1. Introduction {#sec1}
===============

Excited-state proton transfer (ESPT) processes have been attracting extensive attention due to their high potential applications in photoelectronic materials, photostabilizers, and fluorescent probes.^[@ref1]−[@ref4]^ It is well established that the electron density redistribution for the proton donor and proton acceptor after photoexcitation provides a driving force for ESPT.^[@ref5],[@ref6]^ The feasibility of ESPT is related to the strength of the hydrogen bond, that is, the stronger the hydrogen bond, the more likely the proton transfer to occur.^[@ref7]−[@ref9]^ Due to the large Stokes shift accompanied by the ESPT process, the molecules exhibiting intramolecular ESPT are widely used in fluorescence sensors and imaging agents. Especially, the near-infrared (NIR) fluorescent sensor is particularly significant in vitro and in vivo. Thus, proper design and development of NIR fluorescent sensors is a great challenge and very desirable.

10-Hydroxybenzo\[*h*\]quinoline (HBQ) is a typical molecule exhibiting excited-state intramolecular proton transfer.^[@ref10]−[@ref13]^ It gains particular attention for its potential applications as a probe in biological systems and organic light-emitting devices.^[@ref14]−[@ref16]^ So far, many efforts have been made on improving the photophysical properties and achieved several results. It has been well established both experimentally and theoretically that, in the ground state, only the normal isomer (Enol) exists, while after photoexcitation, ESPT occurs along the intramolecular hydrogen bond O--H···N, forming Keto, from where orange--red fluorescence is observed.^[@ref17]−[@ref20]^ The electronic properties, photophysical properties, and dynamics of proton transfer in HBQ could change when chemical modification is introduced. Recently, Antonov et al.^[@ref21]^ studied the substitution effect of an electron-withdrawing group (−NO~2~) at site 7 on the proton transfer process via experiments combined with theoretical calculations and revealed that the potential energy profiles along the proton transfer could be changed substantially from a single well to a double well. The Gryko group^[@ref22]^ studied the substitution effect of electron-rich and electron-poor groups on ESPT behavior in HBQ and showed that the near IR emission and large Stokes shift can be obtained through introducing an electron-donating group and electron-withdrawing group to the pyridine ring. Particularly, they observed the emission at 745 nm by introducing a CN group and through π expansion simultaneously, while the effect on photophysical properties and the proton transfer behavior via substitution to the phenol ring were rarely examined. Chen et al.^[@ref23]^ reported that photophysical properties of HBQ can be tuned from 600 to 655 nm by introducing an electron-withdrawing or -donating substituent at the phenol ring or through the elongation of the p-conjugation length. In particular, they pointed out that the electron-donating (HC=O) group and electron-withdrawing group (OH) could lead to a decrease and increase of the energy gap for emission. In their work, HC=O, CN, and OH substituent groups are considered, while other groups are not examined. So far, as we know, the systematic and comprehensive study of both electron-donating and electron-withdrawing groups at the phenol ring on the photophysical properties as well as proton transfer behavior is not reported.

To reveal whether the HBQ backbone could be used as an NIR fluorescent sensor, we further investigated the substituent effects of electron-donating groups (−NH~2~, −CH~3~, −C(CH~3~)~3~) and electron-withdrawing groups (−NO~2~, −CN, −F, −Cl, −CF~3~) at proton donor site 7 on the photophysics and the dynamics of proton transfer in HBQ. The corresponding fluorescence properties, as well as the environmental effect of polar and nonpolar solvents on the emission and intensity, are also explored in the HBQ-based chromophore. According to the calculated results, we successfully screened the 7NH~2~-HBQ derivative as a potential NIR fluorescent sensor. Next, we theoretically examined the capability of 7NH~2~-HBQ for F^--^ detection among anions (F^--^, Cl^--^, Br^--^), and our results showed that the 7NH~2~-HBQ sensor is very sensitive^[@ref24]^ and selective toward F^--^ based on the hydrogen bonding of F^--^ and O--H in the S~0~ state and F^--^ with N--H in the S~1~ state, leading to a ratiometric change in the fluorescence intensity. This result is very meaningful for rational design of fluorescent probes based on the HBQ molecule.

