Paintable Room-Temperature Phosphorescent Liquid Formulations of Alkylated Bromonaphthalimide

Article abstract of DOI:10.1002/anie.201811834

Organic phosphors have been widely explored with an understanding that crystalline molecular ordering is a requisite for enhanced intersystem crossing. In this context, we explored the room-temperature phosphorescence features of a solvent-free organic liquid phosphor in air. While alkyl chain substitution varied the physical states of the bromonaphthalimides, the phosphorescence remained unaltered for the solvent-free liquid in air. As the first report, a solvent-free liquid of a long swallow-tailed bromonaphthalimide exhibits room-temperature phosphorescence in air. Doping of the phosphor with carbonyl guests resulted in enhanced phosphorescence, and hence a large-area paintable phosphorescent liquid composite with improved lifetime and quantum yield was developed.

Full text of DOI:10.1002/anie.201811834

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Accepted Article
Title: Paintable Room Temperature Phosphorescent Liquid
Formulations of Alkylated Bromonaphthalimide
Authors: Goudappagouda - -, Ananthakrishnan Manthanath, Vivek
Chandrakant Wakchaure, Kayaramkodath Chandran
Ranjeesh, Tamal - Das, Kumar - Vanka, Takashi Nakanishi,
and Santhosh Babu Sukumaran
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811834
Angew. Chem. 10.1002/ange.201811834
Link to VoR: http://dx.doi.org/10.1002/anie.201811834
http://dx.doi.org/10.1002/ange.201811834

Angewandte Chemie International Edition
10.1002/anie.201811834
COMMUNICATION
Paintable Room Temperature Phosphorescent Liquid
Formulations of Alkylated Bromonaphthalimide
[
a,b]
[c]
[a,b]
Goudappagouda,
Ananthakrishnan Manthanath, Vivek Chandrakant Wakchaure,
[
a,b]
[b,d]
[b,d]
[e]
Kayaramkodath Chandran Ranjeesh, Tamal Das,
Kumar Vanka,
Takashi Nakanishi,
[
a,b]
Sukumaran Santhosh Babu*
Abstract: Organic phosphors have been widely explored with an
understanding that crystalline molecular ordering is a requisite for
enhanced intersystem crossing. In this context, we explore the room
temperature phosphorescence features of a solvent-free organic
liquid phosphor in air. Given alkyl chain substitution varied the
physical states of bromonaphthalimides, phosphorescence remained
unaltered for the solvent-free liquid in air. As the first report, a
solvent-free liquid of a long swallow tailed bromonaphthalimide
exhibits room temperature phosphorescence in air. Doping of the
triplet emission is established with the help of several other
methods such as doping with molecules containing heavy atoms,
matrix assisted isolation using polymers, micelles or
[5,6]
cavitants.
The availability of a support medium rigidifies the
emitter and in turn strengthens the radiative relaxation process.
Despite all the developments, still many key points to be
addressed in the design of efficient organic phosphors.
Moreover, a challenging problem related to the processability of
these crystalline phosphors still persists. In this context, a new
processable soft material called functional molecular liquid has
phosphor
with
carbonyl
guests
resulted
in
enhanced
phosphorescence and hence a large area paintable phosphorescent
liquid composite with improved lifetime and quantum yield is
developed.
been introduced as
a
replacement for solid luminescent
[7-9]
materials.
Here the viscous medium supports luminescence
[
8e]
quantum yield via suppressing the nonradiative decay. Given
organic phosphors have been mostly tested in the crystalline
form under inert conditions, RTP liquids have never been
explored.
Metal-free organic materials exhibiting room temperature
[
1,2]
phosphorescence (RTP) found useful in various applications.
Easy synthetic protocols, structural tunability and air stability
Naphthalimides are a class of rhylene dyes, which have
been widely exploited in self-assembly, light harvesting, sensors,
organic electronics etc. Here we have chosen two alkylated 4-
bromo-1,8-naphthlimides showing RTP in both crystalline and
liquid states for a narrow singlet-triplet energy gap (ΔEST) that is
essential for π-π* transitions and a heavy atom effect, which
[
1-6]
urge the use of organic phosphors.
