Viscosity sensitive fluorescent coumarin-carbazole chalcones and their BF2 complexes containing carboxylic acid – Synthesis and solvatochromism

Article abstract of DOI:10.1016/j.molliq.2018.05.074

Red emitting coumarin-carbazole chalcones of the D- π –A and D–π–A–π–D type were synthesized with the coumarin core as acceptor and carbazole with anchoring carboxylic group at the end of N-alkyl chain as donor. The introduction of extra donor on the coumarin core led to bathochromic shift. Further, the BF₂ complexation of the synthesized chalcones led to emission in red region due to efficient increase in the electron attracting capacity of coumarin acceptor. A strong emission solvatochromism than absorption was observed for the studied dyes which is well understood with the help of the multi-linear regression analysis. The frontier molecular orbital analysis of the dyes suggest strong intramolecular charge transfer characteristics. Viscosity induced emission enhancement was observed for all dyes in polar-protic (ethanol: PEG 400) solvent mixtures. As the dyes 6 and 8 emit at 612 nm and 647 nm in polar solvents these can be used as red emitting fluorescent molecular rotors.

Full text of DOI:10.1016/j.molliq.2018.05.074

Accepted Manuscript
Viscosity sensitive fluorescent coumarin-carbazole chalcones and
their BF2 complexes containing carboxylic acid – Synthesis and
solvatochromism
Manali Rajeshirke, Abhinav B. Tathe, Nagaiyan Sekar
PII:
S0167-7322(18)31566-6
doi:10.1016/j.molliq.2018.05.074
MOLLIQ 9129
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
24 March 2018
7 May 2018
17 May 2018
Please cite this article as: Manali Rajeshirke, Abhinav B. Tathe, Nagaiyan Sekar ,
Viscosity sensitive fluorescent coumarin-carbazole chalcones and their BF2 complexes
containing carboxylic acid – Synthesis and solvatochromism. The address for the
corresponding author was captured as affiliation for all authors. Please check if
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ACCEPTED MANUSCRIPT
Viscosity sensitive fluorescent coumarin-carbazole chalcones and their BF
2
complexes containing carboxylic acid – synthesis and solvatochromism
Manali Rajeshirke, Abhinav B. Tathe, and Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology,
Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India)
*
Corresponding author. Tel.: +91 22 3361 1111/2222, 2707(direct), Fax. +91 22 3361 1020.
e-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
1

ACCEPTED MANUSCRIPT
Viscosity sensitive fluorescent coumarin-carbazole chalcones and their BF
2
complexes containing carboxylic acid – synthesis and solvatochromism
Manali Rajeshirke, Abhinav B. Tathe, and Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology,
Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India)
*Corresponding author. Tel.: +91 22 3361 1111/2222, 2707(direct), Fax. +91 22 3361 1020.
E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Abstract
Red emitting coumarin-carbazole chalcones of the D- π –A and D–π–A–π–D type were synthesized with the coumarin
core as acceptor and carbazole with anchoring carboxylic group at the end of N-alkyl chain as donor. The introduction
2
of extra donor on the coumarin core led to bathochromic shift. Further, the BF complexation of the synthesized
chalcones led to emission in red region due to efficient increase in the electron attracting capacity of coumarin
acceptor. A strong emission solvatochromism than absorption was observed for the studied dyes which is well
understood with the help of the multi-linear regression analysis. The frontier molecular orbital analysis of the dyes
suggest strong intramolecular charge transfer characteristics. Viscosity induced emission enhancement was observed
for all dyes in polar-protic (ethanol: PEG 400) solvent mixtures. As the dyes 6 and 8 emit at 612nm and 647nm in
polar solvents these can be used as red emitting fluorescent molecular rotors.
Keywords
Carbazole, BF
rotors.
2
complex, Radiative and non-radiative rate constant, multi-linear regression, Fluorescent molecular
Introduction
One of the widely studied classes of flavonoids is chalcones which is investigated largely with better NLO properties
over urea and thus could be found applicable for applications like in information storage[1], optoelectronic devices[2],
and also in biological systems [3]. The chalcone have structural flexibility in synthesis and it mainly consist of two
planar aromatic ring units connected covalently via three carbon bridge consisting of carbonyl unit and α,β-
unsaturation [4]. The dyes with good florescence in the red region referred to as biological window, are of recent
interest in biology[5]. The restricted intramolecular rotation in such molecules due to bulk property of environment
viscosity, result in molecular rotors development[6]. The BODIPYs, chalcone derivatives, and boronates are the
2

