Fluorescence quenching phenomena facilitated by excited-state hydrogen bond strengthening for fluorenone derivatives in alcohols

2-Phenylquinoline-polyamine conjugate (QPC): Interaction with bovine serum albumin (BSA)

A novel 2-phenylquinoline-polyamine conjugate (QPC) was synthesized and characterized, its interaction with bovine serum albumin (BSA) was evaluated using UV-Vis, fluorescence and circular dichroism (CD) spectroscopy. The results showed that QPC caused a whole train of spectral variation, including enhancement of UV-vis absorption and reduction of fluorescence (FL), indicating QPC-BSA complex formed. FL results showed that the type of FL quenching waslarge static quenching, which was also accompanied with a process of dynamic quenching. Binding constants, thermodynamic parameters and docking results showed that the interaction between QPC and BSA was basically a Van der Waals, hydrogen bond and hydrophobic interaction. Synchronous and 3D-FL analysis revealed that QPC resulted in unapparent conformational alteration of BSA. The docking study suggested QPC was situated at the binding sites II of BSA, and 2-phenylquinoline moiety contributed to the hydrophobic interaction. The results of molecular dynamics revealed QPC altered the conformation of BSA, which showed that the inconsistency between experimental data and theoretical calculation results may be due to the instability of the compound.

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.

Coupling between tautomerism and radiationless deactivation in porphycenes

Quantum yields of fluorescence of porphycenes – porphyrin isomers – can vary by orders of magnitude, even for very similar derivatives, such as meso-dimethyl- vs. meso-tetramethylporphycene. In weakly emitting porphycenes the fluorescence intensity strongly depends on viscosity and can be recovered by placing a molecule in a rigid environment. We postulate that the efficient nonradiative deactivation is due to the quantum effect, delocalization of the inner protons. The delocalization, which increases with the strength of intramolecular hydrogen bonds may induce structural changes that lead to distortion from planarity and, as a result, efficient S0 ← S1 internal conversion. The effect seems to be general, as indicated by good correlation between the quantum yield of fluorescence and the distance between H-bonded nitrogen atoms, the latter being a reliable measure of hydrogen bonding strength. Based on the available photophysical and X-ray data, such correlation was found so far for over 20 differently substituted porphycenes.

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen-Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H₂O, D₂O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen-Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

Investigation of the interaction of a polyamine-modified flavonoid with bovine serum albumin (BSA) by spectroscopic methods and molecular simulation

The interaction between BSA and compound 1 was studied by UV-vis, fluorescence and circular dichroism spectroscopy under physiological conditions (pH = 7.4). Molecular docking and molecular dynamics analyses were also performed. The results showed that compound 1 could bind to BSA. When compound 1 bound to BSA, there were a series of changes in the spectral properties of BSA, which were an enhancement effect of the UV-Vis spectrum of BSA, fluorescence quenching and a weak conformational change in the CD spectrum. The results of the fluorescence experiments at 298, 303 and 310 K showed that fluorescence quenching caused by the addition of compound 1 to BSA was generally static quenching accompanied by a dynamic quenching process, which was shown by the quenching constants of 2.010 × 10⁴ L∙M^-1, 1.850 × 10⁴ L∙M^-1, and 1.970 × 10⁴ L∙M^-1 at the three different temperatures, respectively. From the obtained binding constants and thermodynamic parameters, it was found that hydrophobic forces played an important role in the binding process of 1 to BSA. The results of synchronous fluorescence and three-dimensional fluorescence showed that compound 1 caused a weak conformational change in BSA. Docking results showed that compound 1 was located at binding site II of bovine serum albumin protease. In addition, the flavonoid moiety of compound 1 contributes to the hydrophobic binding of compound 1 to BSA. The results of molecular dynamics, including the root-mean-square deviation (RMSD) and RMS fluctuation (RMSF) values, showed that the binding of compound 1 to BSA did not cause a significant conformational change in BSA.

Revelation solvent effects: excited state hydrogen bond and proton transfer of 2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol

ESIPT reaction of an MMT molecule is gradually inhibited with increasing solvent polarity.

