Electronic Coupling–Decoupling-Dependent Single-Molecule Interfacial Electron Transfer Dynamics in Electrostatically Attached Porphyrin on TiO₂ Nanoparticles

Factors controlling the interfacial electron-transfer (ET) dynamics in molecule–semiconductor systems have been intensively investigated in recent years. Here, we study dynamics of interfacial ET on Zn­(II) meso-tetra (<i>N</i>-methyl-4-pyridyl) porphine tetrachloride (ZnTMPyP)–TiO₂ nanoparticle (NP) system using single-molecule photon-stamping spectroscopy while electrostatically controlling the coupling between ZnTMPyP and TiO₂ NP by changing the surface charge of the TiO₂ NP. The single-molecule fluorescence trajectories show strong fluctuation and blinking between bright and dark states providing clear indication for the binding affinity between ZnTMPyP and the TiO₂ NP via electrostatic interaction. By changing the surface charge on the TiO₂ NP, positive or negative, we are able to change the coupling between ZnTMPyP and the TiO₂ NP, which is revealed from the dominant dark states distribution in fluorescence trajectories and shorter fluorescence lifetime of ZnTMPyP attached on negatively charged TiO₂ NP surface compared to positively charged TiO₂ NP surface. The observed difference in fluorescence trajectories and lifetime of ZnTMPyP can be qualitatively accounted for by considering the change in purely electronic coupling factor caused by the positively or negatively charged TiO₂ NP surface in electrostatically bound dye-sensitized TiO₂ systems. Strong binding interaction between ZnTMPyP and negatively charged TiO₂ NP is further observed by higher fluorescence anisotropy compared to ZnTMPyP on positively charged TiO₂ NP

Single-Molecule Interfacial Electron Transfer Dynamics of Porphyrin on TiO₂ Nanoparticles: Dissecting the Complex Electronic Coupling Dependent Dynamics

The photosensitized interfacial electron transfer (ET) dynamics of the zinc­(II)–5,10,15,20-tetra­(3-carboxyphenyl)­porphyrin (<i>m</i>-ZnTCPP)–TiO₂ nanoparticle (NP) system has been studied using single-molecule photon-stamping spectroscopy. The single-molecule fluorescence intensity trajectories of <i>m</i>-ZnTCPP on TiO₂ NP surface show fluctuations and blinking between bright and dark states, which are attributed to the variations in the reactivity of interfacial ET, i.e., intermittent interfacial electron transfer dynamics. Comparing the results with that from our earlier studied <i>p</i>-ZnTCPP–TiO₂ nanoparticle system, we show the effect of anchoring group binding geometry (meta or para), hence electronic coupling of sensitizer (<i>m</i>-/<i>p</i>-ZnTCPP) and TiO₂ substrate, on interfacial ET dynamics. Compared to <i>p</i>-ZnTCPP on TiO₂ NP surface, with <i>m</i>-ZnTCPP, dark states are observed to dominate in single-molecule fluorescence intensity trajectories. This observation coupled with the large difference in lifetime derived from bright and dark states of <i>m</i>-ZnTCPP demonstrates higher charge injection efficiency of <i>m</i>-ZnTCPP than <i>p</i>-ZnTCPP. The nonexponential autocorrelation function decay and the power-law distribution of the dark-time probability density provide a detailed characterization of the inhomogeneous interfacial ET dynamics. The distribution of autocorrelation function decay times (τ) and power-law exponents (<i>m</i>_dark) for <i>m</i>-ZnTCPP are found to be different from those for <i>p</i>-ZnTCPP, which indicates the sensitivity of τ and <i>m</i>_dark on the molecular structure, molecular environment, and molecule–substrate electronic coupling of the interfacial electron transfer dynamics. Overall, our results strongly suggest that the fluctuation and even intermittency of excited-state chemical reactivity are intrinsic and general properties of molecular systems that involve strong molecule–substrate interactions

Nanocavity Effect On Photophysical Properties Of Colchicine: A Proof by Circular Dichroism Study and Picosecond Time-Resolved Analysis in Various Reverse Micellar Assemblies

In August 2009, colchicine won Food and Drug Administration (FDA) approval in the United States as a stand-alone drug for the treatment of acute flares of gout and familial Mediterranean fever. Recently, it is now the center of attraction in medicinal research. In this present paper, we have employed two other analogues of colchicine for exploring the photophysical properties inside nanocavity environment in details. Here we have a series of interesting results that have interesting similarity with the colchinoid–tubulin interaction. To monitor fluorescence properties of colchinoids, we have used absorption, emission, and time-resolved spectroscopy and to monitor structural properties we have measured circular dichroism. Steady-state anisotropy and dynamic light scattering results give an idea about the microenvironment sensed by the colchinoids molecules. A sharp increment for colchicine, very small increment for isocolchicine and no increment for colcemid in fluorescence and different circular dichroism (CD) spectra of all of these colchinoids upon embedment inside nanocavity of reverse micelle made a supposition that all these changes of fluorescence properties and CD results of colchinoids is not solely due to viscosity effect but also the constraint, that is, very narrow space to spread over, given by the nanocavity of reverse micelle. Moreover, we have noticed that the B ring of the colchinoids also have a pronounced effect on the interaction nature as well as on conformational change of these compounds after entrapment

