Evidence of Short-Lived High-Energy Emissive State and Triplet Character of the Self-Trapped Exciton in Cs3Cu2I5 Perovskite.

Cs3Cu2I5 perovskite displays a Stokes-shifted photoluminescence (PL) at 445 nm, attributed to the self-trapped excitons (STEs). Unlike that observed in other perovskite materials, the free-exciton emission is not evidenced in this case. Herein, we reveal the existence of a short-lived high-energy emission centered around 375 nm through the reconstruction of time-resolved emission spectra (TRES), which is independent of the shape/size of Cs3Cu2I5 perovskite. This high-energy emission is proposed to originate from the free-exciton-derived distorted S1 state of the 0D Cs3Cu2I5 moiety. Moreover, STE PL (∼445 nm) was found to have phosphorescence characteristics. Theoretical calculation confirms a facile intersystem crossing at the Franck-Condon geometry, indicating the high lifetime of the STE and its triplet nature. The existence of a high-energy emissive state and the phosphorescent nature of the STE PL band provide valuable insights that could advance our understanding of the photophysics in these materials.

Osmolyte induced protein stabilization: modulation of associated water dynamics might be a key factor.

The mechanism of protein stabilization by osmolytes remains one of the most important and long-standing puzzles. The traditional explanation of osmolyte-induced stability through the preferential exclusion of osmolytes from the protein surface has been seriously challenged by the observations like the concentration-dependent reversal of osmolyte-induced stabilization/destabilization. The more modern explanation of protein stabilization/destabilization by osmolytes considers an indirect effect due to osmolyte-induced distortion of the water structure. It provides a general mechanism, but there are numerous examples of protein-specific effects, i.e., a particular osmolyte might stabilize one protein, but destabilize the other, that could not be rationalized through such an explanation. Herein, we hypothesized that osmolyte-induced modulation of associated water might be a critical factor in controlling protein stability in such a medium. Taking different osmolytes and papain as a protein, we proved that our proposal could explain protein stability in osmolyte media. Stabilizing osmolytes rigidify associated water structures around the protein, whereas destabilizing osmolytes make them flexible. The strong correlation between the stability and the associated water dynamics, and the fact that such dynamics are very much protein specific, established the importance of considering the modulation of associated water structures in explaining the osmolyte-induced stabilization/destabilization of proteins. More interestingly, we took another protein, bromelain, for which a traditionally stabilizing osmolyte, sucrose, acts as a stabilizer at higher concentrations but as a destabilizer at lower concentrations. Our proposal successfully explains such observations, which is probably impossible by any known mechanisms. We believe this report will trigger much research in this area.

Understanding the intricacy of protein in hydrated deep eutectic solvent: Solvation dynamics, conformational fluctuation dynamics, and stability.

Deep eutectic solvents (DESs) are potential biocatalytic media due to their easy preparation, fine-tuneability, biocompatibility, and most importantly, due to their ability to keep protein stable and active. However, there are many unanswered questions and gaps in our knowledge about how proteins behave in these alternate media. Herein, we investigated solvation dynamics, conformational fluctuation dynamics, and stability of human serum albumin (HSA) in 0.5 Acetamide/0.3 Urea/0.2 Sorbitol (0.5Ac/0.3Ur/0.2Sor) DES of varying concentrations to understand the intricacy of protein behaviour in DES. Our result revealed a gradual decrease in the side-chain flexibility and thermal stability of HSA beyond 30 % DES. On the other hand, the associated water dynamics around domain-I of HSA decelerate only marginally with increasing DES content, although viscosity rises considerably. We propose that even though macroscopic solvent properties are altered, a protein feels only an aqueous type of environment in the presence of DES. This is probably the first experimental study to delineate the role of the associated water structure of the enzyme for maintaining its stability inside DES. Although considerable effort is necessary to generalize such claims, it might serve as the basis for understanding why proteins remain stable and active in DES.

Does Viscosity Decoupling Guarantee Dynamic Heterogeneity? A Clue through an Excitation and Emission Wavelength-Dependent Time-Resolved Fluorescence Anisotropy Study.