Spectroscopic Study of Solvent Polarity on the Optical and Photo-Physical Properties of Novel 9,10-bis(coumarinyl)anthracene

Novel 7,7'-((anthracene-9,10-diylbis(methylene))bis(oxy))bis(4-methyl-2H-chromen-2-one) (BisCA) was prepared as fluorescent probe. The chemical structure of the novel BisCA was confirmed by spectroscopic data as well as elemental analyses. The solvatochromic characteristics of the new proble and its precursors were investigated in different solvents including, ethanol, DMF and toluene as protic polar, aprotic polar and non-polar solvents, respectively. Photo-physical parameters of probes, such as fluorescence quantum yields, fluorescence lifetime of excited state, radiative and non-radiative decay, were assessed in different media. The intermolecular H-bond effect on absorption and excitation spectra of the novel probe was reported in different solvents. Also, Onsager cavity radius and dipole moment of ground state and excited state of the probe were calculated as described by Bakhshiev and Reichardt methods.

Effect of different alkyl groups on excited-state tautomerization of 7AI-azaindole-H2O: A theoretical study

The effect of substituted alkyl groups at different substituted position on the first excited-state proton transfer of nR7AI-H₂O (n = 2-6; R = CH₃, C₂H₅, CF₃) complexes were theoretically investigated at the TD-M06-2X/6-31 + G(d, p) level. Here n value denoted the substituted position C_n of R group. The replacement of alkyl R group had no effect on the features of HOMO and LUMO, but influenced the S₀ → S₁ adiabatic transition energies of the nR7AI-H₂O complex. Through computation, we found that the double proton transfer took place in a concerted but asynchronous protolysis pattern regardless of substituted group R and substituted position in the nR7AI-H₂O complex. The vibrational-mode specific nature of ESPT was verified. The alkyl group R changed the geometrical parameters of TS, and resulted in enlarging/narrowing the asynchronousity of ESPT. The ESPT barrier height was also affected by the substituted group and position.

Elaborating a new excited state intramolecular proton transfer (ESPT) mechanism for a new π-conjugated dye 2, 2′-((5-(2-(4-methoxyphenyl)ethenyl)-benzene-1,1-diyl)-bis-(nitrilomethylylidene)-diphenol)

In this work, the excited state dynamical behavior of a novel π-conjugated dye 2,2′-((5-(2-(4-methoxyphenyl)ethenyl)-benzene-1,1-diyl)-bis-(nitrilomethylylidene)-diphenol) (C1) has been investigated. Two intramolecular hydrogen bonds of C1 are tested to pre-existing in the ground state via AIM and reduced density gradient. Using a time-dependent density functional theory (TDDFT) method, it has been substantiated that the intramolecular hydrogen bonds of C1 should be strengthened in the S1 state via analyzing fundamental bond length, bond angles, and corresponding infrared vibrational modes. The most obvious variation of these two hydrogen bonds is the O4–H5···N6 bond, which might play important roles in excited state behavior for the C1 system. Furthermore, based on electronic excitation, charge transfer could occur. Just due to this kind of charge re-distribution, two hydrogen bonds should be tighter in the first excited state, which is consistent with the variation of hydrogen bond lengths. Thus, the phenomenon of charge transfer is reasonable evidence for confirming the occurrence of the excited state proton transfer (ESPT) process in the S1 state. Our theoretically constructed potential energy surfaces of C1 show that excited state single proton transfer should occur along with the O4–H5···N6 hydrogen bond rather than the O1–H2···N3 bond. We not only clarify the ESIPT mechanism for C1 but put forward new affiliation and explain a previous experiment successfully.

A recognition mechanism study: Luminescent metal-organic framework for the detection of nitro-explosives

This article presents a recognition mechanism for nitro-explosives by the luminescent metal-organic framework 1 (LMOF-1) with the aid of density functional theory (DFT) and time-dependent density functional theory (TDDFT). The behavior of hydrogen bonding between the LMOF-1 and nitro-explosives in the S₁ state is closely associated with the fluorescence properties of the LMOF-1. In our research, we calculated the geometric configuration, ¹H NMR and IR spectra of the Complex 2 formed by LMOF-1 and nitrobenzene in the S₀ and S₁ states. The results showed that the hydrogen bond in the S₁ state was increased, which was unfavorable for the luminescence of LMOF-1. Furthermore, the fluorescence rate of LMOF-1 decreased after encapsulating nitrobenzene into it. These calculated results collectively suggest that LMOF-1 is a potential fluorescence sensor for the detection of nitro-explosives. This research was aiming to provide a better understanding of the recognition mechanism by LMOFs for nitro-explosives.

