Synthesis, photophysics of a novel green light emitting 1,3,4-oxadiazole and its application in FRET with ZnSe/ZnS QDs donor

Article abstract of DOI:10.1016/j.molliq.2017.10.066

The design and synthesis of a novel molecule 2-(4-((E)-2-(5-((E)-4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)styryl)thiophen-2-yl)vinyl)-5-(4-tert-butylphenyl)1,3,4-oxadiazole (2TVPO) based on Donor-π-Acceptor strategy through the Wittig reaction as well as characterization by ¹H, ¹³C NMR spectroscopy, high resolution mass spectrometry and FT-IR spectroscopy form the central theme of the present work. Thermal (TGA and DSC), photophysical and electrochemical properties have been studied in detail for this novel probe. In addition, fluorescence resonance energy transfer (FRET) employing steady state and time resolved spectroscopy methods has been studied using core-shell ZnSe/ZnS QDs (7 and 8 nm) and 2TVPO fluorescent probe, respectively, as the energy donor and acceptor. Results of these studies reveal that (i) FRET pairs show a strong dipole-dipole interaction tendency to transfer energy from donor to acceptor; (ii) they possess high FRET efficiency and (iii) strong dependence of the acceptor emission on the overlap integral, quantum dot size and donor-acceptor distance. Further, the type of quenching is analyzed by Stern–Volmer plots.

Full text of DOI:10.1016/j.molliq.2017.10.066

Accepted Manuscript
Synthesis, photophysics of a novel green light emitting
1,3,4-oxadiazole and its application in FRET with ZnSe/ZnS QDs
donor
G.H. Pujar, Narahari Deshapande, M.S. Sannaikar, M.N. Wari,
I.M. Khazi, S.R. Inamdar
PII:
DOI:
Reference:
S0167-7322(17)33110-0
doi:10.1016/j.molliq.2017.10.066
MOLLIQ 8028
To appear in:
Journal of Molecular Liquids
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Revised date:
Accepted date:
12 July 2017
11 October 2017
13 October 2017
Please cite this article as: G.H. Pujar, Narahari Deshapande, M.S. Sannaikar, M.N. Wari,
I.M. Khazi, S.R. Inamdar , Synthesis, photophysics of a novel green light emitting
1,3,4-oxadiazole and its application in FRET with ZnSe/ZnS QDs donor. The address
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ACCEPTED MANUSCRIPT
Synthesis, Photophysics of a Novel Green Light Emitting 1,3,4-
oxadiazole and its application in FRET with ZnSe/ZnS QDs donor
G.H. Pujar[a], Narahari Deshapande[b], M.S. Sannaikar[a], M.N. Wari[a], I.M. Khazi[b] and S.R.
Inamdar [a],*
aLaser Spectroscopy Programme, Department of Physics and UGC-CPEPA, Karnatak University, Dharwad
580003, India.
bCPEPA, Department of Chemistry, Karnatak University, Dharwad 580003, Karnataka, India.
*Corresponding author: him_lax3@yahoo.com
Abstract: The design and synthesis of a novel molecule 2-(4-((E)-2-(5-((E)-4-(5-(4-tert-
butylphenyl)-1,3,4-oxadiazol-2-yl)styryl)thiophen-2-yl)vinyl)-5-(4-tert-butylphenyl)1,3,4-
oxadiazole (2TVPO) based on Donor-π-Acceptor strategy through the Wittig reaction as well
as characterization by 1H, 13C NMR spectroscopy, high resolution mass spectrometry and FT-
IR spectroscopy forms the central theme of the present work. Thermal (TGA and DSC),
photophysical and electrochemical properties have been studied in detail for this novel probe.
In addition, fluorescence resonance energy transfer (FRET) employing steady state and time
resolved spectroscopy methods has been studied using core-shell ZnSe/ZnS QDs (7 and 8
nm) and 2TVPO fluorescent probe, respectively, as the energy donor and acceptor. Results of
these studies reveal that (i) FRET pairs show a strong dipole-dipole interaction tendency to
transfer energy from donor to acceptor; (ii) they possess high FRET efficiency and (iii) strong
dependence of the acceptor emission on the overlap integral, quantum dot size and donor-
acceptor distance. Further, the type of quenching is analyzed by Stern–Volmer plots.
Keywords: Wittig reaction, FRET, oxadiazole, Solvatochromism, ZnSe/ZnS QDs, Stern–
Volmer plots.

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1. Introduction
One of the important and intriguing aspects of modern nanotechnology is to develop
new nanomaterials and systems, which can considerably enhance the effects of organic
molecules traditionally applied in photosensitization [1-2], energy conversion [3],
optoelectronics, etc. On the other hand, the synthesis, design and development of novel
organic chromophores with efficient photophysical and optoelectronic properties have caught
increasing attention because of their potential applications in organic light-emitting diodes
(OLEDs), organic solid-state lasers, organic solid-state sensors and light-emitting
electrochemical cells [4-8]. At present, the incorporation of donor group (D - electron-
donating) and acceptor group (A - electron deficient) is the most appealed strategy in organic
electronics field. In particular, π-conjugated donor-acceptor (D-π-A) fluorescent
chromophores are very much useful in variety of applications [6-8] due to their efficient
absorption/emission properties originated through intramolecular charge transfer (ICT) from
D to A via π-bridge. Oxadiazoles being electron deficient are known for hole-
blocking/electron-transporting materials in OLEDs [9-10 and references therein] which
restrict the π-conjugation to have deeply laying highest occupied molecular orbitals
(HOMOs) along with high triplet energies [11-13]. Such promising class oxadiazoles are also
being used in various biological activities [14].
In the last couple of decades, semiconductor nanocrystals and their applications have
dramatically diverted science in the direction of new world due to their unique and
remarkable optical, electrical, chemical and mechanical properties. Such nanostructures have
found potential applications in the fields of luminescence tagging, biomedical applications
like drug delivery, biological imaging, diagnostics and therapeutic purposes, multiplexing
and recently biosensing [15-18]. Specifically, core/shell nanocrystals are interesting, in that,
they exhibit enhanced fluorescence intensity and increased photostability. Over the past
decade, extensive experimental work has been carried out on the synthesis of core/shell
nanostructures, with tailored properties [19-21] that include CdSe/ZnSe, CdSe/CdS,
CdSe/ZnS, CdS/HgS, ZnS/CdS, ZnSe/ZnS, HgS/CdS, CdS/PbS, etc., [22-28] which show
reduced lifetimes, enhanced quantum yields and advantages in the tuning of relative
bandgaps between the core and shell materials. Zinc Selenide (ZnSe) an interesting n-type, II-
VI class, direct bandgap semiconductor has caught great attention owing to its key
applications in photodetectors, full color displays and light emitting diodes (LEDs) [29-30].
Variety of core-shell nanocrystals have been synthesized with ZnS as an inorganic

