3.1 Geometric structures and IR vibrational spectra
Calculations were performed at the B3LYP/TZVP level to determine the geometric configurations of BPI-OH (N), BPI-OH (T1), and BPI-OH (T2) in DCM; the results are shown in Fig. 1. The optimized geometric structures of BPI-OH-F, BPI-OH-Cl, and BPI-OH-Br are show in Fig. 2. The optimized geometries of BPI-OH and its derivatives all have the same symmetry. The bond length of O1-H2 is the same as the bond length of O4-H5, and the O1-H2…N3 bond angle is the same as the O4-H5…N6 bond angle. Table 1 lists the hydrogen-bond lengths in BPI-OH and its derivatives (normal structure) in the S0 and S1 states. The O1-H2 hydrogen bond length in BPI-OH (N) is 0.9813 Å in S0 state, and 0.9975 Å in S1 state. For BPI-OH-F (N), BPI-OH-Cl (N), and BPI-OH-Br (N), the O1-H2 bond lengths are 0.9832, 0.9843, and 0.9847 Å, respectively, in the S0 state. These increase by 0.0189, 0.0189, and 0.0190 Å, respectively, to 1.0021, 1.0032, and 1.0037 Å, respectively, in the S1 state. In BPI-OH-F (N), BPI-OH-Cl (N), and BPI-OH-Br (N), the distances between hydroxyl groups are greater than the corresponding distances in BPI-OH (N) in both the S0 and S1 states. This shows that the further apart the hydroxyl group is, the easier proton loss is; this is conducive to proton transfer. The H2-N3 hydrogen-bond length in BPI-OH (N) is 2.0590 Å in S0 state and 1.9486 Å in S1 state. For BPI-OH-F (N), BPI-OH-Cl (N), and BPI-OH-Br (N), the H2-N3 bond lengths are 2.0538, 2.0217, and 2.0104 Å, respectively, in the S0 state. These decrease by 0.1263, 0.1172, and 0.1147 Å, respectively, to 1.9275, 1.9045, and 1.8957 Å, respectively, in the S1 state. For the BPI-OH (N), BPI-OH-F (N), BPI-OH-Cl (N), and BPI-OH-Br (N), the O1-H2…N3 bond angles change from 143.77°, 143.42°, 144.59°, and 144.99° in the S0 state to 147.27°, 147.32°, 148.21°, and 148.52°, respectively, in the S1 state, i.e., they increase by 3.50°, 3.90°, 3.62°, and 3.53°, respectively. In BPI-OH-Br (N), BPI-OH-Cl (N), and BPI-OH-F (N), the bond angles tend to be larger than those in BPI-OH (N). This shows that the presence of a ligand increases the ability of the proton acceptor to capture a proton.
Table 1
Calculated primary bond lengths (Å) and angles (°) for BPI-OH (N), BPI-OH-F (N), BPI-OH-Cl (N), and BPI-OH-Br (N) in S0 and S1 states.
