Multiple-State Emissions from Neat, Single-Component Molecular Solids: Suppression of Kasha's Rule

Article abstract of DOI:10.1002/anie.202000608

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3′H-[1,1′-biisobenzofuranylidene]-3,3′-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3′H-[1,1′-bibenzo[c] thiophenylidene]-3,3′-dione, are found to fluoresce in their neat solid phases, from upper (S₂) and lowest (S₁) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by “suppressing” Kasha's rule.

Full text of DOI:10.1002/anie.202000608

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Multiple State Emissions from Neat, Single-Component Molecular
Solids. Suppression of Kasha's Rule
Authors: Li-Zhu Wu, Ya-Hang Wu, Hongyan Xiao, Bin Chen, Richard
G. Weiss, Yu-Zhe Chen, and Chen-Ho Tung
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202000608
Angew. Chem. 10.1002/ange.202000608
Link to VoR: http://dx.doi.org/10.1002/anie.202000608
http://dx.doi.org/10.1002/ange.202000608

10.1002/anie.202000608
Angewandte Chemie International Edition
RESEARCH ARTICLE
Multiple State Emissions from Neat, Single-Component Molecular
Solids. Suppression of Kasha’s Rule
Ya-Hang Wu+, Hongyan Xiao+, Bin Chen, Richard G. Weiss, Yu-Zhe Chen,* Chen-Ho Tung and Li-Zhu
Wu*
[*] Y.-H. Wu, Prof. Dr. B. Chen, Prof. Dr. Y.-Z. Chen, Prof. Dr. C.-H. Tung, Prof. Dr. L.-Z. Wu
Key Laboratory of Photochemical Conversion and Optoelectronic Materials
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190, (China)
E-mail: chenyuzhe@mail.ipc.ac.cn and lzwu@mail.ipc.ac.cn
Prof. Dr. H. Xiao
Key Laboratory of Bio-inspired Materials and Interfacial Science
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190, (China)
Prof. Dr. R. G. Weiss
Department of Chemistry and Institute for Soft Matter Synthesis and Metrology
Georgetown University
Washington, D.C. 20057-1227, United States
[+] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Contrary to Kasha’s rule, three rigid and structurally-
Kasha’s rule[3]). In particular, emission from other than the lowest
singlet and triplet excited states is extremely difficult to observe
in neat crystals, due to significant amounts of fast vibrational or
collisional relaxation of upper excited states in solid or
condensed phases. A few documented exceptions to Kasha’s
rule have employed some derivatives of azulenes, thioketones
and polyenes in collision-free conditions (i.e., in the vapor phase
at low pressure) or in very dilute solutions.[4] To the best our
knowledge, there are no definitive examples of dual
fluorescence (and/or dual phosphorescence) from upper and
lowest singlet excited states in neat solids, and certainly not at
room temperature in the air. Mechanistic understanding of how
to obtain upper excited state emissions in single-component
simple heterocyclic stilbene derivatives, (E)-3H, 3'H-[1,1'-
biisobenzofuranylidene]-3,3'-dione
(SDPE-OO),
(E)-3-(3-
oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one
(SDPE-OS), and (E)-3H,3'H-[1,1'-bibenzo[c] thiophenylidene]-3,3'-
dione (SDPE-SS), are found to fluoresce in their neat solid phases,
from upper (S2) and lowest (S1) singlet excited states, even at room
temperature in air. Photophysical studies, single-crystal structure
analyses and theoretical calculations indicate that large energy gaps
between S2 and S1 states (T2 and T1 states) as well as an
abundance of intramolecular and intermolecular hydrogen bonds
suppress internal conversions of the upper excited states in the
solids and make possible the occurrence of fluorescence from S2
excited state (phosphorescence from T2 excited state). These results, solids are largely unknown, as well.
including unprecedented fluorescence quantum yields (2.3-9.6%)
from the S2 states in the neat solids, establish a unique molecular
skeleton for achieving multi-colored emissions from upper excited
states by ‘suppressing’ Kasha’s rule.
