Push-pull fluorophores with viscosity dependent and aggregation induced emissions insensitive to polarity

Article abstract of DOI:10.1016/j.dyepig.2015.07.017

Abstract A series of push-pull chromophoric extended styryls from 5,5′-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-morpholinothiazole-4-carbaldehyde) were synthesized by Knoevenagal condensation reaction with active methylene compounds. The intermediate carbaldehyde was synthesized from carbazole through multistep reactions. The intramolecular charge transfer of synthesized highly conjugated symmetrical D-π-A (D-donor, A-acceptor) extended styryls with rigid structure have been investigated by means of photophysical properties. The photophysical properties like absorption, emission and quantum yield of styryl derivatives were evaluated in various solvents of different polarities. All these synthesized extended styryls have exhibited aggregation induced emission with enhanced fluorescence intensity. This series of compounds can also be used as fluorescence molecular rotors for viscosity sensing. The sensitivity of viscosity towards UV absorption as well as fluorescence emission has also been investigated.

Full text of DOI:10.1016/j.dyepig.2015.07.017

Accepted Manuscript
Push-Pull fluorophores with viscosity dependent and aggregation induced emissions
insensitive to polarity
Rahul D. Telore, Manjaree A. Satam, Nagaiyan Sekar
PII:
S0143-7208(15)00285-5
10.1016/j.dyepig.2015.07.017
DYPI 4857
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 June 2015
Revised Date: 7 July 2015
Accepted Date: 13 July 2015
Please cite this article as: Telore RD, Satam MA, Sekar N, Push-Pull fluorophores with viscosity
dependent and aggregation induced emissions insensitive to polarity, Dyes and Pigments (2015), doi:
10.1016/j.dyepig.2015.07.017.
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ACCEPTED MANUSCRIPT
Push-Pull fluorophores with viscosity dependent and aggregation induced emissions
insensitive to polarity
Rahul D. Telore, Manjaree A. Satam, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, Matunga,
Mumbai-400 019, India.
E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel: +91 22 3361 2707; Fax: +91 22 3361 1020.
Graphical Abstract:

