Novel Rhodafluors: Synthesis, Photophysical, pH and TD-DFT Studies

Article abstract of DOI:10.1007/s10895-016-1915-z

An efficient protocol for the synthesis of new rhodol derivatives has been developed. The synthesis involves condensation of 2-hydroxy benzophenone derivatives with 1, 3-dihydroxy benzene derivatives in solvents such as ionic liquid (N-methyl-2-pyrrolidonium hydrogen sulfate) and methane sulphonic acid. Both ionic liquid and methane sulphonic acid were found to be promising self-catalyzed solvents to bring out the conversion to form desired rhodols in excellent yields. In N-methyl-2-pyrrolidonium hydrogen sulfate reaction proceeds faster compared to methane sulphonic acid. The new fluorophores are investigated for their photophysical properties in various microenvironments and systematically characterized by means of density functional theory and time dependent density functional theory. Photophysical properties of the new rhodafluors found sensitive towards change in the pH of media and thus can be used as efficient pH sensors.

Full text of DOI:10.1007/s10895-016-1915-z

J Fluoresc
DOI 10.1007/s10895-016-1915-z
ORIGINAL ARTICLE
Novel Rhodafluors: Synthesis, Photophysical, pH
and TD-DFT Studies
Supriya S. Patil1 & Kishor G. Thorat1 & Ramnath Mallah1 & Nagaiyan Sekar1
Received: 10 February 2016 /Accepted: 26 August 2016
#
Springer Science+Business Media New York 2016
Abstract An efficient protocol for the synthesis of new
rhodol derivatives has been developed. The synthesis
involves condensation of 2-hydroxy benzophenone de-
rivatives with 1, 3-dihydroxy benzene derivatives in sol-
vents such as ionic liquid (N-methyl-2-pyrrolidonium
hydrogen sulfate) and methane sulphonic acid. Both
ionic liquid and methane sulphonic acid were found to
be promising self-catalyzed solvents to bring out the
conversion to form desired rhodols in excellent yields.
In N-methyl-2-pyrrolidonium hydrogen sulfate reaction
proceeds faster compared to methane sulphonic acid.
The new fluorophores are investigated for their
photophysical properties in various microenvironments
and systematically characterized by means of density
functional theory and time dependent density functional
theory. Photophysical properties of the new rhodafluors
found sensitive towards change in the pH of media and
thus can be used as efficient pH sensors.
Introduction
Rhodol fluorophores are the hybrid structure of fluores-
cein and rhodamine, and therefore are also named as
BRhodafluor^ [1] (Fig. 1). They inherit all the excellent
photophysical properties of parents such as high extinc-
tion coefficients, quantum yields, photostability, and sol-
ubility in a variety of solvents [2]. Moreover the
photophysical properties of rhodol fluorophores can be
fine-tuned through different substitution patterns on the
nitrogen atom [3] and therefore they are subject of in-
terest as fluorescent probes [4].
The synthesis of rhodol derivatives can be achieved
by various conventional methods such as condensation
using acid catalysts [2, 5], palladium-catalyzed
amination reaction [6], Buchwald-Hartwig coupling re-
action [7], etc.
Despite their excellent photophysical properties there
are only few examples about the applications of rhodol
fluorophores [1, 2, 5, 8–11], probably due to the lack of
a convenient synthetic methods. Here, we report a high-
ly efficient and environment friendly protocols, mediat-
ed by ionic liquid (IL) and methane sulphonic acid
(MSA). Ionic liquids are considered as ideal alternatives
to volatile organic solvents and widely regarded as en-
vironment friendly alternatives to many commonly used
solvents [12]. They are good solvents for wide range of
both inorganic and organic reactions [13]. Ionic liquids
are non-volatile, hence they may be used in high-
vacuum systems and eliminate many containment
problems.
+
−
.
.
Keywords DFT ([HNMP] [HSO4] ) Methane sulphonic
.
.
.
acid pH study Rhodafluors Td-DFT
Supriya S. Patil and Kishor G. Thorat contributed equally equally to this
work.
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-016-1915-z) contains supplementary material,
which is available to authorized users.
