A BODIPY sensor for water based on a photo-induced electron transfer method with fluorescence enhancement and attenuation systems

The development of fluorescence sensing methods for the detection and quantification of a trace amount of water in solutions, solids, the atmosphere and products has received considerable attention in recent years from the viewpoint of fundamental study in analytical chemistry and photochemistry, and is eagerly anticipated in industry because of their potential applications in environmental and quality control monitoring systems, and sanitary and medical materials. Most of fluorescent sensors for a trace amount of water developed so far, including conjugated polymer and organic fluorescent dyes, are based on fluorescence quenching systems, that is, the fluorescence intensity of the sensor decreases as a function of water content in organic solvents. This feature can be attributed to the solvatochromic properties of sensors, the aggregation of sensors or the formation of hydrogen bonding between the sensor and water molecules with the increase in the water content. However, this fluorescence quenching system makes it difficult to detect trace amounts of water. Therefore, in our previous work, as a new approach for improving the detection limit of water, we have devised the fluorescence enhancement system based on PET (photo-induced electron transfer), that is, anthracene-aminomethyl-phenylboronic acid pinacol ester (PhenylBPin) OF-2 was designed and synthesized as a fluorescent PET sensor for a trace amount of water (Scheme 1a). In the low water content region below 1.0 wt%, the addition of water to organic solvents containing OF-2 causes an efficient formation of fluorescent ionic structure OF-2a with a protonated tertiary amino group, resulting in the occurrence of fluorescence by the suppression of the PET process from the electron-donating tertiary amino group to the photoexcited anthracene fluorophore due to the lowering of the HOMO level of the tertiary amino group. The detection limits (DLs) were as low as 0.009 wt% for water in acetonitrile. Thus, we demonstrated that the fluorescence enhancement system based on the PET method is useful for the detection and quantification of a trace amount of water in organic solvents. On the other hand, as another fluorescence enhancement system for the detection of water, an AIE (aggregation-induced emission)-based fluorescent sensor is attributed to the emission enhancement induced by aggregate formation upon addition of large amounts of water (over 40 wt% in almost every case) into the solution, leading to restricted intramolecular rotation (RIR) in the molecular structure, that is, the aggregation can restrict the intramolecular Scheme 1 Proposed mechanisms of fluorescent sensors: (a) OF-2 and (b) MH-1 with a PET (photo-induced electron transfer) structure for the detection of water in non-polar solvent and polar solvent.

The development of fluorescence sensing methods for the detection and quantification of a trace amount of water in solutions, solids, the atmosphere and products has received considerable attention in recent years from the viewpoint of fundamental study in analytical chemistry and photochemistry, and is eagerly anticipated in industry because of their potential applications in environmental and quality control monitoring systems, and sanitary and medical materials. [1][2][3][4] Most of fluorescent sensors for a trace amount of water developed so far, including conjugated polymer and organic fluorescent dyes, are based on fluorescence quenching systems, that is, the fluorescence intensity of the sensor decreases as a function of water content in organic solvents. [5][6][7][8] This feature can be attributed to the solvatochromic properties of sensors, 5 the aggregation of sensors 6 or the formation of hydrogen bonding 7 between the sensor and water molecules with the increase in the water content. However, this fluorescence quenching system makes it difficult to detect trace amounts of water. Therefore, in our previous work, as a new approach for improving the detection limit of water, we have devised the fluorescence enhancement system based on PET (photo-induced electron transfer), 9 that is, anthracene-aminomethyl-phenylboronic acid pinacol ester (PhenylBPin) OF-2 was designed and synthesized as a fluorescent PET sensor for a trace amount of water (Scheme 1a). 10a In the low water content region below 1.0 wt%, the addition of water to organic solvents containing OF-2 causes an efficient formation of fluorescent ionic structure OF-2a with a protonated tertiary amino group, resulting in the occurrence of fluorescence by the suppression of the PET process from the electron-donating tertiary amino group to the photoexcited anthracene fluorophore due to the lowering of the HOMO level of the tertiary amino group. The detection limits (DLs) were as low as 0.009 wt% for water in acetonitrile. Thus, we demonstrated that the fluorescence enhancement system based on the PET method is useful for the detection and quantification of a trace amount of water in organic solvents. 10 On the other hand, as another fluorescence enhancement system for the detection of water, an AIE (aggregation-induced emission)-based fluorescent sensor is attributed to the emission enhancement induced by aggregate formation upon addition of large amounts of water (over 40 wt% in almost every case) into the solution, leading to restricted intramolecular rotation (RIR) in the molecular structure, that is, the aggregation can restrict the intramolecular free rotation, resulting in the elimination of radiationless (nonemissive) relaxation of the photoexcited fluorophore. [11][12][13][14] Meanwhile, in some cases, the fluorescence attenuation with the increase in the water content was observed in the low water content region below 1.0-2.0 wt% due to the increase in polarity of the solution. 13,14 However, the fluorescence enhancement system based on the AIE sensors developed so far is not sufficient for the detection of a trace amount of water because the addition of large amounts of water into the solution is necessary for being AIE active.
