Excited-State Proton Transfer from Pyranine to Acetate in γ-Cyclodextrin and Hydroxypropyl γ-Cyclodextrin

Excited-State Dynamics of a Photoacid: A Potential Probe for Recognizing Transition from Lamellar to Nonlamellar Inverted Structures of Liposome based Nanocarriers

Liposomes of a cationic lipid dioctadecyldimethylammonium bromide (DODAB) are efficient nanocarriers of nucleic acids. Incorporation of a neutral lipid monoolein (MO) in excess (xMO >0.5) changes the lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes facilitating nucleic acid delivery to cells. Photoexcitation of 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS), a photoacid, initiates an excited state proton transfer (ESPT) reaction in its protonated form (ROH*) generating the deprotonated anionic form (RO- *). The fluorescence intensity ratio (IROH* /IRO-* ) of these two forms is governed by the ESPT dynamics, and increases with increasing MO content (xMO ) in the cationic liposomes of DODAB. Transition from lamellar organization of DODAB liposomes into non-lamellar inverted structures of DODAB/MO liposomes, due to incorporation of MO (xMO ~0.7), is manifested by a significant increase of ESPT time (τPT ) and the time constant of wobbling motion (τW ) of HPTS. Thus, the lamellar organizations of DODAB or DODAB-rich (xMO 0.2) liposomes and the non-lamellar organizations of MO-rich (xMO ~0.7) liposomes are recognized by significantly different excited state dynamics of the photoacid.

Novel Approach for Detecting Vapors of Acids and Bases with Proton-Transfer Luminescent Dyes Encapsulated within Metal-Organic Frameworks

Luminescent metal-organic frameworks (LMOFs) are one of the most promising materials for being implemented as active layers in the fabrication of photonic devices such as luminescent sensors of harmful chemicals. It is highly desirable that these materials undergo quantifiable spectroscopic (absorption or emission) changes in the presence of vapors of those analytes, as in many industrial processes, these toxic compounds are in the gas phase. Although great progresses have been achieved in the field, in most of the examples reported hitherto, the detection of chemicals by LMOFs is attained in solution. Herein, we present a novel approach consisting of the encapsulation of proton transfer dyes (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS, and 3-hydroxyflavone, 3-HF) within the pores of two distinct MOFs. The trapped proton transfer dyes (PT-dyes) may exist as different structures (enol, anion, or zwitterion), each of these exhibiting unique optical properties. Indeed, our findings reveal that the dyes can be encapsulated as anionic or enol species. Remarkably, the PT-dye@MOF composites exhibit a high luminescence quantum yield (up to 30%), which is sensitive (showing shifting in the emission wavelengths with a concomitant quenching/enhancement of the intensity) in the presence of vapors of an acid (HCl) and a base (triethylamine). These results open a novel avenue for the development of smarter vapoluminescent MOF-based materials.

pH Jumps in a Protic Ionic Liquid Proceed by Vehicular Proton Transport

The dynamics of excess protons in the protic ionic liquid (PIL) ethylammonium formate (EAF) have been investigated from femtoseconds to microseconds using visible pump mid-infrared probe spectroscopy. The pH jump following the visible photoexcitation of a photoacid (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS) results in proton transfer to the formate of the EAF. The proton transfer predominantly (∼70%) occurs over picoseconds through a preformed hydrogen-bonded tight complex between HPTS and EAF. We investigate the longer-range and longer-time-scale proton-transport processes in the PIL by obtaining the ground-state conjugate base (RO^-) dynamics from the congested transient-infrared spectra. The spectral kinetics indicate that the protons diffuse only a few solvent shells from the parent photoacid before recombining with RO^-. A kinetic isotope effect of nearly unity (k_H/k_D ≈ 1) suggests vehicular transfer and the transport of excess protons in this PIL. Our findings provide comprehensive insight into the complete photoprotolytic cycle of excess protons in a PIL.