Theoretical study of excited-state proton transfer of 2,7-diazaindole·(H2O)2 cluster via hydrogen bonding dynamics

A new chromophore, 2,7-diazaindole (2,7-DAI), has been designed to surpass the limitation of 7-azaindole (7AI). It exhibits remarkable water catalyzed proton-transfer properties. Excited-state proton transfer (ESPT) has been investigated based on the time-dependent density functional theory method. The calculated vertical excitation energies in the S₀ and S₁ states agree well with the experimental values. Proton transfer couples with hydrogen-bonding dynamics between the 2,7-diazaindole and the surrounding water molecules. Hydrogen bond strengthening has been testified in the S₁ state based on a comparison of primary bond lengths and hydrogen bond energy that is involved in the intermolecular hydrogen bond between the S₀ and S₁ states. Frontier molecular further suggest that the electron density changes between the ground and excited states serve as basic driving forces for proton transfer. We determined the potential-energy curves of the S₀ and S₁ states to characterize the ESPT process. This work explains that the ESPT process for 2,7-DAI·(H₂O)₂ clusters at the molecular level, and highlights the importance of hydrogen bonding in ESPT.

The new competitive mechanism of hydrogen bonding interactions and transition process for the hydroxyphenyl imidazo [1, 2-a] pyridine in mixed liquid solution

The new competitive mechanism of intermolecular and intramolecular hydrogen bond can be proposed with an improved mixed model. Upon the photoinduced process, the twisting intramolecular charge transfer (TICT) structure of the hydroxyphenyl imidazo [1, 2-a] pyridine (HPIP) can be obtained. TICT character prompts the fluorescent inactivation via non-radiative decay process. For exploring the photochemical and photophysical properties, the electronic spectra and the infrared (IR) vibrational spectra of titled compounds have been detailedly investigated. In addition, the frontier molecular orbitals (MOs) analysis visually reveals that the unbalanced electron population can give rise to the torsion of molecular structure. To further give an attractive insight into the non-radiative decay process, the potential energy curves have been depicted on the ground state (S₀), the first excited state (S₁) and the triple excited state (T₁). Minimum energy crossing point (MECP) has been found in the S₁ and T₁ state. On the MECP, the intersystem crossing (ISC) might be dominant channel. The density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods have been throughout employed in the S₀ state, T₁ state and S₁ state, respectively. The theoretical results are consistent with experiment in mixed and PCM model.

N,S co-doped carbon dots as a stable bio-imaging probe for detection of intracellular temperature and tetracycline

N,S-CDs display an unambiguous bioimaging ability in the detection of intracellular temperature and tetracycline with satisfactory results.

Formulation of saponin stabilized nanoemulsion by ultrasonic method and its role to protect the degradation of quercitin from UV light

The objective of the present study was to prepare quercitin (QT) loaded o/w nanoemulsion using food grade surfactants (saponin and tween 80). The prepared nanoemulsion) was stable up to 30 days. The average particle size of the nanoemulsion was 52 ± 10 nm. The formation of saponin stabilized nanoemulsion was confirmed by transmission electron microscopy. Quercitin (QT) trapped nanoemulsion showed higher stability on exposure to UV light (254 nm) as compared to water/ethanol system. The degradation rate was found to decrease from 9 ± 1%, 11 ± 1% at pH 7.4, 8.0 respectively as compared to 42 ± 2% in water/ethanol system. Attempt was also made to study the interaction of QT with two different bile salts (sodium cholate and sodium taurocholate). The free radical scavenging activity of DPPH quercitin and curcumin was compared in NEm media. The obtained IC50 value of quercitin, curcumin and ascorbic acid are 28.88 ± 1, 45.53 ± 2 and 51.51 ± 2 μM respectively. The values of binding constant for sodium cholate (NaC) and sodium taurocholate (NaTC) are 2.66 × 10(5) and 2.72 × 10(4) M(-1) respectively. Sodium cholate (NaC) was found to show strong interaction towards quercitin (QT) due to more electron density on oxygen atom of carboxylate ion.

Synthesis of a novel trinuclear palladium complex: the influence of an oxime chelate ligand on biological evaluation towards double-strand DNA, BSA protein and molecular modeling studies

Synthesis of a novel cluster palladium complex containing an oxime ligand. Study of the ability of the complex to interact with DNA via groove mechanism. The molecular docking indicates high binding affinity between DNA and BSA with the Pd complex.

