Design, synthesis and photophysical investigation of triphenylamine-bithiophene dyes as RGB emitters for white light applications

Article abstract of DOI:10.1016/j.dyepig.2018.06.001

A set of three triphenylamine-bithiophene based Donor-Acceptor (D-A) molecules viz. TPT, TPT-Ben and TPT-Ac were designed, synthesized and studied for their photophysical properties. Systematic density functional theory (DFT) calculations have been perfor

Full text of DOI:10.1016/j.dyepig.2018.06.001

Accepted Manuscript
Design, synthesis and photophysical investigation of triphenylamine-bithiophene dyes
as RGB emitters for white light applications
Athira Krishna, E. Varathan, Padincharapad Sreedevi, V. Subramanian, Venugopal
Karunakaran, Ramavarma Luxmi Varma
PII:
S0143-7208(18)31011-8
10.1016/j.dyepig.2018.06.001
DYPI 6806
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 4 May 2018
Revised Date: 1 June 2018
Accepted Date: 1 June 2018
Please cite this article as: Krishna A, Varathan E, Sreedevi P, Subramanian V, Karunakaran V, Varma
RL, Design, synthesis and photophysical investigation of triphenylamine-bithiophene dyes as RGB
emitters for white light applications, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.06.001.
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ACCEPTED MANUSCRIPT
Design, synthesis and photophysical investigation of
triphenylamine-bithiophene dyes as RGB emitters
for white light applications
Athira Krishnaab E. Varathanbc, Padincharapad Sreedevia, V. Subramanianbc, Venugopal
Karunakaranab, Ramavarma Luxmi Varmaab*
aChemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary
Science and Technology, Thiruvananthapuram 695019, Kerala, India
bAcademy of Scientific and Innovative Research (AcSIR), CSIR-NIIST, Thiruvananthapuram
695019, Kerala, India
cCSIR- central leather research institute, Chennai
*lux_varma@rediffmail.com

ACCEPTED MANUSCRIPT
Design, synthesis and photophysical investigation of
triphenylamine-bithiophene dyes as RGB emitters for
white light applications
Athira Krishnaab E. Varathanbc, Padincharapad Sreedevia, V. Subramanianbc, Venugopal
Karunakaranab, Ramavarma Luxmi Varmaab*
aChemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary
Science and Technology, Thiruvananthapuram 695019, Kerala, India
bAcademy of Scientific and Innovative Research (AcSIR), New Delhi, 110020, India
cCSIR- Central Leather Research Institute, Chennai,600020, Tamil Nadu, India
*lux_varma@rediffmail.com

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1. Abstract
A set of three triphenylamine-bithiophene based Donor-Acceptor (D-A) molecules viz. TPT,
TPT-Ben and TPT-Ac were designed, synthesized and studied for their photophysical
properties. Systematic density functional theory (DFT) calculations have been performed on the
molecules to gain insights into the structural, electronic and optical properties. The nanosecond
transient absorption spectra showed broad absorption due to the presence of triphenylamine
radical cation with an average time constant of 3.5 µs. The molecules were found to emit in the
Blue, Green and Orange-Red regions of the visible spectrum due to the differences in their
Stokes shift values. A 3:2:1 mixture of TPT, TPT-Ben and TPT-Ac gave white light emission
with a colour coordinate of (0.32, 0.36) even at an µM level concentration in both solution state
(DMSO) as well as in the solid state (0.33,0.37).
Keywords: Triphenylamine, Thiophene, Donor-acceptor, Stokes shift, White light emission.
2. Introduction
Developing red, green and blue (RGB) emitters with high luminescence efficiency is the most
important aspect for generating white light emission. Different methods used so far for
achieving white light emission include, mixing RGB emitters, [1,2] synthesizing white light
emitting polymers with RGB monomers,[3] and using a single molecule as emitter with the aid
of properties like ESIPT (excited state intramolecular proton transfer).[4,7] But still developing
an efficient white organic light emission is an active area of research, mainly due to its
applications in display and lighting. Donor-Acceptor systems based on triphenylamine have
gained considerable interest over the period of time mainly due to their inherent properties like
low-oxidation potential, propeller like geometry etc. They are widely used for applications
towards 3D microfabrication, [8,9] two photon bioimaging, [10-12] organic photovoltaics, [13-
15] optical data storage [16,17] and as hole transporting materials. [18-24] Besides this, another
class of molecules that are extensively studied for their better photophysical properties are
conjugated oligothiophenes. [25, 26] Though conjugated thiophenes are attractive for a wide
range of applications including conductive layers in material applications, [27] photoactive
layers in photovoltaics, [28] and in anti-static packaging, [29] their insolubility in organic

