White light emission in water through admixtures of donor-π-acceptor siblings: Experiment and simulation

Article abstract of DOI:10.1039/c9nj02389e

Donor-acceptor π-conjugated molecules with triphenylamine donor and different acceptor (H, cyano, and pyridinium) units with a double bond spacer were synthesized. These compounds exhibit bathochromic shifts in absorption and emission with an increase in the acceptor strength. Solvatochromic measurements in water reveal emitting states characterized by a polar nature with three distinct emission spectral regions from blue to red. Interestingly, binary mixtures of the stilbenes in acetonitrile and water gave white light emission. MD simulations of the admixtures reveal that the emission data directly correlate to the structural arrangements of the molecules driven by intermolecular and solvent interactions with micelle-like structural arrangements for one set and uniform homogenous mixing for the other two sets. Such a tunable emission strategy using simple structural siblings could offer great potential towards designing novel emitting systems.

Full text of DOI:10.1039/c9nj02389e

View Article Online
View Journal
NJC
New Journal of Chemistry
Accepted Manuscript
A journal for new directions in chemistry
This article can be cited before page numbers have been issued, to do this please use: B. Kumari, A.
Singh, P. Jana, M. Radhakrishna and S. Kanvah, New J. Chem., 2019, DOI: 10.1039/C9NJ02389E.
Volume 42
This is an Accepted Manuscript, which has been through the
Number
6
21 March 2018
Pages 3963-4776
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
New Journal of Chemistry
rsc.li/njc
A journal for new directions in chemistry
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
rsc.li/njc

Page 1 of 18
New Journal of Chemistry
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
5
6
7
8
White Light Emission in Water through Admixtures of
9
Donor--Acceptor Siblings: Experiment and Simulation
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Beena Kumari‡, Amit Singh‡, Palash Jana‡, Mithun Radhakrishna† and Sriram Kanvah‡*.
*‡Department of Chemistry, Indian Institute of Technology Gandhinagar Palaj Gandhinagar
382 355
†Department of Chemical Engineering, Indian Institute of Technology Gandhinagar Palaj
Gandhinagar 382 355
*sriram@iitgn.ac.in, kanvah@gatech.edu, mithunr@iitgn.ac.in
Abstract:
Donor-acceptor π-conjugated molecules with triphenylamine donor and different acceptor (H,
Cyano, and Pyridinium) units with a double bond spacer were synthesized. These compounds
exhibit bathochromic shifts in absorption and emission with an increase in the acceptor
strength. Solvatochromic measurements in water reveal emitting states characterized by a
polar nature with three distinct emission spectral regions from blue to red. Interestingly,
binary mixtures of the stilbenes in acetonitrile and water gave white light emission. MD
simulations of the admixtures reveal that the emission data directly correlates to the structural
arrangements of the molecules driven by intermolecular and solvent interactions with
micelle-like structural arrangements for one set and uniform homogenous mixing for the
other two sets. Such tunable emitting strategy using simple structural siblings could offer
great potential towards designing novel emitting systems.
1

New Journal of Chemistry
Page 2 of 18
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
Introduction
5
6
7
8
9
Conjugated organic molecules with a delocalized -system of electrons represent attractive
targets for application in advanced functional materials1-3. The ease of synthesis, tunable
structure, and photoresponsive behavior of the π-conjugated systems allowed their extensive
use in materials science for organic and biological applications4-9. The delocalization of -
electrons in such organic molecules with donor (D) acceptor (A) framework causes
intramolecular charge transfer yielding substituent dependent solvatochromic emission
behavior with emission in blue, green, and red regions. This substituent dependent emission
can be an ideal strategy to obtain white light emission through careful admixtures of different
chromophores encompassing the visible range. White light emission is usually obtained by
the blending of materials exhibiting complementary emission. Previously white light
emission was demonstrated through the use of single molecular system bearing luminescent
units10, 11, binary or ternary mixtures12, 13, phosphorescent systems14, modulation of pH15,
use of organic-inorganic hybrid materials16, a combination of vegetable extracts17, 18, self-
assembly19-21, through supramolecular interactions22 and, covalent organic frameworks23.
However, issues about color quality, costs, stability, toxicity meant that research to achieve
organic white-light-emitting molecular systems is still dominant24, 25. Such fluorescent or
luminescent materials have also been extensively utilized for biological applications. In this
study, our strategy was based on the utilization of intramolecular charge transfer behavior of
simple D--A molecular siblings to achieve tunable emission. In particular, we have used
triphenylamine (TPA) as an electron donating group due to its favorable electron donating
properties and facile synthetic strategies towards the incorporation of other substituents. The
triphenylamine based molecular systems were a dominant scaffold for many organic
electronic applications26-30. Incorporation of suitable electron acceptor unit combined with -
bridge results in differential emission contributed by the extent of intramolecular charge
transfer exerted by the donor or the acceptor group. Our study reveals that both substituents
and solvents influence the emission properties resulting in blue-red emission. When
combined in specific proportions, we were able to achieve white light emission in acetonitrile
and water. To rationalize the experimental observations, MD simulations were carried out to
understand the effect of substituted acceptor arm on molecular interactions in aqueous
admixtures. Simulations revealed that emission spectra of admixtures correlate to the
structural arrangement of molecules in admixtures. Such a strategy based on simple stilbene
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2

