Amino substituted 4-pyridylbutadienes: Synthesis and fluorescence investigations

Article abstract of DOI:10.1016/j.dyepig.2015.08.018

Synthesis and spectroscopic investigations of a series of donor-π-acceptor systems containing pyridine as the electron withdrawing group and an amino derivative (dimethylamino, diphenylamino, carbazole and julolidine) as electron donating group, separated by a π-spacer are described. The effect of varying donors on absorption and emission properties was studied in different solvents. All the molecules investigated exhibit pronounced positive polarity dependent solvatochromic shifts of up to ～141 nm. Strong fluorescence quantum yields are also observed for dienes containing carbazole and diphenylamine donors. This behavior suggests the presence of highly polar emitting states as a result of π -π?intramolecular charge-transfer (ICT). The observations were corroborated by a linear relation of the fluorescence maximum (ν_max) versus the solvent polarity function (Δ_f) from the Lippert-Mataga correlation. The emission lifetime shows a decay profile consistent with the formation of one species (1 and 3) and two species (2 and 4) in the excited state.

Full text of DOI:10.1016/j.dyepig.2015.08.018

Dyes and Pigments 123 (2015) 341e348
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Amino substituted 4-pyridylbutadienes: Synthesis and fluorescence
investigations
Harsha Agnihotri b, Paramasivam Mahalingavelar b, Hemant Mande a, Prasanna Ghalsasi a,
,
*
Sriram Kanvah b
a Department of Chemistry, MS University of Baroda, Pratapgunj, Vadodara, Gujarat 390002, India
b Department of Chemistry, Indian Institute of Technology Gandhinagar, Chandkheda, Ahmedabad 382 424, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis and spectroscopic investigations of a series of donor-p-acceptor systems containing pyridine as
Received 4 May 2015
Received in revised form
27 July 2015
Accepted 17 August 2015
Available online 25 August 2015
the electron withdrawing group and an amino derivative (dimethylamino, diphenylamino, carbazole and
julolidine) as electron donating group, separated by a -spacer are described. The effect of varying do-
nors on absorption and emission properties was studied in different solvents. All the molecules inves-
tigated exhibit pronounced positive polarity dependent solvatochromic shifts of up to ~141 nm. Strong
fluorescence quantum yields are also observed for dienes containing carbazole and diphenylamine do-
p
nors. This behavior suggests the presence of highly polar emitting states as a result of
p -p* intra-
Keywords:
Solvatochromism
Pyridyl dyes
Donor-acceptor systems
Julolidine
molecular charge-transfer (ICT). The observations were corroborated by a linear relation of the
fluorescence maximum (nmax) versus the solvent polarity function (Df) from the LipperteMataga cor-
relation. The emission lifetime shows a decay profile consistent with the formation of one species (1 and
3) and two species (2 and 4) in the excited state.
Intramolecular charge transfer
Diarylpolyenes
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
towards photochemical transformations, the lone pair of electrons
on the nitrogen atom of the pyridine contributes to a coordinate
p
-conjugated substrates bearing
p
-electron donor (D) and a
p
-
covalent bonding with Lewis acid sites of TiO2 surface lending its
utility as anchoring group for dye-sensitized solar cells applications
[19e21]. Because of the facile protonation of pyridine moiety and
electron acceptor substitutions exhibiting intramolecular charge
transfer (ICT) have gained prominence for use as functional organic
materials [1e6]. Among many known substrates, derivatives such
as donor-acceptor substituted diphenylpolyenes have attracted
attention as model compounds to understand the light induced
conformational processes of biological substrates [7] and also as
fluorescence probes [8]. Replacement of CH of the phenyl ring of
diphenylpolyenes by nitrogen (N) leads to an isoelectronic species
that is expected to have significant influence on the electronic
its role in deactivating effect of the (n,
p*) state [22], these sub-
strates have also been utilized as potential fluorescence probes for
biological applications [23e28] and also for non-linear optical
(NLO) applications [29,30]. In contrast to research investigations on
unsubstituted pyridylbutadiene derivatives, the emission proper-
ties of the electron donor or acceptor substituted heterocyclic
(pyridine) derivatives of butadienes are scarce. Here in, we report
synthesis and solvatochromic properties of novel amino [dime-
thylamino, diphenylamino, carbazole and julolidine] substituted 4-
pyridylbutadiene derivatives (Fig. 1). Through this study, we also
properties due to the presence of (n,p*) states [9]. Consequently,
neutral styryl and butadienyl pyridine analogs have been synthe-
sized and were investigated for their solution and/or solid state
photochemical [10e12] and photophysical properties [13e15]. Ex-
amples of photochemical investigations include regioselective
photodimerization [16,17], formation of pre-organized molecular
ladderane like structures through hydrogen bonding interactions
[18] and photoisomerization studies [10,12,14]. Apart from its role
intend to examine influence of extended
p-conjugation on the
steady state and time resolved emission properties of the sub-
strates. The results suggest remarkable fluorescence properties
characterized by intramolecular charge transfer (ICT).