ACCEPTED MANUSCRIPT
Push-Pull fluorophores with viscosity dependent and aggregation induced emissions
insensitive to polarity
Rahul D. Telore, Manjaree A. Satam, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, Matunga,
Mumbai-400 019, India.
E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel: +91 22 3361 2707; Fax: +91 22 3361 1020.
Abstract:
A series of push-pull chromophoric extended styryls from 5,5'-(9-ethyl-9H-carbazole-
3,6-diyl)bis(2-morpholinothiazole-4-carbaldehyde) were synthesized by Knoevenagal
condensation reaction with active methylene compounds. The intermediate carbaldehyde
was synthesized from carbazole through multistep reactions. The intramolecular charge
transfer of synthesized highly conjugated symmetrical D-π-A (D- donor, A- acceptor)
extended styryls with rigid structure have been investigated by means of photophysical
properties. The photophysical properties like absorption, emission and quantum yield of
styryl derivatives were evaluated in various solvents of different polarities. All these
synthesized extended styryls have exhibited aggregation induced emission with enhanced
fluorescence intensity. This series of compounds can also be used as fluorescence molecular
rotors for viscosity sensing. The sensitivity of viscosity towards UV absorption as well as
fluorescence emission has also been investigated.
Keywords: Extended styryl, Photophysical properties, Aggregation induced emission (AIE),
Fluorescent Molecular Rotors (FMR).
1. Introduction:
The carbazole-based compounds are having respectable chemical and physical
properties, such as highly emissive and electron conducting properties, chemical and
thermal stability and also capability with various transformation, polymerization, metal
catalyzed cross coupling reaction [1–3]. The carbazole containing compounds have many
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applications in various fields like organic light emitting diodes (OLED) [4], dye-sensitized
solar cells [5,6], non linear optics [7], photoconductors [8], solid state lasers [9], and bio-
imaging [10]. The carbazole derivatives are also appealing molecules because of their
intense luminescence and electron efficiency [11].
Planar structure of the carbazole ring helps to maintain good conjugation within the
molecule. The carbazole ring is having two electron rich active sites 3 and 6 positions with
respect to the bridge nitrogen. The introduction of electron withdrawing groups at these
position leads to intramolecular charge transfer [12]. The carbazole moiety being rigid with
a donor-rigidised residue improves π-electron delocalization resulting in a better two-photon
absorbing property [13]. The N-alkyl carbazole compounds show solid-state fluorescence
with good photophysical properties, and electro-optical properties [12]. A wide variety of
structural modifications on the donor, acceptor and π-conjugated moieties have been
reported such as carbazole extended with thiazole or thiophene moieties [5].
The aggregation induced emission (AIE), first reported by Tang and co-workers in 2001
[14–19], has great importance due to its promising applications such as OLEDs [20–22],
optoelectronic devices [23,24], biosensor [25–28], photo memory [29], and chemosensors
[30,31]. The AIE phenomenon is mainly caused by restriction of intramolecular rotation
[32] accompanied by restriction of intramolecular charge transfer [33], and twisted
intramolecular charge transfer [34]. Many long conjugated molecules with twisted skeleton
conformation have been established to show AIE phenomenon [18]. The restriction of the
intramolecular rotation (RIR) and the non-planar configuration with π– π stacking
interactions are mainly responsible for the AIE effect [35].
Much attention has also been paid to fluorescent molecular rotors (FMRs), which show
solvent viscosity dependent fluorescence emission [36–39]. These rotors emit with low
intensity in fluid solution (low viscosity) due to the free rotation of the C–C/C=C bonds in
the excited state, which serves as an efficient overflow for the photo excited molecules [40].
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However, in a viscous environment the intramolecular rotation is restricted, and thus the
emission is highly intensified [39].
In this paper, electron donating carbazole nucleus functionalized at 3 and 6 positions
linked to the various acceptor residues originating from the active methylene compounds by
Knoevenagel condensation to give extended styryls are reported. Photophysical properties
of the synthesized compounds in several solvents have been studied.
2. Experimental :
2.1. Materials and equipment:
All the reagents and solvents were purchased from the S. D. Fine Chemicals Pvt. Ltd.
and used without purification. All the solvents were used of spectroscopic grade. Melting
points were recorded by open capillary on Sunder Industrial Product and are uncorrected.
UV-Visible absorption measurements were carried out using Perkin Elmer
spectrophotometer with 1 cm quartz cells. The excitation wavelength was taken as λmax of
compound. The scan range was 250 to 650 nm. Fluorescence emission measurements were
recorded on Cary Eclipse fluorescence spectrophotometer (Varian, Australia) using 1 cm
1
13
quartz cells. H NMR and C NMR spectra were measured on an Agilent 500 MHz NMR
spectrometer with tetramethylsilane as the internal standard.
2.2. Synthesis and characterization:
2.2.1. Preparation of 1,1'-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-bromoethanone) 4:
N-Ethylcarbazole 2 (5.0 g, 25.6 mmol) was dissolved in dichloromethane (20 mL) and
then AlCl3 (8.5 g, 64.1 mmol) was added followed by drop wise addition of bromoacetyl
bromide 3 (12.9g, 64.1 mmol, d = 2.3 g/mL) at 0 oC. The reaction mixture was stirred for 5-
o
6 h at 0 oC and then poured over ice bath at 0-5 C. The aqueous layer was extracted in
dichloromethane. The organic layer was dried over anhydrous sodium sulphate and
evaporated under vacuum. The precipitate was recrystallized from ethanol to obtained
desired compound 2 as yellowish green crystal.
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Yield: 60%; M.P.: 186-190oC.
1H-NMR(DMSO): 1.34 (t, 3H, CH3), 4.5 (q, 16H), 5.01 (s, 4H), 7.81 (d, 2H, J = 8.6 Hz,
ArH), 8.1 (d, 2H, J = 6.7 Hz), 9.1 (s, 2H).
FT-IR (cm-1): 1663 (-C=O); 1489 (-Ar); 824 (-C-Br).
Mass: 438 (m+1).
2.2.2.
Preparation
of
4,4'-(5,5'-(9-ethyl-9H-carbazole-3,6-diyl)bis(thiazole-5,2-
diyl))dimorpholine 6:
Bis-haloacetyl N-ethylcarbazole 4 (5.0 g, 0.011 mmol) was heated with substituted
thiourea 5 (3.6g, 0.025 mmol) in ethanol (25 mL) at 80 oC for 4 h. The reaction mixture was
poured over ice-cold water. A pale yellow solid 6 was obtained, filtered and recrystallized
using ethanol.
Yield: 72%; M.P.: 220-222 oC.
1H-NMR (DMSO): 1.3 (t, 3H), 3.7 (m, 16H), 4.4 (q, 2H), 7.2 (s, 2H), 7.5 (d, 2H, J = 8.4
Hz), 7.9 (d, 2H, J = 8.5 Hz), 8.6 (s, 2H).
FT-IR (cm-1): 1531 (-C=N); 1475 (-Ar); 1112 (-C-O); 699 (C-S).
Mass: 532 (m+1).
2.2.3. Preparation of 5,5'-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-morpholinothiazole-4-
carbaldehyde 7:
o
POCl3 (4.3 g, 28.0 mmol, d=1.64g/mL) was added drop wise at 0 C DMF (2.3g 28.0
mmol, D = 0.9 g/mL) followed by the addition of the compound 6 (5.0 g, 9.4 mmol) in
o
DMF at 0 C. The reaction mixture was stirred for 30 min at room temperature and heated
o
to 70 C for 5-6 h. Water (100 mL) was added into the reaction mixture and neutralized by
2N (25 mL × 3) sodium bicarbonate solution. A green solid was obtained, filtered and
purified by column chromatography on silica with dichloromethane/methanol as eluent.
Yield: 68%; M.P.: >300oC;
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1H-NMR (CDCl3): 1.43 (t, 3H), 3.7 (m, 16H), 4.38 (q, 2H), 7.4 (s, 2H, J = 6.42 Hz), 7.8 (d,
2H, J = 1.72 Hz), 8.4 (s, 2H), 9.7 (s, 2H).
FT-IR (cm-1): 2969 (-C-H stretch, Ald), 1737 (-C=O), 1513 (-C=N), 1445 (-Ar), 1114 (-C-
O), 697 (-C-S).
Mass: 589 (M+1).
2.2.4. Preparation of extended styryls 9a-9d:
The carbaldehyde 7 (0.2 g, 0.33 mmol) was stirred with various active methylene
compounds 8a-8d (0.74 mmol) in ethanol. Then piperidine was added in catalytic amount
(0.1 mL) and the reaction mixture was refluxed for 24 h. After cooling the precipitated solid
was filtered and purified by column chromatography on silica with n-hexane/ethyl acetate
as eluent.
2.2.4.1.
2,2'-((5,5'-(9-Ethyl-9H-carbazole-3,6-diyl)bis(2-morpholinothiazole-5,4-
diyl))bis(methanylylidene))dimalononitrile 9a:
Color: Orange; Yield: 66%; M.P.: >300oC.
1H-NMR (CDCl3): 1.52 (t, 3H), 3.8 (m, 16H), 4.5 (q, 2H), 7.57 (d, 2H, J = 8 Hz), 7.72 (d,
2H, J = 9 Hz), 7.6 (s, 2H), 8.35 (s, 2H).
13C NMR (CDCl3, δ): 13.8, 30.9, 66.0, 109.5, 114.8, 115.9, 122.8, 123.2, 124.5, 128.4,
141.8,150.1, 169.3, 173.4
FT-IR (cm-1): 2206 (–CN), 1533 (C=N), 1425 (-Ar), 1105 (-C-O), 1025 (-C-N).
Mass: 684 (M+1).
HRMS: Calculated for C36H29N9O2S2Na [M+Na]+ 706.1783, found 706.1717.
2.2.4.2.
(2Z,2'Z)-Diethyl
3,3'-(5,5'-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-
morpholinothiazole-5,4-diyl))bis(2-methylacrylate) 9b:
Color: Yellow; Yield: 59%; M.P.: >212oC.
1H-NMR: 1.31 (t, 6H), 1.47 (t, 3H), 3.80 (m, 16H), 4.20 (q, 4H), 4.40 (q, 2H), 7.52 (d, 2H, J
= 8.5 Hz), 7.78 (d, 2H, J = 7.5 Hz), 8.39 (s, 2H). 8.45 (s, 2H).
5

