Probing Local Effects in Silica Sol−Gel Media by Fluorescence Spectroscopy of p-DASPMI

A thiol-anchored solvatochromic and fluorogenic molecular rotor for covalent protein labeling in SDS-PAGE and mitochondria specific fluorescence imaging

Fluorescent labeling is a widely used method for protein detection and fluorescence imaging. A solvatochromic and fluorogenic molecular rotor DASPBCl was developed for covalent protein labeling in solution and SDS-PAGE, and also for stable mitochondria labeling and fluorescence imaging. The dye DASPBCl consisted of a 4-(N,N-dimethylamino)phenyl moiety as the electron donor and a positively charged N-benzylpyridinium moiety as the electron acceptor. A benzyl chloride group was introduced into the pyridine moiety for covalent labeling of thiol in proteins. When the fluorescent dye DASPBCl is covalently labeled to the thiol of proteins, significantly enhanced fluorescence was obtained, which is attributed to the polarity sensitivity caused solvatochromic effect from the hydrophobic protein structure and the viscosity sensitivity caused fluorogenic effect from the restriction of single bond rotation. DASPBCl exhibits high sensitivity and good linear response for protein detection in SDS-PAGE analysis with both the pre-staining method and post-staining method. DASPBCl was also successfully used for covalently protein-anchored fluorescence imaging of mitochondria in living cells.

Visualization of the Sol-Gel Transition in Porous Networks Using Fluorescent Viscosity-Sensitive Probes

The sol-gel transition involves the transformation of a colloidal suspension into a system-spanning, interconnected gel. This process is widely used to reinforce mechanically weakened porous artifacts, such as sculptures but the impact of the restricted geometry (porous network) on the gelation dynamics of the solution remains unclear. Here, using fluorescent viscosity-sensitive molecular rotors, confocal microscopy, and model pores, we visualize the local viscosity changes at the microscale that accompany the sol-gel transition of a methyltriethoxysilane solution into a gel network. We show that, with evaporation of the solvent, a viscosity gradient develops near the free surface, triggering the sol-gel transition inside small pores near the surface. In homogeneous porous media, this leads to skin formation, which reduces the evaporation rate. In heterogeneous porous media, a gradient in gel density develops toward the heart of the porous material, where the gel formation mainly occurs as capillary bridges within smaller pores.

Multiple Effects of an Anionic Cyclodextrin Macrocycle on the Reversible Isomerization of a Photoactive Guest Dye

Understanding and controlling the reversible isomerization of photoactive molecules in order to obtain a tunable optical response is desirable for many photofunctional applications. This study describes the interesting effects of an anionic cyclodextrin host (sulfated-βCD, SCD) on the photoisomerization and protonation equilibrium of an important hemicyanine dye (trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide, DSP). The SCD host assists in unlocking the photoisomerization potential of DSP by promoting protonation of the dye. It also assists in stabilizing the cis isomer of the protonated dye, thereby significantly delaying the reverse cis to trans isomerization of DSPH⁺. Furthermore, the interplay of both hydrophobic and electrostatic interactions in the complex formation of SCD with DSPH⁺ makes the reverse cis to trans isomerization of DSPH⁺ amenable to influence by the added salt. The stimuli-responsive reversible isomerization of SCD-DSPH⁺ is an interesting case from the perspective of chemical sensing or light operated functional materials with host-guest systems.

Slip/Stick Viscosity Models of Nanoconfined Liquids: Solvent-Dependent Rotation in Metal-Organic Frameworks

Artificial molecular machines are expected to operate in environments where viscous forces impact molecules significantly. With that, it is well-known that solvent behaviors dramatically change upon confinement into limited spaces as compared to bulk solvents. In this study, we demonstrate the utility of an amphidynamic metal-organic framework with pillars consisting of ²H-labeled dialkynyltriptycene and dialkynylphenylene barrierless rotators that operate as NMR sensors for solvent viscosity. Using line-shape analysis of quadrupolar spin echo spectra we showed that solvents such as dimethylformamide, diethylformamide, 2-octanone, bromobenzene, o-dichlorobenzene, and benzonitrile slow down their Brownian rotational motion (10³-10⁶ s^-1) to values consistent with confined viscosity values (ca. 10⁰-10³ pa s) that are up to 10000 greater than those in the bulk. Magic angle spinning assisted ¹H T₂ measurements of included solvents revealed relaxation times of approximately 100-1000 ms over the explored temperature ranges, and MAS-assisted ¹H T₁ measurements of included solvents suggested a much lower activation energy for rotational dynamics as compared to those measured by the rotating pillars using ²H measurements. Finally, translational diffusion measurements of DMF using pulsed-field gradient methods revealed intermediate dynamics for the translational motion of the solvent molecules in MOFs.

