Red emitting triphenylamine based rhodamine analogous with enhanced Stokes shift and viscosity sensitive emission

Article abstract of DOI:10.1016/j.dyepig.2016.11.021

Four novel structural hybrid analogues of Rhodamine B and Rhodamine 101 are synthesized by condensing N-substituted amino phenols with keto-acids of N-substituted phenols in presence of trifluoroacetic acid and are characterized by spectroscopic methods. Triphenylamine based derivatives show large Stokes shift (47 nm–69 nm) and red shifted emission (close to Near Infrared region) as compared to parent Rhodamine B and Rhodamine 101. These N-phenyl substituted dyes exhibited negative solvatochromism and pronounced viscosity sensitivity (14–24 folds increase in emission intensity) as compared to parent rhodamines. Polarity graphs and mathematically calculated charge transfer descriptors are in good correlations with observed trends. Computed values obtained by Density Functional Theory are in good agreement with the experimental results.

Full text of DOI:10.1016/j.dyepig.2016.11.021

Accepted Manuscript
Red emitting Triphenylamine based Rhodamine analogous with enhanced Stokes
shift and viscosity sensitive emission
Amol G. Jadhav, Shantaram Kothavale, Nagaiyan Sekar
PII:
S0143-7208(16)30567-8
10.1016/j.dyepig.2016.11.021
DYPI 5590
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 August 2016
Revised Date: 11 October 2016
Accepted Date: 18 November 2016
Please cite this article as: Jadhav AG, Kothavale S, Sekar N, Red emitting Triphenylamine based
Rhodamine analogous with enhanced Stokes shift and viscosity sensitive emission, Dyes and Pigments
(2016), doi: 10.1016/j.dyepig.2016.11.021.
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ACCEPTED MANUSCRIPT
Red emitting Triphenylamine based Rhodamine analogous with enhanced
Stokes shift and viscosity sensitive emission
Amol G. Jadhav, Shantaram Kothavale and Nagaiyan Sekar*
Department Dyestuff Technology
Institute of Chemical Technology (Formerly UDCT),
N. P. Marg, Matunga, Mumbai - 400 019.
Maharashtra, India.
Email: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel.: +22 3361 1111/ 2707; Fax: +22 3361 1020
Mumbai-400 019

