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A biphenyl photosensitizer axle was implanted into the cavities of native and capped γ-cyclodextrins through rotaxanation using a cucubit[6]uril-templated azide-alkyne 1,3-dipolar cycloaddition, resulting in the construction of highly defined chiral binding/sensitizing sites. The orientation and interaction of the axle and capping moieties at the ground and excited states were interrogated by NMR, UV-vis, circular dichroism, and fluorescence spectroscopic studies. In situ photoisomerization of (Z,Z)-1,3-cyclooctadiene sensitized in the cavity of these [4]rotaxanes afforded (Z,E)-1,3-cyclooctadiene in up to 15.3% ee, which represents the highest level of enantiodifferentiation obtained to date for this supramolecular photochirogenesis.

6-O-(2-Naphthoyl)curdlan was newly synthesized as a sensitizing polysaccharide host to examine the chiroptical properties, supramolecular complexation, and photochirogenic behavior with (Z,Z)-1,3-cyclooctadiene (1ZZ). The enantiodifferentiating photoisomerization of 1ZZ included and sensitized by this polysaccharide host gave a highly strained chiral (E,Z)-isomer in up to 8.7% enantiomeric excess (ee) in solution and 11.7% ee in the solid state, which are the highest values ever reported for a supramolecular photochirogenesis of 1EZ.

Enantiodifferentiating photoisomerizations of (Z,Z)-1,3-cyclooctadiene (1ZZ) to the chiral E,Z-isomer 1EZ were performed at varying temperatures in the presence of some benzenepoly-, naphthalene(di)-, and anthracenecarboxylates. Of these chiral sensitizers, (-)-menthyl benzenehexacarboxylate afforded the highest enantiomeric excesses (ee's) up to 10% and 18% in pentane at 25 and -40°C, respectively. In contrast, the use of polar solvents greatly diminished the product's ee, suggesting intervention of a radical ionic rather than exciplex intermediate in these solvents. Optically pure 1EZ is shown to possess anomalously high specific rotation and circular dichroism as a simple diene chromophore: [α]25(D) 1380°(CH2Cl2), Δε 12.8 M-1 cm-1 (λ(ext) 228 nm, cyclohexane). The chiroptical properties observed not only correct the previous data but also present the conclusive evidence for the recent theoretical predictions.

Direct photolysis of alkyl-substituted cyclobutene derivatives in solution with monochromatic far-UV light sources results in competing ring opening to the corresponding substituted 1,3-butadiene derivatives and fragmentation to the corresponding alkene and alkyne via formal cycloreversion.The excited states leading to the two types of products have been identified.Cycloreversion occurs stereospecifically and arises as a result of excitation of the ?,R(3s) Rydberg state, which is of comparable energy to the ?,?* state in alkylcyclobutenes.In spite of its stereospecificity, the reaction is nonconcerted; evidence is presented to suggest that it occurs by a mechanism involving migration (ring contraction) to yield a cyclopropylcarbene intermediate, which then fragments to yield the alkyne and alkene.The stereochemistry at C3 and C4 in the starting cyclobutene is retained in the alkene produced in the reaction.Ring opening proceeds nonstereoselectively (from the ?,?* singlet state) in well over a dozen systems that have been investigated, and possible mechanisms to account for this are discussed.It is proposed that orbital symmetry plays a role in the reaction, but only in the early stages of ring opening.Bicyclic cyclobutene derivatives in which the cyclobutene double bond is located across the bridge undergo ring opening with a high degree of disrotatory stereoselectivity.Possible reasons for this unusual behaviour are discussed in light of recent ab initio theoretical results.

Two substituted bicyclic cyclobutene derivatives - 7-methyl- and 7-(trifluoromethyl)bicyclo[4.2.0]oct-7-ene - have been prepared. Gas- and solution-phase UV absorption and Hel UV photoelectron spectra have been recorded for the two compounds as well as for the parent hydrocarbon bicyclo[4.2.0]oct-7-ene. The gas-phase spectra suggest that the π,R(3s) state is the lowest energy state in bicyclo[4.2.0]octene and the 7-methyl derivative but is raised to higher energies than the π,π* state in the 7-trifluoromethyl derivative. Direct photolysis of the three compounds in hydrocarbon solution with monochromatic far-UV (193 and 214 nm) light leads to competitive ring opening to the corresponding cis,cis- and cis,trans-1,3-cyclooctadiene derivatives, as well as fragmentation to cyclohexene and alkyne in all three cases. Product quantum yields (193-nm excitation) have been measured for both substituted derivatives relative to those for the parent compound. The quantum yields of fragmentation products are highest for the methyl- and unsubstituted compounds, suggesting that these products arise from a Rydberg-like excited state. In contrast, ring opening is most efficient for the trifluoromethyl-substituted compound, although the diene distributions obtained from the reaction do not vary throughout the series. These results indicate that nonstereospecific ring opening arises largely from the π,π* state of cyclobutene. The contribution to the fragmentation process from a concerted [σ2s+σ2s] cycloreversion pathway is minor, at best. The wavelength dependence of the product distributions from photolysis of the three derivatives is in perfect accord with these assignments.

The photochemistry of bicyclohept-6-ene, bicyclooct-7-ene, and cis- and trans-3,4-dimetylcyclobutene has been investigated in hydrocarbon solution with monochromatic far-ultraviolet (185 and 193 nm) light sources.All of these simple cyclobutene derivatives undergo ring opening to yield the isomeric 1,3-dienes, and the latter three open nonstereospecifically to yield mixtures of the possible geometric isomers.The isomeric 3,4-dimethylcyclobutenes yield different mixtures of the three 2,4-hexadiene isomers, and in each case the mixtures are weighed in favor of the orbital symmetry forbidden isomer(s).Attempts have been made to analyze the relative isomeric diene yields from ring opening of bicyclooctene and the isomeric 3,4-dimethylcyclobutenes within the context of the purely disrotatory, adiabatic ring-opening mechanism that recent ab initio calculations suggest should be possible.While the results for the former compound are consistant with this mechanism, analysis of the relative yields of the isomeric 2,4-hexadienes from photolysis of the latter two compounds indicates that photochemical ring opening by the formally forbidden, conrotatory pathway may compete to some extent with disrotatory ring opening.

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In sharp contrast to the wavelength-dependent photobehaviour of simple mono-alkenes, the photochemistry of conjugated dienes was found to be independent of excitation wavelength in the region 185-254 nm.

Bicyclohexenyl anion (1) and bicycloheptenyl anion (2) rearrange in THF to monocyclic pentadienyl anions, whereas bicyclooctenyl anion 3 is stable under the reaction conditions. 3 in contrary is formed by the known electrocyclic ring closure of cyclooctadienyl anion 7.Rearrangements of cyclopentenyl anion and pentadienyl anion are not detected.Cyclic allyl anions are alkylated by ethene, formed by cleavage of THF with base or independently added. 6,6-Dimethylcyclohexadienyl anion undergoes slow fragmentation to toluene at room temperature.

Direct photolyses at 185 nm of bicyclohept-6-ene and bicyclooct-7-ene in pentane afforded acetylene and the fragment cycloalkenes as the major photoproducts, whereas the Woodward-Hoffmann allowed ring-opening giving rise to 1,3-diene was a minor process.