Supporting Information
Triplet harvesting in poly(9-vinylcarbazole) and poly(9-(2,3-epoxypropyl)
carbazole) doped with CdSe/ZnS quantum dots encapsulated with 16-(Ncarbazolyl) hexadecanoic acid ligands
Adis Khetubol,1 Sven Van Snick,2 Egle Stanislovaityte,3 Antti Hassinen,4 Eduardo Coutiño-González,1
Willem Vanderlinden,1 Yuliar Firdaus,1 Eduard Fron,1 Maarten Vlasselaer,2 Jurate Simokaitiene,3
Steven De Feyter,1 Zeger Hens,4 Juozas V. Grazulevicius,3 Wim Dehaen,2 and Mark Van der Auweraer1
1
Molecular Imaging and Photonics, Chemistry Department, Katholieke Universiteit Leuven, Leuven, B3001, Belgium
2
Molecular Design and Synthesis, Chemistry Department, Katholieke Universiteit Leuven, Leuven, B3001, Belgium
3
Department of Organic Technology, Kaunas University of Technology, Kaunas, LT 3028, Lithuania
4
Physics and Chemistry of Nanostructures, Department of Inorganic and Physical Chemistry, Gent, B9000, Belgium
Correspondence to: Mark Van der Auweraer (E-mail: mark.vanderauweraer@chem.kuleuven.be)
1
S1. Synthesis and Characterization of 16-(N-carbazolyl) hexadecanoic acid ligands
General Procedure 1. Synthesis of methyl-16-bromohexadecanoate
To a well-stirred solution of 16-bromohexadecanoic acid (1670 mg, 5.0 mmol) and 50 mL methanol was
added a catalytic amount of concentrated sulphuric acid. The reaction mixture was heated to reflux and
stirred overnight. To the mixture was then added a saturated solution of NaHCO3 until no more CO2
evolved, followed by extraction with Et2O (3x50 mL). The organic layers were combined, washed with
brine, dried on anhydrous MgSO4 and concentrated by rotary evaporation. This yielded the methyl-16bromohexadecanoate as a white solid (1132 mg, 65%). 1H NMR (300 MHz, CDCl3): δ 1.26 (br, 20 H), 1.42
(br, 2 H), 1.62 (br, 2 H), 1.85 (m, 2 H), 2.30 (t, 2 H), 3.40 (t, 2 H), 3.66 (s, 3 H).
16-Carbazol-9-yl- hexadecanoic acid (C16)
To a suspension of K2CO3 (1591 mg, 8.9 mmol) in CH3CN (25 mL) were added carbazole (500 mg, 3.0
mmol) and methyl-16-bromohexadecanoate (1002 mg, 3.6 mmol) in one portion. The reaction mixture
was heated to reflux and stirred for 3 days. The inorganic salts were removed by filtration and the
filtrate was concentrated by rotary evaporation. Purification by column chromatography (petroleum
ether ethylacetate 8/2) yielded a yellow oil, to which 10 mL NaOH (2M) was added. Stirring overnight at
100°C resulted in a transparent solution from which C16 was precipitated as a grey solid by acidifying
the reaction mixture. This precipitate was filtered, washed with H2O (3 x 10 mL) and dried in vacuo to
give 212 mg (17 %) of C16. Mp: 89.7 °C. 1H NMR (400 MHz, CDCl3): δ 1.24 (br, 22 H), 1.60 (m, 2 H), 1.86
(m, 2 H), 2.34 (t, J = 7.5 Hz, 2 H), 4.29 (t, J = 7.2 Hz, 2 H), 7.22 (m, 2 H), 7.45 (m, 4 H), 8.09 (d, J = 7.72 Hz,
2 H).
S2. Synthesis of poly(9-(2-3-epoxypropyl) carbazole) (PEPK)
Cationic polymerization of 9-(2-3-epoxypropyl)carbazole initiated with BF3•O(C2H5)2 (0.1 mol% with
respect to monomer) was carried out in dry 1,2-dichloromethane solution at room temperature for 20 h.
The initial concentration of a monomer was 1 mol/l. The polymer was isolated by precipitation in
methanol containing aqua ammonia which was added for neutralization of the initiator. The low
molecular weight fraction of the polymer was removed by Soxhlet extraction with methanol for 70 h.