ACCEPTED MANUSCRIPT
Red emitting Triphenylamine based Rhodamine analogues with enhanced
Stokes shift and viscosity sensitive emission
Amol G. Jadhav, Shantaram Kothavale and Nagaiyan Sekar*
Department Dyestuff Technology
Institute of Chemical Technology (Formerly UDCT),
N. P. Marg, Matunga, Mumbai - 400 019.
Maharashtra, India.
Email: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel.: +22 3361 1111/ 2707; Fax: +22 3361 1020
Mumbai-400 019
Abstract:
Four novel structural hybrid analogues of Rhodamine B and Rhodamine 101 are synthesized by
condensing N-substituted amino phenols with keto-acids of N-substituted phenols in presence of
trifluoroacetic acid and are characterized by spectroscopic methods. Triphenylamine based
derivatives show large Stokes shift (47 nm to 69 nm) and red shifted emission (close to Near
Infrared region) as compared to parent Rhodamine B and Rhodamine 101. These N-phenyl
substituted dyes exhibited negative solvatochromism and pronounced viscosity sensitivity (14 to
24 folds increase in emission intensity) as compared to parent rhodamines. Polarity graphs and
mathematically calculated charge transfer descriptors are in good correlations with observed
trends. Computed values obtained by Density Functional Theory are in good agreement with the
experimental results.
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Keywords: Co-planarity, Negative solvatochromism, Rhodamine analogues, Triphenylamine,
Viscosity induced emission.
1. Introduction:
The photophysical properties of xanthenes, particularly rhodamines have been extensively
studied [1]. Rhodamines are widely used as fluorescent probes [2], chemosensors in detection of
small molecules [2], sensitizers in dye sensitized solar cells [3] and as laser dyes [4].
Photophysical properties of rhodamine derivatives can be modified by three ways: (i)
modification of the amino groups of xanthene moiety (positions 3 and 6) [5]; (ii) modification of
the carboxyphenyl ring at positions 4’and/or 5‘ [5] or (iii) modification of the carboxylic acid
group (position 2’) [5].
Comparative studies on fluorescence properties of the structural analogues of rhodamine
dyes such as Pyronin B (C.I. 45010), Pyronin Y (C.I. 45005) and Acridine Red (C.I. 45000), have
been reported by Xian-Fu Zhang et al [6]. V. N. Belov, S. W. Hell et al. reported novel red
emitting rhodamine dyes excitable with 630 nm laser light and emitting at around 660 nm [7]. Y.-
H. Chu et al. reported rhodamine analogues containing biaryl linkers to impart rigidity [8].
Triphenylamine is widely used donating group for its charge transfer characteristic [9] and
very good electron donating ability [10]. Triphenylamine has been applied in a number molecules
designed for aggregation induced enhancement [11], organic light emitting devices [12] and dye
sensitized solar cells [13]. Co-planarity between nitrogen atom and the three adjacent carbon
atoms in triphenylamine maintain uninterrupted conjugation [14,15] between the lone pair of
nitrogen and the dendrimer arms [16]. To our motivation, effect of phenyl groups on
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photophysical properties of some phenanthroline derivatives has also recently explained by Taro
Tsubomura et al. [17]. Here, we are interested in understanding the role of phenyl substituent of
triphenylamine group in alternating photophysical properties of our novel rhodamine dyes as
compared to parent dyes.
In rhodamine derivatives apart from fluorescence emission, the only other significant
decay path-way for the excited state is internal conversion to the ground state [18]. The rate of
internal conversion is determined by a number of factors including the substituents at 3 and/or 6
position on xanthene moiety of dyes [18,19], solvent viscosity [20] and solvent polarity [20]. Rate
of internal conversion is dramatically reduced by rigidizing the group at 3 and/or 6 position on
xanthene moiety of dyes, either by chemical bonds or by using a viscous solvent [18,20].
In this work we are presenting synthesis of novel Rhodamine (Rho1-Rho4) dyes. Phenyl
substitution at 3 and/or 6 position on amino groups in xanthene moiety of dyes, Rho 1, Rho 3 and
Rho 4 facilitated significant Stokes shift and red shift in emission as compared to Rhodamine B
and Rhodamine 101. Unfortunately Rho 1, Rho 3 and Rho 4 have very low quantum yields in
solvents. This may be due to unfavorable steric interactions caused by the modification of both
amino groups to enhance the nucleophilicity of the phenolic oxygen and lead to lactone formation
[5]. So, viscosity induced study has been performed to overcome fluorescence quenching caused
by such unfavorable steric interactions.
2. Experimental
2.1 Materials and methods
Sodium bicarbonate, 3-methoxyaniline, phthalic anhydride, toluene, anhydrous aluminium
chloride, ethylene dichloride, pyridine hydrochloride, Iodobenzene, 1,10-phenanthroline and
Cu(I)I were purchased from S.D. fine chemicals Ltd., Mumbai, India. The solid reagents were
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characterized by their melting points and used without further purification. Commercially
available Rhodamine B and Rhodamine 101 were procured from Sigma-Aldrich India and used
without further purification. All reactions were monitored on precoated silica gel aluminum based
plates Kiesel gel 60 F254 Merck, India. Purification was achieved by silica gel 100-200 mesh
size. Melting points were recorded on instrument from Sunder Industrial Product Mumbai. The
absorption spectra of the compounds were recorded on a Perkin-Elmer Lamda 25 UV-visible
spectrophotometer. Fluorescence emission spectra were recorded on Varian Cary Eclipse
fluorescence spectrometer. FT-IR spectra were recorded on a Jasco 4100 Fourier Transform IR
1
instrument (ATR accessories). H NMR and 13C NMR spectra were recorded on a 500
MHz and 125 MHz respectively on Agilent Technology instrument. Chemical shifts are
expressed in 1 (ppm) using TMS as an internal standard.
2.2 Synthesis and characterization
2-(4-(Diethylamino)-2-hydroxybenzoyl)benzoic acid 2 [21], 8-hydroxyjulolidine 7 [22]
and 2-(8-hydroxy-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-carbonyl)benzoic acid 8 [23]
were synthesized by reported methods.
2.2.1 Synthesis of 3-methoxy-N,N-diphenylaniline 4
To a stirred solution of m-anisidine 3 (10 g, 81.20 mmol), iodobenzene (19.9 ml, 178.64
mmol), and 1,10-phenanthroline (0.59 g, 3.25 mmol) in toluene (175 ml) were added potassium
tert-butoxide (27 g, 56.11 mmol) and copper (I) iodide (0.62 g, 3.25 mmol). The reaction mixture
was heated under reflux for 24 hours at 125 °C. On completion of the reaction the mixture was
cooled to room temperature, filtered to remove copper metal. The product was extracted from the
filtrate using ethyl acetate. The combined organic phases were concentrated under vacuum and
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purified by column chromatography using 5 % EtOAc in hexane as the eluent to get the pure
product. (Scheme 1)
Yield: 66.10 %, Boiling point = 130-135 0C
0
1H NMR (500 MHz, CDCl3, 25 C, TMS) δ 7.24 (t, J = 7.9 Hz, 4H), 7.14 (t, J = 8.5 Hz, 1H),
7.09 (d, J = 7.9 Hz, 4H), 7.01 (d, J = 7.3 Hz, 2H) , 6.65 (d, J = 8.5 Hz, 1H) ,6.62 (s, 1H) , 6.56 (d,
J = 8.5 Hz, 1H), 3.71 (s, 3H);
0
13C NMR (125 MHz, CDCl3, 25 C, TMS) δ 160.4, 149.1, 147.7, 129.8, 129.2, 124.4, 122.8,
116.4, 109.7, 107.9, 55.2 ppm.
2.2.2 Synthesis of 3-(diphenylamino)phenol 5
3-Methoxy-N,N-diphenylaniline 4 (8 g, 29.09 mmol) was dissolved in 40 g of pyridine.HCl
o
and heated to 200 C for 10 hrs. After cooling to room temperature, water was added and solid
came out was filtered, dried well and collected as crude product, which was further purified on
column chromatography using 10 % EtOAc in hexane as the eluent to get the pure product.
(Scheme 1)
Yield: 79 %; Melting point = 98-100 0C
1H NMR (500 MHz, CDCl3, 25 0C, TMS) δ 4.60 (s, 1H), 6.46 (dd, J = 8.0 and 1.5 Hz, 1H), 6.52
(t, J =1.5 Hz, 1H), 6.64 (dd, J = 8.0 and 1.5 Hz, 1H), 7.01-7.07 (m, 2H), 7.09-7.11 (m, 4H), 7.23-
7.27 (m, 4H) ppm.
0
13C NMR (125 MHz, CDCl3, 25 C, TMS) δ 109.4, 110.4, 116.1, 123.1, 124.7, 129.3, 130.1,
147.6, 149.3, 156.2 ppm.
2.2.3 Synthesis of 2-(4-(diphenylamino)-2-hydroxybenzoyl)benzoic acid 6
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3-(Diphenylamino)phenol 5 (0.25 g, 0.95 mmol), phthalic anhydride (0.18 g, 1.20 mmol),
anhydrous aluminium chloride (0.19 g, 1.43 mmol) were mixed in ethylene dichloride (8 ml)
under nitrogen atmosphere at 0 °C. The mixture was stirred at 0 °C for 1 hour and then warmed to
room temperature and maintained at room temperature for 2 hours. After 2 hours reaction mass
was stirred at 50 °C for 12 hours and then it was cooled to room temperature, poured into 100 ml
ice water with continuous stirring. The product was extracted with chloroform and the aqueous
layer was separated. Organic layer was washed with water, dried using anhydrous sodium sulfate
and then solvent was removed on rotatory vacuum. The product was purified by silica gel (100-
200 mesh size) column chromatography using 3 % MeOH in CHCl3 as eluent to give the pure
product (Scheme 1).
Yield: 55.20 %; Melting Point = 188 – 192 0C.
1H NMR (500 MHz, CDCl3, 25 0C, TMS) δ 6.22-6.25 (dd, J = 9 and 2.5, 1H), 6.42-6.43(d, J =2,
1H), 6.82-6.84 (d, J = 9, 1H), 7.14-7.19 (m, 6H), 7.30-7.33 (m, 4H), 7.35-7.37 (dd, J = 7.5 and 1,
1H), 12.30 (s, 1H) ppm.
0
13C NMR (125 MHz, CDCl3, 25 C, TMS) δ 105.2, 110.0, 113.1, 125.5, 126.9, 127.4, 127.8,
129.4, 129.6, 131.1, 132.9, 133.7, 140.9, 145.4, 155.0, 164.6, 170.1, 199.4 ppm.
2.2.4 Procedure for synthesis of novel Rhodamine derivatives (Rho 1 – Rho 4)
Rho 1 was obtained by refluxing 1 (1mmol) with 6 (1mmol) in trifluoroacetic acid. Rho 2
was obtained by refluxing 7 (1mmol) with 2 (1mmol) in trifluoroacetic acid. Rho 3 was obtained
by refluxing 5 (1mmol) with 8 (1mmol) in trifluoroacetic acid. Rho 4 was obtained by refluxing 5
(1mmol) with 6 (1mmol) in trifluoroacetic acid. Temperature of reaction mass was maintained at
90-95 0C for 12 hours (Scheme 2). After completion of the reaction, the reaction mass was cooled
to room temperature and water was added with continuous stirring. Reaction mass was then
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neutralized by using NaHCO3. Precipitate formed was then filtered and washed with water twice.
Recrystallization in ethanol yield the desired product (Rho 1 – Rho 4) in moderate to good yields.
Characterizations (Rho 1 – Rho 4) (Supporting Information)
Rho 1: N-[9-(Isobenzofuran-1(3H)-one)-6-(dipnenylamino)-xanthene]-N-diethyl amine
Yield: 82 %, Melting point: 182 - 184 0C
0
1H NMR (500 MHz, CDCl3, 25 C, TMS) δ 7.98 (d, J = 7.5 Hz, 1H), 7.64 (t, J = 7.5 Hz, 1H),
7.57 (t, J = 7.5 Hz, 1H), 7.28 (s, 1H), 7.25 (s, 1H), 7.24 (s, 2H), 7.22 (s, 1H), 7.12 (s, 2H), 7.11
(s, 2H), 7.06 (t, J = 7.5 Hz, 2H), 6.81 (s, 1H), 6.63 (d, J = 8.5 Hz, 1H), 6.55 (t, J = 7 Hz, 2H), 6.37
– 6.31 (m, 2H), 3.31 (q, J = 7 Hz, 4H), 1.13 (t, J = 7 Hz, 6H) ppm.
0
13C NMR (125 MHz, CDCl3, 25 C, TMS) δ 168.5, 152.1, 151.7, 151.3, 151.3, 148.9, 148.6,
145.8, 133.5, 128.4, 128.3, 127.9, 127.5, 124.4, 122.9, 116.2, 111.2, 107.6, 107.3, 104.5, 101.5,
96.4, 89.5, 84.4, 43.5, 11.4 ppm.
FT-IR: 1750 (C=O stretch), 1516 (Ar-C=C stretch), 1270 (C-N stretch), 1146 (C-O strech) cm-1.
Mass: Calculated 538.2 for C36H30N2O3 found 539.1 (M+1)
Elemental analysis calcd. (%) for C36H30N2O3: C 80.27, H 5.61, N 5.20, O 8.91; found C 80.22,
H 5.60, N 5.21, O 8.92.
Rho 2: N-[9-(2-Carboxyphenyl)-6-(octahydro-1H-quinolizine)-3H-xanthen-3-ylidene]-N-
ethylethanaminium chloride
Yield: 68%, Melting point: 194 - 196 °C
1H NMR (500 MHz, CDCl3, 25 0C, TMS) δ 12.66 (s, 1H), 8.09 (d, J = 7 Hz, 1H), 7.59 – 7.56 (m,
1H), 7.54 (d, J = 7.5 Hz, 1H), 7.16 (d, J = 7 Hz, 1H), 6.73 (d, J = 9 Hz, 1H), 6.49 (s, 1H), 6.43 (d,
J = 9 Hz, 1H), 6.35 (s, 1H), 3.40 (dd, J = 14 and 7 Hz, 4H), 3.28 – 3.20 (m, 4H), 3.00 – 2.94 (m,
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2H), 2.59 (dd, J = 14.5 and 7.5 Hz, 2H), 2.56 – 2.47 (m, 2H), 2.04 (dd, J = 11 and 6 Hz, 2H), 1.89
(dd, J = 11 and 6 Hz, 2H), 1.20 (t, J = 7 Hz, 6H) ppm.
0
13C NMR (125 MHz, CDCl3, 25 C, TMS) δ 185.6, 169.2, 166.1, 165.8, 162.9, 150.6, 145.5,
145.5, 145.1, 144.7, 141.6, 140.3, 140.1, 133.3, 128.4, 124.7, 121.6, 113.6, 101.5, 90.5, 60.6,
49.6, 45.8, 28.6 ppm.
FT-IR: 3550 (Acidic O-H stretch), 1750 (C=O stretch), 1491 (Ar-C=C stretch), 1272 (C-N
stretch) cm-1.
Mass: Calculated 502.2 for C30H31ClN2O3 found 503.3 (M+1).
Elemental analysis calcd. (%) for C30H31ClN2O3: C 71.63, H 6.21, Cl 7.05, N 5.57, O 9.54;
found C 71.62, H 6.15, Cl 7.00, N 5.55, O 10.44.
Rho 3: N-[9-(Isobenzofuran-1(3H)-one)-6-(octahydro-1H-quinolizine)-xanthene]-N-diphenyl
amine
Yield: 77%, Melting point: 220 - 222 °C
0
1H NMR (500 MHz, CDCl3, 25 C, TMS) δ 8.01 (d, J = 7.5 Hz, 1H), 7.67 (t, J = 7.5 Hz, 1H),
7.60 (t, J = 7.5 Hz, 1H), 7.30 (d, J = 7.5 Hz, 2H), 7.28 (s, 2H), 7.25 (d, J = 7.5 Hz, 2H), 7.14 (s,
2H), 7.13 (s, 1H), 7.09 (t, J = 7 Hz, 2H), 6.87 (s, 1H), 6.64 (d, J = 8.5 Hz, 1H), 6.55 (d, J = 8.5
Hz, 1H), 6.16 (s, 1H), 3.16 (dt, J = 11 and 5.5 Hz, 4H), 2.53 (dtd, J = 22, 15.5 and 6 Hz, 4H), 1.98
(dd, J = 12 and 6 Hz, 2H), 1.88 (d, J = 4 Hz, 2H) ppm.
0
13C NMR (125 MHz, CDCl3, 25 C, TMS) δ 169.9, 153.5, 153.1, 152.6, 150.3, 150.1, 147.1,
134.8, 129.8, 129.7, 129.3, 128.9, 128.1, 125.8, 125.3, 124.6, 117.5, 112.6, 108.9, 108.7, 105.9,
97.8, 85.8, 44.8, 30.0, 14.4, 12.8 ppm.
FT-IR: 1744 (C=O stretch), 1482 (Ar-C=C stretch), 1278 (C-N stretch), 1161 (C-O stretch)
cm-1.
8