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13C NMR (CDCl3, δ): 13.8, 38.0, 48.5, 61.7, 66.1, 91.2, 108.8, 115.6, 117.5, 122.8, 123.2,
124.9, 128.1, 141.5, 147.3, 165.6, 168.1, 173.0, 183.0.
FT-IR (cm-1): 2203 (-CN), 1701 (-C=O), 1521 (-C=N), 1435 (-Ar), 1113 (-C-O), 1014 (-C-
N).
Mass: 778 (M+1).
HRMS: Calculated for C40H39N7O6S2Na [M+] 778.2481, found 778.2410.
2.2.4.3.
(2Z,2'Z)-3,3'-(5,5'-(9-Ethyl-9H-carbazole-3,6-diyl)bis(2-morpholinothiazole-5,4-
diyl))bis(2-(benzo[d]thiazol-2-yl)acrylonitrile) 9c:
Color: Red; Yield: 48%; M.P.: >300oC.
1H-NMR: 4.87 (q, 2H), 7.29 (t, 2H), 7.39 (t, 2H), 7.6 (d, 2H, J = 8.3Hz), 7.7 (d, 2H, J = 7.5
Hz), 7.84 (d, 2H, J = 6.7Hz), 7.89 (d, 2H, J = 7.8Hz), 8.39 (s, 2H).
13C NMR (CDCl3, δ): 13.9, 30.9, 48.4, 66.0, 96.4, 109.1, 116.3, 117.7, 121.3 122.8, 123.4,
124.9, 125.5, 126.4, 128.4, 134.5, 139.6, 141.4, 153.9, 164.2, 165.6, 172.1.
FT-IR: 2201 (-CN), 1525 (-C=N), 1425 (-Ar), 1112 (-C-O), 1027 (-C-N);
Mass: 900(M+1).
HRMS: Calculated for C48H38N9O2S4 [M+] 900.1265, found 900.1082.
2.2.4.4.
(2Z,2'Z)-3,3'-(5,5'-(9-Ethyl-9H-carbazole-3,6-diyl)bis(2-morpholinothiazole-5,4-
diyl))bis(2-(1H-benzo[d]imidazol-2-yl)acrylonitrile) 9d:
Color: Yellow; Yield: 51%; M.P.: Decompose 260oC.
1H-NMR (DMSO δ): 1.39 (t, 3), 3.65 (m, 16H), 4.59 (q, 2H), 7.08 (m, 4H), 7.32 (d, 2H, J =
6.67 Hz), 7.42 (d, 2H, J = 6.65 Hz), 7.82 (m, 4H), 8.24 (s, 2H), 8.58 (s, 2H), 12.69 (s, 2H).
13C NMR (DMSO, δ): 14.4, 48.5, 65.8, 92.6, 110.1, 111.6, 115.6, 118.0, 118.7, 122.9, 123.5,
125.5, 128.7, 135.4, 138.2, 141.2, 143.9, 148.9, 164.4, 171.2.
FT-IR: 2207 (-CN), 1524 (-C=N), 1418 (-Ar), 1120 (-C-O), 1024 (-C-N).
Mass: 867 (M+1).
HRMS: Calculated for C48H40N11O2S2 [M+H]+ 866.0258, found 866.1890.
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3. Results and Discussion:
3.1. Design of extended styryl:
<< Please insert Figure 1: Design of extended styryl.>>
In continuation of our previous work [12], we planned to synthesize novel highly
conjugated symmetrical D-π-A (D- donor, A- acceptor) extended styryls with rigid structure
exhibiting intramolecular charge transfer. Design of such extended styryls with introduction
of the thiazole ring between the acceptor and donor for rigidity may lead to enhancement of
photophysical properties (Figure 1). The thiazole ring is responsible for restriction of cis-
trans polarization of extended double bond, which may improve the fluorescence. The
thiazole bridge also gives rigidity to double bond due to which the terminal cyano groups
rotate spontaneously which leads to the Fluorescent Molecular Rotors (FMR). At the same
time thiazole ring gives more planarity to the molecules that may lead to Aggregation
Induced Emission (AIE) phenomenon.
3.2. Synthesis of carbaldehyde 7 and extended styryls 9a-9d:
We first synthesized the carbaldehyde 7 by multistep reactions on carbazole 1
(Scheme 1). The 9H-carbazole 1 was N-ethylated using diethyl sulphate, which was further
bis-acetylated using bromo acetyl bromide at 3,6-position by Friedel-Craft reaction to
obtain the compound 4. The compound 4 was condensed with morpholine-substituted
thiourea 5 to give the intermediate 6. This intermediate 6 was formylated by Vilsmeier-
Haack reaction to give the carbaldehyde 7.
All the extended styryls were synthesized by simple classical Knoevenagel
condensation of the carbaldehyde 7 with the active methylene compounds 8a-8d as shown
in Scheme 2. The carbaldehyde 7 and the various active methylene compounds 8a-8d were
refluxed in absolute ethanol containing a catalytic amount of piperidine (2 drops) to yield
the desired extended styryls 9a-9d linked through morpholine-thiazole to the carbazole core.
The carbaldehyde 7 and the extended styryls 9a-9d were purified by column
7

