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In this work, based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we theoretically explore the excited state behavior for a novel dye molecule 3,3′‐(5,5′‐((1E,1E′)‐hydrazine‐1,2‐diylidenebis (methanylylidene))bis(2‐morpholinothiazole‐5,4‐diyl))bis(4‐hydroxy‐2H‐chromen‐2‐one) (CS‐Azine). Coupling with atoms in molecules, we investigate the intramolecular dual hydrogen bonds of CS‐Azine system and verify the formation of them. Via study the primary bond distances, bond angle, and infrared vibrational spectra involved in hydrogen bonding moieties, we find O1‐H2···N3 and O4‐H5···N6 of CS‐Azine should be strengthened in the S1 state. When exploring the photo‐excitation process, we confirm that the charge redistribution around hydrogen bonding moieties reveals the tendency of ESIPT reaction. To further investigate whether single or double proton transfer occurs in the S1 state, we consider two kinds of reaction paths (ie, the stepwise and synergetic ESIPT reactions). And the constructed potential energy curves demonstrate that only the single proton transfer reaction should be the most supported in the S1 state from CS‐Azine to CS‐Azine‐SPT form due to the low potential energy barrier. This work not only clarifies the excited state behavior and mechanism about CS‐Azine system but also paves the way for further applying CS‐Azine dye in future. Highlights: Although two hydrogen bonds exist for CS‐Azine system, only the single proton transfer reaction should be the most supported one in the S1 state. [ABSTRACT FROM AUTHOR]

Abstract: In this present work, using density functional theory and time‐dependent density functional theory methods, we theoretically study the excited‐state hydrogen bonding dynamics and the excited state intramolecular proton transfer mechanism of a new 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol (2PYP) system. Via exploring the reduced density gradient versus sign(λ2(r))ρ(r), we affirm that the intramolecular hydrogen bond O1‐H2⋯N3 is formed in the ground state. Based on photoexcitation, comparing bond lengths, bond angles, and infrared vibrational spectra involved in hydrogen bond, we confirm that the hydrogen bond O1‐H2⋯N3 of 2PYP should be strengthened in the S1 state. Analyses about frontier molecular orbitals prove that charge redistribution of 2PYP facilitates excited state intramolecular proton transfer process. Via constructing potential energy curves and searching transition state structure, we clarify the excited state intramolecular proton transfer mechanism of 2PYP in detail, which may make contributions for the applications of such kinds of system in future. [ABSTRACT FROM AUTHOR]

In the present work, using density functional theory and time-dependent density functional theory methods, we investigated and presented the excited-state intramolecular proton transfer (ESIPT) mechanisms of a novel Compound 1 theoretically. Analyses of electrostatic potential surfaces and reduced density gradient (RDG) versus sign(λ2)ρ, we confirm the existence of intramolecular hydrogen bond O1-H2···N3 for Compound 1 in the S0 state. Comparing the primary structural variations of Compound 1 involved in the intramolecular hydrogen bond, we find that O1-H2···N3 should be strengthened in the S1 state, which may facilitate the ESIPT process. Concomitantly, infrared (IR) vibrational spectra analyses further verify the stability of hydrogen bond. In addition, the role of charge transfer interaction has been addressed under the frontier molecular orbitals, which depicts the nature of electronical excited state and supports the ESIPT reaction. The theoretically scanned and optimized potential energy curves according to variational O1-H2 coordinate demonstrate that the proton transfer process should occur spontaneously in the S1 state. It further explains why the emission peak of Compound 1-enol was not reported in previous experiment. This work not only presents the ESIPT mechanism of Compound 1 but also promotes the understanding of this kind of molecules for further applications in future. [ABSTRACT FROM AUTHOR]