|
BPI-OH(N)
|
BPI-OH-F(N)
|
BPI-OH-Cl(N)
|
BPI-OH-Br(N)
|
parameter
|
S0
|
S1
|
S0
|
S1
|
S0
|
S1
|
S0
|
S1
|
O1-H2
|
0.9813
|
0.9975
|
0.9832
|
1.0021
|
0.9843
|
1.0032
|
0.9847
|
1.0037
|
H2-N3
|
2.0590
|
1.9486
|
2.0538
|
1.9275
|
2.0217
|
1.9045
|
2.0104
|
1.8957
|
δ(O1-H2-N3)
|
143.77
|
147.27
|
143.42
|
147.32
|
144.59
|
148.21
|
144.99
|
148.52
|
O4-H5
|
0.9813
|
0.9975
|
0.9832
|
1.0021
|
0.9843
|
1.0032
|
0.9848
|
1.0038
|
H5-N6
|
2.0590
|
1.9486
|
2.0539
|
1.9275
|
2.0218
|
1.9044
|
2.0103
|
1.8968
|
δ(O4-H5-N6)
|
143.80
|
147.37
|
143.42
|
147.32
|
144.59
|
148.21
|
144.99
|
148.47
|
Table 2 shows the parameters for the BPI-OH (T1), BPI-OH-F (T1), BPI-OH-Cl (T1), and BPI-OH-Br (T1) structures, which were formed via ESIPT. In hydrogen-bonded O1-H2···N3, the O1-H2 bond lengths are 2.1388, 2.1723, 2.1320, and 2.1244 Å, respectively, in S0 state, and 2.1637, 2.2158, 2.1838, and 2.1753 Å, respectively, in the S1 state. The H2···N3 bond lengths are 1.0218, 1.0207, 1.0217, and 1.0220 Å, respectively, in S0 state, and 1.0206, 1.0188, 1.0194, 1.0196 Å, respectively, in the S1 state. The O1…H2-N3 bond angle changes from 133.46°, 132.73°, 133.09°,and 133.14° in S0 state to 132.48°, 131.43°, 131.46°,and 131.48°,respectively, in the S1 state. This indicates that the hydrogen bond in O1…H2-N3 is stronger in the S0 state. In hydrogen-bonded O4-H5…N6, the O4-H5 bond lengths increase from 0.9806, 0.9819, 0.9829, and 0.9835 Å in S0 state to 0.9861, 0.9890, 0.9902, and 0.9908 Å, respectively, in the S1 state. The H5…N6 bond lengths decrease from 2.0947, 2.0875, 2.0528, and 2.0372 Å, respectively, in the S0 state to 2.0362, 2.0099, 1.9817, and 1.9696 Å, respectively, in the S1 state. The O4-H5…N6 hydrogen-bond angles in BPI-OH (T1), BPI-OH-Br (T1), BPI-OH-Cl (T1), and BPI-OH-F (T1) are 2.61°,2.67°,2.73°,and 2.88°, respectively, higher in the S1 state than in the S0 state. This data show that the bond angle increases with increasing electron absorption intensity after addition of a coordinating group. These observations indicate that the intramolecular hydrogen bonds become stronger with increasing electron absorption intensity, and are stronger in the S1 state than in the S0 state.
BPI-OH (T2), BPI-OH-F (T2), BPI-OH-Cl (T2), and BPI-OH-Br (T2) are formed via two ESIPT processes, along with two corresponding hydrogen bonds. The O1···H2-N3 is discussed because of its symmetrical structure. The data in Table 3 show that in the S0 state, the O1···H2 and H2-N3 bond lengths in BPI-OH (T2), BPI-OH-F (T2), BPI-OH-Cl (T2), and BPI-OH-Br (T2) are 2.2752, 2.2878, 2.2512, and 2.2406 Å, respectively, and 1.0155, 1.0152, 1.0157, and 1.0159 Å, respectively. These are 2.2184, 2.2380, 2.2104, and 2.2009 Å, respectively, and 1.0186, 1.0181, 1.0183Å, and 1.0185Å, respectively, higher in the S1 state. The O1···H2-N3 bond angles change from 128.70°, 128.88°, and 128.97°,respectively, in the S0 state to 130.13°, 129.96°, and 130.00°, respectively, in the S1 state. These changes in the bond parameters prove that two intramolecular hydrogen bonds are strengthened on photoexcitation. The effect of hydrogen-bond enhancement is clearest when the coordinating group contains F. ESIPT is therefore promoted by the presence of a coordinating base and excited states.
