Search Results (100)

Tb³⁺-doped CaF₂ single crystals are attractive materials for green photonic applications due to their low phonon energy, high optical transparency, and efficient Tb³⁺ emission. In this work, CaF₂ single crystals doped with different TbF₃ concentrations (1, 5, and 10 mol%) were grown and systematically investigated in order to clarify the concentration-dependent spectroscopic behavior of Tb³⁺ ions in a fluorite host. Optical absorption spectroscopy, Judd–Ofelt analysis, steady-state and time-resolved photoluminescence, colorimetric evaluation, and emission cross-section and gain calculations were employed. Judd–Ofelt intensity parameters typical of fluoride hosts were obtained, enabling the calculation of radiative transition probabilities and lifetimes. The emission spectra are dominated by intense green luminescence from the ⁵D₄ → ⁷F₅ transition, while the absence of ⁵D₃ emission is attributed to efficient cross-relaxation processes. Fluorescence lifetimes in the millisecond range show slight changes with Tb³⁺ concentration. Quantum efficiency increases from low to intermediate concentrations and tends to saturate at higher doping levels. CIE 1931 chromaticity coordinates confirm stable green emission, while emission cross-sections and gain parameters reveal a highest value for orange emission of 10 mol% TbF₃-doped CaF₂ crystal. These results indicate that CaF₂:Tb³⁺ single crystals are promising materials for photonic applications. Full article

The photophysical properties of a BODIPY derivative with the highly twisted molecular structure of anthracene-fused boron–dipyrromethene (AN-BDP) were studied with steady-state and time-resolved spectroscopic methods. The fused anthryl and the BDP units in AN-BDP units both adopt distorted geometry (with ca. 10° of torsion), and there is large dihedral angle between the two units (ca. 49.7°). Interestingly, the fluorescence quantum yields are highly dependent on the solvent polarity (59~3%, from toluene to acetonitrile), yet the fluorescence emission wavelength does not change in different solvents. Nanosecond transient absorption spectra indicate that the triplet state is long-lived, with an intrinsic triplet state lifetime of 551 μs. Interestingly the severely twisted structure only shows a moderate intersystem crossing (ISC) yield (10%). Femtosecond transient absorption spectra indicate slow ISC (>1.5 ns), which is in agreement with the fluorescence lifetime (2.3 ns). Time-resolved electron paramagnetic resonance (TREPR) spectra show smaller zero-field-splitting D and E tensors as (−71.4 mT, 16.7 mT, respectively) compared to the triplet state of the iodinated native BDP (D = −104.6 mT, E = 22.8 mT), inferring that the triplet-state wave function of the new compound is delocalized over the twisted molecular framework. The theoretical computation indicated a solvent-polarity-dependent energy barrier for the relaxed S₁ state to a conical interaction (CI) of the S₁ and the S₀ state potential curves, which agrees with the weaker fluorescence in polar solvents. Full article

Liposomal systems are advanced carriers of active substances which, thanks to their ability to encapsulate these substances, significantly improve their pharmacokinetics, bioavailability, and selectivity. This article presents the results of spectroscopic studies for a selected compound from the 1,3,4-thiadiazole group, namely 4-[5-(naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol (NTBD, see below in the text), in selected liposomal systems formed from the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Detailed spectroscopic analyses were carried out using electronic absorption and fluorescence spectroscopy; resonance light scattering (RLS) spectra measurements; dynamic light scattering (DLS); as well as time-resolved methods—fluorescence lifetime measurements using the TCSPC technique. Subsequently, based on the interpretation of spectra obtained by FTIR infrared spectroscopy, the preliminary molecular organization of the above-mentioned compounds within lipid multilayers was determined. It was found that NTBD preferentially occupies the region of polar lipid headgroups in the lipid multilayer, although it also noticeably interacts with the hydrocarbon chains of the lipids. Furthermore, X-ray diffraction (XRD) techniques were used to study the effect of NTBD on the molecular organization of DPPC lipid multilayers. Monomeric structures and aggregated forms of the above-mentioned 1,3,4-thiadiazole analogue were characterized using X-ray crystallography. Interesting dual fluorescence effects observed in steady-state fluorescence measurements were linked to the excited-state intramolecular proton transfer (ESIPT) effect (based on our earlier studies), which, in the obtained biophysical systems—liposomal systems with strong hydrophobicity—is greatly enhanced by aggregation-induced emission (AIE) effects. In summary, the research presented in this study, concerning the novel 1,3,4-thiadiazole derivative NTBD, is highly relevant to drug delivery systems, such as various model liposomal systems, as it demonstrates that depending on the concentration of the selected fluorophore, different forms may be present, allowing for appropriate modulation of its biological activity. Full article

