Influence of Solvents and Halogenation on ESIPT of Benzimidazole Derivatives for Designing Turn-on Fluorescence Probes

This work reports a theoretical investigation of the solvent polarity as well as the halogenation of benzimidazole derivatives during excited state intramolecular proton transfer (ESIPT). It details how the environment and halogen substitution may contribute to the efficiency of ESIPT upon keto–enol tautomerism and exploits this effect to design fluorescence sensing. For this purpose, we first examine the conformational equilibrium of benzimidazole derivatives containing different halogen atoms, which results in intramolecular proton transfer, using density-functional theory (DFT) combined with the polarizable continuum model (PCM). Then we evaluate the fluorescence of the benzimidazole derivatives in different dielectric constants within time-dependent DFT (TD-DFT) approaches. Our results quantitatively allow the determination of large Stokes shifts in nonpolar solvents around 100 nm. These theoretical results are in agreement with experimental solvatochromism studies of benzimidazoles. The effect of halogenation, with fluorine, chlorine, and bromine, is less important than solvent polarization when ESIPT takes place. Thus, halogenation can be properly chosen depending on the interest of the synthesis of benzimidazole-based turn-on fluorescence in appropriate solvents.


INTRODUCTION
−8 In general, the IPT mechanism appears to be simple, although it cannot always be easily observed spectroscopically. 9However, IPT may occur in excited states (ES) leading to the so-called ESIPT process.This phenomenon naturally occurs in some aromatic molecules that exhibit structural flexibility to deliver a proton upon electronic excitation and is useful to design turn-on fluorescent probes; 10−18 for example, when involving molecules that contain an enolic (or phenolic) group able to form an intramolecular H-bond with a nearby heteroatom. 19,20−24 To understand external/internal influences on the ESIPT process, we first consider a molecule, capable of forming an intramolecular H-bond, in its singlet electronic ground state, S 0 , in a specific solvent at room temperature.Then, this molecule is excited, for example, to the electronic singlet state S 1 , which enhances the acidity of the proton involved in the intramolecular H-bond.This allows, after a vibrational relaxation to a lower vibrational state within the S 1 electronic state, for the process to be followed via fluorescence. 25,26sually, this process occurs on a time scale of femtoseconds 27,28 and its spectral band is quite sensitive to the environment (an external effect) 29−31 as well as to a substituent (an internal effect) in the chromophore.The analysis of such effects can help to design efficient ESIPT processes.
A current application of ESIPT in organic compounds considers molecules containing an imidazole group; 32 thanks to their superlative optical properties, giving rise to laser dyes. 33,34However, unlike the behavior of organic molecules in solution that provides a high fluorescence quantum yield, 35 in the solid state, the fluorescence weakens or even quenches, 36,37 limiting the applications of ESIPT in this condition.An important group of organic molecules with which we can investigate the effects of solvent, as well as substituents 38−41 on ESIPT, is the benzimidazole derivatives. 15,42,43,44Experimentally, it has been observed that a bathochromic shift occurs in the fluorescence band of certain benzimidazole derivatives with increasing solvent polarity.However, high-polarity solvents also can inhibit ESIPT.Furthermore, protic solvents can prevent intramolecular H-bond formation, avoiding ESIPT. 45n the present work, we have investigated the role of solvent polarity as well as halogenation on the ESIPT process of benzimidazole derivatives and their implications for designing optimized molecules for fluorescence sensing.We employ DFT combined with the solvent-implicit PCM scheme to simulate the different dielectric constants of a set of solvents and calculate their effect on the UV−vis absorption and emission spectra via TD-DFT approaches.First, we determine the optimized S 0 states of the benzimidazole derivatives, within DFT methods for different solvent models (varying the dielectric constant from n-hexane to water), considering their enolic forms.Second, we model the tautomerism processes in the excited S 1 states within TD-DFT.Finally, after vibrational relaxation, we examine ESIPT by simulating the photophysical cycle with fluorescence.

