Photoswitchable quantum dots by controlling the photoinduced electron transfers

Article abstract of DOI:10.1039/c2cc34002j

We report photoluminescence (PL) modulation of quantum dots (QDs) by photoinduced electron transfers from acridine-1,8-dione derivative surface ligands. Reversible PL switching upon many repeated cycles was demonstrated, as alternating on and off of the U

Full text of DOI:10.1039/c2cc34002j

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COMMUNICATION
Photoswitchable quantum dots by controlling the photoinduced electron
transfersw
Jiwon Bang,a Joonhyuck Park,b Ranganathan Velu,a Eunjin Yoon,a Kyunghoon Lee,c
Sunghee Cho,c Soonwook Cha,c GeeSung Chae,c Taiha Jooa and Sungjee Kim*a
Received 5th June 2012, Accepted 25th July 2012
DOI: 10.1039/c2cc34002j
We report photoluminescence (PL) modulation of quantum dots
(QDs) by photoinduced electron transfers from acridine-1,8-dione
derivative surface ligands. Reversible PL switching upon many
repeated cycles was demonstrated, as alternating on and off of the
UV excitation for the surface ligand has successfully resulted in the
QD PL modulation.
acridine-1,8-dione (AD) dye derivative functionalized QDs by
photo-induced electron transfer (PET). PET has been widely
used for molecule-based optical modulators especially for
sensors.12 PET-based optical modulations of QDs are not
accompanied by the structural changes, and thus can guarantee
the long-term stability. The electron transfer dynamics can also
allow fast QD modulations. To the best of our knowledge, this is
the first case reporting the PET-based QD emission modulation.
An AD disulfide derivative molecule (denoted ADD) was
synthesized by modified procedures from our previous report13
(see ESIw for details). Schematic energy levels for 1.9 nm radius
CdSe QD and our ADD are shown in Scheme 1. The LUMO
level of the AD14 lies at a higher energy level than the conduction
band of our QD.15 Electrons from the photoexcited ADDs are
designed to transfer to the QD, which should result in quenching
of the QD PL. 1.9 nm radius CdSe QDs (denoted b-QD) and
1.9 nm radius/1.0 nm thick shell CdSe/CdS/ZnS (core/shell/
shell) overcoated QDs (denoted o-QD) were synthesized using
a method previously described.16 TEM images confirm the
thick CdS/ZnS shells over the b-QDs (Fig. 1c, d, and Fig. S1
for their high-resolution TEM images).
Reversible switching of the emission from fluorescent probes
can offer many emerging applications including optoelectronic
devices,1 chemical sensing,2 contrast-enhanced imaging,3 and
super-resolution imaging.4 Quantum dots (QDs) are fluorescent
nanocrystals which provide distinct advantages over conventional
organic dyes by the large absorption coefficient, tunable and bright
photoluminescence (PL) emissions with narrow and symmetric
emission profiles, and high photochemical stability.5 They can
promise many potential applications such as probes for cellular
and whole-body imaging,6 optical modulators,7 and light emitting
diodes.8 Modulating or reversibly switching the QD emission is
often crucial before QDs can be applied for such applications and
outperform conventional technologies. Typically, QD emissions are
modulated by energy transfers or charge transfers with molecules
in proximity.2b,9 Spectral switching of QDs has been also
demonstrated by charging/decharging type-II QDs.7b However,
optically triggered modulations of QD PL have been only
recently demonstrated by introducing photochromic compounds
which can isomerize by light.7a,10 Optical modulations of QDs
can provide unique opportunities for information processing,
data storage, and next-generation imaging probes.11 The photo-
isomerizable molecules can effectively modulate QD PL, however
this is often hampered by the slow modulation dynamics
(up to Bseconds) and limited switching cycles due to the limited
stability of the isomerizable molecules under continuous photo-
excitations. Herein, we report reversible photo-switching of
Both b-QD and o-QD show broad absorption profiles and
their PL peaks at around 600 nm (Fig. 1a). PL quantum yields
(QYs) of b-QD and o-QD were 4.9% and 10%, respectively.
