Substituent effects on the ESIPT process and the potential applications in materials transport field of 2'-aminochalcone derivatives

This work investigates the different charge transfer characteristics and excited state intramolecular proton transfer process (ESIPT) of 2'-aminochalcones derivatives carrying different electron-withdrawing groups. Four new molecules are designed in the experiment and named as 2c, 3c, 4c and 5c, respectively. (Dyes and Pigments, 2022, 202.) Based on these four molecules, the effect of substituents on the ESIPT process and the charge transfer process are discussed in detail in our work. According to the study of the related parameters at the hydrogen bond site, infrared vibration spectrum, interaction region indicator isosurface (IRI) and scatter plots, it is concluded that the hydrogen bond interaction is enhanced under photoexcitation, and the descending order of the excited state hydrogen bond strength is 3c > 5c > 4c > 2c. The hydrogen bond energy is calculated by atoms in moleculs (AIM) topological analysis and core-valence bifurcation (CVB) index. The potential energy curve reveals the ESIPT mechanism. Frontier molecular orbital and electron-hole analyses explain the reasons for the changes in the ESIPT process at the electronic level. In addition, the ionization potentials (IPa and IPv), affinity energies (EAa and EAv) and reorganization energies are calculated to evaluate the potential application value of organic molecules in material transport field.

Fluorescence quenching of deprotonated phenylurea through twisting motion induced by an electron-donating substituent group

The excited-state proton transfer (ESPT) reaction between anthracen-2-yl-3-phenylurea (PUA) derivatives and tetrabutylammonium acetate (TBAAc) in dimethyl sulfoxide (DMSO) solvent was theoretically investigated using time-dependent density functional theory. The electron-donating methoxy group (OMe) and electron-withdrawing trifluoromethyl group (CF3) were bonded to 2PUA to form OMe-2PUA and CF3-2PUA, respectively. Two hydrogen bonds formed in the 1 : 1 hydrogen-bonded complexes between the 2PUA derivative and acetate ion (AcO-), namely N1-H1⋯O1 and N2-H2⋯O2. Strong charge transfer (CT) due to the electron-donating OMe group led to H1 transfer in the S1 state for the OMe-2PUA:AcO- hydrogen-bonded complex. On the contrary, weak CT due to the electron-withdrawing CF3 group led to H2 transfer in the S1 state for CF3-2PUA. After the ESPT reaction, the binding energies of the hydrogen-bonded complexes strongly decreased in both cases, and this promoted the separation of contact-ion pairs (CIPs*) and formed different types of anionic species. CF3-2PUA- could keep its nearly planar structure in the S1 state and emit "abnormal" fluorescence. On the other hand, the anionic OMe-2PUA- underwent a twisting motion to form a twisted structure in the S1 state with very low energy, and this led to a rapid internal conversion (IC) to quench long-wave fluorescence in the emission spectra.

Axial H-Bonding Solvent Controls Inhomogeneous Spectral Broadening, While Peripheral H-Bonding Solvent Controls Vibronic Broadening: Cresyl Violet in Methanol

The dynamics of the nuclei of both a chromophore and its condensed-phase environment control many spectral features, including the vibronic and inhomogeneous broadening present in spectral line shapes. For the cresyl violet chromophore in methanol, we here analyze and isolate the effect of specific chromophore-solvent interactions on simulated spectral densities, reorganization energies, and linear absorption spectra. Employing both chromophore and its condensed-phase environment control many spectral features, including the vibronic and inhomogeneous broadening present in spectral line shapes. For the cresyl violet chromophore in methanol, we here analyze and isolate the effect of specific chromophore-solvent interactions on simulated spectral densities, reorganization energies, and linear absorption spectra. Employing both force field and ab initio molecular dynamics trajectories along with the inclusion of only certain solvent molecules in the excited-state calculations, we determine that the methanol molecules axial to the chromophore are responsible for the majority of inhomogeneous broadening, with a single methanol molecule that forms an axial hydrogen bond dominating the response. The strong peripheral hydrogen bonds do not contribute to spectral broadening, as they are very stable throughout the dynamics and do not lead to increased energy-gap fluctuations. We also find that treating the strong peripheral hydrogen bonds as molecular mechanical point charges during the molecular dynamics simulation underestimates the vibronic coupling. Including these peripheral hydrogen bonding methanol molecules in the quantum-mechanical region in a geometry optimization increases the vibronic coupling, suggesting that a more advanced treatment of these strongly interacting solvent molecules during the molecular dynamics trajectory may be necessary to capture the full vibronic spectral broadening.