2. Experimental
The reagents required for the synthesis of pyridyl substituted
* Corresponding author.
E-mail addresses: kanvah@gatech.edu, sriram@iitgn.ac.in (S. Kanvah).
derivatives were obtained from SigmaeAldrich, Alfa Aesar, Acros
http://dx.doi.org/10.1016/j.dyepig.2015.08.018
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

342
H. Agnihotri et al. / Dyes and Pigments 123 (2015) 341e348
125 MHz, ppm)
d 150.56, 149.89, 136.54, 134.68, 128.03, 126.95,
123.93, 120.42, 112.29, 40.34. HRMS [Mþ1]þ 251.1550.
2.2.2. N,N-diphenyl-4-((1E,3E)-4-(pyridin-4-yl)buta-1,3-dienyl)
aniline (4)
Yellow solid, 1H NMR (CDCl3, 500 MHz, ppm)
d
8.51-8.50 (d, 2H),
7.05-7.00 (m, 4H), 6.86-
6.53-6.49
d
7.32-7.26 (m, 8H),
d
7.14-7.09 (m, 5H),
d
d
6.80 (m, 1H, J ¼ 15.5 Hz),
d
6.73-6.70 (d, 1H, J ¼ 15.5 Hz),
d
(d, 1H, J ¼ 15.5 Hz); 13C NMR (CDCl3, 125 MHz, ppm)
d
d
149.96,
129.36,
d
d
148.06,
128.58,
d
d
147.36,
127.65,
d
d
144.9,
126.39,
d
135.56,
d
134.06,
d 130.58,
d
124.81,
d
123.39,
d
123.02, 120.54.
HRMS [ESI] [Mþ1]þ 375.1859.
Fig. 1. Structures of synthesized 4-pyridyl-4-phenylbutadiene derivatives.
2.2.3. 9-((1E,3E)-4-(pyridin-4-yl)buta-1,3-dienyl)-1,2,3,5,6,7-
hexahydropyrido[3,2,1-ij]quinoline (5)
and S.D. Fine. The solvents utilized for the synthesis and spectral
studies were dried using reported procedures. 1H and 13C NMR
spectra were carried out using 500 MHz Bruker Avance spectrom-
eter in CDCl3 with tetramethylsilane as an internal standard. Ac-
curate mass analysis was performed using Waters-Synapt G2S (ESI-
QToF) mass spectrometer. Absorption spectra were recorded on
Analytik Jena, Specord 210 model UVevis spectrophotometer.
Steady state fluorescence studies were performed utilizing Horiba
Jobin-Yvon fluorolog-3 spectrofluorimeter and relative fluores-
cence quantum yields were measured using quinine sulfate (0.545
in 0.5 N H2SO4) as a standard [31]. The concentrations used for
fluorescence experiments are typically in the order of 10ꢀ5 M. The
excitation wavelengths were set at the absorption maxima (la) of
the compounds while recording their emission spectra. Fluores-
cence life-times were determined by Edinburgh Life Spec II in-
strument at excitation wavelengths of 360 nm for (2), 405 nm for
(3e5). The percent error associated with the lifetime studies is
0.1e0.3%.
Brown solid; 1H NMR (CDCl3, 500 MHz, ppm)
d8.48-8.47 (d, 2H),
7.23-7.22 (2H, d), 7.12-7.07 (m, 1H, J ¼ 15.5 Hz), 6.91 (s, 2H), 6.73-
6.68 (m, 1H, J ¼ 15.5 Hz), 6.62-6.59 (d, 1H, J ¼ 15 Hz), 6.42-6.39 (d,
1H, J ¼ 15 Hz), 3.19-3.17 (4H, t), 2.76-2.73(4H, t), 1.97-1.95 (4H, t);
13C NMR (CDCl3, 125 MHz, ppm)
d151.7, 149.8, 145.4, 143.3, 136.9,
134.8, 126.2, 125.8, 124.0, 123.0, 121.2, 120.3, 49.9, 27.7, 21.8 HRMS]
[Mþ1]þ 303.1868.