* Nagaiyan Sekar
The synthesized rhodofluors are investigated for their
photophysical properties in various microenvironments
experimentally and systematically characterized by
means of DFT and TD-DFT. The photophysical
n.sekar@ictmumbai.edu.in; nethi.sekar@gmail.com
1
Dyestuff Technology Department, Institute of Chemical Technology,
Matunga, Mumbai 400 019, India

J Fluoresc
Fig. 1 General structures of the
Rhodol, Rhodamine and
Fluorescein dyes
sample and standard respectively, η2X is refractive index
of solvent in which quantum yield of sample is being
calculated and η2ST is refractive index of solvent in
which quantum yield of standard is taken as reference.
Oscillator strength ( f ) was obtained from integrated ab-
sorption coefficient of the absorption band by using the Eq. 2,
Z
properties of the new rhodofluors owing to formation of
two different forms are found to be sensitive towards
change in pH of phosphate buffer.
Experimental
f ¼ 4:32 Â 10−9 εðϑÞdv
ð2Þ
Reagents and Apparatus
All the chemicals and spectroscopic grade solvents were
obtained from the local suppliers like S. D. Fine
chemicals and SPECTROCHEM PVT. LTD., unless oth-
erwise mentioned, and used as such without further pu-
rifications. All the common chemicals used were of an-
alytical grade. All the reactions were monitored by TLC
(thin-layer chromatography) with detection by UV light.
1H NMR and 13C NMR spectra were recorded on
Varian 500 MHz instrument using TMS as an internal
standard. LCMS spectra were recorded on Finnegan
mass spectrometer.
The transition dipole moment (μeg) is related to oscillator
strength ( f ) and calculated from Eq. 3 [15],
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
u
3e2h
8π2mc
f
t
μeg
¼
Â
ð3Þ
ϑeg
Where, m, ϑeg and e are the mass of electron, absorption
frequency, and charge on electron, respectively.
Preparation of Ionic Liquid, N-Methyl-2-Pyrrolidonium
Hydrogen Sulphate (1)
Photophysical Studies
The ionic liquid was prepared by following reported protocol
[16] in quantitative yields (Scheme 1).
The photophysical studies such as absorption spectra,
emission spectra, molar extinction coefficients and quan-
tum yields of fluorescence are determined using dye
solutions of 4–10 μM concentrations. The absorption
spectra were recorded at room temperature on the
Perkin-Elmer UV-VIS spectrometer, Lambda 25. The
emission spectra were recorded at room temperature on
VARIAN CARY Eclipse fluorescence spectrophotometer
using 10 mm cuvette with 2.5 nm to 5 nm slit width.
The comparative quantum yields of fluorescence were
calculated by using Rhodamine B in methanol (quantum
yield = 0.7) as reference [14] using following Eq. 1.
General Procedure for the Preparation
of 2-Carboxyl-4′-Diethylamino-2′-Hydroxy
Benzophenone (2)
The compound 2 was prepared by using reported procedure
[17] (Scheme 2) in good yields (70 %); melting point-202 °C.
ꢀ
ꢁꢀ
ꢁ
GradX
ηX2
η2ST
ϕX ¼ ϕST
ð1Þ
GradST
Where,
and
are quantum yield of fluorescence
ST
X
of the sample and reference respectively, GradX and
GradST are slope of the plots of absorbance vs area
under curve of corresponding emission spectra of
Scheme 1 Synthesis of N-methyl-2-pyrrolidonium hydrogen sulphate
(1)

J Fluoresc
Scheme 2 Synthesis of 2-
carboxyl-4′-diethylamino-2′-
hydroxy benzophenone (2)
General Procedure for Synthesis of Rhodol Derivatives
3a-3c
N-(7-Acetyl-9-(2-Carboxyphenyl)
-6-Hydroxy-3H-Xanthen-3-Ylidene)-N-Ethylethanaminium
(3b)
A solution compound 2 (1 g, 3.2 mmol) and a-c (3.2 mmol,
1 eq.) in the solvent (IL or MSA) (10 mL) was stirred at 85 °C
under N2 atmosphere for 4–15 h. The hot reaction mixture was
subsequently poured onto ice cold water (50 vol.). The obtain-
ed solid was isolated by filtration, dried and purified by silica
gel column chromatography using methanol (2 to 5 %) in
chloroform as the eluent. (Scheme 3).