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passivation shell because of its wide bandgap (2.7 eV in bulk) and considerably large binding
energy (i.e., 21 meV) [31] with a view to improve the emission properties and stability of
core nanocrystals while acting as a protective layer to reduce the toxicity of core materials
such as CdS. Norris et. al. have reported that ZnSe acts as an attractive host in formation of
the doped nanocrystals [32]. Further, there is a good match between ZnSe and ZnS as it gives
high photoluminescence (PL) efficiency and high chemical stability. The PL is trapped in the
ZnSe core quantum dot (QD) when ZnSe and ZnS form core/shell structures. The core/shell
ZnSe/ZnS QD structures have high luminescence efficiencies, less surface defect states and
less toxicity in their variety of applications [22-28,33-34]. Therefore it is imperative to know
the mechanisms of interaction between ZnSe/ZnS QDs and other chromophores.
Semiconductor quantum dots have been increasingly employed in the study of
fluorescence resonance energy transfer (FRET) due to its ample applications in light
harvesting in photosynthesis, biolabeling, structure determination and multiplexing
experiments, design of sensors [35-38], etc,. Earlier reports [1,34,39] have demonstrated the
possibility of effective energy transfer in such quantum dots, in particular, ZnSe/ZnS QDs
donor–acceptor systems. Thus, we have initiated research on heavy metal free core-shell
ZnSe/ZnS QDs as energy donors and a novel organic dye as acceptor by FRET phenomenon.
Owing to great potential and vast applications of highly conjugated oxadiazoles
containing donor-π-acceptor chromophores, good deal of work has been dedicated to the
study of sensitizers in OLEDs, dye-sensitized solar cells and hardly as fluorescence sensors
[40]. Most of these reports confine their focus on optoelectronic applications. However, to the
best of our knowledge, the applicability of 1,3,4-oxadiazole chromophore itself as an
acceptor counterpart in the FRET course with core-shell ZnSe/ZnS QDs has not been
reported earlier. The present work deals with design and synthesis of novel 2TVPO
chromophore along with detailed study of spectroscopic, optoelectronic and thermal
properties. Apart from this, the spectroscopic signature of this optoelectronically important
bipolar molecule (2TVPO) during excitation via FRET from ZnSe/ZnS QDs is also explored.
2. Experimental Section
2.1. Materials: All chemicals/starting materials of reagent/analytical grade are purchased
from Sigma-Aldrich and used without further purification. All solvents were freshly distilled
over appropriate drying reagents prior to use during the synthesis. Heavy metal free
ZnSe/ZnS-405 and 425 (7 and 8 nm) core-shell QDs dispersed in toluene were purchased
from Mesolight LLC., USA. All the organic solvents used were of spectroscopic grade

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(HPLC grade, Fluka), transparent, nonfluorescent in the range of excitation and fluorescence
emission and used as received.
2.2. Instrumentation and experimental methods
Physical Measurements: FT-IR spectra were measured as KBr pellets on a ThermoFisher
Nicolet 6700 FT-IR spectrometer in the region of 4000-400 cm-1 at 2 cm-1 resolution with
1
13
100 scans. H and C NMR were recorded on Bruker 400-MHz FT NMR spectrometer in
CDCl3 by using TMS as internal standard. Chemical shifts are reported in ppm downfield (δ)
from TMS. Melting points were determined by open capillary method and are uncorrected.
High resolution mass spectra (HRMS) were obtained on Agilent Technologies 6538 UHD
Accurate-Mass Q-TOF LC/MS spectrometer. Cyclic voltammetry experiments were
conducted in a standard one-compartment cell in acetonitrile solvent (Spectrochem, HPLC
grade). Thin films on ITO used as working electrode, platinum as counter electrode and
Ag/AgCl as reference electrode respectively at a scan rate of 50 mV s-1. DSC and TGA were
performed with a TA Instruments DSC Q20 V24.10 Build 122 and TA Instruments SDT
Q600 V20.9 Build 20, respectively, under nitrogen with heating rates of 10 oC min–1.
Steady state measurements: The absorption spectra were measured with a commercial dual-
beam JASCO UV-Vis NIR spectrophotometer (Model V-670). Photoluminescence spectra
(PL) were collected with a spectrofluorometer (JY Horiba, Floromax-4) using an integration
time of 1 s nm-1, resolution of 1 nm, an excitation slit width of 2 nm and emission slit width
of 2 nm. For PL measurements in solid phase, the 2TVPO chromophore film was prepared by
using 5% 2TVPO solutions in chloroform, which was spin-coated on quartz slides at about
1000 rpm, for 40 s, at room temperature.
Time resolved measurements: The fluorescence lifetimes were measured employing
picosecond time domain spectrometer based on Time Correlated Single Photon Counting
(TCSPC) technique (IBH Jobin Yvon 6.1). The decays were collected over 1024 channels
with at least 10,000 photons in the peak maximum for the lifetime experiments. The samples
of core-shell ZnSe/ZnS quantum dots were excited at 280 nm and 2TVPO dye in all solvents
at 415 nm, using pulsed excitation source (NanoLED) in an IBH Fluorocube apparatus. The
fluorescence emission at the magic angle of 54.7o was dispersed in a monochromator (f/3)
aperture to eliminate the contribution from anisotropy decay and counted by a Hamamatsu
Micro Channel Plate Photo-multiplier tube (R3809 MCP-PMT). The instrument response
function (IRF) for this system is ∼844 ps. The fluorescence decay curves were deconvoluted

