Development of optical sensor for water in acetonitrile based on propeller-structured BODIPY-type pyridine–boron trifluoride complex

A propeller-structured 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex, ST-3-BF3, which has three units of 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile at the 3-, 5-, and 8-positions on the BODIPY skeleton, was designed and developed as an intramolecular charge transfer (ICT)-type optical sensor for the detection of a trace amount of water in acetonitrile. The characterization of ST-3-BF3 was successfully determined by FTIR, 1H and 11B NMR measurements, high-resolution mass spectrometry (HRMS) analysis, thermogravimetry-differential thermal analysis (TG-DTA), photoabsorption and fluorescence spectral measurements, and density functional theory (DFT) calculations. ST-3-BF3 showed a broad photoabsorption band in the range of 600 to 800 nm, which is assigned to the S0 → S1 transition of the BODIPY skeleton with the expanded π-conjugated system over the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at the 3-, 5-, and 8-positions onto the BODIPY core. In addition, a photoabsorption band was also observed in the range of 300 to 550 nm, which can be assigned to the ICT band between the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at 3-, 5-, and 8-positions and the BODIPY core. ST-3-BF3 exhibited a characteristic fluorescence band originating from the BODIPY skeleton at around 730 nm. It was found that by addition of a trace amount of water to the acetonitrile solution of ST-3-BF3, the photoabsorption band at around 415 nm and the fluorescence band at around 730 nm increased linearly as a function of the water content below only 0.2 wt%, which could be ascribed to the change in the ICT characteristics due to the dissociation of ST-3-BF3 into ST-3 by water molecules. Thus, this work demonstrated that the 3,5,8-trithienyl-BODIPY-type pyridine–boron trifluoride complex can act as a highly-sensitive optical sensor for the detection of a trace amount of water in acetonitrile.


Introduction
2][3][4][5][6][7][8][9] In fact, to date, some kinds of colorimetric and uorescent sensors for water based on ICT (intramolecular charge transfer), 10,11 PET (photo-induced electron transfer), 12,13 or ESIP (excited state intramolecular proton transfer) 14 have been designed and developed.Among them, the ICT-type sensor, which has a donor-p-acceptor (D-p-A) structure with photoabsorption and uorescence properties originating from the ICT excitation from the electron-donating (D) moiety to the electron-accepting (A) moiety, allows colorimetric and ratiometric uorescence measurements, which are preferable because the ratio of photoabsorption or uorescence intensities at two wavelengths is in fact independent of the total concentration of the sensor, photobleaching, uctuations in light source intensity, sensitivity of the instrument, etc.Indeed, in ICT-type sensors based on a D-p-A structure for detecting cations, anions, and neutral organic species, the dipole moment and electronic structure changed due to the intermolecular interaction (electrostatic interaction) between the electron-donating or electron-accepting moiety of the sensors and the species, resulting in changes in photoabsorption, uorescence (intensity and wavelength), and electrochemical properties (oxidation and reduction potentials) and enabling the detection (recognition) of the analytes.For this reason, we recently focused on D-p-A-type pyridine-boron triuoride (BF 3 ) complexes as colorimetric and uorescent sensors for water. 11n our previous work, we have designed and actually developed This journal is © The Royal Society of Chemistry 2020