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chromatography and confirmed by FT-IR, mass spectrometry, 1H-NMR spectroscopy.
<< Please insert Reaction Scheme 1: Synthesis of 5,5'-(9-ethyl-9H-carbazole-3,6-
diyl)bis(2-morpholinothiazole-4-carbaldehyde 7 >>
<< Please insert Reaction Scheme 2: Synthesis of extended styryls (9a-9d) >>
3.3. Effect of solvent polarity on photophysical properties of carbaldehyde 7 and extended
styryls 9a-9d:
All the compounds show fluorescence in various solvents under irradiation of UV
light except the carbaldehyde 7. The carbaldehyde 7 has less solubility in organic solvents
and hence fluoresces only in highly polar aprotic solvents, DMF and DMSO (Figure 2).
The carbaldehyde 7 showed slight red shift in the absorption as well as in the emission from
DMF to DMSO (Table 1). In the case of DMF, the carbaldehyde 7 showed a triple
absorption but in DMSO it showed single absorption. A study of the photophysical
properties of the carbaldehyde 7 in the different mixtures of DMSO and DMF has also been
carried out to understand the effect of change in solvent polarity. There was no extreme
change observed in the absorption and emission values at various compositions of solvent
mixtures (Table S1).
<< Please insert Figure 2: Effect of solvent polarity on photophysical properties of
carbaldehyde 7.>>
<< Please insert Table 1: Effect of solvent polarity on photophysical properties of
carbaldehyde 7. >>
To evaluate the effect of the solvent polarity on the photophysical properties of the
extended styryls 9a-9d, the absorption and emission properties of all the extended styryls
were measured in different solvents of differing polarities. All the analyses were carried out
at room temperature and concentrations of solutions were 1x10-6M. The photophysical data
of the extended styryls 9a-9d were given in (Figure 3a-3d), (Tables 2 and S2-S5).
8

