Blue-red emitting 2,12-disubstituted [5]helicenes for high fluorescence efficiency and sensing application

Article abstract of DOI:10.1016/j.jphotochem.2021.113203

We report photochemical synthesis of two new fluorene and carbazole-based 2,12-disubstituted [5]helicenes namely FLU-HEL and CBZ-HEL respectively. Both FLU-HEL and CBZ-HEL exhibit very intense fluorescence property in solution as well as solid state. The fluorescence quantum yield (Φ_f) = 27 % and 30 % and life time ?τ_f?=9.7 ns and 4.2 ns were observe for both [5]helicenes, respectively. Also, both the [5]helicene dyes have shown very good sensing properties towards Fe³⁺ transition metal ion than other transition metal ions. We have also explored the feasibility of these compounds for security applications.

Full text of DOI:10.1016/j.jphotochem.2021.113203

Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113203
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Blue-red emitting 2,12-disubstituted [5]helicenes for high fluorescence
efficiency and sensing application
a,
b
a
a,b,
B. Yadagiri , Prem Jyoti Singh Rana , Surya Prakash Singh
*
a
Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, 500007, India
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
We report photochemical synthesis of two new fluorene and carbazole-based 2,12-disubstituted [5]helicenes
namely FLU-HEL and CBZ-HEL respectively. Both FLU-HEL and CBZ-HEL exhibit very intense fluorescence
Helicene
Fluorophores
property in solution as well as solid state. The fluorescence quantum yield (Φ ) = 27 % and 30 % and life time
f
Dual application
Sensing application
Security application
〈τ 〉=9.7 ns and 4.2 ns were observe for both [5]helicenes, respectively. Also, both the [5]helicene dyes have
f
3
+
shown very good sensing properties towards Fe transition metal ion than other transition metal ions. We have
also explored the feasibility of these compounds for security applications.
1
. Introduction
Organic molecules with extended delocalized
Recently, Chuan-Feng Chen et al. reported a new tetrahydro[5]hel-
icene based imides dye having intense fluorescence and large Stokes
π-electron systems
shifts in both solution and solid state (Fig.1; A and B) [10]. Kenji Mat-
suda and co-workers demonstrated the fluorescence properties of 5,
10-disubstituted [5]helicene derivative (Fig. 1; C) in solutions state
and its self-assembly behaviours [9]. Further, Julian M. W. Chan et al.,
reported sultam-based hetero[5]helicene for a typical blue shifting of its
solid state emission relative to the solution phase (Fig. 1; D).
have been extensively used for fluorescence and non-linear applications.
Generally high fluorescence phenomenon occurs by the electron
donating and electron withdrawing groups [1]. Even though a number
of organic dyes show intense fluorescence in solution state, only a few
reports are available on full-colour emission organic dyes both in solu-
tion and solid state [2]. This is due to the quenching of the fluorescence
arising from the presence of intermolecular interaction and aggregation
of molecular assembly [3].
The fluorescence phenomenon also useful for the sensitive detection
of micro levels of analysing agents such as small molecules, anions and
cations. A fluorescence sensor consists of two parts, ionophore and flu-
orophore. The ionophore has binding ability towards analyte and fluo-
rophore emits the fluorescence signal. Recently, [5]helicene based
organic dyes gained good attention as a fluorescence sensors in biolog-
ical and other applications, because of its chiral structure and more
To improve the fluorescence behaviour and resolve above mentioned
demerits of organic dyes, several approaches were adopted i.e J-aggre-
gate formation [4], enhanced intramolecular charge transfer transition
[
5], and aggregation induced emission for the development of new small
molecular organic dyes [6]. In yet another approach, the full-colour
emission spectra can be achieved by systematic design of a molecule
via incorporation of the various chromophores at different positions to
the molecular backbone of organic dyes [7].
π-electron cloud on the molecular backbone [11–13]. The development
of fluorescent probes that were used for the sensing of different haz-
ardous chemicals and elements in living cells is important, because the
abnormal production of hazardous chemicals and elements can damage
cellular biomolecules, which was closely connected with numerous
diseases [21–23]. Previously some research groups reported applica-
tions of [5]helicenes for sensing metal ions and discuss about their
binding phenomenon [16]. Nantanit Wanichacheva et al. developed a
Helically ortho-fused polyaromatic hydrocarbons are generally
referred as helicenes [8]. The recent reports show that [5]helicene based
organic materials display intense fluorescence behaviour in both solu-
tion and solid state. In Fig. 1 we have shown some [5]helicenes and its
analogues reported earlier and investigated by us in the present work
[5]helicene derivative for water-soluble Fe3 fluorescence sensor with a
+
[
9].
large Stokes shift (192 nm) [13]. This helicene based sensor was used to
*
Corresponding author at: Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, 500007,
India.
E-mail address: spsingh@iict.res.in (S.P. Singh).
https://doi.org/10.1016/j.jphotochem.2021.113203
Received 4 November 2020; Received in revised form 23 January 2021; Accepted 6 February 2021
Available online 12 February 2021
1
010-6030/© 2021 Elsevier B.V. All rights reserved.