In the present work, we report three stiff, structurally related
heterocyclic 1,2-diphenyl ethylene derivatives (SDPE-OO,
SDPE-OS and SDPE-SS, Scheme 1), which exhibit excitation
wavelength-dependent, multiple emissions in their neat solid
states at room temperature and in the air (i.e., the presence of
molecular oxygen). Unexpectedly high fluorescence quantum
yields from the upper excited states (most likely, S2; vide infra)
are observed in these solids at room temperature in the air:
2.3%, 6.0% and 9.6% for SDPE-OO, SDPE-OS and SDPE-SS,
respectively. Also, a logarithmic relationship between the S2-S1
energy gap and the S2 emission quantum yields was found for
this series of compounds; an analogous energy gap relationship
has been found for azulene derivatives in the gas phase[4b]
Detailed analyses of the results from photophysical studies,
theoretical calculations and single crystal structures indicate that
the large energy gaps between the two excited states of the
molecules, multiple intra- and inter-molecular hydrogen bonding
interactions of the molecules in their crystalline lattices, and the
lack of easily abstractable hydrogen atoms suppress
nonradiative deactivation of the upper excited states to lowest
excited states (including trans-cis isomerizations and
photodimerizations involving the central double bonds and
photo-dehydrocyclizations), make possible the observed
Introduction
Solid-state, multi-emissive organic materials are interesting both
for their fundamental electronic properties and for their wide
range of potential applications, such as in flexible displays, solid-
state lighting and sensing.[1] To have in hand solid multiple state
light emitters, comprised of organic molecules with the same
structure, would be extremely useful.[2] Their simplicity would
obviate problems associated with phase separation and device
fabrication. If the properties were also to include good stability to
light and heat, and low fabrication costs, the utility of such
materials would be enhanced even further. However,
development of such materials has been very challenging
because photoemission from organic molecules originates
normally from only the lowest excited state of a given spin
multiplicity, irrespective of the excitation wavelength (N. B.,
1
This article is protected by copyright. All rights reserved.

10.1002/anie.202000608
Angewandte Chemie International Edition
RESEARCH ARTICLE
fluorescence from upper excited singlet states and
phosphorescence from upper excited triplet states. Moreover,
the multiple emissions of these single component systems can
be manipulated to achieve white-light emission at low
temperature. In this way, the research provides an
unprecedented molecular skeleton for achieving multi-colored
emissions from upper excited states of single components in
solids by ‘suppressing’ Kasha’s rule.
Analogous mirror-image relationships were observed for the
emissions and excitations of neat solid SDPE-OS and SDPE-SS
(Figure 1b). Changing the heteroatom from oxygen (as in SDPE-
OO) to the less electronegative element, sulfur (as in SDPE-SS),
allows greater dispersion of the π-electrons. Consequently, the
decreased excitation energy in SDPE-SS, as observed, lead to
bathochromic shifts in its absorption and emission bands.
Importantly, the wavelength difference between the maxima of
the two emission peaks (60, 109, and 131 nm for SDPE-OO,
SDPE-OS and SDPE-SS, respectively) demonstrate that the
energy gap increases between the excited states: from 3260 to
5525 and to 6305 cm-1. The fluorescence quantum yields from
the upper excited states in these solids are unexpectedly high
(2.3%, 6.0% and 9.6%) for SDPE-OO, SDPE-OS and SDPE-SS,
respectively; which shows the linear relationship between the Sn-
S1 energy gap and the Sn emission quantum yield ratios (like
that found for azulenes in the gas phase[6]) (the inset of Table 1).
This trend is also in good agreement with the energy gap law
that the non-radiative rate from Sn to S1 decreases (and, thus,
the probability for radiative emission increases) upon increasing
the separation energy between the two states.[4b]
Scheme 1. Molecular structures of the multi-emissive luminogens investigated.
Results and Discussion
Syntheses. (E)-3H, 3'H-[1,1'-biisobenzofuranylidene]-3,3'-
dione
(SDPE-OO),
(E)-3-(3-oxobenzo[c]thiophen-1(3H)-
When the onset time for measuring the emission spectra was
delayed by 0.5 ms, the upper fluorescence at 391, 393 and 395
nm for SDPE-OO, SDPE-OS and SDPE-SS was still detectable
(Figures 2a and S5). The biexponential decays of nano-second
range components (17.2, 9.2, and 13.7 ns for SDPE-OO, SDPE-
OS and SDPE-SS, respectively) and microsecond range
ylidene)isobenzofuran-1(3H)-one (SDPE-OS), and (E)-3H,3'H-
[1,1'-bibenzo[c]thiophenylidene]-3,3'-dione (SDPE-SS) were
synthesized using
a cross-coupling condensation reaction
between phthalic anhydrides and their sulfur derivatives in the
presence of triethyl phosphite;[5] the structures and purities of the
compounds were fully characterized by nuclear magnetic
resonance (NMR) spectroscopy, high resolution mass
spectrometry (HR-MS), and single crystal analyses; see
Supporting Information for details.
