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Luminescent water-soluble cycloplatinated complexes: Structural, photophysical, electrochemical and chiroptical properties
Accepted Manuscript Research paper Luminescent water-soluble cycloplatinated complexes: structural, photophysical, electrochemical and chiroptical properties Andreea Ionescu, Nicolas Godbert, Loredana Ricciardi, Massimo La Deda, Iolinda Aiello, Mauro Ghedini, Isabella Rimoldi, Edoardo Cesarotti, Giorgio Facchetti, Giuseppe Mazzeo, Giovanna Longhi, Sergio Abbate, Marco Fusè PII: DOI: Reference:
S0020-1693(16)30592-8 http://dx.doi.org/10.1016/j.ica.2017.02.026 ICA 17452
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
13 October 2016 12 January 2017 24 February 2017
Please cite this article as: A. Ionescu, N. Godbert, L. Ricciardi, M. La Deda, I. Aiello, M. Ghedini, I. Rimoldi, E. Cesarotti, G. Facchetti, G. Mazzeo, G. Longhi, S. Abbate, M. Fusè, Luminescent water-soluble cycloplatinated complexes: structural, photophysical, electrochemical and chiroptical properties, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.02.026
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1 2 3
Luminescent water-soluble cycloplatinated complexes: structural, photophysical, electrochemical and chiroptical properties
4 5 6 7 8 9 10 11 12 13
Andreea Ionescu,*a Nicolas Godbert,a,b Loredana Ricciardi,b Massimo La Deda,a,b Iolinda Aiello,a,b Mauro Ghedini,a,b Isabella Rimoldi,c Edoardo Cesarotti,c Giorgio Facchetti,c Giuseppe Mazzeo,d Giovanna Longhi,d,e Sergio Abbate,d,e Marco Fusè* c,f a
MAT-INLAB(Laboratorio di Materiali Molecolari Inorganici), LASCAMM and CR-INSTM, Unità INSTM della Calabria, Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, 87036 Arcavacata di Rende (CS),Italy b CNR NANOTEC-Istituto di Nanotecnologia U.O.S. Cosenza, 87036 Arcavacata di Rende (CS), Italy c Dipartimento di Scienze Farmaceutiche, Università di Milano Via Golgi 19, 20133 Milano, Italy
14
d
15
25123 Brescia, Italy
16
e
17
Vasca Navale, 84, 00146 Roma, Italy
18
f
19
*Corresponding authors: Andreea Ionescu ([email protected]),
Dipartimento di Medicina Molecolare e Traslazionale, Università di Brescia, Viale Europa 11,
CNISM Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Via della
current address: Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
Marco Fusè ([email protected])
20 21 22
Abstract
23
The luminescent square-planar Pt(II) complexes of the type [Pt(C^N)(N^N)]+X- have been
24
synthesised, where (C^N) is cyclometallated 2-phenylpyridine ligand, (N^N) a chiral diamine and
25
BF4- or CH3COO- as counterion (X-) have been synthesised. The chiral diamine Campy has been
26
used, resulting in a mixture of regioisomers. The complexes were characterised and their structures
27
in solution were determined by a combination of different spectroscopic and chiroptical techniques
28
(NMR, UV-Vis, ECD, IR, VCD and CPL) associated with electrochemical, DFT and TDDFT
29
studies. Remarkably, the acetate derivatives proved to be soluble and stable in water for at least a
30
month. The electrochemical behaviour of the [Pt(C^N)(N^N)]BF4 was investigated and the LUMO
31
energies were estimated. The photophysical properties of the new compounds were deeply
32
investigated in water or dimethyl sulfoxide solution by UV-Vis, steady state emission and time-
33
correlated single-photon counting spectroscopy.
34 35
Keywords: Luminescence, Pt(II) Chiral Complexes, Electronic Circular Dichroism , Vibrational
36
Circular Dichroism, Circularly Polarized Luminescence
37 38 1
1
1. Introduction
2 3
Photoactive ionic transition-metal complexes find extensive application in optoelectronics [1]-[4]
4
and bio-related fields [5],[6] especially for the high photoluminescent quantum yields they display,
5
for their long-lived excited states and for the possibility of tuning their emission colour across the
6
whole UV-vis-near IR range.[7]
7
Among the most studied luminescent complexes, square-planar Pt(II) complexes exhibit a
8
noteworthy tendency to form self-assemblies by efficient stacking through metallophilic
9
interactions. These assemblies may result in enhanced properties as recently reviewed for imaging
10
applications.[8] In this regard but also for all bio-related applications, water soluble highly
11
luminescent Pt(II) complexes may play an important role.
12
The photophysical and photoconductive properties of Pt(II) complexes are influenced by the ligand
13
field and thus can be modulated via ligand exchange and/or substitution.[9],[10] Although Even if
14
the synthesis and the study of properties of luminescent cyclometallated neutral [11]-[13] or ionic
15
Pt(II) complexes embedded with tridentate ligands [14]-[16] had been already reported, fewer
16
cationic
17
characterized.[17],[18]
18
functionalization by varying either the (N^N) ligand and the counterion separately, in order to tune
19
both luminescence and solubility. Indeed, water solubility can be achieved for Pt(II) complexes by
20
selecting specific counterions [19],[20] and/or introducing hydrophilic or charged groups onto the
21
ligand molecular structures.[21] However, the pool of water-soluble Pt(II) complexes comprises
22
few examples so far.
23
Since chirality is an important issue in bio-related systems, the design of synthetic pathways leading
24
to enantiomerically pure complexes is highly relevant. In this context, we present a series of novel
25
chiral cyclometallated luminescent Pt(II) complexes (Fig. 1). These are square-planar complexes
26
bearing a 2-phenylpyridine (ppy) fragment as a (C^N) cyclometallated ligand and a chiral (N^N)
27
ligand 8-amino-5,6,7,8-tetrahydroquinolines (Campy).
Pt(II)
complexes
of
the
type
[Pt(C^N)(N^N)]X
Such complexes feature
28 29 30 31 32 33 34 2
the
advantage
were
synthesized
of
potential
a
and
double
1 2 3 4 5
Figure 1. Molecular structures and the two possible regioisomers of the synthesised complexes
6
(trans on the left, cis on the right, relative to the pyridine rings).
7 8 9
The positive charge of complexes 1 is balanced by a BF4- counterion, chosen in order to favour
10
solubility in organic solvents. Water solubility was induced instead in complexes 2 by introducing a
11
CH3COO- counterion, analogously to the already reported chiral ionic Ir(III) complexes.[22]
12
Although the strong trans-influence of the C-donor is expected to direct the reaction towards the
13
formation of the cis isomer,[23] the introduction of a dissymmetric chelating ligand could give rise
14
to two possible regioisomers with the pyridine of the ppy ligand and that of the diamine ligand in
15
trans or in cis position.
16
Furthermore, complexation with sterical hindered bidentate ligands leads to a distortion of the
17
square planar geometry in a helical shape and thus introducing a second chirality at the metal
18
centre.[23],[24] Helical chirality can be described by ∆/Λ notation (Fig. 2).[25],[26] As a
19
consequence, due to the presence of an enantiopure chiral diamine ligand, each regioisomer can
20
exist in two diastereomeric forms ∆ S-N^N and Λ S-N^N.
21
In absence of the crystallographic data, the composition, the structure and the regio- and
22
diasteroselectivity was investigated by NMR spectroscopy. However this technique does not always
23
allow to assign the absolute configuration even in the presence of diastereomeric interaction;[22]
24
therefore the configuration of complexes in solution has been proven by a combination of
25
chiroptical techniques: electronic (ECD) and vibrational (VCD) circular dichroism. These
26
techniques provide complementary/supplementary information [27] and together with DFT and
27
TDDFT calculations proved to be effective in several cases.[28]-[31]
28 29 30
3
1
Figure 2. Schematic representation of the four possible isomers and their chiral descriptor in the
2
skew-lines (top) and in the oriented-skew-lines (bottom) reference systems.[25],[26] The oriented-
3
lines have been directed towards the pyridine ring of the substituent. See Table 8.SM for the
4
representation with molecular models.
5 6
In this works we present the components characterisation of an unresolved mixture of isomers via
7
several spectroscopic techniques.
8 9
2. Materials and methods
10 11
Unless otherwise stated, reagents and solvents were purchased from commercial sources and used
12
without further purification. Enantiomerically pure Campy was obtained as reported in the
13
literature.[32] All synthesis involving Pt(II) complexes were carried out under nitrogen atmosphere
14
using standard Schlenk techniques. The [(ppy)Pt(µ-Cl)]2 precursor was prepared according to
15
literature procedures.[33] 1H and 13C NMR spectra were recorded in CDCl3, CD3OD or in a mixture
16
of them on Bruker DRX Advance 300 MHz equipped with a non-reverse probe or Bruker DRX
17
Avance 400 MHz. Chemical shifts (in ppm) were referenced to residual solvent proton/carbon peak.
18
Fast Atom Bombardment Spectra (FAB) were acquired on a VG Autospec M246 spectrometer
19
using p-nitrobenzyl alcohol as the matrix. Elemental analyses (EA) were recorded on Perkin Elmer
20
Series II CHNS/O Analyzer 2400.