Then the polymer was re-precipitated. Poly(9-(2-3-epoxypropyl)carbazole) (PEPK) was obtained as white
amorphous powder with the yield of 80%. 1H NMR (300 MHz, CDCl3), δ (ppm): 2.27-4.53 (m, 5H, CH2,
CH2, CH), 6.76-7.56 (m, 6H, Ar), 7.81-8.09 (m, 2H, Ar).IR (KBr, cm-1): 3048  (C-H arene); 2928, 2870  (C-
2
H aliphatic); 1626, 1597, 1484, 1453  (C=C arene); 1325  (C=C, C-N arene); 1130, 1121  (C-O-C); 749,
722 γ (C-H arene).
S3. Sample Preparation and Characterization
Sample Preparation
All the solution samples used for the determination of the fluorescence quantum yields and for the
time-resolved PL measurements were diluted until the optical density was approximately 0.1 at the
excitation wavelength. The relative fluorescence quantum yields (QYs) of the solution samples upon
excitation at 330 and 440 nm were determined using respectively a solution of quinine sulfate in 0.1 N
H2SO4 and fluorescein in 0.1 N NaOH as a reference standard. Quartz substrates for the thin film samples
were cleaned by acetone, alkaline solution for cleaning, and water and then dried with air flow. Polymer
and doped polymer (with QDs) films were prepared by spin-coating from the solutions in CB with a fixed
concentration of the polymer (20 mg/ml) on the prepared substrates. The spin-coating rotation rate and
the duration amounted to respectively 1000 rpm and 60 seconds for the deposition of the films.
Sample Characterization
Steady-state absorption and corrected PL spectra were recorded with a Lambda 40 (Perkin-Elmer) and
SPEX Fluorolog spectrophotometer, respectively. The fluorescence spectra were corrected for the
wavelength dependence of the sensitivity of the detection system and of the intensity of the excitation
beam.
The fluorescence decays were determined by single photon timing (SPT). The excitation wavelength of
330 nm was obtained using the third harmonic of a Ti:sapphire laser (Tsunami, Spectra Physics) with a
repetition rate of 8.1 MHz using a pulse picker (GWU). The Ti:sapphire laser was pumped by an
intracavity frequency doubled Millenia laser (Spectra Physics). For the excitation at 440 nm the second
harmonic of the Ti:sapphire laser (Tsunami, Spectra Physics) was used. The detection system consists of
a subtractive double monochromator (9030DS, Sciencetech) and a microchannel plate photomultiplier
(R3809U, Hamamatsu). Fluorescence decays were recorded under magic angle polarization. A time-toamplitude converter (TAC), and an analog-to-digital converter (ADC) on a PC board (PICOQUANT) were
used to obtain the fluorescence decay histograms in 4096 channels with time increments of 20-30
ps/channel. The fluorescence decays were globally analyzed as a sum of exponentials with the timeresolved fluorescence analysis (TRFA) software. The quality of the fit was controlled by χ2 < 1.1 and
visual inspection of the residuals.1-2
3
All AFM experiments were performed on a commercial AFM system (Multimode AFM equipped with a
Nanoscope IV controller and a type E scanner; Veeco Instruments) in amplitude modulation mode using
Si cantilevers (AC160TS; spring constant 21-60 /m, resonance frequency of ca. 300 kHz, Olympus Japan).
Imaging (tapping mode in ambient air condition) was performed at a 5 percent reduction of the free air
amplitude using optimized feedback parameters. Topography images were acquired at a scan rate of 1.5
scan lines per second and with a pixel size of ~ 8 nm. Image processing and analysis was performed using
the SPIP software package (v.6.0.6.; Image Metrology ApS). Processing involved plane correction using a
third degree polynomial global correction procedure as well as line-wise correction via histogram
alignment. The z-offset method was performed by setting the minimum z value to zero. The root-mean
square roughness Sq and the height distribution was determined using the SPIP software to
quantitatively evaluate the surface morphology.3
S4. Characterization of C16 covered QDs by NMR spectroscopy
The C16 capped QD sample for the NMR measurement was prepared by the procedure for the postsynthesis encapsulation detailed in the Experimental Section. In contrast to the QDs used for optical
spectroscopy experiments the QDs were purified two times and redissolved in CDCl3 after the reaction in
chlorobenzene.
