Design of thermally activated delayed fluorescent emitters for organic solid-state microlasers

Article abstract of DOI:10.1039/d0tc05562j

A small energy gap between charge transfer (CT) singlet and triplet states enables thermally activated delayed fluorescence (TADF). Nevertheless, the small oscillator strength associated with CT states and their long exciton lifetimes are detrimental to establishing a population inversion for stimulated emission (SE), hindering the application of a TADF material in organic lasers. Here, we demonstrated that a TADF molecule of sulfide-substituted difluoroboron derivatives can achieve stimulated emission in microcrystals by employing a new molecular design, in which an ultrafast reverse intersystem crossing (RISC) process was achieved between a hybrid locally excited CT (HLECT) singlet S₁and a high-lying triplet T₂(³HLECT) state. Femtosecond transient abaorption and time-reolved PL spectra reveal that the two states of S₁and T₂equilibrate within a time of 180 ps. In addition, the energetic spacing of ΔE_S1-T2= 0.11 eV enables delayed fluorescence involving the T₂state at room temperature. Besides, the extremely fast exciton lifetime (0.31 μs) that decreases the probability of carrier annihilation, the HLECT singlet provides larger oscillator strength and therefore larger SE cross-section than those of the CT state. A multimode TADF laser was realized based on the good optical feedback (cavity quality factorQ≈ 2000) provided by Fabry Perot (FP) microcrystal microcavity. Our results not only confirm that the high-lying T_nstate plays a key role in the RISC process of TADF, but also provides a design of TADF gain materials.

Full text of DOI:10.1039/d0tc05562j

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Design of thermally activated delayed fluorescent
emitters for organic solid-state microlasers†
Cite this: J. Mater. Chem. C, 2021,
, 7400
9
a
b
b
c
c
c
Shuai Li,‡ Xue Jin,‡ Zhenyi Yu,* Xiaoxiao Xiao, Hua Geng,
Qing Liao,
c
c
b
ac
Yi Liao, Yishi Wu, Wenping Hu and Hongbing Fu
*
A small energy gap between charge transfer (CT) singlet and triplet states enables thermally activated
delayed fluorescence (TADF). Nevertheless, the small oscillator strength associated with CT states and
their long exciton lifetimes are detrimental to establishing a population inversion for stimulated emission
(SE), hindering the application of a TADF material in organic lasers. Here, we demonstrated that a TADF
molecule of sulfide-substituted difluoroboron derivatives can achieve stimulated emission in
microcrystals by employing a new molecular design, in which an ultrafast reverse intersystem crossing
(
1
RISC) process was achieved between a hybrid locally excited CT (HLECT) singlet S and a high-lying
3
triplet T
2
( HLECT) state. Femtosecond transient abaorption and time-reolved PL spectra reveal that the
two states of S
1
and T
2
equilibrate within a time of 180 ps. In addition, the energetic spacing of
state at room temperature. Besides, the
DES1–T2 = 0.11 eV enables delayed fluorescence involving the T
2
extremely fast exciton lifetime (0.31 ms) that decreases the probability of carrier annihilation, the HLECT
singlet provides larger oscillator strength and therefore larger SE cross-section than those of the CT
state. A multimode TADF laser was realized based on the good optical feedback (cavity quality factor
Q E 2000) provided by Fabry Perot (FP) microcrystal microcavity. Our results not only confirm that the
Received 26th November 2020,
Accepted 13th May 2021
DOI: 10.1039/d0tc05562j
high-lying T
materials.
n
state plays a key role in the RISC process of TADF, but also provides a design of TADF gain
rsc.li/materials-c
OSSLs remain a formidable challenge. Owing to the spin statistics in
the electroluminescence process, recombination of injected elec-
Introduction
Organic semiconductors have attracted extensive attention in opto- trons and holes would generate 25% singlet and 75% triplet
electronic applications, due to their flexible molecular design, excitons. However, both OSSLs and dye lasers are based on a
adjustable optoelectronic properties, and the ease of device fluorescence gain mechanism. The widely recognized bottleneck
1–3
fabrication. In particular, organic light-emitting diodes (OLEDs) in realizing electrically driven OSSLs is ascribed to the accumulation
4
are of industry interest in flat-panel displays and lighting markets.
of long-lived triplets. On the one hand, a serious overlap of lasing
In the past decades, organic solid-state lasers (OSSLs) have been wavelength with triplet absorption leads to the so-called re-
demonstrated in a variety of resonators, including whispering- absorption, and therefore, photon loss; on the other hand, sing-
5
6
1,9–11
gallery mode (WGM) microresonator, distributed feedback (DFB)
let–triplet annihilation (STA) leads to the loss of singlet excitons.
7,8
and photonic band-gap structures. Nevertheless, electrically driven One strategy adopted so far is to remove triplet excitons by energy
12,13
transfer to a ‘‘scavenger’’.
This, however, is obviously not the best
a
b
way, because 75% of triplet excitons generated under electric
excitation are not fully utilized. Therefore, it is crucial to develop
organic gain materials capable of improving the utilization efficiency
of triplet excitons.