21 22 23 24 25
2.1. Synthesis of complexes
26 27
The complexes were synthesised by treating [(ppy)Pt(µ-Cl)]2 (50 mg, 0.065 mmol) with 1.1 eq of
28
optically pure diamine ligand in the presence of 5 eq of sodium salt (NaBF4 or NaCH3COO) in a
29
degassed solution (N2) of MeOH and irradiating this mixture with 150 W-microwave radiation in a
30
sealed vial. Temperature was prevented from rising above 65 °C by stopping irradiation and quickly
31
cooling the reaction mixture. Three irradiation/cooling cycles of 1 min were performed. Upon
32
completion, the black Pt(0) precipitate was filtered off and the solvent removed by reduced
33
pressure. The solid was then washed with diethylether and dissolved in the minimum volume of
34
CH2Cl2 and filtered. The products were obtained by evaporation of the solvent. 4
1 2
2.1.1 [(ppy)Pt((R)-Campy)]+BF4- (1)
3
Grey/white solid, yield 55 mg (72%), slightly soluble in CH2Cl2 and MeOH. Soluble in DMSO with
4
isomerisation to the major cis isomer. 1H-NMR (300 MHz, DMSO-d6) δ: 8.97 (d, J = 5.3 Hz,
5
0.25H, c2trans), 8.81 (d, J = 5.4 Hz, 1.25H, c2cis+a2trans), 8.73 (d, J = 5.5 Hz, 1H, a2cis), 8.26 -
6
8.10 (m, 2.5H, a3,a5cis+a3,a5trans), 8.08 - 7.98 (m, 1.25H, c4cis+c4trans), 7.85 - 7.74 (m, 1.5H,
7
b5cis+b5trans), 7.74 - 7.66 (m, 1H, c3cis), 7.66 - 7.60 (m, 0.25H, c3trans), 7.52 - 7.45 (m, 1H,
8
a3cis), 7.45 - 7.38 (m, 1.25H, b2cis+a3trans), 7.34 - 7.22 (m, 1H, b2trans), 7.22 - 7.11 (m, 2.5H,
9
b4, b3cis+b4, b3trans), 6.76 (m, 1H, NHa cis), 6.47 - 6.32 (m, 1H,NHb cis), 6.32 - 6.17 (m,
10
0.25H,NHa trans), 5.56 - 5.42 (m, 0.25H, NHb trans), 4.58- 4.37 (m, 1H, c8cis), 4.37 - 4.26 (m,
11
0.25H, c8trans), 2.86 (s, 2.5H, c5cis+c5trans), 2.46 - 2.33 (m, 1H, c7cis), 2.34 - 2.24 (m, 0.25H,
12
c7trans), 2.00 (m, 1.25H, c6’cis+c6’trans), 1.76 (m, 2.5H, c6,c7’cis+c6,c7’cis);
13
MHz, CD3OD) δ: 159.80, 151.29 (a2trans), 150.23 (c2trans), 149.54 (a2cis), 147.02 (c2cis),
14
145.83, 140.33, 140.25, 140.14, 136.79, 133.74, 132.64, 130.11, 129.75, 125.49, 124.75, 124.16,
15
123.85, 123.60, 123.38, 123.05, 119.73, 119.52, 59.91 (c8cis), 57.36 (c8trans), 31.86 (c7cis), 30.64
16
(c7cis), 27.44, 27.30, 21.10; MS (FAB): m/z 497[M]+; Elemental analysis: calculated C 41,11% H
17
3.45% N 7.19%; found C 40.8% H 3.43% N 7.1%.
13
C NMR (75
18 19
2.1.2 [(ppy)Pt((R)-Campy)]+CH3COO- (2)
20
Grey/white solid, yield 60 mg (83%), soluble in water. The complex results stable in H2O, less in
21
chlorinated solvents. 1H-NMR (400 MHz, CD3OD) δ: 9.04 (d, J = 5.5 Hz, 0.25H), 8.77 (d, J = 5.0
22
Hz, 1.25H), 8.71 (d, J = 5.4 Hz, 1H), 8.17 - 8.05 (m, 2.5H), 7.99 (m, 2.5H), 7.76 - 7.60 (m, 2.25H),
23
7.59 - 7.51 (m, 0.25H), 7.41 (m, 2H), 7.33 (m, 0.25H), 7.29 - 7.24 (m, 25H), 7.24 - 7.11 (m, 2.25H),
24
4.52 (m, 1H), 4.41 (s, 0.25H), 2.95 (s, 2.5H), 2.59 - 2.39 (m, 1.25H), 2.10 (m, 1.25H), 1.92 (s,
25
3.75H), 1.90 - 1.71 (m, 2.5H);
26
47.48% H 4.17% N 7.55%; found C 47.68% H 4.21% N 7.3%.
13
C NMR mirrored that of 1. Elemental analysis: calculated C
27 28 29
2.2 Electrochemical Measurements
30
Electrochemical measurements were carried out by cyclic voltammetry, performed with an Autolab
31
Potentiostat-Galvanostat controlled by the NOVA 1.1 software. A conventional 3 mL three-
32
electrode cell was employed, with a Pt wire as counter-electrode, an Ag wire as pseudo-reference
33
electrode and a Pt disk as working electrode. NBu4PF6 (0.1 M) was used as supporting electrolyte
34
and experiments were performed in a dry, and degassed (Ar) dimethylformamide solution. 5
1 2
2.3. Photophysical Measurements
3
Spectrofluorimetric grade solvents were used for the photophysical investigations in solution
4
without further purification. A Perkin Elmer Lambda 900 spectrophotometer was employed to
5
obtain the UV/Vis absorption spectra, using quartz cuvettes of 1 cm path length. Steady-state
6
emission spectra were recorded on a Horiba Jobin Yvon Fluorolog 3 spectrofluorimeter, equipped
7
with a Hamamatsu R-928 photomultiplier tube. The luminescence quantum yields were determined
8
using the optical dilution method [34] using Ru(bpy)3Cl2 in air-equilibrated water solution as a
9
reference standard (Φ= 0.028).[35] Solutions were degassed by bubbling argon into quartz cells
10
prior to measurements. Time-resolved measurements were performed using the time-correlated
11
single-photon counting (TCSPC) on the Fluorolog-3 apparatus. A pulsed NanoLED centred at 379
12
nm (FWHM 750 ps with 1 MHz repetition rate) was used as excitation source and fixed directly on
13
the sample chamber at 90° to a single-grating emission monochromator (2.1 nm/mm dispersion;
14
1200 grooves/mm). Data analysis was performed using the commercially available DAS6 software
15
(HORIBA Jobin Yvon IBH). The quality of the fit was assessed by minimizing the reduced χ2
16
function.
17 18
2.4. Electronic Circular Dichroism (ECD) Spectra
19
Spectra were recorded with a Jasco 815SE spectrometer employing 2 mm quartz cells; with CHCl3
20
or H2O solutions in concentration range from 0.23 mM to 0.26 mM. Spectra were recorded at 25
21
°C.
22 23 24 25
2.5. VCD and IR (Vibrational Absorption = VA) Spectra
26
All spectra were recorded with a Jasco FVS6000 VCD spectrometer with liquid N2-cooled MCT
27
detector. Spectra were recorded at room temperature (25 °C) in CDCl3 or DMSO-d6 solutions in the
28
range 0.037-0.1 M concentration, in 200 mm BaF2 cells. In some cases a drop of CD3OD was added
29
to the solution, for better solubilization of the sample. Repeated sets of 5000 scans were taken for
30
each sample and series of 5000 scan-average spectra were co-added, if necessary; subtraction of
31
solvent signals was performed in VA and VCD spectra.
32 33
2.6. Circularly Polarized Luminescence (CPL) Spectra
6
1
Spectra were recorded with a home-built apparatus described in ref. [36]-[38] with excitation
2
radiation brought in through an optical fibre from a Jasco FP8200 fluorimeter (90° geometry). The
3
incident light is polarized parallel to the exit beam direction. Twenty scans were accumulated for
4
each spectrum. The excitation wavelength has been 367 nm. All spectra were taken at 25 °C.
5 6
2.7 Computational Details
7
DFT calculations were carried out with Gaussian package.[39] The four diastereomers were
8
modelled starting from the crystallographic structure of the diamine of previous reported
9
complexes,[40] then the ligand was coordinated to Pt(II) centre in a square planar geometry with an
10
ortho-metallated ppy as a second coordinated ligand. The conformation of the C6-ring of the
11
tetrahydroquinoline moiety was also taken into account. The most stable conformation was
12
confirmed to be a half-chair on C(7), other possible geometries (boat or half chair on C(6))
13
converged to the C(7) half-chair during the optimization processes.
14
We screened four promising functionals (B3LYP,[41]-[43] B3PW91,[42]-[45] MPW1PW91,[45]-
15
[47] PBE0,[48],[49]) with two basis sets, LANL2DZ and Pople basis sets and pseudo potential on
16
Pt atom. Bulk solvent effects (CHCl3, H2O, DMSO) were taken into account using the polarizable
17
continuum model (PCM),[51]-[52] the same solvents used in the experiments were retained in the
18
calculations.
19
The analysis was only conducted on the cationic moiety of the molecules and results were
20
compared with the experimental data of BF4– salts in DMSO or CHCl3, assuming the interaction of
21
the counterion to be negligible.
22
Calculation of Dipole and Rotational Strengths through the field-response approach[53] was carried
23
out; VA and VCD spectra were generated by assigning 5 cm-1 bandwidth Lorentzian bandshape to
24
each vibrational transition. The scaling factor 0.975 was applied to the calculated frequencies.