Through our continuing efforts to gain insight into the molecular design toward developing a high-sensitivity optical sensor for water based on the PET method, we found that boron-dipyrromethene (BODIPY) MH-1 with a PhenylBPin unit can act as a fluorescent sensor for trace amounts of water based on the PET method with both fluorescence enhancement and attenuation systems (Scheme 1b). Herein, we report that MH-1 is a dual function-based fluorescent sensor possessing fluorescence intensity and fluorescence quantum yield (F F ) as a function of detection of water.
The absorption and fluorescence spectra of MH-1 in THF and various solvents are shown in Fig. 1a and Fig. S1 (ESI †), respectively, and their optical data in various solvents are summarized in Table 1. In all the solvents, the absorption maximum wavelength (l abs ) of MH-1 was observed at around 535 nm, which is assigned to the S 0 -S 1 transition of the BODIPY core. The molar extinction coefficient (e) at l abs is 67 000-78 000 M À1 cm À1 . The corresponding fluorescence maximum wavelength (l fl ) appeared at around 550 nm, which arises from the BODIPY locally excited (LE) state. It is worth mentioning here that a significant dependence of F F value on the solvent polarity was observed (Fig. 1b), that is, the F F value significantly decreases with increasing the empirical solvent polarity parameter (E N T ) from toluene (E N T = 0.099, F F = 62%) to acetonitrile (E N T = 0.460, F F = 2%). Thus, on the basis of the fact that there is no change in l fl by changing the solvent polarity, which reveals the decrease in F F value with increasing the solvent polarity is not attributed to the intramolecular charge transfer (ICT) properties and the change of photoexcited state, 15 this result indicates that in non-polar solvents such as toluene and 1,4-dioxane the formation of strong boron-nitrogen (B-N) interaction 16 between BPin and a tertiary amino group in MH-1(BN) can lead to the suppression of the PET process from the electron-donating tertiary amino group to the photoexcited BODIPY core due to the lowering of the HOMO levels of the tertiary amino group, resulting in a high F F value (Scheme 1b). Moreover, it is revealed by the fact that 2,6-diethyl-1,3,5,7- 17 suffers from a drastic fluorescence attenuation due to the PET process from the electron-donating tertiary amino group to the photoexcited BODIPY core when compared to the corresponding 1,3,5,7,8-pentamethyl-2,6-diethyl BODIPY (PM567) without the amino group (F F = 75% in THF). 18 On the other hand, in polar solvents such as acetone and acetonitrile the weakened B-N interaction accompanied with the promotion of intramolecular rotation of PhenylBPin in MH-1 due to the polar environment can lead to the activation of the PET process, resulting in a low F F value.
