Modulating the ESIPT Mechanism and Luminescence Characteristics of Two Reversible Fluorescent Probes by Solvent Polarity: A Novel Perspective

As reversible fluorescent probes, HTP-1 and HTP-2 have favourable applications for the detection of Zn2+ and H2S. Herein, the impact of solvent on the excited-state intramolecular proton transfer (ESIPT) of HTP-1 and HTP-2 was comprehensively investigated. The obtained geometric parameters and infrared (IR) vibrational analysis associated with the intramolecular hydrogen bond (IHB) indicated that the strength of IHB for HTP-1 was weakened in the excited state. Moreover, structural torsion and almost no ICT behaviour indicated that the ESIPT process did not occur in HTP-1. Nevertheless, when the 7-nitro-1,2,3-benzoxadiazole (NBD) group replaced the H atom, the IHB strength of HTP-2 was enhanced after photoexcitation, which inhibited the twisting of tetraphenylethylene, thereby opening the ESIPT channel. Notably, hole-electron analysis and frontier molecular orbitals revealed that the charge decoupling effect was the reason for the fluorescence quenching of HTP-2. Furthermore, the potential energy curves (PECs) revealed that HTP-2 was more inclined to the ESIPT process in polar solvents than in nonpolar solvents. With a decrease in solvent polarity, it was more conducive to the ESIPT process. Our study systematically presents the ESIPT process and different detection mechanisms of the two reversible probe molecules regulated by solvent polarity, providing new insights into the design and development of novel fluorescent probes.

In the past few years, 2-(2-hydroxyphenyl) benzoazole (HBX) derivatives had been widely studied as ESIPT molecules with high research value as organic luminescent and fluorescent probe materials [32][33][34][35].Many previous studies have shown that solvent polarity has a vital effect on the ESIPT process of HBX derivatives in the excited state [36][37][38].Xiao et al. theoretically studied the ESIPT process and the photophysical mechanism of 2-benzo [b] thiphen-3-yl-3-hydroxy -6-methoxy -chroman-4-one (CHBT) in different solvents.It was confirmed that the ESIPT reaction for CHBT in nonpolar solvents was easier [39].Du et al. discussed the ESIPT reaction of 6-amino-2-(2 ′ -hydroxyphenyl) benzoxazole (6A-HBO) in different solvents.It was verified that the ESIPT reaction was more prone to occur as the solvent polarity increased [40].Zhao [42].HTP-1 and HTP-2 can detect Zn 2+ and H 2 S, respectively.HTP-1 exhibits obvious blue fluorescence.When HTP-1 detects Zn 2+ , the combination of HTP-1 and Zn 2+ significantly enhances the fluorescence, resulting in the production of HTP-2.Pure HTP-2 has no fluorescence, but it releases HTP-1 when detecting H 2 S and exhibits blue fluorescence.The structural difference between HTP-1 and HTP-2 is that the H atom in HTP-1 is replaced by the 7-nitro-1,2,3-benzoxadiazole (NBD), which is correspondingly converted to HTP-2 (Scheme 1).However, the underlying reason for the different luminescence mechanisms of these two molecules remained unclear.It cannot be ignored that there are flexible rotating tetraphenylethylene and ESIPT sites in both molecules.The corresponding photophysical characteristics and luminescence mechanism are worth exploring.In addition, the authors speculate that solvent polarity may further alter the photophysical properties of the molecules [42].Therefore, it is imperative to promptly investigate the internal mechanism of the excited state for HTP-1 and HTP-2 by solvent polarity.This will theoretically validate the detection mechanism of Zn 2+ and H 2 S using the two probes.[42].HTP-1 and HTP-2 can detect Zn 2+ and H2S, respectively.HTP-1 exhibits obvious blue fluorescence.When HTP-1 detects Zn 2+ , the combination of HTP-1 and Zn 2+ significantly enhances the fluorescence, resulting in the production of HTP-2.Pure HTP-2 has no fluorescence, but it releases HTP-1 when detecting H2S and exhibits blue fluorescence.The structural difference between HTP-1 and HTP-2 is that the H atom in HTP-1 is replaced by the 7-nitro-1,2,3-benzoxadiazole (NBD), which is correspondingly converted to HTP-2 (Scheme 1).However, the underlying reason for the different luminescence mechanisms of these two molecules remained unclear.It cannot be ignored that there are flexible rotating tetraphenylethylene and ESIPT sites in both molecules.The corresponding photophysical characteristics and luminescence mechanism are worth exploring.In addition, the authors speculate that solvent polarity may further alter the photophysical properties of the molecules [42].Therefore, it is imperative to promptly investigate the internal mechanism of the excited state for HTP-1 and HTP-2 by solvent polarity.This will theoretically validate the detection mechanism of Zn 2+ and H2S using the two probes.
This study comprehensively investigated the impact of solvent polarity on the ESIPT process of HTP-1 and HTP-2.We calculated the geometric parameters and infrared (IR) vibrational spectra which were associated with the HB interactions.In addition, we simulated the absorption and fluorescence spectra for HTP-1 and HTP-2.According to holeelectron analysis and frontier molecular orbitals (FMOs) diagrams, the internal causes of the different luminescence mechanisms of the two molecules were determined.To provide a more intuitive description for the different ESIPT processes of the two molecules, we constructed potential energy curves (PECs) of four different solvents in the S0 and S1 states.This study comprehensively investigated the impact of solvent polarity on the ESIPT process of HTP-1 and HTP-2.We calculated the geometric parameters and infrared (IR) vibrational spectra which were associated with the HB interactions.In addition, we simulated the absorption and fluorescence spectra for HTP-1 and HTP-2.According to hole-electron analysis and frontier molecular orbitals (FMOs) diagrams, the internal causes of the different luminescence mechanisms of the two molecules were determined.To provide a more intuitive description for the different ESIPT processes of the two molecules, we constructed potential energy curves (PECs) of four different solvents in the S 0 and S 1 states.

