transfer to the PhenH+ moiety. The energy transfer efficiency is
difficult to estimate, since some residual 3PV fluorescence
( < 0.5%, relative to the initial value) is present, even when a
large excess of acid is added. Such residual emission, likely
arising from unprotonated 3PV-Phen in the acid–base equilib-
rium, overlaps the much weaker PhenH+ emission, thus making
difficult clean excitation spectroscopy. In any case one can
conclude that, in an acid environment, the OPV luminescence of
3PV-Phen is off due to an intercomponent quenching process
displaying a 3PV ? Phen direction [Fig. 1].
In terms of electronic energy levels our bipartite system has
been designed in order to achieve the following: in one
component (3PV) the energy of the fluorescent level (E1) is
insensitive to protons; by contrast, in the other component
(Phen), a proton input tunes the fluorescent levels between two
energy values (E2 and E3). The key feature is that the energy of
E1 is intermediate between that of E2 and E3, thus enabling an
energy transfer in the desired direction via the chemical (proton)
input. The energy scaling (E2 > E1 > E3) is reflected in the
spectral position of the corresponding fluorescence bands
[Fig. 2(a), inset].
In conclusion, we have shown a simple way to reversibly
switch on and off the widely exploited fluorescence of OPVs.
Also, it is worth pointing out that this is one of the rare cases of
multicomponent systems where the direction of the photo-
induced energy transfer can be controlled.11–12 To the best of
our knowledge, this is the first example where such control can
be reversibly accomplished by chemical inputs.
This work was supported by the Italian CNR and the French
CNRS. We thank L. Oswald for technical help and Professor
J.-F. Nicoud for his interest and support.
Fig. 2 (a) Absorption and (inset) fluorescence spectra of 3PV (……),
Phen (––––), and PhenH+ (– – –). (b) Absorption and (inset) fluorescence
(lexc = 314 nm, isosbestic point) spectra of a 1.5 3 1025 M CH2Cl2
solution of 3PV-Phen containing 0, 1, 2, 4 or 20 equivalents of
trifluoroacetic acid. The weak red-shifted band is obtained for the solution
containing 20 equiv., after subtraction of the residual 3PV luminescence
(see text).
Notes and references
† Fluorescence spectra, lifetimes (time resolution 0.5 ns), and quantum
yields were obtained as described in detail in ref. 7; experimental
uncertainties are ±2 nm, ±8% and ±20%, respectively. For emission
quantum yields anthracene in cyclohexane (F = 0.34) and quinine sulfate
in 0.05 M H2SO4 (F = 0.546) were used as standards.
‡ In this case the decay is biexponential and 12.6 ns refers to the longest
component. A shorter component (1.3 ns) accounts for the residual 3PV
fluorescence (see text).
570 nm, with t = 12.2 ns and Ffl = 0.048; the luminescence
color switches from purple (Phen) to yellow (PhenH+). Addition
of an organic base [diazabicyclo[4.3.0]non-5-ene (DBN)]
restores the initial absorption and luminescence properties.
Importantly, no changes of such properties are observed upon
addition of a large excess of TFA or DBN to 3PV solutions.
The absorption spectrum of 3PV-Phen matches the sum of
the spectra of the reference compounds within ±10% error,
showing one diagnostic band for each component unit with
maxima at 286 (Phen) and 360 nm (3PV). Only the typical
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fluorescence band of the 3PV fragment is observed (lmax
=
460 nm, t = 1.3 ns, Ffl = 0.70); the excitation spectrum (lem
= 460 nm) matches the absorption profile, indicating that
excitation of the Phen moiety is followed by quantitative energy
transfer to the 3PV unit. One can thus conclude that, in 3PV-
Phen, only the OPV luminescence is on since a photoinduced
quenching process featuring a Phen ? 3PV direction is active
(Fig. 1).
Addition of increasing amounts of (TFA) to a 1.5 3 1025
M
solution of 3PV-Phen causes dramatic changes in the absorption
spectrum [Fig. 2(b)]. A clear analogy with the protonation
reaction of Phen is found: the same amount of acid is required
to complete the reaction, isosbestic points are located at 296,
314 and 359 nm. Importantly, the final spectrum matches the
sum of the spectra of 3PV and Phen.H+ within ±10% error, and
the reaction is reversible upon addition of DBN. These findings
suggest that the 3PV-PhenH+ species is formed.
The addition of acid also leads to dramatic changes in the
emission properties (lexc = 359 nm, isosbestic point): the
characteristic blue 3PV luminescence is progressively sup-
pressed and a much weaker green-yellow emission (lmax
=
556 nm, t = 12.7 ns‡) is detected, attributable to the PhenH+
moiety (see above). The luminescence quenching of 3PV,
observed also in a rigid matrix at 77 K, is attributable to energy
12 S. Serroni, S. Campagna, R. Pistone Nascone, G. S. Hanan, G. J. E.
Davidson and J.-M. Lehn, Chem. Eur. J., 1999, 5, 3523.
2106
Chem. Commun., 2000, 2105–2106