A combined solvatochromic shift and TDDFT study probing solute-solvent interactions of blue fluorescent Alexa Fluor 350 dye: Evaluation of ground and excited state dipole moments

Conformational analysis and spectroscopic properties of antichagasic nifurtimox

Considering the health relevance of Chagas' disease, recent research efforts have focused on developing more efficient drug delivery systems containing nifurtimox (NFX). This paper comprehensively investigates NFX through conformational analysis and spectroscopic characterization. Using a conformer-rotamer ensemble sampling tool (CREST-xtb), five distinct conformers of NFX were sampled within a 3.0 kcal mol-1 relative energy window. Subsequently, such structures were used as inputs for geometry optimization by density functional theory (DFT) at B3LYP-def2-TZVP level of theory. Notably, harmonic vibrational frequencies were calculated to establish an in-depth comparison with experimental results and existing literature for the NFX or similar molecules and functional groups, thereby achieving a widely reasoned assignment of the mid-infrared band absorptions for the first time. Moreover, UV-VIS spectra of NFX were obtained in several solvents, enabling the determination of the molar absorptivity coefficient for the two electronic transitions observed for NFX. Among the aprotic solvents, a bathochromic effect was observed in the function of the dielectric constants. Furthermore, a hypochromic effect was observed when the drug was dissolved in protic solvents. These findings offer crucial support for new drug delivery systems containing NFX while demonstrating the potential of spectrophotometric studies in establishing quality control assays for NFX drug products.

Comparative Study of Two Spectral Methods for Estimating the Excited State Dipole Moment of Non-Fluorescent Molecules

The electronic absorption spectral characteristics of cycloimmonium ylids with a zwitterionic structure have been analyzed in forty-three solvents with different hydrogen bonding abilities. The two ylids lack fluorescence emission but are very dynamic in electronic absorption spectra. Using the maximum of the ICT band, the goal was to establish an accurate relationship between the shift of the ICT visible band and the solvent parameters and to estimate two of the descriptors of the first (the) excited state: the dipole moment and the polarizability. Two procedures were involved: the variational method and the relationships of the Abe model. The results indicate that the excited state dipole moment of the two methylids decreases in the absorption process in comparison with the ground state. The introduction of a correction term in the Abe model that neglects the intermolecular H-bonding interactions leads to a more accurate determination of the two descriptors. The strong solvatochromic response of both ylids has been further applied in distinguishing the solvents as a function of their specific parameters. Principal component analysis was applied to five selected properties, including the maximum of the charge transfer band. The results were further applied to discriminate several binary solvent mixtures.

Ground and Excited State Dipole Moments of Metformin Hydrochloride using Solvatochromic Effects and Density Functional Theory

In this research, the ground (µg) and excited (µe) state dipole moments of metformin hydrochlorides were determined using Lippert-Mataga, Bakhshiev's, Kawski-Chamma-Viallet, and Reichardt models from fluorescence emission and UV-Vis absorption spectra in various solvents. From solvatochromic effects the calculated excited (µe ) dipole moment of metformin hydrochloride were, 8.55 D, 8.34 D, 6.08 D, and 6.40 D using the Lippert-Mataga, Bakhshiev's, Kawski-Chamma-Viallet and Reichardt models respectively. The results also indicated that the dipole moment at the ground state is smaller than the excited state. This is due to solvent polarity having a stronger effect on fluorescence emission than absorption spectra. Similarly, from density functional theory, the calculated ground and excited states dipole moments of metformin hydrochloride using (DFT-B3LYP- 3-21+G*(μg = 10.02 D and μe = 11.94 D), DFT-B3LYP- 6-31+G (d, p) (μg = 8.44 D and μe = 10.87 D), and DFT-B3LYP- 6-311+G (d, p) (μg = 8.24 D and μe = 18.74 D)) analyzed by Gaussian 09W. From the optimized geometry of the molecule, the HOMO-LUMO energy band gap of metformin hydrochloride were computed using DFT [DFT-B3LYP- 3-21+G*(5.51 eV), DFT-B3LYP- 6-31+G (d, p) (5.66 eV), and DFT-B3LYP- 6-311+G (d, p) (5.70 eV)] respectively.