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All the extended styryls 9a-9d (A-π-D-π-A) were having N-ethylcarbazole unit as
the electron donor and cyano group as the electron acceptor. Due to the presence of electron
accepting group, these extended styryls show intramolecular charge transfer. Thus
photophysical properties of these extended styryls were highly influenced by the solvent
polarity against the parent carbaldehyde 7.
<< Please insert Figure 3a: Effect of solvent polarity on photophysical properties of
extended styryl 9a. >>
<< Please insert Figure 3b: Effect of solvent polarity on photophysical properties of
extended styryl 9b. >>
<< Please insert Figure 3c: Effect of solvent polarity on photophysical properties of
extended styryl 9c. >>
<< Please insert Figure 3d: Effect of solvent polarity on photophysical properties of
extended styryl 9d. >>
<< Please insert Table 2: Effect of solvent polarity on photophysical properties of
extended styryl 9a. >>
All the extended styryls 9a-9d showed a broad absorption band in all the solvents. The
short-wavelength absorption at around 380 nm can be attributed to the π-π* transitions from
the carbazole core to the morpholine-thiazole moiety. This large absorption band in the
visible region was corresponding to ICT transition from the central electron donating N-
ethylcarbazole unit to both the electron accepting cyano groups at the terminal via the
morpholine-thiazole moiety. The absorption maxima of all the extended styryls 9a-9d
varied in different solvents but independent of solvent polarity. The absorption band
observed was from 443 nm to 452 nm for 9a, from 433 nm to 446 nm for 9b. 9a showed
little bathochromic shift than the 9b as 9a having malanonitrile group which is more
electron withdrawing as compared to ethylcyanoacetate in 9b. Similar trend was observed
in the case of 9c and 9d; the absorption band observed was from 471 nm to 482 nm for 9c,
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from 450 nm to 463 nm for 9d. 9c showed bathochromic shift as compared to the 9d due to
the vacant d-orbital of benzothiazole sulfur atom, which exhibits strong electron
withdrawing effect than the benzimidazole unit. The absorbance values of extended styryls
9a-9d were higher in chlorinated solvents.
The fluorescence emission wavelength of the synthesized extended styryls 9a-9d is
independent on solvent polarity. The dye 9b observed a red shift in chlorinated solvents like
DCM and chloroform as compared to the other solvents. The quantum yield of all the
extended styryls 9a-9d also varied in different solvents, but it was independent of the
solvent polarity. The 9a showed the highest quantum yield in 1,4-dioxane and lowered in
toluene. The 9b and 9c showed the highest quantum yield in DMSO and lower in acetone
and acetonitrile respectively. While 9d observed the highest quantum yield in DCM and
lowered in DMF.
The main curiosity of this work is to enhance the photophysical properties of
previously reported extended styryls [12]. Cis-trans polarization may possible across the
double bond of previously synthesized molecules as shown in Figure 4a. Introduction of
the thiazole ring gives more stability as well as restricts cis-trans polarization in double
bond. Also introduction of morpholine ring at terminal position plays an important role to
enhance color and stability of compounds [41] but failed to give an enhancement in the
photophysical properties (Figure 4b). Selection of morpholine moiety is due to its donating
ability intermediate between dimethylamino and diethylamino groups.
The morpholine moiety presented at the thiazole ring has the donating ability, which
may disturb the main conjugation between the donor carbazole to the acceptor cyano groups.
Due to the two donors present in the system, there is competition between two types of
conjugation, conjugation from donor carbazole to acceptor cyano and other from
morpholine nitrogen to acceptor cyano moiety (Figure 4b).
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<< Please insert Figure 4a: Possible cis-trans polarization between -C=C- of previously
reported extended styryl. >>
<< Please insert Figure 4b: Restricted cis-trans polarization due to presence of thiazole
ring. >>
3.3.1. Solvent polarity function plots:
Effects of solvent polarity on the synthesized molecules were also studied with the
help of Lippert-Mataga plots [42]. Lippert-Mataga equation (eq. 1) is used to estimate
change in dipole moment on photoexcitation as a function of the solvent polarity. Lippert-
Mataga plot generally shows specific solvent effect such as dipole moment changes of the
dyes with photoexcitation and this clarifies that the solvent effect such as hydrogen bonding
may lead to quench the fluorescence [43,44].
ꢀꢁꢂ
Δν = ꢃꢄꢅ ℏꢇꢈ (ꢊꢋꢌꢊꢍ)ꢀ + ꢎ
(1)
ꢉ
ꢆ
ꢒ
ꢅꢌꢏ
ꢑ ꢌꢏ
∆ f = ꢀꢅꢐꢏ
−
ꢒ
ꢀꢑ ꢐꢏ
Where, ∆ν = νabs - νem stands for Stokes shift,
abs and νem are absorption and emission (cm-1),
ν
ℏ = Planck’s constant,
c = velocity of light in vacuum,
a = Onsager cavity radius,
b = constant,
∆f = orientation polarizability,
µg = ground-state dipole in the ground-state geometry,
µe = excited-state dipole in the excited-state geometry,
ꢓꢔ = permittivity of the vacuum,
(µe-µg)2 = proportional to the slope of the Lippert-Mataga plot.
Lippert-Mataga plots were constructed for all the extended styryls 9a-9d, which
concluded that there was no specific solvent effect exists (Figure S1). Slope of 9c was
11

