Switching of reverse charge transfers for a rational design of an OFF-ON phosphorescent chemodosimeter of cyanide anions

Article abstract of DOI:10.1021/ic302478e

A rational approach to luminescence turn-on sensing of cyanide by a dicyanovinyl-substituted acetylide Pt(II) complex, which primarily relies on the nucleophilic addition reaction of cyanide anions to the α-position of the dicyanovinyl group, is described. The strategy used for the design of this cyanide-selective sensor takes advantage of a switch of charge transfer from ML′CT to MLCT/L′LCT in this acetylide Pt(II) sensor. As a result, this chromophore that exhibits almost no basal luminescence displays observable changes in its UV-visible spectrum and acquires strong phosphorescence upon addition of cyanide anions. DFT computations reveal that the frontier molecular orbitals of the anionic system obtained after addition of CN^- are drastically different from those of the neutral initial species. TD-DFT computations permitted a full assignment of the observed absorption bands and explained well the emissive properties of the species under consideration.

Full text of DOI:10.1021/ic302478e

Article
pubs.acs.org/IC
Switching of Reverse Charge Transfers for a Rational Design of an
OFF−ON Phosphorescent Chemodosimeter of Cyanide Anions
Jean-Luc Fillaut,* Huriye Akdas-Kilig, Edouard Dean, Camille Latouche, and Abdou Boucekkine*
́
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite de Rennes 1, 35042 Rennes Cedex, France
S
* Supporting Information
ABSTRACT: A rational approach to luminescence turn-on
sensing of cyanide by a dicyanovinyl-substituted acetylide
Pt(II) complex, which primarily relies on the nucleophilic
addition reaction of cyanide anions to the α-position of the
dicyanovinyl group, is described. The strategy used for the
design of this cyanide-selective sensor takes advantage of a
switch of charge transfer from ML′CT to MLCT/L′LCT in
this acetylide Pt(II) sensor. As a result, this chromophore that
exhibits almost no basal luminescence displays observable
changes in its UV−visible spectrum and acquires strong
phosphorescence upon addition of cyanide anions. DFT
computations reveal that the frontier molecular orbitals of the anionic system obtained after addition of CN− are drastically
different from those of the neutral initial species. TD-DFT computations permitted a full assignment of the observed absorption
bands and explained well the emissive properties of the species under consideration.
L′LCT (ligand-to-ligand charge transfer) and MLCT or
ML′CT (metal-to-ligand charge transfer) excited states.15
Such binding results in significant perturbation of the various
photophysical properties of the Pt−acetylide complexes giving
rise to changes in wavelengths, lifetimes, or quantum yields,
which can be advantageous in the field of sensing.8 Conversely,
reports concerning anion recognition with alkynyl Pt(II)
systems are rare.16
Here, we describe a new luminescence-enhanced anion
sensing mechanism based on an inversion of metal-to-ligand
and ligand-to-ligand charge transfers in a neutral cyclometalated
phenylbipyridyl Pt(II) alkynyl complex9,13,14 subsequent to the
irreversible nucleophilic addition of a cyanide anion to the α-
position of a dicyanovinyl group. The sensor was designed so
that its luminescence is dramatically activated concomitantly
with selective and sensitive responses to various concentrations
of cyanide anion. Density functional theory (DFT) and time-
dependent DFT (TD-DFT) calculations were carried out to
help rationalize the observed properties.
INTRODUCTION
■
Molecular recognition and sensing of anions is a major concern
because of the roles that anions play in a wide range of
biological, environmental, and chemical processes.1−3 Thus, the
development of sensitive anion luminescence sensors continues
to be an important field of research.4−8 In this field, transition
metal complexes have attracted increasing interest due to their
unique advantages that make them suitable for luminescent
sensing.8 The properties of the excited states are highly
sensitive to the nature of the metal center, the type of ligands,
and the local environment, allowing the photophysical
properties (such as emission wavelength, lifetime, and
intensity) of the metal complexes to be tailored for sensing
applications.