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Mass: Calculated 562.2 for C38H30N2O3 found 563.1 (M+1).
Elemental analysis calcd. (%) for C38H30N2O3: C 81.12, H 5.37, N 4.98, O 8.53; found C 81.09,
H 5.36, N 4.97, O 8.52.
Rho 4: N-[9-(Isobenzofuran-1(3H)-one)-6-(dipnenylamino)-xanthene]-N-diphenyl amine
Yield: 62%, Melting point: 225 - 227 °C
1H NMR (500 MHz, CDCl3, 25 0C, TMS) δ 7.92 (d, J = 7.5 Hz, 1H), 7.62 (td, J = 7.5 and 1 Hz,
1H), 7.53 (td, J = 7.5 and 1 Hz, 1H), 7.21 (d, J = 2.5 Hz, 1H), 7.21 – 7.17 (m, 8H), 7.04 (dd, J =
8.5 and 1 Hz, 8H), 7.01 (s, 1H), 6.99 (s, 2H), 6.98 (d, J = 1 Hz, 1H), 6.68 (d, J = 2.5 Hz, 2H), 6.59
(d, J = 2.5 Hz, 1H), 6.58 (d, J = 2.5 Hz, 1H), 6.49 (s, 1H), 6.47 (s, 1H) ppm.
0
13C NMR (125 MHz, CDCl3, 25 C, TMS) δ 169.4, 152.6, 152.3, 150.1, 146.7, 134.8, 129.5,
129.4, 128.4, 127.2, 125.4, 124.9, 124.1, 117.2, 111.5, 108.5, 83.7 ppm.
FT-IR: 1749 (C=O stretch), 1493 (Ar-C=C stretch), 1284 (C-N stretch), 1177 (C-O stretch) cm-1.
Mass: Calculated 634.2 for C44H30N2O3 found 635.2 (M+1).
Elemental analysis calcd. (%) for C44H30N2O3: C 83.26, H 4.76, N 4.41, O 7.56; found C 83.21,
H 4.75, N 4.42, O 7.57.
<<Please insert Scheme1: Synthesis of N-substituted amino phenol (5) and Keto-acids of N-
substituted phenol derivatives (2, 6 or 8) >>
<<Please insert Scheme2: Synthesis of novel Rhodamine derivatives (Rho 1 – Rho 4) >>
2.3 Absorption and fluorescence experiments
The absorption and emission spectra were recorded using a quartz cell of 1-cm
path length by using freshly prepared solutions. In solution Rhodamines exist in the lactone and
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zwitterion forms [1,24]. Zwitterion form of dyes were obtained by dissolving these dyes in
methanolic HCl. The emission spectra were recorded in the range from 550 nm to 850 nm
by exciting these dyes between 280-700 nm. Excitation and emission slit width was kept at 5.
The data were smoothened by using Savitzky–Golay smoothing filter. All the spectroscopic
studies were carried out by keeping constant concentration of 5 x10-6 mol L-1 and at room
temperature. All the parameters were kept constant throughout the experiment in order to
have precision and accuracy.
2.4 Computational method
All the DFT computations were performed using the Gaussian 09 (G09) program [25].
DFT calculations were performed on a HP workstation XW 8600 with 4 GB RAM, Xeon
processor and Windows Vista as operating system. DFT and TD-DFT methods were employed
for the ground state and excited state optimizations respectively. The hybrid functional B3LYP
(Becke3-Lee-YangPar) and 6-311+G(d,p) basis set were used for all the atoms [26,27]. Vertical
excitation energies, oscillator strengths and orbital contributions for the lowest 10 singlet
transitions at the optimized geometry in the ground state were obtained by TD-DFT calculations.
The solvents used were toluene, chloroform (CHCl3), ethyl acetate (EtOAc), tetrahydrofuran
(THF), methanol (MeOH), dimethyl sulfoxide (DMSO). Optimization under solvent
environments were done using Polarizable Continuum Model (PCM) [28].
3. Results and discussion
3.1 Photo physical properties:
Absorption and emission properties of all the dyes were obtained in six solvents of various
polarities to evaluate the effect of different N-substituted groups at 3 and 6 position in xanthene
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moiety of novel dyes (Figure S1 and Figure1). Normalized spectra of these all dyes (Rho 1- Rho
4) clearly indicates the bathochromic shift in both the absorption and emission as compared to
Rhodamine B and Rhodamine 101 (Figure S1 and Figure1).
<<Please insert Figure 1: (a) Comparative normalized emission spectra of all dyes in different
solvents>>
The synthesized dyes (Rho 1 – Rho 4) showed absorption and emission in the range of
565-589 nm and 594-658 nm respectively in solvents of varying polarities (Table 1). There is
significantly large Stokes shift observed in Rho 1, Rho 3 and Rho 4 compared to Rhodamine B
and Rhodamine 101 in all the solvents (Table 1). Molar absorptivity (ε) is very high for Rho 3
dyes in all the solvents. Rhodamine 101 (Фf = 0.98 in MeOH) been used as reference standard
for quantum yield calculations. Fluorescence Quantum yield (Фf) of Rho 2 is more than
Rhodamine B in most solvents and is comparable with Rhodamine 101 in CHCl3, MeOH and
DMSO. Fluorescence Quantum yield of Rho 1, Rho 3 and Rho 4 is very low (0.001-0.021)
(Table 1).
<<Please insert Table 1. Photophysical data of all dyes in different solvents>>
3.2 Polarity graphs:
Mathematical quantification of solvatochromic properties of the triphenyl based dyes is done by
using Lippert-Mataga and McRae equation [29]. Lippert-Mataga and McRae plot have shown
very good linearity of Stokes shift vs. ƒLM(ε,η) and ƒMacRae(ε,η) functions. Both the plots have shown
distinct difference between N-Phenyl substituted dyes and the parental Rhodamine B and
Rhodamine 101 (Figure 2). Rho 1, Rho 3 and Rho 4 dyes have shown negative
solvatochromism with excellent regression coefficient (R), while Rhodamine B, Rho 2 and
Rhodamine 101 showed positive solvatochromism.
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<<Please insert Figure 2: Lippert-Mataga and McRae plots>>
3.3 Charge transfer characteristics
An efficient charge transfer in these dyes can be studied by Mulliken-Hush (GMH)
analysis. Degree of delocalization or fractional degree of localization of the excess charge (C2 )
b
and donor–acceptor coupling matrix value (HDA) was calculated by equation 1 and 2 respectively
[30,31].
ꢈ
ꢅꢆ
ꢀꢁꢂ= ꢃꢂ (1-ꢄꢅꢆ +ꢉꢆ
)
ꢈ
……………….(1)
ꢇ
ꢈ
ꢇ
ꢇ
ꢅꢍ ꢅꢆ
ꢊꢋꢌ= ꢅꢍ ꢅꢆ
=
…………..(2)
ꢇ
ꢎ
ꢅꢆ
ꢇ
ꢇ
ꢇ
ꢇ
ꢈ
ꢈ
ꢄ
ꢅꢆ ꢏꢉꢆ
ꢇ
ꢇ
where,
ꢀE≥ = vertical excitation energy
ꢀꢁ≥ = difference between the adiabatic dipole moments of the ground and excited states
ꢀꢁD = difference in diabatic state dipole moments
≥
ꢁ≥ = transition dipole moment
Donor–acceptor coupling matrix distance (RDA) was calculated by equation 3 [32]
ꢓ
ꢗ
ꢅꢓ
ꢒ
ꢔꢕꢖ ꢔꢕꢖ
ꢐꢋꢌ= 2.06 x 10ꢑꢂ
ꢘꢈ…………………(3)
ꢙ
ꢎꢚ
where,
ϵmax= molar extinction coefficient at maximum absorption
ꢀν12 = full width at halfmaximum (FWHM) of charge-transfer band
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<<Please insert Table 2 Degree of delocalization (C2 ), donor–acceptor coupling value (HDA)
b
and donor–acceptor coupling distance (RDA) for all dyes in various solvent>>
Degree of delocalization (C2 ) and donor–acceptor coupling value (HDA) for Rhodamine
b
B, Rho 2 and Rhodamaine 101 increased in polar solvent than non-polar solvents suggesting
good charge transfer in polar solvents for these dyes, while reverse trend was observed for Rho 1,
Rho 3 and Rho 4 (Table 2). Similarly, donor–acceptor coupling distance (RDA) values decreased
in polar solvents for Rhodamine B, Rho 2 and Rhodamine 101, while decreased in non-polar
solvents for Rho 1, Rho 3 and Rho 4. It can be concluded that Rhodamine B, Rho 2 and
Rhodamine 101 showed positive solvatochromism while Rho 1, Rho 3 and Rho 4 showed
negative solvatochromism.
3.4 Viscosity sensitivity (effect of triphenylamine group):
Rhodamine dyes exhibit very small triplet-state quantum yields. The main pathway for non-
radiative deactivation of the excited state is internal conversion and is dependent upon the So-S1
energy gap [18]. In order to avoid unfavorable steric interactions caused by N-Phenyl substituent
and to reduce internal conversion rate, rigidity to the phenyl groups has been provided by solvent
viscosity.
There are possible rotation sites on both donor and acceptor side in these rhodamine
analogues (Figure 3). So, we are interested to study Fluorescent Molecular Rotor (FMR)
properties in theses dyes. Various FMRs available in the literature include 9-(dicyanovinyl)-
julolidine (DCVJ) [33], 4,4-dimethylaminobenzonitrile (DMABN) [34], ((7-amino-4-
methylcoumarin-3-acetyl) amino) hexanoic acid (AMCA) and some coumarin derivatives [34].
13