Photophysical properties of SDPE-OO, SDPE-OS and
SDPE-SS. The steady-state emission spectra of SDPE-OO,
SDPE-OS and SDPE-SS crystalline powders, prepared by the
rapid cooling of xylene solutions, were investigated initially
(Figure 1). Upon electronic excitation at 250 nm, dual emissions
consisting of both a short- and a long-wavelength band are
clearly resolved with maxima at 391 and 451 nm for SDPE-OO,
at 393 and 502 nm for SDPE-OS, and at 395 and 526 nm for
SDPE-SS. The relative intensities of the two emission bands are
very dependent on the excitation wavelength, with higher energy
excitations resulting in enhanced emissions at shorter
wavelengths (Figure 1a). These results strongly indicate that the
emissions emanate from more than one excited state. To
explore further this hypothesis, the excitation and absorption
spectra were compared (Figures 1b and S1-S2 in Supporting
Information). For example, the upper excited state fluorescence
from SDPE-OO centered at 391 nm, was easily detected when
the excitation wavelength was less than 250 nm, and the
emission centered at 451 nm arose primarily when the excitation
was within the broad absorption band at 265-440 nm. Also, there
are mirror-image relationships between the short wavelength
emission bands at 391 nm and their excitation spectra, and for
the long wavelength emission bands at 451 nm and their
excitation spectra. These data provide strong evidence for
emissions derived from both upper and lowest excited states; as
noted, this is a rare occurrence in neat molecular solids. The fact
that the same photophysical behavior was found for solids
produced in different batches is additional evidence that these
observations are not a result of the presence of small amounts
of impurities.
Figure 1. (a) Excitation-wavelength-dependent fluorescence spectra of SDPE-
OO, SDPE-OS and SDPE-SS crystalline powders at room temperature under
air. (b) Excitation spectra of SDPE-OO from emissions monitored at 400 nm
(blue dashed line) and at 460 nm (red dashed lines); excitation spectra of
SDPE-OS from emissions monitored at 400 nm (blue dashed line) and at 500
nm (red dashed lines); Excitation spectra of SDPE-SS from emissions
monitored at 400 nm (blue dashed line) and at 530 nm (red dotted lines);
emission spectra (solid lines) in the powders of SDPE-OO (λex = 250 nm),
SDPE-OS (λex = 250 nm) and SDPE-SS (λex = 255 nm) at room temperature.
Evidence that these emissions are from fluorescence will be presented (vide
infra).
2
This article is protected by copyright. All rights reserved.

10.1002/anie.202000608
Angewandte Chemie International Edition
RESEARCH ARTICLE
Table 1. Luminescence properties of SDPE-OO, SDPE-OS and SDPE-SS solids at 298 K in air.
Tentative emission
λem
Sample
Φ (%)[b]
Φ (%)[c]
assignments

(nm)[a]
(vide infra)
17.2 ns,
391
S2-S0
104.0 μs (TADF)
3.2 ns
63.5[d]
2.3% [d]
451
453
548
S1-S0
T2-S0
T1-S0
SDPE-OO
1.4 ms (RTP)
1.0 ms (RTP)
9.2 ns,
393
S2-S0
107.9 μs (TADF)
9.3 ms (RTP)
3.5 ns
SDPE-OS
SDPE-SS
27.7[e]
6.0%[e]
457
502
T2-S0
S1-S0
13.7 ns,
395
S2-S0
107.4 μs (TADF)
7.9 ms (RTP)
4.2 ns
46.2[e]
9.6% [e]
458
526
T2-S0
S1-S0
[a] λex = 250 nm. [b] Total emission quantum yield. [c] Upper excited state fluorescence quantum yields. [d] λex = 270 nm. [e] λex = 290 nm. The inset is the
fluorescence quantum yields of upper excited states versus energy gap between upper and lowest singlet excited states of SDPE-OO, SDPE-OS and SDPE-SS
powders.
components (104.0, 107.9 and 107.4 μs for SDPE-OO, SDPE-
OS and SDPE-SS, respectively) (Figures 2b and S4-S6)
suggested the presence of delayed fluorescence. When the
onsets of the spectral measurements were delayed further to 2
ms, new emission peaks at ca. 450 nm with lifetimes of ca. 1.4,
9.3, and 7.9 ms for SDPE-OO, SDPE-OS and SDPE-SS,
respectively, became apparent (Figures 2a and S7). The
millisecond range decays are attributed reasonably to
phosphorescence from triplet excited states. We also measured
the emission spectra of the three compounds as crystalline
We believe that the emission from upper excited states is
closely related to the inherent structural properties of the
molecules. For that reason, excitation wavelength-dependent
emission spectra of SDPE-OO, SDPE-OS and SDPE-SS in
frozen, aerated 2-methyl tetrahydrofuran (2-MTHF) solutions at
77 K were examined since the emission intensities at room
temperature were very weak (quantum yields < 0.2%) (Figures
S12-S14). Excitation of SDPE-OO at 250 nm resulted in
emission centered at 366 nm and 420 nm. Excitation at 350 nm
led to emission dominated by the 420 nm band. Again, the two
emission bands are consistent with their origins being the
higher and lowest energy excited singlet states. The emission
bands from higher excited states became weaker since TADF
was inhibited at low temperature. Similar excitation wavelength
dependent fluorescence was observed for SDPE-OS and
SDPE-SS. The excitation wavelength-dependent emissions in
the dilute 2-MTHF solutions at 77 K indicate that restriction of
molecules in the frozen environment is effective in achieving
dual emissions from both higher and lowest excited states of
one molecule.