2. Results and Discussion {#sec2}
=========================

2.1. Optimized Geometric Structures in the S~0~ and S~1~ States {#sec2.1}
---------------------------------------------------------------

In this part, we study the substitution effect of different functional groups on the geometric structures of HBQ. The derivatives with electron-donating groups and -withdrawing groups on proton donor ring site 7 are referred to as 7D-HBQ and 7 W-HBQ, respectively. For a detailed functional group, D or W is replaced with the functional group in 7D-HBQ or 7 W-HBQ. For example, the derivative with the NH~2~ group introduced is referred to as 7NH~2~-HBQ. [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"} shows all the HBQ derivatives we considered in this work. The optimized ground-state equilibrium geometries were obtained at the B3LYP/6-31G(d) level. In the S~0~ state, only Enol can be located in HBQ and all HBQ derivatives except 7NO~2~-HBQ and 7CN-HBQ. We tried to optimize the Keto structures for these derivatives, but all attempted structures were optimized to Enol, which indicates that Keto in HBQ and the HBQ derivatives (except 7NO~2~-HBQ and 7CN-HBQ) is not a stationary point and the reverse proton transfer is very feasible. [Figure S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf) shows the optimized structures of HBQ and 7D(W)-HBQ derivatives in the S~0~ state where Enol and Keto are shortened as E and K, respectively. The optimized geometry of HBQ in the current computational level is very similar to the one calculated at the B3LYP/TZVP level in the previous study,^[@ref13]^ which indicates that our computational method is reliable. All the optimized geometries for HBQ and the derivatives belong to the *C~s~* symmetry, except 7NO~2~-HBQ where one of the O atoms in NO~2~ is out of the molecular plane. The geometric parameters of HBQ derivatives do not change much except for the hydrogen bond distance. The detailed hydrogen bond distances in the Enol (or Keto) structures for HBQ and HBQ derivatives in the S~0~ state are listed in [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}. The hydrogen bond distance is 1.716 Å in HBQ and ranges from 1.659 in 7NO~2~-HBQ to 1.725 Å in 7F-HBQ. We note that the smaller the distance of the hydrogen bond, the stronger the hydrogen bond, which could facilitate the proton transfer. For Keto, the hydrogen bond distances are 1.453 and 1.478 Å in 7NO~2~-HBQ and 7CN-HBQ, respectively. The equilibrium structures of Enol and Keto in the S~1~ state were determined by TD-B3LYP/6-31G(d), referred to as Enol~S~1~~ and Keto~S~1~~, respectively. [Figure S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf) shows the optimized structures of Enol~S~1~~ and Keto~S~1~~. Only the Enol~S~1~~ equilibrium structure was obtained in 7NH~2~-HBQ. [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"} also lists the detailed hydrogen bond distances in the Keto (or Enol) structures for HBQ and HBQ derivatives in the excited state. For the optimized Enol~S~1~~ structure in 7NH~2~-HBQ, the hydrogen bond distance is significantly decreased compared with that in Enol~S~0~~. The large decrease in the hydrogen bond indicates the enhancement in the strength of the hydrogen bond, which is expected to facilitate the proton transfer in the S~1~ state. We note that the optimized geometric parameters for Keto~S~1~~ in HBQ are very similar to the one in the previous study,^[@ref13]^ which again indicates that the method we used is suitable for this system. For example, the calculated hydrogen bond distance and OHN angle in Keto~S~1~~ are 1.721 Å and 141.68° in this study, respectively, and very consistent with previously calculated values of 1.72 Å and 141.2°.

![Scheme of HBQ derivatives.](ao-2019-006937_0008){#fig1}

###### Bond Distances of O--H and N--H in the Enol and Keto Structures for HBQ and HBQ Derivatives in the S~0~ and S~1~ States[a](#t1fn1){ref-type="table-fn"}

  ground state                                                                                   
  -------------- ------- ------- ------- ------- ------- ------- ------- ------- ------- ------- -------
  *r*~O--H~      0.997   0.995   0.997   0.999   1.005   1.002   0.996   0.998   1.000   1.478   1.453
  *r*~N--H~      1.716   1.719   1.708   1.680   1.659   1.693   1.725   1.699   1.687   1.101   1.105

  excited state                                                                           
  --------------- ------- ------- ------- ------- ------- ------- ------- ------- ------- -------
  *r*~O--H~       1.721   1.064   1.693   1.666   1.641   1.706   1.702   1.730   1.746   1.747
  *r*~N--H~       1.041   1.507   1.045   1.045   1.056   1.044   1.042   1.037   1.035   1.030

Bond lengths are in angstrom.

