Photophysical behavior of a novel 4-aza-indole derivative in different solvents: reverse solvatochromism

Article abstract of DOI:10.1007/s11164-018-3648-x

The photophysical properties of a new 4-aza-indole derivative [ethyl 1-((2-(2-ethoxy-2-oxoethyl)pyridin-3-yl)carbamoyl)-2-hydroxy-1H-pyrrolo-[3,2-b]pyridine-3-carboxylate, 12] were determined in different solvents. Compound 12 exhibited an absorbance peak at 340–360?nm with high fluorescence intensity in the wavelength range from 405 to 417?nm in all solvents except N,N-dimethylformamide (DMF). Compound 12 exhibited reverse solvatochromism behavior depending on the solvent polarity. Furthermore, compound 12 showed very high quantum yield in all solvents independent of their polarity. The results suggest that this novel dye could be used for many applications, e.g., as a labeling agent and in bio- or analytical sensors and/or optoelectronic devices.

Full text of DOI:10.1007/s11164-018-3648-x

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-3648-x
Photophysical behavior of a novel 4‑aza‑indole derivative
in diferent solvents: reverse solvatochromism
Ebru Bozkurt1 · Sengul Dilem Dogan2
Received: 23 August 2018 / Accepted: 15 October 2018
© Springer Nature B.V. 2018
Abstract
The photophysical properties of a new 4-aza-indole derivative [ethyl 1-((2-(2-eth-
oxy-2-oxoethyl)pyridin-3-yl)carbamoyl)-2-hydroxy-1H-pyrrolo-[3,2-b]pyridine-
3-carboxylate, 12] were determined in diferent solvents. Compound 12 exhibited an
absorbance peak at 340–360 nm with high fuorescence intensity in the wavelength
range from 405 to 417 nm in all solvents except N,N-dimethylformamide (DMF).
Compound 12 exhibited reverse solvatochromism behavior depending on the solvent
polarity. Furthermore, compound 12 showed very high quantum yield in all solvents
independent of their polarity. The results suggest that this novel dye could be used
for many applications, e.g., as a labeling agent and in bio- or analytical sensors and/
or optoelectronic devices.
Keywords 4-Aza-indole · Absorption · Fluorescence · Solvent · Reverse
solvatochromism
Introduction
An aromatic amino acid, tryptophan, is widely used to study segmental elasticity,
structure–function relationships, and interactions of proteins due to its intrinsic
fuorescence property. However, tryptophan cannot always be used as a probe,
since it has complex photophysics. Therefore, synthesis and optical characteri-
zation of novel tryptophan analogs is a very interesting research area. In recent
years, many azatryptophan derivatives have been synthesized as fuorescence
probes to solve this problem. In such investigations of the photophysical proper-
ties of azaindole derivatives, the side chain of azatryptophan is mainly the focus
*
Ebru Bozkurt
ebrubozkurt@atauni.edu.tr
1
2
Program of Occupational Health and Safety, Erzurum Vocational Training School, Ataturk
University, 25240 Erzurum, Turkey
Department of Pharmaceutical Basic Sciences, Faculty of Pharmacy, Erciyes University,
38039 Kayseri, Turkey
Vol.:(0123456789)
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E. Bozkurt, S. D. Dogan
[1, 2]. For this purpose, researchers have focused on azaindole isomers such as
4-azaindole, 5-azaindole, 6-azaindole, and 7-azaindole (Scheme 1). Among these
isomers, 4-azaindole has been identifed as a promising candidate [1]. Therefore,
synthesis of new 4-azaindole derivatives and characterization of their photophysi-
cal properties is a very important area of study.
Solvatochromism refers to changes in the absorption and fuorescence spectra
of fuorescent molecules depending on factors such as the hydrogen-bonding abil-
ity, pH, and polarity of the solvent. Since the ground- and excited-state polarities
of fuorescent molecules are diferent, the stability of these states may change
depending on solvent properties, altering the energy gap between them and
thereby the position, intensity, and shape of the absorption or fuorescence spec-
tra. A shift in the absorption or fuorescence band maximum to longer wavelength
due to the efect of a solvent is called positive solvatochromism, whereas a shift
to shorter wavelength is called negative (reverse) solvatochromism [3, 4].
