An investigation of the effect of conjugation on fluorene based chromophores; Optoelectronic and electrochemical behavior

Article abstract of DOI:10.1016/j.dyepig.2017.08.019

The requisition of small light emitting organic molecules is on the rise in the field of organic electronics. This study is aimed at synthesis of fluorene based chromophores which possess promising optoelectronic properties. The characterization of synthesized derivatives was carried out by spectroscopy (¹H-NMR and ¹³C-NMR) and spectrometry (EI-MS). The optical properties evaluated by using UV-Vis spectroscopy, were from 394 to 420 nm, Fluorescence ranged 48 nm-113 nm, HOMO energy levels are ?6.75 eV to ?7.15 eV LUMO -4.08 to ?4.52eV with a band gap energy values 2.63–2.67 eV. The band gap energy values suggest that these synthesized molecules can be manipulated in the designing of blue and green OLEDS.

Full text of DOI:10.1016/j.dyepig.2017.08.019

Accepted Manuscript
An investigation of the effect of conjugation on fluorene based chromophores;
optoelectronic and electrochemical behavior
Jamaluddin Mahar, Aamer Saeed, Kevin D. Belfield, Anila Iqbal, Madiha Irfan,
Ghulam Shabbir, Fayaz Ali Larik, Pervaiz Ali Channar
PII:
S0143-7208(17)31286-X
10.1016/j.dyepig.2017.08.019
DYPI 6181
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 4 June 2017
Revised Date: 12 August 2017
Accepted Date: 12 August 2017
Please cite this article as: Mahar J, Saeed A, Belfield KD, Iqbal A, Irfan M, Shabbir G, Larik FA, Channar
PA, An investigation of the effect of conjugation on fluorene based chromophores; optoelectronic and
electrochemical behavior, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.08.019.
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ACCEPTED MANUSCRIPT
An Investigation of the effect of conjugation on fluorene based chromophores;
Optoelectronic and electrochemical behavior
Jamaluddin Mahara,b, Aamer Saeeda*, Kevin. D. Belfieldb, Anila Iqbala,c, Madiha Irfana,d,
Ghulam Shabbira, Fayaz Ali Larika, Pervaiz Ali Channara
aDepartment of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan, 45320
bDepartment of Chemistry School of Optics/Center for Research and Education in Optics and
Lasers University of Central Florida, P.O. Box 162366 Orlando, Florida 32816
cNano Science and Catalysis Division, National Center for Physics, Quaid-i-Azam University
Campus, Islamabad
dDepartment of Basic Sciences and Humanties, Khwaja Fareed University Engineering and
Information Technology, Rahim Yar Khan. (KFUEIT, RYK).
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Graphical Abstract
An Investigation of the effect of conjugation on fluorene based chromophores; Optoelectronic
and electrochemical behavior
Jamaluddin Mahara,b, Aamer Saeeda*, Kevin. D. Belfieldb, Anila Iqbala,c, Madiha Irfana,d,
Ghulam Shabbira, Fayaz Ali Larika, Pervaiz Ali Channara
The band gap energy values for newly synthesized fluorine derivatives suggest that these
molecules can be employed in the design of blue and green OLEDS.
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Abstract
The requisition of small light emitting organic molecules is on the rise in the field of organic
electronics. This study is aimed at synthesis of fluorene based chromophores which possess
promising optoelectronic properties. The characterization of synthesized derivatives was carried
out by spectroscopy (1H-NMR and 13C-NMR) and spectrometry (EI-MS). The optical properties
evaluated by using UV-Vis spectroscopy, were from 394-420 nm, Fluorescence ranged 48 nm-
113 nm, HOMO energy levels are -6.75 eV to -7.15 eV LUMO -4.08 to -4.52eV with a band gap
energy values 2.63 to 2.67 eV. The band gap energy values suggest that these synthesized
molecules can be manipulated in the designing of blue and green OLEDS.
Keywords: fluorene based chromophores; Stoke’s shift; optical properties; electrochemical
behavior; band gap;
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1. Introduction
The wide band gap for blue OLEDS (organic light emitting diodes) is the major cause behind
their low commercial success rate [1-3]. The quest to develop and explore efficient small organic
molecules for the display devices such as OLEDS is in the continuous process of development
since last two decades. To achieve high external quantum efficiencies (EQE) is considered as a
daunting task in case of deep-blue OLEDS [4-6]. Materials with strong push-pull aptitude along
with efficient hole transporting ability can certainly balance the charges in emitting layers and
consequently can improve the performance of OLEDS. However, to achieve materials with high
band energy value and blue emission is still a work in progress.
