“Dial-In” Emission from a Unique Flexible Material with Polarization Tuneable Spectral Intensity

Article abstract of DOI:10.1002/chem.201902333

The development of organic photoluminescent materials, which show promising roles as catalysts, sensors, organic light-emitting diodes, logic gates, etc., is a major demand and challenge for the global scientific community. In this context, a photoclick polymerization method is adopted for the growth of a unique photoluminescent three-dimensional (3D) polymer film, E, as a model system that shows emission tunability over the range 350–650 nm against the excitation range 295–425 nm. The DFT analysis of energy calculations and π-stacking supports the spectroscopic observations for the material exhibiting a broad range of emission owing to newly formed chromophoric units within the film. Full polarization spectroscopic Mueller matrix studies were employed to extract and quantify the molecular orientational order of both the ground (excitation) and excited (emission) state anisotropies through a set of newly defined parameters, namely the fluorescence diattenuation and fluorescence polarizance. The information contained in the recorded fluorescence Mueller matrix of the organic polymer material provided a useful way to control the spectral intensity of emission by using pre- and post-selection of polarization states. The observation was based on the assumption that the longer lifetime of the excited dipolar orientation is attributed to the compactness of the film.

Full text of DOI:10.1002/chem.201902333

A Journal of
Accepted Article
Title: “Dial-In” Emission from Unique Flexible Material with Polarization
Tuneable Spectral Intensity
Authors: Rajan Kumar, Subir Kumar Ray, Saikat Mukherjee, Sudipta
Saha, Arjit Bag, Pradip K Ghorai, Nirmalya Ghosh, and Raja
Shunmugam
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.201902333
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Chemistry - A European Journal
10.1002/chem.201902333
“Dial-In” Emission from Unique Flexible Material with
Polarization Tuneable Spectral Intensity
b
b
c
Rajan Kumarᵃ, Subir Kumar Ray , Saikat Mukherjeeᵃ, Sudipta Saha , Arijit Bag , Pradip Kr.
c#
b$
Ghorai , Nirmalya Ghosh , and Raja Shunmugamᵃ*
*Polymer Research Centre (PRC), Centre for Advanced Functional Materials (CAFM),
Department of Chemical Sciences, Indian Institute of Science Education and Research
Kolkata (IISER K), Mohanpur, West Bengal, India – 741 246.
$
Department of physical Sciences, Indian Institute of Science Education and Research
Kolkata (IISER K), Mohanpur, West Bengal, India – 741 246.
#
Department of Chemical Sciences, Indian Institute of Science Education and Research
Kolkata (IISER K), Mohanpur, West Bengal, India – 741 246.
ABSTRACT
The growth of an organic photoluminiscent material which could show a promising role as a
catalyst, sensors, organic light-emitting diode, logic gate, etc is a major demand and
challenge for the global scientific society. In context to this, we adopted a photoclick
polymerization method for the growth of unique photoluminescent three dimensional (3D)
polymer film E as a model system that showed emission tunability over the range 350-650
nm against excitation range 295-425 nm. The DFT analysis on energy calculation and pi
stacking supported well to the spectroscopic observation for the material to exhibit broad
range of emission due to newly formed chromophoric units within the film. Full polarization
spectroscopic Mueller Matrix studies employed to extract and quantify the molecular
orientational order of both the ground (excitation) and excited (emission) state anisotropies
via a set of newly defined parameters, namely the fluorescence diattenuation and
fluorescence polarizance. The information contained in the recorded fluorescence Mueller
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10.1002/chem.201902333
matrix of the organic polymer material enabled a useful way to control the spectral intensity
of emission using pre and post-selection of polarization states. The observation was on the
assumption for the longer lifetime of the excited dipolar orientation attributed to compactness
of the film.
Key Words: Thiol-alkene photoclick reaction, pi-pi interaction, crosslinked framework,
pyrene, photoluminescent, anisotropy, polarization, molecular orientation order, Density
Function Theory (DFT).
INTRODUCTION
A recent survey reports the global world to have overflown with photoluminescent
molecules. Such luminescent molecules are either comprising of inorganic constituents or the
[1]
organic – inorganic hybrid constituents. Though both inorganic, as well as hybrid organo-
metallic luminescent material, show their dominance, however their limitations couldn’t be
overcome. New approaches are invoked with the input of modified organic-inorganic hybrid,
