Magdalena Małecka1, Agata Szlapa-Kula1, Anna M Maroń1, Przemyslaw Ledwon2, Mariola Siwy3, Ewa Schab-Balcerzak1,3, Karolina Sulowska4, Sebastian Maćkowski4, Karol Erfurt5, Barbara Machura1. 1. Institute of Chemistry, University of Silesia, 9th Szkolna Street, 40-006 Katowice, Poland. 2. Department of Physical Chemistry and Technology of Polymers, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland. 3. Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Sklodowska Street, 41-819 Zabrze, Poland. 4. Nanophotonics Group, Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, 5 Grudziadzka Street, 87-100 Torun, Poland. 5. Department of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland.
Abstract
Rhenium(I) complexes with 2,2':6',2″-terpyridines (terpy) substituted with 9-anthryl (1) and 2-anthryl (2) were synthesized, and the impact of the anthryl linking mode on the ground- and excited-state properties of resulting complexes [ReCl(CO)3(4'-An-terpy-κ2N)] (An─anthryl) was investigated using a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. Different attachment positions of anthracene modify the overlap between the molecular orbitals and thus the electronic coupling of the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores. Following the femtosecond transient absorption, the lowest triplet excited state of both complexes was found to be localized on the anthracene chromophore. The striking difference between 1 and 2 concerns the triplet-state formation dynamics. A more planar geometry of 2-anthryl-terpy (2), and thus better electronic communication between the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores, facilitates the formation of the 3An triplet state. In steady-state photoluminescence spectra, the population ratio of 3MLCT and 3An was found to be dependent not only on the anthryl linking mode but also on solvent polarity and excitation wavelengths. In dimethyl sulfoxide (DMSO), compounds 1 and 2 excited with λexc > 410 nm show both 3MLCT and 3An emissions, which are rarely observed. Additionally, the abilities of the designed complexes for 1O2 generation and light emission under the external voltage were preliminary examined.
Rhenium(I) complexes with 2,2':6',2″-terpyridines (terpy) substituted with 9-anthryl (1) and 2-anthryl (2) were synthesized, and the impact of the anthryl linking mode on the ground- and excited-state properties of resulting complexes [ReCl(CO)3(4'-An-terpy-κ2N)] (An─anthryl) was investigated using a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. Different attachment positions of anthracene modify the overlap between the molecular orbitals and thus the electronic coupling of the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores. Following the femtosecond transient absorption, the lowest triplet excited state of both complexes was found to be localized on the anthracene chromophore. The striking difference between 1 and 2 concerns the triplet-state formation dynamics. A more planar geometry of 2-anthryl-terpy (2), and thus better electronic communication between the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores, facilitates the formation of the 3An triplet state. In steady-state photoluminescence spectra, the population ratio of 3MLCT and 3An was found to be dependent not only on the anthryl linking mode but also on solvent polarity and excitation wavelengths. In dimethyl sulfoxide (DMSO), compounds 1 and 2 excited with λexc > 410 nm show both 3MLCT and 3An emissions, which are rarely observed. Additionally, the abilities of the designed complexes for 1O2 generation and light emission under the external voltage were preliminary examined.
Since the first report on Re(I) carbonyl
diamine complexes by Wrighton
and Morse,[1] a large number of experimental
and theoretical studies have been carried out to understand the ground-
and excited-state properties of [Re(CO)3X(N∩N)] (X—ancillary ligand, n = 0 or 1). Structural modifications of the chelating organic
ligand (N∩N) and variations of the ancillary one
(X = halogens, carboxylates, phosphines, nitriles, isonitriles, etc.)
have been found to be effective tools for fine-tuning the metal-to-ligand
charge-transfer (MLCT) state of these systems or switching it to ligand-to-ligand
charge transfer (LLCT), halogen-to-ligand charge transfer (XLCT),
intraligand charge transfer (ILCT), intraligand (IL), and σ-bond-to-ligand
charge transfer (SBLCT) ones.[2−8] Relatively simple synthetic procedure, thermal stability, and photophysical
properties make Re(I) carbonyl polypyridyl complexes very promising
agents for bioimaging and photodynamic therapy (PDT) of cancer,[9−18] organic light-emitting diodes (OLED),[19−23] and photocatalysis for selective reduction of CO2 to CO.[24−29] The control of electronically excited states in [Re(CO)3X(N∩N)] systems,
and thus the ability to tune their photophysical properties in the
context of such many applications, is generally realized by the manipulation
of the diimine core, substitution of the diimine ligand with π-conjugated
organic chromophores, electron-donating or -withdrawing substituents,
and variations of the ancillary X ligand.[1−29] More specifically, Re(I) carbonyl chromophores with prolonged excited-state
lifetimes have been successfully obtained using two popular concepts:
(i) via the attachment of π-conjugated organic chromophores
to the diimine ligand and formation of bichromophoric systems featuring
the triplet-state equilibrium between 3MLCT and 3IL,[30−32] and (ii) via introduction of electron-rich groups
into the diimine unit and switching the emitting state from 3MLCT to 3ILCT.[33,34] Excellent examples
of such systems are Re(I) complexes with N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide[30] and 1,10-phenanthroline substituted with triphenylamine
at the 5-position,[33] with approximately
3000-fold and 30-fold longer excited-state lifetimes compared to the
parent complex [ReCl(CO)3(phen)], respectively. Dramatic
enhancement of the excited-state lifetime has also been reported among
terpyridine Re(I) systems. As a result of the decoration of C6H5-terpy with the dimethylamine group and formation
of the 3ILCT excited state in [ReCl(CO)3(Me2N-C6H4-terpy-κ2N)],