Fluorene is blessed with enriched organic electronic properties such as high thermal and
photophysical stabilities along with high emission quantum yields [7]. Fluorene represents the
class of polycyclic aromatic hydrocarbon (PAH) and serve as an essential structural unit in the
field of organic electronics [8-10]. Fluorene apart from extended utility in organic electronics
also exhibits applications in the synthesis of peptides, pharmaceuticals and dyes. Fluorene
generally possess three reactive positions 2, 7 and 9 [11,12]. The derivatization at 2 and 7
positions allows adjusting electronic groups. Fluorene serve as π spacer who paves the way for
fine tuning of electronic groups at 2 and 7 positions [13]. Herein, we explore the structure-
property relationship (SPR) of electron deficient and electron rich groups attached at 2 and 7
position of fluorine. We have synthesized three A-π−A molecules and one A- π−D compound to
evaluate their photophysical properties and electrochemical behavior. In the course of current
study, fluorine moiety was locked with electron accepting motif of benzo[d]thiazole (Figure 1).
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Figure 1. Design of A-π-A and A- π-D fluorene based molecules for applicability in blue
OLEDS
1,3-diketone linked with electron withdrawing group CF3 (6) served as a platform to carry out
synthetic transformations to obtain various group bearing fluorine derivatives (9, 10 and 11). The
SPR revealed that incorporating the 1,3-diketone played the role of accepting electrons. While in
case of compound 9 which possessed extended conjugation having para N,N-dimethylstyryl
moiety as an electron donating group. Compound 10 and 11 bear electron deficient moiety which
also enhanced the extended conjugation in the system. The structure-property relationship played
pivotal role in determining the photo physical and electrochemical behavior. The results of
fluorescence, UV-Vis and cyclic voltammetry showed variation in nature of photo physical and
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electrochemical properties. The band gap energy values were found to lie in the region of blue-
green OLEDS.
2. Experimental
2.1. Materials
Fluorene, ethyl bromide, iodine, tetrabutyl ammonium bromide (TBAB) were purchased from
Fluka. Aluminum chloride, acetyl chloride, 2-(tributylstannyl)benzo[d]thiazole were obtained
from Sigma-Aldrich. Tetrakis(triphenylphosphine)palladium(0), potassium tertiary butoxide, 4-
(dimethylamino)benzaldehyde, hydrazine hydrate, pyrene-1-carbaldehyde, piperidine,
Acetonitrile (AcCN), dichloromethane (DCM), ethyl 2,2,2-trifluoroacetate (CF3COOC2H5) were
purchased from Sigma-Aldrich. Purification of the reagents was carried wherever necessary.
2.2. Methods
1
All the chemicals used for analysis were of 99–100% purity. H-NMR spectra of all the
compounds were conducted on a 300 MHz Bruker NMR spectrometer in DMSO solvent.
Splitting patterns were as follows: s (singlet), d (doublet), dd, (double doublet), t (triplet), m
(multiplet) and br (broad). Chemical shifts were represented in d (ppm). The FTIR spectra were
taken in the single beam Nicolet FT-IR 100. Ultraviolet-visible (UV-vis) spectra were recorded
on a double beam Perkin-Elmer Lambda 900 UV-vis-NIR spectrophotometer. The data were
used to calculate the molar extinction coefficients for the compounds. TLC plate Silica Gel 60
F254 was utilized to test the progress of fluorene chromophore. Mobile phase for TLC was n-
hexane: ethyl acetate 5:2 ratio. Electrochemical analysis was made by Electrochemical Analyzer
CH1830C. Fluorescence quantum yields were determined using reference fluorescein having
quantum yield 94% in DCM.
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Preparation of monoiodofluorene (1)
Fluorene (5.0 g., 0.30 mole) was treated with iodic acid. The mixture was
poured into water, and the product was filtered off, washed with water, and
dried (8.1 g.). A portion (1.02 g.) was chromatographed on silica gel in cyclohexane, giving
almost colourless 2-iodofluorene (0.73 g., 65%), m.p. 127-128 [14].