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solvents limits their potential usage. [30] Herein, we report a set of three molecules, based on
triphenylamine-bithiophene conjugate that proved to circumvent the problem of solubility of the
latter without compromising on the electronic properties of both the components. The synthetic
route for the construction of the molecules is depicted in Scheme 1. The basic molecule TPT
was synthesised initially and was derivatised at its third position in order to get TPT-Ben and
TPT-Ac (Figure 1). The 5’ position of bithiophene in TPT was particularly chosen since this
will mask the most reactive site of the thiophene moiety, subsequently making the molecule
chemically and photochemically inert. [30]
.In our set of systems, in the case of TPT, triphenylamine acted as a donor and bithiophene part
served as an acceptor. In TPT-Ben the dibenzothiophene served the role of a second donor,
whereas the acetophenone moiety acted as an acceptor in TPT-Ac (Figure 1). The selection of
the substituents were made according to the electron donor and accepting property of the
moieties.
The photophysical studies of the molecules indicated that, substitution on TPT with either donor
or acceptor group would result in a red shift in its emission maxima. It was observed that while
TPT showed emission in the blue region, it got shifted to the green region upon substitution with
a donor unit (TPT-Ben), while the acceptor appended TPT-Ac showed a shift towards the red
region of the visible light spectrum. Due to this observation of primary colour generation we
examined the emission spectrum of a mixture of all the three compounds in solution state (in
DMSO) at micromolar (µM) concentrations as well as in the solid state using PMMA matrix.
The results showed that, at an optimum composition of 3:2:1 ratio of TPT:TPT-Ben:TPT-Ac
(weight/ weight), the mixture gave rise to white light emission with a CIE colour coordinate of
(0.32, 0.36) in the solution state and (0.33, 0.37) in the solid state.

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Fig. 1. Triphenylamine-bithiophene derivatives a) TPT, b)TPT-Ben, c) TPT-Ac.
3. Experimental Section
3.1 Methods
All the solvents were purchased from Merck Millipore.
Boronic acids and 4-
bromotriphenylamine were purchased from Sigma Aldrich. The catalyst Pd(PPh3)4 was
purchased from Alfa Aesar and used without further purification. Reactions were carried out
under inert atmosphere of argon in oven dried glasswares. Progress of the reaction was
monitored using Silica Gel TLC plates (Merck-Millipore, 60 F254) and UV detection (254 nm
and 365 nm). Column chromatography was done using activated aluminium oxide.
1H and 13C NMR spectra were recorded using Bruker NMR spectrometer (AV500) with working
1
frequency of 500 MHz for H NMR and 125 MHz for 13C NMR. The chemical shifts were
reported in δ (ppm) relative to TMS as internal standard. Mass spectra were recorded using ESI/
HRMS at 60000 resolution (Thermo Scientific Exactive). Cyclic voltammetry experiments were
carried out with a BAS 50W voltametric analyzer using three electrode cell assemblies. Pt wires
were used as counter electrodes and quasi Ag/Ag+ wires as reference electrode. Measurements
were carried out with tetrabutylammonium hexafluorophosphate as a supporting electrolyte at a
scan rate of 100 mVs-1. Oxidation potentials were determined using ferrocene as internal
standard.
3.2. General Procedure for the Suzuki-Miyaura Coupling [31]
The boronic acid (1 equivalent) and bromo compound (1 equivalent) were taken in reaction
vessel along with Pd(PPh3)4. THF along with 2N Na2CO3 dissolved in water (2:1) was added to

