by a factor of four as compared with the pristine nanorods. As
discussed above, the counter ions of Dye 1, I2, form a diffuse layer
extending into the pristine nanorod. Note that iodine can be an
effective fluorescence quencher due to the static quenching (in this
process the fluorescence decay of the unquenched fluorophore
does not change).9 Therefore, both FRET to Dye 1 and iodine
quenching are occurring in DPNP/Dye 1 core/diffuse nanorods.
In conclusion, organic core/diffuse-shell nanorods have been
successfully fabricated by two steps: (i) the preparation of uniform
DPNP nanorods by a simple reprecipitation method; (ii) the
adsorption of Dye 1 on the surface of DPNP nanorods through
hydrophobic interactions. The FRET from the DPNP core to the
Dye 1 shell can be adjusted by controlling the aging time. Such
organic composite nanostructure with tunable FRET from the
core to the shell might be promising for multifunctional
nanomaterial in optoelectronic nanodevices.
Table 1 The decay times of the fluorescence of DPNP at 475 nm in
the pristine nanorods and the core/diffuse-shell nanorods obtained at
different aging intervals. The excitation wavelength is 375 nm
t1
t2
Aging time/min
ns
%
ns
%
2
ta/ns
1
0
5
1.36
1.16
0.97
0.75
0.53
86
82
79
77
77
4.53
3.75
3.31
2.78
2.55
14
18
21
23
23
1.82
1.62
1.47
1.22
0.99
30
60
180
a
Average fluorescence lifetime, t = (t1%1 + t2%2)/100.
adsorb on the pristine nanorods with increasing the aging time.
Under conditions of the aging time .180 min, the absorbance of
Dye 1 reaches a constant value since the adsorption and
desorption of Dye 1 on DPNP nanorods reaches an equilibrium
in water.
This work was supported by the National Natural Science
Foundation of China (Nos. 90301010, 20373077, 90606004), the
Chinese Academy of Sciences (‘‘100 Talents’’ program), and the
National Research Fund for Fundamental Key Project 973
(2006CB806200). We also express our appreciation to the referees
for their helpful and valuable comments and suggestions.
Fig. 3(A) shows that the diffuse reflectance UV-vis absorption
spectrum of Dye 1 powder (curve 3) overlaps very well with the
steady-state emission spectrum of the pristine nanorods of DPNP
(curve 2) in the wavelength range of 450–600 nm. This is essential
for the occurrence of FRET.9 It is known that besides the spectral
overlap, the rate of FRET also depends upon the relative
orientation of the donor and acceptor transition dipoles, and the
distance between these molecules.9 The latter, i.e. the so-called
Fo¨rster radius, is about 2–5 nm9 at the same order of the diffuse-
shell thickness. Therefore, FRET from the DPNP core to the Dye
1 shell is possible in our core/diffuse-shell nanorods. Indeed,
although the absorbance of Dye 1 at 358 nm is very weak (y5% of
that at 530 nm, see the arrow in Fig. 3(A)), the emission spectra of
Dye 1/DPNP core/diffuse-shell nanorods excited at 358 nm
(Fig. 3(C)) shows that the contribution of Dye 1 around 590 nm
gradually increases with increasing the aging time, meanwhile, the
contribution of DPNP around 470 nm decreases. The correspond-
ing excitation spectra monitored at 590 nm shows that the
enhanced emission of Dye 1 results from the energy levels of
DPNP (Fig. 3(D)). Thus, FRET from the DPNP core to the Dye 1
shell is responsible for the enhanced emission of Dye 1 in the core/
diffuse-shell nanorods.
Notes and references
{ By using the same procedure, DPNP molecules form spherical particles/
cubic nanocrystals at low chemical potential,11 rather than the nanorods in
the present study at high chemical potential. This might relate to the kinetic
growth of one-dimensional nanostructures of organic semiconductors, and
is under investigation in this laboratory.
