Fluorescent phenanthroimidazoles functionalized with heterocyclic spacers: Synthesis, optical chemosensory ability and two-photon absorption (TPA) properties

Article abstract of DOI:10.1039/c7nj02113e

Three series of fluorescent phenanthroimidazoles bearing heterocyclic spacers were synthesized in moderate to excellent yields and evaluated as optical chemosensors for ions as well as two-photon absorbing chromophores. Interaction of compounds 5-7 with anions and cations in acetonitrile and acetonitrile/H₂O (95:5) showed them to be selective receptors for several anions (AcO^-, CN^- and F^-) and cations (Fe³⁺, Cu²⁺ and Pd²⁺), with compound 7a being the most sensitive receptor for Fe³⁺ and Cu²⁺. On the other hand, compounds 5a, 7b and 7c were the most sensitive receptors for AcO^-, CN^- and F^-. The binding stoichiometry between the receptors and the anions and cations was found to be 1:2 (ligand to anion/metal cation). The binding process was also followed by ¹H NMR titrations. The evaluation of the TPA properties of chosen phenanthroimidazoles 7a-c by the two-photon induced fluorescence method revealed that compound 7c, which contains a bithienyl spacer, exhibited the highest TPA cross-section (σ₂) value.

Full text of DOI:10.1039/c7nj02113e

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Fluorescent phenanthroimidazoles functionalized with
heterocyclic spacers: synthesis, optical chemosensory ability and
Two-Photon Absorption (TPA) properties
Received 00th January 20xx,
Accepted 00th January 20xx
Rosa Cristina M. Ferreira,a Susana P. G. Costa,a Hugo Gonçalves,b Michael Belsley,b and Maria
Manuela M. Raposo*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Three series of fluorescent phenanthroimidazoles bearing heterocyclic spacers were synthesized in moderate to excellent
yields and evaluated as optical chemosensors for ions as well as two-photon absorbing chromophores. Interaction of
compounds 5-7 with anions and cations in acetonitrile and acetonitrile/H2O (95:5) showed them to be selective receptors
for several anions (AcO-, CN-and F-) and cations (Fe3+, Cu2+ and Pd2+), with compound 7a being the most sensitive receptor
for Fe3+ and Cu2+. On the other hand, compounds 5a, 7b and 7c were the most sensitive receptors for AcO-, CN- and F-. The
binding stoichiometry between the receptors and the anions and cations was found to be 1:2 (ligand to anion/metal
cation). The binding process was also followed by 1H NMR titrations. The evaluation of the TPA properties of chosen
phenanthroimidazoles 7a-c by the two-photon induced fluorescent method revealed that compound 7c, which contains a
bithienyl spacer, exhibited the highest TPA cross-section (σ2) value.
presence of heterocycles containing N, O and S atoms such as
electron rich heterocycles (thiophene, pyrrole, furan), electron
deficient azines (pyridine, phenanthroline) and azoles
(imidazole, thiazole, oxazole) in a particular system may
Introduction
π
-Conjugated heterocyclic systems are versatile organic
contribute to enhanced photophysical properties, especially
increasing the fluorescence efficiency.2
components for the preparation of several devices with
application in nonlinear optics, DSSCs, supramolecular
chemistry and others.1
Additionally, inclusion of heterocycles can promote higher
polarizability as well as thermal and chemical stabilities,4 while
also constituting a site for further modification (acid/base and
coordination properties). For example, heterocyclic systems
containing chelating groups have the ability to act as both the
recognition unit for anions and cations, as well as the signalling
unit, since variation of their absorption/fluorescence
properties can happen during the recognition event.5
During the last decades, experimental and theoretical studies
have confirmed that in an organic push-pull system the
substitution of a benzene ring by a heterocycle results in
improved intramolecular charge transfer (ICT).2a-p Due to their
electronic nature and low aromaticity, they can act efficiently
as
π-bridges as well as auxiliary donors (electron-rich
heterocycles: e.g. thiophene, furan) or as auxiliary acceptors
(electron-deficient heterocycles: e.g. imidazole, pyrazine). The
Generally, π-conjugated heterocyclic systems increase
intramolecular electron delocalisation, enhancing
main advantage of the CT chromophores with a D-π-A
photophysical properties. With careful design, the inclusion of
heterocycles can optimize analyte recognition while
simultaneously leading to higher fluorescence and, therefore,
higher sensitivity.2
arrangement over other organic dyes is the well-defined and
tuneable structures and predictable properties. The longest
wavelength absorption maxima (CT band) and dipole moment
can be modulated by alternating the A (acceptor), D (donor)
and
π
-bridge.2,3 Moreover, the optical and electronic
The design and synthesis of selective chromo-fluorogenic
sensors for anions, cations and neutral molecules is an
interesting and exciting field of research with many recent
developments,6 such as the possibility of using
colorimetric/fluorimetric probes to sense both anions and
cations in aqueous solution. 7
properties of these heteroaromatic systems depends not only
on the electronic nature of the aromatic rings, but also on the
location of these heterocycles in the system. In fact, the
Trivalent metal cations play important roles in biological
processes. Therefore, their detection is an interesting topic of
research in supramolecular chemistry. For example, iron is the
most abundant transition metal in cellular systems. More
specifically, Fe3+ is an essential element in the growth and
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development of living systems as well as in many biochemical imidazole, etc.) which preserves the chemical and
processes at cellular level,8 and its deficiency is associated with photochemical stabilities and additionally allows the fine-
several diseases (anemia, hemochromatosis, diabetes, tuning of the electro-optical properties with the aim of
Parkinson´s and dysfunction of heart, pancreas and liver.9 producing strong fluorescence, an essential prerequisite for
Palladium is widely used as catalyst in organic synthesis and certain TPA-based applications.14,16-18
the development of novel systems for its easy detection is Imidazole based chromophores have received increasing
especially important; a high level of residual palladium attention recently due to their distinctive optical properties
(typically 300-2000 ppm) is often found in the resultant and excellent thermal stabilities making them versatile
products, which can be a health hazard.10 Finally, the systems for several applications such as optical
development of optical chemosensors for Cu2+ has also chemosensors,17,19 and two-photon absorbing molecules,17-18
received great attention because it is as an essential trace among others.
element, playing a critical role as a catalytic cofactor for many Motivated by our previous studies,16c,16g,19 as well as by the
metalloenzymes and transcriptional events.11 An alteration of work reported by several groups,16-18 we decided to explore
intracellular copper ion homeostasis can cause oxidative stress the potential of fluorescent phenanthroimidazole derivatives
and neurodegenerative disorders including Alzheimer’s, as novel TPA chromophores and optical chemosensors for the
Parkinson’s, Wilson’s, Menkes’ diseases and amyotrophic detection of ions with environmental and medicinal interest.
lateral sclerosis.9a-b,12 Selective recognition of anions is also a The functionalization of the imidazole core at position 4 and 5
very active topic of research and among the anions of by a fused planar phenanthrene moiety as well the
biological, clinical, environmental, and waste management introduction of extra UV-absorbing and fluorescent
interest, fluoride is of particular importance due to its heterocycles at position 2 of the imidazole heterocycle was
established role in dental care and treatment of osteoporosis. expected to provide additional binding sites for a variety of
Due to the toxicity of the cyanide anion, highly harmful to the ions through the heterocycle donor atoms, as well as improved
environment and human health, there is also an interest to photophysical properties (e.g. TPA properties) and enhanced
develop new and more selective chemosensors for this optical chemosensory ability. It was also intended to assess the
analyte.6a-b,6d-f,7a
influence of the structure in the TPA properties and
Very recently, several reports on phenanthroimidazole chemosensing ability of anions and cations.
derivatives as chemosensors were found in the literature, but We report herein the synthesis, characterization and
the derivatives reported in the present manuscript differ from evaluation of the optical (linear and TPA) properties as well as
the previously reported by using different π-spacers of the chemosensory ability of three new series of
heterocyclic nature, namely oxygen and sulphur five- phenanthroimidazole derivatives
5-7 bearing an imidazole
membered heterocycles (furan and thiophene). Such electro-deficient heterocycle fused with an emissive
heterocycles contribute to the overall photophysical phenanthrene moiety in the central core, and different spacers
properties and provide additional binding sites for certain with diverse electronic nature, constituted by thiophene and
ions.13
furan heterocycles.