25
Time-Dependent DFT (TDDFT) calculations have been employed, within the same Gaussian
26
package, to obtain ECD spectra: the same level of theory was adopted and 100 excited states were
27
admitted. Spectra were then plotted by associating Gaussian bands to each transition with 0.15 eV
28
bandwidth. Percentage composition of the molecular orbital (MO) was computed using the
29
GaussSum3 program.[54]3. Results and discussion
30 31
The newly synthesized complexes were fully characterized by a combination of different
32
spectroscopic and chiroptical techniques (NMR, UV-vis, ECD, IR, VCD and CPL) associated with
33
electrochemical, DFT and TDDFT studies chiroptical tecniques. For the sake of brevity, the
34
electrochemical results are reported in the supplementary information. 7
1 2
3.1. Synthesis
3 4
The complexes were synthesized starting from the binuclear complex [(ppy)Pt(µ-Cl)]2 via an
5
adapted microwave assisted procedure reported in literature.[33] In order to force the counterion
6
outside the coordination sphere, chlorine was replaced via metathesis reaction including fivefold
7
times sodium tetrafluoroborate or sodium acetate in the reaction medium. The choice to synthesize
8
acetate derivatives was addressed to increase the solubility in the aqueous medium.[55]
9
While complexes 1 were only slightly soluble in CH2Cl2, MeOH, in a mixture CH2Cl2 :MeOH 3:1
10
and DMSO, the acetate complexes 2 presented a good solubility also in water. The obtained
11
complexes were fully characterized by NMR, FAB spectrometry and EA. The reaction of the
12
binuclear Pt(II) complex with enantiopure Campy gave rise to a mixture of two complexes, which
13
were identified as the two cis/trans regioisomers by NOESY experiments. The major product was
14
the cis isomer as pointed out by NOE-crosspeak between the two ortho protons of the pyridine
15
rings. In the case of the minor isomer, the trans one, the key NOE correlation was between the
16
ortho-hydrogen of the Campy pyridine ring and that of the phenyl one (Figure 3.ESI).
17
The counterion was found to influence the ratio between the two regioisomers, indeed when BF4-
18
was used as counterion the reaction results in a 60:40 mixture, whereas when the reaction was
19
performed in presence of CH3COO- the ratio was found to be 75:25. The polar coordinating solvent
20
DMSO was able to move the isomerisation equilibrium towards the prevailing cis form, reaching
21
the final ratio of 85:15.
22
Thereafter, for the sake of brevity, the cis/trans isomeric mixture of the S-Campy complexes will be
23
reported as product 1 for the BF4- or product 2 for the CH3COO- salts respectively.
24
3.2 Electrochemistry
25 26
The electrochemical behaviour of complexes 1 was investigated by cyclic voltammetry in a ca. 0.1
27
M NBu 4PF6 dry DMF electrolytic solution using an Fc/Fc+ redox couple as internal standard.
28
Since the electrochemical activity is displayed just by the Pt(II) cation, only complexes 1 were
29
studied. This choice was due to their better solubility in organic solvent such as DMF, allowing a
30
wider electrochemical potential window than water. Reduction and oxidation potentials and
31
estimated LUMO levels are reported in Table 2.SM together with literature data of similar
32
[(ppy)Pt(N^N)]+ cations with (N^N) = ethylenediamine (en), and 2,2’-bipyridine (bpy) and 1,10-
33
phenanthroline (phen),[56] for direct comparison.
8
1
First, two irreversible reduction waves are observed when applying negative potentials. For cationic
2
Pt (II) complexes, reversible or quasi reversible reduction waves were expected to be ligand centred
3
involving the π*-orbitals of the pyridine rings.
4
As previously concluded on similar [(ppy)Pt(N^N)]+ cations, the first reduction wave is attributed
5
to the reduction of the pyridine ring of the cyclometallated ligand and the second wave to the
6
reduction of the pyridine fragment on the ancillary N^N ligand. In agreement with this hypothesis,
7
the LUMO resulted predominantly localized onto the cyclometallated ligand.
8
Secondly, concerning oxidation, a single irreversible wave was recorded and assigned to the
9
oxidation of the Pt(II) metal centre, leading to an unstable species in turns stabilized by chemical
10
reaction.[56] This behaviour prevents the estimation of the HOMO energy by electrochemical
11
measurements, since the oxidation induces the complex degradation.
12 13
3.3. Photophysical properties
14 15
The photophysical properties of the complexes were investigated in H2O or DMSO solution at 298
16
K. The electronic absorption spectra of the compounds (Fig. 3 and Table 1) were dominated,
17
regardless of the solvent, by bands in the UV region arising from LC (π-π*) transitions involving
18
the cyclometallated ligand. At low-energy (350-410 nm) an unresolved and less-intense absorption
19
band was present, that could be assigned to a spin allowed MLCT transition. Bands ascribable to the
20
ancillary ligand were excluded by comparison with the absorption spectrum of the related complex
21
[(ppy)Pt(en)]+,[56] in which all features were MLCT or ppy-located transition.
22 23 24
27 28 29 30 31 32
35000
b) 1. DMSO
30000 25000 20000 15000 10000
7000
5000 4000 3000
1000 300
325
350
375
400
0 225 250 275 300 325 350 375 400 425
425
Wavelength (nm)
Wavelength (nm)
Figure 3. Absorption spectra of complexes 1-2, the first one in DMSO the second one in H2O.
33 34
2. H2O
2000
5000 0 275
8000
6000
ε M -1cm-1
26
a) ε M-1cm-1
25
Table 1. Photophysical properties of complexes 1 and 2 9
1
Complex
Solvent
1
DMSO
2
H2O
a
Absorption -1 -1 λ max/nm (ε/M cm ) 279(32691)-303(sh)-327(12707)340(9929)-378(4008) 235(7389)-251(6558)-272(6313)299(sh)-311(3018)-323(2979)-368 (774)
Emission λ max/ nm 486, 520, 553 481, 513, 548
Ф(%)
Lifetime τ(α)/ns(%) 12.1 (20.42%), 47.5 (79.58%) 53.1 (28.69%), 268.3 (71.31%)
0.7 0.6
Фa(%) 1.4 0.7
a
Lifetime τ(α)/ns(%) 85.1 (2.55%), 1906.2 (97.45%) 72.6 (28.10%), 378.8 (71.90%)
deaerated solution
2 3
The absorption spectrum of BF4- compounds 1 slightly differed from the analogous ones with
5
CH3COO-, in particular for the extinction coefficient and the band position: bands of 1 (dissolved in
6
DMSO) were red-shifted and less intense than those of 2 (dissolved in water). To discriminate
7
between the effects of either solvent or counterion, absorption spectrum of 2 was recorded in
8
DMSO solutions (Fig. 8.SM); while the band shift was attributed to a solvatochromic effect, it
9
could be concluded that the replacement of BF4- by CH3COO- as the counterion led to a lower
10
extinction coefficient. This feature could be due to a greater proximity and interaction of cations
11
and anions in acetate pair compared to BF4-, leading to a change in the dipole moment on going
12
from the ground to the excited state, reducing the electronic transition probability.
13
All complexes were phosphorescent, showing similar emission bands with a well-defined vibronic
14
structure (Fig. 4). The emission intensities were weak in all cases, with luminescence quantum
15
yields, ranging from 0.7% in degassed water to 2.95% in degassed DMSO (Table 2). Luminescence
16
was most efficiently quenched by oxygen in the latter solvent, a fact which could be explained on
17
account of the different oxygen concentration in the two solvents (0.46 mmolL-1 in DMSO vs 0.29
18
mmolL-1 in water).[57] Excited state decay of all complexes displayed bi-exponential kinetics
19
(Table 1), which could be attributed to the presence of dimeric aggregates in solution[58]
20
responsible for the short lifetimes.
21 22 23 24 25 26
Emission Intensity (a. u.)
4
60
1. DMSO 2. H2O
50 40 30 20 10 0 500
27 28
550
600
650
700
Wavelength (nm)
Figure 4. Emission spectra of complexes 1-2 in DMSO and H2O solution (4·10-5 M)
29 30 10
1
3.4. Experimental and calculated ECD
2 3
Chiroptical methodologies are the only experimental techniques capable of directly probing
4
molecular chirality and thus in order to assign the absolute configuration of the complexes, a
5
chiroptical study comprised of ECD, VCD and CPL spectroscpy was carried out.
6
The calculated and experimental ECD spectra of complexes in CH2Cl2 and water were reported in
7
Fig. 5. Despite differences in the band shape, in complexes with (S) configured diamine, the
8
following sequence of bands was observed: ca. 380 nm (-, medium); ca. 340 (+, medium/shoulder);
9
ca. 300 nm (+, medium), ca. 280 nm (-, medium/strong).
10 11
Figure 5. a) Experimental and calculated ECD spectrum of S-1 in CHCl3 (a). The pure forms of
12
cis and trans isomers are also reported as dashed lines; b) Experimental ECD spectra of S-2 and R-2
13
are recorded in H2O.
14 15
In the spectra collected in water, the wider transparency region allows to monitor the presence of
16
two additional mirror shaped bands. These bands can be assigned to LC (π-π*) transitions involving
17
the aromatic ligands. On the other hand, the band at ca. 380 nm, assigned to a spin allowed MLCT
18
transition, in the literature has been related to the helicity of the complex,[59] indeed a negative
19
Cotton effect could be related to a left-handed helical path as induced by ∆ configuration. Thus the
20
prevailing configuration in solution should be the ∆ one.
