Structural control of the triplet-singlet equilibrium in cyclophane diarylcarbenes

Persistent Triplet Carbenes

“…D values decrease upon increasing n from 9 to 12. 64 Similar decreases in the D values are observed for a series of diarylcarbenes where two phenyl rings are bridged at the ortho position with various types of units (32-35). Anthronylidene (34) has a small D value, which indicates that one of the unpaired electrons is mainly delocalized in the σ-plane at the carbene center, as in the phenyl radical, while the other is delocalized in the π-system, as in the phenoxy radical.…”

Generation, Characterization, and Kinetics of Triplet Di[1,2,3,4,5,6,7,8-octahydro-1,4:5,8-di(ethano)anthryl]carbene

“…Preparation of Di[1,2,3,4,5,6,7,8-octahydro-1,4:5,8-di-(ethano)anthryl]diazomethane (1a). 9, 2,3,4,5,6,7,4:5,anthracene dihydrobromide (500 mg, 1.16 mmol), 36 prepared by bishydroxyamination of 1,2,3,4,5,6,7,8-octahydro-1,4:5,8-di(ethano)anthraquinone ( 7) 37 followed by treatment with SnCl2 in EtOH and HBr aqueous solution, was added to a solution of NaBr (5.0 g, 48.6 mmol) in 48% hydrobromic acid (50 mL) for 5 h. To the stirred suspension was added dropwise a solution of sodium nitrite (4.8 g, 69.6 mmol) in water (20 mL) at 0 °C over 30 min. After the solution was stirred at 0 °C for 3 h, activated Cu powder (0.22 g, 6.96 mmol) was added, followed by stirring at room temperature overnight.…”

“…This geometric factor tends to stabilize the carbene thermodynamically, since the wider the angle the more stable the triplet state becomes with respect to the singlet. [6][7][8] This strategy, however, encounters a limitation when alkyl groups are employed as protecting groups, since as a result of the highly reactive nature of carbenes, they can abstract even from very poor sources of electrons such as C-H bonds. For instance, di(2,4,6-trimethylphenyl)carbene was shown 9 to decay by undergoing dimerization and have a half-life of 160 ms, some 5 orders of magnitude longer than "parent" triplet diphenylcarbene (DPC).…”

“…At the same time such groups tend to increase the angle at the divalent center simply because of steric congestion. This geometric factor tends to stabilize the carbene thermodynamically, since the wider the angle the more stable the triplet state becomes with respect to the singlet. − …”

Theoretical Prediction of Triplet–Triplet Energy Transfer Rates in a Benzophenone–Fluorene–Naphthalene System

“…This tendency is again consistent with the experimental measurements. The explicit deviation in the 3 Bp–F may come from the delocalization of the carbonyl π* into the phenyl ring π* orbital in Bp which causes the decrease of the steric interaction of phenyl rings. − Another interesting property is that the carbonyl stretching frequency increases from 1260 cm –1 in the 3 Bp–F state to 1667 cm –1 in the Bp– 3 F state because of the different equilibrium bond lengths. These data agree with the experimental IR spectra data 1222 and 1665 cm –1 well. , The frequency change in the different spin state of oxygen has also been detected by Gorman and Darmanyan …”