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the reaction mixture. The reaction mixture was degassed with argon and allowed to stir at 65-70
oC for required time.
3.2.1 Synthesis of 4-(2,2'-bithiophen-5-yl)-N,N diphenylaniline (TPT) [32]
4-bromotriphenylamine (500 mg, 0.002 mol) and 2,2'-bithiophen-5-ylboronic acid (0.3718 ml,
0.002 mol) were added to the Schlenk tube along with the Pd(PPh3)4 as the catalyst. THF (2 mL)
along with 2N Na2CO3 (1 mL) dissolved in water (2:1 ratio) was added to the reaction mixture.
o
The reaction mixture was degassed with argon and allowed to stir at 65-70 C for 12 h. The
extent of the reaction was monitored by the TLC and on completion, the reaction mixture was
evaporated and the residue was subjected to column chromatography on activated aluminium
oxide using hexane as eluent to afford the product in moderate yield.
3.2.2 Spectral characterization of TPT
Yield: 63% as a yellow crystal.
1H NMR (500 MHz, CDCl3): δ (ppm) 7.45 (d, 2H, J = 9 Hz), 7.29 (t, J = 7.5 Hz, 4H), (d, J = 7.8
Hz, 1H), 7.17 (d, J = 3.5 Hz, 1H), 7.12 (t, J = 5 Hz, 6H), 7.07-7.01 (m, 5H).
13C NMR (125 MHz, CDCl3): δ (ppm) 147.4, 147.3, 143.1, 137.6, 135.8, 129.3, 128.1, 127.8,
126.4, 124.6, 124.5, 124.2, 123.6, 123.4, 123.2, 122.8.
Mass Spectrometric Analysis: HRMS (ESI) m/z calculated for C26H19NS2, (M+H)+: 410.1037
Found: 410.1024
3.2.3 Synthesis of 4-(5'-bromo-2,2'-bithiophen-5-yl)-N,N-diphenylaniline (TPT-Br) [32]
In the absence of light, a solution of NBS (220 mg, 0.00122 mol) in DMF (4 mL) was added
drop wise to a solution of TPT (500 mg, 0.00122 mol) in DMF (10 mL) and cooled to 0 °C. The
reaction was allowed to be stirred for 2 h at this temperature followed by 1 h at room
temperature. After completion, 1 M aqueous solution of HCl was added to the reaction mixture
(30 mL) and the product was extracted with dichloromethane (100 mL) and the resulting organic
layer was washed with water (4 x 300 mL) and dried over Na2SO3 and concentrated. The
product was purified by column chromatography on activated aluminium oxide using hexane as
the eluent.

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3.2.4 Synthesis of 4-(5'-(dibenzo[b,d]thiophen-2-yl)-2,2'-bithiophen-5-yl)-N diphenylaniline
(TPT-Ben)
4-(5,-bromo-2,2,-bithiophen-5-yl)-N,N-diphenylaniline and 4-dibenzothienyl boronic acid were
added in a three necked round bottom flask along with the Pd(PPh3)4 as the catalyst. The
mixture was degassed with argon. THF along with 2N Na2CO3 at a 2:1 ratio was added to the
reaction mixture, again degassed with argon and allowed to stir at 65-70 °C for 12 h. The
reaction mixture was evaporated and the residue was subjected to column chromatography on
silica using ethyl acetate /hexane (1:4) mixture as eluent to afford the product in good yield.
3.2.5 Spectral Characters of TPT-Ben
Yield: 73% as green solid.
1H NMR (500 MHz, CDCl3): δ (ppm) 8.218-8.2 (m, 1H), 8.15 (d, J = 8Hz, 1H), 7.92-7.90 (m,
1H), 7.70 (d, J = 7 Hz, 1H), 7.58 (d, J = 3.5 Hz, 1H), 7.55 (d, J = 8 Hz, 1H), 7.527-7.492 (m,
4H), 7.30 (d, J = 8Hz, 4H), 7.23 (d, J = 3.5Hz, 1H), 7.18 (d, J = 3.5Hz, 1H), 7.15 (d, J = 8Hz,
4H), 7.10- 7.05 (m, 4H).
13C NMR (125 MHz, CDCl3): δ (ppm) 147.5, 131.8, 130.9, 129.4, 128.0, 127.0, 126.4, 125.0,
124.9, 124.6, 123.6, 123.2, 121.8.
Mass Spectrometric Analysis: HRMS (ESI) m/z calculated for C38H25NS3, (M+H)+: 592.1227
Found: 592.1228
3.2.6 Synthesis of the compound 1-(4-(5'-(4-(diphenylamino)phenyl)-2,2'-bithiophen-5-
yl)phenyl)ethanone (TPT-Ac)
4-(5’-bromo-2,2’,-bithiophen-5-yl)-N,N-diphenylaniline and 4-acetylphenylboronic acid were
added in a three necked round bottom flask along with the Pd(PPh3)4 as the catalyst. The
mixture was degassed with argon. THF along with 2N Na2CO3 at a 2:1 ratio was added to the
reaction mixture, again degassed with argon and allowed to stir at 65-70 °C for 12 h. The
reaction mixture was evaporated and the residue was subjected to column chromatography on
silica using ethyl acetate/hexane (1:5) mixture as eluent to afford the product in good yield.

