Photoactive cyclophanes, I. Synthesis, photophysical and photochemical properties of cinnamophanes

Abstract

In order to transfer the so-called topochemical control established by Schmidt and coworkers to fluid solutions, the [2.2]paracyclophane framework was substituted with two acryloyl substituents, one on each of the two decks. The 4 products thus formed are termed cinnamophanes. The pseudo-gem derivative, 1, was shown to undergo a very efficient stereospecific [2 + 21 photocycloaddition to form 5 with the highest quantum yield (=0.8) ever observed for such a Cinnamates are of great interest and under current investigation because of their use as chromophores for the design of photoactive polymers, molecular assemblies, supramolecular devices and UV It is well-known that cinnamic acids and cinnamate derivatives photodimerize or photopolymerize in the crystalline state according to the topochemical rules established by Schmidt and coworkers [*]. In contrast to the solid, the fluid solutions (and low viscosity media) are conducive mainly to trans-cis isomerizati~n [~]. Dimerization becomes significant only when the chromophores are held together by a However, even then, several intramolecular photodimers are generally observed, associated with the dynamic and conformational properties of the molecular skeleton[g-' 1. Furthermore, in the crystal, for a P-type stacking, the yield of the dimerization cannot be quantitative for statistical reasonsL2I; the maximum value would never exceed 86% even for a perfect single crystal transformation occurring without any destruction of the crystalline habitrn.It was anticipated that topochemical reaction control could be transferred to fluid solutions if the reactive centres were held in a given mutual orientation within a rigid molecular framework.The [2.2]paracyclophane skeleton fulfills this requirement: with acryloyl substituents, the pseudo-gem compound 1 is reminiscent of the P-type stacking of trans-cinnamic acids in the crystal. Comparison of the photochemical properties of 1 with those of its isomers 2, 3 and 4 should bring new insights into bimolecular interactions between cinnamates.

One‐Pot Preparation of [n]Ladderanes by [2π + 2π] Photocycloaddition

“…The spectra of the pseudo‐ gem diesters 1 , 7 , 11 , and 15 in acetonitrile are represented in Figure 3. In line with preceding studies,2 the long wavelength absorption bands presumably result from a Davydov splitting between the polarized transitions of the two polyenic ester substituents which form an angle close to 0°;2 the two new transitions are then blue‐ and red‐shifted, respectively, whereby the shorter wavelength bands ( λ max. at 289, 311, 344 and 366 nm respectively, are more intense ( ϵ = 3.1 × 10 4 to 1.1 × 10 5 M –1 cm –1 ) than the longer wavelength absorption ( ϵ ca.…”

“…As developed in preceding publications,1,2 cinnamophane 1 was prepared from 4,5‐diformyl[2.2]paracyclophane, obtained by chromatographic separation from a mixture of the four regioisomeric bis‐formyl paracyclophanes (resulting from the Hopf synthesis of [2.2] para ‐cyclophanes) 16. To avoid the lengthy separation it was desirable to prepare the diene system 7 directly (see Scheme below) using the appropriate Wittig–Horner reaction shown in Scheme .…”

“…In preceding publications1,2 dealing with the photochemistry of cinnamophane 1 and its dimethoxyvinylogue 3 , it was shown that both compounds 1 and 3 undergo intramolecular [2π+2π] photocyclization in high chemicalyield (Scheme ). The first reaction was found to be stereo‐specific2 but the second led to a mixture of stereoisomers ( 4 ), probably because of the presence of the methoxy substituents 1…”

Photocapture of dynamic hydrogen‐bonded assemblies

“…This species varies from the previous examples, not only in the choice of photodimerizable unit, but also because the photodimerizable substituent is introduced on the barbituric acid moiety. A degree of distortion is induced in the barbituric acid unit due to the close approach of the fullerene cage to the barbituric acid carbonyl oxygens in the 4-and 6-positions, as suggested by the molecular modelling (PM3 calculations) and confirmed by 13 C-NMR spectra [27]. A similar molecular geometry was proposed for structurally related Nalkylated fullerene-barbiturates in separate reports by Hirsch and Diederich [28].…”

“…In the case of [2 + 2] photodimerization reactions, the formation of a rigid cyclobutane structure effectively "locks in" the geometry of the reagents, thus allowing the covalent capture of the dynamic supramolecular architecture. This general approach is summarised in Figure 1b, where the face-to-face (syn) geometry is anticipated to lead to the most efficient photodimerization reactions [13]. Provided the reaction in presence of template is faster, the equilibrium mixture will shift towards the templated products.…”