ACCEPTED MANUSCRIPT
organoboron compounds of D-π-A design [7], which find application as fluorescent molecular rotors (FMR) [8].
FMRs are responsive towards environment thus useful to measure the viscosity in biological systems as sensor in bio-
fluid, polymerization processes, and micro viscosity[9, 10]. The red emissive FMRs prove useful over the
conventional methods for background-free detection of biomolecules in the biological systems, in fluorescence
imaging to study viscosity studies on cellular scale [11–13].
2
The main objective in our research was to study chalcones and their BF complexes of D-π-A and D-π-A-π-D type
respectively, consisting of coumarin core as acceptor and carbazole donor. The D-π-A and D-π-A-π-D type
chromophores are widely studied for application in information storage, optoelectronics and NLO responses as the
properties of such systems could be easily improved by altering the donor/acceptor part [14–16]. The coumarin and
its derivatives are excellent fluorophore and have applications in several photonic applications, as sensors, as optical
brighteners, fluorescent probes in biology and as light emitting devices [17, 18]. Also, the coumarin dioxaborine push-
pull dye is known to show good FMR properties[19]. Carbazole is a good donor found applicable as hole-transporting
material due its strong intramolecular charge transfer (ICT) characteristic attributed to larger π-conjugation [20, 21].
Carbazole based dyes show good FMR properties[22]. The carbazole donor here is substituted by alkyl carboxyl
group at the 9 position. The carboxyl group at the terminal end of alkyl chain on carbazole have application as a
derivatization reagents in HPLC analysis, in Langmuir Blodgett film[23], in photo exposure delay (PED)[24]. In our
earlier work, we have studies the synthesis of coumarin-carbazole chalcone, whoes photophysical properties along
with DFT computations showed greater first order static hyperpolarizability than the urea which was 435 to 1360
times greater than urea [25].
Thus, our research interest was to study the FMR properties of dyes containing both coumarin and carbazole core
linked via chalcone linkage. We have focused here on the study of photophysical properties, the solvatochromism
2
behavior and viscosity sensitivity of the newly synthesized coumarin-carbazole chalcone and their BF complexes,
wherein the carbazole donor is substituted at the 9 position by alkyl chain with hydrophobic polar carboxylic group at
the end. The spectroscopic data is utilized to study the radiative and non-radiative rate constant, the polarity function
plots, multilinear regression analysis which help to understand the photophysical, solvatochromism behavior of all the
dyes along with the aid of frontier molecular orbital analysis as obtained from TD-DFT computations. Further, the
FMR properties of the synthesized dyes are also studied in this paper. From the result, they act as promising candidate
to red emissive FMR.
3

ACCEPTED MANUSCRIPT
CHO
Br(CH ) COOH , KOH
DMF,POCl3
2
2
Acetone, reflux
5oC
8
N
N
N
H
COOH
COOH
1
2
Chloroform, 24h, Reflux,Piperidine
OH
O
O
O
F
F
B
O
O
4
a R = H
R
4b R = NEt2
3
3
a R = H
b R = NEt2
R
O
O
F
F
O
O
HO
O
B
O
O
Red shift
R
N
HOOCH CH C
N
O
O
R
2
2
5 R = H
7
HOOCH CH C
2 2
6
8
R = H
R = NEt2
R = NEt2
Day Light
UV Light
<< Please insert Scheme 1 >>
2
. Experimental
2
.1. Materials and equipment
The chemical reagents used in the experiments were obtained from S.D. fine chemicals Ltd., Mumbai, India. The solid
reagents were used directly without purification on checking the melting points. A pre-coated silica gel aluminum
based plates Kiesel gel 60 F254 from Merck, India was used to monitor the progress of the reactions. Purification of
the synthesized compounds was done by recrystallization and column chromatography wherever necessary. Melting
point instrument from Sunder Industrial Product Mumbai was used to record the melting points. 500 MHz and 125
1
MHz on Agilent Technology instrument was used to record the H NMR and 13C NMR spectra respectively wherein
the TMS was used as an internal standard. The Perkin-Elmer Lambda 25 UV-visible spectrophotometer was used to
record the UV-Vis absorption spectra of the dyes. The solvents used in the study were toluene (purity ≥ 99.8 %), ethyl
acetate (purity ≥ 99.5 %), dichloromethane (DCM) (purity ≥ 99.9 %), methanol (purity ≥ 99.9 %), acetonitrile (purity
≥
99.9 %), and N, N-dimethyl formamide (DMF) (purity ≥ 99.8 %) all high-performance liquid chromatography
4

