Insights into the Aggregation Behaviour of a Benzoselenadiazole-Based Compound and Generation of White Light Emission

Article abstract of DOI:10.1002/cphc.201900476

We have designed and synthesized the benzoselenadiazole (BDS) based donor-acceptor-donor (D-A-D) π-conjugated compound 4,7-di((E)styryl)benzo[2,1,3]selenadiazole (1). A single-crystal study of 1 shows J-type molecular aggregation in the solid state. The crystal packing of 1 shows head-to-head dimeric intermolecular assembly via Se???N interactions while staircase-type interlock molecular packing has occurred via Se???π interaction. The staircase-type interlock packing of dimeric molecular arrangement induces sheet-type, herringbone type architecture along crystallographic a axis and ab plane via CH???π interactions. Interestingly, the J-type aggregation of 1 in solid state changes to H-type aggregation upon UV-irradiation. Moreover, our spectroscopic findings in solution state reveal H-type of aggregation of 1 in 90 % aqueous THF. We have further demonstrated white light emission in the binary mixture of 1 and 1-pyrenemethanol (2) in 90 % aqueous THF. Our study reveals solvent specific co-assembly of H-aggregated 1 and 2 in 90 % aqueous THF solution, which shows white light emissive properties with the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (0.32, 0.31). The observed white light emission arises mainly due to the combination of red light from H-aggregated 1, blue light from monomeric 2 and green light from excimers of 2.

Full text of DOI:10.1002/cphc.201900476

Accepted Article
Title: Insights into the Aggregation Behaviour of Benzoselenadiazole-
based Compound and Generation of White Light Emission
Authors: Apurba K Das, Sayan Maiti, Rohit G. Jadhav, Shaikh M.
Mobin, and Tushar K. Mukherjee
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To be cited as: ChemPhysChem 10.1002/cphc.201900476
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10.1002/cphc.201900476
ChemPhysChem
Insights into the Aggregation Behaviour of Benzoselenadiazole-based
Compound and Generation of White Light Emission
Sayan Maiti,[a] Rohit G. Jadhav,[a] Dr. Shaikh M. Mobin,[a] Dr. Tushar K. Mukherjee[a] and
Dr. Apurba K. Das*,[a]
[a]Department of Chemistry, Indian Institute of Technology Indore, Khandwa Road, Indore
453552, India
*E-mail: apurba.das@iiti.ac.in
Abstract
We have designed and synthesized benzoselenadiazole (BDS) based donor-acceptor-donor
(D-A-D) type π-conjugated 4,7-di((E)styryl)benzo[2,1,3]selenadiazole (1). The single crystal
study of 1 shows J-type molecular aggregation in solid state. The crystal packing of 1 shows
head to head dimeric intermolecular assembly via SeN interactions while staircase-type
interlock molecular packing has occurred via Se interaction. The staircase-type interlock
packing of dimeric molecular arrangement induces sheet-type, herringbone type architecture
along crystallographic a axis and ab plane via CH interactions. Interestingly, the J-type
aggregation of 1 in solid state changes to H-type aggregation upon UV-irradiation. Moreover,
our spectroscopic findings in solution state reveal H-type of aggregation of 1 in 90% aqueous
THF. We have further demonstrated white light emission in the binary mixture of 1 and 1-
pyrenemethanol (2) in 90% aqueous THF. Our study reveals solvent specific co-assembly of
H-aggregated 1 and 2 in 90% aqueous THF solution, which shows white light emissive
properties with the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates
(0.32, 0.31). The observed white light emission arises mainly due to the combination of red
light from H-aggregated 1, blue light from monomeric 2 and green light from excimers of 2.
Keywords: J-aggregation, H-aggregation, white light emission, solvent induced emission,
aggregation caused quenching (ACQ).
1
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1. Introduction
Organic chromophores have drawn a great attention due to their applications in electronics,[1-
3] digital displays,[4] sensors[5] and light emitting diodes (LEDs).[6-11] The emission colour of
organic chromophores at an appropriate wavelength is controlled by tuning of their
aggregation pattern. It is important to develop next generation light emitting materials.
