Highly fluorescent blue-green emitting phenanthroimidazole derivatives: Detail experimental and DFT study of structural and donating group effects on fluorescence properties

Article abstract of DOI:10.1016/j.dyepig.2018.06.020

A series of highly fluorescent blue-green emitting phenanthrene and 3,6-diphenylphenanthrene based compounds have been synthesized to study the effect of different electron donating groups on photophysical properties. A very unique and different fluorescence properties are observed in the case of N-phenyl carbazole donor based compounds as compared to the other electron donating groups. Observed intramolecular charge transfer and highly polar excited states of these compounds are elucidated by Lipert-Mataga correlations. The positive and negative solvatochromism observed from non-polar to polar solvents is also supported using Kamlet-Taft and Catalan multilinear regression analysis. Comparative study of photophysical properties between benzil, phenanthrene and 3,6-diphenylphenanthrene based hydroxyl imidazole derivatives is presented to study the effect of rigidity and extended conjugation. Observed experimental results are also correlated theoretically using Density Functional theory computations.

Full text of DOI:10.1016/j.dyepig.2018.06.020

Dyes and Pigments 159 (2018) 209–221
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Highly fluorescent blue-green emitting phenanthroimidazole derivatives:
Detail experimental and DFT study of structural and donating group effects
on fluorescence properties
Shantaram Kothavale, Sulochana Bhalekar, Nagaiyan Sekar∗
Department of Dyestuff Technology, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Phenanthrene imidazole
Solvatochromism
Multi-linear regression analysis
Density function theory
A series of highly fluorescent blue-green emitting phenanthrene and 3,6-diphenylphenanthrene based com-
pounds have been synthesized to study the effect of different electron donating groups on photophysical
properties. A very unique and different fluorescence properties are observed in the case of N-phenyl carbazole
donor based compounds as compared to the other electron donating groups. Observed intramolecular charge
transfer and highly polar excited states of these compounds are elucidated by Lipert-Mataga correlations. The
positive and negative solvatochromism observed from non-polar to polar solvents is also supported using
Kamlet-Taft and Catalan multilinear regression analysis. Comparative study of photophysical properties between
benzil, phenanthrene and 3,6-diphenylphenanthrene based hydroxyl imidazole derivatives is presented to study
the effect of rigidity and extended conjugation. Observed experimental results are also correlated theoretically
using Density Functional theory computations.
1. Introduction
spectral change [25].
Phenanthrene based imidazole compounds with hydroxyl group
Due to its rigid, conjugated aromatic structure phenanthrene based
attached on phenyl ring are studied in detail for their ESIPT phenom-
enon [15,26], while different phenanthrene based imidazole deriva-
tives are reported as charge transfer compounds where phenanthrene
unit acts as electron donor while imidazole acts as electron acceptor.
Proton coupled electron transfer reactions have attracted much atten-
tion due to their biological applications [27–33]. Kasha et al. proved
that in such type of compounds emission can occur through all three
states i.e. locally excited state, intramolecular charge transfer (ICT)
state or proton transfer state and also they can compete with each other
[34–36].
organic compounds are reported to show improved fluorescence prop-
erties with excellent thermal stability, long fluorescence lifetime, high
quantum yield as well as solid state fluorescence [1–5]. Phenanthrene
and related derivatives have been widely applied in the field of organic
light emitting devices (OLED), mainly as blue organic light emitting
material [6–12]. Number of conjugated organic molecules were re-
cently reported for their high fluorescence quantum yield in solid state
without concentration quenching problem [13–17]. By replacing
phenyl unit with phenanthrene moiety different near infra red emitting
(
NIR) emitting dyes were synthesized and reported to show red shifted
absorption-emissions as well as high fluorescence quantum yields
18,19].
Excited state intramolecular proton transfer (ESIPT) molecules have
We synthesized different phenanthrene based imidazole derivatives
using commercially available phenanthrene-9, 10-dione to study the
effect of rigidity on ICT as well as ESIPT properties. Further, to find out
the effect of elongated conjugation on fluorescence properties, 3 and 6
position of phenanthrene moiety are substituted with the extra phenyl
rings. Both by increasing rigidity and extending the conjugation, im-
proved photophysical properties are observed compared to the pre-
viously reported benzil based ESIPT compounds [37]. Similar to the
benzil derivatives, typically red shifted absorption-emission peaks are
observed for N-phenyl carbazole derivatives; probably due to the keto
emission which is absent in other derivatives. Fig. 1 represents all the
[
attracted much attention in recent years due to their unique photo-
physical properties such as large Stokes shift with no self absorption
and are used as UV photo stabilizers [20], proton transfer lasers [21],
photo switches [22] or fluorescent probes [23] and also as potential
materials for organic light emitting diodes [24]. However the use of
ESIPT compounds is limited due to its low emission efficiency, con-
centration quenching of the keto emission and environment sensitive
∗
Corresponding author.
E-mail addresses: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com (N. Sekar).
https://doi.org/10.1016/j.dyepig.2018.06.020
Received 19 December 2017; Received in revised form 13 May 2018; Accepted 14 June 2018
Available online 20 June 2018
0143-7208/ © 2018 Elsevier Ltd. All rights reserved.

S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Fig. 1. Structure of the synthesized compounds.
Fig. 2. Photograph of compounds DHPhI, JHPhI, THPhI, CHPhI, DHPPhI, JHPPhI, THPPhI and CHPPhI.
structures of the synthesized derivatives. Compound DHPhI, JHPhI,
THPhI and CHPhI are based on phenanthrene moiety while compounds
DHPPhI, JHPPhI, THPPhI and CHPPhI are based on the 3,6-diphe-
nylphenanthrene moiety (see Fig. 2).
filtered, washed with water and dried well as crude product, which was
further purified by using column chromatography (10–40% EtOAc in
hexane) to get the pure products.
