Introduction

Conjugated π systems are not only used as core materials for optoelectronic devices, such as organic light-emitting diodes and organic photovoltaic solar cells, but are also being applied as fluorescent sensors that sensitively respond to external stimuli, such as temperature and chemicals [1,2,3,4]. It is very important to thoroughly understand their photophysical properties in developing them into advanced optoelectronic materials [5]. Fluorenone is particularly interesting to hold a potential application as a sensor due to its very unique fluorescence (FL) behavior [6,7,8,9]. Fluorenone exhibits significant FL quenching in alcohol, which can be explained as a phenomenon due to vibronic coupling caused by alcohol molecules [10, 11]. FL quenching appears very dramatically; therefore, it can be said to be an inherent photophysical property of fluorenone compared to other fluorophores. However, since fluorenone does not have a sufficiently extended conjugation length, the electron transition occurs only in ultraviolet region; thus, FL quenching cannot be recognized with the naked eye, which may be a drawback for sensor application.

This study attempted to address this issue and further explore new sensor applications. Organic compounds with an extended conjugated structure composed of aromatic and unsaturated hydrocarbon groups can exhibit FL emission in the visible region mainly due to π–π* electronic transition [12]. This study used the Sonogashira coupling reaction to synthesize a fluorenone derivative that can exhibit FL in the visible region by connecting fluorenone and fluorene groups to each other via acetylene linkage. FL behavior was investigated in various matrix solvents for this extended π-conjugation system. It was identified that due to intramolecular charge transfer and vibronic coupling, the fluorenone derivative exhibited not only a remarkable solvatochromic FL but also FL quenching, depending on physicochemical properties of the solvents, such as polarity and hydrogen bond donor (HBD) acidity. Based on such a solvent-dependent FL behavior, the fluorenone derivative responded to a very small amount of water in organic solvents, resulting in considerable FL quenching.

Material and Method

Materials

Fluorene, iodomethane, 2-methyl-3-butyn-2-ol, iodine, triphenylphosphine, bis(triphenylphosphine)palladium(II) dichloride, copper(I) iodide, and the organic solvents were obtained from Fujifilm Wako Pure Chemical Co., Ltd and were used as received without further purification. All the solvents were stored in a dry state using anhydrous sodium sulfate until immediately before use. These compounds were used to synthesize 2-ethyl-9,9-dimethylfluorene according to the literature [13]. 2-Bromo-9-fluorenone was obtained from Tokyo Chemical Industry Co., Ltd.

Synthesis of 1-(Fluorenone-2-yl)-2-(9,9-dimethylfluorene-2-yl)Acetylene (FDMFA)

A 500-mL round-bottom three-neck flask was equipped with a reflux condenser, a three-way stopcock, and a magnetic stirring bar and was flushed with dry nitrogen. 2-Bromo-9-fluorenone (11.7 g, 45 mmol), bis(triphenylphosphine)palladium dichloride (0.112 g, 0.16 mmol), cupper iodide (0.183 g, 0.96 mmol), triphenylphosphine (0.168 g, 0.64 mmol), and piperidine (250 mL) were placed in the flask. Subsequently, 2-ethynyl-9,9-dimethylfluorene (9.82 g, 45 mmol) in piperidine (50 mL) solution was added, and the reaction mixture was stirred at 90 °C for 24 h. After the piperidine in the reaction mixture was evaporated, ether (~ 500 mL) was added, and the insoluble salt was then filtered off. The filtrate was washed with 1 M hydrochloric acid and then with water. The solution was dried over anhydrous sodium sulfate and then concentrated at reduced pressure. Silica gel column chromatography (eluent: hexane/chloroform = 2:1) was performed to purify the crude product, obtaining the desired yellow solid product (9.32 g, 52% yield).

1H Nuclear magnetic resonance (NMR) (500 MHz, CDCl3, ppm): 7.81 (s, 1H, ArCH), 7.75–7.62 (m, 5 H, ArCH), 7.54–7.41 (m, 5 H, ArCH), 7.36–7.25 (m, 3 H, ArCH), 1.51 (s, 6 H, CH3). 13C NMR (125 MHz, CDCl3, ppm): 193.0, 153.9, 153.6, 143.9, 143.5, 139.7, 138.4, 137.5, 134.8, 134.3, 134.2, 130.8, 129.3, 127.8, 127.2, 127.1, 125.9, 124.4, 124.3, 122.6, 121.1, 120.5, 120.31, 120.27, 120.0, 92.0, 88.7, 46.9, 27.0. HR–MS (DART): for C30H21O (M + H)+ (calc.: 397.1592; found: 397.1582).