ADD shows a strong absorption at around 360 nm due to the
charge transfer from the ring nitrogen to the ring carbonyl
oxygen centre and emission at around 445 nm (Fig. 1b).17 The
QD–ADD complex was prepared by adding a 2–100 fold
molar excess of reduced form of ADDs to QDs (see ESIw
for details). Two different excitation wavelengths of 365 nm
and 532 nm were used. The 365 nm at UV excites both QD and
ADD. The excited electrons in ADDs are expected to transfer
a Department of Chemistry, Pohang University of Science and
Technology (POSTECH), San 31, Hyojadong, Namgu,
Pohang 790-784, South Korea. E-mail: sungjee@postech.ac.kr;
Fax: +82-54-2791484; Tel: +82-54-2792108
b School of Interdisciplinary Bioscience and Bioengineering,
POSTECH, South Korea
c Material Team 2, LG Display Co., Ltd. 1007, Deogeun-ri,
Wollong-myeon, Paju 413-811, South Korea
w Electronic supplementary information (ESI) available: Synthesis of
acridinedione ligand and quantum dots, characterizations including
NMR, mass, and spectroscopy data. See DOI: 10.1039/c2cc34002j
Scheme 1 Schematic illustration of QD photoswitching. Potential
diagrams of HOMO–LUMO levels of the ADD surface molecule
and 1S electron–hole levels of the CdSe QD (1.9 nm radius) are shown.
c
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efficient quenching considering the inorganic double shell
layers of CdS and ZnS which form 1.0 nm thickness. This
efficient PL quenching via PET can be obtained because of the
energy level cascades from ADD LUMO to the conduction
band of CdSe via 1S electron levels of CdS and ZnS shell
layers.14,21 In addition, the efficient quenching of o-QD-ADD
suggests that the PET electrons should occupy delocalized
states of the QD in the conduction band rather than surface
states because the protective CdS and ZnS shell layers should
better passivate the surface defect sites than the bare QD
counterpart. When o-QD-ADD was excited by the visible
light, the complex retained the PL as high as 84% of that of
QD before surface modification. This makes a sharp contrast
to the case of b-QD-ADD. The CdS and ZnS shell layers seem
to effectively block the charge transfer quenching mechanism.
The quenched PL of o-QD-ADD under 365 nm excitation can be
partially restored by intentional exposure of the sample to air,
which is similar to the case of charged QDs via chemical electron
injection (Fig. S6a, ESIw).7b,22 The photoexcited electrons in ADD
can transfer to QD in o-QD-ADD, and as a result the ADD
emission was efficiently suppressed (Fig. S6b, ESIw). As a control,
a physical mixture of identical QDs with disulfide form of ADD
which cannot anchor on QD surfaces was used. The control
sample showed more than 5 times brighter ADD emission, which
confirms the inability to efficiently interact with QDs by the lack of
anchoring unit (Fig. S6b, ESIw). Fig. 2c and f show time-resolved
PL measurements for b-QD-ADD and o-QD-ADD under the UV
excitation (Table S1, ESIw). Both b-QD-ADD and o-QD-ADD
showed significantly shortened PL lifetimes, from 1.4 ns to 0.64 ns
and from 5.5 ns to 3.0 ns, respectively, when compared with b-QD
and o-QD control samples. For the controls, a surface molecule
that lacks the ADD chromophore but has the identical anchoring
unit, dihydrolipoic acid (DHLA), was used for the surface
functionalization. The faster PL decays confirm the efficient
PET from the ADD to QD in the QD–ADD complex.
Fig. 1 UV-vis spectra (closed circles) and normalized PL spectra
(open circles) of (a) b-QD (black) and o-QD (red), and (b) ADD in
THF solvent. TEM images of (c) the b-QD, and (d) the o-QD.
from the LUMO to conduction bands of QDs, which should
result in negatively charged QDs of which PL can be effectively
quenched by fast Auger processes.7b,18 Whereas, the 532 nm
at visible was chosen to selectively excite QDs. Fig. 2 shows
the emission spectra of the b-QD–ADD complex (denoted
b-QD-ADD) and the overcoated counterpart of the o-QD–
ADD complex (denoted o-QD-ADD) excited either by the UV
or visible light. The complex between QD and ADD was set
at a 1 : 2 molar ratio. As complexing with ADDs, the b-QD-
ADD showed significantly reduced PL intensity. Under UV,
the PL quenched to 6% before the ADD surface functionalization.