the lifetime of the resulting complex became ca. 260 times longer
than that for [ReCl(CO)3(C6H5-terpy-κ2N)] with the lowest triplet state of the 3MLCT
character.[34]In our recent work,[35] we investigated
the impact of selected π-conjugated aryl chromophores on the
photophysical behavior of [ReCl(CO)3(4′-Ar-terpy-κ2N)]. We demonstrated
that Re(I) complexes with 2,2′:6′,2″-terpyridines
(terpy) substituted with 1-naphthyl-, 2-naphthyl-, and 9-phenanthryl
are typical 3MLCT emitters, and naphthyl- and phenanthryl
substituents have a negligible effect on the energies of the 1MLCT absorption and 3MLCT emission bands. On the
contrary, the attachment of the electron-rich pyrenyl group leads
to a bathochromic shift of the visible absorption accompanied by a
significant increase in its intensity relative to the parent complex
[ReCl(CO)3(terpy-κ2N)],[36,37] as well as substantial enhancement of the room-temperature (RT)
photoluminescence lifetime of [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] (4.4 μs) in comparison to [ReCl(CO)3(terpy-κ2N)] (3 ns) due to the establishment of the triplet-state equilibrium
between the pyrenyl and Re(I) coordination framework. Further, we
demonstrated that the excitation of [ReCl(CO)3(4′-pyrenyl-terpy-κ2N)] populates predominately the 1ILCT state, which
undergoes the energy transfer to the 1MLCT* state via the
Förster resonance energy transfer (FRET) mechanism. The 1MLCT state formed in this process is transformed into 3MLCT* by femtosecond intersystem crossing (ISC). In the next
step, triplet–triplet energy transfer occurs from the relaxed 3MLCT state to a lower energy 3IL/3ILCT
state localized on the pyrenyl-terpy ligand.Continuing exploration
of this field, two Re(I) complexes with
2,2′:6′,2″-terpyridines (terpy) substituted with
9-anthryl (1) and 2-anthryl (2) were synthesized
to investigate the impact of the anthryl group and its linking mode
on the ground- and excited-state properties of resulting complexes
[ReCl(CO)3(4′-An-terpy-κ2N)] (Scheme ). In similarity
to pyrene, the anthracene chromophore (An) is well known for its valuable
optical and electronic properties. Its attachment to polypyridyl ligands
may lead to the energy transfer from 3MLCT to a long-lived 3IL excited state or, if 3MLCT and 3IL
states have similar energies, formation of an excited-state equilibrium
in metal complexes.[38−45] Anthracene is also a rare example of a compound, which shows ISC
via the upper excited state, that is T2, which is isoenergetic
to the S1 state.[46,47]
Scheme 1
Structural Formulas
of Re(I) Complexes Investigated in This Study
Most remarkably, the only difference between
designed complexes 1 and 2 is the relative
orientation of anthracene
and {ReCl(CO)3(terpy-κ2N)} chromophores,
which was achieved by appending anthracene via its 9-position or 2-position
to the terpy ligand. Therefore, by elimination of additional structural
factors, this unique pair gives the opportunity to explore the impact
of the mutual chromophore orientation and thus their electronic communication
on light-triggered processes in such bichromophoric systems. The optimization
of the photophysical properties of transition metal complexes by spatial
effects still remains a great challenge, and comprehensive studies
in this area are strongly desirable for the rational design of new
photoluminescent materials with predefined photophysical properties.In the present work, an insight into the ground- and excited-state
properties of 1 and 2 was achieved using
a combination of steady-state and time-resolved optical techniques
accompanied by theoretical calculations. In addition, the abilities
of the designed complexes for 1O2 generation
and light emission under the external voltage were preliminary examined.
Experimental Section
Materials
The ligands 9-anthryl-terpy and 2-anthryl-terpy
were prepared according to the literature method.[48] Re(CO)5Cl, solvents for synthesis (of reagent
grade) and for spectroscopic studies (of high-performance liquid chromatography
(HPLC) grade), diphenylisobenzofuran (DPBF), [Ru(bipy)3](PF6)2, poly(9-vinylcarbazole) (PVK, Mn = 25 000–50 000; Sigma-Aldrich),
poly(3,4-(ethylenedioxy)thiophene):poly-(styrenesulfonate) (PEDOT:PSS)
(0.1–1.0 S·cm–1), and substrates with
pixilated indium tin oxide (ITO) anodes (Ossila) were all commercially
available and used without further purification.
Preparation of [ReCl(CO)3(4′-(9-anthryl)-terpy-κ2N)] (1) and [ReCl(CO)3(4′-(2-anthryl)-terpy)]
(2)
The corresponding 4′-(anthracen-9-yl)-2,2′:6′,2″-terpyridine
(9-anthryl-terpy) or 4′-(anthracen-2-yl)-2,2′:6′,2″-terpyridine
(2-anthryl-terpy) ligand (0.27 mmol) was added to a suspension of
[Re(CO)5Cl] (0.27 mmol) and toluene (35 mL). The reaction
mixture was heated at reflux for 8 h under an argon atmosphere. The
formed precipitate was filtered off and washed with diethyl ether,
dried in the air, and then purified by repeated recrystallization
from toluene.
High-resolution mass spectrometry
analyses were performed on a Waters Xevo G2 Q-TOF mass spectrometer
(Waters Corporation) equipped with an ESI source operating in positive-ion
modes. Elemental analysis was recorded on a Vario EL Cube apparatus.
IR spectra were measured using a Nicolet iS5 Fourier transform infrared
(FTIR) spectrophotometer (4000–400 cm–1)
in the form of KBr pellets. Absorption spectra were recorded on a
Thermo Scientific Evolution 220 (solution) and a Jasco V-570 (solid
state as a film deposited on a glass substrate and as blends with
poly(N-vinylcarbazole) (PVK): 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) on a glass
substrate). NMR spectra were recorded on a Bruker Avance 400 NMR spectrometer
(298 K) using DMSO-d6 as the solvent.
Resonance frequencies of 400 and 100 MHz were used for 1H NMR and 13C NMR spectra, respectively.Cyclic
voltammetry (CV) experiments were carried out in a classic three-electrode
system. A Pt wire and a Pt spiral were used as working and counter
electrodes, respectively. A leakless Ag/AgCl electrode (eDAQ) was
used as a reference electrode. The electrode potential was calibrated
against ferrocene. Bu4NPF6 in CH2Cl2 with a concentration of 0.1 mol·dm–3 was used as a supporting electrolyte. Measurements were performed
with an Autolab PGSTAT 100N with a scan rate of 50 mV·s–1.
