Photo-induced intermolecular electron transfer from electron donating solvents to Coumarin dyes in bile salt aggregates: role of diffusion in electron transfer reaction

Thermo-Reversible Persistent Phosphorescence Modulation Reveals the Large Contribution Made by Rigidity to the Suppression of Endothermic Intermolecular Triplet Quenching

The suppression of thermally driven triplet deactivation is crucial for efficient persistent room-temperature phosphorescence (pRTP). However, the mechanism by which triplet deactivation occurs in metal-free molecular solids at room temperature (RT) remains unclear. Herein, we report a large pRTP intensity change in a molecular guest that depended on the reversible amorphous–crystal phase change in the molecular host, and we confirm the large contribution made by the rigidity of the host in suppressing intermolecular triplet quenching in the guest. (S)-(−)-2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl ((S)-BINAP) was doped as a guest into a highly purified (S)-bis(diphenylphosphino)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl ((S)-H₈-BINAP) host. It was possible to reversibly form the amorphous and crystalline states of the solid by cooling to RT from various temperatures. The RTP yield (Φ_p) originating from the (S)-BINAP was 6.7% in the crystalline state of the (S)-H₈-BINAP host, whereas it decreased to 0.31% in the amorphous state. Arrhenius plots showing the rate of nonradiative deactivation from the lowest triplet excited state (T₁) of the amorphous and crystalline solids indicated that the large difference in Φ_p between the crystalline and amorphous states was mostly due to the discrepancy in the magnitude of quenching of intermolecular triplet energy transfer from the (S)-BINAP guest to the (S)-H₈-BINAP host. Controlled analyses of the T₁ energy of the guest and host, and of the reorganization energy of the intermolecular triplet energy transfer from the guest to the host, confirmed that the large difference in intermolecular triplet quenching was due to the discrepancy in the magnitude of the diffusion constant of the (S)-H₈-BINAP host between its amorphous and crystalline states. Quantification of both the T₁ energy and the diffusion constant of molecules used in solid materials is crucial for a meaningful discussion of the intermolecular triplet deactivation of various metal-free solid materials.

Characterization of a New Electron Donor-Acceptor Dyad in Conventional Solvents and Ionic Liquids

Ionic liquids are being tested as potential replacements for current electrolytes in energy-related applications. Electron transfer (ET) plays a central role in these applications, making it essential to understand how ET in ionic liquids differs from ET in conventional organic solvents and how these differences affect reaction kinetics. A new intramolecular electron donor-acceptor probe was synthesized by covalently linking the popular photoacceptor coumarin 152 with the donor dimethylaniline to create the dyad "C152-DMA" for potential use in probing dynamical solvent effects in ionic liquids. Molecular dynamics simulations of this dyad show the considerable conformational flexibility of the linker group but over a range of geometries in which the ET rate parameters vary little and should have minimal effect on reaction times >100 ps. Steady-state and time-resolved fluorescence methods show the spectra of C152-DMA to be highly responsive to solvent polarity, with ET rates varying over the range of 10⁸ to 10¹² s^-1 between nonpolar and high-polarity conventional solvents. The sensitivity to hydrolysis in the presence of acidic impurities limits the dyad's use to ionic liquids of high purity. The results in the few ionic liquids examined here suggest that in addition to solvent polarity, electron transfer in C152-DMA also depends on solvent fluidity or solvation times.

Luminescence, absorption, and Stern-Volmer studies of cerium chloride and nitrate compounds in acidic and neutral aqueous, and non-aqueous solutions