ACCEPTED MANUSCRIPT
(
HPLC) grade obtained from Merck India. The Varian Cary Eclipse fluorescence spectrometer was used to record the
fluorescence emission spectra of the dyes. Rhodamine 6G was used as the reference standard to calculate the quantum
yield of dyes 5-8.
2
.2. Synthesis and characterization
The Claisen-Schmidt reaction was employed to obtain the coumarin-carbazole chalcones by reacting the aldehyde of
-(9H-carbazol-9-yl) propionic acid (2) with 3-acetyl, 4-hydroxy-coumarin (3a) and 3-acetyl, 4-hydroxy, 7-N, N-
diethyl coumarin (3b) to get the chalcone 5 and 7 respectively. Synthesis is as follow:
3
2
.2.1. Synthesis of 3-(9H-carbazol-9-yl) propionic acid 1 was prepared using the reported method[26, 27].
2
.2.2. Synthesis of 3-(3-formyl-9H-carbazol-9-yl) propionic acid 2
3
Vilsmeier Haack reaction was employed on 3-(9H-carbazol-9-yl) propionic acid 1. POCl (9.8 mL, 105.02 mmol) was
0
o
added drop wise to DMF (20 mL, 273.97 mmol) at 0-5 C, and stirred for 30 min at 0-5 C. Solution of 3-(9H-carbazol-
0
9
-yl) propionic acid 1 (5g, 20.92 mmol) in 15 mL DMF was added dropwise over 30 min at 0-5 C temperature. The
reaction mixture was stirred for next 25-30 min and then brought to room temperature. The reaction mixture was then
0
heated for one hour at 75-85 C. The reaction was monitored by TLC. After completion of reaction, the reaction mass
was poured into crushed ice stirred well. The product precipitate obtained was filtered off, dried and was purified by
column chromatography on silica 100-200 mesh and using petroleum ether: ethyl acetate (60:40) as eluent. Yield 2.23
g (39.81%) pale white solid.
o
2
.2.3. The compounds 4a-4b were synthesized from the compounds 3a-3b by heating in benzene at 60 C [28].
2
.2.4. General Procedure for the synthesis of 5, 6, 7, and 8:
The reaction mixture of 3-(3-formyl-9H-carbazol-9-yl) propionic acid 2 (0.5mmol), 3-acetyl, 4-hydroxy-coumarin 3a,
and catalytic amount of piperidine was refluxed in chloroform for 24h. The progress of the reaction was monitored by
TLC. The solvent was evaporated from the reaction mixture and residue obtained was recrystallize from 5%
chloroform-petroleum ether. The product was filtered and dried to give product 5. Same procedure was used for
synthesis of compounds 6, 7, and 8.
o
1
(
5) Yield: 27%, M.P. 226 C, H NMR (500 MHz, DMSO) δ 12.30 (s, 1H), 8.54 (s, 1H), 8.23 (dd, J = 7.2 Hz, 3H),
7
7
1
1
4
.98 (d, J = 6.9 Hz, 1H), 7.86 (d, J = 10.5 Hz, 1H), 7.77 – 7.62 (m, 3H), 7.52 – 7.44 (m, 1H), 7.36 (d, J = 7.2 Hz, 2H),
.30 (d, J = 10.5Hz 1H), 4.61 (t, 2H), 2.71 (t, 2H). 13C NMR (126 MHz, DMSO) δ190.84, 181.52, 172.85, 154.50,
42.45, 140.73, 136.60, 127.22, 125.07, 123.87, 123.65, 123.37, 122.64, 121.14, 120.68, 120.38, 119.12, 117.35,
+
11.13, 110.86, 110.66, 110.37, 100.76, 40.70, 33.68. HRMS: 454.1257 (M + H ) (Calculated for: C27
53.119).
6
H19NO :
o
1
(
6) Yield: 20%, M.P. 218 C, H NMR (500 MHz, DMSO) δ 8.34 (s, 1H), 8.11 (d, J = 8.5 Hz, 1H), 7.72 (d, J = 16.3 Hz, 1H),
7
.65 – 7.52 (m, 5H), 7.51 – 7.41 (m, 3H), 7.25 – 7.18 (m, 1H), 7.02 (d, J = 16.3 Hz, 1H), 4.64 (t, 2H), 2.79 (t, 2H).
13C NMR (126 MHz, DMSO) δ 192.39, 170.24, 144.55, 141.73, 129.14, 127.24, 125.88, 124.21, 122.79, 121.29,
+
1
20.83, 110.45, 44.72, 33.52. HRMS: 502.1136 (M + H ) (Calculated for: C27
H
18BF
2
NO
6
: 501.1195).
o
1
(
7) Yield: 30%, M.P. 253 C, H NMR (500 MHz, DMSO) δ 8.54 (s, 1H), 8.33 (d, J = 8.3 Hz, 1H), 8.23 (d, J = 7.7
Hz, 1H), 8.16 (d, J = 2.1 Hz, 1H), 7.86 (dd, J = 8.7, 0.8 Hz, 1H), 7.76 – 7.72 (m, 2H), 7.67 (d, J = 15.6 Hz, 1H), 7.50
5

