Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation

Article abstract of DOI:10.1002/anie.201601252

Purely organic materials with room-temperature phosphorescence (RTP) are currently under intense investigation because of their potential applications in sensing, imaging, and displaying. Inspired by certain organometallic systems, where ligand-localized

Full text of DOI:10.1002/anie.201601252

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201601252
German Edition: DOI: 10.1002/ange.201601252
Phosphorescence
Versatile Room-Temperature-Phosphorescent Materials Prepared
from N-Substituted Naphthalimides: Emission Enhancement and
Chemical Conjugation
+
+
Xiaofeng Chen , Cheng Xu , Tao Wang, Cao Zhou, Jiajun Du, Zhongping Wang, Hangxun Xu,
Tongqing Xie, Guoqiang Bi, Jun Jiang, Xuepeng Zhang,* James N. Demas, Carl O. Trindle,
Yi Luo, and Guoqing Zhang*
Abstract: Purely organic materials with room-temperature
phosphorescence (RTP) are currently under intense investiga-
tion because of their potential applications in sensing, imaging,
and displaying. Inspired by certain organometallic systems,
(LMCT) or metal-to-ligand charge transfer (MLCT) states
( LMCT/ LMCT or MLCT/ MLCT); the poor superposition
of an orbital from the metal and another from the ligand
1
3
1
3
results in very small singlet–triplet energy splitting (DE ). If
ST
3
3
3
where ligand-localized phosphorescence ( p-p*) is mediated
the ligand-localized triplet state ( IL* or p-p*) happens to be
the lowest in the metal complex, then LMCT or MLCT
readily relaxes to p-p* and may give rise to ligand-localized
RTP under suitable conditions. Alternatively, metal ions
simply induce an intraligand CT state that leads to more
efficient ISC, as we and others recently demonstrated that
fluorescence/RTP dual emission can be achieved for many
3
3
by ligand-to-metal or metal-to-ligand charge transfer (CT)
states, we now show that donor-to-acceptor CT states from the
same organic molecule can also mediate p-localized RTP. In
the model system of N-substituted naphthalimides (NNIs), the
3
[9]
1
relatively large energy gap between the NNI-localized p-p*
3
and p-p* states of the aromatic ring can be bridged by
[
10,11]
intramolecular CT states when the NNI is chemically modified
with an electron donor. These NNI-based RTP materials can
be easily conjugated to both synthetic and natural macro-
molecules, which can be used for RTP microscopy.
aromatic ketone/Lewis acid systems.
Organometallic systems have some intrinsic problems,
such as toxicity and instability in aqueous environments. Thus
an alternative approach to achieve RTP with purely organic,
water-stable compounds is highly desirable. Many purely
organic RTP systems based on carbonyl or heavy-atom
R
oom temperature phosphorescent (RTP) materials are
useful in a plethora of important applications such as sensing,
chemistry have been developed and have found important
[
1–4]
[12–16]
imaging, and displaying.
is to enhance the efficiency of intersystem crossing (ISC).
Generally speaking, ISC is efficient in organometallic sys-
A key method for promoting RTP
applications.
However, for these systems, the chemistry is
[5]
very limited (i.e., carbonyl groups or halogen substituents).
Herein, we report a molecular RTP system that is based on N-
substituted naphthalimides (NNIs; Scheme 1), which com-
bine many benefits, such as easy synthesis, facile chemical
modification, and water stability. Although derivatives of
NNIs have been frequently reported owing to their popularity
[
6,7]
tems,
possibly because of 1) heavy and/or paramagnetic
[
8]
metal ions, which can enhance spin–orbit coupling, or 2) the
presence of closely spaced singlet and triplet ligand-to-metal
[
17,18]
in optoelectronic applications,
RTP
the observation of
[
+]
[+]
[
19,20]
[
*] X. Chen, Dr. C. Xu, T. Wang, C. Zhou, T. Xie, Prof. G. Bi,
Prof. J. Jiang, Dr. X. Zhang, Prof. Y. Luo, Prof. G. Zhang
Hefei National Laboratory for Physical Sciences at the Microscale
University of Science and Technology of China
in this type of molecules is rare. We show here
that heavy-atom induction of RTP is not necessary for NNIs
when an electron donor is introduced.
