Tuning optical properties of phenanthroline derivatives through varying excitation wavelength and pH values

Article abstract of DOI:10.1039/c3tc32025a

Organic emissive materials equipped with tuneable optical properties have gained serious attention. Herein, two phenanthroline derivatives (ImPhTz and ImPhOz) have been prepared to possess an imidazole-thiadiazole/oxadiazole core and their tuneable emission behaviors have been investigated. As a result, ImPhTz is described with emission wavelengths spanning the visible region (λ_em = 481-611 nm) with different excitation wavelengths. The absorption (359 nm) and emission (478 nm) wavelengths of ImPhTz were red-shifted 52 nm and 93 nm when the pH value increased from 1.70 to 13.29, respectively. It is worth noting that the fluorescence quantum yields of ImPhTz did not reduce with the increase in pH values. Thus, multicolor photoluminescence has been achieved based on a single organic chromophore. Comparatively, ImPhOz was less sensitive to environmental stimulus due to a lower degree of conjugation. The emission wavelength of ImPhOz is red-shifted 82 nm and 69 nm when the excitation wavelengths and pH values increased, respectively. Moreover, the maximum two photon absorption cross-section value of ImPhTz (190 GM) is also higher than ImPhOz (27 GM). In other words, the optical properties of molecules equipped with thiadiazole may be more sensitive to the environment than those equipped with oxadiazole.

Full text of DOI:10.1039/c3tc32025a

Journal of
Materials Chemistry C
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PAPER
Tuning optical properties of phenanthroline
derivatives through varying excitation
Cite this: J. Mater. Chem. C, 2014, 2,
wavelength and pH values†
1539
Pei-Yang Gu,a Junkuo Gao,b Qichun Zhang,b Gaoyan Liu,a Feng Zhou,a
a
a
*
*
Qing-Feng Xu and Jian-Mei Lu
Organic emissive materials equipped with tuneable optical properties have gained serious attention.
Herein, two phenanthroline derivatives (ImPhTz and ImPhOz) have been prepared to possess an
imidazole-thiadiazole/oxadiazole core and their tuneable emission behaviors have been investigated. As
a result, ImPhTz is described with emission wavelengths spanning the visible region (lem ¼ 481–611
nm) with different excitation wavelengths. The absorption (359 nm) and emission (478 nm)
wavelengths of ImPhTz were red-shifted 52 nm and 93 nm when the pH value increased from 1.70 to
13.29, respectively. It is worth noting that the fluorescence quantum yields of ImPhTz did not reduce
with the increase in pH values. Thus, multicolor photoluminescence has been achieved based on a
single organic chromophore. Comparatively, ImPhOz was less sensitive to environmental stimulus due
to a lower degree of conjugation. The emission wavelength of ImPhOz is red-shifted 82 nm and 69
nm when the excitation wavelengths and pH values increased, respectively. Moreover, the maximum
two photon absorption cross-section value of ImPhTz (190 GM) is also higher than ImPhOz (27 GM). In
other words, the optical properties of molecules equipped with thiadiazole may be more sensitive to
the environment than those equipped with oxadiazole.
Received 13th October 2013
Accepted 19th November 2013
DOI: 10.1039/c3tc32025a
www.rsc.org/MaterialsC
are few reports on the systematic study of excitation-wave-
lengths or pH dependent optical properties based on this
1 Introduction
type of material.