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and analyzed on IBH (DAS-6) decay analysis software. From the deconvoluted fluorescence
lifetime data the average fluorescence lifetime <τ> was calculated from the following
equation [35]:
A
  
i
i
(1)
A

i
in which Ai is the relative amplitude corresponding to the ith decay component having the
characteristic decay time constant τi. The acceptability of the fits was judged from the χ2
criterion and the residual of the fitted functions to the actual data.
2.3. Quantum yield calculation
Fluorescence quantum yield (Φ) was measured at room temperature using Coumarin
480 (2,3,5,6-1H,4H-Tetrahydro-8-methylquinolizino-[9,9a,1-gh]-coumarin) as standard
reference (λabs=389, λem=457 nm, ΦR=0.95 in ethanol [41]). For all the samples, Φ, was
calculated in the manner detailed by Lakowicz and Rhys Williams et al. [35,42], accounting
for the refractive index of the solvent, the integrated fluorescence intensity and the
absorbance at the excitation wavelength, with respect to the reference in the following
equation.
I ODR n2
IR OD nR2
(2)
  R
where I is the integrated fluorescence intensity, OD is the optical density and n is the
refractive index of the solution, the subscript R refers to the standard reference fluorophore of
known quantum yield.
2.4. Synthesis and Characterization of novel fluorescent derivative
The required intermediates, 2-(4-tert-butylphenyl)-5-p-tolyl-1,3,4-oxadiazole (OXD),
2-(4-(bromomethyl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (1) and Wittig salt (2)
were synthesised according to our previous work [10 and references therein ]. The detailed
procedure is given in the ESI†.
Synthesis of (4-((E)-2-(5-((E)-4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)styryl)thioph
en-2-yl)vinyl)phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (2TVPO):
A mixture of Phosphonium salt (2) (1g, 0.00158 mmol) and thiophene-2,5-
dicarbaldehyde (0.42 g, 0.00315 mmol) were taken in Water:DCM (40:60) mixture under
vigorous stirring. Sodium hydroxide solution (50%, 0.65 ml) was added to the mixture by
syringe and stirred for 5-6 h at room temperature. Completion of reaction was confirmed by

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TLC. The reaction mixture was diluted with dichloromethane and water. The organic layer
was separated and the solvent was removed under reduced pressure. The resulting solid was
dried and recrystallized from ethanol to get the desired product in 70-80% yield. The
synthetic approach is highly convenient and provides compound 2TVPO in the trans
1
configuration across the double bond. The trans configuration is evidenced by H-NMR
spectroscopy. The compound 2TVPO displayed a doublet signal around 6.5-7.5 ppm with a
typical coupling of ca. 16.00-16.60 Hz. This value corresponds with the expected value for a
3Jtrans Coupling of the vinylic protons. Further evidence concerning the structure of 2TVPO
was obtained by 13C-NMR. The good accordance between calculated and expected values for
microanalysis gives a proof of the purity of the material.
Yellow solid; Yield: 80 %; m.p. 210-215 °C; FT-IR (KBr, cm-1): 3082 (-C=C-H), 3028 (C-H
aromatic), 2957, 2928 (C-H aliphatic), 1605, 1547, 1490, 1415 (C=C and C=N Aromatic),
1
1016, 962 (1,3,4-oxadiazole ring), 933 (Thiophene ring), 842 (tert-Butylbenzene); H NMR
(400 MHz, CDCl3) δ 8.12 (d, J = 8.4 Hz, 4H, Ar-CH), 8.08 (d, J = 8.6 Hz, 4H, Ar-CH), 7.63
(d, J = 8.4 Hz, 4H, Ar-CH), 7.56 (d, J = 8.5 Hz, 4H, Ar-CH), 7.33 (d, Jtrans = 16.0 Hz, 2H,
trans-CH), 7.06 (s, 2H, Thiophene-CH), 6.97 (d, Jtrans = 15.8 Hz, 2H, trans-CH), 1.38 (s,
13
18H, Ar-C(CH3)3); C NMR (101 MHz, CDCl3) δ 164.65, 164.26, 155.27, 140.12, 129.62,
128.28, 127.62, 127.33, 127.23, 126.83, 126.10, 124.64, 123.84, 121.15, 31.15, 29.72;
HRMS: m/z [M + H]+ calc’d for C44H40N4O2S 688.8790, found 688.8935.
3. Results and discussion
Firstly, it is important to overview the spectroscopic/photophysical behavior of the
novel compound (2TVPO), which is being used as an acceptor counterpart in the FRET
study, with a view to understand its optoelectronic importance.
3.1. Synthesis and characterization
A quite efficient and simple synthetic procedure was used for the synthesis of compound
2TVPO as shown in scheme 1. The required starting material 2-(4-tert-butylphenyl)-5-p-
tolyl-1,3,4-oxadiazole (OXD) was synthesized by the treatment of 4-methylbenzohydrazide
with 4-tert-butylbenzoic acid in refluxing POCl3. The allylic bromination of OXD with N-
Bromosuccinimide in the presence of dibenzoyl peroxide at refluxing carbon tetrachloride
(CCl4) results in the formation of target compound 2-(4-(bromomethyl)phenyl)-5-(4-tert-