Melting point: 214 °C;
1H NMR (500 MHz, dmso-d6) δ 8.21 (d, J = 8.5 Hz, 1H),
7.83 (t, J = 7.4 Hz, 1H), 7.77 (t, J = 7.1 Hz, 1H), 7.47 (s, 1H),
7.42 (d, J = 7.4 Hz, 1H), 7.21 (s, 1H), 7.18 (d, J = 11.4 Hz,
1H), 7.14 (s, 1H), 7.07 (d, J = 9.4 Hz, 1H), 3.69 (q, J = 7.1 Hz,
4H), 2.45 (s, 3H), 1.19 (t, J = 7.1 Hz, 6H);
13C NMR (126 MHz, dmso-d6) δ 200.9, 162.0, 156.2,
152.2, 149.8, 136.1, 132.1, 130.7, 129.0, 126.8, 125.2,
124.5, 119.7, 109.4, 104.2, 97.6, 44.2, 39.8, 39.6, 39.5,
29.1, 12.7;
N-(7-Benzoyl-9-(2-Carboxyphenyl)
-6-Hydroxy-3H-Xanthen-3-Ylidene) -N-Ethylethanaminium
(3a)
FTIR (KBr-cm−1): 3429(OH/COOH), 1601, 1353, 764;
Mass m/z: 429 (M+), 430 (M + 1).
Melting point: 212 °C;
Elemental analysis:
Calculated: C, 75.7489; H, 5.1265; N, 2.8496; O, 16.2750.
Found: C, 75.7499; H, 5.1288; N, 2.8499; O, 16.2749.
1H NMR (500 MHz, dmso-d6) δ 8.15 (d, J = 7.7 Hz,
1H), 7.80–7.71 (m, 1H), 7.71–7.64 (m, 1H), 7.61 (d,
J = 6.7 Hz, 2H), 7.55 (d, J = 6.2 Hz, 1H), 7.41 (s,
3H), 7.28 (s, 1H), 7.21 (s, 2H), 7.14 (t, J = 10.3 Hz,
2H), 3.70 (q, J = 5.0 Hz, 4H), 1.21 (t, 6H).
13C NMR (126 MHz, dmso-d6) δ 159.3, 159.0, 158.7,
158.3, 118.7, 116.4, 114.2, 111.7, 39.1, 38.6, 18.4, 39.1, 38.6.
FTIR (KBr-cm−1): 3420 (OH/COOH), 1761.48 (lactone),
1609, 1356, 675–870.
N-(9-(2-Carboxyphenyl)
-7-Formyl-6-Hydroxy-3H-Xanthen-3-Ylidene)-N-
Ethylethanaminium(3c)
Melting point: > 350 °C;
Mass m/z: 492 (M+), 493 (M + 1).
Elemental analysis:
Calculated: C, 72.7134; H, 5.3980; N, 3.2614; O, 18.6271.
Found: C, 72.7211; H, 5.3890; N, 3.2615; O, 18.6295.
1H NMR (500 MHz, dmso-d6) δ 8.21 (d, J = 7.6 Hz, 1H),
7.84 (t, J = 8.0 Hz, 1H), 7.78 (t, J = 7.6 Hz, 1H), 7.57 (s, 1H),
7.43 (d, J = 7.4 Hz, 1H), 7.12 (m, 4H), 3.68 (q, J = 13.5,
7.1 Hz, 4H), 1.18 (t, J = 7.1 Hz, 6H);
Scheme 3 Synthesis of rhodol
derivatives (3a-3c)

J Fluoresc
13C NMR (126 MHz, dmso-d6) δ 167.6, 159.3, 159.0,
158.7, 158.4, 118.8, 116.5, 114.2, 111.9, 46.8, 37.0, 12.5;
FTIR (KBr-cm−1): 3435(−OH/COOH), 1598, 1383,
1108; Mass m/z: 430 (M+), 432 (M + 1).
Results and Discussion
Choice of Solvent-Catalyst
Elemental analysis:
Calculated: C, 69.5970; H, 4.9061; N, 3.2465; O, 22.2504.
