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We theoretically investigate the excited state intramolecular proton transfer (ESIPT) mechanism about a novel doxorubicin (DOX) system based on density functional theory (DFT) and time-dependent DFT methods. We mainly focus on the double proton transfer process regarding the stepwise versus synchronous dual proton transfer mechanism. The changes in our calculated primary bond lengths and bond angles show that the intramolecular hydrogen bonds (O1-H2···O3 and O4-H5···O6) are both strengthened in the S1 state, which provides the possibility of the ESIPT process. Though our computational simulations, the DOX indicates the lowest absorption at around 490 nm. After excitation, we find three emission bands maximized at 656, 663 and 687 nm, which are ascribed to the tautomer. To further elucidate the ESIPT mechanism, we construct the potential energy surfaces (PESs) of both S0 and S1 states. Clearly, three kinds of stable structures could be located on the S1-state PES. We establish a new mechanism of DOX that excludes synchronous double proton transfer, while the stepwise double proton transfer mechanism is confirmed theoretically. We believe that the systematic investigation for photo-physical and photochemical properties of the antineoplastic drug DOX might be significant to obtain fundamental insight into the transport mechanism of DOX inside cultured cells. [ABSTRACT FROM AUTHOR]

Given the tremendous potential of fluorescence sensors in recent years, in this present work, we theoretically explore a novel fluorescence chemosensor [2‐(2‐Hydroxy‐phenyl)‐1H‐benzoimidazol‐5‐yl]‐phenyl‐methanone (HBPM) about its excited state behaviors and probe‐response mechanism. Using density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods, we explore the S0‐state and S1‐state hydrogen bond dynamical behaviors and confirm that the strengthening intramolecular hydrogen bond in the S1 state may promote the excited state intramolecular proton transfer (ESIPT) reaction. In view of the photoexcitation process, we find that the charge redistribution around the hydroxyl moiety plays important roles in providing driving force for ESIPT. And the constructed potential energy curves further verify that the ESIPT process of HBPM should be ultrafast. That is the reason why the normal HBPM fluorescence cannot be detected in previous experiment. Furthermore, with the addition of fluoride anions, the exothermal deprotonation process occurs spontaneously along with the intermolecular hydrogen bond O–H⋯F. It reveals the uniqueness of detecting fluoride anions using HBPM molecules. As a whole, the fluoride anions inhibit the initial ESIPT process of HBPM, which results in different fluorescence behaviors. This work presents the clear ESIPT process and fluoride anion‐sensing mechanism of a novel HBPM chemosensor. [ABSTRACT FROM AUTHOR]

In the present work, three novel phenols (10a,11‐dihydro‐4bH‐indeno[1,2‐b]quinolin‐4‐ol (1), 5,6‐dihydro‐benzo[c]acridin‐1‐ol (2), and 5,5,7,7a‐tetrahydro‐4aH‐13‐aza‐benzo[3,4]cyclohepta[1,2‐b]naphthalene‐1‐ol (3)) have been explored theoretically in detail. Using density functional theory (DFT) and time‐dependent DFT (TDDFT) methods, we inquire into the intramolecular hydrogen‐bonding interactions and the excited‐state intramolecular proton transfer (ESIPT) process. Exploring the steady‐state absorption and emission spectra under TDDFT/B3LYP/TZVP theoretical level in acetonitrile solvent, our calculated results demonstrate an experimental phenomenon. Based on analysis of the variations of geometrical parameters and infrared (IR) vibrational spectra, we confirm that O–H⋯N should be strengthened in the S1 state. Investigating the frontier molecular orbitals (MOs) and the charge density difference (CDD) maps, it can be confirmed that the charge redistribution facilitates the tendency of the ESIPT process for 1, 2, and 3 systems. By constructing potential energy curves, we confirm that the proton transfer should occur in the S1 state. In particular, the ESIPT for 2 and 3 systems are nonbarrier processes in the S1 state, which confirms that ESIPT should be exothermal spontaneously. This work explains previous experimental results and makes a reasonable assumption about the ESIPT mechanism for 1, 2 and 3 systems. We sincerely hope our work can facilitate understanding and promoting applications about them in future. The intramolecular hydrogen bond of 1, 2, and 3 compounds should be strengthened in the S1 state. By exploring frontier molecular orbitals and charge density difference, the charge redistribution can be confirmed to facilitate the excited‐state intramolecular proton transfer (ESIPT) tendency. The ESIPT reaction for 1 compound demonstrates a low potential barrier, while that for 2 and 3 compounds should be exothermal spontaneously. [ABSTRACT FROM AUTHOR]

