3.1 Geometric studies
To examine the sensing process for fluoride anion, the ground state and excited state geometries of the chemosensors (APTSC and ATSC), receptor-fluoride complex have been investigated in detail. The optimized geometric structures of APTSC and ATSC receptors and receptor-fluoride complexes (APTSC−F and ATSC−F) at the ground state and excited state are as shown in Figure 1 and Figure 2. The important structural parameters of ATSC and APTSC are listed in Table 1 and Table 2. The vibrational frequency of the ground state structures were positive, hence, all the structures are in local minima. The ground state energies of ATSC and APTSC are -43.34 and -51.824 eV kcal/mol respectively. Whereas, the receptor-fluoride complex such as APTSC−F and ATSC−F have less SCF energy than pure receptor.
In the ground state of APTSC, the dihedral angle between C5−C4−N3−N2 was 34.39o. The dihedral angle between anthracene moiety and imine group were115.99 and 1.248 for APTSC and ATSC receptors respectively. These results revealed that the receptors, APTSC and ATSC were not in coplanar. The N1−H1 and N2−H2 bond distance in APTSC are found to be 1.01 and 1.02 Å respectively. The bond angle between C1-N1-C2 is 130.900. The C-N bond distance in the APTSC is 1.28 Å. However, in the case of excited state of APTSC-F complex, the C-N bond distance found 1.32 Å, The high bond length points the formation of single bond between C-N. In the ground state of receptor-fluoride complex, the bond distance between F-H1 and F-H2 were found in the range of 1.50 and 1.46 Å, which indicated that the fluoride ion made hydrogen bond with receptor via N−H…F…H−N. The dihedral angles between phenyl group, imine group and anthracene moiety- imine group were found same in the APTSC and APTSC-F complex.
In the excited state of APTSC-fluoride (APTSC-F*) complex, the calculated dihedral angle between anthracene moiety and imine group is 159.81, which is higher than the ground state structure of APTSC-F complex. In the APTSC-F*, the bond distance between F−H1 and F−H2 are found to be 1.91 and 0.99 Å respectively. The lower bond length between fluoride and H2 implies the presence of strong bond between them, which indicates the proton transfer was happened at excited state. While, in the case of excited state ATSC−F (ATSC−F*) structure, the F−H1, F−H2, N−H1 and N−H2 bond distance are calculated as1.97, 1.05, 1.01 and 1.40 Å respectively. The higher bond distance between N-H2 and lower bond distance between F-H2 implies the deprotonation happened first at ‘H2’ proton of ATSC. Hence, it is evident of the excited state proton transfer (ESPT) process. The dihedral angle between anthracene moiety and imine group was twisted to 28.6800 in ATSC−F*, which provides higher planarity and conjugation than pure receptor. The extension of the conjugation might help to delocalize the negative charge of the fluoride ion, hence, the photophysical property of the receptor-F complex varies from pure receptor.
3.2 Absorption and molecular orbital analysis
The absorption spectral studies were carried out for receptors and receptor-fluoride complexes to understand the change in optical behaviour in the host-guest process. The molecular orbitals involved in the major electronic transitions with the largest oscillator strength have been shown in Figure 3 and Figure 4. Theoretical calculations predict six absorption transitions for each compounds, and the dominant absorption transition of APTSC, APTSC-F complex, ATC and ATSC-F complex are given in Table 3 and Table 4. The intense absorption transition for APTSC is calculated at 367.48 nm, with large oscillating strength 0.2957, which is assigned as π→π* transition from the highest occupied molecular orbital (HOMO, H) to the lowest unoccupied molecular orbital (LUMO, L). In HOMO, the electron delocalized over anthracene moiety and imine group. Whereas, in LUMO, the electron cloud only delocalized at anthracene moiety. The second dominant absorption peak was at 310 nm, which was corresponds to HOMO, third lowest unoccupied molecular orbital (LUMO+2, L+2), where the electron delocalized throughout the anthracene moiety. While in APTSC-F complex, the absorption peak at 367 nm decreases and accompanies with the formation of a new band at 374 nm. The high oscillating strength and percentage π→π* transition was between HOMO to LUMO. In the HOMO of APTSC-F complex, the electron was delocalized only on imine group. Whereas, the electron was delocalized on anthracene moiety in LUMO.
In the case of ATSC, the high oscillated frequency wavelength at 411 nm with 3.0136 eV energy. The high percentage orbital transition was in between HOMO-1 to LUMO. The second major absorption transition was at HOMO to LUMO orbitals. In HOMO, the electrons delocalized in anthracene moiety and thiosemicarbazide group. While, the HOMO-1 and HOMO-2 orbitals had electron cloud only on imine group, not in anthracene moiety. Whereas, in LUMO, LUMO+1 and LUMO+2 orbitals the electron clouded only at anthracene moiety. The resulting electron cloud rearrangement is cause for the changes in the photophysical properties of receptor in the host-guest interaction. Figure 5 displays the calculated orbital energies of APTSC and APTSC-F complex. The energy gap between HOMO and LUMO orbitals in emission profile is higher than that of absorption profile. The complete charge separation from thiosemicarbazone group to anthracene in the relaxation process is in good accordance with the definition of PET. The absorption profiles are calculated using the Gaussian models and compared with the experimental results.
