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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. 

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. 

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. 

Low temperature matrix isolation method is the most popular one to generate and store reactive molecules and characterize them by in-situ IR spectroscopy. Recognizing the need for a simpler method to trap and store such molecules and characterize by NMR spectroscopy at room temperature in solution we have performed experiments exploring the value of water-soluble octa acid (OA) capsule as a storage vessel. The molecule we have chosen to illustrate the feasibility is the highly hindered 7-cis--ionone, which has been established to exist in equilibrium with its cyclic form with the later favored at room temperature. In this study we have shown that confined space can be an alternative to temperature to tilt an equilibrium towards higher energy isomer. During the course of the study, we were surprised to note that 7-trans--ionone aggregates in water and have distinct 1H NMR spectra. Ability to assemble characterizable organic aggregates in water reveals the value of water as a reaction medium that is yet to be fully explored by photochemists. Finally, we have clarified the likely mechanism of secondary photoreaction of -pyran to the final photoproduct that involves 1,5-hydrogen migration. 

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of trans -azobenzene ( t -Az) and several alkyl-substituted t -Az derivatives encapsulated in a water-soluble supramolecular host–guest complex. Encapsulation increases the excited-state lifetimes and alters the yields of the trans → cis photoisomerization reaction compared with solution. Kinetic modeling of the transient spectra for unsubstituted t -Az following nπ* and ππ* excitation reveals steric trapping of excited-state species, as well as an adiabatic excited-state trans → cis isomerization pathway for confined molecules that is not observed in solution. Analysis of the transient spectra following ππ* excitation for a series of 4-alkyl and 4,4′-dialkyl substituted t -Az molecules suggests that additional crowding due to lengthening of the alkyl tails results in deeper trapping of the excited-state species, including distorted trans and cis structures. The variation of the dynamics due to crowding in the confined environment provides new evidence to explain the violation of Kasha's rule for nπ* and ππ* excitation of azobenzenes based on competition between in-plane inversion and out-of-plane rotation channels.