Can di-4-ANEPPDHQ reveal the structural differences between nanodiscs and liposomes?

The potential-sensitive di-4-ANEPPDHQ dye is presently gaining popularity in structural studies of the lipid bilayer. Within the bilayer, dye environmental sensitivity originates from the excitation induced charge redistribution and is usually attributed to solvent relaxation. Here, di-4-ANEPPDHQ is utilized to compare the structure of neutral and negatively charged lipid bilayers between two model systems: the nanodiscs and the liposomes. Using the well-established approach of measuring solvatochromic shifts of the steady-state spectra to study the bilayer structural changes has proved insufficient in this case. By applying an in-depth analysis of time-resolved fluorescence decays and emission spectra, we distinguished and characterized two and three distinct emissive di-4-ANEPPDHQ species in the liposomes and the nanodiscs, respectively. These emissive species were ascribed to the dual emission of the dye rather than to solvent relaxation. An additional, long-lived component present in the nanodiscs was associated with a unique domain of high order, postulated recently. Our results reveal that the di-4-ANEPPDHQ steady-state fluorescence should be interpreted with caution. With the experimental approach presented here, the di-4-ANEPPDHQ sensitivity was improved. We confirmed that the bilayer structure is, indeed, altered in the nanodiscs. Moreover, molecular dynamic simulations showed a distribution of the probe in the nanodiscs plane, which is sensitive to lipid composition. In POPC nanodiscs, probe frequently interacts with MSP, while in POPC-POPG nanodiscs, such interactions are rare. We did not observe, however, any impact of those interactions on the probe fluorescence.

Probing the Aggregation and Signaling Behavior of Some Twisted 9,9'-Bianthryl Derivatives: Observation of Aggregation-Induced Blue-Shifted Emission

With an aim to understand the photophysical behavior of twisted organic fluorescent molecules in their aggregated state, two twisted biaryl molecules, namely, 9,9'-bianthryl and 10,10'-dicyano-9,9'-bianthryl, have been synthesized and characterized by conventional spectroscopic methods. To understand the role of C-C bond twisting on the photophysical response of biaryl aggregates, monoaryl counterparts (anthracene and 9-anthracenecarbonitrile) of the biaryl systems are also investigated. Photophysical behaviors of these systems along with their monoaryl counterpart are investigated in both solution and aggregated state. Investigations reveal that fluorescence spectra of the biaryl compounds show blue-shifted emission upon aggregation. Interestingly, no blue shift of the emission has been observed for monoaryl aggregates. Photophysical data of biaryl systems compared to monoaryl unit reveal that change in geometry, during self-assembly process, disfavors the formation of charge-transfer state, which eventually causes blue shift in the emission upon aggregation. In addition to this, potential of these systems toward signaling of nitroaromatic explosive has also been explored. Among all of the nitroaromatics, the highest fluorescence quenching is observed for nitrophenols (say picric acid (PA)). The investigation also reveals that compared to monoaryl systems, biaryl systems are more responsive to fluorescence quenching by nitroaromatics. Perrin's model of quenching sphere action has been attributed to nitrophenol (PA) selective signaling behavior of biaryl systems.