As shown in Scheme 2, the inefficient RTP of NNI
derivatives may be due to the large singlet–triplet energy
230026 Hefei (P.R. China)
E-mail: zhangxp@mail.ustc.edu.cn
gzhang@ustc.edu.cn
gap (DE ) that is intrinsic to p–p* transitions. As previously
ST
[
+]
X. Chen, J. Du, Prof. H. Xu
reported, the poor overlap between the HOMO and LUMO
of intramolecular charge transfer (ICT) states could result in
Department of Polymer Science and Engineering
University of Science and Technology of China
[4]
a reduced DEST and thus enhanced ISC processes. There-
fore, ICT states introduced into molecules with dominant p–
p* transitions may serve to bridge the large DEST and resulted
in RTP under suitable conditions (e.g., in deoxygenated
polymers). Compounds 1–4 (see the Supporting Information
for their synthesis) were co-dissolved with PMMA in CH Cl
230026 Hefei (P.R. China)
Prof. Z. Wang
Physics Experiment Teaching Center
University of Science and Technology of China
230026 Hefei (P.R. China)
2
2
Prof. J. N. Demas, Prof. C. O. Trindle
Department of Chemistry, University of Virginia
Charlottesville, VA 22903 (USA)
(0.5%, dye/PMMA, w/w), and the solution mixtures were
then dried in glass tubes to form clear homogenous films (ca.
0.2 mm). The steady-state emission spectra of 1 and 2 in the
films under air show featureless bands around 450 nm while 3
and 4 are almost non-fluorescent (Figure 1b). When the tubes
were sealed under vacuum, however, the emissions of 1 and 2
+
[
] These authors contributed equally to this work.
Supporting information and the ORCID identification numbers for
the authors of this article can be found under http://dx.doi.org/10.
1002/anie.201601252.
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Figure 1. a) Photographs of 1–4 in PMMA films in vacuo under 365 nm UV
excitation. b,c) Steady-state emission spectra of 1–4 in PMMA under air (b)
and in vacuo (c). d) Delayed emission spectra of 1–4 in PMMA in vacuo
(
Dt=100 ms, l =365 nm). e,f) Calculated lowest singlet transitions for
ex
simplified 1 (e) and 4 (f).
exhibited little visual alterations but strong yellow emissions
from 3 and 4 were “turned on”. As shown in Figure 1c, the
steady-state emission spectra of 1 and 2 in PMMA films under
vacuum do not show dramatic changes compared to those in
air, whereas two emission peaks centered at l = 541 nm and
Scheme 1. Chemical structures of the naphthalimide-based organic
dyes.
5
88 nm emerged for both 3 and 4 in vacuo. The blue
luminescence centered at lem = 450 nm has a lifetime of
16.7 ns (Table 1, F = 0.046) and 19.7 ns (F = 0.032) for 1 and
2, respectively. In contrast, the lifetime of the fluorescence of
3 and 4 at lem = 541 nm was measured to be approximately
0.23 s (F = 0.031 and 0.039). All lifetimes can be fit to a triple-
exponential decay presumably owing to the heterogeneous
nature of the polymer matrix and possible dye–dye inter-
actions. The emissions with ultralong lifetimes were convinc-
Table 1: Lifetimes and quantum yields of 1–5a in PMMA under vacuum.
1
2
3
4
5a
[
a]
[b]
[b]
tF
16.7 ns
19.7 ns
–
–
4.2 ns
(450 nm)
(450 nm)
(425 nm)
[
a]
[b]
[b]
tRTP
–
–
0.23 s
0.23 s
5.1 ms
(541 nm) (541 nm) (611 nm)
[
[
c]
F
F
0.046
<0.001
0.032
0.0013
0.0011
0.031
<0.001 0.013
0.039 0.050
d]
[a] Excitation source: 369 nm light-emitting diode; lifetime fit to triple-
exponential decay (see the Supporting Information). [b] Emission too
weak for reliable lifetime measurements. [c] Absolute photolumines-
cence quantum yield from 400–500 nm. [d] Absolute photoluminescence
quantum yield from 500–750 nm.