Y. B. et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113203
identify Fe3 in human cancer cells. In this work, we have developed [5]
+
spectrometer at 25 ⁰C unless otherwise noted. H and 13C NMR spectra
are referenced to SiMe using the residual solvent peak impurity of the
solvent CDCl and corresponding data described as follows: chemical
shift in parts per million (PPM) measured from tetramethylsilane (TMS)
referenced and CDCl
1
helicene based sensors for Fe3 ion sensing property and security
+
4
applications.
3
Herein, we describe the design and synthesis by the photo-
cyclodehydrogenation reaction of two novel symmetric 2,12-disubsti-
tuted [5]helicenes namely 9,12-bis(9,9-dioctyl-9H-fluorene-2 yl)
dibenzo[c,g]phenanthrene-1,6-dicarbonitrile (FLU-HEL) and 9,12-bis
3
(δ = 7.26 ppm) as a residual isotopomer solvent,
13
multiplicity, integration. C NMR spectra analysis also well studied by
using 400 MHz and corresponding data described as follows: chemical
(
(
9-octyl-9H-carbazol-3-yl)dibenzo[c,g]phenanthrene-1,6-dicarbonitrile
CBZ-HEL) (Fig. 1). These materials have an ability to show good
shift in PPM from TMS reference and CDCl
3
(δ = 77.16 ppm). Mass
details of synthesized molecules were obtained by high resolution mass
spectra such as electrospray ionization (ESI), MALDI-TOF mass and high
resolution mass spectrometry (HRMS).
quantum yield (Φ = 27 % and 30 %) and excellent sensing and security
f
applications. These symmetric [5]helicene based organic dyes were
substituted with fluorene (FLU-HEL) and carbazole (CBZ-HEL) at 2,12
positions of the [5]helicene skeleton along with substitution of two
cyano (-CN) groups at selected positions, respectively. In addition,
aliphatic alkyl chains have also been appended to increase the solubility
of helicene dyes in all common organic solvents, which is beneficial to
study the different applications in different solvents. With this study, we
analysed that the substitution on 2,12 positions at [5]helicene core
moiety with fluorene and carbazole chomophores can boost the fluo-
rescence performance of [5]helicene at monomeric state.
2.2. Synthesis
Synthetic pathways of FLU-HEL and CBZ-HEL are represented in
Scheme 1. Synthetic procedure for both [5]helicene dyes is nearly
similar. The compounds 1 and 2 were subjected to Miyauraborylation
reaction to give compound 3 and 4, followed by compounds 3 and 4
were subjected to Suzuki-Miyaura cross-coupling reaction to obtain
compound 5 and 6. Further, the compound 5 and 6 was reacted with
terepthaldehyde in the presence of ethanol via Knoevenagel condensa-
tion reaction to get compound 7 and 8. Finally, the target compound 9
and 10 was prepared via photocyclodehydrogenation reaction by using
2
. Results and discussion
2
.1. Experimental section
2
compound 7, 8 and iodine (I ) in dry toluene under irradiation of
General Methods: All reagents from commercial sources were used
450 nm high-pressure mercury lamp in very good yield [15].
without further purification unless otherwise noted. Tetrahydrofuran
THF) and 1, 4-dioxane were purchased from Alfa Aeser, dried over
sodium/benzophenone, distilled under nitrogen and stored over 4 Å
molecular sieves. Chloroform (CHCl ) and dichloroethane (DCE), were
dried over calcium hydride (CaH ), distilled under nitrogen and stored
2-Bromo-9,9-dioctyl-9H-fluorene (1): A dry flask (100 mL) was
charged with 2-bromofluorene (3.0 g, 0.012 mol), KOH (4.12 g,
0.073 mol) and DMSO (30 mL) under nitrogen gas atmosphere. After
stirred for 20 min, 1-bromoctane (7.09 g, 0.036 mol) was slowly added
and stirred at 110 ⁰C for 24 h. After completion of the reaction, the re-
action mixture was diluted with ice-water/ water (more than two times)
and extracted with EtOAc by separatory funnel. The combined organic
layer was washed with brine and dried over anhydrous sodium sulphate
(
3
2
over molecular sieves (4 Å). N,N-dimethylformamide (DMF) was dried
and purified using standard techniques. Reactions were carried out
under a nitrogen atmosphere; Glass TLC plate Silica gel 60G F254 from
Merck Millipore was used to monitor the reaction. Compounds were
2 4
(Na SO ) and then the organic solvents were completely removed by
1
13
detected by UV irradiation, staining with I
2
and/or KMnO
4
. H and
C
rotary evaporation. The residue was purified by column chromatog-
1
nuclear magnetic resonance (NMR) spectroscopy spectra were recorded
on a Bruker Avance-300 MHz, Avance-400 MHz and Avance-500 MHz
raphy using petroleum as eluent, to afford product 1 yield (5 g, 75 %). H
NMR (400.1 MHz, CDCl
3
): δ 7.67ꢀ 7.63 (m, 1 H), 7.54 (d, 1 H),
7
.45ꢀ 7.42 (m, 2 H), 7.32ꢀ 7.28 (m, 3 H), 1.98ꢀ 1.86 (m, 4 H),
Fig. 1. Previous and present work of [5]helicenes.