Spectroscopic Study on the Interaction between Naphthalimide-Polyamine Conjugates and Bovine Serum Albumin (BSA)

The effect of a naphthalimide pharmacophore coupled with diverse substituents on the interaction between naphthalimide-polyamine conjugates 1–4 and bovine serum albumin (BSA) was studied by UV absorption, fluorescence and circular dichroism (CD) spectroscopy under physiological conditions (pH = 7.4). The observed spectral quenching of BSA by the compounds indicated that they could bind to BSA. Furthermore, caloric fluorescent tests revealed that the quenching mechanisms of compounds 1–3 were basically static type, but that of compound 4 was closer to a classical type. The K_sv values at room temperature for compound-BSA complexes-1-BSA, 2-BSA, 3-BSA and 4-BSA were 1.438 × 10⁴, 3.190 × 10⁴, 5.700 × 10⁴ and 4.745 × 10⁵, respectively, compared with the value of MINS, 2.863 × 10⁴ at Ex = 280 nm. The obtained quenching constant, binding constant and thermodynamic parameter suggested that the binding between compounds 1–4 with BSA protein, significantly affected by the substituted groups on the naphthalene backbone, was formed by hydrogen bonds, and other principle forces mainly consisting of charged and hydrophobic interactions. Based on results from the analysis of synchronous three-dimensional fluorescence and CD spectra, we can conclude that the interaction between compounds 1–4 and BSA protein has little impact on the BSA conformation. Calculated results obtained from in silico molecular simulation showed that compound 1 did not prefer either enzymatic drug sites I or II over the other. However, DSII in BSA was more beneficial than DSI for the binding between compounds 2–4 and BSA protein. The binding between compounds 1–3 and BSA was hydrophobic in nature, compared with the electrostatic interaction between compound 4 and BSA.

Visibly emissive and responsive extended 6-aza-uridines

A family of extended 5-modified-6-aza-uridines was obtained via Suzuki coupling reactions with a common brominated precursor. Extending the conjugated-6-aza-uridines with substituted aryl rings increases the push–pull interactions yielding enhanced bathochromic shifts and solvatochromism compared to the parent nucleosides. For example, the methoxy substituted derivative 1d displays λ_max abs around 375 nm, with visible emission maxima at 486 nm (Φ = 0.74) and 525 nm (Φ = 0.02) in dioxane and water, respectively.

Deactivation of 6-aminocoumarin intramolecular charge transfer excited state through hydrogen bonding

This paper presents results of the spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in various aprotic hydrogen-bond forming solvents. It was established that solvent polarity as well as hydrogen-bonding ability influence solute properties. The hydrogen-bonding interactions between S₁-electronic excited solute and solvent molecules were found to facilitate the nonradiative deactivation processes. The energy-gap dependence on radiationless deactivation in aprotic solvents was found to be similar to that in protic solvents.

TDDFT study of the polarity controlled ion-pair separation in an excited-state proton transfer reaction

6-Hydroxyquinoline (6HQ) is an ideal photoacid system for exploring excited-state proton transfer (ESPT) reactions. We have previously (Mahata et al. (2002)) shown that the ESPT reaction between 6HQ and trimethylamine (TMA) leads to an "unusual" emission in the 440-450 nm range, containing two decay components (∼5 ns and ∼12 ns). The observed results suggest the presence of a contact ion-pair and a solvent separated ion-pair. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been employed to study the nature of the contact ion-pair formed between 6HQ and TMA and to determine why the decay component ∼12 ns is absent in a non-polar solvent. Calculations of the hydrogen-bonded complexes formed between 6HQ and TMA and its ESPT reaction product, namely 6HQ-TMA and 6HQ-TMA-PT, respectively, have been carried out, both in the electronic ground and excited states. Moreover, by using the CPCM model, different dielectric constants have been introduced into the calculations. On increasing the dielectric constant, the hydrogen bond in 6HQ-TMA-PT becomes weaker and the hydrogen bond length becomes larger; this effectively facilitates the proton transfer reaction and formation of separated ion-pair. Thus, the separation and diffusion of the contact ion-pair can be controlled by changing the polarity of the surroundings.

Intermolecular hydrogen bond and twisted intramolecular charge transfer in excited state of fast violet B (FVB) in methanol solution

The excited state hydrogen bonding dynamics and corresponding photophysical processes of fast violet B (FVB) in hydrogen-donating methanol (MeOH) solution are investigated by using time-dependent density functional theory (TDDFT) method. In the FVB molecule, there are -C=O, -N-H groups which could act as hydrogen acceptor and donor. It is demonstrated that both the intramolecular hydrogen bond O⋯H-N in FVB and intermolecular hydrogen bond C=O⋯H-O between FVB and MeOH are formed in the ground state S0 and strengthened in the excited state S1. The absorption spectra are obviously red shifted for the hydrogen-bonded complex in comparison with FVB monomer in the low energy range. The theoretical investigation demonstrates that the twisted intramolecular charge transfer takes place in the excited states for both isolated FVB and hydrogen-bonded complex, and the dominant twisting is along N2-C3 bond. The potential energy curve is investigated to understand the photophysics process of FVB and hydrogen-bonded complex.