Institute of Molecule Plus, Tianjin University, and Collaborative Innovation Center
of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China
Tianjin Key Laboratory of Molecular Optoelectronic Sciences,
Department of Chemistry, School of Sciences, Tianjin University,
Tianjin 300072, P. R. China. E-mail: zhenyi.yu@tju.edu.cn
c
In recent years, phosphorescence and thermally activated delayed
fluorescence (TADF) materials have received increasing attention
because they can use both singlet excitons and triplet excitons to
Beijing Key Laboratory for Optical Materials and Photonic Devices,
Department of Chemistry, Beijing Advanced Innovation Center for Imaging
Technology, Capital Normal University, Beijing 100048, P. R. China.
E-mail: hbfu@cnu.edu.cn
14
obtain nearly 100% exciton utilization under electrical excitation.
†
Electronic supplementary information (ESI) available: Synthetic procedures,
However, the use of phosphorescent materials as an optical gain
medium still faces enormous challenges. We have recently dis-
covered a luminescent rotor as a phosphorescent gain material for
light amplification by stimulated emission (SE) from triplet
experimental details and supplemental figures. CCDC 1914187. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0tc05562j
‡
These authors contributed equally to this work.
7
400
|
J. Mater. Chem. C, 2021, 9, 7400–7406
This journal is © The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
1
5,16
3
states.
2
Recently, there have been several studies on amplified ( HLECT) states of the SBF BTF molecule exhibit a small DES1–T2
ꢁ1
spontaneous emission (ASE) of TADF emitters doped in thin value (0.11 eV), large SOC constant x(S
1
,T
As compared with doped thin-films, organic molecular small reorganization energy (86 meV), thus promoting the kISC
crystals (OMCs) provide a perfect platform for electrically pumped and kRISC of S 2 T , which is 1–2 orders of magnitude larger
2
) = 21.53 cm and
17–20
films.
1
2
19,25,26
OSSLs, becaue their highly ordered strucutre and well-faced shape than those of traditional TADF compounds.
Extremely fast
make it possible to combine together high carrier transport, efficient TADF life (0.31 ms) was realized in SBF BTF microcrystals, with the
2
7
ꢁ1
1
23,24
optical gain and bulit-in microresonators, such as nanowire Fabry– radiative decay rate of 10
s
typical for HLECT state.
Perot (FP) and microplate WGM microresonators. Nevertheless, Combining with the optical feedback provided by the FP micro-
OSSLs based on OMC microresonators made up of pure TADF cavity (cavity quality factor Q B 2000), SBF BTF microcrystals
molecules have remained largely unexplored. enable the observation of high-performance multimode TADF
21
2
Conventional design guideline of TADF emitters relies on lasers. Our results not only confirm that the high-lying T state
n
electron-donor (D)–electron-acceptor (A) systems, in which plays a key role in the RISC process of TADF, but also provides a
spatial separation between highest occupied and lowest unoc- basis for the design of TADF gain materials based on
cupied molecular orbitals (HOMOs and LUMOs) produces a HLECT state.
1 1
small energy gap between the singlet (S ) and triplet (T ),
DES1–T1 r 0.1 eV, which promotes intersystem crossing (ISC)
and reverse intersystem crossing (RISC) probabilities. Never- Results and discussion
1
theless, the small oscillator strength associated with CT S
1
Photophysical properties
state will result in a small stimulated emission (SE) cross
section. The hybrid locally excited (LE) CT (HLECT) state
exhibits large oscillator strength and large SE cross section,
2
2
Fig. 1b depicts the steady-state absorption (black line) and
photoluminescence (PL) spectra (red line) of SBF BTF in dilute
dichloromethane (DCM) solution (10 M) at room tempera-
ture. And the related photophysical data are listed in Table 1.
The absorption spectrum of SBF
2
ꢁ
5
but the large DES1–T 4 0.5 eV hinders the application of HLECT
1
2
3,24
emission for TADF.
efficient TADF through ultrafast RISC between a HLECT S
In this study, we demonstrated highly
1
2
BTF exhibits a strong
1
3
S
0
- S band at 374 nm with a molar extinction coefficient
2
state and a high-lying HLECT T
2
state. The compound of (E)-3-
((4-trifluoromethyl)imino)methyl)-2H-thiochroman-4-olateꢀBF
SBF BTF, Fig. 1a) was composed of sulfidesubstituted difluo-
4
ꢁ1
ꢁ1
of e = 3.28 ꢂ 10 M cm (Fig. S1 and S2, ESI†), which is
ascribed to LE p–p* transition, and a relative weak S0 - S1
band at 430 nm, which is assigned to n–p* transition with ICT
(
(
2
2
roboron part (SBF ) with lone-pair electrons that offers large
2
characteristics. Furthermore, the SBF BTF in DCM solution
2
spin-orbital coupling (SOC), and benzotrifluoride (BTF) part
that introduces weak intramolecuar charge transfer (ICT) char-
exhibits a broad PL spectrum with a main peak at 560 nm,
and a moderate PLQY of 12% measured by using an integration
acter. Experimental and theoretical results indicate that the
2
7
1
sphere. By measuring the efficiency (F
D
) of singlet oxygen
lowest singlet S
1
( HLECT) and the high-lying triplet T
2
1
28
(
O
2
) generation, we found that the value of F of SBF BTF is
D
2
as high as 83% (Fig. S3, ESI†), indicating that the ISC in