Complexation of cerium chloride and nitrate in neutral and acidic aqueous solutions as well as in anhydrous alcohol solutions were investigated using emission, excitation, and absorption spectroscopic techniques. In aqueous solution cerium chloride shows a strong, and broad emission centering at 365 nm. The excitation spectra are observed at 266 and 296 nm with the shorter wavelength showing the highest intensity. Cerium chloride compound also strongly emits in methanol (MeOH), where the broad emission spectrum is red shifted by ∼10-375 nm. The excitation spectrum in MeOH shows bands at 255 and 309 nm, respectively with the longer wavelength band (at 309 nm) dominating. The relative intensities of these two excitation bands are reversed in protic aqueous solution. In contrast, solutions of cerium nitrate are only weakly luminescent in aqueous media, while the emission is totally quenched in MeOH solution. These observations indicate that the spectral profiles are largely influenced by the extent of inner-sphere coordination and the type of the dominant species in solutions. Both nitrate and chloride anions show enhanced inner-sphere coordination in MeOH when compared with that of the aqueous media. However, enhanced inner-sphere complexation of the NO(3)(-) ion quenches the emission, while the reverse effect is observed upon Cl(-) coordination. Stern-Volmer studies provide quenching constant, K(sv), value of 577M(-1). The calculated rate constant k(r) is 1.3×10(10)M(-1)s(-1) indicating diffusion controlled bimolecular process as the major mode of interaction.

Modulation of the photophysical properties of 2,2'-bipyridine-3,3'-diol inside bile salt aggregates: a fluorescence-based study for the molecular recognition of bile salts

2,2'-Bipyridine-3,3'-diol (BP(OH)(2)) has been used as a sensitive excited-state intramolecular proton transfer fluorophore to assess different bile salt aggregates as one of the potential biologically relevant host systems useful for carrying many sparingly water-soluble drug molecules. The formation of inclusion complexes, complex-induced fluorescence behavior, and their binding ability have been investigated from the modulated photophysics of BP(OH)(2) by means of photophysical techniques. The constrained hydrophobic environment provided by the aggregates significantly reduces the water-assisted nonradiative decay channels and lengthens the fluorescence lifetime of the proton-transferred DK tautomer. Both the absorption and fluorescence properties of BP(OH)(2) are found to be sensitive to the change in the structure, size, and hydrophobicity of the aggregates. Fluorescence quenching experiments were performed to gain insight into the differential distribution of the probe molecules between bulk aqueous phase and nanocavities of various aggregates. The observation of longer fluorescence lifetime and rotational relaxation time in NaDC aggregates compared to that in NaCh and NaTC aggregates indicates that the binding structures of NaDC aggregates are more rigid due to its greater hydrophobicity and larger size and therefore provide better protection to the bound guest. It is noteworthy to mention that the hydrophobic microenvironments provided by bile salt aggregates are much stronger than that provided by micelles and cyclodextrins. The accessibility of water to the aggregate-bound guest can significantly be enhanced with the addition of organic cosolvents. However, the efficiency decreases in the order of dimethylformamide, acetonitrile, and methanol.

Determination of Reaction and Reorganization Free Energies of Electron Transfer Reactions under Restricted Geometry Conditions

In this paper, different methods of obtaining the two parameters controlling the rate of electron transfer processes (reaction and reorganization free energies, Λ and Δ G0’, respectively) under restricted geometry conditions are considered. The main difficulty of accomplishing this comes from lack of knowledge of the properties in the interfacial region, where the reaction occurs. A general method has been presented and illustrated with the study of intermolecular processes in micelles. This method is optimized when the free energies for (at least) the three reactions required are quite different. For excited state electron transfer, the general approach is based on the appearance of the so-called Marcus inverted region: at the starting point of this region the value of Δ G0’ gives the value of Λ directly. These reaction free energies also present some uncertainties because in their calculation it is necessary to know the value of the local dielectric constant. Finally, it should be mentioned that some authors have suggested that the treatments for electron transfer reactions could not be applicable under restricted conditions. However, experiments do seem to show the applicability of the Marcus-Hush treatment under these conditions.

