Templation of the Excited-State Chemistry of α-(n-Alkyl) Dibenzyl Ketones:  How Guest Packing within a Nanoscale Supramolecular Capsule Influences Photochemistry

Abstract

Excited-state behavior of eight alpha-alkyl dibenzyl ketones (alkyl = CH3 through n-C8H17) that are capable of undergoing type II and/or type I photoreactions has been explored in isotropic solution and within a water-soluble capsule. The study consisted of two parts: photochemistry that explored the excited-state chemistry and an NMR analysis that revealed the packing of each guest within the capsule. The NMR data (COSY, NOESY, and TOCSY experiments) revealed that ternary complexes between alpha-alkyl dibenzyl ketones and the capsule formed by two cavitands are kinetically stable, and the guests fall into three packing motifs modulated by the length of the alpha-alkyl chain. In essence, the host is acting as an external template to promote the formation of distinct guest conformers. The major products from all eight guests upon irradiation either in hexane or in buffer solution resulted from the well-known Norrish type I reaction. However, within the capsule the excited-state chemistry of the eight ketones was dependent on the alkyl chain length. The first group consisted of alpha-hexyl, alpha-heptyl, and alpha-octyl dibenzyl ketones that yielded large amounts of Norrish type II products within the host, while in solution the major products were from Norrish type I reaction. The second group consists of alpha-butyl and alpha-pentyl dibenzyl ketones that yield equimolar amounts of two rearranged starting ketones within the capsule (combined yield of ca 60%), while in solution no such products were formed. The third group consisted of alpha-methyl, alpha-ethyl, and alpha-propyl dibenzyl ketones that within the capsule yielded only one (not two) rearranged starting ketone in larger amounts (21-35%) while in solution no rearrangement product was obtained. Variation in the photochemistry of the guest within the capsule, with respect to the alpha-alkyl chain length of the guest, highlights the importance of how a small variation in supramolecular structure can influence the selectivity within a confined nanoscale reactor.

Funktionale molekulare Reaktionskolben: neuartige Eigenschaften und Reaktionen in diskreten, selbstorganisierten Wirtmolekülen

“…[66] Bei Alkylgruppen mit R ! Propyl wurden neben den Norrish-I-Produkten von Decarbonylierungen und Umlagerungen auch die Norrish-II-Produkte 26 und 27 erhalten (Gesamtumsatz 30-50 %).…”

Simulation optimization of spherical non-polar guest recognition by deep-cavity cavitands

“…3 Second, the pocket entrance is rimmed with aromatic rings that bestows OA a predisposition to assemble into dimeric capsules. 4 These capsules have been used as yoctoliter reaction vessels, [5][6][7] separation devices, 8,9 for modulating the properties of redox-active 10 or fluorescence guests, 11,12 as well as controlling electron transfer 13 and electron-electron communication. 14 In addition to this capsule chemistry, three classes of guests bind to OA without triggering assembly and consequently allow the study of a range of 1:1 complexes.…”

Size-Selective Phase-Transfer Catalysis with Interfacially Cross-Linked Reverse Micelles

“…3 Ramamurthy and Gibb were able to control the photoreactions of ketones using water-soluble molecular capsules. 4 Fujita and colleagues demonstrated that, when packed in a metalÀ organic nanocapsule, unreactive naphthalenes could undergo regio-and stereoselective DielsÀAlder reactions. 5 Badjic and co-workers developed cavitand "baskets" with conformational gating to control reactivity.…”