1-(6-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)-4,4,4-trifluorobutane-1,3-dione (06)
Synthesis of 1-(6-(benzo[d]thiazol-2-yl)-9,9-diethyl-
9H-fluoren-2-yl)-4,4,4-trifluorobutane-1,3-dione
was
achieved by condensation of 1-(6-(benzo[d]thiazol-2-
yl)-9,9-diethyl-9H-fluoren-2-yl)ethanone (3.97 g, 10 mmol), with ethyl 2,2,2-trifluoroacetate
(CF3COOC2H5)(2.84 g, 0.02 mol) and potassium tertiary butoxide (t-C4H9OK )(1.46 g, 12
mmol) for 6 h in toluene. The mixture was neutralized by diluted hydrochloric acid and extracted
with toluene. The solvent was evaporated under vacuum and pure product was obtained by
recrystallization from ethanol.
Dark red solid; Yield: 73%; Rf: 0.49; m.p.165-167oC; FTIR (Neat, cm-1) max: 3102(C=C-H),
2956(C-Hstr), 2837 (C-Hstr), 1727 (CF3-C=O) 1698 (Ar-C=O), 1615, 1573, 1501 (Aromatic,
C=Cstr), 1452 (CH2bend), 1372 (CH3), 1628 (C=N),941 (C-S), 1246 (C-F).
1H-NMR (300 MHz, CDCl3) δ 8.24-8.03 (m, 4H), 7.92 (dd, J = 7.5, 6.9 Hz, 1H), 7.59-7.40 (m,
5H), 3.71 (s, 2H), 1.89 (q, J = 6.7 Hz, 4H), 1.00 (t, J = 6.7 Hz, 6H). 13C-NMR (75 MHz, CDCl3)
δ 194.8, 182.0, 162.6, 153.9, 152.5, 150.4, 142.7, 139.7, 139.2, 138.2, 136.3, 126.9, 126.0, 125.6,
125.5, 125.3, 123.7, 123.7, 122.7, 122.4, 121.8, 121.0, 55.0, ,32.2, 7.9; MS (m/z, APCI): calcd
493.13(M+), found 494.13[M+H]+.
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General procedure for the Knoevenagel condensation of 1-(6-(benzo[d]thiazol-2-yl)-9,9-
diethyl-9H-fluoren-2-yl)-4,4,4-trifluorobutane-1,3-dione with aromatic aldehydes
Knoevenagel condensation [15] were achieved by using equivalent ratio of 1-(6-(benzo[d]thiazol
-2-yl)-9,9-diethyl-9H-fluoren-2-yl)-4,4,4-trifluorobutane-1,3-dione and aromatic aldehydes in
EtOH (25 mL). Piperidine (0.5 mL) was added slowly through a syringe with stirring, the
reaction mixture was refluxed for 72 h. Purification of the products was obtained through a silica
gel column using hexanes/ethyl acetate (5/2) as eluent.
(Z)-1-(7-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)-2-(4-(dimethylamino)
benzylidene)-4,4,4-trifluorobutane-1,3-dione (09)
Reddish solid; Yield: 73%; Rf: 0.45; m.p.171-172oC;
FTIR (Neat, cm-1) max: 3095(C=C-H), 2930(C-Hstr),
2851(C-Hstr), 1714(CF3-C=O), 1687(Ar-C=O), 1600,
1567, (Aromatic, C=Cstr), 1448(CH2bend), 1378(CH3),
1624(C=N), 1325(C-N), 939(C-S), 1237(C-F). 1H
NMR (300 MHz, CDCl3) δ 8.87 (s, 1H), 8.79-7.74 (m, 8H), 7.81 (ddd, J = 7.5, 3.8, 1.4 Hz, 2H),
7.81 (ddd, J = 7.5, 3.8, 1.4 Hz, 2H), 7.58-7.33 (m, 3H), 7.33 (s, 1H), 6.72 (d, J = 7.5 Hz, 2H),
2.91 (s, 6H), 1.89 (q, J = 6.7 Hz, 4H), 1.01 (t, J = 6.7 Hz, 6H). 13C NMR (75 MHz, CDCl3δ
192.2, 175.8, 175.6, 162.6, 153.9, 152.5, 151.0, 150.4, 142.7, 139.2, 138.9, 138.5, 136.3, 131.4,
128.5, 126.9, 126.5, 126.0, 125.5, 125.3, 123.7, 123.3, 122.7, 121.8, 121.1, 120.9, 118.0, 112.3,
54.4, 41.9, 32.2, 7.9; MS (m/z, APCI): calcd 624.21 (M+), found 626.21 [M+2H]+.