Photo- (PL) and Electroluminescence (EL) spectra
Steady-state
luminescence spectra were recorded on an FLS-980 fluorescence spectrophotometer
equipped with a 450 W Xe lamp and a high-gain photomultiplier PMT
+ 500 nm (Hamamatsu, R928P) detector. The emission spectra at 77 K
were registered in a butyronitrile rigid matrix. The PL lifetime measurements
were performed with the time-correlated single photon counting (TCSPC)
method. The TCSPC measurements were carried out in optically diluted
solutions using a picosecond laser diode (EPL 405 nm) as the excitation
light source and PMT (Hamamatsu, R928P, Japan) as a detector. The
IRF was designated using the Ludox solution. Decay curves were fitted
using deconvolution fit analysis. For samples with long-lived phosphorescence,
PL lifetime measurements were performed with the multichannel scaling
(MCS) method, and a 60 W microsecond Xe flash lamp was used for sample
excitation. The quantum yields of fluorescence were determined by
the absolute method using the integrating sphere with the solvent
(for argon-saturated solution samples) or the Spectralon reflectance
standard (for powdered samples) as blanks. FLS-980 software was used
to perform the emission correction and calculation of the quantum
yield. To collect electroluminescence (EL) spectra, a precise voltage
supply (Gw Instek PSP-405) with the sample fixed to an XYZ stage was applied. Light from the OLED device was collected through
a 30 mm lens, focused on the entrance slit (50 μm) of a monochromator
(Shamrock SR-303i), and detected using a charge-coupled device (CCD)
detector (Andor iDus 12305). Typical acquisition times were equal
to 10 s. The prealignment of the setup was done using a 405 nm laser.Singlet oxygen generation efficiency was determined by monitoring
the photooxidation of 1,3-diphenylisobenzofuran (DPBF) sensitized
by Re(I) complexes in DMSO. Singlet oxygen quantum yields (ΦΔ) of Re(I) complexes were estimated using [Ru(bipy)3](PF6)2 as the standard (ΦΔO = 0.66 in DMSO).[49,50]
Femtosecond Transient Absorption (fsTA)
Femtosecond
TA spectra were recorded using a pump–probe transient absorption
spectroscopy system (Ultrafast Systems, Helios) described in our previous
work.[35] TA experiments were carried out
for the solution samples (in CHCl3, MeCN, and DMSO), stirred
during the experiments to avoid photoproduct interference. The absorbance
range was equal to 0.20–0.70 in the pumping wavelengths (corresponding
to a concentration of 2.5 × 10–4 mol·dm–3). The 405 and 355 nm pump pulses were used to excite
the samples. Transient absorption data were analyzed using Surface
Xplorer (Ultrafast Systems) and OPTIMUS software.[51,52] A more detailed description of the fsTA studies is given in Figures S20–S25 in the Supporting Information
(SI).
Computational Details
Theoretical calculations were
performed using the Gaussian 09 program package[53] at the density functional theory (DFT) or time-dependent
(TD)-DFT level with the PBE1PBE[54,55] hybrid exchange–correlation
functional, the def2-TZVPD basis set for rhenium, and the def2-TZVP
basis set for other elements.[56,57] In all calculations,
the acetonitrile solvent environment was simulated using the polarizable
continuum model (PCM).[58,59]
Results and Discussion
Synthesis, Molecular Structures, and Stability
The
complexes [ReCl(CO)3(4′-An-terpy-κ2N)] (An = 9-anthryl (1) and 2-anthryl (2)) were obtained in satisfactory yields by reacting 4′-An-terpy
with [Re(CO)5Cl] under reflux in toluene. The identity
and purity of 1 and 2 were determined by
NMR spectroscopy (Figures S1 and S2), FTIR
technique (Figures S3 and S4), HRMS (Figures S5 and S6), and elemental analysis. 1H NMR spectra of complexes 1 and 2 demonstrate the splitting of signals for the protons of the peripheral
pyridine rings, which confirms the κ2N coordination
of the 4′-An-terpy ligand. For 1, the full assignment
of 1H and 13C NMR signals was provided with
the aid of two-dimensional techniques 1H–1H correlated spectroscopy (COSY), 1H–13C heteronuclear multiple quantum correlation (HMQC), and 1H–13C heteronuclear multiple bond correlation (HMBC)
(Figures S1 and S2). The facial geometry
of carbonyl ligands in 1 and 2 was evidenced
by solid-state IR spectroscopy, where the characteristic band pattern
for CO vibrations was observed. Namely, a sharp intense C≡O
stretching band (2024 cm–1 for 1 and
2021 cm–1 for 2) was present in the
spectra along with two poorly resolved bands in the lower energy range
(1927 and 1897 cm–1 for 1 and 1916
and 1893 cm–1 for 2; Figures S3 and S4).[1−8,35]Both Re(I) complexes are
stable in solution (Figure S7) and show
acceptable photostability (Figure S8).Regarding the thermal properties, the complexes showed melting
temperatures of 177 and 217 °C for 1 and 2, respectively, as detected using the differential scanning calorimetry
(DSC) technique. These values are relatively high. After rapid cooling,
the compounds form a stable amorphous phase with glass transition
registered during the second heating scan at 206 and 198 °C for 1 and 2, respectively.
Ground-State Properties
To determine the impact of
the anthryl molecular geometry on the ground-state properties of [ReCl(CO)3(4′-An-terpy-κ2N)], complexes 1 and 2 were investigated by cyclic voltammetry
and absorption spectroscopy.The basic electrochemical parameters,
such as oxidation onset potential (Eox onset), reduction onset potential (Ered onset), ionization potential (IP), and electron affinity (EA) are summarized
in Table . Cyclic
voltammetry curves of 1 and 2 are shown
in Figure . Bu4NPF6/CH2Cl2 was chosen as
an electrolyte due to the good solubility of the studied compounds
in this solvent. Both complexes present multistage oxidation and reduction
characteristics.
Table 1
Electrochemical Properties of Complexes 1 and 2
Eox onseta (V)
Ered onsetb (V)
IPc (eV)
EAd (eV)
Egele (eV)
Egoptf (eV)
1
0.67
–1.76
5.77
3.34
2.43
2.61
2
0.61
–1.69
5.71
3.41
2.30
2.46
Eox onset: oxidation onset potential.
Ered onset: reduction onset potential.
IP: ionization potential estimated
from the equation IP = |e–|(5.1 + Eox onset).
EA: electron affinity estimated
from the equation EA = |e–|(5.1 + Ered onset).
Egel: electrochemical
band gap estimated from the equation ΔEgel = Eox onset – Ered onset.
.
Figure 1
Cyclic voltammetry curves of complexes 1 and 2. Concentration: 2 mmol·dm–3, electrolyte:
Bu4NPF6/CH2Cl2, and scan
rate: 50 mV·s–1.