powders at 77
K
(Figure S8). The very similar
phosphorescence bands at 450 nm and 550 nm confirmed that
the band at 450 nm in Figure 2a at room temperature is
phosphorescence. The dual phosphorescence at 450 nm and
550 nm probably originate from T2 and T1, respectively (vide
infra).
The weak emissions at ca. 391 nm at 77 K imply delayed
emission from Sn is strongly suppressed at the low temperature.
To explore the origin of delayed fluorescence, we investigated
the emission decay curves at different temperatures. As shown
in Figures 2c and S9, the proportion of the delayed
fluorescence at 391 nm for SDPE-OO, SDPE-OS and SDPE-
SS powders decreased significantly as the temperature was
reduced, indicating the emission from S2 is a result of thermally
activated delayed fluorescence (TADF). The decay profiles for
the delayed fluorescence of the crystalline powders were very
similar when 10,000 counts were accumulated in the peak
channel and measured under vacuum or under air at room
temperature (Figure S10). Resistance to O2 quenching, which
might be due to the dense packing of molecules in the
crystalline powder (vide infra), is an additional asset of these
systems. Interestingly, thin films of the three compounds made
from evaporation of chloroform solution also exhibit similar
excitation-wavelength dependent multiple emissions (Figure
S11).
The compounds can adopt different emission colors by
changing their excitation wavelength or temperature. Upon
excitation at 250 nm at room temperature, the SDPE-OO,
SDPE-OS and SDPE-SS crystalline powder have Commission
7
Internationale de L'Eclairage (CIE) 1931 coordinates [ ] near to
the blue area. In contrast, upon excitation at 350 nm at room
temperature, the SDPE-OO crystalline powder has CIE
coordinates of (0.19, 0.20), near to the blue area; the SDPE-OS
and SDPE-SS crystalline powders have CIE coordinates of
(0.22, 0.50) and (0.32, 0.55), respectively, in the green region
(Figure S15). Since low temperature can suppress triplet non-
radiative decays and promote phosphorescence, the neat
solids at 77 K with CIE coordinates for SDPE-OO (0.27, 0.33),
for SDPE-OS (0.29, 0.33), and for SDPE-SS (0.32, 0.34), can
produce white light (Figure 2d). This is the first example of
3
This article is protected by copyright. All rights reserved.

10.1002/anie.202000608
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 2. (a) Prompt and delayed emission spectra of SDPE-OO, SDPE-OS and SDPE-SS crystalline powders under air (λex = 250 nm, delay time: 0.5 ms, 2.0
ms). (b) Biexponential fluorescence decay curves from the emission of SDPE-OO, SDPE-OS and SDPE-SS crystalline powders at 391 nm. A greater number of
counts in the decay histograms did not alter the fits. (c) Temperature-dependent luminescence decay curves of SDPE-OO crystalline powders at 391 nm (λex
=
250 nm). (d) Calculated photoluminescence (PL) emission color coordinates of solid SDPE-OO (0.27, 0.33), SDPE-OS (0.29, 0.33), and SDPE-SS (0.32, 0.34) at
77 K in the CIE 1931 chromaticity diagram. (λex = 250 nm) (In this figure, SDPE-OO, SDPE-OS and SDPE-SS are labeled as OO, OS and SS, respectively.
