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

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

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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,

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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^-1at 2 cm^-1resolution with

1

13

100 scans. H and C NMR were recorded on Bruker 400-MHz FT NMR spectrometer in

CDCl₃by 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 54.7^owas 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

(1)

A



i

in which A_iis the relative amplitude corresponding to the i^thdecay component having the

characteristic decay time constant τ_i. 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 (λ_abs=389, λ_em=457 nm, Φ_R=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 OD_Rn²

I_ROD n_R²

(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

³J_transCoupling 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, J_trans= 16.0 Hz, 2H,

trans-CH), 7.06 (s, 2H, Thiophene-CH), 6.97 (d, J_trans= 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

N N

NH₂

POCl_3,Reflux, 12 h

N

HO

O

H

OXD

NBS, DBP

CCl₄, 80 ^o

C

N

N N

TPP, DMF - 80 ^o

C

Br^-P

O

Br

O

1

2

50% aqu. NaOH

MDC/H₂O

H

S

O

N

S

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 λ_em

around 485 nm indicating that fluorescence occurs only from the S₁state 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 λ_absand λ_emvalues 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 ×10⁵M⁻¹cm⁻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., µ_e>µ_g). 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., µ_e> µ_g). 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 (T_g) 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 T_5dand T_gas 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 N₂with heating rate of 10^oC/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 (T_g) at 254.4 ^oC. It is interesting to note that, the T_gof 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 (T_g=60^oC), 1,3-

bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (T_g=77^oC) and also than the nonbipolar

host emitters that include oxadiazoles (BOBP, T_g=45^oC; TPOs and PPOs, T_gsup to 121^oC)

[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 (T_5d) 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 R₀is 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 k_T(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

R₀obtained 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 k_T(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 λ_exc= 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) + A₂exp(−t/τ₂), where τ₁and τ₂represent the fast and slow decay times,

respectively and A₁and A₂are 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:

F_D

_D

_DA

(3)

1 K_D[Q] 1 k_q_D[Q] 

F_DA

where F_Dand F_DAare the PL intensity of donor (QDs) in the absence and presence of

quencher (2TVPO), K_Dis the SV quenching constant, k_qis the rate constant of the

bimolecular process, [Q], the quencher concentration while τ_Dand τ_DAare 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 F_D/F_DAand τ_D/τ_DAversus [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 (K_D) obtained from the slopes of these

graphs are 6.02×10⁴M^–1, 11.15 × 10⁴M^–1and 3.20×10⁴M^–1, 7.31 × 10⁴M^–1for QD1 and QD2

to 2TVPO in steadystate and time resolved measurements, respectively. The estimated

quenching rate constant (k_q) reflects on the efficiency of quenching or the accessibility of the

probes to the quencher. The k_qvalues thus determined are 8.9×10¹², 16.1×10¹²M^–1S^–1and

4.7×10¹², 10.5×10¹²M^–1S^–1for these FRET pairs in steady state and time resolved

ACCEPTED MANUSCRIPT

measurements, respectively. Further, the diffusion controlled quenching usually results in

values of k_q~10¹⁰M^–1S^–1as expected from the Smoluchowski eqn. (k_diff=8RT/3η) [61].

Values of k_qsmaller 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 k_q

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

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

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.

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A

B

a

ZnSe/Zns 405-2TVPO

ZnSe/Zns 425-2TVPO

2x10⁵

6x10⁵

5x10⁵

3x10⁵

Q

D 1

5

Q

2x10

D 2

h

1x10⁵

2 T

h

V P O

h

2

5x10⁴

2x10⁵

0

T

V

P

O

a

0

400

450

500

550

600

400

450

500

550

600

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 (λ_exc=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

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)

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.

ACCEPTED MANUSCRIPT

(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)

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)

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.

ACCEPTED MANUSCRIPT

Table 1. Spectroscopic properties of 2TVPO in various solvents at 300 K.

[b]

[a]

ε_abs

λ_abs

λ_em

Δλ Φ_f

τ_f

(ns)

Solvent

(×10¹M^-1cm^-1)

(nm)

(nm) (nm) (%)

Methanol

Ethanol

Butanol

Pentanol

Hexanol

Heptanol

Octanol

Nonanol

Decanol

435

433

431

430

429

428

427

26100

26000

27300

27100

28900

29000

29500

29600

501

496

494

491

490

488

487

486

66

63

61

60

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

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.

ACCEPTED MANUSCRIPT

Table 2.

Photophysical properties of ZnSe/ZnS QDs.

Φ_f(%)

QD

Size (nm)

λ_exc(nm)

λ_emi(nm)

ZnSe/ZnS-405

ZnSe/ZnS-425

7

8

330

406

418

62

70

29

Article Doi

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

DOI: 10.1016/j.molliq.2017.10.066

Source and publish data:

Authors:

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

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

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