Self-Assembled Helical Arrays for the Stabilization of the Triplet State

Article abstract of DOI:10.1002/anie.202005105

Room-temperature phosphorescence of metal and heavy atom-free organic molecules has emerged as an area of great potential in recent years. A rational design played a critical role in controlling the molecular ordering to impart efficient intersystem crossing and stabilize the triplet state to achieve room-temperature ultralong phosphorescence. However, in most cases, the strategies to strengthen phosphorescence efficiency have resulted in a reduced lifetime, and the available nearly degenerate singlet-triplet energy levels impart a natural competition between delayed fluorescence and phosphorescence, with the former one having the advantage. Herein, an organic helical assembly supports the exhibition of an ultralong phosphorescence lifetime. In contrary to other molecules, 3,6-phenylmethanone functionalized 9-hexylcarbazole exhibits a remarkable improvement in phosphorescence lifetime (>4.1 s) and quantum yield (11 percent) owing to an efficient molecular packing in the crystal state. A right-handed helical molecular array act as a trap and exhibits triplet exciton migration to support the exceptionally longer phosphorescence lifetime.

Full text of DOI:10.1002/anie.202005105

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Accepted Article
Title: Self-assembled Helical Arrays for Stabilizing the Triplet State
Authors: Aakash D. Nidhankar, Goudappagouda - -, Divya S. Mohana
Kumari, Shailendra Kumar Chaubey, Rashmi Nayak, Rajesh
G. Gonnade, G. V. Pavan Kumar, Retheesh Krishnan, and
Santhosh Babu Sukumaran
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RESEARCH ARTICLE
Self-assembled Helical Arrays for Stabilizing the Triplet State
[
a,b]
[a,b]
[d]
Aakash D. Nidhankar, Goudappagouda, Divya S. Mohana Kumari, Shailendra Kumar
[
e]
[a]
[b,c]
[e]
[d]
Chaubey, Rashmi Nayak, Rajesh G. Gonnade, G. V. Pavan Kumar, Retheesh Krishnan,
[
a,b]
Sukumaran Santhosh Babu*
[
[
[
[
[
a] Organic Chemistry Division, National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411 008, India.
b] Academy of Scientific and Innovative Research (AcSIR), Ghaziabad-201 002, India.
c] Center for Materials Characterization (CMC), National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411 008, India.
d] Department of Chemistry, Government College for Women, Thiruvananthapuram-695 014, Kerala, India.
e] Department of Physics, Indian Institute of Science Education and Research, Pune-411 008, Maharashtra, India.
Abstract: Room-temperature phosphorescence of metal and heavy
atom-free organic molecules has emerged as an area of great
potential in the recent past. A rational design played a critical role in
controlling the molecular ordering to impart efficient intersystem
crossing and stabilize the triplet state to achieve room-temperature
ultralong phosphorescence. However, in most of the cases, the
strategies followed to strengthen phosphorescence efficiency have
resulted in a reduced lifetime, and the available nearly degenerate
singlet-triplet energy levels impart a natural competition between
delayed fluorescence and phosphorescence, with former one having
the advantage. Here, an organic helical assembly supports it
towards phosphorescence in this competitive pathway to exhibit an
ultralong phosphorescence lifetime. In contrary to other molecules,
facilitate reverse intersystem crossing (RISC) to exhibit delayed
[
12]
fluorescence (DF). Hence it demands a new alternate plan of
action to control the competitive decay pathways and efficiently
manage the triplet excitons to enhance phosphorescence
lifetime. It can be achieved only through unexplored molecular
designs and with the rigorous assistance of molecular packing.
As known, helical arrays of organic small molecules and
polymers exhibit exciting optoelectronic properties by controlling
[
13]
the spatial organization of the structural units.
design of a helical assembly to modulate the triplet state to
achieve longer phosphorescence lifetime will be highly
Hence, the
a
appreciated. Our continuous attempts in this direction enabled
us to successfully utilize the crystalline helical arrays of
phenylmethanone functionalized alkylated carbazole to achieve
a longer lifetime of above 4 s (Figure 1b).
3
,6-phenylmethanone functionalized 9-hexylcarbazole exhibits a
remarkable improvement in phosphorescence lifetime ( 4.1 sec)
and quantum yield (11 %) due to an efficient molecular packing in
the crystal state. A right-handed helical molecular array act as a trap
and exhibits triplet exciton migration to support the exceptionally
longer phosphorescence lifetime. The present work will urge new
molecular designs to achieve ultralong organic phosphorescence
under ambient conditions.