Page 3 of 18
New Journal of Chemistry
1
2
3
4
5
6
7
View Article Online
molecular scaffold introduces a prospective way for white light emission with intDraOmI: 1o0.l1e03c9u/Cla9NrJ02389E
charge transfer characteristics without the need for identifying blue or red emitting materials.
The results are detailed below.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1: The synthetic pathways towards the synthesis of formyltriphenylamine and
stilbene derivatives (1-3).
Experimental
All the chemicals, reagents used in this study were purchased from Sigma Aldrich, Alfa
Aesar, Acros, and S. D. Fine Chemicals. UV-Vis absorption spectra were recorded using
Analytik Jena specord 210 plus, and the fluorescence emission studies were performed using
1
13
Horiba-Jobin Yvon Fluolorog-3 spectrofluorometer with a slit-width of 1 nm. H and C
NMR spectra were obtained with a Bruker Avance 500 MHz NMR spectrometer. For the
absorption or fluorescence studies, 10 μL of the desired fluorophore-stock solution in dioxane
(∼10−3 M) was added to 1 mL (effective concentration ~10 µM) of the desired solvent media.
The excitation wavelengths were set at the absorption maxima (λa) of the compounds under
investigation and the emission (λf) reported is uncorrected, and measurements were
performed at room temperature. Scanning electron microscopy (SEM) analysis was carried
out using a field emission SEM (JSM 7600F JEOL). For this purpose, one drop of the
sample [~10 µM solutions] was deposited on a Si-wafer mounted on an aluminum stub with
the help of double-sided adhesive carbon tape. The samples were dried at room temperature
for 12 h and coated with platinum before being analyzed. For powdered sample analysis,
dried powered sample was sprinkled on the surface of carbon tape and coated with platinum
3

New Journal of Chemistry
Page 4 of 18
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
before analysis.
5
6
7
8
9
Computational Methodology: The 3D coordinates for all three molecules, were generated
by using GaussView31 software followed by geometry optimization at HF/6-31G* level. The
partial atomic charges and the parameters for potential energy function were derived from
OPLS-AA force field32. All MD simulations were carried out by using Gromacs package33.
Water molecules were represented by the TIP3P water model, which describes water as
three-point with rigid pair potentials34. After performing energy minimization using steepest
descent algorithm, each system undergoes NVT for 300ps followed by NPT equilibration run
for 300 ps. With a step size of 1 fs (10-15s) throughout the simulations, the temperature was
controlled at 300K using Langevin dynamics and pressure is maintained at 1 bar using
isotropic pressure coupling with Parrinello-Rahman35 barostat at each 2 ps. Long-range
interactions were evaluated by particle mesh Ewald36 in reciprocal space while short-range
van der Waals interactions were truncated at 1 nm. To eliminate the wall effect, periodic
boundary conditions were employed in XYZ directions, and all bond lengths were
constrained via the LINCS algorithm37. Single molecule dynamics were studied in a cubical
box with 4334 water molecules whereas one counter ion was added to the charged molecular
system to maintain electrical neutrality. For studying molecular interactions in water, we
choose a total of 60 molecules in the 8x8x8 nm3 cubical box with around 50000 water
molecules and a respective number of counter ions to neutralize the system. Similarly, all
three binary mixtures were simulated with a 1:1 ratio of participating molecules with a total
of 60 molecules. The emission spectra were calculated by using M062x DFT functionals38
with 6-311++G(2d,2p) basis set whereas instead of representing a huge number of water
molecules explicitly, polarizable continuum model calculations using the integral equation
formalism approach39 was employed in Gaussian.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Synthetic Methods
The synthetic strategy for the synthesis of the stilbenes is given in scheme-1.
Synthesis
4- (diphenylamino) benzaldehyde (4): POCl3 (53 mmol, 5 mL) was added dropwise to 20
mL of dry DMF at 0C with continuous stirring and after raising the temperature to 75C,
triphenylamine (40.7 mmol, 10 g) and stirred for 2.5 h. After allowing the contents to warm
4