Molecular aggregation of organic chromophores in solid and solution states plays crucial role
in bulk emission characteristics. Moreover, white light emissive system from organic
building blocks is a great discovery by researchers. Organic light emissive materials have
attracted exceptional attention due to their solubility in wide range of solvents and sufficient
flexibility over inorganic materials for various applications. Several approaches have been
reported for the design and development of white light emitting molecular systems.[12]
Recently, combination of trichromic[13] or bichromic[14] molecular systems has been
developed for the generation of white light emission. The multi-component light emissive
systems facilitate molecular co-assembly by non-covalent molecular interactions which
results in white light emission. A bicomponent white light emitting material has been
developed by co-assembly of pyrene butyric acid and dipeptide functionalized perylene based
amphiphilic molecules.[15] An efficient white light emitting diode has been fabricated by
bicomponent molecular system.[16] Multicomponent white light emission through the energy
transfer among trichromatic system has also been reported.[17] A stable white light emission
has been developed via photo-facilitated aggregation in solution.[18] The mechanistic aspects
of white light emission have been explained by various processes such as host guest
interaction,[19-20] supramolecular interactions,[21-22] mixing of two room temperature
phosphorescent compounds,[23] photo-modulating multicolor photoluminescence systems,[24]
Forster resonance energy transfer (FRET),[25-29] excited state proton transfer (ESIPT),[30-31]
photoinduced electron transfer (PET)[32] and molecular aggregation.[33-36] However,
multicomponent white light emission with CIE coordinates (0.33, 0.33) remains a
challenge.[37]
Benzoselenadiazole (BDS)[38-40] core as an acceptor unit has crucial contribution in tuning the
optical and electrochemical properties of a molecular system. Incorporation of Se atom in
BDS core results in better optical properties over O and S atom based corresponding
molecular units[41-46] due to larger in size, less electronegativity and more soften nature of
selenium. Incorporation of Se and imine (CN) groups can lower the band gap, which shows
various aggregation behaviours and increases the thermal stability.[47-48] Further,
2
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10.1002/cphc.201900476
ChemPhysChem
benzoselenadiazole based derivatives show a distinct aggregation behaviour in organic-
aqueous solvent and show better optical properties.[49] Thus, it is interesting to investigate the
aggregation and photophysical properties of benzoselenadiazole based molecular system.
Here, our objectives are 1) to synthesize benzoselenadiazole based D-A-D molecule, 2) to
study the aggregation behaviour of BDS based D-A-D molecule in solution and solid states
and 3) to generate white light emission using co-assembly of bicomponent molecular system
by tuning solvent responsive aggregation properties.
In this work, we have synthesized
a
-conjugated D-A-D type 4,7-di((E)
styryl)benzo[2,1,3]selenadiazole (1) and studied optical properties in organic-aqueous solvent
mixture. Single crystal of 1 reveals the staircase type J-aggregation arrangement in solid state
which is the origin of intense fluorescence colour. However, the fluorescence intensity of 1 is
quenched upon irradiation of UV light. Tuning of J to H-type aggregation is observed in solid
state thin film after UV irradiation. Emission property in solid state is directly dependent on
the molecular aggregation.[50-55] Optical properties of 1 originate from different extent of
aggregation in organic-aqueous solvent mixture that changes the electronic distribution and
shows different absorption and fluorescence behaviours in comparison to the monomer.
Moreover, a trichromic system has been developed for the generation of white light emission
from the co-assembly of two components (synthesized 1 and 1-pyrenemethanol 2) in a
particular percentage of organic-aqueous solvent mixture.
H2O
1 in THF
solution
H- aggregation of 1 in
90 % aq. THF solution
Co-assembled
H2O
White Light Emission
2 in THF
solution
2 in 90 % aq. THF
Solution
Scheme 1. Schematic representation of white light emissive co-assembled system.
3
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10.1002/cphc.201900476
ChemPhysChem
2. Results and Discussion
Compound was
1
synthesized
by the
Heck
coupling
between
4,7-
dibromobenzo[2,1,3]selenadiazole and styrene. 1-Pyrenemethanol 2 is commercially
available. Single crystal packing and photophysical property of 1 are investigated. White light
emissive system is developed from binary mixing of 1 and 2.
2.1 Molecular Packing of 1
The single crystals of 1 were grown upon slow evaporation at room temperature in DMSO
solution. Single crystal X-ray diffraction of 1 shows the triclinic type crystal packing with the
P¯1 space group. The crystal structure of 1 (Figure 1) consists two asymmetric head-to-head
dimers (dimer 1 and 2) with intermolecular Se···N and N···N interactions. The strong
Se2N2 (3.069 Å), Se1N3 (2.972 Å), N2N3 (3.071 Å) and Se3N7 (2.977 Å), Se4N6
(2.946 Å), N6N7 (3.007 Å) interactions are responsible for formation of the asymmetric
dimeric molecular arrangement. Two dimers are conformationally different. Two trans
phenyl rings are not coplanar with the central BDS unit. For dimer 1, phenyl rings are twisted
around 30.70and 15.21 with respect to central BDS unit. In case of dimer 2, phenyl rings
are twisted around 15.82 and 27.57 with respect to central BDS unit. BDS units in the two
dimers are not exactly parallel to each other. Two BDS units in dimer 1 are deviated with an
angle of 6.39 where as two BDS units are deviated with an angle of 4.61 in dimer 2. Strong
Se interactions (3.730 Å, 3.616 Å Se to  clouds of BDS unit) help to form staircase-type
packing. For dimer 2, strong Se interactions (3.466 Å, 3.741 Å, 3.680 Å, Se to  clouds of
BDS units) lead to form staircase-type (Figure 1) packing along with the crystallographic a
axis. Compound 1 forms sheet-type[57] of structure along crystallographic a axis (Figure S1).