2.2. Calculated Absorption and Emission Spectra {#sec2.2}
-----------------------------------------------

We calculated the absorption and fluorescence spectra of HBQ and the derivatives shown in [Figures [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"} and [[3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}, respectively. The absorption spectra of Enol~S~0~~ are dominated by ππ\* transitions from HOMO to LUMO within the aromatic ring in the S~1~ state. The peak of the absorption spectra is 363 nm in HBQ in this work, in agreement with experimental absorption maximum at 370 nm^[@ref10]^ and also consistent with a previous study at 364 nm.^[@ref20]^ In HBQ derivatives, the peak of absorption spectra becomes blue or red shifted depending on the functional groups. In particular, we noticed that the absorption peak becomes largely red shifted toward the visible region in 7NH~2~-HBQ. The calculated emission peak for the Keto~S~1~~ is located at 623.0 nm in HBQ. The value is very consistent with the experimental value at 609 nm^[@ref10]^ and also in agreement with the previous theoretical study at 621 nm.^[@ref20]^ The emission peaks for Enol~S~1~~ and Keto~S~1~~ in 7NH~2~-HBQ were located at 566.6 and 739.5 nm in 7NH~2~-HBQ. We note that the emission peaks for the Keto~S~1~~ are significantly red shifted to the IR region, which deeply widens the spectra of HBQ. The emission peaks for Keto~S~1~~ range from 595.5 to 659 nm in other 7D(W)HBQ derivatives. Basically, the absorption and emission spectra are significantly expanded in HBQ derivatives compared with the ones in HBQ. The large Stokes shift is very meaningful in optical electronic materials and probes. Thus, we estimated the values of the Stokes shift in HBQ and HBQ derivatives. The values are very large in HBQ (259.6 nm) and HBQ derivatives, ranging from 237.5 to 313.0 nm. Combined with the large potential application of emission spectra in the near IR region, we point out that 7NH~2~-HBQ is a very promising material for this purpose. The solvent effect on the absorption and emission spectra was also studied for 7NH~2~-HBQ. The peaks of the absorption and emission do not change much, calculated to be 565.2 and 727.6 in THF and 561.2 and 728.39 nm in toluene, respectively.

![Calculated Enol absorption spectra of HBQ and HBQ derivatives at the (TD)-B3LYP/6-31G(d) level in the gas phase.](ao-2019-006937_0001){#fig2}

![Calculated Enol~S~1~~ and Keto~S~1~~ emission spectra of HBQ and HBQ derivatives at the (TD)-B3LYP/6-31G(d) level in the gas phase.](ao-2019-006937_0003){#fig3}

2.3. Molecular Orbital Analysis {#sec2.3}
-------------------------------

The calculated frontier molecular orbitals of Enol~S~0~~ in HBQ and HBQ derivatives are shown in [Figure S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been shown since these orbitals are involved in photoexcitation. In HBQ, HOMO and LUMO have a π (π\*) character, with electrons distributed on the proton donor ring, middle ring, and proton acceptor ring. In HBQ derivatives, due to the introduction of functional groups, electron distribution on HOMO and LUMO changes considerably. In 7D(W)HBQ derivatives, electron distribution on HOMO and LUMO is very similar to that in HBQ except that the HOMO orbital expands toward the functional group. Particularly, in 7NH~2~-HBQ, the HOMO to LUMO transition shows a significant charge transfer character from the proton donor ring to the proton acceptor ring. Besides, in all cases, after photoexcitation, the electron density becomes redistributed, shifted from the proton donor O atom in HOMO to the proton acceptor N atom in LUMO, providing a driving force for proton transfer from the Enol structure to Keto structure in the S~1~ state.