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<<Please insert Figure 3 Possible rotation sites of General tautomers (lactone and zwiterionic)
forms >>
It has been observed that all N-phenyl dyes are more sensitive to viscosity. Rho 1 showed
20 fold increase in emission intensity with increase in viscosity (MeOH-glycerol mixture).
Similarly for Rho 3 and Rho 4 has 24 and 14 fold increase in emission intensity (Figure 4).
<<Please insert Figure 4: (a) Emission spectra of Rhodamine B in varying viscosity media; (b)
Emission spectra of Rho 2 in varying viscosity media; (c) Emission spectra of Rhodamine 101 in
varying viscosity media; (d) Emission spectra of Rho 1 in varying viscosity media; (e) Emission
spectra of Rho 3 in varying viscosity media; (f) Emission spectra of Rho 4 in varying viscosity
media>>
Comparative viscosity sensitivity (mPa s) as a function of normalized emission intensity
has been plotted for all dyes (Figure S2). Viscosity increases from 0.55 mPa s to 93.03 mPa s in
MeOH-glycerol mixture. Rate of increase of emission intensity with increase in viscosity is much
more for N-Phenyl substituent as compare to Rhodamine B and Rhodamine 101. Viscosity
sensitivity trend of dyes has given in decreasing order as Rho 3 > Rho 1 > Rho 4 > Rhodamine
B ≥ Rhodamine 101 ≥ Rho 2.The emission intensity of FMR and effect of viscosity of the
solvent was described by using Forster-Hoffmann Eq.
Log I = C + x log η
where,
14

