Photoswitchable Dihydroazulene Macrocycles for Solar Energy Storage: The Effects of Ring Strain

Efficient
energy storage and release are two major challenges of
solar energy harvesting technologies. The development of molecular
solar thermal systems presents one approach to address these issues
by tuning the isomerization reactions of photo/thermoswitches. Here
we show that the incorporation of photoswitches into macrocyclic structures
is a particularly attractive solution for increasing the storage time.
We present the synthesis and properties of a series of macrocycles
incorporating two dihydroazulene (DHA) photoswitching subunits, bridged
by linkers of varying chain length. Independent of ring size, all
macrocycles exhibit stepwise, light-induced, ring-opening reactions
(DHA-DHA to DHA-VHF to VHF-VHF; VHF = vinylheptafulvene) with the
first DHA undergoing isomerization with a similar efficiency as the
uncyclized parent system while the second (DHA-VHF to VHF-VHF) is
significantly slower. The energy-releasing, VHF-to-DHA, ring closures
also occur in a stepwise manner and are systematically found to proceed
slower in the more strained (smaller) cycles, but in all cases with
a remarkably slow conversion of the second VHF to DHA. We managed
to increase the half-life of the second VHF-to-DHA conversion from
65 to 202 h at room temperature by simply decreasing the ring size.
A computational study reveals the smallest macrocycle to have the
most energetic VHF-VHF state and hence highest energy density.