2

Y. B. et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113203
Scheme 1. Synthetic route for the preparation of FLU-HEL and CBZ-HEL [5]helicene dyes.
1
.32ꢀ 1.16 (m, 6 H), 1.16ꢀ 0.98 (m, 18 H), 0.82 (t, 6 H). 13C (75.4 MHz,
138.9, 128.6, 128.3, 127.9, 127.2, 126.8, 125.9, 122.9, 121.4, 120.0,
CDCl
3
): δ 152.9, 150.5, 140.1, 140.0, 129.8, 127.4, 126.9, 126.1, 122.9,
119.8, 117.9, 55.2, 40.4, 31.8, 30.0, 29.2, 23.8, 23.3, 22.6, 14.1. HRMS
+
′
1
21.0, 119.7, 55.4, 40.3, 31.9, 29.9, 29.2, 23.7, 22.6, 14.1. MALDI-TOF-
(m/z): [M+H] calcd for C37
H47N: 505.3704; found 505.3701.
+
+
′
MS m/z [M+H] calcd for C29
H
41Br [M+H] m/z 468.54; found 468.11.
(2E,2 E)-3,3 -(1,4-Phenylene)bis(2-(4-(9,9-dioctyl-9H-fluoren-2 yl)
phenyl)acrylonitrile) (7): In a 100 mL of round bottom flask was charged
with terephthalaldehyde (100 mg, 0.74 mmol), compound-5 (1.50 g,
0.003 mol) and ethanol (10 mL), after formation of homogeneous solu-
tion sodium ethoxide (152 mg, 2.23 mmol) was added. The reaction
2
-(9,9-Dioctyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane (3): To a dry flask (100 mL) was charged with compound 1 (4.65 g,
.009 mol), Bis(pinacalato)diboron (3.27 g, 0.011 mol), and KOAc
5.82 g, 0.054 mol) in 1,4-dioxane (50 mL), Pd(dppf)Cl (10 %) was
added. The reaction mixture was stirred under nitrogen atmosphere at
00 ⁰C for 24 h. After cooling to room temperature, the solvent was
0
(
2
◦
were stirred at 50 C for 4 h, after the completion of reaction the crude
1
material was filtered using Buchner funnel and washed with cold
ethanol. The crude solid was washed with brine water and dried over
removed under reduced pressure. The residue was purified by column
chromatography using silica gel with petroleum ether/ethyl acetate
2 4
Na SO . The organic solvent was evaporated using reduced pressure and
(
(
2:3) to give 3 as colour less liquid yield (3.5 g, 69 %). 1H NMR
purified by column chromatography using petroleum ether/DCM (2:3)
to afford the product-7 (500 mg, 60 % yield). 1H NMR (500.1 MHz,
500.1 MHz, CDCl
3
): δ 7.82 (d, J =7.8 Hz, 1 H), 7.76ꢀ 7.65 (m, 3 H),
7
4
1
8
.37ꢀ 7.27 (m, 3 H), 2.02ꢀ 1.90 (m, 4 H), 1.38 (s, 12 H), 1.34ꢀ 1.14 (m,
CDCl
7.63ꢀ 7.59 (m, 3 H), 7.38ꢀ 7.32 (m, 3 H), 2.04–2.18 (m, 4 H), 1.33ꢀ 0.99
(m, 24 H), 0.80 (t, 6 H). 13C NMR (125.7 MHz, CDCl
): δ 151.6, 151.0,
3
): δ 8.05 (s, 2 H), 7.83ꢀ 7.80 (m, 2 H), 7.80ꢀ 7.73 (m, 4 H),
1
3
H), 1.14ꢀ 0.85 (m, 16 H), 0.80 (s, 6 H). C (75.4 MHz, CDCl
3
): δ 151.3,
49.8, 144.1, 140.8, 133.7, 128.8, 127.4, 126.6, 122.9, 120.1, 118.9,
3
3.7, 55.1, 40.25, 31.8, 30.0, 29.2, 24.9, 23.7, 22.6, 14.1. MALDI-TOF-
142.9, 141.1, 140.5, 140.0, 138.5, 135.5, 132.8, 129.8, 127.8, 127.2,
+
[M+H]+ m/z 516.61; found
MS m/z [M+H]
16.32.
calcd for C35H53BO
2
126.8, 126.5, 125.9, 122.9, 121.3, 120.1, 119.9, 117.8, 112.6, 55.2,
+
5
40.4, 31.8, 30.0, 29.2, 23.8, 22.6, 14.1. HRMS (m/z): [M+H] calcd for
2
-(4-(9,9-Dioctyl-9H-fluoren-2-yl)phenyl)acetonitrile (5):
A
dry
82 96 2
C H N : 1108.7574; found 1108.7571.
flask (50 mL) was charged with compound-3 (2.94 g, 0.005 mol), 4-bro-
mophenylacetonitrile (1.40 g, 0.006 mol), DME (30 mL) and 2 M sodium
carbonate (1.06 g) in inert nitrogen condition. De-gassed the reaction
mixture up to 20 min in nitrogen atmosphere followed by addition of Pd
9,12-Bis(9,9-dioctyl-9H-fluoren-2-yl)dibenzo[c,g]phenanthrene-1,6-
dicarbonitrile (9): To a dry toluene solution (100 mL) of compound 7
(100 mg, 0.09 mmol), iodine (92 mg, 0.36 mmol) was added. The solu-
tion was irradiated using 400 nm UV-lamp for 48 h. After completion of
reaction the crude product was washed with sodium thiosulfate, brine
(
3 4
PPh ) (10 %) catalyst and reflux the reaction mixture for 12 h at 70 ⁰C.