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(Z)-1-(7-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)-4,4,4-trifluoro-2-(pyren-1-
methylene)butane-1,3-dione (10)
Reddish solid; Yield: 73%; Rf: 0.49; m.p.194-196oC;
O
N
O
FTIR (Neat, cm-1) max: 3090 (C=C-H), 2938 (C-Hstr),
S
C2H5
C2H5
CF3
2856 (C-Hstr), 1717 (CF3-C=O) 1689 (Ar-C=O), 1601,
1565, (Aromatic, C=Cstr), 1453 (CH2bend), 1375 (CH3),
1
1624 (C=N), 943 (C-S), 1237 (C-F); H-NMR (300
MHz, CDCl3): δ 9.26 (s, 1H), 8.34 (dd, J = 7.5, 1.4 Hz, 1H), 8.19-8.07 (m, 3H), 8.05-7.74 (m,
13
13H), 7.63-7.43 (m, 2H), 1.89 (q, J = 6.7 Hz, 4H), 0.98 (t, J = 6.7 Hz, 6H); C NMR (75 MHz,
CDCl3): δ 192.2, 175.6, 162.6, 161.7, 153.9, 152.5, 150.4, 142.7, 139.2, 138.9, 138.5, 136.3,
131.8, 130.9, 130.3, 129.5, 129.0, 128.3, 128.2, 127.7, 127.2, 126.9, 126.5, 126.0, 125.7, 125.5,
125.3, 124.7, 123.8, 123.7, 123.3, 122.7, 121.8, 120.9, 118.0, 54.4, 32.2, 7.9; MS (m/z, APCI):
calcd 705.19 (M+), found 706.30[M+H]+.
2-(9,9-diethyl-7-(5-(trifluoromethyl)-1H-pyrazol-3-yl)-9H-fluoren-2-yl)benzo[d]thiazole (11)
1-(6-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-2-
yl)-4,4,4-trifluorobutane-1,3-dione (0.31g, 0.62 mmol)
dropwise added into a flask containing hydrazine
hydrate (0.62 mmol) in EtOH (5 mL) catalytic amount of piperidine was added. The reaction
proceeded at room temperature with continuous stirring for 6h, the solvent was diluted with 100
mL of ice cold distilled water and neutralized with dilute HCl at neutral pH crude product was
precipitated. After recrystallization with the ethanol purified product was obtained. Light red
solid; Yield: 84%; Rf: 0.65; m.p.141-142oC; FTIR (Neat, cm-1) max: 3095(C=C-H), 2942(C-
Hstr), 2851 (C-Hstr), 1670(C=N), 1620, 1570, (Aromatic, C=Cstr), 1465 (CH2bend), 1378 (CH3),
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1
942 (C-S), 1245 (C-F), 1112 (N-N); H NMR (300 MHz, CDCl3); δ 14.14 (s, 1H), 8.19 – 8.04
(m, 6H), 7.89 (d, J = 7.8 Hz, 1H), 7.59-7.36 (m, 3H), 2.15 (q, J = 7.2 Hz, 4H), 0.28 (t, J = 7.2 Hz,
6H); 13C NMR (75 MHz, CDCl3); δ 168.1, 154.1, 151.6, 151.0, 144.8, 144.0, 142.3, 141.0,
134.9, 132.5, 128.2, 127.6, 127.2, 126.0, 125.2, 124.0, 123.2, 122.8, 121.8, 121.6, 121.6, 120.7,
120.5, 116.5, 101.8, 56.6, 32.2, 8.9; MS (m/z, APCI): calcd 489.15 (M+), found 490.01 [M+H]+.
3. Results and Discussions
3.1 Synthesis
Synthesis of the compound 06 (Figure 2) started with commercially available fluorene and
iodine. Selective iodination of fluorene yields 2-Iodofluorene [14], alkylation of this 2-
iodofluorene was carried out with bromoethane by using KOH as base, TBAB as phase transfer
catalyst in DMSO at room temperature to give 2-Iodo-9,9-diethylfluorene This compound was
subjected to Friedel-Crafts acylation reaction with acetyl chloride, which yields the ketone, 1-
(9,9-diethyl-7-iodo-9H-fluoren-2-yl)ethanone after a simple recrystallization. 1-(9,9-diethyl-7-
iodo-9H-fluoren-2-yl)ethanone and 2-(tributylstannyl)benzo[d]thiazole were subjected for C-C
coupling by using renowned Stille coupling reaction to furnish 1-(6-(benzo[d]thiazol-2-yl)-9,9-
diethyl-9H- fluoren-2-yl)ethanone [15].