Cyclometalated phenylbipyridyl complexes Pt(II) [Pt(tBu2-
C∧N∧N)(CC−R)]9−14 (tBu2-C∧N∧N = 4,4′-di(tert-butyl)-6-
phenyl-2,2′-bipyridine), incorporating σ-alkynyl ligands, are
characterized by high quantum efficiency and convenient
absorption and emission maxima located in the visible spectrum
with a large Stokes shift and relatively long lifetime (∼1−20
μs). It is also noteworthy that the MLCT/LLCT transitions
and 3MLCT [Pt(5d) → π*(tBu2-C∧N∧N)] emission are
sensitive to electronic effects due to the substituents at the σ-
alkynyl ligands.13 Taking advantage of the properties of neutral
cyclometalated polypyridyl Pt(II) complexes incorporating
phenylacetylide ligands, it was previously demonstrated that
the para-positions of the phenylacetylide group can be utilized
as cation receptors, which are covalently bound to the
cyclometalated Pt(II) signaling centers through Pt−acetylide
σ-coordination.10−12,14,15 The binding of cations to the
receptors can induce optical signal switching between the
EXPERIMENTAL SECTION
■
Synthesis: General Procedure. All manipulations were per-
formed using Schlenk techniques under an Ar atmosphere. All
commercially available starting materials were used as received. All
solvents were dried and purified by standard procedures. The starting
complex, [Pt(tBu2-C∧N∧N)Cl], was prepared according to previously
published methods.13
Physical Measurements and Instrumentation. NMR spectra
were recorded on Bruker DPX-200, AV 300, or AV 500 MHz
Received: November 13, 2012
Published: April 8, 2013
© 2013 American Chemical Society
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Scheme 1
1
spectrometers. H and 13C chemical shifts are given versus SiMe4 and
60.30, H 4.59, N 7.76. Calcd for C36H32N4Pt (715.34): C 60.39, H
4.51, N 7.83.
were determined by reference to residual H and 13C solvent signals.
1
Synthesis of 2. Compound 2 was prepared in situ according to a
general procedure: to the solution of 1 [Pt(tBu2-C∧N∧N)(4-CC−
C6H4−CHC(CN)2] (5 mg, 0.149 mmol) in 0.3 mL of
CD2Cl2(CDCl3) is added n-Bu4NCN (1.9 mg, 0.149 mmol) in 0.3
High-resolution mass spectra (HRMS) were performed on a MS/MS
ZABSpec TOF at the CRMPO (Centre de Mesures Physiques de
l’Ouest) in Rennes. UV−vis absorption spectra were recorded using a
UVIKON 9413 or Biotek Instruments XS spectrophotometer using
quartz cuvettes of 1 cm path length.
Luminescence spectra were measured in dilute dichloromethane
solutions (∼10−6 M) using a FS920 steady-state fluorometer
(Edinburgh Instruments). The spectra shown are corrected for the
wavelength dependence of the detector, and the quoted emission
maxima refer to the values after correction. Luminescence quantum
yields were determined using the method of continuous dilution, using
[Ru(bpy)3]Cl2 as the standard (Φ = 0.028 in air-equilibrated aqueous
solution)17 and correcting for the refractive index.
1
mL CD2Cl2(CDCl3). H NMR (400 MHz; CD2Cl2): 9.07 (dd, 1H,
3
4
3JH−H = 5.73 Hz, JPt−H = 17.8 Hz); 7.87 (d, 1H, JH−H = 1.72 Hz);
3
3
4
7.82 (ddd, 1H, JH−H = 6.95 Hz, JPt−H = 64 Hz, JH−H = 1.07 Hz);
7.64−7.60 (m, 2H); 7.57 (dd, 1H, 3JH−H = 7.89 Hz, 4JH−H = 1.28 Hz);
3
3
7.40 (m, 3H); 7.42 (d, JH−H = 7.42 Hz, 1H); 7.34 (d, JH−H = 7.42
Hz, 1H); 7.13 (td, 1H, 3JH−H = 7.36 Hz, 3JH−H = 6.2 Hz, 4JH−H = 1.36
3
4
Hz); 7.08 (td, 1H, JH−H = 7.48 Hz, JH−H = 1.36 Hz); 4.27 (s, 1H);
1.47 (s, 9H); 1.45 (s, 9H). 13C {1H} NMR (75 MHz; CDCl3): 165.2;
163.5; 163.4; 158.1; 154.5; 151.5; 147.1; 142.2; 138.5; 136.6; 133.6;
131.6; 130.9; 128.9; 124.5; 124.1; 123.5; 122.7; 121.3; 121.2; 119.1;
115.5; 114.4; 108.0; 105.1; 58.8; 37.2; 36.0; 35.7; 30.6; 30.4; 29.0;
19.7; 17.6; 13.6. The identity of the complex obtained was not able to
be confirmed by mass spectrometry.
Synthesis of 4. [Pt(tBu2-C∧N∧N)Cl]13 (100 mg, 0.171 mmol)
was added to a suspension of 4-(trimethylsilylethynyl)benzaldehyde18
(50 mg, 0.23 mmol), CuI (5.0 mg, 25 μmol), and K2CO3 (300 mg) in
20 mL of deoxygenated methanol and methylene chloride (1:1 v/v).
