The effect of water in THF/water mixtures on CMC, aggregation sizes, and fluorescence quenching of a new calix[4]resorcinarene macrocycle.

This study explores how water content modulates the self-assembly and fluorescence behavior of a novel calixarene, C1. C1 forms large, flattened structures in pure THF, but water addition triggers a transition to smaller, unimodal clusters. A critical micellar concentration (CMC) is identified, decreasing with increasing water content. Fluorescence quenching is observed upon water addition, attributed to nonradiative deactivation. These findings highlight water as a key regulator of C1's assembly and fluorescence, paving the way for further development of water-responsive calixarene systems.

Exploring potooxidative degradation pathways of harmol and harmalol alkaloids in water: effects of pH, excitation sources and atmospheric conditions.

This work explores the photochemical degradation of cationic species of 7-hydroxy-1-methyl-2H-pyrido[3,4-b]indole or harmol (1C) and the corresponding partially hydrogenated derivative 7-hydroxy-1-methyl-3,4-dihydro-2H-pyrido[3,4-b]indole or harmalol (2C) in aqueous solution. UV-visible absorption and fluorescence emission spectroscopy coupled with multivariate data analysis (MCR-ALS and PARAFAC), HPLC and HRESI-MS techniques were used for both quantitative and qualitative analysis. The formation of hydrogen peroxide reactive oxygen species (ROS) was quantified, and the influence of pH, oxygen partial pressure and photoexcitation source on the photochemical degradation of both compounds was assessed. The potential implications on the biosynthesis of ßCs and their biological role in living systems are discussed.

Neutral and Cationic Re(I) Complexes with Pnictogen-Based Coligands and Tunable Functionality: From Phosphorescence to Photoinduced CO Release.

In this work, we have explored Re(I) complexes featuring triphenylpnictogen (PnPh3, Pn = P, As, or Sb)-based coligands and bidentate (neutral or monoanionic) luminophores derived from 1,10-phenantroline (phen), as well as from 2-(3-(tert-butyl)-1H-1,2,4-triazol-5-yl)pyridine (H(N-tBu)). The effect of the increasingly heavy elements on the structural parameters, photoexcited-state properties, and electrochemical behavior as well as the hybridization defects and polarization of the Pn atoms was related to the charges of the main luminophores (i.e., phen vs N-tBu) and explored in terms of photoluminescence spectroscopy, X-ray diffractometry, and quantum-chemical methods. Therefore, an in-depth analysis of the bonding, crystal packing, excited-state energies, and lifetimes was assessed in liquid solutions, frozen glassy matrices, and crystalline phases along with a semiquantitative photoactivation study. Notably, by changing the main ligand from phen to N-tBu, an increase in radiative and radiationless deactivation rates (kr and knr, respectively) at 77 K together with a faster photoinduced CO release and fragmentation at room temperature was detected. In addition, a progressively red-shifted phosphorescence was observed with the growing atomic number of the pnictogen atom, along with a boost in kr and knr at 77 K. Down the Vth main group and upon coordination of the Pn atom to the Re(I) center, an increasingly prominent jump of s-orbital participation on the binding sxp3.00-orbitals of the Pn atoms is evidenced. Based on these findings, the ability of these complexes to act as tunable photoluminescent labels able to perform as light-driven CO-releasing molecules is envisioned.

Rhenium(I) Complexes with Neutral Monodentate Coligands and Monoanionic 2-(1,2,4-Triazol-5-yl)pyridine-Based Chelators as Bidentate Luminophores with Tunable Color and Photosensitized Generation of 1O2: An Integrated Case Study Involving Photophysics and Theory.