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x is the viscosity sensitivity parameter, η is viscosity in mPa s, C is constant and I is the maximum
emission intensity.
<<Please insert Figure 5: Comparative logarithmic spectrum of intensity against viscosity>>
Very high x values for Rho 1, Rho 3 and Rho 4 as 0.42 (R2 = 0.99), 0.41 (R2 = 0.98) and
0.45 (R2 = 0.90) respectively were observed. On the other hand very low x values were found for
Rhodamine B, Rho 2 and Rhodamine 101 as 0.04 (R2 = 0.93), 0.09 (R2 = 0.90) and 0.04 (R2 =
0.95) respectively (Figure 5). x values clearly indicate the effect of phenyl substituent for
rigidization of the system in viscous medium.
3.5 DFT study
3.5.1 Optimization of molecular structure
To gain insight into the geometry of the structure of dyes, computational studies were
performed using density functional theory (DFT) at the B3LYP/6-311+ G(d,p) level. Ground state
geometries of all the dyes have shown variation in the dihedral angles (Figure 6, S3).
<<Please insert Figure 6 Optimized structures of Rho 2 and Rho 4 dyes at B3LYP 6-311+G
(d,p) in CHCl3 solvent>>
All the dyes showed small variation of dihedral angles (62.65 0 to 72.42 0) between xanthene
moiety and carboxyphenyl group at meso position (C8-C7-C21-C23), but dihedral angles at 3 and 6
position of xanthene moiety and N-substituents showed significant variations. Rho 4 dye with N-
phenyl substituents at both 3 and 6 positions of xanthene core has not shown significant variation of
dihedral angles (37.84 0 – 38.82 0). Dyes Rho 1 and Rho 3 also have N-phenyl substituents showed
0
0
dihedral angles ranges from 39.66 to 40.63 (C13-C12-N35-C37 and C10-C12-N35-C48) clearly
15

