Self-Assembled Microdisk Lasers of Perylenediimides

Article abstract of DOI:10.1021/jacs.5b10353

Organic solid-state lasers (OSSLs) have been a topic of intensive investigations. Perylenediimide (PDI) derivatives are widely used in organic thin-film transistors and solar cells. However, OSSLs based on neat PDIs have not been achieved yet, owing to the formation of H-aggregates and excimer trap-states. Here, we demonstrated the first PDI-based OSSL from whispering-gallery mode (WGM) hexagonal microdisk (hMD) microcavity of N,N′-bis(1-ethylpropyl)-2,5,8,11-tetrakis(p-methyl-phenyl)-perylenediimide (mp-PDI) self-assembled from solution. Single-crystal data reveal that mp-PDI molecules stack into a loosely packed twisted brickstone arrangement, resulting in J-type aggregates that exhibit a solid-state photoluminescence (PL) efficiency σ > 15%. Moreover, we found that exciton-vibration coupling in J-aggregates leads to an exceptional ultrafast radiative decay, which reduces the exciton diffusion length, in turn, suppresses bimolecular exciton annihilation (bmEA) process. These spectral features, plus the optical feedback provided by WGM-hMD microcavity, enable the observation of multimode lasing as evidenced by nonlinear output, spectral narrowing, and temporal coherence of laser emission. With consideration of high carrier-mobility associated with PDIs, hMDs of mp-PDI are attractive candidates on the way to achieve electrically driven OSSL.

Full text of DOI:10.1021/jacs.5b10353

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Article
Self-assembled Microdisk Lasers of Perylenediimides
Zhenyi Yu, Yishi Wu, Qing Liao, Haihua Zhang, Shuming Bai, Hui Li,
Zhenzhen Xu, Chunlin Sun, Xuedong Wang, Jiannian Yao, and Hongbing Fu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b10353 • Publication Date (Web): 18 Nov 2015
Downloaded from http://pubs.acs.org on November 18, 2015
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Self-assembled Microdisk Lasers of Perylenediimides
Zhenyi Yu,†,‡ Yishi Wu,*,† Qing Liao,§ Haihua Zhang,§ Shuming Bai,§ Hui Li,§ Zhenzhen Xu,§
Chunlin Sun,† Xuedong Wang,†,‡ Jiannian Yao†,⊥ and Hongbing Fu*,†,§,⊥
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†Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of chemistry, Chinese Academy of Sciences,
Beijing 100190, People's Republic of China
‡Graduate University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
§Beijing Key Laboratory for Optical Materials and Photonic Devices (BKLOMPD), Department of Chemistry, Capital
Normal University, Beijing 100048, People's Republic of China
⊥Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, People's Republic of China
ABSTRACT: Organic solid-state lasers (OSSLs) have been a topic under intensive investigations. Perylenediimide (PDI)
derivatives are widely used in organic thin-film transistors and solar cells. However, OSSLs based on neat PDIs have not
been achieved yet, owing to the formation of H-aggregates and excimer trap-states. Here, we demonstrated the first PDI-
based OSSL from whispering-gallery mode (WGM) hexagonal microdisk (hMD) microcavity of N,N’-bis(1-ethylpropyl)-
2,5,8,11-tetrakis(p-methyl-phenyl)-perylenediimide (mp-PDI) self-assembled from solution. Single-crystal data reveal that
mp-PDI molecules stack into a loosely packed twisted brickstone arrangement, resulting in J-type aggregates that exhibit
a solid-state photoluminescence (PL) efficiency
ϕ > 15%. Moreover, we found that exciton-vibration coupling in J-
aggregates leads to an exceptional ultrafast radiative decay, which reduces the exciton diffusion length, in turn, suppress-
es bimolecular exciton annihilation (bmEA) process. These spectral features, plus the optical feedback provided by WGM-
hMD microcavity, enable the observation of multimode lasing as evidenced by nonlinear output, spectral narrowing, and
temporal coherence of laser emission. With consideration of high carrier-mobility associated with PDIs, hMDs of mp-PDI
are attractive candidates on the way to achieve electrically driven OSSL.