In this work, we describe the synthesis as well as structural, photophysical, and theoretical investigation of a new coordination chemical concept involving rhenium(I) complexes bearing monoanionic 1,2,4-triazolylpyridine-based bidentate chromophores. The X-ray diffractometric analysis of single crystals revealed particular packing features: the trifluoromethylated exemplar displayed two kinds of arrangements of the coordination centers, where the bidentate ligands at the edges of the unit cell are staggered parallel to each other, whereas those inside show antiparallel stacking with respect to the external ligands. On the other hand, the complexes bearing an adamantyl substituent yield a linear arrangement, where the bulky moiety of one luminophore points to the pyridine center of the adjacent ligand of the neighboring complex while including methanol molecules hydrogen-bonded to the triazolato unit. We observed that the photophysical properties of the complexes (photoexcited-state lifetimes, photoluminescence maxima and quantum yields) can be adjusted by tuning of the substitution pattern at the bidentate luminophore as well as by variation of the monodentate coligand. The photoluminescence spectra and photoexcited-state lifetimes of the crystalline phases were measured by phosphorescence lifetime micro(spectro)scopy. Interestingly, the vibrationally resolved emission spectra of the crystals closely resemble those of diluted frozen glassy matrixes at 77 K, in contrast with the broad bands observed in amorphous solids and in fluid solutions, where the charge-transfer character is enhanced. While the photoluminescence quantum yields (ΦL) reach up to 15%, the complexes are able to attain up to 55% efficiency regarding the photosensitization of 1O2 (ΦΔ), depending on the combination of luminophore and coligand. Theoretical calculations showed that the photoexcited triplet (T1) state has a metal-ligand-to-ligand charge-transfer character, where promotion to the excited electronic configuration shortens the Re(I)-N bond involving the bidentate triazolylpyridine while stretching the three fac-CO-Re(I) bonds as well as the linkage to the axial monodentate coligand. The calculated vertical (Evl) and 0-0 (E(0-0)) radiative transition energies are in very good agreement with the experimental values (Eexplum).

Spectroscopic and quantum chemical characterization of the ground and lowest electronically excited singlet and triplet states of halo- and nitro-harmines in aqueous media.

Halogenated and nitro ß-carboline (ßCs) alkaloids have garnered increasing interest for their role in a broad range of biological, pharmacological and biotechnological processes. Addressing their spectroscopic and photophysical properties provide tools to further explore the presence of these alkaloids in complex biological matrices. In addition, these studies help to elucidate processes where these alkaloids are involved. The UV-visible and steady-state room temperature fluorescence of bromo- and nitro-harmines in an aqueous environment at different pHs, low-temperature phosphorescence (at 77 K) and quantum yields of singlet oxygen production are reported herein. Singlet (S0 and S1) and triplet (T1) electronic states are further analyzed using density functional theory (DFT) and the results compared with experimental data. Data are discussed in the framework of potential biotechnological applications of these ßC alkaloids.

Photophysical and spectroscopic features of 3,4-dihydro-ß-carbolines: a combined experimental and theoretical approach.

3,4-Dihydro-ß-carbolines (DHßCs) are a set of endogenously synthesized alkaloids spread over a great variety of living species (e.g., plants, animals and microorganisms), playing a broad spectrum of biological, biochemical and/or pharmacological roles, in a structure-dependent manner. Addressing unresolved fundamental aspects related to the photophysical properties of DHßCs might help to gain further insights into the molecular basis of the mechanisms of the biological processes where these alkaloids are involved. In this work, the UV-visible spectroscopic features of DHßCs are revisited and they are further analyzed by calculations at the Density Functional Theory (DFT) level. In addition, steady-state and time-resolved fluorescence spectroscopy, as well as quantitative singlet oxygen production analysis is reported. Data obtained herein are discussed in the framework of the potential biological role of these alkaloids.

Facile synthesis and exploration of excited state assisted two-photon absorption properties of D-A-D type thiophene-pyridine derivatives.

A series of trigonal-shaped molecules (TPyT and TPyP) and a polymer (P2TPy) with donor-acceptor-donor (D-A-D) structural arrangement were designed with thiophene and 2,4,6-trisubstituted pyridine as the backbone i.e., a 4-(4-(decyloxy)phenyl)-2,6-di(thiophen-2-yl)pyridine core structure, for the first time and synthesized via a microwave assisted one pot reaction. The extension of conjugation of the system by substituting thiophene acrylonitrile (TPyT) and phenyl acrylonitriles (TPyP and P2TPy) via low cost condensation reactions improved the linear optical, electronic and thermal properties of the synthesized compounds. Furthermore, the nonlinear absorption (NLA) was also enriched appreciably by increasing the conjugation. The three compounds showed an "effective two-photon absorption" phenomenon with the NLA coefficient (ßeff) of the order of 10-10 m W-1. The suitability to be utilized in the fabrication of all-optical limiting devices is best achieved by polymer P2TPy as it exhibits substantially higher ßeff (7.02 × 10-10 m W-1) and an extremely low optical limiting threshold (1.42 J cm-2).

