C. Poriel et al.
the same range than the lifetimes of 1c–g (0.90–1.30 ns),
and the third one A3 is very short (<75 ps). This situation is
actually a strong indication of the kinetics of excimer forma-
tion. In addition, the fluorescence decays of 2c–g are wave-
length-dependent as exemplified in Figure 8, bottom for 2c.
From the full set of decay curves recorded at several emis-
sion wavelengths, a general trend can be drawn (see Fig-
ure S21): i) for each given compound, the three time-con-
stants were found identical whatever the emission wave-
length, ii) the relative contribution of the first time-constant
(A1, slow contribution) increases as the emission wavelength
increases, whereas the relative contribution of the third
time-constant (A3, short contribution) decreases as the emis-
sion wavelength increases, and iii) the relative contribution
of the second time-constant (A2, intermediate contribution)
remains almost constant whatever the emission wavelength.
Then, the slower time-constant t1, mostly present at higher
emission wavelength, is representative of the excimer emis-
sion of the “aryl-fluorene-aryl” species. Most of the intensity
fraction is actually contained in this slow excimer contribu-
tion, as shown by the f1 values, greater than 0.85 except for
2g where f1 is in the range 0.49–0.76. The short time-con-
stant t3 represents the deactivation of the “aryl-fluorene-
aryl” monomers, forming the intramolecular excimers in a
very short time after excitation and leading to a very limited
intensity fraction (f3 <0.04 in all cases). One can emphasize
that no rise time were observed experimentally for 2c–g.
Indeed, at the emission wavelength of the excimers, one
could expect a short rise time,[56,57] corresponding to the ki-
netics of the intramolecular excimer formation process from
one excited “aryl-fluorene-aryl” moiety and its neighbor. In
the present case, as shown by the very short decay time t3
corresponding to the decay of monomers forming excimers,
this rise time takes probably place in a few picoseconds or
tens of picoseconds at most. Such a rise time would repre-
sent a very limited intensity fraction,[56] and may be hidden
by the fast decay time t3 itself, that is nevertheless almost
beyond the time-resolution of our instrument (~10 ps).
Indeed, several examples in the literature show that the rise
time corresponding to fluorene excimer formation is not
always observed.[57,58] The very fast process of intramolecu-
lar excimer formation strongly suggests that the “aryl-fluo-
rene-aryl” chromophores in 2c–g are located closely, in a fa-
vorable conformation, and require only a very slight spatial
reorganization to form excimers. This close vicinity between
the two “aryl-fluorene-aryl” chromophores in the ground
very specific conformation, due to the relative flexibility of
the molecules, such that the conformational reorganization
to form excimers is too large to take place within the life-
time of the monomer.
Interestingly, derivatives 2c, 2d, and 2 f display very simi-
lar behaviors: excimer emission wavelength is located be-
tween 450 and 457 nm, the slower decay time t1 is in a
narrow range 11.16–12.99 ns, corresponding to an intensity
fraction greater than 0.87. For 2c, 2d and 2 f, excimer for-
mation represents the predominant process leading to a
well-defined red-shifted emission.
In the case of 2e, the slower time-constant t1 is also pre-
dominant (f1 >0.93), showing that despite the existence of
monomer, most of the emission intensity is due to excimer
species. However, the emission wavelength of 2e is shorter
(lem =413 nm, see Table 2), and its main time-constant t1 is
also shorter (t1 =7.56 ns, see Table 3) than that of its conge-
ners 2c–d and 2 f–g. In the specific situation of 2e, the
nature of the excimers could be different than for the other
derivatives. Indeed, due to the steric hindrance of the tBu-
aryl substituents, the p-stacking mode between the two con-
jugated “aryl-fluorene-aryl” moieties would be less efficient,
leading to a less-stabilized excimer geometry. Such a sche-
matic interpretation would explain that the excimer emis-
sion is less red-shifted, compared to 2c–d and 2 f–g (see Sec-
tion on Theoretical modeling below).
Compared to 2c–f derivatives, 2g shows a unique excimer
feature (Table 3, Figure 7, bottom). Its slower decay time t1
(10.75 ns) is in the same range than 2c–d and 2 f, but its rel-
ative contribution is drastically reduced (A1 =0.05–0.17).
This observation applies also for the shorter time-constant
t3, for which A3 <0.37, whereas its relative contribution is
noticeably larger than 0.37 in all other derivatives 2c–f. This
tendency is accompanied by a stronger contribution of the
intermediate time-constant t2 (A2 >0.58, f2 =0.24–0.51)
compared to 2c–f. One can easily conclude that the emission
of 2g is composed of two overlapped contributions: the blue
part of the emission spectrum arises mainly from monomers
that are unable to form excimers (larger contribution of the
intermediate time-constant in the decay curves, structured
“monomer-like” emission shoulder in the emission spectrum
below 410 nm), whereas the red part of the emission spec-
trum results from the excimer emission (larger contribution
of the slower time-constant, structureless “excimer-like”
emission in the spectrum at l >410 nm). Then, 2g repre-
sents a borderline situation where both monomers (that
cannot form excimers) and excimers can emit. The two
bulky tBu substituents borne by the phenyl rings of 2g are
surely responsible for this effect. Indeed, the steric hin-
drance is quite large, inhibits the spatial approach between
two neighboring “aryl-fluorene-aryl” moieties, and allows
monomer emission to occur. Nevertheless, due to large de-
grees of freedom of this “aryl-fluorene-aryl” molecular
structure, some very specific conformations may probably
lead to excimer formation.
1
state in the 2c–g series was already highlighted by H NMR
spectroscopy and electrochemical studies (see above).
The second decay time of 2c–g (t2 =0.90–1.30 ns) corre-
sponds quite closely to the lifetimes of the 1c–g derivatives
(1.09–1.34 ns). Its relative contribution to the decay (A2)
stays almost constant over the emission wavelength, but its
intensity fraction (f2) decreases significantly as the emission
wavelength increases. Then, it is reasonable to conclude that
this intermediate decay represents some very small propor-
tion of emitting monomers which cannot form excimers. It
could originate from “aryl-fluorene-aryl” chromophores in a
Geometry optimization of 2d and 2e in the first singlet
excited state indicates a relevant conformational change
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Chem. Eur. J. 2011, 17, 10272 – 10287