Results and Discussion
Introduction
Room-temperature phosphorescence (RTP) of organic
molecules has been enthusiastically investigated recently for
various applications such as electroluminescence, sensing, bio-
[
1,2]
imaging etc.
of heavy atoms and functional moieties equipped with lone pair
of electrons facilitate intersystem crossing (ISC) rate (kST
through strong spin-orbit coupling (SOC), leading to efficient
It has been demonstrated that the incorporation
)
[
3-6]
RTP.
In this direction, diverse molecular designs have been
explored for ultralong organic phosphorescence (UOP).
[
7-10]
Hence, successful strategies such as molecular units with n
orbitals, which can reduce the singlet-triplet (S-T) splitting
energy (ΔEST) and stabilize the triplet excited state through
Figure 1. (a) Recent developments in the area of small molecule-based
organic phosphors with lifetime above 2 s and the present work with above
4.17 s. (b) Schematic of the stabilization of the triplet state, leading to ultralong
phosphorescence.
[
5f]
various interactions, have been widely employed. Significant
advancement has been noticed in the area of UOP materials
and the maximum lifetime reported for crystalline small
molecule-based organic phosphor is ~2.5 s (Figure 1a, Table
Among the potential RTP candidates, carbazole has undergone
diverse functionalization using various combinations of heavy
[
11]
S1).
Mostly, longer lifetimes under ambient condition is
atoms and other functional groups to achieve UOP (Table
achieved by stabilization of the triplet state through H-
aggregation (Figure 1b). However, the already established
strategies to improve the phosphorescence efficiency often lead
to shortening the lifetime. Moreover, the molecular designs to
enhance the nearly degenerate triplet manifolds can, in turn,
[8c,9b,10d,10e]
S2).
We planned to study the effect of
phenylmethanone functionalization on the RTP features of 9-
[14]
hexylcarbazole, which exhibits very low RTP lifetime.
Our
design strategy comprised of, i. N-alkylation of carbazole to
enhance the solubility and impart alkyl-π interaction to promote
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
crystallization, ii. phenylmethanone unit to support a (strain-
induced) crystalline assembly formation and stabilize the triplet
state by providing hybrid (n,π*) and (π,π*) configurations for
in MTHF show that the higher lifetime component varied as 4.11
ns (1), 6.39 ns (2), 6.04 ns (3) and 6.38 ns (4) (Figure S5, Table
1 and Table S4). Both red-shifted fluorescence maximum and
enhanced fluorescence lifetime for molecules 2-3b points to the
effect of benzoylation on carbazole. A comparison of the
fluorescence spectra of the MTHF solution at RT (298 K) and 77
K indicates that 1 exhibits additional long-wavelength peaks
(400-550 nm) along with fluorescence peak at 370 nm, whereas
[
15]
UOP.
with phenylmethanone units at
synthesized and unambiguously characterized by H and
Accordingly, a series of 9-hexylcarbazole molecules
3
and 3,6-positions were
1
13
C
nuclear magnetic resonance spectroscopies and high-resolution
mass spectrometry (Figure 2a, Scheme 1). While our study was
progressing, Chen et al., reported the effect of impurity on the
a
completely red-shifted fluorescence peak appeared for
[
16]
afterglow property of carbazoles. In the purview of this report,
molecules 2-3b (Figure S6, Table 1). The fluorescence spectra
of 2-3b both at RT and 77 K match with the red-shifted peak of 1
between 400-550 nm at 77 K (Figure S6).
[17]
we synthesized carbazole in our lab using reported protocols
and confirmed the purity by high-performance liquid
chromatography (HPLC) using acetonitrile-water mixture
(Scheme 1). All the final molecules were also purified similarly
and hence, we would like to mention that the concerns related to
the effect of impurity on the present work, if any, are not valid.
Figure 2. (a) Chemical structure of 1-3b. Normalized steady-state
fluorescence spectra of 1-3b in (b) MTHF solution and (c) crystals at RT. (d)
Photographs of 1-3b crystals under UV light (365 nm).