ACCEPTED MANUSCRIPT
(
t, J = 7.2 Hz, 1H), 7.27 (t, J = 7.4 Hz, 1H), 6.77 (d, J = 9.1 Hz, 1H), 6.48 (d, J = 15.6 Hz, 1H), 4.65 (t, J = 6.9 Hz,
2
1
1
H), 3.48 (q, 4H), 2.77 (t, J = 6.9 Hz, 2H), 1.14 (t, 6H). 13C NMR (126 MHz, DMSO) δ 190.39, 179.94, 172.87,
60.77, 157.31, 157.18, 157.10, 154.29, 151.71, 147.39, 147.27, 142.16, 140.73, 127.17, 126.78, 126.20, 123.32,
+
22.65, 122.64, 121.14, 119.71, 103.42, 97.88, 44.84, 33.69, 12.71. HRMS: 525.0247 (M + H ) (Calculated for:
31 28 2 6
C H N O : 524.1943).
o
1
(
8) Yield: 21%, M.P. 240 C, H NMR (500 MHz, DMSO) δ 8.72 (s, 1H), 8.46 (s, 1H), 8.37 (d, J = 15.4 Hz, 1H), 8.24
d, J = 7.7 Hz, 1H), 7.99 – 7.96 (m, 1H), 7.80 (dd, J = 14.2, 9.0 Hz, 2H), 7.70 (d, J = 8.2 Hz, 1H), 7.55 – 7.49 (m,
(
1
2
1
1
H), 7.30 (t, J = 7.5 Hz, 1H), 6.88 (dd, J = 9.4, 2.2 Hz, 1H), 6.61 (d, J = 2.2 Hz, 1H), 5.73 (s, 1H), 4.67 (t, J = 6.6 Hz,
H), 3.56 (q, 4H), 2.78 (t, J = 6.8 Hz, 2H), 1.17 (t, 6H). 13C NMR (126 MHz, DMSO) δ 179.79, 172.81, 159.38,
58.71, 156.07, 152.86, 143.29, 140.86, 128.45, 128.06, 127.36, 126.00, 123.67, 122.72, 121.29, 120.93, 116.10,
+
11.63, 109.99, 102.28, 97.89, 55.35, 45.37, 33.68, 13.96. HRMS: 595.1795 (M + Na ) (Calculated for:
31 2 2 6
C H27BF N O : 571.1967).
3
. Results and discussion
The fluorescent coumarin-carbazole chalcones and their BF
2
complexes were prepared by Claisen-Schmidt reaction
as shown in scheme 1. The photophysical, solvatochromism and molecular properties of the synthesized dyes were
studied.
3
.1. Photo-physical properties of Dyes 5-8
Electronic absorption and fluorescence spectra of chalcones (5, 7) and their BF
2
complexes (6, 8) were measured in
solvents of varying polarity, and the results are summarized in the Table 1. Results showed that the dyes exhibited
good emission solvatochromism than absorption in different microenvironment. Fig. (1) provides a clear comparison
of absorption and emission behavior of dyes 5-8 in dichloromethane solvent. A representative absorption and emission
solvatochromism plot in different solvents for dye 8 are presented in Fig. (2), while the absorption and emission plots
for 5, 6, and 7 are presented in supplementary information (S. Fig. 1). The absorption maxima of all the dyes was in
the range of 420-576 nm. The order of the absorption and emission maxima of the dyes is 5 (432nm, 517nm) < 7
(
452nm, 546nm) < 6(535nm, 583nm) <8 (559nm, 621nm) in chloroform Table 1 respectively. A little red shift in
absorption maxima was observed with the increase in the solvent polarity from 421 nm to 440 nm for dye 5, 530 nm
to 542 nm for dye 6, 442 nm to 454 nm for dye 7, and 555 nm to 576 nm for dye 8 (Table 1). The chalcone 5 has D-
π -A design consisting of an electron-donor 3-(9H-carbazol-9-yl) propionic acid unit and electron-withdrawing 3-
acetyl, 4-hydroxy-coumarin group whereas chalcone 7 consists of donor-π-acceptor-π-donor (D–π–A–π–D) type
design with 7-N, N-diethyl as extra donor at the 7 position of coumarin ring. As expected with introduction of
additional donor in the coumarin-carbazole chalcone led to bathochromic shift evident from Table 1. Red shift in
absorption as well as in emission maxima was also observed on BF
shift in optical properties is attributed to efficient charge transfer (CT) as the electron withdrawing nature of carbonyl
carbon of 3-acetyl, 4-hydroxy-coumarin is enhanced on complexation with BF (Fig. 1)
2
complexation of the chalcones. This bathochromic
2
.
6

ACCEPTED MANUSCRIPT
Table 1 Experimental photophysical properties dyes 5-8 in various solvents.
abs[a]
emi[b]
[c]
[d]
[e]
[f]
[g]
Dye
Solvent
λ
λ
∆ ῡ
ε
max
Φ
f
K
r
K
nr
cm-1
nm
79
90
85
82
81
61
97
60
67
51
53
79
63
58
98
95
94
91
5
Toluene
DCM
421
430
432
433
428
430
440
521
525
532
530
535
536
547
442
447
452
447
445
447
454
559
562
559
555
562
559
576
500
520
517
515
509
491
537
581
592
583
583
614
599
605
540
542
546
538
557
565
559
612
627
621
615
646
647
645
3752.97
4025.04
3805.79
3677.21
3718.12
2889.22
4105.30
1982.15
2155.73
1644.33
1715.27
2404.94
1962.23
1752.61
4105.92
3921.18
3808.88
3784.00
4518.59
4672.25
4137.34
1549.22
1844.63
1786.03
1757.86
2313.72
2433.14
1857.24
17216.40 0.31 0.24
18563.20 0.37 4.4
41669.20 0.36 1.5
14420.40 0.13 1.3
16612.80 0.38 1.6
16138.00 0.26 2.9
7.15
7.40
2.70
8.90
2.60
8.20
20.60
5.01
1.72
0.76
3.02
3.88
22.30
-
6.94
5.21
2.77
5.66
12.50
18.50
Chloroform
Ethyl acetate
Acetonitrile
Methanol
DMF
7571.50
0.11 2.5
6
7
8
Toluene
DCM
43633.00 0.27 1.8
37190.00 0.47 1.5
21309.00 0.52 0.81
36354.00 0.32 1.4
31542.33 0.22 1.1
20609.67 0.03 0.72
Chloroform
Ethyl acetate
Acetonitrile
Methanol
DMF
29271.67
-
-
Toluene
DCM
26223.67 0.23 2.1
43682.67 0.37 3.1
30600.00 0.44 2.1
31140.00 0.25 1.9
Chloroform
Ethyl acetate
Acetonitrile
Methanol
DMF
112 25226.00 0.11 1.5
118 31281.67 0.10 1.9
105 13733.00
53
65
62
60
84
88
69
-
-
Toluene
DCM
46722.00 0.23 2.1
44155.67 0.43 1.7
43668.67 0.48 1.7
40465.67 0.23 1.5
31543.00 0.02 1.4
40082.00 0.05 1.4
6.98
2.18
1.85
5.13
59.90
28.20
Chloroform
Ethyl acetate
Acetonitrile
Methanol
DMF
29465.33
-
-
-
1
[
a] the absorption maxima in nm, [b] the emission maxima in nm, [c] Stokes Shift in cm and
nm, [d] Molar extinction coefficient in L M cm , [e] Fluorescence quantum yield (estimated
using Rhodamine 6G in ethanol), [f] Radiative rate constant (x10 s ), [g] Non-Radiative rate
-
1
-1
-
8 -1
-
8 -1
constant (x10 s ).
<<Please insert Table 1 Experimental photophysical properties dyes 5-8 in various solvents >>
The strong solvent effect was observed in emission spectra of the dyes due to dipole stabilization in high polar solvents
like DMF. The shift in emission maxima with solvent polarity from toluene to DMF was as follows: 5/42 nm, 6/33
nm, 7/31 nm, 8/35 nm. Among the dyes studied, dye 7 and 8 particularly showed a bathochromic shift in emission
maxima moving from toluene to methanol with decrease in emission intensity (Table 1).This is due the fact that
emission in non-polar solvent occurs from the local excited state whereas in polar solvent like methanol it occurs from
the ICT state which is supported from the trend in quantum yields of these dyes in non-polar to polar solvents. The
quantum yield for all the compounds was very low in polar solvents like methanol and almost negligible in DMF
compared to nonpolar solvents like chloroform despite of bathochromic shift in absorption and emission maxima of
f
all compounds (Table 1). Rhodamine-6G (ɸ 0.94 in ethanol) was used as reference to obtain the quantum yield. The
lowering of quantum yield in polar solvents is due the solvent stabilization of non-emissive twisted intramolecular
7