21
In order to confirm the hypothesized configuration, a DTF and TDDFT investigation of the S0
22
electronic states has been performed at the optimised geometries. The focus has been set on the
23
lowest energy transitions, i.e. the ones in which the metal orbitals are more involved. The
24
bandshape of these transitions being similar in H2O and CHCl3, suggests a low influence of the
25
counterion. That fact provided a further motivation to limit the study of the system to the cationic
26
moiety of the complexes and to compare the results with the experimental spectra of 1 in DMSO,
27
where the interactions with the counterion are expected to be negligible. The best matching of 11
1
calculated and experimental spectra was achieved with the well-known B3LYP[60] functional at 6-
2
311+(2d,p)[LANL2DZ] level of theory. While in Fig. 5a the experimental and calculated spectra of
3
complex (S)-1 were displayed together, in Table 6SM a simplified assignment of the main relevant
4
transition in the ECD spectra of complex (S)-1 was reported.
5
A first observation concerned the relative energies of the isomers (see Table 3.SM): according to
6
the experimental NMR data, the results highlighted that the cis isomer is more stable for both
7
diamine ligands; in both cis and trans isomers a remarkable energy difference between the two
8
possible helicities of the complexes occurred, these results suggest that the rigidity of the chiral
9
ligand prompted the complexes to a preferred helicity at the metal.
10
Despite of a small shift (ca. 10 nm), calculations provide spectra in fair agreement with the
11
experimental ones. The analysis of frontier orbitals revealed that the LUMO is mainly located on
12
the ppy ligand, whereas the LUMO+1 is located on the diamine one. On the other hand most of the
13
HOMO-n (n=1,2,3) are localised on the metal and on the ppy ligand. Indeed the lowest energy
14
absorption and ECD band observed at 380 nm is related to the calculated HOMO to LUMO
15
excitation and thus to a MLCT that slightly involves the chiral ligand.
16
The calculated spectrum of complex (S)-1 has been obtained by the NMR averaged contributions of
17
the cis/trans isomers. In Fig. 5 the spectra of the cis and trans complexes in their most stable
18
configuration are also shown for comparison. The major contribution of cis-∆(S) complex perfectly
19
agrees with the negative sign of the experimental band at 380 nm, while the trans-∆(S) isomer
20
shows a positive sign.
21
The opposite signs of cis-∆(S) and trans-∆(S) is explained through the inversion of the electric
22
dipole moment associated with the substituents and consequently to the sign of charge flow. In this
23
respect, the oriented-skew-line reference system is able to describe the opposite configuration of the
24
isomers: arbitrarily orienting the lines from the non-pyridine ring of the ligands to the pyridines one,
25
the isomers present a different configuration as depicted in Fig. 2. In order to confirm the sign of the
26
transition at 380 in the minor isomer and, in turn, its configuration at the metal centre, the
27
isomerization process in DMSO was followed by NMR and ECD spectroscopy. The ration between
28
the regioisomers was determined by NMR, moving from a ratio of 60:40 to 80:20, the most
29
significant change was an increased intensity of the transition at 380 nm in ECD spectra. This result
30
is compatible with the calculated spectrum and so to a trans-∆ configuration of the minor isomer
31
(see Fig. 5a).
32 33
3.5. Experimental and calculated VCD
12
1 2
Figure 6 a) IR (bottom) and VCD (top) spectra of S-1 in DMSO-d6; b) IR (bottom) and VCD (top)
3
spectra of S-2 in DMSO-d6 and CDCl3.
4 5
The complexes have been investigated also by VCD spectroscopy. In Fig. 6 the experimental VCD
6
and IR spectra of 1 and 2 in CDCl3 and DMSO-d 6 are reported. Spectra are shown in the 1650-1200
7
cm range region, which is transparent to both deuterated solvents. Complex 1 was soluble at
8
concentration values, which were suitable for VCD analysis only in DMSO. In general, throughout
9
the infrared spectra, the complexes did not exhibit large absorptivity, featuring only five signals
10
between 1500 cm-1 and 1350 cm-1, with g factors of ca. 10-4.
11
Interestingly, the different types of counterions and of solvents highly influenced the shape of the
12
spectrum; in fact the spectra of 1 and 2 in DMSO-d6 and CDCl3 present different shapes.
13
In particular the couplet at ca. 1450 cm-1 was enhanced by a factor of two in intensity in the same
14
way as in the IR spectra, whereas other signals change their sign or position. The origin of this
15
phenomenon could be ascribed to an increased ion association and to the donor-acceptor charge
16
transfer mechanism between the ions.[61] A clue to the assumption and characterization of the
17
effects of ion association in different solvents may be traced in the spectrum of 2 in DMSO (dashed
18
2b). Although the spectra are shown in small region due to the high noise, features around 1450
19
cm-1 are definitely more similar to those manifesting in 1 (e.g. S -,+,+,-) . This result suggests a less 13
1
strong interaction as experienced with BF4. A related effect may be also traced in the ECD spectra
2
(see Fig. 10.SM); indeed by increasing the amount of DMSO in solution, the spectrum became
3
similar to that of 1. As stated above, several functionals have been tested and the best results in the
4
prediction of VCD spectra have been achieved with the mPW1PW91 functional (see Fig. 14.SM for
5
a comparison) taking into account the contributions from the cis and trans isomers to generate the
6
theoretical spectra.
7
In Fig. 6a the calculated and experimental spectra of complex (S)-1 are superimposed and, with the
8
exception of the transition band at 1270 cm-1, the experimental and calculated VCD spectra are in
9
good agreement. On the other hand the predicted IR bands are four or five times more intense than
10
the observed ones, a result noticed before for charged species.[62]
11
The analysis of the normal modes revealed that the first two bands of opposite sign (from high to
12
low wavenumbers) are the result of a combination of CC stretchings and CH in plane bending of the
13
two pyridine rings. The second couplet at ca. 1480-1450 cm-1 is instead due to the combination of
14
CH2 scissoring on the chiral ligand and CC stretching on the ppy one. Finally, the strong
15
monosignated band at ca. 1380 cm-1 is related to the HC*-bending at the asymmetric carbon atom
16
#8 (C*) of Campy.[40]
17
Based on the above assignment, we were able to
18
signatures in the VCD spectra of the Pt-complexes: at 1480 cm-1 the +/- bisignate VCD features
19
(from low to high wavenumbers) are only compatible with a ⃗∆ configuration of the system (see
20
Fig. 15.SM); at 1450 cm-1 the -/+ bisignate VCD features appear to be related to the chair
21
conformation of the aliphatic ring and finally the monosignate strong feature at 1380 cm-1 is
22
representative of the stereogenic carbon.
23 24 25 26
3.6. CPL spectra
14
recognize several different configurational
1 2
Figure 7. CPL spectra of (S)-2 and (R)-2 in water; Solid noisy lines represent the experimental data,
3
while dashed lines may be used as helpful trails and correspond to the simple moving average (100
4
points) of the experimental data.
5 6
Since the acetate complexes exhibited good solubility and, most importantly, good stability in water
7
we decided to record CPL spectra on water solutions. In this case no effect related to an organized
8
supramolecular structure was observed, so the signal and the dissymmetry factor presented pure
9
molecular origin and were quite low contrary to what observed on other systems undergoing
10
intramolecular helical structure or intermolecular helical aggregation.[63],[64] The observed sign
11
for (S)-2 and (R)-2 is the same as the sign of the lowest energy ECD band of the corresponding
12
compounds (Fig. 5 and 7)
13 14
4. Conclusions
15 16
In this work we have synthesised and characterised a mixture of cationic Pt(II) complexes of the
17
type [Pt(C^N)(N^N)]X , where (C^N) is cyclometallated 2-phenylpyridine ligand, (N^N) is a chiral
18
diamine and BF4- or CH3COO- are the counterion (X). Reaction with Campy gave rise to a mixture
19
of two complexes. Their structural and photochemical properties in solution have been investigated
20
by several spectroscopic techniques. In particular, the assignment of trans or cis isomer has been
21
achieved by NMR experiments.
15
1
The complexes presented a distorted square planar geometry and the rigid structure of the chiral
2
ligands prompted a preferred helical configuration. Comparison of experimental and calculated
3
ECD spectra allowed to discriminate the helicity of the complex.
4
All the investigated compounds displayed similar photophysical properties in water or DMSO at
5
room temperature, with luminescence quantum yields up to 2.95% and lifetimes in the microsecond
6
range.
7
Use of VCD spectroscopy on the complexes 1 and 2 with the Campy diamine has offered the
8
possibility to identify signatures characteristic of the helicity, has confirmed the chirality of the
9
diamine ligands and has helped us to assign the conformation of the chiral ligands.
10
Moreover IR and VCD spectra of 2 in CHCl3 highlighted the effect of the ion paring in solution,
11
that resulted into a modification of the spectrum shape and an enhancement of signals. Finally the
12
CPL spectra of 2 and their dissymmetry factor in water solution pointed out that likely no
13
supramolecular organized structures occurred under these conditions.
14
Thus, together with electrochemical and computational studies performed on the newly synthesized
15
complexes, chiroptical studies have shed light both on the helicity of complexes as well as on the
16
influence of ion pairing interaction in solution. This outcome is particularly relevant when Pt(II)
17
ionic species are introduced into bioactive environments, also chiral and ionic in nature.
18 19
Acknowledgements
20
Part of this work was supported by the Ministero dell’Istruzione, dell’Università e della Ricerca
21
through PON 2007/2013 ELIOTROPO (PON03PE_00092_2) and through Italian PRIN 2012 (N.
22
2012JHFYMC) and by Regione Calabria (POR CALABRIA FESR 2007/2013, ASSE I, Progetto
23
NUOVALUCE).