The mixture was stirred at room temperature overnight. After
evaporation of the solvents, the crude residue was purified by column
chromatography on silica gel with dichloromethane/pentane/ethyl
acetate (49/49/2) as eluent to give 4 as a yellow solid (60 mg, 52%
Computational Studies. Density functional theory (DFT)
calculations were performed using the standard B3LYP functional19−21
and a double-ζ LANL2DZ basis set,22 augmented with polarization
functions on all atoms except hydrogen (exponents equal to 0.587 and
0.736, respectively, for the d functions of C and N and 0.8018 for the f
function of Pt) with the Gaussian0923 program. At the very beginning
of the computations, the geometries of complexes 1 and 2 in their
singlet ground state were optimized. Solvent (CH2Cl2) effects have
been taken into account using the PCM model.24,25 Then, the normal
modes of vibration of the molecules were computed; all vibration
frequencies were found to be real, confirming that the optimized
geometries are minima on the potential energy surface. Finally, the
computations of the 13C NMR chemical shifts, as well as the electronic
absorption spectra using time-dependent DFT (TD-DFT), were
carried out at the same level of theory with the same Gaussian09
package, using the previously optimized ground-state geometries. In
order to estimate the phosphorescence wavelengths, the relaxed
triplet-state geometries of the complexes were obtained using both
TD-DFT and unrestricted DFT computations. Drawings of molecular
structures and orbitals were done using the Molekel program,26 while
theoretical absorption spectra were plotted using Swizard,27 the half-
bandwidths for the Gaussian model being taken equal to 2000 cm−1.
Percentage compositions of molecular orbitals (MOs) were analyzed
using the AOMix program.28
1
yield). H NMR (400 MHz; CDCl3): 9.99 (s, 1H); 9.06 (dd, 1H,
3
3
3JH−H = 5.71 Hz, JPt−H = 17.9 Hz); 7.88 (ddd, 1H, JH−H = 7.0 Hz,
3JPt−H = 64 Hz, 4JH−H = 1.05 Hz); 7.81 (d, 1H, 4JH−H = 1.70 Hz); 7.80
(dd, JH−H = 7.42 Hz, JH−H = 1.26 Hz, 2H); 7.68 (dd, JH−H = 7.42
Hz, JH−H = 1.26 Hz, 2H); 7.58 (m, 2H); 7.51 (dd, 1H, JH−H = 7.90
Hz, JH−H = 1.27 Hz); 7.40 (dd, 1H, JH−H = 5.74 Hz, JH−H = 1.92
3
5
3
5
3
4
3
4
3
4
Hz); 7.17 (td, 1H, JH−H = 7.34 Hz, JH−H = 1.36 Hz); 7.08 (td, 1H,
4
3JH−H = 7.50 Hz, JH−H = 1.36 Hz); 1.47 (s, 9H); 1.45 (s, 9H). 13C
{1H} NMR (75 MHz; CDCl3): 191.4; 164.7; 164.2; 164.1; 158.1;
154.5; 151.3; 147.1; 142.1; 138.1; 135.8; 133.0; 132.1; 131.2; 129.8;
124.8; 124.2; 123.6; 119.3; 115.6; 115.2; 115.0; 106.5; 36.0; 35.7; 30.2;
30.0. m/z: 690.2061 ([M + Na]+. C33H32N2ONa195Pt requires
690.2060). Elemental Anal. Found: C 59.30, H 5.09, N 3.88. Calcd
for C33H32N2OPt (667.33): C 59.34, H 4.83, N 4.20. Calcd for
C33H32N2OPt·0.5C4H8O2 (711.355) (signals for ethylacetate were
1
observed by H NMR): C 59.04, H 5.10, N 3.94.
Synthesis of 1. [Pt(tBu2-C∧N∧N)(4-CC−C6H4−CHO] (100
mg, 0.149 mmol) was added to a suspension of malononitrile (20 mg,
0.30 mmol) and basic alumina (100 mg) in 20 mL of deoxygenated
toluene. After stirring at 60 °C overnight, the suspension was cooled to
room temperature and filtered. Removal of the solvent in vacuo and
trituration of the solid residue by diethylether gave the desired product
RESULTS AND DISCUSSION
■
1
1 as a reddish powder (95 mg, 84%). H NMR (400 MHz; CD2Cl2):
Synthesis and Characterization. Our approach primarily
relies on the nucleophilic addition reaction of cyanide to the
dicyanovinyl-substituted acetylide Pt(II) complex 1 (Scheme
1). This dicyanovinyl-substituted Pt(II) chromophore exhibits
almost no basal luminescence but acquires strong phosphor-
escence upon addition of cyanide anions.