Molecular-Level Control over Ionic Conduction and Ionic Current Direction by Designing Macrocycle-Based Ionomers

Poor ionic conductivity of the catalyst-binding, sub-micrometer-thick ionomer layers in energy conversion and storage devices is a huge challenge. However, ionomers are rarely designed keeping in mind the specific issues associated with nanoconfinement. Here, we designed nature-inspired ionomers (calix-2) having hollow, macrocyclic, calix[4]arene-based repeat units with precise, sub-nanometer diameter. In ≤100 nm-thick films, the in-plane proton conductivity of calix-2 was up to 8 times higher than the current benchmark ionomer Nafion at 85% relative humidity (RH), while it was 1-2 orders of magnitude higher than Nafion at 20-25% RH. Confocal laser scanning microscopy and other synthetic techniques allowed us to demonstrate the role of macrocyclic cavities in boosting the proton conductivity. The systematic self-assembly of calix-2 chains into ellipsoids in thin films was evidenced from atomic force microscopy and grazing incidence small-angle X-ray scattering measurements. Moreover, the likelihood of alignment and stacking of macrocyclic units, the presence of one-dimensional water wires across this macrocycle stacks, and thus the formation of long-range proton conduction pathways were suggested by atomistic simulations. We not only did see an unprecedented improvement in thin-film proton conductivity but also saw an improvement in proton conductivity of bulk membranes when calix-2 was added to the Nafion matrices. Nafion-calix-2 composite membranes also took advantage of the asymmetric charge distribution across calix[4]arene repeat units collectively and exhibited voltage-gating behavior. The inclusion of molecular macrocyclic cavities into the ionomer chemical structure can thus emerge as a promising design concept for highly efficient ion-conducting and ion-permselective materials for sustainable energy applications.

A computational approach on the stereoselective binding of peptides from aqueous medium with endo-functionalized molecular tubes

The need to obtain enantiomerically pure isomers of amino acids and peptides is often realized in the field of biology and in the pharmaceutical industry. Research is underway to devise simple methods for the chiral resolution of amino acids from their racemic mixtures. Inspired by this objective, in our present work, we have computationally shown the possibility of chiral separation of the enantiomeric pairs of two model peptides, namely, (D,L)-aspargine and (D,L)-phenylalanine, in the presence of water. For this purpose, we have used two synthetic supramolecular receptors named host-1a and host-1b, respectively. Molecular dynamics simulations and quantum chemical methods are employed to analyze the structural features and the energy aspects involved in the separation process. The information obtained at the molecular level helps us gain better insights into the key interactions that operate to produce such enantioselectivity. We have also investigated the dynamics and changes in the water structure in the vicinity of the host molecules, both in the presence and absence of the model peptides. The D- and L-isomers of the same peptide undergo complexation with a particular host molecule registering a difference of more than 1.5 kcal mol^-1 (obtained from PMF and MM-PBSA analyses) in their respective energies. This indicates that the chiral separation of the peptides with the help of these endo-fuctionalized molecular tube receptors may be energetically feasible. The connection between the peptide stereochemistry and its interaction with the endo-functionalized hosts would be instrumental in designing novel segregation techniques that can be further extended to separate larger model peptides or proteins.

Excited-State Proton Transfer on the Surface of a Therapeutic Protein, Protamine

Proton transfer reactions on biosurfaces play an important role in a myriad of biological processes. Herein, the excited-state proton transfer reaction of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) has been investigated in the presence of an important therapeutic protein, Protamine (PrS), using ground-state absorption, steady-state, and detailed time-resolved emission measurements. HPTS forms a 1:1 complex with Protamine with a high association constant of 2.6 × 10⁴ M^-1. The binding of HPTS with Protamine leads to a significant modulation in the ground-state prototropic equilibrium causing a downward shift of 1.1 unit in the acidity constant (pK_a). In contrast to a large number of reports of slow proton transfer of HPTS on biosurfaces, interestingly, HPTS registers a faster proton transfer event in the presence of Protamine as compared to that of even the bulk aqueous buffer medium. Furthermore, the dimensionality of the proton diffusion process is also significantly reduced on the surface of Protamine that is in contrast to the behavior of HPTS in the bulk aqueous buffer medium, where the proton diffusion process is three-dimensional. The effect of ionic strength on the binding of HPTS toward PrS suggests a predominant role of electrostatic interaction between anionic HPTS and cationic Protamine, which is further supported by molecular docking simulations which predict that the most preferable binding site for HPTS on the surface of Protamine is surrounded by multiple cationic arginine residues.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Coherent intermolecular proton transfer in the acid–base reaction of excited state pyranine

The acidic proton in pyranine is transferred coherently to acetate through the stretching motion of the whole molecule.

