Synthesis , Crystal Structure , Spectroscopic Properties , and DFT Studies of 7 , 9-Dibromobenzo [ h ] quinolin-10-ol

7,9-Dibromobenzo[h]quinolin-10-ol (1), a benzo[h]quinolin-10-ol derivative, was synthesized and characterized by single-crystal X-ray diffraction. The crystal belongs to monoclinic space group P21/n, with a = 3.9573(4), b = 18.0416(18), c = 15.8210(16) Å, α = 90◦, β = 96.139(3)◦, and γ = 90◦. Compound 1 exhibits an intramolecular six-membered-ring hydrogen bond, from which excited-state intramolecular proton transfer takes place, resulting in a proton-transfer tautomer emission of 625 nm in cyclohexane. The crystal structure is stabilized by intermolecular π–π interactions, which links a pair of molecules into a cyclic centrosymmetric dimer. Furthermore, the geometric structures, frontier molecular orbitals, and potential energy curves (PECs) for 1 in the ground and the first singlet excited state were fully rationalized by density functional theory (DFT) and time-dependent DFT calculations.


Synthesis and Characterization
Scheme 1 depicts the synthetic route and the chemical structure of 1.The bromination of 2 was carried out by the reaction of 2 with bromine in the presence of acetic acid, giving 1 in a good yield of 95% after purification.The structure of 7,9-dibromobenzo[h]quinolin-10-ol (1) can be verified by the presence of only seven signals (two singlet and two doublet signals and three doublet of doublets signals) at δ 7.0-17.0ppm in the 1 H NMR spectrum, which indicates that the bromination at the 7,9-positions of 2 was achieved.To confirm its structure, a single crystal of 1 was obtained from a dichloromethane solution, and the molecular structure was determined by X-ray diffraction analysis.Moreover, its X-ray structure is compared with that of 2 [27].

Synthesis and Characterization
Scheme 1 depicts the synthetic route and the chemical structure of 1.The bromination of 2 was carried out by the reaction of 2 with bromine in the presence of acetic acid, giving 1 in a good yield of 95% after purification.The structure of 7,9-dibromobenzo[h]quinolin-10-ol (1) can be verified by the presence of only seven signals (two singlet and two doublet signals and three doublet of doublets signals) at δ 7.0-17.0ppm in the 1 H NMR spectrum, which indicates that the bromination at the 7,9-positions of 2 was achieved.To confirm its structure, a single crystal of 1 was obtained from a dichloromethane solution, and the molecular structure was determined by X-ray diffraction analysis.Moreover, its X-ray structure is compared with that of 2 [27].
Scheme 1.The synthetic route and the structure for 1.

Hydrogen Bond Studies
The dominance of an enol-form for compounds 1 and 2 is supported by a combination of 1 H NMR and X-ray single-crystal analyses.In the 1 H NMR studies, the presence of a strong hydrogen bond between O-H and N is evidenced by the observation of a large downfield shift of the proton peak at δ > 14 ppm (in CDCl3) for both compounds 1 (16.65 ppm) and 2 (14.86 ppm).According to Schaefer's equation [28], the hydrogen bonding energies (∆E in kcal/mol) of 1 and 2 can be estimated to be as large as 12.8 ± 0.2 kcal/mol and 11.0 ± 0.2 kcal/mol, respectively.Note that the substitution of the hydrogen atoms at the 7,9-positions in 2 by bromine atoms, forming 1, seems to increase the acidity of phenol (O-H) through an inductive effect.Thus, 1 exhibits a downfield shift of the O-H proton, and hence, a stronger hydrogen bond relative to 2.

Hydrogen Bond Studies
The dominance of an enol-form for compounds 1 and 2 is supported by a combination of 1 H NMR and X-ray single-crystal analyses.In the 1 H NMR studies, the presence of a strong hydrogen bond between O-H and N is evidenced by the observation of a large downfield shift of the proton peak at δ > 14 ppm (in CDCl 3 ) for both compounds 1 (16.65 ppm) and 2 (14.86 ppm).According to Schaefer's equation [28], the hydrogen bonding energies (∆E in kcal/mol) of 1 and 2 can be estimated to be as large as 12.8 ± 0.2 kcal/mol and 11.0 ± 0.2 kcal/mol, respectively.Note that the substitution of the hydrogen atoms at the 7,9-positions in 2 by bromine atoms, forming 1, seems to increase the acidity of phenol (O-H) through an inductive effect.Thus, 1 exhibits a downfield shift of the O-H proton, and hence, a stronger hydrogen bond relative to 2.