8.97 (dd, 1H, 3JH−H = 5.75 Hz, 3JPt−H = 17.9 Hz); 7.87 (ddd, 1H, 3JH−H
= 6.95 Hz, 3JPt−H = 64 Hz, 4JH−H = 1.07 Hz); 7.82 (d, 1H, 4JH−H = 1.72
3
Hz); 7.81 (d, 2H, JH−H = 7.78 Hz); 7.68 (s, 1H); 7.62 (s, 2H); 7.54
3
4
(m, 3H); 7.45 (dd, 1H, JH−H = 5.75 Hz, JH−H = 1.90 Hz); 7.11 (td,
1H, 3JH−H = 7.36 Hz, 3JH−H = 6.2 Hz, 4JH−H = 1.36 Hz); 7.07 (td, 1H,
3JH−H = 7.48 Hz, JH−H = 1.36 Hz); 1.47 (s, 9H); 1.45 (s, 9H). 13C
4
The platinum(II) compound [Pt(tBu2-C∧N∧N)(CC−
C6H4−CHC(CN)2)] (tBu2-C∧N∧N = 4,4′-di(tert-butyl)-6-
phenyl-2,2′-bipyridine), 1, incorporating a dicyano-vinyl moiety
was obtained in an overall yield of 30% in two steps from
[Pt(tBu2-C∧N∧N)Cl] as depicted in Scheme 2. The first step
{1H} NMR (75 MHz; CDCl3): 165.2; 164.1; 163.9; 159.1; 158.1;
154.4; 151.6; 147.1; 141.7; 138.4; 136.8; 132.8; 132.7; 131.5; 131.0;
127.4; 124.7; 124.3; 123.9; 119.9; 119.3; 115.7; 114.8; 114.5; 113.6;
107.7; 78.8; 36.0; 35.8; 30.6; 30.4. m/z: 738.2170 ([M + Na]+,
C36H32N4Na195Pt requires 738.2167). Elemental Anal. Found: C
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Scheme 2
Figure 1. Changes in the UV−vis absorption spectrum of 1 (2.0 × 10−5 M in CH2Cl2) upon the addition of cyanide ((0−2.0) × 10−5 M). Inset
shows the absorption titration of 1 (2.0 × 10−5 M) at 450 nm with addition of n-Bu4NCN in CH2Cl2 at 293 K. [n-Bu4NCN] = (0, 0.06, 0.1, 0.2, 0.6,
0.8, 1.0, 1.2, 1.6, 2.0) × 10−5 M.
involves the one-pot reaction of 4-trimethylsilyl-ethynylbenzal-
dehyde and [Pt(tBu2-C∧N∧N)Cl] in presence of CuI (0.1
equiv) and an excess K2CO3 in CH3OH−CH2Cl2 under
anaerobic conditions. This step results in both the removal of
the trimethylsilyl group and the coordination of the free alkyne
to the [Pt(tBu2-C∧N∧N)Cl] precursor. Finally, 1 was obtained
by a Knoevenagel reaction of malononitrile in presence of basic
alumina in toluene at 60 °C.
UV−Visible Absorption Spectroscopy. The UV−vis
absorption characteristics of complex 1 strongly differ from
the other [Pt(tBu2-C∧N∧N)(CC−C6H4−R)] com-
pounds.9−15,29 The electronic absorption spectra of [Pt(tBu2-
C∧N∧N)(CC−C6H4−R)] exhibited intense absorption
bands in the 330−360 nm range and less intense bands (ε ≈
5 × 103 dm3 mol−1 cm−1) in the 400−450 nm range. The broad
low-energy bands in the 400−450 nm range were assigned as
[dπ(Pt) → π*(tBu2-C∧N∧N)] MLCT transitions with mixing
of some acetylide to diimine ligand [π(CC) → π*(tBu2-
C∧N∧N)] ligand-to-ligand charge-transfer (L′LCT) transition.
In contrast, complex 1 shows a very intense broad absorption
band at 450 nm (ε ≈ 1.9 × 104 dm3 mol−1 cm−1) and a
shoulder at 400 nm (ε ≈ 1.35 × 104 dm3 mol−1 cm−1). The
strongest absorption band found at 450 nm was tentatively
ascribed to ligand-centered (1L′C) transitions, while the
shoulder was assigned as a ML′CT (metal-to-ligand charge
transfer to the alkynyl ligand).12,15
Not only does compound 1 display unusual absorption
features, but excitation into these bands does not result in
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Figure 2. Changes in photoluminescence intensity of 1 (2.0 × 10−5 M in CH2Cl2) upon the addition of cyanide ((0−2.4) × 10−5 M n-Bu4NCN).