Cyclic voltammetry curves of complexes 1 and 2. Concentration: 2 mmol·dm–3, electrolyte:
Bu4NPF6/CH2Cl2, and scan
rate: 50 mV·s–1.Eox onset: oxidation onset potential.Ered onset: reduction onset potential.IP: ionization potential estimated
from the equation IP = |e–|(5.1 + Eox onset).EA: electron affinity estimated
from the equation EA = |e–|(5.1 + Ered onset).Egel: electrochemical
band gap estimated from the equation ΔEgel = Eox onset – Ered onset..By analyzing the values of IP and EA, which are related
to the
removal of an electron from the highest occupied molecular orbital
(HOMO) or the addition of an electron to the lowest unoccupied molecular
orbital (LUMO), slight differences between 1 and 2 can be noticed. Most remarkably, complex 2 exhibits
a narrower electrochemical band gap (Egel), indicating
the increase of the effective coupling in comparison to complex 1. This is in line with the results of the DFT calculations
discussed below and optical energy gap values (Table ). Similar relationships were also reported
for other complexes containing anthracene-based ligands, for example
bis(benzo[h]quinolinato) Ir(III) complexes[60] and anthracene-bridged bimetallic ruthenium
vinyl complexes.[61]For both complexes,
the shape of CV curves indicates the electrochemical
irreversibility of reduction and oxidation processes. A noticeable
impact of the anthryl linking mode on the shape of the CV recorded
during the oxidation leads to the assumption that the HOMO is largely
associated with the anthracene substituent. On the other hand, weaker
dependence of the reduction process on the anthryl molecular geometry
may indicate that the LUMO of these systems is located predominantly
on the terpy core.The electronic absorption spectra of 1 and 2 were recorded in solvents of different
polarities and as thin films
(Table S1 and Figure ).
Figure 2
UV–vis absorption spectra of 1 and 2 in solutions (a) and as thin films (b).
UV–vis absorption spectra of 1 and 2 in solutions (a) and as thin films (b).A comparison of spectral features of 1 and 2 in a lower energy region (330–470 nm)
clearly indicates that
the anthryl linking mode has a noticeable impact on the absorption
properties of [ReCl(CO)3(4′-An-terpy-κ2N)]. For complex 1, the absorption in this region
is in principle a sum of πAn → π*An and Re → terpy* transitions. The structured absorption
in the range of 330–390 nm is typical of that of anthracene,
while 1MLCT (Re → terpy*) charge transfer contribution
is visible in the form of a low-energy shoulder above 390 nm.[41,62,63] These observations are supportive
of the weak electronic coupling between the 9-anthryl and terpy units
owing to the strong steric hindrance of the rotation about the C–C
linker in 9-anthryl-terpy.[48,64] On the contrary, complex 2 does not show typical anthracene-like absorption with clear
vibronic progression. In agreement with a more coplanar geometry and
thus more extended delocalization of 2-anthryl-terpy, the 1MLCT/1ILAn absorption band of 2 shifts to lower energies and becomes slightly more intense compared
to that of 1.[48,64,65] A bathochromic shift of the lowest energy absorption of 1 and 2 caused by replacing acetonitrile with less polar
chloroform is due to negative solvatochromism, which is well recognized
for rhenium(I) tricarbonyl diimine complexes.[35,66−70] Intensive bands in the high-energy region of 1 and 2 are attributed to π–π*(terpy) and π–π*(anthracene)
transitions (Figure S9).The impact
of the anthryl group directly attached to the terpy
core via its 9-position (1) and 2-position (2) on the electronic structure of [ReCl(CO)3(4′-An-terpy-κ2N)] was also investigated theoretically at the DFT/PBE0/def2-TZVPD/def2-TZVP
level. The geometry optimization shows good agreement with previously
reported experimental X-ray data for structurally related Re(I) complexes.[35,68,70−73] The 9-anthryl and 2-anthryl groups
introduced in the terpy ligand do not generate any noticeable structural
changes in the {ReClN2C3} coordination core.
The bond lengths and bond angles around the Re(I) ion are almost the
same for 1 and 2 (Table S2). Also, the electrochemical data of 1 and 2 are satisfactorily reproduced by the calculated ionization
potentials and electronic affinities[74] (Table S3). The striking difference between 1 and 2 concerns the dihedral angle between the
plane of the anthryl group and the central pyridine ring (72°
for 1 and 32° for 2).For both
compounds, the lowest unoccupied molecular orbital (LUMO)
largely resides on the terpy core and the highest occupied molecular
orbital (HOMO) is localized on the anthryl substituent, while HOMO–1,
HOMO–2, and HOMO–3 orbitals spread over the {Re(CO)3Cl} moiety (Figures and S10). Depending on the anthryl
linking mode, no clear energy variations are observed for HOMO, HOMO–1,
HOMO–2, and HOMO–3 energy levels between compounds 1 and 2. When considering the LUMO levels of 1 and 2, the replacement of 9-anthryl by 2-anthryl
leads to a slight stabilization of the LUMO and thus causes a decrease
in the HOMO–LUMO gap of 2 (3.28 eV) relative to 1 (3.37 eV). These changes can be rationalized as resulting
from a more coplanar geometry of 2-anthryl-terpy, which leads to a
stronger coupling between the 2-anthryl group and the terpy core.
As a result of more extended overlapping orbitals of the 2-anthryl
and terpy moieties, molecular orbitals LUMO+1 and LUMO+2 of 2 also have higher contributions of the 2-anthryl orbitals
and are better energetically stabilized compared to those of 1. Most importantly, the HOMO–LUMO energy gaps of compounds 1 and 2 become noticeably lower in comparison
to the previously reported [ReCl(CO)3(4′-Ar-terpy-κ2N)] complex bearing the structural isomer 4′-(phenanthren-9-yl)-2,2′:6′,2″-terpyridine
(Figure S11). In contrast to 1 and 2, the HOMO of [ReCl(CO)3(4′-Ar-terpy-κ2N)] with 9-phenanthryl-terpy is distributed over the {ReCl(CO)3} unit,[35] which also confirms a
pivotal role of the molecular configuration of the appended aryl group.
In contrast to linear anthracene, phenanthrene is nonlinear.
Figure 3
Partial molecular
orbital energy-level diagrams for compounds 1 and 2.