Scheme 1. Synthetic scheme for carbazole and target molecules 1-3e, a)
o
Pd(PPh
3
)
4
, K
3
PO
4
, EtOH, 90 C, 12 h, 91%, b) Pd(OAc)
2
, Imes HCl, H
2
O
2
o
o
(35%), AcOH, 120 C, 20 min, MW, 80%, c) TBAB, KOH, DMF, 70 C, 24 h,
o
o
89%, d) anhy. AlCl
3
(1 eq.), anhy. CH
Cl
2
Cl
2
, 0 C - 30 min, 30 C - 12 h, 62%, e)
o
o
anhy. AlCl
Pd(PPh , Zn(CN)
3
(2.5 eq.), anhy. CH
2
2
, 0 C - 30 min, 30 C - 12 h, 70-75%, f)
, DMF, 110 C, 24 h, 61%.
o
3
)
4
2
To study further, all molecules were crystallized from CH
the morphology was visualized by fluorescence microscopy (FM)
Figure S7), scanning and transmission electron microscopy
2 2
Cl and
(
After the complete characterization of the molecules, we
checked the thermal stability and followed by optical properties.
Thermogravimetric analysis indicated the enhanced stability
upon moving from molecule 1 to 3b, indicating the importance of
substitution on 9-hexylcarbazole (Figure S2). We observed a
similar trend in the melting point of 1-3b in differential scanning
calorimetry experiments as well (Figure S3). While absorption
features of all the luminogens 1-3b in 2-methyltetrahydrofuran
(SEM and TEM) images (Figure S8). It was found that molecule
1-3a formed crystalline rods, whereas flat sheets were formed
by 3b (Figure S7,8). The bulk purity and ordering of the
molecules were confirmed by matching the experimental and
theoretical powder X-ray diffraction patterns (Figure S9). The
diffuse reflectance spectra of crystals of 1-3b exhibited a broad
band between 300-450 nm, having an additional absorption
band located around 360-380 nm (Figure S10). It is clear from
the fluorescence spectra of crystals at RT that molecule 3a
exhibits a far red-shifted peak (λmax = 517 nm) than that of 1
(λmax = 390 nm), 2 and 3b (λmax = 425 and 490 nm) (Figure 2c,d).
Similarly, compared to 1, molecules 2-3b exhibited a marked
difference in fluorescence features at 77 K in the crystalline state
(Figure S11). The presence of a long-lived component in the
(
MTHF) have not varied significantly (Figure S4), compared to 1,
the fluorescence spectra of 2-3b showed a bathochromic shift of
0 nm due to the increasing electron-withdrawing ability of the
5
carbonyl groups (Figure 2b and Table S3). An increase in the
number of benzoyl groups did not alter the fluorescence
maximum of 2-3b in MTHF. Fluorescence lifetime decay profiles
2
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RESEARCH ARTICLE
fluorescence lifetime decay of 2-3b was noticed when the
lifetime was monitored at the fluorescence maxima. Hence, to
find out the fluorescence lifetime, the measurement was carried
out by monitoring at lower wavelengths and the longer lifetime
component varied as 11.1 ns (87%) for 1, 1.88 ns (17%) for 2,
a)
3
.37 ns (59%) for 3a, and 3.23 ns (75%) for 3b (Figure S12 and
b)
c)
d)
Table S4).
π···π = 3.34 Å
>C=O···π = 3.17 Å
>C=O···H = 2.67 Å
Ar C-H···π = 2.73 Å
Alkyl C-H···π = 2.88 Å
f)
0
10
20
e)
2
Ѳ = 29.82ꢀ
1
Ѳ = 29.84ꢀ
-
-
3
57 nm
2
89 nm
3
00
400
 / nm
500
1
Ѳ = 29.67ꢀ
2
Ѳ = 29.44ꢀ
Figure 4. (a) The extended molecular packing of 3a forming the helical 1D-
array in the b-axis with the help of (b) - interaction between carbazole and
phenylmethanone unit of the adjacent molecule. (c) Six adjacent helical arrays
of 3a leading to extended columnar packing in the c-axis. (d) All possible
intermolecular interactions in a unit cell of 3a. (e) Crystal structure of 3a
showing the dihedral angles. (f) The solid-state CD spectrum of 3a crystals at
RT.
Figure 3. (a) Phosphorescence spectra of 1-3b crystals at RT in air. (b)
Phosphorescence lifetime decay profile of 3a crystal recorded at RT in air. (c)
The frontier molecular orbitals of 3a from DFT calculations.
Time-dependent
density
functional
theory
(TD-DFT)
The red-shifted fluorescence of the crystals of 2-3b at RT nearly
matches with the corresponding spectra at 77 K, implying RTP
for 2-3b in the crystalline assembly (Figure S11, S13-16). To
clarify the presence of RTP for crystals of 2-3b,
phosphorescence spectra were measured and confirmed that
the observed red-shifted peak at RT is due to phosphorescence
computations at the B3LYP/6-31G(d) level of theory for single-
molecule in vacuum show that the electron density of the HOMO
is located on the carbazole moiety and the LUMO is delocalized
over the entire molecule with a considerable extension to the
phenylmethanone group (Figure 3c, S18-21 and Table S6-9).