Inset shows luminescence titration of 1 (2.0 × 10−5 M) with n-Bu4NCN in CH2Cl2 at 293 K. [n-Bu4NCN] = (0, 0.06, 0.1, 0.2, 0.3, 0.6, 0.8, 1.0, 1.2,
1.6, 2.0, 2.2, 2.4) × 10−5 M; λex = 413 nm.
luminescence. We assume these characteristics to be caused by
the presence of the dicyanovinyl fragment as a strong electron-
withdrawing group (Hammett constant, σp = 0.84),30 which
makes the ML′CT transfer toward the electron-deficient
ancillary acetylide ligand the lowest.12,15
Concomitantly, the luminescent intensity (λem = 590 nm; λex
= 413 nm) is enhanced in response to the increase in the
concentration of the added CN− ion in dichloromethane at
room temperature (Figure 2). This emission (τ = 500 ns) may
3
be due to emission from the MLCT state related to the
[Pt(tBu2-C∧N∧N)(CC−C6H4−R)] acetylide system. To
examine the sensitivity of 1 toward CN−, its detection limit
was evaluated. As shown in Figure 2 (inset), the emission
titration profile of 1 (2.0 × 10−5 M) with CN− demonstrated
that CN− could be detected at least down to 6 × 10−6 M, and
the emission intensity of 1 increased linearly with the
concentration of CN− ((0−2.0) × 10−5 M) (R2 = 0.9972).
To confirm the formation of the anionic species 2 (Scheme
2), the addition of cyanide anions was monitored by NMR. In
the 1H NMR (CD2Cl2) spectrum of 1 (Figure 3a), the
dicyanovinylic CH proton signal is at 7.60 ppm. Upon addition
of cyanide anions, 1.0 equiv, this signal disappeared, while a
new singlet at 4.27 ppm was observed (Figure 3b). This singlet
Addition of Cyanide Anions. The cyanide anion is
strongly nucleophilic; this property has been used advanta-
geously in the design of fluorescent and colorimetric chemo-
sensors.7,31−37 Here, the nucleophilic addition reaction of
cyanide to the α-position of the dicyanovinyl fragment was
expected to result in the formation of the anionic species 2
(Scheme 1). This addition would not only reduce the extent of
conjugation but also make the ancillary acetylide ligand less
electron deficient. The anion binding affinity of 1 was thus
evaluated by monitoring its UV−vis and steady-state emission
properties as a function of anion concentration. Preliminary
investigations revealed that the addition of cyanide anions, as
tetra-n-butylammonium salts in CH2Cl2 to 2.0 × 10−5
M
−
is consistent with the formation of a CH(CN)−C(CN)2
solutions of 1 caused significant changes to the UV−vis and
steady-state emission properties. In control experiments, the
addition of other anions (10 equiv of fluoride, chloride,
dihydrogenphosphate, and acetate as tetra-n-butylammonium
salts) did not have a marked effect on the absorption spectrum
and emission of 1. Finally, spectroscopic and luminescent
responses of 1 toward hydroxyl ion (as tetramethylammonium
hydroxide) as another strong nucleophile were studied. These
control experiments clearly indicated that 1 does not respond
to hydroxyl ion (see Figures S1 and S2, Supporting
Information).
fragment. In parallel, two doublets in the aromatic region at
Figure 1 presents the changes in the UV−vis absorption
spectrum of 1 as a function of CN− concentration. The low-
lying ML′CT absorption and the ligand-based absorptions
decrease monotonically throughout the addition with saturation
observed toward the end of the titration. Two new broad low-
energy bands (λ = 420 nm, ε ≈ 4.6 × 103 dm3 mol−1 cm−1; λ =
452 nm, ε ≈ 3.9 × 103 dm3 mol−1 cm−1) were assigned as the
[dπ(Pt) → π*(tBu2-C∧N∧N)] (MLCT) and [π(CC) →
π*(tBu2-C∧N∧N)] (L′LCT) charge-transfer transitions of
complex 2. The titration reaction curve of 1 toward the CN−
ion was investigated as shown in Figure 1 (inset).
1
Figure 3. Partial H NMR (CD2Cl2, 300 MHz) spectra of 1 (a) and
that upon titration of 1 equiv of cyanide ions (as n-Bu4N+ salt) (b).
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withdrawing dicyanovinylic unit.15 Other LUMOs are mainly localized
(about 93% (LUMO + 1) and 96% (LUMO + 2)) on the bipyridine
part of the C∧N∧N moiety (ligand L).
7.81 (b) and 7.54 ppm (c) are upfield shifted at 7.42 and 7.34
ppm (b′ and c′, respectively). These doublets were attributed
to the aromatic protons of the phenylacetylide ligand, which are
likely to be significantly affected upon addition of the cyanide
anion to the dicyanovinyl moiety. On the other hand, the
signals of the phenyl-bipyridine ligand were weakly affected.