To further reveal the substitution effect on the electron density distribution in HBQ and HBQ derivatives, the Mulliken charge has been examined. We found that the charges for the atoms are slightly changed except for the atoms connecting with the functional group. Basically, the charges become less negative and more negative for the proton donor O and the proton acceptor N atoms, respectively, when 7NO~2~, 7CN, and 7CF~3~ functional groups are introduced, which may facilitate the proton transfer from the O to N atom. This result is consistent with the predicted trend for the strength of the hydrogen bond reflected from the hydrogen bond distance. On the other hand, the charges become more and less negative on the O and N atoms, respectively, in other derivatives exhibiting longer hydrogen bond distances.

2.4. Proton Transfer Reaction Path in the S~0~ and S~1~ States {#sec2.4}
--------------------------------------------------------------

We consider the substitution effect on the dynamics of proton transfer. [Figure S4](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf) shows the constrained minimum-energy paths for proton transfer in HBQ and several HBQ derivatives in the S~0~ state. In HBQ, the potential energy increases with the proton transfer from the O atom to the N atom, which indicates that the proton transfer is infeasible thermodynamically in the S~0~ state, and Keto could not be formed stably. In the derivatives, the potential energy curves are very similar to that in HBQ but lie slightly higher or lower than the one in HBQ. The higher the potential energy, the more difficult for proton transfer to occur. We point out that the potential energy profile of 7NH~2~-HBQ lie on the upper part of the figure and those of 7NO~2~-HBQ and 7CN-HBQ lie on the lower part of the figure. This result is very consistent with the strength of the hydrogen bond in the HBQ derivatives. In [Section [2.1](#sec2.1){ref-type="other"}](#sec2.1){ref-type="other"}, we located the stable structure Keto in derivatives 7NO~2~-HBQ and 7CN-HBQ. However, Keto would tautomerize to Enol through the reverse proton transfer because this process is both dynamically and thermodynamically feasible. Thus, Enol could be the only isomer detected in the S~0~ state in HBQ and all the derivatives.

[Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"} shows the potential energy curves along the proton transfer coordinate in the S~1~ state in HBQ and some typical derivatives. The energy of the Enol structure in the S~1~ state of HBQ (HBQ derivatives) is taken to be 0.0 kcal/mol as reference. In HBQ, after photoexcitation, the structure relaxes toward the proton-transferred structure Keto~S~1~~, which lies ∼12.7 kcal/mol lower than the Enol region. The proton transfer process in HBQ should be very fast because this process is a barrier-free process. This result is in agreement with a previous study with a larger basis set^[@ref8]^ and also consistent with the geometry optimization result discussed above. In 7CH~3~-HBQ, 7C(CH~3~)~3~-HBQ, 7NO~2~-HBQ, 7F-HBQ, 7Cl-HBQ, and 7CF~3~-HBQ derivatives, the potential energy curves are very similar to the one in HBQ, which suggests a single-well proton transfer process in these derivatives, with proton transfer in 7CF~3~-HBQ and 7CN-HBQ being the fastest followed by 7Cl-HBQ and other derivatives. However, in 7NH~2~-HBQ, the potential energy curves are slightly different; both Enol~S~1~~ and Keto~S~1~~ exist along the proton transfer exhibiting double wells. The energy barriers for the proton transfer processes are very small, estimated to be 0.24 kcal/mol in 7NH~2~-HBQ; thus, fast proton transfer processes are expected.

![Potential energy profiles of ESPT in HBQ and derivatives in the S~1~ state at the (TD)-B3LYP/6-31G(d) level in the gas phase.](ao-2019-006937_0002){#fig4}

Solvent effects play important roles not only in the dynamics for proton transfer but also in the stability order for isomers. In this part, we also investigate the solvent effect on the ESPT behavior in the 7NH~2~-HBQ derivative, which shows the potential uses for probes in biological systems. In nonpolar solvent toluene and polar solvent THF, Keto~S~1~~ becomes more stable than Enol~S~1~~ compared with the case in the gas phase. For example, the energy difference between Keto~S~1~~ and Enol~S~1~~ in 7NH~2~-HBQ increases from 3.8 in the gas phase to 5.39 in toluene and 6.64 kcal/mol in THF. In 7NH~2~-HBQ, the proton transfer becomes barrier-free or almost zero in both nonpolar and polar solvents. Thus, proton transfer becomes more favorable in solvents than in the gas phase.