In recent years, organic
[
10]
small molecules decorated with various functional moieties such
[
3]
as halogens, boronate ester, carbonyl group etc. have been
explored in RTP materials. These structural modifications
eventually resulted in efficient intersystem crossing towards
stable phosphorescence. However, organic phosphors mostly
exhibit excellent RTP in crystalline state, than in the solution
[
6b,11]
populate the triplet excitons by spin-orbit coupling.
These
advantages found effective in delivering RTP not only in
crystalline solid but in a new solvent-free liquid as well. Alkylated
4-bromo-1,8-naphthlimides 1 and 2 (Figure 1a) are synthesized
and characterized by H, C NMR and MALDI-TOF MS.
Photophysical characterization of 1 and 2 are provided in Figure
[4]
state. A strong control over the nonradiative decay in the rigid
crystalline molecular packing supports intersystem crossing.
Moreover, it reduces the triplet quenching by placing the
molecules at finite distances. In short, RTP in organic phosphors
is mainly controlled by the intermolecular interactions and overall
organization of molecules in the crystal. In addition, an efficient
1
13
1b, S1-S3 and Table S1. The melting (T
(T ) temperatures of 1 and 2 are determined by differential
scanning calorimetry (DSC) (Figure 1c, S4). 2 shows Tg, offset of -
8.7 °C, which is different from the solid to isotropic state phase
transition of 1 (101.8 °C). The high T
m
) and glass transition
g
3
[
[
a]
b]
Goudappagouda, V. C. Wakchaure, K. C. Ranjeesh, S. S. Babu
Organic Chemistry Division, National Chemical Laboratory (CSIR-
NCL), Dr. Homi Bhabha Road, Pune-411008, India
E-mail: sb.sukumaran@ncl.res.in
Goudappagouda, V. C. Wakchaure, K. C. Ranjeesh, Tamal Das,
Kumar Vanka, S. S. Babu
m
of 1 and low Tg, offset of 2,
allow 1 and 2 to exist as a crystal and a solvent-free liquid,
respectively (inset; Figure 1c, S5). The melting transition of 1
originates from the favourable crystallization facilitated by the
molecular skeleton of the alkyl chains whereas the random
orientation of long aliphatic chains prevents crystallization of 2
Academy of Scientific and Innovative Research (AcSIR), New Delhi,
India
and hence it remains as
a
RT solvent-free liquid.
[
c]
d]
A. Manthanath
Amrita Vishwa Vidyapeetham University, Amritapuri, Kerala-690525,
India.
Tamal Das, Kumar Vanka,
Thermogravimetric analysis (TGA) indicates that 2 extends
stability to higher temperatures than 1 and 5% mass loss occurs
[
o
o
at 267 C and 212 C, respectively (Figure 1d). The presence of
Physical and Materials Chemistry Division, National Chemical
Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008,
India.
o
large halo at around 2ѳ = 20 in X-ray diffraction (XRD) pattern
of 2 corresponds to branched alkyl chains in the molten state
[
8]
[
e]
Takashi Nakanishi
(Figure 1e). In addition, the small and broad halo at around 2ѳ
Frontier Molecules Group, International Centre for Materials
Nanoarchitectonics (WPI-MANA), National Institute for Materials
Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan.
o
=
4 might be the random distance among the aromatic moieties.
The amorphous features seen in 2 is distinctly different from the
reflection peaks of relatively ordered solid 1 (Figure 1e). It points
to the direct effect of alkyl modification in tuning the physical
Supporting information for this article is given via a link at the end of
the document.
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characteristics of molecules. As a result, 1 forms needle-like
crystal and 2 stays as RT liquid upon evaporation from CH Cl
2 2
solution (Figure S5-S7, inset; Figure 1c).
which normally suppress the nonradiative decay processes.
