Real Time Quantification of Ultrafast Photoinduced Bimolecular Electron Transfer Rate: Direct Probing of the Transient Intermediate

Fluorescence quenching studies through steady-state and time-resolved measurements are inadequate to quantify the bimolecular electron transfer rate in bulk homogeneous solution due to constraints from diffusion. To nullify the effect of diffusion, direct evaluation of the rate of formation of a transient intermediate produced upon the electron transfer is essential. Methyl viologen, a well-known electron acceptor, produces a radical cation after accepting an electron, which has a characteristic strong and broad absorption band centered at 600 nm. Hence it is a good choice to evaluate the rate of photoinduced electron transfer reaction employing femtosecond broadband transient absorption spectroscopy. The time constant of the aforementioned process between pyrene and methyl viologen in methanol has been estimated to be 2.5 ± 0.4 ps using the same technique. The time constant for the backward reaction was found to be 14 ± 1 ps. These values did not change with variation of concentration of quencher, i.e., methyl viologen. Hence, we can infer that diffusion has no contribution in the estimation of rate constants. However, on changing the solvent from methanol to ethanol, the time constant of the electron transfer reaction has been found to increase and has accounted for the change in solvent reorganization energy

Dynamics of Solvent Response in Methanol–Chloroform Binary Solvent Mixture: A Case of Synergistic Solvation

Steady-state absorption, emission, and femtosecond transient absorption spectroscopies were used to ascertain the static and dynamic nature of the solvent response of methanol–chloroform binary solvent mixtures of different stoichiometric ratios using 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4<i>H</i>-pyran (DCM) as the probe molecule. The appearance of synergistic solvation behavior in the steady-state absorption measurements can be explained in terms of solvent–solvent interactions through an extended hydrogen-bonding network. The disappearance of such synergistic behavior in the excited state of the DCM dye was recently proposed by us to be due to the weak nature of the intermolecular interactions present in binary solvent mixtures (J. Phys. Chem. B 2012, 116, 1345). It was anticipated and subsequently confirmed by the dynamics of the solvent response that the disruption of the weak interactive solvent network is the main reason for the absence of the synergism in the excited state. As expected, we observed the slowest dynamics for the mixture with <i>X</i>_MeOH = 0.45, with an average solvation time of 12.03 ps, which is much higher than the values for the pure bulk counterparts (⟨τ_s⟩_Methanol = 4.32 ps and ⟨τ_s⟩_Chloroform = 1.32 ps). The unprecedented slowing of solvation for DCM is probably due to the rigid interactive methanol-chloroform solvent system in the first solvation shell, followed by solvent rearrangements around the solute dipole. Overall interactions present within the methanol-chloroform binary solvent mixture furnish clear evidence of solvent association through weak hydrogen bonding

Conformational Fluctuation Dynamics of Domain I of Human Serum Albumin in the Course of Chemically and Thermally Induced Unfolding Using Fluorescence Correlation Spectroscopy

The present study elucidates the involvement of conformational fluctuation dynamics during chemically and thermally induced unfolding of human serum albumin (HSA) by fluorescence correlation spectroscopic (FCS) study, time-resolved fluorescence measurements, and circular dichroism (CD) spectroscopic methods. Two fluorescent probes, tetramethylrhodamine-5-maleimide (TMR) and <i>N</i>-(7-dimethylamino-4-methylcoumarin-3-yl) iodoacetamide (DACIA) were used to selectively label the domain I of HSA through the reaction with cys-34 for these studies. The guanidine hydrochloride (GnHCl) induced global structural change of HSA is monitored through its hydrodynamic radius (<i>r</i>_H) and CD response, which is found to be two step in nature. In FCS experiment, along with the diffusion time component we have observed an exponential relaxation time component (τ_R) that has been ascribed to the concerted chain dynamics of HSA. Unlike in the global structural change, we found that the τ_R value changes in a different manner in the course of the unfolding. The dependence of τ_R on the concentration of GnHCl was best fitted with a four state model, indicating the involvement of two intermediate states during the unfolding process, which were not observed through the CD response and <i>r</i>_H data. The fluorescence lifetime measurement also supports our observation of intermediate states during the unfolding of HSA. However, no such intermediate states were observed during thermally induced unfolding of HSA