The low-lying excited electronic states mainly result from well-
described π-π* transitions along with a significant contribution
from the n-π* character. On examining the relative computed
(Figure 3a). In the crystalline state, the phosphorescence
intensity of 3a was found to be highest among all, and it was ~27
times higher than that of 1. The enhanced RTP for 3a was
visualized by phosphorescence lifetime measurement and it
varied as 5.1, 16.6, 4171 and 114.5 ms for 1-3b, respectively
energies for the singlet (S
been found that there are many triplet states (T
degenerate with the first singlet excited state (S ) (Figure S20
n
) and triplet (T
n
) states for 3a, it has
4
9
-T ) nearly
1
(Figure 3b, S17 and Table S5). Molecule 3a exhibited a longer
and Table S8). As the phenylmethanone functionalization
increases, an intense mixing of nearly degenerate excited
singlet and triplet states were observed, which could bring out a
natural competition between ISC and RISC (Figure S20). The
energetic proximity of the singlet and the triplet manifold
potentials established a small ΔEST. Thus it appears that
molecules 2-3b have energetically well-matched states that
enable efficient singlet-triplet or triplet-singlet crossings. Once
phosphorescence lifetime with afterglow for  12 s upon
excitation after just dipping in liquid nitrogen (Figure S15). The
phosphorescence quantum yield of the crystals varied as 1
(
3.36 %), 2 (0.85 %), 3a (11.25 %) and 3b (5.15 %) (Table S6,7).
The observed enhancement in the phosphorescence lifetime of
a raised curiosity about the excited triplet state of this particular
molecule, and hence we decided to study it in detail.
3
T
4-
T
9
levels are populated, relaxation through the triplet manifold
to the T state leads to phosphorescence (Figure S18-21).
1
Besides, we have to consider the strong possibility of RISC in a
competitive pathway to exhibit DF. The ΔES1T1 value calculated
for the monomer from DFT and phosphorescence spectra were
varied as 0.72 and 0.32 eV, respectively (Figure 3a, S20, and
3
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Table S8). It shows that the aggregation of 3a with the
assistance of intermolecular interactions reduces ΔES1T1 to
facilitate both ISC and RISC.
5.7 Å (Figure S24). Hence, the helical array of 3a is responsible
for the enhanced RTP through an efficient space filled molecular
packing by making use of maximum intermolecular interactions
(Figure 4a-c).
a)
b)
Body
End
a)
b)
c)
6
3
Ar C-H···π = 2.85 Å
C=O···π = 3.17 Å
Alkyl C-H ···π = 2.78 Å
Alkyl C-H···C-H = 2.32 Å
Ar C-H···π = 2.82 Å
Alkyl C-H···π = 2.82 Å
25 m
600
 / nm
700
800
c)
d)
d)
Cl···Cl = 3.45 Å
C=O···H = 2.54 Å
C=O···π = 3.17 Å
Ar C-H···π = 2.87 Å
Alkyl C-H···Cl = 2.92 Å
C=O···π = 3.17 Å
C-Br ···π = 3.46 Å
Ar C-H···π = 2.82 Å
Alkyl C-H···π = 2.82 Å
e)
f)
C=O···H = 2.49 Å
C=O···π = 3.17 Å
Ar C-H···π = 2.82 Å
Ar C-H···Alkyl C-H = 2.09 Å
C=O···H = 2.52 Å
Ar C-H···π = 2.81 Å
Ar C-N···π = 3.19 Å
Figure 6. (a) Passive waveguiding properties of the crystal of 3a upon
excitation at 532 (above) and 633 nm (below) wavelength, the colors green
and red are attributed to the excitation sources, respectively. (b)
Phosphorescence waveguiding by the single crystalline rods of 3a after
filtering out the excitation wavelength (532 nm). (c) The red-shifted
phosphorescence observed at the distal end of the crystal compared to the
body (λex = 532 nm). (d) Schematic of the helical array of 3a leading to triplet
exciton migration.
Figure 5. The molecular packing of (a) 1 (b) 2 (c) 3b (d) 3c (e) 3d and (f) 3e in
the unit cell.
Single-crystal X-ray analysis has been employed to get a deeper
structure-property correlation of 1-3b. (Figure 4 and S22-27).