ACCEPTED MANUSCRIPT
charge transfer (TICT) excited state which is further explained by the larger non-radiative rate constant of the
compounds in polar solvents like methanol (Table 1).
0.2
80
5
6
7
8
- Abs
-Abs
-Abs
-Abs
0.18
0.16
0.14
0.12
70
60
50
5-emi
6-emi
7-emi
8-emi
0
.1
40
0.08
0.06
0.04
0.02
0
30
20
10
0
324 354 385 416 444 475 505 536 566 597 628 658 689
<< Please insert Fig 1 Absorption and emission graph of 5μM solution of compounds 5-8 in DCM solvent >>
<< Please insert Fig 2 (a) Absorption graph of 5μM solution of compounds 8 in different solvents (b) Emission
graph of 5μM solution 8 in different solvents >>
3
.2. Radiative and Non-radiative rate constants
8

ACCEPTED MANUSCRIPT
The increased loss of excitation energy with increase in polarity leads to reduction in fluorescence quantum yield in
highly polar solvents like methanol, DMF for all derivatives. Thus, the radiative rate constant (K
r
) and non-radiative
rate constant (Knr)[29] were obtained by Strickler Berg equation (1 and 2), and values are tabulated in Table 1.
∫
푓(ῡ)
ꢁ3
퐾푟 = 2.88 × 10−9푛ꢀ
∆휐푒
∫ 휖(ῡ)ꢂῡ -- (1)
∫
푓(ῡ)ῡ 푑ῡ
∆
휐푎
ꢄ−휙퐹
퐾ꢃ푟
=
-- (2)
휏
where τ fluorescence lifetime, n represents the refractive index of the solvent, ∆ꢅ and ∆ꢅ are the experimental limits
ꢆ
ꢇ
-
1
0 1
of absorption and emission bands (S –S transitions), 휖 the molar extinction co-efficient, ῡ the wavenumber (cm )
and f(ῡ) is the maximum fluorescence intensity measured in atomic unit (a.u.). The area under the curve of plot
intensity in a.u. versus per wavelength interval is shown by term ∫∆ꢉ푒 ꢈ(ῡ) whereas the term ∫∆ꢉ푎 ꢈ( ῡ)ῡ−ꢊꢂῡ is the
−
ꢊ
area under the curve of the ꢈ(ῡ)ῡ vs wavenumber interval and the term ∫ 휖(ῡ)ꢂῡ is the area under the curve of the
plot molar absorptivity vs wavenumber interval.
The larger non-radiative rate constant (Knr) of all the dyes in polar solvents indicate the predominance of radiationless
relaxation (Table 1). For dyes 7 and 8, these observation suggest that the non-fluorescent TICT excited state of the
dye is stabilized by polar solvent resulting into low quantum yield whereas in non-polar solvents the emission occur
from the local excited (LE) state as evident from the high quantum yield (Table 1).
3
.3. Solvatochromism/Polarity functions
The charge transfer characteristics and solvatochromic behavior of the synthesized dyes was studied by the Lippert-
Mataga [30], McRae [31], and Weller’s [32] polarity function plots. The theoretical description and the equations for
all models are given in the supporting information. A good regression coefficient was observed for dyes 5-8 from the
-
1
straight line plot of Stokes shift (cm ) against orientation polarizability i.e. Lippert function (ƒLM(ε,η)) which states
the dependence of optical properties of the dyes on orientation polarizability (Fig. 3).
<< Please insert Fig 3 Plot of Lippert-Mataga function Vs Stokes shift for dyes 5-8 >>
9