In the last two decades, novel two-photon-active
chromophores,14 which play an important role in the current
field of non-linear optical materials, have also attracted a large
attention from chemists, material scientists and biophysicists.
They are finding applications in 3D optical data storage, optical
limiting, microfabrication, upconversion lasing, TPA probes,
sensors, fluorescence imaging, two-photon microscopy (TPM),
photodynamic therapy, photoactivation and drug delivery.15
A large quantity of studies concerning the synthesis, structure-
property relationships and theory revealed the importance of
certain straightforward structural motifs for TPA-active organic
materials such as (D–π–A) dipoles, (D–π–D) and (A–π–A)
quadrupoles, octupoles, etc. In general, the most important
factors in the optimization of TPA properties are the efficiency
of intramolecular charge transfer (ICT), conjugation length,
Results and discussion
Synthesis of phenanthroimidazoles 5-7
Aldehydes
electronic nature (aryl, thienyl, furyl) were used as precursors
of phenanthroimidazoles in order to evaluate the effect of
2-4 bearing (hetero)aromatic groups with diverse
5-7
the π-spacer (length, electronic nature and relative position of
the heterocyclic moieties) on the optical (linear and TPA)
properties and on the chemosensory ability (selectivity and
sensitivity). Therefore, compounds
5-7 bearing a rigid
phenanthroimidazole core and (hetero)aromatic π-bridges
linked to position 2 of the imidazole were synthesized in
moderate to excellent yields (53-89%, Table 1), through the
molecular planarity, dimensionality of the charge-transfer Radziszewski reaction,20 of 9,10-phenanthrenedione
1
with
and ammonium acetate in refluxing
glacial acetic acid for 8 h, (Scheme 1). Compounds 5a have
network, and the donating and withdrawing abilities of the
component moieties.14 Despite extensive experimental and
theoretical efforts, no single unified strategy for the
construction of molecules with large TPA cross sections has yet
emerged. Therefore, there still remains significant space for
structural modification by incorporation of easily delocalizable
π-excessive or π-deficient heterocycles (e.g. thiophene, furan,
formyl precursors
2-4
-b
been already reported by other investigators having in mind
their preparation using different synthetic methodologies as
well the study of their biological activity 21a-c
Compounds 5-7
.
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were characterized by the usual spectroscopic techniques
(Table 1).
N
N
+
N
H
X
N
H
5a
5b X = O
5c X = S
CHO
X
CHO
2b X = O
2c X = S
2a
O
O
1
+
NH4OAc/AcOH/reflux
X
CHO
OHC
X
S
S
3a X = O
3b X = S
OHC
4b X = O
4c X = S
O
4a
N
N
X
N
+
S
N
O
N
H
X
N
H
S
H
6a X = O
6b X = S
7b X = O
7c X = S
7a
Scheme 1. Synthesis of phenanthroimidazoles 5-7.
1
The H NMR spectra of phenanthroimidazoles
5
-7
exhibited Electronic absorption spectra of imidazole derivatives 5-7 in ACN
solutions showed an intense lowest energy charge-transfer
absorption band in the UV-visible region which can be ascribed to
an intramolecular (ICT) transition. The position of this band
depended on the π-spacer (length, electronic nature of the
aromatic or heteroaromatic rings and relative position of the
heterocyclic moieties on the π-bridge). Therefore, substitution of
the phenyl ring in 5a by the electron-rich thiophene heterocycle
(e.g. 5c) induced a batochromic shift of 23 nm. On the other hand,
modification of the electronic nature as well as increasing the
length of the π-spacer by introduction of furan or thiophene
induced bathocromic shifts in the range of 28-31 nm, for imidazoles
broad singlets at about
δ 13.44-13.67 ppm which were
attributed to the NH of the imidazole. A broad correlation was
observed between the electronic nature of the π-spacer and
the chemical shift of the nitrogen proton of the imidazole ring
in compounds 5-7. From the data in Table 1, one may conclude
that an increase in the chemical shift of the NH proton in the
1H NMR spectra results from the substitution of a phenyl ring
by thiophene or furan heterocycles bearing electronegative S
and O heteroatoms (e.g. 5a versus 5b and 5c) or by the
introduction of a second heterocyclic ring at the π-bridge (e.g.
5c versus 7b and 7c). IR spectroscopy was also used to identify
the typical NH absorption bands in compounds
appeared as sharp bands between 3122-3438 cm-1.
5-
7
which bearing phenylfuran (6a; λmax = 340 nm) and phenylthiophene (6b;
max = 343 nm), compared to the phenyl derivative (5a; λmax = 312
nm). As expected, substitution of the phenyl ring by a second
heterocyclic ring in compounds 6a and 6b also induced
λ
a
Photophysical properties of phenanthroimidazoles 5-7
bathochromic shift of 41 nm (7a) or 42 nm (7b), as seen in Table 1
for compound 6b versus 7c.
The photophysical properties (absorption and emission) of
phenanthroimidazoles 5-7 were measured in ACN solutions and are
presented in Table 1 and Figure 1.
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The position of the electron-rich five-membered heterocycles furan
absorption band (e.g 7a, λmax = 371 nm versus 7b, λmax = 381 nm).
and thiophene on the π-bridges influenced the λmax of the
Table 1. Yields, IR absorption, 1H NMR, UV-visible absorption and fluorescence data for phenanthroimidazoles 5-7 in acetonitrile solution.
UV/Vis
Fluorescence
Yield
(%)
IR ν
(cm-1)a
δH
(ppm)b
Cpd.
π-spacer
λmax
λem
Stokes’
log ε
4.7
ΦF
(nm)
(nm)
shift (nm)
5a
5b
5c
6a
6b
7a
7b
7c
54
87
82
86
89
53
60
88
3122
3438
3236
3437
3246
3438
3423
3168
13.44
13.67
13.53
13.48
13.51
13.59
13.63
13.59
312
389
394
396
410
423
427
445
455
0.38
77
312
335
340
343
371
381
385
4.5
4.5
4.4
4.6
4.6
4.6
4.7
0.31
0.55
0.90
0.79
0.70
0.41
0.50
82
61
70
80
56
64
70
O
S
O
S
O
S
S
S
O
S
a For the NH stretching band of the imidazole (recorded in KBr).
b For the NH proton of the imidazole (in DMSO-d6).