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butylphenyl)-1,3,4-oxadiazole (1). The nucleophilic substitution reaction of
triphenylphosphine at the functionalized allylic carbon leads to the formation of
o
phosphonium ion (2) at 80 C in DMF. Deprotonation of (2) by aq. NaOH gives ylide
(Wittig reagent) in DCM and water. The ylide, acting as a nucleophile, adds to carbonyl
carbon of thiophene-2,5-dicarbaldehyde. The resulting alkoxide oxygen added to the
phosphorus in an intramolecular closure reaction to form four-membered ring
(oxaphosphetane). The ring fragmentation leads to the formation of triphenylphosphine oxide
and the desired product 2TVPO. The resulting molecule was then purified using column
chromatography on silica gel followed by recrystallization in ethanol. The nature of 2TVPO
is amorphous and can be preserved for longer time under ambient conditions without any
1
detectable decomposition. The structural identities and purities were characterized by H
NMR, 13C NMR, FT-IR, HRMS (ESI†) which were found to corroborate well with the
proposed structure.
O
O
N N
NH2
POCl3, Reflux, 12 h
N
HO
O
H
OXD
NBS, DBP
CCl4, 80 o
C
N
N
N N
TPP, DMF - 80 o
C
Br- P
O
Br
O
1
2
50% aqu. NaOH
MDC/H2O
H
H
S
O
O
O
N
S
N
N
N
O
2TVPO
Scheme 1. Synthetic route for the target compound 2TVPO
3.2. Spectroscopic properties of 2TVPO
Absorption and fluorescence spectra: The photophysical properties of novel compound

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2TVPO are studied in several organic solvents using UV-Vis absorption, photoluminescence
(PL) and time resolved spectroscopy. The complete details are given in Fig. 1 and 2 and
Table 1. 2TVPO exhibited solvent polarity independent, broad and structureless absorption
band with two distinct peaks in all the solvents. The broad absorption bands observed at
longer wavelength around 370-450 nm were assigned to the π→ π* electronic transition of
whole bipolar molecule and those observed around shorter wavelength were attributed to π→
π* transition of 2,5-di(4-tert-butylphenyl)-1,3,4-oxadiazole units of the compound [43].
Broad absorption bands indicate that multiple optical transitions are contributing to the
intense band in the region 375–455 nm [44]. The electronic structure of the newly
synthesized molecule can be studied by PL spectra. The compound 2TVPO has exhibited
spectral profiles that are independent of excitation (i.e. ~240, 300 or 425 nm), with λem
around 485 nm indicating that fluorescence occurs only from the S1 state in 2TVPO probe.
The maximum fluorescence intensity was noticed under the excitation at around 425 nm. In
addition, significant Stokes shift is observed for the compound 2TVPO which might be
attributed to its intramolecular rotation upon photo-excitation. It has been demonstrated that
incorporating a rotational substituent in a fluorophore molecule with tailored steric hindrance
effects and resonance effects leads to a substantial increase in the Stokes shift in many dye
molecules [45].
A typical PL spectrum of 2TVPO in both solution (toluene) and solid phase is shown
in Fig. 3. Solution phase emission spectrum is broader and structureless, having FWHM of 65
nm. The compound is highly green emissive solid with emission maxima at 552 nm. A red
shift of around 77 nm in the solid phase is observed when compared with toluene solution of
the 2TVPO compound. Thus the compound emits intense green color in solution and yellow
color in solid phase. The red shift could probably be related to aggregation effects and certain
solid state molecular morphology imposed by the donor and acceptor units of 2TVPO.
Salvatochromism of 2TVPO: Solvatochromic behavior of 2TVPO has been studied in polar
protic solvents (methanol-decanol), nonpolar solvents (chloroform, toluene, dichloromethane
(DCM)) and polar aprotic solvents (dimethylsulfoxide (DMSO), dimethyleformamide
(DMF)) in order to understand the intramolecular charge transfer (ICT) effect as summarized
in Table 1 and Figs. 1 and 2. The shift in λabs and λem values with respect to solvent polarity
reflects the molecular interactions between solute and solvent. The molar absorptivities, ε, of
aqueous 2TVPO were determined at around 425 nm. An absorption having high molar
absorptivities (ε = 2.60–3.57 ×105 M−1 cm−1) at lower energies, occurs for 2TVPO with