24 25
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19
1
Highlights
2
•
Water-soluble Pt(II) chiral complexes have been synthesised
3
•
The complexes were characterized by chiroptical techniques
4
•
Electrochemical, DFT and TDDFT studies have been performed
5
•
The photophysical properties of the complexes were deeply investigated in water
6 7 8 9 10
20
1 2 3 4
Graphical abstract
21
S0020-1693(16)30592-8 http://dx.doi.org/10.1016/j.ica.2017.02.026 ICA 17452
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
13 October 2016 12 January 2017 24 February 2017
Please cite this article as: A. Ionescu, N. Godbert, L. Ricciardi, M. La Deda, I. Aiello, M. Ghedini, I. Rimoldi, E. Cesarotti, G. Facchetti, G. Mazzeo, G. Longhi, S. Abbate, M. Fusè, Luminescent water-soluble cycloplatinated complexes: structural, photophysical, electrochemical and chiroptical properties, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.02.026
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1 2 3
Luminescent water-soluble cycloplatinated complexes: structural, photophysical, electrochemical and chiroptical properties
4 5 6 7 8 9 10 11 12 13
Andreea Ionescu,*a Nicolas Godbert,a,b Loredana Ricciardi,b Massimo La Deda,a,b Iolinda Aiello,a,b Mauro Ghedini,a,b Isabella Rimoldi,c Edoardo Cesarotti,c Giorgio Facchetti,c Giuseppe Mazzeo,d Giovanna Longhi,d,e Sergio Abbate,d,e Marco Fusè* c,f a
MAT-INLAB(Laboratorio di Materiali Molecolari Inorganici), LASCAMM and CR-INSTM, Unità INSTM della Calabria, Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, 87036 Arcavacata di Rende (CS),Italy b CNR NANOTEC-Istituto di Nanotecnologia U.O.S. Cosenza, 87036 Arcavacata di Rende (CS), Italy c Dipartimento di Scienze Farmaceutiche, Università di Milano Via Golgi 19, 20133 Milano, Italy
14
d
15
25123 Brescia, Italy
16
e
17
Vasca Navale, 84, 00146 Roma, Italy
18
f
19
*Corresponding authors: Andreea Ionescu ([email protected]),
Dipartimento di Medicina Molecolare e Traslazionale, Università di Brescia, Viale Europa 11,
CNISM Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Via della
current address: Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
Marco Fusè ([email protected])
20 21 22
Abstract
23
The luminescent square-planar Pt(II) complexes of the type [Pt(C^N)(N^N)]+X- have been
24
synthesised, where (C^N) is cyclometallated 2-phenylpyridine ligand, (N^N) a chiral diamine and
25
BF4- or CH3COO- as counterion (X-) have been synthesised. The chiral diamine Campy has been
26
used, resulting in a mixture of regioisomers. The complexes were characterised and their structures
27
in solution were determined by a combination of different spectroscopic and chiroptical techniques
28
(NMR, UV-Vis, ECD, IR, VCD and CPL) associated with electrochemical, DFT and TDDFT
29
studies. Remarkably, the acetate derivatives proved to be soluble and stable in water for at least a
30
month. The electrochemical behaviour of the [Pt(C^N)(N^N)]BF4 was investigated and the LUMO
31
energies were estimated. The photophysical properties of the new compounds were deeply
32
investigated in water or dimethyl sulfoxide solution by UV-Vis, steady state emission and time-
33
correlated single-photon counting spectroscopy.
34 35
Keywords: Luminescence, Pt(II) Chiral Complexes, Electronic Circular Dichroism , Vibrational
36
Circular Dichroism, Circularly Polarized Luminescence
37 38 1
1
1. Introduction
2 3
Photoactive ionic transition-metal complexes find extensive application in optoelectronics [1]-[4]
4
and bio-related fields [5],[6] especially for the high photoluminescent quantum yields they display,
5
for their long-lived excited states and for the possibility of tuning their emission colour across the
6
whole UV-vis-near IR range.[7]
7
Among the most studied luminescent complexes, square-planar Pt(II) complexes exhibit a
8
noteworthy tendency to form self-assemblies by efficient stacking through metallophilic
9
interactions. These assemblies may result in enhanced properties as recently reviewed for imaging
10
applications.[8] In this regard but also for all bio-related applications, water soluble highly
11
luminescent Pt(II) complexes may play an important role.
12
The photophysical and photoconductive properties of Pt(II) complexes are influenced by the ligand
13
field and thus can be modulated via ligand exchange and/or substitution.[9],[10] Although Even if
14
the synthesis and the study of properties of luminescent cyclometallated neutral [11]-[13] or ionic
15
Pt(II) complexes embedded with tridentate ligands [14]-[16] had been already reported, fewer
16
cationic
17
characterized.[17],[18]
18
functionalization by varying either the (N^N) ligand and the counterion separately, in order to tune
19
both luminescence and solubility. Indeed, water solubility can be achieved for Pt(II) complexes by
20
selecting specific counterions [19],[20] and/or introducing hydrophilic or charged groups onto the
21
ligand molecular structures.[21] However, the pool of water-soluble Pt(II) complexes comprises
22
few examples so far.
23
Since chirality is an important issue in bio-related systems, the design of synthetic pathways leading
24
to enantiomerically pure complexes is highly relevant. In this context, we present a series of novel
25
chiral cyclometallated luminescent Pt(II) complexes (Fig. 1). These are square-planar complexes
26
bearing a 2-phenylpyridine (ppy) fragment as a (C^N) cyclometallated ligand and a chiral (N^N)
27
ligand 8-amino-5,6,7,8-tetrahydroquinolines (Campy).
Pt(II)
complexes
of
the
type
[Pt(C^N)(N^N)]X
Such complexes feature
28 29 30 31 32 33 34 2
the
advantage
were
synthesized
of
potential
a
and
double
1 2 3 4 5
Figure 1. Molecular structures and the two possible regioisomers of the synthesised complexes
6
(trans on the left, cis on the right, relative to the pyridine rings).
7 8 9
The positive charge of complexes 1 is balanced by a BF4- counterion, chosen in order to favour
10
solubility in organic solvents. Water solubility was induced instead in complexes 2 by introducing a
11
CH3COO- counterion, analogously to the already reported chiral ionic Ir(III) complexes.[22]
12
Although the strong trans-influence of the C-donor is expected to direct the reaction towards the
13
formation of the cis isomer,[23] the introduction of a dissymmetric chelating ligand could give rise
14
to two possible regioisomers with the pyridine of the ppy ligand and that of the diamine ligand in
15
trans or in cis position.
16
Furthermore, complexation with sterical hindered bidentate ligands leads to a distortion of the
17
square planar geometry in a helical shape and thus introducing a second chirality at the metal
18
centre.[23],[24] Helical chirality can be described by ∆/Λ notation (Fig. 2).[25],[26] As a
19
consequence, due to the presence of an enantiopure chiral diamine ligand, each regioisomer can
20
exist in two diastereomeric forms ∆ S-N^N and Λ S-N^N.
21
In absence of the crystallographic data, the composition, the structure and the regio- and
22
diasteroselectivity was investigated by NMR spectroscopy. However this technique does not always
23
allow to assign the absolute configuration even in the presence of diastereomeric interaction;[22]
24
therefore the configuration of complexes in solution has been proven by a combination of
25
chiroptical techniques: electronic (ECD) and vibrational (VCD) circular dichroism. These
26
techniques provide complementary/supplementary information [27] and together with DFT and
27
TDDFT calculations proved to be effective in several cases.[28]-[31]
28 29 30
3
1
Figure 2. Schematic representation of the four possible isomers and their chiral descriptor in the
2
skew-lines (top) and in the oriented-skew-lines (bottom) reference systems.[25],[26] The oriented-
3
lines have been directed towards the pyridine ring of the substituent. See Table 8.SM for the
4
representation with molecular models.
5 6
In this works we present the components characterisation of an unresolved mixture of isomers via
7
several spectroscopic techniques.
8 9
2. Materials and methods
10 11
Unless otherwise stated, reagents and solvents were purchased from commercial sources and used
12
without further purification. Enantiomerically pure Campy was obtained as reported in the
13
literature.[32] All synthesis involving Pt(II) complexes were carried out under nitrogen atmosphere
14
using standard Schlenk techniques. The [(ppy)Pt(µ-Cl)]2 precursor was prepared according to
15
literature procedures.[33] 1H and 13C NMR spectra were recorded in CDCl3, CD3OD or in a mixture
16
of them on Bruker DRX Advance 300 MHz equipped with a non-reverse probe or Bruker DRX
17
Avance 400 MHz. Chemical shifts (in ppm) were referenced to residual solvent proton/carbon peak.
18
Fast Atom Bombardment Spectra (FAB) were acquired on a VG Autospec M246 spectrometer
19
using p-nitrobenzyl alcohol as the matrix. Elemental analyses (EA) were recorded on Perkin Elmer
20
Series II CHNS/O Analyzer 2400.