COMPUTATIONAL METHODS
We obtain the ground state optimized geometries of the benzimidazole derivatives I, II, and III (Figure 1) within DFT using the B3LYP 46−48 /6-31G(d,p) level of approximation, where X = H, F, Cl, and Br, which is a reasonable method for describing these sizes of systems. 15After performing the conformational analysis of the enolic forms in the different solvent environments (n-hexane, 1,4-dioxane, ethanol, acetonitrile, and water) within PCM, 49 we calculate the UV−vis absorption properties of the conformers with TD-DFT 50 using Coulomb-attenuating method CAM-B3LYP 51 /6-31G(d,p) in all these solvents.All solvents considered here exhibit increasing polarity indices that increase with their dielectric constants so that low (high) polarity coincides with their low (high) dielectric constant.The optimized structures were checked by performing frequency calculations within the harmonic approximation.We perform all calculations and analyses using the Gaussian 09 52 suite of programs.

Conformational Analysis of the Benzimidazole
Derivatives in the Ground State.In the following, we discuss the molecular properties and structural features for analyzing the occurrence of tautomeric equilibrium in the excited state for solvents with increasing polarity.Figure 1 displays the optimized geometries of benzimidazole derivative I−III in their S 0 states.After obtaining enol tautomer III, the possible keto tautomer is obtained in its S 1 excited state in different solvents.We will discuss the excited state structures later in this paper.The atom indicated by X represents hydrogen or halogens (F, Cl, or Br) and we provide the Cartesian coordinates for each structure in the Supporting Information.The relaxation process in S 0 consists of two steps: (a) formation of an intramolecular H-bond, O−H•••N, by rotating the O−H group (cf.indicated motion in Figure 1a); and (b) rotation of the perpendicular aromatic ring to the chromophore plane (cf.indicated motion in Figure 1b).The relaxed dihedral angles ϕ and ψ after (a) and (b), illustrated in Figure 2, are reported in Table 1, together with other structural parameters for the different substituents and solvents.
As summarized in Table 1, these geometric parameters are not very sensitive either to the type of halogen or to the type of solvent.In general, the H 1 −N 1 distance in III is around 1.73 Å in low-polarity solvents and 1.72 Å in high-polarity solvents, considering X = H, Cl, and Br, with the exception of X = F that maintains the H 1 −N 1 distance around 1.73 Å in all considered solvents.Similarly, the equilibrium dihedral angle (ψ) in III is almost constant under these conditions.The great changes in the structures of these conformers, however, are observed in the dipole moments of III and dipole moment variations  between structures II and III.As reported in Table 1, there is a small variation Δμ for X = H of 1.00 D in n-hexane to 1.22 D in water.Interestingly, this variation becomes maximal for X = Cl (Br), that is, Δμ = 3.21 D (Δμ = 3.06 D) in n-hexane and Δμ = 4.23 D (Δμ = 4.09 D) in water.These changes indicate that a relatively higher dipole moment variation, as a function of the dielectric constant, prevents ESIPT, since III is the conformer in which IPT may occur.The relative orientation of the dipole moments of II and III is illustrated in Figure 3, which are calculated as 3.37 and 6.43 D, respectively, for X =  Another property that may be related to ESIPT is the shift in the vibrational stretching mode of the O−H group forming a H-bond with N at the imidazole ring (cf. Figure 2).In order to have a first estimate of the medium impact on this mode, we report in Table 2 the calculated redshift associated with the vibrational stretching upon transferring the molecules to a specific solvent (i.e., from vacuum to a specific dielectric constant), considering the different types of halogens.Considering the purely hydrogenated systems, the redshift increases toward higher polarity.For example, we determine an amount of 18.3 cm −1 in n-hexane to 43.7 cm −1 in water in this derivative with X = H.At low polarity, these redshifts slightly increase in X = H and Br, being maximal in X = Cl with 27.4 cm −1 , from vacuum to 1,4-dioxane.From moderate to high polarity, these redshifts increase in X = H and Br, reaching the maximum value of 58.9 cm −1 for X = Br in water.
To evaluate the energetics of conformers I, II, and III, we have considered steps (a) and (b) described in Figure 1.As  reported in Table 3, the H-bond formation leading to conformer II accounts for a great amount of energy (∼10 kcal/mol) in low-polarity solvents.This relative energy decreases to ∼7 kcal/mol from ethanol to water, also indicating that protic solvents could compete with the intramolecular H-bond.Consequently, the ESIPT could be hampered by a high-polarity medium.In addition, in lowpolarity solvents, we find that the relative energy between structures III and II is higher for X = H and F (0.6 and 0.5 kcal/mol, respectively) and tends to decrease for X = Cl and Br (about 0.2 kcal/mol).On the other hand, this difference is lower (0.2−0.4 kcal/mol) in more polar solvents, independently of the type of the substituted halogen.