2
.2.1.1. 5-(Diethylamino)-2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol
1
(
DHPhI). Yield = 0.29 g (52%); Melting point = 158–162 °C. H NMR
500 MHz, DMSO‑d ) δ 13.52 (s, 1H), 9.14 (d, J = 8 Hz, 2H), 8.76 (s,
H), 8.24 (d, J = 8.5 Hz, 2H), 8.00 (t, J = 7.5 Hz, 2H), 7.90 (t,
2. Experimental section
(
1
6
2
.1. Materials and equipments
J = 8.5 Hz, 2H), 6.68 (dd, J = 8.5 and 2 Hz, 1H), 6.50 (d, J = 2 Hz,
1
3
1H), 3.63–3.67 (q, J = 7 Hz, 4H), 1.39 (t, J = 7 Hz, 6H). C NMR
All the required chemicals were purchased from local supplier, S D
(
126 MHz, DMSO‑d ) δ 172.7, 159.8, 150.6, 128.1, 127.9, 125.9, 124.7,
6
Fine Chemicals, Mumbai, India and also from Alfa aesar Ltd and are
used as received without any further purification. The solvents used for
synthesis and analytical measurements were also obtained from the
local suppliers, dried by following standard procedures and distilled
prior to use. All the reactions were monitored by TLC (thin layer
chromatography) with detection by UV light. Compound purification
was obtained by using column chromatography, which was performed
on 100–200 mesh silica as the stationary phase. H and C NMR
spectral data were recorded on a 500 MHz Varian Cary Eclipse Australia
5
was done using a QTOF LC/MS spectrometer. The absorption recorded
on PerkinElmer UV- visible spectrophotometer Lambda 25 and emission
spectra were recorded on Varian Cary Eclipse fluorescence spectro-
photometer at room temperature using a 10 mm cuvette with a 2.5 nm
slit width. Emission quantum yields were obtained by using 9,10-di-
+
1
22.5, 104.2, 98.6, 44.4, 13.3. HRMS (ESI): m/z calcd for (M + H)
O 383.1919; found 383.1286. Elemental analysis calcd (%);
Mol. Formula: C25 O (C: 78.71, H: 6.08, N: 11.02, O: 4.19; Found:
C: 78.93, H: 5.96, N: 11.05, O: 4.04).
25 23 3
C H N
23 3
H N
2
.2.1.2. 9-(1H-Phenanthro[9,10-d]imidazol-2-yl)-1,2,3,5,6,7-
hexahydropyrido[3,2,1-ij]quinolin-8-ol (JHPhI). Yield = 0.35 g (62%);
Melting point = 160–164 °C. 1H NMR (500 MHz, DMSO‑d
1
13
6
) δ 13.43
s, 1H), 13.39 (s, 1H), 9.09–9.13 (m, 2H), 8.78 (d, J = 7.5 Hz, 1H), 8.68
d, J = 7.5 Hz, 1H), 7.96–8.02 (m, 2H), 7.87–7.89 (m, 3H), 3.40–3.44
m, 4H), 3.01 (t, J = 7 Hz, 2H), 2.95 (t, J = 7 Hz, 2H), 2.15–2.18 (m,
00 MHz instrument using TMS as an internal standard. HRMS analysis
(
(
(
4
1
1
H). 13C NMR (126 MHz, DMSO‑d
) δ 154.8, 151.8, 145.5, 134.6,
6
28.1, 128.1, 127.9, 127.8, 126.1, 125.8, 124.8, 124.5, 123.5, 122.6,
22.5, 122.3, 112.9, 107.5, 101.2, 49.9, 49.5, 27.5, 22.5, 21.6, 21.4.
+
F
phenylanthracene (Φ = 0.95 in cyclohexane) as reference standard.
HRMS (ESI): m/z calcd for (M + H)
27
C H
23
N
3
O 406.1919; found
O (C:
4
7
3
06.0000. Elemental analysis calcd (%); Mol. Formula: C27
9.97, H: 5.72, N: 10.36, O: 3.95; Found: C: 80.43, H: 5.55, N: 10.32, O:
.67).
H
23
N
3
2
2
.2. Synthesis
.2.1. General procedure for the synthesis of DHPhI, JHPhI, THPhI, CHPhI,
DHPPhI, JHPPhI, THPPhI and CHPPhI
2.2.1.3. 5-(Diphenylamino)-2-(1H-phenanthro[9,10-d]imidazol-2-yl)
phenol (THPhI). Yield = 0.25 g (76%); Melting point = 233–236 °C. H
NMR (500 MHz, DMSO‑d
1
Different hydroxyl aldehyde compounds (1 eq) and phenanthrene or
3,6-diphenylphenanthrene diketo compounds (1.1 eq) were dissolved
6
) δ 13.79 (s, 1H), 13.45 (s, 1H), 9.13–9.15 (m,
in minimum amount of acetic acid. Ammonium acetate (20 eq) was
added and the mixture was refluxed for 4–5 h. After completion the
reaction mixture was cooled to room temperature, quenched with water
and neutralized with aq. ammonia. The solid precipitated out was
2H), 8.71–8.80 (m, 2H), 8.33 (d, J = 8.5 Hz, 1H), 7.89–8.00 (m, 4H),
7.63 (t, J = 7.5 Hz, 4H), 7.39–7.41 (m, 6H), 6.88 (dd, J = 8.5 and
2.5 Hz, 1H), 6.71 (d, J = 2.5 Hz, 1H). HRMS (ESI): m/z calcd for
(M + H)+
33 23 3
C H N O 478.1919; found 478.1900. Elemental analysis
210

S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Scheme 1. Synthesis of compounds DHPhI, JHPhI, THPhI, CHPhI, DHPPhI, JHPPhI, THPPhI and CHPPhI.
calcd (%); Mol. Formula: C33
H
23
N
3
O (C: 83.00, H: 4.85, N: 8.80, O:
J = 7.5 Hz, 4H), 7.86 (d, J = 7.5 Hz, 1H), 7.48–7.52 (m, 4H), 7.34–7.40
(m, 6H), 7.11–7.14 (m, 5H), 6.60 (dd, J = 8.5 and 2 Hz, 1H), 6.43 (d,
3.35; Found: C: 83.33, H: 4.74, N: 8.82, O: 3.10).
