Arylene Diimide Phosphors: Aggregation Modulated Twin Room Temperature Phosphorescence from Pyromellitic Diimides

Article abstract of DOI:10.1002/anie.202101538

Arylene diimide derived ambient organic phosphors are seldom reported despite their potential structural characteristics to facilitate the triplet harvesting. In this context, highly efficient room temperature phosphorescence (RTP) from simple, heavy-atom

Full text of DOI:10.1002/anie.202101538

Angewandte
Communications
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International Edition: doi.org/10.1002/anie.202101538
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Tuneable Phosphorescence
Hot Paper
Arylene Diimide Phosphors: Aggregation Modulated Twin Room
Temperature Phosphorescence from Pyromellitic Diimides
Swadhin Garain, Suman Kuila, Bidhan Chandra Garain, Meenal Kataria, Aditya Borah,
Swapan K. Pati, and Subi J. George*
Abstract: Arylene diimide derived ambient organic phosphors
are seldom reported despite their potential structural character-
istics to facilitate the triplet harvesting. In this context, highly
efficient room temperature phosphorescence (RTP) from
simple, heavy-atom substituted pyromellitic diimide derivatives
in amorphous matrix and crystalline state is reported here.
Multiple intermolecular halogen bonding interactions among
these phosphors, such as halogen-carbonyl and halogen-p
resulted in the modulation of phosphorescence, cyan emission
from monomeric state and orange-red emission from its
aggregated state, to yield twin RTP emission. Remarkably,
the air-stable phosphorescence presented here own one of the
highest quantum yield ( ꢀ 48%) among various organics in
orange-red emissive region.
molecules to be explored, with its multiple carbonyl groups in
the backbone and with the diverse synthetic strategies
available for their core-substitution with heavy atoms to
increase the SOC and thus the resultant ISC efficiency.[6]
Although arylene diimides have been extensively investigated
for various optoelectronic applications as electron-deficient
semiconductors, their ambient triplet harvesting properties
are rarely explored.[7] While arylene monoimides such as
naphthalene monoimides and phthalimide derivatives are
recently shown to exhibit ambient RTP, corresponding
diimide derivatives are seldom exploited for triplet harvest-
ing.[8] In this context, we have recently reported the ambient
triplet harvesting of core-substituted naphthalene diimides
via RTP[4h] and thermally activated delayed fluorescence
(TADF).[9]
D
esign of ambient organic phosphors is one of the most
Further we envision that, well-studied supramolecular
chemistry of arylene diimides would give an opportunity to
realize multimode phosphorescence via tunable intermolec-
ular interactions. Recently, (supramolecular) aggregation of
organic phosphors has been elegantly used by the groups of
Huang[2c,10] and Yang[11] for realizing dual-mode phosphor-
escence, which is of great importance for various applications
such as white-light phosphorescence.[3i,12] Herein we report
novel heavy-atom core-substituted pyromellitic diimide
(PmDI) derivatives, as efficient orange-red ambient RTP
emitters stable in air (Figure 1). Such intense RTP emission
actively pursued research in recent times as promising
alternative to the toxic, metal-based organometallic phos-
phors for various applications.[1] One of the important
challenges in realizing the RTP from organic molecules is to
stabilize the triplet states by minimizing the vibrational and
oxygen mediated triplet quenching.[1a] Recently, very elegant
design strategies in this direction have been reported to
achieve ambient RTP from organic phosphors both in their
crystalline states[2] and by embedding them in polymeric
matrix[3] or in various supramolecular scaffolds.[4] However,
new molecular designs with high quantum yield phosphors are
required for the further advancement in this field. Since
phosphorescence is a spin-forbidden process, a strong spin-
orbit coupling (SOC) is considered as a basic design to
promote the intersystem crossing process (ISC) between
a singlet (Sn) and triplet state (Tn).[1a,5] In this respect, we
envisage that arylene diimides present an important class of
[*] S. Garain, S. Kuila, B. C. Garain, M. Kataria, Prof. Dr. S. K. Pati,
Prof. Dr. S. J. George
New Chemistry Unit and School of Advanced Material (SAMat),
Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR)
Jakkur, Bangalore 560064 (India)
E-mail: george@jncasr.ac.in
subijg@gmail.com
B. C. Garain, Prof. Dr. S. K. Pati
Theoretical Science Unit, Jawaharlal Nehru Centre for Advanced
Scientific Research (JNCASR) (India)
Figure 1. a) Molecular structures of BrPmDI and IPmDI. b) Simplified
Jablonski diagram to explain the monomer and aggregated phosphor-
escence and other photophysical process. Inset: Monomeric and
dimeric optimized geometry of BrPmDI at the first triplet excited state.
Photographs of the monomer fluorescence in solution, phosphores-
cence in the polymer matrix and phosphorescence in crystal of
BrPmDI under 365 nm light is also shown.