In order to investigate the sensing ability of MH-1 for water in solvents, the absorption and fluorescence spectra of MH-1 were measured in 1,4-dioxane, THF, acetone and acetonitrile that contained various concentrations of water ( Fig. 2 for 1,4-dioxane and acetonitrile, Fig. S2, ESI †). In the water content region below 20 wt% for all the four solvents, the absorption spectra of MH-1 did not undergo appreciable changes upon addition of water to the solution. On the other hand, for 1,4-dioxane and THF the florescence spectra of MH-1 underwent a decrease in fluorescence intensity with a slight red-shift (ca. 5 nm) of the fluorescence peak wavelength at 552 nm upon addition of water to the solution. The decrease in fluorescence peak intensity levels off in the water content region greater than 4.0-5.0 wt%. In contrast, for acetone and acetonitrile the fluorescence spectra exhibited an enhancement of fluorescence intensity with a slight red-shift (ca. 8 nm) of the fluorescence peak wavelength at around 548 nm upon addition of water to acetone or acetonitrile solution (Fig. 2d). The enhancement of the fluorescence peak intensity levels off in the water content region greater than 2.0-5.0 wt%. These results indicate that the addition of water to a polar solvent    20 Therefore, this work reveals that for MH-1 the formation of B-N interaction can effectively inactivate the PET process, rather than the formation of an ionic structure between the protonated tertiary amino group and the hydroxylated PhenylBPin, that is, the HOMO level of the tertiary amino group in MH-1(BN) due to the B-N interaction is lower than that in MH-1(H 2 O). Consequently, this work demonstrates that MH-1 can act as a fluorescent sensor for trace amounts of water based on the PET method with both fluorescence enhancement and attenuation systems.
To estimate the sensitivity and accuracy characteristics of MH-1 for the detection of water in the low water content region, the changes in fluorescence intensity (at ca. 550 nm) and F F value are plotted against the water fraction in these four organic solvents (Fig. 3 for 1,4-dioxane and acetonitrile, and Fig. S3 (ESI †) for THF and acetone). These plots demonstrated that for 1,4-dioxane and THF the fluorescence intensity and F F value decreased almost linearly as a function of the water content, but for acetone and acetonitrile the fluorescence intensity and F F value increased linearly as a function of the water content. Moreover, it is worth mentioning here that the F F value levels off at 20-30% in the water content region greater than 2.0-5.0 wt% for all the four solvents, that is, this result strongly indicates the formation of MH-1(H 2 O) upon addition of water to all the four solvents. The DL determined from the plot of the fluorescence intensity as a function of the water content is 0.3 wt% for acetonitrile and over 1.0 wt% for 1,4-dioxane, respectively (DL = 3.3s/m s , where s is the standard deviation of the blank sample and m s is the slope of the calibration curve in the low water content region below 5.0 wt% for 1,4-dioxane and 2.0 wt% for acetonitrile, respectively). Therefore, this work indicates that the fluorescence behavior of MH-1 for the detection of water based on the PET method with both fluorescence enhancement and attenuation systems exhibited a good linear response in the low water content region, indicating that BODIPY MH-1 with a PET structure can act as a dual function-based fluorescent sensor for trace amounts of water possessing fluorescence intensity and F F value as a function of detection of water, although the DL for water based on the PET characteristics of MH-1 is not superior to that based on the PET method in our previous work. 9  In conclusion, we revealed that BODIPY MH-1 with a PET structure can act as a dual function-based fluorescent sensor possessing fluorescence intensity and F F value as a function of detection of trace amounts of water. It was found that the fluorescence enhancement of MH-1 with increasing water content in a polar solvent can be attributed to the suppression of the PET process associated with the formation of fluorescent ionic species MH-1(H 2 O) by the addition of water molecules. In contrast, the fluorescence attenuation of MH-1 with increasing water content in a non-polar solvent can be attributed to the disruption of boron-nitrogen (B-N) interaction between BPin and a tertiary amino group in MH-1(BN) associated with the formation of MH-1(H 2 O) by the addition of water molecules. We propose that this organic fluorescent dye with a PET structure which is capable of efficiently forming the fluorescence ionic structure by the addition of water molecules could be a promising candidate as a fluorescent sensor for a trace amount of water based on the PET method. Consequently, this work provides new insight into the molecular design for the development of high-sensitivity and -accuracy optical sensors for water based on the PET method with both fluorescence enhancement and attenuation systems.
The absorption spectra were recorded using a Shimadzu UV-2910 spectrophotometer. The fluorescence spectra were recorded using a Hitachi F-4500 spectrophotometer. The addition of water to organic solvents containing MH-1 was made in terms of weight percent (wt%). The determination of water in 1,4-dioxane, THF, acetone and acetonitrile was done using a MKC-610 and MKA-610 Karl Fischer moisture titrator (Kyoto Electronics manufacturing Co., Ltd) based on Karl Fischer coulometric titration for below 1.0 wt% and volumetric titration for above 1.0 wt%, respectively. This work was supported by an Iketani Science and Technology Foundation's research grant.