UV Absorption Bands of the Enol Conformers from Vertical
Excitations.First, we consider the vertical excitation from S 0 to S 1 for the three conformers displayed in Figure 1, using the B3LYP/6-31G(d,p) scheme within TD-DFT.In Table 4, we report the absorption wavelengths (λ) and oscillator strength (f) for these structures with X = H, F, Cl, and Br in different solvents.From structure I, we examine the effect of H-bonding during the photoabsorption in structure II.For example, before the intramolecular H-bond formation, the absorption of I takes place in the limit between the UVC and UVB regions (∼280 nm).For X = H, the absorption band varies from 277.9 nm (in n-hexane) to 280.8 nm (in water), increasing by ∼10 nm for X = F. Upon halogenation with X = Cl or Br, the absorption band becomes constant at ∼288 nm for all solvents.There is, however, a large redshift in the absorption band upon H-bond formation in II.This occurs now in the UVA region (above 315 nm), with the values being more sensitive to the solvent polarity.For example, considering X = F, the absorption band reads 324− 325 nm in low polarity solvents and 329−330 nm in solvents with higher polarities.
Upon dihedral relaxation leading to halogenated structures III, which correspond to an experimental structure in the case of X = Br 45 and are candidates that exhibit ESIPT, the UV absorption trend remains in the same region, although reading ∼2 nm above the absorption profile of structures II.As expected at this level of approximation, because of the lack of Coulomb attenuation during the electronic excitation calculations, the absorption is overestimated, mainly in higher dielectric constants.This result, however, does not invalidate the preliminary analysis of the role of the solvent and impact of halogenation on these benzimidazole derivatives.Moreover, as we discuss in the following, the inclusion of Coulomb attenuation into B3LYP can resolve this issue.
In Figure 4, we display the separate impact of both substituent and solvent polarity for structure III.Now, it is clear to notice a significant redshift (max 12.7 nm for X = F from the hydrogenated structure) upon halogenation of the benzimidazole derivatives, as indicated in the left panel for the  structures in n-hexane.In the right panel, the solvent effect is also clearly observed with an appreciable redshift to highpolarity solvents (3.8 nm for X = Br from n-hexane to water).Furthermore, we notice an increase in the absorption bands for this structure in solvents with high dielectric constants, as indicated by the oscillator strengths in Table 4.These are around 0.4 in low polarity and 0.6 in high polarity for X = Br.In turn, the absorption intensities of the purely hydrogenated benzimidazole derivative are higher (f = 0.5−0.7)than the halogenated species.As a complement to this analysis, we display the frontier molecular orbitals involved in the vertical excitations of III (with X = Br) from S 0 to S 1 in Figure 5.As displayed within TD-DFT, the HOMO and LUMO are delocalized on the chromophore, exhibiting πand π*-type characters for all benzimidazole derivatives.We notice that the halogen atom at the chromophore contributes to the HOMO but is completely devoid of charge in the LUMO density.Additionally, we report the Mulliken charges in the O−H•••N bond in both ground and excited states to estimate the charge transfer (CT) during the π → π* excitation (cf.Table S2).Atomic charges are not true observables, and the results used here, of course, are for qualitative analysis and trends.We notice, in all cases, the same CT trend from S 0 to S 1 , where O 1 loses and N 1 gains electron charge, even in solvents with low polarities, such as n-hexane and 1,4-dioxane.
At this point, it is important to mention that the results obtained for the electronic excitations can be improved by using the CAM-B3LYP/6-31G(d,p) level of approximation.Indeed, according to this calculation, the UV bands are in better agreement with the available experimental data. 42For the case of X = Br in n-hexane, the experimental value is 296.4 nm, whereas the theoretical prediction using CAM-B3LYP is 296.3 nm.A similar agreement is obtained in 1,4-dioxane, for which the experimental band reads 295.6 nm, and the calculated value is 296.7 nm.The theoretical result is somewhat less accurate in the case of more polar solvents, as noticed for the case of acetonitrile.Now, the experimental band is 288.5 nm, whereas the theoretical value is 299.6 nm.From these new results, we proceeded to investigate the ESIPT of these systems using CAM-B3LYP.
3.3.Turning on the ESIPT Process from III during the Keto−Enol Tautomerism.To simulate ESIPT, we consider the excited states S 1 of enol tautomers III (X = H, F, Cl, and Br) using the CAM-B3LYP scheme in the different solvents, leading to structure IV.In Figure 6, we illustrate a case where the IPT takes place, resulting in the keto tautomer IV in S 1 (K*).In Table 5, we summarize all relaxed structures to IV, indicating the structures in which the keto−enol tautomerism actually takes place by the changes in the O 1 −H 1 and H 1 −N 1 distances.Additionally, we report the dipole moment variation in the excited states.Now, it is clear that IPT does occur only in low-polarity solvents, such as n-hexane and 1,4-dioxane, which is in agreement with experimental results for correlated benzimidazole derivatives. 45n high-polarity environments, such as ethanol and acetonitrile, we notice only a small increase in the O 1 −H 1 distance (∼0.04 Å) and a large decrease in the H 1 −N 1 distance (∼0.17 Å) from III to IV.Indeed, considering the high-polarity solvents, both structures III and IV remain as enol tautomers upon relaxation of their excited states (cf. Figure 7).The variations in these distances, however, reflect the charge redistribution in the excited states.From this analysis, the solvent with high dielectric constants do not favor ESIPT, which is in line with available experimental data for benzimidazole derivatives. 45ased on this investigation about the environment and substituent effects, it is now possible to investigate the fluorescence process for the benzimidazole derivatives that exhibit ESIPT.Thus, we consider (i) a vertical excitation from III (S 0 ) → III (S 1 ), (ii) the relaxation in the excited state from III (S 1 ) → IV (S 1 ), and (iii) IV (S 1 ) → V (S 0 ).We illustrate the general scheme in Figure 7 for the case where X = Br in nhexane.The calculated emission bands and the corresponding Stokes shifts, that is, the difference between the absorbed and emitted wavelengths of a fluorescent molecule, are reported in Table 6.The smallest emission is calculated for the benzimidazole derivative with X = H (∼387.3 nm), and largest emission occurs for X = F, around 402.0 nm.For X = Cl and Br, the emission bands are 393.1 and 393.2 nm, respectively.
Considering that ESIPT is favored in low polarities, the obtained values for the fluorescence and Stokes shift,  calculated using CAM-B3LYP, were found to be similar in nhexane and 1,4-dioxane.For instance, in the case of X = Br, in n-Hexane, we observe emission at 393.2 nm with a Stokes shift of 96.9 nm, while in 1,4-dioxane, the corresponding values were 392.9 and 96.2 nm, respectively.These results suggest that in cases where ESIPT occurs both fluorescence and Stokes shift appear to be less sensitive to variations of solvent polarity.On the other hand, these properties appear to be more sensitive to the substituent.For instance, when X = F, in nhexane, the calculated fluorescence and Stokes shift yields the highest values, that is, 402.0 and 102.8 nm, respectively.Considering practical applications, the choice of a suitable substituent may contribute to the reduction of self-absorption.Furthermore, an interesting approach to further investigate the ESIPT in benzimidazole derivatives is by using infrared spectroscopy (IR).We can note that the N−H stretching of tautomer V is absent in III, as realized in Figure 8.
As indicated in Table 2, the O−H stretching vibrational frequencies of conformer III decrease with increasing solvent polarity.On the contrary, considering that the tautomer V exists only in low polarity, the N−H vibrational mode is less sensitive to the medium effect.For instance, the purely hydrogenated system reads values between 3672 and 3674 cm −1 in low polarity for this vibrational mode.This is illustrated in Figure 8. Table 7 exhibits the calculated values for the N−H stretching mode and the corresponding differences between this mode and the O−H•••N stretching mode of tautomer III for different substituents in low-polarity solvents.
We notice now that the N−H stretching modes appear to be more sensitive to halogenation than the medium.For instance, in n-hexane, it reads 3673.8 and 3670.1 cm −1 for X = F and Br, respectively.On the other hand, their differences with respect to the O−H•••N stretching of tautomer III are only slightly sensitive to halogenation.In the case of X = Br, the highest values are 452.2 and 454.8 cm −1 for n-hexane and 1,4-dioxane, respectively; while for the purely hydrogenated system, the corresponding values are 443.9 and 444.9 cm −1 .In this sense, this vibrational difference can be used to follow the ESIPT mechanism in benzimidazole derivatives.

