3698-15-5Relevant academic research and scientific papers
Intramolecular triplet energy transfer in flexible molecules: Electronic, dynamic, and structural aspects
Wagner, Peter J.,Klan, Petr
, p. 9626 - 9635 (2007/10/03)
Exothermic intramolecular triplet energy transfer (TET) rate constants in various flexible bichromophoric systems D-(CH2)n-O-A (D = benzoyl, 4-methylbenzoyl; A = 2-naphthyl, 4-, 3-, 2-biphenyl; n = 3-14) have been determined from steady-state quenching and quantum yield measurements. The magnitude of the rate constants in molecules where n = 3 is comparable to those in molecules with a rigid spacer between chromophores, so that a through-bond mechanism is presumed to remain important. A very gradual drop in TET rate constants as the connecting polymethylene chain becomes longer indicates that through-space interactions compete and apparently provide the only mechanism responsible for transfer when n ≥ 5. Rate constants in long molecules (n = 11-14) remain remarkably high (～108 s-1) - lower than in those with four-atom tethers by only 1 order of magnitude. This effect is explained on the basis of rapid conformational equilibria always keeping a sufficient fraction of the molecules coiled so that the two chromophores are close enough to interact within 10 ns, the time required for the competing γ-hydrogen abstraction used to monitor triplet lifetimes. Energy transfer accounts for 40-75% of triplet decay for the longer molecules. This high efficiency indicates that only a small fraction involves static quenching in ground-state conformers with the two ends within 4 A. The majority must represent a combination of rate-determining bond rotations to such geometries and equilibrated conformations with their ends farther apart but still able to undergo energy transfer within 10 ns. Thus, the measured rate constants are, in fact, a weighted average of three different conformational mechanisms. The decrease in rate constant with tether length is not monotonic: a relative increase in rate for medium-chain-length molecules is explained by a larger number of favorable conformers and further, in biphenyl derivatives, by a rotation along the terminal O-C bond between the tether and the aromatic ring. As was expected, replacement of the polymethylene tether with poly(ethylene oxide) promotes better flexibility and thus higher transfer rates. Rate constants were found to be lower by a factor of ～2 when biphenyl rather than naphthyl is the acceptor, in agreement with earlier bimolecular measurements. With the 4-methylbenzoyl group (π,π* lowest triplet) as donor instead of benzoyl (n,π* lowest triplet), a small (～1.5x) but consistent rate increase occurred for all tether lengths.
Photolyses of (3-Naphthoxypropyl)-, (4-Naphthylbutyl)-, and (4-Naphthyl-4-oxobutyl)cobaloxime
Tada, Masaru,Hiratsuka, Mitsunori,Goto, Hiroyuki
, p. 4364 - 4370 (2007/10/02)
The cobalt-carbon bond of the titled compounds is photochemically cleaved to generate an organoradical and a cobaloxime(II) radical pair. 3-(1- or 2-naphtoxy)propyl, 4-(1- or 2-naphthyl)butyl, and 4-(1-or 2-napthyl)-4-oxobutyl radicals thus formed undergo three types of reactions: (a) hydrogen abstraction to give a saturated terminal, (b) hydrogen elimination to give a terminal olefin, and (c) substitution on the naphthalene ring.In benzene and radicals follow process b exclusively (the radicals from (3-(2-napthoxy)propyl)cobaloxime (1a), (3-(1-napthoxy)propyl)cobaloxime (2a), and (4-(1-napthyl)butyl)cobaloxime (2 b)) or preferentially (the radicals from (4-(2-napthyl)butyl)cobaloxime (1 b), (4-(2-napthyl)-4-oxobutyl)cobaloxime (1c), and 4-(1-napthyl)-4-oxobutyl)cobaloxime (2c)).In chloroform, process a becomes important to the extent as the sum of the other two processes.In water-acetonitril (4:1), process c becomes important and even takes precedence of others for the radicals from 1b and 1c.This feature is accounted for by the folding of the side chain of hydrophobic radicals.Encapsulation of the radicals in β-cyclodextrin stimulates process c except for the case of the radical from 2c.In the case of cobaloxime 2c, α-cyclodextrin does not effect the partition process of the intermediate radical.This feature is accounted for by the shallow inclusion of the radical due to the hydrogen bonding as depicted in Figure 1d.