rious concentration quenching effect.11 ii) Under intense-
INTRODUCTION
ly optical or electrical excitation, high density Frenkel
excitons will accumulate in the excited state and overlap
Organic solid-state lasers (OSSLs) have been widely in-
with each other, thus bimolecular exciton–exciton anni-
vestigated during the past decades,1 owing to their ame-
hilation (bmEA) takes place.1a,12 As the bmEA process can
nability to low-cost and low-temperature processing,2
cause very serious efficiency roll-off at high carrier density
compatibility with plastic substrates,3 and broad spectral
in OLED,12 it is considered as one of major blocks on the
tunability.4 A variety of optical resonators have been ap-
way to achieve electrically pumped OSSLs, in addition to
plied for optically pumped OSSLs,1a including planar
other problematic processes, such as triplet absorption,
waveguide Fabry-Pérot (FP) microcavity,5 distributed
triplet-triplet annihilation, polaron absorption or annihi-
feedback (DFB),6 whispering-gallery mode (WGM) mi-
lation, and so on.12,13 Therefore, development of organic
croring microresonator,7 and photonic band-gap struc-
semiconductor gain materials, which not only exhibit
tures.8 In these devices, the gain medium generally com-
high carrier mobility and photoluminescence (PL) quan-
posed of a polycrystalline thin-film of either polymer or
tum yield (ϕ) simultaneously but also possess optimized
small molecule semiconductors. OSSLs have already
been demonstrated in three primary colors of blue, green,
and red, which are essential for realization of full-color
laser displays.9 Nevertheless, electrically driven OSSLs
remain still a great challenge hindered mainly by two
stumbling blocks.10 i) The conflicting requirement be-
tween large stimulated emission and high charge carrier
mobility narrows the range of organic semiconductor
gain materials available for electrically driven OSSLs.
This is because most of organic semiconductors, which
exhibit high carrier mobilities, such as pentacene and
rubrene, are poorly emissive in the solid state due to se-
energy levels that can help decrease bmEA process, is of
crucial importance.
Perylenediimide (PDI) derivatives are one of the super-
star organic semiconductors widely used in organic thin-
film transistors (OTFTs)14 and photovoltaic solar cells,15
due to their excellent thermal- and/or photo-stability,
strong electron-accepting character and their chemically
tunable structures via modification at either imide or bay
positions.16 For instance, electron mobility as high as 4.65
cm2V-1s-1 was reported in n-channel OFETs of tetrachlo-
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rinated diperylenebisimide.17 Moreover, power conver-
ronic acid neopentyl glycol ester and bis(N-ethylpropyl)-
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sion efficiency > 6.0% was achieved in a solution-
processed non-fullerene solar cell using a helical PDI di-
mer as the electron acceptor.18 Note that PDI monomers
in the dilute solution are highly emissive with ϕmonomer
close to 100%, enabling applications, such as dye laser,
optical sensing, and single molecule spectroscopy.19
Nonetheless, strong π-π intermolecular interactions drive
PDI molecules form cofacially stacked H-aggregates in
the solid state, which exhibit unfavorable, strongly
quenched PL properties with ϕH < 1%.16 This, plus the
formation of excimer trap-states in thin film,20 limits the
application of PDIs in light-emitting devices. Although
DFB lasers have been demonstrated by doping PDIs into
an optically (also electrically) inert polystyrene (PS) film,
the concentration of PDIs was typically kept low at 0.5–1
wt% of PDI with respect to PS in order to avoid the con-
centration quenching effect.21 To the best of our
knowledge, OSSLs based on neat PDIs have not been
achieved yet. Here, we demonstrated the first PDI-based
OSSL from hexagonal microdisk (hMD) WGM microcavity
perylenediimidee (PDI-1) was heated in refluxing mesity-
lene and pinacolone at 140°C for 48h in the presence of
25 mol% of [RuH2(CO)(PPh3)3] (see supporting infor-
mation for details). After silica-gel separation, mp-PDI
1
was obtained in 59% yield and was characterized by H-
NMR and MALDI-MS (see supporting information).
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of
N,N’-bis(1-ethylpropyl)-2,5,8,11-tetrakis(p-methyl-
phenyl)-perylenediimide (mp-PDI) self-assembled from
solution. Single-crystal X-ray diffraction (XRD) analysis
revealed that steric hindrance of methylphenyl groups
substituted at head-positions drives mp-PDI molecules
stacking in a loosely packed twisted brickwork arrange-
ment of J-type aggregates, with a solid-state PL efficiency
Figure 1. (a) SEM and (b) PL microscopy images of as-
prepared hMDs of mp-PDI. (d) SAED pattern recorded by
directing the electron beam perpendicular to the top-
surface of a single hMD shown in (c). (e) XRD spectrum of
a mat of hMDs deposited on a silicon wafer. The inset
illustrates a cartoon of hMD.
of
ϕ > 15%. This, plus the optical feedback provided by
hMD-WGM microcavity, enables the observation of mul-
timode lasing as evidenced by nonlinear output, spectral
narrowing, and temporal coherence of laser emission.
More importantly, we found that ultrafast radiative decay
due to exciton-vibration coupling in J-aggregates signifi-
cantly reduces the exciton diffusion length (LD) and
therefore suppresses the bmEA process. These features,
such as the moderate optical gain and the absence of
bmEA, combine with the high carrier-mobility associated
with PDIs, making mp-PDI attractive candidates on the
way to achieve electrically driven OSSL.