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indicated very weak/no involvement of nitrogen as donor in these dyes. Rhodamine B, Rho 2 and
Rho 1 have N-ethyl substituent have very small dihedral angles from 1.47 0 to 7.20 0 (C6-C1-N34-C59
and C2-C1-N34-C64) indicating good donating capacity leading to co-planarity in the molecules
(Table 3).
<< Please insert Table 3 Dihedral angles of the selected atoms of dyes optimized at B3LYP 6-
311+G (d,p) in CHCl3 solvent>>
3.5.2 TD-DFT
In order to understand absorption properties of all rhodamine dyes, the ground state optimized
geometries in solvents of various polarities were subjected to TD at least first 10 states to get
vertical excitations using B3LYP/6-311+G(d,p) method and the results are collected in Table 4.
Results by DFT and TD-DFT suggested pronounced effect of polarities of the solvents on these
dyes.
<<Please insert Table 4. Comparative experimental data and computed data at B3LYP 6-311+G
(d,p) for different dyes in CHCl3>>
3.5.3 Frontier Molecular orbitals
Electronic density at HOMO is situated on the xanthene core and nitrogen atom in all the
dyes. There is a complete transfer of electronic density to the carboxyphenyl moiety at LUMO
level in all dyes, but interestingly LUMO+1 (which has major orbital contribution) showed
further delocalization of electronic density to N-phenyl core only in Rho 1, Rho 3 and Rho 4
supporting the red shifting absorption and emissions maxima in these dyes (Figure 7). Such
further transfer of electronic density at LUMO+1 is absent in dyes Rhodamine B, Rhodamine
101 and Rho 2.
16

