The photochemical coupling of various stilbenes (S) and chloranil (Q) is ef
fected by the specific charge-transfer (CT) activation of the precursor ele
ctron donor-acceptor (EDA) complex [S, Q], and the [2+2] cycloaddition is e
stablished by X-ray structure elucidation of the crystalline trans-oxetanes
formed selectively in high yields. Time-resolved (fs/ps) spectroscopy reve
als the (singlet) ion-radical pair (1)[S.+, Q(.-)] to be the primary reacti
on intermediate and thus unambiguously establishes for the first time the e
lectron-transfer pathway for this typical Paterno-Buchi transformation. The
alternative cycloaddition via the specific activation of the carbonyl comp
onent (as a commonly applied procedure in Paterno-Buchi couplings) leads to
the same oxetane regioisomers in identical molar ratios. As such, we concl
ude that a common electron-transfer mechanism applies via the quenching of
the photoactivated quinone acceptor by the stilbene donor to afford triplet
ion-radical pairs (3)[S.+, Q(.-)] which appear on the ns/mu s time scale.
The spin multiplicities of the critical ion-pair intermediate [S.+, Q(.-)]
in the two photoactivation methodologies determine the time scale of the re
action sequences (which are otherwise the same), and thus the efficiency of
the relatively slow ion-pair collapses (k(c) similar or equal to 10(8) s(-
1)) to the 1,4-biradical that ultimately leads to the oxetane product.