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3.2.7 Spectral characterization of TPT-Ac
Yield: 70% as orange solid.
1H NMR (500 MHz, CDCl3) δ: 7.98 (d, J = 8.5 Hz, 2H), 7.69 (d, J = 8.5 Hz, 2H), 7.47 (d, J = 5
Hz, 2H),7.37 (d, J = 3.5 Hz, 1H), 7.30 (d, J = 7.5 Hz, 4H), 7.19 (d, J = 3.5 Hz, 3H), 7.20-7.13
(m, 5H), 7.09-7.05 (m, 4H).
13C NMR (125 MHz, CDCl3) δ: 197.3, 147.6, 147.4, 143.8, 141.1, 138.7, 138.5, 135.6, 135.2,
129.4, 129.2, 129.0, 127.7, 127.5, 126.6, 126.4, 125.5, 125.2, 125.0, 124.6, 124.4, 123.5, 123.3,
122.9, 26.6.
Mass Spectrometric Analysis: HRMS (ESI) m/z calculated for C34H25NOS2, (M+): 527.1377
Found: 527.1366
3.3. Instrumentation
Absorption spectra were recorded using a Shimadzu UV-2600 UV-Visible spectrophotometer.
The optical diffuse reflectance spectra were measured for solid samples using the above
mentioned spectrophotometer equipped with an integrating sphere. BaSO4 was used as the
reference material, and the solid samples were ground well before the measurement. The
absorption (a/S) data were calculated from the reflectance spectra using the Kubelka–Munk
function: α/S = (1 - R) 2/2R, where R = reflectance at a given wavelength, α = absorption
coefficient, and S = scattering coefficient (practically wavelength independent when the particle
size is larger than 5 mm)
.
Steady-state fluorescence experiments were performed using
FluoroLog-322 (Horiba) which was equipped with a 450 W Xe arc lamp by using optically dilute
solutions. The fluorescence quantum yields in various solvents were determined by the relative
method employing an optically matched solution of quinine sulphate in 0.1N sulphuric acid as
the reference (ΦR = 0.54). The following equation (1) was used for calculating the quantum
yield, [33]
ꢋ
ꢁꢂꢃ
ꢁꢆꢇꢈ
ꢊꢀ
ꢄ
ꢉ
Φꢀ =
×
×
× Φꢌ...............................................................................................(2)
ꢋ
ꢁꢂꢃ
ꢁꢆꢇꢈ
ꢊꢌ
ꢅ
ꢄ

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where the subscript R and S refer to the reference and samples respectively. Abs, Area and n are
the absorbance at the excitation wavelength, area under the fluorescence spectrum and refractive
index of the solvent respectively. Time-resolved fluorescence spectra and lifetime experiments
were performed by using an IBH picosecond single photon counting system employing the 375
nm nano LED as excitation sources and a Hamamatsu C4878-02 microchannel plate (MCP)
detector. Decay in the fluorescence intensity (I) with time (t) was fitted either by a double/triple-
exponential function:
I = A1e-t/τ1 + A2e-t/τ2........................................................................................................(3)
I = A1e-t/τ1 + A2e-t/τ2 + A3e-t/τ3........................................................................................(4)
whereτ1, τ2, andτ3 are the lifetimes of different species, and A1, A2, and A3 are their respective
amplitudes. The weighted mean lifetime (〈τ〉) was calculated according to equation (5):
〈 〉
ꢍ = Σꢍꢎꢏꢎ..........................................................................................................................(5)
The quality of the fits was checked by examining the residual distribution and the χ2 value. The
solid state samples were recorded with the front face mode. All the experiments were conducted
at room temperature.
4. Results and discussions
4.1. Synthesis of TPT, TPT-Ben, and TPT-Ac
TPT was synthesized from 4-bromotriphenylamine and 2,2'-bithiophen-5-ylboronic acid using
THF/water system at 65-70 °C for 12 h. It was then subjected to a controlled NBS bromination
to get TPT-Br. TPT-Ben was synthesized from TPT-Br and 4-dibenzothienyl boronic acid
whereas TPT-Ac was synthesized from TPT-Br and 4-acetylphenylboronic acid using the same
conditions employed for synthesis of TPT. (Scheme 1) All the compounds were characterized
by 1H and 13C NMR and were further confirmed by mass spectrometric analyses.

