Theoretical Investigation of Excited-State Intramolecular Double-Proton Transfer Mechanism of Substituent Modified 1, 3-Bis (2-pyridylimino)-4,7-dihydroxyisoindole in Dichloromethane Solution

In this paper, density functional theory (DFT) and time-dependent DFT (TDDFT) methods were used to investigate substituent effects and excited-state intramolecular double-proton transfer (ESIDPT) in 1, 3-bis (2-pyridylimino)-4, 7-dihydroxyisoindole (BPI–OH) and its derivatives. The results of a systematic study of the substituent effects of electron-withdrawing groups (F, Cl and Br) on the adjacent sites of the benzene ring were used to regulate the photophysical properties of the molecules and the dynamics of the proton-transfer process. Geometric structure comparisons and infrared (IR) spectroscopic analysis confirmed that strengthening of the intramolecular hydrogen bond in the first excited state (S1) facilitated proton transfer. Functional analysis of the reduced density gradient confirmed these conclusions. Double-proton transfer in BPI–OH is considered to occur in two steps, i.e., BPI–OH (N) [Formula: see text] BPI–OH (T1) [Formula: see text] BPI–OH (T2), in the ground state (S0) and the S1 state. The potential-energy curves (PECs) for two-step proton transfer were scanned for both the S0 and S1 states to clarify the mechanisms and pathways of proton transfer. The stepwise path in which two protons are consecutively transferred has a low energy barrier and is more rational and favorable. This study shows that the presence or absence of coordinating groups, and the type of coordinating group, affect the hydrogen-bond strength. A coordinating group enhances hydrogen-bond formation, i.e., it promotes excited-state intramolecular proton transfer (ESIPT).

Hydrogen-bonding interactions are typically present in the microscopic frameworks of molecules and supramolecules, e.g., in proteins, DNA, and some polymers [22,23]. Intramolecular hydrogen bonds are also formed between proton donors and acceptors, and are prerequisites for ESIPT reactions. The theory of excited-state hydrogen-bond strengthening, which was proposed by Han et al., has attracted signi cant attention in recent years [24][25][26][27][28][29]. In addition to hydrogen bonding, the substituents on donor and acceptor units, solvent polarity, and pH values of the surrounding media can affect the ESIPT rate [30][31][32][33]. Some progress has been made in improving the photophysical properties of probes and sensors by using halogen atoms as coordinating groups. Piechowska's group reported that large Stokes shifts could be obtained by introducing electron-donating and electron-accepting groups into pyridine rings, and investigated the effects of such groups on the ESPT behavior of 10-hydroxy-11H-benzo[b] uoren-11-one (HBQ) [34]. Yu et al. reported that the photophysical properties could be adjusted by introducing electronaccepting or electron-donating substituents into a phenolic ring [35].
Recently, the photo-physical behavior of 5,6-dichloro-1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole and the absorption and emission properties of ten isomers have been reported [36]. However, in this previous study, the effects of substituents on the ESIDPT process were not considered. Here, we systematically investigated the effects of different electron-withdrawing substituents (F, Cl, and Br) in BPI-OH on proton transfer in the S 0 and S 1 states. We used DFT and TDDFT methods to explore the effects of electronwithdrawing groups (F, Cl, and Br); the geometric parameters, infrared (IR) spectra, and reduction density gradient (RDG) function were examined. The frontier molecular orbitals (FMOs) [37] and Mulliken's charge were used to predict the charge distribution. Most importantly, details of the ESIDPT mechanism were clari ed by constructing the potential-energy curves (PECs) for the S 0 and S 1 states.

Computational Details
DFT/TDDFT calculations were performed using the Gaussian 16 program suite [38]. The geometric structures and reaction pathways were calculated without constraint by using the B3LYP function (Hybrid Becke, three-parameter, with 20% Hartree-Fock exchange energy) with the TZVP basis set [39,40].
Dichloromethane (DCM) was selected as the solvent in all caiculations on the basis of the integral equation formalism variant in the polarizable continuum model (IEFPCM) [41][42][43]. Vibration frequency calculations were performed to con rm each optimized structure corresponding to a local minimum (no imaginary frequency). The RDG function was introduced to investigate non-covalent interactions by using the Multiwfn program [44][45][46]. The PECs for the S 0 and S 1 states in double-proton transfer were obtained at the B3LYP/TZVP level.