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<<Please insert Figure 7 Molecular orbitals showing electronic distribution in all dyes in CHCl3
solvent>>
HOMO-LUMO and LUMO+1 of dyes Rho 1, Rho 3 and Rho 4 are lower in energy as
compared to Rhodamine B, Rho 2 and Rhodamine 101 suggesting the red shift in the absorption
and emission of the dyes. Moreover, the energy gap of HOMO-LUMO decreased from 2.229 eV
(Rhodamine B) to 1.905 eV (Rho 4) suggesting the enhanced Stokes shift in Rho 4 (Figure 8).
The energy levels of HOMO, LUMO and LUMO+1 have followed the trend as Rhodamine B >
Rho 2 > Rhodamine 101 > Rho 1 > Rho 3 > Rho 4. Similarly, energy band gap between HOMO
and LUMO followed the Rhodamine B > Rho 2 > Rhodamine 101 > Rho 1 > Rho 3 > Rho 4.
These observations suggested both red shifted emission and enhanced stokes shift in fused
triphenyl amine derivatives.
<<Please insert Figure 8 HOMO-LUMO energy level diagram of all dyes in CHCl3>>
Conclusion:
Novel structural analogues of Rhodamine B and Rhodamine 101 are synthesized by
condensing N-substituted amino phenols with keto-acids of N-substituted phenols. Three dyes
(Rho 1, Rho 3 and Rho 4) show good Stoke shift (47-70 nm) and red shifted emission as
compared to parent analogues Rhodamine B and Rhodamine 101 dyes. Moreover Rho 1, Rho 3
and Rho 4 show negative solvatochromism while Rho 2, Rhodamine B and Rhodamine 101
show positive solvatochromism. Dye Rho 3 showed pronounced viscosity sensitivity of 24 folds
increase in emission intensity. Such triphenylamine based rhodamines can found applications as
the viscosity probes in the Near Infrared region.
17

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Acknowledgements
The authors Amol G. Jadhav and Shantaram Kothavale are grateful to UGC, New Delhi, India for
financial support by way of Junior Research and Senior Research Fellowships.
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Figures and Schemes
Figure 1: (a) Comparative normalized emission spectra of all dyes in different solvents
Figure 2: Lippert-Mataga and McRae plots
22

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Figure 3 Possible rotation sites of General tautomers (lactone and zwiterionic) forms
Figure 4: (a) Emission spectra of Rhodamine B in varying viscosity media; (b) Emission spectra
of Rho 2 in varying viscosity media; (c) Emission spectra of Rhodamine 101 in varying viscosity
media; (d) Emission spectra of Rho 1 in varying viscosity media; (e) Emission spectra of Rho 3
in varying viscosity media; (f) Emission spectra of Rho 4 in varying viscosity media
Figure 5: Comparative logarithmic spectrum of intensity against viscosity
Figure 6 Optimized structures of Rho 2 and Rho 4 dyes at B3LYP 6-311+G (d,p) in CHCl3
solvent
Figure 7 Molecular orbitals showing electronic distribution in all dyes in CHCl3 solvent
Figure 8 HOMO-LUMO energy level diagram of all dyes in CHCl3.
Scheme1: Synthesis of N-substituted amino phenol (5) and Keto-acids of N-substituted phenol
derivatives (2, 6 or 8)
Scheme2: Synthesis of novel rhodamine derivatives (Rho 1 – Rho 4)
23

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Tables
Table 1. Photophysical data of all dyes in different solvents
Table 2 Degree of delocalization (C2 ), donor–acceptor coupling value (HDA) and donor–acceptor
b
coupling distance (RDA) for all dyes in various solvent
Table 3 Dihedral angles of the selected atoms of dyes optimized at B3LYP 6-311+G (d,p) in CHCl3
solvent
Table 4. Comparative experimental data and computed data at B3LYP 6-311+G (d,p) for different
dyes in CHCl3.

ACCEPTED MANUSCRIPT
Table 1. Photophysical data of all dyes in different solvents
ꢀϑ
Solvent
Dye
λabs (nm)
ε (M-1 cm-1)
λemm (nm) ꢀϑ (nm)
Ф
(cm-1)
Rhodamine B
Rho 2
564
573
47634
74516
590
600
26
27
781
785
0.235
0.421
Rhodamine 101
574
109626
600
26
754
0.863
Rho 1
Rho 3
570
581
589
557
567
576
571
579
586
557
567
577
566
575
575
558
568
578
565
577
579
554
565
567
561
573
573
565
576
586
565
578
96193
210201
31481
624
639
658
583
595
603
625
637
651
585
596
604
615
630
635
587
597
607
614
630
634
583
594
595
609
622
626
597
607
617
612
628
54
58
69
26
28
27
54
58
65
28
29
27
49
55
60
29
29
29
49
53
55
29
29
28
48
49
53
32
31
31
47
50
1518
1562
1780
800
0.007
0.002
0.009
0.360
0.471
0.392
0.010
0.004
0.008
0.257
0.492
0.881
0.002
0.001
0.002
0.432
0.514
0.893
0.003
0.001
0.004
0.703
0.947
0.988
0.016
0.011
0.021
0.457
0.765
0.541
0.002
0.001
Rho 4
Rhodamine B
Rho 2
123117
104112
180178
98713
829
Rhodamine 101
Rho 1
777
1513
1572
1703
859
Rho 3
200841
55251
Rho 4
Rhodamine B
Rho 2
111947
100823
38868
858
Rhodamine 101
Rho 1
774
79011
1407
1518
1643
885
Rho 3
203484
51278
Rho 4
Rhodamine B
Rho 2
107219
101736
48056
855
Rhodamine 101
Rho 1
826
64341
1412
1458
1498
898
Rho 3
199507
28162
Rho 4
Rhodamine B
Rho 2
91445
76224
864
Rhodamine 101
Rho 1
192433
84223
830
1405
1375
1375
949
Rho 3
123984
34953
Rho 4
Rhodamine B
Rho 2
105902
97348
887
Rhodamine 101
Rho 1
167427
112226
220565
857
1359
1377
Rho 3