compound SBF BTF is highly efficient. We further investigated
2
the photophysical properties of SBF BTF in a number of
2
solvents with different polarities (Fig. S4, ESI†). A significant
red-shift of PL maxima from 538 nm in low-polarity n-hexane to
5
94 nm in high-polarity acetonitrile is observed. The Lippert–
2
9
Mataga relationship, as shown in Fig. S4c (ESI†), exhibits two
linear regions, indicating two different types of excited states
2 2 2
Table 1 Photophysical parameters of SBF BTF compound in CH Cl
solution
S
1
–T
1
S
1
–T
2
S
1
T
1
T
2
k
ISC
ꢂ
k
RISC DEST
ꢁ1
k
ISC
ꢂ
k
RISC
ꢂ
DEST
[eV]
9
ꢁ1
9
ꢁ1
9
ꢁ1
[
eV] [eV] [eV] 10 [s ] [s
]
[eV] 10 [s ] 10 [s
]
a
a
b
d
c
a
d
c
d
2
.525b 2.138 2.495 2.44c
2.73 0.387c 2.44c
0.565 22.9
14.7
0.12c
b
Fig. 1 (a) Molecule structure of compound SBF
2
BTF. (b) Steady-state 2.595 2.03
6.24
0.11
absorption (black line) and PL spectra (red line) of SBF
2
BTF in DCM
a
ꢁ5
Energy level and difference estimated based on 10% of the starting
solutions (ca. 1 ꢂ 10 M) at room temperature (RT). Transient PL spectra:
wavelength of the fluorescence and phosphorescence spectra measured
at 77 K. Calculated adiabatic excitation energy upon geometry opti-
mization based on the B3LYP/6-31G* level. Calculated intersystem
crossing decay rate (kISC), reverse intersystem crossing decay rate (kRISC
prompt component (blue line), delayed component (orange line). (c) Streak
b
camera image of SBF
decay of SBF BTF in degassed DCM at RT (blue line) and 77 K (red line). The
transient decay curve was integrated over the spectral range of 520 and
20 nm. (e) Normalized fluorescence spectrum (green line) and the Momap program package. The kISC and DEST are calculated from
phospherescence spectrum (red line) of SBF BTF in DCM solutions at 77 K. experimental date.
2
BTF in degassed DCM solution at RT. (d) The PL
c
2
)
and adiabatic excitation energy (DEST) obtained by Gaussian and
d
6
2
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 7400–7406 | 7401

View Article Online
Journal of Materials Chemistry C
Paper
dependent on the solvent polarity, i.e., LE in low-polarity method we used is suitable to calculate the excited state
solvents and CT in high-polarity solvents. In particular, in properties of SBF BTF. It can be seen from Table 1 that the
2
high-polarity solvents, the excited state dipole moment (m ) is calculated adiabatic energy difference between S1 and T1 is
e
calculated to be 14.37 D, verifying the HLECT characters of 0.565 eV, slightly larger than the experimental value of
SBF BTF. In low-polarity solvents, the m decreases to 7.87 D, DES1–T1 = 0.387 eV. Notably, the calculated adiabatic energy
2
e
which is attributed to the LE state.
difference between S
BTF with the exepriemntally determined DEST = 0.12 eV deduced
molecules in degassed DCM solution by nitrogen bubbling, from prompt and delayed fluorescence components.
recorded by a streak camera upon 400 nm femtosecond laser Hole–electron analysis (Fig. 2a and Fig. S8, ESI†) by the
excitation. It can be seen that both prompt and delayed MULTIWFN program reveals that the S
components were detectable in the wavelength range of HLECT state containing both LE (63.3%) and CT components
10–625 nm over a time window of 10 s. As shown in Fig. 1d, (36.7%). Moreover, the T state is dominated by LE character
1 2
and T is 0.115 eV, in good agreement
Fig. 1c depicts the time-resolved PL (TRPL) of SBF
2
3
2
1
2
state of SBF BTF is a
5
1
the PL of SBF BTF in degassed DCM solution at 298 K decays (90.86%), while the high-lying T state is also HLECT with both
2
2
bi-exponentially (blue line), containing a short lifetime of LE (69.8%) and CT (30.2%) contributions. According to the
3
3
0
.798 ꢃ 0.014 ns and a long lifetime of 0.89 ꢃ 0.02 ms, El-Sayed rule, effective SOC occurs once the n orbital is
respectively (see Fig. S5, ESI†). The blue and orange lines in involved in the ISC process. As shown in Fig. 2b, the SOC
ꢁ
1
Fig. 1b show the prompt spectrum integrated over the nanose- constants are calculated to be large, x(S
cond time range and the delayed one recorded upon time x(S ,T ) = 17.57 cm , respectively, owing to the lone-pair
1
2
,T ) = 21.53 cm and
ꢁ1
1 1
gating (after ca. 0.1 ms), respectively. The similarity between electrons of the sulfur atom, which leads to contributes of
them suggests that the prompt and delayed PL should originate n - p* transition to the S1 (6.7%) and T2 states (11.4%).
from the same singlet excited state. We further measured the According to the first-order perturbation theory, large SOC
n m
dependence of PL decay on temperature, and found that the constant between S and T and small DEST can promote kISC
3
4
delayed component decreases significantly at 77 K (red line in and kRISC as expressed by eqn (1),
Fig. 1d). Combined with time-resolved spectroscopy, we con-
clude that SBF BTF can undergo an effective RISC process at
2
room temperature, giving rise to TADF. Based on the PL
2
jhSnjHSOjTmij
kISCðkRISCÞ /
(1)
2
ðESꢁTÞ
quantum yields of prompt fluorescence (F ) and delayed
F
fluorescence (FDF) as well as their lifetimes (t and tDF), the where hS
n
|HSO|T
m
i is the spin–orbit matrix element between
F
DEST value was estimated to be 0.12 eV (see Table S1, ESI†).