Optical Properties
Figure 3 depicts the absorption and fluorescence spectra of 1 in cyclohexane.The longest wavelength absorption band of 1 appears at 376 nm, which is assigned to the π-π* transition (vide infra).Another higher energy absorption band is also observed at 314 nm.As for the steady-state emission, compound 1 exhibits a long wavelength emission at 625 nm in cyclohexane.Figure 3 also shows a large separation of the energy gap between the 0-0 onset of the absorption and emission.The Stokes shift of the emission, defined by the peak (absorption)-to-peak (emission) gap in terms of frequency, is calculated to be as large as 10,596 cm −1 .Accordingly, the assignment of 625 nm in cyclohexane to a proton-transfer tautomer emission is unambiguous [1,2], and ESIPT takes place from the phenolic proton to the pyridinic nitrogen, forming the keto-amine tautomeric species.This viewpoint can be further supported by a theoretical approach based on density functional theory (vide infra).

Quantum Chemistry Computation
To gain more insight into the molecular structure and optical properties of 1, quantum chemical calculations were performed using density functional theory (DFT) and time-dependent DFT (TD-DFT) at the B3LYP/LanL2DZ level.The geometric parameters were compared with the experimental data (Table 2).One can clearly see that there are no significant differences between the

Optical Properties
Figure 3 depicts the absorption and fluorescence spectra of 1 in cyclohexane.The longest wavelength absorption band of 1 appears at 376 nm, which is assigned to the π-π* transition (vide infra).Another higher energy absorption band is also observed at 314 nm.As for the steady-state emission, compound 1 exhibits a long wavelength emission at 625 nm in cyclohexane.Figure 3 also shows a large separation of the energy gap between the 0-0 onset of the absorption and emission.The Stokes shift of the emission, defined by the peak (absorption)-to-peak (emission) gap in terms of frequency, is calculated to be as large as 10,596 cm −1 .Accordingly, the assignment of 625 nm in cyclohexane to a proton-transfer tautomer emission is unambiguous [1,2], and ESIPT takes place from the phenolic proton to the pyridinic nitrogen, forming the keto-amine tautomeric species.This viewpoint can be further supported by a theoretical approach based on density functional theory (vide infra).

Optical Properties
Figure 3 depicts the absorption and fluorescence spectra of 1 in cyclohexane.The longest wavelength absorption band of 1 appears at 376 nm, which is assigned to the π-π* transition (vide infra).Another higher energy absorption band is also observed at 314 nm.As for the steady-state emission, compound 1 exhibits a long wavelength emission at 625 nm in cyclohexane.Figure 3 also shows a large separation of the energy gap between the 0-0 onset of the absorption and emission.The Stokes shift of the emission, defined by the peak (absorption)-to-peak (emission) gap in terms of frequency, is calculated to be as large as 10,596 cm −1 .Accordingly, the assignment of 625 nm in cyclohexane to a proton-transfer tautomer emission is unambiguous [1,2], and ESIPT takes place from the phenolic proton to the pyridinic nitrogen, forming the keto-amine tautomeric species.This viewpoint can be further supported by a theoretical approach based on density functional theory (vide infra).