Dimer 1 and dimer 2 are interconnected through CH··· (2.891 Å and 2.775 Å) interaction to
the ac plane. A group of staircases are arranged in a herringbone-like structure to form layer
structure parallel to the ab plane (Figure S2). 1 shows staircase, herringbone and sheet type of
structure along the crystallographic a axis, ab plane and c axis. The crystal structure is
formed due to J- type of aggregation. Single crystal analysis of 1 shows J-aggregation via
head to tail stacking between two dimers which enhances the radiative decay and enhance the
fluorescence in solid state.[58-62] Crystal packing and several non-covalent interactions deeply
affect the photophysical properties of 1 in solid state due to electronic redistribution in the
various electronic states. Solid state UV-visible absorption spectrum of solid powder of 1
(Figure S3) shows a broad absorption band in the wavelength range of 400-600 nm. Solid
4
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10.1002/cphc.201900476
ChemPhysChem
state fluorescence spectrum of solid powder of 1 (Figure S4) shows an intense broad emission
(580-750 nm) centered at 622 nm.
(a)
(c)
(b)
a
b
c
Figure 1. (a) Crystal structure of 1 (asymmetric unit contains four monomers) and intermolecular
interactions are responsible for head-to-head dimer formation. (b) and (c) staircase like representation
of two dimeric forms (interactions are indicated as dotted lines with distance parameters in Å).
2.2 Photophysical properties of 1 in aqueous THF
Absorption spectrum of 1 in THF displays two absorption bands centred at 341 and 474 nm
due to π-π* and internal charge transfer transitions (Figure 2a), respectively.48 A broad
fluorescence band centred at 609 nm has been observed upon excitation at 341 nm (Figure
2a). The estimated quantum yield of 1 in pure THF is 23%. Fluorescence decay measurement
of 1 in THF reveals a single exponential decay with a lifetime of 8.9 ns (Figure 2b). The
fluorescence peak position shifts from 598 to 626 nm upon increase in the solvent polarity
(Figure S5). Fluorescence lifetime decay measurement has been performed in different
solvents by TCSPC setup (Table 1 and Table S1). Notably, the absorption peak and lifetime
does not change significantly with the polarity of the solvent indicating non polar nature of 1
(Figure S6, S7).
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ChemPhysChem
Table 1: Spectroscopic and photophysical properties of 1 in different solvents
max (M-1cm-1) em (nm) -b (cm-1) f
 (ns)
S. no. Solvent
abs (nm)

1
2
3
4
Benzene 481
9033.2
598
609
621
626
4067.6
4676.6
4949.5
4989.9
0.001
8.3
8.9
9.2
9.2
THF
474
475
477
8505.2
8886.4
8444.1
0.209
0.274
0.263
DMF
DMSO
Where, abs = absorption maxima, max= molar extinction coefficient of solvent, em= emission
maxima, -b= stokes shift, f = polarizability factor,  = average lifetime. Concentration of sample
was 66.2 M
104
0.5
(a)
(b)
IRF
UV-vis of 1 in THF
1 In THF
Emission of 1 in THF
0.4
0.3
0.2
0.1
0.0
103
102
300
400
500
600
700
0
10
20
30
Time (ns)
Wavelength (nm)
Figure 2. (a) Absorption and emission spectra of 1 in THF. (b) Fluorescence life time decay
curve of 1 in THF (ex = 375 nm, em = 620 nm).
Next, the photophysical behaviour of 1 in aqueous THF mixture was investigated. A
remarkable deviation in the aggregation behaviour of 1 has been observed in solution state
compared to crystalline solid state behaviour. The two absorption peaks at 341 and 474 nm
become broad and blue shift to 335 and 470 nm with increase in the amount of water (Figure
3a). The fluorescence of 1 gets quenched significantly upon increasing the amount of water
with noticeable red shift (Figure 3b). In 90% aqueous THF, the fluorescence peak of 1 shifts
to 630 nm with 85% fluorescence quenching (Figure 3b). The fluorescence quantum yield of
1 decreases from 23% in pure THF to 4.6% in 90% aqueous THF solution. Notably, the
emission colour is also changed from orange to red with varying water content in THF
solvent (Figure 3b inset). Similar fluorescence quenching with blue shift in the absorption
band has also been observed earlier for various H-type of aggregates.63-64 For example,
Pandey et al observed H-type aggregation in cyanine dye upon addition of 2 wt% 1M
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10.1002/cphc.201900476
ChemPhysChem
aqueous NaOH in 1L[bmim][BF4].65 Similarly, Jassen et al.66 synthesized thiophene end-
capped pyridine DPP derivatives which shows H-type aggregation with the blue shifted
absorption spectrum. Therefore, the decrease in fluorescence intensity along with blue shift in
absorption bands of 1 in aqueous THF mixture suggests H-type of aggregation. To gain
further knowledge into the photophysical behaviour of 1 in aqueous THF mixture, we have
performed fluorescence decay measurement.