Found: C, 69.5988; H, 4.9059; N, 3.2470; O, 22.2511.
The synthesis of the dyes 3a to 3c were performed using two
different solvent-catalyst systems, IL and MSA, and results
such as reaction time, and percent yields are summarized in
Table 1. It was observed that at 85 °C both in IL (10 mL) and
MSA (10 mL) the complete conversion of the starting mate-
rials is observed but in IL reaction proceeds faster compared to
the MSA with superior yields of the desired products
(Table 1). Also the formation of 3c both in IL and MSA sys-
tems took place shorter time compared to 3a and 3b owing to
lower electron withdrawing effects of carboxylic acid group in
c compared to that of ketone in a and b (Scheme 3).
Recyclability of Ionic Liquid, N-Methyl-2-Pyrrolidonium
Hydrogen Sulphate (1)
The recyclability study was carried out for the com-
pound 3a, as representative rhodol. After the completion
of the reaction as indicated by TLC, the reaction mix-
ture was cooled to r. t. and then poured on to the ice
cold water and precipitated solid was separated by fil-
tration under vacuum. Water was then removed from the
filtrate under vacuum using rotary evaporator and the
reddish viscous liquid obtained was used for the next
cycle without any further purification. The same proce-
dure was then repeated for the next two cycles.
Recyclability Study
In organic synthesis recyclability or reusability of solvent-
catalyst is a very important factor to make the process eco-
nomical and environment friendly. We have studied the ability
of ionic liquid to recycle without purification for the synthesis
of novel rhodol derivative 3a and results are summarized in
Table 2. From Table 2 it is seen that the solvent, IL can be
recycled up to 4 cycles without considerable loss in its reac-
tivity which makes the process economical and eco-friendly.
Thus the ionic liquid ([NMP]+[HSO4]−) is a good candidate as
a solvent-catalyst system for the formation of rhodafluors un-
der study.
Computational Strategy
The present calculations include the optimization of the
dyes in the ground (S0) and first excited (S1) state in
various solvents. Vertical excitation and emissions were
evaluated by single point energy (TD) calculations from
S0 and S1 optimized geometries, respectively [18]. Present
calculations were accomplished using of the Gaussian 09
program package [19]. The geometries of the dyes in their
S0 state were optimized using DFT method [20]. The
popular hybrid functional, B3LYP, (the B3LYP combines
Becke’s three parameter exchange functional (B3) [21]
with the non-local correlation functional by Lee, Yang
and Parr (LYP) [22] in combination with the basis set 6-
31G(d) is used in both, DFT and TD-DFT methods for all
the atoms. The vertical excitation energies and oscillator
strengths were calculated for the first twenty S0 → S1
transitions at the optimized ground (S0) state equilibrium
geometries by using TD-DFT with the same hybrid func-
tional and basis set [23–27]. To obtain their lowest energy
geometries which correspond to the emissive state the
low-lying first singlet excited states (S1) of the dyes were
relaxed using the TD-DFT. The emission wavelengths and
oscillator strengths were gained for the first ten S1 → S0
transitions at the optimized excited state equilibrium geom-
etries by using the TD-DFT at B3LYP/6-31G(d) level [15,
27, 28]. All the computations in the different solvents
were carried out using the Self-Consistent Reaction Field
(SCRF) under the Polarisable Continuum Model (PCM)
[29, 30].
Photophysical Properties
The photophysical properties of the rhodafluors, 3a-3c such as
absorption maxima, emission maxima, molar absorptivity, os-
cillator strength, transition dipole moments and quantum
yields of fluorescence, etc. in different solvents were investi-
gated and summarized in Table 3. The normalized absorption
and emission spectra of the dyes 3a-3c representative solvent
(water) is shown for illustration in Fig. 2 and in different
solvents is shown in Figs. S1 to S3 (supporting information).
Table 1 Optimization of reaction conditions for synthesis of the
rhodafluors (3a-c)
Sr.
Compound Catalyst/solvent
Yield Time Temperature
no.