but their prolonged efficiency remains one of the major concerns. Therefore with the focus
onto such drawbacks, the field is slowly switching over to the growth of luminescent organic
material. Extensive research is done on conjugated fluorophore based diode whereas reports
[2-11]
on intact pi stacked 3D photoluminescent polymer film is limited.
After thorough
learning from the literature examples, we believed that with modification on the existing
systems would provide superior material with excellent features related to sensing, the
organic light-emitting diode (OLED), catalysis, etc. First, we wanted to adopt a simple and
facile synthetic approach. Secondly, this approach should give an intact pi-stacked polymer
film where chromophore constituents introduced should gain a static phase. The additional
features that we expected to mold and incorporate within the modeled luminescent
macromolecular film are flexibility, stability, insolubility, easy handling, and many more.
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Considering all these points, herein we report on the design, synthesis, and complete
demonstration of a newly developed photoluminescent polymer material. We have utilized
[13-15]
the thiol-alkene photo click polymerization reaction to grow such polymer material.
polymerization is two plane polymerization process, which gives rise to three dimensional
3D) polymer material in the form of film. This polymer film so grown via proposed
The
(
photopolymerization technique is likely to be stable. Our proposed photopolymerization
method has been highlighted in Scheme 2a.
Further, we have explored the fluorescence Mueller matrix spectroscopy technique for
probing the polarization anisotropies of the film arising due to its cross-linked organization.
The fluorescence polarization spectroscopy has shown its considerable promise for
characterization of different chemical, biological, and complex physical systems.
Traditionally fluorescence polarization anisotropy is quantified by the anisotropy parameter
r= (I||-I
parallel to and perpendicular to the excitation linear polarization. The anisotropy parameter
r) contains information about the inherent depolarization associated with the fluorescence
⊥
)/ (I||+2 I
⊥
) where I|| and I
⊥
are respectively the measured fluorescent intensities
[16]
(
emission. Depolarization in case of fluorescence is caused due to the random orientation of
fluorophores, radiationless energy transfer among fluorophores and extrinsic causes like
multiple scattering effects, etc. Thus the resulting anisotropy parameter contains lumped
information about all the contributing polarizing and depolarizing effects associated with the
fluorescence scattering process. In addition to the intrinsic and extrinsic causes of
depolarization, it is useful to capture and quantify exclusive information of the molecular
orientation and organization of the fluorescent molecules. For this purpose, we have
employed Mueller matrix (transfer function of an optical system) measurement for
quantification of organizational anisotropies of both the excitation and emission states of the
molecule. These anisotropies manifest themselves as differential excitation of fluorescence
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with orthogonal polarization (both linear and circular) and differential emission of
fluorescence for orthogonal polarization (both linear and circular). These anisotropies are
quantified through the various elements of the recorded fluorescence Mueller matrix. Note
that the Mueller matrix polarimetry is usually studied and well-explored for elastic scattering
[17-18]
processes like reflection, refraction etc.
Recent theoretical and experimental studies of
our group and others have extended the Muller matrix measurement for inelastic scattering
[19-20]
processes like fluorescence.
The inverse analysis of fluorescence Mueller matrix has
enabled us to quantify the excitation and emission state anisotropies via a set of newly
defined polarimetry parameters, namely, the fluorescence diattenuation (differential
excitation of fluorescence) and fluorescence polarizance (differential emission of
[21]
fluorescence).
These are then used to quantify the molecular orientational order of the
organic polymer domain, and finally, the polarization anisotropies of fluorescence emission
showed useful application involving polarization based tuning of spectral intensity.
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Scheme a:
O
H
O
Et O
N
2
O
O
+
+
O
NH
o
9
0 C
O
A
Scheme b:
O
O
OH toluene
O
H N
N
2
o
100 C
OH
O
reflux
O
A'
Scheme c:
O
O
O
O
N
NaHCO3
O
+
Br
Br
O
NH
o
N
5
0 C
O
O
O
B
Scheme d:
O
O
O
DCC-DMAP
DMSO
+
OH
N
N
OH
O
O
O
O
C
Scheme 1(a-d). Synthesis of compounds - A, A’, B and C