2x106
1.0
(a)
(b)
Water (%)
0
Water (%)
0
10
20
30
40
50
10
20
30
40
50
90
1x106
0.5
90
0.0
300
0
450
400
500
600
700
800
500
550
600
650
Wavelength (nm)
Wavelength (nm)
4
0.5
0.4
0.3
0.2
0.1
0.0
(c) 10
(d)
IRF
0
Water (%)
1.0
a2
a1
10
20
30
40
50
90
0.9
0.8
0.7
0.6
0.5
103
102
101
0
20
40
60
80
100
0
10
20
30
40
50
Water (%)
Time (ns)
Figure. 3. (a) Absorption and (b) emission spectra (λex=340 nm) of 1 (6.6 M) with different
volume percentage (%) of water in THF. (c) Fluorescence lifetime decay curves of 1 in
different percentage of water in THF (ex = 375 nm, em = 620 nm). (d) Plot of the amplitudes
of the two components of 1 in the biexponential fits with the different water content (%) in
THF.
Figure 3c shows the fluorescence decay curves of 1 in the solvent mixtures. In pure THF, the
decay fits well with a single exponential function. The estimated lifetime of 1 in THF is 8.90
ns. However, a significant change in the decay kinetics has been observed upon addition of
water. The decays do not fit with single exponential function rather fit well with a
biexponential function in the presence of water. Moreover, the fluorescence decay of 1
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10.1002/cphc.201900476
ChemPhysChem
becomes shorter upon addition of gradual amount of water (Figure 3c). At 10% aqueous
THF, the average lifetime of 1 is 7.00 ns with lifetime components of 2.2 (0.08%) and 7.4 ns
(92%) (Table S2). Notably, the contribution of the shorter lifetime component increases with
increase in the amount of water (Table S2). At 90% aqueous THF, the average lifetime
decreases to 0.9 ns with lifetime components of 0.34 (45%) and 1.3 ns (55%). These results
clearly indicate that upon addition of water, a new kind of emissive component forms in
solution. The single exponential decay kinetics of 1 in pure THF indicates the presence of
only one component, which is monomeric non-aggregated form of 1. However, the
appearance of biexponential decay in aqueous THF mixture indicates the formation of a new
kind of emissive component with shorter lifetime. While the contribution of the shorter
lifetime component increases with increase in the water content of the system, the longer
lifetime component decreases (Figure 3d and Table S2). This clearly indicates a dynamic
equilibrium between two kind of emissive species in the solvent mixture. On the basis of our
steady-state observations, we can assign the new shorter lifetime component to H-type of
aggregates. We propose that a displacement in long molecular axis of 1 occurs upon addition
of water and molecules are stacked side-by-side that reduced the radiative decay and show H-
aggregation.
To know whether the same phenomenon appears in solid state, we have performed
solid state absorption and emission measurements of drop-casted thin film of 1 in THF and in
90% aqueous THF. Thin film absorption spectrum of 1 in THF shows two bands at 355 and
460 nm (Figure 4a and b). In contrast, the thin film of 1 in 90% aqueous THF shows a broad
structureless absorption in the region of 300-500 nm. Thin film emission spectrum of 1 in
THF shows intense red emission at 623 nm, which is almost quenched in 90% aqueous THF.
This quenching of thin film emission intensity of 1 in aqueous THF is similar to that observed
earlier in solution state and arises due to the formation of H-type of aggregates (Figure 4a and
b).
8
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10.1002/cphc.201900476
ChemPhysChem
2
(b)
1.2x106
1.0x106
8.0x105
6.0x105
4.0x105
(a)
THF
90% aq THF
THF
90% Aq THF
1
2.0x105
0.0
0
300
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
2
1
0
(c)
(d)
1.2x106
1.0x106
8.0x105
6.0x105
4.0x105
1 before UV
1 in THF before UV irradiaton
1 in THF after UV irradiation
irradiation
1 after UV
irradiation
2.0x105
0.0
500
300
400
500
600
700
800
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
(e)
After UV (max = 254 nm)
irradiation
Emission from thin film of 1
Emission from thin film of 1
under 365 nm UV light
under 365 nm UV light
Figure 4. Solid state (a) UV-Vis and (b) fluorescence (ex= 340 nm) spectra of 1 in THF and
90% aqueous THF. Thin film (c) absorption and (d) emission spectra (ex= 340 nm) of 1 in
THF solution prior and after UV irradiation. (e) Emission of 1 before and after UV irradiation
under 365 nm UV lamp.