Donor–π–acceptor (D–π–A) dyes have garnered significant attention due to their unique optical properties and potential applications in various fields, including optoelectronics, chemical sensing and bioimaging. This study presents the design, synthesis, and comprehensive photophysical investigation of a series of ionic dyes incorporating five- and six-membered heterocyclic rings as electron-donating and electron-withdrawing units, respectively. The influence of the dye structure, i.e., (a) the systematically varied heteroatom (NMe, S and O) in donor moiety, (b) benzannulation of the acceptor part and (c) position of the donor vs. acceptor, on the photophysical properties was evaluated by steady-state and time-resolved spectroscopy across solvents of varying polarity. To probe solvatochromic behavior, the Reichardt parameters and the Catalán four-parameter scale, including polarizability (SP), dipolarity (SdP), acidity (SA) and basicity (SB) parameters, were applied. Emission dynamics were further analyzed through time-resolved fluorescence spectroscopy employing multi-exponential decay models to accurately describe fluorescence lifetimes. Time-dependent density functional theory (TDDFT) calculations supported the experimental findings by elucidating electronic structures, charge-transfer character, and dipole moments in the ground and excited states. The experimental results show the introduction of O or S instead of NMe causes substantial hypsochromic shifts in the absorption and emission bands. Benzannulation enhances the photoinduced charge transfer and causes red-shifted absorption spectra to be obtained without deteriorating the emission properties. Hence, by introducing an appropriate modification, it is possible to design materials with tunable photophysical properties for practical applications, e.g., in opto-electronics or sensing. Full article

The Z-isomer of N,N’-diammoniopropyl derivative of di(3-pyridyl)ethylene was synthesized. The structure and stability of complexes between this non-planar weak acceptor (A, (Z)-2) and a planar strong donor, the E-isomer of bis(18-crown-6)stilbene (D, (E)-1), were studied using X-ray diffraction, ¹H NMR spectroscopy, and optical spectroscopy, including ¹H NMR and spectrofluorimetric titrations. In MeCN, the components form a very stable pseudocyclic bimolecular complex (logK_D·A = 8.48) due to homoditopic coordination of the ammonium groups of the acceptor to the crown moieties of the donor through numerous hydrogen bonds. Intrasupramolecular photo-driven electron transfer (ET) in the isomeric complexes of (E)-1 with (E)- and (Z)-2 was studied using steady-state absorption and fluorescence spectroscopy with time-resolved pulse absorption spectroscopy. It was found that back ET is approximately two times faster in complex (E)-1·(Z)-2 than in closely related (E)-1·(E)-2. Meanwhile, it is ~67 times slower in complex (E)-1·(E)-2 than in the isomeric complex based on N,N’-diammoniopropyl derivative of (E)-di(4-pyridyl)ethylene. Quantum chemical (DFT, TD-DFT) calculations suggest the actual photorelaxation pathway for the complexes under study. Full article

Bi-phasic (with a local minimum) response of fluorescence to scattering when probed by a single fiber (SF) was first observed in 2003. Subsequent experiments and Monte Carlo studies have shown the bi-phasic turning of SF fluorescence to occur at a dimensionless reduced scattering of ~1 and vary with absorption. The bi-phase of SF fluorescence received semi-empirical explanations; however, better understandings of the bi-phase and its dependence on absorption are necessary. This work demonstrates a quasi-analytical projection of a bi-phasic pattern comparable to that of SF fluorescence via photon-transport analyses of fluorescence in a center-illuminated-area-detection (CIAD) geometry. This model-approach is principled upon scaling of the diffuse fluorescence between CIAD and a SF of the same size of collection, which expands the scaling of diffuse reflectance between CIAD and a SF discovered for steady-state and time-domain cases. Analytical fluorescence for CIAD is then developed via radial-integration of radially resolved fluorescence. The radiance of excitation is decomposed to surface, collimated, and diffusive portions to account for the surface, near the point-of-entry, and diffuse portion of fluorescence associated with a centered illumination. Radiative or diffuse transport methods are then used to quasi-analytically deduce fluorescence excited by the three portions of radiance. The resulting model of fluorescence for CIAD, while limiting to iso-transport properties at the excitation and emission wavelengths, is compared against the semi-empirical model for SF, revealing bi-phasic turning [0.5~2.6] at various geometric sizes [0.2, 0.4, 0.6, 0.8, 1.0 mm] and a change of three orders of magnitude in the absorption of the background medium. This model projects a strong reduction in fluorescence versus strong absorption at high scattering, which differs from the semi-empirical SF model’s projection of a saturating pattern unresponsive to further increases in the absorption. This framework of modeling fluorescence may be useful to project frequency-domain and lifetime pattens of fluorescence in an SF and CIAD. Full article

