Substituent and intramolecular hydrogen-bond effect on the fluorescent emission of two easy-synthesizable fused rigid bicyclic octadiene derivatives

Tetra-tert-butyl-cis,cis-3,7-dihydroxybicyclo[3.3.0]octa-2,6-diene-2,4-exo-6,8-exo-tetracarboxylate (I) and tetra-tert-butyl-cis,cis-3,7-dimethoxybicyclo[3.3.0]octa-2,6-diene-2,4-exo-6,8-exo-tetracarboxylate (II) were prepared in a three step synthetic procedure. These compounds show two central five carbon atom rings, fused in such a way to define a central bicyclo[3.3.0]octa-2,6-diene core. The carbon atoms which fuse both rings have sp³ hybridization, then they are not coplanar. A dihedral angle of about 63° corresponds to butterfly conformation. UV–Vis spectra of I and II measured in solution show symmetrical bands centred around 245 nm (ε ∼ 10⁴ M⁻¹cm⁻¹). These bands are consistent with π→π* transitions. TD-DFT simulated spectra over the DFT optimized in gas phase confirms this hypothesis, and additionally suggest a non-negligible contribution of n→π* transition for II. The slight dependence of λ_max on solvent polarity experimentally observed for II is consistent with some nπ* character. After excitation at 250 nm, a weak emission around 400 nm was detected for both compounds, with quantum yield values below detection limit for I. The value of λ_em of II was observed to be sensible to the solvent polarity, confirming some relevant n→π* character. The almost fully quenching of the emission of I in solution would be attributed to a rather strong intramolecular hydrogen bond established between the hydroxyl group and the oxo-oxygen atom from tert-butoxy group, which is observed in the crystal structure of the compound (O⋯O ranges from 2.635(2) Å to 2.672(2) Å). We hypothesize that it is probably preserved in solution due to the molecular rigidity, and would be the responsible for the quenching of the emission in solvent solution.