A. Borah
Department of Chemistry, Indian Institute of Technology Bombay
Mumbai 400076 (India)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202101538.
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with one of the highest quantum yield ( ꢀ 48%) in the long-
wavelength region of the visible spectrum is unprecedented.
Further, we show that the phosphorescence emission could be
modulated from cyan of the monomeric state to orange-red of
aggregated state of the phosphors, by exploiting various
intermolecular halogen bonding interactions such as halogen-
p and halogen-carbonyl interactions, which facilitates the
stacking of the phosphors and increases the SOC significantly.
Although triplet harvesting of PmDI has recently been
explored as polymer phosphors, small molecule-based phos-
phors are not yet reported.[13]
First, we have synthesized BrPmDI and IPmDI, from
a durene derivative and were fully characterized by NMR,
mass spectrometry, single crystal X-ray diffraction and HPLC
(See Supporting Information, Scheme S1). Spectroscopic
characterization of these molecules in solution and solid
states was subsequently performed using steady-state and
time-resolved emission experiments. In solution state (THF,
0.05 mM), absorption spectra of BrPmDI and IPmDI showed
characteristic S0-S2 (p-p*) transition with a maximum at
360 nm and 380 nm, respectively (Figure S1).[13] In solution,
both molecules exhibited only weak fluorescence emission in
the 400–550 nm regions with a short average lifetime of
ꢀ 0.8 ns (Figure S1, Table S1). On the other hand, when the
experiments were performed at 77 K (THF) in the glassy
matrix to reduce the vibrational dissipation, red-shifted
emission in the 425 to 700 nm region with an emission
maximum at 504 and 507 nm for BrPmDI (tavg = 7.7 ms) and
IPmDI, (tavg = 1.9 ms), respectively was observed (Fig-
ure 2b,e, S1 and Table S2). The high lifetime of this red-
shifted emission suggests its phosphorescence origin. In order
to harvest the triplets by minimizing the vibrational dissipa-
tion under ambient conditions, we first dispersed the mole-
cules in poly(methylmethacrylate) (PMMA) matrix, an
amorphous polymeric host known to stabilize RTP, at very
low concentrations (1 wt.% with respect to PMMA).[3a]
Remarkably, a strong cyan emission was observed for both
PMMA films with maximum at 480 and 487 nm along with
very weak fluorescence emission for BrPmDI and IPmDI,
respectively under ambient conditions (Figure 2b,e, Fig-
ure S2, Table S3). High lifetime of these red-shifted emissions
(1.4 ms and 6.3 ms for BrPmDI and IPmDI, respectively) and
its similarity with that of the emission of the pure molecules in
frozen THF at 77 K clearly suggests the phosphorescence
origin from monomeric states (Figures 2c,f, S1 and Tables S2,
S4). The absolute phosphorescence quantum efficiencies of
BrPmDI and IPmDI in 1 wt.% PMMA matrix were 18% and
2%, respectively under air (Table S5). Therefore, it is evident
that a strong internal heavy-atom effect is operative in these
derivatives to facilitate substantial ISC and subsequent
efficient RTP in amorphous matrix (Table S6). We hypothe-
sized that higher phosphorescence efficiency of the BrPmDI
compared to the IPmDI in 1 wt.% PMMA matrix is due to
higher ISC rate (kISC), lower non-radiative decay rate (knr)
and the more number of ISC channels present in monomeric
state (Figure S3, Table S6). It is worth mentioning that
1 wt.% amorphous films of BrPmDI when under vacuum
displayed an impressive phosphorescence quantum yield of
54% (Table S5). Interestingly, on increasing the concentra-
tion of these phosphors in the PMMA matrix (5 to 50 wt.%
Figure 2. Steady-state emission spectra of PMMA films with different wt.% of a) BrPmDI and d) IPmDI. Normalized emission spectra of
b) BrPmDI and e) IPmDI in frozen THF at 77 K, in PMMA films with 1 wt.% and 50 wt.% of phosphors and in crystalline state (Inset:
photographs of BrPmDI and IPmDI in PMMA films with 1 wt.% and 50 wt.% of phosphors and in crystalline state under 365 nm UV-lamp
excitation, lexc.=330 and 350 nm for BrPmDI and IPmDI, respectively). Phosphorescence lifetime decay profiles of c) BrPmDI and f) IPmDI in
PMMA films with 1 wt.% and 50 wt.% of phosphors and in crystalline state (lexc. =330 and 350 nm for BrPmDI, IPmDI respectively,
lcollected =500 nm for monomer and 650 nm for aggregate/crystal, all experiments were performed in air).