Geometric Parameters
The configurations for HTP-1 and HTP-2 in different electronic excited states were optimized at the theoretical level of MN15/6-31G (d, p).In the excited state, we labelled the E* structure as the S 1 state and the K* structure as the S 1 state.The atoms involved were numerically labelled (see Figures 1 and 2), and the calculated geometric parameters for the four solvents (ACN, ACE, DCM and n-Hexane) are recorded in Tables 1 and S1.For HTP-2 (Figure 2), in the S0 state, the O1-H1 bond lengths were 0.990 Å, 0.990 Å, 0.989 Å and 0.988 Å in four solvents (ACN, ACE, DCM, n-Hexane), respectively.After photoexcitation, the O1-H1 bond lengths elongated to 0.999 Å, 1.000 Å, 1.000 Å and 1.003 Å, respectively.Analogously, the N1-H1 bond lengths were 1.775 Å (ACN), 1.776 Å (ACE), 1.778 Å (DCM) and 1.789 Å (n-Hexane) in the S0 state, respectively.In the S1 state, the N1-H1 bond lengths shrunk to 1.687 Å (ACN), 1.685 Å (ACE), 1.682 Å (DCM) and 1.660 Å (n-Hexane).In addition, after photoexcitation, the bond angle (O1-H1...N1) increased in four solvents.The above analysis indicated that the IHBs (O1-H1...N1) were significantly strengthened in the S1 state.This facilitated the ESIPT process of HTP-2.The strength of the IHBs increased with decreasing solvent polarity in the excited state.Especially, the degree of IHB strengthening (in DCM solvent) was the most obvious in polar solvents.In addition, we obtained the tautomer configuration (S1′ state).In the S1′ state, a new IHB (N1-H1...O1) was formed, the O1-H1 bond length was 1.881 Å.In contrast to that in HTP-1, the introduction of the NBD group replaced the H atom to suppress the structural torsion in HTP-2, causing the molecular structure to tend to be flat.For HTP-1, from the S 0 to the S 1 state, the O 1 -H 1 bond lengths were almost unchanged in four solvents.The N 1 ...H 1 bond lengths from the S 0 to the S 1 state elongated by 0.002 Å (ACN), 0.003 Å (ACE), 0.003 Å (DCM) and 0.001 Å (n-Hexane), respectively (see Figure 1).This indicated that the IHBs (O 1 -H 1 ...N 1 ) did not change significantly in the S 1 state.Remarkably, after photoexcitation, HTP-1 underwent torsion at multiple positions in different solvents (see Table S1).The dihedral angle α (C 5 -C 1 -C 2 -C 3 ) had the most severe torsion.It varied from 12.35  (DCM) and 37.97 • (n-Hexane), respectively.This indicated that the structure of HTP-1 was distorted after photoexcitation, and the degree of torsion was more intense with increasing solvent polarity.