ESIPT mechanism of triple emission with hydroxy-oxadiazole compound in DMSO: A theoretical reconsideration

The compound in solvents with triple fluorescence feature of excited state intramolecular proton transfer (ESIPT) has a broad prospect in fluorescent probes, dye sensors and molecular synthesis of photosensitive dyes. An ESIPT molecule hydroxy-bis-2,5-disubstituted-1,3,4-oxadiazoles (compound 1a) emits two fluorescence peaks in dichloromethane (DCM) and three fluorescence peaks in dimethyl sulfoxide (DMSO). [Dyes and Pigments 197 (2022) 109927]. Two longer peaks were attributed to enol and keto emission in both solvents and the shortest third peak in DMSO was just attributed simply. However, there is a significant difference in proton affinity between DCM and DMSO solvents which has influence on the position of emission peaks. Therefore, the correctness of this conclusion needs to be further verified. In this research, density functional theory and time-dependent density functional theory method are used to explore ESIPT process. Optimized structures indicate ESIPT occurs through molecular bridge assisted by DMSO. The calculated fluorescence spectra demonstrate two peaks indeed originated from enol and keto in DCM, while interestingly three peaks are originated from enol, keto and intermediate in DMSO. Infrared spectrum, electrostatic potential and potential energy curves further prove existence of three structures. We reveal the mechanisms that compound 1a molecule occurs ESIPT in DCM solvent and undergoes an ESIPT through assisted by DMSO molecular bridge. Additionally, three fluorescence peaks in DMSO are reattributed. Our work is expected to provide an insight for understanding intra- and intermolecular interactions and synthesis of efficient organic lighting-emitting molecule.

Unveiling and regulating the solvent-polarity-associated excited state intramolecular double proton transfer behavior for 1-bis(benzothiazolyl)naphthalene-diol fluorophore

Inspired by the regulatory luminescence properties of HBT derivatives, in this work, we mainly conduct a detailed theoretical exploration on the photoinduced excitation behavior of a novel di-proton-transfer type HBT derivative 1-bis(benzothiazolyl)naphthalene-diol (1-BBTND). The intramolecular double hydrogen bonding interaction and the excited state intramolecular double proton transfer (ESDPT) behavior of 1-BBTND fluorophore are investigated in combination with different polar solvent environments. From the structural changes and charge recombination induced by photoexcitation, we can conclude that strong polar solvent environment promotes the excited state dynamical reaction for 1-BBTND compound. By constructing potential energy surfaces (PESs) in S₀ and S₁ states, we clarify that 1-BBTND fluorophore should undergo a stepwise ESDPT reaction after photoexcitation. Combined with the size of potential energy barriers along with reaction paths in different solvents, we finally propose a new solvent-polarity-dependent stepwise ESDPT for 1-BBTND fluorophore.

Jellybean Quantum Dots in Silicon for Qubit Coupling and On-Chip Quantum Chemistry

The small size and excellent integrability of silicon metal-oxide-semiconductor (SiMOS) quantum dot spin qubits make them an attractive system for mass-manufacturable, scaled-up quantum processors. Furthermore, classical control electronics can be integrated on-chip, in-between the qubits, if an architecture with sparse arrays of qubits is chosen. In such an architecture qubits are either transported across the chip via shuttling or coupled via mediating quantum systems over short-to-intermediate distances. This paper investigates the charge and spin characteristics of an elongated quantum dot-a so-called jellybean quantum dot-for the prospects of acting as a qubit-qubit coupler. Charge transport, charge sensing, and magneto-spectroscopy measurements are performed on a SiMOS quantum dot device at mK temperature and compared to Hartree-Fock multi-electron simulations. At low electron occupancies where disorder effects and strong electron-electron interaction dominate over the electrostatic confinement potential, the data reveals the formation of three coupled dots, akin to a tunable, artificial molecule. One dot is formed centrally under the gate and two are formed at the edges. At high electron occupancies, these dots merge into one large dot with well-defined spin states, verifying that jellybean dots have the potential to be used as qubit couplers in future quantum computing architectures.