Generally, the lack of molecular ordering in noncrystalline
materials weakens intersystem crossing. However, in the case
of 2, the viscous nature plays a key role in controlling the
a)
[
8e]
nonradiative decay processes
and promotes the triplet
emission. The complex viscosity of 2 was found to be 0.79 Pa·s
-1
at 1 rads , which is in the range of reported organic liquids
1
2
[8]
(
Figure S13). Sada et al., have recently demonstrated the high
b)
c)
fluorescence quantum yield of tetraphenylethene based solvent-
free liquids through controlled nonradiative decay by the virtue of
1
2
[8e]
inherent viscous medium.
a)
d)
S1
T2
T1
d)
e)
So
ΔEST = 0.13 eV for 1
ΔEST = 0.12 eV for 2
b)
c)
R2 = 0.99
= 5.7 ms
Figure 1. a) Chemical structure of 1 and 2. b) Absorption and steady state

-
5
emission spectra of 1 and 2 in CH
42 nm). c) DSC thermograms of 1 and 2 at a scanning rate of 10 C/min;
insets show the photographs of 1 and 2. d) TGA and e) XRD pattern of 1 and
2 2
Cl solution (C = 1 x 10 M, l = 1 cm, λex =
o
LUMO
HOMO
3
2.
Even though many RTP organic molecules have been
exploited with the advantage of crystalline assembly, till date, no
RTP in a solvent-free liquid has been described in literature.
Zhang et al., have reported phosphorescence features of 1-
poly(methyl methacrylate) (PMMA) films in vacuum, but no
Figure 2. a) Normalized phosphorescence spectra of solid 1 and solvent-free
o
liquid 2 at 25 C in air (λex = 345 nm). b) Phosphorescence lifetime decay
o
profile of neat 2 recorded at 25 C (λex = 345 nm and λmon = 594 nm). c) HOMO
and LUMO of 2 and d) energy level diagram showing ΔEST for 1 and 2 from
detailed study on tunable luminescence features of
1
is
DFT calculations.
[
11b]
available.
Hence to begin with, we measured the
phosphorescence of 1 and 2 in 2-methyltetrahydrofuran (MTHF)
solution (Figure S8) and in neat (Figure 2a) and found that both
derivatives are phosphorescence active under tested conditions.
When solid 1 is excited at 345 nm, phosphorescence spectrum
exhibited two peaks with λmax around 546 nm and 580 nm
To get a deeper understanding of the phosphorescence from
an amorphous solvent-free liquid, luminescence features of 2 is
monitored in the presence of PMMA (Figure S14).
Phosphorescence measurements of 2-PMMA mixture clearly
point out the requisite of Br···O interaction to exhibit RTP in air.
The reduced nonradiative decay due to rigidification of the
molecule by wrapping with polymer is not favouring intersystem
crossing. Monomer emission (350-500 nm) dominates over
(
λ
Figure 2a). Compared to 1, 2 showed a red shifted peak with
max around 594 nm (Figure 2a), which is identical to that of
o
[12]
MTHF solution and neat form at -196 C (Figure S8, S9).
It
supports the molecular design of 2 lacking crystalline ordering to
exhibit RTP. Aa a direct evidence for the control of molecular
structure on photophysical properties, the bulky alkyl substituent
on 2 doesn’t allow to form excimer even at low temperature
o
phosphorescence (550-650 nm) even at -196 C (Figure S14).