Tuning the Probe Location on Zwitterionic Micellar System with Variation of pH and Addition of Surfactants with Different Alkyl Chains: Solvent and Rotational Relaxation Studies

In this manuscript, we have modulated the location of an anionic probe, Coumarin-343 (C-343) in a zwitterionic (<i>N</i>-hexadecyl-<i>N</i>,<i>N</i>-dimethylammonio-1-propanesulfonate (SB-16)) micellar system by three different approaches. The effect of addition of the surfactant sodium dodecyl sulfate (SDS) and the room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium octylsulfate (EmimOs) and <i>N</i>,<i>N</i>-dimethylethanol hexanoate (DAH), to the micellar solution has been studied. The effect of pH variation has been studied as well using solvent and rotational measurements. Migration of the anionic probe, C-343, from the palisade layer of SB-16 micelle to the bulk water has been observed to varying extents with the addition of SDS and EmimOs. The effect is much more pronounced in the presence of SDS and can be ascribed to the presence of the long alkyl (dodecyl) chain on SDS which can easily orient itself and fuse inside the SB-16 micelle and facilitate the observed migration of the probe molecule. This phenomenon is confirmed by faster solvation and rotational relaxation of the investigated probe molecule. The analogous fusion process is difficult in case of EmimOs and DAH because of their comparatively smaller alkyl (octyl and hexanoate) chain. However, the direction of C-343 migration is reversed with the decrease of pH of the SB-16 micellar medium. An increase in the average solvation and rotational relaxation time of the probe in acidic medium has been observed. Since experimental conditions are maintained such that the probe molecules and the zwitterionic SB-16 micelles remain oppositely charged, the observed results can be attributed to the increased electrostatic interaction (attractive) between them. Temperature dependent study also supports this finding

Solvation Dynamics and Rotational Relaxation Study Inside Niosome, A Nonionic Innocuous Poly(ethylene Glycol)-Based Surfactant Assembly: An Excitation Wavelength Dependent Experiment

Excitation wavelength dependence of solvation and rotational relaxation dynamics has been investigated inside niosome, a biologically stable, nontoxic to our body, multilamellar vesicle system, by using steady state and time-resolved fluorescence spectroscopy to explore the heterogeneity of such a system. Red edge excitation shifts (REES) of 7 nm for Coumarin-153 (C-153) and 11 nm for C-480 were observed with change in λ_ex. Average solvation dynamics is composed of two types of slow components and one fast component. There are two distinct restricted regions, one at the bilayer headgroup region and the other on the two extreme surfaces, which are responsible for the slow components. An unaltered fast component is reported for the segmental chain dynamics of poly(ethylene glycol) (PEG) located at the headgroup region of niosome. The trend in λ_ex dependence obtained for C-153 is found to be similar to that obtained for C-480. Such hindered solvation is attributed to the presence of a strong H-bonding environment of water molecules in the headgroup region, and movement of these highly bound water molecules along with a hydrated oxyethyelene moiety control the observed slow relaxation

Probing Electric Field Effect on Covalent Interactions at a Molecule–Semiconductor Interface

Fundamental understanding of the energetic coupling properties of a molecule–semiconductor interface is of great importance. The changes in molecular conformations and vibrational modes can have significant impact on the interfacial charge transfer reactions. Here, we have probed the change in the interface properties of alizarin–TiO₂ system as a result of the externally applied electric field using single-hot spot microscopic surface-enhanced Raman spectroscopy (SMSERS) and provided a theoretical understanding of our experimental results by density functional theory (DFT) calculations. The perturbation, caused by the external potential, has been observed as a shift and splitting of the 648 cm^–1 peak, typical indicator of the strong coupling between alizarin and TiO₂, at SMSERS. On the basis of our experimental results and DFT calculations, we suggest that electric field has significant effects on vibrational coupling at the molecule–TiO₂ interface. The presence of perturbed alizarin–TiO₂ coupling under interfacial electric potential may lead to changes in the interfacial electron transfer dynamics. Additionally, heterogeneously distributed dye molecules at the interface on nanometer length scale and different chromophore-semiconductor binding interactions under charge accumulation associated interfacial electric field changes create intrinsically inhomogeneous interfacial ET dynamics associated with both static and dynamic disorders

Room Temperature Ionic Liquid in Confined Media: A Temperature Dependence Solvation Study in [bmim][BF₄]/BHDC/Benzene Reverse Micelles