Structural and fluorescence quenching characterization of hematite nanoparticles

Nanoparticles of the dominant hematite form (α-Fe(2)O(3)) of iron oxide have been prepared by a simple route of dropping FeCl(3) solution into boiling water. The nanoparticles have been characterized by transmission electron microscopy (TEM), UV-visible electronic absorption spectroscopy, chemical stoichiometry, thermal analysis methods (TGA, DSC and DTA), XRD, FTIR and magnetic susceptibility measurements. Kinetic analysis of the DSC calorigram of thermal dehydration of the nanoparticles reveals one stage of the dehydration process of energy of activation of 29.0 kJ mol(-1). The role of iron oxide nanoparticles in fluorescence quenching of coumarin thiourea derivatives (I-IV) was investigated at room temperature (296 K) by means of steady-state fluorescence spectroscopy. The quenching process was characterized by Stern-Volmer (S-V) plots which display a positive deviation from linearity. This could be explained by static and dynamic quenching models. The positive deviation in the S-V plot is interpreted in terms of ground-state complex formation model and sphere of action static quenching model. Various rate parameters for the fluorescence quenching process were determined by using the modified Stern-Volmer equation. The sphere of action static quenching model agrees very well with experimental results. Quenching constants for iron oxide nanoparticles are about four orders of magnitudes higher than quenching by Fe(3+) ions.

Roles of Diffusion and Activation Barrier on the Appearance of Marcus Inversion Behavior: A Study of a Photoinduced Electron-Transfer Reaction in Aqueous Triblock Copolymer (P123) Micellar Solutions

Photoinduced electron transfer (ET) reactions between amines and a series of coumarins have been investigated using fluorescence-quenching measurements in aqueous P123 triblock copolymer micellar solutions. Fluorescence spectral characteristics and fluorescence anisotropy measurements indicated a nearly similar microenvironment for all of the coumarins used in P123 micelles. Substantial quenching of coumarin fluorescence in the presence of amines has been observed. The quenching rates (k(q)(TR)) are largely reduced in the P123 micelle as compared to those in other micelles (sodium dodecyl sulfate (SDS), Triton-X 100 (TX-100), cetyl trimethyl ammonium bromide (CTAB), and dodecyl trimethyl ammonium bromide (DTAB)), which is probably due to larger coumarin-amine separations in the micellar phase. The k(q)(TR) values, when plotted against free energy changes (DeltaG degrees), follow a Marcus predicted bell-shaped correlation. The estimated activation energy for the ET reactions follow an inverse bell-shaped correlation with DeltaG degrees. Present results indicate that the appearance of Marcus inversion is primarily related to the changes in the activation barrier, as predicted from the Marcus ET theory. As the k(q)(TR) values are higher than the estimated bimolecular diffusional rate constant, the role of reactant diffusion on the quenching kinetics in the P123 micelle is negligible. The appearance of Marcus inversion at unexpectedly lower exergonicity has been rationalized on the basis of slow solvent relaxation and by the application of the two-dimensional ET (2DET) theory. Critical analysis of the present results establishes that the inversion in the ET rates at high exergonicity is not due to the alteration in the diffusion parameters of the reactants, as has been suggested in some recent reports. Instead, it is evident that the inversion in quenching rates at high exergonicity is due to the alteration in the activation barrier for the ET reactions.

Photoinduced Electron Transfer in a Protein−Surfactant Complex: Probing the Interaction of SDS with BSA

Photoinduced fluorescence quenching electron transfer from N,N-dimethyl aniline to different 7-amino coumarin dyes has been investigated in sodium dodecyl sulfate (SDS) micelles and in bovine serum albumin (BSA)-SDS protein-surfactant complexes using steady state and picosecond time resolved fluorescence spectroscopy. The electron transfer rate has been found to be slower in BSA-SDS protein-surfactant complexes compared to that in SDS micelles. This observation has been explained with the help of the "necklace-and-bead" structure formed by the protein-surfactant complex due to coiling of protein molecules around the micelles. In the correlation of free energy change to the fluorescence quenching electron transfer rate, we have observed that coumarin 151 deviates from the normal Marcus region, showing retardation in the electron transfer rate at higher negative free energy region. We endeavored to establish that the retardation in the fluorescence quenching electron transfer rate for coumarin 151 at higher free energy region is a result of slower rotational relaxation and slower translational diffusion of coumarin 151 (C-151) compared to its analogues coumarin 152 and coumarin 481 in micelles and in protein-surfactant complexes. The slower rotational relaxation and translational diffusion of C-151 are supposed to be arising from the different location of coumarin 151 compared to coumarin 152 and coumarin 481.