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Br
R
S
S
S
B(OH)2
S
S
S
S
a
c
b
N
Br
N
S
N
N
TPT
R =
R =
S
O
TPT- Ben
TPT- Ac
Scheme 1. The synthetic route for the preparation of triphenylamine derivatives, TPT,TPT-Ben, and TPT-Ac. (a) Pd(PPh3)4,
Na2CO3, THF-Water, 65-70 oC, 12 h, (b) N- bromo succinimide (recrystallized), 0 oC, DMF, dark, (c) Pd(PPh3)4, Na2CO3, THF-
Water, 65-70oC, 12 h.
4.2. Photophysical Characterization
Fig.2. a) Absorption and b) Emission spectra of TPT (black), TPT-Ben (red), andTPT-Ac (green) in DMSO at room
temperature. λexcTPT= 380 nm, TPT-Ben= 410 nm, TPT-Ac= 420 nm.
Figure 2 depicts the absorption and emission profiles of TPT, TPT-Ben and TPT-Ac in DMSO.
TPT showed an absorption maximum at 383 nm while TPT-Ben and TPT-Ac exhibited the
same at 411 nm and 425 nm respectively. The high energy absorption band at 300 nm can be
˜
attributed to the n-π* transition of the triphenylamine core [34] and the broad structure-less

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absorption can be assigned to the CT band. [32] The molar extinction coefficients (ε) of the low
energy bands of the three molecules were calculated as 38,000 M-1 cm-1, 48,000 M-1 cm-1, 20,000
M-1 cm-1 for TPT, TPT-Ben, TPT-Ac respectively (Supporting information, Figure S10).
Studies indicated that, the dependence of the absorption spectra of the molecules on the polarity
of the different solvents used for the experiment like toluene, THF, ethyl acetate, 1, 4 dioxane,
methanol, acetonitrile and DMSO are negligible (
≤ 11 nm) (Supporting information, Figure
S10). The molecules TPT, TPT-Ben and TPT-Ac showed emission maxima at 481 nm (blue),
541 nm (green) and at 617 nm (orange-red) respectively. Unlike solvent independent absorption
profile, all the three molecules showed an emission maximum that are greatly influenced by the
polarity of the solvent. In DMSO, the Stoke shift values for TPT, TPT-Ben and TPT-Ac were
calculated as 5319.64 cm-1, 5846.61 cm-1 and 7321.96 cm-1 respectively. In figure 3b the Stokes
shifts (in wavenumber) are plotted vs. solvent orientation polarizability (∆
f), ∆f = (ε - 1)/(2ε + 1)
- (n2 -1)/(2n2 + 1), where
ε
is the dielectric constant and n is the refractive index). [35–37] It
shows a nonlinear relationship indicating the presence of an intramolecular charge transfer state
in the molecules. [38]
Hexane
Toluene
1,4 doxane
EA
CHCl3
THF
DMSO
MeOH
ACN
a)
b)
TPT
0.8
4500
0.4
3000
6000
0.0
0.8
TPT-Ben
TPT-Ac
4500
0.4
3000
6000
4000
0.0
0.8
0.4
0.0
0.0
0.1
0.2
0.3
400
500
600
700
800
∆
f
Wavelength, nm
Fig. 3. a) The change in the emission spectra of TPT, TPT-Ben and TPT-Ac with respect to different solvents and their polarity.
λexc TPT = 380 nm, TPT-Ben = 410 nm, TPT-Ac = 420 nm. (b) Relationship between solvent orientation polarizability (∆f) and
Stokes shift (in wavenumber)
.