21 22 23 24 25
2.1. Synthesis of complexes
26 27
The complexes were synthesised by treating [(ppy)Pt(µ-Cl)]2 (50 mg, 0.065 mmol) with 1.1 eq of
28
optically pure diamine ligand in the presence of 5 eq of sodium salt (NaBF4 or NaCH3COO) in a
29
degassed solution (N2) of MeOH and irradiating this mixture with 150 W-microwave radiation in a
30
sealed vial. Temperature was prevented from rising above 65 °C by stopping irradiation and quickly
31
cooling the reaction mixture. Three irradiation/cooling cycles of 1 min were performed. Upon
32
completion, the black Pt(0) precipitate was filtered off and the solvent removed by reduced
33
pressure. The solid was then washed with diethylether and dissolved in the minimum volume of
34
CH2Cl2 and filtered. The products were obtained by evaporation of the solvent. 4
1 2
2.1.1 [(ppy)Pt((R)-Campy)]+BF4- (1)
3
Grey/white solid, yield 55 mg (72%), slightly soluble in CH2Cl2 and MeOH. Soluble in DMSO with
4
isomerisation to the major cis isomer. 1H-NMR (300 MHz, DMSO-d6) δ: 8.97 (d, J = 5.3 Hz,
5
0.25H, c2trans), 8.81 (d, J = 5.4 Hz, 1.25H, c2cis+a2trans), 8.73 (d, J = 5.5 Hz, 1H, a2cis), 8.26 -
6
8.10 (m, 2.5H, a3,a5cis+a3,a5trans), 8.08 - 7.98 (m, 1.25H, c4cis+c4trans), 7.85 - 7.74 (m, 1.5H,
7
b5cis+b5trans), 7.74 - 7.66 (m, 1H, c3cis), 7.66 - 7.60 (m, 0.25H, c3trans), 7.52 - 7.45 (m, 1H,
8
a3cis), 7.45 - 7.38 (m, 1.25H, b2cis+a3trans), 7.34 - 7.22 (m, 1H, b2trans), 7.22 - 7.11 (m, 2.5H,
9
b4, b3cis+b4, b3trans), 6.76 (m, 1H, NHa cis), 6.47 - 6.32 (m, 1H,NHb cis), 6.32 - 6.17 (m,
10
0.25H,NHa trans), 5.56 - 5.42 (m, 0.25H, NHb trans), 4.58- 4.37 (m, 1H, c8cis), 4.37 - 4.26 (m,
11
0.25H, c8trans), 2.86 (s, 2.5H, c5cis+c5trans), 2.46 - 2.33 (m, 1H, c7cis), 2.34 - 2.24 (m, 0.25H,
12
c7trans), 2.00 (m, 1.25H, c6’cis+c6’trans), 1.76 (m, 2.5H, c6,c7’cis+c6,c7’cis);
13
MHz, CD3OD) δ: 159.80, 151.29 (a2trans), 150.23 (c2trans), 149.54 (a2cis), 147.02 (c2cis),
14
145.83, 140.33, 140.25, 140.14, 136.79, 133.74, 132.64, 130.11, 129.75, 125.49, 124.75, 124.16,
15
123.85, 123.60, 123.38, 123.05, 119.73, 119.52, 59.91 (c8cis), 57.36 (c8trans), 31.86 (c7cis), 30.64
16
(c7cis), 27.44, 27.30, 21.10; MS (FAB): m/z 497[M]+; Elemental analysis: calculated C 41,11% H
17
3.45% N 7.19%; found C 40.8% H 3.43% N 7.1%.
13
C NMR (75
18 19
2.1.2 [(ppy)Pt((R)-Campy)]+CH3COO- (2)
20
Grey/white solid, yield 60 mg (83%), soluble in water. The complex results stable in H2O, less in
21
chlorinated solvents. 1H-NMR (400 MHz, CD3OD) δ: 9.04 (d, J = 5.5 Hz, 0.25H), 8.77 (d, J = 5.0
22
Hz, 1.25H), 8.71 (d, J = 5.4 Hz, 1H), 8.17 - 8.05 (m, 2.5H), 7.99 (m, 2.5H), 7.76 - 7.60 (m, 2.25H),
23
7.59 - 7.51 (m, 0.25H), 7.41 (m, 2H), 7.33 (m, 0.25H), 7.29 - 7.24 (m, 25H), 7.24 - 7.11 (m, 2.25H),
24
4.52 (m, 1H), 4.41 (s, 0.25H), 2.95 (s, 2.5H), 2.59 - 2.39 (m, 1.25H), 2.10 (m, 1.25H), 1.92 (s,
25
3.75H), 1.90 - 1.71 (m, 2.5H);
26
47.48% H 4.17% N 7.55%; found C 47.68% H 4.21% N 7.3%.
13
C NMR mirrored that of 1. Elemental analysis: calculated C
27 28 29
2.2 Electrochemical Measurements
30
Electrochemical measurements were carried out by cyclic voltammetry, performed with an Autolab
31
Potentiostat-Galvanostat controlled by the NOVA 1.1 software. A conventional 3 mL three-
32
electrode cell was employed, with a Pt wire as counter-electrode, an Ag wire as pseudo-reference
33
electrode and a Pt disk as working electrode. NBu4PF6 (0.1 M) was used as supporting electrolyte
34
and experiments were performed in a dry, and degassed (Ar) dimethylformamide solution. 5
1 2
2.3. Photophysical Measurements
3
Spectrofluorimetric grade solvents were used for the photophysical investigations in solution
4
without further purification. A Perkin Elmer Lambda 900 spectrophotometer was employed to
5
obtain the UV/Vis absorption spectra, using quartz cuvettes of 1 cm path length. Steady-state
6
emission spectra were recorded on a Horiba Jobin Yvon Fluorolog 3 spectrofluorimeter, equipped
7
with a Hamamatsu R-928 photomultiplier tube. The luminescence quantum yields were determined
8
using the optical dilution method [34] using Ru(bpy)3Cl2 in air-equilibrated water solution as a
9
reference standard (Φ= 0.028).[35] Solutions were degassed by bubbling argon into quartz cells
10
prior to measurements. Time-resolved measurements were performed using the time-correlated
11
single-photon counting (TCSPC) on the Fluorolog-3 apparatus. A pulsed NanoLED centred at 379
12
nm (FWHM 750 ps with 1 MHz repetition rate) was used as excitation source and fixed directly on
13
the sample chamber at 90° to a single-grating emission monochromator (2.1 nm/mm dispersion;
14
1200 grooves/mm). Data analysis was performed using the commercially available DAS6 software
15
(HORIBA Jobin Yvon IBH). The quality of the fit was assessed by minimizing the reduced χ2
16
function.
17 18
2.4. Electronic Circular Dichroism (ECD) Spectra
19
Spectra were recorded with a Jasco 815SE spectrometer employing 2 mm quartz cells; with CHCl3
20
or H2O solutions in concentration range from 0.23 mM to 0.26 mM. Spectra were recorded at 25
21
°C.
22 23 24 25
2.5. VCD and IR (Vibrational Absorption = VA) Spectra
26
All spectra were recorded with a Jasco FVS6000 VCD spectrometer with liquid N2-cooled MCT
27
detector. Spectra were recorded at room temperature (25 °C) in CDCl3 or DMSO-d6 solutions in the
28
range 0.037-0.1 M concentration, in 200 mm BaF2 cells. In some cases a drop of CD3OD was added
29
to the solution, for better solubilization of the sample. Repeated sets of 5000 scans were taken for
30
each sample and series of 5000 scan-average spectra were co-added, if necessary; subtraction of
31
solvent signals was performed in VA and VCD spectra.
32 33
2.6. Circularly Polarized Luminescence (CPL) Spectra
6
1
Spectra were recorded with a home-built apparatus described in ref. [36]-[38] with excitation
2
radiation brought in through an optical fibre from a Jasco FP8200 fluorimeter (90° geometry). The
3
incident light is polarized parallel to the exit beam direction. Twenty scans were accumulated for
4
each spectrum. The excitation wavelength has been 367 nm. All spectra were taken at 25 °C.
5 6
2.7 Computational Details
7
DFT calculations were carried out with Gaussian package.[39] The four diastereomers were
8
modelled starting from the crystallographic structure of the diamine of previous reported
9
complexes,[40] then the ligand was coordinated to Pt(II) centre in a square planar geometry with an
10
ortho-metallated ppy as a second coordinated ligand. The conformation of the C6-ring of the
11
tetrahydroquinoline moiety was also taken into account. The most stable conformation was
12
confirmed to be a half-chair on C(7), other possible geometries (boat or half chair on C(6))
13
converged to the C(7) half-chair during the optimization processes.
14
We screened four promising functionals (B3LYP,[41]-[43] B3PW91,[42]-[45] MPW1PW91,[45]-
15
[47] PBE0,[48],[49]) with two basis sets, LANL2DZ and Pople basis sets and pseudo potential on
16
Pt atom. Bulk solvent effects (CHCl3, H2O, DMSO) were taken into account using the polarizable
17
continuum model (PCM),[51]-[52] the same solvents used in the experiments were retained in the
18
calculations.
19
The analysis was only conducted on the cationic moiety of the molecules and results were
20
compared with the experimental data of BF4– salts in DMSO or CHCl3, assuming the interaction of
21
the counterion to be negligible.
22
Calculation of Dipole and Rotational Strengths through the field-response approach[53] was carried
23
out; VA and VCD spectra were generated by assigning 5 cm-1 bandwidth Lorentzian bandshape to
24
each vibrational transition. The scaling factor 0.975 was applied to the calculated frequencies.