1
H nuclear magnetic resonance (NMR) was used to confirm the binding of the C16 ligands to the
dissolved QDs. Figure S1 shows the 1H NMR spectra of (a) a solution in CDCl3 of the free C16 ligand and
(b) a solution in CDCl3 of the QDs capped with the C16 ligand after addition of 2 μl CH2Br2. Upon binding
to the QDs the T2 of the carbazole protons decreases drastically due to decreased mobility leading to
broad lines. The sharp peak at ~ 7.26 ppm, which is seen in both spectra, is attributed to the NMR signal
of residual CHCl3 in the solvent (CDCl3).4
4
FIGURE S1 1H spectra of (a) pristine C16 ligands and (b) the QDs capped with the C16 ligand in CDCl 3. To
the QD sample 2 μl of CH2Br2 was added corresponding to a molecular ratio of 1:363 for the ratio
QDs:CH2Br2.
By adding a known amount of CH2Br2 to the a known volume of a solution with a known
concentration of the QDs the number of ligands per QD can be estimated.5 In this case, 2 μl of
CH2Br2 was added to the C16 capped QD solution corresponding to a final molecular ratio of
1:363 of QD:CH2Br2. The sharp peak at ~ 4.8 ppm in the solution of the QDs is attributed to
CH2Br2. From the ratio of the area under the CH2Br2 resonance and the area under one of the
carbazole resonances (8.09 ppm) which was normalized to one, a number of 62 ligands/QD or
5
1.7 ligands/nm2 was estimated. The density of ligands per unit area (nm2) was determined
corresponding to the average size of the core-shell QDs estimated from a TEM image in [ref. 3].
S5. Fluorescence quantum yield of C16 capped QDs in polymer films
The fluorescence quantum yield of the C16 capped QDs in the polymer was estimated in the
following way;
4
Counts
10
10
3
10 20 30 40 50 60 70 80 90
Time (ns)
FIGURE S2 Fluorescence decays of C16 capped CdSe/ZnS QDs in chlorobenzene solution (grey) and PVK
film (red). Excitation occurred at 440 nm and the decay of the emission was recorded at 490 nm.
TABLE S1 Fluorescence decay times and amplitudes recovered from the fluorescence decays of C16
capped CdSe/ZnS QDs in Figure S2.
Samples
A1 (%)
A2 (%)
A3 (%)
τ1 (ns)
τ2 (ns)
τ3 (ns)
<τ> ns
QDs (C16) solution
21.97
38.77
39.26
1.88
10.33
20.14
12.33
Doped PVK film
29.51
53.85
16.64
1.83
6.60
18.52
7.18
The fluorescence decays were analyzed as a sum of three exponentials [Equation (S1)]. As the exact
origin of this non-exponential decay is outside the scope of this manuscript no direct physical meaning
should be given to decay times and amplitudes. A possible explanation for the multi-exponential decay
could be trapping of electrons or holes in shallow intrabandgap states.6
6
𝐼 (𝑡) = 𝐴1 𝑒𝑥𝑝(−𝑡⁄𝜏1 ) + 𝐴2 𝑒𝑥𝑝(−𝑡⁄𝜏2 ) + 𝐴3 𝑒𝑥𝑝(−𝑡⁄𝜏3 )
(S1)
Those parameters are just used to calculate the amplitude weighted average decay time [Equation (S2)]
⟨𝜏⟩ =
∑𝑖 𝐴𝑖 𝜏𝑖
∑ 𝑖 𝐴𝑖
(S2)
Assuming an identical radiative decay rate for the QDs in a solution in CB and in the PVK and PEPK films
the combination of the average decay times with the fluorescence quantum yield of 0.45±0.05 for the
C16 capped QDs dissolved in CB yields a fluorescence quantum yield of 0.26±0.03 for C16 capped QDs in
PVK film. As we have seen before in Khetubol et al.,1 the decrease of the average fluorescence decay
time of the QDs in a polymer film compared to a solution in CB was also observed for QDs incorporated
in an “inert” polymer film as polystyrene. Hence it is unlikely that this decreased decay time is due to
hole injection from the valence band of the excited QD into the HOMO of polymer. In the following
calculations for triplet energy transfer efficiencies, we shall also use this estimated value (0.26±0.03) as
the QY of the C16 capped QDs in PEPK film.