Page 5 of 18
New Journal of Chemistry
1
2
3
4
up to room temperature, the reaction mixture was neutralized by NaOH and extracted VwiewitAhrticle Online
DOI: 10.1039/C9NJ02389E
5
6
7
8
dichloromethane. The organic layer was dried using anhydrous sodium sulfate and
concentrated under reduced pressure. The desired product is obtained after chromatography
purification (silica gel with 1:10 v/v CH2Cl2: petroleum ether). Yield 8.60 g (70%). (White
color powder, 0.838 gm, yield 67.2%). 1HNMR (500 MHz, CDCl3, δ ppm) δ 8.79-8.78 (d, J =
6.5 Hz, 2H), 8.165-8.152 (d, J = 6.5 Hz, 2H), 4.66-4.63 (t, J = 15 Hz, 2H) 3.51 – 3.48 (m, J =
15 Hz, 2H), 3.15 (s, 9H), 2.64 (s, 3H), 2.59-2.53 (m, 2H), 3.08 (s, 6H); 13C NMR (125 MHz,
CDCl3, δ ppm) 153.58, 150.09, 145.24, 142.05, 136.42, 130.68, 130.13, 128.84, 127.27,
125.97, 124.42, 123.41, 121.19, 117.01, 105.89, 47.8.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
General procedure for Stilbenes 1 & 2: para-substituted benzyl bromide and triethyl
phosphite [P(OEt)3] was dissolved in 20 ml of dry THF and stirred for 30 h at 75C. The
reaction mixture was cooled to room temperature, and the solvent was concentrated under
reduced pressure. Sodium hydride (5 equiv) in ~20 ml of dry THF was added to the reaction
contents at 0C and stirred for 15 min. The temperature was raised to 75C, and
formyltriphenylamine was added, and the reaction was stirred for 30 h. The contents were
then cooled and extracted using dichloromethane, dried using anhydrous Na2SO4 and
concentrated under reduced pressure. Purification using silica gel column chromatography
(2.5:10 v/v with CH2Cl2: Petroleum ether) yielded the desired products.
1
N,N diphenyl 4-styrene (1): (whitish-green powder, 0.838g, yield 67%). H NMR (500
MHz, DMSO-D6, δ ppm) 7.581-7.566 (2H, d, J=7.5 Hz), 7.531-7.514 (2H, d, J=8.5Hz)
7.384-7.353 (t, J =7.5 Hz, 2H), 7.342-7.31 (t, J=8Hz, 4H), 7.262-7.23 (t, J = 7.5 Hz, 1H)
7.224-7.191 (d, J=16.5 Hz, 1H), 7.138-7.106 (d, J-16 Hz, 1H), 7.075-7.039 (m, J = 8 Hz,
13
6H); 6.977-6.96 (d, J=8.5 Hz, 2H)., C NMR (125 MHz, CDCl3, δ ppm) 147.45, 147.22,
137.77, 131.82, 130.06, 129.16, 128.37, 128.10, 127.79, 127.34, 126.72, 124.60, 123.74,
123.50. Calcd for C26H21N, 347.1674; Found, 347.1706.
N,N diphenyl 4-cyano-styryl benzene (2): (yellow color powder, 0.87 g, yield 65%).
1HNMR (500 MHz, CDCl3, δ ppm): 7.611-7.596 (d, J = 7.5 Hz, 2H), 7.546-7.531 (d, J = 7.5
Hz, 2H), 7.392-7.377 (d, J = 7.5 Hz, 2H) 7.288-7.260 (m, J = 6.5 Hz, 4H), 7.166-7.122 (m,
13
5H), 7.073-7.042 (t, J = 7.5 Hz, 4H), 6.967-6.935 (d, J = 16 Hz, 1H); C NMR (125 MHz,
CDCl3, δ ppm) 148.38, 147.30, 142.27, 132.48, 131.94, 130.00, 129.40, 127.86, 126.59,
5