The discovery and development of novel organic emissive
materials with dual and multiple photoresponses to exotic
stimuli, such as pH, temperature, light or mechanical inter-
action, is of great interest in both foundation research and
technical applications.1–10 Although Kasha's rule has already
predicted that the uorescence of organic emissive
compounds should be independent of excitation wavelength,
several exceptional cases for polar molecules have been
reported.11–17 For example, benzobis(imidazolium) salts
display red-shied emission wavelengths and exhibit large
multiphoton uorescence action cross sections when the
excitation wavelength is increased.12 In addition, the unsub-
stituted N-atom in imidazole is sensitive to pH and can
induce pH-dependent uorescence.18–20 Although imidazole
derivatives display many attractive optical properties, there
It is our interest to investigate the optical properties of
multifunctional phenanthroline compounds: two phenan-
throline derivatives containing imidazole and thiadiazole/
oxadiazole (Chart 1). Although ImPhOz has been reported to
be an active element in organic light-emitting diodes
(OLEDs),21 the optical properties of ImPhOz tuned by excita-
tion wavelengths or pH values have not been determined. On
the other hand, Jansson et al. employed density functional
theory to investigate a series of thiadiazole-based molecules
as electron transport materials and found that 2,5-diphenyl-
1,3,4-thiadiazole displayed stabilized LUMO levels and its
structure was the one closest to linearity when compared to
the triazole and oxadiazole analogues.22 We are interested in
aCollege of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou 215123, China. E-mail: lujm@suda.edu.cn; xuqingfeng@suda.edu.cn; Fax:
+86 512 65880367; Tel: +86 512 65880368
bSchool of Materials Science and Engineering, Nanyang Technological University,
Singapore 639798, Singapore
† Electronic supplementary information (ESI) available: Details of the synthesis
and
characterization
of
ImPhOz
and
2-(4-(bromomethyl)
phenyl)-5-phenyl-1,3,4-thiadiazole. See DOI: 10.1039/c3tc32025a
Chart 1 Chemical structures of ImPhOz and ImPhTz.
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the possibility of combining the advantages of imidazole and each case to be quadratic. Then, the TPA cross section d
thiadiazole into one molecule to achieve multifunctional values were calculated on the basis of the following
performance, which might have broad applications in a lot of expression:
detection elds.
Cr nr Fs Fr
ds ¼ dr
(1)
Herein, two phenanthroline derivatives containing imid-
azole and thiadiazole/oxadiazole have been successfully
synthesized and their tuneable emission behaviors have been
investigated. Our results show that both compounds exhibit
three modes of pH sensitivity in the range of pH 1–14 due to
the protonated, neutral, and deprotonated forms of imidazole
with different emission wavelengths. Moreover, ImPhTz is
described with emission wavelengths spanning the visible
Cs ns Fr Fs
where d is the TPA cross section, C and n are the con-
centration and refractive index of the sample solution
respectively, and F is the integrated area under the TPEF
spectrum.
2.3 Synthesis
region (lem
wavelengths.
¼
481–611 nm) with different excitation
4-(5-Phenyl-1,3,4-thiadiazol-2-yl)benzaldehyde was synthesized
according to our previous report.27 The synthesis of ImPhTz
(Scheme 1) was similar to the reported method for ImPhOz.
The detailed characterizations are provided in the ESI.†
4-(5-Phenyl-1,3,4-thiadiazol-2-yl)benzaldehyde (3). 2-(4-(Bro-
momethyl)phenyl)-5-phenyl-1,3,4-thiadiazole 4 (3.3 g, 10 mmol)
and hexamethylenetetramine (1.4 g, 10 mmol) were dissolved in
dry CHCl3 (75 mL) and heated to reux for 2–3 h. The mixture
was then cooled to room temperature and stirred for another
1.5 h. The precipitate was ltered off and washed three times
with CHCl3. The crude intermediate was then mixed with 75%
glacial acetic acid (75 mL) and reuxed for 3 h. The reaction
mixture was subsequently ltered off and washed with 75%
glacial acetic acid three times to give 1.32 g, 4 mmol (yield, 40%)
white solid.
2 Experimental
2.1 Materials
Benzoylhydrazine, p-toluoyl chloride, 1,10-phenanthroline-5,6-
dione, and Lawesson's reagent were purchased from Tokyo
chemical industry (TCI) company. Other chemicals and solvents
were purchased from Sinopharm Chemical Reagent Co, Ltd.
2.2 General information
Using CDCl3, CF3COOD (TFA) or DMSO-d6 as the solvent and
tetramethylsilane (TMS) as the internal standard, 1H NMR and
13C NMR spectra were measured on an INOVA 300 or 400 MHz
NMR spectrometer at ambient temperature. UV-vis absorption
spectra were determined on a Shimadzu RF540 spectropho-
tometer. The emission and excitation spectra were produced at
room temperature using an Edinburgh-920 uorescence spectra
photometer. The ground-state geometry of the two compounds
was optimized via the hybrid density functional theory (B3LYP)
with the 6-31G* basis set using the Gaussian 03 program
package.23–25 The uorescent quantum yields (QYs) of the two
compounds under acidic and neutral conditions were deter-
mined using quinine bisulfate (FF ¼ 0.546 in 0.1 mol Lꢀ1
H2SO4) as the standard, whereas under basic conditions uo-
rescein (FF ¼ 0.79 in 0.1 mol Lꢀ1 NaOH) was used as the
standard.
