Modulation of lifetimes and diastereomeric discrimination in triplet-excited substituted butane-1,4-diones through intramolecular charge-transfer quenching

Triplet lifetimes have been determined for the diastereomers of a broad set of butane-l,4-dione derivatives (1-3). A remarkable dependence of lifetime s on conformational preferences is revealed in that the lifetimes are short er for the meso diastereomers of 1-3 than those for the racemic ones. The i ntramolecular beta-phenyl quenching is promoted in the case of meso diaster eomers by virtue of the gauche relationship between the excited carbonyl gr oup and the beta-aryl ring, while a distal arrangement in the lowest energy conformation (H-anti) in racemic diastereomers prevents such a deactivatio n. The involvement of charge transfer in the intramolecular beta-phenyl que nching is suggested by the correlation of the triplet lifetimes of the meso diastereomers of compounds 2 with the nature of the substituent on the bet a-phenyl rings. In the case of racemic diastereomers, beta-methoxy substitu tion on the beta-phenyl ring (2-OCH3, 3-OCH3) also led to a decrease of the triplet lifetimes when compared to those of the nonsubstituted compounds ( 2-H, 3-H). This shortening is accounted for by the deactivation of a small proportion of the excited molecules through beta-phenyl quenching. In addit ion to the above factors, the lifetimes in the case of meso diastereomers c an further be controlled by increasing the energy spacing between the T-1 a nd T-2 states, since beta-phenyl quenching occurs from the latter for compo unds 2 and 3. Through a rational conformational control, a surprisingly lon g triplet lifetime (300 ns) has been measured for the first time for a pure ly n,pi* triplet-excited beta-phenylpropiophenone dimer (1-rac).