Intramolecular Triplet Energy Transfer in Flexible Molecules:  Electronic, Dynamic, and Structural Aspects

Abstract

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 Å. 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 (∼1.5×) but consistent rate increase occurred for all tether lengths.

Exploiting Long-Lived Molecular Fluorescence

“…However, quenching by electron transfer, which presents an alternative quenching mechanism, could operate through bond (superexchange mechanism); it could also occur over a considerable distance through space or through the solvent (up to 5-8 Å) and must therefore be excluded [40]. The same applies for Dexter-type triplet energy transfer, which has been employed in other intramolecular probe/quencher pairs to assess end-to-end contact formation [26] [41] [42]. Quenching over larger distances than van-der-Waals contact would result in a continuum of distance-dependent rate constants, which could not be analyzed in terms of a diffusion-controlled collision process.…”

Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

“…Bimolecular sensitization is achieved through diffusive encounters. When the donor is covalently bound to the acceptor molecule, intramolecular energy transfer enables a more specific control of the transfer; 419 in such a case, only “equimolar” amounts of a sensitizer are needed. This strategy, benefiting from an increased “uncaging cross section” ( Φ rel ε(λ irr ); see ref (22) for discussion) of release using triplet sensitization, has already been demonstrated on several examples.…”

Study of a Two-Stage Photobase Generator for Photolithography in Microelectronics

“…10 The values of E T for the benzoyl and naphthyl groups are ∼73 and 62 kcal mol −1 , respectively. Therefore, intramolecular triplet−triplet energy transfer is strongly exothermic.…”

“…The highest energy transfer rate constant has been observed for the shortest linker (n = 3; k ET = 1.6 × 10 9 s −1 ). 10 This rate constant serves us as an estimate of the triplet energy transfer rate constant from the AP moiety to the CH moiety in 1. Since energy transfer occurs by the exchange mechanism for both S 1 and T 1 , we can assume k ET ∼ 2 × 10 9 s −1 is the rate constant for energy transfer from either S 1 or T 1 of AP to CH.…”