the nth singlet excited state and the adjacent mth triplet excited
Fig. 1e presents fluorescence (green) and phosphorescence state, and DES–T is the splitting energy between singlet and
(red) spectra measured at 77 K. The phosphorescence at
6
0
00 nm decays mono-exponentially with a lifetime constant of
.98 ꢃ 0.03 ms (Fig. S6b, ESI†), much slower than the decay of
fluorescence at 545 nm (0.543 ꢃ 0.002 ns, Fig. S6a, ESI†). From
the onset (at which the intensity is 10% of the maximum
intensity) of fluorescence and phosphorescence spectra at
7
7 K (Fig. 1e), the values of E(S ) and E(T ) were determined
1 1
to be 2.525 and 2.138 eV (Table 1), respectively, giving rise to
DES1–T1 = 0.387 eV, three to four times larger than that observed
in CT-based TADF molecules (B0.1 eV). According to the
energy-gap law, this value of DES1–T = 0.387 is too large to
1
enable efficient RISC. In contrast, as abovementioned, the DEST
value determined based on prompt and delayed fluorescence
components is only about 0.12 eV.
Theoretical calculation
To further understand how an efficient RISC process occurs,
the energy levels of SBF BTF in DCM were calculated by using
2
the time-dependent density functional theory (TD-DFT) method
at the B3LYP/6-31G* level. The solvent effect of DCM was
simulated by using the polarizable continuum model (PCM) adiabatic energy level of compound SBF
Fig. 2 (a) Nature transition orbital analysis for S
The predominant hole and electron wave functions are shown. (b) The
BTF. Energy level diagram
1 1 2 2
, T , and T of SBF BTF.
2
showing the reverse intersystem crossing processes taking place in the
molecule, the inner value is the spin–orbit coupling constant calculated by
Gaussian and Pysoc. (c) Femtosecond transient absorption spectra of
(
see the Note S5, ESI†). The calculated absorption (vertical
excitation energy) and emission spectra (adibatic energy upon
geometry optimization) by the Gaussian 09 and MOMAP
program package match well with those experimental ones
(
3
0
SBF
tored at different wavelengths. The pump laser wavelength is at 400 nm.
see Table 1 and Fig. S7, ESI†), indicating that the theoretical (e) PL decay of prompt components of SBF BTF in degassed DCM.
2 2 2
BTF in CH Cl at room temperature. (d) Decay kinetic traces moni-
3
1
2
7
402
|
J. Mater. Chem. C, 2021, 9, 7400–7406
This journal is © The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
triplet states. Moreover, from a dynamic point of view, ISC is a (353.2 ꢃ 6.3 ps) followed by decay (798.6 ꢃ 14 ps). And the
forward process, and RISC is a reverse process affected by more slowing rising at RT is also an evidence of S 2 T1,2 equili-
1
factors, such as temperature and non-radiation attenuation, brium at the early stage, consistent with the fsTA data.
thereofre kRISC is smaller than kISC. We calculated the kISC and Furthermore, the actual energy spacing between T2 and T1,
kRISC constants by using the MOMAP program package (Fig. 2b DET1–T2 = DE
ꢁ DE
= 0.267 eV, is smaller than the
S –T ,exp
S –T ,exp
1
1
1
2
and Table 1). Strikingly, the T
2
- S
1
RISC rate is calculated to theoretically calculated value 0.465 eV. Therefore, we speculate
- T that T - T upconversion followed by T - S RISC also
ISC rate of 22.9 ꢂ 10 s (Fig. 2b and Table 1). Meanwhile, the contributes to a delayed fluorescence process at RT (Fig. 1b).
9
ꢁ1
be 14.7 ꢂ 10 s , which is only slightly slower than the S
1
2
1
2
2
1
9
ꢁ1
9
ꢁ1
S
1
- T
- S
1
ISC rate is calculated to be 6.24 ꢂ 10 s , but the Returning to Fig. 1c, we observed a negative gain signal near
T
1
1
RISC rate is 2.73, significantly slower than T - S 580 nm in the earlier time window, which formed within
2
1
RISC rate (Table 1). These results show that the RISC process 1.2 ꢃ 0.2 ps and lasted 4300 ps. Most importantly, we observed
contributing to TADF in SBF BTF seems to be implemented significant ASE of the degassed SBF BTF/DCM solution, indi-
2
2
through the T2
- S1 process. On the basis of this cating that the SBF BTF molecules can be used as a gain
2
understanding, it can be found that owing to the small medium for the laser (Fig. S9, ESI†).
ꢁ
1
DES1–T2 = 0.11 eV, large SOC x(S
reorganization energy (86 meV, 694.01 cm , Table S2, ESI†),
the kISC and kRISC of the S 2 T are much larger than those of
traditional CT-based TADF molecules in previous reports,
1 2
,T ) = 21.53 cm and small
ꢁ1
Preparation, structural characterization and photophysical
properties of MRs
1
2
2
5,26
We obtained a single crystal microribbons (MRs) of compound
enabling extremely fast TADF lifetime of SBF BTF. Neverthe-
2
37,38
SBF BTF by
a facile solution self-assembly method.