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Rho 4
569
31242
617
48
1367
0.006
Table 2 Degree of delocalization (C2 ), donor–acceptor coupling value (HDA) and donor–acceptor
b
coupling distance (RDA) for all dyes in various solvent
Rhodamine B
Rho 2
Rhodamine 101
Rho 1
Rho 3
Rho 4
Solvent
HDA
RDA
HDA
RDA
HDA
RDA
HDA
RDA
HDA
RDA
HDA
RDA
C2b
C2b
C2b
C2b
C2b
C2b
(cm-1)
(Ao)
(cm-1)
(Ao)
(cm-1)
(Ao)
(cm-1)
(Ao)
(cm-1)
(Ao)
(cm-1)
(Ao)
Toluene
CHCl3
EtOAc
THF
0.091
0.262
0.275
0.284
0.282
0.234
5098
7934
8005
8094
8076
7511
9.63
0.160
0.259
0.277
0.283
0.276
0.290
6369
7755
7882
7943
7883
7918
7.11
5.45
6.28
6.11
6.81
4.69
0.242
0.322
0.194
0.176
0.301
0.289
7311
8239
6843
6596
7967
7898
8.29
0.342
0.310
0.293
0.271
0.296
0.187
8325
8099
8039
7869
8137
6903
3.49
4.32
3.88
3.64
4.04
5.48
0.390
0.358
0.364
0.361
0.328
0.282
8394
8278
8367
8325
8191
7788
5.22
5.84
5.42
5.50
4.42
6.01
0.221
0.266
0.259
0.208
0.236
0.125
7045
7544
7617
7013
7413
5820
2.32
2.86
2.80
2.22
2.33
2.93
6.09
6.41
6.48
7.30
4.32
8.14
4.93
4.53
8.64
5.70
MeOH
DMSO
Table 3 Dihedral angles of the selected atoms of dyes optimized at B3LYP 6-311+G (d,p) in CHCl3
solvent
Compound
C8-C7-C21-C23 C13-C12-N35-C37
C10-C12-N35-C48
(0)
C6-C1-N34-C59 C2-C1-N34-C64
(0)
(0)
(0)
(0)
68.56698
60.11645
110.847
7.20771
5.46866
Rhodamine B
Rho 2
62.65031
62.78011
67.07833
68.70447
72.4202
13.81554
13.95179
40.10561
40.63504
38.25814
16.82875
17.11534
39.66384
40.51022
37.86349
3.84412
14.23323
1.47503
2.75891
17.01033
4.54981
Rhodamine 101
Rho 1
13.10442
38.42828
15.63374
37.84132
Rho 3
Rho 4

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Table 4. Comparative experimental data and computed data at B3LYP 6-311+G (d,p) for different
dyes in CHCl3
Experimental
Theoretical (TD-DFT)
Orbital
Verticalb
Contribution
µge
(Debye)
a
f c
f c
λmax
Compound
Excitation
Band gap (eV)
(nm)
(nm)
535
586
586
568
577
590
557
567
576
571
579
586
0.7134
0.6561
0.9974
0.9693
1.9979
0.6768
1.23
1.39
1.18
2.09
2.12
2.61
0.0027
0.0133
0.0142
0.0045
0.0089
0.0044
H
H
H
H
H
H
→
→
→
→
→
→
L+1 (96 %)
L+1 (97 %)
L+1 (97 %)
L+1 (79 %)
L+1 (66 %)
L+1 (75 %)
Rhodamine B
Rho 2
Rhodamine 101
Rho 1
Rho 3
Rho 4

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Figures and Schemes
Figure 1: (a) Comparative normalized emission spectra of all dyes in different solvents
Figure 2: Lippert-Mataga and McRae plots
Figure 3 Possible rotation sites of General tautomers (lactone and zwiterionic) forms
Figure 4: (a) Emission spectra of Rhodamine B in varying viscosity media; (b) Emission spectra of
Rho 2 in varying viscosity media; (c) Emission spectra of Rhodamine 101 in varying viscosity media;
(d) Emission spectra of Rho 1 in varying viscosity media; (e) Emission spectra of Rho 3 in varying
viscosity media; (f) Emission spectra of Rho 4 in varying viscosity media
Figure 5: Comparative logarithmic spectrum of intensity against viscosity
Figure 6 Optimized structures of Rho 2 and Rho 4 dyes at B3LYP 6-311+G (d,p) in CHCl3 solvent
Figure 7 Molecular orbitals showing electronic distribution in all dyes in CHCl3 solvent
Figure 8 HOMO-LUMO energy level diagram of all dyes in CHCl3.
Scheme1: Synthesis of N-substituted amino phenol (5) and Keto-acids of N-substituted phenol
derivatives (2, 6 or 8)
Scheme2: Synthesis of novel rhodamine derivatives (Rho 1 – Rho 4)