Measurements

NMR spectra were recorded on ECX-500 (JEOL, Japan) spectrometer at room temperature with CDCl3 as the solvent. Mass spectra were recorded on an AccuTOF-DART (JEOL, Japan) mass spectrometer, with poly(ethylene glycol) (MW: 400) used for the calibration. Ultraviolet–visible (UV–Vis) absorption spectra were recorded on a JASCO V-650 spectrophotometer at room temperature, and the FL spectroscopy was performed on a JASCO FP-6500 spectrofluorometer at an excitation wavelength of 350 nm. The FL quantum efficiencies (FLQEs) of FDMFA in various solvents were determined relative to a quinine sulfate solution in a 0.5-M H2SO4 at room temperature, assuming a quantum yield of 0.546 when excited at 365 nm.

Test for Detection of Alcohols and Water in THF

FL emission spectra were measured upon adding alcohol and water as an analyte to the THF solution of FDMFA.

The Stern–Volmer constant (Ksv) was determined by plotting the FL intensity change as a function of the molar concentration of the analyte according to the following equation [14].

$$\frac{{F}_{0}}{F}=1+{K}_{SV}\left[Q\right]$$
(1)

where F0 and F are the initial FL intensity without the analyte and the FL intensity with the analyte, respectively, and [Q] is the analyte molar concentration.

Similarly, limit of detection (LOD) and limit of quantitation (LOQ) were determined by plotting the FL intensity change as a function of the weight concentration of the analyte according to the following equations [15]:

$$LOD=\frac{3.3\sigma }{S}$$
(2)
$$LOQ=\frac{10\sigma }{S}$$
(3)

where σ and S are the standard deviation of y-intercept and slope of the calibration curve, respectively.

Fig. 1
figure 1

Synthetic route of FDMFA.

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Fig. 2
figure 2

(a) UV–Vis absorption and (b) FL emission spectra of 1.0 × 10− 5 M FDMFA solution in various solvents when excited at 350 nm

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Results and Discussion

To impart an extended conjugated structure to fluorenone, a derivative, in which the fluorene group is connected via acetylene linkage, was synthesized through the Sonogashira coupling reaction (FDMFA in Fig. 1). The chemical structure was identified by 1H and 13C NMR spectra (Fig. S1). UV–Vis absorption and FL emission spectra of this compound were measured in various solvents (Fig. 2). FDMFA showed a maximum absorption at around 350 nm and a relatively weak and broad absorption band around 425 nm (Fig. 2a). Table 1 summarizes the spectroscopic results. These two characteristic bands are due to π–π* and n–π* electronic transitions, respectively. In particular, in nonpolar hydrocarbon (hexane), aprotic polar (acetone), and protic polar (methanol) solvents, the absorption band appeared at a relatively shorter wavelength and shifted to a longer wavelength in aprotic polar solvents (DMF, NMP) and nonpolar aromatic solvent (toluene). This indicates that the interaction with the matrix molecules diversely affects the electronic energy level of FDMFA in the ground state.