Like HBQ, excited-state intramolecular proton transfer is very fast in all HBQ derivatives we investigated; thus, HBQ derivatives could be used for designing probes or organic optical materials based on the intramolecular proton transfer process. By introducing the functional group at the proton donor site, we obtained the emission spectra of the HBQ-based chromophore ranging from 567 to 739 nm in the gas phase. Through systematical investigation, we obtained promising candidates that exhibit distinctive emission spectra toward the near IR region. In 7NH~2~-HBQ, the peak of emission spectra is 739.5 nm in the gas phase and is not dependent on the solvent polarity, with peaks located at 727.6 and 728.4 nm in THF and toluene, respectively. Clearly, the emission spectra have been successfully expanded to the near IR region. Thus, we expect that 7NH~2~-HBQ is a potential candidate as a fluorescent probe in biological systems.

2.5. Sensitivity and Selectivity of 7NH~2~-HBQ Sensor to F Anion {#sec2.5}
----------------------------------------------------------------

In order to examine whether 7NH~2~-HBQ could be used as a fluorescent sensor, we theoretically investigate the fluorescence properties of 7NH~2~-HBQ for detecting anions, such as F^--^. The sensitivity and the selectivity toward F^--^ among anions (F^--^, Cl^--^, and Br^--^) are discussed based on the calculated results, and the sensing mechanism^[@ref24]^ is further included.

### 2.5.1. Sensitivity of 7NH~2~-HBQ toward F^--^ {#sec2.5.1}

[Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"} shows the optimized geometries of F-anion-attached complex 7NH~2~-HBQ and the deprotonated anionic form in the S~0~ state, referred to as FAC~S~0~~ and DA~S~0~~. Compared with 7NH~2~-HBQ, the intramolecular hydrogen bond O--H···N is disturbed upon addition of the F anion, and the H atom in the hydrogen bond is directly transferred from the O atom to the F atom, forming a new intermolecular hydrogen bond O···H--F. In FAC~S~0~~, bond distances of O···H--F and N--H···F are calculated to be 1.65 and 2.20 Å; clearly, F--H is bonded with the O atom. Meanwhile, the NH~2~ group is rotated with one of the N--H bonds located in the HBQ plane, and the other N--H bond is almost vertical to the HBQ plane. The structural parameters are basically not changed except for the bonds neighboring the new intermolecular hydrogen bond. We note that no stable tautomerization formed with F--H bonded to the N atom could be located. All attempted optimizations from the tautomer structures finally went to the FAC~S~0~~ structure, which indicates that FAC~S~0~~ is the only stable form in the S~0~ state. In DA~S~0~~, the structure parameters are basically similar to FAC~S~0~~. [Figure S5](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf) shows the optimized geometries for FAC~S~0~~ and DA~S~0~~ at the B3LYP/6-31+G(d) level. Including the diffuse function, the structural parameters for FAC~S~0~~ and DA~S~0~~ are rarely changed except the hydrogen bond distance. The bond distances of O···H--F and N--H···F are calculated to be 1.54 and 2.40 Å at this level.

![Minimum-energy geometries of 7NH~2~-HBQ upon addition of anions in the S~0~ state optimized at the B3LYP/6-31G(d) level in the gas phase: (a) FAC~S~0~~, (b) DA~S~0~~, (c) ClAC~S~0~~, and (d) BrAC~S~0~~. Bond lengths are in angstrom.](ao-2019-006937_0007){#fig5}