The 1-(6-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)ethanone was treated with ethyl
2,2,2-trifluoroacetate by Claisen condensation method to yield 1-(6-(benzo[d]thiazol-2-yl)-9,9-
diethyl-9H-fluoren-2-yl)-4,4,4-trifluorobutane-1,3-dione 06 [16]. Knoevenagel Condensation of
06 with 4-(dimethylamino)benzaldehyde using piperidine as base in acetonitrile afford excellent
yield of (Z)-1-(7-(benzo[d]thiazol-2-yl)-9,9-diethyl-9H-fluoren-2-yl)-2-(4-(dimethylamino)
benzylidene)-4,4,4-trifluoro butane-1,3-dione 09 [17,18]. By following same method when
compound 06 was treated with pyrene-1-carbaldehyde offered (Z)-1-(7-(benzo[d]thiazol-2-yl)-
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9,9-diethyl-9H-fluoren-2-yl)-4,4,4-trifluoro-2-(pyren-1-methylene)butane-1,3-dione 10 in good
yield. When hydrazine hydrate was treated with compound 6 the cyclization of diketone occurred
and product 2-(9,9-diethyl-7-(5-(trifluoromethyl)-1H-pyrazol-3-yl)-9H-fluoren-2-yl)benzo[d]
thiazole 11 was obtained in good yield. (Scheme 1)
C2H5Br, KOH
AlCl3, CH3COCl
I
I
Bu4NBr, DMSO
DCM, 20h
C2H5
C2H5
1
C2H5
2
C2H5
N
O
O
N
I
Pd(PPh3)4, Toluene
Ar, 72 h, reflux
SnBu3
S
S
C2H5
C2H5
4
5
3
O
N
Tert-BuOK, CF3CO2C2H5
Toluene
O
S
C2H5
C2H5
CF3
6
Piperidine,
CH3CN
O
N
O
Piperidine,
CH3CN
7
N
N
NH
S
C2H5
C2H5
CF3
O
8
N
O
11
S
O
C2H5
C2H5
N
CF3
O
S
C2H5
C2H5
CF3
9
N
10
Figure 2. Synthesis of Fluorene based various π acceptor dyes (06, 09-11)
3.2 UV-Visible, FTIR and NMR studies of fluorene A-π-A dyes (06, 09-11)
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The characterization of newly synthesized Fluorene derivatives was achieved by UV-Visible,
FTIR and NMR spectroscopy. The UV-visible absorption spectra of these dyes were recorded in
dichloromethane at room temperature using (1×10−7 M) solution (Figure 3). The selected
spectral data are tabulated below in Table 1.
06
11
10
09
2.0
1.5
1.0
0.5
0.0
400
600
Wavelength (nm)
Figure 3. UV-Visible spectra of fluorene A-π-A chromophores (6, 9-11)
Table 1. Wavelength of maximum absorption λmax of Fluorene A-π-A dyes in dichloromethane
(6, 9-11)
Dye
06
λmax (nm)
362, 394
393, 420
391, 420
09
10
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11
361, 395
The absorption studies of fluorene acceptor-π-acceptor (A-π-A) dyes were investigated in
dichloromethane solution. The solution displayed two band pattern absorption maxima (λmax),
first at 361-393 nm and the other at 394-420 nm, respectively. The high absorption λmax for all
the compounds at 397-420 nm are attributed to π-π* transition of the compounds. This transition
is the characteristic of the C=C, of fluorene conjugated architecture. Absorption spectroscopic
results of these derivatives showed that Knoevenagel reaction at the active methylene effect the
absorption maxima and also the wavelength. The cyclisation of 1,3-diketone did not affect in a
pronounced manner on the absorption.