After completion of reaction, the reaction mixture was workup using
EtOAc. The combined organic layer was washed with brine and dried
2 4
solution and dried over Na SO . The organic solution was evaporated
under reduced pressure. The residue was purified by column chroma-
tography using petroleum/DCM (3:2) to afford the final product-9 as a
pale green colour solid (50 mg, 35 % yield). 1H NMR (500.1 MHz,
2 4
over Na SO and organic solvents were completely removed by rotary
evaporation. The residue was purified by column chromatography using
petroleum and EtOAc (3:2) are elutes, to afford product-5 yield (1.5 g 81
CDCl
3
): δ 8.95 (s, 2 H), 8.60 (d, J =8.5 Hz, 2 H), 8.38 (s, 2 H), 8.26 (d, J
1
%
). H NMR (500.1 MHz, CDCl
3
): δ 7.75 (d, J =7.7 Hz, 1 H), 7.72 (d, J
=9.5 Hz, 2 H), 8.0 (s, 2 H), 7.63ꢀ 7.52 (m, 6 H), 7.24ꢀ 7.21 (m, 8 H),
1.79ꢀ 1.66 (m, 8 H), 1.33ꢀ 1.20 (m, 7 H), 1.11ꢀ 1.02 (m, 9 H),
=
6.5 Hz, 1 H), 7.65 (d, J =8.2 Hz, 2 H), 7.54 (d, J =7.6 Hz, 1 H), 7.52 (s,
1
3
1
H), 7.41 (d, J =8.3 Hz, 2 H), 7.35ꢀ 7.29 (m, 3 H), 3.84 (s, 2 H),
0.99ꢀ 0.78 (m, 32 H), 0.75 (t, 6 H), 0.67 (t, 6 H). C NMR (125.7 MHz,
2
6
.07ꢀ 1.90 (m, 4 H), 1.27ꢀ 1.12 (m, 6 H), 1.12ꢀ 0.99 (m, 16 H), 0.80 (t,
CDCl
3
): δ 151.8, 150.9, 141.4, 140.1, 139.9, 138.0, 133.6, 132.1, 130.9,
1
3
3
H). C NMR (125.7 MHz, CDCl ): δ 151.5, 151.0, 141.6, 140.8, 140.6,
129.2, 129.1, 127.8, 127.7, 127.3, 126.7, 126.4, 126.1, 122.8, 121.4,
3

Y. B. et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113203
1
2
20.1, 119.8, 117.3, 110.8, 55.0, 40.1, 40.0, 31.8, 31.6, 30.1, 29.7, 29.3,
yield (100 mg, 60 % yield). 1H NMR (500.1 MHz, CDCl
): δ 8.36 (s,
3
9.0, 28.9, 23.9, 23.5, 22.6, 22.5, 14.1, 13.9. HRMS (m/z):
2 H), 8.17 (d, J =7.5 Hz, 2 H), 8.04 (s, 3 H), 8.01ꢀ 7.99 (m, 1 H), 7.82 (s,
6 H), 7.77ꢀ 7.74 (m, 3 H), 7.62ꢀ 7.58 (m, 3 H), 7.53ꢀ 7.48 (m, 5 H),
7.45ꢀ 7.42 (m, 2 H), 7.26 (m, 1 H), 4.34 (t, 4 H), 1.93ꢀ 1.88 (m, 4 H),
+
[
M+H] calcd for C82
92 2
H N : 1104.7261; found: 1104.7269.
3
-Bromo-9-octyl-9H-carbazole (2): To a dry flask (50 mL) was
charged with 3-bromocarbazole (1.0 g, 0.004 mol), K CO (2.25 g,
.016 mol) and DMF (30 mL) under nitrogen gas atmosphere. After
1
3
2
3
1.43ꢀ 1.31 (m, 8 H), 1.25ꢀ 1.23 (m, 12 H), 0.87 (t, 6 H). C (75.4 MHz,
0
3
CDCl ): δ 143.5, 141.0, 140.9, 140.8, 140.3, 139.5, 135.4, 134.9, 132.8,
stirring for 10 min, 1-bromoctane (2.35 g, 0.012 mol) was slowly added
and refluxed at 110 ⁰C for 24 h. After completion of the reaction, the
DMF solvent was evaporated by rota evaporator and extracted with
EtOAc. The combined organic layer was washed with brine and dried
129.8, 129.8, 127.7, 127.5, 126.5, 126.0, 124.9, 123.4, 122.8, 120.4,
119.1, 118.8, 109.1, 108.9, 43.3, 31.8, 29.7, 29.4, 29.2, 27.3, 22.6, 14.0.