2.3. Synthesis of 9-(4-((1E,3E)-4-(pyridin-4-yl)buta-1,3-dien-1-yl)
phenyl)-9H-carbazole (2)
Derivative (2) was synthesized by a method [35] showed in
Scheme 1d. A mixture of carbazole (60 mg, 0.350 mmol), 4-
((1E,3E)-4-(4-bromophenyl)buta-1,3-dienyl) pyridine diene (13)
(100 mg, 0.350 mmol), CuI (0.0350 mmol), 18-Crown-6
(0.012 mmol), K2CO3 (100 mg, 0.7 mmol) and 1 ml of 1,3-Dimethyl-
3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) was put into a
round bottom flask, and then heated at 170 ꢁC for 13 h under ni-
trogen atmosphere. After cooling to room temperature, the mixture
was quenched with 1 N HCl, and the precipitate was washed with
NH3eH2O and water. The residue was extracted with ethyl acetate,
dried over Na2SO4 and then concentrated under vacuum. The re-
action mixture was purified on silica-gel column chromatography
using 35% ethylacetate/hexane as eluent.
2.1. X-ray crystallographic details
Crystallographic data for the compound (4) was collected on
Xcalibur, Gemini with EOS detector diffractometer. The reflection
data was integrated and reduced using CrysAlisPro and Superflip
[32] and the structure was refined using SHELXS97 [33]. ORTEP
diagrams of the compound generated using ORTEP-3. The single
crystals of (4) were grown in acetone solvent system. The crystal-
lographic data reveal that the compound (4) crystallized into a
triclinic crystal system with space group P-1 with Z ¼ 2 and shows a
trans-configuration. Crystallographic data and the ORTEP diagram
are given in the supporting information [Tables C1eC3 and Fig. C1].
Characterization data: Orange solid 1H NMR (CDCl3, 500 MHz,
ppm)
d 8.57 (m, 2H), d 8.15-8.14 (d, 2H), d 7.69-7.67 (d, 2H), d 7.57-
7.56 (d, 2H), 7.46-7.40 (m, 4H), 7.31-7.29 (m, 4H), 7.21-7.16 (m, 1H,
J ¼ 15.5 Hz), 7.07-7.02 (m, 1H, J ¼ 15.5 Hz), 6.89-6.85 (d, 1H,
J ¼ 15.5 Hz), 6.66-6.62 (d, 1H, J ¼ 15.5 Hz); 13C NMR (CDCl3,
125 MHz, ppm)
d 151.73, 150.21, 144.48, 140.69, 137.49, 135.83,
134.63, 133.34, 130.37, 128.93, 128.03, 127.21, 126.00, 123.51, 120.70,
120.36, 120.11, 109.79. HRMS [Mþ1]þ 373.1699.
2.2. Synthesis of (1), (3), (4) and (5)
2.4. Computational details
The synthetic scheme for the preparation of dienes (1), (3e5) is
given in Scheme 1c. In a typical procedure [15], a mixture of
diphenyl (4-picolyl)phosphane oxide [34] (1 equiv), NaH (2.5
equiv), 18-crown-6 (0.5 equiv) was stirred in 40 mL of dry THF at
Density functional theory (DFT) calculations have been per-
formed using Gaussian 09 ab initio quantum chemical software
package. DFT has been used for the ground-state properties and
time-dependent DFT (TDDFT) for the estimation of ground to
excited-state transitions. The geometry optimization of the mole-
cules obtained using non-local functional B3LYP with 6-31G (d, p)
basis set without any symmetry constraints. The minimized ge-
ometry was further confirmed by vibrational analysis, resulting in
no imaginary frequencies and used as the input for further calcu-
lations to obtain the frontier molecular orbitals (FMOs) and single-
point TDDFT studies (first 15 vertical singletesinglet transitions) to
obtain the UVeVis spectra. The integral equation formalism
polarizable continuum model (PCM) within the self-consistent re-
action field (SCRF) theory, has been used for TDDFT calculations to
describe the solvation of the dye in acetonitrile solvent. The
0
ꢁC. After 30 min of stirring, 4-substituted cinnamaldehyde (1
equiv) in dry THF was added drop wise and allowed to stir for
10 h at room temperature. The reaction mixture was then filtered
over celite and the desired product was purified by column chro-
matography using neutral silica gel using 30% ethyl acetate/petro-
leum ether.