2
less, internal conversion (IC) from T2 states might also take
A
solution of 100 mL of SBF BTF in dichloromethane
2
place, consuming T excitons and generating non-radiative T
2
1
ꢁ1
(0.5 mg mL ) was quickly injected into 0.5 mL of n-hexane.
excitons, leading to a low PLQY.
After aging 1 hour at room temperature, microstructures of
SBF BTF were obtained (Fig. S11, ESI†). Fig. 3a shows fluores-
cence microscopy images of SBF BTF MRs. These MRs exhibit
Excited-state dynamics of SBF BTF
2
2
2
In order to reveal the TADF process, femtosecond transient
absorption (fsTA) spectra were recorded in CH Cl solution
bright green PL emission at the two end faces and weaker PL
2
2
upon 400 nm femtosecond laser excitation at room tempera-
ture. Fig. 2c presents fsTA spectra at various pump–probe delay
ꢁ2
times at a low pump fluence of 10 mJ cm . Fig. 2d shows decay
kinetic traces monitored at different wavelengths. At 1.8 ps
after laser excitation, an intense positive absorption band at
4
7
80 nm appears and then decays with a time constant of
24.6 ꢃ 6.3 ps (red dot and line in Fig. 2d), in line with the
time-resolved fluorescence data, which is attributed to the
generation of lowest singlet-excited, i.e., S1 - Sn absorption
(
Fig. 2c). Meanwhile, very broad fsTA between 500 and 650 nm
gradually appears and and persists across the entire time
window (Fig. 2c). Note that the S - T ISC and subsequent
- T IC as well as the S - T ISC together make the co-
existence of T and T highly possible in our system. Therefore,
we ascribed the broad fsTA feature (between 500 and 650 nm
detected at the late delay time window) to T1,2 - T excited
state absorption (Fig. 2c). The rising time of T1,2 - Tn at
75 nm, 409.8 ꢃ 78.3 ps (green trace in Fig. 2d), is comparable
to the decay of S - S absorption at 480 nm (724.6 ꢃ 6.3 ps,
1
2
T
2
1
1
1
1
2
n
5
1
n
red trace in Fig. 2d), indicating a close correlation between
the S decay and the 575 nm band formation. Therefore,
the apparaent ISC rate of S - T1,2 is determined to be
.44 ꢂ 10 s (Table 1 and Fig. 2d). Note that an isosbestic
point can be found around 520 nm, indicating that the S and
1
1
9
ꢁ1
2
Fig. 3 (a) m-PL images of the MRs excited with unfocused UV light
1
(
330–380 nm) from a mercury lamp. (b) SEM images of ensemble MRs
T1,2 states equilibrate within this time constant, serving as a on a quartz substrate (c) The SAED patterns of a single microribbon. The
inset shows the corresponding TEM images. (d) XRD spectrum of a mat of
direct evidence of the ISC process between S and T1,2
1
3
5,36
MRs deposited on a quartz substrate and the inset illustrates a cartoon of
microribbons. (e) Molecule packing arrangement of SBF BTF viewed along
states.
Fig. 2e shows the early-stage decay profile of a
2
prompt fluorescence component at RT, recorded by
a
the crystal a-axis. (f) The diffuse reflection absorption (solid line) and PL
streak camera with a time-resolution o50 ps. The decay
spectra (dash line) of microribbons. (g) PL decay of microribbons at 298 K
profile of prompt fluorescence at RT comprises a slow rising and 77 K.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 7400–7406 | 7403

View Article Online
Journal of Materials Chemistry C
Paper
from the bodies when excited by unfocused ultraviolet light
(
330–380 nm), exhibiting typical features of an active optical
waveguide. Scanning electron microscopy (SEM) results
Fig. 3b) confirm that microcrystals are indeed two-
(
dimensional (2D) MRs with smooth surfaces and sharp edges.
In combination with atomic force microscopy (AFM) measure-
ments, these 2D-MRs have regular shape with an average width
(W) of 5 mm uniformly distributed along the entire length (L) of
10–50 mm, and a thickness (H) of about 355 nm (Fig. S12, ESI†).
In order to understand the solid-state molecule arrangement
of the MRs, the selected area electron diffraction (SAED) pattern
is obtained by directing the electron beam perpendicular to the
top facet of a single microribbon (Fig. 3c). The sharp spots in
SAED images reveal that the MR is single crystalline rather than
polycrystalline. Monoclinic SBF
space group of P2 /c, with cell parameters of a = 8.2610(5) Å,
b = 12.9941(3) Å, c = 31.0168(9) Å, a = g = 901, and b = 93.377(4)1
CCDC No. 1914187†). Further analysis of the SAED pattern
shows that the red circle and yellow square labelled spots
2
BTF crystals belong to the
1
Fig. 4 (a) PL spectrum of an individual MR with L = 12.5 mm under
different pump pulse densities at RT. Inset: The corresponding m-PL
images of a single MR below (upper) and above (lower) lasing threshold.