J = 2 Hz, 1H). 13C NMR (126 MHz, DMSO‑d
) δ 179.2, 158.9, 150.3,
6
2
4
.2.1.4. 3-(1H-Phenanthro[9,10-d]imidazol-2-yl)-9-phenyl-9H-carbazol-
-ol (CHPhI). Yield = 0.26 g (80%); Melting point = 198–201 °C. 1
) δ 14.00 (s, 1H), 13.77 (s, 1H), 9.31 (s, 1H),
.14 (d, J = 8.5 Hz, 1H), 9.12 (d, J = 8.5 Hz, 1H), 8.88 (d, J = 8 Hz,
H), 8.75 (d, J = 8 Hz, 1H), 8.40 (d, J = 7.5 Hz, 1H), 8.07 (t,
150.2, 147.4, 146.9, 140.8, 139.2, 137.8, 136.1, 130.2, 129.3, 128.6,
H
127.8, 126.8, 125.8, 124.6, 122.5, 112.7, 108.9, 107.2. HRMS (ESI): m/
z calcd for (M + H)+
C
45
H
31
N
3
O 630.2545; found 630.2478. Elemental
NMR (500 MHz, DMSO‑d
9
1
6
analysis calcd (%); Mol. Formula: C45 O (C: 85.83, H: 4.96, N:
31 3
H N
6.67, O: 2.54; Found: C: 85.99, H: 4.81, N: 6.69, O: 2.33).
J = 7.5 Hz, 1H), 7.89–8.01 (m, 7H), 7.82 (t, J = 7.5 Hz, 1H),
1
3
7
1
1
1
.58–7.66 (m, 3H), 7.13 (s, 1H). C NMR (126 MHz, DMSO‑d
6
) δ
2.2.1.8. 3-(6,9-Diphenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-9-phenyl-
58.1, 150.9, 143.2, 141.5, 137.2, 134.7, 130.9, 128.6, 128.5, 128.3,
28.1, 127.9, 127.3, 126.8, 126.4, 126.2, 124.9, 124.6, 123.8, 122.7,
22.6, 122.4, 121.4, 119.9, 118.6, 116.4, 110.4, 108.1, 97.1. HRMS
9H-carbazol-4-ol
(CHPPhI). Yield = 0.27 g
(82%);
Melting
point = 175–179 °C. 1H NMR (500 MHz, DMSO‑d ) δ 13.62 (s, 1H),
6
13.45 (s, 1H), 9.13 (s, 1H), 9.09 (s, 1H), 8.96 (s, 1H), 8.62 (d, J = 8 Hz,
1H), 8.45 (d, J = 8.5 Hz, 1H), 8.06–8.10 (m, 2H), 7.94 (d, J = 7.5 Hz,
3H), 7.88 (d, J = 7.5 Hz, 2H), 7.66 (t, J = 7.5 Hz, 2H), 7.47–7.58 (m,
(
ESI): m/z calcd for (M + H)+
C
33
H
21
N
3
O 476.1763; found 476.0000.
Elemental analysis calcd (%); Mol. Formula: C33 O (C: 83.35, H:
.45, N: 8.84, O: 3.36; Found: C: 83.53, H: 4.39, N: 8.85, O: 3.21).
21 3
H N
7H), 7.27–7.41 (m, 5H), 6.80 (s, 1H). 13C NMR (126 MHz, DMSO‑d ) δ
4
6
157.8, 150.8, 142.8, 141.3, 140.9, 140.8, 137.8, 137.6, 136.9, 134.4,
2
.2.1.5. 5-(Diethylamino)-2-(6,9-diphenyl-1H-phenanthro[9,10-d]
(DHPPhI). Yield = 0.28 g (51%); Melting
) δ 9.50 (s, 2H),
.98 (d, J = 8 Hz, 2H), 8.36 (d, J = 8.5 Hz, 2H), 8.15–8.24 (m, 6H),
130.6, 129.3, 128.7, 128.5, 128.3, 127.7, 127.0, 126.7, 126.6, 126.5,
125.8, 125.2, 123.6, 123.1, 122.7, 122.6, 122.4, 121.6, 121.1, 119.6,
118.3, 116.2, 110.1, 107.8, 96.6. HRMS (ESI): m/z calcd for (M + H)
imidazol-2-yl)phenol
point = 162–166 °C. H NMR (500 MHz, DMSO‑d
8
7
3
1
+
6
C
45
H
29
N
3
O 628.2389; found 628.2322. Elemental analysis calcd (%);
Mol. Formula: C45 O (C: 86.10, H: 4.66, N: 6.69, O: 2.55; Found:
C: 86.29, H: 4.53, N: 6.73, O: 2.41).
.79–7.82 (m, 5H), 7.70 (t, J = 7 Hz, 2H), 6.76 (s, 1H), 6.65 (s, 1H),
29 3
H N
.67–3.70 (q, J = 7 Hz, 4H), 1.42 (t, J = 7 Hz, 6H). 13C NMR (126 MHz,
DMSO‑d
6
) δ 159.0, 140.7, 139.2, 132.2, 132.1, 129.6, 129.5, 129.4,
129.3, 128.4, 128.2, 123.7, 122.9, 79.9, 79.8, 79.6, 79.4, 46.4, 13.2.
3. Result and discussion
+
HRMS (ESI): m/z calcd for (M + H)
C
37
H
33
N
3
O 534.2545; found
5
8
2
34.2535. Elemental analysis calcd (%); Mol. Formula: C37 O (C:
3.27, H: 5.86, N: 7.87, O: 3.00; Found: C: 83.39, H: 5.78, N: 7.86, O:
.95).