In our experiment, single-crystalline hMDs of mp-PDI
were prepared via the reprecipitation method,23 by injec-
tion of 100 ꢀL of a stock solution (0.5 mg/mL) in di-
chloromethane into 0.5 mL of hexane. Within 1 h, hMDs
of mp-PDI were formed and then centrifugally separated
from the suspension and washed twice using hexane pri-
or to vacuum drying.
Figure 1a shows a scanning electron microscope (SEM)
image of typical products deposited on a silicon wafer. It
can be seen that hMDs of mp-PDI were successfully pre-
pared, which exhibit smooth outer surfaces and sharp
edges (also see Figure S1a). The edge-length (W) of hMDs
is found to range between 1 and 10 ꢀm, with 80% of MDs
having edge-lengths between 3 and 8 ꢀm (Figure S1b).
The thickness (T) of hMDs of mp-PDI is about 600 nm as
shown by a vertically grown hMD at the upper-left of
Figure 1a, and is further confirmed by atomic force mi-
croscopy (AFM) measurements (Figure S2). Figure 1c de-
picts a transmission electron microscopy (TEM) image of
a single hMD. Selected area electron diffraction (SAED)
pattern was recorded by directing the electron beam
perpendicular to the flat surface of the single hMD (Fig-
ure 1d). The clearly observed rectangular symmetry of
SAED spots in Figure 1d clarifies that hMDs of mp-PDI
are single crystalline in nature.
RESULTS AND DISCUSSION
Scheme 1. Ru-catalyzed four-fold arylation of mp-
PDI.
We have synthesized mp-PDI via a Ru-catalyzed 4-fold
arylation reaction (Scheme 1).22 A mixture of phenyl bo-
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(011), (0-11), (01-1) and (0-1-1) crystal planes on the six
side-faces (also see Figure S3b)
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It should be pointed out that different from bay-
substituted PDIs, which generally exhibit a highly twist-
ed perylene backbone, head-substituted mp-PDI mole-
cule maintains the planarity of the perylene core with
four carbonyl oxygen atoms sticking out the perylene
plane up and/or down (Figure 2a and b). Figure 2c de-
picts the packing arrangement of mp-PDI molecules
within the crystal bc plane [that is, the (100) crystal
plane], viewed perpendicular to the top-surface of hMDs.
It can be seen that mp-PDI molecules are packed in a
brickwork arrangement,24 similar to the cases of parent
PDI-NH and bay-substituted tetrachloro- and octachlo-
ro-PDIs.25 Specifically, each mp-PDI molecule labeled 1 is
intercalated between four molecules labeled 2 and vice
versa, through molecular contacts between carbonyl oxy-
gen and perylene bay-position carbon atoms, such as,
C(1)⋅⋅⋅O'(3), O(1)⋅⋅⋅C'(7), C(7)⋅⋅⋅O'(1) and O(3)⋅⋅⋅C'(1) con-
tacts (3.12 Å, see pink dashed lines in Figure 2c). Because
mp-PDI molecules 1 and 2 form a dihedral angle about
19.8°, an average π-π distance of dπ-π = 4.27 Å is obtained
as indicated by blue double-arrows between (040) and
(0-40) crystal planes in Figure 2c, which is much larger
than 3.34 Å reported for parent PDI-NH and 3.40−3.45Å
reported for bay-substituted tetrachloro- and octachloro-
PDIs PDIs.25,26 Therefore, our mp-PDI molecules in sin-
gle-crystalline hMDs arrange in a loosely packed twisted
brickstone structure. Furthermore, it can be seen from
Figure 2d that orthogonal projections of mp-PDI mole-
cules 1 and 2 on the crystal ac plane are oriented exactly
in the same direction, with their N-N axis tilted at an an-
gle of 42.12° with respect to the crystal c-axis. The longi-
tudinal (along the molecular long-axis) and transverse
(along the molecular short-axis) displacements between
mp-PDI 1 and 2 are measured to be dL-L = 5.88 Å (Figure
2d and S4a) and dS-S = 6.17 Å (Figure 2d and S4b), respec-
tively.27 Therefore, π-π overlap between each mp-PDI
molecule and its four neighbors is negligible in the loose-
ly packed twisted brickstone structure (Figure 2d). This
is in sharp contrast with most PDI derivatives reported in
literatures, which generally exhibit strong π-π overlap
owing to small values of dπ-π, dL-L and dS-S.14-18
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Figure 2. Molecular structures of mp-PDI obtained from
single-crystal X-ray analysis, viewed perpendicular (a) and
parallel (b) to the perylene backbone. Packing arrange-
ments of mp-PDI molecules within the crystal bc plane (c)
and ac plane (d). See text for details.