25
Time-Dependent DFT (TDDFT) calculations have been employed, within the same Gaussian
26
package, to obtain ECD spectra: the same level of theory was adopted and 100 excited states were
27
admitted. Spectra were then plotted by associating Gaussian bands to each transition with 0.15 eV
28
bandwidth. Percentage composition of the molecular orbital (MO) was computed using the
29
GaussSum3 program.[54]3. Results and discussion
30 31
The newly synthesized complexes were fully characterized by a combination of different
32
spectroscopic and chiroptical techniques (NMR, UV-vis, ECD, IR, VCD and CPL) associated with
33
electrochemical, DFT and TDDFT studies chiroptical tecniques. For the sake of brevity, the
34
electrochemical results are reported in the supplementary information. 7
1 2
3.1. Synthesis
3 4
The complexes were synthesized starting from the binuclear complex [(ppy)Pt(µ-Cl)]2 via an
5
adapted microwave assisted procedure reported in literature.[33] In order to force the counterion
6
outside the coordination sphere, chlorine was replaced via metathesis reaction including fivefold
7
times sodium tetrafluoroborate or sodium acetate in the reaction medium. The choice to synthesize
8
acetate derivatives was addressed to increase the solubility in the aqueous medium.[55]
9
While complexes 1 were only slightly soluble in CH2Cl2, MeOH, in a mixture CH2Cl2 :MeOH 3:1
10
and DMSO, the acetate complexes 2 presented a good solubility also in water. The obtained
11
complexes were fully characterized by NMR, FAB spectrometry and EA. The reaction of the
12
binuclear Pt(II) complex with enantiopure Campy gave rise to a mixture of two complexes, which
13
were identified as the two cis/trans regioisomers by NOESY experiments. The major product was
14
the cis isomer as pointed out by NOE-crosspeak between the two ortho protons of the pyridine
15
rings. In the case of the minor isomer, the trans one, the key NOE correlation was between the
16
ortho-hydrogen of the Campy pyridine ring and that of the phenyl one (Figure 3.ESI).
17
The counterion was found to influence the ratio between the two regioisomers, indeed when BF4-
18
was used as counterion the reaction results in a 60:40 mixture, whereas when the reaction was
19
performed in presence of CH3COO- the ratio was found to be 75:25. The polar coordinating solvent
20
DMSO was able to move the isomerisation equilibrium towards the prevailing cis form, reaching
21
the final ratio of 85:15.
22
Thereafter, for the sake of brevity, the cis/trans isomeric mixture of the S-Campy complexes will be
23
reported as product 1 for the BF4- or product 2 for the CH3COO- salts respectively.
24
3.2 Electrochemistry
25 26
The electrochemical behaviour of complexes 1 was investigated by cyclic voltammetry in a ca. 0.1
27
M NBu 4PF6 dry DMF electrolytic solution using an Fc/Fc+ redox couple as internal standard.
28
Since the electrochemical activity is displayed just by the Pt(II) cation, only complexes 1 were
29
studied. This choice was due to their better solubility in organic solvent such as DMF, allowing a
30
wider electrochemical potential window than water. Reduction and oxidation potentials and
31
estimated LUMO levels are reported in Table 2.SM together with literature data of similar
32
[(ppy)Pt(N^N)]+ cations with (N^N) = ethylenediamine (en), and 2,2’-bipyridine (bpy) and 1,10-
33
phenanthroline (phen),[56] for direct comparison.
8
1
First, two irreversible reduction waves are observed when applying negative potentials. For cationic
2
Pt (II) complexes, reversible or quasi reversible reduction waves were expected to be ligand centred
3
involving the π*-orbitals of the pyridine rings.
4
As previously concluded on similar [(ppy)Pt(N^N)]+ cations, the first reduction wave is attributed
5
to the reduction of the pyridine ring of the cyclometallated ligand and the second wave to the
6
reduction of the pyridine fragment on the ancillary N^N ligand. In agreement with this hypothesis,
7
the LUMO resulted predominantly localized onto the cyclometallated ligand.
8
Secondly, concerning oxidation, a single irreversible wave was recorded and assigned to the
9
oxidation of the Pt(II) metal centre, leading to an unstable species in turns stabilized by chemical
10
reaction.[56] This behaviour prevents the estimation of the HOMO energy by electrochemical
11
measurements, since the oxidation induces the complex degradation.
12 13
3.3. Photophysical properties
14 15
The photophysical properties of the complexes were investigated in H2O or DMSO solution at 298
16
K. The electronic absorption spectra of the compounds (Fig. 3 and Table 1) were dominated,
17
regardless of the solvent, by bands in the UV region arising from LC (π-π*) transitions involving
18
the cyclometallated ligand. At low-energy (350-410 nm) an unresolved and less-intense absorption
19
band was present, that could be assigned to a spin allowed MLCT transition. Bands ascribable to the
20
ancillary ligand were excluded by comparison with the absorption spectrum of the related complex
21
[(ppy)Pt(en)]+,[56] in which all features were MLCT or ppy-located transition.
22 23 24
27 28 29 30 31 32
35000
b) 1. DMSO
30000 25000 20000 15000 10000
7000
5000 4000 3000
1000 300
325
350
375
400
0 225 250 275 300 325 350 375 400 425
425
Wavelength (nm)
Wavelength (nm)
Figure 3. Absorption spectra of complexes 1-2, the first one in DMSO the second one in H2O.
33 34
2. H2O
2000
5000 0 275
8000
6000
ε M -1cm-1
26
a) ε M-1cm-1
25
Table 1. Photophysical properties of complexes 1 and 2 9
1
Complex
Solvent
1
DMSO
2
H2O
a
Absorption -1 -1 λ max/nm (ε/M cm ) 279(32691)-303(sh)-327(12707)340(9929)-378(4008) 235(7389)-251(6558)-272(6313)299(sh)-311(3018)-323(2979)-368 (774)
Emission λ max/ nm 486, 520, 553 481, 513, 548
Ф(%)
Lifetime τ(α)/ns(%) 12.1 (20.42%), 47.5 (79.58%) 53.1 (28.69%), 268.3 (71.31%)
0.7 0.6
Фa(%) 1.4 0.7
a
Lifetime τ(α)/ns(%) 85.1 (2.55%), 1906.2 (97.45%) 72.6 (28.10%), 378.8 (71.90%)
deaerated solution
2 3
The absorption spectrum of BF4- compounds 1 slightly differed from the analogous ones with
5
CH3COO-, in particular for the extinction coefficient and the band position: bands of 1 (dissolved in
6
DMSO) were red-shifted and less intense than those of 2 (dissolved in water). To discriminate
7
between the effects of either solvent or counterion, absorption spectrum of 2 was recorded in
8
DMSO solutions (Fig. 8.SM); while the band shift was attributed to a solvatochromic effect, it
9
could be concluded that the replacement of BF4- by CH3COO- as the counterion led to a lower
10
extinction coefficient. This feature could be due to a greater proximity and interaction of cations
11
and anions in acetate pair compared to BF4-, leading to a change in the dipole moment on going
12
from the ground to the excited state, reducing the electronic transition probability.
13
All complexes were phosphorescent, showing similar emission bands with a well-defined vibronic
14
structure (Fig. 4). The emission intensities were weak in all cases, with luminescence quantum
15
yields, ranging from 0.7% in degassed water to 2.95% in degassed DMSO (Table 2). Luminescence
16
was most efficiently quenched by oxygen in the latter solvent, a fact which could be explained on
17
account of the different oxygen concentration in the two solvents (0.46 mmolL-1 in DMSO vs 0.29
18
mmolL-1 in water).[57] Excited state decay of all complexes displayed bi-exponential kinetics
19
(Table 1), which could be attributed to the presence of dimeric aggregates in solution[58]
20
responsible for the short lifetimes.
21 22 23 24 25 26
Emission Intensity (a. u.)
4
60
1. DMSO 2. H2O
50 40 30 20 10 0 500
27 28
550
600
650
700
Wavelength (nm)
Figure 4. Emission spectra of complexes 1-2 in DMSO and H2O solution (4·10-5 M)
29 30 10
1
3.4. Experimental and calculated ECD
2 3
Chiroptical methodologies are the only experimental techniques capable of directly probing
4
molecular chirality and thus in order to assign the absolute configuration of the complexes, a
5
chiroptical study comprised of ECD, VCD and CPL spectroscpy was carried out.
6
The calculated and experimental ECD spectra of complexes in CH2Cl2 and water were reported in
7
Fig. 5. Despite differences in the band shape, in complexes with (S) configured diamine, the
8
following sequence of bands was observed: ca. 380 nm (-, medium); ca. 340 (+, medium/shoulder);
9
ca. 300 nm (+, medium), ca. 280 nm (-, medium/strong).
10 11
Figure 5. a) Experimental and calculated ECD spectrum of S-1 in CHCl3 (a). The pure forms of
12
cis and trans isomers are also reported as dashed lines; b) Experimental ECD spectra of S-2 and R-2
13
are recorded in H2O.
14 15
In the spectra collected in water, the wider transparency region allows to monitor the presence of
16
two additional mirror shaped bands. These bands can be assigned to LC (π-π*) transitions involving
17
the aromatic ligands. On the other hand, the band at ca. 380 nm, assigned to a spin allowed MLCT
18
transition, in the literature has been related to the helicity of the complex,[59] indeed a negative
19
Cotton effect could be related to a left-handed helical path as induced by ∆ configuration. Thus the
20
prevailing configuration in solution should be the ∆ one.
21
In order to confirm the hypothesized configuration, a DTF and TDDFT investigation of the S0
22
electronic states has been performed at the optimised geometries. The focus has been set on the
23
lowest energy transitions, i.e. the ones in which the metal orbitals are more involved. The
24
bandshape of these transitions being similar in H2O and CHCl3, suggests a low influence of the
25
counterion. That fact provided a further motivation to limit the study of the system to the cationic
26
moiety of the complexes and to compare the results with the experimental spectra of 1 in DMSO,
27
where the interactions with the counterion are expected to be negligible. The best matching of 11
1
calculated and experimental spectra was achieved with the well-known B3LYP[60] functional at 6-
2
311+(2d,p)[LANL2DZ] level of theory. While in Fig. 5a the experimental and calculated spectra of
3
complex (S)-1 were displayed together, in Table 6SM a simplified assignment of the main relevant
4
transition in the ECD spectra of complex (S)-1 was reported.