Potential Energy Curves (PECs)
To explore the degree of difficulty of the ESIPT reaction for HTP-1 and HTP-2, we plotted the S0and S1-stated PECs for HTP-1 and HTP-2 in four solvents by fixing the O1-H1 bond length.As shown in Figure 3, the high barrier and the consistently rising energy of HTP-1 in four solvents made the ESIPT process impracticable in the S1 state, indicating that the enol * configuration was more stable.In the S0 state, the energy of HTP-2 gradually increased with the extension of the O1-H1 bond length (see Figure 4).This indicated that the ESIPT reaction was not feasible in the S0 state.In the S1 state, the energy barriers were 4.56 kcal/mol (ACN), 4.54 kcal/mol (ACE), 2.79 kcal/mol (DCM) and 6.11 kcal/mol (n-Hexane).This result indicated that the ESIPT process tended to occur after photoexcitation.

Potential Energy Curves (PECs)
To explore the degree of difficulty of the ESIPT reaction for HTP-1 and HTP-2, we plotted the S 0 -and S 1 -stated PECs for HTP-1 and HTP-2 in four solvents by fixing the O 1 -H 1 bond length.As shown in Figure 3, the high barrier and the consistently rising energy of HTP-1 in four solvents made the ESIPT process impracticable in the S 1 state, indicating that the enol * configuration was more stable.In the S 0 state, the energy of HTP-2 gradually increased with the extension of the O 1 -H 1 bond length (see Figure 4).This indicated that the ESIPT reaction was not feasible in the S 0 state.In the S 1 state, the energy barriers were 4.56 kcal/mol (ACN), 4.54 kcal/mol (ACE), 2.79 kcal/mol (DCM) and 6.11 kcal/mol (n-Hexane).This result indicated that the ESIPT process tended to occur after photoexcitation.The energy barrier gradually increased with increasing solvent polarity in polar solvents (ACN, ACE and DCM), which was consistent with the order of HB strength.Whereas the energy barrier in nonpolar solvent (n-Hexane) was the highest.This result indicated that the ESIPT process of HTP-2 was more prone to occur in low-polarity solvents.
The energy barrier gradually increased with increasing solvent polarity in polar s (ACN, ACE and DCM), which was consistent with the order of HB strength.Whe energy barrier in nonpolar solvent (n-Hexane) was the highest.This result indica the ESIPT process of HTP-2 was more prone to occur in low-polarity solvents.

Infrared (IR) Spectra Analysis
Infrared vibrational frequency is a vital indicator to analyse the strength of IH 46].Figures 5 and 6 show the IR vibrational frequencies of O1-H1 and N1-H1 for HT HTP-2 in four solvents.The energy barrier gradually increased with increasing solvent polarity in polar s (ACN, ACE and DCM), which was consistent with the order of HB strength.Wher energy barrier in nonpolar solvent (n-Hexane) was the highest.This result indicat the ESIPT process of HTP-2 was more prone to occur in low-polarity solvents.