Moreover, 2-PMMA thin film also exhibits major contribution from
o
the monomer emission at 25 C. The presence of PMMA
(
Figure S9, S10). 1 exhibited a reversible tunable luminescence
isolates the molecules and hence narrows down the possible
intermolecular interaction. Even though 2 exists as a RT liquid,
the presence of weak Br···O interaction cannot be ruled out and
is supported by the broad UV-Vis absorption of the molecule in
consists of phosphorescence, excimer and monomer emissions,
demonstrating a luminescence based temperature thermometer
for low temperature monitoring (Figure S10). Phosphorescence
lifetime of 1 and 2 were found to be 6.2, 5.7 ms at 25 C and 319, neat form (Figure S2). Phosphorescence studies of 2-PMMA
2
o
o
10 ms at -196 C, respectively, (Figure 2b, S11, S12). Here we
conclude the requisite of available Br···O interaction to support
have demonstrated RTP in a solvent-free liquid, for the first time,
without the assistance of very strong intermolecular interactions,
RTP in 2.
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Single crystal X-ray analysis enables to study the molecular
arrangements of 1, which crystallizes from ethanol in the triclinic
space group P-1 (CCDC: 1873219) (Figure S15). A self-
assembled molecular arrangement is anticipated to reduce
nonradiative decay, additionally provides charge-transfer
character through halogen, hydrogen bonding and leads to
Supramolecular two component assemblies are proved to be
[
13]
an excellent way to heighten RTP. An ample way of improving
phosphorescence by mixing with carbonyl compounds such as
A1-3 (Inset; Figure 3a), which has intrinsic tendency to support
[
14]
RTP via halogen bonding,
has been adopted. A significant
increment of phosphorescence intensity is observed for 2 upon
mixing with increasing the equivalents of A3 and A2 at RT in air
(Figure 3a-c, S18). Among all the combinations, 2+A2 (1:1)
showed maximum phosphorescence enhancement (Figure 3b).
The coassembly formation suppresses monomer emission of 2
and improves phosphorescence (Figure 3c, S19). A slight
[
11b]
efficient intersystem crossing.
However, 2 remain as a free
flowing RT liquid, devoid of strong molecular ordering. It has to
be noted that the steady decrease in crystallinity from 1 to 2
imparted only a marginal effect on phosphorescence of 2 (Figure
2a, 2b). Phosphorescence of amorphous 1 prepared by
o
o
mechanical grinding retained similar phosphorescence features,
but with decreased intensity (Figure S16). This control
experiment shows that crystallinity is not highly required
variation of Tg,offset of 2 from -38.7 C (0 eqv.) to -33.4 C (1.0
eqv.) upon increasing the ratio of A2 was observed (Figure 4a,
S20). IR spectral variations of the composites confirm the
whereas minimum intermolecular interactions as in the case of 2, presence of multiple halogen bonding interactions (Figure S21,
which enhance the spin orbit coupling might result in efficient
intersystem crossing (Figure S16).
Table S4). Similar phosphorescence enhancement was
exhibited by 1 also in combination with A3 and A2 (Figure S22-
2
4) and fluorescent microscope images showed the formation of
In order to probe the phosphorescence of 2 further, a series
of TD-DFT computations were carried out at the B3LYP/6-
luminescent crystalline rods (Figure S25).
a) d)
31+G(d) level of theory. The UV-Vis absorption of 2 originates
from HOMO to LUMO electronic transition, where HOMO is
located on the naphthalene ring (π-type) and LUMO is a
delocalized p-type orbital with significant Br-C P-orbital overlap
2
alone
2
2
2
+A2 (1:0.25)
+A2 (1:0.5)
+A2 (1:1)
(
Figure 2c, S17, Table S2, S3). On examining the relative
computed energies for the singlet (S ) and triplet (T ) states for 2,
there is a triplet state (T ) that is nearly degenerate with the first
singlet excited state (S ) (Figure 2d). Thus it appears that both
the compounds have energetically well-matched states to
enable efficient singlet-triplet crossing to occur; once T is
populated, then relaxation through the triplet manifold to the T
state is possible, followed by emission (Figure 2d). Here it
n
n
2
1
2
cm
-50
-25
T / C
0
25
50
c)
o
2
b)
1
[
8e]
confirms that along with the viscous medium, a low singlet-
triplet energy difference (∆EST
)
facilitates the intersystem
crossing leading to RTP of 2.
a)
Figure 4. a) DSC thermograms in the heating trace of RTP composite of 2
o
with varying ratio of A2 at a scanning rate of 10 C/min. Variation of b)
phosphorescence quantum yield and c) lifetime of 2 upon composite formation
with A3 and A2. d) Photograph of large area (10x10 cm) coating of 2+A2 (1:1)
RTP liquid composite in air.