Quantum Chemistry Computation
To gain more insight into the molecular structure and optical properties of 1, quantum chemical calculations were performed using density functional theory (DFT) and time-dependent DFT (TD-DFT) at the B3LYP/LanL2DZ level.The geometric parameters were compared with the experimental data (Table 2).One can clearly see that there are no significant differences between the

Quantum Chemistry Computation
To gain more insight into the molecular structure and optical properties of 1, quantum chemical calculations were performed using density functional theory (DFT) and time-dependent DFT (TD-DFT) at the B3LYP/LanL2DZ level.The geometric parameters were compared with the experimental data (Table 2).One can clearly see that there are no significant differences between the experimental and DFT/B3LYP calculated geometric parameters.Therefore, we can conclude that basis set LanL2DZ is suited in its approach to the experimental results.
Figure 4 depicts the optimized geometric structures (Table S2) and the corresponding hydrogen bond lengths of the enol form for 1 and 2 in the ground (S 0 ) and the first singlet excited state (S 1 ).From E to E*, we can see that the intramolecular hydrogen bond length (red dashed lines) decreases from 1.58 (1.77) Å to 1.46 (1.66) Å for 1 (2).The results distinctly provide the evidence for the strengthening of the intramolecular hydrogen bond from S 0 → S 1 .Consequently, there is no doubt that the decrease of the intramolecular hydrogen bond lengths from E to E* is a very important positive factor for the ESIPT reaction.
Crystals 2017, 7, 60 6 of 11 experimental and DFT/B3LYP calculated geometric parameters.Therefore, we can conclude that basis set LanL2DZ is suited in its approach to the experimental results.Figure 4 depicts the optimized geometric structures (Table S2) and the corresponding hydrogen bond lengths of the enol form for 1 and 2 in the ground (S0) and the first singlet excited state (S1).From E to E*, we can see that the intramolecular hydrogen bond length (red dashed lines) decreases from 1.58 (1.77) Å to 1.46 (1.66) Å for 1 (2).The results distinctly provide the evidence for the strengthening of the intramolecular hydrogen bond from S0 → S1.Consequently, there is no doubt that the decrease of the intramolecular hydrogen bond lengths from E to E* is a very important positive factor for the ESIPT reaction.The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of the enol and keto forms of 1 are shown in Figure 5.The first excited states for both the normal (enol) and tautomer (keto) forms are a dominant π → π* transition from the HOMO to the LUMO. Figure 5 also shows that the electron density around the intramolecular hydrogen bonding system is mainly populated with hydroxyl oxygen and pyridinic nitrogen in the HOMO and LUMO, respectively.The results clearly indicate that, upon electronic excitation of 1, the hydroxyl proton (O-H) is expected to be more acidic, whereas the pyridinic nitrogen is more basic with respect to its ground state, driving the excited-state intramolecular proton transfer reaction.Moreover, the absorption and fluorescence spectra of 1 were calculated by time-dependent DFT calculations (Franck-Condon principle).The calculated excitation (emission) wavelength for the S0 → S1 (S1 → S0) transition is 392 (631) nm, which is slightly higher but in good agreement with experimental results.
In an effort to explain the ESIPT reaction of 1, the potential energy curves (PECs) of the intramolecular proton transfer at both the ground state and the excited state were scanned, keeping the O-H bond lengths fixed at values in the range from 1.03 Å to 1.83 Å in steps of 0.1 Å, as shown in Figure 6.The energy of the S0 state increases along with the lengthening of the O-H bond and the potential barrier is 4.8 kcal/mol.The potential energy curve of the S1 state indicates that the ESIPT reaction is a barrierless process and the tautomeric structure of the reaction product is the only The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of the enol and keto forms of 1 are shown in Figure 5.The first excited states for both the normal (enol) and tautomer (keto) forms are a dominant π → π* transition from the HOMO to the LUMO. Figure 5 also shows that the electron density around the intramolecular hydrogen bonding system is mainly populated with hydroxyl oxygen and pyridinic nitrogen in the HOMO and LUMO, respectively.The results clearly indicate that, upon electronic excitation of 1, the hydroxyl proton (O-H) is expected to be more acidic, whereas the pyridinic nitrogen is more basic with respect to its ground state, driving the excited-state intramolecular proton transfer reaction.Moreover, the absorption and fluorescence spectra of 1 were calculated by time-dependent DFT calculations (Franck-Condon principle).The calculated excitation (emission) wavelength for the S 0 → S 1 (S 1 → S 0 ) transition is 392 (631) nm, which is slightly higher but in good agreement with experimental results.
1 H NMR spectra were recorded in CDCl3 on a Bruker 400 MHz.Mass spectra were recorded on a VG70-250S mass spectrometer (Hitachi, Tokyo, Japan).The absorption and emission spectra were measured using a Jasco V-570 UV-Vis spectrophotometer (Jasco, Tokyo, Japan) and a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan), respectively.The single-crystal X-ray diffraction data were collected on a Bruker Smart 1000CCD area-detector diffractometer (Bruker, Billerica, MA, USA).In an effort to explain the ESIPT reaction of 1, the potential energy curves (PECs) of the intramolecular proton transfer at both the ground state and the excited state were scanned, keeping the O-H bond lengths fixed at values in the range from 1.03 Å to 1.83 Å in steps of 0.1 Å, as shown in Figure 6.The energy of the S 0 state increases along with the lengthening of the O-H bond and the potential barrier is 4.8 kcal/mol.The potential energy curve of the S 1 state indicates that the ESIPT reaction is a barrierless process and the tautomeric structure of the reaction product is the only minimum on the excited state surface.The results clearly show that the ESIPT for 1 is thermodynamically favorable, which is consistent with the experimental results.
1 H NMR spectra were recorded in CDCl3 on a Bruker 400 MHz.Mass spectra were recorded on a VG70-250S mass spectrometer (Hitachi, Tokyo, Japan).The absorption and emission spectra were measured using a Jasco V-570 UV-Vis spectrophotometer (Jasco, Tokyo, Japan) and a Hitachi F-4500
1 H NMR spectra were recorded in CDCl 3 on a Bruker 400 MHz.Mass spectra were recorded on a VG70-250S mass spectrometer (Hitachi, Tokyo, Japan).The absorption and emission spectra were measured using a Jasco V-570 UV-Vis spectrophotometer (Jasco, Tokyo, Japan) and a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan), respectively.The single-crystal X-ray diffraction data were collected on a Bruker Smart 1000CCD area-detector diffractometer (Bruker, Billerica, MA, USA).