Solvent Effects on the Singlet-Triplet Couplings in Nitroaromatic Compounds

Nitrated polycyclic molecules can present the largest singlet-triplet crossing rates among organic molecules. This implies that most of these compounds have no detectable steady-state fluorescence. In addition, some nitroaromatics undergo a complex series of photoinduced atom rearrangements that result in nitric oxide dissociation. The overall photochemistry of these systems depends critically on the competition between the rapid intersystem crossing channel and other excited-state pathways. In this contribution, we sought to characterize the degree of stabilization of the S₁ state due to solute-solvent interactions, and to quantify the effect of such stabilization on their photophysical pathways. We studied 2- and 4-nitropyrene (2-NP and 4-NP), which are atypically emissive nitroaromatics in a series of solvents. From steady-state and time-resolved measurements, the S₁ state of these molecules shows significant stabilization as the solvent polarity is increased. On the other hand, specific triplet states that are iso-energetic with the emissive singlet (T₃ for 2-NP and T₂ for 4-NP) in nonpolar solvents become slightly de-stabilized upon increasing the solvent polarity. These combined effects result in rapid singlet-triplet population transfer in nonpolar solvents for both molecules. In contrast, for solvents with even slightly higher polarities, the first excited singlet is stabilized in relation to the specific triplet states, leading to much longer S₁ lifetimes. These effects can be summarized as a highly solvent-dependent coupling/decoupling of the manifolds. Similar effects are also likely to be present in other nitroaromatics where there is a dynamic competition between nitric oxide dissociation and intersystem crossing. The drastic effects of the solvent polarity in the manifold crossing pathway should be taken into consideration in both theoretical and experimental studies of nitroaromatics.

Experimental and Theoretical Spectroscopic Investigation on Coumarin Based Derivatives for Non-Linear Optoelectronics Application

The spectral properties of MBTC (4-((4-((Benzo[d]oxazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)-7-methoxy-2H-chromen-2-one), CBTC (4-((4(((5Chlorobenzo[d]oxazol-2-yl)thio)methyl)-1H-1,2,3-triazol-1-yl)methyl)-2H-benzo[h]chromen-2-one) and TBTC (4-((4-((Benzo[d]oxazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)6(tertbutyl)2Hchromen-2-one) were studied in series of solvents with different polarity at room temperature to explore their solvatochromic effect and dipole moment. Stokes shift revealed a bathochromic shift with varying solvent polarity for all molecules which implies π-π^*transition. The ground state and excited state dipole moment of the molecules are calculated experimentally using salvatochromic methods like Lippert-Mataga, Bakhshiev, Kawaski-chamma-viallet, and Reichardt's microscopic solvent polarity functions and computationally by density functional theory (DFT) method. It is observed that the excited state dipole moment is higher than the ground state so synthesized molecules are more polar in the excited state than in the ground state. Using the DFT method HOMO and LUMO energy values were obtained and compared with values obtained by the cyclic voltammetry. Using the values of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) we have estimated energy gap, chemical hardness (ɳ), chemical softness (s), ionization potential (IP), electron affinity (EA), electronegativity (χ), electrophilicity (ω), and chemical potential (μ) of the molecules were estimated. The energy gap of MBTC, CBTC, and TBTC were found to be low, that is 3.861 eV, 3.822 eV, and 3.801 eV respectively, this indicates molecules are more reactive and it has the easiest π-π^* transition. Further electrophilic and nucleophilic sites were figured out using molecular electrostatic potential (MESP) which is useful in photochemical reactions. Hence the quantum chemical calculation and spectroscopic properties of the molecules can give a better understanding of their use in an optoelectronic device.

Synthesis and Strong Solvatochromism of Push-Pull Thienylthiazole Boron Complexes

The solvatochromic behavior of two donor-π bridge-acceptor (D-π-A) compounds based on the 2-(3-boryl-2-thienyl)thiazole π-linker and indandione acceptor moiety are investigated. DFT/TD-DFT calculations were performed in combination with steady-state absorption and emission measurements, along with electrochemical studies, to elucidate the effect of two different strongly electron-donating hydrazonyl units on the solvatochromic and fluorescence behavior of these compounds. The Lippert–Mataga equation was used to estimate the change in dipole moments (Δµ) between ground and excited states based on the measured spectroscopic properties in solvents of varying polarity with the data being supported by theoretical studies. The two asymmetrical D-π-A molecules feature strong solvatochromic shifts in fluorescence of up to ~4300 cm⁻¹ and a concomitant change of the emission color from yellow to red. These changes were accompanied by an increase in Stokes shift to reach values as large as ~5700–5800 cm⁻¹. Quantum yields of ca. 0.75 could be observed for the N,N-dimethylhydrazonyl derivative in nonpolar solvents, which gradually decreased along with increasing solvent polarity, as opposed to the consistently reduced values obtained for the N,N-diphenylhydrazonyl derivative of up to ca. 0.20 in nonpolar solvents. These two push–pull molecules are contrasted with a structurally similar acceptor-π bridge-acceptor (A-π-A) compound.