Partial molecular
orbital energy-level diagrams for compounds 1 and 2.To examine the nature of electronic transitions
of obtained complexes,
time-dependent DFT (TD-DFT) calculations were performed using optimized
structures of 1 and 2. The calculated transitions
along with the experimental spectra and natural transition orbitals
for the lowest energy band of 1 and 2 are
presented in Figure S12. For both complexes,
the excitation S0 → S1 can be assigned
to the intraligand πAn → π*terpy and πAn → π*An/π*terpy transition for 1 and 2, respectively.
A twist of the 2-anthryl group, which is smaller than that of the
9-anthryl one, results in an increase in the oscillator strength of
the S0 → S1 transition for complex 2, which correlates with the stronger absorptivity (ε)
of 2 relative to that of 1 in the visible
region. The character πAn → π*terpy/π*An can also be assigned to the S0 →
S4 transition, while the transitions S0 →
S2 and S0 → S3 are of metal-to-ligand
charge transfer (MLCT) nature (Table S4).
Excited-State Properties
Photoluminescence spectra
of 1 and 2 are displayed in Figures and S13–S19. Upon excitation into the red side of the lowest energy absorption,
with λexc > 400 nm, the complexes in acetonitrile
solution show broad and unstructured emission with maximum at 627
nm for 1 and 609 nm for 2 (Table ). With reference to previously
reported compounds [ReCl(CO)3(4′-Ar-terpy-κ2N)] incorporating 2,2′:6′,2″-terpyridines
functionalized with 1-naphthyl, 2-naphthyl, and 9-phenanthryl groups,
it can be derived that the emission of 1 and 2 in MeCN originates predominately from the 3MLCT excited
state (Figure S15). However, as shown in Figure S15, the emission bands of 1 and 2 are not completely superimposed over those for
[ReCl(CO)3(terpy-κ2N)] and [ReCl(CO)3(4′-Ar-terpy-κ2N)] substituted with
1-naphthyl, 2-naphthyl, and 9-phenanthryl. Both maxima and onsets
of the 3MLCT emission of 1 and 2 are slightly blue-shifted relative to those previously reported,
which may suggest some contribution of residual fluorescence due to
any incomplete FRET from 1IL to 1MLCT, as reported
previously for other bichromophoric systems.[31,32,75−77]
Figure 4
Photoluminescence spectra
of 1 and 2 in
different environments upon excitation at the red side of the lowest
energy absorption (a) and 355 nm (b).
Table 2
Summary of the Photoluminescence Properties
of Complexes 1 and 2a
CHCl3
MeCN
DMSO
CH3OH:C2H5OH (77 K)
λexc (nm)
λem (nm)
τav (ns)
φ (%)
λexc (nm)
λem (nm)
τav (ns)
φ (%)
λexc (nm)
λem (nm)
τav
φ (%)
λexc (nm)
λem (nm)
τav (μs)
1
445
511
4.91
3.85
430
627
2.26
4.08
430
I627
2.83 ns
0.02
430
I587,
I3.6
II711
14.28 μs
II688, 761
II12 069
2
450
580
7.30
1.78
430
609
3.97
0.05
430
I593
5.75 ns
1.40
470
707, 784
2590
II712
22.71 μs
τav: average lifetime
of multiexponential fits of decay curves (see Figure S11 in the SI). φ: luminescence quantum yield.
Photoluminescence spectra
of 1 and 2 in
different environments upon excitation at the red side of the lowest
energy absorption (a) and 355 nm (b).τav: average lifetime
of multiexponential fits of decay curves (see Figure S11 in the SI). φ: luminescence quantum yield.The contribution of the residual fluorescence becomes
more noticeable
when acetonitrile solutions of 1 and 2 are
excited at the blue side of the lowest energy absorption. Irradiation
with λexc = 355 nm predominately populates the 1An state. Higher energy excitation results in dual fluorescence–phosphorescence
emission for 1 and a significant hypsochromic shift of
the structureless emission band for 2 (Figure and Table S5). The replacement of acetonitrile by less polar chloroform
induces noticeable changes in the photophysical properties of 1 and 2. In this case, the emission of 1 excited at 355 nm and λexc > 410 nm
appears
at significantly shorter wavelengths (∼510 nm), and it can
be assigned to 1IL/1ILCT fluorescence. The phosphorescence
of the 3MLCT origin of 1 may be effectively
masked by brighter and faster fluorescence or even quenched by the
lower-lying triplet state localized on the anthracene moiety, which
is well recognized for bichromophoric systems with the anthracene
unit.[32,41,78,79] The structureless emission band of 2 in CHCl3 upon excitation λexc > 410
nm appears at ∼580 nm, which is bathochromically shifted by
∼135 nm compared to the free ligand in CHCl3 (Figure S16). In addition, compared to the emission
from the 3MLCT excited state of 2 in MeCN,
it is blue-shifted by ∼30 nm. This allows us to assume that
the photophysics of 2 in CHCl3 is determined
by both the 1IL/1ILCT fluorescence and 3MLCT phosphorescence. The presence of two components is clearly
noticeable upon excitation of 2 in CHCl3 at
410 nm (Figure S14). In DMSO, two emission
bands are observed for both complexes 1 and 2 excited with λexc > 410 nm. The broad and weak
component at higher energies falls in the range of 3MLCT
phosphorescence, while the structured emission at longer wavelengths
(>680 nm) can be safely assigned to 3An. Regarding excited-state
lifetimes of 3MLCT and 3An, it can be assumed
that they decay independently, there is no evidence of formation of
the triplet-state equilibrium between the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores.Higher energy
excitation (λexc = 355 nm) of 1 in DMSO
results in dual fluorescence–phosphorescence
ligand-centered emission for 1, while the emission spectrum
of 2 still shows both 3MLCT and 3An components. However, compared to the spectra measured for lower
energy excitation (λexc = 430 nm), the relative contribution
of the 3MLCT component is considerably enhanced. Most importantly,
the anthracene-related room-temperature phosphorescence is rarely
observed. For the first time, this was reported for Pt(II) bisacetylide
complexes,[80] and its presence was rationalized
by the enhanced effect of the heavy atom on anthracene.To obtain
further insights, the photoluminescence of [ReCl(CO)3(4′-An-terpy-κ2N)] was examined at
a low temperature (77 K). The frozen-state emission spectra also revealed
noticeable differences between 1 and 2 (Figure ). For compound 1, the emission spectrum at 77 K consists of two components:
a broad and slightly structured emission band in the range of 480–670
nm, which overlaps with the phosphorescence of [ReCl(CO)3(4′-Ar-terpy-κ2N)] incorporating 2,2′:6′,2″-terpyridines
functionalized with 1-naphthyl, 2-naphthyl, and 9-phenanthrenyl groups
(Figure S17), and a well-structured emission,
which is superimposed over the phosphorescence of the free ligand
and anthracene (Figure S18). A weak vibronic
structure of the high-energy emission is assigned to a small contribution
of 3ILterpy, as reported for Re(I) carbonyl
complexes with naphthyl- and phenanthrenyl-substituted terpyridine
ligands.[35] In contrast, complex 2 shows mainly the emission band with a well-resolved vibronic structure
in the range of 680–830 nm, originating from the 3ILAn excited state. Relative to complex 1, 3An emission of 2 appears in a lower energy
region, which can be rationalized by the negligible
population of the 3MLCT excited state, contrary to complex 1 (Figure ).The formation of the 3MLCT state upon excitation
of 2 was evidenced more clearly by time-resolved emission
spectra
(TRES) recorded at 77 K. As demonstrated in Figure , the 3MLCT excited state of 2 almost immediately undergoes triplet–triplet energy
transfer into 3IL localized on the 2-anthryl-terpy ligand.