The corresponding structures in the S~1~ state, referred to as FAC~S~1~~ and DA~S~1~~, are shown in [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}. In FAC~S~1~~, the bond distances for F--H···N and F--H···O bonds are calculated to be 1.81 and 2.12 Å, respectively. The F--H bond is already transferred from the O atom to the N atom forming a F--H···N intermolecular hydrogen bond. Besides, the whole plane for the three six-membered rings is a little distorted, with C1--C2--C3--N1 calculated to be 7.4°. We note that the structure with F--H bonded to the O atom is not located, and the optimization directly went to the FAC~S~1~~ structure, which means that, once FAC~S~0~~ is excited, the F--H bond transfers from the O atom to the N atom, just like the proton transfer from O to N in an intramolecular hydrogen bond. In DA~S~1~~, the structure parameters are very similar to the ones in FAC~S~1~~, except that the plane becomes even more distorted, with C1--C2--C3--N1 calculated to be 13.7°. The electronic character for FAC~S~1~~ and DA~S~1~~ is very similar to the one in 7NH~2~-HBQ, showing a considerable charge transfer character from the phenol ring to the pyridine ring. Evidently, F is rarely involved in the electronic transition. [Figure S6](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf) shows the optimized geometries for FAC~S~1~~ and DA~S~1~~ at the (TD-)B3LYP/6-31+G(d) level. For FAC~S~1~~, the structural parameters are basically unchanged except the hydrogen bond distance. Besides, the rigid plane of HBQ becomes distorted with the phenol ring out of the plane of the other two rings. The optimized structure for DA~S~1~~ is very similar to the one without the diffuse function.

![Minimum-energy geometries of 7NH~2~-HBQ upon addition of F^--^ in the S~1~ state optimized at the (TD)-B3LYP/6-31G(d) level in the gas phase: (a) FAC~S~1~~ and (b) DA~S~1~~. Bond lengths are in angstrom.](ao-2019-006937_0006){#fig6}

The simulated absorption spectra for FAC~S~0~~ and DA~S~0~~, as well as the simulated emission spectra for FAC~S~1~~ and DA~S~1~~, are shown in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}a,b, respectively. The simulated absorption and emission spectra for HBQ and 7NH~2~-HBQ are also included for comparison. For FAC~S~0~~, the shape of absorption spectra is very similar to the one for 7NH~2~-HBQ but is widely expanded toward 600 nm. The absorption peaks for FAC~S~0~~ are located at 285, 404, and 506 nm, which correspond to the electron transition from HOMO-2 to LUMO (mainly local excitation character), HOMO to LUMO+1 (mainly charge transfer character), and HOMO to LUMO (mainly charge transfer character), respectively. Including the diffuse function, the absorption peaks are only slightly red shifted, calculated to be 292, 416, and 513 nm for FAC~S~0~~. The shape for DA~S~0~~ is basically similar to the one in FAC~S~0~~ toward the long wavenumber. The main absorption peaks appear at 319, 419, and 540 nm, which correspond to HOMO to LUMO+2 (mainly charge transfer character), HOMO to LUMO+1 (mainly charge transfer character combined with local excitation), and HOMO to LUMO (mainly charge transfer character) transition, respectively. The main absorption peaks are calculated to be 355, 435, and 554 nm for DA~S~0~~ when the diffuse function is included. Evidently, upon addition of F^--^ or forming the deprotonated form, the absorption spectra in FAC~S~0~~ or DA~S~0~~ are not only red shifted but also broadened. The involved frontier orbitals are shown in [Figure S7](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf). Compared with the absorption spectra for FAC~S~0~~ or DA~S~0~~, the emission spectra for FAC~S~1~~ and DA~S~1~~ are largely red shifted towards the IR region. The emission energy for FAC~S~1~~ (or DA~S~1~~) is calculated to be 1.42 (or 1.28) eV, which is smaller than the one for 7NH~2~-HBQ without including F^--^. The values are slightly changed considering the diffuse effect, calculated to be 1.28 and 1.31 eV at the (TD)-B3LYP/6-31+G(d) level for FAC~S~1~~ and DA~S~1~~, respectively. Except this, the fluorescent intensity is also changed upon addition of F^--^. Thus, 7NH~2~-HBQ is very sensitive toward F^--^ from both absorption and emission spectra; no matter, the final product is FAC~S~1~~ or DA~S~1~~. Considering the solvent effect in THF, the fluorescence intensity becomes slightly increased and the Stokes shift becomes a little smaller.