[10]
white light emission based on dual fluorescence and dual
phosphorescence from a single component system in the
solid state.
state intramolecular proton transfer (ESIPT) states
are
excluded by the planar crystal structures and the absence of
active hydrogen atoms. The presence of strong inter- and intra-
molecular hydrogen bonds also makes unlikely the occurrence
of trans-cis isomerization about the central C=C bonds or other
Single crystal structure analyses of SDPE-OO, SDPE-OS
and SDPE-SS. Single-crystal structures of SDPE-OO, SDPE-
OS and SDPE-SS were obtained by very slow evaporation of
solutions of xylene, the same solvent that was employed to
prepare the crystalline powders (Figures S16-S17). The SDPE-
OO crystal grew preferentially along [200] and [002] directions;
the SDPE-OS and SDPE-SS grew along the [011] direction (like
planar layered structures) with larger dihedral angles. Also,
these molecules adopt a typical slip-stacking mode in their
crystal lattices and have molecular layer distances of 3.73, 3.76,
photochemical reactions of SDPE-OO, SDPE-OS and SDPE-SS
[11]
in their solid states
(Figures S21-23). Therefore, the
suppressed photochemical reactions and triplet excited state
facilitate fluorescence from upper excited states and enhance
room temperature phosphorescence.
and 3.81
Å
for SDPE-OO, SDPE-OS and SDPE-SS,
respectively (Figures 3, and S18-S20). In the single crystal
structures, the formation of hydrogen bonds due to the presence
of O/S atoms enhance the propensity of the 3 molecules to
remain planar and lead to overall denser molecular packing than
would be found otherwise. The presence of intramolecular
hydrogen bonds by sulfur atoms with the nearby hydrogen on
the phenyl (C-S…H, 2.45 Å in SDPE-SS) and by oxygen atoms
(C-O…H, 2.61 Å in SDPE-OO) are evidenced by their short
distances (Figure 3a). The SDPE-OS single crystal contains two
different intramolecular hydrogen bonds (C-S…H, 2.43 Å and C-
O…H, 2.69 Å). An abundance of intermolecular hydrogen bonds
is also apparent in the crystal structures (Figures 3b, and S18-
S20).[8] These multiple intra- and inter-molecular hydrogen-
bonding forces, as well as the molecular packing density in the
crystalline states, are expected to inhibit vibrational deactivation
of the upper singlet excited state. Furthermore, the emissions
Figure 3. (a) Important intramolecular hydrogen bonding interactions of
SDPE-OO, SDPE-OS and SDPE-SS. (b) Top view of the crystal structure with
multiple intermolecular hydrogen bonding interactions in the unit cells of the
SDPE-SS single crystal structure.
[9]
from twisted intramolecular charge transfer (TICT) or excited-
4
This article is protected by copyright. All rights reserved.

10.1002/anie.202000608
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 4. (a) Suggested mechanism for the upper excited state fluorescence from neat solid SDPE-OO, SDPE-OS and SDPE-SS that is based on DFT
calculations. IC is internal conversion, ISC is intersystem crossing, rISC is reverse intersystem crossing (i.e., nonradiative Tn-S2), F is fluorescence and P is
phosphorescence. (b) Pertinent calculated orbitals of the S2 state for SDPE-OO, SDPE-OS and SDPE-SS.
Theoretical calculations pertaining to the emission
mechanism. To gain deeper insights into the possible origin of
the multiple emissions of these luminogens, time-dependent
density functional theory (TD-DFT) calculations were carried out.
The aggregation effect and multiple hydrogen bonds were
considered by using the quantum mechanics/molecular
mechanics (QM/MM) theory by the two-layer ONIOM
method.[2a,10c,12] The computational models were built from
5×5×5 molecular units extracted from the crystal structures
(Figure S24); see Supporting Information for details. The
calculated energy levels, electronic transition characters, and
molecular orbitals for the upper and lowest singlet excited states
(S2 and S1) in crystals of SDPE-OO, SDPE-OS and SDPE-SS
are summarized in Figures 4 and S25-S40. As expected, there
is a large difference in excitation energies (0.41-0.47 eV)
between S2 and S1 for SDPE-OO, SDPE-OS and SDPE-SS,
which can greatly reduce the rate of internal conversion between
S2 and S1 (Figure 4a and Tables S1-S6). Since excitation at 250
nm produces transient and delayed upper excited-state
fluorescence, excitation into much higher singlet excited state(s)
(Sm) occurs at this excitation energy.[13] The resultant Sm state(s)
will deactivate to S2 directly through internal conversion due to
small energy difference between Sm and S2, or undergo
intersystem crossing (ISC) to Tn states, which can populate an
energetically matched S2 state directly or through reverse
intersystem crossing (rISC) back to the S2 state. To probe
further this point and to understand better the characteristics of
the transition, we have calculated the orbitals and energy levels
of S2 and its energetically similar triplet excited states (Figures
4b and S26). According to El-Sayed’s rules,[14] transitions
between S and T states are facilitated by a change of molecular
orbital type. The S2 state of SDPE-OO is mainly π, π* in
character, and its energetically similar triplet states are a mixture
of n, π* and π, π*; see Supporting Information for details. For
SDPE-OS and SDPE-SS, the S2 state is mainly n, π* in
character, and its adjacent triplet states are mainly π, π*.