ACCEPTED MANUSCRIPT
Similarly, a good regression coefficient for dyes 5-8 was observed in the McRae plot (S. Fig. 2), which is the plot of
-
1
ƒ
McRae versus Stokes shift in cm an improved version of Lippert-Mataga model [31, 33] suggesting that red shift in
emission arise from dipoles created by the dye molecules. Weller's theory which considers the electron transfer in a
D-π-A systems, where in only emission wavelength is considered, also behaves linearly for dyes 5-8. All the equations
used for McRae and Weller’s theory are shown in supporting material. Good regression coefficient is suggestive of
the ICT characteristics of all the dyes in excited state (S. Fig. 2).
3
.4. Multilinear regression analysis of photophysical properties on solvent polarity scales using Kamlet-Taft and
Catalan parameters
The solute-solvent interaction as done above by Lippert-Mataga and Mac-Rae are based on the single parameter scale
which appears to be an inappropriate choice since it lacks to explain which parameter is precisely responsible for the
solvent dependent physiochemical changes. Kamlet-Taft and Catalan, a multi-parameter approach is applied to study
the effect of solvent property on the absorption and fluorescence spectra by means of linear solvation energy
relationship (LSER) using a Kamlet-Taft and Catalan solvent parameters. The multi-linear expression, in general, is
given as;
푦 = 푦 + ꢋ퐴 + 푏퐵 + 푐퐶 + ꢂ퐷 -- (9)
표
Where solvent affected physiochemical property represented as y, and the studied physiochemical property in the gas
0 0
phase presented as y . When all other solvent parameters are absent, then y = y . a, b, c and d are adjustable coefficients
which reflect the dependence of y to the various {A, B, C, D} solvent parameters. Herein we have studied three
different physiochemical properties (y) namely absorption maxima (ῡabs), emission maxima (ῡemi) and Stokes shift
(
∆ῡ). Kamlet and Taft [34–36] put forward the π*, α, and β parameters which stand for the polarity/polarizability, the
acidity, and the basicity of a solvent respectively as shown by Eq. (5).
∗
푦 = 푦 + ꢋ ꢌ + 푏 ꢍ + 푐
∗
ꢎ (퐾ꢋ푚푙ꢏ푡 ꢐ 푇ꢋꢈ푡) -- (5)
표
훼
훽
휋
If the value of y is not affected by acidity or basicity of solvent then by Kamlet-Taft it is difficult to differentiate
between among polarity and polarizability factor, thus, Catalan et al. [37–39] proposed an empirical solvent scales
wherein total four solvent parameters solvent acidity (SA), solvent basicity (SB), solvent polarizability (SP) and
solvent dipolarity (SdP) based on well-defined reference processes were introduced and given as;
푦 = 푦 + ꢋ ꢒ퐴 + 푏 ꢒ퐵 + 푐 ꢒꢔ + ꢂ푆푑푃ꢒꢂꢔ (퐶ꢋ푡ꢋ푙ꢋ푛) -- (6)
표
푆ꢑ
푆ꢓ
푆푃
The solvent parameters behavior is studied for the dyes 5-8 in different solvents and is presented in Table 2, which is
complex in both the scales. The negative values of solvent parameters in indicative of the bathochromic shift in highly
polar medium. The absorption data of dyes 5-8 by Kamlet-Taft comparatively poor fit than Catalan parameter as
shown by correlation coefficient r = 0.89-0.93 by Catalan whereas r = 0.5-0.8 by Kamlet-Taft (Table 2). The negative
value of cπ* coefficient of polarity/polarizability in Kamlet-Taft model for all the dyes indicates that
polarity/polarizability solvent parameter is responsible for the bathochromic shift in observed dyes parameter. Further,
1
0