Table 2
Calculated primary bond lengths (Å) and angles (°) for BPI-OH (T1), BPI-OH-F (T1), BPI-OH-Cl (T1), and BPI-OH-Br (T1) in S0 and S1 states.
|
BPI-OH(T1)
|
BPI-OH-F(T1)
|
BPI-OH-Cl(T1)
|
BPI-OH-Br(T1)
|
parameter
|
S0
|
S1
|
S0
|
S1
|
S0
|
S1
|
S0
|
S1
|
O1-H2
|
2.1388
|
2.1637
|
2.1723
|
2.2158
|
2.1320
|
2.1838
|
2.1244
|
2.1753
|
H2-N3
|
1.0218
|
1.0206
|
1.0207
|
1.0188
|
1.0217
|
1.0194
|
1.0220
|
1.0196
|
δ(O1-H2-N3)
|
133.46
|
132.48
|
132.73
|
131.43
|
133.09
|
131.46
|
133.14
|
131.48
|
O4-H5
|
0.9806
|
0.9861
|
0.9819
|
0.9890
|
0.9829
|
0.9902
|
0.9835
|
0.9908
|
H5-N6
|
2.0947
|
2.0362
|
2.0875
|
2.0099
|
2.0528
|
1.9817
|
2.0372
|
1.9696
|
δ(O4-H5-N6)
|
142.84
|
145.45
|
142.52
|
145.40
|
143.81
|
146.54
|
144.26
|
146.93
|
Table 3
Calculated primary bond lengths (Å) and angles (°) for BPI-OH (T2), BPI-OH-F (T2), BPI-OH-Cl (T2), and BPI-OH-Br (T2) in S0 and S1 states.
|
BPI-OH(T2)
|
BPI-OH-F(T2)
|
BPI-OH-Cl(T2)
|
BPI-OH-Br(T2)
|
parameter
|
S0
|
S1
|
S0
|
S1
|
S0
|
S1
|
S0
|
S1
|
O1-H2
|
2.2752
|
2.2184
|
2.2878
|
2.2380
|
2.2512
|
2.2104
|
2.2406
|
2.2009
|
H2-N3
|
1.0155
|
1.0186
|
1.0152
|
1.0181
|
1.0157
|
1.0183
|
1.0159
|
1.0185
|
δ(O1-H2-N3)
|
128.99
|
130.46
|
128.70
|
130.13
|
128.88
|
129.96
|
128.97
|
130.00
|
O4-H5
|
2.2750
|
2.2181
|
2.2882
|
2.2383
|
2.2512
|
2.2102
|
2.2407
|
2.2008
|
H5-N6
|
1.0155
|
1.0186
|
1.0152
|
1.0180
|
1.0157
|
1.0183
|
1.0159
|
1.0185
|
δ(O4-H5-N6)
|
128.99
|
130.47
|
128.70
|
130.12
|
128.89
|
129.96
|
128.96
|
130.00
|
The IR vibrational spectra of BPI-OH and its derivatives were calculated. The stretching vibration frequencies on the S0 and S1 states for BPI-OH, BPI-OH-F, BPI-OH-Cl, and BPI-OH-Br are shown in Figs. 3 and 4. For BPI-OH (N), the O1-H2 stretching frequency is 3484 cm− 1 in the S0 state and 3198 cm− 1 in the S1 state. A red shift of 286 cm− 1 was observed for the S1 state; this shows that enhanced hydrogen bonding results in a red shift. Similarly, the spectra of BPI-OH-F (N), BPI-OH-Cl (N), and BPI-OH-Br (N) show gradually increasing red shifts from the S0 to S1 state, namely 335, 333, and 333 cm− 1, respectively. Among the three derivatives of BPI-OH, BPI-OH-F (N) gives the highest red shift from the S0 to the S1 state. These data confirm an excited-state hydrogen-bond enhancing mechanism. The presence or absence of coordinating groups and different coordinating groups affect the hydrogen-bond strength. The presence of coordinating groups enhances hydrogen bonding, i.e., it promotes ESIPT.