2.2.1. N,N-dimethyl-4-((1E,3E)-4-(pyridin-4-yl)buta-1,3-dienyl)
aniline (3)
Brown solid, 1H NMR (CDCl3, 500 MHz, ppm)
d
d
8.50 (d, 2H),
6.80-6.68 (m,
d
7.36-7.35 (d, 2H),
d
7.15-7.09 (dd, 1H, J ¼ 15.5 Hz),
4H),
d
6.47-6.44 (d, 1H, J ¼ 15.5 Hz),
d
2.99 (s, 6H); 13C NMR (CDCl3,

H. Agnihotri et al. / Dyes and Pigments 123 (2015) 341e348
343
Scheme 1. Synthetic steps for preparation of dienes (1)e(5). Reagents and conditions: (i) (a) dry DMF, addition of POCl3 at 0 ꢁC. (b) after 30 min at 90 ꢁC for 3 h (ii) 1,3-dioxan-2-yl-
tributylphosphonium bromide, NaH, 18-crown-6, dry THF, RT, 24 h. (iii) 10% HCl, THF at RT, 1 h.
software GaussSum 2.2.5 was employed to simulate the major
portion of the absorption spectrum and to interpret the nature of
transitions.
changes are observed. The lowest shift (þ18 nm) is obtained for
diene (2) containing a carbazole group and a maximum shift
(þ81 nm) was observed for diene (5) containing julolidine [Fig. 2b].
Despite increase in the electron donating strength, only a small
(þ4 nm) wavelength shift was observed for diene (4) as compared
to diene (3). This small increase is due to sharing of the lone pair of
electrons on nitrogen between the adjacent phenyl rings and rest of
the molecule. In case of diene (2), the carbazole ring system twists
out of the plane with respect to the aryl group resulting in
decreased electron donating strength. The absorption at ~300 nm
3. Results and discussion
3.1. Absorption and fluorescence properties
Fig. 1 gives structural details of the dienes investigated. Diene
(1) is a simple un-substituted 4-pyridyl-4-phenylbutadiene deriv-
ative. Diene (2) is substituted with carbazole while dienes (3), (4)
and (5) have dimethylamine, diphenylamine and julolidine sub-
stituents respectively. These amino derivatives function as electron
donors while the heterocyclic pyridine ring behaves as the electron
acceptor. Table 1 summarizes the absorption and fluorescence data
of dienes (1e5) in different solvents. The absorption spectrum of
diene (1) in different solvents is shown in Fig. 2a. This unsub-
stituted diene absorbs (la) at ~332 nm in acetonitrile. Upon intro-
duction of electron donating moieties, significant absorption
in dienes (2) and (4) is attributed to the
pep* transition of the
triphenylamine or carbazole moiety. The other band located at
about 360e390 nm with stronger intensity is due to the intra-
molecular charge transfer (ICT) absorption band [Fig. 2b and
Fig. S2]. The alkyl substitution on nitrogen (5) increases the
donating capacity through positive inductive effect (þI) effect
resulting in larger absorption shifts. The possibility of planarization
in the system is also another contributing factor [36]. Based on the
observed absorbance data, the strength of electron donors can be

344
H. Agnihotri et al. / Dyes and Pigments 123 (2015) 341e348
Table 1
Absorption and emission data of diene (1e5) in various nonpolar to polar solvents.