RSC Advances
PAPER a D-(p-A) 2 -type pyridine-BF 3 complex YNI-2-BF 3 composed of a carbazole skeleton as a donor moiety and two pyridine-BF 3 units as acceptor moieties (Fig. 1a).11a It was found that the blue-shi of the photoabsorption and the enhancement of the uorescence intensity in the low-water-content region could be attributed to the change in the ICT characteristics due to the dissociation of YNI-2-BF 3 into the D-(p-A) 2 -type pyridine dye YNI-2 by water molecules.Furthermore, a red-shi of uorescence bands with a decrease in the uorescence intensity in the high-water-content region was observed because of the formation of the hydrogen-bonded proton transfer (PTC) complex YNI-2-H 2 O with water molecules.Moreover, 9-methyl pyrido [3,4-b]indole-BF 3 complex, 9-MP-BF 3 , was designed and developed as a colorimetric and ratiometric uorescent sensor for the detection of water in the low-, moderate-, and high-watercontent regions in solvents (Fig. 1b).11b It was found that in the low-water-content region, the blue-shis of photoabsorption bands with an isosbestic point and uorescence bands with an isoemissive point could be attributed to the dissociation of 9-MP-BF 3 into 9-methyl pyrido [3,4-b]indole (9-MP) by water molecules.In the moderate-water-content region, the photoabsorption and the uorescence bands of 9-MP gradually shied to a longer wavelength region with the increase in the uorescence intensity, which can be ascribed to the formation of the hydrogen-bonded complex (9-MP-H 2 O) with water molecules.Furthermore, in the high-water-content region, two photoabsorption bands and one uorescence band gradually reappeared in a longer wavelength region with simultaneous decreases in the photoabsorption and the uorescence bands of 9-MP-H 2 O, which was attributed to the formation of the PTC complex (9-MP-H + ) with water molecules.Consequently, our previous works proposed that the ICT-type pyridine-BF 3 complexes can act as colorimetric and uorescent sensors for the detection of water in the low-, moderate-, and high-water-content regions in solvents.
In this work, in order to gain a further insight into the impacts of uorophore and molecular structure on the optical sensing properties of ICT-type pyridine-BF 3 complexes for the detection of water, we designed and developed propellerstructured 3,5,8-trithienyl-BODIPY ST-3 (ref.15) and its pyridine-BF 3 complex ST-3-BF 3 , which have three units of 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile as strong electronwithdrawing moiety at the 3-, 5-, and 8-positions on the BOD-IPY skeleton, leading to the bathochromic shi of the photoabsorption band due to the enhancement of the ICT characteristics (Fig. 1c).4,4-Diuoro-4-bora-3a,4a-diaza-s-indacenes (boron dipyrromethene: BODIPY) dyes have created considerable interest as optical sensors and probes, 16 photosensitizers 17 for photodynamic therapy (PDT), and emitters 18 and dye-sensitizers 19 for optoelectronic devices such as organic light-emitting diodes (OLEDs) and dye-sensitized solar cells (DSSCs).It is expected that the addition of a trace amount of water to the solution of ST-3-BF 3 causes the dissociation of ST-3-BF 3 into ST-3 by water molecules, resulting in the photoabsorption and uorescence spectral changes.Herein we report the preparation, the characterization, and the optical sensing properties of the propeller-structured 3,5,8-trithienyl-BODIPY-type pyridine-BF 3 complex for the detection of a trace amount of water in acetonitrile based on FTIR, 1 H and 11 B NMR measurements, high-resolution mass spectrometry (HRMS) analysis, thermogravimetry-differential thermal analysis (TG-DTA), photoabsorption and uorescence spectral measurements of ST-3-BF 3 in acetonitrile containing various concentrations of water, and density functional theory (DFT) calculations.