On the other hand, FDMFA showed an FL behavior that highly depends on the polarity (dipole moment, dielectric constant) of the matrix solvent (Fig. 2b, Table 1). This compound showed maximum FL intensity at 474 nm in hexane, a nonpolar aliphatic hydrocarbon compound. However, as the polarity of the solvent increased, the compound underwent a large bathochromic shift to show maximum FL at 518 and 539 nm in THF and in DMF, respectively. Consistent with this spectral result, FDMFA presented as sky blue, green, and yellow in hexane, THF, and DMF, respectively (inset of Fig. 2b). Simultaneously with the red shift, FL intensity decreased significantly, corresponding to a typical solvatochromic FL, which indicates the occurrence of an intramolecular charge transfer in the excited state. This should be due to a donor–π–acceptor (D–π–A) electronic structure for the following reasons. Since FDMFA is all composed of sp2 and sp hybridized carbons, coplanarity is very high for the molecular geometry; therefore, the π electrons are estimated to be highly delocalized throughout the overlapped p orbitals in the molecule [12]. Accordingly, the fluorenone moiety can act as an electron acceptor (A) because the carbonyl group serves as a strong electron-withdrawing group, while the π electron-rich fluorene moiety acts as the electron donor (D) (Fig. 3a) [16]. Notably, in methanol and chloroform, the FL of FDMFA seemed to be greatly affected by another factor in addition to charge transfer. FDMFA showed exceptionally large bathochromic shift and FL quenching in these two solvents even though these solvents are relatively less polar than DMF. This was quite different from the typical FL behavior due to intramolecular charge transfer. FL quenching in methanol occurred critically, so the FLQE was close to zero, and it was difficult to identify the maximum FL wavelength from the very weak band. This should be due to the vibronic coupling caused by methanol molecules even in the present fluorenone derivative with an extended conjugated structure [6,7,8,9]. Similarly, FDMFA showed a very large bathochromic shift even in chloroform, displaying a maximum FL at 547 nm, which was shifted to a longer wavelength by 8 nm compared to that in DMF with higher polarity. FL was significantly attenuated even in chloroform. Moreover, the FLQE was determined to be 1.6%, corresponding to only 1/8–1/10 of the quantum efficiencies in hexane, toluene, and THF and about 1/3–1/4 of quantum efficiencies in NMP and DMF. Accordingly, chloroform can also be said to cause vibronic coupling to FDMFA, resulting in significant FL quenching. Even when the excitation wavelength was changed in each solvent, only the FL intensity of FDMFA changed, but the maximum FL wavelength and band shape did not change (Fig. S2). This indicates a single excited species. Moreover, similar solvent dependence as above was shown at other excitation wavelengths (Fig. S3). The succeeding description thoroughly discusses the intramolecular charge transfer and vibronic coupling phenomena on the FL solvatochromism and quenching of FDMFA with the Jablonski energy diagram.

Table 1 Properties of solvents used in this study and the spectral results of FDMFA measured in these solvents
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Fig. 3
figure 3

(a) Schematic illustration (↑ and ↓ indicate “increase” and “decrease,” respectively) and (b) Jablonski energy diagram on electronic transition change by interaction between FDMFA and solvent

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The unique FL behavior of FDMFA based on “intramolecular charge transfer” and “vibronic coupling” phenomena is highly related to non-covalent interactions with matrix solvent molecules. That is, the higher the polarity of the matrix solvent, the greater the polarization of FDMFA, resulting in a lower charge transfer level (CT1) in the excited state, as shown in Fig. 3. Consequently, a significant FL redshift appears along with the matrix solvent polarity. In particular, methanol causes intramolecular charge transfer of FDMFA and simultaneously generates internal conversion in the electronic transition through vibronic coupling. This leads to significant non-radiative decay in addition to FL redshift. The carbonyl group of fluorenone moiety has a large dipole moment and a partially negative charge on the oxygen atom [17]. Therefore, FDMFA can intrinsically act as a hydrogen bond acceptor in hydrogen bonding with matrix molecules (Fig. 3a) [18, 19]. During this time, the matrix molecule is expected to act as an HBD. For this reason, the hydrogen bond donor acidity (HBDA) of solvent molecules will affect the vibronic coupling caused by the intermolecular interaction. That is, the higher this value, the more pronounced the FL quenching. As mentioned previously, the FLQE reached almost zero in a matrix with a very high HBDA, such as alcohol (0.93 for methanol in Table 1). On the other hand, in hydrocarbon (hexane, toluene), ether (THF), and amide (NMP, DMF) solvents, which all have zero HBDA value, solvatochromism appears more dominant than FL quenching. Therefore, it can be said that in these nonalcoholic solvents, the intramolecular charge transfer has a more dominant effect on the FL behavior of FDMFA. It should also be noted that although chloroform is a nonalcoholic solvent, the FL quenching was exceptionally remarkable. This can be attributed to chloroform’s much higher HBDA value (0.44 for CHCl3) relative to other nonalcoholic solvents. This high HBDA comes from an induction effect, which is due to the high electronegativity of chlorine atom.