New Journal of Chemistry
Page 6 of 18
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
124.90, 124.72, 123.50, 122.89, 119.18, 110.16. m/z (EI MS): Calcd for C27H20N2, 372.1626;
5
6
Found, 373.1733.
7
8
9
4-(4-(diphenylamino)styryl)-1-methylpyridin-1-ium (3): The pyridinium derivative (3)
was obtained by the condensation of 1-methyl-4-pyridinium iodide (2.9 mmol, 0.7 g) with (2)
(2.9 mmol, 0.59 g) in ethanol (20 mL) containing piperidine and Zinc acetate (0.1mmol, 0.03
g). The reaction mixture was refluxed for 8h at 80°C and reaction was monitored with TLC
till the complete consumption of starting material. Cooling of the reaction to room
temperature yielded a red precipitate that is purified using column chromatography (dark red
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
color powder, 0.42 g, yield 67%). HNMR (500 MHz, CDCl3, δ ppm) δ 8.864-8.851 (d, J =
6.5 Hz, 2H), 7.894-7.881 (d, J = 6.5 Hz, 2H), 7.637-7.605 (d, J = 16 Hz, 1H) 7.474-7.456 (d,
J = 9 Hz, 2H), 7.344-7.313 (t, J = 8 Hz, 4H), 7.165-7.135 (m, J = 7.5 Hz 6H), 7.022-7.004 (t,
13
J = 9 Hz, 2H), 6.959-6.927 (d, 1H, J-16 Hz), 4.486 (s, 3H); C NMR (125 MHz, DMSO, δ
ppm) 153.90, 150.80, 146.41, 144.41, 142.14, 129.99, 129.64, 127.03, 125.86, 124.68,
123.29, 120.86, 118.90, 48.14. m/z (EI MS): Calcd for C26H23N2, 363.1856; Found,
363.1866. The 1H and 13 C NMR spectra are provided in the supporting information (Fig S7-
S9)
Results and discussion
Absorption and emission properties
The absorption spectra of stilbenes 1-3 (Scheme-1) are given in Fig 1, and Fig S1 and Table 1
summarizes the absorption and emission data of the synthesized stilbenes. All the stilbenes
are characterized by an electron donating triphenylamine moiety, connected to an aromatic
ring by a -bridge. Stilbene 1 has a phenyl group, stilbene 2 an electron withdrawing cyano
group, and stilbene 3 has an electron withdrawing pyridinium group. In dioxane, stilbene 1
has maximum absorption (ab) at ~368 nm, 2 absorb slightly longer (~390 nm), and 3 absorbs
longest at 459 nm [Fig 1A]. All the stilbenes are characterized by a shorter wavelength band
at 300 nm that is attributed to electronic transitions of the triphenylamine moiety alone40. The
order of the absorption maxima (3 >2> 1) directly correlates to the strength of the electron
withdrawing group and reflects intramolecular charge transfer from donor to acceptor40.
Solvent polarity changes exert weaker (~2 to 7 nm) effects on the absorption maxima for the
stilbenes. For 1 and 2, a broad absorption band is noted in water and this may be due to the
aggregate formation41.
6

Page 7 of 18
New Journal of Chemistry
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig 1: a) Overlapped absorption spectra of stilbenes (1-3) in dioxane and b) absorption of
stilbene 2 in various solvents. Absorption spectra of 1 and 3 in different solvents are given in
the SI.
Table-1. Absorption and emission data of 1-3 in homogeneous solvents and solid state
Solvent
1
2
3
abs
f (nm)
*f
abs
(nm)
392
390
390
387
393
391
400
-
f (nm)
*f
abs
f (nm)
*f
(nm)
366
367
367
364
368
(nm)
462
459
461
459
458
468
452
Heptane
THF
Dioxane
CH3CN
DMF
399, 415 0.044
427, 454 0.351
-
437
425
457
457
452
442
466
0.058
0.069
0.056
0.056
0.055
0.020
494
462
528
528
520
505
540
0.271
0.341
0.194
0.239
0.179
0.020
626
592
665
652
639
615
625
0.131
0.364
0.009
0.031
0.017
0.007
-
Methanol 364
Water
Solid
377
-
*Fluorescence Quantum Yields were calculated using Quinine Sulfate (0.546 in 1N H2SO4) as a
relative standard (for 1 & 2) and Rhodamine 6G, QY = 0.94 in EtOH for 3.
Emission in Solvents
In contrast to the weak absorption maxima changes, the stilbenes show strong solvatochromic
emission due to intramolecular charge transfer (ICT) from a donor, triphenylamine group, to
the acceptor (phenyl, cyano or pyridinium groups). The trends observed in the emission are
also consistent with the strength of the electron withdrawing ability. Structured emission is
noted for stilbenes 1 (399 and 415 nm) and 2 (427 and 454 nm) in heptane. Upon increasing
the solvent polarity variable shifts in the emission, maxima were observed with smaller
bathochromic shifts (~22 nm comparing dioxane to acetonitrile) for 1 and higher shifts (~54
nm) for 2 (Fig 2A & 2B). Stilbene 3 with pyridinium group (Fig 2C) shows strongest
bathochromic emission shifts (~73 nm) with emission maxima > 600 nm in polar solvents.
7

New Journal of Chemistry
Page 8 of 18
1
2
3
4
5
View Article Online
The solvent polarity bathochromic shifts are attributed to the strong ICT charDaOctI:e1r0.1o03f9/tCh9eNJ02389E
excited state and its stabilization through solvent-solute interactions42.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig 2. Emission spectra of stilbenes 1(A), 2 (B) and 3 (C) in solvents of different polarity
Emission in binary solvent mixtures and Scanning Electron Microscopy studies
To understand the effect of solvent polarity changes and to learn of the aggregation behavior,
we measured the emission of the stilbenes in dioxane (non-polar) & water (polar) binary
solvent mixture. With an increase in solvent polarity by addition of water in dioxane, gradual
emission shifts were observed (Fig S2). For 1, the emission maxima shifts from 427 nm in
dioxane to 459 nm (70% water) and 445 nm at 100% water with overall emission shifts of 20
nm. Moreover, a significant drop (~40 fold) in intensity is noted indicating emission
quenching. Stilbene 2 also shows a similar behavior: 469 nm in dioxane to 540 nm at 70%
water that is lowered to 505 nm in 100% water with a ~27 fold decrease in emission
intensity. These emission losses are due to aggregation induced quenching43 unlike their
structural siblings, α-cyanostilbenes, where the opposite aggregation induced emission, is
8