Scheme 2. Design strategy for enhancing the room-temperature phos-
phorescence of naphthalimide-based organic dyes.
2
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Angewandte
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ingly ascribed to room-temperature phosphorescence (RTP).
The RTP spectra for all four molecules (Figure 1d) are almost
identical and show vibration progressions of the aromatic
ring, that is, the emission peaks at 541 nm, 588 nm, and
ꢀ1
6
42 nm are separated by approximately 1500 cm , which
indicates that the lowest excited triplet states are p-localized
3
excited states p-p* for all molecules. This is also consistent
with the observed sub-second lifetimes. All four molecules
share the same electron-deficient, p-conjugated NNI ring.
However, the N substituents exhibit increasing electron-
donating capability, and thus the DEST values of the ICT
states in the four compounds are expected to be in the order
DE (4) < DE (3) < DE (2) < DE (1). To confirm this
ST
ST
ST
ST
hypothesis, using the S1-optimized geometry computed with
[
21]
wB97XD/cc-pVDZ and the Tomasi model for the CH Cl2
2
[
22]
solvent,
GAMESS.
we conducted TDDFT calculations in
The lambda statistic (L) employed by Peach
which measures the locality extent excitations,
[
23,24]
[
25]
et al.,
enables the determination of their CT character. The
lambda value can range from 0 to 1, where the limits indicate
disjoint (L!0) and common (L!1) spaces occupied by the
origin and destination states of an excitation. The calculations
show that the fluorescence is an emission from an allowed
state at about 3.3 eV for each N-aryl species (see the
Supporting Information, Table S1). These are well-localized
excitations with L > 0.8 in each case. Orbital representations
confirm that these transitions are local to the NI frame
Figure 2. a) Steady-state emission spectra of 5a in PMMA under air
black) and in vacuo (red). b) Photographs of 5a in PMMA film under
(
air and in vacuo (l =365 nm). c) Steady-state emission spectra of 5b
ex
in a pH 6.8 buffer before (black) and after deoxygenation (red).
d) Photographs of 5b buffer solutions in air and after purging with
nitrogen gas (l =365 nm).
ex
potential of NNI-based phosphorescent molecules in biolog-
ical imaging, we also synthesized a series of NNI-based ROP
initiators I1–I6 (Scheme S1), from which the luminescent
PLAs 6–11 (Scheme 1) were made. NMR spectra and gel
permeation chromatography (GPC) data (see the Supporting
Information) indicate that polymers 6–11 are well-defined
(PDI < 1.15), and their molecular weights are given in
Table 2.
(
Figure 1e). The lowest singlet transitions from 3 and 4 are
unambiguously CT states (Figure 1 f, L = 0.099 and 0.083)
with a forbidden nature (dark state). This explains the lack of
fluorescence for 3 and 4 even in the solid-state films.
It is well-known that heavy atoms can help populate
triplet excitons by spin–orbit coupling. As shown in Figure 2a,
the steady-state emission spectra of compound 5a in PMMA
film (0.5% by weight) in air exhibit two major bands with
their maxima at l = 420 nm (4.2 ns) and l = 611 nm
F
RTP
Table 2: Molecular weight data of polymers 6–11.
(
(
5.1 ms). Compared to the ultralong “afterglow” of 3 and 4
tRTP ꢁ 0.23 s), the RTP of 5a has a much shorter lifetime and
6
7
8
9
10
11
[
a]
could not be entirely quenched even when exposed to air (see
Figure 2a,b). This is likely due to the enhanced triplet to
M (NMR)
5200
11000
1.09
4900
9300
1.06
7000
10400
1.04
6500
17400
1.14
10000
14500
1.11
11000
12800
1.10
n
[
b]
M
n
(GPC)
[
c]
PDI (GPC)
(
singlet) ground state decay rate from the heavy atom effect,
which outcompetes the relatively slow oxygen diffusion in
PMMA film (ca. 0.2 mm). When a hydrophilic group is used
to replace the butyl group (5b), RTP could also be detected in
deoxygenated buffer solution (Figure 2c,d) but with a shorter
lifetime (0.5 ms) and lower relative intensity, which is
presumably due to activated collisional quenching.