Geometric structures and IR vibrational spectra
Calculations were performed at the B3LYP/TZVP level to determine the geometric con gurations of BPI- Å, respectively, in the S 1 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 S 0 and S 1 states.
This shows that the further apart the hydroxyl group is, the easier proton loss is; this is conducive to proton transfer. The  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 S 1 state than in the S 0 state.
BPI-OH (T 2 ), BPI-OH-F (T 2 ), BPI-OH-Cl (T 2 ), and BPI-OH-Br (T 2 ) are formed via two ESIPT processes, along with two corresponding hydrogen bonds. The O 1 ···H 2 -N 3 is discussed because of its symmetrical structure. The data in Table 3 show that in the S 0 state, the O 1 ···H 2 and H 2 -N 3 bond lengths in BPI-OH (T 2 ),   Table 3 Calculated primary bond lengths (Å) and angles (°) for BPI-OH (T 2 ), BPI-OH-F (T 2 ), BPI-OH-Cl (T 2 ), and BPI-OH-Br (T 2 ) in S 0 and S 1 states. The IR vibrational spectra of BPI-OH and its derivatives were calculated. The stretching vibration frequencies on the S 0 and S 1 states for BPI-OH, BPI-OH-F, BPI-OH-Cl, and BPI-OH-Br are shown in Figs show gradually increasing red shifts from the S 0 to S 1 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 S 0 to the S 1 state. These data con rm 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.  Fig. 7. This shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO); the S 0 -S 1 transitions can be described as predominantly ππ*-type transitions. The data in Table 4  The data in Table 5 show that for BPI-OH-F (N), the Mulliken's charge on O 1 decreases from − 0.275 to -0.270 and that on N 3 increases from − 0.226 to -0.236. This indicates that the electronegativity of N 3 increases and that of O 1 decreases. The attraction between N 3 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.

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 O 1 -H 2 and O 4 -H 5 bond distances xed 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 S 1 state.
The electronic energies of the keto products (T 1 ) 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 S 0 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 S 0 state than in the S 1 state. Single-proton transfer is exothermic in the S 1 state but endothermic in the S 0 state. The results show that proton transfer may occur in S 1 state.
Previous studies have shown that two protons in the BPI-OH molecule are transferred step-by-step in the S 0 and S 1 states, i.e., proton transfer along the O 1 -H 2 bond occurs only after completion of transfer of the rst proton along the O 4 -H 5 bond [47]. This represents a proton-relay process. Figure 9 shows that for T 1 →T 2 , 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 S 1 state. Compared with the energy barriers for proton transfer along the O 1 -H 2 bond in the rst step, the corresponding values for the O 4 -H 5 bond are much higher in both the S 0 and S 1 states. Reaction barrier is lowest for BPI-OH-F, therefore the ESIPT reaction occurs more easily. In summary, substituent modi cation facilitates ESIPT, and the higher the electron absorption intensity, the lower the energy barrier for a proton-transfer reaction.

Conclusions
In summary, new insights into substituent modi cation and the ESIDPT mechanism for symmetric structures of BPI-OH and its derivatives were obtained by using DFT/TDDFT methods. Comparisons of the changes in the primary bond parameters con rmed that substituent modi cation strengthened excited-state intermolecular hydrogen bonds. Analysis of PECs in the S 0 and S 1 states showed that proton transfer involved two steps in the S 1 state, and was di cult to achieve in the S 0 state. Our results suggest that strengthening of intramolecular hydrogen bonds can effectively decrease the energy barriers and promote the proton-transfer process. Modi cation with electron-withdrawing groups (F, Cl, and Br) favors proton-transfer reactions. These distinctive properties will enable control of ESIDPT processes and provide guidance for the design and synthesis of similar molecules.

Declarations
Funding The open fund of the state key laboratory of molecular reaction dynamics in DICP, CAS is provided by Yi Wang.