The photophysics of the 2,6 dialkoxy anthracenes: Evidence for excited state side-chain conformational relaxation

Three 2,6 dialkoxyanthracenes with varying ether sidechain lengths (methyl, n-propyl and n-hexyl) are synthesized and characterized. When compared to unsubstituted anthracene, the spectroscopic properties of the alkoxy-anthracenes are significantly different. The oscillator strength appears to be evenly distributed between the La and Lb states, rather than concentrated in the low-energy La state, and the transition dipole moments are rotated by ∼30° in the molecular frame. More importantly, all three derivatives undergo an intramolecular conformational change in the excited state that gives rise to a reshaping of the fluorescence spectrum on the nanosecond timescale. This process has an activation energy of 8±1 kJ/mol. By ∼150 K the fluorescence spectrum reflects only the high energy conformer, whose emission lineshape is similar to that of the unsubstituted anthracene. The temperature-dependent changes in the monomeric fluorescence spectra of these compounds will have to be taken into account in future studies of their solid-state spectroscopy.