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Results and Discussions
Scheme 2: (a) Synthesis of polymer E (sample). (b) and (c) respectively represent polymer E
to exhibit flexibility and luminescence. (d) Represents a schematic of the experimental
spectroscopic Fluorescence Mueller matrix setup. F: long-pass filter; P1 and P2: linear
polarizers; Q1 and Q2: achromatic quarter-wave retarders. P1 and Q1 form the polarization
state generator (PSG) unit. P2 and Q2 form the polarization state analyzer (PSA) unit. The
inverse analysis of the recorded Fluorescence Muller matrix gives anisotropies of the ground
and excited molecular states, namely fluorescence diattenuation, and polarizance
respectively. The 4×4 fluorescence spectroscopic Mueller matrix R (  =450−800nm)
recorded from the gel sample. The first element ( R11 ) represents the polarization
independent fluorescence spectra. The other three elements in the first row (blue colored)
represent excitation anisotropy related to the ground molecular state while the three elements
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in the first column (red colored) represent emission anisotropy related to the excited
molecular state, respectively.
Photoluminescent material is a typical example for future utilization in a scientific and global
society. With this into account, the need to work on up-gradation towards its efficiency in
terms of tunability and mechanical properties has been invoked. We approached for the
photopolymerization of the synthesized monomers B, and C, with the commercial monomer -
penterthritoltetrakis (3-mercaptopropionate) tetrathiol (4SH) to grow the photoluminescent
3D polymer film E (abbreviated as polymer E) as showed in Scheme 2a. To check the film
formation, the non luminescent 3D polymer film D (abbreviated as polymer D) was
synthesized first. The synthetic scheme for the polymer D is showed in supporting
information, SI. The monomers required for the growth of polymer E is showed in Scheme 1.
All the monomers were well characterized using NMR spectroscopy, mass spectrometer, and
FTIR-ATR spectroscopy techniques (Figure S1-8, S13-16, S17, SI). The formation of
Polymer D and E was characterized via NMR (Figure S9-S10, SI) and FTIR-ATR
1
spectroscopy techniques (Figure S18, SI). The comparative H NMR spectra confirmed the
propagation of polymerization and product formation (Figure S11-12, SI). Comparative
FTIR-ATR spectra supported well. The thiol (-SH) signal at 2535 cm⁻¹ disappeared for the
polymers (Figure S18, SI). This confirmed thiolation to have occurred during
photopolymerization process. The thermogravimetric analysis (TGA) curves for both the
polymers D and E confirmed their thermal stability (Figure S20, SI). The morphology for
the materials was observed too (Figure S22, SI). EDX and mapping for the respective
polymers were analysed as well (Figure S23-24, SI). The observation supported the presence
of elements- carbon; C, oxygen; O, nitrogen; N and sulphur; S. Comparative UV for the
monomers were done as showed in Figure S19, SI. This confirmed only monomer C showing
absorbance up to 350 nm. The physical observation under ultraviolet (UV) light reveals the
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polymer E contained with chromophore C emits white light against excitation at a
wavelength of 365 nm (Scheme 2c). The basic principle that governs photoluminescence in
devices is a combination of charge injection (CI) and charge recombination (CR) i.e., back
[22]
electron transfer (BET). Based on this, prolonged analysis has been undertaken to check
the tunability of the polymer E. During the process towards growth of pyrene based polymer
E, the reaction mixture was allowed to stabilize. The reorientation and interlocking due to pi
stacking occurred within the reaction mixture. The polymer E as film so grown was flexible,
transparent and compressible (Scheme 2b). The probable mechanism to formation of polymer
E has been showed in Figure S21, SI.
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Figure 1: a) represent UV spectrum, b) represent fluorescence spectrum for excitation at 355
nm, c) represents CIE coordinate diagram, d) represents tunability to emission with switch in
excitation energy, e) represent computed absorption spectra of aggregated pyrene derivative,
and f) represents lifetime decay for the polymer E with lifetime of 17.7 µs.
The proposed mechanism has been highlighted for single thiol-alkene moiety. The thiol
radical (-Sº) generated in step1 reduces the alkenes B, and C in substages. The free radical
crosslinking proceed repeatedly unless the saturation and the randomness of the monomers
ceases. The polymer E with white light emission was analysed using solid state ultraviolet