Next, we have investigated the effect of UV irradiation (λmax = 254 nm) on the photophysical
properties of 1 in solid thin film. Upon UV irradiation for 1 h, the -* and charge transfer
bands shift from 355 to 345 nm and 460 to 450 nm, respectively (Figure 4c). The thin film
emission spectrum of 1 exhibits a strong red emission at 623 nm upon excitation at 340 nm
(Figure 4c and d). Interestingly, this red emission quenches significantly upon UV irradiation
(λmax = 254 nm) for 1 h (Figure 4d and e). The UV irradiation induced changes in the
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10.1002/cphc.201900476
ChemPhysChem
photophysical behaviours of 1 are similar to our earlier observations in aqueous THF
mixtures and may originate due to the structural changes from J- to H-type of aggregation.
Here, it is important to note that UV light may assists - stacking and/or strong dipole-
dipole interactions, which results in lower fluorescence in the H-aggregated form.
3x106
0.50
(b)
(a)
Water ()
0
Water (%)
10
20
30
40
50
90
0
2x106
10
20
30
40
50
90
0.25
0.00
1x106
0
350
300
350
400
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 5. (a) Absorption and (b) emission spectra (λex = 340 nm) of 2 (6.6 M) with different
volume percentage of water (%) in THF.
2.3 Photophysical properties 2 in aqueous THF
Next, we have explored the photophysical behaviour of 2 in aqueous THF mixture by using
steady-state and time-resolved fluorescence techniques. Multiple absorption peaks of 2 are
observed in THF at 311, 325 and 340 nm (Figure 5a) due to transitions from S0(v0)-S1(v2),
S0(v0)-S1(v1) and S0(v0)-S1(v0) respectively. Fluorescence peaks are appeared at 375, 394 and
415 nm (Figure 5b) due to emission from S1(v0)-S0(v0), S1(v0)-S0(v1) and S1(v0)-S0(v2),
respectively. The absorption intensity as well as fluorescence intensity increases with
increase in the amount of water in THF solution (Figure 5). Notably, the absorption and
emission peak positions remain unchanged with increase in the percentage of water in THF
suggesting absence of any specific aggregation in aqueous THF mixture. The fluorescence
quantum yield of 2 increases from 1.9% in pure THF to 15% in 90% aqueous THF solution.
The measured fluorescence lifetime of 2 in pure THF is 17.7 ns (Figure S8). However, the
lifetime increases to 27.5 ns in 90% aqueous THF, which is similar to the results, observed in
steady-state experiment. To account the observed photophysical behaviour of 2 in aqueous
THF mixture, we have calculated the radiative (kr) and non-radiative (knr) decay rates from
steady-state and time-resolved spectroscopic data (Table 2). While the radiative decay rate of
2 increases from 1.07×106 to 5.4×106 s-1 in 90% aqueous THF, the non-radiative decay rate
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10.1002/cphc.201900476
ChemPhysChem
decreases from 5.5×107 to 3.06×107 s-1. The high value of knr in pure THF is attributed to the
free rotation of the -CH2OH group, which provides a rapid non-radiative deactivation
pathway to the ground state.[67-68] However, in aqueous THF mixture this free rotation is
restricted due to the H-bonding interaction with water molecules. As a consequence, the
fluorescence intensity as well as the lifetime of 2 are enhanced in 90% aqueous THF.
Table 2. Radiative and nonradiative rates of 2 in different volume% of aqueous THF
kr =  (s-1)
1.07×106
5.4×106
knr = 1− (s-1)
% of water in THF
 (ns)


0
0.019
0.15
17.7
27.5
5.5×107
3.0×107
90
(a)
(b)
1
4.0x106
1.0
2
1+2
1 + 2
2
1
1+2
1
2
2.0x106
0.5
0.0
nm
0.0
350
300
400
500
600
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
(c)
(d)
5x103
IRF
Decay of 2
Decay of 2
in presence of 1
4x103
3x103
1
1+2
2
2x103
0
10
20
30
40
50
Time (ns)
Figure 6. (a) Absorption and (b) emission spectra of 1 (6.6 M), 2 (47.6 M) and their
mixture in 90% aqueous THF solvent mixture. (c) Corresponding CIE diagram. (d)
Fluorescence lifetime decay curves (λex = 334 nm, em = 484 nm) of 2 in the absence and
presence of 1.