S6. Analysis of the fluorescence decays of pristine polymer films and polymer films doped with C16
capped QDs
The fluorescence decays were analyzed as a sum of three exponentials [Equation (S1)] yielding the
amplitudes and decay times shown in Table S2 and Table S3. From the amplitude weighted average
fluorescence decay time [Table S2 and Table S3] the efficiency of the singlet energy transfer (  SENT )
[Table S2 and Table S3] can be estimated by following expression;
𝑆
Φ𝐸𝑁𝑇
=1−
⟨𝜏𝐷𝐴 ⟩
⁄⟨𝜏 ⟩
𝐷
(S3)
𝑆
where Φ𝐸𝑁𝑇
is the singlet energy transfer efficiency, ⟨𝜏𝐷 ⟩ and ⟨𝜏𝐷𝐴 ⟩ are the amplitude-weighted
fluorescence decay time of the donor (carbazole) in the absence and presence of the acceptor (QDs),
respectively.1,7,8
7
TABLE S2 Fluorescence decay amplitudes and decay times as well as amplitude weighted average
fluorescence decay times [Equation (S2)] corresponding to the fluorescence intensity decays in Figure
6(a) and 6(b) of the pristine PVK film and doped PVK films with C16 capped QDs.
⟨𝜏⟩
 SENT
𝜆𝑒𝑚 = 370 nm
A1 (%)
A2 (%)
A3 (%)
𝜏1 (ns)
𝜏2 (ns)
𝜏3 (ns)
PVK
47.86
26.07
26.07
0.88
4.31
23.83
7.76
5%
50.64
25.36
24.00
0.87
4.45
23.37
7.18
0.075±0.004
10%
53.48
24.53
22.00
0.86
4.59
23.03
6.65
0.143±0.007
30%
53.97
26.66
19.37
0.55
3.58
22.02
5.52
0.29±0.01
𝜆𝑒𝑚 = 410 nm
A1 (%)
A2 (%)
A3 (%)
𝜏1 (ns)
𝜏2 (ns)
𝜏3 (ns)
PVK
37.13
0.11.53
51.34
2.11
6.64
26.68
15.25
5%
34.25
15.07
50.68
1.55
5.61
26.34
14.72
0.034±0.002
10%
35.79
15.38
48.82
1.50
6.03
26.02
14.17
0.071±0.004
30%
35.04
17.68
47.27
1.38
5.67
25.18
13.39
0.122±0.006
⟨𝜏⟩
 SENT
TABLE S3 The fluorescence decay amplitudes and fluorescence decay times as well as amplitude
weighted average fluorescence decay times [Equation (S2)] corresponding to the fluorescence intensity
decays in Figure 6(c) and 6(d) of the pristine PEPK and doped PEPK films with C16 capped QDs.
⟨𝜏⟩
 SENT
𝜆𝑒𝑚 = 368 nm
A1 (%)
A2 (%)
A3 (%)
𝜏1 (ns)
𝜏2 (ns)
𝜏3 (ns)
PEPK
65.21
28.88
5.91
2.15
3.90
12.81
3.29
5%
64.34
30.73
4.92
1.86
3.39
11.65
2.81
0.144±0.007
10%
58.26
37.72
4.02
1.47
2.73
10.28
2.30
0.300±0.015
30%
70.83
26.26
2.91
1.28
2.73
11.00
1.94
0.41±0.02
𝜆𝑒𝑚 = 405 nm
A1 (%)
A2 (%)
A3 (%)
𝜏1 (ns)
𝜏2 (ns)
𝜏3 (ns)
PEPK
0.002
83.74
16.26
2.38
3.31
13.37
4.94
5%
2.43
83.87
13.70
3.13
3.07
13.01
4.43
0.104±0.005
10%
12.63
74.02
13.35
2.34
2.70
11.76
3.86
0.22±0.01
30%
31.05
53.51
15.44
1.88
2.52
10.64
3.58
0.28±0.01
⟨𝜏⟩
 SENT
S7. Estimation for the efficiency of triplet energy transfer to the QDs in the doped polymer films1
As an example we show the calculation of the fluorescence quantum efficiency for triplet transfer in a
PVK film with a loading of 5 wt% of C16 capped QDs. Upon excitation at 330 nm the fluorescence
quantum yield of the carbazole emission in the PVK film loaded with the QDs amounts to
8


 f   0f 1   SENT . With  0f = 0.16 ±0.031 and  SENT = 0.034±0.002 (λem = 410 nm), the latter
