Donor-Pi-Acceptor Fluorene Conjugates, Based on Chalcone and Pyrimidine Derivatives: an Insight into Structure-Property Relationship, Photophysical and Electrochemical Properties

Article abstract of DOI:10.1007/s10895-020-02516-z

A small set of four new fluorenyl chromophores (5-5a-c) was accomplished by stepwise nucleophilic substitution, Friedel-Crafts acylation, Ullman coupling, aldol condensation and cyclization reactions. The fluorene moiety was substituted at 2,7,9 and 9′ positions with diverse groups. The synthesized derivatives were characterized by FTIR, ¹H-NMR and ¹³C-NMR spectroscopic techniques. The optical properties were evaluated by by UV-VIS absorption and Fluorescence studies. HOMO and LUMO energy levels were evaluated by electrochemical studies and were found at ?5.37-5.83 eV and ? 2.47-2.94 eV respectively with band gap energy values 2.88 to 2.91 eV. The band gap energy values suggested that these synthesized molecules can be manipulated in the designing of blue and green OLEDS. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10895-020-02516-z

Journal of Fluorescence
https://doi.org/10.1007/s10895-020-02516-z
ORIGINAL ARTICLE
Donor-Pi-Acceptor Fluorene Conjugates, Based on Chalcone
and Pyrimidine Derivatives: an Insight into Structure-Property
Relationship, Photophysical and Electrochemical Properties
1
1
1
1
2
1,3
Jamaluddin Mahar & Ghulam Shabir & Pervaiz Ali Channar & Aamer Saeed
Anwar Ul-Hamid
& Kevin D. Belfield & Madiha Irfan
&
4
Received: 16 January 2020 /Accepted: 14 February 2020
#
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A small set of four new fluorenyl chromophores (5-5a-c) was accomplished by stepwise nucleophilic substitution, Friedel-Crafts
acylation, Ullman coupling, aldol condensation and cyclization reactions. The fluorene moiety was substituted at 2,7,9 and 9′
1
13
positions with diverse groups. The synthesized derivatives were characterized by FTIR, H-NMR and C-NMR spectroscopic
techniques. The optical properties were evaluated by by UV-VIS absorption and Fluorescence studies. HOMO and LUMO
energy levels were evaluated by electrochemical studies and were found at −5.37-5.83 eVand − 2.47-2.94 eV respectively with
band gap energy values 2.88 to 2.91 eV. The band gap energy values suggested that these synthesized molecules can be
manipulated in the designing of blue and green OLEDS.
.
.
.
.
Keywords Fluorene conjugates OLEDS Optoelectronic properties HOMO and LUMO Band gap energy
Introduction
derivatives serves as guideline for the designing efficient light
emitting materials [3]. The ease of modification in fluorene
moiety paves the way for synthesis of new molecules having
distinct optical characteristics. The rigidly planar biphenyl unit
of fluorene provides the prospect for designing controlled
organic electronic devices [4]. The aromatic rings of fluorene
are electron rich and play key role in the enhancement of
Fluorene derivatives have observed unprecedented privilege in
the field of optoelectronics and organic electronics [1].
Conjugated fluorene small organic molecules play pivotal role
in designing of organic light emitting diodes (OLEDs) [2].
Investigation of structure-property relationship in fluorene
Jamaluddin Mahar and Ghulam Shabir both are first authors.
Research Highlights
•
Facile synthesis of Pyrimidine derivatives of Fluorene chromophore by
multistep strategy.
•
•
Large stokes shift values and high fluorescence quantum yields.
Chromophores exhibiting oxidation potential.
Electronic supplementary material The online version of this article
https://doi.org/10.1007/s10895-020-02516-z) contains supplementary
material, which is available to authorized users.