Tomato waste (TW) was employed as a sustainable source for the synthesis of fluorescent carbon dots (CDs) via a microwave-assisted hydrothermal carbonization (Mw-HTC) method, aiming at its valorization. Several amines were used as nitrogen additives to enhance the fluorescence quantum yield (QY) of CDs, and a set of reaction conditions, including additive/TW mass ratio (0.04–0.32), dwell time (15–60 min), and temperature (200–230 °C) of the HTC process, were scrutinized. The structural analysis of the tomato waste carbon dots (TWCDs) was undertaken by FTIR and ¹H NMR techniques, revealing their most relevant features. In solid state, transmission electron microscopy (TEM) analysis showed the presence of nearly spherical nanoparticles with an average lateral size of 8.1 nm. Likewise, the topographical assessment by atomic force microscopy (AFM) also indicated particles’ heights between 3 and 10 nm. Their photophysical properties, revealed by UV–Vis, steady-state, and time-resolved fluorescence spectroscopies, are fully discussed. Higher photoluminescent quantum yields (up to 0.08) were attained when the biomass residues were mixed with organic aliphatic amines during the Mw-HTC process. Emission tunability is a characteristic feature of these CDs, which display an intensity average fluorescence lifetime of 8 ns. The new TWCDs demonstrated good antioxidant properties by the ABTS radical cation method (75% inhibition at TWCDs’ concentration of 5 mg/mL), which proved to be related to the dwell time used in the CDs synthesis. Moreover, the synthesized TWCDs suppressed the growth of Escherichia coli and Staphylococcus aureus at concentrations higher than 2000 μg/mL, encouraging future antibacterial applications. Full article

Ketoprofen (KTF) and ketorolac (KTL) are among the most primarily used non-steroidal anti-inflammatory drugs (NSAIDs) in humans to alleviate moderate pain and to treat inflammation. Their binding affinity with albumin (the main globular protein responsible for the biodistribution of drugs in the bloodstream) was previously determined by spectroscopy without considering some conventional pitfalls. Thus, the present work updates the biophysical characterization of the interactions of HSA:KTF and HSA:KTL by ¹H saturation-transfer difference nuclear magnetic resonance (¹H STD-NMR), ultraviolet (UV) absorption, circular dichroism (CD), steady-state, and time-resolved fluorescence spectroscopies combined with in silico calculations. The binding of HSA:NSAIDs is spontaneous, endothermic, and entropically driven, leading to a conformational rearrangement of HSA with a slight decrease in the α-helix content (7.1% to 7.6%). The predominance of the static quenching mechanism (ground-state association) was identified. Thus, both Stern–Volmer quenching constant (K_SV) and binding constant (K_b) values enabled the determination of the binding affinity. In this sense, the K_SV and K_b values were found in the order of 10⁴ M⁻¹ at human body temperature, indicating moderate binding affinity with differences in the range of 0.7- and 3.4-fold between KTF and KTL, which agree with the previously reported experimental pharmacokinetic profile. According to ¹H STD-NMR data combined with in silico calculations, the aromatic groups in relation to the aliphatic moiety of the drugs interact preferentially with HSA into subdomain IIIA (site II) and are stabilized by interactions via hydrogen bonding and hydrophobic forces. In general, the data obtained in this study have been revised and updated in comparison to those previously reported by other authors who did not account for inner filter corrections, spectral backgrounds, or the identification of the primary mathematical approach for determining the binding affinity of HSA:KTF and HSA:KTL. Full article

Diblock conjugated oligomers are π-conjugated molecules that contain two segments having distinct frontier orbital energies and HOMO-LUMO gap offsets. These oligomers are of fundamental interest to understand how the distinct π-conjugated segments interact and modify their excited state properties. The current paper reports a study of two series of diblock oligomers that contain oligothiophene (T_n) and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) segments that are coupled by either ethynyl (-C≡C-) or trans-(-C≡C-)₂Pt(II)(PBu₃)₂ acetylide linkers. In these structures, the T_n segment is electron rich (donor), and the TBT is electron poor (acceptor). The diblock oligomers are characterized by steady-state and time-resolved spectroscopy, including UV-visible absorption, fluorescence, fluorescence lifetimes, and ultrafast transient absorption spectroscopy. Studies are compared in several solvents of different polarity and with different excitation wavelengths. The results reveal that the (-C≡C-) linked oligomers feature a delocalized excited state that takes on a charge transfer (CT) character in more polar media. In the (-C≡C-)₂Pt(II)(PBu₃)₂-linked oligomers, there is weak coupling between the T_n and TBT segments. Consequently, short wavelength excitation selectively excites the T_n segment, which then undergoes ultrafast energy transfer (~1 ps) to afford a TBT-localized excited state. Full article