Finally, substitution of the furan by a thiophene in the π-spacer also = 0.79 and 7a ФF = 0.70 versus 7c; ФF = 0.50). Furan derivatives do
induced bathochromic shifts in the range of 3-23 nm due to the not suffer from the heavy atom effect observed in thiophene-based
enhancement of their charge-transfer properties. These results are fluorophores, thus, they tend to exhibit stronger luminescence. For
not unexpected, and can be explained by a combination of the example, compound 6a (0.90) showed a great enhancement of the
bathochromic effect of sulphur and the partial increase of aromatic quantum yield, compared with that of 7b (0.41) due to the
character of the thiophene heterocycle compared to furan, which quenching of fluorescence associated with the heavy atom induced
allows an increase of conjugation.22 The relative fluorescence spin-orbit coupling by the sulphur atoms, which gives rise to a very
quantum yields for phenanthroimidazoles 5-7 were determined by efficient intersystem crossing mechanism that lowers the
using 10-6 M solutions of DPA in ethanol as standard (ФF = 0.95).23 As emission.22,25-26 Compounds 5-7 showed moderate to good Stokes’
noted in Table 1, compounds 5-7 exhibited good to excellent shifts in the range of 56-82 nm.
emissive properties with relative fluorescence quantum yields Having in mind these photophysical properties, derivatives 5-7
between 0.31 and 0.90.
The planarity of the fused could be potential candidates as fluorescent chemosensors and TPA
phenanthroimidazoles 5-7, the usually high quantum yields of chromophores due to the high fluorescence quantum yields,
imidazole derivatives as well as the extent of these π-conjugated important for maximization of the response in the analysis of very
systems all contribute to these results.19,24 The position of λem dilute samples.
depended also on the π-spacer (length, electronic nature of the
aromatic or heteroaromatic rings and relative position of the Two-photon absorption of phenanthroimidazoles 7a-c
heterocyclic moieties on the π-bridge) following the same trend Given the high fluorescent quantum yields, the two-photon
described above for the absorption properties (Table 1). Therefore,
bathochromic shifts in the emission bands were observed by
substitution of aryl by heteroaryl groups on the spacers as well as
by increase of the π-spacer due to the addition of five-membered
heterocycles. Noteworthy is also the effect of the heteroatom of
the five membered heterocycles. π-Spacers bearing furan generally
had higher relative fluorescence quantum yields compared to the
corresponding thiophene π-spacers (e.g. 6a; ФF = 0.90 versus 6b; ФF
absorption (TPA) cross-sections
(σ2) of phenanthroimidazole
derivatives could be determined by investigating the two-photon
induced fluorescence using the well-established method described
by Xu and Webb.27 Rhodamine 590 (Rhodamine 6G) was employed
as the reference,28 for measurements of the TPA properties of the
selected imidazoles 7a-c. The TPA spectra were recorded in the
750-850 nm spectral range using ethanol solutions (ca. 1.0 × 10-4 M)
of the chosen phenanthroimidazoles 7a-c (Table 2) and the
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corresponding two-photon absorption cross-sections were found to Furthermore, these modifications provide an opportunity to assess
lie between 1.5 and 5.3 GM.
the influence of the structure on the chemosensing ability of anions
and cations.
Table 2. Photophysical and two-photon absorption properties of
imidazoles 7a-c in ethanol.
a
b
σ2
Cpd.
π-spacer
λmax
λem
ΦF
(nm)
(nm)
(GM)
7a
7b
7c
372
382
385
426
0.78
0.42
0.47
1.5
444
455
4.3
5.3
a
Emission maximum wavelength excited at the single photon absorption
maximum.
b
Figure 1. Acetonitrile solutions of phenanthroimidazoles 5-7: (top)
Two-photon absorption cross-section given in GM at an excitation
wavelength of 750 nm, 1 GM = 1 x 10 -50 cm4 s photon-1.
naked-eye colour; (down) visualization under a UV lamp at 365 nm.
A prospective study of the TPA properties of phenanthroimidazoles
7a-c showed that the position of the furan and thiophene on the π-
bridge lead to marked differences on the two-photon absorption
Several studies have demonstrated that imidazole derivatives are
good binding groups for the coordination of anions.19,30 In fact, the
coordination ability of the imidazole group depends on the acidity
of the NH proton that can be modulated,31 for example by the
presence in the structure of easily delocalizable heteroaromatic
rings, such as thiophene and furan. On the other hand, the
presence in the imidazole nucleus of two nitrogen atoms, one of
them with a free electron pair, also supports the coordination of
metal cations by this heterocycle.19,32
cross-sections (σ2) values. Therefore, compound 7b, in which the
thiophene heterocycle is directly linked to the electron-deficient
imidazole ring, exhibited an almost 3 times higher σ2 (4.3 GM)
compared to 7a (σ2 = 1.5 GM) (Table 2). On the other hand,
comparison between phenanthroimidazoles 7b and 7c showed that
replacing a thiophene heterocycle by a furan also increased the σ2
value of 7c compared to 7b. In both cases the results obtained for
σ2 follow the same trend described for the linear optical (absorption
Therefore, we decided to design and synthesize several
phenanthromidazoles bearing different π-spacers (constituted by
furan and thiophene heterocycles) in order to modulate their
photophysical properties having in mind also their application as
optical chemosensors for specific anions and metal cations.
and emission properties) described above.
At first sight it seems strange that the two-photon absorption cross-
section for compound 7a is less than half that for molecules 7b and
7c given the very similar chemical structure. However, one point
worth keeping in mind is that two-photon absorption is a third
order nonlinear effect and within the unsold approximation the
second hyperpolarizability is proportional to the fourth moment of
the electron distribution in the HOMO state.29 As molecule 7a has
the furan moiety closed to the imidazole, one expects the extent of
the π-electron density to be partially localized around the more
central oxygen atom in 7a, leading to lower values of the higher
moments in the π-electron distribution. In contrast, for molecule
7b, the furan moiety is at the far end, creating a less central π-
electron distribution and consequently somewhat greater second
hyperpolarizability. In the case of molecule 7c with two thiophene
moieties, one would also expect a wider π-electron distribution.
The interaction of phenanthroimidazoles 5-7 with anions (AcO-, F-,
-
-
-
Cl-, Br-, CN-, NO3 , BzO-, H2PO4 , HSO4 ) and metal cations (Cu2+, Cd2+
,
Pd2+
, , , ) with biological,
Ni2+, Hg2+, Zn2+ Fe2+ Fe3+ and Cr3+
environmental and analytical relevance, was evaluated through
spectrophotometric and spectrofluorimetric titrations in ACN and
ACN/H2O (95:5) solutions.
A preliminary test was carried out by addition of up to 50 equiv of
each ion to the solutions of compounds 5-7. Noticeable changes
occurred in the colour of the solutions (from pale yellow to dark
yellow) for compound 7c upon interaction with CN- and for 7b with
CN- and F- anions. Subsequent spectrophotometric titrations of
phenanthroimidazoles 7b and 7c in acetonitrile with CN- and F-
revealed a trend in the UV-vis spectra: the intensity of longest
wavelength absorption band (at 380 and 386 nm, respectively)
decreased progressively upon addition of CN- and F- (7b) or upon
addition of CN- (7c), with the simultaneous growth of a new red-
shifted absorption band located at 404/405 and 417 nm,
respectively. Simultaneously, there was also the appearance of a
new absorption band located at about 300 nm for compounds 7b
and 7c after the interaction with CN- and F- (Figure 2 and Figures S1-
S2 in supplementary information). This observation can be
Spectrophotometric/spectrofluorimetric
titrations
and
chemosensing studies of phenanthroimidazoles 5-7 with anions
and metallic cations
The modification of imidazole through the introduction of extra UV-
absorbing and fluorescent heterocycles within its structure was
expected to provide additional binding sites for a variety of ions
through the heterocycle donor atoms, as well as improved
photophysical properties for the chemosensing studies.