Fig. 4
figure 4

FL emission spectra upon addition of (a) methanol and (b) water to a solution of 1.0 × 10− 5 M FDMFA in THF (excited at 350 nm, adding 2 µL each up to 30 µL)

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Table 2 Test results for the detection of alcohols and water in THF using FL quenching of FDMFA
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FDMFA has an extended D–π–A conjugated structure composed of fluorenone acceptor, acetylene linkage, and fluorene donor. FDMFA exhibits FL in the visible region and is readily dissolved in common organic solvents due to the two methyl groups attached to the fluorene moiety. Therefore, the FL variation caused by the non-covalent interaction with the matrix molecules can be easily recognized with the naked eye. In this respect, FDMFA holds potential as a probe material. To demonstrate this, the FL spectral change according to the methanol content in THF was investigated (Fig. 4a). As the methanol content increased, the FL intensity decreased as expected. Moreover, it was possible to detect methanol in THF at several mg/mL levels. Other alcohols were also examined for the same test (Fig. S4). Similar FL quenching was observed in all the alcohols, although there was no tendency along with the alkyl length of the alcohol molecule. The Stern–Volmer constants were all quite high (> 9.9 mol− 1∙L) (Table 2, Fig. S5). This indicates that the vibronic coupling due to hydrogen bonding between FDMFA and alcohol occurs regardless of the size of the alcohol molecule. The LOD and LOQ were quite low (0.40 and 1.23 for methanol, 0.32 and 0.97 for ethanol, 0.54 and 1.64 for n-propanol, and 0.60 and 1.81 for n-butanol, respectively) (Table 2, Fig. S6).

Organic solvents often absorb moisture from the air during manufacturing and used to be contaminated. Accordingly, water in organic solvents acts as an impurity in most chemical reactions, causing a decrease in yield and purity of the product. Fluorescent sensor materials that can detect moisture in organic solvents with a high sensitivity have recently been developed [20,21,22,23,24]. It is expected that water, like alcohols, can act as HBD and cause FL quenching. Therefore, FDMFA was also evaluated for its ability to detect water in organic solvents. As in the case of alcohol, significant FL quenching was observed as the water content in THF increased (Fig. 4b). Since water is a non-solvent for FDMFA, it might cause aggregation when present in a high content. In spectroscopy, an isosbestic point is a specific wavelength at which the total absorbance of a sample does not change during a chemical reaction or a physical change of the sample. In our case, if aggregation occurs during the process of adding water, the total absorbance of FDMFA and its complex (FDMFA-water) will change and the isosbestic point will not appear. However, isosbestic point was observed at 422 nm in the absorption spectra even when the water content increased. This indicates that aggregation did not occur at several mg/mL levels examined in this study (Fig. S7). Therefore, FL quenching is not due to the formation of excimers by aggregation but is due to vibronic coupling by water molecules. This idea is supported by the fact that water has an extremely high HBDA of 1.17 [25,26,27]. FDMFA still has a quite high Stern–Volmer constant of 7.74 for the detection of water in THF, despite being slightly lower than those for the detection of alcohols (Table 2, Fig. S5). The LOD and LOQ were also quite low, which are determined to be 0.43 and 1.30, respectively (Table 2, Fig. S6). This indicates the possibility of use as an FL sensor to detect water in various organic solvents.

Conclusion

The Sonogashira coupling reaction was used to synthesize FDMFA that is composed of fluorenone and fluorene, which serve as electron acceptor and electron donor, respectively, and are connected via acetylene linkage. This compound exhibited a variety of visible light FL colors in matrix solvents. The solvatochromic FL was thought to result from the intramolecular charge transfer in an excited state based on the D–π–A electronic structure. In particular, FDMFA showed a remarkable FL quenching in alcohol and chloroform, which was thought to be due to the vibronic coupling caused by the solvent molecules through hydrogen bonding. In fact, these two solvents have quite high HBDA values compared to other solvents used in this study. This meant that the two solvents greatly caused internal conversion via vibronic coupling, leading to non-radiative decay. FDMFA also responded to a small amount of water at several mg/mL levels in organic solvents, resulting in FL quenching. Consequently, the present fluorenone derivative can be used as a fluorescent probe sensor capable of detecting water in various organic solvents.