An official website of the United States government
This 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 S¬2. 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.
more »
« less
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.
more »
« less
- Award ID(s):
- 1807729
- PAR ID:
- 10179893
- Date Published:
- Journal Name:
- Photochemical & Photobiological Sciences
- Volume:
- 19
- Issue:
- 7
- ISSN:
- 1474-905X
- Page Range / eLocation ID:
- 976 to 986
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
Excited-state chemistry relies on the communication between molecules, making it a crucial aspect of the field. One important question that arises is whether intermolecular communication and its rate can be modified when a molecule is confined. To explore the interaction in such systems, we investigated the ground and excited states of 4′-N,N-diethylaminoflavonol (DEA3HF) in an octa acid-based (OA) confined medium and in ethanolic solution, both in the presence of Rhodamine 6G (R6G). Despite the observed spectral overlap between the flavonol emission and the R6G absorption, as well as the fluorescence quenching of the flavonol in the presence of R6G, the almost constant fluorescence lifetime at different amounts of R6G discards the presence of FRET in the studied systems. Steady-state and time-resolved fluorescence indicate the formation of an emissive complex between the proton transfer dye encapsulated within water-soluble supramolecular host octa acid (DEA3HF@(OA)2) and R6G. A similar result was observed between DEA3HF:R6G in ethanolic solution. The respective Stern–Volmer plots corroborate with these observations, suggesting a static quenching mechanism for both systems.more » « less
-
Photochemical valence bond isomerization of a crystalline Dewar benzene diacid monoanion salt with an acetophenone-linked piperazinium cation that serves as an intramolecular triplet energy sensitizer (DB-AcPh-Pz) exhibits a quantum chain reaction with as many as 450 product molecules per photon absorbed (F ≈ 450). By contrast, isomorphous crystals of the Dewar benzene diacid monosalt of an ethylbenzene-linked piperazinium (DB-EtPh-Pz) lacking a triplet sensitizer showed a less impressive quantum yield of ca. F ≈ 22. To establish the critical importance of a triplet excited state carrier in the adiabatic photochemical reaction we prepared mixed crystals with DB-AcPh-Pz as a dilute triplet sensitizer guest in crystals of DB-EtPh-Pz. As expected from the their high structural similarities, solid solutions were easily formed with the triplet sensitizer salt in the range of 0.1% to 10%. Experiments carried out under conditions where light is absorbed by the triplet sensitizer-linked DB-AcPh-Pz can be used to initiate a triplet state adiabatic reaction from 3DB-AcPh-Pz to 3HB*-AcPh-Pz, which can serve as a chain carrier and transfer energy to an unreacted DB-EtPh-Pz where exciton delocalization in the crystalline solid solution can help carry out an efficient energy transfer and enable a quantum chain employing the photoproduct as a triplet chain carrier. Excitation of mixed crystals with a little as 0.1% triplet sensitizer resulted in an extraordinarily high quantum yield F ≈ 517.more » « less
-
Exerting control on excited state processes has been a long-held goal in photochemistry. One approach to achieve control has been to mimic biological systems in Nature ( e.g. , photosynthesis) that has perfected it over millions of years by performing the reactions in highly organized assemblies such as membranes and proteins by restricting the freedom of reactants and directing them to pursue a select pathway. The duplication of this concept at a smaller scale in the laboratory involves the use of highly confined and organized assemblies as reaction containers. This article summarizes the studies in the author's laboratory using a synthetic, well-defined reaction container known as octa acid (OA). OA, unlike most commonly known cavitands, forms a capsule in water and remains closed during the lifetime of the excited states of included molecules. Thus, the described excited state chemistry occurs in a small space with hydrophobic characteristics. Examples where the photophysical and photochemical properties are dramatically altered, compared to that in organic solvents wherein the molecules are freely soluble, are presented to illustrate the value of a restricted environment in controlling the dynamics of molecules on an excited state surface. While the ground state complexation of the guest and host is controlled by well-known concepts of tight-fit, lock and key, complementarity, etc. , free space around the guest is necessary for it to be able to undergo structural transformations in the excited state, where the time is short. This article highlights the role of free space during the dynamics of molecules within a confined, inflexible reaction cavity.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0pp00115e