Traditionally, deviation from Stokes-Einstein-Debye (SED) relation in terms of viscosity dependence of medium dynamics, i.e., τx∝(ηT)p with p ≠ 1, is taken as a signature of dynamic heterogeneity. However, it does not guarantee medium heterogeneity, as the decoupling may also originate from the deviation of the basic assumption of SED. Here, we developed a method to find a stronger relation between viscosity decoupling (p ≠ 1) and dynamic heterogeneity in terms of rotational motion. Our approach exploited the fact that in heterogeneous media, a solvatochromic probe will be solvated to a different extent at different microdomains (subpopulations), and photoselection of these subpopulations can be achieved by excitation or emission wavelength-dependent measurements. We hypothesized that the dynamics of a homogeneous system might show viscosity decoupling, but the extent of decoupling at different excitations (or at different emissions) should not be different. On the other hand, in a heterogeneous medium, this extent of viscosity decoupling (p-value) should be different at different excitations (or at different emissions). As proof of concept, we investigated three versatile solvent media: squalane (viscous molecular liquid), 1-ethyle-3-methylimidazolium ethyl sulfate ionic liquid (IL), and [0.78 acetamide + 0.22 LiNO3] deep eutectic solvent (DES). We found that squalane is homogeneous, although it shows fractional viscosity dependence (p ≠ 1). Interestingly, mild heterogeneity in IL and significant heterogeneity in the DES were observed. Overall, we conclude that the difference in the p-value as a function of excitation (or emission) wavelength-dependent might be a superior way for the detection of dynamic heterogeneity.

Associated Water Dynamics Might Be a Key Factor Affecting Protein Stability in the Crowded Milieu.

Over the past 20 years, the most studied and debated aspect of macromolecular crowding is how it affects protein stability. Traditionally, it is explained by a delicate balance between the stabilizing entropic effect and the stabilizing or destabilizing enthalpic effect. However, this traditional crowding theory cannot explain experimental observations like (i) negative entropic effect and (ii) entropy-enthalpy compensation. Herein, we provide experimental evidence that associated water dynamics plays a crucial role in controlling protein stability in the crowded milieu for the first time. We have correlated the modulation of associated water dynamics with the overall stability and its individual components. We showed that rigid associated water would stabilize the protein through entropy but destabilize it through enthalpy. In contrast, flexible associated water destabilizes the protein through entropy but stabilizes through enthalpy. Consideration of entropic and enthalpic modulation through crowder-induced distortion of associated water successfully explains the negative entropic part and entropy-enthalpy compensation. Furthermore, we argued that the relationship between the associated water structure and protein stability should be better understood by individual entropic and enthalpic components instead of the overall stability. Although a huge effort is necessary to generalize the mechanism, this report provides a unique way of understanding the relationship between protein stability and associated water dynamics, which might be a generic phenomenon and should trigger much research in this area.

Site-specific Heterogeneity of Multi-domain Human Serum Albumin and its Origin: A Red Edge Excitation Shift Study†.

Conformational heterogeneity is a defining characteristic of a protein and is vital in understanding its function and folding landscape. In the present work, we interrogated the presence of conformational heterogeneity in multi-domain human serum albumin in a domain-specific manner using red edge excitation shift (REES) in its native state and also monitored its variation along the unfolding transition. We also looked into the origin of such conformational heterogeneity by varying the solution viscosity. We observed (1) even in the native state, the heterogeneity and dynamics of the side chain exhibit varied behaviors depending on which domain of the multi-domain human serum albumin (HSA) is being examined. (2) When the protein is in the unfolded state, the extent of REES is rendered unimportant since there is a greater quantity of free water present, in addition to the disruption of the protein's structure. (3) While the rigid protein matrix provides the rigidity of domain-I and domain-III, the rigidity of domain-II is provided by water molecules, which indicates that the role of water molecules in providing the rigidity is significant. Overall, our results provide direct evidence of the rigidity and alternate side chain packing arrangement of protein core that varies domain-wise in multi-domain HSA.

Red-emitting polyaniline-based nanoparticle probe for pH-sensitive fluorescence imaging.