Characterization of ST-3-BF 3
The propeller-structured 3,5,8-trithienyl-BODIPY-type pyridine-BF 3 complex ST-3-BF 3 studied in this work was prepared by treating ST-3 (ref.15) with boron triuoride diethyl etherate  (BF 3 -OEt 2 ) and fully characterized by FTIR, 1 H and 11 B NMR measurements, HRMS, and TG-DTA, although we could not obtain the 13 C NMR spectrum that is clear enough to be assigned, due to the low solubility of ST-3-BF 3 into solvent (Fig. 2-4).In the FTIR spectra, the B-F and B-N stretching bands originating from the BODIPY core were observed at 1082 and 1522 cm À1 for ST-3 and 1047 and 1504 cm À1 for ST-3-BF 3 , respectively (Fig. 2a).In addition, for ST-3-BF 3 , the characteristic C]N stretching band of the pyridyl group coordinated to BF 3 , the B-N stretching band of the pyridine-BF 3 complex, and the B-F stretching band of BF 3 were clearly observed at 1636, 1429, and 1024 cm À1 , respectively.The TG-DTA of ST-3-BF 3 indicated the decreased weight loss by 7.78% in comparison with that of ST-3 at around 275 C, which is in good agreement with the calculated weight loss of 7.61% for the release of a BF 3 unit from ST-3-BF 3 (Fig. 2b).Moreover, the 11 B NMR spectrum of ST-3-BF 3 in acetonitrile-d 3 showed a singlet at À0.21 ppm, which can be assigned to BF 3 coordinated to the pyridyl group, and a characteristic triplet with coupling constant (J B-F ) of 33 Hz at around 2-3 ppm, which indicates the presence of the BF 2 group in BODIPY (Fig. 3).Based on this result, the ratio of the peak integrals of BF For the 1 H NMR spectrum of the propeller-structured 3,5,8trithienyl-BODIPY-type pyridine-BF 3 complex, if it is assumed that BF 3 coordinates to a pyridyl group at the end of the 3-or 5position on the BODIPY core, the 1 H NMR spectrum of ST-3-BF 3 is expected to be more complex than that of ST-3.For example, the 1-position protons on the pyridyl groups at the end of the 3-, 5-, and 8-positions on the BODIPY core will appear as three different signals.On the other hand, if it is assumed that BF 3 coordinates to the pyridyl group at the end of the 8-position on the BODIPY core, the signal pattern in the 1 H NMR spectrum of ST-3-BF 3 is expected to be similar to that of ST-3.In fact, the 1 H NMR spectrum of ST-3-BF 3 demonstrated that the chemical shis and signal pattern of the 1-position protons (H a 0 and H l 0 ) on the pyridyl groups of ST-3-BF 3 show little change from those (H a and H l ) of ST-3, indicating the formation of the pyridine-BF 3 complex coordinated to the pyridyl group at the end of the 8position on the BODIPY core (Fig. 4), although the comparison of the 1 H NMR spectra between ST-3 and ST-3-BF 3 might be difficult because different deuterated solvents were used for ST-3 (in CDCl 3 ) and ST-3-BF 3 (in acetonitrile-d 3 ).
The photoabsorption spectra of ST-3 and ST-3-BF 3 in acetonitrile revealed that the two dyes show a strong and broad photoabsorption band in the range of 600 to 800 nm, which is assigned to the S 0 / S 1 transition of the BODIPY skeleton with the expanded p-conjugated system over the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at the 3-, 5-, and 8-positions onto the BODIPY core (Fig. 5a).In addition, a photoabsorption band was also observed in the range of 300 to 550 nm, which can be assigned to the ICT band between the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile units at 3-, 5-, and 8-positions and the BODIPY core. 15,20It is worth noting here that for ST-3, the peak absorbance of the former photoabsorption band at 695 nm is comparable with that of the latter ICT band at 415 nm, while for ST-3-BF 3 , the peak absorbance of the former band at 695 nm is lower than that of the latter band at 415 nm, which is attributed to the enhanced ICT characteristics.Moreover, for ST-3, the peak absorbance at 415 nm is higher than that at 450 nm, whereas for ST-3-BF 3 , the peak absorbance at 415 nm is lower than that at 465 nm.The corresponding uorescence spectra of the two dyes show a characteristic uorescence band at around 730 nm originating from the BODIPY skeleton, and the uorescence band of ST-3-BF 3 is broader than that of ST-3 (Fig. 5b).Consequently, the characterization of the propeller-structured 3,5,8-trithienyl-  BODIPY-type pyridine-BF 3 complex is successfully determined by the photoabsorption and uorescence spectral measurements as well as FTIR, 1 H and 11 B NMR, HRMS, and TG-DTA.In order to examine the electronic structures of the propeller-structured 3,5,8-trithienyl-BODIPY dyes, the molecular structures and molecular orbitals of ST-3 and ST-3-BF 3 were calculated using DFT at the B3LYP/6-31G(d,p) level (Fig. 6).For the two dyes, the HOMOs are mostly localized on the BODIPY core and the two thienyl groups at the 3-and 5-positions.On the other hand, the LUMO of ST-3 is mostly localized on the BODIPY core and the three thienyl groups at the 3-, 5-, and 8-positions, but that of ST-3-BF 3 is mostly localized not only on the BODIPY core and the two thienyl groups at the 3-and 5-positions but also over the 2-(pyridin-4-yl)-3-(thiophen-2-yl)acrylonitrile unit at the 8-position.Thus, the DFT calculations suggest that the dissociation of ST-3-BF 3 into ST-3 by water molecules results in the photoabsorption and uorescence spectral changes based on their ICT characteristics due to the perturbation in the LUMO over the 2-(pyridin-4yl)-3-(thiophen-2-yl)acrylonitrile unit of ST-3-BF 3 .