(
3
*
Aamer Saeed
Department of Basic Sciences and Humanities, Khwaja Fareed
aamersaeed@yahoo.com
University of Engineering and Information Technology (KFUEIT),
Rahim Yar Khan 64200, Pakistan
1
2
Department of Chemistry, Quaid-i-Azam University,
Islamabad 44000, Pakistan
4
Center of Engineering Research, KFUPM, Dhahran 31261,
Kingdom of Saudi Arabia
College of Science and Liberal Arts, New Jersey Institute of
Technology, University Heights, NJ, Newark, NJ 07102, USA

J Fluoresc
absorption properties and lowering the oxidation potential [5–7].
The 2, 7 and 9 positions of fluorene ring are activated and can be
readily functionalized to introduce desired motifs [8].
tetrabutylammonium bromide (TBAB), urea, thiourea, and
guanidine hydrochloride were purchased from Sigma-
Aldrich. Dimethylformamide (DMF) and Potassium hydrox-
ide (KOH), were commercial products from Daejing, Korea.
Solvents such as ethanol, ethyl acetate, and methanol were
common laboratory grade chemicals.
The judicious choice of heterocycles stimulate manipula-
tion of HOMO-LUMO levels and emission colors in organic
molecule seems to be attractive strategy [9, 10]. In the regard
pyrimidine derivatives have observed resurgence in the past
decade as efficient electron accepting chromophores.
Pyrimidine, a six membered highly pi-deficient heterocyclic
unit, has the ability to accept electrons and this feature makes
pyrimidine a suitable architect for designing of fluorene-based
materials with desirable push-pull properties [11, 12]. To ex-
plore the effect of electron donating groups linked with py-
Methods
All the chemicals used for analysis were of 99–100% purity.
1
HNMR spectra of all the compounds were conducted using a
3
00 MHz Bruker NMR spectrometer in CDCl solvent.
3
Splitting patterns were as follows: s (singlet), d (doublet),
dd, (double, doublet), t (triplet), m (multiplet) and br (broad).
Chemical shifts were represented in δ (ppm). The FTIR spec-
tra were taken in the single beam Nicolet FTR 100.
Ultraviolet-visible (UV-VIS) spectroscopic studies were done
by a double beam Cecil UV-VIS spectrophotometer 7400 se-
ries. Molar extinction coefficients of the compounds were
obtained from (UV-VIS) spectroscopic data. Purification of
the compounds was carried out by silica gel column chroma-
tography. Electrochemical study was made by
Electrochemical Analyzer CH1830C. Fluorescence quantum
yields were determined using reference fluorescein having
quantum yield 95% in water.
rimidine ring, we incorporated NH , SH and OH as electron
2
donor moieties (Fig. 1). Fluorene nucleus is locked with thio-
phene ring which greatly influences the optoelectronic prop-
erties because it assists in reducing the band gap and tunes the
colour in device [13–15]. Charge separation in these com-
pounds can be controlled by modifying the chemical structure,
i.e., extending the π-conjugation path or alternating the
strength of donating and accepting substances. In order to
undermine the effect of various electron donating and electron
withdrawing groups attached to fluorene nucleus, we have
carried out UV-visible studies, Fluorescence studies and elec-
trochemical studies.
General Procedure for the Synthesis of Fluorene Dyes
5-5a-c
Experimental
Materials
Synthesis of fluorene derivatives was accomplished through
five step procedure which is as follows:
Fluorene, iodine, hexylbromide, acetic acid, sulphuric acid,
tetrahydrofuran, thiophene-2-carbaldehyde, diphenylamine,
a) Preparation of monoiodoflourene (1)
Fig. 1 Design of donor-pi-
acceptor Fluorene-pyrimidine
conjugates

J Fluoresc
Scheme 1 Synthetic route for the
targeted Fluorene derivatives (5,
5
a-c)
Fluorene (4 g, 23.6 mmol) was dissolved in acetic acid
AcOH) at 40 °C in a 100 mL two neck round bottom flask.
