charge transfer.6-8 Giant σ2 values can be obtained with
DAAD or ADDA systems having strong D and A terminal
and median groups but unfortunately often at the drastic
expense of fluorescence quantum yield. On the basis of these
observations, we have designed novel push-push fluorophores
consisting of nanometric systems in which two electron-
donating end groups are connected to a conjugated core via
rigid and/or semirigid elongated conjugated rods (Figure 1).
Scheme 1a
a Reagents and conditions: (a) 2-methyl-3-butyn-2-ol, Pd(PPh3)2Cl2,
CuI, Et3N, 40 °C, 12 h (82%); (b) NaOH, toluene/i-PrOH, reflux,
1 h (87%); (c) 1,4-diiodobenzene (3 equiv), Pd(PPh3)2Cl2, CuI,
toluene/Et3N, 30 °C, 6 h (71% of 2a); (d) 4-bromobenzaldehyde
(1.2 equiv), Pd(PPh3)2Cl2, CuI, toluene/Et3N, 40 °C, 15 h (74% of
2b); (e) 4-iodobenzaldehyde, t-BuOK, CH2Cl2, 20 °C, 5 h, then I2
cat., hν, 2 h (85%).
means of a Wittig condensation between phosphonium salt
3 and 4-iodobenzaldehyde, followed by catalytic isomeriza-
tion. Double Sonogashira coupling of 4,4′-diethynylbiphenyl
(5) with 1a, 2a, and 4 afforded fluorophores 6a, 6b, and 7,
respectively, whereas 9 was prepared by a double Wittig-
Horner-Emmons condensation of bisphophonate 8 with
aldehyde 2b (Scheme 2).
Figure 1. Molecular engineering of quadrupolar fluorophores
designed for TPEF.
The synthesis of the fluorene core 10d was achieved from
9,9-dinonylfluorene (10a) in a three-step sequence (diiodi-
nation, Pd(II)-catalyzed cross-coupling reaction with 2-meth-
yl-3-butyn-2-ol and base-promoted deprotection). Push-push
molecules 11 and 12 were finally obtained by means of
double Sonogashira coupling with 2a and 4, respectively.
These derivatives show spectacularly higher solubility than
their biphenyl analogues 6b and 7. All new fluorophores (6,
7, 9, 11, 12) have been fully characterized by NMR, HRMS,
and elemental analysis data.
We selected biphenyl or fluorene central units, which allow
the tuning of the electronic delocalization along the conju-
gated backbone by modulation of the twist angle between
the two sections of the molecules. Conjugated rods built from
phenylene-ethynylene and/or phenylene-vinylene oligo-
mers were investigated in order to preserve fluorescence and
modulate the electronic communication between the end and
the center of the molecules. One of our goals was to ascertain
the appropriate combination of core and rods moieties for
optimized luminescence and/or TPA properties. Long alkyl
chains could be added on the end groups and on the central
block in order to obtain highly soluble derivatives.
End groups, linkers, and cores were assembled by means
of Sonogashira, Wittig, and Wittig-Horner-Emmons reac-
tions. The electron-donating moiety 1a bearing an iodo group
was converted to the ethynyl derivative 1c in a two-step
cross-coupling/deprotection sequence (Scheme 1). Extended
rigid building blocks (tolanes 2a and 2b) were prepared by
palladium(II)-catalyzed reaction of 1c with 1,4-diiodobenzene
and 4-bromobenzaldehyde, respectively. In parallel, the
semirigid stilbene building block 4 was readily obtained by
As shown in Figure 2a, the novel fluorophores exhibit an
intense absorption band in the UV blue region and show
high transparency in the remaining range of the visible
region. In addition, all molecules exhibit high fluorescence
quantum yields, ranging between 0.50 and 0.87 (Table 1).9
Changing the biphenyl core to a fluorene one results in a
slight red-shift of the absorption band (Figure 2a), indicative
of improved electronic conjugation, but in nearly no shift of
the emission band (Figure 2b). In contrast, modulation of
the linkers allows the spectral tuning of both the absorption
and emission characteristics, as noted from Figure 2 and
Table 1. Increasing the conjugated rods’ length induces a
bathochromic and hyperchromic shift of both the absorption
and emission bands, in agreement with an extended electronic
conjugation. A similar effect is obtained when replacing a
triple bond with a double bond in the conjugated linkers.
(6) Albota, M.; Beljonne, D.; Bre´das, J.-L.; Ehrlich, J. E.; Fu, J.-Y.;
Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-
Maughon, D.; Perry, J. W.; Ro¨ckel, H.; Rumi, M.; Subramaniam, G.; Webb,
W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653-1656.
(7) Rumi, M.; Ehrlich, J. E.; Heikal, A. A.; Perry, J. W.; Barlow, S.;
Hu, Z.-Y.; McCord-Maughon, D.; Parker, T. C.; Ro¨ckel, H.; Thayumanavan,
S.; Marder, S. R.; Beljonne, D.; Bre´das, J.-L. J. Am. Chem. Soc. 2000,
122, 9500-9510.
Lengthening of the conjugated rods based on phenylene-
ethynylene oligomers does not decrease the high fluorescence
(8) (a) Ventelon, L.; Moreaux, L.; Mertz, J.; Blanchard-Desce, M. Chem.
Commun. 1999, 2055-2056. (b) Ventelon, L.; Charier, S.; Moreaux, L.;
Mertz, J.; Blanchard-Desce, M. Angew. Chem. 2001, 113, 2156-2159;
Angew. Chem., Int. Ed. 2001, 40, 2098-2101.
(9) Comparative experiments conducted in degassed, nondegassed, and
aerated solutions of fluorophores in toluene revealed that the presence of
oxygen has no effect on the fluorescence properties.
720
Org. Lett., Vol. 4, No. 5, 2002
Mongin, Olivier
