Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character

Abstract

An extremely long-lived localised singlet diradical with π-single bonding character is found in a macrocyclic structure that retards the radical–radical coupling reaction by the “stretch and solvent-dynamic effects”.

Direct Detection of Singlet Cyclopentane-1,3-diyl Diradicals By Infrared and Ultraviolet–Visible Spectroscopy at Cryogenic Temperature and Their Photoreactivity

“…This experimentally observed peak at 520 nm is also consistent with the reported value of 530 nm in EPA glass at 77 K . Notably, several other singlet cyclopentane-1,3-diyl diradical systems showed similar broad absorption peaks in the range of 500–650 nm. ,− ,,, …”

“…This experimentally observed peak at 520 nm is also consistent with the reported value of 530 nm in EPA glass at 77 K. 23 Notably, several other singlet cyclopentane-1,3diyl diradical systems showed similar broad absorption peaks in the range of 500−650 nm. 17,[24][25][26]28,30,31 Along with UV−vis spectroscopy, IR spectroscopy was also used to further characterize S-DR1-pl. An additional experiment Scheme 2.…”

“…This type of singlet diradical is experimentally challenging to directly detect due to shorter lifetime and higher reactivity than its triplet counterpart. Optimization of the right substituent, ,− the stretch effect, − and the N atom effect resulted in the increase in lifetime, which provided stabilization and characterization of the singlet state (Scheme ).…”

“…There are some examples of singlet cyclopentane-1,3-diyl diradical derivatives, which have been detected by transient electronic absorption, ,− steady-state UV–vis spectroscopy in an organic matrix at low temperature, ,,,,, and time-resolved IR (TRIR) spectroscopy. , The TRIR detections were achieved based only on the peaks of CN and CO functional groups present in the studied derivatives, therefore lacking information on (i) the fingerprint region of this system and most importantly (ii) the identification of the characteristic frequency that involves the two radical centers. Moreover, to the best of our knowledge, a singlet cyclopentane-1,3-diyl diradical system is yet to be directly detected by steady-state IR spectroscopy.…”

How Long Can A C–C σ-Single Bond Be?

“…There have been considerable efforts to design, synthesize, and isolate molecules with an extremely long (bond lengths ca. 2.3–2.4 Å) π-single bond between two sp 2 -hybridized carbon atoms, but the resultant π-single-bonded species are all short-lived intermediates that readily isomerize to more stable ring-closed products with an ordinary C–C σ-bond between two sp 3 -hybridized carbon atoms. − Making stable π-single bonds remains an immensely challenging task for synthetic organic chemistry.…”