Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Supramolecular-Surface Photochemistry: Assembly and Photochemistry of Host−Guest Capsules on Silica Surface Elamparuthi Ramasamy and V. Ramamurthy* Department of Chemistry, University of Miami, Coral Gables, Miami, Florida 33146, United States
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S Supporting Information *
ABSTRACT: Host cavitands and organic guest molecules independently adsorbed on silica particles when mixed and shaken in the presence of a few drops of water underwent intra- and interparticle migration to form capsular complexes that were not formed either in water or organic solvents. Importance of cavitand migration and tumbling on silica surface was established by demonstrating that covalently linked cavitands do not form capsular complexes. The encapsulated guests exhibited selective photochemistry as they do within an organic capsule in solution.
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been established.20,21 In a classic study, de Mayo and coworkers established the intra- and intergranular migration of aromatic molecules on silica surfaces.22−24 Also, adsorption of a preassembled host−guest complexes on the silica surface has been achieved.25 However, to our knowledge, a host−guest capsular complex from individual molecules has not been previously assembled on a surface. In this study, we show that a guest−host complex comprising up to four molecules (one or two host molecules and one or two guest molecules) could be assembled on the surface of silica in the presence of water. In these examples, water not only facilitated the translational mobility of these molecules but also provided the driving force needed for the guest inclusion within an organic capsule. Our results establish that the silica surface can serve as a medium to assemble water-insoluble synthetic organic hosts 1 and 2 and hydrophobic guests 4−9. The results presented in this paper highlight the importance of merging the principles of supramolecular chemistry and surface chemistry to carry out selective phototransformations. Our interest in organic solvent soluble hosts that would form capsular assemblies in nonaqueous media prompted us to synthesize 1 and 2 and explore their complexation abilities. The two hosts were synthesized by modification of the procedures used for octa acid (OA)26 and resorcinol-capped tetraacid.25 Detailed synthetic procedures and spectral data are included in the Supporting Information (Pages 3−8 and Figures S1−S4). Hosts 1 and 2 have structural similarities with OA, a cavitand well-known to include a large number of organic molecules in borate buffer solution.27,28 The eight COOH groups of OA are replaced in 1 and 2 by hydrogen or methyl at the top and by OH at the bottom pendants. Replacement of COOH groups in 1 and 2 resulted in their solubility in DMSO and THF and insolubility in water.
olecules confined in restricted spaces have been established to exhibit photochemical and photophysical behaviors often unpredictable from their inherent electronic and steric features.1−3 In this context, micelles, cyclodextrins and related cavitands, organic and inorganic capsules, zeolites, silica, and crystals have been identified as effective organized assemblies that can alter the behavior of guest molecules by restricting their freedom.4−16 In this paper, we report the results of our studies that exploit the combined features of supramolecular assemblies in solution and on solid surfaces to achieve selectivity in photoprocesses. Because of the failure of cavitands 1 and 2 (Scheme 1) in including guests in water or Scheme 1. Structures of Hosts and Guests Used in This Study
organic solvents, they could not be exploited in solution as supramolecular hosts. Looking for an alternate approach led us to the use of photochemically inert, chromatographic silica as solid matrix to hold the above organic cavitands during the host−guest capsular assembly process. The value of functionalized silica through chemical modification and physical adsorption of organic and inorganic molecules has been recognized in the past.17 In recent times, anchoring photocatalysts on silica surfaces has attracted attention.18,19 The feasibility of assembling the donor and acceptor components of energy and electron-transfer systems in an organized manner on the silica surface, especially in the presence of water, has © XXXX American Chemical Society
Received: May 16, 2018
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DOI: 10.1021/acs.orglett.8b01497 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
(80 °C). In a similar manner, a separate batch of silica adsorbed with a given guest (known amount corresponding to 2:1 or 2:2 complex) was prepared. The two dry silica samples, one adsorbed with the host and the other with the guest, were combined and shaken for about 6 h using a mechanical shaker. UV-diffuse reflectance spectra of the thus-prepared samples (Figures S15 and S16) confirmed their adsorption on the silica surface. However, the emission spectra of these solid samples corresponded to that of the probes adsorbed alone on the silica surface (Figure 1, green traces), making clear that the host and
To examine the complexation abilities of 1 and 2, we used five probe molecules 5−9 (Scheme 1) that have been established to show unique photophysical and photochemical behavior when confined within an OA capsule in aqueous solution.27 When encapsulated within OA the normally nonphosphorescent 5 and 7 phosphoresce; anthracene exhibiting only monomer emission in solution exhibits excimer-type emission; pyrene showing a mixture of monomer and excimer emission in solution, independent of the concentration, shows only monomer emission; and 1-phenyl3-tolyl-2-propanone (4-methyl dibenzylketone) that undergoes the Norrish type I cleavage both in solution and within the OA capsule shows significantly different product distributions between these media. Within OA, the cage effect (enhanced AB, see Scheme 2) and formation of rearrangement product 10 are the norm. We expected similar changes to occur when guests 5−9 are encapsulated within hosts 1 and 2. Scheme 2. Products and Mechanism of Norrish Type I Reaction of 1-Phenyl-3-tolylpropanone
Figure 1. Normalized emission spectra of solid silica samples accommodating host encapsulated guests adsorbed on the silica surface: (i) 5 (λex = 310 nm), (ii) 6 (λex = 350 nm), (iii) 7 (λex = 254 nm), (iv) 8 (λex = 337 nm) (I1/I3 = 1.01). Blue: host 1. Red: host 2. Green: guest alone, no host.