The hypothesis that the observed changes would be caused by
the nucleophilic addition of cyanide is also supported by in situ
13C NMR and DEPT experiments (CDCl3): 13C signals at 37.2
and 17.6 ppm in 2 can be tentatively attributed to the CH and
C− carbon atoms of the CH(CN)−C−(CN)2 fragment, while
the presence of three CN carbon atoms is confirmed (122.7,
121.3, and 121.2 ppm) (Figures S3 and S4, Supporting
The HOMO of 2 is fully localized on the H(CN)C−C(CN)2
anionic moiety of the acetylide ligand L′ resulting from the addition of
CN−. This anionic moiety is clearly the strongest negative/electron-
donating part of the complex. The other HOMOs of 2 are
combinations of the Pt(II) orbitals (16% (HOMO − 1) and 28%
(HOMO − 2)) and L′ moiety orbitals (79% (HOMO − 1) and 63%
2
(HOMO − 2)). An almost pure dz orbital is present for both
compounds in low-lying occupied frontier MOs (HOMO − 3 of 1 and
HOMO − 4 of 2). The latter MOs could be involved in high-energy
excitations.
The LUMO, LUMO + 1, and LUMO + 2 of 2 are localized on the
C∧N∧N ligand L. After addition of cyanide anions, the modified
acetylide ligand cannot compete with the electron-withdrawing
character of the bipyridine moiety: this leads to a drastic change of
the electron distribution in the lowest unoccupied frontier MOs from
1 to 2 (Table 1).38
Table 2 provides the computed excitation energies, wavelengths,
and oscillators strengths, as well as the λmax of the bands of the
simulated and experimental absorption spectra. The S0 → Sn
excitations exhibiting a non-negligible oscillator strength are reported
in this table. The corresponding electronic transitions and their
weights in the excitations are given in the last column of the table. The
calculated absorption spectra for 1 and 2 are displayed in Figure 5. A
good agreement with the observed UV−visible absorption spectra of
the two complexes is found, permitting a detailed assignment of the
observed bands.
1
Information). DFT computations of the H and 13C chemical
shifts fully confirm these assignments.
Computational Results. The computed 13C NMR spectrum
confirms the experimental assignments; in particular, the cyanovinylic
carbon atom, C#29 (numbering of atoms and computed chemical
shifts in Supporting Information) is found to experience a strong
chemical shift from 71 ppm (carbon sp2 hybridization, 1) to 25 ppm
(sp3 hybridization, 2), while C#28 is expected to shift from 171 (CH,
sp2 hybridization) to 46 ppm (CH, sp3 hybridization), upon addition
of CN−. These values are in good agreement with the experimental
ones (from 78.8 to 13.6 ppm for C#29 and from 159.1 to 58.8 ppm for
C#28).
The frontier molecular orbitals (MOs) of complexes 1 and 2 are
dramatically different (Figure 4). The highest occupied MOs
The simulated absorption spectrum of 1 exhibits an intense band at
467 nm and another one at 336 nm probably corresponding to the
observed bands at 450 and 330 nm. Moreover, a very slight shoulder is
present at 408 nm, which is likely to correspond to the experimental
one at 400 nm. The highest computed wavelength of complex 1 at 467
nm (oscillator strength 1.37) corresponds to the HOMO → LUMO
transition and is 1L′L′CT/ML′CT in character (L′ = acetylide ligand;
L = C∧N∧N). It can be also assigned as an intraligand (IL)/ML′CT
transition. Computed transitions at 408 nm corresponding to
conventional ML′CT/L′LCT transitions should contribute little to
the absorption spectra because of their small oscillator strength
(0.025). The second transition of significance (oscillator strength
0.25) at 336 nm is a mixed LLCT/MLCT from HOMO − 4, which
has a slight metallic character.
For complex 2, the calculations reveal a superposition of different
types of excitations (IL, ligand−ligand charge transfer, MLCT) in the
UV range. The lower energy excitations in the visible region appear at
529, 475, and 431 nm for 2, leading to a large absorption band at λmax
roughly around 440 nm. These excitations do not lead to intense
bands in the absorption spectra due to their small oscillator strengths.
This is in line with the discoloration of the solution experimentally
observed upon addition of cyanide anions (Figure 1). The computed
bands at 475 and 431 nm (corresponding to the observed bands at 452
and 420 nm) involve several transitions from HOMO and HOMO − 1
to LUMO and LUMO + 1 and correspond to combined [π(CC) →
π*(C∧N∧N)] ligand-to-ligand charge-transfer (L′LCT) and [dπ(Pt)
→ π*(C∧N∧N)] metal-to-ligand charge transfer (MLCT) transitions,
while the computed transition at 529 nm (HOMO to LUMO)
corresponds to a L′LCT transition with no metal character. It could
correspond to the tail observed in the experimental spectra from 500
to 575 nm. A higher energy band is computed at 349 nm (330 nm
experimentally) that involves several excitations, is more intense than
the first ones, in agreement with experiment, and is a mix of MLCT/
LLCT and L′LCT transitions.