Fluorescent probes based on semiconducting polymer nanoparticles (NPs) such as polyaniline (PANI) usually require external fluorophore doping to provide fluorescence function. Direct use of PANI-based NPs for bioimaging applications has been limited by PANI's weak blue fluorescence and aggregation-induced quenching in physiological medium. In this report, we developed a facile solid-state synthesis method to produce fluorescent polyaniline nanoparticles (FPNs) that are not only water-soluble but also exhibit high intensity and pH-sensitive red fluorescence. The FPNs showed high photoluminescence quantum yield (PLQY) of 19.3 % at physiological pH, which makes FPNs ideal for application as fluorescent nanoprobes in bioimaging. Moreover, we performed an in-depth study of photoluminescence dependence on pH and the phenomena of exciton-polaron quenching at low pH was highlighted. We also found that the ratio of emission intensity at 600 nm and 650 nm increased from 0.04 to 1.65 as pH was raised from 2.6 to 11.8, which could find its application in ratiometric pH sensing. FPNs exhibited excellent biocompatibility with >85 % cell viability for fibroblasts NIH/3 T3 and prostate cancer 22RV1 cells even at concentrations as high as 1000 µg/mL. In addition, fluorescence microscopy demonstrated concentration-dependent red fluorescence in the cytoplasm owing to the cellular uptake of FPNs in prostate cancer cells.

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.

Macromolecular crowding: how shape and interaction affect the structure, function, conformational dynamics and relative domain movement of a multi-domain protein.

The cellular environment is crowded by macromolecules of various sizes, shapes, and charges, which modulate protein structure, function and dynamics. Herein, we contemplated the effect of three different macromolecular crowders: dextran-40, Ficoll-70 and PEG-35 on the structure, active-site conformational dynamics, function and relative domain movement of multi-domain human serum albumin (HSA). All the crowders used in this study have zero charges and similar sizes (at least in the dilute region) but different shapes and compositions. Some observations follow the traditional crowding theory. For example, all the crowders increased the α-helicity of HSA and hindered the conformational fluctuation dynamics. However, some observations are not in line with the expectations, such as an increase in the size of HSA with PEG-35 and uncorrelated domain movement of HSA with Ficoll-70 and PEG-35. The relative domain movement is correlated with the activity, suggesting that such moves are essential for protein function. The interaction between HSA and Ficoll-70 is proposed to be hydrophobic in nature. Overall, our results provide a somewhat systematic study of the shape-dependent macromolecular crowding effect on various protein properties and present a possible new insight into the mechanism of macromolecular crowding.

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 (ϕf) drastically as expected, whereas bromine substitution at the same position increased the ϕf. 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.

Tracking Wormlike Micelle Formation in Solution: Unique Insight through Fluorescence Correlation Spectroscopic Study.

Although worm-like micelles were invented 35 years ago, its formation pathway remains unclear. Inspired by the fact that a single molecular level experiment could provide meaningful and additional information, especially in a heterogeneous subpopulation, herein, we present a single molecular level study on the formation of wormlike micelles by cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water. Our results indicated a coexistence of normal spherical micelles along with a big wormlike micelle in its formation path. More interestingly, we have two unique insights into the formation mechanism, which are inaccessible in ensemble averaged experiments: (i) at extremely low concentrations of the surfactant, [CTAB]/[NaSal] ∼ 0.06, the wormlike micelle attains the highest size; and (ii) the relative concentration of wormlike micelles is highest when [CTAB]/[NaSal] ∼ 2.

Does Microsecond Active-Site Dynamics Primarily control Proteolytic Activity of Bromelain? Clues from Single Molecular Level Study with a Denaturant, a Stabilizer and a Macromolecular Crowder.

Proteins are dynamic entity with various molecular motions at different timescale and length scale. Molecular motions are crucial for the optimal function of an enzyme. It seems intuitive that these motions are crucial for optimal enzyme activity. However, it is not easy to directly correlate an enzyme's dynamics and activity due to biosystems' enormous complexity. amongst many factors, structure and dynamics are two prime aspects that combinedly control the activity. Therefore, having a direct correlation between protein dynamics and activity is not straightforward. Herein, we observed and correlated the structural, functional, and dynamical responses of an industrially crucial proteolytic enzyme, bromelain with three versatile classes of chemicals: GnHCl (protein denaturant), sucrose (protein stabilizer), and Ficoll-70 (macromolecular crowder). The only free cysteine (Cys-25 at the active-site) of bromelain has been tagged with a cysteine-specific dye to unveil the structural and dynamical changes through various spectroscopic studies both at bulk and at the single molecular level. Proteolytic activity is carried out using casein as the substrate. GnHCl and sucrose shows remarkable structure-dynamics-activity relationships. Interestingly, with Ficoll-70, structure and activity are not correlated. However, microsecond dynamics and activity are beautifully correlated in this case also. Overall, our result demonstrates that bromelain dynamics in the microsecond timescale around the active-site is probably a key factor in controlling its proteolytic activity.

Dynamic heterogeneity and viscosity decoupling: origin and analytical prediction.