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2.4 Generation of White Light Emission from Co-assembly of 1 and 2
Here, we have used two components system for the generation of white light in solution. We
have chosen 1-pyrenemethanol (2) as one component which has dual emission bands and 1 as
another component. The intensity of these emission bands can be easily tuned by changing
the local environments. In order to achieve white light emission, varying proportions of 1 and
2 were mixed in different ratios of THF-water solvent mixture (Figure S9). Figure 6 displays
the absorption and emission spectra of individual molecules and their mixture. In 90%
aqueous THF, a mixture of 1 (6.6 M) and 2 (47.6 M) shows white light emission with CIE
coordinates of 0.32, 0.31 (Figure 6c) at an excitation wavelength of 365 nm. While the
emission spectrum of 1 remains unchanged, a new broad emission band of 2 appears at 480
nm in this concentration range of 47.6 M (Figure 6b). The excitation spectrum of 2 at 480
nm is similar with the excitation spectrum of 2 at 394 nm. These results signify that the origin
of the new emission band at around 480 nm of 2 is attributed to the formation of excimer
(Figure S10). In 90% aqueous THF, this excimer of 2 is not observed at a concentration of 6.6
M. Moreover, a rise time of 0.52 ns has been observed at 484 nm emission wavelength,
which unambiguously indicates the presence of excimers. Therefore, this broad emission
(460-550 nm) of 2 at 480 nm can be assigned due to the formation of excimer, which is a
common phenomenon for pyrene containing molecules.69 Moreover, the intensity of excimer
band increases gradually with water percentage in THF (Figure S11). Notably, no excimer
emission of 2 has been observed in pure THF, indicating the role of water in excimer
formation (Figure S11). We assume that flexibility of 2 is gradually decreased and enter into
a rigid environment with increasing the water percentage in THF due to H-bonding. That
creation of rigid environment might be the cause of generation of excimer at mentioned
concentration (47.6 M). The emission spectrum of the binary mixture (1 and 2) shows
characteristic emission of both the molecules in the wavelength range of 370-650 nm (Figure
6b). While a noticeable red shift of 5 nm in the excimer band of 2 has been observed in the
mixture, the peak positions of 2 in the wavelength range of 370-420 nm remain unaltered
suggesting stabilization of excimers upon association with the H-type aggregates of 1. Next,
we have performed TCSPC experiment (Figure 6d, Figure S12, Table S3, S4) to clarify
whether this white light emission is a consequence of energy transfer or photoelectron
transfer between 1 and 2. No significant change in the fluorescence decay as well as in
lifetime of 2 has been observed in the presence of 1, indicating no energy or electron transfer
between 1 and 2. These observations clearly indicate that the white light from the binary
12
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10.1002/cphc.201900476
ChemPhysChem
mixture arises mainly due to the combination of blue light from the monomeric 2, green light
from the excimers of 2, and red light from H-aggregated 1. Here it is important to mention
that 2 shows prominent green excimer emission only at high concentration in 90% aqueous
THF and an appropriate combination of blue, green, and red emission is required to generate
pure white light. Moreover, our findings reveal that the excimers of 2 are associated with the
H-aggregates of 1 and play a vital role for the white light generation in the present two-
component system. CIE coordinates of corresponding emissive colour of physical mixture of
1 and 2 at different water% in THF (Table S5).
3. Conclusion
In summary, a benzoselenadiazole (BDS) based donor-acceptor-donor (D-A-D) type π-
conjugated 4,7-di((E)styryl)benzo[2,1,3]selenadiazole (1) was synthesized. The aggregation
properties of 1 were investigated in solid and solution states which are characterized by UV-
vis, fluorescence spectroscopy and solid state single crystal analysis. The solid-state
photophysical properties of 1 were strongly controlled by its molecular packing
arrangements. In solid state, 1 aggregates via J-aggregation through non-covalent interactions
(Se, CH), whereas in solution state H-type of aggregation was observed. J-type
aggregation of 1 was tuned to H- type upon UV irradiation. White light emission was
achieved from the solvent induced co-operative assembly of 1 and 2 in 90% aqueous THF
solvent mixture with CIE coordinates of (0.32, 0.31), which is very close with the CIE
coordinates (0.33, 0.33) of pure white light. Importantly, appearance of green light from
excimer of 2 at 90% aqueous THF is responsible to generate white light emission by the co-
assembly of 1 and 2.