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strong allowed π→ π* transitions. However, the spectra show moderate band shift (~10 nm)
up on changing solvents from methanol to toluene (blue shift) as illustrated in Fig. 1 and
Table 1, though the molar absorptivities differ slightly. It should be noted that the observed
differences in molar absorptivity of 2TVPO is mainly due to its solubility in aqueous
solutions. The small shift in absorption band as a function of solvent polarity is due to the
small change in polarity of the ground state of solvated 2TVPO (see Table 1). It indicates
that the ground state energy distribution is not affected to greater extent as 2TVPO is less
polar in that state. However, solvent polarity has a slight effect on the peak positions of the
electronic emission maxima (~24 nm). Increasing solvent polarity leads to a red shift in the
PL maxima owing perhaps to the reasonable difference between charge distributions in the
ground and excited states of solute which results in a strong interaction between polar
solvents and 2TVPO in the excited state. This sensitivity of dye to the selected organic
solvents is due to ICT between the thiophene (electron donor) unit and the oxadiazole
(electron acceptors) units in the excited state. This results a slight increase in excited state
dipole moment (i.e., µe>µg). This dipole interacts with the dipole of polar solvent molecules
to reduce the excited state energy and hence results in bathochromic shift. During ICT,
bathochromic shift also occurs due to net stabilization of the excited state as compared to the
ground state with increasing solvent polarity [46]. Generally, a small Stokes shift leads to
larger overlap between the absorption and emission spectra causing self absorption of the
emitted light and decreased fluorescence efficiency. Interestingly, 2TVPO chromophore
exhibited large Stokes shift values (~52-66 nm), indicating substantially reduced self-
absorption. Stokes shift value is found to increase with polarity of solvent, implying a charge
transfer transition and an increased dipole moment in the excited state (i.e., µe> µg). This
indicates that the PL emission is from a more polar ICT state.
Generally, for ICT chromophores, the solvent polarity has significant effect on the
fluorescence quantum efficiency (ability to emit light) because ICT in itself is a quenching
factor which promotes the nonradiative decay rate. Particularly, in more polar solvents, ICT
stabilizes charge separation and results in low quantum yields (QY). This kind of effect is
observed for 2TVPO fluorophore, the QY of which is around 47–62% in solvents such as
DMSO, DMF, chloroform, DCM and toluene. Further, the fluorescence QY decreased
drastically in polar protic solvents like methanol and ethanol (Table 1). This is possibly due
to the interaction between the protic solvents and the excited state and fluorescence
quenching. It may be noted that the fluorescence quenching of many molecules having ICT

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states results from the deactivation via internal conversion ensuing from hydrogen bonding
[44, 47-49].
The abrupt change in the dipole moment is one of the principal reasons for spectral
shifts in steady state solvatochromism, whereas the conformational stability of excited state is
determined by fluorescence lifetimes [50-51]. Usually, the lifetimes increase with solvent
polarity for stabilized excited state. Fluorescence lifetimes were measured in solvents of
different polarity varying from polar to nonpolar (methanol to toluene) (Fig. 2). The
fluorescence lifetime of 2TVPO appears to decrease as the solvent polarity increases in
selected solvents from decanol to methanol (see Table 1). However, in DMSO, DMF,
chloroform and DCM, it is shorter than those in alcohols (butanol-decanol) and showing
slight nonlinear trend with solvent polarizibility, indicating that the stability of the excited
state also could be affected by solvent viscosity [52]. The shorter lifetimes in methanol and
ethanol are due to a strong interaction between protic solvents and 2TVPO probe (hydrogen
bonding between the ICT state of 2TVPO and solvents) [47-48].
3.3. Thermal properties
The formation of morphologically stable material films is a key requirement for the operation
of organic electronic devices. For the creation of stable organic devices, it is necessary to
synthesize materials exhibiting high glass transition temperatures (Tg) which ensures that no
phase transition will occur upon operation. It is obvious that the morphological and thermal
stability are closely correlated to the molecular size and weights. The corporation of the tert-
butylphenyl unit significantly improves the T5d and Tg as compared to previously reported
bipolar molecules without such substitution [9]. The morphological and thermal stabilities of
compound 2TVPO were characterized by thermal gravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The results are depicted in Fig. S18 and Table S1 (ESI). DSC
o
was carried out from 25 to 400 C under N2 with heating rate of 10oC/min. The phase
transition properties of 2TVPO were analyzed using DSC which reveals that the molecule is
crystalline in nature. The DSC curve exhibits an endothermic baseline shift related to the
glass transition temperature (Tg) at 254.4 oC. It is interesting to note that, the Tg of 2TVPO is
significantly higher than those of well-known oxadiazole based electron transporting
materials, like 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (Tg=60oC), 1,3-
bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (Tg=77oC) and also than the nonbipolar
host emitters that include oxadiazoles (BOBP, Tg=45oC; TPOs and PPOs, Tgs up to 121oC)
[9]. The 2TVPO is a material, crystalline nature and having sharp endothermic melting peaks

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o
at 313.6 and 322.3 C in the thermogram. The peaks corresponds to exothermic were absent
at higher temperature which indicates that 2TVPO has crystallization nature with excellent
amorphous glass state stability. TGA result revealed that the 2TVPO is thermally quite stable
o
with 5% weight loss temperatures (T5d) at 378 C and hence 2TVPO is exhibiting good
thermal stability. Thus, the compound is potential material for making stable devices in
photovoltaic and optoelectronic applications.
3.4. FRET from core-shell ZnSe/ZnS QDs to novel fluorescent derivative 2TVPO
3.4.1. Steady-state measurements
The ZnSe/ZnS nanocrystals exhibit unique photophysical properties like broad absorption
spectra, narrow and symmetric emission spectra and high PL quantum yields [39,53]. These
were selected as donors with 2TVPO as acceptor in order to enhance the overlap of the donor
emission and acceptor absorption spectra as it is a prerequisite to get any measurable FRET
(eqn. (S3)). The size of QDs offers a unique advantage to control this parameter, which in
turn can regulate the efficiency of FRET. Further, this overlap can also be controlled by
altering the composition of QD and thereby varying its emission when paired with an
acceptor dye [53-55]. This distinct aspect is illustrated in Fig. 4 where the PL emissions of
two different sizes (7 and 8 nm) of heavy metal free ZnSe/ZnS-405 and 425 core-shell
quantum dots (QD1 and QD2) emitting violet color at 406 and 418 nm are plotted along with
the absorption spectrum of 2TVPO dye. The 2TVPO probe has absorption peak at 425 nm,
thus making the present pair perfectly suited for FRET process. The photophysical properties
of two FRET pairs selected based on the changing size of core/shell QD donors are listed in
Table 2. The excitation wavelength was fixed at 330 nm for all samples to avoid direct
excitation of the acceptor (see Fig. 4), as the absorption of 2TVPO is negligible at 330 nm
and therefore a clear photoinduced resonance energy transfer is possible from donor to
acceptor. Inset of Fig. 4 shows the extent of overlap functions, J(λ) which describes the
effects of size of the QDs and its emission on the degree of spectral overlap for a given
acceptor chromophore. Evidently, absorption spectra of 2TVPO overlaps well with the QD2
donor fluorescence spectra as compared to QD1 (Table 3). With increasing overlap integral
an expected proportional increase in the value of R0 is seen (eqn. (S5) which together with
the high QY of the QDs helps in efficient FRET at longer distances of separation.
Fig. 5 depicts gradual decrease in the PL intensity of donor ZnSe/ZnS QDs and a
simultaneous enhancement of the emission intensity of acceptor 2TVPO with increasing the
concentration of 2TVPO (1 to 9 µM). The reduction in QDs emission intensity and increase