(
(
b) Integrated PL intensity as a function of pump density. The laser
shown in Fig. 3c are due to (100) and (010) Bragg reflections. threshold is identified as the intersection between linear and superlinear
regions. (c) PL decay profiles of MRs monitored at lasing wavelength under
Fig. 3d shows that the XRD spectrum of MRs is dominated by a
series of peaks corresponding to the crystal plane (002) with
d = 30.97 Å. Combined with SAED, XRD and TEM images, we
different pump densities energies, showing the evolution from
unquenched spontaneous emission to lasing emission as the power of
the pump laser increases. (d) The dependence of laser threshold on the
2
can conclude that MRs of SBF BTF grow preferentially along
temperature. As the temperature decreases (increases), the laser threshold
the crystal (100) direction, and are bound by (001) and (010) rises (drops) sharply. The error bars represent the standard deviation of
three measurements.
crystal planes on the top and side faces (inset of Fig. 3d). Fig. 3e
reveals the molecular packing arrangement of SBF BTF viewed
2
along the crystal a-axis. Due to the twisted molecular conforma- a 1 kHz Ti:sapphire regenerative amplifier was focused to a
tion, the intermolecular p–p distances are measured to be 50 mm diameter spot to uniformly excite the selected isolated
d
p–p = 4.33 Å and 4.9 Å (see Fig. 3e), large enough to avoid MR on a 2D movable table. The pump density (P) of the
the formation of excimers.
excitation laser is adjusted by using neutral density filters.
Fig. 3f presents the diffused reflectance absorption (solid Spatially resolved PL spectra were recorded underneath by
line) and fluorescence spectra (dashed line) of SBF
BTF MRs using a three dimensional movable objective (50 ꢂ 0.9 NA)
2
deposed on a quartz plate. The two absorption bands of coupled to an optical fiber and detected using a liquid-
SBF BTF MRs at 366 and 444 nm and the emission maximum nitrogen-cooled charge-coupled device (CCD).
2
at 556 nm resemble those of the dilute solution (see Table S3,
2
Fig. 4a shows the PL spectrum of a single MR of SBF BTF
ESI†). To obtain further information on the nature of the (L = 12.5 mm, W = 3.3 mm). The PL spectrum is a broad
excited states, we also measured the PL lifetimes by using the spontaneous emission (red curve) at a low pump density of
ꢁ
2
streak camera (Fig. 3g). The PL decay of MRs at 556 nm decays P = 10.62 mJ cm . When the pump density reaches a value from
ꢁ
2
bi-exponentially with a short lifetime of 0.42 ꢃ 0.002 ns and a P = 11.9 to 16.2 mJ cm , strong laser emission appears as a set
long lifetime of 0.31 ꢃ 0.015 ms, respectively. Furthermore, the of spikes at 560 nm. Fig. 4b shows the integrated intensities of
long-lived component vanished at 77 K. Fig. S12a (ESI†) depicts emission as a function of P, demonstrating a threshold
ꢁ
2
the fluorescence and phosphorescence spectra of MRs mea-
Pth = 10.62 mJ cm (black line in Fig. 4b). We also plotted the
sured at 77 K. The phosphorescence spectrum shows clearly full-width at half maximum (FWHM) of the emission profile as
vibronic bands at 610 and 650 nm (Fig. S13a, ESI†), respec- a function of pump density, as indicated by the blue line of
tively, which decay with lifetime constants of 0.16 ꢃ 0.005 and Fig. 4b. The FWHM has a nearly constant value of about 58 nm
0
.24 ꢃ 0.008 ms (Fig. S13b, ESI†). These results confirm that below the threshold and abruptly narrows to 1.52 nm for
the PL decay behavior of MR is consistent with the behavior in individual lasing modes above the threshold. The sharp
solution, and can be referred to as TADF.
decrease of the FWHM value and the clear threshold confirm
the laser behavior in the two-dimensional cavity of MRs. The
m-PL image of a 12.5 mm-long MR above the laser threshold
3
9
Lasing characterization of MR lasers
It is worth noting that MRs exhibit active waveguiding behav- shows a significantly brighter green PL at the tips of the MRs
iour and may serve as optical microresonators. To further and a weaker PL from the mid-end (inset of Fig. 4a), suggesting
investigate this possibility, we characterized an isolated single the formation of the Fabry–Perot microcavity, which is vali-
MR using a home-made optical microscope system (Fig. S14, dated by the linear relationship between Dl and the reciprocal
ESI†). The second harmonic (l = 400 nm, pulse width 150 fs) of of the length of the microrod (Fig. S15, ESI†). We also measured
7
404
|
J. Mater. Chem. C, 2021, 9, 7400–7406
This journal is © The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
the cavity quality factor (Q) experimentally according to the which plays a key role in population inversion. Overall, our
definition Q = l/dl, where l and dl are the wavelength of the work provides a guideline for designing TADF molecules, which
laser peak and the FWHM. The dl at a laser wavelength of not only promotes the development of OLEDs, but also offers
562 nm is 0.26 nm, thereby obtaining a Q = 2161, which also the possibility of electrically driving organic lasers.
certified the good optical properties of our FP resonators.
To further confirm the laser behavior, we studied the PL
lifetime under different pump intensities. Fig. 4c shows the Conflicts of interest
decay curve from spontaneous emission to stimulated emis-
There are no conflicts to declare.
sion. At low excitation energy (i.e. 0.4Pth), the spontaneous
emission of MRs exhibits a single exponential with a lifetime
constant of about 0.4 ns. Howener, above the threshold, for
Acknowledgements
example, at P = 2.1P , ultra-fast decay with a lifetime of about
th
3
5 ps dominates, indicating that stimulated emission occurs in
This work was supported by the Ministry of Science and
Technology of China (Grant No. 2017YFA0204503 and
the MR microcavity. Most importantly, during the evolution
from spontaneous emission to stimulated emission, the slow
rising component of B180 ps (blue dots of Fig. 4c) can be
clearly observed, indicating that singlet excitons generated
from the T - S RISC process indeed contribute to the
2 1
establishment of population inversion.