§ Considering the aggregate absorption of the nanorods at 425 nm, the
donor–donor Fo¨rster overlap for nanorods (between spectra 1 and 2 in
Fig. 3(A)) is not poor. Therefore, energy migration through the aggregate
state in nanorods might be possible, and amplifies the shell quenching.
ˇ
1 F. Qian, S. Gradecak, Y. Li, C. Y. Wen and C. M. Lieber, Nano Lett.,
2005, 5, 2287.
2 M. Green, Small, 2005, 1, 684.
3 C. W. Chan and S. Nie, Science, 1998, 281, 2016.
4 J. Jang, J. Ha and B. Lim, Chem. Commun., 2006, 15, 1622.
5 H. G. Zhu and M. J. McSchane, J. Am. Chem. Soc., 2005, 127, 13448.
6 H. Y. Huang, E. E. Remsen and K. L. Wooley, Chem. Commun., 1998,
1415.
7 E. A. Silinsh, Organic Molecular Crystals: Their Electronic States,
Springer-Verlag, Berlin, 1980.
8 A. D. Peng, D. B. Xiao, Y. Ma, W. S. Yang and J. N. Yao, Adv.
Mater., 2005, 17, 2070; C. W. Tang, D. J. Williams and J. C. Change,
US Pat., #5 294 870 1944; C. W. Tang, S. A. VanSlyke and C. H. Chem,
J. Appl. Phys., 1989, 65, 3610.
9 J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press,
New York, 1999; N. J. Turro, Modern Molecular Photochemistry,
Benjamin Cummings Publishing Co., Menlo Park, CA, 1978.
10 H. Kasai, H. S. Nalwa, H. Oikawa, S. Okda, H. Matsuda, N. Minami,
A. Kakuta, K. Ono, A. Mukoh and H. Nakanishi, Jpn. J. Appl. Phys.,
Part 2, 1992, 31, L1132.
11 H. B. Fu, X. H. Ji, X. H. Zhang, S. K. Wu and J. N. Yao, J. Colloid
Interface Sci., 1999, 220, 177.
12 Z. Y. Tian, Y. Chen, W. S. Yang, J. N. Yao, L. Y. Zhu and Z. G. Shuai,
Angew. Chem., Int. Ed., 2004, 43, 4060.
13 K. Balakrishnan, A. Datar, T. Naddo, J. L. Huang, R. Oitker, M. Yen,
J. C. Zhao and L. Zang, J. Am. Chem. Soc., 2006, 128, 7390;
P. Jonkheijm, P. V. D. Schoot, P. H. J. Schenning and E. W. Meijer,
Science, 2006, 313, 80.
14 J. Leja, Surface Chemistry of Froth Flotation, Plenum Press, New York,
1982; D. Myers, Surfactant Science and Technology, 2nd edn, VCH,
1992.
Table 1 shows the fluorescence lifetimes for the pristine and
core/diffuse-shell nanorods. The fluorescence decay of DPNP
monomers in ethanol solution is monoexponential with a lifetime
of 1.43 ns at 500 nm. However, the fluorescence of the pristine
nanorods shows biexponential decay. The short lifetime (t1) of
1.36 ns is comparable to the value of the monomer, thus is ascribed
to the molecular state, while the long one (t2) might be relative to
the aggregate state of DPNP molecules in the nanorod.15 As
shown in Table 1, both t1 and t2 of the core/diffuse-shell nanorod
become shorter as compared with those of the pristine nanorods.
Therefore both the molecular and aggregate states of DPNP
molecules in the core/diffuse-shell nanorod might be involved in
the FRET process from the DPNP core to the Dye 1 shell. The
average fluorescence lifetimes (t) of the core/diffuse-shell and
pristine nanorods are 0.99 and 1.82 ns at 0 and 180 min,
respectively, indicating that the DPNP fluorescence is quenched by
a factor of two due to the FRET process.§ However, the steady-
state measurement in Fig. 3(C) discloses that the DPNP
fluorescence in the core/diffuse-shell nanorods is quenched totally
15 H. B. Fu and J. N. Yao, J. Am. Chem. Soc., 2001, 123, 1434.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 2695–2697 | 2697