CONCLUSIONS
We theoretically investigated the ESIPT process of some benzimidazole derivatives using TD-DFT approaches.Our examination focused on understanding the influence of solvent polarity and the substitution of different halogen atoms in facilitating ESIPT, with implications for the design of turn-on fluorescent probes for sensing.
Initially, we assessed the conformational equilibrium and the role of the dipole moment variation of benzimidazole derivatives in the ground state across different solvent polarities and different halogenations.Our findings highlight that slight variation in dipole moment, that is, around 1.0 D, in low polarity environments under different halogenations is sufficient to favor ESIPT.However, when the same magnitude of variation occurs in higher polarities, the process is not achieved.This dipole moment variation proves to be of relevance to ESIPT, particularly for enolic conformers, which are amenable to intermolecular proton transfer.
Another ground-state property influencing the ESIPT is the redshift of the O−H stretching band upon forming a hydrogen bond with the N atom of the imidazole ring.Our results also reveal a significant variation in the redshift from vacuum to high-polarity environments, suggesting a potential correlation with ESIPT.For example, in the case of Br − benzimidazole derivatives in low-polarity solvents, such as n-hexane and 1,4dioxane, the calculated redshifts of the OH stretching were of 22.6 and 27.0 cm −1 , respectively, which increased dramatically to 57.1 cm −1 in ethanol, indicating that higher-polarity solvents can inhibit ESIPT.
Regarding halogen substitution, its effect is found to exert a subtle influence on ESIPT, providing a valuable perspective in the synthesis of benzimidazole derivatives for turn-on fluorescence probes.These theoretical results are in line with spectral studies and solvatochromism of some benzimidazole derivatives and suggest that ESIPT is facilitated in very lowpolarity solvents such as n-hexane and 1,4-dioxane.Conversely, our study indicates that ESIPT should not take place in highpolarity environments, such as ethanol, acetonitrile, and water.In this sense, this theoretical study provides useful information for the synthesis of benzimidazole-based turn-on fluorescence probes.
Cartesian coordinates of the optimized structures at the B3LYP/6-31G(d,p) level of theory and Mulliken