(%)
(h)
(°C)
1
2
3
3a
3b
3c
([HNMP]+[HSO4]−) 65
13.5 85
15 85
12.5 85
MSA
49
([HNMP]+[HSO4]−) 68
MSA 53
([HNMP]+[HSO4]−) 65
MSA 52
15
85
85
85
3.5
4.0

J Fluoresc
Table 2 Recyclability study of ionic liquid ([HNMP]+[HSO4]−)
aprotic solvents such as DMF and DMSO because lactone
form predominates over carboxylate form in aprotic solvents.
The values of oscillator strength and transition dipole mo-
ments follow the similar trend as that of molar absorptivity
being highest in water for the dyes 3a-3c. The compounds 3a-
3c show good quantum yields of fluorescence in different
solvents ranging from 0.44 to 0.89.
Cycle no.
mL of Ionic liquid recovered
Yield (%)
Fresh
10
93
91
88
84
79
1
2
3
4
9.4
8.9
8.6
8.0
Effect of Change in pH on Photophysical Properties
of the Dyes 3a-3c
From Table 3 and Figs. S1 to S3 it is seen that the dyes 3a-
3c absorbs at around 518 nm to 539 nm with no or little effect
of change in polarity of microenvironment and emits at around
543 nm to 573 nm in different solvents. The emission maxima
in case of all the dyes are influenced more due to change in
solvent polarity compared to the absorption maxima and show
positive solvatochromism suggest that the dyes 3a-3c in its
excited state are more polar and thus more polar solvents
provide the stability to polar forms leading to the red shift in
respective emission maxima.
The position and intensity of the absorption maxima of the
dyes 3a-3c were found to be very sensitive towards change in
the pH of microenvironment and it is expected that the dyes
3a-3c appear in three different forms (A, B and C, Fig. 3)
depending on the pH of surrounding media. Thus we observed
the changes in absorption peaks of the dyes 3a-3c with respect
to change in pH of the microenvironment and results are illus-
trated in Figs. 4-6, respectively. The pH of the medium deter-
mines as to which form predominates over the other. When the
dyes were treated/ dissolved in PBS buffer of different pH it
was observed that at certain pH there is sudden enhancement
in the absorption intensity of the dyes 3a-3c.
From Table 3 it is seen that the dyes 3a-3c possess the
highest values of molar absorptivity in water followed in
methanol. Dyes shows poor values of molar absorptivity in
Table 3 Photophysical properties of rhodol derivatives 3a, 3b and 3c, in various solvents
h
i
Solvent
λabsa nm
FWHMb cm−1
λemc nm
Δ d cm−1
εmaxe M−1 cm−1
∫ε(ϑ)dvf /108 M−1 cm−2
fg
μeg
фfl
Dye 3a
Water
533
528
526
533
1325
1297
1399
1942
564
558
560
573
1031
1018
1154
1310
84,716
38,221
10,835
6895
1.89
0.70
0.18
0.15
0.8297
0.3073
0.0790
0.0659
9.70
5.88
2.97
2.73
0.70
0.66
0.80
0.55
MeOH
EtOH
DMSO
Dye 3b
Water
536
534
533
539
1367
1415
1868
2044
566
564
564
570
989
102,176
5854
1.99
0.18
0.14
0.13
0.8736
0.0790
0.0615
0.0571
9.98
3.00
2.64
2.56
0.74
0.45
0.67
0.44
MeOH
EtOH
996
1031
1009
6198
DMF
1549
Dye 3c
Water
529
520
518
2774
1903
2103
555
543
543
1176
815
53,484
4843
1.66
0.18
0.07
0.7287
0.0790
0.0307
7.56
2.50
1.56
0.45
0.88
0.89
MeOH
EtOH
889
1954
a Wavelength absorption maximum
b Full width at half absorption maxima
c Wavelength emission maximum
d Stokes shift
e Molar absorptivity at absorption maximum
f Integrated area of absorption coefficient
g Oscillator strength
h Transition dipole moment
i Quantum yield of fluorescence

J Fluoresc
Fig. 2 Normalized absorption
(solid lines) and emission (dashed
lines) spectra of the dyes 3a-3c in
water
From Fig. 4 it is seen that at pH 4 the dye 3a show dual
absorptions with almost similar intensity, at 340 nm and
533 nm, corresponds to form A and B or C. As the pH in-
creases there is enhancement in intensity of the band at
533 nm and at pH change from 5 to 6 the dye 3a shows 4
folds increase in the absorption intensity at its absorption max-
ima (533 nm) and after pH 6 there is no further considerable
change in the absorption intensity (Fig. 4). On the other hand it
was observed that there is a decrease in the intensity of ab-
sorption band at 340 nm, which corresponds to the form A,
with the increase in pH (Fig. 4). Thus complete opening of
spiro ring occurs at pH 6, conversion of form A to form B or C
(Fig. 3).