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explained by the interaction of the anions with the electron-
withdrawing part of the molecule.
Abs
2
Abs
2
1,5
1
1
Abs
1
0,5
0
Abs
1A
1B
0,5
0
380 nm
404 nm
1,5
1
380 nm
405 nm
0
5
[CN-]/[7b]
0
2
4
[F-]/[7b]
0,5
0
0,5
0
250
300
350
400
450
500
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Abs
1,5
1
1
Abs
1C
0,5
386 nm
417 nm
0
0
1
[CN-]/[7c]
0,5
0
250
300
350
400
450
500
Wavelength (nm)
Figure 2. Spectrophotometric titrations of 7b (1A, 1B) and 7c (1C) with addition of increasing amounts of CN- and F- in ACN. The inset
represents the normalized absorption a 380 and 404 nm (1A); 380 and 405 nm (1B) and 386 and 417 nm (1C) ([7b-c] = 1x10-5 M, T = 298 K).
Regarding the fluorimetric response, a preliminary test was also decreased progressively upon addition of CN- (5b-c, 6a and 7a-c) or
carried out by addition of up to 50 equiv of each ion to the upon addition of F- (5-7), with the simultaneous growth of a new
acetonitrile solutions of compounds 5-7. Noticeable changes red-shifted emission band. In Figure 3, representative examples are
occurred in the fluorescence of the solutions for compounds 5-7 shown,
namely
the
spectrofluorimetric
titrations
of
upon interaction with several anions (AcO-, F-, CN-) and cations phenanthroimidazole 7c with F- and CN- in which the intensity of the
(Cu2+, Pd2+ and Fe3+). Consequently, spectrofluorimetric titrations of emission band centred at 456 nm was reduced (a chelation
phenanthroimidazoles 5-7 in acetonitrile with the ions described enhanced quenching, CHEQ effect), and a new red-shifted emission
above were also performed. Spectrofluorimetric titrations of band appeared at 516 nm due to a fluorescence enhancement upon
phenanthroimidazoles 5-7 in acetonitrile with CN- and F- and AcO- chelation effect (CHEF). An isoemissive point was also observed at
revealed the same trend in the emission spectra: the intensity of 493 nm. In the case of F-, 2.8 equivalents were necessary to achieve
longest wavelength emission band (between 389 and 455 nm) a plateau, while for CN- 3.2 equivalents were required.
I/a.u
.
I/a.u
.
I/a.u 1
.
I/a.u
.
1
1
1
456 nm
516 nm
456 nm
515 nm
0,5
0
0,5
0
0,8
0,6
0,4
0,2
0
0,8
0,6
0,4
0,2
0
0
2
0
2
4
[F-]/[7c]
[CN-]/[7c]
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
6 | J. Name., 2012, 00, 1-3
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Figure 3. Spectrofluorimetric titrations of 7c with addition of increasing amounts of CN- (right) and F- (left) in ACN. The inset represents the
normalized emission ([7c] = 1x10-5 M, T = 298 K).
I/a.u
1
.
I/a.u.
I/a.u
.
1
I/a.u
.
1
1
0,8
0,6
0,4
0,2
0
396 nm
0,5
0
396 nm
0,5
0
0,8
0,6
0,4
0,2
0
0
20
40
0
200
400
[Cu2+]/[5b]
[Pd2+]/[5c]
351
401
451
501
360
410
460
510
Wavelength (nm)
Wavelength (nm)
Figure 4. Spectrofluorimetric titrations of 5b and 5c with addition of increasing amounts of Cu2+ and Pd2+, respectively, in ACN. The inset
represents the normalized emission ([5b] = [5c] = 1x10-5 M, T = 298 K).
Comparison between the spectrofluorometric titrations of number of equivalents for each cation was as follows: Fe3+: 112
compounds 5-7 showed that the only difference was the number of equiv. for 5a, 89 equiv. for 5b, 199 equiv. for 5c, 36 equiv. for 6a; 65
equivalents necessary to quench at least 90% of the initial equiv. for 6b, 14 equiv. for 7a; 56 equiv. for 7c; for Cu2+: 63 equiv.
fluorescence intensity of each phenanthroimidazole solution. The for 5b; 3.7 equiv. for 7a; 5.6 equiv. for 7b, 44.2 equiv. for 7c, and for
number of equivalents for each anion was as follows: CN-: 13.3 Pd2+: 401 equiv for 5c. Therefore, imidazole 7a was the most
equiv. for 5b, 23.6 equiv. for 5c, 53 equiv. for 6a; 5.5 equiv. for 7a; sensitive receptor for Fe3+ (14 equiv.) and Cu2+ (3.7 equiv.) in ACN
3.5 equiv. for 7b and 3.2 equiv. for 7c; F-: 10 equiv. for 5a, 3.1 equiv. solutions due to the smaller number of equivalents needed to
for 5b, 3 equiv. for 5c, 42 equiv. for 6a; 3 equiv. for 6b, 2.3 equiv. achieve 90% of quenching of fluorescence. In Figure 4,
for 7a; 1.1 equiv. for 7b and 2.8 equiv. for 7c; AcO-: 86 equiv. for 5a, representative examples of the spectrofluorimetric titrations of
310 equiv. for 5b; 123 equiv. for 5c; 120 equiv. for 7a and 92 equiv. phenanthroimidazoles 5b with Cu2+ and 5c with Pd2+ are shown. The
for 7c. Therefore, the most sensitive receptors for CN-, F- and AcO- drastic effect of cation complexation is evident in the band centred
were imidazoles 7c (3.2 equiv.), 7b (1.1 equiv.) and 5a (86 equiv.) at the wavelength of maximum emission at 396 nm for both
respectively, due to the smaller number of equivalents need to compounds.
achieve 90% of fluorescence quenching.