Excited-state hydrogen-bonding dynamics of camphorsulfonic acid doped polyaniline: a theoretical study

We present a TDDFT study on excited-state hydrogen-bonding dynamics of a camphorsulfonic acid doped polyaniline complex.

Multiple hydrogen bonding in excited states of aminopyrazine in methanol solution: time-dependent density functional theory study

Aminopyrazine (AP) and AP-methanol complexes have been theoretically studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The excited-state hydrogen bonds are discussed in detail. In the ground state the intermolecular multiple hydrogen bonds can be formed between AP molecule and protic solvents. The AP monomer and hydrogen-bonded complex of AP with one methanol are photoexcited initially to the S2 state, and then transferred to the S1 state via internal conversion. However the complex of AP with two methanol molecules is directly excited to the S1 state. From the calculated electronic excited energies and simulated absorption spectra, we find that the intermolecular hydrogen bonds are strengthened in the electronic excited states. The strengthening is confirmed by the optimized excited-state geometries. The photochemical processes in the electronic excited states are significantly influenced by the excited-state hydrogen bond strengthening.

An intermolecular hydrogen-bonding effect on spectral and photophysical properties of 6-aminocoumarin in protic solvents

The paper presents results of the spectral (absorption and emission) and photophysical study of 6-aminocoumarin (6AC) in strong protic solvents with different hydrogen-bond acidity. The measurements of the fluorescence lifetime and quantum yield clearly show that in protic solvents intermolecular hydrogen-bonding interactions between solute and solvent molecules facilitate the nonradiative processes. It is suggested that in these solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. A large isotope effect is found for 6AC in deuterated water.

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

In this work, the dynamics of hydrogen bonds (as well as the hydrogen-bonded wire) in excited-state tautomerization of 7-hydroxyquinoline·(NH3)3 (7HQ·(NH3)3) cluster has been investigated by using time-dependent density functional theory (TDDFT). It shows that upon an excitation, the hydrogen bond between -OH group in 7-hydroxyquinoline (7HQ) and NH3 moiety would extremely strengthened in S1 state, which could effectively facilitate the releasing of the proton from the phenolic group of 7HQ moiety to the hydrogen-bonded wire and the forming an Eigen-like cationic wire (NH3···NH4(+)···NH3) in the cluster. To fulfill the different optimal angles of NH4(+) in the wire, a wagging motion of hydrogen-bonded wire would occur in excited state. Moreover, the wagging motion of the hydrogen-bonded wire would effectively promote excited-state proton transfer reaction. As the results, an excited-state multiple proton transfer (ESMPT) mechanism containing two concerted and asymmetrical processes has been proposed for the proton transfer dynamics of 7HQ·(NH3)3 cluster.

Hydrogen bonding and coordination bonding in the electronically excited states of Cu2(L)2 (L=5-(4-pyridyl)tetrazole)MeOH: a TDDFT study

The luminescent metal organic framework (MOF), Cu2(L)2·MeOH (L=5-(4-pyridyl)tetrazole), was studied using time-dependent density functional theory (TDDFT). A combination of frontier molecular orbitals and electronic configuration analysis revealed that the emission mechanism was a ligand to metal charge transition (LMCT) rather than a metal to ligand charge transfer (MLCT). Hydrogen bonding significantly changed the nature of the frontier orbital and the luminescence. Electronic transition energies predicted that the hydrogen bonding in excited state would become weaker with an electronic spectral blue-shift. The bond lengths, frequencies, and binding energies indicated weakening of the hydrogen bonding in the excited state, which can affect emissions in two ways, including: (i) a decrease in the electronic coupling between methanol and the motif and suppressing the occurrence of the photo-induced electron transfer (PET); and (ii) increasing the energy gap between S1 and S0, leading to radiative transition. Coordination bonding was also investigated in the excited state through bond lengths, frequencies, and bond orders. Coordination bonds were found to become stronger in the excited state leading to an enhancement of the luminescence.