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(UV) and Fluorescence spectroscopy. The solid-state UV and fluorescence spectroscopic
analysis supported our physical observation. UV curve reflects the polymer E to cover the
range over 250-600 nm, whereas the emission spectrum covered the range for red, green, and
blue (RGB) (Figure 1a-b). To value the white color and emissive property, Commission
[23-25]
International de I’Eclairage (CIE) chromaticity coordinates (x,y) were calculated.
The
color co-ordinate was 0.34 and 0.37, falling in range of white light CIE coordinates (x = 0.33,
y = 0.33). The correlated color temperature, Tcp for the polymer E is 4936 K. The Tcp value
is around light source of a cool compact fluorescent lamp (CFL).
Further to check the emission tunability, emission spectra over range 400 - 650 nm for
excitation over range 295-425 nm was observed. The lifetime decay for the polymer E was
found to be 17.8 µs (Figure 1f). The emission tunability can now be attributed to the
formation of pi stacked pyrene units within the polymer E. To support the growth of pi
stacked energy states in polymer E responsible to emission tunability, theoretical analysis on
pi stacking was undertaken.
[26]
We performed quantum computation in density functional approach using Gaussian 09
package for molecular-level understanding of the observed absorption phenomenon of our
polymer E. It is previously reported that pyrene or pyrene derivatives show fluorescence due
[27-30]
to the formation of excimer.
In recent time it is reported that intermolecular
photoinduced electron transfer (PET) and BET is also possible for pyrene which causes
[31]
intense fluorescence.
But, in all these cases, emission occurs in the blue region. At the
same time, it is believed that the formation of excimer is not possible in polymer E. There are
possibilities of electron transfer from the pyrene unit to any other unit. Similarly, it is
expected in both the units exist in our pyrene based polymer film E. To test these possibilities
we compute absorption spectra of compounds formed from unit-c (pyrene derivative) with
unit-b, unit-c with unit-a separately and unit-c with both unit-a and unit-b. Computed results
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are presented in Figure-S25, ESI. We observe that all lowest energy absorption peaks are
located near 338 nm where the lowest absorption peak of unit-c exists. Thus, emission of
white light is not due to intramolecular charge transfer. There is only one possibility left
which may be responsible for white light emission- pi-pi stacking of pi-electron of pyrene
rings which exist in parallel orientation within the polymer film E. To test this possibility, we
compute absorption spectra of aggregated form of unit-c. We studied dimer, tetramer and
hexamer of unit-c. A computed spectrum is presented in Figure 1e. Absorption spectra of
monomer of pyrene derivative i.e. unit-c consists of three different peaks located at 338 nm,
266 nm and 225 nm. Here lowest energy transition (338 nm) occurs for HOMO to LUMO
electron transition. Both HOMO and LUMO are located over pyrene ring (Figure S26, ESI).
Thus, derivatization of pyrene doesn't help to shift of absorption spectra. Transitions
correspond to the other two peaks (266 nm and 225 nm) are dominated by HOMO-1 to
LUMO and HOMO-1 to LUMO+2 electron transfer. All these MOs are also located on the
pyrene ring (Figure S26, SI). Thus, other peaks also don't change due to derivatization. But,
peaks shifted due to aggregation. It is observed that with an increase of aggregation, peaks
merge into two peaks for dimer and tetramer. It further reduces to one peak for hexamer. In
hexamer, only one broad peak appears which shows very good absorbance (more than 5000)
in the visible region (400 nm to 650 nm). Thus, it would emit white light. To understand the
redshift of spectra on aggregation, we studied key electron transitions of the hexamer. It is
observed that the electron transition for the lowest energy absorption (710 nm) which is in the
red color zone, is dominated by HOMO-1 to LUMO+6. This is, in fact, the charge transfer
from one pyrene ring to its nearest pyrene ring (Figure S27-Y1, SI). The key absorption in
the green color zone (542 nm) is dominated by HOMO to LUMO+14 electron transfer. This
transition is the charge transfer from the pyrene ring to the carbonyl pi-electron (Figure S27-
Y2, SI). Absorption in the blue color zone (399 nm) is also due to pyrene ring electron
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transfer but it is quite complicated. Here, the pi-electron cloud of two closest pyrene rings is
transferred to their nearest pyrene ring simultaneously (Figure S27-Y3, SI). The important
nature of these transitions is that they depend on aggregation and the compactness of
