14 research outputs found
Unraveling factors leading to efficient norbornadiene-quadricyclane molecular solar-thermal energy storage systems
Developing norbornadiene-quadricyclane (NBD-QC) systems for molecular solar-thermal (MOST) energy storage is often a process of trial and error. By studying a series of norbornadienes (NBD-R-2) doubly substituted at the C7-position with R = H, Me, and iPr, we untangle the interrelated factors affecting MOST performance through a combination of experiment and theory. Increasing the steric bulk along the NBD-R-2 series gave higher quantum yields, slightly red-shifted absorptions, and longer thermal lifetimes of the energy-rich QC isomer. However, these advantages are counterbalanced by lower energy storage capacities, and overall R = Me appears most promising for short-term MOST applications. Computationally we find that it is the destabilization of the NBD isomer over the QC isomer with increasing steric bulk that is responsible for most of the observed trends and we can also predict the relative quantum yields by characterizing the S-1/S-0 conical intersections. The significantly increased thermal half-life of NBD-iPr(2) is caused by a higher activation entropy, highlighting a novel strategy to improve thermal half-lives of MOST compounds and other photo-switchable molecules without affecting their electronic properties. The potential of the NBD-R-2 compounds in devices is also explored, demonstrating a solar energy storage efficiency of up to 0.2%. Finally, we show how the insights gained in this study can be used to identify strategies to improve already existing NBD-QC systems
The Missing C<sub>1</sub>–C<sub>5</sub> Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes
The last missing example of the four
archetypical cycloaromatizations
of enediynes and enynes was discovered by combining a twisted alkene
excited state with a new self-terminating path for intramolecular
conversion of diradicals into closed-shell products. Photoexcitation
of aromatic enynes to a twisted alkene triplet state creates a unique
stereoelectronic situation, which is facilitated by the relief of
excited state antiaromaticity of the benzene ring. This enables the
usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig
closure. The 1,4-diradical product of the C<sub>1</sub>–C<sub>5</sub> cyclization undergoes internal H atom transfer that is coupled
with the fragmentation of an exocyclic C–C bond. This sequence
provides efficient access to benzofulvenes from enynes and expands
the utility of self-terminating aromatizing enyne cascades to photochemical
reactions. The key feature of this self-terminating reaction is that,
despite the involvement of radical species in the key cyclization
step, no external radical sources or quenchers are needed to provide
the products. In these cascades, both radical centers are formed transiently
and converted to the closed-shell products via intramolecular H-transfer
and C–C bond fragmentation. Furthermore, incorporating C–C
bond cleavage into the photochemical self-terminating cyclizations
of enynes opens a new way for the use of alkenes as alkyne equivalents
in organic synthesis