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much smaller than other extended styryls, which concluded that 9c has smaller dipole
moment changes on photoexcitation.
The Wellers plots [45] can be used to study ICT. These plots of emission wave
numbers of emission versus the respective polarity functions were screened for the linearity
(figure S2). The polar solvents were used to plot these functions and linear plots were
obtained which concluded that these extended styryls 9a-9d observed good charge transfers
within the molecules at the excited state.
3.3.2. Oscillator strength and Transition state dipole moment:
Oscillator strength is dimensionless quantity that expresses the probability of absorption and
emission properties in energy levels, which helps to understand charge transfer within the
molecules. It simply describes number of electron transition from ground to excited state.
Oscillator strength (f) can be calculated using the following Equation 2 [46]. Where ε is the
extinction coefficient (Lmol-1cm-1), and ꢕ represents the wavenumber (cm-1). From this
equation we have calculated oscillator strength for extended styryls and tabulated in Table
3.
ꢖ = 4.42 × 10ꢌꢘ ꢓ ꢕ ꢙꢕ
( )
2
ꢗ
ꢗ
By using the value of f, we have calculated transition dipole moment that was the
differences in electric charge distribution between the ground and excited state of the
molecule. The transition dipole moment for absorption (ꢀa) is a measure of the probability
of radiative transitions which have been calculated for the extended styryls in different
solvent environments using the equation 3 [46]. The transition dipole moment increased
with increase in the oscillator strength.
ꢂ
ꢊꢈꢀ = ꢀ.ꢏꢚ×ꢏꢔ
3
ꢛꢉꢆ
× ꢜ
Where,
ꢊ
a is transition dipole moment (D),
f is oscillator strength,
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ꢕ is wavenumber (cm-1).
The extended styryls 9a (4.6-5.9 D), 9b (6.5-9.8 D), 9c (7.3-8.2 D) and 9d (6.2-7.4)
have significant transition dipole moment. Observed results were concluded that the
conjugation of 3- and 6- position of carbazole gave better charge transfer through the
carbazole to cyano groups.
<< Please insert Table 3: Oscillator strength (f) and Transition dipole moments (TDM)
obtained from absorption properties of extended styryls in various solvents.>>
3.3.3. Solid state fluorescence:
The carbaldehyde 7 and all the extended styryls 9a-9d showed solid state
fluorescence under long UV (365 nm). The carbaldehyde 7 was excited at 340 nm while the
extended styryls 9a-9d were excited at 400 nm keeping all the parameters constant such as
source of light, slit width, sample holder. The carbaldehyde 7 showed emission maxima at
486 nm while the extended styryls 9a-9d showed emission maxima at 540 nm, 541 nm, 612
nm, 539 nm respectively (Figure 5). All the extended styryls 9a-9d observed a
bathochromic shift as compared to the carbaldehyde 7. The extended styryl 9c showed solid
state fluorescence in NIR region with the highest red shift.
<< Please insert Figure 5: Solid state fluorescence spectra for carbaldehyde 7 and
extended styryls 9a-9d.>>
Solid state fluorescence emissions of all the synthesized styryls except 9d were
observed red shift as compared to the corresponding emission in solutions. This may be due
to molecular staking in the solid state. This red shift was also depending upon presence of
neighboring polar substituents. The extent of red shift in carbaldehyde 7 (4 nm/170.75cm-1)
was smaller compared to extended styryls 9a-9c since these styryls contained more polar
groups such as malanonitrile, ethylcyanoacetate and benzthiazolylacetonitrile respectively.
The red shift was large in the case of the extended styryls 9a (28 nm/1012.73 cm-1), 9b (28
nm/1008.83cm-1), 9c (49 nm/1422.12 cm-1). The extended styryl 9c showed the highest red
13

ACCEPTED MANUSCRIPT
shift at 612 nm (16339.86 cm-1) may due to the presence of empty d-orbital of sulphur of
benzothiazole ring. These groups may responsible for different types of aggregations.
Carbaldehyde 7 and extended styryls 9a-9c were undergo J-aggregation in solid state
resulting in red shift whereas 9d was formed H-aggregates with blue shift.
3.4. Aggregation-induced emission (AIE):
The study of AIE of the extended styryls 9a-9d has been carried out in the mixture
of DMF and water. Different amount of water has been added to the pure solution of DMF
to get 10-90% volume of water-DMF solution. The absorption and emission spectra have
been recorded immediately after the sample preparation (Figure S3-S6).
When water was added, absorption maxima of synthesized extended styryls 9a-9d
remained unchanged but a dramatic change was observed in the emission maxima. These
extended styryls showed almost similar absorption maxima in different combination of
water-DMF solutions at around 448 nm, 443 nm, 474 nm and 479 nm respectively.
In 40-50% of water-DMF solution, emission wavelength of 9a was observed a red
shift from 606 nm to 612 nm with enhanced fluorescence due to the presence of two cyano
groups, which act as strong electron withdrawing groups. In the case of 9b anomalous
results were obtained. In 40-50% water-DMF solution, emission of 9b showed
hypsochromic shift from 567 nm to 547 nm. This hypsochromic shift may due to the
presence of the ester group, which acts as a weak withdrawing group. A large red shift was
observed in the case of 9c in 30-40% water-DMF solution from 560 nm to 605 nm.
Similarly 9d showed a red shift in 50-60% water-DMF solution from 580 nm to 602 nm.
The Stokes shift also increases with increasing water content. The shift observed in
emission peak of these extended styryls 9a-9d with increasing water content may due to the
strong electronic interaction through the π-stacking in aggregate that present at carbazole
and thiazole unit. All these synthesized extended styryls 9a-9d showed aggregation induced
emission with enhanced fluorescence intensity. The quantum efficiency of extended styryls
14