31 3
H N
3
.1. Discussion of synthesis
Electron donating hydroxyl aldehyde such as julolidine hydroxyl
aldehyde was synthesized by using the reported procedure [38], while
triphenylamine and N-phenyl carbazole based hydroxyl aldehydes were
synthesized using procedures reported from our group [37]. We syn-
thesized derivatives such as DEMAP hydroxyl phenanthrene imidazole
(DHPhI), julolidine hydroxyl phenanthrene imidazole (JHPhI), tri-
phenylamine hydroxyl phenanthrene imidazole (THPhI) and carbazole
hydroxyl phenanthrene imidazole (CHPhI) by reacting all available
aldehydes with commercially available phenanthrene-9,10-dione in
ammonium acetate and acetic acid (Scheme 1). To study the effect of
extra phenyl rings at 3, 6 position of phenanthrene-9, 10-dione, 3, 6-
dibromophenanthrene-9, 10-dione (intermediate 1) was synthesized by
reacting phenanthrene-9, 10-dione with bromine in nitrobenzene
(Scheme S1 in the supporting information). Further desired inter-
2
1
.2.1.6. 9-(6,9-Diphenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-
,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-8-ol
1
(JHPPhI). Yield = 0.03 g (25%); H NMR (500 MHz, DMSO‑d
6
) δ 9.48
(
s, 2H), 8.96 (d, J = 8 Hz, 2H), 8.33 (d, J = 8.5 Hz, 2H), 8.13–8.22 (m,
H), 7.77–7.80 (m, 5H), 7.67 (t, J = 7 Hz, 2H), 3.38–3.42 (m, 4H), 3.02
t, J = 7 Hz, 2H), 2.93 (t, J = 7 Hz, 2H), 2.13–2.16 (m, 4H). Elemental
analysis calcd (%); Mol. Formula: C39 O (C: 83.99, H: 5.60, N:
.53, O: 2.87; Found: C: 84.16, H: 5.51, N: 7.51, O: 2.81).
6
(
31 3
H N
7
2.2.1.7. 2-(6,9-Diphenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-5-
(diphenylamino)phenol (THPPhI). Yield = 0.26 g (81%); Melting
1
point = 245–248 °C. H NMR (500 MHz, DMSO‑d
1
6
) δ 13.45 (s, 1H),
3.18 (s, 1H), 9.17 (s, 2H), 8.53 (s, 2H), 8.01–8.05 (m, 3H), 7.93 (d,
mediate
2
was synthesized by reacting compound 3,6-
211

S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Fig. 3. Absorption (a) and emission (b) spectra of DHPhI, JHPhI, THPhI and CHPhI in THF solvent.
Table 1
Photophysical properties of all compounds in THF solvent.
Solvent
λ
abs
ε
max × 104
fwhm
(nm)
λ
ems
Stokes shift
(nm)
Φ
F
f
μ
eg
(
nm)
(nm)
−1
cm−1
(cm−1
(
M
)
)
(debye)
DHPhI
JHPhI
THPhI
CHPhI
DHPPhI
JHPPhI
THPPhI
CHPPhI
355
367
386
381
362
367
398
392
3.9
77
52
46
85
111
123
91
425
439
420
504
431
454
420
506
70
72
34
123
69
87
22
114
4639
4469
2097
6405
4422
5222
1316
5747
0.63
0.43
0.91
0.91
0.51
0.55
0.93
0.98
0.90
0.80
1.11
0.82
1.95
1.20
1.62
1.08
8.26
7.89
9.55
8.17
12.27
9.70
11.73
9.48
3.56
5.26
3.83
5.17
2.95
6.38
3.85
66
dibromophenanthrene-9,10-dione with phenyl boronic acid using Su-
zuki coupling reaction conditions, which was further reacted with all
four different hydroxyl aldehydes to acquire derivatives DHPPhI,
JHPPhI, THPPhI and CHPPhI in good yields (Scheme 1).
dominated in THPhI and CHPhI compounds for their better in-
tramolecular charge transfer ability over DHPhI and JHPhI derivatives.
Comparatively sharper absorption peaks as well as high molar extinc-
tion coefficients were observed for compounds THPhI and CHPhI than
compounds DHPhI and JHPhI, which show typically broad absorption
peaks. In their emission spectra (Fig. 3b), compound DHPhI, JHPhI and
3.2. Photophysical properties
THPhI are blue emitting compounds and exhibited emission λ
max
va-
lues at 425, 439 and 420 nm respectively, while compound CHPhI is
green emitting compound and emits at 504 nm. Compound THPhI
shows comparatively very high emission intensity than other three
derivatives due to effective charge transfer from the triphenylamine
donor. In all these derivatives as proton transferring OH group is pre-
sent in vicinity with the proton acceptor imidazole nitrogen, dual
emission peaks for their keto tautomer was expected, but they exhibited
single emission peaks in non-polar as well as polar solvents. Ex-
ceptionally highly red shifted emission peak at 503 nm observed for
compound CHPhI with 6405 cm Stokes shift, which can be attributed
to its keto tautomer formed in the excited state (Table 1). Fig. 4 re-
presents combined normalized absorption and emission spectra of
compounds DHPhI, JHPhI, THPhI and CHPhI in THF solvents, where
To study the effect of different electron donating groups on ab-
sorption and emission properties of phenanthene based imidazole de-
rivatives in different solvent, we recorded the absorption as well as
emission spectra of all these newly synthesized derivatives in solvents
of different dielectric constants. The comparative absorption as well as
emission spectra of all phenanthrene based ESIPT imidazole derivatives
in THF solvent are represented in Fig. 3. Due to their structural simi-
larity, compound DHPhI and JHPhI are exhibiting similar type of ab-
sorption spectra, while same is true for compound THPhI and CHPhI.
Compound CHPhI typically exhibited one extra high energetic ab-
sorption peak for its rigid carbazole unit (Fig. 3a). Low energetic sharp
absorption peak observed for all compounds at around 380 nm can be
attributed to the intramolecular charge transfer (ICT) peak, which gets
−
1
212

S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Table 2
Photophysical properties of DHPhI in all solvents.
Solvent
λ
abs
ε
max × 104
fwhm
(nm)
λ
ems
Stokes shift
(nm)
Φ
F
f
μ
eg
(
nm)
(nm)
−1
cm−1
(cm−1
(
M
)
)
(debye)
Hexane
Toluene
THF
DCM
MeOH
356
361
355
360
352
355
359
3.8
4.0
3.9
4.0
4.2
4.1
4.1
80
77
77
73
75
75
75
407
412
425
428
433
446
454
51
51
70
68
81
91
94
3519
3428
4639
4413
5314
5747
5780
0.58
0.38
0.63
0.14
0.32
0.39
0.51
0.88
0.93
0.90
0.93
0.96
0.93
0.99
8.16
8.47
8.26
8.43
8.49
8.39
8.72
Acetonitrile
DMSO
compounds DHPhI, JHPhI and THPhI exhibited small Stokes shift of
−
1
−1
−1
4
639 cm , 4469 cm
and 2097 cm , while compound CHPhI ex-
−1
hibited large Stokes shift of 6405 cm
(see Table 2).