Thin films of carbazole (Cz) derivatives are frequently used in organic electronics, such as organic light-emitting diodes (OLEDs). Because of the proximity of the Cz units, the excited-state relaxation in such films is complicated, as intermolecular pathways, such as singlet–singlet annihilation (SSA), kinetically compete with the emission. Here, we provide an investigation of two benchmark systems employing neat carbazole and 3,6-di-tert-butylcarbazole (t-Bu-Cz) films and also their thin film blends with poly(methyl methacrylate) (PMMA). These are investigated by a combination of atomic force microscopy (AFM), femtosecond and nanosecond transient absorption spectroscopy (fs-TA and ns-TA) and time-resolved fluorescence. Excitonic J-aggregate-type features are observed in the steady-state absorption and emission spectra of the neat films. The S₁ state shows a broad excited-state absorption (ESA) spanning the entire UV–Vis–NIR range. At high S₁ exciton number densities of about 4 × 10¹⁸ cm⁻³, bimolecular diffusive S₁–S₁ annihilation is found to be the dominant SSA process in the neat films with a rate constant in the range of 1–2 × 10⁻⁸ cm³ s⁻¹. SSA produces highly vibrationally excited molecules in the electronic ground state (S₀*), which cool down slowly by heat transfer to the quartz substrate. The results provide relevant photophysical insight for a better microscopic understanding of carbazole relaxation in thin-film environments. Full article

Tenofovir (TFV) is the active form of the prodrugs tenofovir disoproxil fumarate (TDF) and tenofovir alafenamide (TAF), both clinically prescribed as HIV reverse transcriptase inhibitors. The biophysical interactions between these compounds and human serum albumin (HSA), the primary carrier of exogenous compounds in the human bloodstream, have not yet been thoroughly characterized. Thus, the present study reports the interaction profile between HSA and TFV, TDF, and TAF via UV–Vis, steady-state, and time-resolved fluorescence techniques combined with isothermal titration calorimetry (ITC) and in silico calculations. A spontaneous interaction in the ground state, which does not perturb the microenvironment close to the Trp-214 residue, is classified as weak. In the case of HSA/TFV and HSA/TDF, the binding is both enthalpically and entropically driven, while for HSA/TAF, the binding is only entropically dominated. The binding constant (K_a) and thermodynamic parameters obtained via ITC assays agree with those obtained using steady-state fluorescence quenching measurements, reinforcing the reliability of the data. The small internal cavity known as site I is probably the main binding pocket for TFV due to the low steric volume of the drug. In contrast, most external sites (II and III) can better accommodate TAF due to the high steric volume of this prodrug. The cross-docking approach corroborated experimental drug-displacement assays, indicating that the binding affinity of TFV and TAF might be impacted by the presence of different compounds bound to albumin. Overall, the weak binding capacity of albumin to TFV, TDF, and TAF is one of the main factors for the low residence time of these antiretrovirals in the human bloodstream; however, positive cooperativity for TAF and TDF was detected in the presence of some drugs, which might improve their residence time (pharmacokinetic profile). Full article

Metal phthalocyaninates and their higher homologues are recognized as deep-red luminophores emitting from their lowest excited singlet state. Herein, we report on the design, synthesis, and in-depth characterization of a new class of dual-emissive (visible and NIR) metal naphthalocyaninates. A 4-N,N-dimethylaminophen-4-yl-substituted naphthalocyaninato zinc(II) complex (Zn-NMe₂Nc) and the derived water-soluble coordination compound (Zn-NMe₃Nc) exhibit a near-infrared fluorescence from the lowest ligand-centered state, along with a unique push–pull-supported luminescence in the visible region of the electromagnetic spectrum. An unprecedentedly broad structural (2D-NMR spectroscopy and mass spectrometry) as well as photophysical characterization (steady-state state and time-resolved photoluminescence spectroscopy) is presented. The unique dual emission was assigned to two independent sets of singlet states related to the intrinsic Q-band of the macrocycle and to the push–pull substituents in the molecular periphery, respectively, as predicted by TD-DFT calculations. In general, the elusive chemical aspects of these macrocyclic compounds are addressed, involving both reaction conditions, thorough purification, and in-depth characterization. Besides the fundamental aspects that are investigated herein, the photoacoustic properties were exemplarily examined using phantom gels to assess their tomographic imaging capabilities. Finally, the robust luminescence in the visible range arising from the push–pull character of the peripheral moieties demonstrated a notable independence from aggregation and was exemplarily implemented for optical imaging (FLIM) through time-resolved multiphoton micro(spectro)scopy. Full article

