Ultrafast Solvation Dynamics Reveal that Octa Acid Capsule's Interior Dryness Depends on the Guest

A supramolecular approach towards the photorelease of encapsulated caged acids in water: 7-diethylaminothio-4-coumarinyl molecules as triggers

Herein, we establish the release of aliphatic acids in water upon excitation of 7-diethylaminothio-4-coumarinyl derivatives encapsulated within the organic host octa acid (OA). The 7-diethylaminothio-4-coumarinyl skeleton, employed here as the trigger, photoreleases caged molecules from the excited triplet state, in contrast to its carbonyl analogue, where the same reaction is known to occur from the excited singlet state. Encapsulation in OA solubilizes molecules in water that are otherwise water-insoluble, and retains the used trigger within itself following the release of the aliphatic acid. Such supramolecular characteristics usher in new features to the photorelease methodology. The thiocarbonyl chromophore extends the absorption of coumarinyl trigger to visible range while enhancing the intersystem crossing (ISC) to the triplet state, making it the reactive state. Despite the non-polar environment within the OA capsules the photocleavage occurs in a heterolytic fashion to release the conjugate base and the used trigger as triplet carbocation in an adiabatic process. Interestingly, the triplet carbocation crosses to the ground singlet surface (closed shell singlet carbocation) with the help of water molecules, possibly aided by C = S chromophore. Utilizing the known excited state dynamics of related thiocoumarinyl and coumarinyl systems, we have identified a few of the important mechanistic features of the photorelease process of 7-diethylaminothio-4-coumarinyl derivatives. Ultrafast excited state dynamic studies and quantum chemical calculations planned should help us better understand the photorelease process so as to effectively exploit the proposed system for potential applications.

Ultrafast Processes in Upper Excited Singlet States of Free and Caged 7-Diethylaminothiocoumarin

The photochemistry and photophysics of thiocarbonyl compounds, analogues of carbonyl compounds with sulfur, have long been overshadowed by their counterparts. However, recent interest in visible light reactions has reignited attention toward these compounds due to their unique excited-state properties. This study delves into the ultrafast dynamics of 7-diethylaminothiocoumarin (TC1), a close analogue of the well-known probe molecule coumarin 1 (C1), to estimate intersystem crossing rates, understand the mechanisms of fluorescence and phosphorescence, and evaluate TC1's potential as a solvation dynamics probe. Enclosing TC1 within an organic capsule indicates its potential applications, even in aqueous environments. Ultrafast studies reveal a dominant subpicosecond intersystem crossing process, indicating the importance of upper excited singlet and triplet states in the molecule's photochemistry. The distinct fluorescence and phosphorescence origins, along with the presence of closely spaced singlet excited states, support the observed efficient intersystem crossing. The sulfur atom alters the excited-state behavior, shedding light on reactive triplet states and paving the way for further investigations.

Structure, Dynamics, and Reactivity of Encapsulated Molecules in Restricted Spaces: Arylazoisoxazoles within an Octa Acid Capsule

In this study, a well-defined organic capsule assembled from two octa acid (OA) molecules acting as host and select arylazoisoxazoles (AAIO) acting as guests were employed to demonstrate that confined molecules have restricted freedom that translates into reaction selectivity in both ground and excited states. The behavior of these AAIO guests in confined capsules was found to be different from that found in both crystals, where there is very little freedom, and in isotropic solvents, where there is complete freedom. Through one-dimensional (1D) and two-dimensional (2D) 1H NMR spectroscopic experiments, we have established a relationship between structure, dynamics and reactivity of molecules confined in an OA capsule. Introduction of CF3 and CH3 substitution at the 4-position of the aryl group of AAIO reveals that in addition to space confinement, weak interactions between the guest and the OA capsule control the dynamics and reactivity of guest molecules. 1H NMR studies revealed that there is a temperature-dependence to guest molecules tumbling (180° rotation along the capsular short axis) within an OA capsule. While 1H NMR points to the occurrence of tumbling motion, MD simulations and simulation of the temperature-dependent NMR signals provide an insight into the mechanism of tumbling within OA capsules. Thermal and photochemical isomerization of AAIO were found to occur within an OA capsule just as in organic solvents. The observed selectivity noted during thermal and photo induced isomerization of OA encapsulated AAIOs can be qualitatively understood in terms of the well-known concepts due to Bell-Evans-Polanyi (BEP principle), Hammond and Zimmerman.