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in 2TVPO dye emission (at around 500 nm) confirms the nonradiative energy transfer from
QDs to 2TVPO dye. The new emission peak at around 500 nm corresponds to the 2TVPO
dye arising via FRET from QDs to dye. The fluorescence intensity of 2TVPO fluorophore
increases due to an increase in the FRET efficiency from QD to dye. Interstingly, the
emission intensity of the 2TVPO dye bound to the QDs is more than nearly six times larger
than when excited directly, indicating sensitized acceptor fluorescence due to FRET. All
these observations demonstrate that the steady state emission of 2TVPO in the presence of
QD1 and QD2 originates through FRET. Additionally, this Förster type resonance energy
transfer from QDs donor to 2TVPO acceptor bears excellent consistency with earlier reports
of Martynenko et al. [1], Hering et al. [34], Lakowicz [35], Shu et al. [39] and our own works
[53-55].
Further, it is of interest to note that the concentration of the acceptor (2TVPO)
required is rather too low (~µM) in the medium for FRET to occur with estimable efficiency
(Fig. 5). However, this can be explained as due to high quantum yield of 2TVPO in toluene
medium (Table 1). Now, remembering the essential role of FRET on the immensely
significant aspects of photo-sensitization [1-2], energy-conversion processes [3] etc., it is
logical to focus the present investigation on the highly fluorescent 2TVPO chromophore to
elucidate the key role of FRET in its emission at longer wavelength via FRET. Hence, the use
of fluorescent 2TVPO probe in FRET based applications and photosensitization in aqueous
medium could be interesting.
It is well known that FRET efficiency (ET) varies as function of overlap integral of
QDs and dye fluorophore [35, 54-56] and the molecular proximity of donor–acceptor in the
mixture [57]. It has been observed that the ET varies almost linearly with increase in acceptor
(2TVPO) concentration. This linear variation is due to the possible contribution of Förster
type energy transfer and close proximity of the QDs and 2TVPO. The FRET parameters viz.,
spectral overlap integral J(λ) (eqn. (S3), Förster radius R0 (eqn. (S5), the donor-acceptor
distance r, ET (eqn. (S8) and transfer rate kT(r) (eqn. (S1) have been calculated from the
spectral characteristics of donor-acceptor mixture and listed in Table 3. We observed that ET
is 34.83% for core–shell QD1-2TVPO pair and it is less compared to 55.02% noted in case of
QD2-2TVPO pair. This difference can be well explained by FRET parameters. The values of
R0 obtained for present FRET pairs QD1-2TVPO and QD2-2TVPO are 6.20 and 6.59 nm
respectively, which strongly depend on the QY of the donor and overlap. It is noticed that R0
linearly increases with the increase in the overlap and that r decreases with the increasing J(λ)
that results in increase in the kT(r) and hence increase in ET. From the donor–acceptor

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distances, it is evident that these FRET couples are well suited for investigation of
conformational changes in the molecules and/or detecting the interactions between the
molecules [35].Therefore, all the FRET parameters obtained for donor-acceptor pair as well
as size of donor QDs play a significant role in influencing the ET.
3.4.2. Time resolved fluorescence study
The steady state fluorescence measurements of the donor-acceptor pairs give an average and
relative representation about FRET. A better confirmation is, however, could be derived from
time resolved measurements which give an absolute measure (independent of concentration
and fluctuations in excitation source intensity) and provide a clear dynamic picture of the
fluorescence. In order to obtain quantitative confirmation for the FRET from QDs to 2TVPO,
the PL decays of the donor (QD1 and QD2) were examined in the absence and presence of
2TVPO. Fig. 6 shows PL decays dynamics of QDs alone and QDs-2TVPO mixture when
excited with λexc= 280 nm at 300 K. The fluorescence decays contain two components (fast
and slow) (see Table 4). Therefore, decay profiles are best stated by bi-exponential function,
A1exp(−t/τ1) + A2 exp(−t/τ2), where τ1 and τ2 represent the fast and slow decay times,
respectively and A1 and A2 are the corresponding contributions to the total fluorescence
intensity and accordingly their average lifetime is calculated using by eqn.(S9). This origin of
bi/tri/multiexponential PL decay profile of semiconductor quantum dots is still not clear. The
faster decay is due to radiative relaxation of the excited electron and is attributed to initially
populated core-shell recombination [54-56, 58-59]. However, the possible origin of long and
longer radiative decays remains relatively uncertain. Thus multiexponential nature of the PL
decays and the average lifetimes of the QDs are consistent with the literature.
As listed in Table 4, the PL decay profiles of the donor (QDs) in the absence and
presence of the acceptor (2TVPO) are analyzed by a bi-exponential function with the average
lifetimes of donor QD1 and QD2 without the 2TVPO dye being 6.78 and 6.92 ns,
respectively. The acceptor induced changes in lifetimes of QD1 and QD2 are found to
conform by decreasing lifetimes in presence of 2TVPO (9 µM), reaching to 5.28 and 4.14 ns
in QD1 and QD2, respectively (Table 4). Shortening of decay time is perceived with
decrease in both components (τ1and τ2) which confirms that nonradiative energy is
transferred via FRET to 2TVPO dye. Thus FRET offers an alternative pathway for donor
relaxation, reducing its lifetime in the presence of acceptor 2TVPO. For QDs with high
fluorescence QY, the amplitude (A2) with the longest lifetime component dominates the total
PL. In other words, holes and electrons have greater probability of being on the QDs surfaces

