Fluorescence Behavior of Fluorenone Derivative in Chlorinated Hydrocarbons: Verification of Solvent Proticity via Fluorescence Spectroscopy

Abstract

In this study, the fluorescence (FL) behavior of a fluorenone derivative (FDMFA) in four chlorinated hydrocarbon solvents was investigated. While all four solvents have low polarities, their proticities are considerably different. Therefore, the FL properties of FDMFA could be considered to depend solely on the solvent’s proticity, with any polarity effects being insignificant. The hydrogen bond donor acidity was used as a measure of proticity, with higher values representing greater FL quenching due to vibronic coupling. The hydrogen bonding between FDMFA and the solvents could be thermodynamically controlled; thus, the FL emission was substantially enhanced during the heating process and quenched again during the cooling process. This change occurred reversibly and repeatedly. Because chlorinated hydrocarbon solvents are widely used for reaction and cleaning purposes in industrial applications, the findings of this study will be helpful in ensuring that such solvents are appropriately handled.

1. Introduction

Protic solvents are liquid compounds in which a hydrogen atom is directly attached to a highly electronegative element, such as nitrogen (N) or oxygen (O) [1]. Water and alcohols are typical examples of protic polar solvents. Halogenated hydrocarbons are also widely used as organic solvents for various purposes; however, unlike water and alcohols, they are not recognized as protic solvents [2,3,4,5]. This is because, since the molecular structure of these solvents is composed of an X–C–H sequence (where X is a halogen), the proticity of hydrogen atoms is relatively low. In fluorinated hydrocarbons, the 2nd-period halogen atom (fluorine, F) has very high electronegativity but very low polarizability. In brominated and iodinated hydrocarbons, the 4th-period (bromine, Br) and 5th-period (iodine, I) halogen atoms, with considerably large atomic radii, have minimal orbital overlap with the carbon atom. Accordingly, the reduced negative charge delocalization of their conjugate bases (C_nX_mH_2n+1−m⁻, where X is F/Br/I) leads to low proticity. In contrast, chlorinated hydrocarbons (C_nCl_mH_2n+2−m) are highly protic [6] and exhibit a strong inductive effect, although it depends on the number of chlorine atoms attached to carbon. This is attributed to the conjugate base (C_nCl_mH_2n+1−m⁻) becoming highly stable since the 2nd-period halogen atom (chlorine, Cl) is strongly electronegative and because its negative charge is effectively delocalized in the empty d orbital. Thus, chlorinated hydrocarbon solvents tend to possess partially positively charged hydrogen atoms, which results in their proticity being higher than that of other halogenated hydrocarbon solvents. Despite this unique physicochemical behavior, however, chlorinated hydrocarbons have received minimal research attention [7].

When using solvent compounds for reaction, degreasing, and cleaning purposes, their physicochemical properties must be clearly understood to avoid unexpected accidents, prevent environmental pollution, and ensure efficient recovery [8,9,10,11,12]. In organic chemistry, the proticity of solvents has not been discussed in as much depth as other properties, such as polarity and boiling point [13]. Specifically, proticity must be distinguished from polarity. While the dielectric constant quantifies the polarity of a compound, proticity can be measured in terms of the hydrogen bond donor acidity (HBDA) [14,15]. Since chlorinated hydrocarbons have a tetrahedral geometry, where the difference in electronegativity between the two atoms in the Cl–C bond is not substantial, dipole moments are often largely canceled out, according to the VSEPR theory [16,17,18]. Accordingly, when chlorinated hydrocarbons are used as solvents, the effect of their polarity on the solute may often be ignored. However, since chlorinated hydrocarbons can exhibit a fairly high HBDA depending on the number of chlorine atoms, the influence of their proticity cannot be ignored, even in a hydrophobic environment.