Solvent
la (nm)
lf (nm)
Stokes shift (cmꢀ1
)
Ff
la (nm)
lf (nm)
Stokes shift (cmꢀ1
)
Ff
(2)
(3)
Heptane
360
356
356
356
355
354
350
353
(4)
392
436
439
445
454
460
492
496
2267
5154
5310
5618
6063
6409
8246
8167
0.78
0.55
0.47
0.50
0.50
0.45
0.32
0.02
383
392
393
393
392
394
392
394
(5)
446
488
495
504
508
522
540
551
3678
4019
5243
5604
5825
6223
6992
7232
0.023
0.021
0.017
0.019
0.019
0.019
0.018
0.016
Dioxane (D)
1% AcN/D
5% AcN/D
10% AcN/D
20% AcN/D
AcN
Methanol
Heptane
393
395
397
397
396
397
396
399
472
481
491
498
504
513
534
540
4259
4526
4822
5108
5411
5695
6526
6544
0.40
0.30
0.16
0.17
0.17
0.15
0.16
0.02
403
413
e
442
518
e
2189
4998
0.033
0.031
0.031
0.032
0.033
0.035
0.034
0.016
Dioxane (D)
1% AcN/Da
5% AcN/D
10% AcN/D
20% AcN/D
AcN
414
414
416
413
419
520
530
547
569
583
4924
5387
5757
6639
6714
Methanol
a
AcN is acetonitrile. Quinine sulfate (0.545 in 0.5 N H2SO4 is used as standard).
Fig. 2. Normalized UV spectra of (a): (1) in varying solvent polarity; (b): (1e5) in acetonitrile (ACN is acetonitrile).
given as
diphenyl < Julolidine.
H
<
Carbazole
<
N, N-dimethyl
ꢂ
N, N-
lowered absorption maxima, contrary to expected absorption
maxima based solely on electron donating strength. This is sub-
stantiated from TDDFT simulation studies. In all the molecules, the
HOMOs are completely delocalized over the entire molecule,
whereas the LUMOs are mainly populated over the unsaturated
chain and the pyridine unit. This charge separation observed from
the distribution of HOMO and LUMO indicates that the HOMO-
/LUMO transition bears a considerable intramolecular charge-
transfer (ICT) character.
TDDFT calculations were used to examine the vertical excitation
energy of the dienes using B3LYP functional. The absorption spectra
show a small underestimation in gas phase (not shown) and used
the same functional further in the framework of polarizable con-
tinuum model (PCM) with acetonitrile as solvent. The trend in
these computed values is in reasonable agreement with the
experimentally observed wavelength maximum. The plot of TDDFT
simulated spectra of the dienes (1e5) is depicted in Fig. 4. The
comparison of computed HOMO-LUMO energy difference, lowest
excitation energies along with their oscillator strength, nature of
transitions and the ground state dipole moment of all the dienes
using the B3LYP level of theory with experimental observations
were summarized in Table S1. The relative tendency in the ab-
sorption wavelength is interpreted by means of molecular orbitals
composition which is essential to determine the extent of charge-
separation. The HOMO to LUMO transition for all the dienes
(1e5) are calculated to be 97%, 79%, 93%, 86% and 92% respectively.
The dienes show vibrational structure in non-polar heptane and
a smooth structure less absorption spectrum in solvents of higher
polarity. This is attributed to involvement of charge transfer and
strong soluteesolvent interactions. Solvent polarity variations lead
to small absorption wavelength shifts (~4e16 nm) [Fig. 2b and
Fig. S1 & S2]. Among the dienes, (5) shows a maximum shift of
16 nm from non-polar heptane to acetonitrile, and carbazole con-
taining (2) shows an absorption shift of about 7 nm.
To understand the effect of the amino donating groups on
electronic, optical and geometrical properties of the dienes (1e5),
density functional theory (DFT) calculations were performed using
Gaussian 09 ab initio quantum chemical software package. The
optimized geometry with dihedral angles, molecular orbital
amplitude plot of the highest occupied molecular orbitals (HOMOs)
and lowest unoccupied molecular orbitals (LUMOs) of the dienes
(1e5) are shown in Fig. 3. The optimized geometries obtained from
B3LYP/6-31g (d, p) level of theory shows that all the dienes except
(2 & 4) are almost planar in nature. In diene (2), carbazole is twisted
out of the plane of the diene system with the dihedral angle of 54.1ꢁ.
Similarly in diene (4), a large torsion angle of ~47ꢁ observed be-
tween the phenyl rings present in the starburst triphenylamine
(TPA) segment. The steric repulsion induced by the diphenylamine
unit in TPA derivative twisted out the plane of the diene system to
the dihedral angle of 37.1ꢁ. This out of plane twisting results in

H. Agnihotri et al. / Dyes and Pigments 123 (2015) 341e348
345
Fig. 3. Optimized geometries and their computed isodensity (0.02) surfaces of HOMO and LUMO of the dienes (1)e(5).
