C O M M U N I C A T I O N S
Blue-Br is insoluble in EtOH so the microstructure is preserved
in the process (Table 1, condition IV, and Figure S3), while
Red-OH is soluble (1.0 mM) in EtOH in the presence of K2CO3.
Powder X-ray Diffraction (XRD) patterns also indicate that the
crystal structures do not change significantly after modification
(Figure S9). Three additional evidential observations confirm
that Red-OH had been covalently bonded onto the surface of
Blue-Br: (1) product Red-O-Blue on modified microwires was
detected by MALDI-TOF MS measurements (Figure S5); (2)
under the same reaction conditions, Blue-H cannot be modified
(condition III); (3) Although Red-OH can also physically adsorb
onto the surface of Blue-Br without K2CO3 (condition II),7 they
are readily washed away after resuspending the microwires in
hot EtOH (Figure S6). In contrast, the covalent linkage between
Red-OH and Blue-Br can withstand such conditions and the
WLE wires are stable.
than that of the body (Figure S8). This is indeed what we
observed. Figure 2d shows that relatively long lifetime spots
(green) regularly circle around the wire, which indicates a
core-shell structure. To quantify such a distribution, both the
intensity and averaged lifetime are plotted versus position across
the wire, as shown in Figure 2e. Clearly, the position of the
intensity peak coincides with the minimum of the average
lifetime, where the ratio of Blue-Br is highest. In contrast, the
average lifetime reaches maxima where the intensity starts to
decay, because the ratio of Red-OH is much higher at the wall.8
Apart from heterogeneity across the wire, we also observed a
higher intensity at the end of the wire, indicating a wave-guiding
phenomenon.
Neglecting the light distribution on the wire, the whole wire
appears to be a bright white color under illumination. It has CIE
coordinates of (0.32, 0.36), very close to those of the standard
white light (0.33, 0.33). Compared with other methods,9 our
strategy only needs a very small amount of the acceptor
(estimated to be 10-5 compared to that of the bulk from
geometric calculation), because the acceptors are exposed at the
surface and contribute better to the spectrum when viewing from
the outside of the wire. In addition, after redissolving WLE
microwires in chloroform and recrystallizing in ethanol, the new
assembled microwires were blue-emissive, which demonstrated
the advantage of postmodification over coassembly in this WLE
system.
In conclusion, we have developed and demonstrated a new
concept in the design of organic 1D heterostructures, i.e. via
postmodification of their surface. A proof of concept example
demonstrates its feasibility and usefulness: WLE structures are
readily obtained from normal blue-emitting wires after modifica-
tion with second red-emitting materials on their surface. We
envisage many situations where such a modification will be
desirable. For example, biosensors can be grafted onto the
surface, with only a very small amount needed. Moreover,
core-shell P-N structures can also be readily obtained in this
way. Finally, the ability to control the surface property is
important to devices such as FET, which is extremely sensitive
to the surface layers.10 We hope our strategy would open a new
avenue to explore in this area.
The state of the modifiers on the surface was revealed by the
steady state and transient fluorescent emission features of the
modified wires (as shown in Figure 2a-b). The emission maximum
(580 nm) and lifetime (5.2 ns) of Red on the surface show excellent
agreement with those in the molecular state (10-6 M in hexane,
550 nm, 5.5 ns), which are drastically different from those in the
aggregated state of Red-OH (625 nm, 2.4 ns). This indicates that
Red chromophores do not form aggregates on the surface. In other
words, the surface density of Red is low, which is consistent with
the fact that the amount of nitrogen element is below the detection
limit of the surface sensitive X-ray photoelectron spectrum (XPS)
measurement.
Acknowledgment. This work was financially supported by
the Major State Basic Research Development Program (No.
2009CB623601) and by the National Natural Science Foundation
of China (NSFC). We thank Professor Lindong Sun for discus-
sions and Jiacai Zhou from Peking University for lifetime
mapping experiments. We also thank Dr. Xiaoyun Li from Akron
University for mass spectra experiments.
Figure 2. (a) Fluorescence spectra of Blue-Br microwires before and after
modification; (b) transient fluorescence spectra of Blue-Br microwires before
and after modification; (c) intensity mapping of the WLE microwires. Red
color indicates the region with high intensity. (d) Lifetime mapping of WLE
microwires excited at 485 nm; blue color indicates the region with a short
lifetime. The white rectangle specifies the region used for analysis; (e)
distribution of lifetime and intensity across the wire. Average lifetime is
defined as the time it takes for the intensity at a certain pixel to decay to
1/e of its initial value after excitation. Each point is averaged over 57 µm
along the wire in a specified region.
Supporting Information Available: Detailed experimental proce-
dures and characterization data of all new compounds. This material
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To further prove the success of the surface modification, we
mapped the spatial distribution of Red-OH and Blue-Br wires
by measuring the lifetime of different locations (Figure 2d). The
lifetime of Red-OH (5.5 ns) is longer than that of Blue-Br (0.8
ns), so the average lifetime on one pixel depends on the ratio of
Red-OH and Blue-Br in that region. It is expected that the
average lifetime on the outer circle of the 1D structure be longer
9
J. AM. CHEM. SOC. VOL. 132, NO. 45, 2010 15873