The Role of H-Bonds in the Excited-State Properties of Multichromophoric Systems: Static and Dynamic Aspects

Given their importance, hydrogen bonds (H-bonds) have been the subject of intense investigation since their discovery. Indeed, H-bonds play a fundamental role in determining the structure, the electronic properties, and the dynamics of complex systems, including biologically relevant materials such as DNA and proteins. While H-bonds have been largely investigated for systems in their electronic ground state, fewer studies have focused on how the presence of H-bonds could affect the static and dynamic properties of electronic excited states. This review presents an overview of the more relevant progress in studying the role of H-bond interactions in modulating excited-state features in multichromophoric biomimetic complex systems. The most promising spectroscopic techniques that can be used for investigating the H-bond effects in excited states and for characterizing the ultrafast processes associated with their dynamics are briefly summarized. Then, experimental insights into the modulation of the electronic properties resulting from the presence of H-bond interactions are provided, and the role of the H-bond in tuning the excited-state dynamics and the related photophysical processes is discussed.

Experimental and theoretical analyses and investigation of intermolecular interactions and antibacterial activity of a novel proton transfer compound:8-hydroxyquinolinium oxalate monohydrate

A novel proton transfer compound, 8-hydroxyquinolinium oxalate monohydrate was synthesised by solid state grinding of 8-hydroxyquinoline and oxalic acid. The resulting compound is characterised by single crystal X-ray diffraction (SXRD), FT-IR, UV-Visible, TG/DTG, DTA and DSC analyses. The compound crystallizes in monoclinic crystal system with space group P21/n. The carboxylate oxygen O₂ which acts as a tetrafurcated acceptor of four hydrogen bonds is the main feature of the crystal structure. The molecules are linked together by O-H⋯O, N-H⋯O and C-H⋯O hydrogen bonds. Carbonyl-carbonyl interactions play a crucial role in stabilising the crystal packing. Hirshfeld surface analysis and the associated finger print plots facilitates the comparison of intermolecular interactions. The nature of charge density distribution and topological parameters of the proton transfer region N₁-H_1A⋯O₂ hydrogen bond reveals that the bond has considerable covalent character. Natural Bond Orbital (NBO) has been extended to analyse the nature and strength of intermolecular interactions. Topology analysis using ELF and LOL reveals electron localisation and depletion regions. ADMET analysis reveals that the compound satisfies Lipinski's rule of five and drug likeness. Antibacterial activity was screened against 3 g positive - Bacillus subtilis, Enterococcus faecalis, Staphylococcus aureus and 2 g negative strains- Klebsiella pneumonia and Salmonella typhi by employing disc diffusion method.

Circularly Polarized Luminescence of Achiral Carbon Dots in Bi-Solvent Systems Triggered by Supramolecular Self-Assembly

An innovative strategy for circularly polarized luminescence (CPL) of carbon dots (CDs) has been developed: The achiral CDs displayed contrary supramolecular chirality and opposite CPL in two different bi-solvent systems, which are due to the formation of self-assembled helical nanostructures with two diverse assembly modes (P and M) in aggregate state via intermolecular π-π interactions and differential hydrogen bonding (H-bonding) without the need of any additional element of chirality.

The Journey of 1-Keto-1,2,3,4-Tetrahydrocarbazole Based Fluorophores: From Inception to Implementation

Carbazole is a unique template associated with several biological activities. It is due to the diverse and versatile biological properties of carbazole derivatives that they are of immense interest to the research community. 1-keto-1,2,3,4-tetrahydrocarbazoles are important synthetic intermediates to obtain carbazole derivatives. Several members of this family emit fluorescence on photoexcitation. In the context of biochemical and biophysical research, designing and characterising small molecule environment sensitive fluorophores is extremely significant. This article aims to be a state of the art review with synthetic and photophysical details of a variety of fluorophores based on 1-keto-1,2,3,4-tetrahydrocarbazole skeleton.