Crystal Structural Determination
A single crystal of the title compound with dimensions of 0.38 mm × 0.08 mm × 0.07 mm was selected.The lattice constants and diffraction intensities were measured with a Bruker Smart 1000CCD area detector (Bruker, Billerica MA, USA) radiation (λ = 0.71073 Å) at 302(2) K.An ω-2θ scan mode was used for data collection in the range of 3.437 ≤ θ ≤ 26.378 • .A total of 16148 reflections were collected and 2266 were independent (R int = 0.0438), of which 1679 were considered to be observed with I > 2σ(I) and used in the succeeding refinement.The structure was solved by direct methods with SHELXS-97 [34] and refined on F 2 by a full-matrix least-squares procedure with Bruker SHELXL-97 packing [35].All non-hydrogen atoms were refined with anisotropic thermal parameters.The hydrogen atoms, refined isotropically with riding model position parameters, were located from a difference Fourier map and added theoretically.At the final cycle of refinement, R = 0.0472 and wR = 0.1296 (w = 1/[σ 2 (F o 2 ) + (0.0856P) 2 + 3.9902P], where P = (F o 2 + 2F c 2 )/3).S = 1.095, (∆/σ) max = 0.007, (∆/ ) max = 3.932 and (∆/ ) min = −1.923e/ Å 3 , were observed.Crystallographic data for the structure reported in this article have been deposited in the Cambridge Crystallographic Data Center with a supplementary publication number of CCDC 1486418.Copies of this information can be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Computational Methods
In the present work, all theoretical calculations were carried out using the Gaussian 03 program suite [36].The electronic geometric optimizations of the enol and keto forms for 1 in the ground (S 0 ), and the first singlet excited state (S 1 ) was performed using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods with the LanL2DZ basis set and the B3LYP functional.There were no constraints to all the atoms, bonds, angles, and dihedral angles during the geometric optimization.The vibrational frequencies were calculated using the same method to verify that the optimized structures correspond to the local minima on the energy surface.After obtaining the converged geometries, the TD-B3LYP/LanL2DZ was used to calculate the vertical excitation energy, and the tautomer emission energy was obtained from TD-DFT/B3LYP/LanL2DZ calculations performed on the S 1 optimized geometry.As observed by the only slightly solvent polarity dependent shift of the emission (absorption) spectra in 1, the charge transfer character of tautomer emission for 1 is slim.Thus, solvent effects are not considered throughout these computations [37].In addition to the unconstrained optimization, constrained calculations on the S 0 and S 1 potential energy curves of 1 were scanned, keeping the O−H bond lengths fixed at values in the range from 1.03 Å to 1.83 Å in steps of 0.1 Å.The energies shown in the curves are relative values, with the lowest point on the curve as zero.