Page 9 of 18
New Journal of Chemistry
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
observed13, 44, 45. Stilbene 3 with cationic pyridinium group showed greater emission shifts
5
6
7
8
with solvent polarity increase along with concomitant emission quenching in higher water
fractions with an ~80 fold emission intensity drop. The emission quenching in polar solvents
is due to solute-solvent interactions coupled with charge orientation46. The drop-cast
scanning electron microscopy images measured at a concentration of 10 µM are shown in
Figure 3. Dispersed aggregates for 1, irregularly shaped aggregates for 2 and flat popsicle
stick-shaped structures for 3 were observed in water. The structures in 1 and 2 in water are
guided by strong hydrophobic interactions that allow molecules to come together forming
aggregates while in 3 a combination of non-covalent interactions such as hydrophobic and H-
bonding interactions play a role in the formation of much-ordered structures. The SEM
images of mixtures (1-2, 1-3, 2-3 and 1-2-3) were also taken (Fig S4) to get the better insight
into its interactions. The concentration of each component was 10 µM in mixture. The
morphology of the mixture was different to the individual morphology indicating structural
changes after addition of the second component.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
Fig 3. Scanning Electron Microscopy (SEM) images of stilbenes 1-3 (10µM) in water.
9

New Journal of Chemistry
Page 10 of 18
1
2
3
4
View Article Online
Emission of molecular mixtures in acetonitrile and water: White Light EmissDioOnI: 10.1039/C9NJ02389E
5
6
7
8
The emission data (Table-1) reveals that Stilbene 1 with no electron withdrawing substituents
emits in the violet, 2 with nitrile substituent emits in the cyan and 3 in the red region. We
implied that the mixing of these fluorophores should enable tunable emission behavior. To
test this, we performed a titration of the molecules with one another in water and recorded
their emission spectrum. Titration of 1 with 2 shows gradual emission shifts with increased
addition of the second fluorophore. Stilbene 1 emits at ~445 nm in water and stilbene 2 emits
at ~505 nm. Gradual addition of 2 yields emission shifts with emission culminating at ~496
nm that is 10 nm adrift from the individual emission of 2. Individual emission bands were not
observed corresponding to the two molecules for the mixtures [Fig 4]. The emission titration
with 2 and 3 reveal similar features. Stilbene 2 emits at ~505 nm in the water while stilbene 3
emits at 614 nm. Gradual emission shifts with distinct emission maxima different to either
molecule culminating at 530 nm are observed.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig 4: Binary mixture titrations of the molecules in water A (1 and 2), B (2 and 3) and C (1
and 3). Individual emission spectra obtained for 1, 2 and 3 are given as dotted lines.
10

Page 11 of 18
New Journal of Chemistry
1
2
3
4
Titration of stilbene 1 with 3 shows slightly different behavior. Increased addition of 3 yVieiewldAsrticle Online
DOI: 10.1039/C9NJ02389E
5
6
7
8
intensity losses with the appearance of a second emission band at a longer wavelength. The
445 nm shows a slight hypsochromic shift to ~435 nm while the longer wavelength band
shows emission maxima at ~530 nm which is still adrift by 80 nm from the individual
emission maxima of 3. The intermolecular interactions of the fluorophores coupled with
solvent interactions result in the observed unique emission. The binary mixture titration
experiments were also repeated in acetonitrile where all the stilbenes are readily soluble (Fig
S3). Titration of 1 with 2 shows the initial emission at 459 nm that shifts to ~519 nm upon
addition of 2 with the appearance of shoulder band at 460 nm. Titration of 1 with 3 yield
emission maxima at 553 nm (3 alone emits at 665 nm) culminating with distinct bands at 1:2
proportions of 1 and 3 with a shorter wavelength band at ~452 nm and the titration of 2 and 3
yield distinct emission maxima at 567 nm.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
B
A
4
6
1
3
2
2
5
5
6
1
3
7
4
Figure 5 (A). CIE plots of binary mixtures 1 & 3 obtained through the mixing of two
different components 1 and 3 in water. 5B. CIE plot for stilbenes 1-2 mixtures in acetonitrile
at different concentrations (data points 1 to 6 are for different concentrations). The
coordinates and the concentrations used are given in the supporting information [Table-S2].
The white light emission shown is visualized under UV light (using 312 nm light on a
transilluminator).
The commission internationale de l’e´clairage (CIE) chromaticity index for the binary
11

