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

Abstract

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 λmax 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 λem 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.