Another striking difference between 1 and 2 concerns the relative contributions of the residual fluorescence,
which is significantly larger for complex 1.
Figure 5
(a) Time-resolved
emission spectra for 1 and 2 in the BuCN
rigid matrix at 77 K (excitation wavelength:
405 nm and time window: 80 ms for 1, and excitation wavelength:
405 nm and time window: 20 ms for 2). (b) Decay curves
for the maxima of emission bands observed in time-resolved emission
spectra for 1 and 2 in the BuCN rigid matrix
at 77 K.
(a) Time-resolved
emission spectra for 1 and 2 in the BuCN
rigid matrix at 77 K (excitation wavelength:
405 nm and time window: 80 ms for 1, and excitation wavelength:
405 nm and time window: 20 ms for 2). (b) Decay curves
for the maxima of emission bands observed in time-resolved emission
spectra for 1 and 2 in the BuCN rigid matrix
at 77 K.The nature of the triplet excited state of [ReCl(CO)3(4′-An-terpy-κ2N)] was also investigated
theoretically. In this regard, structures of 1 and 2 were optimized in their triplet states (T1).
The character of the lowest energy triplet excited state was assigned
using the spin density surfaces generated from the lowest energy triplet
state, and the phosphorescence energies were determined as the difference
between the ground singlet and triplet states ΔET.As shown in Figure , the spin density
surfaces of both 1 and 2 were distributed
on the anthryl substituent and central pyridine
of terpy, supporting the 3An/3(An-terpy) character
of the lowest energy triplet state. Calculated phosphorescence energies,
905 nm for 1 and 886 nm for 2, correlate
well with the lowest energy shoulder of the structured triplet emission
band of anthracene.[81]
Figure 6
Spin density surface
plots for 1 and 2. Gray and green colors
show regions of positive and negative spin
density values, respectively (see also Table S6 in the SI).
Spin density surface
plots for 1 and 2. Gray and green colors
show regions of positive and negative spin
density values, respectively (see also Table S6 in the SI).
Femtosecond Transient Absorption—The Nature of the Lowest
Triplet-State and Excited-State Dynamics
The nature of the
lowest triplet-state and excited-state dynamics of 1 and 2 was investigated by applying pump–probe transient
absorption with femtosecond (fsTA) resolution. The measurements were
performed for solutions of 1 and 2 in CHCl3, MeCN, and DMSO upon excitation at 355 nm, as well as for 1 and 2 in CHCl3 excited at 405 nm.
After photoexcitations at 355 and 405 nm, both 1An and 1MLCT excited states are simultaneously populated, but a higher 1An/1MLCT population ratio is expected in the case
of 355 nm excitation.The results of fsTA experiments of complexes 1 and 2 together with global fitting analysis
are summarized in Figures and S21–S25.
Figure 7
Summary of
fsTA measurements for complexes 1 and 2 excited
at 355 nm: fsTA 2D maps (A), TA spectra at selected
time delays (B), and decay-associated spectra (DAS) (C) (see also Figures S21–S25).
Summary of
fsTA measurements for complexes 1 and 2 excited
at 355 nm: fsTA 2D maps (A), TA spectra at selected
time delays (B), and decay-associated spectra (DAS) (C) (see also Figures S21–S25).The most intense excited-state absorption (ESA)
of 1 in the range of 410–460 nm is a typical spectral
feature
corresponding to the T1 → T transitions of anthracene.[46,47,65,78,82,83] It is the only signal at longer time delays,
confirming that the lowest triplet state is localized on the anthracene
chromophore. The ESA band starts rising almost immediately after photoexcitation
and persists up to the end of the delay stage, as the 3An lifetime is much longer than the maximum pump–probe time
delay of the fsTA setup.[81] Regarding the
spectral changes occurring at early time delays and previous reports
on related systems,[37,47,84,85] we can assume that the final spectral component
(3An) of 1 is formed via two paths, 1MLCT → 3MLCT → 3An and S1(1An) → T2(3An) →
T1(3An). The population of 3An via
two channels is clearly evidenced by the biphasic kinetics of growth
displayed on time trace at 426 nm (see Figures , S23, and S24 showing time traces for selected wavelengths along with fitting
curves obtained via global analysis). The 3MLCT state is
represented by ESA bands with maxima at ∼385 and ∼495
nm, in agreement with the results for the parent chromophore [ReCl(CO)3(terpy-κ2N)][37] (Figure S25). The signal in the UV region
is attributed to the absorption of polypyridine anion radicals, while
ESA in the visible part corresponds to ligand-to-metal charge transfer
(LMCT, Cl/L•– → Re) transitions.[37,84,85] As reported by Yang,[47] spectral features attributable to T2(3An) → T(3An) transitions are expected in the range of 500–600 nm, partly
overlapping with the ESA of LMCT.
Figure 8
Time traces for representative wavelengths
of 426 nm (maximum of
T1(3An) → T(3An) ESA), 510 nm (red edge of 3LMCT ESA),
and 570 nm (maximum of T2(3An) → T(3An) ESA) for 1 in
MeCN excited at 355 nm.