[a] Number-average molecular weight calculated from the integration
ratio of the CH (PLA) proton and residual initiator protons in NMR
spectra recorded in CDCl . [b] Number-average molecular weight
3
determined by GPC using THF as the eluent at 258C; the system was
calibrated with linear polystyrene standards. A correction factor of 0.58
[
26]
for PLA was applied to all data. [c] Polydispersity index (PDI=M /M ).
w
n
The experimental luminescence results for 5a and 5b are
[
20]
consistent with a recent report by Taddei and co-workers,
As shown in Figure 3a, films of 6 (PLA-Br), 7 (PLA-OPh-
Br), 9 (PLA-OPh-Cl), and 11 (1,2-OPh-PLA) exhibit barely
detectable steady-state emission under 365 nm excitation in
air, whereas emissions centered at 412 nm and 385 nm were
observed for 8 (PLA-Cl) and 10 (1,2-PLA), respectively.
After sealing the PLA films under vacuum, a dramatic
increase in the emission intensity of the lower-energy band
(500–700 nm) was observed for all polymers except for 10,
whose emission hardly changed. The luminescence lifetimes
of the newly added emissions for 6, 7, 8, 9, and 11 were
where RTP in both organic solution and the crystalline state
was observed for a Br-substituted NNI derivative. However,
we show in the following experiments that the RTP/fluores-
cence ratio can be further increased by the ICT enhancement
strategy (6–11). Introducing a heavy halogen atom (6–9) or
extending the p-conjugation (10 and 11) can affect the
miscibility between the NNI dye and the polymer matrix; we
thus resorted to the method of covalent dye conjugation with
a polymer by ring-opening polymerization (ROP), which was
[
26]
developed by Fraser and co-workers.
To extend the
measured to be t = 5.6 ms (lem = 611 nm), 3.6 ms (lem
=
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shorter than that of 9. On the other hand, 7 and 9
benefit from ISC enhancement by both heavy-
atom effects and ICT, whereas 6 and 8 only take
advantage of heavy-atom effects; thus the
observed RTP of 7 (or 9) is more prominent
than that of 6 (or 8), as indicated by the
normalized steady-state emission spectra (Fig-
ure 3b,c). Compared to polymers 6–9, the RTP
of 11, which was synthesized to show the most
dramatic CT effect given its large DE (ca.
ST
ꢀ1
5
6 kcalmol ) and thus inefficient ISC, is
enhanced only by ICT. This exerts a substantial
effect in increasing the rate of ISC (S*!T ) but
1
plays a minor role (e.g., vibrational coupling) on
3
the transition from p-p* to the ground state
(
T !S ). As a result, the lifetime of the phos-
1
0
phorescence of 11 is as long as 1.12 s, one of the
longest RTP lifetimes for purely organic mole-
cules ever reported.
Figure 3. a) Steady-state emission spectra of 6–11 in air. b) Steady-state emission
Aside from the synthetic polymers, we also
conducted facile modifications to these NNI-
based dyes with natural biomacromolecules,
using chitosan and bovine serum albumin
(BSA) as two examples. Visually, NNI-modified
chitosan 12 showed red RTP when illuminated
with 365 nm UV light at room temperature even
spectra of 6 and 7 in vacuo normalized to the fluorescence band. Inset: Corresponding
photographs of 6 and 7. c) Steady-state emission spectra of 8 and 9 in vacuo
normalized to the fluorescence band. Inset: Corresponding photographs of 8 and 9.
d) Steady-state emission spectra of 10 and 11 in vacuo normalized to the fluorescence
band. Inset: Corresponding photographs of 10 and 11 (l =365 nm).