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with respect to PMMA) gradual appearance of a red-shifted
the highest phosphorescence quantum yield reported in the
emission (orange-red region) with maximum at 576 and
585 nm for BrPmDI and IPmDI respectively with the
concomitant disappearance of monomeric cyan phosphores-
cence, was observed (Figures 2a and d). The lifetime in the
millisecond time scale for these films suggests the phosphor-
escence modulation of these phosphors via intermolecular
interactions (tavg. = 5.2, 0.32 ms and Fp = 17.5%, 30% for
BrPmDI and IPmDI respectively in 50 wt.% with respect to
PMMA under air) (Figures 2c,f, S4 and Tables S5, S7). Drop-
casted films of BrPmDI and IPmDI showed similar emission
features with lower quantum yield, which confirms the
emission from aggregated phosphors (Figure S5 and
Table S8). Aggregation of phosphors in the polymer matrix
at higher wt.% is further evident from the increased scatter-
ing and red-shift in the absorption spectra as well as red-
shifted excitation spectra monitored at 650 nm (Figure S6,
S7). We envisage that the presence of multiple non-covalent
interactions in the aggregated state stabilizes the triplet
energy level.[10,11]
orange-red region among various ambient organic phosphors
(Tables S5, S12 and Scheme S2). Remarkably, the phosphor-
escence quantum yield of IPmDI crystals has increased to
68% under vacuum (Tables S5). Higher phosphorescence
quantum efficiency of IPmDI crystals compared to the
BrPmDI could be attributed to a stronger SOC and high
ISC rate (kISC) in the former, which is also supported by its
faster phosphorescence lifetime (Figures 2c,f and Table S6,
S9), as expected in heavy-atom organic phosphors.
X-ray diffraction analysis of the crystals further provided
insights into the molecular organization of the phosphors in
the aggregated state. Single crystals of BrPmDI exhibited
a layered slip-stacked molecular organization with an inter-
layer distance of 3.8 ꢀ (Figures 3a,b and Table S13) which
justifies the stabilization of orange-red emissive, red-shifted
phosphorescence of the aggregated state in the crystalline
phase. Further stacked arrangement of phosphors is stabilized
by the halogen-p (C···Br, 3.5 ꢀ) and the interdigitated alkyl
chains between molecules of adjacent layers. In addition,
=
In an attempt to characterize the various intermolecular
interactions in the stacked state of these phosphors, single
crystals of BrPmDI and IPmDI were further investigated.
Interestingly, single crystals of BrPmDI and IPmDI showed
intense orange-red phosphorescence emission with a maxi-
mum at 576 and 585 nm for BrPmDI and IPmDI, respectively
similar to that of the PMMA films with aggregated phosphors
(Figure 2b,e). The high lifetimes of 11.4 ms and 0.61 ms for
BrPmDI and IPmDI crystals, respectively and gated emission
spectra (delay time = 0.5 ms) confirms the phosphorescent
nature of the emission (Figures 2c,f, S8 and Table S9).
Selective excitation of the aggregated absorption also resulted
in similar red-sifted phosphorescence emission suggesting the
assembled state of the phosphors in crystals (Figure S9 and
Table S10). Remarkably, both BrPmDI and IPmDI crystals
showed high absolute phosphorescence quantum efficiencies
of 26% and 48% (statistical data presented in Table S11),
respectively under air, which is the highest phosphorescence
quantum yield in the arylene diimide family and also one of
within each layer, these molecules are connected by C O···Br
intermolecular non-covalent interactions (3.1 ꢀ) (Figure 3c).
It has been well-established that halogen bonding can
enhance the SOC by external heavy-atom effect.[2a] We
envisage that, rigid 3D network due to the various non-
covalent interactions specially halogen bonding help the
triplet stabilization via minimizing the vibrational dissipation
and by enhancing the SOC. In addition, strong intra-
=
molecular C O···Br (3.2 ꢀ) (Figure 3b) interactions can
also contribute to high SOC even in the molecular dispersed
and glassy matrix. Similar organization was also observed in
the crystals of IPmDI (Figure S10, Table S13). Since the
crystal packing is similar in both molecules with closely
matching short-contact distances, we envision that the extent
of external/internal heavy-atom effect determines the differ-
ences in their RTP intensity. To validate the stabilization of
triplets in the crystal state we have performed time-depen-
dent density functional theory (TDDFT) calculations. The
calculated energy levels of the optimized geometry of the first
Figure 3. a) Unit cell, b) slipped stacked arrangement showing various intermolecular interactions, c) intramolecular and intermolecular halogen-
carbonyl interaction of BrPmDI single crystal.
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excited triplet state of monomeric and dimeric BrPmDIs
supports the observed red shift in phosphorescence in the
aggregated state (Figure S11, Table S14). Further, reduced
density gradient (RDG) plot of the dimeric triplet state also
clearly shows the intra- and intermolecular non-covalent
interactions observed in the crystalline state, which can
enhance the ISC/SOC efficiency (Figure S12).
times higher than BrPmDI (Figure S3, S16 and Table S16).