The unit cell of 3a contains six molecules arranged in a helical
way in two rows along b axis using various intermolecular
interactions. An extended molecular array was formed by the
arrangement of molecules (Figure 4a) mainly through π-π
interaction (3.34 Å) between carbazole and phenylmethanone
units of adjacent molecules (Figure 4b) along b-axis. This one-
dimensional (1D) helical array is stabilized with the help of
CH···π interaction (2.88 Å) between the alkyl chain on carbazole
and phenylmethanone unit in the adjacent helical columns to
form a three-dimensional network structure (Figure 4c). We
strongly believe that this kind of space filled packing (Figure
Recently, helical/twisted aromatic systems have been of interest
[
18]
to promote intersystem crossing.
Hence, the stabilization of
the triplet state of 3a is strongly supported by the presence of
strained helical arrays. To ascertain the impact of helical arrays
on such a longer phosphorescence lifetime, we prepared a
series of carbazole molecules by varying the functional moieties
on the phenylmethanone unit (3c-e) (Scheme S1 and Figures
S25-37). However, molecules 3c-e packed differently (Figures 5,
S25-27 and Table S5, S10-12) and hence exhibited lower
phosphorescence lifetimes (Figure S34 and Table S5). It
indicates the critical supportive role of helical arrays to impart a
longer lifetime for 3a. Besides, we have tested the emission of
4
c,d) rigidifies the molecular conformations and remarkably
block the nonradiative decay pathways. The dihedral angle
between the aryl groups and carbonyl subunits was found to be
around 30° and it enables efficient electronic communication
between carbazole and phenylmethanone via conjugation
3
a in poly(methyl methacrylate) (PMMA) matrix and found that
the spectral feature has been significantly changed and the
lifetime also drastically decreased (8.15 ms) due to the lack of
specific molecular ordering (Figure S38).
(Figure 4e). It can be anticipated that better communication
A
perfectly aligned and densely packed fluorescent pi-
between the (n,π*) from the ˃C=O and (π,π*) of carbazole could
result in hybrid triplet states. The helical array of 3a was
confirmed by the solid-state circular dichroism (CD) spectrum
conjugated molecular array with a large Stokes shift is known for
[
2a,19]
optical waveguiding (WG).
Hence, to check the possibility of
WG, one end of the crystals of 3a was excited at 532 and 633
(Figure 4f). A bisignated CD signal observed points to the right-
[
20]
nm using a 10X, 0.3 NA objective lens
and found that the
handed helical array of molecules (Figure 4a,f). However, no
such helical array is present in the case of 1, 2 and 3b, and
mostly, a loosely packed crystalline assembly was observed
micro rods exhibit intense green and red emissions at the distal
end, respectively, with characteristic features of the optical WG
(Figure 6a). The maximum phosphorescence intensity at the
(Figure 5, S21-23). Especially, 3b packs in a kind of slipped
distal end was observed when it is excited with polarization
molecular arrangement with a carbazole-carbazole distance of
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
along the long axis of the rod, and it has been significantly
reduced in the perpendicular polarization (Figure S39). Since
molecule 3a is RTP active, it is obvious that phosphorescence of
between the optical WG and exciton migration, we checked the
difference in emission from the body and tip of the rods. A red-
shifted phosphorescence at the distal end of the crystal
[
21]
3
a propagates along the long axis of the single crystal rods
compared to the body at RT was observed (Figure 6c). The
[
21,22]
(Figure 6b). The self-waveguided phosphorescence was imaged
red-shifted emission is due to the triplet exciton migration
on a rod with an excitation spot (λ = 532 nm) positioned at
different distances from the end of the rod (Figure 6b). To
examine the secondary emission we filtered out the excitation
wavelength using an edge filter. The propagation loss coefficient
calculated for phosphorescence of 3a by measuring the intensity
at the distal end and the distance from the excitation point is
approximately 0.028 dB/m (Figure S40). In order to distinguish
from the body to the tip of 3a single crystal rods and thereby
actively supports the long-lived phosphorescence (Figure 6d). In
the case of other molecules, only waveguiding and no exciton
migration was observed, and it points to the critical supportive
role of molecular packing assisted exciton migration in achieving
longer phosphorescence lifetime.
Table 1. Photophysical parameters of compounds 1-3e based on fluorescence and phosphorescence experiments in the crystal state.
