To compare the properties of the metallosurfactants upon
aggregation, the photophysical characteristics of the complexes
9 – 13 were investigated in air-equilibratedn-hexane and ethanol
conditions. Both absorption and emission spectra of all the
complexes are closely related to the ones of [Ru(bpy) 3 ]^2 þ.
However, contrary to the reference complex, a pronounced and
unexpected bathochromic shift was detected in the^1 MLCT
absorption band inn-hexane when compared to ethanol. A simi-
lar and more evident behavior was observed for the emission
spectra indicating that in hexane, the ruthenium moieties expe-
rience a more polar environment.
These results suggest the formation of aggregates in the lower
polarity solvent, being most likely reversed micelles or reversed
vesicles. The cationic heads of the ruthenium complexes being
in the core of the aggregates are organized close to each other
feeling a higher polarity environment despite the low polarity
of the solvent.
The time-resolved analysis of the excited state responsible for
such photoluminescence gave further evidence of the formation
of aggregates inn-hexane, in particular, when compared to etha-
nol. Figure 6 shows the decays of the complex 12 in air-
equilibrated solutions, as monomeric species and in the aggre-
gate form. The long component was assigned to the monomeric
nonaggregated ruthenium amphiphilic molecules, while the short
component would arise from the aggregated and closely-packed
molecules. The quenching effect is due to the triplet–triplet
annihilation, which strongly reduces the luminescence excited-
state population of the complexes. To support such explanation,
0
110010Counts1000200 400 600 800
channels9 R = C 11 H 23 1000 1200 1400 1600 1800 2000RRN
NNR2CI−RRRu
NN
N
N 2 CI−12 R = (C 15 H 31 ) 2 CH
C 16 H 33 OC 16 H 33 O
O13 R = C 16 H 33 ON
NR 2+ 2+NN
N
Ru10 R = C 15 H 31
11 R = i-C 7 H 14FIG. 6. Chemical structure of the investigated metallosurfactants with
two (left) and one (middle) disubstituted bipyridines. Right: room temper-
ature time-resolved emission decays of complex 12 inn-hexane (solid line,
—) and upon addition of one drop of ethanol (dotted line, ---). Emission
was monitored at 644 and 623 nm, respectively.
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 63