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contributing to the longest lifetime. Thus, it implies that the surface of QDs is certainly
involved in the process of energy transfer via FRET [54-56]. The transfer efficiency values
determined using the time resolved data from eqn. (S8) are 22.08 and 40.25% for QD1 and
QD2 to 2TVPO dye, respectively. Further, it has been observed that these values are smaller
than ones obtained from steady state studies. This is due to the fact that time resolved data are
independent of fluctuations in the excitation source intensity and concentration (see Table 4).
3.4.3. Fluorescence quenching of QDs in the presence of 2TVPO
The Stern–Volmer (SV) plots were analyzed for both steady state and time resolved
measurements to get better insight into the type of quenching in the FRET. The SV equation
(eqn. (3) relates concentration of quencher and excited states quenching [56, 60] as follows:
FD
D
DA
(3)
1 KD[Q] 1 kqD[Q] 
FDA
where FD and FDA are the PL intensity of donor (QDs) in the absence and presence of
quencher (2TVPO), KD is the SV quenching constant, kq is the rate constant of the
bimolecular process, [Q], the quencher concentration while τD and τDA are the average
lifetimes of the donor with and without the quencher, respectively. In many cases, the
chromophore may be quenched either by static or by dynamic quenching. In case of static
quenching, complex formation occurs between the chromophore and quenchers in the ground
state. This certainly affects PL intensity but not on the lifetimes. However, dynamic
quenching signifies collision between the chromophore and quenchers during the excitation
of the chromophore. Consequently, the PL lifetime is shortened.
When 2TVPO dye (from 1 to 9 µM) was gradually added to the ZnSe/ZnS QDs
solution of fixed concentration, a decrease in the PL intensity of QDs was noticed. This
quenching is described in the SV plots for both steady state and time resolved studies (Fig. 7
(A, B, C and D)). Note that FD/FDA and τD/τDA versus [Q] vary linearly with increasing
concentration of quencher, suggesting that dynamic quenching is involved in the ZnSe/ZnS
QDs-2TVPO systems. The SV quenching constant (KD) obtained from the slopes of these
graphs are 6.02×104 M–1, 11.15 × 104 M–1 and 3.20×104 M–1, 7.31 × 104 M–1for QD1 and QD2
to 2TVPO in steadystate and time resolved measurements, respectively. The estimated
quenching rate constant (kq) reflects on the efficiency of quenching or the accessibility of the
probes to the quencher. The kq values thus determined are 8.9×1012, 16.1×1012 M–1S–1 and
4.7×1012, 10.5×1012 M–1S–1 for these FRET pairs in steady state and time resolved

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measurements, respectively. Further, the diffusion controlled quenching usually results in
values of kq ~1010 M–1S–1 as expected from the Smoluchowski eqn. (kdiff =8RT/3η) [61].
Values of kq smaller than the diffusion limited rate constant can result from steric shielding of
the fluorophore or a low quenching efficiency. However, in the present study the values of kq
estimated from the SV equation are quite larger than the diffusion limited rate constant.
Hence, it is inferred that this is not a diffusion controlled process [35].
4. Concluding remarks
A novel fluorescent highly conjugated 1,3,4-oxadiazole containing donor-π-acceptor
chromophore molecular system viz., 2TVPO has been synthesized through a relatively simple
synthetic protocol (Wittig reaction) and its spectroscopic and electrochemical aspects are
investigated in detail. The structure of this newly synthesized compound was confirmed by
their analytical and spectral data. The thermal and morphological stabilities of this compound
were found to be good at higher temperatures. The compound 2TVPO is then employed as
acceptor moiety in the study of FRET from core-shell ZnSe/ZnS QD as donors. A combined
application of steady-state and time-resolved fluorescence spectroscopic tools has been used
to confirm the occurrence of FRET process via long-range dipole–dipole interaction. This
study clearly illustrates that the energy transfer efficiency linearly varies with spectral
overlap, size of QDs and quantum yield of the donor. It is found that transfer efficiency is
higher for ZnSe/ZnS-425 QD (8 nm) compared to that of ZnSe/ZnS-405 QD (7nm).
Additionally, the values of SV and bimolecular quenching constants determined
experimentally inferred that quenching is not a diffusion controlled process. The results
demonstrate that the novel D-π-A chromophore might play an important role in organic
optoelectronics and FRET based sensing applications. Further, ZnSe/ZnS QDs are free from
toxic heavy metals like cadmium, selenium, etc., which allows the exploration of their
possible applications as safer donors for FRET in various biological applications.
Supporting Information Summary
Supporting information includes the following details;
(a) Experimental section describing the synthesis of intermediates, (b) Analytical and
spectroscopic data viz.,1H NMR, 13C NMR, HR-MS, FT-IR, for all intermediates and titled
compound (c) optical band gap study, (d) electrochemical properties, CV measurements, (e)
DSC and TGA thermograms and (f) theory of FRET measurements.
Acknowledgments

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The authors thank the University Grants Commission (UGC), New Delhi, India for
the financial support under Centre with Potential for Excellence in Particular Area
(CPEPA;No.8-2/2008(NS/PE)). They are indebted to Prof. S.M. Shivaprasad and Dr. Subi
Jacob George, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR),
Bangalore, for HRMS Spectra. The timely help of Prof. P. Ramamurthy and Dr. Selvaraju,
National Centre for Ultrafast Processes, Chennai, in time-resolved measurements is gratefully
acknowledged.