In the reaction mixture sulphuric acid (H SO , 2.6 mL,
Dihexyliodo Fluorene (3.0 g, 6.3 mmol), Potassium hy-
droxide (KOH)(0.353 g, 6.3 mmol), and a catalytic amount
of tetrabutylammonium bromide (0.20 g, 0.62 mmol) were
added to a flask. The flask was degassed three times by ap-
plying freeze-thaw cycles. With the help of syringe n-Hexyl
bromide (8.32 g, 7.07 mL, 50.4 mmol) was added (degassed)
which was followed by the addition of DMSO. This mixture
(
2
4
4
.2 mmol) and iodine (2.52 g, 1.0 mmol) were added followed
by iodic acid (1.4 g, 0.8 mmol) aqueous solution (5 mL).
Continued heating at 70 °C for 1 h. On completion of the
reaction, the mixture after cooling to room temperature was
poured into ice cold water (250 ml), and the precipitates were
obtained which were separated and neutralized by washing
2.5
2.0
1.5
1.0
0.5
0.0
with 2% sodium bicarbonate solution (NaHCO ).
3
5
5
5
Precipitates were dried in vacuum desiccator. Purified com-
pound was obtained by recrystallization of crude product in
methanol.
a
b
5c
b) Preparation of 9,9’-dihexyl monoiodoflourene (2)
Table 1 Wavelength of maximum absorption λmax of D-π-A fluorene
dyes in DCM (5, 5a-5c)
Dye
λmax (nm)
5
430
428
428
426
5
5
5
a
b
c
250
300
350
Wavelength (nm)
Fig. 2 UV-Visible spectrum of fluorene Dyes 5, 5a-c
400
450
500
550

J Fluoresc
Table 3 Fluorescence values of D-π-A fluorene dyes in DCM (5, 5a-5c)
stirred continuously for 24 h. The product was purified by
silica gel column chromatography using a mixture of ethyl
acetate: hexane (1:4) to afford the product as a white solid.
Dye
λex (nm)
λem(nm)
Stokes Shift (nm)
ɸ
f
5
430
428
426
428
645
650
657
653
215
222
231
225
13.3
15.7
14.5
15.1
c) Synthesis of 2-Iodo-9,9-dihexyl-9H-fluoren-7-yl)-
5a
5b
ethanone (3).
5
c
To a solution of acetyl chloride (0.52 mL) in25mL of
CH Cl was added AlCl (1.02 g, 7.67 mmol) under inert
2
2
3
Results and Discussion
atmosphere at 0 °C. Followed by the addition of 2-iodo-9,9-
dihexyl-9H-fluorene(3.09 g, 6.72 mmol) dissolved in 10 mL
of CH Cl at the same temperature. Then was stirred at ambi-
The synthesis of the different fluorene derivatives was execut-
ed as shown in Scheme 1. Mono iodination of fluorene was
achieved by reacting it with stoichiometric amount of iodine
in the presence of iodic acid [16, 17], then alkylation with n-
hexyl bromide yielded compound 2 [18] which on Friedel-
Crafts acylation provided the acetylated product 1-(9,9-
dihexyl-7-iodo-9H-fluoren-2-yl)ethanone 3. Arylamination
of 3 was achieved by Buchwald-Hartwig amination coupling
method to obtain acetophenone derivative 4 [19, 20]. The
intermediate 4 on Claisen-Schmidt condensation with thio-
phene 2-carboxaldehyde furnished the chromophore (E)-
2
2
ent temperature for 20 h, the solution was treated with cold
water. The organic phase was dried over Na SO and concen-
trated by rotary evaporator. The product was purified by silica
gel column chromatography using a mixture of ethyl acetate:
hexane (1:4). A white solid was obtained.
2
4
d) Synthesis of 1-(2-(Diphenylamino)-9,9-dihexyl-9H-
fluoren-7-yl)ethanone (4).