ex
6
1
11 nm), 84 ms (lem = 607 nm), 70 ms (lem = 607 nm), and
in air. As shown in Figure 4a, the emission spectra of 12
exhibit an RTP band around 600 nm (5.3 ms) and a weaker
fluorescence band around 410 nm (0.5 ns). Similarly, a sus-
.12 s (lem = 510 nm), respectively (Table 3). These millisec-
3
ond to second lifetimes are characteristic of RTP of p-p*
origin. The lifetimes of the shorter-wavelength emissions for
pension of 14 exhibited bimodal emission at 450 nm (t =
F
6
7
3
, 8, 9, and 10 were measured to be t = 1.75 ns (lem = 413 nm),
0.4 ns) and 575 nm (tRTP = 0.24 ms). Shown in Figure 4c and
4d are two preliminary examples to demonstrate their
application potential. Figure 4c shows that the RTP from
a deoxygenated film of 9 slowly decays over a period of 80 ms;
this method has great potential for investigating the 3D
spatial topology of solid materials over a small area. Figure 4d
is an RTP microscopy image of neuronal cells stained by 13 to
show that such NNI derivatives are very convenient for
biochemical conjugation and could be easily developed as
low-cost, time-resolved bioimaging agents.
.0 ns (lem = 425 nm), 7.1 ns (lem = 450 nm), and 7.3 ns (lem
=
85 nm), respectively, which are ascribed to fluorescence.
The presence of either a heavy atom (Cl or Br) or ICT
provides 6, 7, 8, 9, and 11 with sufficient ISC efficiency to
allow for strong RTP. Compared to Cl, the heavier Br should
have a more significant heavy-atom effect on the NNI
derivatives by stronger spin–orbit coupling. Therefore, con-
sidering the comparable photoluminescence quantum yields
(
ca. 0.03–0.06 in PLA; Table 3), 6 has a larger RTP/
fluorescence intensity ratio (IRTP/I ) as well as a shorter
FL
RTP lifetime than 8. Similarly, the IRTP/IFL ratio of 7 is larger
than that of 9, and the RTP lifetime of 7 is also about 10 times Acknowledgements
We acknowledge support from the Fundamental Research
Table 3: Lifetime and quantum yield data of 6–11 under vacuum.
Funds for the Central Universities (WK2340000068 and
CX2060200022) and the National Natural Science Foundation
of China (21222405, 51402282).
6
7
8
9
10
11
[
a]
[b]
[b]
tF
1.75 ns
–
7.0 ns
7.1 ns
7.3 ns
–
(
413 nm)
(425 nm) (450 nm) (385 nm)
Keywords: charge transfer · fluorescence · naphthalimides ·
organic dyes · phosphorescence
tRTP[a] 5.6 ms
3.6 ms
84 ms
70 ms
–
[b]
1.2 s
(510 nm)
(
600 nm) (600 nm) (600 nm) (600 nm)
[
[
c]
F
F
0.0020
0.050
0.0016
0.046
0.027
0.0057
0.0042
0.021
0.068
<0.001 0.039
0.0035
d]
[a] Excitation source: 369 nm light-emitting diode; lifetime fit to triple
exponential decay (see the Supporting Information). [b] Emission too
weak for reliable lifetime measurements. [c] Absolute photoluminescence
quantum yield from 400–500 nm. [d] Absolute photoluminescence
quantum yield from 525–750 nm.
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Communications
Chemie
Communications
Phosphorescence
Purely organic: N-Substituted naphthal-
imides (NNIs) are purely organic mate-
rials that may show room-temperature
phosphorescence (RTP). The relatively
large energy gap between the NNI-local-
ized p-p* and p-p* states can be
bridged by intramolecular charge transfer
(CT) states when the NNI is chemically
modified with an electron donor.
X. Chen, C. Xu, T. Wang, C. Zhou, J. Du,
Z. Wang, H. Xu, T. Xie, G. Bi, J. Jiang,
X. Zhang,* J. N. Demas, C. O. Trindle,
1
3
Y. Luo, G. Zhang*
&&&& — &&&&
Versatile Room-Temperature-
Phosphorescent Materials Prepared from
N-Substituted Naphthalimides: Emission
Enhancement and Chemical Conjugation
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!