Dienes (2e5) show considerable ICT nature besides
p
-
p
* transition,
character compared to the ground state processes and also due to
strong non-radiative losses through processes such as internal
conversion [39,40]. In comparison, the amino derivatives (2e5)
generally result in broad, structure-less emission band with strong
solvatochromic emission shifts (lf) with the increase in solvent
polarity. Among the dienes investigated, diene (5) containing the
julolidine group has a maximum bathochromic shift (þ141 nm)
from heptane to acetonitrile while diene (4) bearing a diphenyla-
mino group leads to a minimum (þ68 nm) shift [Fig. 5]. The
enhanced strength of electron donating group due to the presence
of alkyl groups in julolidine result in huge solvatochromic emission
shifts. Such a large solvatochromic shifts observed can be attributed
to twisted intramolecular charge transfer (TICT) and this attribute
was utilized as viscosity or polarity dependent fluorescence probes
[41,42]. Diene (3), with dimethylamino groups also exhibit large
solvent polarity dependent emission shifts. In comparison, a related
derivative of (3) containing a phenyl ring in place of a pyridine ring
shows an emission shift of 56 nm [43]. Despite twisting out of
plane, carbazole containing diene (2) shows a bathochromic shift of
~100 nm. This strong bathochromic shift in dienes clearly shows
that fluorescence originates as a consequence of pronounced in-
crease in charge separation due to intramolecular charge transfer
(ICT). This ICT state was also reflected through observance of large a
Stokes shift for these dienes in polar solvents. Large Stokes shift
indicates least overlap of absorption and emission and this char-
acteristic enables their use as biological labels [44]. Among the
dienes, (2) containing carbazole exhibits larger Stokes shift as
compared to other dienes. Although more twisting was expected in
carbazole substituted compound, it can experience large electron
delocalization resulting in a greater ICT character [45]. The calcu-
lated quantum yields are also higher for carbazole containing (2)
than other dienes. The quantum yields for diphenylamine con-
taining (4) are also higher indicating the potential utility of these
whereas in diene (1), * transition is more predominant [37,38].
p-p
It is worth noting that the HOMO to LUMO transition of dienes (2)
and (4) containing carbazole and triphenylamine as electron do-
nors are comparatively lower (Table S1). This may be attributed to
the non-planar donor-acceptor interaction which leads to poor
overlap of orbital interactions over the system.
3.2. Fluorescence properties in solution
Table 1 detail the absorption and fluorescence data of the
investigated dienes. Diene (1) has very weak fluorescence and the
spectrum is highly noisy (Fig. S3). The weak emission is due to fast
deactivation of the molecules in the excited state through free
rotation of the molecules as the excited state has less double bond
Fig. 4. Simulated absorption spectra obtained with TD-DFT/B3LYP/6-31G (d, p) level of
theory.

346
H. Agnihotri et al. / Dyes and Pigments 123 (2015) 341e348
Fig. 5. Emission spectra of (a) 2, (b) 4, (c) 3 and (d) 5.
chromophores for biological or optical applications. The dienes
containing aliphatic amino substitutions (3 and 5) both show lower
quantum yield of fluorescence. Despite the restriction caused by the
aliphatic ring, diene (5) shows comparable quantum yields with
diene (3). To further examine the solvatochromic shift, the excited
state dipole moment (me) of dienes (2e5) was calculated using
LipperteMataga equation [46]. The dipole moment (me) of the ICT
excited states can be calculated from the slope (mf) of the plot of the
In this, (N) is Avogadro number, (M) is the molecular weight, and
(d) is the density and taken as 1 g/cm3 [47]. The estimated dipole
moments (me) are listed in Table 2 along with calculated values of a,
mf, mg, for dienes (2e5). The plots of △
n
(n
A ꢀ
nF) vs △f are shown
in Fig. S4. In all the cases, a linear correlation with a positive sol-
vatochromism was observed indicating their solvent sensitive
behavior due to presence of intramolecular charge transfer (ICT)
emissive states.