9
-1
9
-1
7
-1
-1
-1
No
1
QYFl (%)
59.00
03.68
13.58
27.14
4.14

Fl (ns)
k
nr, Fl 10 S
k
r,Fl10 S
k
isc 10 S
QYPhos (%)
3.36

Phos (ms)
k
nr, Phos
S
r, Phos
k S
11.09
1.36
2.92
2.53
1.06
6.23
1.11
0.033
0.701
0.257
0.267
0.879
0.148
0.842
0.053
0.027
0.046
0.107
0.039
0.007
0.040
0.302
0.625
3.85
5.1
189.4
59.72
0.212
8.28
6.58
0.51
0.026
0.44
0.57
4.08
1.68
2
0.85
16.6
4171
114.5
45.3
6.59
11.9
3a
3b
3c
3d
3e
11.25
5.15
2.03
2.46
2.61
21.49
147.6
82.34
4.52
0.431
1.81
2.69
4.47
2.01
TD-DFT studies revealed a qualitative picture of the equal
possibility of ISC and RISC for 3a due to the presence of
energetically matching sing let and triplet levels. Even though a
shorter component is noticed, the unambiguously confirmed
long-lived phosphorescence decay supersedes it with the
support of triplet exciton migration through helical molecular
arrays. The afterglow observed for more than 12 s after just
dipping in liquid nitrogen supports the suppression of RISC and
thereby endorses improved radiative decay through the triplet
manifolds at low temperature. The efficient molecular ordering in
the helical array of 3a ensures that the populated triplet decays
stabilization of the triplet state by the helical arrays and the
presence of triplet exciton migration results in the longer
phosphorescence lifetime, so far reported for a single molecule
crystal. The findings put forth a new strategy of helical arrays to
support long-lived phosphorescence under ambient conditions
and definitely will invoke the development of new organic
phosphors.
Acknowledgements
1
very slowly. Here T acts as a trap state and plays a vital role in
ADN and Goudappagouda acknowledge University Grants
Commission (UGC), India, for fellowship. RK thanks the
Department of Higher Education, Government of Kerala for the
funding of instrumentation facility in the Government College for
Women, Thiruvananthapuram. This work is supported by SERB,
Govt. of India, CRG/2019/002539.
triplet exciton migration as revealed by the WG results. Hence, it
can be concluded that the peculiar helical molecular packing
results in a long phosphorescence lifetime of 3a compared to
other molecules in the series and it is substantiated by the
crystal structure and phosphorescence lifetime. In all other
cases, the triplet states mostly undergo nonradiative internal
conversions compared to radiative transitions.
Keywords: ultralong phosphorescence • carbazole •
waveguiding • phenylmethanone • helicity
CONCLUSION
[
1]
a) A. Forni, E. Lucenti, C. Botta, E. Cariati, J. Mater. Chem. C. 2018 6,
In summary, we have reported
phenylmethanone functionalized N-alkylated
exhibiting UOP. A helical array by the peculiar molecular
packing of 3,6-bis(phenylmethanone) substituted 9-
a
new series of
4603; b) M. Baroncini, G. Bergamini, P. Ceroni, Chem. Commun. 2017,
53, 2081; c) G. Baryshnikov, B. Minaev, H. Ågren, Chem. Rev. 2017,
117, 6500; d) X. Yang, G. Zhou, W-Y. Wong, Chem. Soc. Rev. 2015,
carbazoles
44, 8484.
hexylcarbazole in the crystal state enabled to mix up the singlet-
triplet states to create hybrid triplets to enhance the intersystem
crossings. By optimizing the molecular structure and a strained
[2]
a) Z. Yu, Y. Wu, L. Xiao, J. Chen, Q. Liao, J. Yao, H. Fu, J. Am. Chem.
Soc. 2017, 139, 6376; b) S. M. A. Fateminia, Z. Mao, S. Xu, Z. Yang, Z.
Chi, B. Liu, Angew. Chem. Int. Ed. 2017, 56, 12160; Angew. Chem.
2017, 129, 12328; c) G. Zhang, G. M. Palmer, M. Dewhirst, C. L. Fraser,
crystal packing,
derivative resulted in
a
metal- and heavy atom-free carbazole
significant improvement of
Nat. Mater. 2009, 8, 747.
a
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Table of Contents
̶
+
Exciton migration
̶ +
Ultralong Phosphorescent Organic Helical Arrays




Stabilization of Triplet State by Helical Arrays
Exciton Migration Along the Aggregates
Ultralong Phosphorescence Lifetime of ~4 sec
Enhanced Phosphorescence Quantum Yield of 11%
A helical array of phenylmethanone functionalized carbazole with ultralong phosphorescence.
7
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