Abstract A novel fluorescence molecule (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine (HMPM) has been explored theoretically in this present work. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we investigate the excited state hydrogen bonding behaviors and excite state intramolecular proton transfer (ESIPT) process for HMPM molecule. Via simulating the reduced density gradient (RDG) versus sign(λ 2)ρ, we firstly verify the double intramolecular hydrogen bonds (O1 H2⋯N3 and O4 H5⋯N6) for HMPM system. Comparing with the changes about these two hydrogen bonds (i.e., bond distances, bond angles and infrared (IR) vibrational spectra), we find that they should be enhanced in the first excited state upon the photo-excitation. The shortened hydrogen bonding distance of H2⋯N3 and H5⋯N6 provide the possibility for ESIPT reaction. Given the photo-excitation process, we confirm the charge redistribution around the hydrogen bonding moieties plays an important role as a driving force for the ESIPT process. Further, via constructing S 0 -state and S 1 -state potential energy surfaces (PESs), we confirm the excited state double proton transfer (ESDPT) is excludable since the high optimized energy and high potential energy barrier. While the low potential barrier for excited state single proton transfer path results in the ultrafast ESIPT reaction, which explains why the initial HMPM fluorescence peak cannot be detected in previous experimental phenomenon. This work not only clarifies the excited state dynamical behavior for HMPM system, but also explains previous experimental phenomenon and attributions about steady state spectra. We hope this work can facilitate novel applications based on the novel HMPM system in future. Graphical Abstract Unlabelled Image Highlights • Hydrogen bond of HMPM should be strengthened in the first excited state. • Charge transfer facilitates the ESIPT process for HMPM molecule. • Only the excited state single proton transfer process occurs for HMPM molecule. [ABSTRACT FROM AUTHOR]

In this work, density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are used to explore the excited‐state intramolecular proton transfer (ESIPT) mechanism of a novel system 4′‐dimethylaminoflavonol (DAF). By analyzing the molecular electrostatic potential (MEP) surface, we verify that the intramolecular hydrogen bond in DAF exists in both the S0 and S1 states. We calculate the absorption and emission spectra of DAF in two solvents, which reproduce the experimental results. By comparing the bond lengths, bond angles, and relative infrared (IR) vibrational spectra involved in the hydrogen bonding of DAF, we confirm the hydrogen‐bond strengthening in the S1 state. For further exploring the photoexcitation, we use frontier molecular orbitals to analyze the charge redistribution properties, which indicate that the charge transfer in the hydrogen‐bond moiety may be facilitating the ESIPT process. The constructed potential energy curves in acetonitrile and methylcyclohexane solvents with shortened hydrogen bond distances demonstrate that proton transfer is more likely to occur in the S1 state due to the lower potential barrier. Comparing the results in the two solvents, we find that aprotic polar and nonpolar solvents seem to play similar roles. This work not only clarifies the excited‐state behaviors of the DAF system but also successfully explains its spectral characteristics. Hydrogen bond should be formed in the S0 state for 4′‐dimethylaminoflavonol (DAF), as shown via Atoms in Molecules (AIM) analysis. The intramolecular hydrogen bond of DAF should be strengthened in the S1 state, which provides the tendency for the excited‐state intramolecular proton transfer (ESIPT) reaction. The charge redistribution and low potential energy barriers of DAF facilitate the ESIPT reaction. [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]

The excited-state intramolecular proton transfer (ESIPT) mechanism of a new compound ( E)-1-((2,2-diphenylhydrazono)methyl)naphthalen-2-ol ( EDMN) sensor, reported and synthesized by Mukherjee et al. [ Sensors Actuat. B-Chem. 2014, 202, 1190], is investigated in detail theoretically. The calculations on primary bond lengths, bond angles, and the corresponding infrared (IR) vibrational spectra and hydrogen-bond energy involved in intramolecular hydrogen bond between the S0 and S1 states confirm that the intramolecular hydrogen bond is strengthened in the S1 state, which reveals the tendency of ESIPT reaction. The fact that the experimental absorption and emission spectra are well reproduced demonstrates the rationality and effectiveness of the time-dependent density functional theory (TDDFT) level of theory we adopt here. Furthermore, intramolecular charge transfer based on the frontier molecular orbitals (MOs) gives indication of the ESIPT reaction. The constructed potential energy curves of both the S0 and S1 states while keeping the O─H distance of EDMN fixed at a series of values are used to illustrate the ESIPT process. The lower barrier of ~3.934 kcal/mol in the S1 state potential energy curve (lower than the 8.254 kcal/mol in the S0 state) provides the transfer mechanism. [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]