ACCEPTED MANUSCRIPT
9a-9d increases with increasing water fraction, indicating that the aggregated state has
higher quantum yield as compared to the non-aggregated state. The aggregates formed with
40 to 50% of water fraction showed the highest quantum yield (Figure S10).
The change in dipole moment at excited state and ground state can be explained by
using Bilot-Kawski function (eq. 4) that give the ratio of ground state and excited state
dipole moment (Figure S7). Ratio of excited state and ground state dipole moment
drastically increases in 9a (19 fold) and 9d (32 fold). These results concluded that dipole
moment increases in excited state and unaffected in ground state.
ꢒ
ꢝꢈꢌꢝꢂ = ꢞꢟ ꢌꢟ ꢢ ∆ fBK+ constant
(4)
ꢠ
ꢡ
ꢉ
ꢄꢅꢣꢤꢥ
ꢒ
ꢒ
ꢑ ꢌꢏ
∆ fBK = ꢀꢑ ꢐꢏ ꢦꢅꢌꢏ
−
ꢨ
ꢒ
ꢒ
ꢑ ꢐꢀ ꢧꢐꢀ
ꢑ ꢐꢀ
3.5. Fluorescent Molecular Rotors (FMR):
FMR needs good intramolecular charge transfer (ICT) within the molecule, which
also requires planarity with rigidity. Keeping this concept in mind, previously reported
extended styryls [14] has been modified by introduction of morpholine-thiazole moiety to
avoid cis-trans polarization which responsible for fluorescence quenching.
The carbaldehyde 7 showed absorption around 330 nm and extended styryl 9a-9d
having ICT showed absorption in the range of 450-480 nm. This proves that molecules
showed good charge transfer from carbazole to acceptor cyano moiety. According to the
concept of FMR, polarity dependent emission molecules are FMR inactive. Emissions of
synthesized extended styryls 9a-9d are independent on solvent polarity, which can be used
as FMR.
Effects of viscosity on emission properties of synthesized molecules 9a-9d have also
been studied to check compatibility as FMR. These synthesized molecules 9a-9d have less
solubility in the mixture of ethylene glycol and glycerol. Thus, these synthesized molecules
15

ACCEPTED MANUSCRIPT
9a-9d have been solubilized in highly polar aprotic solvent DMF and increased their
viscosity using glycerol (Figure S8). Similar study has been carried out in non-polar solvent
DCM and increased their polarity with PEG-400 (Figure S9). The pattern of increase in
emission intensity was the same in both the studies. This result proved that the change in
emission intensity was due to the change in viscosity and not because of polarity. Thus
synthesized molecules 9a-9d are good candidates for FMR.
logI = C + x logη
(5)
The emission intensity of FMR and viscosity of the solvent was described by using Förster–
Hoffmann equation (5), in which η is viscosity of the solvents, I is the emission intensity of
the rotors, C is a constant on temperature and x is the viscosity sensitivity of the rotors. The
higher the x value proves that emission intensities are more sensitive towards the viscosity
change. The x values of the rotors 9a-9d in DMF-Glycerol system were 0.35, 0.35, 0.37 and
0.54 respectively, while x values of the rotors 9a-9d in DCM: PEG-400 system was 0.42,
0.52, 0.67, and 0.64 respectively. Rotors 9c and 9d in DCM: PEG-400 system showed
better x value as compared to the historical rotor 4-(dicyanovinyl)julolidine (x values is 0.6)
[39]. The higher values of x in DCM: PEG-400 may due to the effect of solvent polarity on
the transition from the local excited state to the intramolecular charge transfer (ICT) state.
4. Conclusion:
In this paper, we have extended our previous work and succeeded to synthesize four
extended styryls containing carbazole core as a donor and electron withdrawing cyano
moiety linked through π bridge of morpholine-thiazole moiety with intramolecular charge
transfer. All synthesized extended styryls were showed solid state fluorescence. The aim
was to observe bathochromic shift in photophysical properties as compared to previously
reported extended styryls by introduction of more conjugation and rigidity via thiazole ring.
But anomalous results were obtained. Morpholine ring presented on thiazole moiety may
disturb the conjugation. Photophysical properties of all synthesized extended styryls were in
16