Tissue clearing may alter emission and absorption properties of common fluorophores

In recent years, 3D cell culture has been gaining a more widespread following across many fields of biology. Tissue clearing enables optical analysis of intact 3D samples and investigation of molecular and structural mechanisms by homogenizing the refractive indices of tissues to make them nearly transparent. Here, we describe and quantify that common clearing solutions including benzyl alcohol/benzyl benzoate (BABB), PEG-associated solvent system (PEGASOS), immunolabeling-enabled imaging of solvent-cleared organs (iDISCO), clear, unobstructed brain/body imaging cocktails and computational analysis (CUBIC), and ScaleS4 alter the emission spectra of Alexa Fluor fluorophores and fluorescent dyes. Clearing modifies not only the emitted light intensity but also alters the absorption and emission peaks, at times to several tens of nanometers. The resulting shifts depend on the interplay of solvent, fluorophore, and the presence of cells. For biological applications, this increases the risk for unexpected channel crosstalk, as filter sets are usually not optimized for altered fluorophore emission spectra in clearing solutions. This becomes especially problematic in high throughput/high content campaigns, which often rely on multiband excitation to increase acquisition speed. Consequently, researchers relying on clearing in quantitative multiband excitation experiments should crosscheck their fluorescent signal after clearing in order to inform the proper selection of filter sets and fluorophores for analysis.

On the physical-chemical nature of solvent polarizability and dipolarity

The positive solvatochromism of three dyes, with a spectral behavior strongly dependents on the medium dipolarity/polarizability, was studied theoretically. Both a polarizable continuum-solvent model (CSM) and explicit solvent molecules were employed to model solvent effects. The CSM approach, coupled with ten different TDDFT methods, yielded unsatisfactory results in eleven solvents. The explicit-solvation calculations, thought of much higher computational cost, yielded excellent results. As CSM schemes are known correctly model non-specific electrostatic effects, our results indicate that the traditionally considered non-specific nature of solvent dipolarity needs to be reconsidered, requiring the explicit consideration of the solute-solvent interactions for their accurate theoretical description.

Resolved Structural States of Calmodulin in Regulation of Skeletal Muscle Calcium Release

Calmodulin (CaM) is proposed to modulate activity of the skeletal muscle sarcoplasmic reticulum (SR) calcium release channel (ryanodine receptor, RyR1 isoform) via a mechanism dependent on the conformation of RyR1-bound CaM. However, the correlation between CaM structure and functional regulation of RyR in physiologically relevant conditions is largely unknown. Here, we have used time-resolved fluorescence resonance energy transfer (TR-FRET) to study structural changes in CaM that may play a role in the regulation of RyR1. We covalently labeled each lobe of CaM (N and C) with fluorescent probes and used intramolecular TR-FRET to assess interlobe distances when CaM is bound to RyR1 in SR membranes, purified RyR1, or a peptide corresponding to the CaM-binding domain of RyR (RyRp). TR-FRET resolved an equilibrium between two distinct structural states (conformations) of CaM, each characterized by an interlobe distance and Gaussian distribution width (disorder). In isolated CaM, at low Ca²⁺, the two conformations of CaM are resolved, centered at 5 nm (closed) and 7 nm (open). At high Ca²⁺, the equilibrium shifts to favor the open conformation. In the presence of RyRp at high Ca²⁺, the closed conformation shifts to a more compact conformation and is the major component. When CaM is bound to full-length RyR1, either purified or in SR membranes, strikingly different results were obtained: 1) the two conformations are resolved and more ordered, 2) the open state is the major component, and 3) Ca²⁺ stabilized the closed conformation by a factor of two. We conclude that the Ca²⁺-dependent structural distribution of CaM bound to RyR1 is distinct from that of CaM bound to RyRp. We propose that the function of RyR1 is tuned to the Ca²⁺-dependent structural dynamics of bound CaM.

Fluorescent Probes from Aromatic Polycyclic Nitrile Oxides: Isoxazoles versus Dihydro-1λ3,3,2λ4-Oxazaborinines

Anthracenenitrile oxide undergoes 1,3‐dipolar cycloaddition reaction with propargyl bromide affording the expected isoxazole as single regioisomer, suitably synthetically elaborated and functionalized with a protected triple bond. The introduction of a bromine atom at the position C10 of the anthracene moiety allows for inserting a variety of aromatic and heterocyclic substituents through Suzuki coupling. A two‐way synthetic route can lead to simple isoxazole derivatives or, after N−O bond reductive cleavage and BF₃ complexation, enamino ketone boron complexes. The photophysical properties of both the substituted isoxazoles and the corresponding boron complexes were investigated to show the potentialities for the employment as fluorescent tags in imaging techniques. The quite good quantum yield values confirm the suitability of these compounds in the cellular environment. Scope and limitations of the methodology are discussed.