI (m/z): [M]+ calcd. for C24H26BF2N3: 405.2184; found: 405.2182. 2: 1H NMR (CDCl3,400 MHz): δ 7.51 (d,2H,J = 8.8 Hz),7.52-7.45 (m,1H),7.36 (d,2H,J = 8.8 Hz),7.27-6.93 (m,5H),6.85 (s,1H),6.72-6.65 (m, 2H),6.69 (d,4H,J = 8.8 Hz),6.61 (s,2H),3.03 (s,6H),3.01 (s,6H), 2.27 (s,3H),2.26 (s,3H). 13C NMR (CDCl3,100 MHz): δ 154.5,150.9, 137.1,136.4,129.1,128.2,125.4,120.8,114.5,112.0,40.3,29.6, 22.6,14.1,11.3. HRMS-MALDI (m/z): [M]+ calcd. for C35H37BF2N4: 562.3078; found: 562.3074. 3. Results and discussion

Mono- and bis-dialkylaminophenylbuta-1,3-dienyl BODIPYs 1- 12 were successfully synthesized in 36%-42% yields by a Knoevenagel-type condensation reaction [47, 48, 49, 50] between the representative BODIPYs (R = H or Ph) and the corresponding acrolein derivatives in the presence of acetic acid,piperidine,and molecular sieves in toluene (Scheme 1).

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Scheme 1.Synthesis of mono- and bis-dialkylaminophenylbuta-1,3-dienyl BODIPYs 112.

To gain insight into the spectroscopic properties of BODIPYs bearing the dialkylaminophenylbuta-1,3-dienyl group,the absorption and emission spectra for 1-12 were measured and outlined in Fig. 2 and Table 1. The absorption and emission maxima (λabs = 614-739 nm; λem = 655-776 nm in CHCl3) of dyes 1-12 covered from the visible to the near-infrared region. The absorption maxima of dyes 1-6 without substitution in meso position are slightly longer than (about 2-5 nm) those of the corresponding dyes 7-12 with a phenyl group in meso position. In each sub-series,the absorption maxima relies on the electrondonating ability of the amine group and have shifting ability of NEt2 > NMe2 > morpholino (λmax(3) = 626 nm,λmax(1) = 614 nm and λmax(5) = 599 nm; λmax(9) = 622 nm,λmax(7) = 614 nm, λmax(11) = 595 nm). These dyes have relatively large extinction coefficients (60,000-150,000 L mol-1 cm-1) and possess a broad full width at half maximum (FWHM = 57-90 nm). As expected, dyes 1-12 exhibit relatively low fluorescence quantum yields due to the ICT effect. The fluorescent quantum yields of monosubstituted BODIPYs (Фf = 0.23-0.26 in CHCl3) are higher than those (Фf = 0.07-0.10) of the corresponding bis-dialkylaminophenylbuta- 1,3-dienyl BODIPYs. Such amine-containing dyes are expected to be a good system for pH sensing study.

We chose dyes 1 as the representative compound to investigate the pH-responsive properties. Photoimage of 1 were taken under normal room illumination and UV light,and notable changes of relatively vivid bright colors of 1 with the pH can be easily observed with naked eye (Fig. 3). Upon addition of hydrochloric acid to the mono-substituted dye 1 with a NMe2 group as a pHsensitive functionality,1 was protonated at relatively low pH value [58, 59]. A stepwise decrease of the absorption intensity was observed in the 617 nm band,and this peak disappeared completely at pH 1 (Fig. 4). The formation of a new band at 584 nm was first observed in pH 2.8 in partial enlarged view in Fig. 4,and another new peak subsequently arose at 565 nm. The sharp isosbestic point was found at 590 nmduring the formation of the protonated compound 1-H+. The ratiometric nature of the distinct and well-separated spectral bands could facilitate in situ monitoring of acid concentration and remain impervious to environmental interference. The absorption intensity of 1-H+ reached the maximum when 2 mol/L HCl was used and gradually decreased along with increase of the concentration of HCl (up to 6 mol/L). The fluorescence quantum yield of 1 in DMSO/H2O (4:1, v/v) is low (Фf = 0.01,ε = 30,000 L mol-1 cm-1 at pH 7) with the emission maximum at 727 nm in partial enlarged view (Fig. 5); however,with decreasing pH the twin emission maxima were blue-shifted to 573 and 592 nm and a dramatic increase in fluorescence intensity at 573 nm with I573 nm/I727 nm by one thousand folds (Фf = 0.97,ε = 34,000 L mol-1 cm-1,when treated with HCl to 2 mol/L) (inset of Fig. 5).

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Fig. 2.Absorption (up) and fluorescence spectra (down) of 112 in CHCl3 at 298 K.

Table 1
Spectroscopic data of 112 in CHCl3 at 298 K.

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Fig. 3.Photograph of solutions of BODIPY dyes without (upper row) and under UV irradiation (bottom row) for 1 at pH 1, 2, 3 and 7 in DMSO/H2O (4:1, v/v), respectively.

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Fig. 4.Absorption spectra (up: 6 mol/L-pH 7; down: pH 1–4 for clarity) of 1 in DMSO/H2O (4:1, v/v) as a function of pH. Isosbestic point at 590 nm.

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Fig. 5.Corresponding fluorescence spectra (up: full figure, λex = 540 nm; down: partial enlarged view, λex = 600 nm) of 1 in DMSO/H2O (4:1, v/v) as a function of pH. Inset: the relationship of fluorescence intensity ratio (I573 nm/I727 nm) and pH.
4. Conclusions

We successfully synthesized mono- and bis-dialkylaminophenylbuta- 1,3-dienyl BODIPYs 1-12 in 36%-42% yields by a Knoevenagel-type condensation reaction. The absorption and emission maxima (λabs = 614-739 nm; λem = 655-776 nm in CHCl3) of 1-12 covered from the visible to the near-infrared region. The electron donation amine led to the redshift of the BODIPY dye in shifting ability of NEt2 > NMe2 > morpholino. These dyes exhibited relatively high extinction coefficients (60,000- 150,000 L mol-1 cm-1) and possessed a broad full width at half maximum (FWHM = 57-90 nm). In the investigation of the pHresponsive properties,probe 1 was weakly fluorescent (Φf = 0.01) in DMSO/H2O (4:1,v/v). Upon the protonation of the tertiary amine function of 1,the strong fluorescence (Φf = 0.97) was released and the fluorescence intensity was dramatically increased by up to one thousand fold. The sharp isosbestic point was discovered at 590 nm for 1,which was a ratiometric probe. The ratiometric nature of the distinct and well-separated spectral bands could facilitate in situ monitoring of acid concentration and remain impervious to environmental interference.

Acknowledgments

This work was supported by NNSFC (No. 21372063),Program for Changjiang Scholars and Innovative Research Team in University (No. PCS IRT1126),the Foundation of the Education Department of Henan Province for Science and Technology Research Projects (No. 13A150046) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars,State Education Ministry.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.11.010.

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