We further extended the conjugation of phenanthrene moiety by
substituting its 3, 6 position hydrogen atoms with phenyl rings to ac-
quire derivatives such as DHPPhI, JHPPhI, THPPhI and CHPPhI.
Fig. 5 represents their comparative absorption and emission spectra in
THF solvents. In their absorption spectra, same type of scenario as like
compounds DHPhI, JHPhI, THPhI and CHPhI was observed, except
one additional high energetic peak was observed for the extra phenyl
rings at 3, 6 position of phenanthrene moiety of these compounds.
Hence compound CHPPhI exhibited total four absorption peaks
(
Fig. 5a) as compared to the three absorption peaks of compound
CHPhI. Compounds DHPPhI and JHPPhI exhibited very broad ab-
sorption spectra than comparatively sharper absorption spectra ob-
served for compound THPPhI and CHPPhI. In their emission spectra
again compound CHPPhI exhibited highly red shifted emission peak for
its keto tautomer with highest emission intensity than other three de-
rivatives (Fig. 5b).
Fig. 5. Absorption (a) and emission (b) spectra of DHPPhI, JHPPhI, THPPhI
and CHPPhI in THF solvent.
3.3. Effect of solvents on absorption and emission spectra
The polarity of organic solvents strongly affects the excited state of
exhibited very slight positive solvatochromism while compounds
CHPhI and CHPPhI exhibited negative solvatochromism i.e. blue shift
from non-polar to polar solvent in their emission spectra. Fig. 7 re-
presents emission spectra of compounds DHPhI, JHPhI, THPhI and
CHPhI in different solvents. It is observed that derivatives containing
DEMAP and Julolidine aldehyde i.e. DHPhI and JHPhI exhibited
quenching of fluorescence from non-polar to polar solvents, while tri-
phenyl and carbazole based derivatives such as THPhI and CHPhI, no
quenching of fluorescence from non-polar to polar solvents was ob-
served. Similar types of emission spectra were observed for remaining
derivatives such as DHPPhI, JHPPhI, THPhI and CHPPhI. In general
all these derivatives exhibited very high fluorescence quantum yield in
non-polar solvents such as hexane, toluene and THF.
donor-acceptor compounds; hence we studied absorption and emission
spectra of these compounds in different non-polar to polar solvents.
Fig. 6 represents absorption and emission spectra of DHPhI in different
solvents. When we go from non-polar to polar solvents, slightly red shift
in their absorption spectra and highly red shifted emission spectra were
observed suggesting that the excited state is more stabilized by polar
solvents than ground state. The values of molar extinction coefficient,
Stokes shift, oscillator strength and transition dipole moment are
slightly increased when we go from non-polar to polar solvents for all
compounds (Table 2, S1 to S7). Fig. S1 to S7 represents the absorption
and emission spectra of compound JHPhI, THPhI, CHPhI, DHPPhI,
JHPPhI, THPPhI and CHPPhI in all solvents. Compounds DHPhI,
JHPhI, DHPPhI and JHPPhI exhibited very good positive solvato-
chromism in their emission spectra. Compounds THPhI and THPPhI
Fig. 4. Combined normalized absorption and emission spectra of compounds DHPhI, JHPhI, THPhI and CHPhI in THF solvent.
213

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Dyes and Pigments 159 (2018) 209–221
Fig. 6. Absorption (a) and emission (b) spectra of DHPhI in different solvents.
Fig. 7. Emission spectra of (a)DHPhI, (b)JHPhI, (c)THPhI and (d)CHPhI in different solvents.
3
.4. Solvatochromism and Lippert-Mataga analysis
to polar solvents; hence it can be presented as the example of in-
tramolecular charge transfer (ICT). Again similar type of scenario was
observed for compound DHPPhI, JHPPhI, THPPhI and CHPPhI, where
highest positive solvatochromism (83 nm) is observed for JHPPhI and
negative solvatochromism is observed for compound CHPPhI (23 nm).
Very small red shift in the absorption spectra but highly red shifted
emission spectra of these compounds, suggest that the excited state of
these compounds (except CHPhI and CHPPhI) is highly stabilized as
compared to the ground state by the polar solvents and opposite is true
for the compounds CHPhI and CHPPhI, which shows negative solva-
tochromism. Hence the role of dielectric constant as well as refractive
index of the solvents is very important for the observed shift in emission
spectra from non-polar to polar solvents. Consequently we also ob-
served gradual increase in Stokes shift with the increased solvent po-
larity and which can be correlated by using Lippert-Mataga equation.
According to Lippert-Mataga correlation physical parameters such as
dielectric constant and refractive index are directly correlated with the
observed Stokes shift. It is very general equation and does not account
Fig. 8 represents normalized emission spectra of compounds DHPhI,
JHPhI, THPhI and CHPhI in all solvents. We observed positive solva-
tochromism for compound DHPhI, JHPhI and THPhI, while negative
solvatochromism for compound CHPhI from non-polar to polar sol-
vents. This can be correlated with the presence of keto tautomer
emission in CHPhI, while enol tautomer emission in all remaining three
derivatives. Negative solvatochromism is already reported for the
compounds which demonstrate ESIPT process [37]. Rigidized donor
derivative (JHPhI) exhibited very large positive solvatochromism
(
85 nm) and the triphenylamine donor derivative (THPhI) exhibited a
very small positive solvatochromism of 21 nm. Quenching of fluores-
cence from non-polar to polar solvent with positive solvatochromism
was observed for compound DHPhI and JHPhI, can be correlated with
the twist of the molecule in the excited state to exhibit twisted in-
tramolecular charge transfer (TICT) state. In the case of compound
THPhI no any quenching of fluorescence was observed from non-polar
Fig. 8. Normalized emission spectra of compound DHPhI, JHPhI, THPhI and CHPhI (positive and negative solvatochromism).
214

S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Fig. 9. Lippert-Mataga correlations between Stokes shift and Lippert-Mataga function (Δf) for compounds DHPhI, JHPhI, THPhI and CHPhI.
the observed spectral shift. Hence multilinear approach is proposed by
Kamlet and Taft [39,40] as represented in equation (1)
y = y
0
+ a
α
α + b
β
β + cπ* π*
(1)
where, ‘y’ is the solvent affected physiochemical property (e.g. ab-
sorption maxima (ῡabs), emission maxima (ῡemi), Stokes shift (Δῡ), etc.).