In a reaction flask a mixture of diphenylamine (4.43 g,
1
3
-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)-
-(thiophen-2-yl)prop-2-en-1-one 5 as a yellow solid. The
2
6.20 mmol), 1-(2-iodo-9,9-dihexyl-9H-fluoren-7-
yl)ethanonecarbaldehyde (2.4 g, 6.98 mmol), P(tBu)3
0.08 g, 0.42 mmol), Cs CO (3.33 g, 10.22 mmol) and
synthesis of compounds 5a-c was accomplished by conden-
sation of compound 5 with urea, thiourea and guanidine in the
presence of potassium hydroxide in ethanol at room tempera-
ture. The purified products were obtained after re-
crystallization from ethanol.
(
2
3
Pd(OAc) (0.05 g, 0.24 mmol) in toluene (20 mL) was
refluxed for 36 h. After that it was cooled to ambient temper-
ature, the reaction mixture was filtered through a short plug
2
(
silica gel) and after was concentrated by rotary evaporated, a
brownish yellow oil obtained.
UV-Visible, FTIR and NMR Studies of Fluorene Dyes
5-5a-c
e) Synthesis of (E)-3-(2-(Diphenylamino)-9,9-dihexyl-9H-
fluoren-7-yl)-1-(thiophen-2-yl)prop-2-en-1-one (5).
The structures of newly-synthesized fluorene derivatives were
5-5a-c were investigated by UV-Visible, FTIR and NMR
spectroscopy. The UV-visible absorption studies of the
fluorene dyes were conducted in DCM solution (1 × 10 M)
and the selected spectral data are summarized in Table 1, Fig.
2.
The electronic transitions in dyes 5-5a-c in DCM provided
two absorption bands with maxima at 310-315 nm and 426-
430 nm respectively (Fig. 1). The λmax for all the compounds
at 426-430 nm is the result of π-π* transition of the com-
pounds due to the presence of C=C, which is the characteristic
of fluorine conjugated motif. From UV results, it is revealed
that all fluorene derivatives show approximately identical
λmax. This evidence indicates that cyclization at α,β unsatu-
rated position of D-π-A fluorene dyes does not affect much
the λmax of dyes [21].
Into the reaction mixture of 1-(2-(diphenylamino)-9,9-
dihexyl-9H-fluoren-7-yl)-ethanone (2 g, 3.68 mmol) was
added the solution of KOH (0.25 g, 4.5 mmol) in 20 mL
−5
MeOH: H O (5:1). After mixing, Thiophene-2-carbaldehyde
2
(
4
0.41 g, 3.68 mmol) was added to the solution and refluxed for
8 h. Precipitated product was filtered, washed with n-hexane
and dried in vacuum desiccator. Recrystallization was carried
in n-hexane and yellow solid was obtained. In this way 5a-c
fluorene derivatives were synthesized and characterization.
Table 2 Molar extinction coefficients of D-π-A fluorene dyes (5, 5a-5c)
1
−1
Dyes
λmax(nm)
A
ϵ
max(L mol− cm )
5
430
428
426
428
0.90101
0.99082
2.45808
1.62007
18,020.2
19,816.4
49,161.6
32,401.4
The FTIR spectra of fluorene chromophores provided ab-
sorption bands due to C=C-H, C-H, S-H, α, β-unsaturated
5
5
5
a
b
c
C=O, Aromatic C=C, C=N, CH , (C-N ), C-S, Ar-H. In case
2
str
of compound 5 characteristic IR absorption band at 1680/cm

J Fluoresc
Fig. 3 a Excitation spectrum of
fluorene dye 5, (b) Emission
spectrum of D-π-A fluorene dyes
a
b
800
700
600
1
000
280nm
3
3
3
3
3
4
4
4
00nm
20nm
40nm
60nm
80nm
00nm
20nm
30nm
5, 5a-c
8
6
4
2
00
00
00
00
0
5
5
00
00
5a
5b
5c
4
300
440nm
2
00
00
0
1
600
700
4
50
500
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
is due to α,β-unsaturated C=O moiety developed as a result of
reaction between intermediate 4 and thiophene-1-aldehyde.