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DMSO
DMF
chloroform
DCM
1.0
0.5
0.0
1.0
0.5
0.0
1.0
0.5
0.0
1.0
0.5
0.0
B
methanol
ethanol
butanol
pentanol
hexanol
heptanol
octanol
nonanol
decanol
A
toluene
300
400
500
600
700
300
400
500
600
Wavelength/nm
Wavelength/nm
Figure 1.Normalized absorption and emission spectra of 2TVPO in (A) alcohols (methanol-
decanol) and (B) general solvents at room temperature.

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10000
10000
1000
100
IRF
B
A
IRF
DMSO
DMF
chloroform
DCM
toluene
methanol
ethanol
butanol
pentanol
1000
hexanol
heptanol
octanol
nonanol
decanol
100
800
900
1000
1100
1200
1300
825
900
975
Channel/ns
1050
Channel/ns
Figure 2. Typical time resolved fluorescence decays profiles of 2TVPO in (A) alcohols
(methanol-decanol) and (B) general solvents. The sharp gray profile on the extreme left
represents the instrument response function (IRF). 1 Channel=0.02743484 ns.

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1
2
1.0
0.5
0.0
450
500
550
600
650
700
750
Wavelength/nm
Figure 3. Typical normalized PL spectra of 2TVPO in (1) toluene solution (solid black line)
and (2) Solid phase (dashed red line), excited at 425 nm.

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b
a
b
-14
5x10
a
-14
3x10

-14
2x10
0
360
380
400
420
440
460
Wavelength(nm)
300
400
500
600
700
Wavelength/nm
Figure 4. Overlap between absorption spectra of 2TVPO (dotted line) and PL spectra of [a]
ZnSe/ZnS-405 QD (solid black line) and [b] ZnSe/ZnS-425 QD (solid red line) in toluene at
room temperature. The inset shows plots of the resulting overlap functions J(λ). The size of
QDs can substantially improve the degree of spectral overlap with the 2TVPO.

ACCEPTED MANUSCRIPT
A
B
a
a
ZnSe/Zns 405-2TVPO
ZnSe/Zns 425-2TVPO
2x105
6x105
5x105
3x105
Q
D 1
5
Q
2x10
D 2
h
h
1x105
2 T
h
V P O
h
2
5x104
2x105
0
T
V
P
O
a
a
0
400
450
500
550
600
400
450
500
550
600
Wavelength/nm
Wavelength/nm
Figure 5. Representative steady-state PL profile of [A] ZnSe/ZnS-405 QD and [B]
ZnSe/ZnS-425 with incremental addition of 2TVPO (λexc=330 nm). Curves (a-h) correspond
to 2TVPO [(a) QD alone, (b) 1 µM, (c) 2 µM, (d) 3 µM, (e) 5 µM, (f) 7 µM, (g) 8 µM, (h) 9
µM]. The enhanced emission above 450 nm corresponds to the 2TVPO resulting from energy
transfer from the QDs

ACCEPTED MANUSCRIPT
B
A
10000
10000
1000
100
1000
Q
D2
QD1
Q
D
Q
1 w
D2
100
i
t
with 2TVPO
h 2T
V
PO
5
10 15 20 25 30 35 40 45
5
10 15 20 25 30 35 40 45
Time (ns)
Time (ns)
Figure 6. Log scale plot of time-resolved PL traces for [A] ZnSe/ZnS-405 QD and [B]
ZnSe/ZnS-425 QD with and without 2TVPO dye.

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(A) QD1-2TVPO
(B) QD2-2TVPO
1.56
1.44
1.32
1.20
1.08
2.0
1.8
1.6
1.4
1.2
1.0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Conc. of Quencher (M)
Conc. of Quencher (M)
1.30
1.25
1.20
1.15
1.10
1.05
1.00
(C) QD1-2TVPO
(D) QD2-2TVPO
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Conc. of Quencher (M)
Conc. of Quencher (M)
Figure 7. (A) & (B) are steady state Stern-Volmer plot of F0\ F vs. concentration of
quencher, (C) & (D) are time resolved Stern-Volmer plot of 0/ vs. concentration of
quencher for FRET pairs ZnSe/ZnS-405 (QD1)-2TVPO and ZnSe/ZnS-425 (QD2)-2TVPO.

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Table 1. Spectroscopic properties of 2TVPO in various solvents at 300 K.
[b]
[a]
[a]
εabs
λabs
λem
Δλ Φf
τf
(ns)
Solvent
(×101 M-1 cm-1)
(nm)
(nm) (nm) (%)
Methanol
Ethanol
Butanol
Pentanol
Hexanol
Heptanol
Octanol
Nonanol
Decanol
435
433
431
430
430
429
428
427
427
26100
26000
27300
27100
28900
29000
29500
29500
29600
501
496
494
491
490
488
487
486
486
66
63
63
61
60
59
59
59
59
22 0.536
31 0.640
34 0.725
32 0.759
36 0.787
41 0.811
41 0.836
40 0.849
43 0.858
DMSO
DMF
432
430
29900
28500
32000
31100
35700
490
488
482
481
477
58
58
53
53
52
48 0.672
47 0.649
53 0.717
51 0.688
62 0.878
Chloroform 429
DCM
428
425
Toluene
[a] The spectra were measured in selected organic solvents at 1µM concentration.
[b]The molar absorption coefficient was measured in solution.

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Table 2.
Photophysical properties of ZnSe/ZnS QDs.
Φf (%)
QD
Size (nm)
λexc(nm)
λemi (nm)
ZnSe/ZnS-405
ZnSe/ZnS-425
7
8
330
330
406
418
62
70
29