Meanwhile, the S2 and adjacent T states of the three compounds
have very small energy differences (ΔEST =0.08, 0.04 and 0.02
eV for SDPE-OO, SDPE-OS and SDPE-SS, respectively. These
results can rationalize well the generation of delayed
fluorescence from S2 excited states at room temperature [15] and
the experimental observation that the probability of emission
from S2 excited states increases gradually from SDPE-OO to
SDPE-SS. However, the S1 state and its adjacent triplet states of
SDPE-OO, SDPE-OS and SDPE-SS are mainly π, π* in
character (Figure S26 and Tables S1-S6), which is consistent
with the absent delayed emission of the S1 state in experiments.
In addition, a large energy gap between T1 and T2 (1.40, 1.31
and 1.31 eV for SDPE-OO, SDPE-OS and SDPE-SS,
respectively), might be closely related to the observation of dual
phosphorescence of SDPE-OO, SDPE-OS and SDPE-SS.
Conclusion
A simple, one-component, solid-state system is shown to
produce multiple emissions, even at room temperature in the air
by ‘suppressing’ Kasha’s rule. The observation of excitation
wavelength dependent fluorescence, the mirror-image
relationship between the two emissions and their excitation
spectra, the time-resolved emission decay times, and the results
from the emissions at different temperatures are compelling
evidence that the dual fluorescence and dual phosphorescence
emanate from emissions from the first and upper excited singlet
and triplet states. Moreover, the qualitative trends in the
comportment of the emission of the neat solid phases of SDPE-
OO, SDPE-OS and SDPE-SS are completely analogous to the
energy rules derived for azulene derivatives in the gas phase; as
with azulene, there seems to be a logarithmic relationship
between the Sn-S1 energy gap and Sn emission quantum yields.
Single crystal X-ray analyses and calculations reveal the large
energy gaps and an abundance of intra- and inter-molecular
hydrogen bonds whose presence can be related to suppression
of internal conversion of the high-level excited states to the
lowest energy excited singlet states. Together with the matched
energy levels between S2 with its adjacent triplet excited states,
these factors enhance the probability of delayed fluorescence
from upper excited states and are responsible for the high
fluorescence quantum yields from the upper excited states.
Thus, this system not only provides a simple model for
suppression of Kasha’s rule in the solid state, but also opens
new approaches for the design of other single-component,
5
This article is protected by copyright. All rights reserved.

10.1002/anie.202000608
Angewandte Chemie International Edition
RESEARCH ARTICLE
2019, 13, 406-411; f) H. Wang, H. F. Shi, W. P. Ye, X. K. Yao, Q. Wang,
C. M. Dong, W. Y. Jia, H. L. Ma, S. Z. Cai, K. W. Huang, L. Gu, Y. L.
Zhao, Z. F. An, W. Huang, Angew. Chem. Int. Ed. 2019, 58, 18776-
18782; Angew. Chem. 2019, 131, 18952-18958.
multiple-emissive, solid-state organic materials, as exemplified
by the white-light emission produced at low temperature from
these neat solids. The results provided here may lead to the
development of organic solid-state structures with other
intriguing luminous behaviors that include multiple emissions in
wavelength regimes different from those found here, even longer
lifetimes of upper excited states, and resistance to quenching by
common excited state quenchers besides molecular oxygen that
may be useful for various applications.
[3]
[4]
M. Kasha, Discuss. Faraday Soc. 1950, 9, 14-19.
a) N. J. Turro, V. Ramamurthy, W. Cherry, W. Farneth, Chem. Rev.
1978, 78, 125-145; b) T. Itoh, Chem. Rev. 2012, 112, 4541-4568; c) A.
P. Demchenko, V. I. Tomin, P.-T. Chou, Chem. Rev. 2017, 117, 13353-
13381.
[5]
[6]
a) F. Ramirez, H. Yamanaka, O. H. Basedow, J. Am. Chem. Soc. 1961,
83, 173-178; b) C. F. Morrison, D. J. Burnell, Org. Lett. 2000, 2, 3891-
3892.
a) S. Murata, C. Iwanaga, T. Toda, H. Kokubun, Chem. Phys. Lett.
1972, 13, 101-104; b) H. Griesser, U. Wild, Chem. Phys. 1980, 52, 117-
131.
