*2.3. Structural Optimizations*

All optimized E forms of 3HF, 3HFW, and 3HF encapsulated in the γ-CD cavity (Form I, Form II, Form I-W and Form II-W) with important labeled atoms and distances (O−H covalent bonds, intraHB, and interHBs) in the S<sup>0</sup> state are shown in Figure 4, where the measured distances in the S<sup>0</sup> and S<sup>1</sup> states are summarized in Table 1.

states.

**Compound** 

**Figure 4.** All S0 optimized structures of 3HF, 3HFW, and the different conformations of the 3HF/γ-CD inclusion complexes (Form I and Form II) as well as its inclusion complex with a water molecule (Form I-W and Form II-W) computed by PBE0/def2-SVP level of theory. The blue and green dot lines represent intraHB in 3HF and interHBs between 3HF and a water molecule, respectively. **Figure 4.** All S<sup>0</sup> optimized structures of 3HF, 3HFW, and the different conformations of the 3HF/γ-CD inclusion complexes (Form I and Form II) as well as its inclusion complex with a water molecule (Form I-W and Form II-W) computed by PBE0/def2-SVP level of theory. The blue and green dot lines represent intraHB in 3HF and interHBs between 3HF and a water molecule, respectively.

**Table 1.** A summary of the important bonds and distances involving PT process of enol form for all complexes in the S0 and S1 **Important Bond Distance (Å) S0 State S1 State**  From Table 1, for the compounds without a water molecule (3HF, Form I, and Form II), the O1–H1 covalent bond at the 3HF hydroxyl group slightly increases from the S<sup>0</sup> to S<sup>1</sup> states around 0.015–0.034 Å. Whereas, the length of O2· · · H1 intraHB between the hydroxyl group and carbonyl group of 3HF significantly decreases in the S<sup>1</sup> state at 0.214,

> From Table 1, for the compounds without a water molecule (3HF, Form I, and Form II), the O1–H1 covalent bond at the 3HF hydroxyl group slightly increases from the S0 to S1 states around 0.015–0.034 Å. Whereas, the length of O2···H1 intraHB between the hydroxyl group and carbonyl group of 3HF significantly decreases in the S1 state at 0.214, 0.174, and 0.097 Å for 3HF, Form I, and Form II, respectively. The length of O2···H1 intraHB of Form I and Form II is longer than that of the isolated 3HF because the O2 of 3HF forms a stronger interHB with a hydroxy group at the primary rim of γ-CD (O6–H) for Form I and at the secondary rim of γ-CD (O2–H) for Form II in the S1 state. Overall, these results indicate that the strength of an intraHB in the S1 state of 3HF, Form I, and Form II is stronger than that in the S0 state. Consequently, the ESIntraPT process might easily oc-

> For the compounds with a water molecule (3HFW, Form I-W, and Form II-W), a water molecule forms interHBs with 3HF, therefore an O2···H1 intraHB of these complexes is dramatically longer than that of the compounds without a water molecule. It can be stated that a water molecule might block ESIntraPT process and support ESInterPT process. The O1–H1 and Ow–Hw covalent bonds of 3HFW/Form II-W are slightly increased from the S0 to S1 states at 0.069/0.074 Å for O1–H1 and 0.024/0.023 Å for Ow–Hw covalent

**O1–H1 O2···H1 Ow···H1 Ow–Hw O2···Hw O1–H1 O2···H1 Ow···H1 Ow–Hw O2···Hw** 

**Form I-W** 1.009 2.390 1.585 0.977 1.808 1.030 2.415 1.503 0.978 1.903 **Form II-W** 1.004 2.429 1.612 0.983 1.733 1.078 2.452 1.375 1.006 1.581

**3HF** 0.983 1.920 1.017 1.706

**Form II** 0.981 2.177 0.996 2.080

cur in the S1 state.

**Form I** 0.979 1.978 1.002 1.804

0.174, and 0.097 Å for 3HF, Form I, and Form II, respectively. The length of O2· · · H1 intraHB of Form I and Form II is longer than that of the isolated 3HF because the O2 of 3HF forms a stronger interHB with a hydroxy group at the primary rim of γ-CD (O6–H) for Form I and at the secondary rim of γ-CD (O2–H) for Form II in the S<sup>1</sup> state. Overall, these results indicate that the strength of an intraHB in the S<sup>1</sup> state of 3HF, Form I, and Form II is stronger than that in the S<sup>0</sup> state. Consequently, the ESIntraPT process might easily occur in the S<sup>1</sup> state.

**Table 1.** A summary of the important bonds and distances involving PT process of enol form for all complexes in the S<sup>0</sup> and S<sup>1</sup> states.


For the compounds with a water molecule (3HFW, Form I-W, and Form II-W), a water molecule forms interHBs with 3HF, therefore an O2· · · H1 intraHB of these complexes is dramatically longer than that of the compounds without a water molecule. It can be stated that a water molecule might block ESIntraPT process and support ESInterPT process. The O1–H1 and Ow–Hw covalent bonds of 3HFW/Form II-W are slightly increased from the S<sup>0</sup> to S<sup>1</sup> states at 0.069/0.074 Å for O1–H1 and 0.024/0.023 Å for Ow–Hw covalent bonds. While the lengths of Ow· · · H1 and O2· · · Hw interHBs of 3HFW/Form II-W are significantly decreased from the S<sup>0</sup> to S<sup>1</sup> states around 0.236/0.237 Å for Ow· · · H1 and 0.151/0.152 Å for O2· · · Hw interHBs, so ESInterPT may easily take place more than ESIntraPT for 3HFW and Form II-W due to the strong interHBs in the S<sup>1</sup> state. For Form I-W, the length of Ow· · · H1 interHB decreases (0.082 Å), while the length of O2· · · Hw interHB increases (0.095 Å) in the S<sup>1</sup> state because Hw of a water molecule forms a stronger interHB with O6 at a primary rim of γ-CD instead of O2 acceptor of 3HF in the S<sup>1</sup> state. Consequently, ESInterPT of Form I-W is quite difficult to occur because the O2· · · Hw interHB between 3HF and a water molecule is weaker (elongated length) in the S<sup>1</sup> state. Furthermore, the strength of intraHBs and interHBs in the S<sup>1</sup> state will be further discussed in the next section.