Effects of azole rings with different chalcogen atoms on ESIPT behavior for benzochalcogenazolyl-substituted hydroxyfluorenes

ESIPT behavior has attracted a lot of eyes of researchers in recent years because of its unique optical properties. Due to its large Stokes shift and double emission fluorescence, white light can be generated in the fluorophore based on the excited state intramolecular proton transfer (ESIPT) principle. The excited state proton transfer behavior of hydroxylated benzoxazole (BO-OH), benzothiazole (BS-OH) and benzoselenazole (BSe-OH) have been investigated in heptane, chloroform and DMF solvents. By comparing the infrared vibration spectra and the variation of bond parameters from the S₀ to S₁ states, and analyzing the frontier molecular orbitals, the influence of hydrogen bond dynamics, the solvent polarity, charge redistribution and the effects of different proton acceptors on proton transfer were observed. The only structural difference among the three substituted hydroxyfluorenes is the heteroatom in the azole ring (oxygen, sulfur and selenium, respectively). We have scanned the potential energy curve of the ESIPT process, and compared the potential barrier, it is found that the heavier chalcogen atoms are more favorable for proton transfer. At the same time, the potential application of changing heteroatoms in the azole ring by walking down the chalcogenic group in crystal luminescence color regulation is also discussed.

Excited-State Dependent Hydrogen Bond Natures and Their Critical Role in Determining the Photophysical Properties of Aromatic Thioketones

In this work, how the excited-state dependent hydrogen bond (H-bond) interactions control photophysical processes have been uncovered by accurate electronic structure calculations for the five lowest-lying states (S0, S1, S2,...

Excited-state intramolecular proton transfer with and without the assistance of vibronic-transition-induced skeletal deformation in phenol-quinoline

The excited-state intramolecular proton transfer (ESIPT) reaction of two phenol-quinoline molecules (namely PQ-1 and PQ-2) were investigated using time-dependent density functional theory. The five-(six-) membered-ring carbocycle between the phenol and quinolone moieties in PQ-1 (PQ-2) actually causes a relatively loose (tight) hydrogen bond, which results in a small-barrier (barrier-less) on an excited-state potential energy surface with a slow (fast) ESIPT process with (without) involving the skeletal deformation motion up to the electronic excitation. The skeletal deformation motion that is induced from the largest vibronic excitation with low frequency can assist in decreasing the donor-acceptor distance and lowering the reaction barrier in the excited-state potential energy surface, and thus effectively enhance the ESIPT reaction for PQ-1. The Franck-Condon simulation indicated that the low-frequency mode with vibronic excitation 0 → 1' is an original source of the skeletal deformation vibration. The present simulation presents physical insights for phenol-quinoline molecules in which relatively tight or loose hydrogen bonds can influence the ESIPT reaction process with and without the assistance of the skeletal deformation motion.

Photoexcitation of oxazine 170 dye in aqueous solution: TD-DFT study

The vibronic absorption spectra of OX170 dye in an aqueous solution using 40 hybrid functionals, the 6-31 +  + G(d,p) basis set, and the SMD solvent model were calculated. It turned out that the long-range corrected ωB97XD functional provided the best agreement with the experiment in the positions of the main maximum and the short-wavelength shoulder. Calculations showed that this shoulder is vibronic and is not caused by a separate electronic transition. At the same time, the shoulder intensity in the calculated spectrum turned out to be lower than in the experimental one. Various parameters of the OX170 cation in the ground and excited states (IR spectra, atomic charges, dipole moments, and transition moment) were calculated. Maps of the distribution of electron density and electrostatic potential have been built. The influence of four strong hydrogen bonds of the dye with water molecules on the absorption spectrum was analyzed. It was shown that these bonds are strengthened upon OX170 excitation. It was found that explicit assignment of water molecules strongly bound to the dye leads to a redshift of the calculated spectrum by ≈15 nm as a whole, and worsened its shape. Photoexcitation of the dye leads to a noticeable polarization of only one of the four considered water molecules (associated with the endocyclic nitrogen atom in the central ring of the chromophore, the electron density on which increases the most).