its highest electron withdrawing ability compared to benzoyl
and carboxyl groups in 3b and 3c respectively, facilitates com-
plete opening of the lactone to carboxylate at relatively lower
pH.
DFT and TD-DFT Studies
The density functional theory (DFT) [20] and time dependent
density functional theory (TD-DFT) [20] with B3LYP/6-
31G(d) [20–22] level were used to get more understanding
of the structural, and photophysical properties of rhodafluors
under study. The unconstrained geometries of the dyes in their
S0 and S1 states in vacuum as well as in different solvents were
optimized by DFT using the Becke’s three-parameter func-
tional [21] hybridized with the Lee–Yang–Parr correlation
functional [22] and the 6-31G(d,) basis set [23, 24, 31].
Similarly the dye 3b shows 2.5 fold increase in its absorp-
tion intensity when pH changes from 5 to 7 with no further
change in the intensity upon increasing the pH (Fig. 5) and the
dye 3c show gradual increase in the intensity after pH 7 till
pH 12 (Fig. 6).
Optimized Geometries of the Dyes 3a-3c
The dyes 3a-3c show different response towards change in
the pH that could be attributed to the electron withdrawing
ability of the substituent present on the phenol ring of the
fluorophore (Fig.3). As acetyl substituent in dye 3a owing to
The TD-DFT optimized geometries and main geometrical pa-
rameters of the rhodol derivatives (dyes 3a-3c) for keto form
(Form B, Fig. 3) in its S0 and S1 states are shown in Figs. 7-9
A
B
C
Fig. 3 Possible forms of rhodafluors (a-c) at different pH conditions

J Fluoresc
Fig. 4 Effect of change in pH on absorption intensity of compound 3a in
PBS buffer
Fig. 6 Effect of change in pH on absorption intensity of compound 3c in
PBS buffer
and collected in Table S1. From Figs. 7-9 it is seen that the
main rhodol chromophore (Figs. 7-9) possesses perfectly pla-
nar arrangement thus allowing effective p-orbital overlap
throughout, N1 to O1 and O1 to O2.
The dyes 3b and 3c (Figs. 8 and 9) in its S0 and S1 states
show similar observations except the acetyl and carboxy sub-
stituents in the dyes 3b and 3c, respectively, possesses perfect-
ly planar arrangement with main chromophore making effec-
tive p-orbital overlap supports the red shifted absorption and
emission of the dye 3b compared to that of the dyes 3a and 3c
(Table 3).
In the dye 3a (Fig. 7) in S0 state the phenyl substit-
uent at C6 is oriented with main chromophore almost
orthogonally making a dihedral angle of 88.4° but in
first excited (S1) it becomes twisted from its orthogonal
arrangement by 22° making final dihedral angle of
66.4°. The benzoyl substituent at C9 show twisting
from planar arrangement by 48.2° in S0 state but the
twisting decreases by 16.7° in S1 state making dihedral
angle of 31.3°. From Fig. 7 it is seen that there occurs
elongation of the bonds in S1 state compared to the S0
in the main chromophore, except C5-C6, C7-C8 and
C9-C10, suggests the polarization of the chromophore
in its excited state.
TD-DFT Absorption and Emissions
The TD-DFT absorption and emission energies, major elec-
tronic configurations, and transition nature of the selected S0
and S1 states with large oscillator strengths (ƒ) along with
respective experimental values such as absorptions and emis-
sions of keto forms of the dyes 3a-3c in different solvent
media for comparison are listed in Table 4.
