Web Release Date: March 6,
Templation of the Excited-State Chemistry of 
-(n-Alkyl)
Dibenzyl Ketones: How Guest Packing within a Nanoscale
Supramolecular Capsule Influences Photochemistry
and
Department of Chemistry, University of Miami, Coral Gables, Florida 33124, and Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
Received December 3, 2007
Abstract:
Excited-state behavior of eight -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 -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 -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 -hexyl, -heptyl, and -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 -butyl and -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 -methyl, -ethyl, and -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 -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.
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