Figure 1 .
Figure 1.Schematic view of the conformational equilibrium for the benzimidazole derivatives in the ground (S 0 ) state: (a) Conformer I, (b) Conformer II, and (c) Conformer III.The main motions are indicated with blue arrows.

Figure 3 .
Figure 3. Relative orientation of the dipole moments in enolic (E) conformers II (a) and III (b) in S 0 for X = Br in n-hexane.

Figure 7 .
Figure 7. Photophysical cycle for the ESIPT process of the benzimidazole derivatives.

Figure 8 .
Figure 8. Calculated infrared spectra of III and V in the stretching region for X = H in n-hexane.Lines in blue and red refer to III and V tautomers in S 0 , respectively.

Table 1 .
Calculated O 1 −H 1 and H 1 −N 1 Bond Lengths (in Å), Dihedral Angles ϕ and ψ (in Degrees), Dipole Moments (μ), and Dipole Moment Variations (in D) for Structures II and III in S 0 at the B3LYP/6-31G(d,p) Level of Theory

Table 2 .
Calculated Shifts of the O−H Vibrational Stretching (in cm −1 ) of Conformer III, Related to the Corresponding Vacuum Equilibrium Structure at the B3LYP/6-31G(d,p) Level of Theory

Table 3 .
Calculated Energy Difference (in kcal/mol) for the O−H Bond Torsion (I → II) and Conformational Equilibrium (II → III) for Different Substituents and Solvents, at the B3LYP/6-31G(d,p) Level of Theory (cf.Figure1)

Table 4 .
Calculated Absorption Wavelengths (λ in nm) and Oscillator Strengths (f) of Conformers I, II, and III with X = H, F, Cl, and Br, in Different Solvents, with TD-B3LYP/6-31G(d,p)

Table 5 .
Calculated O 1 −H 1 and H 1 −N 1 Bond Lengths (in Å) and Dipole Moment Variation Δμ* (in D) for III in S 0 and Relaxed to IV in the S 1 Excited State.Bond Lengths Calculated Using B3LYP and Δμ* Calculated with CAM-B3LYP

Table 7 .
Calculated N−H Vibrational Stretching Modes (in cm −1 ) for Tautomer V in Different Solvents and Shifts (in cm −1 ) Related to O−H Stretching Vibrational Mode Using B3LYP/6-31G(d,p) analysis of atomic charges in the O1−H1••• N1 bond of conformers III in S0 and S1 states for different substituents and solvents (PDF) population