Time traces for representative wavelengths
of 426 nm (maximum of
T1(3An) → T(3An) ESA), 510 nm (red edge of 3LMCT ESA),
and 570 nm (maximum of T2(3An) → T(3An) ESA) for 1 in
MeCN excited at 355 nm.Time constants determined from global fit analysis
are summarized
in Table , and decay-associated
spectra (DAS) are shown in column C in Figure . The ultrafast intersystem crossings, which
occur in the time range shorter than the instrument response, are
not included in Table . Besides the discarded ultrafast time constants, which can be safely
assigned to the ultrafast ISC from the initially populated 1MLCT to 3MLCT,[37,84,85] four components are involved in the fsTA kinetics of 1. The lifetimes t2, t3, and t4 can be attributed
to S1(1An) → T2(3An), T2(3An) → T1(3An), and 3MLCT → 3An processes, respectively.
DAS5 (with t5 = inf) corresponds
to the absorption spectrum of the fully relaxed lowest triplet state 3An, which does not completely decay within the time scale
of the measurement. The photophysical processes occurring upon photoexcitation
of 1 are summarized in Scheme .
Table 3
Summary of the Time Constants (ps)
from Global Fit Analysis
405 nm
355 nm
t (ps)
CHCl3
CHCl3
MeCN
DMSO
1
t1
0.18
t2
1.53
1.61
1.93
2.18
t3
92.9
97.37
52.0
269.6
t4
1964
1360
1730
1621
t5
inf
inf
inf
inf
2
t1
0.58
0.23
0.18
0.29
t2
168.8
172.5
105.1
465.3
t3
inf
inf
inf
inf
Scheme 2
Representative Energy-Level Diagram along
with Photophysical Processes
Occurring upon Photoexcitation of 1
The energies of 1LC
and 1MLCT were estimated from onsets of the lowest energy
absorption bands of the corresponding ligand and parental [ReCl(CO)3(terpy-κ2N)] in CHCl3, respectively,
while 3LC and 3MLCT energies were obtained from
onsets of 77 K phosphorescence of the organic ligand and the room-temperature
emission of [ReCl(CO)3(terpy-κ2N)] in
CHCl3, respectively.
Representative Energy-Level Diagram along
with Photophysical Processes
Occurring upon Photoexcitation of 1
The energies of 1LC
and 1MLCT were estimated from onsets of the lowest energy
absorption bands of the corresponding ligand and parental [ReCl(CO)3(terpy-κ2N)] in CHCl3, respectively,
while 3LC and 3MLCT energies were obtained from
onsets of 77 K phosphorescence of the organic ligand and the room-temperature
emission of [ReCl(CO)3(terpy-κ2N)] in
CHCl3, respectively.Excitation of 2 at both 355 and 405 nm
results in
an instant appearance of excited-state absorption (ESA) with the main
peak at 460 nm and a minor one at 640 nm. Within about 2–5
ps, the intensity of the ESA at 460 nm increases and then starts to
slowly decrease. The rise of the higher energy band is accompanied
by a slight intensity decrease of the minor peak within this time
scale. In analogy to 1, the dominant ESA corresponding
to the T1 → T transitions
of the anthracene unit does not completely decay within the time scale
of the measurements, but a more planar geometry of 2-anthryl-terpy,
and thus stronger electronic communication between the anthracene
and {ReCl(CO)3(terpy-κ2N)} chromophores,
facilitates substantial population of the 3An triplet state.
Compared to 1, however, T1 → T absorption band shows noticeable red shift
and broadening, which can be attributed to the stronger electronic
coupling between An and terpy moieties in complex 2.For the best fitting of transient spectra kinetics of 2 upon excitation at 355 and 405 nm, three time constants were required
(Table ). Over the
course of the first time constant (Table ), the vibrationally hot 3An excited
state is formed. DAS2 (corresponding to t2), which is positive in the region corresponding to the
fully relaxed lowest triplet state 3An, reflects the vibrational
relaxation of the lowest triplet state 3An, comprising
reorganization within the [ReCl(CO)3(4′-An-terpy-κ2N)] bichromophore and interacting solvent molecules. DAS3 with infinite lifetime corresponds to ground state recovery.
Abilities for 1O2 Generation and Light
Emission under the External Voltage
Since both complexes
have long excited-state lifetimes (14 μs for 1 and
22 μs for 2 in DMSO), it can be anticipated that
they are suitable for transferring the excited triplet-state energy
to molecular oxygen, generating singlet oxygen. The ability of Re(I)
complexes for photosensitized generation of singlet oxygen was examined
by an indirect method with the use of diphenylisobenzofuran (DPBF),
which is highly sensitive to 1O2. Reacting with 1O2, diphenylisobenzofuran forms endoperoxide, which
spontaneously decomposes to 1,2-dibenzoylbenzene.[49,86−88] As shown in Figure , both complexes 1 and 2 exhibit
singlet oxygen sensitizing ability. The insignificantly enhanced singlet
oxygen sensitizing ability of 2 (ΦΔO = 0.45) in relation to 1 (ΦΔO = 0.42) correlates with its longer triplet
excited-state lifetime in DMSO at room temperature (Table ). For both complexes, the values
of ΦΔO are lower than that for
anthracene (0.57) but ∼2 times greater than those for free
ligands (0.21 for 9-anthryl-terpy and 0.23 for 2-anthryl-terpy; Figures S27 and S28). Relative to the efficient
[Ru(bipy)3](PF6)2 photosensitizer
(ΦΔO: 0.66), however, the absorbance
plots of DPBF in the presence of 1 and 2 display lower slopes, indicating their weaker abilities for 1O2 generation.
Figure 9
UV–vis absorption spectra of diphenylisobenzofuran
(DPBF)
in DMSO (c: 50 μM) treated with complex 1 (c: 50 μM) upon exposure to visible
light at 420 nm recorded over 240 s (a) and relative changes in the
absorbance of DPBF at 417 nm (A/A0) with time (b).