Subtleties of catanionic surfactant reverse micelle assemblies revealed by a fluorescent molecular probe

In this work, the absorption and emission behavior of the cationic hemicyanine trans-4-[4-(dimethylamino)styryl]-N-methylpyridinium iodide (HC) in reverse micelles (RMs) formed by the catanionic surfactants benzyl-n-hexadecyldimethylammonium-1,4-bis-2-ethylhexylsulfosuccinate (AOT-BHD) and cetyltrimethylammonium-1,4-bis-2-ethylhexylsulfosuccinate (AOT-CTA) have been investigated. Our results show that the spectroscopic behavior of HC changes when the dye is dissolved in AOT-BHD or in AOT-CTA RMs. While HC undergoes an intramolecular charge-transfer process upon excitation in AOT-CTA RMs, in AOT-BHD RMs this process is inhibited due to a specific interaction between HC and the polar head group of the BHD⁺ cation. This implies that the chemical structure of CTA⁺ and BHD⁺ cations has a large impact on the excited stated from which HC emission occurs. Additionally, the structural difference between the two cations impacts on the water-RM interface interaction, which provides a way of controlling the solvation process in these RMs. Furthermore, differences in the interfacial fluidity between the two catanionic RMs is observed, a result that is particularly interesting with regard to these systems being used as nanoreactors.

Long-living optical gain induced by solvent viscosity in a push–pull molecule

The combination of continuum and ultrafast pump–probe spectroscopy, in viscous and non-viscous environments, with DFT and TDDFT calculations, is effective in unraveling important features of the twisted intramolecular charge transfer mechanism in a new push–pull molecule that possesses aggregation induced emission properties.

Inclusion of push–pull N-methylpyridinium salts within surfactant hydrogels: is their excited state intramolecular charge transfer mediated by twisting?

By confining two push–pull N-methylpyridinium derivatives within the rigid domains of surfactant hydrogels, the twisted nature of their intramolecular charge transfer state (formed in water upon excitation) was unravelled.

Probing the Microenvironment in the Confined Pores of Mesoporous Silica Nanoparticles

The microenvironment inside of the pores of mesoporous silica nanoparticles is probed using spectroscopic techniques. The probe molecules are sealed inside of the pores by a nanovalve system that is capable of controlling the access to the pore and ensuring the exclusive probing of the pore environment without any interference from the probe molecules on the outer surface of the particles or from the surrounding solution. Rigidochromism studies are used to evaluate the rigidity of the solvent matrix inside of the pore, and dynamic fluorescence anisotropy experiments are employed to determine the rotational diffusion freedom of the probe molecule. The data show that those probe molecules are neither completely free to move nor tightly attached to the pore wall, and their mobility is changed by altering the charge of the pore wall.

Volume increment effect on the photoisomerization of hemicyanine dyes in oligo(ethylene glycol)s

We studied the excited-state dynamics of three hemicyanine dyes that undergo internal twisting from the localized excited state to the twisted intramolecular charge-transfer state. The dyes differ in the length of the alkyl chain in the aniline moiety and, thus, the volume of the motional moiety increases without having much of an effect on the excited-state potential surface. By employing oligo(ethylene glycol)s as a new homologous series of solvents that covers a high viscosity region, we showed that the excited-state lifetime of the hemicyanines gradually increases at any given viscosity when the size of the substituent increases. We describe our results for the solution-phase photoisomerization processes in terms of the breakdown of Stokes' law, multidimensionality, and the Hubbard relation.

Probing Local Environments by Time-Resolved Stimulated Emission Spectroscopy

Time-resolved stimulated emission spectroscopy was employed to probe the local environment of DASPMI (4-(4-(dimethylamino)styryl)-N-methyl-pyridinium iodide) in binary solvents of different viscosity and in a sol-gel matrix. DASPMI is one of the molecules of choice to probe local environments, and the dependence of its fluorescence emission decay on viscosity has been previously used for this purpose in biological samples, solid matrices as well as in solution. The results presented in this paper show that time-resolved stimulated emission of DASPMI is a suitable means to probe the viscosity of local environments. Having the advantage of a higher time resolution, stimulated emission can provide information that is complementary to that obtained from fluorescence decay measurements, making it feasible to probe systems with lower viscosity.