![(a) Calculated absorption spectra of FAC~S~0~~, DA~S~0~~, ClAC~S~0~~, and BrAC~S~0~~ at the (TD)-B3LYP/6-31G(d) level in the gas phase. (b) Calculated emission spectra of FAC~S~1~~ and DA~S~1~~ at the (TD)-B3LYP/6-31G(d) level in the gas phase.](ao-2019-006937_0005){#fig7}

### 2.5.2. Selectivity of Detection of F^--^ among F^--^, Cl^--^, and Br^--^ Anions {#sec2.5.2}

In this part, in order to examine whether 7NH~2~-HBQ is selective to F^--^, we investigated the equilibrium structures and absorption spectra for 7NH~2~-HBQ with Cl^--^ and Br^--^. The optimized structures for Cl^--^- and Br^--^-attached 7NH~2~-HBQ are shown in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}, referred to as ClAC~S~0~~ and BrAC~S~0~~, respectively. Structurally, ClAC~S~0~~ and BrAC~S~0~~ are very similar to each other. Unlike FAC~S~0~~, the intramolecular hydrogen bond is not disturbed in these complexes, and Cl^--^ (Br^--^) is located at least 2.5 Å away from the pyridine ring. The simulated absorption spectra for ClAC~S~0~~ and BrAC~S~0~~ are also shown in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}a. In contrast to the absorption spectra for FAC~S~0~~, there is no absorption for both ClAC~S~0~~ and BrAC~S~0~~ in the UV region, while very small absorption appears near 786.8 nm for ClAC~S~0~~ and 908.7 nm for BrAC~S~0~~, which corresponds to the charge transfer from Cl^--^ (Br^--^) to 7NH~2~-HBQ. We tried to optimize the equilibrium structure in the S~1~ state for the complexes ClAC~S~0~~ and BrAC~S~0~~, but all the optimizations lead to the dissociation for the complex, which might result from the charge transfer from Cl^--^ (Br^--^) to 7NH~2~-HBQ. Thus, 7NH~2~-HBQ is very sensitive to F^--^. All the above results lead us to deduce that 7NH~2~-HBQ is selective to F^--^ due to the hydrogen bond.

In summary, upon addition of F^--^, 7NH~2~-HBQ could interact with F^--^, forming a stable complex FAC~S~0~~, with F--H bonded with the O atom. The absorption spectrum for FAC~S~0~~ is widely expanded compared with the one for 7NH~2~-HBQ. Once it is excited, the F--H bond is transferred from the O atom to the N atom, forming the FAC~S~1~~ structure, from where the fluorescence of 875 nm could be observed. However, 7NH~2~-HBQ could only interact with Cl^--^ and Br^--^ weakly, forming ClAC~S~0~~ and BrAC~S~0~~ without disturbing the intramolecular hydrogen bond, respectively. The absorption spectra for ClAC~S~0~~ and BrAC~S~0~~ are very different from the one in FAC~S~0~~, which indicates that 7NH~2~-HBQ is sensitive and selective to F^--^.

3. Conclusions {#sec3}
==============

In this work, we have theoretically designed a ratiometric and NIR fluorescent sensor for F-anion detection based on the HBQ backbone with DFT and TDDFT methods. First, we systematically investigated the substitution effect of a series of electron-withdrawing groups (−NO~2~, −CN, −F, −Cl, −CF~3~) and electron-donating groups (−NH~2~, −CH~3~, −C(CH~3~)~3~) at the site 7 on the photophysical properties and proton transfer process in HBQ. We found that not only the proton transfer process but also the photophysical properties could be modulated via the introduced groups. It is revealed that proton transfer in the S~1~ state is very fast and favorable in HBQ and all HBQ derivatives due to the barrier-free or almost barrier-free process. Besides, the energy gap of HOMO and LUMO could either be increased or decreased, leading to the blue or red shift of the absorption and emission spectra. The emission spectra can be tuned from 567 to 739 nm in the derivatives. In particular, we found that 7NH~2~-HBQ has very small emission energy, which is a promising candidate for NIR fluorescent probes in biological systems.