+
HRMS (m/z): [M+H ] calcd for C64
62 4
H N : 886.4974; found 886.4971.
9,12-Bis(9-octyl-9H-carbazol-3-yl)dibenzo[c,g]phenanthrene-1,6-
dicarbonitrile (10): To a toluene solution (120 mL) of compound 8
(100 mg, 0.10 mmol), iodine (115 mg, 0.40 mmol) was added. The so-
lution was irradiated by 400 nm UV-lamps for 64 h, after completion of
reaction mixture. The crude material was washed with sodium thiosul-
over Na
petroleum as eluent, to afford product 2 yield (850 mg, 75 %). H NMR
400.1 MHz, CDCl
2 4
SO . The residue was purified by column chromatography using
1
(
3
): δ 8.17 (s, 1 H), 8.01 (d, J =7.82 Hz, 1 H), 7.51 (m,
H), 7.49 (m, 1 H), 7.43 (m, 1 H), 7.34 (d, J =8.31 Hz, 1 H), 7.20 (m,
H), 4.19 (t, 2 H), 1.83ꢀ 1.78 (m, 2 H), 1.35ꢀ 1.27 (m, 4 H), 1.25ꢀ 1.15
): δ 140.7, 139.0, 128.2,
1
1
2 4
fate and brine solution dried over Na SO . The organic solvent was
(
m, 6 H), 0.85 (t, 3 H). 13C (75.4 MHz, CDCl
3
evaporated under reduced pressure. The residue was purified by column
chromatography using petroleum/DCM (3:2) to afford the final product-
1
3
26.3, 124.5, 123.0, 121.8, 120.5, 119.2, 111.5, 110.0, 108.9, 43.4,
+
1
1.8, 29.4, 29.2, 28.9, 27.322.6, 14.1. MALDI-TOF-MS m/z [M+H ]
10 as an orange colour solid yield (50 mg, 35 %). H NMR (500.1 MHz,
+
calcd for C20
H
24BrN [M+H] m/z 358.11; found 358.83.
CDCl
3
): δ 8.37 (s, 2 H), 8.18 (d, J =7.5 Hz, 2 H), 8.05 (s, 4 H), 7.83 (m,
9
-Octyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carba-
zole (4): A dry flask (50 mL) was charged with compound 2 (950 mg,
.65 mmol), Bis(pinacalato)diboron (876 mg, 3.44 mmol) and KOAc
1.60 g, 0.015 mol) in dry 1,4-dioxane (40 mL), after de-gassed for
0 min Pd(dppf)Cl (10 %) was added. The reaction mixture was stirred
6 H), 7.76 (d, J =8.4 Hz, 2 H), 7.63 (s, 2 H), 7.50 (d, J =8.3 Hz, 2 H), 7.44
(d, J =8.1 Hz, 2 H), 4.34 (t, 4 H), 1.94ꢀ 1.87 (m, 4 H), 1.48ꢀ 1.34 (m,
1
3
2
8 H), 1.28ꢀ 1.23 (m, 12 H), 0.86 (t, 6 H). C (75.4 MHz, CDCl
3
): δ 143.4,
(
140.9, 140.2, 139.7, 139.3, 135.5, 132.3, 132.1, 130.7, 129.7, 127.7,
2
2
126.5, 126.0, 124.9, 123.4, 122.9, 120.5, 119.1, 118.8, 117.9, 114.1,
under nitrogen atmosphere at 100 ⁰C for 24 h. After cooling to room
temperature, the solvent was removed under reduced pressure. The
residue was purified by column chromatography on silica gel with pe-
troleum ether/ethyl acetate (2:3) to afford the product 4 as colour less
112.6, 109.1, 108.9, 43.2, 31.9, 31.8, 29.4, 29.0, 27.3, 22.6, 14.1. HRMS
+
(m/z): [M+H] calcd for C64
58 4
H N : 883.4692; found 883.4690.
Absorption spectra of FLU-HEL and CBZ-HEL (Fig. 2a) [5]helicene
dyes were studied in dichloromethane solution with concentration of
0.02 mol/L, both dyes displayed two peaks in higher energy region (287,
319 nm and 288, 356 nm respectively) and one peak in the lower energy
region (431 and 415 nm respectively). The broad absorption at the lower
energy is associated with the intramolecular charge transitions between
the strong electron acceptor cyano (-CN) groups and strong electron
donor fluorene and carbazole moiety. The parent compounds FLU-TPD
and CBZ-TPD show similar absorption pattern as the final dyes (see
Scheme 1, Fig. S1a in the Supporting Information (SI)). FLU-HEL and
CBZ-HEL dyes shows 20 nm red shift from their precursor FLU-TPD and
CBZ-TPD compounds due to increase in the electronic conjugation
within the molecule. Further we have extended our study to thin film.