In order to explore the contribution of RISC to stimulated
emission more clearly, we measured the laser thresholds as a
function of temperature (Fig. 4d). As the temperature decreases
from 300 K to 77 K, the laser threshold increases rapidly (blue
2
018YFA0704805), the National Natural Science Foundation of
China (Grant No. 22090022, 21790364, 21833005 and
1873065), the Beijing Natural Science Foundation of China
Grant No. 2192011), the High-level Teachers in Beijing Muni-
2
(
cipal Universities in the Period of 13th Five-year Plan (Grant No.
IDHT20180517 and CIT&TCD20180331), the Beijing Talents
Project (2019A23), the Open Fund of the State Key Laboratory
of Integrated Optoelectronics (IOSKL2019KF01), the Capacity
Building for Sci-Tech Innovation-Fundamental Scientific
Research Funds (025185305000/210 and 009/19530050162),
the Youth Innovative Research Team of Capital Normal
University (009/19530050148), and the Beijing Advanced Inno-
vation Center for Imaging Technology (009/19530011009).
ꢁ
2
ꢁ2
line); Pth,77K = 21.2 mJ cm is twice of Pth,300K = 10.3 mJ cm
.
We then increased the temperature from 77 K back to 300 K,
and found that the threshold recovers gradually, following
almost the same path as that obtained during the process of
decreasing the temperature (red line). Therefore, inactivation of
the RISC process at 77 K decreases singlet excitons upconverted
2 1
from T to S , leading to a higher lasing threshold to establish
Notes and references
population inversion for stimulated emission. This is in sharp
contrast to the trend reported for a traditional fluorescence
gain medium, whose laser threshold decreases as the tempera-
ture decreases owing to suppression of nonradiative transition
1 J. Clark and G. Lanzani, Nat. Photonics, 2010, 4, 438.
2 R. Yan, J.-H. Park, Y. Choi, C.-J. Heo, S.-M. Yang, L. P. Lee
and P. Yang, Nat. Nanotechnol., 2012, 7, 191.
3 F. Yang, S. Cheng, X. Zhang, X. Ren, R. Li, H. Dong and
W. Hu, Adv. Mater., 2018, 30, 1702415.
4
0,41
at low temperature.
Compared with the previously reported
TADF material, the ultra-fast krkISC and kRISC of SBF BTF MRs
2
greatly reduce the accumulation of triplet excitons and mini-
mize TTA and TPA effects, which is the key to realize organic
solid-state microlasers.
4 S. Reineke, Nat. Mater., 2015, 14, 459.
5 X. Wang, Q. Liao, Q. Kong, Y. Zhang, Z. Xu, X. Lu and H. Fu,
Angew. Chem., Int. Ed., 2014, 53, 5863.
6
V. Bonal, R. Munoz-Marmol, F. Gordillo Gamez, M. Morales-
Vidal, J. M. Villalvilla, P. G. Boj, J. A. Quintana, Y. Gu, J. Wu,
J. Casado and M. A. Diaz-Garcia, Nat. Commun., 2019,
Conclusions
10, 3327.
In summary, we demonstrated a new type of organic TADF
7 A. Mikosch, S. Ciftci and A. J. C. Kuehne, ACS Nano, 2016,
10, 10195.
molecule SBF BTF, and for the first time realized an organic
2
solid-state laser based on a TADF molecule in self-assembled
single crystal organic microribbons. In particular, the 2D-MRs
obtained by solution self-assembly form the FP microcavity to
provide a good optical feedback, and can exhibit excellent laser
8 Y. Lu, Y. Yang, Y. Wang, L. Wang, J. Ma, L. Zhang, W. Sun
and Y. Liu, Opt. Express, 2018, 26, 3277.
9 M. A. Baldo, R. J. Holmes and S. R. Forrest, Phys. Rev. B:
Condens. Matter Mater. Phys., 2002, 66, 035321.
behavior at 560 nm with a quality factor of up to 2161. 10 C. Grivas and M. Pollnau, Laser Photonics Rev., 2012, 6, 419.
Theoretical calculations and detailed experimental results 11 A. J. C. Kuehne and M. C. Gather, Chem. Rev., 2016,
show that high performance TADF characteristics are achieved
116, 12823.
1
by an ultra-fast RISC process between the S ( HLECT) and the 12 Y. Zhang and S. R. Forrest, Phys. Rev. B: Condens. Matter
1
3
T
2
( HLECT) states. More importantly, the RISC process reduces
Mater. Phys., 2011, 84, 241301.
the accumulation of triplet excitons and continuously converts 13 A. Salehi, C. Dong, D.-H. Shin, L. Zhu, C. Papa, A. T. Bui,
triplet excitons into singlet excitons through up-conversion,
F. N. Castellano and F. So, Nat. Commun., 2019, 10, 2305.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. C, 2021, 9, 7400–7406 | 7405

View Article Online
Journal of Materials Chemistry C
Paper
14 M. A. Baldo, D. F. O’Brien, M. E. Thompson and 27 X. Cao, Y. Wu, H. Fu and J. Yao, J. Phys. Chem. Lett., 2011,
S. R. Forrest, Phys. Rev. B: Condens. Matter Mater. Phys.,
999, 60, 14422.