The photophysical and sensory properties of the donor–acceptor pyrazoloquinoline derivative (PQPc) were investigated using absorption, steady-state, and time-resolved fluorescence measurements. The compound synthesized from commercial, readily available substrates exhibited absorptions in the UV–Vis range, with a maximum of the longwave band around 390 nm. The maximum fluorescence was around 460–480 nm, depending on the solvent. The quantum yield was between 12.87% (for n-hexane) and 0.75% (for acetonitrile) and decreased with increasing solvent polarity. The PET mechanism was implicated as the cause of fluorescence quenching. Divalent ions such as Zn²⁺, Pb²⁺, Cd²⁺, Ca²⁺, Mg²⁺, Co²⁺, Ni²⁺, and Cu²⁺ were introduced to study the fluorescent response of PQPc. A 13-times increase in fluorescence quantum yield was observed after the addition of Zn²⁺ ions. Detailed research was carried out for the PQPc-Zn²⁺ system in order to check the possibility of analytical applications of PQPc as a fluorescent sensor. A detection limit of Zn²⁺ was set at the value level 1.93 × 10⁻⁷ M. PQPc-Zn²⁺ complexes had a stoichiometry of 1:1 with a binding constant of 859 M⁻¹. Biological studies showed that the sensor was localized in cells near the membrane and cytoplasm and may be used to detect zinc ions in eukaryotic cells. Full article

The Photophysical properties, such as fluorescence quenching, and photoexcitation dynamics of bimolecular non-covalent systems consisting of cationic poly[(9,9-di(3,3′-N,N′-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and anionic graphene carboxylate (GC) have been discovered for the first time via steady-state and time-resolved femtosecond transient absorption (TA) spectroscopy with broadband capabilities. The steady-state fluorescence of PFN is quenched with high efficiency by the GC acceptor. Fluorescence lifetime measurements reveal that the quenching mechanism of PFN by GC is static. Here, the quenching mechanisms are well proven via the TA spectra of PFN/GC systems. For PFN/GC systems, the photo electron transfer (PET) and charge recombination (CR) processes are ultrafast (within a few tens of ps) compared to static interactions, whereas for PFN/1,4-dicyanobenzene DCB systems, the PET takes place in a few hundreds of ps (217.50 ps), suggesting a diffusion-controlled PET process. In the latter case, the PFN^+•–DCB^−• radical ion pairs as the result of the PET from the PFN to DCB are clearly resolved, and they are long-lived. The slow CR process (in 30 ns time scales) suggests that PFN^+• and DCB^−• may already form separated radical ion pairs through the charge separation (CS) process, which recombine back to the initial state with a characteristic time constant of 30 ns. The advantage of the present positively charged polyfluorene used in this work is the control over the electrostatic interactions and electron transfers in non-covalent polyfluorene/quencher systems in DMSO solution. Full article

Dipeptides 1 and 2 were synthesized from unnatural amino acids containing pyrene as a fluorescent label and polynucleotide binding unit, and modified tyrosine as a photochemically reactive unit. Photophysical properties of the peptides were investigated by steady-state and time-resolved fluorescence. Both peptides are fluorescent (Φ_f = 0.3–0.4) and do not show a tendency to form pyrene excimers in the concentration range < 10⁻⁵ M, which is important for their application in the fluorescent labeling of polynucleotides. Furthermore, both peptides are photochemically reactive and undergo deamination delivering quinone methides (QMs) (Φ_R = 0.01–0.02), as indicated from the preparative photomethanolysis study of the corresponding N-Boc protected derivatives 7 and 8. Both peptides form stable complexes with polynucleotides (log K_a > 6) by noncovalent interactions and similar affinities, binding to minor grooves, preferably to the AT reach regions. Peptide 2 with a longer spacer between the fluorophore and the photo-activable unit undergoes a more efficient deamination reaction, based on the comparison with the N-Boc protected derivatives. Upon light excitation of the complex 2·oligoAT₁₀, the photo-generation of QM initiates the alkylation, which results in the fluorescent labeling of the oligonucleotide. This study demonstrated, as a proof of principle, that small molecules can combine dual forms of fluorescent labeling of polynucleotides, whereby initial addition of the dye rapidly forms a reversible high-affinity noncovalent complex with ds-DNA/RNA, which can be, upon irradiation by light, converted to the irreversible (covalent) form. Such a dual labeling ability of a dye could have many applications in biomedicinal sciences. Full article