Both the [5]helicene dyes show ~5ꢀ 20 nm red shift compared to the
absorption in solution (Fig. S1b), which might be attributed to strong
liquid yield (780 mg, 69 %). 1H NMR (500.1 MHz, CDCl
): δ 8.60 (s,
3
1
1
1
6
1
2
H), 8.13 (d, J =7.7 Hz, 1 H), 7.92 (d, J =9.4 Hz, 1 H), 7.50ꢀ 7.43 (m,
H), 7.40 (d, J =8.9 Hz, 2 H), 7.20–7.22 (m, 1 H), 4.29 (m, 2 H),
.89ꢀ 1.82 (m, 2 H), 1.40 (s, 12 H), 1.35ꢀ 1.27 (m, 4 H), 1.25ꢀ 1.16 (m,
H), 0.85 (t, 3 H). 13C (75.4 MHz, CDCl
): δ 142.6, 139.7, 132.2, 127.7,
3
25.6, 123.1, 122.6, 120.6, 119.2, 108.7, 108.1, 83.6, 43.1, 31.8, 29.4,
+
9.1, 28.9, 27.3, 24.9, 22.6, 14.1. MALDI-TOF-MS m/z [M+H ] calcd
+
for C26
H36BNO
2
[M+H] m/z 405.38; found 405.10.
2
-(4-(9-Octyl-9H-carbazol-3-yl) phenyl)acetonitrile (6): To a dry
flask (50 mL) compound 4 (780 mg, 1.92 mmol), 4-bromophenylaceto-
nitrile (453 mg, 2.30 mmol), DME (30 mL) and 2 M sodium carbonate
(
1.06 g) was added under inert nitrogen condition. De-gassed the reac-
tion mixture for 20 min in nitrogen atmosphere then added Pd(PPh
10 %) catalyst and refluxed 12 h at 70 ⁰C. After completion of the re-
action, the reaction mixture was workup by using EtOAc. The combined
organic layer was washed with brine and dried over Na SO . The residue
was purified by column chromatography using petroleum and EtOAc
3:2) to afford product-6 yield (1.17 g 81 %). 1H NMR (500.1 MHz,
CDCl
): δ 8.28 (s, 1 H), 8.13 (d, J =7.7 Hz, 1 H), 7.70 (d, J =8.2 Hz, 2 H),
3 4
)
(
intermolecular
π
- stacking interaction, exists between the molecular
π
backbones in the solid film. The optical band gaps of films were esti-
mated from the onset of absorption to be ~2.0–2.5 eV (Table S1 in SI).
To better understand the excitation and intramolecular charge
transfer for the synthetic [5]helicene dyes, we have performed the
fluorescence emission spectra for both FLU-HEL and CBZ-HEL dyes in
dry dichloromethane at room temperature with concentration of
2
4
(
3
7
.66 (d, J =7.4 Hz, 1 H), 7.47 (t, J =5.7 Hz, 1 H), 7.44 (s, 1 H), 7.41 (d, J
ꢀ 1
=
7.0 Hz, 1 H), 7.37 (d, J =4.8 Hz, 2 H), 7.24 (d, J =8.3 Hz, 1 H), 4.28 (t,
0.02 mg mL (Fig. 2b). We have observed the broad emission in the
region ~ 400–750 nm with a single emission maximum at 475 and
581 nm for FLU-HEL and CBZ-HEL respectively. Literature review sug-
gests that the substitution on the most stable [5]helicene aromatic
moiety influences the fluorescence emission of dyes. Here, in the case of
CBZ-HEL we observed more than 100 nm red shift in fluorescence
emission compared to FLU-HEL due to the presence of carbazole moiety,
a strong electron donor. A subtle effect on the molecular absorption
spectra and also a noticeable change on the fluorescence emission
spectra might be occurring due to change of structures in the ground
state and excited state [14,16–17].
2
H), 3.76 (s, 2 H), 1.89ꢀ 1.83 (m, 2 H), 1.40ꢀ 1.31 (m, 4 H), 1.25ꢀ 1.23
(
m, 6 H), 0.86 (t, 3 H). 13C (75.4 MHz, CDCl
3
): δ 142.0, 140.9, 140.1,
1
1
2
32.3, 131.1, 129.6, 128.4, 127.9, 125.9, 125.0, 123.4, 122.9, 120.4,
19.0, 118.8, 118.0, 109.0, 108.9, 43.2, 31.8, 29.4, 29.2, 29.0, 27.3,
3.3, 22.6, 14. HRMS (m/z): [M+H]+ calcd for C28
30 2
H N : 394.240;
found 394.241.
′
′
(
2E,2 E)-3,3 -(1,4-Phenylene)bis(2-(4-(9-octyl-9H-carbazol-3-yl)
phenyl)acrylonitrile) (8): To a 50 mL of dry round bottom flask charged
with terephthalaldehyde (100 mg, 0.74 mmol), compound-6 (1.17 g,
0
.002 mol) and ethanol (10 mL), after formation of homogeneous solu-
tion sodium ethoxide (152 mg, 2.23 mmol) was added. The reaction
We have calculated the photoluminescence quantum yield (QY) by
using standard anthracene dye as a reference having QY of 27 % in
EtOH. The QY of FLU-HEL and CBZ-HEL was found to be 27 % and 30 %
respectively and the relevant data is summarized in Table S1. The high
QY in CBZ-HEL arises probably due to strong electron donating char-
acter of carbazole unit. The average life-time decay for FLU-HEL and
CBZ-HEL dyes was found to be in order of 9.72 and 4.28 ns respectively.