In order to understand the solid-state molecular packing,
we also cultivated bulk-crystals suitable for single-crystal
X-ray diffraction (XRD) analysis using a solvent diffusion
method at the liquid-liquid interface between CH2Cl2 and
hexane. Monoclinic mp-PDI crystals belong to the space
group of C2/c with cell parameters of a = 38.429(16) Å, b
= 8.548(3) Å, c = 17.774(8) Å,
α = γ = 90°, β = 112.242(6)°
(CCDC No. 1407196, Figure S3a). Based on single-crystal
cell parameters, the red circled set of SAED spots in Fig-
ure 1d is ascribed to (010) Bragg reflections with d(010)
=
Figure 3a presents the steady-state absorption (black)
and PL (red) spectra of a mat of mp-PDI hMDs (solid)
deposited on a quartz plate and mp-PDI monomers (dash
dot) in the dilute solution. Table 1 summarizes the relat-
ed photo-physical parameters. The lowest S0→S1 transi-
tion of mp-PDI monomers in the dilute solution exhibits
a vibronic progression with a subband spacing around
1400 cm-1, while the PL spectrum of monomers shows a
mirror structure of its absorption with 0-0, 0-1, o-
2emissions at 542, 584, 635 nm, respectively. The absorp-
tion and PL maxima of mp-PDI monomers in CH2Cl2 are
factually similar to those of monomers of parent PDI-NH
8.5 Å, while the white and yellow squared sets of SAED
spots are ascribed to (001) and (002) Bragg reflections
with d(001) = 17.8 Å and d(002) = 8.9 Å, respectively. Figure
1e presents the XRD spectrum of a mat of hMDs deposit-
ed on a silicon substrate, which is dominated by a series
of peaks corresponding to the crystal plane (100), such as
(200), (300), (400), and (500) peaks. This suggests that
hMDs of mp-PDI adopt a lamellar structure with the crystal
(100) plane parallel to the substrate (Figure S3a). Combin-
ing SAED and XRD results together, we drew a schematic
cartoon of mp-PDI hMDs (see the inset of Figure 1e), which
are single crystals bound by (100) and (-100) crystal
planes on the top and bottom faces and by (002), (00-2),
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Table 1. Photo-physical parameters of mp-PDI monomers in the dilute solution and solid-state hMDs.
kr (108 s-1)
4
5
6
sample
λ
abs (nm)
λ
em (nm)
ϕ
τ (ns)
monomer
460,493,530
542,584,635
0.03
0.15
2.0
7
8
hMDs
464,497,533
560,595,645
0.15
0.05
30.0
9
groups (methylphenyl groups in our mp-PDI) and pre-
sents n-octyl chains at imide nitrogen positions
(ethylpropyl groups in our mp-PDI).28 Single crystal data
reveal that head-substituted p-PDI molecules form face-
to-face slip-stacked columns with dπ-π = 3.40 Å, dL-L = 3.29
Å and dS-S = 0.28 Å. As a result, the strong π-π stacking
makes the absorption and PL spectra of p-PDI aggregates
significantly red-shifted than those of p-PDI monomers.28
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As abovementioned, most PDI derivatives are highly
fluorescent in the dilute solution, but are poorly emissive
in the solid-state owing to the formation of H-aggregates
and excimer traps. By measuring the PL quantum yield
(ϕ) through a relative method for the monomer solution
in CH2Cl2 by using Rhodamine 6G as a standard and
through an absolute method for hMDs by using an inte-
gration sphere, however, we found that mp-pDI mono-
mers are poorly emissive in the dilute solution (ϕm,mp-PDI
= 0.03 ± 0.01), but hMDs exhibit a moderate value of
ϕ
hMD,mp-PDI = 0.15 ± 0.02 (Table 1).
To obtain further information on the nature of the ex-
cited states, we also measured the PL lifetimes by using a
streak camera (Figure 3b). The monomer emission at 542
nm decays mono-exponentially, yielding a lifetime of
τm,mp-PDI = 0.15 ± 0.02 ns (black line in Figure 3b). The PL
decay of hMDs at 595 nm was also fitted mono-
exponentially with a time constant of τhMD,mp-PDI = 0.05 ±
0.01 ns (red line in Figure 3b). According to the equation,
Figure 3. (a) Normalized UV/vis absorption spectra
(black) and PL (red) spectra of a mat of mp-PDI hMDs
(solid) deposited on a quartz plate and mp-PDI mono-
mers (dash dot) in the dilute solution. (b) PL decay and
fitted curves of mp-PDI monomeric solution in CH2Cl2
and hMDs on quartz plate.