Second, the 7NH~2~-HBQ sensor is used to detect F^--^ among anions (F^--^, Cl^--^, and Br^--^) through investigating the absorption and emission spectra before and after including the anions. We found that the 7NH~2~-HBQ sensor is very sensitive and selective to F^--^, showing a ratiometric change in the fluorescence intensity. This phenomenon could be explained that the intramolecular hydrogen bond N···O--H is disturbed upon addition of F^--^ due to forming complex FAC~S~0~~ with a new intermolecular hydrogen bond F--H···O. After excitation, the F--H bond is transferred from the O atom to the N atom, accompanied by the large Stokes shift.

4. Computational Details {#sec4}
========================

Equilibrium geometries of Enol with the H atom bonded to the O atom and Keto with the H atom bonded to the N atom in the S~0~ state were determined by the B3LYP^[@ref25]^ method, while geometries of minima in the S~1~ state were determined by the TDDFT method with the B3LYP (TD-B3LYP) functional. All minimums in the S~0~ and S~1~ states were confirmed by normal mode analysis at the B3LYP and TD-B3LYP levels, respectively. The constrained minimum-energy paths (cMEPs) for the proton transfer were determined by fixing the hydroxyl group (−OH) bond length in increments of 0.1 Å in both the S~0~ and S~1~ states. In this study, 6-31G(d) was used for the optimization in DFT and TDDFT calculations. This basis set has been proven to be reliable for this system. In detail, we have compared the geometries optimized by using the B3LYP/6-31G(d) basis set with the ones obtained with using the TZVP basis set in a previous study,^[@ref13]^ and we found that the geometries are very similar to each other. For example, the distance of the hydrogen bond and the angle of O--H--N are 1.716 Å and 148.2° at the current computational level, and the corresponding values at the TZVP level are 1.71 Å and 148.1°, respectively. Thus, 6-31G(d) is chosen for the S~0~ and S~1~ state calculations. Besides, the consistency of the calculated absorption (emission) energies with the available experimental values for the absorption (emission) peak for some HBQ derivatives again demonstrates that the current computational level is reliable for HBQ derivatives. In the case of the F-anion sensing process, 6-31+G(d) was used to examine the diffuse function effect on the geometry optimization and excitation energy calculations. All calculations were performed using Gaussian 09,^[@ref26]^ and the solvent effect was included using the IEF-PCM^[@ref27]^ model.

The Supporting Information is available free of charge on the [ACS Publications website](http://pubs.acs.org) at DOI: [10.1021/acsomega.9b00693](http://pubs.acs.org/doi/abs/10.1021/acsomega.9b00693).Minimum-energy geometries of HBQ and 7D(W)-HBQ derivatives in the S~0~ state optimized at the B3LYP/6-31G(d) level and minimum-energy geometries in HBQ and other 7D(W)-HBQ derivatives in the S~1~ state optimized at the TD-B3LYP/6-31G(d) level; the mainly involved frontier molecular orbitals of E~7NH~2~ -- HBQ~, E~7CH~3~ -- HBQ~, E~7C(CH~3~)~3~ -- HBQ~, E~7Cl-HBQ~, E~7CF~3~ -- HBQ~, E~7NO~2~ -- HBQ~, E~7CN-HBQ~, and E~7F-HBQ~ at the B3LYP/6-31G(d) level in the gas phase; potential energy profiles of proton transfer in HBQ and derivatives in the S~0~ state at B3LYP/6-31G(d) level in the gas phase; minimum-energy geometries for FAC~S~0~~ and DA~S~0~~ at the B3LYP/6-31+G(d) level; minimum-energy geometries for FAC~S~1~~ and DA~S~1~~ at the (TD)-B3LYP/6-31+G(d) level; the involved frontier molecular orbitals of FAC~S~0~~ and DA~S~0~~ at the B3LYP/6-31G(d) level in the gas phase ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.9b00693/suppl_file/ao9b00693_si_001.pdf))

Supplementary Material
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ao9b00693_si_001.pdf

The authors declare no competing financial interest.

This work was supported by the National Natural Science Foundation of China (21803051) and Natural Science Foundation of Shandong Province (ZR2016BQ06).