◦
contents were stirred under inert atmosphere at 50 C for 4 h, after the
completion of reaction the crude material was filtered by Buchner funnel
and washed with cold ethanol. The crude solid was washed with brine
solution and dried over sodium sulphate. The organic solvent was
evaporated under reduced pressure and purified by column chroma-
tography using petroleum ether/DCM (2:3) to afforded the product-8
4

Y. B. et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113203
Fig. 2. Normalized UV–vis absorption (a) and fluorescence (b) spectra of FLU-HEL and CBZ-HEL in dichloromethane with concentration of 0.02 mol/L.
The radiative and non-radiative decay constant for both the dyes were
calculated (Table S2 in SI). These findings demonstrate that the 2,12-
disubstituted [5]helicene dyes such as FLU-HEL and CBZ-HEL display
large Stokes shifts and high quantum yields. Both dyes showed a sig-
nificant variation in its photophysical properties just by the changing
substituent on stable [5]helicene core. We have also investigated the
variation of the fluorescence properties in dichloromethane under
different wavelength of exciting light (Fig. S2 in SI), which
unambiguously demonstrated their fluorescence variation.
Fig. 3a and b display the absorption and emission spectra of both the
dyes in various solvents. The absorption spectra are insensitive towards
the solvatochromic effect which implies that the ground state is less
polar (see Table S3 and S4 in the Supporting Information). However, the
emission spectra of both dyes were sensitive to the solvent polarity
indicating a moderate positive solvatochromic shift upon increasing the
solvent polarity. The results suggest that the excited states of both
Fig. 3. a) and b) Normalized UV–vis absorption and emission spectra of FLU-HEL and CBZ-HEL in different solvents with concentration of 0.02 mg mLꢀ , (c) Stokes
1
shift versus E
T
(30) and (d) Lippert ꢀ Mataga plots for the dyes and (e) time-correlated single photon counting (TCSPC) analysis for FLU-HEL and (f) for CBZ-HEL in
different solvents.
5

Y. B. et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113203
molecules are more polar than their ground states [10].
HEL and CBZ-HEL can be used as a fluorescent ink under excitation at
365 nm, and can also replace traditional inks for security purposes. We
have chosen the commercially available paper because it is non-
fluorescent under 365 nm excitation. Initially, we have refilled the pen
refill with the FLU-HEL and CBZ-HEL fluorescent dyes shown in Fig. 4d
and used for hand writing purpose on paper. We observed that hand-
written paper under excitation at 365 nm show the written text and in
day light condition it looks like blank paper (Fig. 4c and e). Further,
silica gel mesh used for TLC purpose does not show any fluorescent
under excitation at 365 nm. When the FLU-HEL and CBZ-HEL dyes were
adsorbed on it starts to show the fluorescent (Fig. 4a and b). These are
few basic experiments which can be used for the forensic and security
purpose.
Interestingly, FLU-HEL exhibited poor sensitivity towards sol-
vatochromic shift and indicates a poor polar
π
-π* excited states.
Whereas, CBZ-HEL contains carbazole chromophore, a strong electron
donor, thus exhibiting positive solvent induced emission shift, which
reflects the stabilization of the excited states in polar solvents. However,
both molecules showed a significant dipolar character in the excited
states but not in the ground state [18]. The positive solvatochromic shift
for both dyes might be due to intramolecular charge transfer between
the excited states from donor molecules (fluorene, carbazole) to
acceptor cyano group.
Though, the extent charge transmission is restricted in FLU-HEL as it
is evident from the smaller value of bathochromic shift with increasing
solvent polarity from TOL to DMSO (Δλem = ~30 nm) [18]. Generally,
the charge transfer in bipolar compounds is associated with large Stokes
shifts difference between non-polar (TOL) and the polar solvent (DMSO)
The circular dichroism (CD) spectrum was obtained in a chloroform
ꢀ
5
solution (1 × 10 M) for the both FLU-HEL and CBZ-HEL [5]helicene
molecules (Fig. 5). CBZ-HEL displayed a maximum at 303 nm (Δ = 77
cm ) attributable to the central helicene moiety and a minimum at
ε
-
1
-1
[
19], as is the case with CBZ-HEL (λem = ~100 nm) red shifted from TOL
M
-
1
-1
to DMSO. Unexpectedly the dyes showed a strong red shift in high polar
DMSO solution. Mostly, this kind of behaviour has been observed in
organic fluorophores and can be attributed to the instant stabilization of
polarizable electrons during electronic excitation [20]. The fluorescence
alterations of both dyes in five different solvents were showed in pres-
ence of different wavelength of light (UV-light and fluorescence light)
473 nm (Δε = 111 M cm ) most likely due to the synergistic combi-
nation of the inherent chirality of [5]helicene with the strong absorp-
tivity of the carbazole chromophores [20]. The corresponding peaks in
-
1
-1
FLU-HEL were at 318 nm (Δ
ε
= 80 M cm ) and 488 nm (Δ
ε = 76
-
1
-1
M
cm ) respectively. These observations confirm the inherently
chirality and efficient transference of chirality throughout the mole-
cules. The theoretical geometry study of both FLU-HEL and CBZ-HEL
dyes were briefly studied by using B3LYP exchange correlations func-
tional with 6ꢀ 311 g (d, p) as a basis set and the optimized structures
were showed in supporting information (Fig. S8). To present some
insight into experimentally observed photophysical properties of
FLU-HEL and CBZ-HEL [5]helicene dyes, TDDFT calculations were
performed under the same basis set and briefly discussed in supporting
information (Fig. S9-S11), the corresponding data were tabulated in
table S8.