Association studies to transporting proteins of fac-ReI(CO)3(pterin)(H2O) complex.

A new synthetic route to acquire the water soluble complex fac-ReI(CO)3(pterin)(H2O) was carried out in aqueous solution. The complex has been obtained with success via the fac-[ReI(CO)3(H2O)3]Cl precursor complex. ReI(CO)3(pterin)(H2O) has been found to bind strongly with bovine and human serum albumins (BSA and HSA) with intrinsic-binding constants, Kb, of 6.5 × 105 M-1 and 5.6 × 105 M-1 at 310 K, respectively. The interactions of serum albumins with ReI(CO)3(pterin)(H2O) were evaluated employing UV-vis fluorescence and absorption spectroscopy and circular dichroism. The results suggest that the serum albumins-ReI(CO)3(pterin)(H2O) interactions occurred in the domain IIA-binding pocket without loss of helical stability of the proteins. The comparison of the fluorescence quenching of BSA and HSA due to the binding to the Re(I) complex suggested that local interaction around the Trp 214 residue had taken place. The analysis of the thermodynamic parameters ΔG0, ΔH0, and ΔS0 indicated that the hydrophobic interactions played a major role in both HSA-Re(I) and BSA-Re(I) association processes. All these experimental results suggest that these proteins can be considered as good carriers for transportation of ReI(CO)3(pterin)(H2O) complex. This is of significant importance in relation to the use of this Re(I) complex in several biomedical fields, such as photodynamic therapy and radiopharmacy.

Chemical and photochemical properties of chloroharmine derivatives in aqueous solutions.

Thermal and photochemical stability (Φ(R)), room temperature UV-vis absorption and fluorescence spectra, fluorescence quantum yields (Φ(F)) and lifetimes (τ(F)), quantum yields of hydrogen peroxide (Φ(H2O2)) and singlet oxygen (Φ(Δ)) production, and triplet lifetimes (τ(T)) have been obtained for the neutral and protonated forms of 6-chloroharmine, 8-chloroharmine and 6,8-dichloroharmine, in aqueous media. When it was possible, the effect of pH and oxygen concentration was evaluated. The nature of electronic transitions of protonated and neutral species of the three investigated chloroharmines was established using Time-Dependent Density Functional Theory (TD-DFT) calculations. The impact of all the foregoing observations on the biological role of the studied compounds is discussed.

Norharmane rhenium(I) polypyridyl complexes: synthesis, structural and spectroscopic characterization.

Two novel Re(i) complexes with the general formula fac-[Re(CO)3(L)(nHo)]CF3SO3, where L = 2,2'-bipyridine (bpy) or 1,10 phenanthroline (phen) and nHo (9H-pyrido[3,4-b]indole; norharmane) have been synthesized. The Re(i)-nHo complexes were characterized by structural X-ray diffraction, (1)H and (13)C NMR, UV-vis absorption and FT-IR spectroscopy, and by a combination of two mass spectrometry techniques, namely ESI-MS and UV-MALDI-MS. All characterizations showed that nHo is coordinated to the metal atom by the pyridine nitrogen of the molecule. X-ray structural analysis revealed that the crystal lattices for both complexes are further stabilized by a strong >N-HO bond between the pyrrole NH group of the pyridoindole ligand and one oxygen atom of the trifluoromethanesulfonate counter-ion. Ground state geometry optimization by DFT calculations showed that in fluid solution the nHo ligand may rotate freely. The nature of the electronic transitions of Re(CO)3(bpy)(nHo)(+) were established by TD-DFT calculations. The set of the most important electronic transitions present in this complex are comprised of π→π* electronic transitions centered on bpy and nHo moieties, LLCTnHo→COs, MLLCTRe(CO)3→bpy and LLCTnHo→bpy transitions. Additionally, TD-DFT calculations predict the existence of another two intense MLLCTRe(CO)3→nHo electronic transitions. Calculated UV-vis absorption spectra are in good agreement with the corresponding experimental data for the bpy-containing complex.