Conclusions
A benzo[h]quinolin-10-ol derivative, namely, 7,9-dibromobenzo[h]quinolin-10-ol (1) was synthesized and fully characterized.Compound 1 exhibits an intramolecular six-membered-ring hydrogen bond, from which ESIPT takes place, resulting in a proton-transfer tautomer emission of 625 nm in cyclohexane.The geometric structures, frontier molecular orbitals, and potential energy curves (PECs) for 1 in the ground and the first singlet excited state were fully rationalized by DFT and time-dependent DFT calculations and were in good agreement with the experimental results.In addition, the single-crystal X-ray structure determinations reported here have brought to light many interesting properties between 1 and 2 in the solid state, including intra-and intermolecular hydrogen bonding interactions and π• • • π stacking.This offers the potential to synthesize benzo[h]quinolin-10-ol derivatives with extended molecular architectures and optical properties.

Figure 1 .
Figure 1.The molecular structure of 1, showing the atom-labeling scheme.Displacement ellipsoids are drawn at the 50% probability level.Green and blue dashed lines denote the intramolecular O-H⋅⋅⋅N and C-H⋅⋅⋅Br hydrogen bonds.

Figure 1 .
Figure 1.The molecular structure of 1, showing the atom-labeling scheme.Displacement ellipsoids are drawn at the 50% probability level.Green and blue dashed lines denote the intramolecular O-H• • • N and C-H• • • Br hydrogen bonds.

Figure 3 .
Figure 3. Normalized absorption (blue line) and emission (red line) spectra of 1 in cyclohexane.

Figure 3 .
Figure 3. Normalized absorption (blue line) and emission (red line) spectra of 1 in cyclohexane.

Figure 3 .
Figure 3. Normalized absorption (blue line) and emission (red line) spectra of 1 in cyclohexane.

Figure 4 .
Figure 4.The optimized geometric structures of the enol (E) form for 1 (left) and 2 (right) in the ground (S0) and the first singlet excited state (S1) together with the intramolecular hydrogen bond lengths.The red dashed lines denote the intromolecular O-H⋅⋅⋅N hydrogen bonds.

Figure 4 .
Figure 4.The optimized geometric structures of the enol (E) form for 1 (left) and 2 (right) in the ground (S 0 ) and the first singlet excited state (S 1 ) together with the intramolecular hydrogen bond lengths.The red dashed lines denote the intromolecular O-H• • • N hydrogen bonds.

Figure 5 .
Figure 5. Selected frontier molecular orbitals involved in the excitation and emission of 1.

Figure 6 .
Figure 6.Potential energy curves of S0 and S1 states for 1 along with the O-H bond length.The insert shows the corresponding optimized geometries.

Figure 5 .
Figure 5. Selected frontier molecular orbitals involved in the excitation and emission of 1.

Figure 5 .
Figure 5. Selected frontier molecular orbitals involved in the excitation and emission of 1.

Figure 6 .
Figure 6.Potential energy curves of S0 and S1 states for 1 along with the O-H bond length.The insert shows the corresponding optimized geometries.

Figure 6 .
Figure 6.Potential energy curves of S 0 and S 1 states for 1 along with the O-H bond length.The insert shows the corresponding optimized geometries.

Table 2 .
Comparison of the experimental and optimized geometric parameters of 1 (Å and • ).DFT: Density Functional Theory.