The FTIR spectra of fluorene A-π-A dyes afforded absorption bands due to C=C-H, C-H, S-H,
Aromatic C=C, α,β-unsaturated C=O, C=N, (C-Nstr), CH2, C-S, C-F, Ar-H. Compound 11
showed the characteristic IR absorption band at νmax 1497-1501 cm-1 is due to C=N moiety. This
compound was synthesized by the reaction of compound 06 with hydrazine hydrate, cyclisation
with hydrazine was confirmed by the disappearance of both carbonyl peaks 1701-1738 cm-1
from the synthesized compound. Along this appearance of C=N band in range, from 1497-1501
cm-1. The peak obtained at 1246, 1237, 1052 and 1050 cm-1 showed the existence of C-F
stretching. The FTIR data of stretching and bending absorption bands recognize the Fluorene A-
π-A dyes (06, 09-11). The 1H-NMR spectrum of compound 11 exhibited a high field triplet peak
due to CH3 group at 0.28 ppm, a triplet due to methylene proton at 2.15. The protons of the
aromatic region of are observed from 8.13-7.36 ppm, and singlet at 14.0 ppm due to N-H of
pyrazole ring. 13C-NMR spectra of compound 11 there are signals for twenty eight different
carbon peak at 56.6 is due to the alkylation at 9-position of fluorene, methylene carbon absorbed
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at 32.2 and methyl carbon at 8.9 ppm. The disappearance of carbonyl group peaks from 194.8
and 182.0 and the appearance of the peaks at 168.1, 154.1 are confirming the cyclization of the
compound and evidence for the confirmation of dye 11. While the peaks at 56.6 ppm is for
methine carbon of fluorene and other two peaks at δ 32.2 ppm and 8.9 ppm, confirmed the
alkylation at 9,9’-position of fluorene. The other synthesized compounds of fluorene A-π-A dyes
have been characterized by using the same techniques.
3.3 Optical properties
3.3.1 Maximum extinction coefficients (ϵmax
)
It is an essential property which depends upon the path length (l) absorbance (A) and
concentration (c), of the species (Beer-Lambert law) i.e., A= ϵmax Cl. Molar extinction coefficient
of fluorene A-π-A dyes are presented in Table 2
Table 2 Molar extinction coefficients of fluorene A-π-A dyes (06, 09-11)
Dyes
06
λmax(nm) A
ϵmax(L mol-1 cm-1)
136800.0
394
420
420
395
0.68400
09
0.616240
1.490332
0.868987
123248.0
298066.4
10
173797.4
11
Absorption spectra of these dyes showed that compound 09 and 11 showed almost same
absorption pattern and absorption maxima but there is an increase in the molar extinction
coefficient of 11 which indicated that after socialization there is little difference in conjugation
but a huge increase in stability. Whereas the compound 09 showed bathochromic shift and
hyperchromic shift in UV-visible absorption of the compound, it revealed that the introduction of
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4-(dimethylamino)benzaldehyde at the active methylene of compound 06 had effected the band I
and increases the molar extinction coefficient, due to this enhancement it would be more
beneficial than parent compound 06. Following the same procedure the compound 06 was
coupled with pyrene-1-carboxaldehyude this compound showed same bathochromic shift as
shown by 09 but a drastic enhancement in molar extinction coefficient observed this indicated
that coupling at the active methylene of compound 06 is more beneficial for the optoelectronic
properties of the compound and therefore compound 06 and 10 had shown a high effect in this
regard.
3.3 Fluorescence examination
Fluorescence Spectra of fluorene A-π-A dyes (06, 09-11) in dichloromethane solution were
recorded. These dyes 06, 09, 10 and 11 emit maximum at λmax 422, 527, 533 and 443 nm
respectively. Compounds 09 and 10 showed higher emission value due to coupling of conjugated
aldehyde at active methylene and in case of 10 it also showed a broad range which indicated that
the pyrene group at the active methylene had great influence in electronic transition and bear the
more importance in organic electronics. While compound 06 and 11 showed almost similar
behavior which indicated that the cyclization at 1,3-dicarbonyl carbons did not affect the
electronic transformation in good extent, fluorescence and Stoke’s shift are compared to
literature value which reveals that these compounds are the best candidates for the blue OLEDS
[19] Figure 4 and Table 3.
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30000
25000
20000
15000
10000
5000
0
09
06
10
11
450
500
550
600
650
700
750
800
wavelength (nm)
Figure 4 Fluorescence spectra of fluorene A-π-A dyes in dichloromethane (06, 09-11)
Table 3 Fluorescence values of fluorene A-π-A dyes in dichloromethane (06, 09-11)
Excitation
Emission
Emission
Stoke Shift
(nm)
049
Dye
wavelength (nm) wavelength (nm) Height
394
420
420
395
422
527
533
443
25391.3
14755.9
17393.8
27239.2
06
09
10
11
107
113
048
Singlet Energies (Es)
Singlet energies were calculated by using the same procedure reported above. The data is
represented in Table 4
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Table 4 Singlet energies of fluorene A-π-A dyes in dichloromethane (06, 09-11)
Dye
06
λmax(Å)
3940
Es (kcal/mol)
72.58
4200
68.09
09
4200
68.09
10
3950
72.40
11
The calculated singlet energy of these compounds obtained from the calculations showed that
there is no change in between compound 06 and 11 for compound 09 and 10 both displayed the
same maximum absorption therefore there singlet energy is same. From these results it was
noticed that the change occur in the absorption wavelength when the coupling occurs at active
methylene center of the precursor and cyclization didn’t showed this effect, but it was also noted
that as the compound have two bands pattern, it reflected that after the coupling at active center it
increase the absorption wavelength in some extent in 09 and 10. But there was a promising
change in molar absorptivity, as singlet energy is related to maximum absorption, so therefore
compound 09 and 10 had same singlet energy. From these results it is observed that the enhanced
delocalization couldn’t effect in large extent.