Infrared (IR) Spectra Analysis
Infrared vibrational frequency is a vital indicator to analyse the strength of IH 46].Figures 5 and 6 show the IR vibrational frequencies of O1-H1 and N1-H1 for HTP HTP-2 in four solvents.
stretching modes were 3427 cm −1 (ACN), 3429 cm −1 (ACE), 3435 cm −1 (DCM) and 3465 (n-Hexane) in the S0 state, which moved to 3224 cm −1 , 3221 cm −1 , 3213 cm −1 and 3158 in the S1 state, respectively.Consistent with the geometric analysis, the red shifts in A (203 cm −1 ), ACE (208 cm −1 ) and DCM (222 cm −1 ) increased with decreasing solvent pola The result indicated that the order of the IHB strength was DCM > ACE > ACN.In a tion, the new stretching vibrational mode N1-H1 was found to replace the original O mode in the S1′ state, which situated at 3482 cm −1 (ACN), 3544 cm −1 (ACE), 3527 (DCM) and 3462 cm −1 (n-Hexane), respectively.Through the above analysis, we fo that the IHB strength of HTP-2 was enhanced in four solvents.These results were sistent with the above molecular configuration analysis.stretching modes were 3427 cm −1 (ACN), 3429 cm −1 (ACE), 3435 cm −1 (DCM) and 3465 (n-Hexane) in the S0 state, which moved to 3224 cm −1 , 3221 cm −1 , 3213 cm −1 and 3158 in the S1 state, respectively.Consistent with the geometric analysis, the red shifts in (203 cm −1 ), ACE (208 cm −1 ) and DCM (222 cm −1 ) increased with decreasing solvent pola The result indicated that the order of the IHB strength was DCM > ACE > ACN.In a tion, the new stretching vibrational mode N1-H1 was found to replace the original O mode in the S1′ state, which situated at 3482 cm −1 (ACN), 3544 cm −1 (ACE), 3527 (DCM) and 3462 cm −1 (n-Hexane), respectively.Through the above analysis, we fo that the IHB strength of HTP-2 was enhanced in four solvents.These results were sistent with the above molecular configuration analysis.′ states.
For HTP-1, the O 1 -H 1 stretching modes from the S 0 to the S 1 state were blue-shifted in four solvents, resulting in the IHB intensity being weakened in the S 1 state.The result illustrated that the ESIPT process of HTP-1 was not facile to occur.For HTP-2, the O 1 -H 1 stretching modes were 3427 cm −1 (ACN), 3429 cm −1 (ACE), 3435 cm −1 (DCM) and 3465 cm −1 (n-Hexane) in the S 0 state, which moved to 3224 cm −1 , 3221 cm −1 , 3213 cm −1 and 3158 cm −1 in the S 1 state, respectively.Consistent with the geometric analysis, the red shifts in ACN (203 cm −1 ), ACE (208 cm −1 ) and DCM (222 cm −1 ) increased with decreasing solvent polarity.The result indicated that the order of the IHB strength was DCM > ACE > ACN.In addition, the new stretching vibrational mode N 1 -H 1 was found to replace the original O 1 -H 1 mode in the S 1 ′ state, which situated at 3482 cm −1 (ACN), 3544 cm −1 (ACE), 3527 cm −1 (DCM) and 3462 cm −1 (n-Hexane), respectively.Through the above analysis, we found that the IHB strength of HTP-2 was enhanced in four solvents.These results were consistent with the above molecular configuration analysis.

Frontier Molecular Orbitals (FMOs)
FMOs are an efficient approach for acquiring changes in molecular charge distribution before and after photoexcitation [47,48].Figure 7 visually shows the electron density distribution of HTP-1 and HTP-2 at the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).For HTP-1, the small variation in the charge distribution from HOMO to LUMO indicated that there was almost no ICT process in HTP-1.This finding provided evidence that HTP-1 did not undergo the ESIPT process.For HTP-2, it was obvious that the electron density near the proton donor (O 1 ) decreased, whereas the electron density near the acceptor (N 1 ) increased during photoexcitation.This facilitated the binding of the N 1 atom to the H 1 proton, thus advancing the ESIPT process of HTP-2.Notably, in HTP-2, the degree of overlap for the charge distributions in the two orbitals (HOMO and LUMO) was close to zero, which was the most fundamental reason of the fluorescence quenching in HTP-2.Moreover, the HOMO-LUMO gaps (E gap ) in different solvents was also calculated.As is well known, the energy gap is inversely proportional to the activity of the chemical reaction [49][50][51][52].It was not difficult to find that HTP-2 had higher chemical reactivity than HTP-1, and the E gap of HTP-2 increased with enhancing solvent polarity.In addition, to quantificationally study the redistribution of electron density around the IHBs during photoexcitation, we calculated the natural bond orbital (NBO) layout of HTP-1 and HTP-2.For HTP-1, before and after photoexcitation, the charge distribution on the O 1 and N 1 atoms was almost unchanged (in Table 2).For HTP-2, the charge on the O 1 atom hardly changed before and after photoexcitation.However, the charge on the N 1 atom in ACN, ACE, DCM and n-Hexane solvents were −0.547, −0.546, −0.544 and −0.536 in the S 0 state, which increased to −0.556, −0.556, −0.555 and −0.553 in the S 1 state.The increase in charge on the N 1 atom enhanced its ability to attract protons (H 1 ), which could promote the proton transfer in the excited state.The results indicated that ICT behaviour occurred and facilitated the ESIPT tendency in HTP-2.