Effect of Cosurfactants on the Interfacial Hydration of CTAB Quaternary Reverse Micelle Probed Using Excited State Proton Transfer

It has been proven previously that the negatively charged photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) resides at the interface of the cationic reverse micelle (RM) cetyltrimethylammonium bromide (CTAB)/octanol/water/cyclohexane and is a potential reporter of hydration through the excited state proton transfer (ESPT) process. However, the ESPT dynamics monitored by the pump-probe study was limited to the ultrafast timescale and hence did not report any discernible ESPT signature. Herein, we reinvestigate the ESPT behavior using fluorescence spectroscopy in the nanosecond timescale and at different values of w₀ (=[water]/[surfactant]). We clearly observed distinct w₀-dependent ESPT signatures analogous to conventional ternary cationic RMs implying considerable interfacial hydration. The results agree with a recent molecular simulation study, where significant penetration of water molecules into the interface was predicted for the CTAB quaternary RM. Moreover, we also found that the ESPT dynamics and the fluorescence anisotropy decay of HPTS depend differentially on the octanol/CTAB ratio (p₀). The ESPT process was found to be disfavored, whereas the anisotropy decay accelerates upon the increase in p₀ values. Our analysis indicates that with the increase in the octanol concentration, dehydrated regions enrich gradually at the interface. However, the increase in octanol concentration may reduce the effective electrostatic potential experienced by the probe and thus may result in faster rotational relaxation.

A slowing down of proton motion from HPTS to water adsorbed on the MCM-41 surface

We report on the steady-state and femtosecond-nanosecond (fs-ns) behaviour of 8-hydroxypyrene-1,3,6-trisulfonate (pyranine, HPTS) and its interaction with mesoporous silica based materials (MCM-41) in both solid-state and dichloromethane (DCM) suspensions in the absence and presence of water. In the absence of water, HPTS forms aggregates which are characterized by a broad emission spectrum and multiexponential behavior (τsolid-state/DCM = 120 ps, 600 ps, 2.2 ns). Upon interaction with MCM41, the aggregate population is found to be lower, leading to the formation of adsorbed monomers. In the presence of water (1%), HPTS with and without MCM41 materials in DCM suspensions undergoes an excited-state intermolecular proton-transfer (ESPT) reaction in the protonated form (ROH*) producing a deprotonated species (RO(-)*). The long-time emission decays of the ROH* in different systems in the presence of water are multiexponential, and are analysed using the diffusion-assisted geminate recombination model. The obtained proton-transfer and recombination rate constants for HPTS and HPTS/MCM41 complexes in DCM suspensions in the presence of water are kPT = 13 ns(-1), krec = 7.5 Å ns(-1), and kPT = 5.4 ns(-1), krec = 2.2 Å ns(-1), respectively, The slowing down of both processes in the latter case is explained in terms of specific interactions of the dye and of the water molecules with the silica surface. The ultrafast dynamics (fs-regime) of the HPTS/MCM41 complexes in DCM suspensions, without and with water, shows two components which are assigned to intramolecular vibrational-energy relaxation (IVR) (∼120 fs vs. ∼0.8 ps), and vibrational relaxation/cooling (VC), and charge transfer (CT) processes (∼2 ps without water and ∼5 ps with water) of the adsorbed ROH*. Our results provide new knowledge on the interactions and the proton-transfer reaction dynamics of HPTS adsorbed on mesoporous materials.