3.3 Transition state calculations
In order to capture the dynamics feature of the sensing process, we evaluated the Gibbs free energy profile in DMSO medium. The transition state and intermediate structures were calculated, which are as shown in Figure 6 and Figure 7. In the transition state and intermediate structures, the fluoride ion interacts with receptor via hydrogen boning. The relaxation of the transition state toward the intermediate and the products by IRC calculations did not detect any intermediates, which revealed that the host-guest process is SN2 type of reaction. The sensing process of ATSC and APTSC with fluoride ion had the free energy (ΔG) of 177.17 and 186.22 kcal/mol. The more negative value of ΔG was the reason for quick response of APTSC than ATSC towards fluoride ion. In the transition state, the bond distance between fluoride ion and N2-H2 and F-H2 were about 1.45 and 1.02 Å, which revealed the fluoride ion has higher hydrogen bond with hydrogen atom than in ground state and results the transition state in higher energy state. The binding constant (Ka) of the receptor (APTSC and ATSC) with fluoride ion was calculated (, where, R, T and Ka are the universal gas constant, temperature and the binding constant respectively). The binding constant of APTSC and ATSC towards fluoride ion found to be 0.930 and 0.927 M-1 respectively.
Natural Bond orbital (NBO) calculation
Natural bond orbital (NBO) analysis provides the most accurate possible ‘natural Lewis structure’ picture, because all orbital details are mathematically chosen to include the highest possible percentage of the electron density. Further confirming the ESPT process and the deprotonation of H2 from receptor via hydrogen bond, the NBO analysis were carried out at the ground state and excited state of receptor-fluoride complex. The binding energy (ΔE) between of binding sites of fluoride ion and the residue for receptor were plays an important role in revealing the ability for fluoride anion to attach proton. The binding energies between the two segments separated by H1−F and H2−F of receptor-fluoride complexes were calculated.
The natural atomic hybrids calculation gave the bond order on each hybrid atoms. The hybridisation with single bond between N25−H41 and N27−H42 were listed in S3. In ground state APTSC-F complex, sp3 nitrogen was hybridised to ‘s’ hydrogen, but this bond was not in excited state. Resulting, a strong host–guest interaction between fluoride ion and APTSC occurred, which can be treated by the second-perturbation energy E(2). The interaction between host and guest can be explained by using second perturbation energy, which has been calculated using the equation, , where E(2) is the second perturbation energy, Fij is the off-diagonal element in the NBO Fock matrix, qi is the donor orbital occupancy, and ei and ej are orbital energies. The NBO characters in ground state and excited state were given in the Table 5. The σ-σ* and n-σ* host-guest interactions were seen at the ground state and excited state of ATSC−F and APTSC−F complexes. In the ground state, two mode of interactions were presented via fluoride to N25−H41 and N27−H42. Whereas, in the excited state had only one interaction, which was via fluoride ion with H41. In the ground state of receptor-fluoride complex, the second perturbation energies were varies from 0.3 to 44 kcal/mol, which revealed that both the receptor make hydrogen bonding with fluoride ion in the ground state. But, in the excited state of receptor-fluoride complexes have LP(F)→LP*(H41) interaction with more than 300 kcal/mol energy, which show the fluoride ion has make strong bond with H(41) proton. This interaction generated from lone pair electron (n) of fluoride ion to n* of hydrogen (H41). The interactions were generated from core electron and lone pair electron. More overlap of the electron density confirms the strong donor-acceptor interactions, due to this, the H(41) preferred to interact with Fluoride ion rather than H(43).
3.4 Potential energy curve of excited state geometry
To confirm the ESPT mechanism in host-guest interaction, the potential energy curve investigation of the ground state and excited state receptor-fluoride had been evaluated. The PES calculations employed with only varying the N-H bond length from 0.90 to 1.80 Å in steps of 0.05 Å, which can provide qualitative energetic pathways for the ESIPT process. In the case of APTSC-F complex, the bond distance between N(25)−H(42) and N(27)−H(43) at ground stat and excited state where N(25) is at nearby anthracene moiety and (N27) is nearby Phenyl group. The less energy was seen for the excited state N(25)−H(42) bond distance at 1.00 Å, and the same at ground state was more stable at 1.10 Å bond distance. These results confirm that the fluoride ion form an intermolecular hydrogen bond with receptor’s nitrogen (N2), which is the NH proton near by the anthracene moiety. In the host-guest interaction, the fluoride ion was first react with receptor and form hydrogen bond via N−H…F, not in free H+ ion. Hence, it show a red shift in the fluorescence emission spectra and UV absorption spectra. This change in fluorescence colour signal can directly detect with the naked eye. The PES curve of APTSC-F and ATSC−F complex in ground state and excited state were as shown in S4 and S5 respectively.
3.5 Host-Guest Interaction
The hydrogen bond formation was confirmed in the ground state optimized structures of receptor-fluoride complexes, which has been defined as incipient proton transfer reaction from host to guest molecule. The reported experimental fluorescence quenching behaviour can be interpreted by the PET process when the fluoride anion is coordinated with APTSC. The fluorescence quenching was due to the electron transition from π-orbital of phenyl group to π -orbital of anthracene moiety. The optimized structure of receptor and receptor-fluoride complexes in the ground state and excited state suggested that the deprotonation took place at the excited state of receptor-fluoride complex, along the deprotonation happened at the N−H proton nearby the anthracene moiety. The results from TS calculations, NBO and PES analyses confirm that the deprotonation was takes place from the N-H proton, which is near by the anthracene moiety. The shifts of the signals in absorption and emission spectra are prompted to enhance the strength of host molecules and result to increase the molecular conjugation when deprotonation happens. The great difference between fluorescent properties of the sensor can be used to recognize fluoride anions. The potential energy curve structure of receptor-fluoride in the ground state and the excited state defined the bond parameter for fluoride-sensing mechanism of receptor ATSC and APTSC. The Figure 8 shows the sensing mechanism of receptors towards fluoride ion.