Elaborating the excited-state double proton transfer mechanism and multiple fluorescent characteristics of 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole

Recently, Krishnamoorthy and coworkers reported a new type of proton transfer, which was labeled as 'proton transfer triggered proton transfer', in 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole (bis-HPTA). In this work, the excited-state double proton transfer (ESDPT) mechanism and multiple fluorescent characteristics of bis-HPTA were investigated. Upon photo-excitation, the intramolecular hydrogen bonding strength changed and the electron density of bis-HPTA redistributed. These changes will affect the proton transfer process. In S₀ state, the proton transfer processes of bis-HPTA were prohibited on the stepwise and concerted pathways. After vertical excitation to the S₁ state, the ESIPT-II process was more likely to occur than the ESIPT-I process, which was contrary to the conclusion that the ESIPT-II process is blocked and the ESIPT-II process takes place after the ESIPT-I process proposed by Krishnamoorthy and coworkers. When the K2 tautomer was formed through the ESIPT-II process, the second proton transfer process on the stepwise pathway was prohibited. On another stepwise pathway, after the ESIPT-I process (form the K1 tautomer), the second proton transfer process should overcome a higher potential barrier than the ESIPT-I process to form ESDPT tautomer. On the concerted pathway, the bis-HPTA can synchronous transfer double protons to form the ESDPT tautomer. The ESDPT tautomer was unstable and immediately converted to the K2 tautomer via a barrierless reverse proton transfer process. Thus, the fluorescent maximum at 465 nm from the ESDPT tautomer reported by Krishnamoorthy and coworkers was ascribed to the K2 tautomer. Most of the fluorophores show dual fluorescent properties, while the bis-HPTA undergoing ESDPT process exhibited three well-separated fluorescent peaks, corresponding to its normal form (438 nm), K1 tautomer (462 nm) and K2 tautomer (450 nm), respectively.

Solvent polarity dependent excited state hydrogen bond effects and intramolecular double proton transfer mechanism for 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole system

In this work, we probe into the photo-induced excited state hydrogen bonding interactions and excited state proton transfer (ESPT) behaviors for a representative benzo[1,2-d:4,5-d']bisimidazole derivative (i.e., 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole (HPBB)) compound. In view of aprotic solvents with different polarities, cyclohexane (CYH), dichloromethane (DCM) and acetonitrile (MeCN) solvents are considered. Analyzing hydrogen-bond geometrical parameters, infrared (IR) vibrational spectra, Mayer bond order and predicting hydrogen bonding energy (E_(HB)), we verify dual hydrogen bonds of HPBB are strengthened in S₁ state. Particularly, in nonpolar solvent, the enhanced excited state hydrogen bonds become more obvious. The intriguing charge redistribution and frontier molecular orbitals (MOs) reveal hydrogen bonding acceptance ability of acceptor moieties becomes stronger, which plays a crucial role in capturing hydroxyl proton via photoexcitation. To check and explore ESIPT mechanism, we present the solvent polarity dependent asynchronous excited state intramolecular double proton transfer (ESIDPT) mechanism. That is, nonpolar solvent promotes excited state intramolecular single proton transfer (ESISPT) process for HPBB, while polar solvent contributes to ESIDPT behavior with the primary single proton-transfer product in S₁ state. This work not only makes a rational attribution to experimental phenomena, but also clarifies detailed excited state behaviors for HPBB and presents regulating ESIPT mechanism via solvent polarity.

Excited-State Properties and Relaxation Pathways of Selenium-Substituted Guanine Nucleobase in Aqueous Solution and DNA Duplex