In this work, we reported a detailed study of the solvation dynamics of coumarin-480 in [bmim][BF4]/BHDC/benzene reverse micelles (RMs) with varying [bmim][BF4]/BHDC molar ratio (R) 1.00, 1.25, 1.50, and also study the solvation dynamics at five different temperatures from 15 to 35 °C RMs at [bmim][BF4]/BHDC molar ratio 1.25 for the first time. The average solvation time constant becomes slightly faster with the increase in R values at a temperature 25 °C. The solvation dynamics of the RMs with R value 1.25 becomes faster with the increase in temperature. We have also investigated temperature-dependent solvation dynamics in neat [bmim][BF4]. The solvation dynamics in neat [bmim][BF4] has a substantial temperature effect but for the [bmim][BF4]/BHDC/benzene RMs the temperature effect on the solvation dynamics is not that significant. Time-resolved fluorescence anisotropy studies reveal a decrease in the rotational restriction on the probe with increasing temperature. Wobbling-in-cone analysis of the anisotropy data also supports this finding

An Understanding of the Modulation of Photophysical Properties of Curcumin inside a Micelle Formed by an Ionic Liquid: A New Possibility of Tunable Drug Delivery System

The present study reveals the modulation of photophysical properties of curcumin, an important drug for numerous reasons, inside a micellar environment formed by a surfactant-like ionic liquid (IL-micelle) in aqueous solution. Higher stability of the drug inside IL-micelle in the absence and presence of a simple salt (sodium chloride) as well as considerably large partition coefficient (<i>K</i>_p = 8.59 × 10³) to the micellar phase from water make this system a well behaved drug loading vehicle. Remarkable change in fluorescence intensity with a strong blue-shift implies the gradual perturbation of intramolecular hydrogen bond (H-bond) present within the keto–enol group of curcumin along with considerable formation of intermolecular H-bond between curcumin and the headgroup of surfactant-like IL. Very fast nonradiative decay channels in curcumin mainly caused by the excited state intramolecular proton transfer (ESIPT) are thus depleted remarkably in the presence of IL-micelle of reduced polarity and as a result of restricted rotational and vibrational degrees of freedom when bound to the micelle. Moreover, time-resolved results confirm that not only the keto–enol group of curcumin is playing here but also the phenolic hydroxyl groups are also responsible for such modulation in photophysical properties. From a thermodynamic point of view, our system shows good correlation with its stability parameters (higher binding constant with very less hydrolytic degradation rate ∼1%) and higher negative value of binding enthalpy of interaction (−Δ<i>H</i>) than total free energy change (−Δ<i>G</i>) implies that the nature of binding interaction is enthalpy driven not entropy alone. Summarizing all the above observations, we have concluded that the modulation of the intramolecular proton transfer is due to the presence of both intermolecular proton transfer as well as strong hydrophobic interaction between curcumin and the IL-micelle

Pluronic Micellar Aggregates Loaded with Gold Nanoparticles (Au NPs) and Fluorescent Dyes: A Study of Controlled Nanometal Surface Energy Transfer

In this work we have reported the controlled synthesis of gold nanoparticles into the surface cavities of P123 micellar assemblies together with the fluorescent dye molecules and investigated nanometal surface energy transfer (NSET) from confined donor dye to metal nanoparticles. The formation of hybrid spherical assemblies of P123 combined with fluorescent dyes and gold nanoparticles has been confirmed from HR-TEM, DLS, UV–vis, and fluorescence spectroscopic studies. The observed steady state as well as time-resolved fluorescence quenching of the confined micellar dyes present in the close proximity of gold nanoparticles which are attached to the surface of micellar assemblies, indicates efficient surface energy transfer from dye to gold (Au) nanoparticles. Since the NSET process is strongly dependent on the distance between donor dye and acceptor nanoparticles, successful applications of NSET require the perfect control over their relative location. Herein, we investigate the utilization of nanoparticles embedded self-assemblies of P123 for controlled NSET by tuning the precise location of donor dyes. Through the nanoencapsulation of the different fluorophore having different location inside P123 micelles, we have shown the corona region of P123 micelles as a perfect place for NSET and the core region as a barrier for NSET. Additionally, we have investigated the microenvironment of the confined micellar probe molecules in presence and absence of nanoparticles. This study further reveals that when the system changes from normal micelles to nanoparticles loaded hybrid micelles, unlike the probes C480 and C153, the anionic probe C343 undergoes a change in its location indicating the modulation of the properties of micelles in presence of nanoparticles

Photoinduced Electron Transfer in an Imidazolium Ionic Liquid and in Its Binary Mixtures with Water, Methanol, and 2-Propanol: Appearance of Marcus-Type of Inversion

The photoinduced electron transfer (PET) reaction has been investigated in a room temperature imidazolium ionic liquid (RTIL), 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO₄]) and also in [Emim][EtSO₄]–co-solvents mixtures from <i>N</i>,<i>N</i>-dimethyl aniline (DMA) to different Coumarin dyes using steady state and time-resolved fluorescence quenching measurements. We have used water and methanol and 2-propanol as the cosolvents of RTILs for the PET study. On going from neat ionic liquid to the RTIL–co-solvents mixtures the electron transfer rate has been largely enhanced. In neat RTIL as well as in [Emim][EtSO₄]–co-solvents mixtures, a Marcus type of inversion in the PET rate have been observed