determined from the fluorescence decay at the emission wavelength of 410 nm. The fluorescence
quantum yield of the QD emission upon excitation at 330 nm,  330
f ,QD , is that of the carbazole emission in
the PVK film with QDs multiplied by the ratio between the area under the emission band of the QDs and
that of carbazole which amounts to 0.43±0.02. This gives a fluorescence quantum yield,  330
f ,QD of
0.066±0.017 for the QD emission. The fluorescence quantum yield of the QD emission due to singlet
440
energy transfer,  Sf ,QD , is given by  Sf ,QD   SENT  440
f ,QD where  f ,QD is the fluorescence quantum yield
of the QDs upon direct excitation at 440 nm and which amounts to 0.26±0.04. Hence  Sf ,QD amounts to
0.009±0.002. The fluorescence quantum yield of the QDs upon excitation at 330 nm due to triplet
S
energy transfer amounts then to Tf ,QD  330
f ,QD   f ,QD or 0.057±0.018. This corresponds to
0
S
Tf ,QD   ISC TENT  440
f ,QD . As  ISC   ISC 1   ENT  and assuming that in pristine PVK the excited
state of carbazole decays either by fluorescence or intersystem crossing,9  0ISC  1   0f . With  0f =
0.16±0.03,  ISC amounts to 0.81±0.03. Hence 
T
ENT

 Tf ,QD
 440
f ,QD  ISC
≈ 0.27±0.12.
By applying this method for all the doped film samples, the different parameters obtained by the
𝑇
calculations including the estimated values of Φ𝐸𝑁𝑇
for the PVK and PEPK films doped with 5, 10, and 30
wt% QDs are presented in the Table S4 and Table S5. For the doped PVK films the estimation was
performed based on the fluorescence decays at 410 nm. For the doped PEPK films the calculations were
performed using both the fluorescence decays at 368 and at 405 nm [Table S3]. The calculations were
based on the average fluorescence quantum yield for fluorescence (Φ𝑓0 ) (0.21±0.04) and intersystem
0
crossing (Φ𝐼𝑆𝐶
) (0.79±0.04) determined for a pristine film of PEPK. For PEPK one should note that at 368
nm an important part of the emission is due to carbazole monomers with a smaller (about 1.2 instead of
about 4) ratio between the quantum yields of intersystem crossing and fluorescence compared to the fexcimer. This means that for PEPK the data based on analysis of the fluorescence decays at 368 nm
probably overestimate the quantum yield of intersystem crossing and hence somewhat underestimate
𝑇
Φ𝐸𝑁𝑇
.
9
𝑆
TABLE S4 Singlet transfer quantum yields (Φ𝐸𝑁𝑇
) and parameters used for the calculations of triplet
𝑇
energy transfer efficiencies (Φ𝐸𝑁𝑇 ) to QDs in doped PVK films based on recorded fluorescence decays of
PVK at 370 and 410 nm.
𝝀𝒆𝒎
370
nm
410
nm
f
QD
conc
.
 SENT
S
330
f ,QD  f ,QD
Tf ,QD  ISC
 TENT
5%
0.075±0.
004
10%
0.143
±0.007
30%
0.29
±0.01
5%
0.034
±0.002
0.16±0
.03
0.43±0.02
0.07±0
.02
0.009±0.
001
0.06±0
.02
0.81±0
.03
0.27±0
.12
10%
0.071
±0.004
0.15±0
.03
0.56±0.03
0.08±0
.02
0.019±0.
003
0.06±0
.02
0.78±0
.03
0.31±0
.16
30%
0.122
±0.006
0.14±0
.03
0.74±0.04
0.10±0
.03
0.032±0.