The excited-state properties and relaxation mechanisms after light irradiation of 6-selenoguanine (6SeG) in water and in DNA have been investigated using a quantum mechanics/molecular mechanics (QM/MM) approach with the multistate complete active space second-order perturbation theory (MS-CASPT2) method. In both environments, the S₁ ¹(n_Seπ₅^*) and S₂ ¹(π_Seπ₅^*) states are predicted to be the spectroscopically dark and bright states, respectively. Two triplet states, T₁ ³(π_Seπ₅^*) and T₂ ³(n_Seπ₅^*), are found energetically below the S₂ state. Extending the QM region to include the 6SeG-Cyt base pair slightly stabilizes the S₂ state and destabilizes the S₁, due to hydrogen-bonding interactions, but it does not affect the order of the states. The optimized minima, conical intersections, and singlet-triplet crossings are very similar in water and in DNA, so that the same general mechanism is found. Additionally, for each excited state geometry optimization in DNA, three kind of structures ("up", "down", and "central") are optimized which differ from each other by the orientation of the C═Se group with respect to the surrounding guanine and thymine nucleobases. After irradiation to the S₂ state, 6SeG evolves to the S₂ minimum, near to a S₂/S₁ conical intersection that allows for internal conversion to the S₁ state. Linear interpolation in internal coordinates indicate that the "central" orientation is less favorable since extra energy is needed to surmount the high barrier in order to reach the S₂/S₁ conical intersection. From the S₁ state, 6SeG can further decay to the T₁ ³(π_Seπ₅^*) state via intersystem crossing, where it will be trapped due to the existence of a sizable energy barrier between the T₁ minimum and the T₁/S₀ crossing point. Although this general S₂ → T₁ mechanism takes place in both media, the presence of DNA induces a steeper S₂ potential energy surface, that it is expected to accelerate the S₂ → S₁ internal conversion.

Quinoline Photobasicity: Investigation within Water-Soluble Light-Responsive Copolymers

Quinoline photobases exhibit a distinctly higher pKa in their electronically excited state than in the ground state, thereby enabling light-controlled proton transfer reactions, for example, in molecular catalysis. The absorption of UV light translates to a pKa jump of approximately 10 units, as established for small-molecule photobases. This contribution presents the first synthesis of quinoline-based polymeric photobases prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization. The integration of quinolines as photobase chromophores within copolymers offers new possibilities for light-triggered proton transfer in nanostructured materials, that is, in nanoparticles, at surfaces, membranes and interfaces. To exploit the light-triggered reactivity of photobases within such materials, we first investigated how the ground- and excited-state properties of the quinoline unit changes upon polymer integration. To address this matter, we combined absorption and emission spectroscopy with time-resolved transient-absorption studies to reveal photoinduced proton-transfer dynamics in various solvents. The results yield important insights into the thermodynamic and kinetic properties of these polymeric quinoline photobases.

The effects of electron-withdrawing and electron-donating groups on the photophysical properties and ESIPT of salicylideneaniline

The photophysical properties and excited state intramolecular proton transfer (ESIPT) of salicylideneaniline (1a) and its derivatives (1b-1e) with different substituents have been investigated using the DFT and TD-DFT methods. The calculated results indicate that the introduction of electron-withdrawing group CN weakens the intramolecular hydrogen bond (H···N). However, the introduction of electron-donating group N(CH₃)₂ strengthens it. When the CN and N(CH₃)₂ groups are introduced simultaneously, the intramolecular hydrogen bond (H···N) is weakened. In addition, swapping the CN and N(CH₃)₂ group positions can enhance the intramolecular hydrogen bond (H···N). Compared to 1a, the absorption and emission spectra of compounds 1b-1e are red-shifted. Frontier molecular orbital analyses prove that the more intense intramolecular charge transfer characters caused by CN and N(CH₃)₂ substituents is responsible for the red shift of spectra. Potential energy curves indicate that ESIPT in salicylideneaniline (1a) and the CN substituted derivative (1b) is a non-barrier process, while in the N(CH₃)₂ substituted derivative (1c) and the CN and N(CH₃)₂ co-substituted derivative (1d), ESIPT needs to overcome the energy barriers of 2.32 kcal/mol and 3.38 kcal/mol, respectively. Exchanging the positions of CN and N(CH₃)₂ groups (1e) makes the ESIPT into a barrier-free process. Therefore, the substitution and position of CN and N(CH₃)₂ groups can affect the ESIPT process and induce different photophysical properties.