Figure 3(b) shows that the O1-H2 vibration frequency red shifts by 120 cm− 1 from 3512 cm− 1 in the S0 state to 3392 cm− 1 in S1 state. This indicates that the O1-H2...N3 hydrogen bond is enhanced in the S1 state. The N6-H5 vibration blue shifts by 21 cm− 1 from 3409 cm− 1 in the S0 state to 3430 cm− 1 in the S1 state, i.e., the N6-H5...O4 hydrogen bond is stronger in the S0 state than in the S1 state. Figure 4 (b) shows red shifts of the vibration frequency of 121, 122, and 124 cm− 1 in the spectra of BPI-OH-Br (T1), BPI-OH-Cl (T1), and BPI-OH-F (T1), respectively. This shows that the red shift increases with increasing electron absorption intensity of the coordinating group. Figure 3(c) shows that the stretching vibration frequency of N3-H2 (N6-H5) in BPI-OH (T2) clearly blue shifts by 24 cm− 1. Figure 4 (c) shows vibration frequency blue shifts of 2, 6, and 15 cm− 1 for BPI-OH-Br (T2), BPI-OH-Cl (T2), and BPI-OH-F (T2), respectively. This shows that the N3-H2...O1 (N6-H5...O4) hydrogen bond is stronger in the S0 state than in the S1 state.
3.3 Electronic spectra, FMO analysis, and Mulliken’s charge distributions
The first excited states of BPI-OH-Br, BPI-OH-Cl and BPI-OH-F molecules were fully optimized by TDDFT/B3LYP/TZVP on the basis of their S0 state optimized structures. The absorption and emission spectra are shown in Fig. 6. The calculated absorption values for BPI-OH-Br (N), BPI-OH-Cl (N), and BPI-OH-F (N) are 410, 426, and 431 nm, respectively. The theoretical fluorescence values are 485, 505, and 512 nm for BPI-OH-Br (N), BPI-OH-Cl (N), and BPI-OH-F (N), respectively. These values are close to the experimental emission wavelengths. Figure 6 also shows the fluorescence spectra of BPI-OH-F (T1, T2), BPI-OH-Cl (T1, T2), and BPI-OH-Br (T1, T2) in DCM. During ESIPT, double fluorescence emissions are observed, and large Stokes shifts are observed for BPI-OH-Br (N) (122 nm), BPI-OH-Cl (N) (133 nm) and BPI-OH-F (N) (136 nm). The red shifts of the emission wavelengths and large Stokes shifts favor ESIPT. The addition of an F functional group is therefore most effective in enabling use of BPI-OH as a probe.
Redistribution of the electron densities of proton donors and proton acceptors provide the driving force for electron-spin transfer after photoexcitation. The FMOs of BPI-OH-Br (N), BPI-OH-Cl (N), and BPI-OH-F (N) are shown in Fig. 7. This shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO); the S0-S1 transitions can be described as predominantly ππ*-type transitions. The data in Table 4 show that the contributions to the excited electronic states are BPI-OH-F (N), 98.7%; BPI-OH-Cl (N), 98.9%; and BPI-OH-Br (N), 99.0%. The order of the oscillator strengths in the S1 state is BPI-OH (N), 0.3802; BPI-OH-Br (N), 0.5038; BPI-OH-Cl (N), 0.4659; and BPI-OH-F (N), 0.3832. The molecular optical polarizability and biological activity can be determined from the electron transition energy. The transition energies in the S1 state for BPI-OH-Br (N), BPI-OH-Cl (N), and BPI-OH-F (N) are 2.901, 2.929, and 3.016 eV, respectively (Fig. 7). The calculations show that BPI-OH-Br (N), BPI-OH-Cl (N), and BPI-OH-F (N) all have low kinetic stability and high chemical reactivity in DCM. After light excitation, the electron density is redistributed from the proton donor oxygen (O1) in the HOMO to the proton acceptor nitrogen (N3) in the LUMO. This enhances attraction to H and facilitates proton transfer from the enol structure to the keto structure in the S1 state.
The data in Table 5 show that for BPI-OH-F (N), the Mulliken’s charge on O1 decreases from − 0.275 to -0.270 and that on N3 increases from − 0.226 to -0.236. This indicates that the electronegativity of N3 increases and that of O1 decreases. The attraction between N3 and H is enhanced, which promotes ESIPT. Similar results were obtained for BPI-OH-Cl (N) and BPI-OH-Br (N). The above discussion shows that ESIPT is affected by a change in charge density.