k = ϕ/τ
,25 the radiative decay rates (k) are calculated to be
km,mp-PDI = 2.0 × 108 s-1 and khMD,mp-PDI = 30.0 × 108 s-1 for
mp-PDI monomers and hMDs, respectively (Table 1). i)
Although mp-PDI monomers are poorly emissive in the
dilute solution, the value of km,mp-PDI = 2.0 × 108 s-1 is fac-
tually comparable to those of parent PDI-NH,25a un-
substituted PDI-1 (Table S1), and bay-substituted PDIs.25b
(ref. 25a) and unsubstituted PDI-1 in the dilute solution
(Figure S5a). This suggests that the methylphenyl groups
do not conjugate with the perylene backbone in mp-PDI,
in good agreement with single-crystal structure data,
which reveal
a
dihedral angle of 53° between
Therefore, the low value of ϕm,mp-PDI = 0.03 ± 0.01 is prob-
methylphenyl groups and perylene core (Figure 2a). No-
tably, we found that the diffused reflectance absorption
spectrum of mp-PDI hMDs resembles that of monomers
(Figure 3a). And the 0-0, 0-1, o-2 emission bands of mp-
PDI hMDs at 560, 595, 645 nm, respectively, are only
slightly red-shifted as compared with those of monomers
(Table 1). The similarity between spectral features of mp-
PDI monomers and hMDs suggest weak intermolecular
interactions in the solid state, as evidenced by large π-π
distance of dπ-π = 4.27 Å (Figure 2c) and negligible π-π
overlap between mp-PDI molecules (Figure 2d). Recently,
Wasielewski and co-workers reported a molecule, name-
ly, p-PDI, which is also head-substituted but by phenyl
ably a result of fast nonradiative decay caused by rota-
tional motion of methylphenyl substituents in mp-PDI
(see Figure 2a and b).29 ii) khMD,mp-PDI is about 15 times
higher than km. Such enhanced radiative decay (known
as “superradiance”) is a key characteristic of J-aggregates,
attributed to exciton delocalization as a result of the
coupling of the transition dipole moments of aggregated
chromophores.24 Moreover, the fact of khMD,mp-PDI > km,mp-
excludes the formation of excimer trap-states, proba-
PDI
bly a result of the large π-π distance of dπ-π = 4.27 Å in
mp-PDI hMDs.20 It is well-known that the transition di-
pole of PDIs is along the N−N axis.16c Returning to Figure
2c, within (0-44) [or (0-4-4)] crystal plane, the angle be-
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tween the transition dipole and the packing direction is
42.2° and 53.4° for mp-PDI molecule 1 (or 2) and 2 (or 1)
1
2
(also see Figure S4), respectively, placing the alignment
in J-aggregate regime but close to the magic angle of
54.7°.24a For a typical J-aggregate one would expect the
electronic origin (0-0 transition) to carry the highest os-
cillator strength.24b However, the PL spectrum of mp-PDI
hMDs in Figure 3a exhibits a 0-1 emission peak stronger
than the 0-0 emission peak. This is due to self-absorption
effect as a result of small Stokes shift typical for J-
aggregate (Figure S6a). Furthermore, the 0-0 emission
peak of mp-PDI hMDs is found to be enhanced at 77 K as
compared with that at room temperature (Figure S6c),
consistent with the J-aggregation model developed by F.
C. Spano.24b According to the equation, N = khMD/km, the
size of the coherent domain can be estimated to be N ≈ 15
molecules in J-aggregation hMDs of mp-PDI.30 In sharp
contrast, in the case of un-substituted PDI-1 (Table S1),
the radiative decay rate of weakly emissive H-aggregates,
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kH,PDI-1 = 0.77 × 108 s-1 (
0.15 ns), is slower than that of highly emissive monomers
in the dilute solution, km,PDI-1 = 1.7 × 108 s-1 (
m,PDI-1 = 0.95 ±
0.01,
hMD,PDI-1 = 5.55 ± 0.15 ns).24b
ϕH,PDI-1 = 0.05 ± 0.01, τH,PDI-1 = 0.65 ±
ϕ
Figure 4. (a) The ꢀ-PL spectra of a single hMD with W =
5.5 ꢀm under different pump pulse densities at room
temperature. (b) Integrated area of the 0–1 peak as a func-
tion of pump density. The lasing threshold is identified as
the intersection between the linear and superlinear re-
gions. (c) PL decay profiles of hMD monitored at 595 nm,
showing the evolution from un-quenched spontaneous
emission to lasing emission, with increasing the pumping
laser power. Above the threshold the decay time always
collapses to < 10 ps, indicating an effective stimulated
process. The bmEA is nearly disappeared below the
threshold so that the decay time changes little.
τ
To further explore the PL properties of an individual
hMD, we used home-made optical microscope
a
equipped with a 50 × 0.9 NA objective (Figure S7). The
second harmonic of a 1 KHz Ti: sapphire regenerative
amplifier (λ = 400 nm, pulse width 150 fs) is focused to a
20-μm diameter spot to excite the isolated single hMD
placed on a 2D movable table. Spatially resolved PL spec-
tra with a resolution about 1 μm are then collected un-
derneath by using a 3D movable objective coupled to an
optical fiber and detected using a liquid-nitrogen cooled
charge-coupled device (CCD).
