Aj. Parola et al., PHOTOINDUCED ELECTRON-TRANSFER AND ENERGY-TRANSFER PROCESSES OF BIACETYL IMPRISONED IN A HEMICARCERAND, Journal of the American Chemical Society, 118(46), 1996, pp. 11610-11616
The energy- and electron-transfer quenching processes of the lowest tr
iplet excited state of biacetyl (2,3-butanedione) imprisoned in a hemi
carcerand have been systematically investigated in CH2Cl2 solution at
room temperature. Twenty potential quenchers have been used, including
ten triplet energy accepters (mostly, aromatic hydrocarbons) and nine
electron donors (mostly, aromatic amines). Bimolecular rate constants
for the quenching processes were obtained by Stern-Volmer analysis an
d compared with those found for the quenching of free biacetyl. In the
electron-transfer processes, aromatic amines with oxidation potential
from +0.015 V (N,N,N',N'-tetramethyl-p-phenylenediamine) to +0.83 V (
diphenylamine) quench free biacetyl at the diffusion-controlled limit,
whereas for imprisoned biacetyl the rate constant decreases (roughly
in a linear manner) from 4.0 x 10(8) to 1.2 x 10(5) M(-1) s(-1) As far
as energy-transfer is concerned, the rate constant for the quenching
of free biacetyl increases with decreasing Delta G degrees and reaches
the diffusion-controlled plateau value (k(q) similar to 10(10) M(-1)
s(-1)) for Delta G degrees similar to 0.1 eV, whereas for imprisoned b
iacetyl a scattered, bell-shaped log k(q) vs Delta G degrees plot is o
btained, with a maximum value (similar to 10(6) M(-1) s(-1)) much belo
w the diffusion-controlled limit. The results obtained show that the w
alls of the hemicarcerand allow only very weak electronic interaction
between incarcerated triplet biacetyl and external quenchers. A brief
discussion of the results obtained in the light of current energy- and
electron-transfer theories is presented.