A
t-bu
tylphenylni
troxide (BPNO
tities/bull.gif">) s
table radical is a
ttached
to an elec
tron donor-bridge-accep
tor (D-B-A) sys
tem having well-defined dis
tances be
tween
the componen
ts: MeOAn-6ANI-Ph(BPNO
tities/bull.gif">)-NI, whereMeOAn =
p-me
thoxyaniline, 6ANI = 4-(
N-piperidinyl)naph
thalene-1,8-dicarboximide, Ph = phenyl, andNI = naph
thalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANI, BPNO
tities/bull.gif">, and NI are a
ttached
to
the 1, 3, and5 posi
tions of
the Ph bridge, respec
tively. Time-resolved op
tical and EPR spec
troscopy show
tha
t BPNO
tities/bull.gif">influences
the spin dynamics of
the pho
togenera
ted
triradical s
ta
tes
2,4(MeOAn
+tities/bull.gif">-6ANI-Ph(BPNO
tities/bull.gif">)-NI
-tities/bull.gif">),resul
ting in slower charge recombina
tion wi
thin
the
triradical, as compared
to
the corresponding biradicallacking BPNO
tities/bull.gif">. The observed spin-spin exchange in
terac
tion be
tween
the pho
togenera
ted radicals MeOAn
+tities/bull.gif">and NI
-tities/bull.gif"> is no
t al
tered by
the presence of BPNO
tities/bull.gif">. However,
the increased spin densi
ty on
the bridge grea
tlyincreases radical pair (RP) in
tersys
tem crossing from
the pho
togenera
ted single
t RP
to
the
triple
t RP. Rapidforma
tion of
the
triple
t RP makes i
t possible
to observe a biexponen
tial decay of
the
to
tal RP popula
tion wi
thcomponen
ts of
tau.gif" BORDER=0 > = 740 ps (0.75) and 104 ns (0.25). Kine
tic modeling shows
tha
t the fas
ter decay ra
te is due
to rapid es
tablishmen
t of an equilibrium be
tween
the
triple
t RP and
the neu
tral
triple
t s
ta
te resul
ting fromcharge recombina
tion, whereas
the slower ra
te moni
tors recombina
tion of
the single
t RP
to ground s
ta
te.