To gain further insight, we investigated the PL lifetimes
at different pump intensities. As above mentioned, PL of
individual hMD follows single exponential decay with
Figure 4a shows the PL spectra of an isolated hMD (W =
5.5 μm) with increasing the pump intensity of 400-nm
laser (P). At P = 1.65 ꢀJ/cm2, the PL spectrum of mp-PDI
hMD is presented as a broad spontaneous emission.
When the pump density reaches a threshold from P = 11.8
to 14.6 ꢀJ/cm2, strong laser emission arises as a set of
sharp peaks on the top of the 0–1 transition. This agrees
with the fact that the 0–1 transition has the maximum
net gain in four-level organic laser system,1a because un-
occupied vibronic sublevels of the ground state in ther-
mal equilibrium facilitates the build-up of population
inversion.2 Figure 4b shows the integrated intensities of
the 0−1 peak as a function of P, which demonstrates a
threshold at Pth = 10.8 ꢀJ/cm2. The intensity dependence
is fitted to a power law xp (ref. 2), with p = 1.04 ± 0.05 and
2.93 ± 0.11 below and above the threshold, respectively.
The latter p = 2.93 ± 0.11 above the threshold indicates a
superlinear increase of PL output, characteristic of laser
emission. In previous studies,2,5-8,23 however, p ≈ 0.5 has
been exclusively reported below the threshold owing to
the presence of bmEA process. Therefore, p = 1.04 ± 0.05
obtained for mp-PDI hMD below the threshold suggests
the absence of bmEA.
τhMD,mp-PDI = 0.05 ± 0.01 ns at a very low excitation density
of P = 0.01 Pth. Upon increasing the pump intensity to P =
0.14 Pth and/or 0.8 Pth, the PL decay changes little. This
further confirms the absence of bimolecular quenching.
Note that bmEA process is related to collision of excitons,
controlled by the exciton diffusion according to L = ꢀꢁ,
√
D
where D is the diffusion coefficient,
lifetime and diffusion length, respectively.31 Using D = 5 ×
10-4 cm2s-1 and = 10-9 s (typical for organic semiconduc-
tors),31 LD = 7.1 nm is obtained. In our case, τhMD,mp-PDI
τ and LD the exciton
τ
=
0.05 ± 0.01 ns gives rise to LD = 1.6 nm, just equal to the
size of a single mp-PDI molecule. That is, the fast radia-
tive decay of mp-PDI J-aggregate significantly decreases
the exciton diffusion length, thereby avoiding the exciton
collision, i.e., the bmEA process. In sharp contrast, the
fluorescence intensity dependence of PDI-1 H-
aggregation film is fitted to a power law xp with p = 0.46
± 0.05 (Figure S8), indicating the occurrence of bmEA
even at low excitation density.2,7b Previously, femto-
second transient absorption (fsTA) measurement on con-
jugated polymer thin-film identified extremely short-
lived gain through monitoring the stimulated emission
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process (ref. 1a and references therein). By performing
smallest angle of TIR is
ψTIR = 33.7°. As shown in the inset
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4
5
6
7
8
fsTA on 100-nm thick thin-films (Figure S9), stimulated
emission signal is clearly observed for mp-PDI film, but is
undetectable for PDI-1 film. Therefore, the absence of
bmEA process can greatly help the build-up of optical
gain in J-aggregation hMDs of mp-PDI. In any event, it
can be seen from Figure 4c that above the threshold, for
instance, at P = 1.3 Pth and 1.8 Pth, the PL decay time of
mp-PDI hMD collapses to < 10 ps, demonstrating an ef-
fective stimulated process within the hMD microcavity.
of Figure 5b, = 60° for 6-WGM is larger than ψTIR
ψ
=
33.7°, suggesting that our hMDs can indeed support 6-
WGM microcavities.
To further understand the lasing modes in mp-PDI
hMDs, we performed optical mode simulation for an
hMD (5 ꢀm in width and 600 nm in thickness) placed on
a quartz plate by using the finite-difference time-domain
(FDTD) method with parameters of refractive indexes of
mp-PDI (nr,PDI = 1.8) and quartz (nquartz = 1.4). Figure 5a
shows the absolute electric field distribution |E|2 pat-
terns of a TM-mode, viewed perpendicular to the hexag-
onal top-surface. As one can see, this TM-mode is well-
confined in the hMD and reflected between the six side-
faces. The cavity quality factor (Q) is calculated to be 709
for this TM-mode. We also measured the Q experimen-
tally according to Q = λ/δλ, where λ and δλ are the wave-
length and full-width at half maximum of the lasing
peak.7b,23 And λ = 595 nm and δλ = 1.05 nm from Figure
4a gives rise to Q = 567, which is comparable to the theo-
retical value.