New Journal of Chemistry
Page 12 of 18
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
47
mixtures was generated using RGB (Red, Green, and Blue) color space diagram . The CIE
5
6
7
8
values indicate that observance of good white light emission coordinates for the combination
of the stilbenes (1 and 3) at different concentrations [Fig 5A] while for stilbenes 1 and 2 in
acetonitrile show coordinates close to white light emission [Fig 5b]. This approach of using
simple structural siblings could be of great utility for generating tunable emissions and white
light emission
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Emission in Solid State
Stilbenes with Donor−acceptor substituents with good absorption and emission behavior have
been extensively investigated as organic materials3, 48. We, therefore, measured the emission
of monomers and mixtures in the solid state to check if they yield tunable and or white light
emission. In solid state, 1 emits at 466 nm corresponding to the blue region, 2 at 541 nm in
the green region and 3 emits at 623 nm corresponding to the red region. These emission
maxima are red-shifted concerning the emission maxima in water. Better packing interactions
and increase in effective conjugation length due to greater planarity in the solid state result in
the red-shifted emission as compared to the solution state. For mixtures, 10 mg of each
molecule was ground and dissolved in dioxane. After drying the annealed mixture, the
emission measurements were performed. The emission spectra given in Fig 6b shows an
emission maximum at 564 nm and a band at 505 nm for mixtures of 1-2; 627 nm and a weak
band at 509 nm for 2 & 3; and 644 nm for 1-3. These emission bands show distinct deviations
from the original solid state emission wavelengths. Lastly, the mixture of 1+2+3 yields
emission bands at 599 nm and 507 nm. Thus by mixing of the different substrates, tunable
emission is obtained, but solid-state white light emission was not obtained unlike in water or
acetonitrile in solution. In substrates where intramolecular charge transfer is involved, it is
likely that solvent effects play a cooperative role along with the binary components yielding
tunable emission.
12

Page 13 of 18
New Journal of Chemistry
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig 6. Emission in the solid state (A) of individual molecules and the mixtures (B). The
excitation wavelength used is the absorption maxima of individual fluorophores
Rationalization of observations through MD Simulations: Emission in water
Charge difference across the acceptor arm of all three molecules governs the emission
behavior of molecules. To analyze the distribution of water molecules around each molecule,
we calculated pair distribution function g(r) using trajectories of single molecule MD
simulation. Radial distribution function gij(r) gives the relative probability (compared to that
of an ideal gas) of finding molecule j around a molecule i within a spherical shell of thickness
r at a radial distance r. In the following study, i represent the molecules 1, 2 and 3 and j
represents water molecules. gij(r) from now on is represented as g(r).
Computed g(r) shows significant (Fig 7) difference in water population at the first hydration
shell among three molecules. Compared to 1 and 2, positively charged nitrogen atom in
acceptor's arm in 3 is responsible for attracting water molecules from the intermediate shell to
first hydration shell evident by the higher peak in Fig 7. Intermolecular interaction with water
molecule can affect the conjugation between donor and acceptor arm of the molecule by
altering the angular geometrical between donor and acceptor arm. To understand the binary
mixture interactions, we analyzed the distribution of geometrical angles between donor and
acceptor arm. Superimposition of atomic coordinates (Fig S5) shows significant differences
in the geometry of acceptor arms of all three molecules.
13

New Journal of Chemistry
Page 14 of 18
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7: Radial distribution function g(r) between the stilbenes 1,2 and 3 with water.
Stilbene 3 has a stronger interaction with water due to the presence of charged N group which
is evident from the peak at 0.5nm.
We computed the dihedral angles (Fig S6) for describing the ring-twists and found that the
aromatic rings on the donor's arm have similar geometries, but the presence of the water
molecule significantly affects the planarity of the acceptor's arm. Interaction of positively
charged N atom in acceptor arm of 3 leads to a restriction in the rotational motion of the ring
eventually favoring conjugation between the acceptor and π - linker. We also computed the
absorption and emission wavelength using TDDFT calculations, and the resulting values are
in good agreement with our experimental data (Table S1). Extended conjugation and strong
interaction with the solvent minimizes the gap between the ground and excited state in
molecule 3. Moreover, theoretically computed energy differences among HOMOs and
LUMOs (Fig S7) is decreasing from 1 to 3, which is in line with the electron withdrawing
nature of the acceptor's arm. To understand the structural arrangement of molecules in these
aqueous admixtures, we calculated the radial probability distribution of the molecules in
water.
14

Page 15 of 18
New Journal of Chemistry
1
2
3
4
View Article Online
DOI: 10.1039/C9NJ02389E
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig 8: Radial distribution of molecular admixtures A(1-2), B(2-3) and C(1-3) in their
aggregated state from the center of the aggregate which is obtained from 100 ns equilibrated
MD run.
Equimolar mixtures of 1, 2 and 2, 3 show "like dissolves like" type mixing behavior as we
move from the core to the exterior. The entire admixture in water maintains near spherical
geometry throughout the production run. The probability distribution between 1 and 2 (Fig
8A) and (2 & 3: Fig 8B) matches quantitatively. However, we found that in a binary mixture
of 1 and 3 (Fig 8C), both the molecules have their preferred structural arrangements, with the
positively charged 3 tending to remain at the surface and the neutral 1 forming a hydrophobic
core of the aggregate. This type of segregation in mixing gives rise to two independent bands
as observed in experiments (Fig 4). To our knowledge, this is the first time MD simulations
were used to understand the binary mixture containing Donor--acceptor siblings. Together
15