1H NMR (400 MHz, CDCl3) d: 10.10 (s, 1H), 8.20 (d, J ¼ 8.2 Hz,
2H), 8.03 (t, J ¼ 6.7 Hz, 4H), 7.53 (d, J ¼ 1.5 Hz, 3H).
13C NMR (400 MHz, CDCl3) d: 191.31, 169.22, 166.63, 137.78,
135.31, 131.54, 130.41, 129.82, 129.32, 128.45, 128.07.
2-(4-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenyl)-5-
phenyl-1,3,4-thiadiazole (2). A mixture of 4-(5-phenyl-1,3,4-
thiadiazol-2-yl)benzaldehyde 3 (2.66 g, 10 mmol), 1,10-phe-
nanthroline-5,6-dione (2.1 g, 10 mmol), ammonium acetate
(15.4 g, 200 mmol) and glacial acetic acid (100 mL) was added to
the ask and reuxed for 4 h, and then cooled to room
temperature. The precipitate was collected, washed completely
with H2O three times, and dried under vacuum. A yellow powder
(1.60 g, 3.5 mmol, yield, 35%) was obtained.
1H NMR (400 MHz, CF3COOD) d: 9.49 (s, 2H), 9.24 (s, 2H),
8.43 (s, 2H), 8.31 (d, J ¼ 21.9 Hz, 4H), 7.95 (s, 2H), 7.75 (s, 1H),
7.60 (s, 2H).
The two-photon excitation uorescence (TPEF) measure-
ments were performed using an Avesta femtosecond Ti:
sapphire oscillator as the excitation source. The output laser
pulses have a tuneable centre wavelength from 750 to 870 nm
with a pulse duration of 80 fs and a repetition rate of 85
MHz.26 Two-photon absorption (TPA) cross sections were
determined by the TPEF method and it is assumed that the
quantum efficiency aer two-photon excitation was the same
as those aer one-photon excitation. The TPA cross sections
were obtained by calibration against uorescein with a known
Fd value in aqueous NaOH solution (pH ¼ 11) at concen-
trations of 1.0 ꢁ 10ꢀ4 mol Lꢀ1. The samples were dissolved
13C NMR (400 MHz, CF3COOD) d: 175.68, 166.69, 148.21,
146.56, 135.96, 135.39, 130.10, 129.40, 128.53, 128.35, 127.61,
126.19, 124.80, 124.49, 120.44, 118.63.
HRMS-ESI (m/z): [M + H]+ calcd for C27H17N6S, 457.1230,
found, 457.1230 (Table 1).
in solvents at a concentration of about 1.0 ꢁ 10ꢀ4 mol Lꢀ1
.
The error in TPEF measurements is about 15%. To ensure
that the measured signals were solely due to TPA, the
Scheme 1 Synthesis of ImPhTz: (i) hexamethylenetetramine, CHCl3,
reflux; CH3COOH, reflux, 40%; (ii) 1,10-phenanthroline-5,6-dione,
dependence of TPEF on the incident intensity was veried in CH3COOH, CH3COONH4, reflux, 35%.
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Table 1 Photophysical properties of ImPhTz and ImPhOz in DMFa
3d
4e
dmax
a
b
c
f
Samples labs
lem
lem
ImPhTz 285, 369 479g/611h 456i/524j
3.79 ꢁ 10ꢀ4 0.56 190
ImPhOz 284, 356 442k/524l 444m/456n 3.12 ꢁ 10ꢀ4 0.53
27
a
(a–c) are one-photon absorption, one-photon and two-photon
uorescence maxima peaks (nm), respectively; (d) is the molar
absorption coefficient, molꢀ1 L cmꢀ1; (e) is the uorescence quantum
yield; (f) is the maximum two-photon cross section (GM) and (g–n) are Fig. 4 TPF spectra of (a) ImPhTz and (b) ImPhOz with increasing
the different excitation wavelengths (370, 450, 770, 860, 360, 430, 765
and 860 nm, respectively).
excitation wavelength.
Fig. 1 Normalized absorption and emission spectra of ImPhTz and
ImPhOz in DMF solution (lex ¼ 370 nm for ImPhTz and 360 nm for
ImPhOz).