(
see Fig. S3 in the SI).
FLU-HEL and CBZ-HEL [5]helicene dyes showed solvent dependent
excited state properties which is further supported by Lippert-Mataga
and Stokes shift versus E (30) correlations. We have observed nearly
T
linear movements in Fig. 3c & d with sharper slopes for both dyes, which
explain a general solvent effect for the dyes in the excited state. The dye
CBZ-HEL displays a large change in Stokes shift (λem-λab) confirming
most prominent intramolecular charge transfer in the excited state. The
lifetime study for the [5]helicene dyes in different solvent is shown in
Fig. 3e and f and the supporting data is given in Table S5. As the polarity
of the solvent is increasing the average life time of both the dyes is
increasing due to high solubility and stable excited state.
3
. Conclusions
The HOMO energy levels were calculated to be -5.60 eV and -5.17 eV
for FLU-HEL and CBZ-HEL respectively (briefly discussed in the SI; see
Fig. S4, Table S6). We have observed the fluorescence behaviour using
different metal salts for sensing the metals (see Fig. S6 in the SI). The
UV–vis absorption spectra (see Fig. S5 in the SI) and fluorescence
emission spectra of FLU-HEL and CBZ-HEL with different metal salts (see
Fig. S7a and S7e in the SI) showed drastic change in absorbance as well
In summary, we have designed and synthesized two new fluorene
(
FLU-HEL) and carbazole (CBZ-HEL)-based 2,12-disubstituted [5]heli-
cene via photochemical reaction. Both the [5]helicene dyess exhibited
very intense fluorescence property and fluorescence quantum yield Φ
7 % and 30 % and life time (〈 〉 =9.7 ns and 4.2 ns) was observed. Both
5]helicenes are highly sensitive towards Fe transition metal ion, than
f
=
2
τ
f
3+
[
other transition metal ions. Therefore; these organic dyes can be utilized
as fluorescence. Particularly in the case of Fe3 metal salt both absor-
+
3+
as Fe ion sensors. Furthermore, another interesting features of these
bance and emission were quenched. The interaction of Fe3 ion with
FLU-HEL and CBZ-HEL dyes results in increase in absorption and
quenching of fluorescence. The quenching in fluorescence is related to
non-radiative electron transfer, which involves the partial transfer of an
electron from the excited state of FLU-HEL and CBZ-HEL[5]helicene
+
[
5]helicene for security application was explored. A successful
dyes to the d orbital of Fe3 transition metal ion.
+
The fluorescence intensity of both FLU-HEL and CBZ-HEL [5]heli-
3
+
cene dyes decreases gradually on successive addition of Fe ion with
concentration in the range of 0–426 micromolar (
μ
M) (see Fig. S7b and
S7f in the SI), which reveals that Fe3 metal ion is coordinating with
+
both dyes and reduce the fluorescence so it can be used as a fluorescent
probe for Fe3 metal ion detection [12]. Difference in fluorescence in-
+
tensity of [5]helicene dye is proportional to Fe3 ion concentration so it
was well explained by the Stern–Volmer relationship (see Fig. S7c and
S7g in the SI). This gives a linear regression equation with KSV equal to
+
3
ꢀ 1
3
ꢀ 1
6
.6× 10 L mol and 5.3× 10 L mol and the correlation coefficients
of 0.98 and 0.99 for FLU-HEL and CBZ-HEL respectively. We have
observed the linearity till the 426 M concentration for CBZ-HEL.
μ
We have briefly discussed the quenching phenomenon of both dyes
using KSV values, Stern-Volmer plots and TCSPC etc. are discussed in the
Supporting Information (see Fig. S5 and S7, Table S7 in the SI).
Furthermore, we have extended our study to security purpose. Both FLU-
Fig. 4. a) SiO
2
, SiO
2
+ FLU-HEL, SiO
2
+ CBZ-HEL under daylight. b) SiO
2 2
, SiO
+
FLU-HEL, SiO
2
+ CBZ-HEL under 365 nm excitation c) Handwritten paper
with FLU-HEL and CBZ-HEL dyes under daylight d) Pen refil filled with FLU-
HEL and CBZ-HEL dyes solution e) Handwritten paper with FLU-HEL and
CBZ-HEL dyes under 365 nm excitaiton.
6

Y. B. et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113203
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Author statement
All the authors are aware and agreed about this publication. All the
information provided in this article is correct.
[
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The authors declare no competing financial interest.
Declaration of Competing Interest
The authors declare no conflict of interest.
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