New Journal of Chemistry
Page 16 of 18
1
2
3
4
View Article Online
with our in-silico analysis reveals the role of acceptor's arm in the rearranDOgIe: 1m0.1e0n39t/Co9NfJ02389E
5
6
molecules in binary mixtures that finally govern the emission of the mixtures.
7
8
9
Conclusions
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In summary, we demonstrate that by tuning donor or acceptor substituents on a simple
stilbene scaffold, emissions ranging from blue to red are obtained through intramolecular
charge transfer (ICT). Utilizing this ICT behavior, we observed white-light emission by
careful mixing of the chromophores in water and acetonitrile. MD simulations in water reveal
that the structural arrangements upon mixing the chromophores have a direct correlation on
the emission spectra. Chromophores which interact favorably with each other (1&2 and 2 &
3) exhibit a single peak in the emission spectra governed by simple mixing rule. For
admixtures, with one of the chromophores having preferential interaction with the solvent as
in the case of Compound 1 &3 (Water has a higher affinity towards 3 relative to 1), we
observe two unique peaks in the emission spectra that correlates directly to the formation of a
micelle-type structure wherein we observe a core and shell populated by 1 and 3 respectively.
This strategy of combining experiment and simulation to obtain tunable emission can be
useful in designing newer materials for potential optical applications.
Acknowledgments
Authors (SK, BK) acknowledge a financial grant from BRNS (Board of Research in Nuclear
Sciences), India; 37(2)/14/05/2016) and IIT Gandhinagar for infrastructural facilities. Amit
Singh and Mithun Radhakrishna acknowledge SERB Grant ECR/2017/000097 and High-
Performance Computing facility at IIT Gandhinagar for computational resources.
Electronic Supporting Information: Fig S1-S9 and Tables S1 and S2 are provided.
References
1.
2.
3.
4.
5.
A. Kaeser and A. P. H. J. Schenning, Adv.Mater., 2010, 22, 2985-2997.
D. M. Guldi, Angew. Chem.Intl Ed., 2010, 49, 7844-7846.
Y. Li, T. Liu, H. Liu, M.-Z. Tian and Y. Li, Acc. Chem. Res., 2014, 47, 1186-1198.
M. M. Lerch, W. Szymański and B. L. Feringa, Chem. Soc. Rev., 2018, 47, 1910-1937.
A. Pieper, M. Hohgardt, M. Willich, D. A. Gacek, N. Hafi, D. Pfennig, A. Albrecht and P. J.
Walla, Nature Commun., 2018, 9, 666.
6.
7.
8.
9.
E. Kim and S. B. Park, Chem. Asian J., 2009, 4, 1646-1658.
A. S. Klymchenko, Acc. Chem. Res., 2017, 50, 366-375.
B. N. G. Giepmans, S. R. Adams, M. H. Ellisman and R. Y. Tsien, science, 2006, 312, 217-224.
Y. Luo, Y. Wang, S. Chen, N. Wang, Y. Qi, X. Zhang, M. Yang, Y. Huang, M. Li and J. Yu, Small,
16