Acknowledgements
[7]
[8]
T. Smith, J. Guild, Trans. Opt. Soc. 1931, 33, 73-134.
Financial support from the Ministry of Science and Technology
of China (2017YFA0206903), the National Natural Science
Foundation of China (21871280 and 21861132004), the
Strategic Priority Research Program of the Chinese Academy of
Sciences (Grant XDB17000000), the Key Research Program of
Frontier Sciences of the Chinese Academy of Sciences
(QYZDY-SSW-JSC029), and K. C. Wong Education Foundation
is gratefully acknowledged. The US National Science
Foundation is thanked for its support of the portion of this work
by RGW through grant CHE-1502856.
a) G. R. Dasiraju, Acc. Chem. Res. 2002, 35, 565-573; b) X.-F. Wang,
H. Xiao, P.-Z. Chen, Q.-Z. Yang, B. Chen, C.-H. Tung, Y.-Z. Chen, L.-Z.
Wu, J. Am. Chem. Soc. 2019, 141, 5045-5050; c) Y. J. Huang, Z. R.
Wang, Z. Chen, Q. C. Zhang, Angew. Chem. Int. Ed. 2019, 58, 9696-
9711; Angew. Chem. 2019, 131, 9798-9813;
[9]
a) S. Sasaki, G. P. Drummen, G.-i. Konishi, J. Mate. Chem. C. 2016, 4,
2731-2743; b) H. Qian, M. E. Cousins, E. H. Horak, A. Wakefield, M.
Liptak, I. Aprahamian, Nat. Chem. 2017, 9, 83-87.
[10] a) C.-C. Hsieh, C.-M. Jiang, P.-T. Chou, Acc. Chem. Res. 2010, 43,
1364-1374; b) K. C. Tang, M. J. Chang, T. Y. Lin, H. A. Pan, T. C. Fang,
K. Y. Chen, W. Y. Hung, Y. H Hsu, P. T. Chou, J. Am. Chem. Soc.
2011, 133, 17738-17745; c) Y. Zhang, H. Yang, H. Ma, G. Bian, Q.
Zang, J. Sun, C. Zhang, Z. An, W. Y. Wong, Angew. Chem. Int. Ed.
2019, 58, 8773-8778; Angew. Chem. 2019, 131, 8865–8770.
[11] a) D. H. Waldeck, Chem. Rev. 1991, 91, 415-436; b) J. Catalán, L.
Zimányi, J. Saltiel, J. Am. Chem. Soc. 2000, 122, 2377-2378; c) J.
Saltiel, A. S. Waller, D. F. Sears, E. A. Hoburg, D. M. Zeglinski, D. H.
Waldeck, J. Phys. Chem. 1994, 98, 10689-10698; d) M. F. Budyka,
Russ. Chem. Rev. 2012, 81, 477-493; e) Y.-H Wu, K. Huang, S.-F
Chen, Y.-Z. Chen, C.-H. Tung, L.-Z. Wu, Sci. China Chem. 2019, 62,
1194-1197; f) Y. Wang, C.-L Sun, L.-Y. Niu, L.-Z. Wu, C.-H. Tung, Y.-Z.
Chen, Q.-Z. Yang, Polym. Chem. 2017, 8, 3596-3602.
Keywords: Aggregation-Induced Emission • multi-colored
emission • solid-state organic materials • the upper excited
states • Kasha’s rule.
[1]
a) S. Haldar, D. Chakraborty, B. Roy, G. Banappanavar, K. Rinku, D.
Mullangi, P. Hazra, D. Kabra, R. Vaidhyanathan, J. Am. Chem. Soc.
2018, 140, 13367-13374; b) Z. Zhang, Y. S. Wu, K. C. Tang, C. L.
Chen, H. Tian, P. T Chou, J. Am. Chem. Soc. 2015, 137, 8509-8520; c)
D. Li, F. Lu, J. Wang, W. Hu, X. M. Cao, X. Ma, H. Tian, J. Am. Chem.
Soc. 2018, 140, 1916-1923; d) I. Bhattacharjee, N. Acharya, H. Bhatia,
D. Ray, J. Phys. Chem. Lett. 2018, 9, 2733-2738; e) T. Higuchi, H.
Nakanotani, C. Adachi, Adv. Mater. 2015, 27, 2019-2023; f) C. Zhou, S.
Zhang, Y. Gao, H. Liu, T. Shan, X. Liang, B. Yang, Y. Ma, Adv. Funct.
Mater. 2018, 28, 1802407; g) Z. Xie, C. Chen, S. Xu, J. Li, Y. S. Liu, J.
Xu, Z. Chi, Angew. Chem. Int. Ed. 2015, 54, 7181–7184; Angew. Chem.