Solvent dependent switching of 3MLLCT and 1IL luminescent states in [ClRe(CO)3(bathocuproinedisulfonate)]2-: spectroscopic and computational study.

Steady state and time-resolved luminescence experiments and calorimetric studies, as well as time-dependent density functional theory calculations performed on [ClRe(CO)(3)(Bathocuproinedisulfonate)](2-), show that the photophysical properties of the Re(I) anionic complex are determined by the balance between intraligand ((1)IL) and metal-ligand-to-ligand charge transfer ((3)MLLCT) excited states. In organic solvents, (3)MLLCT states prevail and the usual expected behavior is observed: bathochromic shift of the emission maximum, a reduced luminescence quantum yield and the shortening of the excited-state lifetime upon increasing the polarity of the solvent. In addition, singlet oxygen ((1)O2) is generated with high quantum yields (Φ(Δ) ≈ 0.5 in CH(3)CN) due to the quenching of the (3)MLLCT luminescence by (3)O2. The total quenching rate constant of triplet state by oxygen, k(q), reach values between 2.2 and 2.4 × 10(9) M(-1) s(-1) for the organic solvents studied. In CH(3)CN, the fraction of triplet states quenched by O2 which yield (1)O2, f(O2)T, is nearly unity. In aqueous solution, where no singlet oxygen is generated, the luminescence of the Re(I) complex is of (1)IL character with a emission quantum yield (Φ(em)) strongly pH dependent: Φ(em,(pH=2))/Φ(em,(pH=10)) ≈ 5.6. The variation of the pH of the solution tunes the photophysical properties of the Re(I) complex by changing the relative amount of the different species existing in aqueous solutions: [ClRe(CO)3(BCS)](2-), [(OH)Re(CO)3(BCS)](2-) and [(H2O)Re(CO)3(BCS)](−). TD-DFT calculations show that the percentage of charge transfer character of the electronic transitions is substantially higher in the organic solvents than in aqueous solutions, in agreement with the increase of (1)IL character of HOMO in [(H2O)Re(CO)3(BCS)](−) relative to [ClRe(CO)3(BCS)](2-).

On the origins of the absorption spectroscopy of pterin and Re(CO)3(pterin)(H2O) aqueous solutions. A combined theoretical and experimental study.

The origins of the absorption spectroscopy of pterin and Re(CO)3(pterin)(H2O) complex as a function of pH is studied using the hybrid functional B3LYP and PBE0 in combination with 6-311++G(d,p) and LanL2TZ(f) basis sets. A natural bond analysis was performed to the principal molecular orbitals involved in the electronic transitions of the studied compounds. The low energy band of pterin, which is described as a H→L electronic transition, can be interpreted as an admixture of π→π(*), n→π(*) and n→n electronic transitions involving π(C2-N1, C6-N5, C9-C10) and n(C2) orbitals of the HOMO and π(*)(C6-N5, C7-N8) and n(C4) orbitals of the LUMO. The low energy band of Re(CO)3(pterin)(H2O) can be described as a combination of different MLLCT transitions where electron density residing on different π orbitals of carbonyl-Re bonds and lone pairs of Re is transferred to pterin moiety. Besides MLLCT transitions, IL, LLCT and LLMCT transitions contribute the absorptions of the Re(I) complex in the wavelength region corresponding to the high energy bands. The calculated electronic spectra of the acid and base forms of pterin and Re(CO)3(pterin)(H2O) were simulated from the theoretical results and compared to the experimental absorption spectra with great accuracy both in position and relative intensities of the absorption bands.

Photosensitized generation of singlet oxygen from Re(I) complexes: a photophysical study using LIOAS and luminescence techniques.