Table 4
Calculated of electronic excitation energies (nm), corresponding oscillator strengths (ƒ) of low-lying electronically excited states for BPI-OH (N), BPI-OH-F (N), BPI-OH-Cl (N), and BPI-OH-Br (N), and orbital transition (OT) contributions to electronic excited states (CI).
|
transition
|
λ(nm/eV)
|
f
|
OT
|
CI (%)
|
BPI-OH (N)
|
S0→S1
|
433/2.86
|
0.3802
|
H→L
|
99.2%
|
BPI-OH-F (N)
|
S0→S1
|
410/3.02
|
0.3832
|
H→L
|
98.7%
|
BPI-OH-Cl (N)
|
S0→S1
|
426/2.91
|
0.4659
|
H→L
|
98.9%
|
BPI-OH-Br (N)
|
S0→S1
|
431/2.88
|
0.5038
|
H→L
|
99.0%
|
Table 5
Calculated Mulliken’s charge distributions for BPI-OH (N) and its derivatives in S0 and S1 states.
Mulliken’s charge
|
|
O1
|
H2
|
N3
|
|
S0
|
S1
|
S0
|
S1
|
S0
|
S1
|
BPI-OH-F (N)
|
-0.275
|
-0.270
|
0.320
|
0.328
|
-0.226
|
-0.236
|
BPI-OH-Cl (N)
|
-0.273
|
-0.268
|
0.318
|
0.325
|
-0.227
|
-0.236
|
BPI-OH-Br (N)
|
-0.277
|
-0.272
|
0.318
|
0.325
|
-0.228
|
-0.236
|
3.4 PEC analysis
PECs were constructed by step-by-step and point-to-point optimization to gain a deeper understanding of the effects of various ligands on ESIPT. As shown in Figs. 8 and 9, the PECs were constructed by keeping the O1-H2 and O4-H5 bond distances fixed at given values in steps of 0.05 Å, and optimizing the other atoms in the system without any constraints. As shown in Fig. 8, the reaction barriers for ESIPT are 4.03, 2.82, 2.79, and 2.68 kcal/mol for BPI-OH, BPI-OH-Br, BPI-OH-Cl, and BPI-OH-F, respectively, in the S1 state. The electronic energies of the keto products (T1) are more stable than those of the enol reactants (N), which indicates that the ESIPT reactions are exothermic. The reaction barriers for molecules with coordinating group, i.e., BPI-OH-Br, BPI-OH-Cl, and BPI-OH-F, are much lower than that for BPI-OH, therefore their ESIPT reactions are easier. The energy barriers make proton transfer reactions of BPI-OH, BPI-OH-Br, BPI-OH-Cl, and BPI-OH-F in the S0 state are 10.89, 9.22, 6.20, and 3.71 kcal/mol, respectively. These high energy barriers make proton transfer reactions of BPI-OH, BPI-OH-Br, and BPI-OH-Cl less likely in the S0 state than in the S1 state. Single-proton transfer is exothermic in the S1 state but endothermic in the S0 state. The results show that proton transfer may occur in S1 state.
Previous studies have shown that two protons in the BPI-OH molecule are transferred step-by-step in the S0 and S1 states, i.e., proton transfer along the O1-H2 bond occurs only after completion of transfer of the first proton along the O4-H5 bond [47]. This represents a proton-relay process. Figure 9 shows that for T1→T2, the reaction barriers for ESIPT are 7.88, 6.62, 6.54, and 6.49 kcal/mol for BPI-OH, BPI-OH-Br, BPI-OH-Cl, and BPI-OH-F, respectively, in the S1 state. Compared with the energy barriers for proton transfer along the O1-H2 bond in the first step, the corresponding values for the O4-H5 bond are much higher in both the S0 and S1 states. Reaction barrier is lowest for BPI-OH-F, therefore the ESIPT reaction occurs more easily. In summary, substituent modification facilitates ESIPT, and the higher the electron absorption intensity, the lower the energy barrier for a proton-transfer reaction.