Upon aggregation, number of intersystem crossing channels
increases significantly as evident from the ground state
optimized geometries of BrPmDI dimers (Figure 4, S13 and
S17) which could be one of the reasons for the high
phosphorescence efficiency in the crystalline state compared
to the monomeric state. As a control, we also investigated the
un-substituted HPmDI, and the only possible ISC pathway in
this case is between S1 and T1 state with substantially low
SOCME (Figure S18 and Table S16).
In conclusion, we have introduced a new class of small
organic molecule based efficient, ambient organic phosphors
from the smallest member of arylene diimide family, that is,
pyromellitic diimides, by a rational “heavy-atom” substitution
strategy. The PmDI derivatives reported here showed high
phosphorescence quantum yield ( ꢀ 48% and ꢀ 68%, in air
and vacuum, respectively) with exceptional air stability.
Further the tunable phosphorescence emission could also
be achieved by the stacking of phosphors using multiple
intermolecular halogen bonding interactions. Although many
organic room temperature phosphors are reported recently,
the present system is unique with respect to its new molecular
design, high quantum yield and halogen bonding induced
tunable phosphorescence.[1f] We envisage that arylene diimide
phosphors, with its rich chemistry of core-substitution and
appropriate molecular structure conducive for efficient SOC
and ISC, offers plethora of opportunities in the frontier
research area of organic phosphors.
The fundamental prerequisite for the triplet emission in
arylene diimide based systems is to access the triplet excited
state. Therefore, in order to have a qualitative understanding
of ISC efficiency in PmDI derivatives reported here, we
investigated their excited state characteristics by detailed
TDDFT calculations using CAM-B3LYP exchange-correla-
tion functional (Figure S13–S18, Table S15). The rate of ISC is
dependent on the SOC strength and this is estimated
quantitatively by calculating the SOC matrix element
(SOCME) between S1 and its closely lying triplet states (Tn)
(Table S16). In both these cases, multiple triplet states are
present below the S1 excited state and thus provide a thermo-
dynamically favorable ISC route (Figure S3). In BrPmDI,
there are two main contributing pathways for ISC according
to corresponding SOC matrix element (Figure 4, S3, S13 and
S15). These are S1 (n-p*) to T6 (p-p*) and S1 to T2, where
corresponding calculated SOCME are 16 and 523 cmÀ1
,
respectively (Figures 4, S13 and Table S16). It is important
to note that the closely-lying T5 and T4 possess the similar
molecular orbital (MO) configuration as that of S1 and this
does not follow El-Sayed rule; therefore, the magnitude is
much lower than that between S1 and T2 having different
electron-hole configurations (Figure S15). In addition, both
these transitions are also facilitated by the presence of heavy
Br atoms. For IPmDI monomers, the ISC pathway is
facilitated by close-lying S1-T2 which follow El-Sayed rule
and since Iodine is expected to show a stronger heavy-atom
effect as compared to Br, the magnitude of SOCME is three
Acknowledgements
Funding from SwarnaJayanti Fellowship (DST/SJF/CSA01/
2016–2017) is acknowledged. S.K.P. thanks SERB, DST and
J. C. Bose Fellowship for financial support. S.G. and B.C.G.
thanks CSIR, S.K. and A.B. thanks UGC, Government of
India for fellowship. We thank Prof. R. Murugavel for single
crystal X-ray diffraction studies.
Conflict of interest
The authors declare no conflict of interest.
Keywords: aggregation · dual phosphorescence ·
noncovalent interactions · photochemistry · pyromellitic diimide
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[14] Deposition Number(s) 2014119 (BrPmDI) and 2014124(IPmDI)
contain(s) the supplementary crystallographic data for this
paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinforma-
tionszentrum Karlsruhe Access Structures service www.ccdc.
cam.ac.uk/structures.
Manuscript received: February 1, 2021
Revised manuscript received: February 24, 2021
Accepted manuscript online: March 4, 2021
Version of record online: && &&, &&&&
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢀ 2021 Wiley-VCH GmbH
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5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Tuneable Phosphorescence
Highly efficient room-temperature phos-
phorescence, stable in air, from the sim-
plest class of arylene diimides, i.e., pyro-
mellitic diimides, is achieved. Aggrega-
tion modulated, colour tuneable phos-
phorescence emission from cyan to
orange-red in amorphous film and crys-
talline state is observed. The role of
molecular design and weak inter-
S. Garain, S. Kuila, B. C. Garain,
M. Kataria, A. Borah, S. K. Pati,
S. J. George*
&&&& — &&&&
Arylene Diimide Phosphors: Aggregation
Modulated Twin Room Temperature
Phosphorescence from Pyromellitic
Diimides
molecular interactions in the observed
properties is explained.
6
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
These are not the final page numbers!