Modelling the Encapsulation of 3-Hydroxyflavone with Cyclodextrin and Octa Acid and Comparing Their Differences

The 3-hydroxyflavone (3-HF) is one of the common fluorescence probes. It has two distinct fluorescence bands: normal form and tautomer form. However, 3-hydroxyflavone has poor performance in water because of hydrogen bonding perturbation. The utilization of supramolecular chemistry would improve the fluorescence performance of 3-hydroxyflavone in water. In this paper, it reviews supramolecular chemistry of 3-hydroxyflavone with cyclodextrin and octa acid. Past research has found that the addition of β-cyclodextrin to 3-hydroxyflavone in water would slightly improve the fluorescence intensity of the tautomer form. When adding γ-cyclodextrin to 3-hydroxyflavone in water, the green fluorescence intensity would be enhanced. Finally, the addition of octa acid creates a dry environment for the 3-hydroxyflavone, and it would only have a tautomer form. The ONIOM calculation shows the ways of self-assembly of β- and γ-cyclodextrin. It can explain the difference in ratio between the tautomer form and normal form after understanding the interaction.

Ultrafast Excited State Dynamics of Spatially Confined Organic Molecules

This Feature Article highlights the role of spatial confinement in controlling the fundamental behavior of molecules. Select examples illustrate the value of using space as a tool to control and understand excited-state dynamics through a combination of ultrafast spectroscopy and conventional steady-state methods. Molecules of interest were confined within a closed molecular capsule, derived from a cavitand known as octa acid (OA), whose internal void space is sufficient to accommodate molecules as long as tetracene and as wide as pyrene. The free space, i.e., the space that is left following the occupation of the guest within the host, is shown to play a significant role in altering the behavior of guest molecules in the excited state. The results reported here suggest that in addition to weak interactions that are commonly emphasized in supramolecular chemistry, the extent of empty space (i.e., the remaining void space within the capsule) is important in controlling the excited-state behavior of confined molecules on ultrafast time scales. For example, the role of free space in controlling the excited-state dynamics of guest molecules is highlighted by probing the cis-trans isomerization of stilbenes and azobenzenes within the OA capsule. Isomerization of both types of molecule are slowed when they are confined within a small space, with encapsulated azobenzenes taking a different reaction pathway compared to that in solution upon excitation to S₂. In addition to steric constraints, confinement of reactive molecules in a small space helps to override the need for diffusion to bring the reactants together, thus enabling the measurement of processes that occur faster than the time scale for diffusion. The advantages of reducing free space and confining reactive molecules are illustrated by recording unprecedented excimer emission from anthracene and by measuring ultrafast electron transfer rates across the organic molecular wall. By monitoring the translational motion of anthracene pairs in a restricted space, it has been possible to document the pathway undertaken by excited anthracene from inception to the formation of the excimer on the excited-state surface. Similarly, ultrafast electron transfer experiments pursued here have established that the process is not hindered by a molecular wall. Apparently, the electron can cross the OA capsule wall provided the donor and acceptor are in close proximity. Measurements on the ultrafast time scale provide crucial insights for each of the examples presented here, emphasizing the value of both "space" and "time" in controlling and understanding the dynamics of excited molecules.

Vibration-Assisted Intersystem Crossing in the Ultrafast Excited-State Relaxation Dynamics of Halocoumarins

Due to its numerous applications, triplet formation and resulting phosphorescence remain a frontier area of research for over eight decades. Facile intersystem crossing (ISC) is the primary requirement for triplet formation and observation of phosphorescence. The incorporation of a heavy atom in molecules is one of the common approaches employed to facilitate ISC. A detailed study of the excited state dynamics that governs ISC is necessary to understand the mechanism of heavy atom effect (HAE). Incorporation of iodine at the 3 position of coumarin-1 reduces fluorescence quantum yield (ϕ_f) drastically as expected, whereas bromine substitution at the same position increased the ϕ_f. Such a contrasting effect of the two heavy atoms suggests that there are other features yet to be discovered to fully understand the HAE. Detailed steady state and femtosecond transient absorption studies along with theoretical calculations suggest that the C3-X (X = Br, I) bond vibration plays an important role in the ISC process. The study reveals that while in the case of the iodo-derivative there is no energy barrier in the singlet triplet crossing path, there is a barrier in the case of the bromo-derivative, which slows the ISC process. Such an unexpected phenomenon is not limited to halocoumarins as this rationalizes the photobehavior of 1-bromo-/iodo-substituted naphthalenes as well.

The Role of Hydrogen Bonding in the Preferential Solvation of 5-Aminoquinoline in Binary Solvent Mixtures

5-Aminoquinoline (5AQ) has been used as a fluorescent probe of preferential solvation (PS) in binary solvent mixtures in which the nonpolar component is diethyl ether and the polar component is protic (methanol) or aprotic (acetonitrile). Hence, the roles of solvent polarity and solute-solvent hydrogen bonding have been delineated. Positive deviations of spectral shifts from a linear dependence on the concentration of the polar component, signifying PS, are markedly more pronounced in case of the protic solvent. Solvation dynamics on a nanosecond time scale mark the formation of the solvation shell around the fluorescent probe. Time-resolved area-normalized emission spectra indicate the occurrence of the continuous solvation of the excited state when the polar component is acetonitrile. In contrast, two distinct states were observed when the polar component was methanol, the second state being the hydrogen bonded one. Translational diffusion is the rate-determining step for formation of the solvation shell. The time constant associated with it has been estimated from rise times observed in fluorescence transients monitored at the red end of the fluorescence spectra and also from the time evolution of the spectral width of time-resolved emission spectra.