Origin of Strong Synergism in Weakly Perturbed Binary Solvent System: A Case Study of Primary Alcohols and Chlorinated Methanes

A strong synergistic solvation was observed for the mixtures of hydrogen bond donating and accepting solvent pairs. The nature of the interactions between two solvent pairs was investigated with different dye molecules viz. coumarin 480, coumarin 153, 4-aminophthalimide, and <i>p</i>-nitroaniline. Coumarin 480 in differenet alcohols–CHCl₃ (alcohols: MeOH, EtOH, BuOH) binary mixture shows a strong synergism, which is explained in the backdrop of solvent–solvent interactions. Fluorescence quenching of C480 by 1,2-phenylenediamine in the binary solvent mixture exhibited the maximum deviation in quenching constant corresponding to ∼0.45 mol fraction of MeOH in MeOH–CHCl₃ binary mixture and hence suggested the maximum extent of hydrogen-bonding interactions prevailing at this proportion of mixture. The solvation behavior of MeOH–CHCl₃ mixture shows strong probe dependence with no synergism observed in <i>p</i>-nitroaniline, which is ascribed to its higher ground state dipole moment (8.8 D) relative to C480 (6.3 D). Interestingly, the strong synergistic signature observed through spectrophotometric measurement of C480 in alcohol–CHCl₃ binary mixture is absent when studied by fluorescence measurement. The higher excited state dipole moment of coumarin 480 (13.1 D) is considered to be the driving force for the absence of synergism in the excited state. In such strongly perturbed systems (due to high dipole moment values) the dominant phenomenon is preferential solvation. Analysis of proton NMR of MeOH–CHCl₃ binary solvent mixture indicates the existence of MeOH–CHCl₃ clusters in the stoichiometric ratio of 1:2.15. Refractive index measurement also infers the existence of hydrogen bonded network structure between MeOH and CHCl₃. A modified Bosch solvent exchange model has been used to determine the feasibility of synergistic behavior and polarity parameter of the mixed solvent structure of MeOH–CHCl₃ binary solvent mixture

Bimolecular Photoinduced Electron Transfer in Static Quenching Regime: Illustration of Marcus Inversion in Micelle

Ultrafast bimolecular photoinduced electron transfer (PET) between six coumarin dyes and four viologen molecules in the stern layer of sodium dodecyl sulfate micelle have been studied using femtosecond broadband transient absorption spectroscopy and femtosecond fluorescence up-conversion spectroscopy over a broad reaction exergonicity (Δ<i>G</i>⁰). Emanating the formation of radical cation intermediates of viologen molecules using the transient absorption and the fast decay component of coumarins using the fluorescence up-conversion studies the forward bimolecular electron transfer rate (<i>k</i>_et) have been measured with high accuracy. The relationship of <i>k</i>_et with Δ<i>G</i>⁰ found to follow a Marcus type bell-shaped dependence with an inversion at −1.10 eV. In this report, we have studied PET reaction using ultrafast spectroscopy at the quencher concentration where static quenching regime prevails. Moreover, the incompetency of Stern–Volmer experiments in studying ultrafast PET has been revealed. In contrary to previous claims, here we found that the <i>k</i>_et is lower for lower lifetime coumarins, indicating that static, nonstationary and stationary regime of quenching have the minimal role to play to in the bimolecular electron transfer process. By far, this report is believed to be the most efficient and immaculate way of approaching Marcus inverted region problem in the case of bimolecular PET and settles the long-lasting debate of whether the same can be observed in micellar systems

Excited State Relaxation Dynamics of Model Green Fluorescent Protein Chromophore Analogs: Evidence for <i>Cis–Trans</i> Isomerism

