Non-conjugated bichromophoric systems. Part 4. Synthesis and photochemical study of bis-9-anthryls with a four-membered chain; influence of the replacement of methylene links by oxygen atoms or dimethylsilyl groups on the formation of intramolecular excimers and photocyclomers

Abstract

Bichromophoric systems (I_CC, I_OO, I_O1, I_O2, and I_Sic) incorporating various four-membered links were prepared in order to study the influence of the nature of the linkages upon the photophysical and photochemical properties; these systems are bisanthracenes A–Z–A where A = 9-anthryl and Z =(CH₂)₄(I_CC), O(CH₂)₂O (I_OO), O(CH₂)₃(I_O1), CH₂O(CH₂)₂(I_O2), and SiMe₂(CH₂)₂SiMe₂(I_SiC). By irradiation at high dilution, each of these compounds yields a single photocyclomer P_XX linked through the 9,10 positions of one ring and the 9′,10′ positions of the other, except for I_Sic where the bonds were formed between the 9,10 and 1′,4′ positions. The reaction quantum yields were measured at 365 nm in methylcyclohexane. The fluorescence was examined in the same solvent under stationary conditions at different temperatures and by transient kinetic analysis (single-photon counting). Three of these compounds (I_CC, I_OO, I_O1) did not display any intramolecular excimer fluorescence; a thermodynamic study indicates that the non-radiative decay of the S₁ excited state leads essentially (φ_i.s.c. being very small) to the photoproduct P_XX and the starting material I_XXvia a minimum of the potential surface (pericyclic minimum) with respective partition coefficients α and (1 –α): α= 0.13 (I_CC), 0.28 (I_OO), 0.27 (I_O1). This shows the influence of the replacement of a CH₂ group by oxygen. The other two derivatives (I_O2 and I_SiC) exhibit locally excited and excimer fluorescence, the latter being suggestive of large overlap between the aromatic rings. The excimers were postulated to be on the reaction pathway; the first collapses readily to the photocyclomer P_O2–(φ_R 0.28) whereas the second, due to steric strain, leads to P_SiC with a very low yield (φ_R 8 × 10^–4;k_RDca. 10⁵ s^–1). It was observed that the introduction of an oxygen atom instead of a CH₂ group enhances the cycloaddition efficiency but the second oxygen did not show any further improvement. The presence of the bulky SiMe₂ groups helped the excimer state to be reached (rate of formation at room temperature k_DMca. 10⁹ s^–1) but strongly hindered the formation of the photocyclomer (k_RDca. 10⁵ s^–7). The last result clearly shows that photocyclomerization depends first on the intra-chain interactions controlling the rate of formation of an overlapping conformation and then on possibly more severe inter-linkage and chain-ring (with the so-called ‘peri’ hydrogens) repulsions, which can be a powerful brake in the last step of the build up of cycloadduct.