Electronic Spectra and Hole-Electron Analysis
To preferably comprehend the photophysical properties of HTP-1 and HTP-2 in four solvents, we simulated the absorption and fluorescence spectra using the MN15/6-31G (d, p) method.The main photophysical parameters of HTP-1 and HTP-2 are recorded in Tables 3 and 4, respectively.For HTP-1 (see Figure 8), the simulated absorption peak (351 nm) in ACN solvent was in agreement with the experimental data (348 nm).This indicated that the adopted MN15/6-31G (d, p) method was extremely reliable for further investigation of photophysical properties in other solvents.In addition, the absorption peaks were at 351 nm (ACE), 352 nm (DCM) and 352 nm (n-Hexane), implying that solvent polarity had little effect on them.The fluorescence peaks of the enol * form in ACN, ACE, DCM and n-Hexane corresponded to 733 nm, 687 nm, 683 nm and 661 nm, respectively.The result distinctly implied that the short wavelength Stokes shift was 309-382 nm and that the fluorescence of the HTP-1 came from enol * form.Meanwhile, for HTP-2 (see Figure 9), the absorption peaks in the four solvents were located at 352 nm (ACN), 352 nm (ACE), 354 nm (DCM) and 356 nm (n-Hexane).Moreover, the fluorescence of the enol * and keto * states for HTP-2 was quenched, which was consistent with the fact that no fluorescence was observed in the experiment [42].For HTP-1 (see Figure 8), the simulated absorption peak (351 nm) in ACN solvent was in agreement with the experimental data (348 nm).This indicated that the adopted MN15/6-31G (d, p) method was extremely reliable for further investigation of photophysical properties in other solvents.In addition, the absorption peaks were at 351 nm (ACE), 352 nm (DCM) and 352 nm (n-Hexane), implying that solvent polarity had little effect on them.The fluorescence peaks of the enol * form in ACN, ACE, DCM and n-Hexane corresponded to 733 nm, 687 nm, 683 nm and 661 nm, respectively.The result distinctly implied that the short wavelength Stokes shift was 309-382 nm and that the fluorescence of the HTP-1 came from enol * form.Meanwhile, for HTP-2 (see Figure 9), the absorption peaks in the four solvents were located at 352 nm (ACN), 352 nm (ACE), 354 nm (DCM) and 356 nm (n-Hexane).Moreover, the fluorescence of the enol * and keto * states for HTP-2 was quenched, which was consistent with the fact that no fluorescence was observed in the experiment [42].To explain this phenomenon, we performed the hole-electron analysis shown in Figure 10.The correlative indicators depicting the S1 and S1′ forms of HTP-1 and HTP-2 in four solvents were included in Table 5.The index D symbolizes the distance between the hole centroid and the electron centroid.The index D symbolizes the distance between holes centroid and electrons centroid, H reflects the overall average distribution width for holes and electrons, Sr describes the extent to which holes and electrons overlap and t represents the degree of separation between holes and electrons.In general, the larger the D index was, the lower the Sr index was and the more positive the t index was, which led to a more adequate separation of electrons and holes.Obviously, both D and t indices of HTP-2 were large, whereas the Sr indices were minimal (between 0.1 and 0.2).The above data indicated that the overlap of electrons and holes was small, which enhanced the ICT process of HTP-2.However, compared with that of HTP-2, the t index of HTP-1 was negative (−1.967).This implied that there was little separation between the distributions of holes and electrons.The hole-electron analysis also verified that HTP-1 did not undergo the ICT process and that the extremely small overlap of holes and electrons was the reason for the fluorescence quenching of HTP-2.
Table 5. Indexed characterization of holes and electrons for HTP-1 and HTP-2 in four solvents (D: the distance between the hole centroid and the electron centroid.Sr: the degree of overlap between the holes and the electrons.H: the overall average distribution width between the holes and the electrons.t: the degree of separation between the holes and the electrons).

D (Å) Sr H (Å) T (Å)
HTP-  To explain this phenomenon, we performed the hole-electron analysis shown in Figure 10.The correlative indicators depicting the S 1 and S 1 ′ forms of HTP-1 and HTP-2 in four solvents were included in Table 5.The index D symbolizes the distance between the hole centroid and the electron centroid.The index D symbolizes the distance between holes centroid and electrons centroid, H reflects the overall average distribution width for holes and electrons, Sr describes the extent to which holes and electrons overlap and t represents the degree of separation between holes and electrons.In general, the larger the D index was, the lower the Sr index was and the more positive the t index was, which led to a more adequate separation of electrons and holes.Obviously, both D and t indices of HTP-2 were large, whereas the Sr indices were minimal (between 0.1 and 0.2).The above data indicated that the overlap of electrons and holes was small, which enhanced the ICT process of HTP-2.However, compared with that of HTP-2, the t index of HTP-1 was negative (−1.967).This implied that there was little separation between the distributions of holes and electrons.The hole-electron analysis also verified that HTP-1 did not undergo the ICT process and that the extremely small overlap of holes and electrons was the reason for the fluorescence quenching of HTP-2.