Natural bond orbital analysis shows higher probability for 1
and A2 to interact in plane mode with Br···O distance of 2.9 Å
and 2 kcal/mol higher stabilization interaction energy than that of
out of plane mode (Figure S26, Table S5, S6). Hence halogen
bonding assisted in plane intermolecular interaction is expected
to promote spin-orbit coupling to facilitate efficient triplet
b)
c)
[
4a]
generation. In addition, it will reduce vibrational losses at the
carbonyl and hence activate phosphorescence emission. The
advantage of carbonyl doping is reflected in enhancing both
phosphorescence quantum yield and lifetime (Figure 4b, 4c, S27,
S28, Table S7). However, detailed experiments have shown that
no phosphorescence enhancement is observed for both 1 and 2
with A3-1 in solution (Figure S29-33). Similarly, no variation in
phosphorescence was found for 1 and 2 with A1 (Figure S34).
Figure 3. a) Phosphorescence spectral changes of
2 with increasing
equivalents of A2 at RT in air; inset shows the chemical structure of carbonyl
guests A1-3. b) Variation of phosphorescence intensity at λmax of all the
composites of 1 and 2 (λex = 345 nm). c) Enhanced luminescence of RTP
liquid composite 2+A2 (1:1) in air.
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We extended the carbonyl guest to other molecules A4-15
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(
Scheme 2) and found that only anthraquinone A14 effectively
[
4]
improved phosphorescence (Figure S35, S36). Since 2 exhibited
improved phosphorescence in combination with A2 (1:1), a
liquid phosphor composite was developed by mixing of 2 with A2
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(
1:1) and this combination realised a large area (10x10 cm)
paintable composite with improved RTP (Figure 4d). RTP liquid
composite 2+A2 exhibit stable luminescence features, but when
the ratio of A2 is high (1), composite solidifies immediately. We
strongly believe that RT liquid feature of 2 enabled to deliver a
paintable RTP composite and this demonstration will be a
potential alternate for the tedious and expensive processing
methods of crystalline RT phosphors.
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5]
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In conclusion, hitherto unknown RTP organic liquid is
materialised by introducing a long branched alkyl chain on
bromonaphthalimide. A comparison clearly showed that even
liquefaction of bromonaphthalimide allowed the molecule to
retain RTP. The suppressed nonradiative decay by available
viscous medium, reduced ∆EST and presence of weak Br···O
halogen bonding facilitated RTP for solvent-free liquid in air. As
a new strategy, paintable RTP composite with significantly
improved phosphorescent quantum yield and lifetime is
prepared by mixing the liquid phosphor with carbonyl guests. A
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Keywords: phosphorescence • organic liquids • luminescent
thermometer • liquid phosphor • excimer
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Table of Contents
COMMUNICATION
A new strategy for hitherto unknown
room temperature phosphorescent
liquid and large area paintable
formulations with enhanced
phosphorescence.
Goudappagouda, Ananthakrishnan
Manthanath, Vivek Chandrakant
Wakchaure, Kayaramkodath Chandran
Ranjeesh, Tamal Das, Kumar Vanka,
Takashi Nakanishi, Sukumaran
*
Santhosh Babu
Paintable RTP liquid composite via carbonyl doping
Paintable RTP composite
Enhanced RTP upon doping
6
Page No. – Page No.
3
I
em
2
cm
Paintable Room Temperature
Phosphorescent Liquid Formulations
of Alkylated Bromonaphthalimide
0
550
600
/ nm
650

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