For the dyes 3a-3c in all solvents studied (Table 4) the
vertical excitations originates from the HOMO to LUMO with
good orbital contributions and oscillator strengths. Table 4
revealed that the TD-DFT absorptions for the dyes 3a-3c are
centered around 474.1 nm to 481. 9 nm with 8.3 to 10.8 %
deviations from observed values of absorptions in different
solvents with very good oscillator strengths (0.7737 to
0.9142). From Table 4 it is seen that the TD-DFT account
for emission of the rhodafluors (3a-3c) fairly accurately and
are centered around 532.0 nm to 556.0 nm with 1 % to 5 %
deviations from the observed values of emission maxima. TD-
DFT emissions originated from the LUMO to HOMO transi-
tions with good oscillator strengths supports the good quan-
tum yields of fluorescence of the dyes 8.3 to 3a-3c. From
Table 4 it is seen that the TD-DFT also accounts for the red
shifted absorption and emission maxima of the dye 3b com-
pared to that of the dyes 3a and 3c.
Fig. 5 Effect of change in pH on absorption intensity of compound 3b in
PBS buffer

J Fluoresc
Fig. 7 TD-DFT (B3LYP/6-
31G(d)) S0 and S1 states
optimized geometries of the dyes
3a in water
TD-DFT Analysis of FMOs
plots are shown in Fig. 10. From Fig. 10 it is seen that in
HOMO the major portion of the charge density is localized
on amine part of the chromophore. The charge density at
amine part in HOMO is transferred to keto part of the chro-
mophore in LUMO supports the formation of zwitter ionic
form C (Fig. 3).
In order to gain detailed insight on electronic and
photophysical properties of the rhodafluors under investiga-
tion their frontier molecular orbitals (FMOs), particularly the
HOMO and LUMO were examined and respective counter
Fig. 8 TD-DFT (B3LYP/6-
31G(d)) S0 and S1 states
optimized geometries of the dyes
3b in water

J Fluoresc
Fig. 9 TD-DFT (B3LYP/6-
31G(d)) S0 and S1 states
optimized geometries of the dye
3c in water
Table 4 TD-DFT B3LYP/6-31G(d) calculated absorption and emission parameters of the dyes 3a to 3c in different solvents
a
b
e
f
Solvent
λabs
λabs
f c
% Dd
Major orbital contribution
λem
λem
fg
% Dh
Major orbital contribution
Dye 3a
Water
533
528
526
533
475.4
474.9
475.8
477.9
0.8878
0.8815
0.8909
0.9142
10.8
10.0
9.54
10.3
H → L (94.1 %)
H → L (94.0 %)
H → L (94.2 %)
H → L (94.5 %)
564
558
560
570
537.7
539.8
542.1
541.4
0.6254
0.6084
0.6110
0.6452
4.66
3.26
3.20
5.02
L → H (92.2 %)
L → H (91.9 %)
L → H (92.1 %)
L → H (92.7 %)
MeOH
EtOH
DMSO
Dye 3b
Water
537
536
534
539
478.6
478.3
479.4
481.9
0.8063
0.7993
0.8086
0.8365
10.9
10.8
10.2
10.6
H → L (95.8 %)
H → L (95.7 %)
H → L (95.9 %)
H → L (96.2 %)
566
564
564
570
550.4
553.2
535.6
556.0
0.4794
0.4662
0.5490
0.4976
2.76
1.91
5.04
2.46
L → H (93.6 %)
L → H (93.6 %)
L → H (93.8 %)
L → H (94.3 %)
MeOH
EtOH
DMF
Dye 3c
Water
521
520
518
474.3
474.1
475.3
0.7815
0.7737
0.7825
8.97
8.82
8.25
H → L (95.8 %)
H → L (95.7 %)
H → L (95.8 %)
555
543
543
532.0
534.8
537.5
0.5175
0.5010
0.5040
4.14
1.51
1.01
L → H (93.6 %)
L → H (93.4 %)
L → H (93.6 %)
MeOH
EtOH
a Experimental absorption maximum
b TD-DFT absorption maximum
c Oscillator strength of vertical excitation
d Deviation in TD-DFT absorption maximum from the experimental absorption maximum
e Experimental emission maximum
f TD-DFT emission maximum
g Oscillator strength of vertical emission
h Deviation in TD-DFT emission maximum from the experimental emission maximum

J Fluoresc
Fig. 10 Energy level and electron density distribution of frontier molecular orbitals of dyes 3a-3c in water solvent, calculated by B3LYP/6-31G(d) level
of theory (The orbital diagrams were plotted with the contour value of 0.02 a. u)
TD-DFT Dipole Moments
Photon absorption by chromophore in its S0 state is responsi-
ble for its excitation to the Sn (n is 1, 2, 3, and so on) states
associated with redistribution of the electron density, which
Table 5 TD-DFT B3LYP/6-31G(d) calculated dipole moments of the
dyes 3a to 3c in ground and first excited states in different solvents
a
b
c
Solvent
μg
μe
Δμeg
brings the change in the dipole moment in the Sn state. Dipole
moments of the molecules in the S1 state relative to that in S0
in different microenvironments are useful to extract parame-
ters like molecular polarizability and to evaluate the electron
density distribution in the S1 and T1 states.