ACCEPTED MANUSCRIPT
it was more clearly understood from the Catalan model that polarizability (SP) was main reason for the red shift in
both the spectra. Thus by studying the sign of each estimated coefficient and its extent of standard error of different
absorption maxima (ῡabs), emission maxima (ῡemi) and Stokes shift (∆ῡ) by both the methods, the polarizbility act as
the main factor responsible for the bathochromic shift in absorption and emission spectra.
Table 2 Estimated solvent coefficients (y0, a, b, c, d), their standard errors and correlation coefficients (r) for the
multi-linear analysis of (ῡabs), (ῡemi) and (∆ῡ) of dyes 5-8 on Kamlet-Taft and Catalan scales.
Kamlet-Taft
Catalan
y
1
0
03
x
y
10
0
x
[d]
[e]
[f]
r[g]
3
[h]
[i]
[j]
[k]
r[l]
a
α
b
β
c
π*
a
SA
b
SB
c
SdP
d
SP
5
2
4.03
0.001
(±
0.38)
0.75
(±
0.57)
- 0.75
(±
- 0.49 (±
0.46)
- 0.96
(±
0.95)
- 2.37
(±
0.69 25.68
0.18
(±
0.60)
1.53
(±
0.58)
-1.34
(±
- 0.94
(±
0.61)
- 1.12
(±
0.59)
0.17
(±
- 0.69
(±
0.51)
- 1.25
(±
0.49)
0.56
(±
- 2.31
(±
2.06)
- 5.01
(±
1.98)
2.70
(±
0.87
(
±
(±
1.67)
0.80 24.22
(±
1.61)
0.95 1.46
(±
[
a]
ῡ
abs
0.66)
0.90
2
0.05
(± 0.68)
0.96
0.98
(
±
[
b]
ῡ
emi
0.99)
.13
1.42)
3
- 0.55 (± 1.41
0.24)
(
±
(±
∆ [
ῡ
c]
0.35)
0.20)
0.50)
0.56)
0.20)
0.20)
0.17)
0.69)
6
7
8
1
9.61
- 0.10
(±
0.26)
- 0.23
(±
0.36)
0.13
(±
-0.68 (±
0.31)
- 0.86
(±
0.65)
- 1.68
(±
0.89)
0.81
(±
0.85 20.75
- 0.04
(±
0.51)
0.25
(±
0.62)
-0.29
(±
-0.71
(±
0.52)
0.24
(±
0.63)
- 0.95 0.52
(±
0.77)
- 0.79
(±
0.43)
-1.32
(±
- 1.60
(±
1.75)
-0.90
(±
2.13)
-2.50
(±
0.90
0.81
0.72
(
±
(±
1.42)
0.81 17.06
(±
1.73)
0.44 3.69
(±
[
a]
ῡ
abs
0.45)
8.17
1
- 0.43 (±
0.43)
(
±
[
b]
ῡ
emi
0.62)
.437
0.52)
1
-0.25 (±
0.50)
(
±
(±
0.64)
∆ [
ῡ
c]
0.72)
0.42)
1.04)
2.10)
0.76)
2.59)
2
2.80
- 0.07
(±
0.30)
- 0.48
(±
0.29)
0.41
(±
- 0.14 (±
0.36)
-0.60
(±
0.75)
- 1.85
(±
0.73)
1.24
(±
0.50 24.53
- 0.11
(±
0.49)
-0.22
(±
0.39)
0.11
(±
- 0.41
(±
0.50)
0.001
(±
0.40)
- 0.41 1.07
(±
0.63)
- 047
(±
0.41)
-1.55
(±
- 2.38
(±
1.69)
- 0.79
(±
1.35)
- 1.58
(±
0.87
0.96
0.89
(
±
(±
1.37)
0.90 20.07
(±
1.10)
0.76 4.45
(±
[
a]
ῡ
abs
0.52)
9.90
1
- 0.57 (±
0.35)
(
±
[
b]
ῡ
emi
0.51)
.90
0.33)
2
0.42
(± 0.46)
(
±
(±
0.52)
∆ [
ῡ
c]
0.66)
0.38)
0.95)
1.73)
0.62)
2.13)
1
8.59
0.09
(±
0.17)
-0.56
(±
0.26)
0.65
(±
- 0.20 (±
0.20)
- 1.11
(±
0.42)
- 1.68
(±
0.64)
0.56
(±
0.88 20.32
0.012
(±
0.30)
- 0.47 0.06
(±
0.47)
0.48
(±
- 0.47
(±
0.31)
- 0.60
(±
0.25)
- 1.36
(±
-2.68
(±
1.03)
-0.42
(±
1.61)
-2.25
(±
0.93
0.95
0.99
(
±
(±
0.83)
0.92 17.23
(±
1.31)
0.82 3.11
(±
[
a]
ῡ
abs
0.29)
7.33
1
-0.58 (±
0.31)
(
±
(±
0.48)
- 0.54 0.76
(±
0.18)
[
b]
ῡ
emi
0.45)
.26
0.39)
1
0.38
(±0.37)
(
±
(±
0.15)
∆ [
ῡ
c]
0.54)
0.31)
0.77)
0.49)
0.17)
0.60)
1
1

ACCEPTED MANUSCRIPT
-
1
-1
-1
[
a] the absorption maxima in cm , [b] the emission maxima in cm , [c] Stokes Shift in cm , [d] solvent acidity
estimated coefficient of Kamlet-Taft, [e] solvent basicity estimated coefficient of Kamlet-Taft, [f] collective
parameter of solvent dipolarity and polarizability according to Kamlet-Taft, [g] correlation coefficients (r) for
Kamlet-Taft, [h] solvent acidity estimated coefficient of Catalan, [i] solvent basicity estimated coefficient of
Catalan, [j] solvent dipolarity estimated coefficient of Catalan, [k] solvent polarizability estimated coefficient of
Catalan, and [l] correlation coefficients (r) for Catalan.
<
< Please insert Table 2 Estimated solvent coefficients (y , a, b, c, d), their standard errors and correlation
0
coefficients (r) for the multi-linear analysis of (ῡabs), (ῡemi) and (∆ῡ) of dyes 5-8 on Kamlet-Taft and Catalan
scales >>
3
.4. Frontier Molecular Orbitals (FMOs)
The FMO diagrams are useful to predict the ICT from donor to acceptor in a molecular system. The electron density
in the HOMO to LUMO levels suggest the ICT after excitation. The frontier molecular orbital (FMO) diagrams of
dyes 5-6 and 7-8 presented in Fig 5. The study of energy gap of dipole allowed electronic transitions from HOMO to
LUMO by TD-DFT computations gives insight of the bathochromic shift in both the spectra of the studied dyes.
Figure 4 displays the energy gap of dyes 5, 6, 7, and 8n in toluene solvent. As evident from the figure 4, the energy
gap is in the order of 5< 7< 6< 8 owing for the bathochromic shift in both the spectra of the dyes.
<< Please insert Fig 4 Energy gap of dyes 5-8 in toluene calculated by TD-DFT computations >>
2
From the FMO diagram of the synthesized molecules the coumarin and coumarin-BF -core act as acceptors. The
HOMO level of 5 and 6 indicate the carbazole core act as donor group which donates electron to relatively electron
deficient coumarin core via π-conjugation as evident from the LUMO, whereas in dyes 8 and 7 the extra donor at the
7
position of coumarin ring, N, N-diethyl group, donated electron to electron on coumarin acceptor electron density
1
2