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The formation of aggregates of the derivatives was observed in the THF/water mixture as shown
in supporting information, Figure S12 and S13. In the case of TPT, an increase of fluorescence
intensity was found with an increase in concentration of water up to 80%. But further increase in
the concentration of water decreased the emission intensity tremendously and similar
observations were found in the case of TPT-Ben also. But in the case of TPT-Ac the emission
intensity was found to decrease with respect to the concentration of water.
Quantum yield (Φ) of the triphenylamine derivatives TPT, TPT-Ben and TPT-Ac were
measured in different solvents and was found to increase with an increase in the solvent polarity.
All the three derivatives showed highest quantum yield in DMSO. The fluorescence lifetime of
the derivatives were measured in different solvents using time correlated single photon counting
spectrometer by exciting at 375 nm using a nano second LED source. The results are depicted in
Table 1. The radiative and (kr) non-radiative (knr) rate constants were also calculated using
fluorescence lifetime and quantum yield according to the equation 1 and the results are included
in Table 1.
Φ
ꢔ
ꢐꢑ =
ꢐꢓꢑ = 〈 〉 − ꢐꢓꢑ
(1)
〈 〉
ꢒ
ꢒ
Where the kr and knr are the radiative and non radiative rate constants respectively, Φ is the
〈 〉
quantum yield and
ꢍ is the fluorescence lifetime.
The PL studies on the molecules were extended to the solid state also. Both the three molecules
showed good emission in their solid state also. The solid state spectrum is provided in the
supporting information, Figure S14. The solid state excitation and absorption of the molecules
were also measured and given in supporting information, Figure S15.
Dielectric
Constant
ε
λ max
(Abs)
(nm)
λ max
(Emis)
(nm)
Stokes
Shift
(cm-1)
ꢒ
Compound Solvent
kr
knr
Φ
(ns)
Toluene 2.25
Dioxane 2.38
THF 7.58
380
380
383
440
437
455
3589
3432
4132
0.14
0.14
0.18
0.3 0.47 2.9
0.3 0.47 2.9
0.5 0.36 1.64
TPT

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DMSO
46.7
383
400
400
402
481
5320
0.44
0.9 0.52 0.65
Dioxane 2.25
Toluene 2.38
476,500 4508*
475,503 4508*
0.15# 0.6 0.25 1.4
0.15# 0.4 0.38 2.13
TPT-Ben
THF
7.58
46.7
506
5113
0.16
0.5 0.32 1.7
DMSO
411
417
418
541
519
5847
4713
0.29
0.16
0.5 0.58 1.4
0.6 0.27 1.4
Dioxane 2.25
Toluene 2.38
TPT-Ac
501,525 4430*
0.13# 0.45 0.29 1.9
THF
7.58
46.7
420
425
552
617
5694
7322
0.36
0.48
0.9 0.40 0.71
1.5 0.32 0.34
DMSO
*
Table 1: Summary of photophysical characterization of the molecules. The low energy emission maximum is used for the
Stokes shift value calculations. # Two peaks are not separated for QY calculation
4.3. Computational details
Density functional theory (DFT) at the B3LYP/6-31G* level was carried out to gain a better
insight on the nature of the electronic transition and frontier molecular orbital (FMO)
distributions. [39] The calculated FMO (frontier molecular orbitals) distributions of model
systems are presented in the Figure 4. It is evident from the figure that, in all the model systems
studied, the HOMO is delocalized on the TPA (triphenyalmine) and thiophene moieties. On the
other hand LUMO is delocalized over thiophene and acceptor units. This feature confirms that,
there is an intramolecular charge transfer (ICT) transition between the TPA and end acceptor
groups. It is evident from the figure 4 that substituting -Ac moiety on the 5’-postion of
bithiophene in TPT core leads to a clear charge separated state when compared to substitution by
the -Ben unit at the same position. Hence the molecule TPT-Ac exhibited red shift in the
absorption spectrum when compared to TPT-Ben. Comparing the molecules TPT and TPT-
Ben, the degree of charge transfer is almost the same which indicates that charge transfer state of
these molecules is primarily determined by the TPT and the bithiophene moieties. Moreover,
there is no significant effect of the secondary donor (-Ben moiety) unit on the charge transfer
state. TD-DFT calculations were also performed to find out the orbitals involved in the dominant