005
0.07±0
.03
0.74±0
.03
0.37±0
.22
Emission
ratio
QD:PVK
*  ENT : singlet energy transfer quantum yield,
S
f
: fluorescence quantum yield of PVK (PEPK) in the presence of C16 capped
QDs, Emission ratio QD:PVK (QD:PEPK): ratio of the area under the emission of the QDs and that of PVK (PEPK) in the
S
 330
f ,QD : fluorescence quantum yield of the QD emission upon excitation at 330 nm;  f ,QD
fluorescence spectra,
quantum yield of the QD emission due to singlet energy transfer,
triplet energy transfer,
 ISC :

fluorescence
T
f ,QD : fluorescence quantum yield of the QD emission due to
intersystem crossing quantum yield in a presence of quenching,
 TENT :
triplet energy transfer
efficiency.
𝑆
TABLE S5 Singlet transfer quantum yields (Φ𝐸𝑁𝑇
) and parameters from the calculations of triplet energy
𝑇
transfer efficiencies (Φ𝐸𝑁𝑇 ) to QDs in doped PEPK films based on recorded fluorescence decays of PEPK
at 368 and 405 nm.
𝝀𝒆𝒎
368
nm
405
nm
QD
conc
.
 SENT
f
5%
0.144±0.
0007
0.19±0
.03
10%
0.300±0.
015
30%
S
330
f ,QD  f ,QD
Tf ,QD  ISC
 TENT
0.31±0.02
0.06±0
.01
0.038±0.
002
0.02±0
.01
0.67±0
.03
0.11±0
.09
0.15±0
.02
0.56±0.03
0.09±0
.02
0.079±0.
005
0.01±0
.02
0.55±0
.03
0.05±0
.16
0.41±0.0
2
0.13±0
.02
1.49±0.07
0.19±0
.04
0.107±0.
007
0.09±0
.05
0.46±0
.03
0.72±0
.48
5%
0.104±0.
005
0.20±0
.03
0.31±0.02
0.06±0
.01
0.027±0.
002
0.03±0
.01
0.70±0
.03
0.18±0
.08
10%
0.22±0.0
1
0.17±0
.03
0.56±0.03
0.10±0
.02
0.057±0.
004
0.04±0
.02
0.61±0
.03
0.24±0
.16
30%
0.28±0.0
1
0.16±0
.02
1.49±0.07
0.24±0
.05
0.072±0.
005
0.17±0
.05
0.56±0
.03
1.1±0.
4
* See the caption under Table S4
10
Emission
ratio
QD:PEPK
1. A. Khetubol, S. Van Snick, A. Hassinen, E. Fron, Y. Firdaus, L. Pandey, C. David, K. Duerinckx, W.
Dehaen, Z. Hens, M. Van der Auweraer, J. Appl. Phys. 2013, 113, 083507.
2. M. Maus, E. Rousseau, M. Cotlet, G. Schweitzer, J. Hofkens, M. Van der Auweraer, F.C. De
Schryver, A. Krueger, Rev. Sci. Instrum. 2000, 72, 36.
3. A. Khetubol, A. Hassinen, Y. Firdaus, W. Vanderlinden, S. Van Snick, S. Flamée, B. Li, S. De Feyter,
Z. Hens, W. Dehaen, M. Van der Auweraer, J. Appl. Phys. 2013, 114, 173704.
4. G.R. Fulmer, A.J.M. Miller, N.H. Sherden, H.E. Gottlieb, A. Nudelman, B.M. Stoltz, J.E. Bercaw, K.I.
Goldberg, Organometallics 2010, 29, 2176.
5. a) Z. Hens, I. Moreels, J.C. Martins, ChemPhysChem 2005, 6, 2578; b) I. Moreels, B. Fritzinger, J.C.
Martins, Z. Hens, J. Am. Chem. Soc. 2008, 130, 15081.
6. U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, T. Nann, Nature Methods 2008, 5,
763.
7. A.R. Clapp, I.L. Medintz, J.M. Mauro, B.R. Fisher, M.G. Bawendi, H. Mattoussi, J. Am. Chem. Soc.
2004, 126, 301.
8. a) B. Wardle, Principles and Applications of Photochemistry; John Wiley & Sons Ltd 2009, Ch. 5;
b) Principles of Photochemistry; J.A. Barltrop, J.D. Coyle, Eds.; Wiley, Chichester Eng. and New
York, 1978; c) Excited States in Organic Chemistry; J.A. Barltrop, J.D. Coyle, Eds.; Wiley, London
and New York 1975.
11