Wet Interface of Benzylhexadecyldimethylammonium Chloride Reverse Micelle Revealed by Excited State Proton Transfer of a Localized Probe

Excited state proton transfer (ESPT) of an anionic photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) has been studied inside a cationic reverse micelle (RM), water/benzylhexadecyldimethylammonium chloride (BHDC)/benzene, using steady-state and time-resolved fluorescence spectroscopy. The observed ESPT behavior is found to be remarkably different from the known ESPT trend of HPTS inside anionic AOT and cationic CTAB RMs; the ESPT dynamics approaches that of bulk water at higher w0 (≥10) inside AOT RM while no ESPT was observed for CTAB reverse micelle [ Sedgwick J. Am. Chem. Soc. 2012 , 134 , 11904 - 11907 ]. The ESPT dynamics inside BHDC RM is remarkably slower compared to that of water at all w0 (= [water]/[surfactant]) values and relatively much less sensitive to w0 variation compared to AOT RM. 2D NOESY and fluorescence anisotropy measurements reveal that the probe (HPTS) is embedded inside the positive interface of BHDC RM. Despite its trapped location, HPTS is able to undergo ESPT due to significant penetration of water molecules into the interface. Furthermore, facile ESPT at higher w0 is consistent with higher degree of interface hydration as predicted by a recent MD simulation [ Agazzi Langmuir 2014 , 30 , 9643 - 9653 ]. The study shows that ESPT dynamics inside RM varies not only with the interface charge but also on the nature of the headgroup and solvation.

Supramolecular interaction between a hydrophilic coumarin dye and macrocyclic hosts: spectroscopic and calorimetric study

The photophysics of a hydrophilic molecule, 7-(diethylamino)-coumarin-3-carboxylic acid (7-DCCA), was studied in the presence of two macrocycles, (2-hydroxypropyl)-γ-cyclodextrin and cucurbit[7]uril. We have used steady-state absorption, fluorescence, and time-resolved fluorescence emission spectroscopy; Fourier transform infrared (FTIR) spectroscopy; (1)H NMR spectroscopy; and isothermal titration calorimetry (ITC) to confirm the supramolecular host-guest complex formation. The spectral properties of 7-DCCA were modulated in the presence of both macrocycles. It was assigned that 7-DCCA forms a 1:2 complex with (2-hydroxypropyl)-γ-cyclodextrin and cucurbit[7]uril. The large modulation of the emission properties of 7-DCCA in the presence of the macrocycles indicates the formation of supramolecular complexes. A significant shift in the bond vibration frequencies in the FTIR studies showed encapsulation of the dyes in the hydrophobic cavity of the macrocycles. This is further substantiated by the (1)H NMR studies, in which the upfield and downfield shifts of the protons were observed in both the aliphatic and aromatic region in the presence of macrocycles. The time-resolved anisotropy measurements further reinforce the conception of host-guest supramolecular complex formation because, in both cases, the rotational relaxation time increases significantly compared to that in water. A deeper understanding between the differences in interaction of an anionic molecule with cucurbit[7]uril and (2-hydroxypropyl)-γ-cyclodextrin will be achieved through this work. From the ITC measurement, we have formulated the forces due to complex formation.

Ultrafast conformational dynamics of pyranine during excited state proton transfer in aqueous solution revealed by femtosecond stimulated Raman spectroscopy

Proton transfer reaction plays an essential role in a myriad of chemical and biological processes, and to reveal the choreography of the proton motion intra- and intermolecularly, a spectroscopic technique capable of capturing molecular structural snapshots on the intrinsic time scale of proton transfer motions is needed. The photoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS) serves as a paradigm case to dissect excited state proton transfer (ESPT) events in aqueous solution, triggered precisely by photoexcitation. We have used femtosecond stimulated Raman spectroscopy (FSRS) to yield novel insights into the ultrafast conformational dynamics of photoexcited HPTS in complex with water and acetate molecules. Marker bands attributed to the deprotonated form of HPTS (1139 cm(-1), ∼220 fs rise) appear earlier and faster than the monomer acetic acid peak (864 cm(-1), ∼530 fs rise), indicating that water molecules actively participate in the ESPT chain. Several key low-frequency modes at 106, 150, 195, and 321 cm(-1) have been identified to facilitate ESPT at different stages from 300 fs, 1 ps, to 6 ps and beyond, having distinctive dynamics contributing through hydrogen bonds with 0, 1, and more intervening water molecules. The time-resolved FSRS spectroscopy renders a direct approach to observe the reactive coupling between the vibrational degrees of freedom of photoexcited HPTS in action, therefore revealing the anharmonicity matrix both within HPTS and between HPTS and the neighboring acceptor molecules. The observed excited state conformational dynamics are along the ESPT multidimensional reaction coordinate and are responsible for the photoacidity of HPTS in aqueous solution.