aggregation.
With the theoretical analysis parallel the spectroscopic observation, the concern we looked
for was the flexibility for the polymer E. To analyze the material strength, the polymer E was
mounted onto the Universal Testing Machine (UTM). The respective length (l), breadth (b)
and height (h) of the material were monitored (l=7.01 mm, b=6.9 mm, h=3.09 mm). The
compression curve revealed quality strength and elasticity for the film. The release of
compressed pressure allowed the film to gain its usual form. The compression was performed
over cycles to analyse the existence of elasticity. The polymer material tends to retain its
linear elasticity over cycles as shown in Figure 2. This promising property makes it as a
potential model system towards growth of organic photoluminiscent material that is easy to
handle and can be applied to application such as catalysis, OLED, etc.
Figure 2: Compression analysis for the polymer film E over cycles; Cn.
Further, during our experimental fluorescence Mueller matrix measurement, we selected
wavelength λ= 405 nm beams and excited the polymer E which is close to the absorption
band maxima at λ= 399 nm. The fluorescence is found to be highly anisotropic both during
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excitation and emission. In this situation, the polymer E likely undergoes HOMO-1 to
LUMO+27 transition with negligible contribution from HOMO to LUMO+14 and HOMO-1
to LUMO+6 transitions (see supplementary for the anisotropic configuration obtained using
DFT calculations). This is the reason for the ground state (during excitation) anisotropy of the
fluorescence which is manifested as the fluorescence diattenuation (differential excitation of
fluorescence by orthogonal polarizations). We also saw higher anisotropy during emission of
fluorescence signal from the polymer E implying to have more organized emission of
fluorescence at the excited state of the molecule manifesting as the fluorescence polarizance
(differential emission of fluorescence with orthogonal linear polarization)
Experimental Fluorescence Mueller Matrix studies:
A schematic representation of our home built comprehensive experimental Mueller Matrix
Fluorescence Spectroscopy platform has been introduced in Scheme 2d. The system is
[
32-33]
calibrated using a robust Eigenvalue calibration method.
The specifics of the system and
Recorded spectroscopic 4 x 4
[34]
various steps of this method can be found elsewhere.
fluorescence Mueller matrix (R) from the polymer E has been highlighted in Scheme 2d.
Ground and excited states anisotropic information is extracted from recorded R (λ).
Inverse Analysis of fluorescence Mueller matrix for determination of Molecular
Orientation:
When light interact with an optical medium, the output light is characterised by its stoke
vector SOut = RSIn where S and S are the input and out stokes vector, and R is the
in
Out
transfer function (Mueller Matrix) of the medium which contains all the polarization
information of the system under study. There are several decomposition methods which may
decouple the contributing polarization information from a recorded Mueller Matrix. These,
polarization information have been successfully extracted using several decomposition
methods in case of elastic scattering. However, the existing decomposition methods are not
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valid for inelastic scattering process such as fluorescence. Recent studies of our group and
others have shown that for complex processes such as fluorescence the polarization
information can be extracted from a recorded Mueller matrix as R = M M M where M
d
1
d
0
represents the depolarization effects occurs due to random orientation of the fluorophores and
other extrinsic causes, M & M represents the amplitude anisotropies of the ground and
0
1
3
4
excited states of fluorophore molecules (Figure S28, SI) . Finally, florescence anisotropy
parameters can be obtained from the recorded fluorescence Mueller matrix (R) as:
2
2
13
2
14
2
12
2
13
R + R + R
R + R
R14
R11
1
2
αT =
βT =
;
αL =
βL =
;
;
αC =
βC =
;
R11
R11
2
2
31
2
41
2
2
R + R + R
R + R
31
R41
R11
2
1
21
;
;
……. (1)
R11
R11
Where, α , α , and α are the total, linear, and circular fluorescence diattenuation,
T
L
C
respectively and β , β , and β are the total, linear and circular polarizance, respectively.
T
L
C
Anisotropy parameters contain information about the molecular orientation of the samples.
Mueller Matrix elements R / R and R / R represent average fluorescent dipolar
1
3
12
31
21
orientation angles for the ground (θ ) and the excited (θ ) molecular states and can be
ex
em
[35]
expressed in terms of recorded spectral Mueller Matrix as
1
 R 
1
 R 
-1
13
-1
31
θ = tan
;
θ = tan
……….. (2)
ex