’
‘
y
0
is the studied physiochemical property in the gas phase, while a, b, c
and d are adjustable coefficients which reflect the dependence of ‘y’ to
the various solvent parameters such as α (effect of acidity of solvent), β
(
effect of basicity of solvent) and π* (collective effect of solvent po-
larity/dipolarity and polarizability). However if solvent acidity and
basicity are not affecting the value of y, then among solvent polarity
and polarizability it is difficult to sort out which factor is exactly in-
fluencing the studied physiochemical property y, hence Catalan
[
41–43] proposed another expression as represented in equation (2)
y = y + aSASA + bSBSB + cSPSP + dSdPSdP (2)
where, Catalan utilized same solvent parameters i.e. solvent acidity
Fig. 10. Lippert-Mataga correlations between Stokes shift and Lippert-Mataga
function (Δf) for compounds DHPPhI, JHPPhI, THPPhI and CHPPhI.
0
any specific solute-solvent interactions such as hydrogen bonding.
Figs. 9 and 10 represent the plots of λ shift vs. Lippert-Mataga function
(
SA) and solvent basicity (SB) which already utilized by Kamlet and
Taft, but he divided the last parameter proposed by Kamlet and Taft i.e.
π* into two separate parameters i.e. solvent polarizability (SP) and
solvent dipolarity (SdP). Since the Kamlet-Taft parameters are well
established and Catalan parameters are the modified version of it, we
used both these approaches simultaneously to understand exactly
which factor is mostly affecting the observed spectral shift in absorption
and emission spectra as well as increased or decreased Stokes shift from
non-polar to polar organic solvents for all these imidazole derivatives.
Reported Kamlet-Taft [44] and Catalan [45] parameters are used for
the analysis of observed spectral shift in absorption, emission and
Stokes shift from non-polar to polar solvents.
Table 3 represents multilinear analysis of absorption maxima (ῡabs),
emission maxima (ῡemi) and Stokes shift (Δῡ) for CHPhI in different
organic solvents were carried out by using both Kamlet-Taft (2) and
Catalan (3) parameters. Absorption spectra analysis of this compound
exhibited comparatively higher correlation coefficient by Catalan
method (0.98) than that obtained by Kamlet-Taft method (0.23). We
observed slightly red shifted absorption spectra from non-polar to polar
solvents, hence negative estimated coefficient was expected, which was
observed only for solvent polarizability factor (dSP) by Catalan method
with highest value of estimated coefficient (1.62) and minimum
(
ΔfLM). It shows linear relationship suggesting that the solvent para-
meters such as dielectric constant and refractive index are collectively
responsible for the observed shift in the emission spectra. For com-
pounds DHPhI, JHPhI, THPhI, DHPPhI, JHPPhI and THPPhI Stokes
shift increases with the increasing Lippert-Mataga function (ΔfLM) i.e.
increasing solvent polarity as well as refractive index. In contrast due to
the presence of keto tautomer in the excited state (ESIPT effect),
compounds such as CHPhI and CHPPhI shows decreased Stokes shift
values with the increasing Lippert-Mataga function and they are also
showing very good linear relationship of Stokes shift with the Lippert-
Mataga function (ΔfLM).
3.5. Multilinear regression analysis of observed spectral shift in absorption
and emission spectra using Kamlet-Taft and Catalan parameters
In general linear correlation parameters such as Lippert-Mataga,
N
Mac-Rae or E
T
(30) are used to understand the effect of solvent polarity
on the observed absorption and emission spectra shift, but these para-
meters are not explaining precisely which solvent parameters such as
solvent acidity, basicity, dipolarity or polarizability are responsible for
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S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Table 3
Estimated coefficients (y
0
, a, b, c, d), their standard errors and correlation coefficients (r) for the multi-linear analyis of (ῡabs), (ῡemi) and (Δῡ) of compound CHPhI as
a function of Kamlet-Taft (1) and Catalan (2) solvent scales. Where, α or SA for solvent acidity, β or SB for solvent basicity, π* for collective parameter of solvent
dipolarity and polarizability according to Kamlet-Taft equation, SdP and SP for solvent dipolarity and polarizability respectively according to Catalan equation.
Kamlet-Taft
y
0
× 103
a
α
b
β
c
π*
r
ῡ
ῡ
Δῡ
abs
26.18 ± 0.11
19.78 ± 0.07
6.40 ± 0.11
0.25 ± 0.16
0.78 ± 0.10
−0.53 ± 0.16
0.18 ± 0.28
0.06 ± 0.18
0.12 ± 0.29
−0.20 ± 0.25
0.08 ± 0.17
−0.20 ± 0.26
0.23
0.92
0.62
emi
Catalan
y
0
× 103
a
SA
b
SB
c
SdP
d
SP
r
ῡ
ῡ
Δῡ
abs
27.18 ± 0.07
20.36 ± 0.47
6.81 ± 0.53
0.04 ± 0.04
1.07 ± 0.28
−1.02 ± 0.32
0.00 ± 0.04
−0.25 ± 0.24
0.25 ± 0.28
0.27 ± 0.03
0.38 ± 0.21
−0.11 ± 0.25
−1.62 ± 0.12
−0.94 ± 0.73
−0.68 ± 0.84
0.98
0.86
0.63
emi
standard error (0.12), hence this can be considered as the main factor
responsible for the slightly red shifted absorption spectra. It was further
confirmed by the plot of absorption values (ῡabs) with each individual
solvent parameter.
Multilinear regression analysis of emission spectra of CHPhI ex-
hibited very high values of correlation coefficients by both Kamlet-Taft
(
0.92) and Catalan (0.86) methods. As we observed blue shifted emis-
sion spectra for this compound experimentally from non-polar to polar
solvents, positive estimated coefficients are expected. Though we ob-
served positive values for all solvent parameters by Kamlet-Taft
method, only solvent acidity (aSA) and solvent dipolarity (cSdP) para-
meters show positive value for their estimated coefficients by Catalan
method. Hence parameters such as solvent basicity (bSB) and solvent
polarizability (dSP) can be neglected from the discussion as they showed
negative estimated coefficients. Among solvent acidity (aSA) and sol-
vent dipolarity (cSdP) solvent parameters, higher estimated coefficient
Fig. 11. Correlation between experimental and predicted emission wave
number for DHPhI (a), JHPhI (b), THPhI (c), CHPhI (d), DHPPhI (e), JHPPhI
(f), THPPhI (g) and CHPPhI (f) by Kamlet-Taft method.