Ultrafast Fluorescence Spectroscopy via Upconversion and Its Applications in Biophysics

In this review, the experimental set-up and functional characteristics of single-wavelength and broad-band femtosecond upconversion spectrophotofluorometers developed in our laboratory are described. We discuss applications of this technique to biophysical problems, such as ultrafast fluorescence quenching and solvation dynamics of tryptophan, peptides, proteins, reduced nicotinamide adenine dinucleotide (NADH), and nucleic acids. In the tryptophan dynamics field, especially for proteins, two types of solvation dynamics on different time scales have been well explored: ~1 ps for bulk water, and tens of picoseconds for “biological water”, a term that combines effects of water and macromolecule dynamics. In addition, some proteins also show quasi-static self-quenching (QSSQ) phenomena. Interestingly, in our more recent work, we also find that similar mixtures of quenching and solvation dynamics occur for the metabolic cofactor NADH. In this review, we add a brief overview of the emerging development of fluorescent RNA aptamers and their potential application to live cell imaging, while noting how ultrafast measurement may speed their optimization.

How Well Does a Solvated Octa-acid Capsule Shield the Embedded Chromophore? A Computational Analysis Based on an Anisotropic Dielectric Continuum Model

The optical properties of chromophores embedded in a water-solvated dimer of octa-acid that forms a molecular-shaped capsule are investigated. In particular, we address the anisotropic dielectric environment that appears to blue-shift excitation energies compared to the free aqueous chromophores. Recently we reported that using an effective scalar dielectric constant ε ≈ 3 appears to reproduce the measured spectra of the embedded coumarins, suggesting that the capsule provides a significant, albeit not perfect, screening of the aqueous dielectric environment. Here, we report absorption energies using a theoretical treatment that includes continuum solvation affected by an anisotropic dielectric function reflecting the high-dielectric environment outside of the capsule and the low-dielectric region within. We report time-dependent density functional theory calculations using a range-separated functional with the Poisson boundary conditions that model the anisotropic dielectric environment. Our calculations find that the anisotropic environment due to the water-solvated hydrophobic capsule is equivalent to a homogeneous effective dielectric constant of ≈3. The calculated values also appear to reproduce measured absorption of the embedded coumarin, where we study the effect of the hydrophobic capsule on the excited state.

Marcus Relationship Maintained During Ultrafast Electron Transfer Across a Supramolecular Capsular Wall

Photoinduced electron transfer across an organic capsular wall between excited donors and ground-state acceptors is established to occur with rate constants varying in the range 0.32-4.0 × 10¹¹ s^-1 in aqueous buffer solution. The donor is encapsulated within an anionic supramolecular capsular host, and the cationic acceptor remains closer to the donor separated by the organic frame through Coulombic attraction. Such an arrangement results in electron transfer proceeding without diffusion. Free energy of the reaction (ΔG°) and the rate of electron transfer show Marcus relation with inversion. From the plot, λ and V_el were estimated to be 1.918 and 0.0058 eV, respectively. Given that the donor remains within the nonpolar solvent-free confined space, and there is not much change in the environment around the acceptor, the observed λ is believed to be because of "internal" reorganization rather than "solvent" reorganization. A similarity exists between the capsular assembly investigated here and glass and crystals at low temperature where the medium is rigid. The estimated electronic coupling (V_el) implies the existence of interaction between the donor and the acceptor through the capsular wall. Existence of such an interaction is also suggested by ¹H NMR spectra. Results of this study suggest that molecules present within a confined space could be activated from outside. This provides an opportunity to probe the reactivity and dynamics of radical ions within an organic capsule.

Remote electron and energy transfer sensitized photoisomerization of encapsulated stilbenes

Excited state chemistry and physics of molecules, in addition to their inherent electronic and steric features, depend on their immediate microenvironments. This study explores the influence of an organic capsule, slightly larger than the reactant molecule itself, on the excited state chemistry of the encapsulated molecule. Results presented here show that the confined molecule, in fact, is not isolated and can be manipulated from outside even without direct physical interaction. Examples where communication between a confined molecule and a free molecule present outside is brought about through electronic and energy transfer processes are presented. Geometric isomerization of octa acid encapsulated stilbenes induced by energy and electron transfer by cationic sensitizers that attach themselves to the anionic capsule is examined. The fact that isomerization occurs when the sensitizer present outside is excited illustrates that the reactant and sensitizer are communicating across the molecular wall of the capsule. Ability to remotely activate a confined molecule opens up new opportunities to bring about reactions of confined radical ions and triplet excited molecules generated via long distance energy and electron transfer processes.