Steric-shielding vs sigma-pi orbital interactions in triplet-triplet energy transfer

he influence of non-covalent σ–π orbital interactions on triplet–triplet energy transfer (TTET) through tuning of the donor excitation energy remains basically unexplored. In the present work, we have investigated intermolecular TTET using donor moieties covalently linked to a rigid cholesterol (Ch) scaffold. For this purpose, diaryl ketones of π,π* electronic configuration tethered to α- or β-Ch were prepared from tiaprofenic acid (TPA) and suprofen (SUP). The obtained systems TPA-α-Ch, TPA-β-Ch, SUP-α-Ch and SUP-β-Ch were submitted to photophysical studies (laser flash photolysis and phosphorescence), in order to delineate the influence of steric shielding and σ–π orbital interactions on the rate of TTET to a series of energy acceptors. As a matter of fact, fine tuning of the donor triplet energy significantly modifies the rate constants of TTET in the absence of diffusion control. The experimental results are rationalized by means of theoretical calculations using first principles methods based on DFT as well as molecular dynamics.Financial support from the Generalitat Valenciana (Prometeo Program), the Spanish Government (CTQ2010-19909, SEV-2012-0267 and FPU fellowship for F.P.) and the Carlos III Institute of Health (Grant RIRAAF, RETICS program and Miguel Servet Contract CP11/00154 for I. A.) is gratefully acknowledged. We thank ASIC-UPV for computing time. Dedicated to Prof. Diego Cortes on the occasion of his 60th birthday.Andreu Ros, MI.; Morera Bertomeu, IM.; Palumbo, F.; Sastre Navarro, GI.; Bosca Mayans, F.; Miranda Alonso, MÁ. (2015). Steric-shielding vs sigma-pi orbital interactions in triplet-triplet energy transfer. Chemical Science. 6(7):4035-4041. doi:10.1039/c5sc00823aS403540416