5 Z. Yu, Y. Wu, L. Xiao, J. Chen, Q. Liao, J. Yao and H. Fu,
J. Am. Chem. Soc., 2017, 139, 6376.
2, 2163.
1
28 Z. J. Diwu and J. W. Lown, J. Photochem. Photobiol., A, 1992,
64, 273–287.
29 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev.,
2003, 103, 3899.
1
1
6 S. Li, Z. Yu, X. Xiao, H. Geng, K. Wang, X. Jin, Q. Liao,
Y. Liao, Y. Wu, J. Yao and H. Fu, Laser Photonics Rev., 2019, 30 H. B. Schlegel, G. W. Trucks, M. J. Frisch, G. E. Scuseria,
1
3, 1900036.
M. A. Robb, J. R. Cheeseman and G. Scalmani, Gaussian,
Gaussian, Inc., Wallingford CT, 2009.
31 Y. Niu, W. Li, Q. Peng, H. Geng, Y. Yi, L. Wang, G. Nan,
D. Wang and Z. Shuai, Mol. Phys., 2018, 116, 1078.
1
7 H. Nakanotani, T. Furukawa, T. Hosokai, T. Hatakeyama
and C. Adachi, Adv. Opt. Mater., 2017, 5, 1700051.
8 D.-H. Kim, A. D’Aleo, X.-K. Chen, A. D. S. Sandanayaka,
1
D. Yao, L. Zhao, T. Komino, E. Zaborova, G. Canard, 32 T. Lu and F. Chen, J. Comput. Chem., 2012, 33, 580.
Y. Tsuchiya, E. Choi, J. W. Wu, F. Fages, J.-L. Bredas, 33 M. A. Elsayed, Acc. Chem. Res., 1968, 1, 8.
J.-C. Ribierre and C. Adachi, Nat. Photonics, 2018, 12, 98.
9 H. Huang, Z. Yu, D. Zhou, S. Li, L. Fu, Y. Wu, C. Gu, Q. Liao
and H. Fu, ACS Photonics, 2019, 6, 3208.
34 Y. L. Chen, S. W. Li, Y. Chi, Y. M. Cheng, S. C. Pu, Y. S. Yeh
and P. T. Chou, ChemPhysChem, 2005, 6, 2012.
35 T. Villnow, G. Ryseck, V. Rai-Constapel, C. M. Marian and
P. Gilch, J. Phys. Chem. A, 2014, 118, 11696.
1
2
0 Z. Zhou, C. Qiao, K. Wang, L. Wang, J. Liang, Q. Peng,
Z. Wei, H. Dong, C. Zhang, Z. Shuai, Y. Yan and Y. S. Zhao, 36 C. T. Ziegenbein, S. Froebel, M. Gloess, R. S. Nobuyasu,
Angew. Chem., Int. Ed., 2020, 59, 21677.
1 P. Baronas, G. Kreiza, P. Adomenas, O. Adomeniene,
P. Data, A. Monkman and P. Gilch, ChemPhysChem, 2017,
18, 2314.
2
K. Kazlauskas, J. C. Ribierre, C. Adachi and S. Jursenas, 37 H. Kasai, H. S. Nalwa, H. Oikawa, S. Okada, H. Matsuda,
ACS Appl. Mater. Interfaces, 2018, 10, 2768.
N. Minami, A. Kakuta, K. Ono, A. Mukoh and H. Nakanishi,
22 Q. Ou, Q. Peng and Z. Shuai, Nat. Commun., 2020, 11, 4485.
Jpn. J. Appl. Phys., 1992, 31, L1132.
23 W. Li, Y. Pan, R. Xiao, Q. Peng, S. Zhang, D. Ma, F. Li, 38 Z. Yu, Y. Wu, Q. Liao, H. Zhang, S. Bai, H. Li, Z. Xu, C. Sun,
F. Shen, Y. Wang, B. Yang and Y. Ma, Adv. Funct. Mater.,
014, 24, 1609.
X. Wang, J. Yao and H. Fu, J. Am. Chem. Soc., 2015,
137, 15105.
2
24 L. F. Chen, S. T. Zhang, H. Li, R. F. Chen, L. Jin, K. Yuan, 39 X. He, P. Liu, H. Zhang, Q. Liao, J. Yao and H. Fu,
H. H. Li, P. Lu, B. Yang and W. Huang, J. Phys. Chem. Lett.,
018, 9, 5240.
5 G. Li, W. Lou, D. Wang, C. Deng and Q. Zhang, ACS Appl.
Mater. Interfaces, 2019, 11, 32209.
Adv. Mater., 2017, 29, 1604510.
2
40 V. G. Kozlov, V. Bulovic, P. E. Burrows, M. Baldo,
V. B. Khalfin, G. Parthasarathy and S. R. Forrest, J. Appl.
Phys., 1998, 84, 4096.
2
2
6 L. Gan, K. Gao, X. Cai, D. Chen and S.-J. Su, J. Phys. Chem. 41 C. Spiegelberg, N. Peyghambarian and B. Kippelen, Appl.
Lett., 2018, 9, 4725. Phys. Lett., 1999, 75, 748.
7
406
|
J. Mater. Chem. C, 2021, 9, 7400–7406
This journal is © The Royal Society of Chemistry 2021