em




2
R12
2
R21
-(θ-θex/em )2
2
Here we define a molecular angular distribution function f(θ)=e
/ 2Δθex/ em with
width Δθ around the average fluorescent dipolar orientation angle ( θex and θ ) which
em
contains information on the distribution of the orientation angles of the molecules. Hence, the
measured fluorescence intensity for excitation with a given polarization vector E can be
written as:
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2
π
π
π

0, for H
2
2
I  μ.E f(θ)dθ =

1- η

cos (θ)f(θ)dθ+η Sin (θ)f(θ)dθ ; where, η=
η

η




1, for V
0
0
0
2
π
π
2
IP/ M  μ.E
f(θ)dθµ cos (θ ± π / 4)f(θ)dθ

P/ M

0
0
………….(3)
Figure 3: (a) Normalized fluorescence spectroscopic Mueller matrix; (b)The fluorescence
diattenuation and polarizance parameters; (c)Wavelength variation of the average
fluorescent dipolar orientation angles for the ground (θ , blue dashed line) and the excited
ex
(
θem , solid red line) molecular states. (d) The derived molecular angular distribution function
for the ground ( f (θ) , blue dashed line) θ and the excited ( f (θ) , solid red line)
ex
ex
em
molecular states of the fluorescent molecule. Inverse calculations were performed on the
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Mueller matrix-derived α and β parameters to extract this information. The estimated
L
L
standard deviations of the orientation angle distribution for the ground and excited states
were found to be Δθ = 0.02614 rad and Δθ = 0.05397 rad, respectively. The vertical
ex
em
lines mark the center positions of the distribution functions.
Figure 3a shows the recorded experimental normalized fluorescence Muller matrix, Figure
3
b shows the extracted fluorescence linear ( β ) and circular ( β ) polarizance parameter and
L C
fluorescence linear (α ) and circular (α ) diattenuation using equation (1), Figure 3c shows
L
C
wavelength variation of the average fluorescent dipolar orientation angles for the ground ( θex
blue dashed line) and the excited (θ , red solid line) molecular states using equation (2) ,
,
em
Figure 3d shows the derived molecular angular distribution function for the ground ( f (θ)
ex
blue solid line) θex and the excited ( f (θ) ,red dashed line) molecular states of the
em
fluorescent molecule. Several interesting trends can be obtained from these figures. First
element ( R ) of recorded experimental normalized fluorescence Muller matrix represents
1
1
total fluorescence intensity. The elements in first row colored in blue represents the intrinsic
anisotropic properties of the ground molecular states. Similarly, elements in first column of
MM highlighted in red color represent emission state anisotropy of the molecules in excited
state. The diagonal elements of MM are related to the depolarization effect associated with
the fluorescence process. Elements R , R ; R , R , R , R are related with the phase
23
24
34
32
42
43
anisotropy effect associated with the fluorescence process which is negligible (close to zero)
in case of fluorescence scattering. In Figure 3(b) we have shown the linear and circular
anisotropy of the polymer E. The linear anisotropy of the excited molecular state is higher
compared to that of the ground molecular state which means the excited molecular state is
more organized compared to the ground molecular state. However, the circular anisotropy has
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negligible value implying the fact that molecules possess no chirality in it, which is also
confirmed using DFT simulation. The ground and excited-state molecular orientation angles
are plotted in Figure 3(c). We have plotted the distribution function to infer about the
molecular orientation. As excited molecular state anisotropy parameter ( β ) is significantly
L
larger compared to ground state anisotropy parameter ( α ), the excited molecular states are
L
more organised than ground states, accordingly standard deviation Δθex is larger than Δθem
.
Molecular distribution function f (θ) and f (θ) were calculated near the peak of the
ex
em
fluorescence intensity (520 nm). Note that Pi-Pi stacking of Pyrene moieties and thiol-alkene
photo-click radical polymerization reaction in the norbornene monomers generated a single
one product material within the cross-linked framework. This leads to preferential linear
organization of the fluorescent molecular dipoles (of Pyrene units), which appears to be the
underlying reason for considerably high magnitudes of the fluorescence anisotropy, α and
L
βL parameters.
Polarization controlled tuning of Fluorescence spectra:
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Figure 4: Comparison of polarization projected intensity using different pre and post
selected states: the polarization independent fluorescence spectra is shown by the black
dashed line, the recorded fluorescence intensity for input horizontal pre-selection and
horizontal (vertical) post-selection state is shown by the red(blue) solid curve. The inset
shows spectral variations of area normalized the projected intensities for the same. The