Fig. 5 Absorption (a and b) and emission (c and d) spectra of ImPhTz
under acidic (a and c) and basic (b and d) conditions in DMF–water
(v/v ¼ 9 : 1) with an excitation wavelength of 370 nm.
Fig. 2 Emission spectra of (a) ImPhTz and (b) ImPhOz in DMF solution
with increasing excitation wavelength.
Fig. 6 Absorption (a and b) and emission (c and d) spectra of ImPhOz
under acidic (a and c) and basic (b and d) conditions in DMF–water
(v/v ¼ 9 : 1) with an excitation wavelength of 360 nm.
3 Results and discussion
3.1 One-photon optical properties
The absorption and emission spectra of ImPhTz and ImPhOz
wererecorded inDMFsolution. AsshowninFig. 1, the absorption
spectrumofImPhTzexhibitstwoprominentbandsat285nmand
Fig. 3 TPA spectra of ImPhTz and ImPhOz in DMF solution.
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Scheme 2 Proposed mechanism of the two compounds under acidic,
neutral, and basic conditions.
Table 2 Photophysical properties of ImPhTz and ImPhOz in different
pH DMF–H2O solutions
Fig.
8
DFT molecular simulation results: HOMO and LUMO of
ImPhOz under acidic, neutral, and basic conditions.
a
b
Samples
labs
lem
Dlc
4d
ImPhTz
369
359
411
356
343
393
479
478
581
442
440
509
110
119
170
86
97
116
0.56
0.55
0.58
0.53
0.52
0.57
3.2 Red-edge effect
ImPhTzH+
ImPhTzHꢀ
ImPhOz
A red-edge effect (REE) refers to the shiing of emission wave-
length to longer wavelengths in response to an increase in the
excitation wavelength. Multiple polar organic uorophores
including imidazole derivatives have been found to exhibit
REEs, and this phenomenon has found a lot of applications. As
shown in Fig. 2, the emission wavelength of ImPhTz was red-
shied from 479 nm to 611 nm when the excitation wavelength
was increased from 390 nm to 450 nm. In comparison to
ImPhTz, ImPhOz is less sensitive to the change in excitation
wavelength. The red-shied wavelength (132 nm) in ImPhTz is
higher than that of ImPhOz (82 nm). Upon increasing the
excitation wavelength for ImPhOz, the PL intensity of the short-
wavelength band at 442 nm decreased while the PL intensity of
the long-wavelength band at 524 nm increased. The mechanism
of the REEs in imidazole derivatives is not entirely understood.
Its origins might involve different ground-state orientations of
the associated species. In our systems, the difference in degree
of conjugation in the two compounds might respond to the red-
shied emission when the excitation wavelength changes.
ImPhOzH+
ImPhOzHꢀ
a
b
UV-vis absorption (nm). Photon uorescence maxima peak (nm).
Stoke shied (nm). d Fluorescence quantum yield (Fig. 6).
c
Fig. 7 DFT molecular simulation results: HOMO and LUMO of ImPhTz
under acidic, neutral, and basic conditions.
3.3 Two-photon optical properties
The two-photon absorption spectra of ImPhTz and ImPhOz
were measured using the TPEF method, in which the excitation
369 nm, which can be ascribed to the different p-conjugated wavelength was adjusted from 760 to 840 nm. The two-photon
moieties of the molecule. ImPhTz emits strong blue uorescence cross sections of the two compounds, with the same concen-
with a maximum peak at 479 nm (lex ¼ 370 nm), while the tration in DMF at different excitation wavelengths, are shown in
absorption spectrum of ImPhOz exhibits two prominent bands at Fig. 3. The maximum two photon absorption cross-section
284 nm and 356 nm and its emission wavelength is 442 nm (lex
¼
values of ImPhTz and ImPhOz are 190 GM and 27 GM,
360 nm). Changing oxygen to a sulfur atom could shi the respectively. The difference between the two compounds might
absorption and emission band to the red region because of the come from their different D–A interactions. In addition, the
different donor (D)–acceptor (A) interactions and the degree of degree of conjugation of ImPhTz is higher than ImPhOz, which
conjugation in the two compounds. Generally, the more electro- may also be responsible for the large cross-section.