Having in mind the practical applications of compounds 5-7 in
Due to the obvious changes that occurred also in the fluorescence aqueous media, the chemosensory ability was also evaluated in
of the solutions of compounds 5-7 upon interaction with Cu2+, Pd2+ mixtures of acetonitrile/H2O (95:5). A strong decrease of the
and Fe3+, spectrofluorimetric titrations of phenanthroimidazoles 5-7 fluorescence intensity was observed for all derivatives in the
in acetonitrile with these cations were also performed. They spectrofluorimetric titrations with Fe3+, Cu2+ and Pd2+ cations with
revealed the same trend in the emission spectra: a progressive an almost complete fluorescence quenching, leading to the
quenching of longest wavelength emission band, CHEQ effect, selective fluorescent detection of Fe3+ for compounds 5a, 6a and 6b
between 389 and 455 nm accompanied by a small red-shift of the and Cu2+ for derivative 7b. However,
emission band. equivalents were necessary in order to obtain 90% quenching of
In the spectrofluorimetric titrations with Fe3+, a strong decrease of fluorescence compared to the study in acetonitrile solutions. Thus,
the fluorescence intensity with an almost complete fluorescence the number of equivalents for each cation was as follows: for Fe3+
a higher number of
:
quenching was observed for all phenanthroimidazoles except for 139 equiv. for 5a, 184 equiv. for 5b, 254 equiv. for 5c, 58 equiv. for
7b. Imidazole 5c exhibited a total quenching in the fluorescence in 6a, 267 equiv. for 6b, 333 equiv. for 7a; for Cu2+: 140 equiv. for 5b,
the presence of Pd2+, whereas compounds 5b and 7a-c exhibited 3 equiv. for 7a, 307 equiv. for 7b, 70 equiv. for 7c; and for Pd2+: 361
the same behaviour in the presence of Cu2+ (Figure 4). The equiv for 5c (see also Figures S3-S9 in supplementary information).
quenching effect in the presence of Cu2+ can be attributed to an
I/a.u
.
I/a.u
.
energy transfer quenching of the π* emissive state through low-
lying metal-centered unfilled
-orbitals.33 This result suggests the
1
1
394 nm
d
0,5
0
0,8
0,6
0,4
0,2
0
involvement of the metal ion with donor atoms, the N from the
imidazole and the O or S from the pendant furan or thiophene.
Once again, comparison between the spectrofluorometric titrations
of compounds 5-7 showed that the only difference was the number
of equivalents of cations necessary to quench the initial
fluorescence intensity of each phenanthroimidazole solution. The
0
50
100
150
[Cu2+]/[5b]
350
400
Wavelength (nm)
450
500
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Figure 5. Spectrofluorimetric titration of 5b with addition of formation of the charge transfer complex between the anion and
increasing amounts of Cu2+ in ACN/H2O (95:5). The inset represents the receptor, resulting in an increase in the electron density in the
the normalized emission ([5b] = 1x10-5 M, T = 298 K).
system (Figure 7).
Titration of the same compound with Cu2+ resulted in the
concomitant broadening of the NH signal at 13.6 ppm and the loss
of resolution of the signals related to the aromatic protons,
accompanied by a very subtle upfield shift. These findings can also
corroborate the formation of the already mentioned charge
transfer complex.
I/a.u
.
1
I/a.u
.
1
0,5
457 nm
504 nm
0,8
0,6
0,4
0,2
0
0
0
50
100
150
[CN-]/[7c]
The binding stoichiometry of compounds 5-7 with selected
anions/cations and the binding affinity were calculated by HypSpec
software from the spectrofluorimetric titrations in acetonitrile and
400
450
500
Wavelength (nm)
550
600
aqueous acetonitrile, and
a 1:2 ligand:metal cation/anion
stoichiometry is suggested (Table 3). The 1:2 stoichiometry of the
complex formed between phenanthroimidazoles 5-7 and Fe3+ was
also confirmed by spectrofluorimetry by the method of continuous
variation for the 1:2 binding mode of 5c with Fe3+.
Figure 6. Spectrofluorimetric titration of 7c with addition of
increasing amounts of CN- in ACN/H2O (95:5). The inset represents
the normalized emission ([7c] = 1x10-5 M, T = 298 K).
Table 3. Logarithm of association constants (Kass) for the interaction
of phenanthroimidazoles 5-7 with several anions/cations in
acetonitrile (L:M or L:A stoichiometry suggested from HypSpec is
1:2).
Overall, imidazole 7a was the most sensitive towards Fe3+ (3 equiv.)
and Cu2+ (58 equiv.) in acetonitrile/H2O. Figure 5 shows a
representative example of the spectrofluorimetric titration of
phenanthroimidazole 5b with Cu2+ where the drastic effect of
cation complexation is evident in the band centered at the
wavelength of maximum emission at 394 nm.
Compound
Ion
Fe3+
F-
log Kass
12.1736±0.0451
12.4992±0.1161
13.9239±0.1191
12.0838±0.0258
13.1719±0.1503
12.5790±0.1187
12.2937±0.1310
13.3805±0.0635
12.2449±0.0306
12.6432±0.0313
12.3270±0.1243
11.8473±0.1137
12.9158±0.0431
12.9423±0.1326
13.0154±0.1222
13.2450±0.2381
13.0096±0.0371
12.4786±0.1660
12.1051±0.0355
12.1870±0.1071
12.6623±0.1145
12.1950±0.1366
14.1572±0.1229
12.4124±0.2351
11.5732±0.1265
11.9672±0.1315
11.7641±0.0437
12.8135±0.1757
12.5009±0.1308
12.4308±0.1287
The spectrofluorimetric titrations of compounds 5-7 in
acetonitrile/H2O (95:5) resulted in a marked loss of sensitivity.
Therefore, only compounds 5b, 7a and 7c exhibited noticeable
changes, upon interaction with cyanide, following the same trend
observed in acetonitrile solutions, namely a strong decrease of
fluorescence intensity (CHEQ effect) with the simultaneous growth
of a new red-shifted emission band. The spectrofluorimetric
titration of phenanthroimidazole 7c is shown in Figure 6, as a
representative example where the effect of the cyanide interaction
is evident in the band centred at the wavelength of maximum
emission at 457 nm. A strong quenching of the fluorescence
followed by the formation of a new red-shifted band and a
isoemissive point at about 490 nm, indicating the presence of two
emissive species in solution.
5a
AcO-
Fe3+
Cu2+
CN-
F-
AcO-
Fe3+
Pd2+
CN-
F-
AcO-
Fe3+
F-
CN-
Fe3+
F-
Fe3+
Cu2+
CN-
F-
AcO-
Cu2+
F-
CN-
Fe3+
Cu2+
CN-
F-
5b
5c
1H NMR titrations of phenanthroimidazoles with selected
6a
6b
anions/cations
The sensory behaviour observed with the spectrophotometric and
spectrofluorimetric titrations was also complemented by
performing 1H NMR titrations for compound 7a with F- and Cu2+, as
representative examples, in order to gain further insight into the
binding mode between the receptor and the cations and anions
under study. Due to the limited solubility of compound 7a in
deuterated acetonitrile, the titrations were carried out in DMSO-d6
(Figure 7).
7a
In DMSO-d6, the signal of the imidazole NH appeared downfield at
7b
7c
13.6 ppm, which was further shifted downfield to 16.0 ppm (Δδ ~
2.4 ppm) upon addition of up to 7.0 equiv. of fluoride, thus
suggesting that the interaction is occurring at this site. The aromatic
protons exhibited a slight upfield shift upon continued addition of
fluoride until 7 equiv. This shielding effect is probably due the
8 | J. Name., 2012, 00, 1-3
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ARTICLE
Figure 7. Partial 1H NMR spectra of phenanthroimidazole 7a (2.2 x 10-2 M) in DMSO-d6 in the: (a) absence and presence of (b) 0.1, (c) 1.0,
(d) 3.0 and (e) 7.0 equivalents of F-.