2015, 127, 7287–7290; h) B. Xu, H. Wu, J. Chen, Z. Yang, Z. Yang, Y.
C. Wu, Y. Zhang, C. Jin, P. Y. Lu, Z. Chi, S. Liu, J. Xu, M. Aldred,
Chem. Sci. 2017, 8, 1909-1914; i) J. Yang, Z. Ren, Z. Xie, Y. Liu, C.
Wang, Y. Xie, Q. Peng, B. Xu, W. Tian, F. Zhang, Z. Chi, Q. Li, Z. Li,
Angew. Chem. Int. Ed. 2017, 56, 880-884; Angew. Chem. 2017, 129,
898-902; j) Q. Y. Yang, J. M. Lehn, Angew. Chem. Int. Ed. 2014, 53,
4572-4577; Angew. Chem. 2014, 126, 4660–4665; k) B. P. Yan, S. C. F.
Kui, H. F. Xiang, V. A. Roy, S. J. Xu, C. M. Che, Adv. Mater. 2007, 19,
3599-3603; l) B. W. D'Andrade, R. J. Holmes, S. R Forrest, Adv. Mater.
2004, 16, 624-628; m) G. Q. Zhang, J. B. Chen, S. J. Payne, S. E. Kooi,
J. N. Demas, C. L. Fraser, J. Am. Chem. Soc. 2007, 129, 8942-8943; n)
M. K. Bera, P. Pal, S. Malik, J. Mater. Chem. C. 2020, 8, 788-802; o) H.
Q. Ye, G. F. Liu, S. Liu, D. Casanova, X. Ye, X. T. Tao, Q. C. Zhang, Q.
H. Xiong, Angew. Chem. Int. Ed. 2018, 57, 1928-1932; Angew. Chem.
2018, 130, 1946-1950;
[12] H. Ma, Q. Peng, Z. An, W. Huang, Z. Shuai, J. Am. Chem. Soc. 2019,
141, 1010-1015.
[13] a) J. Liu, Z. Li, T. Hu, X. Wei, R. Wang, X. Hu, Y. Liu, Y. Yi, Y. Yamada-
Takamura, Y. Wang, P. Wang, Adv. Optical Mater. 2019, 7, 1801190;
b) P. Pander, R. Motyka, P. Zassowski, M. K. Etherington, D. Varsano,
T. J. Silva, M. J. Caldas, P. Data, A. P. Monkman, J. Phys. Chem. C.
2018, 122, 23934-23942.
[14] M. A. El-Sayed, Acc. Chem. Res. 1968, 1, 8-16.
[15] Z. Shuai, Q. Peng, Phys. Rep. 2014, 537, 123-156.
.
[2]
a) Z. He, W. Zhao, J. W. Lam, Q. Peng, H. Ma, G. Liang, Z. Shuai, B. Z.
Tang, Nat. Commun. 2017, 8, 416; b) E. Lucenti, A. Forni, C. Botta, L.
Carlucci, C. Giannini, D. Marinotto, A. Pavanello, A. Previtali, S.
Righetto, E. Cariati, Angew. Chem. Int. Ed. 2017, 56, 16302-16307;
Angew. Chem. 2017, 129, 16520–16525; c) J. Guo, J. Fan, L. Lin, J.
Zeng, H. Liu, C. K. Wang, Z. Zhao, B. Z. Tang, Adv. Sci. 2019, 6,
1801629; d) S. Z. Cai, H. L. Ma, H. F. Shi, H. Wang, X. Wang, L. X.
Xiao, W. P. Ye, Z. F. An, W. Huang, Nat. Commun. 2019, 10, 4247-
4254 ; e) L. Gu, H. F. Shi, L. F. Bian, M. X. Gu, K. Ling, X. Wang, H. L.
Ma, S. Z. Cai, W. H. Ning, Z. F. An, X. G. Liu, W. Huang, Nat. Photon.
6
This article is protected by copyright. All rights reserved.

10.1002/anie.202000608
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
A unique molecular skeleton is reported to exhibit multiple state emissions as neat, single-component molecular solids at room
temperature in the air. Mechanistic studies revealed that large energy gaps between S2 and S1 states, and an abundance of
intramolecular/intermolecular hydrogen bonds, suppress internal conversions of the upper excited states in the solids and enable
unprecedented fluorescence quantum yields (2.3-9.6%) from the S2 states.
Y.-H. Wu, H. Xiao, B. Chen, R. G. Weiss, Y.-Z Chen,* C.-H. Tung and L.-Z. Wu*
7
This article is protected by copyright. All rights reserved.