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excited state electronic transitions and their corresponding energies and oscillator strength. It
was found that for the all the molecules the ICT peaks arised from the HOMO to LUMO
transition.
Systems
HOMO
LUMO
TPT-Ac
TPT-Ben
TPT
Fig. 4. Contour plots (isosurface value= 0.025 au) of the HOMO and LUMO levels of the derivatives at B3LYP/6-31G* level.
The hydrogen atoms are omitted for clarity.
The excited and ground state geometries were also computed using B3LYP/6-31G* level. It was
found that TPT-Ben and TPT-Ac showed a twist on excitation from ground to excited state.
They tend to be more planar in their S1 state. Figure 5 shows the ground and excited state
geometries of the molecules. The emission spectrum of TPT-Ac was getting broad in polar
solvents, this observation may be addressed with this conformation change from ground to
excited state. In the case of TPT-Ac twist angle between the thiophene and phenylacetlene is
21.1°, which become 0° in the exited state. TPT-Ben was nearly perpendicular in the ground sate
with a twist angle of 87.6°. In the exited state the calculated angle between thiophene and
dibenzothiophene was 37.7°.

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S0
S1
21.1°
C4
C1
C4
C3
0°
C1
C3
C2
C2
87.6°
37.7°
C4
C2
C4
C3
C2
C1
C1
C3
Fig.5. Optimized molecular structures in ground and excited singlet states at B3LYP/6-31G* level of theory.
4.4. Thermal and electrochemical studies
Thermal stability of a molecule is an important parameter when it comes to DSSC, OLED or any
other optoelectronic applications. The temperature stability of the molecules was tested using
Thermogravimetric (TG) as well as Differential Scanning Calorimetric (DSC) analysis. Among
the three synthesized derivatives TPT-Ben showed maximum temperature stability while TPT
showed the minimum. The decomposition temperature for TPT-Ben was found to be 444 °C.
TPT showed a decomposition temperature of about 268 °C and TPT- Ac showed an initial loss
at around 420 °C but a complete decomposition occurs only at a temperature near 460 °C.
supporting information, Figure S16 and supporting information, Figure S17 shows the TG and
DSC curves for the molecules. The CV curve of all the three molecules showed two oxidation
potentials that can be assigned to the triphenyalamine radicals supporting information, Figure
S18.[40] The HOMO values for the molecules were calculated from the oxidation potentials and
was found to be 5.26 eV, 5.29 eV and 5.14 eV respectively for TPT, TPT-Ben and TPT-Ac.
The LUMO values for the molecules were derived from the HOMO and the optical band gap
obtained from solid state absorption profile. LUMO value for TPT had been calculated as 2.52
eV, for TPT-Ben as 2.62 eV and for TPT-Ac 2.61 eV supporting information, Figure S19. The

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compiled results for electrochemical and thermal analysis of the molecules were given in Table
2.
Compound
HOMO
(eV)
Optical
band gap
(eV)
LUMO
(eV)
Melting
points(oC)
Td.( oC)
5.26
5.29
5.14
2.6
2.5
2.2
2.66
2.79
2.94
161
169
214
268
444
420
TPT
TPT-Ben
TPT-Ac
Table 2: Summary of electrochemical and thermal analysis on the triphenylamine derivatives TPT, TPT-Ben and TPT-Ac
synthesized.
4.5. Nanosecond transient absorption spectra
To get an insight about the excited state properties of the molecules nanosecond transient
absorption studies were carried out. Figure 6 shows the nanosecond transient absorption spectra
of triphenylamine derivatives, TPT, TPT-Ben and TPT-Ac, in Ar saturated THF obtained by
exciting at 355 nm using nanosecond laser flash photolysis. It shows a broad transient
absorption maximum around 600 nm. The first negative signal near 400 nm in the transient
absorption spectrum of the derivatives corresponds to the ground state bleach. In the presence of
oxygen there is no significant change in the spectral dynamics. The time constant of the
transient maximum around 600 nm for TPT, TPT-Ben and TPT-Ac was estimated be around
3.5, 4.88 and 2.9 µs respectively. Based on the literature [41-43] and on the fact that there is no
significant change in the time constant in the oxygen medium, the transient maximum are
attributed to formation of the radical cation of the triphenylamine core.