3.4 Oscillator strengths of fluorene A-π-A dyes in dichloromethane (06, 09-11)
Already documented method was used for the calculation of oscillator strengths. The obtained
results of oscillator strength of these dyes are tabulated below in table 5. The range is in between
0.304 to 1.174, compound 06 which contain two carbonyl moieties in the structure showed the
minimum oscillator strength substitution at active methylene, it increased the oscillator strength
of the molecule with increase in conjugation at active methylene center and an increased in
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oscillator strength, were observed. The increase in oscillator strength was also observed when the
cyclisation was carried out with hydrazine at carbonyl moieties. From this it can be suggested
that the increase in conjugation by coupling at active nuclei or cyclization will increase the
oscillator strength. Compound 10 showed the highest value of oscillator strength, which could be
due to strong π-π* transition, Table 5.
Table 5 Oscillator strengths of fluorene A-π-A dyes in dichloromethane (06, 09-11)
Dye
06
∆V1/2 (cm-1)
515.40
ϵmax (L mol-1 cm-1) Oscillator strengths f
136800.0
123248.0
298066.4
173797.4
0.304
0.364
1.174
0.387
683.65
09
911.70
10
515.40
11
3.5 Theoretical Radiative Lifetimes (Ƭo)
The theoretical radiative life time of these dyes was from 2.75 to 7.13 (ns). Compound 10 has
higher ϵmax value and smaller values of the radiative lifetime (Ƭo). Results of spectroscopic data
showed that those compounds having larger absorptions in UV-visible region exhibited radiative
lifetime (Ƭo) value lower. Calculated theoretical radiative lifetimes are listed in table 6.
Table 6 Theoretical radiative lifet νmax of fluorene A-π-A dyes (06, 09-11)
Dyes
06
∆V1/2(cm-1)
515.4
ϵmax (L mol-1 cm-1)
136800.0
νmax (cm-1)
25380.71
23890.52
23890.52
Ƭo(ns)
7.71
683.65
123248.0
7.28
09
911.70
298066.4
2.26
10
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11
515.4
173797.4
25319.46
6.09
3.6 Fluorescence Rate constant (kf) and theoretical fluorescence lifetime (Ʈf)
The fluorescence rate constants of these dyes were in range from varied from 1.30 x108 s-1 to
4.42 x108 s-1 and theoretical fluorescence lifetime obtained was 0.972 to 4.872. Compound 10
displayed highest fluorescence rate constant value. This compound was obtained by the coupling
of pyrene-1-carboxaldehyde with compound 06 at the active methylene whereas the radiative life
time of the same compound is very low. High fluorescence rate constant of 10 is due to large size
of pyrene ring and extended conjugation due to which increase in the fluorescence rate constant
and decrease in radiative lifetime is observed. The compound 11 also showed the same effect,
but in little extent because here conjugation increased due to cyclization, but this conjugation is
not prominent therefore small effect observed are documented in Table 7.
Table 7 Fluorescence rate constant and theoretical fluorescence lifetime of fluorene A-π-A dyes
(06, 09-11)
Dye
06
Ƭo(ns)
7.71
7.28
2.26
6.09
kf (108/s )
1.30
ɸf
Ʈf(ns)
4.857
4.368
0.972
4.872
0.63
0.60
0.43
0.80
1.37
09
4.42
10
1.64
11
3.7 By conventional method the florescence life time of the dyes also confirmed by time domain
using,
Steady-state and Time-resolved Photoluminescence
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Steady-state and time-resolved photoluminescence (PL) of the samples were carried out by
using standard instrument, Flau Time 300 (FT-300) steady state and life-time spectrometer,
PicoQuant GmbH, Germany. The PL was measured following pulsed LED laser excitation
source, PLS-300, centered at 305 nm with full width half maximum (FWHM) of ~416 ps and
pulse energy 0.077 pJ. (Figure 5)
1.0
09
10
11
06
0.8
0.6
0.4
0.2
0.0
-1
0
1
2
3
4
5
6
Time(ns)
Figure 5
Steady-State florescence life time of fluorene A-π-A dyes
3.8 Thermal properties fluorene A-π-A dyes
These derivatives possess good thermal stability with thermal decomposition temperature
around ˃220°C, determined by TGA under nitrogen measured from 50-750°C at a heating rate of
20 °C/min. . (Figure 6). The results indicate that compound 10 has a highest thermal degradation
temperature 282oC while compound 9 has second highest value 267oC, while compound 6
degraded at 222oC and compound 11 at 233oC. These results revealed that the substitution at the
active methylene enhanced the stability of the compounds this is due to increase in conjugation
of molecule and rigidity due to substitution with bulky group.
