Evidence of the torsion of a polyene chain in a strongly hindered molecular environment: The ttbP4 crystal

Abstract

In this work we provide experimental and theoretical evidence that in a molecular environment as restricted as the crystalline phase, the all-trans ttbP4 (1,1,8,8-tetrakis(tert-butyl)octa-1,3,5,7-tetraene) can undergo a conformational change, by rotating around the second single bond, when it is electronically excited to its 11Bu state. Undoubtedly, this is an interesting step to clarify the viability of the torsional mechanism of retinal pigments in the cavity of bacteriorhodopsin as proposed in the vision mechanism. We show that the fluorescence emission of ttbP4 in the crystalline phase is the combination of two bands corresponding to the emission from two different ttbP4 conformers. Theoretical simulations of absorption and emission spectra allowed us to identify the two conformers as: i) the most stable all-trans ttbP4, giving rise to a structured fluorescence band, and ii) the conformer generated by the torsion around the second single bond of ttbP4 polyene chain, giving rise to an emission band with hardly any structure. Interestingly, experimental data also show that the rotated structure, after deactivating to the ground state, takes a time to return to the all-trans configuration