For the aforementioned reasons, we seek to determine whether the proticity of chlorinated hydrocarbon solvents would be reflected in a fluorescence (FL) change of a dye solute. Studies have revealed that fluorenone derivatives exhibit significant FL quenching due to vibronic coupling via intermolecular hydrogen bonding with alcohols and water as protic solvents (FDMFA in Figure 1) [19,20]. As mentioned earlier, HBDA, which can be considered a measure of solvent proticity, is directly related to FL quenching. However, since the FL behavior of FDMFA also depends heavily on the polarity of the protic solvent, determining the specific influence of proticity is challenging. As noted previously, however, with chlorinated hydrocarbons as solvents, the individual influence of proticity can be examined, without the need to consider polarity. Therefore, we investigated the FL behavior of FDMFA using chlorinated hydrocarbons as a matrix solvent and performed a thermodynamic FL analysis. Based on the results, the FL efficiency varied considerably with the HBDA of the solvent, the FL enhancement during heating and the FL quenching during the cooling occurred reversibly and repeatedly, and the extent of the FL change depended on the solvent’s HBDA. Chlorinated hydrocarbons are still widely used as solvents in fields such as cleaning, coating, paints, and chemical manufacturing [21,22]. By investigating the FL behavior of FDMFA, this study clearly revealed that these solvents are protic, depending on their molecular structure. This information may be of significant importance to engineers and workers in the industrial fields mentioned above. The subsequent sections describe the FL properties of FDMFA in chlorinated hydrocarbons, its relationship with solvent HBDA, and the thermodynamic considerations in temperature-variable FL analysis.

2. Materials and Methods

FDMFA was synthesized using a previously reported methodology [20]. Dichloromethane (DCM) and chloroform (CF) were purchased from Sigma-Aldrich (St. Louis, MO, USA), 1,1,2,2-tetrachloroethane (TCE) was purchased from Tokyo Chemical Industry (Tokyo, Japan), and 1,2-dichloroethane (DCE) and perchloroethylene (PCE) were purchased from Duksan (Gyeonggi-do, South Korea). All the solvents were stored in a dry state using anhydrous sodium sulfate until immediately before use.

Ultraviolet–visible (UV–Vis) spectroscopy was performed using a JASCO V-650 (Jasco, Tokyo, Japan). The FL spectra were obtained at an excitation wavelength of 350 nm using a JASCO FP-6500 spectrofluorometer (Jasco, Tokyo, Japan) equipped with a temperature controller (ETC-273, Jasco, Tokyo, Japan). The FL quantum efficiencies (FLQEs) were determined relative to that of a quinine sulfate solution in a 0.5 M H₂SO₄ at room temperature, assuming a quantum yield of 0.546 under excitation at 365 nm. Nuclear magnetic resonance (NMR) spectroscopy was performed using an Avance III 500 NMR spectrometer (Bruker, Billerica, MA, USA) with CDCl₃ and DMSO-d₆ as solvents. Fourier-transform infrared (FTIR) spectroscopy was performed using an FT-IR 4100 spectrometer (Jasco, Tokyo, Japan) equipped with an attenuated total reflectance (ATR PR0450-S, Jasco, Tokyo, Japan) accessory.

3. Results and Discussion

Among chlorinated hydrocarbon compounds, we selected DCM, CF, DCE, and TCE as solvents and investigated the FL behavior of FDMFA in these compounds (chemical structure in Figure 1). Specifically, we comparatively examined how the FL intensity and FLQE of FDMFA were affected by the HBDA of the four solvents. FDMFA solutions in all four solvents were prepared with the same concentration of 10⁻⁵ M. In the UV–Vis absorption spectra, FDMFA showed almost the same absorption bands, regardless of the solvent (UV–Vis spectra in Figure 2a). A large absorption band was observed at 350 nm, while a weak but broad absorption band appeared around 430 nm. The former is attributed to the π–π* transition in the fluorene moiety, while the latter is ascribed to the n–π* transition in the fluorenone moiety. The molar absorptivities (ε) at the maximum absorption wavelength (λ_{max, abs}) were almost the same in all solvents, approximately 4.3 × 10⁴ M⁻¹ cm⁻¹ (

Table 1. Properties of chlorinated hydrocarbon solvents used in this study and the spectral parameters of FDMFA in these solvents.