4. General Methods and Materials
All solvents and reagents were purchased from Merck and Sigma-Aldrich USA. All the
reactions were performed in dry glassware under nitrogen atmosphere and stirred with
magnetic stirrer. Tetrahydrofuran (THF) was distilled over sodium/benzophenone. Thin layer
chromatography (TLC) was performed using Merck Silica gel 60 F-254 pre-coated plates and
visualized under UV chamber ( = 254/365 nm). Silica gel from Merck (particle size 100-200
1
13
mesh) was used for column chromatography. H and C NMR spectra were recorded using
Bruker 400 MHz spectrometer. Chemical shifts (δ) are reported in ppm relative to the
1
13
residual solvent signal (CDCl3, δ = 7.26 ppm for H NMR and δ = 77.0 ppm for C NMR,
DMSO-d6, δ = 2.5 ppm for 1H NMR and δ = 39.6 ppm for 13C NMR).
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4.1 Synthetic Procedure
Synthesis of Benzo[2,1,3]selenadiazole (3). o-Phenylenediamine (1 g, 9.2 mmol) and
selenium dioxide (1.12 g, 10 mmol) were taken in a 100 mL round bottom flask in 15 mL
ethanol. After refluxing at 65 C for 1 h, solvent was evaporated under reduce pressure to
obtain faint pink coloured solid product (1.6 g, 94%). Benzo[2,1,3]selenadiazole (3) was
synthesized from the previously reported methods.[48] 1H NMR (CDCl3, 400 MHz) 7.76 (d, J
13
= 8 Hz, 2H), 7.38 (d, J = 8 Hz, 2H) ppm. C NMR (CDCl3, 100 MHz):  160.54, 129.08,
123.71 ppm.
Synthesis of 4,7-Dibromobenzo[2,1,3]selenadiazole (4). Benzo[2,1,3]selenadiazole 3 (0.5
g, 2.73 mmol) and silver sulfate (0.84 g, 2.73 mmol) were dissolved in 10 mL concentrated
H2SO4 at 0 C temperature. Liquid bromine (294 L, 0.91 g, 5.70 mmol) was added dropwise
for 20 minutes into the reaction mixture. After stirring for 24 h, the whole reaction mixture
was poured into the cold water and extracted by ethyl acetate and washed with brine (3 × 100
mL) solution. Finally the organic part was concentrated under vacuum. Residue was purified
by column chromatography (silica gel as stationary phase and 1:1 hexane: dichloromethane
as eluent) and yellow powder was obtained (0.4 g, 43%).
4,7-Dibromobenzo[2,1,3]selenadiazole was synthesized from the previously reported
13
methods.[48] 1H NMR (DMSO-d6, 400 MHz): δ 7.83 (s, 2H) ppm. C NMR (DMSO-d6, 100
MHz):  156.82, 132.25, 116.69 ppm.
Synthesis of Compound 1. In a 100 mL two neck round-bottom flask, 4,7-
dibromobenzo[2,1,3]selenadiazole 4 (0.70 g, 2.054 mmol) and styrene (517 L, 4.52 mmol)
were dissolved in dry DMF (25 mL). Tetrabutylammonium bromide (1.30 g, 4.32 mmol) and
potassium carbonate (0.77 g, 5.13 mmol) were dissolved in the reaction mixture. The reaction
mixture was purged with nitrogen gas for 30 min. After that Pd(OAc)2 (0.036 g, 8 mmol%)
was added to the reaction mixture. The reaction mixture was allowed to stir at 110°C for 48
h. The solvent was removed under reduced pressure. The product was extracted using
dichloromethane and washed with 1N HCl solution followed by brine solution (3× 30 mL).
The organic part was dried over sodium sulfate and the solvent was evaporated using rotary
evaporator. The resultant extract was purified by column chromatography using hexane: ethyl
acetate (9:1) as eluent. Compound 1 was obtained as red solid powder (0.6 g, 75% yield).
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10.1002/cphc.201900476
ChemPhysChem
1H NMR (CDCl3, 400 MHz): δ 7.80 (d, J = 16 Hz, 2H), 7.64 (d, J = 16 Hz, 2H), 7.57 (s, 2H),
7.55 (d, J = 8 Hz, 4H), 7.32 (t, J = 8 Hz, 4H), 7.22 (t, J = 8 Hz, 2H), ppm. 13C NMR (CDCl3,
100 MHz): δ 159.66, 137.66, 132.86, 130.78, 128.81, 128.14, 126.99, 126.75, 124.83 ppm.
Stock solution preparation for the spectroscopic experiments. 10 mL of stock solution of
each compound (1 mmol) was prepared for the experiment.
Solid thin film preparation. Thin films of the samples were prepared by drying the glass
slide after drop casting of solution of compound 1 in respective solvent.