Tuning the Binding Strength of Even and Uneven Hydrogen-Bonded Arrays with Remote Substituents

We investigated the tunability of hydrogen bond strength by altering the charge accumulation around the frontier atoms with remote substituents. For pyridine···H2O with NH2 and CN substituted at different positions on pyridine, we find that the electron-withdrawing CN group decreases the negative charge accumulation around the frontier atom N, resulting in weakening of the hydrogen bond, whereas the electron-donating NH2 group increases the charge accumulation around N, resulting in strengthening of the hydrogen bond. By applying these design principles on DDAA-AADD, DADA-ADAD, DAA-ADD, and ADA-DAD hydrogen-bonded dimers, we find that the effect of the substituent is delocalized over the whole molecular system. As a consequence, systems with an equal number of hydrogen bond donor (D) and acceptor (A) atoms are not tunable in a predictable way because of cancellation of counteracting strengthening and weakening effects. Furthermore, we show that the position of the substituent and long-range electrostatics can play an important role as well. Overall, the design principles presented in this work are suitable for monomers with an unequal number of donor and acceptor atoms and can be exploited to tune the binding strength of supramolecular building blocks.

Revised excited-state intramolecular proton transfer of the 3-Aminophthalimide molecule: A TDDFT study

The spectroscopic properties of 3-Aminophthalimide (3AP) molecule were investigated [Chem. Phys. 2002, 283, 249, New J. Chem. 2018, 42, 1181]. The result was that the 3AP molecule was exhibiting excited-state intramolecular proton transfer (ESIPT). In the research, we revised previous result using time-dependent density functional theory (TDDFT) method. The fluorescence spectrum shows that the only fluorescence peak is from initial enol form, which is different from the traditional case of ESIPT. The red shift of characteristic peaks in infrared vibration spectra is not induced by ESIPT process. The change in the vibration mode of the amino group causes the red shift of characteristic peak in the infrared spectrum. Energy curves indicate that the barrier (19.71 kcal/mol) is anomalously high in the first excited state. In addition, there are not stable points to lead the ESIPT to form a keto isomer. Together, these results demonstrate that there is not an ESIPT process happening of 3AP molecule.

Hydrogen bond design principles

Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π-conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (C-H…O, O-H…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance-assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented.

Unraveling the effect of two different polar solvents on the excited-state intramolecular proton transfer of 4'-methoxy-3-hydroxyflavone fluorescent dye

The fluorescence properties of 4'-methoxy-3-hydroxyflavone (M3HF) dye in different solvents were investigated through experimental (Phys. Chem. Chem. Phys., 2018, 20, 7885) and theoretical (Org. Chem. Front., 2019, 6, 218) methods. However, the intermolecular hydrogen bonds between M3HF and solvents were ignored. In this work, we investigated the effect of methanol (MeOH) and N,N-dimethylformamide (DMF) solvents on the excited-state intramolecular proton transfer (ESIPT) of M3HF fluorescent dye. In excited state (S₁), the intramolecular hydrogen bonds are significantly strengthened, which can facilitate the ESIPT processes. The calculated absorption and fluorescence spectra agree well with the experimental date. The fluorescence spectra of M3HF and ESIPT tautomers (T^⁎) were found to be sensitive to the solvent polarity. Upon photo-excitation, the electron density of the M3HF molecular is redistributed, which can provide driving force for the ESIPT. The polar solvents MeOH (hydrogen bond donor) and DMF (hydrogen bond acceptor) can form different types of intermolecular hydrogen bonds with M3HF. The two different bonding modes of intermolecular hydrogen bonds are expected to weaken the intramolecular hydrogen bond of M3HF to varying degrees. The analysis of the potential energy curves indicate that the ESIPT processes of M3HF can be hindered by the intermolecular hydrogen bonds. The intermolecular hydrogen bond of M3HF-DMF complex is weaker than that of M3HF-MeOH complex, while the potential barrier of the ESIPT process in DMF solvent is higher than that of in the MeOH solvent. This is principally because, in DMF solvent, the hydroxyl group H₁ atom of M3HF can be captured by the O₃ atom of DMF and form O₃H₁ bond with O₃ atom in the intermediate process of ESIPT. There appears an energy barrier hopping point on the potential energy curve of M3HF in DMF solvent but does not appear in MeOH solvent.