namely,Ru-CandOs-G, respectively. In polar nonprotic solvent,
that is, CH 2 Cl 2 , these compounds are able to associate in pairs
through mutually complementary triple hydrogen bonds (C G
nucleotide bases) yielding the couple Ru-COs-G. Upon light
excitation, a photoinduced energy transfer process from the
excited ruthenium moiety to the osmium unit is observed.
We have shown that hydrogen bonds can be successfully
employed to decorate the periphery of dendrimers with emitting
Re(I) complexes( 48 ). The dendrimers were used as multivalent
hosts for the barbiturate guests Barbital and for the [Re(Br)
(CO) 3 (barbi-bpy)] (barbi-bpy¼5-[4-(4^0 -methyl)-2,2^0 -bipyridyl]
methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione) (see Fig. 5). The sta-
ble adducts formed between the dendritic architectures (the hosts)
and the barbiturate guests were investigated, and energy transfer
has been shown from the excited dendrimer to the Re complexes.
Further, electron transfer could be investigated in a system
based on the same interaction, in which the electron acceptor,
methyl viologen, was bound to the Re(I) complex, acting as an
excited-state electron donor( 49 ).
R R
RCOCO
ClReOCN NRRRRR
R RRRREThnhn¢RRRHN
HNHN
HN
HNHNHN HNHNHNHN
HNNHNHNHNH
NHNH
NHNHNHNHNHNHNHNHNHHNHN
HNHNHN HNHNHN HN N NH HN
HNH NH
NH NH NHNHHNNH NOO O O O O OO OOO O
O
OOO O
OOO
O
OOO
OOOOOO
OO O
O
O
OOOOOOOO
O
O
O
ONNN
NNN N
N
NN N
N NNNNNNNN
N
N
HH
OOOONHNFIG. 5. Schematic formula of the dendrimer and of the Re(I) guest
which can complex via hydrogen bonds, at its periphery. Upon irradia-
tion atl¼330 nm, where the dendritic structure is almost selectively
excited due to the different absorption profile with respect to the
rhenium complex, an efficient energy transfer can occur from the
dendrimer to the organometallic moiety.
58 CRISTIAN A. STRASSERTet al.