Page 17 of 18
New Journal of Chemistry
1
2
3
4
5
6
7
8
9
View Article Online
DOI: 10.1039/C9NJ02389E
2017, 13, 1604113.
Q. Y. Yang and J. M. Lehn, Angew. Chem.Int. Ed., 2014, 53, 4572-4577.
10.
11.
Y. Yang, T. Wang, P. Yin, W. Yin, S. Zhang, Z. Lei, M. Yang, Y. Ma, W. Duan and H. Ma, Mater.
chem. Front., 2019, 3, 505-512.
J. Thompson, R. I. R. Blyth, M. Mazzeo, M. Anni, G. Gigli and R. Cingolani, Appl. Phys. Lett.
2001, 79, 560-562.
12.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13.
14.
B. Kumari, M. Paramasivam, A. Dutta and S. Kanvah, ACS Omega, 2018, 3, 17376-17385.
J. Wang, X. Gu, H. Ma, Q. Peng, X. Huang, X. Zheng, S. H. P. Sung, G. Shan, J. W. Y. Lam and Z.
Shuai, Nature Commun., 2018, 9, 2963.
Y. Yang, M. Lowry, C. M. Schowalter, S. O. Fakayode, J. O. Escobedo, X. Xu, H. Zhang, T. J.
Jensen, F. R. Fronczek, I. M. Warner and R. M. Strongin, J. Am. Chem. Soc., 2006, 128, 14081-
14092.
15.
16.
17.
18.
19.
20.
21.
22.
A. Samet, S. Triki and Y. Abid, J. Phys.Chem. C, 2019, 123, 6213-6219.
V. Singh and A. K. Mishra, Sci. Rep., 2015, 5, 11118.
V. Singh and A. K. Mishra, J. Mater. Chem. C, 2016, 4, 3131-3137.
Y. Xiao, R. Zhang, H. Gao, H. Zhao and X. Cheng, J. Mater. Chem. C, 2019, 7, 1237-1245.
P. Malakar, D. Modak and E. Prasad, Chem. Commun., 2016, 52, 4309-4312.
E. Ravindran and N. Somanathan, J. Mater. Chem. C, 2017, 5, 4763-4774.
H.-T. Feng, X. Zheng, X. Gu, M. Chen, J. W. Y. Lam, X. Huang and B. Z. Tang, Chem. Mater.,
2018, 30, 1285-1290.
S. Haldar, D. Chakraborty, B. Roy, G. Banappanavar, K. Rinku, D. Mullangi, P. Hazra, D. Kabra
and R. Vaidhyanathan, J. Am. Chem. Soc., 2018, 140, 13367-13374.
S. Mukherjee and P. Thilagar, Dyes Pigm., 2014, 110, 2-27.
D. Liu, Q. Ding, Y. Fu, Z. Song and Y. Peng, Org. lett., 2019, 21,2523-2527.
P. Blanchard, C. Malacrida, C. Cabanetos, J. Roncali and S. Ludwigs, Polymer Int., 2019, 68,
589-606.
Y.-L. Xu, W.-L. Ding and Z.-Z. Sun, Nanoscale, 2018, 10, 20329-20338.
D. Lumpi, B. Holzer, J. Bintinger, E. Horkel, S. Waid, H. D. Wanzenböck, M. Marchetti-
Deschmann, C. Hametner, E. Bertagnolli and I. Kymissis, New J. Chem., 2015, 39, 1840-1851.
W. Zhang, P. Heng, H. Su, T. Ren, L. Wang and J. Zhang, J. Phys.Chem. C, 2018, 122, 25219-
25228.
A. R. Mallia, R. Ramakrishnan, M. A. Niyas and M. Hariharan, CrystEngComm, 2017, 19, 817-
825.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci and G. A. Petersson, Gaussian Inc.: Wallingford, CT, USA,
2013.
W. L. Jorgensen, D. S. Maxwell and J. Tirado-Rives, J. Am. Chem. Soc., 1996, 118, 11225-
11236.
D. Van Der Spoel, E. Lindahl, B. Hess, G. Groenhof, A. E. Mark and H. J. C. Berendsen, J.
Comput.Chem., 2005, 26, 1701-1718.
W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey and M. L. Klein, J. Chem.
Physics, 1983, 79, 926-935.
M. Parrinello and A. Rahman, J. Appl. Phys., 1981, 52, 7182-7190.
U. Essmann, L. Perera, M. L. Berkowitz, T. Darden, H. Lee and L. G. Pedersen, J. Chem. Phys.,
1995, 103, 8577-8593.
B. Hess, H. Bekker, H. J. C. Berendsen and J. G. E. M. Fraaije, J. Comput. Chem., 1997, 18,
1463-1472.
Y. Zhao and D. G. Truhlar, Theor. Chem. Accounts, 2008, 120, 215-241.
S. Miertuš, E. Scrocco and J. Tomasi, Chem. Phys., 1981, 55, 117-129.
J.-S. Yang, C.-M. Wang, C.-Y. Hwang, K.-L. Liau and S.-Y. Chiou, Photochem. Photobiol. Sci.,
2003, 2, 1225-1231.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
17

New Journal of Chemistry
Page 18 of 18
1
2
3
4
5
6
7
8
9
View Article Online
41.
42.
43.
44.
45.
F. Würthner, T. E. Kaiser and C. R. Saha-Möller, Angew. Chem.Int. Ed., 2011, 50, 3376-3410.
Y. Sonoda, M. Goto, S. Tsuzuki and N. Tamaoki, J. Phys. Chem. A, 2006, 110, 133D7O9I-:1103.1308397/.C9NJ02389E
X. Ma, R. Sun, J. Cheng, J. Liu, F. Gou, H. Xiang and X. Zhou, J. Chem. Edu., 2015, 93, 345-350.
Z. He, C. Ke and B. Z. Tang, ACS Omega, 2018, 3, 3267-3277.
Y. Chen, J. W. Y. Lam, R. T. K. Kwok, B. Liu and B. Z. Tang, Materials Horizons, 2019, 6, 428-
433
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
46.
47.
48.
V. S. Pavlovich, ChemPhysChem, 2012, 13, 4081-4093.
T. Smith and J. Guild, Trans. Optic. Society, 1931, 33, 73.
L. Beverina and G. A. Pagani, Acc. Chem. Research, 2014, 47, 319-329.
Graphical Abstract
White light emission through binary admixtures of push-pull stilbenes
18