5
A first observation concerned the relative energies of the isomers (see Table 3.SM): according to
6
the experimental NMR data, the results highlighted that the cis isomer is more stable for both
7
diamine ligands; in both cis and trans isomers a remarkable energy difference between the two
8
possible helicities of the complexes occurred, these results suggest that the rigidity of the chiral
9
ligand prompted the complexes to a preferred helicity at the metal.
10
Despite of a small shift (ca. 10 nm), calculations provide spectra in fair agreement with the
11
experimental ones. The analysis of frontier orbitals revealed that the LUMO is mainly located on
12
the ppy ligand, whereas the LUMO+1 is located on the diamine one. On the other hand most of the
13
HOMO-n (n=1,2,3) are localised on the metal and on the ppy ligand. Indeed the lowest energy
14
absorption and ECD band observed at 380 nm is related to the calculated HOMO to LUMO
15
excitation and thus to a MLCT that slightly involves the chiral ligand.
16
The calculated spectrum of complex (S)-1 has been obtained by the NMR averaged contributions of
17
the cis/trans isomers. In Fig. 5 the spectra of the cis and trans complexes in their most stable
18
configuration are also shown for comparison. The major contribution of cis-∆(S) complex perfectly
19
agrees with the negative sign of the experimental band at 380 nm, while the trans-∆(S) isomer
20
shows a positive sign.
21
The opposite signs of cis-∆(S) and trans-∆(S) is explained through the inversion of the electric
22
dipole moment associated with the substituents and consequently to the sign of charge flow. In this
23
respect, the oriented-skew-line reference system is able to describe the opposite configuration of the
24
isomers: arbitrarily orienting the lines from the non-pyridine ring of the ligands to the pyridines one,
25
the isomers present a different configuration as depicted in Fig. 2. In order to confirm the sign of the
26
transition at 380 in the minor isomer and, in turn, its configuration at the metal centre, the
27
isomerization process in DMSO was followed by NMR and ECD spectroscopy. The ration between
28
the regioisomers was determined by NMR, moving from a ratio of 60:40 to 80:20, the most
29
significant change was an increased intensity of the transition at 380 nm in ECD spectra. This result
30
is compatible with the calculated spectrum and so to a trans-∆ configuration of the minor isomer
31
(see Fig. 5a).
32 33
3.5. Experimental and calculated VCD
12
1 2
Figure 6 a) IR (bottom) and VCD (top) spectra of S-1 in DMSO-d6; b) IR (bottom) and VCD (top)
3
spectra of S-2 in DMSO-d6 and CDCl3.
4 5
The complexes have been investigated also by VCD spectroscopy. In Fig. 6 the experimental VCD
6
and IR spectra of 1 and 2 in CDCl3 and DMSO-d 6 are reported. Spectra are shown in the 1650-1200
7
cm range region, which is transparent to both deuterated solvents. Complex 1 was soluble at
8
concentration values, which were suitable for VCD analysis only in DMSO. In general, throughout
9
the infrared spectra, the complexes did not exhibit large absorptivity, featuring only five signals
10
between 1500 cm-1 and 1350 cm-1, with g factors of ca. 10-4.
11
Interestingly, the different types of counterions and of solvents highly influenced the shape of the
12
spectrum; in fact the spectra of 1 and 2 in DMSO-d6 and CDCl3 present different shapes.
13
In particular the couplet at ca. 1450 cm-1 was enhanced by a factor of two in intensity in the same
14
way as in the IR spectra, whereas other signals change their sign or position. The origin of this
15
phenomenon could be ascribed to an increased ion association and to the donor-acceptor charge
16
transfer mechanism between the ions.[61] A clue to the assumption and characterization of the
17
effects of ion association in different solvents may be traced in the spectrum of 2 in DMSO (dashed
18
2b). Although the spectra are shown in small region due to the high noise, features around 1450
19
cm-1 are definitely more similar to those manifesting in 1 (e.g. S -,+,+,-) . This result suggests a less 13
1
strong interaction as experienced with BF4. A related effect may be also traced in the ECD spectra
2
(see Fig. 10.SM); indeed by increasing the amount of DMSO in solution, the spectrum became
3
similar to that of 1. As stated above, several functionals have been tested and the best results in the
4
prediction of VCD spectra have been achieved with the mPW1PW91 functional (see Fig. 14.SM for
5
a comparison) taking into account the contributions from the cis and trans isomers to generate the
6
theoretical spectra.
7
In Fig. 6a the calculated and experimental spectra of complex (S)-1 are superimposed and, with the
8
exception of the transition band at 1270 cm-1, the experimental and calculated VCD spectra are in
9
good agreement. On the other hand the predicted IR bands are four or five times more intense than
10
the observed ones, a result noticed before for charged species.[62]
11
The analysis of the normal modes revealed that the first two bands of opposite sign (from high to
12
low wavenumbers) are the result of a combination of CC stretchings and CH in plane bending of the
13
two pyridine rings. The second couplet at ca. 1480-1450 cm-1 is instead due to the combination of
14
CH2 scissoring on the chiral ligand and CC stretching on the ppy one. Finally, the strong
15
monosignated band at ca. 1380 cm-1 is related to the HC*-bending at the asymmetric carbon atom
16
#8 (C*) of Campy.[40]
17
Based on the above assignment, we were able to
18
signatures in the VCD spectra of the Pt-complexes: at 1480 cm-1 the +/- bisignate VCD features
19
(from low to high wavenumbers) are only compatible with a ⃗∆ configuration of the system (see
20
Fig. 15.SM); at 1450 cm-1 the -/+ bisignate VCD features appear to be related to the chair
21
conformation of the aliphatic ring and finally the monosignate strong feature at 1380 cm-1 is
22
representative of the stereogenic carbon.
23 24 25 26
3.6. CPL spectra
14
recognize several different configurational
1 2
Figure 7. CPL spectra of (S)-2 and (R)-2 in water; Solid noisy lines represent the experimental data,
3
while dashed lines may be used as helpful trails and correspond to the simple moving average (100
4
points) of the experimental data.
5 6
Since the acetate complexes exhibited good solubility and, most importantly, good stability in water
7
we decided to record CPL spectra on water solutions. In this case no effect related to an organized
8
supramolecular structure was observed, so the signal and the dissymmetry factor presented pure
9
molecular origin and were quite low contrary to what observed on other systems undergoing
10
intramolecular helical structure or intermolecular helical aggregation.[63],[64] The observed sign
11
for (S)-2 and (R)-2 is the same as the sign of the lowest energy ECD band of the corresponding
12
compounds (Fig. 5 and 7)
13 14
4. Conclusions
15 16
In this work we have synthesised and characterised a mixture of cationic Pt(II) complexes of the
17
type [Pt(C^N)(N^N)]X , where (C^N) is cyclometallated 2-phenylpyridine ligand, (N^N) is a chiral
18
diamine and BF4- or CH3COO- are the counterion (X). Reaction with Campy gave rise to a mixture
19
of two complexes. Their structural and photochemical properties in solution have been investigated
20
by several spectroscopic techniques. In particular, the assignment of trans or cis isomer has been
21
achieved by NMR experiments.
15
1
The complexes presented a distorted square planar geometry and the rigid structure of the chiral
2
ligands prompted a preferred helical configuration. Comparison of experimental and calculated
3
ECD spectra allowed to discriminate the helicity of the complex.
4
All the investigated compounds displayed similar photophysical properties in water or DMSO at
5
room temperature, with luminescence quantum yields up to 2.95% and lifetimes in the microsecond
6
range.
7
Use of VCD spectroscopy on the complexes 1 and 2 with the Campy diamine has offered the
8
possibility to identify signatures characteristic of the helicity, has confirmed the chirality of the
9
diamine ligands and has helped us to assign the conformation of the chiral ligands.
10
Moreover IR and VCD spectra of 2 in CHCl3 highlighted the effect of the ion paring in solution,
11
that resulted into a modification of the spectrum shape and an enhancement of signals. Finally the
12
CPL spectra of 2 and their dissymmetry factor in water solution pointed out that likely no
13
supramolecular organized structures occurred under these conditions.
14
Thus, together with electrochemical and computational studies performed on the newly synthesized
15
complexes, chiroptical studies have shed light both on the helicity of complexes as well as on the
16
influence of ion pairing interaction in solution. This outcome is particularly relevant when Pt(II)
17
ionic species are introduced into bioactive environments, also chiral and ionic in nature.
18 19
Acknowledgements
20
Part of this work was supported by the Ministero dell’Istruzione, dell’Università e della Ricerca
21
through PON 2007/2013 ELIOTROPO (PON03PE_00092_2) and through Italian PRIN 2012 (N.
22
2012JHFYMC) and by Regione Calabria (POR CALABRIA FESR 2007/2013, ASSE I, Progetto
23
NUOVALUCE).
24 25
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23 24 25 26 27 28 29
19
1
Highlights
2
•
Water-soluble Pt(II) chiral complexes have been synthesised
3
•
The complexes were characterized by chiroptical techniques
4
•
Electrochemical, DFT and TDDFT studies have been performed
5
•
The photophysical properties of the complexes were deeply investigated in water
6 7 8 9 10
20
1 2 3 4
Graphical abstract
21