From Table 5 it is seen that, for the dyes 3a-3c the dipole
moments in S1 state is higher than that of S0 state (μe > μg),
suggesting the more polar excited state. Also for all the dyes
3a-3c dipole moments both in S0 and S1 states, increases with
increase in the solvent polarity indicates the formation of more
polarized state with increase in solvent polarity being highest
in water, followed by DMSO/DMF, then MeOH and lastly
EtOH (Table 5).
Dye 3a
Water
18.46
18.21
18.08
18.34
19.90
19.68
19.57
19.79
1.44
1.47
1.49
1.45
MeOH
EtOH
DMSO
Dye 3b
Water
19.28
19.01
18.88
19.07
21.30
21.07
20.04
21.12
2.03
2.06
1.17
2.06
MeOH
EtOH
DMF
Dye 3c
Water
19.28
19.00
18.86
20.46
20.23
20.12
1.19
1.23
1.26
MeOH
EtOH
Conclusion
a TD-DFT dipole moment in S0 state
b TD-DFT dipole moment in S1 state
c Difference between the dipole moments in S0 and S1 states
In this paper we report green protocols for synthesis of three
novel rhodol derivatives in good yields. Recyclability of the

J Fluoresc
14. Brouwer AM (2011) Standards for photoluminescence quantum
yield measurements in solution (IUPAC technical report. Pure
Appl Chem 83:2213–2228. doi:10.1351/PAC-REP-10-09-31
15. Thorat KG, Ray AK, Sekar N (2016) Modulating TICT to ICT
characteristics of acid switchable red emitting boradiazaindacene
chromophores: Perspectives from synthesis, photophysical,
hyperpolarizability and TD-DFT studies. Dye Pigment.
doi:10.1016/j.dyepig.2016.08.049
catalyst-solvent, mild reaction conditions, easy workup proce-
dures are some of the striking features of present protocols.
The results of pH study reveals that the new rhodafluors can
be used as efficient pH probes. The structural and
photophysical properties of the dyes under study are rational-
ized by means of DFT and TD-DFT.
16. Chaudhari AS, Parab YS, Patil V, et al. (2012) Intrinsic catalytic
activity of Bronsted acid ionic liquids for the synthesis of triphenyl-
methane and phthalein under microwave irradiation. RSC Adv 2:
12112. doi:10.1039/c2ra21803h
17. Liu Q-H, Liu J, Guo J-C, et al. (2009) Preparation of polystyrene
fluorescent microspheres based on some fluorescent labels. J Mater
Chem 19:2018. doi:10.1039/b816963b
Acknowledgments One of the authors, KGT, is thankful to the Council
of Scientific and Insdustrial Research (CSIR), India for Junior and Senior
Research Fellowships, and RM is thankful to University Grant Comission
for Junior and Senior Research Fellowships under SAP programme. SP is
thankful to TEQIP (Technical Education Quality Improvement Program)
for financial support.
18. Thorat KG, Kamble PS, Ray AK, Sekar N (2015) Novel
Pyrromethene dyes with N-ethyl carbazole at meso position: com-
prehensive photophysical, lasing, photostability and TD-DFT
study. Phys Chem Chem Phys 17:17221–17236. doi:10.1039/C5
CP01741F
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