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a)
b)
0.1
0.0
a)
a)
TPT
470 nm
TPT
0.96
THF
@1
0.72
0.48
0.24
1.05
µ
s
-0.1
0.1
b)
TPT-Ben
520 nm
b)
c)
TPT-Ben
0.0
0.90
0.75
0.60
0.96
0.72
0.48
0.24
-0.1
-0.2
0.1
c)
TPT-Ac
570 nm
TPT-Ac
0.0
-0.1
300 400 500 600 700
Wavelength,nm
0 5 10 15 20 25 30 35 40
Time,
µ
s
Fig. 6. a) Nanosecond transient absorption spectra of TPT derivatives obtained by laser flash photolysis at 355 nm excitation in
Ar saturated THF b) Corresponding decay kinetics.
4.6. Generation of white light using the triphenylamine derivatives
Fig. 7. Pictorial representation of the generation of white light using a 3:2:1 combination of the synthesised triphenylamine
derivatives TPT, TPT-Ben and TPT-Ac.

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Generating white light is always fascinating due to its applications in solid state lighting and
displays. An interesting observation made after the photoluminascent studies of the molecules
was that, the molecules emitted in blue, green and orange regions and also the Stokes shift values
for the derivatives increased from the blue emitter to the orange-red emitter. As a result the
molecules can be excited using same excitation wavelength. Based on this observation we
examined the emission of a mixture of the three triphenylamine-bithiophene derivatives in
solution state (DMSO) as well as in the solid state. In both the cases, the mixture showed white
light emission with a colour coordinate of (0.32, 0.36) and (0.33, 0.37) respectively. The mixture
of TPT, TPT-Ben, and TPT-Ac was excited at a 400 nm and they gave white light for different
permutations of concentrations i.e. the white light generation is independent of the slight
variations in concentration. The best colour coordinate of the white light emission in solution
state was obtained at 400 nm excitation with a ratio of 3:2:1 for TPT, TPT-Ben and TPT–Ac
(Figure 8). The colour coordinates obtained for different permutation combinations of
concentration are given in supporting information. The emission of the mixture of three
molecules in solid state was also checked on a PMMA matrix. The ratio between the three was
kept the same as that used for the solution state studies ie 3:2:1 for TPT, TPT-Ben and TPT-Ac.
Fig. 8. a) Fluorescence emission profile for the mixture of TPT, TPT-Ben and TPT-Ac in solution state and in film state at
micromolar concentration (λexc= 400 nm). Inset shows the photograph of solid state emission for the mixture at the µM
concentration and at 3:2:1 ratio of triphenylamine derivatives under 365 nm excitation b) The CIE colour coordinate for the white
light emission in solid state and in solution state.

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5. Summary
In summary, we demonstrated the synthesis and characterisation of a set of three triphenylamine-
bithiophene based donor-acceptor molecules TPT, TPT-Ben, and TPT-Ac. The emission from
these set of molecules was successfully tuned from blue to orange red. A 3: 2: 1 mixture of the
molecules produced white light emission in both solid and solution state employing the near
lying absorption maxima. We strongly believe that these triphenylamine based donor-acceptor
systems can find applications in the field of white light emitting diodes. Our future work in this
area will focus on the more suitable functionalization of the triphenylamine-bithiophene core for
better photophysical applications.
Acknowledgement
Authors thank Ms. Soumini Mathew, Mr. Saran P Raveendran, Ms. Viji S and Ms. Aathira S
for NMR and mass analysis. AK thanks CSIR-JRF and CSIR-SRF for research fellowship.
Authors thank CSIR and TAPSUN network project for the funding. P. S. thanks KSCSTE for the
research fellowship (No 786/2015/KSCSTE dated 18/12/2015).
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Table of content

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Design, synthesis and photophysical investigation of
triphenylamine-bithiophene dyes as RGB emitters
for white light applications
Athira Krishnaab E. Varathanbc, Padincharapad Sreedevia, V. Subramanianbc, Venugopal
Karunakaranab, Ramavarma Luxmi Varmaab*
aChemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary
Science and Technology, Thiruvananthapuram 695019, Kerala, India
bAcademy of Scientific and Innovative Research (AcSIR), CSIR-NIIST, Thiruvananthapuram
695019, Kerala, India
cCSIR- central leather research institute, Chennai
*lux_varma@rediffmail.com

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Highlight
1) A set of three triphenylamine thiophene derivatives were synthesized and
charecterized
2) Their photophysical studies were carried out
3) The DFT calculations on the molecules were done to have an insight about their
structural, electronic and optical properties
4) Transient absorption studies showed the presence of triphenylamine radical cations in
the systems
5) A mixture of the three molecules in solution and solid state gave white light emission