Stokes shift with
D
f according to eq
The ICT state is also reflected by large change in excited state
dipole moment. The ICT states of diene (2e5) have larger dipole
moments in the excited state than in the ground state as a result of
the charge separation between the donor and acceptors. The order
of me is (5) > (3) > (4) > (2). The observed excited state dipole
moments confirm the earlier observations of greater charge
transfer character for julolidine substituted diene (5). The
competing processes of sharing of lone pair of electrons in (2) and
(4) and the bulkier donating groups in the form of triphenylamine
h
.
i
ꢀ
ꢁ
2
n
¼ 2 m ꢀ m
hca3 Df þ nss0
ss
e
g
where, nss is the Stokes shift, mg is the ground state dipole moment
and were obtained using DFT calculations utilizing Gaussian Soft-
ware, a is the solvent cavity (Onsager) radius. The solvent param-
eter Df represents solvent polarity and polarizability and defined as
hꢂ
ꢃ.ꢂ
ꢃi
Df ¼ ½ðε ꢀ 1Þꢃ=ð2ε þ 1Þꢃ ꢀ n2 ꢀ 1
2n2 þ 1
Table 3
where ε and n are solvent dielectric constant and refractive index
respectively. The Onsager radius (a) calculated by the formula
Fluorescence lifetime data in homogeneous solvents. Diene (2) was excited at
360 nm. Diene (3), (4) and (5) were excited at 405 nm (error percentage-0.1e0.3%).
c2
a ¼ ð3M=4NpdÞ1=3
Diene
Solvent
t1
t2
(2)
Dioxane
1.32
e
1.03
1.08
1.04
0.86
0.88
0.88
0.92
1.04
0.90
0.97
1.02
0.90
20% ACN/Dioxane
Acetonitrile
Dioxane
20% ACN/Dioxane
Acetonitrile
Dioxane
20% ACN/Dioxane
Acetonitrile
Dioxane
20% ACN/Dioxane
Acetonitrile
1.87
2.52
0.11 (98.49%)
0.16 (90.89%)
0.18 (91.75%)
1.24
1.53
1.82
0.25 (91.14%)
0.47 (97.03%)
0.57 (99.80%)
e
e
Table 2
(3)
(4)
(5)
1.13 (1.51%)
0.54 (8.70%)
0.50 (7.87%)
e
Calculated Onsager radius, slope of the curve, ground and excited state dipole mo-
ments of dienes investigated.
Compound
a(Å)
mf
mg(D)
me (D)
e
2
3
4
5
5.28
4.63
5.28
4.93
10,924
7982
7346
6569
1.8522
7.6166
5.0880
8.3659
14.47 0.014
16.47 0.022
15.45 0.005
17.18 0.022
e
2.50 (8.86%)
2.53 (2.96%)
2.77 (0.20%)

H. Agnihotri et al. / Dyes and Pigments 123 (2015) 341e348
347
Fig. 6. Excited state decay profile of dienes (2e5) in different solvents. Excitation of 360 nm for (2) and (b) 405 nm for (3e5).
and carbazole lead to reduction in electron delocalization in excited
state and therefore result in decreased me values in comparison to
(3) and (5). The presence of alkyl (methyl) groups on nitrogen fa-
cilitates the availability of electrons for electron charge transfer
during the excited state resulting in increase in me for diene (3) and
(5).
maximum, indicating efficient intramolecular charge transfer
behavior. It can further be proposed from these studies that such
strongly solvatochromic probes can be useful materials for optical
and biological applications.
Acknowledgements
Room temperature lifetimes of dienes in different solvents are
shown in Table 3. Dienes (2) and (4) bearing carbazole and tri-
phenylamine moieties show single exponential decay while dienes
(3) and (5) show bi-exponential decay with major component
having population greater than 90% with lifetime in the range of
0.11 nse0.57 ns. The minor component (with a population less than
10%) has a lifetime in the range of 0.50 nse2.77 ns. It is possible that
both the dienes containing dimethylamino and julolidine have
excited states that contribute to conformational changes due to
their significant intramolecular charge transfer nature [Fig. 6]. Thus
it can be said that the fluorescence majorly originates from a single
excited state species in the case of (2) and (4) but originates from
two excited states in the case of (3) and (5) due to conformational
changes. Such biexponential decay is also reported in similarly
substituted stilbene derivative [48].
Authors gratefully thank CSIR [Grant no: 01(2487)/11/EMR-II],
Department of Science and Technology and IIT Gandhinagar for
research support.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.08.018.
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