UV–vis absorption spectra of diphenylisobenzofuran
(DPBF)
in DMSO (c: 50 μM) treated with complex 1 (c: 50 μM) upon exposure to visible
light at 420 nm recorded over 240 s (a) and relative changes in the
absorbance of DPBF at 417 nm (A/A0) with time (b).To perform preliminary tests of [ReCl(CO)3(An-terpy-κ2N)] ability for light emission induced
by voltage, two types
of diodes with structures of ITO/PEDOT:PSS/complex/Al and ITO/PEDOT:PSS/PVK:PBD:complex/Al
were fabricated. The matrix consisting of poly(9-vinylcarbazole) (PVK,
50 wt %) and (2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole)
(PBD, 50 wt %) was applied due to its effective charge carrier transport
of ambipolar character. Moreover, diodes with such a guest–host
configuration were also utilized in our previous investigations concerning
Re(I) complexes.[35−37,67,68,70,71,73,82,89] As can be seen in Figure b, absorption of 1 and 2 partially overlaps with the emission spectrum of the host
(matrix PVK:PBD), thus Förster energy transfer can be expected.[90] However, only the diode with complex 1 as an active layer showed weak emission with the maximum electroluminescence
(EL) band (λEL) at 715 nm under 20 V, contrary to
the device based on 2, which was nonemissive. With reference
to the discussion in the previous section, this weak emission can
be assigned to anthracene phosphorescence.In the case of diodes
with guest–host structures, the emission
was dependent on the applied voltage, and an increase of the applied
voltage resulted in an increase in the light emission. Nevertheless,
rather high voltages were required to detect light emission (Figure ).
Figure 10
Electroluminescence
spectra of diodes based on investigated Re(I)
complexes with anthryl groups.
Electroluminescence
spectra of diodes based on investigated Re(I)
complexes with anthryl groups.Most importantly, the electroluminescence (EL)
band of guest–host
diodes covers a broad range from the visible (500 nm) to near-infrared
region (750–900 nm), with λEL located at about
638 and 705 nm for 1 and 600, 705, and 778 for 2. The addition of another component, emitting blue light
from 400 to 500 nm, may yield a diode, which emits white light (WORED).
The EL spectra are similar to the photoluminescence spectra of 1 and 2 registered in DMSO at RT and a rigid
matrix at 77 K in BuCN (Figure ). Regarding the complex content, the addition of 1 wt % gives
the most intense emission. In turn, taking into consideration the
anthryl linking mode, significantly more intense EL showed a device
based on the complex with 2-anthryl substituent (2),
which well correlates with the higher PL quantum yield of 2 in DMSO relative to 1. To the best of our knowledge,
such broad and structured emission, ranging to 900 nm with two or
three maxima, was observed for the first time in diodes based on [ReCl(CO)3(4′-Ar-terpy-κ2N)] compounds.[35−37,67,68,70,71,73,82,89] Indeed, diodes reported previously by our research group, Re(I)
complexes based on 2,2′:6′,2″-terpyridines substituted
with conjugated aryl groups such as 1-naphthyl, 2-naphthyl, 9-phenanthrenyl,
and 1-pyrrenyl applied as guests in the PVK:PBD matrix (1, 2, and
15 wt %), exhibited λEL in ranges of 595–600,
595–610, 585–610, and 640–650 nm, respectively.[35] In addition, devices with molecularly dispersed
Re(I) complexes bearing electron-donating amine units also showed
emission in a significantly narrower spectral range.[72,91]
Conclusions
In summary, in-depth studies of the electrochemical
and optical
properties of two Re(I) complexes with 2,2′:6′,2″-terpyridines
(terpy) substituted with the 9-anthryl (1) and 2-anthryl
(2) confirmed a noticeable impact of the anthryl linking
mode on the ground- and excited-state properties of resulting [ReCl(CO)3(4′-An-terpy-κ2N)]. As a result of
the more extended overlapping orbitals of 2-anthryl and terpy moieties,
the absorption band of 2 is shifted to lower energy and
becomes slightly more intense compared to that of 1 with
a clearly twisted geometry of 9-anthryl-terpy. The stronger electronic
coupling between the anthracene and {ReCl(CO)3(terpy-κ2N)} chromophores also facilitates the population of the 3An triplet state. Following the femtosecond transient absorption,
the excitation of 2 results in an instant appearance
of excited-state absorption (ESA) corresponding to the T1 → T transitions of the anthracene
unit, while the 3An ESA of 1 is formed significantly
slower, and its rise occurs via two independent channels. It is worth
noticing that complexes 1 and 2 are rare
examples that show anthracene-related room-temperature phosphorescence.
However, although complexes 1 and 2 in DMSO
excited with λex > 410 nm show both 3MLCT
and 3An emission bands, there was no evidence of formation
of the equilibrium between the triplet states localized on anthracene
and {ReCl(CO)3(terpy-κ2N)} chromophores.
The excited states 3MLCT and 3An decay independently
with different excited-state lifetimes. The presence of both 3MLCT and 3An emissions was also observed in the
electroluminescence band of guest–host diodes based on investigated
Re(I) complexes. Having long excited-state lifetimes in DMSO, the
examined carbonyls were found to be suitable for transferring the
excited triplet-state energy to molecular oxygen, generating singlet
oxygen. We strongly believe that reported findings will be useful
for understanding and controlling the excited-state nature of transition
metal complexes and thus designing photoluminescent materials with
predefined photophysical properties.
Authors: Nalaka P Liyanage; Weiwei Yang; Stephen Guertin; Sayontani Sinha Roy; Casey A Carpenter; Rebecca E Adams; Russel H Schmehl; Jared H Delcamp; Jonah W Jurss Journal: Chem Commun (Camb) Date: 2019-01-17 Impact factor: 6.222
Authors: Melissa V Werrett; Gregory S Huff; Sara Muzzioli; Valentina Fiorini; Stefano Zacchini; Brian W Skelton; Antonio Maggiore; Joanna M Malicka; Massimo Cocchi; Keith C Gordon; Stefano Stagni; Massimiliano Massi Journal: Dalton Trans Date: 2015-05-14 Impact factor: 4.390
Authors: James E Yarnell; Kaylee A Wells; Jonathan R Palmer; Josué M Breaux; Felix N Castellano Journal: J Phys Chem B Date: 2019-08-23 Impact factor: 2.991
Authors: Amal El Nahhas; Andrea Cannizzo; Frank van Mourik; Ana María Blanco-Rodríguez; Stanislav Zális; Antonín Vlcek; Majed Chergui Journal: J Phys Chem A Date: 2010-06-10 Impact factor: 2.781
Scheme 1
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 8
Scheme 2
Figure 9
Figure 10