ACCEPTED MANUSCRIPT
good agreement and independent on solvent polarity. Morpholine-thiazole moiety presented
in extended styryls were provided rigidity which responsible to act as FMRs. All these
extended styryls were also showed aggregation induced emission (AIE) with enhanced
fluorescence intensity.
Acknowledgements:
Rahul Telore is thankful to University Grants Commission- SAP, India for providing Junior
and Senior Research Fellowships.
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List of figures captions:
Figure 1: Design of extended styryl.
Reaction Scheme 1: Synthesis of 5,5'-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-
morpholinothiazole-4-carbaldehyde 7.
Reaction Scheme 2: Synthesis of extended styryls (9a-9d).
Figure 2: Effect of solvent polarity on photophysical properties of carbaldehyde 7.
Figure 3a: Effect of solvent polarity on photophysical properties of extended styryl 9a.
Figure 3b: Effect of solvent polarity on photophysical properties of extended styryl 9b.
Figure 3c: Effect of solvent polarity on photophysical properties of extended styryl 9c.
Figure 3d: Effect of solvent polarity on photophysical properties of extended styryl 9d.
Figure 4a: Possible cis-trans polarization between -C=C- of previously reported extended
styryl.
Figure 4b: Restricted cis-trans polarization due to presence of thiazole ring.
Figure 5: Solid state fluorescence spectra for carbaldehyde 7 and extended styryls 9a-9d.
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Table 1: Effect of solvent polarity on photophysical properties of carbaldehyde 7.
Absorption (λabs)
Emission (λemi)
Stokes shift
ε (X 104)
7
nm
FWHM
cm-1
nm
cm-1
ϕ
nm
cm-1
( Lmol−1 cm−1)
307
338
63
44
78
32573.2
29585.7
29411.7
4.2
3.8
3.7
DMF
473 21141.6 0.10 166 11432
482 20746.8 0.12 142 8665
DMSO 340
Table 2: Effect of solvent polarity on photophysical properties of extended styryl 9a.
Absorption (λabs)
Emission (λemi)
Stokes shift
ε (X 104)
9a
nm
FWHM
cm-1
nm
cm-1
ϕ
nm
cm-1
( Lmol-1 cm-1)
Toluene
451
73
69
71
71
68
76
80
88
88
22172.9
22371.3
22421.5
22573.3
22471.9
22471.9
22471.9
22321.4
22123.8
2.3
3.1
2.2
1.5
2.2
2.1
2.4
0.8
1.8
508 19685.0 0.001
515 19417.4 0.1
57
68
66
65
62
77
71
76
70
2487.9
2953.9
2890.3
2888.3
2748.1
3314.9
3092.1
3237.5
2966.8
Dioxane 445
DCM
CHCl3
EtOAc
447
446
443
512 19531.2 0.009
508 19685.0 0.004
507 19723.8 0.005
522 19157.0 0.003
516 19379.8 0.009
524 19083.9 0.004
522 19157.0 0.047
Acetone 445
ACN
DMF
445
448
451
DMSO

ACCEPTED MANUSCRIPT
Table 3: Oscillator strength (f) and Transition dipole moments (TDM) obtained from
absorption properties of extended styryls in various solvents.
9a
9b
9c
9c
f
TDM
5.95
5.54
6.71
5.67
4.61
5.62
5.97
3.68
5.29
f
TDM
6.41
9.35
9.55
9.61
6.51
6.53
9.59
9.82
6.37
f
TDM
7.63
7.35
8.10
7.62
7.53
7.92
7.28
7.93
8.24
f
TDM
6.20
6.70
7.45
6.25
6.34
6.44
6.03
6.33
6.65
Toluene
Dioxane
DCM
0.368
0.324
0.472
0.338
0.225
0.334
0.377
0.142
0.291
0.438
0.933
0.970
1.001
0.454
0.459
0.987
1.022
0.427
0.574
0.534
0.651
0.579
0.563
0.624
0.530
0.616
0.663
0.390
0.460
0.565
0.404
0.411
0.429
0.380
0.413
0.451
CHCl3
EA
Acetone
ACN
DMF
DMSO

ACCEPTED MANUSCRIPT
List of Figures
Figure 1: Design of extended styryl.
Reaction Scheme 1: Synthesis of 5,5'-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-morpholinothiazole-
4-carbaldehyde 7.
1

ACCEPTED MANUSCRIPT
Reaction Scheme 2: Synthesis of extended styryls (9a-9d).
Figure 2: Effect of solvent polarity on photophysical properties of carbaldehyde 7.
0.8
DMF
DMF
7
7
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
100
80
60
40
20
0
DMSO
DMSO
370
410
450
490
530
570
610
250
300
350
400
450
Wavelength (nm)
Wavelength (nm)
2

ACCEPTED MANUSCRIPT
Figure 3a: Effect of solvent polarity on photophysical properties of extended styryl 9a.
Toluene
Dioxane
DCM
CHCL3
EA
Acetone
ACN
DMF
Toluene
Dioxane
DCM
CDCL3
EA
Acetone
ACN
DMF
70
60
50
40
30
20
10
0
9a
9a
0.5
0.4
0.3
0.2
0.1
0
DMSO
DMSO
470 495 520 545 570 595 620
Wavelength (nm)
350
400
450
500
550
Wavelength (nm)
Figure 3b: Effect of solvent polarity on photophysical properties of extended styryl 9b.
Toluene
Dioxane
DCM
CHCL3
EA
Acetone
ACN
Toluene
Dioxane
DCM
CHCL3
EA
Acetone
ACN
DMF
200
150
100
50
9b
9b
1
0.8
0.6
0.4
0.2
0
DMF
DMSO
DMSO
0
450
500
550
600
350
400
450
500
Wavelenth (nm)
Wavelength (nm)
Figure 3c: Effect of solvent polarity on photophysical properties of extended styryl 9c.
Toluene
Dioxane
DCM
Toluene
Dioxane
DCM
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
9c
9c
50
40
30
20
10
0
CHCl3
EA
CHCl3
EA
Acetone
ACN
Acetone
ACN
DMF
DMSO
DMF
DMSO
350
400
450
500
550
500
550
600
650
Wavelength (nm)
Wavelength (nm)
3