Figure 4. MO diagrams of 1 and 2.
(HOMOs) of 1 are combinations of Pt(II) orbitals (15% (HOMO),
29% (HOMO − 1)) and L′ moiety orbitals (80% (HOMO), 52%
(HOMO − 2)) or the phenyl-bipyridine ligand (5% (HOMO), 71%
(HOMO − 1), 13% (HOMO − 2)); see Table 1 where the percentage
weights of Pt, L, and L′ ligands in frontier MOs are given.
Interestingly, the lowest unoccupied MO (LUMO) of complex 1 is
mainly localized on the acetylide ligand L′. This is consistent with a
strong stabilization of this LUMO due to the presence of the electron-
Table 1. Percentage Weights of Pt/L/L′ in Frontier MOs
H − 4
H − 3
H − 2
H − 1
H
L
L + 1
L + 2
L + 3
1
2
8/90/2
91/7/2
88/10/2
32/68/0
35/13/52
28/9/63
29/71/0
16/5/79
15/5/80
1/0/99
1/1/98
5/92/3
5/93/2
3/96/1
3/96/1
1/99/0
1/99/0
8/13/79
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Table 2. TD-DFT Calculated Excitation Energies, Computed and Experimental Wavelengths and Weights of Transitions for
Complexes 1 and 2
complex
energy, eV; λ, nm (oscillator strength)
excited states
λmax, calcd [exp], nm
transitions (% weight)
H → L (+84%)
1
2.66; 467 (1.3679)
3.04; 408 (0.0250)
3.69; 336 (0.2540)
2.34; 529 (0.0647)
2.61; 475 (0.0719)
2.88; 431 (0.0707)
3.34; 371 (0.1266)
3.41; 364 (0.0904)
3.67; 338 (0.2015)
S0 → S3
S0 → S6
S0 → S13
S0 → S1
S0 → S2
S0 → S5
S0 → S8
S0 → S9
S0 → S13
467 [450]
[400]
H − 3 → L (+56%); H − 2 → L + 1 (38%)
H − 4 → L + 1 (+85%); H − 2 → L + 2(+9%)
H → L (+89%); H − 1 → L (+10%)
336 [330]
440 [452] [420]
2
H − 1 → L (+86%); H → L (11%)
H → L + 1 (+68%); H − 3 → L (+25%)
H − 2 → L + 1 (+80%); H − 3 → L + 1 (11%)
H − 3 → L + 1 (+85%); H − 2 → L + 1 (+10%)
H − 5 → L (+87%); H − 4 → L + 1 (6%)
349 [330]
Figure 5. Computed absorption spectra of 1 and 2.
Consequently, the observed changes in the absorption spectrum of
1 with addition of cyanide anions to 1 corresponds to a switch of
charge transfers from L′L′CT/ML′CT in 1 (L′ = acetylide ligand) to
L′LCT/MLCT in 2 (L = C∧N∧N). The HOMO → LUMO transition
in 1 is mainly contributed from an electron excitation from the π
orbital of the alkynyl ligand to the π* orbital of the alkynyl ligand
leading to a nonemissive ligand-to-ligand charge-transfer excited state.
The switch-on luminescent response to cyanide is attributed to the
change in the lowest energy excited state of the system from this
nonemissive ligand-to-ligand charge-transfer excited state to a
phosphorescent metal-to-ligand charge-transfer excited state.
Using TD-DFT, we computed the phosphorescence wavelength
from the triplet state arising from the above-mentioned transition of 2.
The obtained value (645 nm) is close to the experimental one (590
nm). These computed phosphorescence wavelengths correspond to
emission from a triplet state with a main component HOMO − 1 to
LUMO excited configuration, the LUMO being mainly localized on L
for complex 2. The nature of the lowest energy phosphorescence is
thus attributed to a combination of 3L′LCT and 3MLCT transitions.15
The differences observed in the spin density plots obtained via
unrestricted DFT computations of the T1 triplet states (Figures S5 and
S6, Supporting Information) of 1 and 2 agree with their contrasted
emission properties. Indeed, a predominant localization of the spin
density on the metal and on the L ligand of Pt(II) cyclometalated
species is, in most cases, needed to warrant emission from the triplet
states.39,40 We observe first that the metal spin population in the T1
state is slightly higher for the anionic species 2, that is, 0.16 vs 0.11 for
1. More significantly, the spin density plots show definitely different
contributions of the alkynyl ligand L′ and of the cyclometalating ligand
L for 1 and 2: the sums of the atomic spin populations of ligand L are
equal to 0.94 for the emissive species 2 and equal to 0.05 for the
nonemissive species 1, so that the unpaired spins are mainly localized
on the L′ moiety for this last species and not on L. The T1 state is thus
essentially L′L′CT in character with a small admixture of MLCT for 1.