Quantum yields and efficiencies of (1)O2 ((1)Δg) production along with photophysical properties for a number of Re(I) complexes in acetonitrile solutions are reported. Two different classes of Re(I) complexes, L(S)-CO2-Re(CO)3(bpy) (L(S) = 2-pyrazine, 2-naphthalene, 9-anthracene, 1-pyrene, 2-anthraquinone) and XRe(CO)3L (X = CF3SO3, py; L = bpy, phen), were probed as photosensitizers for (1)O2 ((1)Δg) production in air-saturated acetonitrile solutions. Depending on the nature of the Re(I) complex, the excited state responsible for the generation of (1)O2 ((1)Δg) is either a metal-to-ligand charge transfer ((3)MLCT) or a ligand centered ((3)LC) state. With L(S)-CO2-Re(CO)3(bpy) complexes, (1)O2 ((1)Δg) is produced by oxygen quenching of (3)LC states of anthracene and pyrene with high quantum yields (ΦΔ between 0.8 and 1.0), while the complexes bearing the ligands L(S) = 2-anthraquinone, 2-pyrazine, and 2-naphthalene did not yield (1)O2. XRe(CO)3L complexes generate (1)O2 ((1)Δg) mainly by oxygen quenching of their (3)MLCT luminescence with an efficiency of (1)O2 ((1)Δg) formation close to unity. Bimolecular rate constants for the quenching of the XRe(CO)3L complexes' emission by molecular oxygen range between 1 × 10(9) and 2 × 10(9) M(-1) s(-1), and they are all ≤ (1/9)kd, in good agreement with the predominance of the singlet channel in the mechanism of (1)O2 ((1)Δg) generation using these Re(I) complexes as photosensitizers. All the experimental singlet oxygen efficiencies are consistent with calorimetric and luminescence data for the studied complexes. With L(S)-CO2-Re(CO)3(bpy) complexes, calorimetric experiments were utilized in the calculation of the quantum yields of triplet formation; namely φT = 0.76 and 0.83 for the triplet states of anthracene and pyrene, respectively.

Mass spectrometry of rhenium complexes: a comparative study by using LDI-MS, MALDI-MS, PESI-MS and ESI-MS.

A group of rhenium (I) complexes including in their structure ligands such as CF(3)SO(3)-, CH(3)CO(2)-, CO, 2,2'-bipyridine, dipyridil[3,2-a:2'3'-c]phenazine, naphthalene-2-carboxylate, anthracene-9-carboxylate, pyrene-1-carboxylate and 1,10-phenanthroline have been studied for the first time by mass spectrometry. The probe electrospray ionization (PESI) is a technique based on electrospray ionization (ESI) that generates electrospray from the tip of a solid metal needle. In this work, mass spectra for organometallic complexes obtained by PESI were compared with those obtained by classical ESI and high flow rate electrospray ionization assisted by corona discharge (HF-ESI-CD), an ideal method to avoid decomposition of the complexes and to induce their oxidation to yield intact molecular cation radicals in gas state [M](+·) and to produce their reduction yielding the gas species [M](-·). It was found that both techniques showed in general the intact molecular ions of the organometallics studied and provided additional structure characteristic diagnostic fragments. As the rhenium complexes studied in the present work showed strong absorption in the UV-visible region, particularly at 355 nm, laser desorption ionization (LDI) mass spectrometry experiments could be conducted. Although intact molecular ions could be detected in a few cases, LDI mass spectra showed diagnostic fragments for characterization of the complexes structure. Furthermore, matrix-assisted laser desorption ionization (MALDI) mass spectra were obtained. Nor-harmane, a compound with basic character, was used as matrix, and the intact molecular ions were detected in two examples, in negative ion mode as the [M](-·) species. Results obtained with 2-[(2E)-3-(4-tert-buthylphenyl)-2-methylprop-2-enylidene] malononitrile (DCTB) as matrix are also described. LDI experiments provided more information about the rhenium complex structures than did the MALDI ones.

On the mechanism of Re(I)-carboxylate bond cleavage by perchloric acid: a kinetic and spectroscopic study.

We have studied the reaction between pz-CO(2)-Re(CO)(3)(bpy) and perchloric acid in acetonitrile by following the UV-vis and IR spectral changes in the reaction mixture. A fast equilibrium was found to be established between solvated protons, pz-CO(2)-Re(CO)(3)(bpy), and the protonated intermediate [pz-C(OH)O-Re(CO)(3)(bpy)](+) which finally yields pz-COOH and Re(CO)(3)(bpy)(CH(3)CN)(+) as reaction products. This intermediate has been characterized by UV-vis and IR spectroscopies and by DFT calculations. The fully optimized DFT/CPCM structures for pz-CO(2)-Re(CO)(3)(bpy) and [pz-C(OH)O-Re(CO)(3)(bpy)](+) were compared with the X-ray structure of pz-CO(2)-Re(CO)(3)(bpy). The structural parameters associated with the carboxyl group in the protonated intermediate are between those of pz-CO(2)-Re(CO)(3)(bpy) and pz-COOH. Multivariate curve resolution methods were employed to obtain the spectrum of the protonated intermediate and the concentration profiles from the full matrix of time-resolved UV-vis spectra. The proposed mechanism was numerically simulated by using Runge-Kutta methods. Model parameters were estimated by nonlinear regression fitting of the concentration profiles, yielding values of log(K) = 4.9 ± 0.3 and k = 0.16 ± 0.03 min(-1) for the formation equilibrium constant and the decay rate constant of the protonated intermediate, respectively.