Reduced Density Gradient (RDG) Analysis
To visually differentiate the different interaction types and analyse the influence of solvent polarity on IHB strength [53], we calculated the RDG scatter plots and isosurface plots of HTP-1 and HTP-2 in four solvents.The expression was: ′ form of HTP-2.
Table 5. Indexed characterization of holes and electrons for HTP-1 and HTP-2 in four solvents (D: the distance between the hole centroid and the electron centroid.Sr: the degree of overlap between the holes and the electrons.H: the overall average distribution width between the holes and the electrons.t: the degree of separation between the holes and the electrons).

Reduced Density Gradient (RDG) Analysis
To visually differentiate the different interaction types and analyse the influence of solvent polarity on IHB strength [53], we calculated the RDG scatter plots and isosurface plots of HTP-1 and HTP-2 in four solvents.The expression was: The second eigenvalue equation of the Hessian matrix was: As shown in the colour scale at the top of Figures 11 and 12, blue represents HB interactions, and the HB intensity increased gradually with the deepening of the colour.Red represents the spatial effects, and green represents the van der Waals interactions.In the S 0 state (see Figures S1-S3), the spike peaks of HTP-1 and HTP-2 in four solvents were located at −0.05 and −0.041~−0.43,respectively.The interaction type was ascribed to the HB interaction, corresponding to IHB O 1 -H 1 ...N 1 .After photoexcitation, the spike peaks of HTP-1 right-shifted to −0.046~−0.049,indicating that IHBs (O 1 -H 1 ...N 1 ) were weakened in the excited state.However, the spike peaks of HTP-2 left-shifted to −0.051~−0.056,indicating that IHBs (O 1 -H 1 ...N 1 ) were enhanced in the excited state.The result was consistent with molecular configuration and IR spectra analysis.The non-covalent interaction analysis confirmed the existence of an HB interaction.Moreover, the strength of IHBs for HTP-1 did not change with the solvent polarity, whereas the IHB strength for HTP-2 in polar solvents followed the order of DCM > ACE > ACN.

Theoretical Methods
All theoretical calculations were conducted using Gaussian 16 software [54].Using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods, all geometric structures of HTP-1 and HTP-2 were completely optimized in four solvents (polarity order of solvents: acetonitrile (ACN) > acetone (ACE) > dichloromethane (DCM) > n-Hexane, see Tables S2 and S3).To simulate the experimental surroundings, we used the polarizable continuum model (PCM) of the formal variant of the integral equation (IEFPCM) [55].In addition, we selected five typical functionals to calculate the absorption spectrum of HTP-1 in ACN solvent.Accordingly, the absorption peak (351 nm) obtained by MN15/6-31G (d, p) was the closest to the experimental data (348 nm) (see Table 6), which proved the dependability of selecting MN15/6-31G (d, p) for further theoretical calculations.To quantitatively analyse the charge distribution around the IHB after photoexcitation, we calculated the natural bond orbital (NBO) charge distribution.In addition, to visually demonstrate the internal mechanism of the ESIPT reaction, we scanned the S 0 -and S 1 stated PECs of HTP-1 and HTP-2 in ACN, ACE, DCM and n-Hexane solvents.The hole-electron analysis, FMOs and the RDG parameters were performed by the Multiwfn program [56][57][58][59], and VMD 1.9.4 software was used for visualization [60].

Figure 5 .
Figure 5. Infrared (IR) vibrational spectra of O 1 -H 1 and N 1 -H 1 bonds for HTP-1 in the S 0 and S 1 states.

2
, the charge on the O1 atom hardly changed before and after photoexcitation.However, the charge on the N1 atom in ACN, ACE, DCM and n-Hexane solvents were −0.547, −0.546, −0.544 and −0.536 in the S0 state, which increased to −0.556, −0.556, −0.555 and −0.553 in the S1 state.The increase in charge on the N1 atom enhanced its ability to attract protons (H1), which could promote the proton transfer in the excited state.The results indicated that ICT behaviour occurred and facilitated the ESIPT tendency in HTP-2.

Figure 8 .
Figure 8. Simulated absorption and emission spectra of HTP-1 in ACN, ACE, DCM and n-Hexane solvents.

Figure 8 .
Figure 8. Simulated absorption and emission spectra of HTP-1 in ACN, ACE, DCM and n-Hexane solvents.

Figure 9 .
Figure 9. Simulated absorption and emission spectra of HTP-2 in ACN, ACE, DCM and n-Hexane solvents.

Figure 9 .
Figure 9. Simulated absorption and emission spectra of HTP-2 in ACN, ACE, DCM and n-Hexane solvents.