Cyclization at α,β-unsaturated C=O position of compound 5
resulted in synthesis of different derivatives which were con-
firmed by disappearance of carbonyl band and appearance of
C=N band in range from 1490 to 1495/cm. The substitution
on pyrimidine ring “common in all derivatives” has been con-
firmed by the bands of different auxochromes like OH at
Optical Properties
Maximum Extinction Coefficients (ϵmax
)
The measurement of how strongly a chemical species absorbs
light at a particular wavelength depending upon the absence or
presence of certain functionalities in the molecule is known as
maximum extinction coefficient. It is fundamental property
which depends upon the actual the concentration (c), absor-
bance (A) and path length (l) of the species (Beer-Lambert
law) i.e., A = ϵmax cl. Molar extinction coefficient of fluorene
dyes is presented in Table 2.
3
2
490/cm, two bands of NH at 3350, 3310/cm and SH at
515/cm. All these stretching and bending absorption bands
2
identify the fluorene derivatives (5-5a-c).
1
The H NMR spectrum of compound 5 exhibited a triplet
peak due to CH group at 0.79 ppm, a multiplet due to meth-
Absorption in UV-visible region by 5-5a-c fluorene dyes is
higher for those dyes which have pyrimidine ring instead of
alpha- beta- unsaturated carbonyl. Dyes with SH
auxochromes showed higher value of molar extinction coeffi-
cients and high absorption intensity. It can be generalized that
fluorene containing pyrimidine derivative instead of α, β-
unsaturated ketone have higher molar extinction coefficient.
Compounds containing mercapto and amino group have large
influence in molar extinction coefficients than hydroxyl as it is
evidenced from high molar extinction coefficients of 5b and
5c as compared to 5a.
3
ylene envelope in the region of 1.39–1.25 ppm and a multi-
plet at 1.99–1.88 ppm due to methylene envelope at 9,9′-
position of fluorine (Figure S1). Similarly, in the aromatic
region signals due to different protons occupy the region
1
3
7
.94–7.05 ppm, common in all compounds. The C NMR
spectrum of compound 5 showed signals for thirty different
carbons which are at 182.1, 153.0, 151.4, 148.1, 147.8,
1
1
45.9, 145.0, 144.0, 135.0, 133.0, 132.7, 131.6, 129.3,
29.2, 129.1, 124.2, 123.2, 122.9, 122.8, 121.0, 120.1,
1
19.5, 118.8, 55.1, 40.2, 31.5, 29.6, 23.8, 22.6, 14.1 ppm
values. The carbonyl group is evidenced from its signal at
82.1 ppm while alkyl chains at 9,9′-positon of fluorine are
confirmed by the signals at 40.2, 31.5, 29.6, 23.8, 22.6 and
4.1 ppm and methine carbon of fluorine is at 55.11 ppm
Figure S2). The other derivatives of fluorene, 5a-c have
1
Fluorescence Examination
1
Fluorescence studies of dyes 5-5a-c were conducted to
(
observe their emission position and intensity.
1
13
−5
been identified from their respective H NMR and
NMR spectra.
C
Fluorescence data recorded for 1 × 10 M solution of
these dyes in DCM are summarized in Table 3.