Solvent	αA	β	π*	μ	ε (104 M−1·cm−1)	λabs, max (nm)	λFL, max (nm)	FLQE (%)
PCE	–	0.00	0.28	0.00	4.55, 0.25	356, 428	492, 513	19.0
DCM	0.07	0.00	0.82	1.14	4.36, 0.08	354, 432	544	2.22
CF	0.15	0.00	0.58	1.15	4.37, 0.13	355, 433	547	1.61
DCE	0.03	0.00	0.81	1.83	4.38, 0.10	354, 430	540	2.17
TCE	0.14	0.00	0.95	1.31	4.30, 0.06	356, 438	550	1.24

–: Not available.

). This indicates that the solvent has minimal effect on the ground-state HOMO energy level of FDMFA. Separately, in the FL emission spectra, FDMFA showed almost the same maximum FL wavelength (λ_{max, FL}) of ~545 nm in all four solvents (FL emission spectra in Figure 2b). Accordingly, the same yellow FL color was observed in all solvents (photographs, inset of Figure 2b). However, the FL intensity of FDMFA varied considerably depending on the solvent: it was weaker in CF and TCE than in DCM and DCE. The FLQE displayed a similar tendency (Table 1): it was lower in CF (1.61%) and TCE (1.24%) than in DCM (2.22%) and DCE (2.17%). The fact that λ_{max, FL} was almost the same in all solvents is attributed to the solvents having similar polarities, which indicates that the solvent does not affect the charge transfer level significantly when FDMFA is in the excited state. By contrast, the differences in FLQE were evident, presumably due to the difference in proticity between the solvents. Specifically, a higher solvent proticity leads to stronger hydrogen bonding interactions between FDMFA and the solvent molecules, ensuring more prominent vibronic coupling. This causes an internal conversion in the electronic transition, leading to greater non-radiative emission decay.

As mentioned earlier, the proticity of a solvent can be quantified by its HBDA. Based on the method proposed by Abraham, HBDA can be calculated using Equation (1), with the chemical shift values obtained from ¹H NMR measurements in two solvents, DMSO-d₆ and CDCl₃ [23].

$${\alpha }_A=0.0066-0.128IS+0.133∆\delta$$

(1)

where IS is an indicator variable, which is 0 for all the chlorinated hydrocarbons, and Dδ is difference in chemical shift values between DMSO-d₆ and CDCl_3.

The HBDA value obtained using the Abraham equation is denoted as α_A herein. According to the literature [23], the α_A values for DCM, CF, and DCE are 0.07, 0.15, and 0.03, respectively (Table 1). However, since the HBDA of TCE is not reported in the literature, we measured the chemical shift values (Figure 3) and calculated α_A using the Abraham method and found it to be 0.14 (Table 1). Regarding the correlation between FL intensity and α_A, CF and TCE, which exhibited relatively low FL intensities, showed α_A values of 0.15 and 0.14, respectively, which far exceed those of DCM (0.07) and DCE (0.03). This suggests that CF and TCE interact much more strongly with FDMFA through hydrogen bonding, which results in more effective vibronic coupling. The indices of dipolarity/polarizability and hydrogen bond acceptor basicity correspond to the Kamlet–Taft parameters π* and β [14]. The β values of chlorinated hydrocarbons used in this study are all 0, and the π* value of CF is somewhat low at 0.58, while the other three solvents have values higher than 0.8 (Table 1). TCE (0.95) has a slightly higher π* value than DCM (0.82) or DCE (0.81), which results in a greater FL quenching effect. On the other hand, CF (0.58) still significantly quenches FL despite having a considerably lower π* value than DCM or DCE. Consequently, the π* value appears to have little to do with the FL behavior of FDMFA.