The aim of this study is to investigate the photophysical properties of a novel
synthesized aza-indole derivative [ethyl 1-((2-(2-ethoxy-2-oxoethyl)pyridin-3-yl)
carbamoyl)-2-hydroxy-1H-pyrrolo-[3,2-b]pyridine-3-carboxylate, compound 12]
(Scheme 2) in diferent solvents. To the best of the authors’ knowledge, such
investigation has not been reported in literature to date. For this purpose, ultra-
violet–visible (UV–Vis) absorption spectroscopy, and steady-state and time-
resolved fuorescence measurements were carried out on the compound 12 in dif-
ferent solvents. Additionally, photophysical parameters of compound 12, such as
the fuorescence quantum yield, fuorescence lifetime, and radiative (kr) and non-
radiative (knr) rate constants, were calculated in diferent solvents. It is hoped that
H
N
H
N
H
N
H
N
N
N
N
N
7-azaindole
4-azaindole
5-azaindole
6-azaindole
Scheme 1 Structure of azaindole isomers
Scheme 2 Structure of com-
pound 12
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Photophysical behavior of a novel 4-aza-indole derivative…
the results of this study will make an important contribution for new tryptophan
analogs.
Experimental
Materials
H2SO4 (98 % purity), all solvents (99 % purity), and quinine sulfate were purchased
from Sigma (USA), Merck (Germany), and Fluka (USA), respectively, and used with-
out further purifcation. Stock solution of compound 12 at 1.0 × 10−3 M was prepared
in ethanol. A certain amount of compound 12 was obtained from this stock solution by
evaporating the solvent. For all measurements, the concentration of compound 12 was
1.0 × 10−6 M. All experiments were performed at room temperature.
Instruments
UV–Vis absorption and fuorescence spectra of the samples were recorded using a
PerkinElmer Lambda 35 UV–Vis spectrophotometer (USA) and Shimadzu RF-5301PC
spectrofuorophotometer (Japan), respectively. Fluorescence and absorption measure-
ments were taken for compound 12 at room temperature. For steady-state fuorescence
measurements, all samples were excited at 350 nm and the fuorescence intensity was
recorded between 360 nm and 600 nm. Fluorescence spectra were measured using slit
width of 1.5–3 and corrected using Origin 8.0 software. Fluorescence lifetime measure-
ments were carried out using a LaserStrobe model TM3 spectrofuorophotometer from
Photon Technology International (PTI, USA) with a combined pulsed nitrogen laser/
tunable dye laser for excitation at 366 nm. Decay curves were collected over 200 chan-
nels using a nonlinear time scale with the time increment increasing according to an
arithmetic progression. The fuorescence decays were analyzed using the lifetime distri-
bution analysis software supplied by the instrument company. Fit quality was assessed
by χ2 values and weighted residuals [5]. Good results typically produce χ2 values of 0.9
to 1.2. In our study, the values were between 0.8 and 1.16.
The fuorescence quantum yields of the molecule were calculated using the
Parker–Rees equation:
(
)
(
)
(
)
ꢁs2
ꢁr2
1 − 10−OD
1 − 10−OD
r
Ds
Dr
ꢀs = ꢀr
,
(1)
s
where D is the integrated area under the corrected fuorescence spectrum, n is the
refractive index of the solution, and OD is the optical density at the excitation wave-
length (λex = 350 nm). The subscripts “s” and “r” refer to the sample and reference
solution, respectively [6]. Quinine sulfate in 0.5 M H2SO4 solution was used as ref-
erence. The fuorescence quantum yield of quinine sulfate was 0.55 in 0.5 M H2SO4
solution [7].
1 3

E. Bozkurt, S. D. Dogan
The rate constants of radiative (kr) and nonradiative (knr) deactivation were calcu-
lated by using the following equations:
ꢀ
kr =
,
(2)
(3)
ꢁav
1
= kr + knr,
ꢀav
where Φ is the fuorescence quantum yield and τav is the average fuorescence life-
time of samples [8].
Scheme 3 Synthesis of the key compound, 2-(2-ethoxy-2-oxoethyl) nicotinic acid (8)
Scheme 4 Synthesis of compound 12
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Photophysical behavior of a novel 4-aza-indole derivative…
Ethyl 1‑((2‑(2‑ethoxy‑2‑oxoethyl)pyridin‑3‑yl)
carbamoyl)‑2‑hydroxy‑1H‑pyrrolo‑[3,2‑b]pyridine‑3‑carboxylate (12)
Compound 12 was synthesized and characterized as described in our previous
study [9] as summarized in Schemes 3 and 4. White solid from chloroform, m.p.
