Ultrafast fluorescence investigation of excitation energy transfer in different dendritic core branched structures

Article abstract of DOI:10.1021/ja035215y

The mechanism of energy transport in branching structures is suggestively related to the geometry of the multichromophore architecture. In organic conjugated dendrimers, both incoherent (hopping) and coherent energy transfer processes have been observed from different dendritic architectures with different building blocks. In this communication, we report the investigation of three fundamental dendritic architectures (G0) with the same attached chromophores, but with different core atoms, C, N, and P. The synthesis of a phosphorus-containing G0 system with distyrylbenzene chromophores is provided. These three systems provide a comparison by which the relative interaction of branching chromophores can be compared on the basis of their different branching centers. Ultrafast fluorescence anisotropy measurements provide a dual measure of the geometry of the chromophores around the different central units as well as the strength of the interactions among chromophores. The nitrogen-cored system appeared to have both the strongest coupling of chromophore excitation as well as the most planar geometry of the three. Interestingly, the phosphorus system appeared to have the least planar geometry, and its interaction strength was found to be stronger than that observed for the carbon system. These results provide a comparison of the energy migration dynamics of the most common and new dendritic architectures with applications for light emission and light harvesting. Copyright

Full text of DOI:10.1021/ja035215y

Published on Web 07/18/2003
Ultrafast Fluorescence Investigation of Excitation Energy Transfer in Different
Dendritic Core Branched Structures
Ying Wang, Mahinda I. Ranasinghe, and Theodore Goodson, III*
Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48201
Received March 18, 2003; E-mail: tgoodson@chem.wayne.edu
The investigation of optical excitations in branched structures
continues to be an important issue as it can provide valuable
information regarding the creation of new organic light-emitting
Scheme 1. Synthesis of Phosphorus-Cored Distyrylbenzene
Dendrimer P(DSB)3, G0
1
and nonlinear optical materials. In particular, the mechanism of
energy transport in dendritic and other multichromophore archi-
tectures plays a key role in the understanding of the enhanced optical
2
effects observed in these systems. Previous reports of a nitrogen-
cored dendritic system illustrated impressive energy transfer ef-
ficiencies as well as very strong electronic interactions of the
3
participating chromophore building blocks. This is explained by
the very good electronic overlap of the nitrogen-cored system where
the attached chromophores are situated in an almost planar
(propeller) geometry. Indeed, the issues of planarity, electronic
coupling, and the degree of functionality are important factors
related to the excitations in dendrimers and should be investigated
systematically. To investigate these questions, we have carried out
the synthesis of new branching structures, all with the same building
blocks (chromophores) but with different core units (C, N, and P).
The dynamics and scale of energy migration are investigated in
each of these model systems to determine the dominance of the
key parameters for superior energy transfer efficiency.
3
is in a sp orbital directed from the apex of the pyramid so that the
bond angle of C-P-C is less than the bond angle of C-C-C in
tetrahedral tetrastilbenoidmethanes (111.3°).5b As a result, the
P(DSB) is more pyramidal than C(DSB) . Shown in Figure 1 are
3
4
A phosphorus-cored distyrylbenzene dendrimer (P(DSB)
3
, G0)
the absorption and the fluorescence spectra of three DSB systems
was synthesized for this purpose. The synthesis of a phosphorus
core conjugated triaryl-dendrimer has not been reported. We initially
and the DSB chromophore. The spectra of P(DSB) are blue-shifted
3
with respect to those for the N(DSB) and red-shifted with respect
3
attempted a method utilizing a Grignard reagent to react with PCl
3
.
to C(DSB) and DSB. This phenomenon can be explained by the
mesomeric effect of the three different cored systems. In com-
4
7
Yet, the lithium/bromine exchange reaction of brominated distyryl-
benzene did not occur and finally appeared to give a tert-butyl-
substituted phosphine. Also, the direct coupling of substituted
parison with the trigonal planar N(DSB) , the overlap of the lone
3
pair orbital on the larger phosphorus with the adjacent p-orbital on
styrenes with tris(p-bromophenyl) phosphine (P(C
due to its weak activation and possible poisoning of active palladium
species by an excess of the phosphine under Heck conditions.4
6
H
4
Br)
3
) failed
the smaller carbon atom is much less efficient. Therefore, the
delocalization of the lone pair on phosphorus with the conjugated
distyrylbenzene units is less extensive. As a result, the spectra of
However, the coupling of an oxidized phosphine (PO(C
