but the interactions between the chromophores can also be governed by changing
the structure of the branches or of the cores. Another essential advantage of the
dendrimer-based design of multichromophores is the complete suppression of
aggregation between the dye molecules. Here we take two dendrimers as examples
to illustrate how to functionalize TDI and PMI for multichromophoric dendrimer
synthesis.
The 4PMI-TDI based dendrimer 31 [26, 27] consists of a terrylene diimide (TDI)
unit acting as energy acceptor in the core and four perylenemonoimide (PMI) units
as energy donors on the periphery. These chromophores (TDI and PMI) were
chosen because of their photostability, high extinction coefficients at convenient
absorption wavelengths (λ max ¼ 495 nm and λ max ¼ 673 nm for PMI and TDI,
respectively), high fluorescence quantum yields, and good overlap between the
emission of PMI and the absorption of TDI. This provides an optimal basis for
efficient intramolecular Fo ¨rster energy transfer. The synthetic approach rests upon
the Diels–Alder cycloaddition of a tetraphenylcyclopentadienone (Cp) system
containing the dye molecule PMI with the ethynyl functionalized TDI. The
monobromo-Cp 28 was firstly coupled with the PMI-boronic ester 10 to generate
the PMI-containing Cp 29, which underwent a fourfold Diels–Alder reaction with a
tetraethynyl-functionalized TDI 30 to afford the target multichromophoric
dendrimer 31 in 92% yield (Scheme 6). The number of bromo groups in compound
28 determined the final number of PMIs in the dendrimer. Moreover, with this
strategy, various chromophore pairs at the rim and the core of the dendrimer could
be established.
Scheme 7 presents another example of a first-generation polyphenylene
dendrimer (G1-4PMI 33) having a rigid tetrahedral core with four PMI
chromophores at the rim [28, 29]. Target compound 33 was synthesized by the
efficient Diels–Alder reaction of tetrakis(4-ethynylphenyl)methane (32) with the
PMI-decorated cyclopentadienenone 29 (Scheme 5) in 90% yield [30]. Moreover,
to systematically study the energy transfer in such multichromophoric systems, the
number of PMI chromophores could be varied from one to four along the branches
of the dendrimer in the direction of the corners of a tetrahedron [30], [31, 32]. In this
way, one could achieve better control over the orientation of the polyphenylene
building blocks around the central core. For asymmetric functionalization of the
periphery of a polyphenylene dendrimer, a stepwise Diels–Alder reaction of the
triisopropylsilyl (TIPS)-protected tetraphenyl methane and Cp with or without PMI
branches was used [30].
PMI-substituted hexa-peri-hexabenzocoronene (HBC) 35 was designed as
model compound for intermolecular energy and electron transfer studies because
of its D 6h symmetry, electronic and self-assembling properties [33–35]. The
multichromophore (HBC-6PMI) 35 has six PMI chromophores attached to HBC
via a 3
0 -dodecyl-4
0 ,5
0 ,6
0 -triphenyl-1,1
0 :2
0 ,1
00 -terphenyl spacer unit (Scheme 8).
HBC-6PMI 35 was obtained by Diels–Alder reaction of hexa-[4-(tetradec-1-yn1-yl)phenyl]-HBC 34 [36] and well-established building block PMI-decorated
Cp 29 (as discussed above) in diphenyl ether at 250
C in 58% yield [33–35]. Indeed,
electronic excitation of the HBC core of this molecule resulted in efficient energy
transfer to the PMI shell [37, 38].
72
T. Basche ´ et al.
the structure of the branches or of the cores. Another essential advantage of the
dendrimer-based design of multichromophores is the complete suppression of
aggregation between the dye molecules. Here we take two dendrimers as examples
to illustrate how to functionalize TDI and PMI for multichromophoric dendrimer
synthesis.
The 4PMI-TDI based dendrimer 31 [26, 27] consists of a terrylene diimide (TDI)
unit acting as energy acceptor in the core and four perylenemonoimide (PMI) units
as energy donors on the periphery. These chromophores (TDI and PMI) were
chosen because of their photostability, high extinction coefficients at convenient
absorption wavelengths (λ max ¼ 495 nm and λ max ¼ 673 nm for PMI and TDI,
respectively), high fluorescence quantum yields, and good overlap between the
emission of PMI and the absorption of TDI. This provides an optimal basis for
efficient intramolecular Fo ¨rster energy transfer. The synthetic approach rests upon
the Diels–Alder cycloaddition of a tetraphenylcyclopentadienone (Cp) system
containing the dye molecule PMI with the ethynyl functionalized TDI. The
monobromo-Cp 28 was firstly coupled with the PMI-boronic ester 10 to generate
the PMI-containing Cp 29, which underwent a fourfold Diels–Alder reaction with a
tetraethynyl-functionalized TDI 30 to afford the target multichromophoric
dendrimer 31 in 92% yield (Scheme 6). The number of bromo groups in compound
28 determined the final number of PMIs in the dendrimer. Moreover, with this
strategy, various chromophore pairs at the rim and the core of the dendrimer could
be established.
Scheme 7 presents another example of a first-generation polyphenylene
dendrimer (G1-4PMI 33) having a rigid tetrahedral core with four PMI
chromophores at the rim [28, 29]. Target compound 33 was synthesized by the
efficient Diels–Alder reaction of tetrakis(4-ethynylphenyl)methane (32) with the
PMI-decorated cyclopentadienenone 29 (Scheme 5) in 90% yield [30]. Moreover,
to systematically study the energy transfer in such multichromophoric systems, the
number of PMI chromophores could be varied from one to four along the branches
of the dendrimer in the direction of the corners of a tetrahedron [30], [31, 32]. In this
way, one could achieve better control over the orientation of the polyphenylene
building blocks around the central core. For asymmetric functionalization of the
periphery of a polyphenylene dendrimer, a stepwise Diels–Alder reaction of the
triisopropylsilyl (TIPS)-protected tetraphenyl methane and Cp with or without PMI
branches was used [30].
PMI-substituted hexa-peri-hexabenzocoronene (HBC) 35 was designed as
model compound for intermolecular energy and electron transfer studies because
of its D 6h symmetry, electronic and self-assembling properties [33–35]. The
multichromophore (HBC-6PMI) 35 has six PMI chromophores attached to HBC
via a 3
0 -dodecyl-4
0 ,5
0 ,6
0 -triphenyl-1,1
0 :2
0 ,1
00 -terphenyl spacer unit (Scheme 8).
HBC-6PMI 35 was obtained by Diels–Alder reaction of hexa-[4-(tetradec-1-yn1-yl)phenyl]-HBC 34 [36] and well-established building block PMI-decorated
Cp 29 (as discussed above) in diphenyl ether at 250
C in 58% yield [33–35]. Indeed,
electronic excitation of the HBC core of this molecule resulted in efficient energy
transfer to the PMI shell [37, 38].
72
T. Basche ´ et al.