(decomposition) 264–265 °C. 1H NMR (400 MHz, CDCl3) δ 11.83 (s, 1H), 11.48
(s, 1H), 8.42 (dd, J = 7.7, 0.7 Hz, 1H), 8.29 (dd, J = 4.7, 1.5 Hz, 1H), 8.26 (dd,
J = 8.2, 1.4 Hz, 1H), 7.55 (t, J = 5.8 Hz, 1H), 7.20 (dd, J = 8.0, 5.0 Hz, 1H), 6.86
(dt, J = 7.1, 1.6 Hz, 1H), 4.32 (q, J = 7.1 Hz, 2H), 4.16 (q, J = 7.1 Hz, 2H), 3.97
(s, 2H), 1.33 (t, J = 7.1 Hz, 3H), 1.21 (t, J = 7.1 Hz, 3H). 13C NMR (100 MHz,
CDCl3) δ 170.1, 165.9, 164.8, 151.1, 147.0, 145.4, 145.3, 132.8, 130.7, 129.5,
127.3, 122.7, 121.8, 113.5, 83.4, 61.2, 60.4, 40.7, 14.6, 14.1. νmax (ATR): 3287,
2979, 1723, 1592, 1557, 1413, 1383, 1329, 1264, 1179, 1131, 1077, 1021.
HRMS: m/z [M-H]− calcd. for C20H19N4O6: 411.13101; found: 411.13273.
Results and discussion
Absorption measurements of compound 12 (1.0 × 10−6 M) were taken in 14 dif-
ferent solvents, such as toluene, diethyl ether, 1,4-dioxane, tetrahydrofuran (THF),
ethyl acetate, chloroform, dichloromethane (DCM), N,N-dimethylformamide
(DMF), dimethylsulfoxide (DMSO), acetonitrile (ACN), isopropanol (i-PrOH),
ethanol (EtOH), methanol (MeOH), and water, at room temperature (Fig. 1a–c).
Compound 12 exhibited two absorption peaks at 250–280 nm and 340–360 nm
in all solvents (except DMF). The bands at 250–280 nm and 340–360 nm were
attributed to the n → π* and π → π* transitions in compound 12, respectively. The
changes in the absorption peaks of compound 12 depending on the solvent polar-
ity are presented in Table 1. Blue-shifts in both peaks were observed. The mag-
nitude of the shifts was related to the strength of the hydrogen bond formed [10].
The steady-state fuorescence and normalized fuorescence spectra of com-
pound 12 are presented in Figs. 2 and 3. As shown in Fig. 2, compound 12 dis-
played a high-intensity fuorescence band between 405 and 417 nm in all the sol-
vents. In addition, formation of shoulders in the fuorescence band of compound
12 was observed in some solvents. It is known that these shoulders can be attrib-
uted to π*–π transitions from diferent vibrational levels of the frst singlet excited
state to the ground state [11, 12]. Blue-shifts in the fuorescence maxima were
observed with increase in the solvent polarity (Table 1). Depending on the sol-
vent polarity, the blue-shifts in the absorption peaks of compound 12 were lower
than those of the fuorescence peaks. It is suggested that compound 12 has no
charge transfer in the ground state and that solvent–solute interactions are greater
in the excited state [13]. This negative solvatochromism observed with increas-
ing solvent polarity indicates that the ground-state molecule is better stabilized
by solvation than the molecule in the excited state [3, 14, 15]. The formation of
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E. Bozkurt, S. D. Dogan
Fig. 1 Absorption spectra of compound 12 in a aprotic, b polar aprotic, and c polar protic solvents
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Photophysical behavior of a novel 4-aza-indole derivative…
Table 1 Spectroscopic and photophysical properties of compound 12 in selected solvents
Solvent
ε (M−1 cm−1
)
λabs (nm) λem (nm)
Φ
τav
(ns)
kr × 10−9 (s−1
)
knr × 10−9 (s−1
)
Toluene
Diethyl ether 2048
1,4-Dioxane 4065
1520
287/358 417
278/357 410
254/354 417
253/354 414
255/355 411
255/356 411
255/355 413
0.72 6.540 0.1104
0.59 8.024 0.0735
0.57 5.403 0.1052
0.63 6.868 0.1060
0.71 6.705 0.1336
0.76 7.140 0.0915
0.96 7.199 0.1053
0.84 7.464 0.1123
0.82 7.486 0.1090
0.71 7.382 0.0967
0.90 7.046 0.1277
0.77 7.051 0.1091
0.76 7.583 0.1000
0.67 8.624 0.0775
0.0425
0.0512
0.0799
0.0341
0.0053
0.0541
0.0439
0.0217
0.0246
0.0388
0.0142
0.0327
0.0319
0.0385
THF
2744
Ethyl acetate 2925
Chloroform
DCM
DMF
DMSO
ACN
i-PrOH
EtOH
MeOH
2402
2307
3146
2760
3064
3269
3212
3455
1854
268
414
262/353 416
254/351 414
254/352 407
254/351 407
254/349 409
225/343 405
Water
Fig. 2 Fluorescence spectra of compound 12 in selected solvents
hydrogen bonds with increasing polarity ensures that the molecule is stable in the
ground state [16].