In this present work, we theoretically investigate a novel system 3‐hydroxy‐2‐(5‐(5‐(5‐(3‐hydroxy‐4‐oxo‐4H‐chromen‐2‐yl)thiophen‐2‐yl)thiophen‐2‐yl)thiophen‐2‐yl)‐4H‐chromen‐4‐one (FT) based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. Via calculating the reduced density gradient (RDG) versus sign(λ2) ρ, we firstly verify the formation of the dual intramolecular hydrogen bonds (O1─H2···O3 and O4─H5···O6) for FT form in the S0 state. Then comparing the primary structural parameters and corresponding infrared (IR) vibrational spectra involved in hydrogen bonds between S0 and S1 state, we demonstrate that these two intramolecular hydrogen bonds should be strengthened in the S1 state. Insights into the vertical excitation process, our theoretical results reproduced experimental absorption nature, which confirms that the theoretical level (B3LYP/TZVP) is reasonable and effective in this work. And frontier molecular orbitals (MOs) depict the nature of electronically excited state and support the excited‐state intramolecular proton transfer (ESIPT) reaction. According to the calculated results of potential energy curves along stepwise and synergetic O1─H2 and O4─H5 coordinates, we verify that only the excited‐state single‐proton transfer could occur for FT molecule in the S1 state, although it possesses two intramolecular hydrogen bonds. We not only investigate the detail excited‐state behaviors for FT system and elaborate the ESIPT mechanism but also explain previous experimental results. Dual intramolecular hydrogen bonds should be strengthened upon photoexcitation. The charge redistribution reveals the tendency of ESIPT reaction. We confirm that only single proton transfer occurs in the S1 state. [ABSTRACT FROM AUTHOR]

Given that molecular excited state dynamical process plays important roles in designing and developing novel applications in recent years. In this work, based on density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we theoretically explored the novel (benzo[d]thiazol‐2‐yl)‐5‐(9H‐carbazol‐9‐yl)phenol (HBT‐Cz) system about its excited state behaviors. Via simulating the electrostatic potential surface (EPS) of HBT‐Cz structure, we confirm the formation of intramolecular hydrogen bond O2—H3···N4 in the S0 state. Our theoretical dominating bond lengths, bond angles, and the infrared (IR) vibrational spectra involved in hydrogen bond demonstrate that O2—H3···N4 should be strengthened in the S1 state. Upon the photoexcitation, we find the charge transfer characteristics around hydrogen bonding moieties play important roles in facilitating excited state intramolecular proton transfer (ESIPT) process. Via constructing potential energy curves, we confirm the ESIPT reaction that further explains previous experimental phenomenon. Moreover, we also search the S1‐state transition state (TS) structure along with ESIPT path, based on which we simulate the intrinsic reaction coordinate (IRC) path. Combing with the atom‐centered density matrix propagation (ADMP) molecular dynamics simulation, we further confirm the ESIPT mechanism presented in this work. We sincerely hope that our theoretical work could guide novel applications based on HBT‐Cz system in future. [ABSTRACT FROM AUTHOR]

Using the density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we theoretically explore a novel fluorescent sensor molecule (abbreviated as "2") (Sensors Actuat B‐Chem. 2018, 263, 585). Because of its symmetry, three stable structures can be located, ie, 2‐enol, 2‐SPT, and 2‐DPT forms in both S0 and S1 states. Via comparing the bond lengths and bond angles involved in the hydrogen bonding moieties, we find the dual intramolecular hydrogen bonds should be strengthened in the S1 state. And based on infrared (IR) vibrational simulations, we further confirm the strengthening dual hydrogen bonds. Upon the photo‐excitation process, the charge redistribution via frontier molecular orbitals (MOs) reveals the tendency of excited state intramolecular proton transfer (ESIPT) reaction. In addition, the constructed S0‐state and S1‐state potential energy curves demonstrate that the excited state single proton transfer (ESSPT) should be the most supported one from 2‐enol to 2‐SPT form. In view of the S1‐state stable 2‐SPT and 2‐DPT structures as well as the fluorescence peaks of them, we can further confirm the ESSPT mechanism for 2 chemosensor. This work not only clarifies the excited state behaviors of 2 system but also successfully explain the previous experimental phenomenon. The strengthening dual intramolecular hydrogen bonds (O1H2···N3 and O4H5···N6) of 2 provides the possibility for ESIPT process.The redistribution of charge involved in the hydrogen bonding moieties facilitates the ESIPT reaction.Potential energy curves and emission spectra confirm that the excited state single proton transfer should occur for 2 system. [ABSTRACT FROM AUTHOR]