The molecular-level structure and dynamics decide the functionality of solvent media. Therefore, a significant amount of effort is being dedicated continually over time in understanding their structural and dynamical features. One intriguing aspect of solvent structure and dynamics is heterogeneity. In these systems, the dynamics follow , where p is the measure of viscosity decoupling. We analytically predicted that in such cases, the Stokes-Einstein relationship is modified to due to microdomain formation, and the second term on the right-hand side leads to viscosity decoupling. We validated our prediction by estimating the p values of a few solvents, and they matched well with the literature. Overall, we believe that our approach gives a simple yet unique physical picture to help us understand the heterogeneity of solvent media.

Correlating Bromelain's activity with its structure and active-site dynamics and the medium's physical properties in a hydrated deep eutectic solvent.

Deep eutectic solvents (DESs) are emerging as new media of choice for biocatalysis due to their environmentally friendly nature, fine-tunability, and potential biocompatibility. This work deciphers the behaviour of bromelain in a ternary DES composed of acetamide, urea, and sorbitol at mole fractions of 0.5, 0.3, and 0.2, respectively (0.5Ac/0.3Ur/0.2Sor), with various degrees of hydration. Bromelain is an essential industrial proteolytic enzyme, and the chosen DES is non-ionic and liquid at room temperature. This provides us with a unique opportunity to contemplate protein behaviour in a non-ionic DES for the very first time. Our results infer that at a low DES concentration (up to 30% V/V DES), bromelain adopts a more compact structural conformation, whereas at higher DES concentrations, it becomes somewhat elongated. The microsecond conformational fluctuation time around the active site of bromelain gradually increases with increasing DES concentration, especially beyond 30% V/V. Interestingly, bromelain retains most of its enzymatic activity in the DES, and at some concentrations, the activity is even higher compared with its native state. Furthermore, we correlate the activity of bromelain with its structure, its active-site dynamics, and the physical properties of the medium. Our results demonstrate that the compact structural conformation and flexibility of the active site of bromelain favour its proteolytic activity. Similarly, a medium with increased polarity and decreased viscosity is favourable for its activity. The presented physical insights into how enzymatic activity depends on the protein structure and dynamics and the physical properties of the medium might provide useful guidelines for the rational design of DESs as biocatalytic media.

Mantis shrimp-inspired organic photodetector for simultaneous hyperspectral and polarimetric imaging.

Combining hyperspectral and polarimetric imaging provides a powerful sensing modality with broad applications from astronomy to biology. Existing methods rely on temporal data acquisition or snapshot imaging of spatially separated detectors. These approaches incur fundamental artifacts that degrade imaging performance. To overcome these limitations, we present a stomatopod-inspired sensor capable of snapshot hyperspectral and polarization sensing in a single pixel. The design consists of stacking polarization-sensitive organic photovoltaics (P-OPVs) and polymer retarders. Multiple spectral and polarization channels are obtained by exploiting the P-OPVs' anisotropic response and the retarders' dispersion. We show that the design can sense 15 spectral channels over a 350-nanometer bandwidth. A detector is also experimentally demonstrated, which simultaneously registers four spectral channels and three polarization channels. The sensor showcases the myriad degrees of freedom offered by organic semiconductors that are not available in inorganics and heralds a fundamentally unexplored route for simultaneous spectral and polarimetric imaging.

Dynamics of Anthracene Excimer Formation within a Water-Soluble Nanocavity at Room Temperature.

Excited anthracene is well-known to photodimerize and not to exhibit excimer emission in isotropic organic solvents. Anthracene (AN) forms two types of supramolecular host-guest complexes (2:1 and 2:2, H:G) with the synthetic host octa acid in aqueous medium. Excitation of the 2:2 complex results in intense excimer emission, as reported previously, while the 2:1 complex, as expected, yields only monomer emission. This study includes confirming of host-guest complexation by NMR, probing the host-guest structure by molecular dynamics simulation, following the dynamics AN molecules in the excited state by ultrafast time-resolved experiments, and mapping of the excited surface through quantum chemical calculations (QM/MM-TDDFT method). Importantly, time-resolved emission experiments revealed the excimer emission maximum to be time dependent. This observation is unique and is not in line with the textbook examples of time-independent monomer-excimer emission maxima of aromatics in solution. The presence of at least one intermediate between the monomer and the excimer is inferred from time-resolved area normalized emission spectra. Potential energy curves calculated for the ground and excited states of two adjacent anthracene molecules via the QM/MM-TDDFT method support the model proposed on the basis of time-resolved experiments. The results presented here on the excited-state behavior of a well-investigated aromatic molecule, namely the parent anthracene, establish that the behavior of a molecule drastically changes under confinement. The results presented here have implications on the behavior of molecules in biological systems.