Aggregation in nanobundles and the effect of diverse environments on the solution-phase photochemistry and photophysics of -Re(CO)3L+ (L = 1,10-phenanthroline, 2,2'-bipyridine) pendants bonded to poly(4-vinylpyridine).

The UV-vis spectroscopy and photochemical properties of {(vpy-[Re(CO)3(2,2'-bpy)])m(vpy-[Re(CO)3(phen)])n(vpy)p}(CF3SO3)(m+n)}, vpy = 4-vinylpyridine, m = 131, n = 131 or m = 200, n = 150, and m + n + p = 600, were investigated in solution phase. The polymers exist in solution as aggregates of polymer strands with radii as large as approximately 10(2) nm. Given the size of the poly-vpy backbone, the aggregates must contain a large number of strands. The luminescence spectrum exhibits a strong resemblance to the emission spectrum of {(vpy-[Re(CO)3(phen)])200(vpy)400}(CF3SO3)200. The existence of Re(I) chromophores in diverse environments was shown by the intrinsic kinetics of the luminescence, the decay kinetics of the MLCT excited states observed by time resolved-absorption spectroscopy, and the quenching of the luminescence by various quenchers. Redox reactions of the MLCT excited states with the quenchers were responsible for the luminescence quenching. While static quenching resulted when Cu(II) and Fe(III) EDTA complexes were the quenchers, a dynamic quenching resulted with Fe(CN)6(4-) or 2,2',2' '-triethanolamine, TEOA. The photochemical and photophysical properties of the mixed-pendant polymers have been discussed in terms of arrays of MLCT excited states whose energies are determined by the diverse environments of the Re(I) chromophores. Conversions (with and without radiation) of the upper-energy MLCT excited states to the ground state and lower-energy MLCT excited states and the latter excited state to the ground state account for the experimental results.

On the quenching of MLCT(Re-->bpy) luminescence by Cu(II) species in Re(I) polymer micelles.

Transmission electron microscopy (TEM) and dynamic light scattering (DLS) studies on acetonitrile solutions of the polymer {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 demonstrated that the Re(I) polymer molecules aggregate to form spherical micelles of radius R = 156 nm. Coordination of Cu(II) species to the Re (I) polymer causes a decrease in the micelle radius and a distortion from the spherical shape. Besides, the coordination of Cu(II) species to the {[(vpy)2-vpyRe(CO)3bpy] CF3SO3}200 polymer produces the quenching of the metal to ligand charge transfer (MLCT) excited state by energy transfer processes that are more efficient than those in the quenching of the monomer pyRe(CO)3bpy+ luminescence by Cu(II). Moreover, the kinetics of the quenching by Cu(II) do not follow a Stern-Volmer behavior. Conversely, the quenching of the MLCT luminescence of the Re(I) polymer by the sacrificial electron donor 2,2',2' '-nitrilotriethanol, TEOA, follows a Stern-Volmer kinetics. A comparison is made between the quenching by CuX2 (X = Cl or CF3SO3) and TEOA.

Temperature effects on the quenching of the 5D0-->7F2 emission of Eu(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)3 by a Cu(II) macrocycle.

Luminescence quenching of Eu(fod)3(fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) by a Cu(II) macrocycle was studied at 25, 35 and 45 degrees C by steady-state and flash luminescence techniques, varying the Cu(II) concentration between 0.2 and 20 mM. Experimental variation of the observed rate constant with the quencher concentration is rationalized in terms of a mechanism involving the quenching of two unequilibrated species by the Cu(II) macrocycle.