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Compound 06
Compound 09
Compound 10
Compound 11
. (Figure 6) Thermal analysis of fluorene A-π-A dyes
3.7 Electrochemical Properties
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3.7.1 Electrochemical studies fluorene A-π-A dyes (06, 09-11)
The electrochemical study of all compounds was carried by using cyclic voltammetry in
dichloromethane solution in the presence of 0.1 M TBAPF6 as a supporting electrolyte. From
their cyclic volatmogram band gap energy, HOMO and LUMO energies were obtained.
Combined cyclic voltamogram of these dyes are displayed in Figure 7, table 2.16.
06
10
11
09
0.000030
0.000025
0.000020
0.000015
0.000010
0.000005
0.000000
-0.000005
-0.000010
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Voltage(V)
Figure 7 Cyclic Voltammogram of fluorene A-π-A dyes (06, 09-11) in dichloromethane
Table 8 Half wave potential (E1/2) and LUMO energy levels of fluorene A-π-A dyes (06, 09-11)
Compounds
E1/2 (V)
-0.712
-0.426
-0.277
LUMO (eV)
-4.08
HOMO (eV)
-6.75
Eg (V)
2.67
06
09
10
-4.37
-7.01
2.64
-4.52
-7.15
2.63
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11
-0.642
-4.15
-6.81
2.66
3.8 Band Gap Energy Values (Eg)
The span of energies between valence and conduction bands for semiconductors and insulators is
known as band gap energy. Every material has its characteristic band gap energy. The formation
of various materials is the results of this variation in band energy. For these dyes band gap
energies were calculated by using the standard procedure [20]. The band gap energy of
compounds is given in table 8. Compound 11 showed little difference in the band gap energy
value compared to the compound 06, which indicating that cyclization at 1,3-diketone position
affect the band gap value but this effect was not significant. Compound 09 has exhibited
comparatively higher value than 11 and slightly lower than 10. These results infer that the
coupling in active methylene center of compound 06 with extended conjugation either by bulky
conjugated group or small group containing electron donating auxochrome will reduce the band
gap energy.
3.9 Highest occupied molecular orbitals (HOMO)
Table 8 shows the highest occupied molecular orbital energy levels, varies from -4.08 to -4.52
eV for these dyes. The value varies with the conjugation with an increase in conjugation as
results showed that compound 10 has greater conjugation compared to all other compounds
therefore it displayed highest value of HOMO energy levels. It was concluded that HOMO
energy levels can be altered only by increasing the delocalization of electrons through
conjugation. It is due to the fact that as conjugation increases the energy difference will decrease
and vice versa.
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4. Conclusions
In summary, we have synthesized fluorene based chromophores (06, 09-11) by using a simple
synthetic protocol. The synthesized compounds were subjected to photo physical studies such as;
fluorescence, UV-visible spectroscopy and cyclic voltammetry. Compound 10 showed maximum
absorption at 420nm with higher stoke shift value 113nm. Conducting behavior of the compound
10 was observed with highest occupied molecular orbital value with -7.15eV and lowest
unoccupied molecular orbital value with -4.52eV having band gap energy value of 2.63eV. From
the results of photo-electrochemical behavior we infer that compound 10 serve as a structural
template for the designing of blue and green OLEDS.
Acknowledgement
We are thankful to PAK-USAID for financial assistance and also thankful to HEC Pakistan for
funding.
Conflict of interest
Authors declare no any conflict of interest
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Highlights
∑ Synthesis of some novel fluorene based chromophores with promising optoelectronic
properties was carried
∑ The UV-Vis absorption and fluorescence were in the range 394-420nm and 48nm-
113nm, respectively.
∑ The band gap energy values 2.63 to 2.67 eV suggest the use of these compounds in blue
and green OLEDS.