The intermolecular hydrogen bonds between the solvent and FDMFA can be broken through heating, which can also affect the vibronic coupling. To verify this assumption, among the four solvents, TCE (b.p. 146.7 °C) and DCE (b.p. 83.5 °C) were chosen since their boiling points are considerably higher than those of CF (b.p. 61.2 °C) and DCM (b.p. 39.6 °C), which allows them to be heated to higher temperatures. Additionally, TCE and DCE represent solvents with high and low HBDAs, respectively. The thermodynamic effects of these solvents were compared by performing a temperature-variable FL analysis of FDMFA (Figure 4). In both solvents, the FL intensity of FDMFA increased significantly during the heating process and returned to the original weak state during the cooling process. This is because the hydrogen bonds between the solvent and FDMFA were broken during heating, which disrupted the vibronic coupling; conversely, during cooling, the hydrogen bonds were regenerated, which restored the vibronic coupling. Fluorophores typically do not exhibit prominent FL enhancement up to temperatures as high as 80 °C and 100 °C, because they usually undergo molecular perturbation at high temperatures, which causes significant FL quenching. Notably, the FL enhancement ratio during the heating process was slightly lower in TCE than in DCE (Figure 5). This is likely because TCE, which has a higher HBDA than DCE, forms stronger hydrogen bonds with FDMFA, which are more difficult to break via heating. Consequently, this unusual heat-induced FL enhancement was caused by the disruption in hydrogen bonding between the chlorinated hydrocarbon solvents and FDMFA. This suggests that the FL intensity of FDMFA in chlorinated hydrocarbon solvents can be readily controlled using thermodynamic techniques.

PCE, which does not contain hydrogen, was used as a perchlorocarbon solvent, and its results were compared with those of the chlorinated hydrocarbon solvents described previously (chemical structure in Figure 6a). FDMFA was found to be highly soluble in PCE, as in the other solvents, and a solution of the same concentration (10⁻⁵ M) was prepared. The λ_{max, abs} and ε values in the UV–Vis absorption spectrum were almost the same as with the other solvents (Figure 6a). However, the FL spectrum highlighted a considerably different FL behavior (Figure 6b–d and Table 1). Specifically, we observed two λ_{max, FL} values, at 492 and 513 nm, which were significantly shifted toward shorter wavelengths compared with the other solvents. Accordingly, the PCE solution appeared to exhibit green FL to the naked eye. The reason why FDMFA in PCE exhibits the two FL bands with a frequency difference of 832 cm⁻¹ should be due to its vibronic structure. Since PCE has zero dipole moment, its polarity is extremely low. Thus, unlike chlorinated hydrocarbons, PCE cannot cause a new charge transfer level in excited-state FDMFA. This is deemed to be the reason behind the blue shift in FL. Similar solvatochromism has also been observed in other fluorenone derivatives [24,25]. Moreover, FDMFA demonstrated a high FL intensity (FLQE 19.0%) in PCE, which was nearly 10 times that in the chlorinated hydrocarbon solvents. This is because PCE possesses no proticity, i.e., it cannot participate in hydrogen bonding with FDMFA, which prevents effective vibronic coupling. Moreover, FTIR spectroscopy confirmed the absence of hydrogen bonding between FDMFA and PCE (Figure 7). In CF or TCE, the C=O stretching signal for FDMFA appears broad around 1715 cm⁻¹, whereas in PCE, it becomes much sharper. This indicates that FDMFA effectively hydrogen bonds with chlorinated hydrocarbons, but not with PCE.

4. Conclusions

In this study, we investigated the FL behavior of FDMFA using chlorinated hydrocarbons as solvents. While all four solvents used had similarly low polarities, their proticities were considerably different. Thus, the FL properties of FDMFA solutions in these solvents could be considered to depend solely on the proticity, with any effect of polarity being negligible. The HBDA values of the solvents were used as a measure of their proticity, with higher values corresponding to greater FL quenching due to vibronic coupling. The hydrogen bonds between FDMFA and the solvents were thermodynamically controllable; moreover, the FL was greatly enhanced during the heating process and quenched again during the cooling process, which is very rarely observed. Although the proticity of chlorinated hydrocarbon solvents has not been extensively studied in chemistry, it must be accounted for when using such solvents in industrial applications involving reaction, degreasing, and cleaning. For example, when chlorinated hydrocarbons are used as solvents in chemical reactions, the basicity of organic bases, such as amines, may be finely tuned by appropriately selecting and mixing the chlorinated hydrocarbons, thereby facilitating the proton transfer step [26,27,28]. In this respect, our study provides helpful practical insights.