Tentative structures of the complexes between compound 7a (as
representative example of the ligand) and F- (as representative of
anions), Cu2+ and Fe3+ (as representatives of cations) were included
in the supplementary material (Figure S10 in supplementary
information). The imidazole nucleus is quite versatile and may be
involved in the complex formation either through hydrogen
bonding of the NH to the fluoride or by coordination of the N with
the lone electron pair to the metal cation (with a cooperative effect
of the other heteroatoms in the structure).
thiophenylfuranyl spacer was found to be the most sensitive
receptor for Fe3+ and Cu2+, while compounds 5a, 7b and 7c were the
most sensitive receptors for AcO-, CN- and F-. The binding
stoichiometry between the receptors and the anions and metal
cations was calculated and found to be 1:2 (L:M or L:A). The binding
process was also followed by 1H NMR titration.
Experimental
Synthesis and characterization general
Reaction progress was monitored by thin layer chromatography
(0.25 mm thick pre-coated silica plates: Merck Fertigplatten
Kieselgel 60 F254), while purification was carried out by silica gel
column chromatography (Merck Kieselgel 60; 230-400 mesh). NMR
spectra were obtained on a Bruker Avance III 400 at an operating
frequency of 400 MHz for 1H and 100.6 MHz for 13C using the
solvent peak as internal reference. The solvents are indicated in
parenthesis before the chemical shift values (δ relative to TMS and
given in ppm). Mps were determined on a Gallenkamp apparatus.
Conclusions
The synthesis of a series of fluorescent phenanthroimidazoles
bearing heterocyclic spacers (furan and thiophene) was achieved,
having in mind their study as optical chemosensors for ions and as
two-photon absorbing chromophores. The photophysical properties
(absorption and fluorescence) were evaluated and the assessment
of the TPA properties of chosen phenanthroimidazoles 7a-c by the
two-photon induced fluorescent method revealed that compound
7c (containing a bithienyl spacer) exhibited the highest TPA cross-
Infrared spectra were recorded on
a BOMEM MB 104
section (σ2) value. The interaction of the synthesised compounds
spectrophotometer. Mass spectrometry analyses were performed
at the “C.A.C.T.I. -Unidad de Espectrometria de Masas” at the
University of Vigo, Spain. All commercially available reagents were
used as received. Dione 1 and aldehydes 2-3, 4c were commercially
with anions and cations was explored in acetonitrile and
acetonitrile/H2O (95:5, v/v) solutions which revealed that higher
sensitivity was achieved in acetonitrile solutions with some of the
title compounds being selective and sensitive to AcO-, CN- and F-
and for Fe3+, Cu2+ and Pd2+. Phenanthroimidazole 7a bearing a
This journal is © The Royal Society of Chemistry 20xx
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available and the syntheses of precursor aldehydes 4a-b have been 127.79, 128.21, 133.90, 136.73, 144.89 ppm. MS (EI) m/z (%):
reported elsewhere.32
300 ([M]+, 100), 190 (11). HRMS: (EI) m/z (%) for C19H12N2S;
calcd 300.0721; found 300.0722.
General procedure for synthesis of phenanthroimidazoles 5-7.
9,10-Phenanthrenedione
1
(1 mmol), heterocyclic aldehydes
2
-
2-(4’-(Furan-2’’-yl)phenyl)-1H-phenanthro[9,10-d]imidazole,
o
(1 mmol), and NH4OAc (20 mmol) were dissolved in glacial 6a. Light brown solid (144 mg, 86%). Mp: 251.8-252.6 C. IR
4
acetic acid (5 mL), followed by stirring and heating at reflux for (KBr): ν = 3434, 2108, 1643, 1455, 1429, 1355, 1283, 1156,
1006, 901, 841, 753, 720 cm-1. H NMR (DMSO-d6):
1
8 h. The mixture was then cooled to room temperature, ethyl
δ = 6.65 –
acetate was added (15 mL) and washed with water (3 x 10 mL).
After drying the organic fraction with anhydrous MgSO4, the
solution was filtered and the solvent was evaporated to
dryness. Recrystallization from acetone/petroleum ether (40-
6.66 (m, 1H, H-4’’), 7.09 (d, J = 3.6 Hz, 1H, H-3’’), 7.63 (dt, J =
8.0 and 1.2 Hz, 2H, H-5 and H-10), 7.74 (t, J = 8.0 Hz, 2H, H-6
and H-9), 7.82 (d, J = 1.6 Hz, 1H, H-5’’), 7.93 (d, J = 8.4 Hz, 2H,
H-2’ and H-6’), 8.36 (d, J = 8.8 Hz, 2H, H-3’ and H-5’), 8.57 –
8.58 (m, 2H, H-4 and H-11), 8.85 (d, J = 8.0 Hz, 2H, H-7 and H-
60oC) afforded the pure imidazole derivatives
5-7.
8), 13.48 (s, 1H, NH) ppm. 13C NMR (DMSO-d6):
δ = 106.83,
2-Phenyl-1H-phenanthro[9,10-d]imidazole, 5a.21a-c Off white
112.35, 121.95, 122.33, 123.83, 124.05, 125.29, 126.61,
127.13, 127.65, 129.06, 130.77, 143.40, 148.71, 152.61 ppm.
MS (EI) m/z (%): 360 ([M]+, 100), 190 (9). HRMS: (EI) m/z (%)
for C25H16ON2; calcd 360.1263; found 360.1261.
o
solid (115 mg, 54%). Mp: 316.6-317.3 C. IR (KBr): ν = 3122,
3054, 1940, 1902, 1704, 1661, 1612, 1544, 1518, 1472, 1456,
1
1382, 1353, 1326, 1230, 1157, 1104, 1037, 750 cm-1. H NMR
(DMSO-d6):
δ = 7.50 (dt, J = 8.4 and 2.0 Hz, 1H, H-4’), 7.57 –
7.65 (m, 4H, H-3’, H-5’, H-5 and H-10), 7.70 – 7.77 (m, 2H, H-6
and H-9), 8.31 (dd, J = 7.2 and 1.2 Hz, 2H, H-2’ and H-6’), 8.55-
8.61 (m, 2H, H-4 and H-11), 8.82 – 8.87 (m, 2H, H-7 and H-8),
2-(4’-(Thiophen-2’’-yl)phenyl)-1H-phenanthro[9,10-d]
imidazole, 6b. Yellow solid (105 mg, 65%). Mp: 216.4-217.3 oC.
IR (KBr): ν = 3246, 3070, 2972, 1658, 1612, 1533, 1479, 1456,
13.45 (s, 1H, NH) ppm. 13C NMR (DMSO-d6):
δ
= 121.87, 1430, 1383, 1312, 1232, 1081, 1051, 754, 700 cm-1. H NMR
1
121.97, 122.40, 123.71, 124.07, 125.12, 125.33, 126.12,
126.97, 127.04, 127.13, 127.51, 127.65, 127.67, 128.91,
129.21, 130.36, 136.96, 149.08 ppm.