negative oxygen atom is expected to increase the D–A interaction
The normalized two-photon emission (TPL) spectra of the
and thus exhibit a lower band gap. However, experimental two compounds are shown in Fig. 4. Upon increasing the exci-
evidence shows the opposite trend due to the lower ionization tation wavelength, TPL wavelengths were red-shied only 10 nm
potential of the heavier chalcogen atoms.28–30 Furthermore, the for ImPhOZ but 68 nm for ImPhTz. There might be two reasons
conjugated degree of the thiadiazole system is higher than the to explain this: rst, due to limitations in our detecting equip-
oxadiazole system according to previous literature.22,31
ment, we cannot measure two-photon emission spectra with
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longer excitation wavelengths because the emission peaks may density distributions of the two compounds on the HOMO
be red-shied to a wavelength more than 860 nm and, second, orbits were mainly concentrated on imidazole.
the emission may occur due to the different state of the one-
photon and two-photon excitation (Fig. 5).
4 Conclusions
3.4 pH-dependent optical properties
In this work, two multifunctional 1,10-phenanthroline deriva-
tives have been successfully synthesized and characterized. The
emission wavelengths of both ImPhOz and ImPhTz were red-
shied when the excitation wavelength was changed. Upon
increasing the excitation wavelength, the TPL wavelengths were
red-shied only 12 nm for ImPhOz but 68 nm for ImPhTz. The
absorption and emission wavelengths for both compounds
were also red-shied 52 nm and 93 nm when the pH values
changed from 1.70 to 13.29, respectively. Comparatively,
ImPhTz was more sensitive to environmental stimulus and the
maximum TPA cross-section value of ImPhTz (190 GM) was also
higher than that of ImPhOz (27 GM). In other words, the uo-
rescence of molecules equipped with a thiadiazole group may
be more sensitive to the environment than those equipped with
an oxadiazole group.
For the analysis of pH-dependent properties, the absorption
and emission spectra of the two compounds were recorded in
DMF–water (9 : 1, by volume) with pH values changing from 1.0
to 14.0. For compound ImPhTz, at lower pH values, the
absorption wavelength changed from 359 nm (pH ¼ 1.70) to
369 nm (pH ¼ 3.94) with an isosbestic point at 369 nm. The
absorption wavelength didn't show any change when the pH
value was between 3.94 and 7.82. However, when the solution
became more basic, the absorption wavelength continued to be
red-shied from 369 nm at pH ¼ 7.82 to 411 nm at pH ¼ 13.29.
These phenomena suggest the existence of three states, which
can be assigned as the protonated, neutral and deprotonated
forms of imidazole (Scheme 2).
Interestingly, under acidic conditions, the emission wave-
length of ImPhTz didn't change (lex ¼ 370 nm). However, under
basic conditions, the emission wavelength (lex ¼ 370 nm)
increased from 479 nm (pH ¼ 7.33) to 581 nm (pH ¼ 13.29). It is
worth noting that the QYs of ImPhTz did not reduce with an
increase in pH value (Table 2). These results may be explained
by the following: in an acidic environment, the imidazole group
was protonated, which is more electron-withdrawing than when
under neutral conditions, while under basic conditions, the
deprotonated imidazole group could serve as an electron donor.
This conclusion was further conrmed by density functional
theory (DFT) calculations.
Acknowledgements
The authors graciously thank Prof. Qing-Hua Xu (National
University of Singapore) for such benecial discussions and Dr
Tingting Zhao (National University of Singapore) for the two-
photon excitation uorescence (TPEF) measurements. The
authors graciously thank the Chinese Natural Science Founda-
tion (21071105, 21371128 and 21336005), Chinese-Singapore
Joint Project (2012DFG41900) and the Specialized Research
Fund for the Doctoral Program of Higher Education of China
(grant no. 20113201130003).
The phenomena of ImPhOz were similar to ImPhTz.
However, the absorption wavelength was red-shied 50 nm
from 343 nm (pH ¼ 1.70) to 356 nm (pH ¼ 3.94–7.82) to 393 nm
(pH ¼ 13.29) and the emission wavelength was red-shied
69 nm from 440 nm (pH ¼ 1.70) to 509 nm (pH ¼ 13.29). In
other words, ImPhTz was more sensitive to environmental
stimuli due to thiadiazole.
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3.5 Calculation
The ground-state geometries of the two compounds were opti-
mized via the density functional theory (B3LYP) with the 6-31G*
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