Two green fluorescent protein (GFP) chromophore analogs (4<i>Z</i>)-4-(<i>N</i>,<i>N</i>-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5<i>H</i>-imidazolin-5-one (DMPI) and (4<i>Z</i>)-4-(<i>N</i>,<i>N</i>-diphenylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5<i>H</i>-imidazolin-5-one (DPMPI) were investigated using femtosecond fluorescence up-conversion spectroscopy and quantum chemical calculations with the results being substantiated by HPLC and NMR measurements. The femtosecond fluorescence transients are found to be biexponential in nature and the time constants exhibit a significant dependence on solvent viscosity and polarity. A multicoordinate relaxation mechanism is proposed for the excited state relaxation behavior of the model GFP analogs. The first time component (τ₁) was assigned to the formation of twisted intramolecular charge transfer (TICT) state along the rotational coordinate of N-substituted amine group. Time resolved intensity normalized and area normalized emission spectra (TRES and TRANES) were constructed to authenticate the occurrence of TICT state in subpicosecond time scale. Another picosecond time component (τ₂) was attributed to internal conversion via large amplitude motion along the exomethylenic double bond which has been enunciated by quantum chemical calculations. Quantum chemical calculation also forbids the involvement of hula-twist because of high activation barrier of twisting. HPLC profiles and proton-NMR measurements of the irradiated analogs confirm the presence of <i>Z</i> and <i>E</i> isomers, whose possibility of formation can be accomplished only by the rotation along the exomethylenic double bond. The present observations can be extended to <i>p</i>-HBDI in order to understand the role of protein scaffold in reducing the nonradiative pathways, leading to highly luminescent nature of GFP

Novel Chemosensor for the Visual Detection of Copper(II) in Aqueous Solution at the ppm Level

A new water-soluble, multisite-coordinating ligand LH₇ was prepared by the condensation of tris­(hydroxymethyl)­aminomethane with 2,6-diformyl-<i>p</i>-cresol. LH₇ is a selective chemosensor for Cu²⁺, under physiological conditions, with visual detection limits of 20 ppm (ambient light conditions) and 4 ppm (UV light conditions). LH₇ can also be used in biological cell lines for the detection of Cu²⁺

Ultrafast Electron Transfer from Upper Excited State of Encapsulated Azulenes to Acceptors across an Organic Molecular Wall

In the context of generating reactive organic radical cations within a confined capsule and exploring photoinduced electron transfer from encapsulated organic molecules to organic and inorganic acceptors through an organic molecular wall, we have investigated electron transfer from the upper excited state (S₂) of azulene (Az) and guaiazulene (GAz) enclosed within an octa acid (OA) capsule to water-soluble 4,4′-dimethyl viologen²⁺ (MV²⁺) and pyridinium⁺ (Py⁺) salts or colloidal TiO₂. S₂ fluorescence of OA encapsulated Az and GAz was quenched by electron acceptors such as MV²⁺ and Py⁺ salts. That electron transfer is responsible for S₂ fluorescence quenching was established by recording the transient absorption spectrum of the MV^●+ in the femtosecond time regime. Femtosecond time-resolved fluorescence experiments suggested that the time constant for the forward and reverse electron transfer from encapsulated Az and GAz to MV²⁺ is 4 and 3.6 ps, and 55.7 and 36.9 ps, respectively. The observed S₂ fluorescence quenching by colloidal TiO₂ in aqueous buffer solution is attributed to electron transfer from encapsulated Az and GAz to TiO₂. Lack of quenching by the wider band gap material ZrO₂ supported the above conclusion. FT-IR-ATR experiments confirmed that OA capsules containing Az and GAz can be adsorbed on TiO₂ films, and excitation of these resulted in S₂ fluorescence quenching. The observations presented here are important in the context of establishing the value of OA type cavitands where charge separation and donor shielding are critical

Calmidazolium Chloride and Its Complex with Serum Albumin Prevent Huntingtin Exon1 Aggregation

Huntington’s disease (HD) is a genetic disorder caused by a CAG expansion mutation in <i>Huntingtin</i> gene leading to polyglutamine (polyQ) expansion in the N-terminus side of Huntingtin (Httex1) protein. Neurodegeneration in HD is linked to aggregates formed by Httex1 bearing an expanded polyQ. Initiation and elongation steps of Httex1 aggregation are potential target steps for the discovery of therapeutic molecules for HD, which is currently untreatable. Here we report Httex1 aggregation inhibition by calmidazolium chloride (CLC) by acting on the initial aggregation event. Because it is hydrophobic, CLC was adsorbed to the vial surface and could not sustain an inhibition effect for a longer duration. The use of bovine serum albumin (BSA) prevented CLC adsorption by forming a BSA–CLC complex. This complex showed improved Httex1 aggregation inhibition by interacting with the aggregation initiator, the NT₁₇ part of Httex1. Furthermore, biocompatible CLC-loaded BSA nanoparticles were made which reduced the polyQ aggregates in HD-150Q cells