ACCEPTED MANUSCRIPT
spread over the carbazole donor and π-conjugation. Thus more efficient electron transfer occur in D- π –A- π –D as
reflected by the red shifted absorption emission maxima of dye 7-8 compared to 5-6 (D- π –A).
<< Please insert Fig 5 FMO diagrams of dyes 5-8 >>
3
.5. Viscosity sensitive emission property in EtOH-PEG 400 solvent mixture
The bathochromic shift in absorption and fluorescence spectra was observed for all the synthesized dyes in polar
solvents like methanol where the solvent stabilizes the excited state (Table 1). The ground state of dye usually prefers
planar conformation energetically while the twisted conformer is preferred in excited state. Therefore, in polar solvents
where excited state is stabilized the molecules goes to the twisted conformation by intramolecular rotation. In viscous
solvents, the emission properties of the dyes were found to be highly sensitive. Thus the emission behavior of all dyes
5
6
-8 is studied in different mixture of polar-protic [EtOH-PEG 400] solvent of varying viscosity. Solution of dyes (5,
, 7, and 8) (concentration 10µM) are studied for the sensitivity of the fluorescence emission towards the viscosity of
microenvironment by using various composition of ethanol and PEG 400 as solvent. Almost all the dyes show red
shifted emission in ethanol solvent with very less fluorescence intensity but with increase in viscosity by PEG 400 the
emission intensity increased (Fig. 6). The maximum increase in emission intensity with rise in viscosity of the medium
is seen for BF
2
complex 8 wherein 4 fold increase in emission intensity at 647nm emission wavelength was observed
while dye 5 showed 2 fold, dye 6 2.5 fold, and 7 dye 2.3 fold increase (Fig. 6).
1
3

ACCEPTED MANUSCRIPT
<< Please insert Fig 6 Viscosity sensitivity of 10μM solution of dyes 5, 6, 7, and 8 in Ethanol-PEG 400 mixture
of varying viscosity >>
In ethanol, the polar twisted intramolecular charge transfer state (TICT) is more stabilized thus leading to quenching
of fluorescence as molecule relaxes via radiationless pathways from this state. With increase in viscosity by addition
of PEG 400 the energy barrier between the LE and TICT state increases as there is restriction of intramolecular
rotational relaxation about the donor and acceptor groups of the fluorophore leading to enhancement in the emission
intensity which is a characteristics of a molecular rotors. The emission intensity of molecular rotors and effect of
viscosity of the solvent was explained by using Forster Hoffmann Eq. (7), in which the viscosity of solvents
represented by η, the emission intensity of the rotors as I, C is an empirical proportionality constant and the viscosity
sensitivity of the rotors represented as x.
log 퐼 = 퐶 + 푥푙ꢕ푔휂 -- (7)
1
4

ACCEPTED MANUSCRIPT
<< Please insert Fig 7 Comparative logarithmic spectrum of emission intensity against viscosity for dyes 5-8 >>
<< Please insert Fig 8 Possible intramolecular rotations affected by viscosity in dye 8 >>
The possible rotations affected by the viscosity are as shown in Fig. 8. The x values for dye 5, 6, 7, and 8 were as 0.16
2
2
2
2
(
R =0.96), 0.18 (R =0.87), 0.19 (R =0.86), and 0.27 (R =0.96) respectively. Though the x values are lower than the
standard rotor DCVJ (0.6)[5] but the red region emission can find application in bio imaging with good signal to
background ratio. Such molecular rotors with acid end group at donor end showed wide application in medicinal and
1
5

ACCEPTED MANUSCRIPT
biological field. The both BF
2
complexes 6 and 8 emit above ≥600nm due to boron complexation and from the x
values they can be considered as red emissive fluorescent molecular rotors in polar-protic solvent mixture.
4
Conclusion
We have successfully synthesized the D- π –A and D–π–A–π–D type chalcone 5 and 7 respectively. With increase in
donor group, 7-N, N-diethyl as extra donor on coumarin ring in the chalcone framework lead to bathochromic shift in
optical properties due to efficient charge transfer character as evident from the FMOs of the synthesized dyes. The
2 2
concept of complexation with BF of the chalcone led to red emitting complexes 6 and 8. The BF complexation
efficiently increased the electron attracting capacity of the 3-acetyl carbonyl carbon of coumarin ring from the donor.
The dyes show emission solvatochromism by responding to the solvent polarity. In highly polar solvents the dyes are
very low fluorescence or non-fluorescent due to relaxation via radiationless pathway. Also, the photophysical
properties of the synthesized compounds with carboxylic anchoring group on the N-alky end are not alter much, thus
these could be further investigated for useful application. The multilinear regression analysis of ῡabs and ῡemi of the 5-
8
dyes has shown that mainly the solvent polarizability factor is responsible for the bathochromic shift. The viscosity
sensing study of the dyes showed the FMR behavior of all dyes in polar-protic (ethanol: PEG 400) solvent mixtures,
wherein the enhancement of emission intensity was observed with increase in viscosity. As the dyes 6 and 8 have
emission maxima above 612nm and 650nm respectively these can be used as red emitting FMRs.
6
Acknowledgement
One of the authors, Manali Rajeshirke, is grateful to UGC-SAP, India for Junior and Senior Research Fellowship.
7
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Graphical Abstract
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Highlights
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Red emitting FMRs with 2- 4 fold increase in emission intensity in polar-protic solvent.
Donor insertion and geometry restriction by BF complexation foster the optical properties.
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Solvent polarizability is mainly responsible for the red shift inabsorption and emission.
Dioxaborines show red shifted emission in dichloromethane compared to chalcones.
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