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In order to understand the effect of the hMD size on the
cavity effect, we investigated a series of hMDs with W = 4,
7, and 9 ꢀm, respectively. It can be seen from Figure 5c
that the lasing spectra of hMDs exhibit more and more
modes with increasing the hMD side-length. Therefore,
the spacing between adjacent modes, ∆λ, decreases with
increasing the value of W, for instance, ∆λ = 3.64, 1.78,
1.25 nm for W = 4, 7, and 9 ꢀm, respectively (Figure 5c).
It is known that ꢁ
with a cavity length of L is given by ꢁ
λ
at wavelength
λ
for a WGM cavity
2/L[n-
(dn/dλ)], where [n-λ(dn/dλ)] is the group refractive in-
Figure 5. (a) Optical mode simulation results for an hMD
with W = 5 ꢀm and T = 0.6 ꢀm. (b) The mode spacing ∆λ
at λ = 596 nm versus 1/W of hMDs, showing clearly a
linear relationship. The inset illustrates a typical optical-
ray analysis within WGM-hMD microcavity. (c) High-
resolution PL spectra of laser emission recorded above
threshold for hMDs with W = 9, 7, and 4 ꢀm, respectively.
λ
= λ
λ
dex.7b,23 According to geometric conditions, L = 3 3W is
√
obtained for an hMD-WGM microcavity of mp-PDI. Fig-
ure 5b plots the value of ꢁλ at λ = 595 nm as a function of
1/W, demonstrating clearly a linear relationship. This fur-
ther confirms the built-in WGM microcavity within mp-
PDI hMDs. Moreover, we found that the Q value increas-
es with increasing the cavity length, Q = 163, 334, 476, for
W = 4, 7, and 9 ꢀm, respectively, in good agreement with
optical simulation results (Figure S10).
Returning to Figure 4a, the upper and lower insets show
the μ-PL images of the 5.5-ꢀm-width hMD below and
above the lasing threshold, respectively. And six bright
edges are observed due to light leakage. This suggests
that six side-faces of mp-PDI hMDs form a WGM micro-
resonator. Note that semiconductor microdisk WGM
resonators, in which photons are strongly confined
through successive total internal reflection along the disk
circumference, have been widely used for acquiring nar-
row-linewidth and low-threshold microlasers.7a Recently,
well-faceted WGM microlasers based on self-assembled
single-crystalline organic microdisks have been also
demonstrated.7b The inset of Figure 5b presents a classi-
cal optical ray analysis within WGM-hMD microcavity of
mp-PDI. It is well-known that total internal reflection
CONCLUSION
In conclusion, we demonstrated the first PDI-based OSSL
from WGM-hMD microcavity of mp-PDI self-assembled
from solution. Single-crystal results reveal that mp-PDI
molecules stack in a loosely packed twisted brickwork
arrangement of J-type aggregates in hMDs, which exhibit
a solid-state PL efficiency of
ϕ > 15%. More importantly,
we found that exceptional fast radiative decay due to ex-
citon-vibration coupling in J-aggregates significantly re-
duces the exciton diffusion length (LD) and therefore
avoids the bmEA process. These features, such as the
moderate optical gain and the absence of bmEA, plus the
optical feedback provided by WGM-hMD microcavity,
enable the observation of multimode lasing. With con-
(TIR) takes place at the hMD/air interfaces if
ψ ≥
arcsin(1/nr), where is the angle of incidence light with
ψ
respect to the normal of the given facet, and nr is relative
phase refraction index.7b With nr,PDI = 1.8 for mp-PDI, the
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Journal of the American Chemical Society
(14) Usta, H.; Facchetti, A.; Marks, T. J. Acc. Chem. Res. 2011, 44,
sideration of high carrier-mobility associated with PDIs,
1
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501-510.
hMDs of mp-PDI are attractive candidates on the way to
achieve electrically driven OSSL.
(15) Hartnett, P. E.; Timalsina, A.; Matte, H. S.; Zhou, N.; Guo, X.;
Zhao, W.; Facchetti, A.; Chang, R. P.; Hersam, M. C.;
Wasielewski, M. R.; Marks, T. J. J. Am. Chem. Soc. 2014, 136,
16345-16356.
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7260-7314. (b) Huang, C.; Barlow, S.; Marder, S. R. J. Org. Chem.
2011, 76, 2386-2407; (c) Wurthner, F. Chem. Commun. 2004,
1564-1579.
ASSOCIATED CONTENT
Supporting Information
General synthesis procedures, spectral data, optical sim-
ulation results, and the CIF file for the X-ray analysis of
mp-PDI single-crystal with CCDC No. 1407196.