The fuorescence quantum yield and lifetime values of compound 12 were also
determined depending on solvent polarity. These values for compound 12 in dif-
ferent solvents are listed in Table 1. The fuorescence quantum yields were rather
high in all the investigated solvents. It was observed that there was no linear rela-
tionship between the fuorescence quantum yield values and the solvent polar-
ity. It is generally observed that the quantum yield increases or decreases with
increasing polarity [17–19]. It is apparently difcult to obtain high quantum yield
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E. Bozkurt, S. D. Dogan
Fig. 3 Normalized fuorescence spectra of compound 12 in selected solvents
Table 2 ET (dye) values for
Solvent
ET(30) (kcal mol−1
)
ET(dye)
compound 12 in selected
(kcal mol−1
)
solvents and the ET(30) values
Toluene
Diethyl ether
1,4-Dioxane
THF
Ethyl acetate
Chloroform
DCM
DMF
DMSO
ACN
i-PrOH
EtOH
33.9
34.5
36.0
37.4
38.1
39.1
40.7
43.2
45.1
45.6
48.4
51.9
55.4
63.1
68.56
69.73
68.56
69.06
69.56
69.56
69.23
69.06
68.76
69.06
70.25
70.25
69.90
70.60
MeOH
Water
in all solvents [20]. As observed for the quantum yield, the fuorescence lifetime
values of compound 12 were also found to be very high in all solvents (Table 1).
The solvatochromism of compound 12 was investigated in 14 solvents with dif-
ferent polarity. Table 2 presents the ET (dye) values for each solvent, calculated
from the fuorescence spectra as ET (dye) = 28,591/Emax, as well as the ET(30)
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Photophysical behavior of a novel 4-aza-indole derivative…
Fig. 4 ET (dye) values for selected solvents as a function of Reichardt’s ET(30) parameter
values of the solvents [14, 21]. Figure 4 shows a plot of ET (dye) as a function
of ET(30) for compound 12. As seen in Fig. 4, there are two regions correspond-
ing to less and more polar solvents. Starting with diethyl ether, the ET values for
compound 12 decrease until DMSO then increase with increasing solvent polarity
from DMSO to water. It is believed that these changes indicate that reverse solva-
tochromism occurs in compound 12 with increasing solvent polarity [22, 23].
Conclusions
The photophysical behavior of a new 4-aza-indole derivative [ethyl 1-((2-(2-eth-
oxy-2-oxoethyl)pyridin-3-yl)carbamoyl)-2-hydroxy-1H-pyrrolo-[3,2-b]pyridine-
3-carboxylate, 12] were determined in diferent solvents. Compound 12 exhibited
high fuorescence intensities at wavelengths ranging from 405 to 417 nm in all
solvents except DMF. Compound 12 displayed reverse solvatochromism behavior
depending on the solvent polarity. Compound 12 exhibited very high quantum
yield values in all solvents, representing a rare result in literature. The results
suggest that this novel dye could be used in many applications, e.g., as a labeling
agent and in bio- or analytical sensors and/or optoelectronic devices.
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