(DMSO-d6):
δ = 7.19-7.21 (m, 1H, H-4’’), 7.60 – 7.67 (m, 4H, H-
3’’, H-5’’, H-5 and H-10), 7.73 (t, J = 7.2 Hz, 2H, H-6 and H-9),
7.90 (dd, J = 8.4 and 1.6 Hz, 2H, H-2’ and H-6’), 8.35 (dd, J = 8.4
and 1.6 Hz, 2H, H-3’ and H-5’), 8.57 (d, J = 8.0 Hz, 2H, H-4 and
H-11), 8.86 (d, J = 8.4 Hz, 2H, H-7 and H-8), 13.51 (s, 1H, NH)
2-(Furan-2’-yl)-1H-phenanthro[9,10-d]imidazole, 5b.21c Light
o
brown solid (205 mg, 87%). Mp: 275.5-275.8 C. IR (KBr): ν =
ppm. 13C NMR (DMSO-d6):
δ = 121.97, 123.91, 124.30, 125.30,
3438, 3222, 3072, 1950, 1773, 1692, 1614, 1538, 1514, 1453,
1409, 1368, 1276, 1233, 1170, 1113, 1040, 1018, 754, 724 cm-
125.76, 126.25, 126.77, 127.11, 127.64, 128.67, 129.21,
134.34, 142.72, 148.62 ppm. MS (EI) m/z (%): 376 ([M]+, 100).
HRMS: (EI) m/z (%) for C25H16N2S; calcd 376.1034; found
376.1030.
1. H NMR (DMSO-d6):
δ = 6.76 (dd, J = 3.2 and 1.6 Hz, 1H, H-
1
4’), 7.23 (dd, J = 3.2 and 0.8 Hz, 1H, H-3’), 7.60 – 7.64 (m, 2H,
H-5 and H-10), 7.69 – 7.72 (m, 2H, H-6 and H-9), 7.96 (dd, J =
1.6 and 0.8 Hz, 1H, H-5’), 8.53 (dd, J = 7.6 and 0.8 Hz, 2H, H-4
and H-11), 8.81 – 8.86 (m, 2H, H-7 and H-8), 13.67 (s, 1H, NH)
2-(2’-(Thiophen-2’’-yl)furan-5’-yl)-1H-phenanthro[9,10-d]
imidazole, 7a. Dark yellow solid (116 mg, 79%). Mp: 285.8-
ppm. 13C NMR (DMSO-d6):
δ
= 109.30, 112.32, 121.86, 121.97, 286.4 C. IR (KBr): ν = 3437, 2111, 1645, 1536, 1502, 1455,
o
122.30, 123.71, 124.04, 125.21, 125.38, 126.82, 126.94, 1425, 1366, 1123, 994, 845, 755, 697 cm-1. 1H NMR (DMSO-d6):
127.11, 127.15, 127.58, 127.71, 136.85, 141.63, 143.98, 145.80
ppm. MS (EI) m/z (%): 284 ([M]+,100), 256 (12), 255 (33).
HRMS: (EI) m/z (%) for C19H12ON2; calcd 284.0950; found
284.0953.
δ
= 7.02 (d, J = 3.6 Hz, 1H, H-3’’), 7.19 – 7.22 (m, 1H, H-4’’),
7.32 (d, J = 3.6 Hz, 1H, H-4’), 7.60 (dd, J = 3.6 and 1.2 Hz, 1H, H-
5’’), 7.62 – 7.66 (m, 3H, H-3’, H-5 and H-10), 7.72 – 7.75 (m,
2H, H-6 and H-9), 8.55 (m, 2H, H-4 and H-11), 8.83-8.89 (m, 2H,
H-7 and H-8), 13.59 (s, 1H, NH) ppm. 13C NMR (DMSO-d6):
δ =
2-(Thiophen-2’-yl)-1H-phenanthro[9,10-d]imidazole, 5c. Beige
o
solid (175 mg, 82%). Mp: 302.1-302.9 C. IR (KBr): ν = 3236,
107.91, 111.75, 122.00, 123.93, 124.22, 125.40, 126.18,
127.14, 127.71, 128.27, 132.11, 141.18, 144.70, 149.53 ppm.
MS (EI) m/z (%): 366 ([M]+, 100), 338 (8), 337 (12). HRMS: (EI)
m/z (%) for C23H14ON2S; calcd 366.0827; found 366.0825.
3073, 1948, 1683, 1614, 1564, 1487, 1456, 1411, 1363, 1294,
1
1239, 1193, 1166, 1123, 1077, 1041, 854, 714, 699 cm-1. H
NMR (DMSO-d6):
δ = 7.26 – 7.28 (m, 1H, H-4’), 7.59 – 7.65 (m,
2H, H-5 and H-10), 7.69 - 7.76 (m, 3H, H-5’, H-6 and H-9), 7.92
(dd, J = 3.6 and 1.2 Hz, 1H, H-3’), 8.45 (d, J = 7.6 Hz, 1H, H-4),
8.50 (dd, J = 7.6 and 0.8 Hz, 1H, H-11), 8.81 – 8.87 (m, 2H, H-7
2-(2’-(Furan-2’’-yl)thiophen-5’-yl)-1H-phenanthro[9,10-
o
d]imidazole, 7b. Dark yellow solid (91 mg, 60%). Mp > 300 C.
IR (KBr): ν = 3583, 3423, 2362, 2340, 1639, 1478, 1407, 1262,
1155, 1016, 998, 765, 696 cm-1. H NMR (DMSO-d6):
and H-8), 13.53 (s, 1H, NH) ppm. 13C NMR (DMSO-d6):
δ =
1
δ = 6.64-
121.78, 121.88, 122.17, 123.72, 124.12, 125.17, 125.37,
125.78, 126.67, 127.08, 127.16, 127.31, 127.49, 127.69,
6.66 (m, 1H, H-4’’), 6.92 (dd, J = 3.6 and 0.8 Hz, 1H, H-3’’), 7.49
(d, J = 4.0 Hz, 1H, H-3’), 7.61 – 7.66 (m, 2H, H-5 and H-10),
10 | J. Name., 2012, 00, 1-3
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7.71- 7.75 (m, 2H, H-6 and H-9), 7.79 (d, J = 3.6 Hz, 1H, H-5’’), was approximately 1 mm. All samples were dissolved in
7.88 (d, J = 4.0 Hz, 1H, H-4’), 8.42 – 8.52 (m, 2H, H-4 and H-11), methanol at a concentration of roughly 10-4 M.
8.82 – 8.88 (m, 2H, H-7 and H-8), 13.63 (s, 1H, NH) ppm. 13C The fluorescence resulting from the two-photon absorption
was collected at right angles using a 5-cm focal length plano-
convex lens to couple the fluorescence into a multimode fiber
NMR (DMSO-d6):
δ = 106.69, 112.44, 121.64, 123.70, 124.17,
125.12, 125.30, 125.51, 126.49, 127.06, 127.36, 127.57,
128.64, 132.28, 133.71, 136.86, 141.41, 143.14, 144.44,
148.26, 149.55 ppm. MS (EI) m/z (%): 366 ([M]+, 100), 337 (17),
305 (8), 300 (10), 227 (9), 191 (9), 190 (29), 189 (8), 183 (9),
165 (12), 164 (11), 163 (14). HRMS: (EI) m/z (%) for
C23H14ON2S; calcd 366.0827; found 366.0830.
bundle connected to the entrance slit of
a 0.3 m
monochromator (Andor Shammrock). The resulting spectrum
was captured using a cooled CCD camera (Andor Newton) and
stored on a desktop computer.