Table 4 Singlet energies of D-π-A fluorene dyes (5, 5a-5c)
Table 5 Oscillator strengths of D-π-A fluorene dyes (5, 5a-5c)
−1
ϵ
max (L mol−1 cm ) Oscillator strengths (f)
−1
Dye
λmax(Å)
E
s
(kcal/mol)
Dye ΔV1/2 (cm )
5
4300
4280
4260
4280
66.51
66.82
67.14
66.82
5
5039.19
4946.59
4697.43
4810.49
18,020.2
0.392
0.423
0.998
0.673
5
5
5
a
b
c
5a
5b
5c
19,816.4
49,161.6
32,401.4

J Fluoresc
0.000020
Table 6 Theoretical radiative lifetime of D-π-A fluorene dyes (5, 5a-5c)
−1
max (L mol−1 cm−1)
max (cm−1)
0.000015
Dye
ΔV1/2(cm )
ϵ
V
Ƭ
o
(ns)
5
5
5
5
a
b
c
0
.000010
0.000005
.000000
5
5039.19
4946.59
4697.43
4810.49
18,020.2
19,816.4
49,161.6
32,401.4
23,255.81
23,364.48
23,474.17
23,364.48
7.13
6.54
2.75
4.11
5
5
5
a
b
c
0
-
0.000005
0.000010
-
Excitation wavelengths were found at 426–430 nm
Fig. 3a). The emission spectra of these dyes show only
(
-0.000015
0.000020
one emission band with maxima at 645-657 nm, after
excitation at different wavelengths (Fig. 3b). All dyes
displayed very similar emission spectrum, which showed
the minor effect of auxochrome on emission intensity
which is seen in small extension of emission wavelength
in dyes 5a-c bearing OH, SH and NH2 groups.
Fluorescence peak of high intensity at 645-657 nm for
diverse dyes corresponds to absorption peak in the range
of 426–430 nm [22, 23].
-
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Volt
Fig. 4 Combined voltammogram of D-π-A fluorene dyes (5, 5a-c)
Theoretical Radiative Lifetimes (Ƭo)
Molar extinction coefficient (ϵmax) is a fundamental entity for
the theoretical radiative lifetimes (Ƭ ) [26]. It is obtained from
o
the half-width of the selected absorption (ΔV ) and mean
1
/2
2
max
Singlet Energies (Es)
frequency (V
). Theoretical radiative lifetimes (Ƭ ) of py-
o
rimidine dyes is low and dependent on the auxochrome pres-
ent on the pyrimidine ring and varies from 2.75 to 7.13 (ns)
(Table 6). The compounds 5b and 5c, have smaller values of
Singlet energies were calculated by using the equation, E =
s
5
2
.86 × 10 / λ
[24] and observed data is summarized
max
Table 4.
radiative lifetime (Ƭ ) and larger ϵ
value. From the data, it
o
max
It is concluded from this study, that there is little bit change
in the singlet energies of fluorene dyes. Singlet energy is de-
pendent on wavelength of maximum absorption, which is al-
most constant. In order to obtain noticeable changes in wave-
length of maximum absorption there must be conjugation ex-
tension through alternate single and double bonds.
is observed that those fluorene derivatives which exhibit lower
values of radiative lifetime (Ƭ ), have larger absorptions inten-
o
sity in UV-visible region. Calculated theoretical radiative life-
times are given in Table 6.
Fluorescence Rate Constants (kf)
Oscillator strengths were calculated by the reported
−
9
procedure using the equation, f = 4.32 × 10 ΔV1/2 ϵmax
25]. The results are shown in Table 5. It is observed that
The fluorescence rate constants of dyes are represented in
[
Table 7. The k values for these dyes are ranging from 1.40
f
−1
oscillator strength varied from 0.392 to 0.998 and is
highest for 5b, having sulphur atom outside of pyrimidine
ring. In compound 5b, the molar extinction coefficient is
higher than other derivatives, therefore its oscillator
strength is high. This can be generalized that oscillator
strength is dependent upon the molar extinction coeffi-
cient (Table 5).
8
to 3.64 × 10 s and the dye 5b shows the highest value
owing to mercapto group present on pyrimidine ring. The
radiative life time of 5b is very low which suggests its striking
high value of fluorescence rate constant. This may also be
credited to large size of sulphur “due to this its polarization
factor” which increases the fluorescence rate constant and
decreases the radiative life time. Same pattern is also shown
Table 7 Fluorescence rate constant and theoretical fluorescence life
time of D-π-A fluorene dyes (5, 5a-5c)
Table 8 Oxidation potential Eox, LUMO, HOMO and band gap energy
of molecular orbital’s of D-π-A fluorene dyes (5, 5a-5c)
8
Dye
o
Ƭ (ns)
k
f
(10 /s)
ɸ
f
f
(ns)
Dye.