vertical line in each curve corresponds to the maximum position of the corresponding
intensity.
The strong polarization anisotropy of fluorescence of the crosslinked polymer E provides a
useful handle to control the emitted spectral intensity of fluorescence by using a proper set of
pre- and post-selection of the polarization states. To get desirable control over emitted
fluorescence intensity without changing the physical or chemical properties of the polymer
matrix, we have thus used a general recipe using polarization state projection on the recorded
1
T
out
fluorescence Mueller Matrix (I(λ)= S M(λ)S ) . The fluorescence intensity can be
in
2
desirably enhanced or suppressed by orders of magnitude using different pre- and post-
selected polarization states, as illustrated in Figure 4. This also leads to systematic changes in
the spectral line shape of the emission (shown in the inset, where the area normalized spectral
line shapes are displayed). The emitted fluorescence spectra with horizontal input and
detection with horizontal and vertical post-selection are shown by red and solid blue curves,
respectively. Using horizontal pre- and post-selection, the emitted fluorescence signal is
observed to be considerably enhanced for unpolarized fluorescence intensity, which is
accompanied by subtle changes in the line shape. For detection with vertical post-selection
for horizontal pre-selection leads to more apparent shifts in the peak position (~ 5 nm) and a
more drastic reduction in the intensity of emission. The fluorescence anisotropy properties,
namely, linear fluorescence diattenuation (differential excitation of fluorescence by
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Chemistry - A European Journal
10.1002/chem.201902333
orthogonal linear polarizations) and the fluorescence polarizance (differential emission of
fluorescence with orthogonal linear polarization) properties of the OLED sample are
responsible for such differential spectral intensity profiles for different linear polarizations.
Thus, in this intrinsically anisotropic systems, pre and post selection of polarization states
provides a unique handle to desirably control both the intensity and the spectral line shape of
emission without changing the chemical composition or molecular organization of the
sample. This remarkable feature of the polymer E may find useful application for developing
polarization control tuneable devices such as diagnostics and imaging, apollo sensors, and
[36-38]
logic gates.
Conclusions
To summarize, in this article, we have successfully demonstrated emission properties of the
newly synthesized photoluminescent 3D polymer film E covering the wide range over RGB
range from spectroscopic analysis. This synthesized polymer E exhibiting physical properties
such as flexibility and mechanical strength in context to compressibility could be useful as an
efficient organic spin-optical device.
Moreover the theoretical analysis confirms its pi-pi stacking via formation to dimer, trimer,
tetramer, pentamer, and so on which has covered the region of our interest. Also, we have
quantified the excited and emission state anisotropies arising from the orientation and
organization of organic polymer film E via the fluorescence diattenuation and fluorescence
polarizance parameters. These parameters are further used to quantify the molecular
orientational order of the organic LED sample. Finally, the information contained in the
fluorescence Mueller matrix is exploited for polarization controlled tuning of spectral
intensity of emission. Thus the modeled organic polymer E holds considerable promise and
which can turn out to be a potential candidate for devices related to catalysis, sensing,
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Chemistry - A European Journal
10.1002/chem.201902333
diagnostic and imaging, light emission as OLED, etc. Particularly the polarization controlled
tuning of the intensity may also have useful application in this domain.
Associate contents
The Supporting Information (SI) contains - Synthetic scheme for polymer D. Synthetic
1
13
procedures for A, A’, B, C, polymer D, and E. H NMR and C NMR for A, A’, B, and C,
1
1
H NMR for the polymer D and E. Comparative H NMR for polymer D and E. ESI-MS for
the synthesized monomers A, A’, B and C. TGA analysis for polymer D and E. ATR-FTIR
curves of 4SH, monomer B, monomer C, polymer D and E. Comparative UV analysis of 4S,
monomer B and monomer C. Mechanism to formation of pyrene crosslinked polymer E.
HOMO-LUMO energy state calculation. Muller Matrix analysis. FE SEM, EDX and
Mapping analysis for polymer D and E.
Author’s contributions
The project is an outcome of effort from all the authors.
Acknowledgement
R. Kumar thanks DBT for research fellowship. S. Mukherjee and S. Ray thanks CSIR and
UGC for research fellowship. S. Saha and A. Bag thanks IISER Kolkata for fellowship. R.
Shunmugam thanks DBT and DST-SERB for research projects. Authors thank IISER-
Kolkata for the infrastructure facility.
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