Fluorescence anisotropy of molecular rotors

We present polarization-resolved fluorescence measurements of fluorescent molecular rotors 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ), 9-(2,2-dicyanovinyl)julolidine (DCVJ), and a meso-substituted boron dipyrromethene (BODIPY-C(12)). The photophysical properties of these molecules are highly dependent on the viscosity of the surrounding solvent. The relationship between their quantum yields and the viscosity of the surrounding medium is given by an equation first described and presented by Förster and Hoffmann and can be used to determine the microviscosity of the environment around a fluorophore. Herein we evaluate the applicability of molecular rotors as probes of apparent viscosity on a microscopic scale based on their viscosity dependent fluorescence depolarization. We develop a theoretical framework, combining the Förster-Hoffmann equation with the Perrin equation and compare the dynamic ranges and usable working regimes for these dyes in terms of utilising fluorescence anisotropy as a measure of viscosity. We present polarization-resolved fluorescence spectra and steady-state fluorescence anisotropy imaging data for measurements of intracellular viscosity. We find that the dynamic range for fluorescence anisotropy for CCVJ and DCVJ is significantly lower than that of BODIPY-C(12) in the viscosity range 0.6<η<600 cP. Moreover, using steady-state anisotropy measurements to probe microviscosity in the low (<3 cP) viscosity regime, the molecular rotors can offer a better dynamic range in anisotropy compared with a rigid dye as a probe of microviscosity, and a higher total working dynamic range in terms of viscosity.

Local viscosity analysis of triblock copolymer micelle with cyanine dyes as a fluorescent probe

The local viscosity of Pluronic F127 triblock copolymer micelles in water was determined with cyanine dyes as fluorescent probes. These dyes show very weak fluorescence at a low temperature, but show enhanced fluorescence at a temperature higher than the critical micellization temperature (T(cm)). This is because a viscous environment within the micelle suppresses the formation of a nonradiative twisted intramolecular charge transfer (TICT) excited state of the dyes. The good correlation between the fluorescence quantum yields of the dyes and the viscosity and the temperature of the media allows a determination of local viscosity of micelle based on the fluorescence quantum yields. The local viscosity of both core and corona regions of micelles increases at >T(cm) and shows a maximum at a temperature 7-9 °C higher than T(cm), and decreases at higher temperature due to the increased fluidity. The core viscosity is larger than that of the corona, and the corona viscosity increases toward the micelle center. The polymer concentration has different effects on the core and corona viscosity: the corona viscosity increases with a polymer concentration increase at the entire temperature range, whereas the core viscosity increases only at a low temperature. The corona viscosity increase is due to the condensation of a large number of polyethylene oxide (PEO) blocks. In contrast, the dehydration degree of polypropylene oxide (PPO) blocks in the core scarcely changes, and the core has a similar composition regardless of polymer concentration. The larger polymer concentration promotes a micelle formation at lower temperature where the fluidity increase is very weak, resulting in larger core viscosity.

Internal motion of an electronically excited molecule in viscoelastic media

The twisting motion of trans-4-[4-(dimethylamino)-styryl]-1-methylpyridinium iodide (4-DASPI) in the excited state was investigated in solutions and various polymers in order to understand dependence of molecular rotor dynamics on viscoelasticity. It was observed that the internal motion of electronically excited 4-DASPI correlates strongly with dynamic viscosity and elastic modulus. Our results also showed that condensed phase dynamics of 4-DASPI are governed by the explicit mode coupling between the rotamerizing coordinate and mechanical properties of viscoelastic media.

Application of Fluorescence Techniques to Characterise the Preparation of Protein-Containing Sol-Gel Derived Hosts for use as Catalytic Media

In this work we collate and review the usage that we have made of fluorescence techniques employed to follow the sol to gel transition and aging in different tetraethylorthosilicate based materials. The sol-gel method allows porous glasslike of good optical quality to be produced at relatively low (ambient) temperatures, which facilitates the incorporation of a range of molecules; from laser dyes to biomolecules. Here the use of “common” viscosity (DASPMI) and polarity (Nile red) sensitive fluorescence probes to monitor the host manufacture is made. Nile red was also used to label two catalytically active proteins (cytochrome c and subtilisin Carlsberg). These were incorporated into the different host media and the dye used to ascertain changes in protein conformation, both upon incorporation and at the end of an aging period. Complementary measurements of catalytic activity were performed. The probe emission was monitored via steady state and time-resolved fluorescence techniques and comparison made with the catalytic activity measurements to elucidate the amount of accessible and active protein. Overall it was found that the hosts became stable after an aging period approaching 20 days and that the major influence on the catalytic reaction rates was that of host mediated mass transport.