9
(17) Lv, A.; Puniredd, S. R.; Zhang, J.; Li, Z.; Zhu, H.; Jiang, W.;
Dong, H. L.; He, Y.; Jiang, L.; Li, Y.; Pisula, W.; Meng, Q.; Hu, W.
P.; Wang, Z. H. Adv. Mater. 2012, 24, 2626-2630.
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AUTHOR INFORMATION
Corresponding Author
(18) Zhong, Y.; Tuan Trinh, M.; Chen, R. S.; Wang, W.; Khlyabich,
P. P.; Kumar, B.; Xu, Q. Z.; Nam, C.-Y.; Sfeir, M. Y.; Black, C.;
Steigerwald, M. L.; Loo, Y.-L.; Xiao S. X.; Ng, F.; Zhu, X.-Y.;
Nuckolls, C. J. Am. Chem. Soc. 2014, 136, 15215-15221.
(19) (a) Calzado, E. M.; Villalvilla, J. M.; Boj, P. G.; Quintana, J. A.;
Gomez, R.; Segura, J. L; Diaz-Garcia, M. A. J. Phys .Chem. C 2007,
111, 13595-13605. (b) Siekierzycka, J. R.; Hippius, C.; Wurthner, F.;
Williams, R. M.; Brouwer, A. M. J. Am. Chem. Soc. 2010, 132,
1240–1242. (c) Weil, T.; Vosch, T.; Hofkens, J.; Peneva, K.; Mullen,
K. Angew. Chem. Int. Ed. 2010, 49, 9068-9093.
(20) Fink, R. F.; Volker Engel, J. S,; Martin Kaupp, M. R.;
Lochbrunner, S,; Zhao, H. M.; Pfister, J.; Wurthner, F.; Engels B.
J. Am. Chem. Soc. 2008, 130, 12858-12859.
(21) (a) Ramirez, M. G.; Pla, S.; Boj, P. G.; Villalvilla, J. M.;
Quintana, J. A.; Diaz-Garcia, M. A.; Fernandez-Lazaro, F.;
Sastre-Santos, A. Adv. Optical Mater. 2013, 1, 933-938. (b)
Ramirez, M. G.; Morales-Vidal, M.; Navarro-Fuster, V.; Boj, P. G.;
Quintana, J. A.; Villalvilla, J. M.; Retolaza, A.; Merino, S.; Diaz-
Garcia, M. A. J. Mater. Chem. C 2013, 1, 1182-1191.
(22) (a) Nakazono, S.; Easwaramoorthi, S.; Kim, D.; Shinokubo,
H.; Osuka, A. Org. Lett. 2009, 11, 5426-5429. (b) Battagliarin, G.;
Zhao, Y.; Li, C.; Müllen, K. Org. Lett. 2011, 13, 3399-3401.
(23) Wang, X. D.; Liao, Q.; Lu, X. M.; Li, H.; Xu, Z. Z.; Fu, H. B.
Sci. Rep. 2014, 4, 7011.
(24) (a) Wurthner, F.; Kaiser, T. E.; Saha-Moller, C. R. Angew.
Chem. Int. Ed.2011, 50, 3376-3411. (b) Spano, F. C. Acc. Chem. Res.
2010, 43, 429-439.
(25) (a) Cao, X. Q.; Wu, Y. S.; Fu, H. B.; Yao, J. N. J. Phys. Chem.
Lett. 2011, 2, 2163-2167. (b) Cao, X. Q.; Bai, S. M.; Wu, Y. S.; Liao,
Q.; Shi, Q.; Fu, H. B.; Yao, J. N. Chem. Commun. 2012, 48, 6402-
6404. (c) Gsanger, M.; Oh, J. H.; Konemann, M.; Hoffken, H. W.;
Krause, A. M.; Bao, Z. N.; Wurthner, F. Angew. Chem. In.t Ed.
2010, 49, 740-743.
(26) (a) Klebe, G.; Graser, F.; Hadicke, E.; Berndt, J. Acta Cryst. B
1989, 45, 69-77. (b) Kaiser, T. E.; Stepanenko, V.; Wurthner, F. J.
Am. Chem. Soc. 2009, 131, 6719-6732.
hongbing.fu@iccas.ac.cn; yswu@iccas.ac.cn
ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (Nos. 91222203, 21273251, 91333111,
21190034, 21221002), project of State Key Laboratory on Inte-
grated Optoelectronics of Jilin University (IOSKL2014KF16),
project of Construction of Innovative Teams and Teacher
Career Development for Universities and Colleges Under
Beijing Municipality (IDHT20140512), the National Basic Re-
search Program of China (973) 2011CB808402, 2013CB933500,
and the Chinese Academy of Sciences.
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