Potassium-Induced Passivation of Deep Traps in Bismuth-Doped Hybrid Lead Bromide Perovskite Nanocrystals: Massive Amplification of Photoluminescence Quantum Yield.

The low photoluminescence quantum yield of Bi3+-doped lead halide perovskite nanocrystals (NCs) is a big challenge to the scientific community. This makes them a weak candidate in the optoelectronics field in spite of their better stability than the pure lead analogue. Herein, the reason behind this reduction of quantum yield in hybrid mixed lead-bismuth bromide (MPBBr) NC is investigated and proposed to be due to ultrafast trapping transfer in the core of the NC, and not due to the surface trap states. Further, we have successfully boosted the quantum yield of MPBBr NC from 9% to 64% by passivating the deep traps within the crystal core by monovalent potassium ion doping. The stability of the developed Bi3+/K+-doped lead halide perovskite NC was found to be extremely high in atmospheric conditions, and this property is sustained up to 100 °C.

Fluorescence correlation spectroscopy as a tool to investigate the directionality of proteolysis.

Enzymatic proteolysis or protein digestion is the fragmentation of protein into smaller peptide units under the action of peptidase enzymes. In this contribution, the directionality of proteolysis has been studied using fluorescence correlation spectroscopy (FCS), taking human serum albumin (HSA) as the model protein and papain, chymotrypsin and trypsin as the model enzymes. Domain-I of HSA has been tagged with tetramethylrhodamine-5-maleimide (TMR) and domain-III with p-nitrophenylcoumarin ester (NPCE) separately and subjected to proteolysis. Following the change in hydrodynamic radius, as monitored by FCS, it has been confirmed that under similar experimental conditions the order of efficiency of digestion is papain > trypsin > chymotrypsin. More interestingly, a faster decrease of hydrodynamic radius was observed when the fluorescence from domain-I was monitored in FCS, compared to that of domain-III. This observation clearly indicates that all these enzymes prefer to start cleaving HSA from domain-I. We assign this preference to the hydrophilic natures of the enzyme active site and domain-I surface. The dependence of the proteolysis on temperature and enzyme concentration has also been studied for papain using the same approach. Reverse-phase HPLC results are found to be in line with the FCS results and validates the applicability of our proposed method.

Partial Viscosity Decoupling of Solute Solvation, Rotation, and Translation Dynamics in Lauric Acid/Menthol Deep Eutectic Solvent: Modulation of Dynamic Heterogeneity with Length Scale.

Deep eutectic solvents (DESs) are new-generation media that can be fine-tuned to have desired properties circumventing economic and environmental issues. Typically, these are ionic, and only recently, nonionic DESs, having interesting properties, are being explored. In this report, we examined the structure and dynamics of a nonionic lauric acid/menthol (LA/Men) DES through steady-state emission, solvation dynamics, time-resolved fluorescence anisotropy, and translational diffusion dynamics. The zero shift in the emission spectra of coumarin 153 (a solvatochromic dye) as a function of the excitation wavelength suggests that LA/Men DES is spatially homogenous. Decoupling (p = 0.63) of the average solvation time, ⟨τs⟩, from medium viscosity suggests the presence of mild dynamic heterogeneity in the system. Rotational time, ⟨τr⟩, which reflects the nature of the first solvation shell, shows little decoupling (p = 0.81), suggesting it to be fairly dynamically homogeneous at a shorter length scale. An Arrhenius-type analysis also proves that rotation is mainly controlled by medium viscosity. Translational diffusion time, ⟨τD⟩, which provides information at a larger length scale, is strongly decoupled from medium viscosity (p = 0.29). This indicates that at a larger length scale, the DES is quite dynamically heterogeneous. The slow component of solvation time, which is believed to originate at a larger length scale, correlates well with the translational diffusion timescale having similar activation energies. This suggests that their origin is same. Expectedly, for the long component of solvation time, the decoupling is quite strong (p = 0.30). Overall, our result demonstrates the structure and dynamics of the nonionic LA/Men DES, and the existence of length scale-dependent heterogeneity has been proposed.

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 Vel 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 (Vel) 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.