4.2 Instrumentation and Methods
Single Crystal XRD. Single crystal X-ray analysis of 1 was performed on a CCD Rigaku
Oxford Diffraction (Earlier Agilent Technologies), Super Nova diffractometer. Data for the
compound were collected at 150(2) K, using graphite-monochromated MoKα radiation (λα =
0.71073). The strategy for the data collection was evaluated by using the CrysAlisPro CCD
software. The data was collected by the standard ‘phi-omega scan techniques and were scaled
and reduced using CrysAlis-Pro RED software. The structure was solved by direct methods
using SHELXS-97 and refined by full matrix least-squares with SHELXL-97, refining on
F2.[56] The positions of all the atoms were obtained by direct methods. All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms were placed in geometrically
constrained positions and refined with isotropic temperature factors, generally 1.2Ueq of their
parent atoms. Data statistics are reported in the Table S6 and cif files. CCDC: 1898373
contains the supplementary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Steady State Absorption and Fluorescence Measurement. Steady state absorption
measurement was performed using Varian Cary 100 Bio UV-Visible Spectrophotometer
using 1 cm path length quartz cuvette. All solvent dependent steady-state fluorescence
measurements were carried out using HORIBA Jobin Yvon Fluorolog fluorimeter using
Origin 8.1 software provided with the instrument. A dilute solution of the sample was taken
for all the measurements to keep the absorption value below the 1 a.u. Fluorescence spectra
were recorded using 1 cm path length quartz cuvette using HORIBA Jobin Yvon, model
(FM-100) and keeping both excitation and emission slit at 1 nm. All the experiments were
performed at room temperature (298 K).
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10.1002/cphc.201900476
ChemPhysChem
Time Resolved Measurement. Time-resolved fluorescence measurements were performed
by using a HORIBA Jobin Yvon picosecond time-correlated single photon counting (TCSPC)
spectrometer (model Fluorocube-01-NL). TCSPC was used for recording PL decays. The
TCSPC setup consists of an Ortec 9327 pico-timing amplifier and using pulse Diode laser
(ex 375 nm and 334 nm) with a setup target 10,000 counts. The emission polarizer at a magic
angle (54.7) by a photomultiplier tube (TBX-07C) was used to collect the PL decays. The
IBH DAS 6.0 software was used to analyse the PL decays by the iterative reconvolution
method, and the goodness of the fit was judged by reduced χ-square (χ2) value. Decays were
fitted with bi-exponential and tri-exponential functions. The instrument response function
(IRF) was measured before and after fluorescence lifetime measurement using a dilute
suspension of Ludox (purchased from Sigma) colloidal silica. All the measurements were
carried out at ambient temperature (298 K). The average lifetime calculated from the
following equation.
푛
<  > = ∑ 푎푖 휏푖
푖=1
Where <  > denotes average lifetime, a stands for the normalized aptitude of the decay
components .
Quantum Yield Measurement. The fluorescence quantum yields of compounds 1 and 2 in
THF and 90% aqueous THF were determined by the Parker-Rees method. Quinine sulfate
was standard fluorophore for the experiments. The Parker-Rees Equation is written as
follows;
퐴s퐼u푛2
퐴u퐼s푛s2
u =
u s
where, ϕs and ϕu are the quantum yield of standard (quinine sulfate in 0.1 M H2SO4) and
unknown fluorophore, respectively. As is the absorbance of standard fluorophore at the
excitation wavelength, Au is the absorbance of unknown fluorophore at the excitation
wavelength, Is is the area of integrated fluorescence intensity of the reference sample when
excited at the same excitation wavelength, and Iu = the total area of integrated fluorescence
intensity for the unknown sample when excited at the same excitation wavelength. The
refractive indices of the solvents for the unknown and the standard samples are denoted.
Their absorption maxima were kept at 0.1.
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Acknowledgement
A.K.D. sincerely thanks Department of Science & Technology, NanoMission, New Delhi,
India (Project: SR/NM/NS-1458/2014), for financial support. S. M. thanks to IIT Indore and
RGJ thanks to UGC, India for their doctoral fellowship. Authors thank Sophisticated
Instrumentation Centre, IIT Indore for providing instrumental facility.
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Graphical Abstract
7x106
6x106
5x106
4x106
3x106
2x106
1
Local excited
state emission
of 2
2
90%
Aqueous
THF
90%
Aqueous
THF
Mixture of local
excited state and
excimer emission
of 2
White light emission
Emission from
H aggregated
solution of 1
Excimer emission of 2
1x106
Blue
Green
Red
0
350
400
450
500
550
600
650
Wavelength (nm)
In this work, we have investigated the aggregation and white light emission behavior in the
binary mixture of π-conjugated 4,7-di((E)styryl)benzo[2,1,3]selenadiazole (1) and 1-
pyrenemethanol (2) in aqueous THF solvent.
21
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