The reported two-photon absorption cross-sections were
determined by using a 10-4 M solution of Rhodamine 590
(Rhodamine 6G) in methanol as a standard. Absolute cross-
sections have been measured for this compound for incident
wavelengths in the range of 620 nm-1090 nm28 and were used
to calibrate our detection system. Specifically, the cross
2-(2’,2'’-Bithiophen-5’-yl)-1H-phenanthro[9,10-d]imidazole,
o
7c. Brown solid (138 mg, 88%). Mp: 258.6-259.1 C. IR (KBr): ν
= 3168, 1936, 1702, 1652, 1614, 1588, 1522, 1486, 1452, 1396,
1
1355, 1230, 1187, 1160, 1086, 1039, 931, 804, 752 cm-1. H
NMR (DMSO-d6):
δ = 7.14 – 7.16 (m, 1H, H-4’’), 7.44 (d, J = 4.0
σs
section of the sample,
is given by
Hz, 1H, H-4’), 7.46 (dd, J = 3.2 and 0.8 Hz, 1H, H-3’’), 7.59 (dd, J
= 5.2 and 1.2 Hz, 1H, H-5’’), 7.61 – 7.66 (m, 2H, H-5 and H-10),
7.71-7.76 (m, 2H, H-6 and H-9), 7.86 (d, J = 3.6 Hz, 1H, H-3’),
8.43 (d, J = 8.0 Hz, 1H, H-4), 8.51 (d, J = 8.0 Hz, 1H, H-11), 8.81
– 8.87 (m, 2H, H-7 and H-8), 13.59 (s, 1H, NH) ppm. 13C NMR
F
C Φ






ref
∫
ref
ref
σs = σref
CsΦs
Fs
∫
.
(1)
Cref /s
Here
represents the concentration of the reference
(DMSO-d6): δ = 121.79, 121.87, 122.07, 123.75, 124.15, 124.71,
124.81, 125.26, 125.49, 126.06, 126.56, 126.59, 127.14,
127.22, 127.55, 127.77, 128.55, 132.33, 136.07, 136.86,
137.63, 144.33 ppm. MS (EI) m/z (%): 382 ([M]+, 100), 381 (8),
190 (7). HRMS: (EI) m/z (%) for C23H14N2S2; calcd 382.0598;
found 382.0593.
Φref /s
(sample),
the fluorescence quantum efficiency
(assumed to be equal for one and two-photon absorption),
Fref / s
∫
while
indicates the total integrated area of the
reference (sample) collected fluorescence. Makarov et al.
estimate the relative uncertainty in their reported values for
the absolute cross-sections of Rhodamine 590 as ±15%. Taking
into account the small fluctuation in laser power registered
over the course of the measurements as well as possible
uncertainties in calculating the integrated fluorescent
intensities, we conservatively place the overall uncertainty in
our reported values as being ±15%.
Photophysical studies
All the photophysical experiments were performed with
freshly prepared, air-equilibrated solutions at room
temperature (293 K). UV-visible absorption spectra (200 – 700
nm) were recorded using
a
Shimadzu UV/2501PC
spectrophotometer. Fluorescence spectra were collected using
a FluoroMax-4 spectrofluorometer and fluorescence quantum
yields were determined according to literature procedures
using dilute solutions (10-6 M) of the compounds and 9,10-
diphenylanthracene (DPA) in ethanol as fluorescence standard
Spectrophotometric and spectrofluorimetric titrations of
compounds 5-7
(ca. 1.0 × 10-5 M) and of
(ФF = 0.95) 23
.
Solutions of phenanthroimidazoles
5
-
7
-
the organic and inorganic anions (H2PO4 , CN-, AcO-, F-, BzO-, Br-
-
-
, Cl-, HSO4 , NO3 ) and cations (in the form of hydrated
tetrafluorborate salts for Cu2+, Ni2+ and Pd2+, and perchlorate
salts for Cd2+, Cr3+, Zn2+, Hg2+, Fe2+ and Fe3+) under study were
prepared in UV-grade ACN and ACN/H2O (95:5) (ca. 1.0 × 10-2
to 1.0 × 10-3 M). Titration of the compounds with several
anions and metallic cations was performed by the sequential
addition of solutions of the ions to the imidazole derivative
solution, in a 10-mm path length quartz cuvette and emission
spectra were measured by excitation at the wavelength of
Two-photon absoprtion
TPA cross-sections (σ2) were measured using the well-
established two-photon induced fluorescence method with
femtosecond laser pulses described by Xu and Webb.23
A
mode-locked Ti:Sapphire (Coherent Mira 900) was used as an
excitation source with a central wavelength tuneable from 750
nm to 860 nm. The pulse duration (FWHM) was typically in the
range of 100-125 fs while the energy per pulse ranged from 5-
10 nJ. A 20-cm focal length plan-convex lens focused the
incident light, polarized in the vertical direction, into a
standard 1 cm cuvette that contained the sample. The distance
between the incident beam and the front wall of the cuvette
maximum absorption for compounds
5
-
7
indicated in Table 1.
with selected
The binding stoichiometry of the compounds
5-7
anions and metal cations was determined by Hyperquad
software.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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ARTICLE
Journal Name
792-798; u) T. Wang, N. Zhang, Q. Li, Z. Li, Y. Bao, R. Bai,
Sensors Actuact. B: Chem. 2015, 225, 81-89.
Acknowledgements
3
4
a) L. Xu, Q. Zhang, SCIENCE CHINA Mater. 1, 2017,
10.1007/s40843-016-5170-2; b) L. Cao, D. Zhang, L. Xu, Z.
Fang, X.-F. Jiang, F. Lu, Eur. J. Org. Chem. 2017, 2495–2500;
c) L. Xu, D. Zhang, Y. Zhou, Y. Zheng, L. Cao, X.-F. Jiang, F. Lu,
Opt. Mater. 2017, 70, 131-137; d) L. Cao, L. Xu, D. Zhang, Y.
Zhou, Y. Zheng, Q. Fu, X.-F. Jiang, F. Lu, Chem. Phys. Lett.
2017, 682, 133–139.
Thanks are due to Fundação para a Ciência e Tecnologia (FCT)
for a PhD grant to R. C. M. Ferreira (SFRH/BD/86408/2012),
and FEDER (European Fund for Regional Development)-
COMPETE-QREN-EU for financial support through the
Chemistry Research Centre of the University of Minho (Ref.
UID/QUI/00686/2013 and UID/ QUI/0686/2016). The NMR
spectrometer Bruker Avance III 400 is part of the National
NMR Network and was purchased within the framework of the
National Program for Scientific Re-equipment, contract
REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER)
and FCT. The femtosecond laser system was acquired within
the framework of the grant REEQ/25/FIS/2005 from the
Portuguese Foundation for Science and Technology (FCT).
a) Z. Wang, P. Gu, G. Liu, H. Yao, Y. Wu, Y. Li, G. Rakesh, J.
Zhu, H. Fu, Q. Zhang, Chem. Commun
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DOI: 10.1039/C7NJ02113E
Table of Contents (TOC) entry
Fluorescent phenanthroimidazoles functionalized with heterocyclic spacers:
synthesis, optical chemosensory ability and Two-Photon Absorption (TPA)
properties
Rosa Cristina M. Ferreira, Susana P. G. Costa, Hugo Gonçalves, Michael Belsley,
Maria Manuela M. Raposo
N
N
N
H
X
N
H
5a
5b-c X = O, S
N
X
N
N
H
Y
N
H
X
6a
X = O
7a-c
6b X = S
X = O, S
Fluorescent phenanthroimidazoles bearing heterocyclic spacers as novel optical
chemosensors and Two-Photon Absorption chromophores