By contrast, for 2, the unpaired spins extend over a large spatial region
of this anionic species, including the metal, the acetylide ligand L′, and
particularly the bipyridine moiety of the L ligand, which is consistent
with a luminescence due to combination of 3L′LCT and 3MLCT
transitions.
Sensing Properties in a Water−Dichloromethane
Biphasic System. Compound 1 has low solubility in water,
which, in principle, could be advantageous in the development
of an anionic sensing set based on a methylene chloride−water
biphasic system. The rationale here is based on the fact that the
1:1 stoichiometric nucleophilic addition of cyanide to vinyl-
substituted derivatives is irreversible and can be performed in
aqueous medium.35,41 It must be stressed that since an
irreversible chemical reaction is used to produce the
colorimetric response, compound 1 acts as cyanide-specific
anion chemodosimeter42−44 and will be subject to all the
caveats that the use of a biphasic system implies (i.e., mass
transfer limitations, fastness and intensity of response being
potentially dependent on concentrations of both CN− and 1,
and hydration of anions). Addition of tetra-n-butylammonium
cyanide (10 equiv) to the aqueous phase of a methylene
chloride−water biphasic system containing 1, followed by
stirring for several minutes, led to a partial discoloration in the
organic phase that became slightly emissive (λem ≈ 590 nm; λex
= 413 nm) corresponding to the cyanide adduct 2. Control
experiments of the responses of 1 toward other anions did not
lead to any detectable visual and spectral changes. Addition of
NaCN (10 equiv in water) did not result in appreciable
changes, but the subsequent addition of tetra-n-butylammo-
nium chloride (10 equiv) to this aqueous phase, followed by
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(7) Yang, Y.; Zhao, Q.; Feng, W.; Li, F. Chem. Rev. 2013, 113, 192−
stirring for 2 min, resulted in the discoloration of the methylene
chloride solution (see Figures S7 and S8, Supporting
Information). These proof of concept experiments suggest
that complexes such as 1 open up the possibility for producing
novel phosphorescent chemodosimeters, with specific advan-
tages. First, the off−on phosphorescent sensor 1/2 exhibits a
significant Stokes shift (almost 150 nm, ∼5800 cm−1) for easy
separation of excitation and emission. Second, its emission
lifetime (τ = 500 ns) can easily discriminate signal from
interference by fluorescent compounds. Finally, it shows high
photostability and pH-independent emission (see Figure S9,
Supporting Information) and open up the possibility of
approaches to sensing based on the use of irreversible reactions.
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CONCLUSION
■
In summary, this investigation has led to the development of a
highly selective and sensitive chemodosimeter for detecting
micromolar concentrations of CN− ions. Compound 1, which
serves as the basis of the detection system, reacts irreversibly
with CN− in a 1:1 stoichiometric manner, a process that
induces a large enhancement in the phosphorescence intensity.
DFT and TD-DFT computations explain very well the
switching of luminescence upon cyanide addition. Indeed, it
has been shown that the frontier MOs that are involved in the
absorption spectra of the neutral 1 and anionic 2 species are
drastically different; only species 2 exhibits a MLCT/L′LCT
excitation needed to ensure phosphorescence. Finally, the
selectivity of this system for CN− over other anions is extremely
high. The strategy for the sensor design presented here may
thus contribute to the development of more efficient “Off−On”
anion phosphorescent sensors.
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ASSOCIATED CONTENT
* Supporting Information
■
S
13C NMR spectra (CDCl3) of 1 and 2, DFT optimized
coordinates, optimized structures of 1 and 2, most
representative computed 13C NMR chemical shifts, spin density
plots of the triplet states, control experiments with other
anionic salts, and tests of sensitivity to pH. This material is
available free of charge via the Internet at http://pubs.acs.org.
(24) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jean-luc.fillaut@univ-rennes1.fr (J.-L.F.), abdou.
boucekkine@univ-rennes1.fr (A.B.).
2002, 117, 43−54.
■
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computing Centre: Lugano, Switzerland.
Notes
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The authors declare no competing financial interest.
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(28) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis;
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ACKNOWLEDGMENTS
■
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The authors are grateful to GENCI-IDRIS and GENCI-CINES
for an allocation of computing time (Grant No. 2011-080649).
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