the guest did not come together to form capsular assemblies. Based on the established translational motions of aromatic molecules on silica surface,22,23 we ruled out mobility as the limiting factor. We presumed that the driving force for host− guest assembly was lacking on the dry silica surface. With the assumption that the hydrophobic effect would favor host−guest assembly, water was added (∼40% by weight) to the above host- and guest-loaded silica samples, and the mixture was shaken in a closed vessel.32 Detailed experimental procedures are presented in the Supporting Information. Although for the sake of completeness we shook all of the samples for 6−10 h, fluorescence analysis of samples prepared by shaking for 1 h indicated the presence of host− guest capsular assembly. Further, TGA analysis of the thusprepared samples indicated the presence of residual water. Residual water was removed under reduced pressure and slightly elevated temperature (2 × 10−3 Torr and 60 °C). The emission spectra of these samples containing 5−8 as guests and 1 and 2 as hosts are presented in Figure 1, and the lifetime decay traces are included in Figures S17 and S18. The above solid silica samples containing hosts 1 and 2 and guests 5 and 7 exhibited intense phosphorescence even in the presence of oxygen and long triplet lifetimes (710 ± 10 and 786 ± 10 μs in 1 and 55 ± 10 and 59 ± 10 μs in 2, respectively; Figures S17 and S18). These features are consistent with 5 and 7 in a highly protected environment that in our case is a capsule.33,34 Anthracene showing only monomer emission in solution as well as when adsorbed alone on silica and when adsorbed with host 1 or 2 on silica showed mainly excimer-type emission (Figure 1; blue and red traces) with long lifetimes (287 ± 10 and 266 ± 10 ns, respectively; Figures S17 and S18).35 These observations similar to anthracene within OA are consistent with two molecules of anthracene confined in a capsule formed by two molecules of
Of the six guests listed in Scheme 1, only the photochemically inert 1-bromoadamantane formed a 1:1 complex with hosts 1 and 2 in DMSO-d6 (1H NMR, see Figures S13 and S14). The snug fit of 1-bromoadamantane in the cavitand must provide a strong van der Waals interaction facilitating the encapsulation in DMSO. However, photochemically active guest molecules 5−9 did not yield capsular assemblies in water and organic solvents. This prompted us to search for a method that would encourage host and guest molecules to form capsules. We have demonstrated previously that OA and octaammonium capsules assembled in aqueous solution and then transferred to the solid surfaces of silica, clay, and TiO2 surfaces were stable.25,29−31 This method was not a viable option as we could not assemble capsules of 1 and 2 in both aqueous and nonaqueous solvents. This led us to explore the feasibility of forming supramolecular complexes of hosts 1 and 2 with guests 5−9 directly on the surface of silica. With the known mobility of aromatics on the silica surface,22 we anticipated that the guest and host molecules would find each other and lead to stable capsules. The experimental protocols for assembling capsules on silica surface are detailed in the SI. A typical procedure of loading the host and the guest on silica consisted of the following: THF solution of the host 1 (or 2; 1 mM) and 30 mg of dry flash chromatographic silica (mesh size 230−400 μm) were stirred together for at least 6 h, and the solvent was evaporated at reduced pressure (2 × 10−3 Torr) and higher temperature B
DOI: 10.1021/acs.orglett.8b01497 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters host 1 or 2 (2:2 complex). Finally, pyrene showing a strong excimer emission (Figure 1) when adsorbed alone on silica showed only monomer emission with I1/I3 (1.03), corresponding to a nonpolar environment when adsorbed on silica in the presence of either host.36 The photophysical features of the above four probes convincingly established that guests and hosts adsorbed independently on different batches of silica upon mixing and shaking in the presence of water migrate and tumble on the surface and form capsular assemblies. Thus, the properties of the capsules formed on the solid silica surface and those formed by OA in aqueous solution are similar. The lack of capsule formation in the dry state highlights the role of water in the assembly of hosts and guests on the surface of silica to form capsules. To confirm the above conclusions, 1-phenyl-3-tolyl-2propanone (4-methyl dibenzylketone, 9), a molecule whose photochemistry has been extensively investigated in organized media, was used as a probe (Scheme 2).37,38 The cage effect, defined as %{[AB] − [AA] − [BB]/[AB] + [AA] + [BB]}, varying between 10 and 50% yield and formation of the rearranged product 10 in