(
1.07) and minimum standard error (0.28) is observed for solvent
acidity parameter as compared to the solvent dipolarity parameter,
hence it can be considered as the main factor responsible for the blue
shifted emission spectra from non-polar to polar solvents. It was further
confirmed by plotting emission values (ῡemi) individually with solvent
acidity and solvent dipolarity parameters.
Table S8 and S9 represents multilinear analysis of absorption
maxima (ῡabs), emission maxima (ῡemi) and Stokes shift (Δῡ) for all
remaining compounds in different solvents by using both Kamlet-Taft
these compounds was recorded at each 30 min time interval. Fig. 12
and Fig S8 to S10 represent absorption and emission spectra of com-
pound JHPhI and DHPhI to CHPhI from 0 to 150 min time interval
respectively. We found these compounds highly photostable under UV
light as no remarkable change in their absorption as well as emission
intensity is recorded up to 150 min time interval. Comparatively com-
pound JHPhI exhibited high photostability, while compound DHPhI
less photostability under UV light.
(
2) and Catalan (3) parameters. All remaining compounds showed
higher values of their correlation coefficient for the analysis of ab-
sorption spectra by Catalan method than that by Kamlet-Taft method.
All the compounds exhibited highest negative estimated coefficient and
minimum standard error for the solvent polarizability factor (dSP) in
comparison with the all other factors, hence this can be considered as
the main factor responsible for the slightly red shifted absorption
spectra. In the case of their emission spectra, solvent acidity (aSA) is
emerged out as the main factor responsible for the blue shifted emission
spectra of CHPPhI from non-polar to polar solvents, while solvent di-
polarity (cSdP) is emerged out as the main factor responsible for the red
shifted emission spectra of compounds DHPhI, JHPhI, THPhI,
DHPPhI, JHPPhI and THPPhI from non-polar to polar solvents.
The graph plotted for the predicted emission data obtained by
Kamlet-Taft method with the experimental data for all compounds
show very good linear fit with slope of 0.79, 0.84, 0.95 and 0.96 for
compounds DHPhI, JHPhI, THPhI and CHPhI, while 0.69, 0.76, 0.92
and 0.95 for compounds DHPPhI, JHPPhI, THPPhI and CHPPhI re-
spectively (Fig. 11).
3
3
.7. Comparative photophysical parameters of benzil, phenanthrene and
,6-diphenylphenanthrene based compounds
Rigidity and extended conjugation are two important factors re-
sponsible for the improved photophysical properties in the case of
fluorescent donor-acceptor compounds. Benzil based free rotating
ESIPT compounds and their detail photophysical properties were re-
ported from our group recently [37]. We compared photophysical
properties of benzil, phenanthrene (rigidized structure) and 3,6-di-
phenylphenanthrene (extended conjugation) as represented in Table 4.
Fig. 13 and Fig. 14 represents normalized absorption and emission
spectra of compounds CHPI, CHPhI and CHPPhI in THF solvent. It can
be observed that when we go from flexible benzil derivative CHPI to-
wards more rigid derivative phenanthrene and 3,6-diphenylphenan-
threne red shifted absorption as well as emission peaks were observed.
Overall all photophysical parameters including quantum yield, oscil-
lator strength (f) and transition dipole moment (μeg) are improved for
rigid phenanthrene based compounds than flexible benzil based com-
pounds.
3.6. Photostability study
Photostability study of compounds DHPhI, JHPhI, THPhI and
CHPhI was carried out under UV light at different time intervals. 10 μM
solutions of these compounds in toluene solvent were irradiated at
254 nm up to 150 min. Both absorption as well as emission spectra of
216

S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Fig. 12. Absorption (a) and emission (b) spectra of compound JHPhI under UV light up to 150 min.
Table 4
Photophysical properties of CHPI, CHPhI and CHPPhI in THF solvent.
Solvent
λ
abs
ε
max × 104
fwhm
(nm)
λ
ems
Stokes shift
(nm)
Φ
F
f
μ
eg
(
nm)
(nm)
−1
cm−1
(cm−1
(
M
)
)
(debye)
CHPI
CHPhI
CHPPhI
365
381
392
1.52
3.83
3.85
27
85
66
489
504
506
124
123
114
6947
6405
5747
0.85
0.91
0.98
0.67
0.82
1.08
7.19
8.17
9.48
Fig. 13. Normalized absorption spectra of compounds CHPI, CHPhI and CHPPhI.
Fig. 14. Normalized emission spectra of compounds CHPI, CHPhI and
CHPPhI.
Fig. 15. Comparative normalized emission spectra of triphenylamine based
compounds (THPI, THPhI, THPPhI) and carbazole based compounds (CHPI,
CHPhI, CHPPhI).
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Dyes and Pigments 159 (2018) 209–221
Fig. 18. HOMO-LUMO FMO diagrams of compound DHPhI, JHPhI, THPhI and
CHPhI in THF solvent.
100 nm shift in emission spectra is observed. This is mainly attributed
to the combined effect of rigidity as well as excited state intramolecular
charge transfer (ESIPT). When we go from non-polar to polar solvents
slight red shift in their emission spectra is observed for triphenylamine
based compounds, while blue shift is observed for N-phenyl carbazole
based compounds as represented in Fig. 8.
Fig. 16. Optimized geometries of THPPhI and CHPPhI in their ground and
excited state in THF solvent.
