A series of Ru(II)-Rh(III) dyads of general formula (ttpy)Ru-tpy-(ph)(
n)-tpy-Rh(ttpy)(5+) with n = 0, 1, 2 [ttpy = 4'-p-tolyl-2,2':6,2 ''-te
rpyridine; tpy-(ph)(n)-tpy = bridging ligand where two 2,2':6',2 ''-te
rpyridine units are connected at the 4'-position through a variable nu
mber of p-phenylene spacers] have been studied, in acetonitrile at roo
m temperature, by picosecond and nanosecond time-resolved emission spe
ctroscopy. When n = 1, excitation of the Ru(II)-based molecular compon
ent is followed by efficient intramolecular quenching by electron tran
sfer to the Rh(III) center. The rate constant, k greater than or equal
to 3 x 10(9) s(-1), is high despite the relatively small driving forc
e of the process (ca. 0.1 eV). When n = 2, with the same driving force
as above, no intramolecular electron transfer quenching is observed (
upper limit for the rate constant of the electron transfer process: k
< 5 x 10(8) s(-1)). The decrease in electron transfer rate obtained in
going from n = 1 to n = 2 is in line with the behavior of other syste
ms containing poly-p-phenylene spacers. The dyad with rt = 0 is defini
tely not homogeneous with the other two: the intercomponent electronic
coupling is much stronger, the Ru(II)-based excited state is lower in
energy, and the electron transfer has a smaller driving force (Delta
G approximate to 0). The lifetime of the Ru(lI)-based emission is 17 n
s. The lack of an obvious model compound makes it difficult to draw fi
rm conclusions for this system.