Optical sensing ability of ST-3-BF 3 for water in acetonitrile
In order to investigate the optical sensing ability of ST-3-BF 3 for water in acetonitrile, the photoabsorption and uorescence spectra of ST-3-BF 3 were measured in acetonitrile that contained various concentrations of water (Fig. 7).With the increase in the water content in acetonitrile solution, a red-shi of the photoabsorption band at 465 nm with a decrease in the absorbance and simultaneous increases in the absorbance of the two photoabsorption bands at around 415 and 695 nm were observed, which could be ascribed to the dissociation of ST-3-BF 3 into ST-3 by water molecules (Fig. 7a).On the other hand, the corresponding uorescence spectra of ST-3-BF 3 underwent an increase in the intensity of the uorescence band at around 730 nm (Fig. 7b).To estimate the sensitivity and accuracy characteristics of ST-3-BF 3 for the detection of water in acetonitrile, the changes in the absorbance and uorescence intensity were plotted against the water fraction in acetonitrile (Fig. 8).The plots of absorbance in the water content region below 1.0 wt% demonstrated that the absorbance at around 415 nm increased linearly as a function of the water content, but the absorbance at around 695 nm slightly increased as a function of the water content (Fig. 8a).Moreover, the plot of uorescence intensity at around 730 nm in the water content region below 1.0 wt% demonstrates that the uorescence peak intensity increases almost linearly as a function of the water content (Fig. 8b).The increases in the absorbance and uorescence intensity leveled off in the water content region above 0.2 wt%.Thus, it was found that the addition of a trace amount of water to the acetonitrile solution of ST-3-BF 3 causes the change in the ICT characteristics due to the dissociation of ST-3-BF 3 into ST-3 by water molecules, and as the result, the photoabsorption band at around 415 nm and the uorescence band at around 730 nm increase linearly as a function of the water content below only 0.2 wt%.Consequently, this work demonstrated that the 3,5,8trithienyl-BODIPY-type pyridine-BF 3 complex can act as a highsensitive optical sensor for the detection of a trace amount of water in acetonitrile.

Conclusions
We have designed and developed the propeller-structured 3,5,8trithienyl-BODIPY-type pyridine-boron triuoride complex, ST-3-BF 3 , which has three units of 2-(pyridin-4-yl)-3-(thiophen-2-yl) acrylonitrile at the 3-, 5-, and 8-positions on the BODIPY skeleton, as an intramolecular charge transfer (ICT)-type optical sensor for the detection of a trace amount of water in acetonitrile.It was found that the addition of a trace amount of water to   the acetonitrile solution of ST-3-BF 3 causes the photoabsorption and uorescence spectral changes based on the ICT characteristics due to the dissociation of ST-3-BF 3 into ST-3 by water molecules.Indeed, the absorbance and uorescence intensity increased linearly as a function of the water content below only 0.2 wt%.Based on the optical sensing mechanism of ST-3-BF 3 , we demonstrated that the 3,5,8-trithienyl-BODIPY-type pyridine-boron triuoride complex can act as a high-sensitive optical sensor for the detection of a trace amount of water in acetonitrile.Thus, our continuous works regarding optical sensors for water conrm that the ICT-type pyridine-boron triuoride complex is one of the most promising colorimetric and uorescent sensors for the detection of water in the low-, moderate-, and high-water-content regions in solvents.Moreover, NIR dyes such as ICT-type pyridine-boron triuoride complex which make it possible to control the intensity of NIR luminescence by the presence or absence of water, may be applicable to the wavelength conversion dye-doped lms for controlling the plant growth (photomorphogenesis).
3 and BF 2 was 1 : 1. Obviously, the FTIR, TG-DTA, and 11 B NMR results demonstrated the presence of one BF 3 unit coordinated to the pyridyl group in ST-3-BF 3 , although HRMS (ESI) of ST-3-BF 3 showed the base peak corresponding to the molecular ion for m/z of [ST-3 + 2H] 2+ (calcd for C 45 H 27 N 8 BF 2 S 3 , 412.07855; found 412.07918) due to the measurement condition.

Fig. 8
Fig. 8 (a) Absorbance at 415 and 695 nm, and (b) fluorescence peak intensity at around 730 nm (l ex ¼ 640 nm) of ST-3-BF 3 as a function of water content below 1.0 wt% in acetonitrile.