Eoxi (V)
LUMO /eV
Eg/eV
HOMO/eV
λ
(nm)
5
7.13
6.54
2.75
4.11
1.4
0.58
0.66
0.74
0.62
4.135
4.316
2.035
2.548
5
1.391
1.433
1.330
0.966
−2.9073
−2.9359
−2.8193
−2.4689
2.8837
2.8971
2.9107
2.8971
−5.791
−5.833
−5.366
−5.730
430
428
426
428
5
5
5
a
b
c
1.53
3.64
2.43
5a
5b
5c

J Fluoresc
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0
.1 M TBAPF6 as a supporting electrolyte (Fig. 4). All redox
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molecular orbital) and LUMO (lowest unoccupied molecular
orbital) were calculated using this technique.
Reduction potentials of reversible processes were calculat-
ed from Cyclic Voltammograms using published procedure
E1/2 = [EHOMO + ELUMO]/2 [27, 28]. Results of redox poten-
tials of dyes are shown in Table 8. Band gap energies of all
dyes are very close to each other and all dyes seems to be
equally proficient for OLEDs.
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and møller plesset (mp2) methods. Karbala Int J Modern Sci 3:18–
Conclusions
2
8
1
1. El Aslaoui Z, Karzazi Y (2017) Theoretical investigation of new
Synthesis of fluorene derivatives 5-5a-c has been achieved
through multistep procedure and their characterization has
been made well through spectroscopic techniques. All these
dyes are highly fluorescent having valuable quantum yields
and exhibit absorptions and emissions in the range 426–430
and 645–657 nm respectively. These chromogens displayed
the oxidation potential Eox at 0.96 to 1.43 eV with LUMO and
HOMO levels in the range − 2.41 to −4.93 and − 5.36 to
pyrimidines π-conjugated based materials for photovoltaic applica-
tions. J Mater Environ Sci 8:1291–1300
12. Cheng Y, Shabir G, Li X, Fang L, Xu L, Zhang H, Li E (2020)
Development of a deep-red fluorescent glucose-conjugated
bioprobe for in vivo tumor targeting. Chem Commun 56:1070–
1
073
13. Lim E, Jung BJ, Lee J, Shim HK, Lee JI, Yang YS, Do LM (2005)
Thin-film morphologies and solution-processable field-effect tran-
sistor behavior of a fluorene− thieno [3, 2-b] thiophene-based con-
jugated copolymer. Macromolecules 38:4531–4535
−5.83 eV, respectively.
1
4. Tang W, Ke L, Tan L, Lin T, Kietzke T, Chen ZK (2007)
Conjugated copolymers based on fluorene− thieno [3, 2-b] thio-
phene for light-emitting diodes and photovoltaic cells.
Macromolecules 40:6164–6171
Acknowledgements We are thankful to PAK-USAID for financial assis-
tance and also thankful to HEC Pakistan for funding.
1
1
5. Yang L, Feng JK, Ren AM (2005) Theoretical studies on the elec-
tronic and optical properties of two thiophene–fluorene based π-
conjugated copolymers. Polymer 46:10970–10981
6. Pasha MA, Myint YY (2004) Acid-catalysed selective
monoiodination of electron-rich arenes by alkali metal iodides.
Syn Commun 34:2829–2833
Compliance with Ethical Standards
Conflict of Interest Authors declare no conflict of interest.
1
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fluorene to 2, 7-diiodofluorene. Org Prep Proced Int 19:258–260
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JW (2004) Metal-ion sensing fluorophores with large two-photon
absorption cross sections: aza-crown ether substituted donor− ac-
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Red-emitting fluorenes as efficient emitting hosts for non-doped,
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