Ultrafast photodynamics of drugs in nanocavities: cyclodextrins and human serum albumin protein

In this feature article, we discuss recent advances in studying ultrafast dynamic and structural aspects of host-guest interactions. Steady-state and time-resolved techniques exploring events from the femto- to nanosecond regime were used to examine the ultrafast photodynamics and subsequent events in selected nanostructures of the formed complexes. These consist of aromatic systems, biologically relevant molecules, and drugs trapped within cyclodextrins (CD) and human serum albumin (HSA) protein pockets. We examine the effects exerted by these chemical and biological cavitands on internal twisting motions, proton transfer and charge transfer, and cis-trans isomerization reactions that may occur in the confined molecular systems. In addition, the influence of a restricting environment on the interaction of guest molecules with biological water is considered. The dynamic details of the complexes (diffusion, early interactions, formation, stability, internal guest diffusion, and conformational changes) and the excited-state relaxation pathways, rate constants of the involved processes, and changes in the electronic distribution within encapsulated guests gave clues to elucidate their photobehavior and are relevant to the photostability and delivery of drugs when using nanocarriers.

Hydration properties of α-, β-, and γ-cyclodextrins from molecular dynamics simulations

Atomistic molecular dynamics (MD) simulations of α-, β-, and γ-cyclodextrins (ACD, BCD, and GCD) in aqueous solutions have been performed. Detailed analyses were carried out to compare the microscopic properties of water confined within the cavities of these macromolecules and in the hydration layers around them. It is noticed that reduced tetrahedral ordering of water in and around the CD molecules are associated with their restricted motions. Interestingly, unlike the translational motions, the rotational motions of cavity water molecules are found to be highly dependent on cavity dimensions. Additionally, it is found that severely hindered rotational motion of cavity water molecules is the origin of drastically restricted structural relaxation of hydrogen bonds involving those water molecules. It is demonstrated that the geometrical constraints within the cavities of the CD molecules enhance the rate of reformation of broken hydrogen bonds, thereby resulting in rapid establishment of the breaking and reformation equilibria for hydrogen bonds involving cavity water molecules.

Spectral resolution of molecular ensembles under ambient conditions using surface plasmon coupled fluorescence emission

We report the ability to spectrally resolve excited-state ensembles of pyranine (Py) utilizing nanometer-thick metal films as a low-cost analytical tool. Surface plasmon coupling allows to mitigate the effect of spectral broadening that is responsible for blurring the emission spectrum at room temperature, a situation common in conventional fluorescence spectroscopy. The approach is especially useful in the case when several excited-state species are present. Fluorescence emission from closely located protonated, deprotonated, and excimer species of Py couple into surface plasmons and are easily separated and observed with 11-14 fold intensity enhancements. Furthermore, the ultranarrowband photon-sorting of emission from microenvironments in a multispecies system is performed in this study using instruments that are readily available in most laboratories without employing any deconvolution procedure and/or additional dispersive optics.

The Effect of Environment on the Dynamics of Proton Dissociation in Water

The effect of the local ordering of water molecules, adjacent to the molecular surface, on the dynamics of excited state proton transfer to bulk was monitored with the pyranine-γ-CD inclusion complex as a model. The bound pyranine (a commonly used photoacid) exhibits slower dissociation dynamics, with activation energy of the proton dissociation reaction that is significantly higher than that of the reaction of the free pyranine. To understand the source of these modulations of the rate constants, the interaction of the pyranine with the water was investigated by molecular dynamics calculations. The solvation patterns of both; the pyranine and γ-CD, differ in the complex from that of the free compounds. In the case of the pyranine´s hydroxyl, the inclusion in the torus of the γ-CD reduces the number of water molecules in its immediate vicinity and their ordering, thus accounting for the variation in the rates of the proton transfer reactions. On increasing the temperature, the water of the pyranine´s hydroxyl third solvation shell, but not those of the first and the second shell, lose their strict orientation, thus facilitating the dissociation of the pyranine.