3.9. Optimization of structures by TD-DFT method
3.8. Shift in emission spectra between triphenylamine and N-phenyl
carbazole attached benzil, phenanthrene and 3,6-diphenylphenanthrene
based compounds
To understand the molecular geometries in detail, all the derivatives
were optimized by Time-dependent density functional theory (TD-DFT)
using 6-31G (d) basis set with B3LYP functional [46–48] in gas phase as
well as THF solvent. Fig. 18 and 19 represents their frontier molecular
orbital pictures at HOMO and LUMO level. At their HOMO level,
electron density is mainly located on the electron donating N,N diethyl
aniline, julolidine, triphenylamine or N-phenyl carbazole phenyl rings,
while on the imidazole heterocycles fused with phenanthrene rings at
their LUMO level. As the N-phenyl ring of carbazole based derivatives
such is CHPhI and CHPPhI is perpendicular to the main carbazole unit,
no any electron density was observed on the N-phenyl ring at their
HOMO level. Comparatively better energy transfer from donor to ac-
ceptor can be expected in the case of compounds DHPPhI, JHPPhI,
THPPhI and CHPPhI than their phenanthrene based counterparts be-
cause minimum overlap between the electron densities between their
Fig. 15 represents normalized emission spectra of compounds THPI,
THPhI, THPPhI and CHPI, CHPhI, CHPPhI in THF solvent. In general
little red shift in the emission spectra is observed when benzil and
phenanthrene based compounds are compared, while opposite is true
for the triphenylamine and N-phenyl carbazole based compounds. Tri-
phenylamine based compounds i.e. THPI, THPhI and THPPhI ex-
hibited their emission spectra in the blue region i.e. from 401 nm to
4
20 nm, while N-phenyl carbazole based compounds i.e. CHPI, CHPhI
and CHPPhI emitted in the bluish green region i.e. from 489 nm to
06 nm. Very slight modification in the structure of electron donating
group resulted into large difference in their emission spectra and almost
5
Fig. 17. HOMO-LUMO energy gaps of compounds CHPhI, DHPhI, JHPhI and THPhI in THF solvent.
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S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
Fig. 19. HOMO-LUMO FMO diagrams of compound DHPPhI, JHPPhI, THPPhI and CHPPhI in THF solvent.
Table 5
Comparative experimental and computational photophysical parameters of all compounds in toluene solvent.
a
b
c
d
e
f
g
%Dh
i
Compound
λ
abs nm
λ
abs nm
λ
ems nm
ƒ
ƒ
μ
eg
μ
eg
Major contribution
DHPhI
JHPhI
THPhI
CHPhI
DHPPhI
JHPPhI
THPPhI
CHPPhI
361
369
371
364
367
369
400
394
370
378.14
387.4
365.65
370.44
401.23
403.47
383.64
412
419
422
506
417
433
423
507
0.93
0.77
1.01
0.89
1.47
1.18
1.64
1.2
0.58
0.57
1.26
1.04
0.82
0.76
1.39
1.17
8.47
7.76
9.15
8.54
10.73
9.79
11.81
10.03
5.12
4.93
4.22
2.91
5.25
4.84
4.25
2.95
2.61
2.47
4.42
0.45
6.93
5.31
0.87
2.63
H →L 87.55
H →L 90.23
H →L 96.91
H →L 94.87
H →L 95.58
H →L 96.27
H →L 92.11
H →L 92.75
a
Experimental absorption λmax
.
b
c
d
e
f
Computational vertical excitation.
Experimental emission λmax
.
Experimentally calculated oscillator strength.
Computational oscillator strength.
Experimentally calculated transition dipole moment.
Computational dipole moment.
Percent deviation from experimental absorption λmax
Major electronic transition.
g
h
i
.
HOMO and LUMO level was observed for these derivatives.
trends (Table 5). Similarly we also compared the experimentally ob-
served oscillator strength and dipole moment values with the compu-
tationally derived one and observed comparatively lower values of
oscillator strength as well as dipole moment than experimental one but
with the same trend. Major electronic transitions for all the compounds
were found from HOMO to LUMO level (87–96%), in which maximum
HOMO-LUMO electronic transition (96.91%) is shown by THPhI com-
pound and minimum one (87.55%) by DHPhI compound.
Fig. 16 represents optimized geometries of compounds THPPhI and
CHPPhI in their ground and excited state. Though there is no much
difference in dihedral angles observed when we go from ground to
excited state for compounds THPPhI and CHPPhI, large difference
between the dihedral angle of C27-N-33-C-35-C37 (46.75°) for THPPhI
and C28-N-33-C36-C38 (56.99°) for CHPPPhI in their ground state as
well as 36.51° and 54.85° in their excited state is observed respectively.
Fig. 17 represents HOMO-LUMO energy difference for compounds
DHPhI, JHPhI, THPhI and CHPhI. Compound JHPhI exhibits lowest
HOMO-LUMO energy difference (3.74 eV), while compound CHPhI
exhibits highest one (3.91 eV) in all four compounds, suggesting that
effective intramolecular charge transfer is observed in the case of
compound JHPhI while opposite is true for compound CHPhI. Instead
of the poor intramolecular charge transfer observed for this compound
highly red shifted emission peak can be supported by the excited state
intramolecular proton transfer (ESIPT) phenomenon. Similar type of
trend is observed for compounds DHPPhI, JHPPhI, THPPhI and
CHPPhI.
4. Conclusion
In conclusion, phenanthrene and 3,6-diphenylphenanthrene based
imidazole derivatives were synthesized to study the effect of different
electron donating groups on photophysical properties. By extending the
conjugation of phenanthrene unit by extra phenyl rings at 3 and 6
positions, red shifted absorption as well as emission peaks and im-
proved quantum yield in different organic solvents were observed.
Further, we noted red shifted emissions for CHPhI and CHPPhI deri-
vatives as compared to the all remaining derivatives due to the extra
rigidity by carbazole unit. Negative solvatochromism observed for
CHPhI and CHPPhI derivatives can be supported by the dominating
excited state intramolecular proton transfer process in these compounds
while positive solvatochromism observed for all remaining compounds
We compared the computationally derived lowest energy transitions
i.e. vertical excitations obtained after exciting the ground state opti-
mized compounds up to at least first 10 states with the experimentally
observed absorption values in toluene solvent and are having similar
219

S. Kothavale et al.
Dyes and Pigments 159 (2018) 209–221
from non-polar to polar solvents can be supported by the dominant
intramolecular charge transfer process. Quenching of fluorescence from
non-polar to polar solvents is observed for all DEMAP and julolidine
based imidazole derivatives but no fluorescence quenching is observed
for triphenylamine and N-phenyl carbazole based derivatives.
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