This quenching is attributed to the efficient PET from ADD to
b-QD with no inorganic shell layer. PL quenching by the inner
filter effect from the absorption of the ADDs should be
insignificant because the molar extinction coefficient of the
QD19 is more than 100-fold higher than that of ADD20 at
the excitation wavelength. The b-QD-ADD also showed
significant quenching (PL intensity down to 37%) when excited
by visible light. This quenching is thought to originate from
charge transfer as ADD functions as an electron acceptor. In
contrast, o-QD-ADD showed more dramatic PL changes by the
excitation wavelengths. When excited by UV, o-QD-ADD
showed the quenched PL down to 17%. The quenching efficiency
is slightly smaller than that of b-QD-ADD, yet it is remarkably
QD PL quenching efficiency via PET from ADD is expected to
increase as a large number of ADDs exist on the QD surface.23 The
ADD/QD molar ratio was varied as 2, 20, or 100 for o-QD-ADD,
and the PL properties were studied. As the ADD ratio increased,
PL intensity of the o-QD-ADD decreased accordingly
(Fig. 3a). To remove the inner filter effect and obtain the
PET-induced quenching efficiencies, control samples were
prepared using physical mixtures of ADDs in their disulfide
form with as-prepared QDs (Fig. S7, ESIw). The PET-induced
PL quenching efficiency was estimated using 1 À (IS,365/IS,532)/
(IR,365/IR,532), where I365 and I532 are the QD PL intensities
Fig. 2 (a, b, d, and e) PL spectra of QD samples before the ADD
functionalization (red) and after the QD–ADD complexation by 1 : 2
molar ratio (black). (a) b-QD/365, (b) b-QD/532, (d) o-QD/365, and
(e) o-QD/532 (sample/excitation wavelength nm). (c and f) Fluorescence
decay profiles of (c) b-QD-ADD (black) and b-QD-DHLA (red), and
(f) o-QD-ADD (black) and o-QD-DHLA (red). All the complexes were
prepared by 1 : 50 molar ratio for the decay profiles and were excited at
375 nm.
Fig. 3 (a) PL spectra of o-QD-ADDs prepared by different QD/ADD
molar ratios of 1 : 2 (red), 1 : 20 (blue), and 1 : 100 (green). QD sample
before the ADD functionalization was measured for a control (black).
Excitation at 365 nm. (b) PET induced QD PL quenching efficiencies of
the o-QD-ADDs.
c
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which calls for further time-resolved spectroscopy studies to
elucidate the dynamics of the turn on and off processes. This
external light triggered PL modulation capability can open a
new window for many applications.
This work was financially supported by a KOSEF grant
funded by MOST (20110018601), the Priority Research Center
Program through NRF (2011-0031405 and 20110027727),
(20110019635), the Basic Science Research Programs
(20110027236), and the Global Research Laboratory Program
(2011-00131).
Fig. 4 QD PL switching upon many repeated cycles for (a) o-QD-ADD
(1 : 50 molar ratio), (b) and CdTe/CdSe type-II QD-ADD (1 : 50 molar
ratio) with alternating on–off cycles of UV light under continuous visible
light illumination. Red and gray squares denote relative PL intensities at
their QD PL on and off status, respectively. Relative PL intensity is
defined as [QD–ADD complex PL intensity]/[non-surface modified QD
PL intensity]. The excitation power is 2.2 mW cmÀ2 for the 532 nm and
0.18 mW cmÀ2 for the 365 nm, respectively.
Notes and references
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when excited by 365 nm and 532 nm, respectively. IS represents
the intensity of o-QD-ADD, and IR the mixture control. The
PET-induced PL quenching efficiencies of 82%, 87%, and 98%
were obtained by increasing the ADD ratio from 2, 20 to 100
(Fig. 3b). This result indicates that a large number of electron
donors (ADDs) per QD can efficiently quench the QD PL when
excited at 365 nm. However, the quenching efficiencies for the
cases of 20 and 100 ADD ratios were actually lower than
expected. The actual numbers of ADDs on QD surfaces do not
seem to reach the nominal amounts presumably because of the
limited surface derivatization sites on QDs and increasing steric
and electrostatic repulsions for the additional ADD anchoring.
Fig. 4a showcases the reversible PL switching capability of
the o-QD-ADD sample (1 : 50 QD : ADD ratio) by repeated
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CdTe/CdSe (core/shell) type-II QDs were prepared to test the PL
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TEM images). The QD PL modulation experiment was repeated
using the type-II QDs (1 : 50 QD : ADD ratio) (Fig. 4b). The
average QD PL quenching efficiency was 40%. The type-II QDs
have unique exciton characteristics, where the electrons and holes
are spatially separated. The electrons reside mostly in the shells,
while the holes stay in the cores. The quenching efficiency was
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were stably reversible over numerous times of switching cycles.
In summary, we have demonstrated a unique optical modulation
of QD PL by controlling the PET using our ADD surface ligands.
The switching response was faster than the milli-second scale,
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