with substituted styrenes was successful. The synthesis of the
phosphorus-cored stilbene dendrimer P(SB) suggested a similar
6
H
4
Br)
3
)
P(DSB) are shifted to the blue. In contrast, there is no lone pair
available from a saturated carbon atom; therefore, the spectra of
3
3
C(DSB) are similar to those of the building block DSB chro-
4
procedure to build up an aldehyde unit containing the central
phosphorus oxide. Such an aldehyde moiety reacts with a dendron
containing benzylphosphonate to obtain the target macromolecules.
mophore.
To investigate the excitation energy transfer dynamics in these
branched macromolecules, we utilize time-resolved fluorescence
upconversion spectroscopy. Our upconversion experimental system
was described in detail in our previous publications. We found
that the P(DSB) is stable to the femtosecond excitation laser pulses.
3
Scheme 1 shows the complete synthetic pathway for P(DSB) . All
structures are well characterized by P, H, and 13C NMR spectra.
The 31P spectrum shows a resonance at -5.5 ppm for the final
phosphine dendritic macromolecule without any peak at 27 ppm
31
1
8
3
Shown in Figure 2 are the fluorescence anisotropy decay dynamics
1
for phosphine oxide. The H NMR spectrum not only displays the
of the branched dendritic macromolecules and of the DSB chro-
mophore. The anisotropy decay dynamics of the branched DSB
macromolecules are different than what is observed for the linear
DSB chromophore. The DSB chromophore shows no significant
anisotropy decay during this experimental time window. We have
shown that the anisotropy decay times can be utilized to explain
energy transfer dynamics in branched structures.9
appearance of the trans-vinyl signal (d, J ) 16.2 Hz), but it also
shows the coupling of phosphorus with aromatic protons. The
coupling of phosphorus with carbons was observed from the 13
C
spectrum. The ESI mass spectrum gave a corresponding molecular
+
ion peak at m/z 1211 (M + H) .
,10
3
The geometry of P(DSB) is pyramidal but not trigonal planar
as in triphenylamine5 due to the increase in size from N (1.71 Å)
a
3
The anisotropy decay of P(DSB) can be fitted by a two-
to P (2.12 Å) and the increase in bond length from N (N-C(aryl)
exponential decay function with decay time constants of 51 and
702 fs. The anisotropy decay times were found to be 37 and 960
6
1.42 Å) to P (P-C(aryl) 1.83 Å). Hence, the lone pair in P(DSB)
3
9562
9
J. AM. CHEM. SOC. 2003, 125, 9562-9563
10.1021/ja035215y CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
of transition dipoles in a branched macromolecular system and
hence gives molecular geometry information as well. As shown in
Figure 2, the fluorescence anisotropy of P(DSB)
3
decays to a
decay to a
residual value of 0.140, while N(DSB) and C(DSB)
3
4
residual value of 0.082 during the same decay time window.
Theoretically, the residual anisotropy value for a planar three-fold
1
1,12
symmetrical dipole arrangement in a branched system is 0.1.
The difference in the residual anisotropy value indicates that the
dipole arrangement around the branching center of P(DSB) deviates
from planarity to a greater extent than that of N(DSB) and
C(DSB) . The pyramidal structure of P(DSB) makes the angle
between absorption and emission transition dipoles of the chro-
mophore system smaller than that of C(DSB) . This changes the
3
3
Figure 1. The normalized UV/vis and fluorescence spectra of DSB,
P(DSB)3, N(DSB)3, and C(DSB)4 in CHCl3.
4
3
4
average angle between transition dipoles, which is strongly related
to the structure of the branched chromophore system, and hence
alters the value of the residual anisotropy.
In conclusion, we have compared the influence of the branching
centers on the energy transfer dynamics in dendritic core macro-
molecules. This investigation provides more insight into the
understanding of the energy delocalization from different cores in
a dendrimer. We have shown that the geometry, structural arrange-
ment of transition dipoles around the branching center, and the
delocalization across the branching center are important in deter-
mining the mode of energy transfer in dendritic architectures.
Acknowledgment. T.G. III acknowledges the NSF (DMR-
0880044) and AFOSR for support.
Figure 2. The fluorescence anisotropy decay for DSB, P(DSB)3, N(DSB)3,
and C(DSB)4 in CHCl3. λex ) 390 nm, and λem ) 480 nm. The solid lines
are fits to the experimental curves.
0
Supporting Information Available: Synthetic procedures of P(SB)
3
and P(DSB) (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
3
fs for C(DSB)
found for N(DSB)
between chromophores in the C(DSB)
the dominant mechanism.10 However, the fast energy migration
processes observed in the N(DSB) system suggest a coherent
mechanism.3 The presence of a slow decay component in the
anisotropy of C(DSB) and P(DSB) before it reaches the residual
4
, and a monoexponential decay time of 57 fs was
. A F o¨ rster-type incoherent energy transfer
system was suggested as
3
4
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