Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties

Article abstract of DOI:10.1002/chem.202001209

A strategy is presented to improve the excited state reactivity of homoleptic copper–bis(diimine) complexes CuL₂⁺ by increasing the steric bulk around Cu^I whereas preserving their stability. Substituting the phenanthroline at the 2-position by a phenyl group allows the implementation of stabilizing intramolecular π stacking within the copper complex, whereas tethering a branched alkyl chain at the 9-position provides enough steric bulk to rise the excited state energy E⁰⁰. Two novel complexes are studied and compared to symmetrical models. The impact of breaking the symmetry of phenanthroline ligands on the photophysical properties of the complexes is analyzed and rationalized thanks to a combined theoretical and experimental study. The importance of fine-tuning the steric bulk of the N–N chelate in order to stabilize the coordination sphere is demonstrated. Importantly, the excited state reactivity of the newly developed complexes is improved as demonstrated in the frame of a reductive quenching step, evidencing the relevance of our strategy.

Full text of DOI:10.1002/chem.202001209

Accepted Article
Accepted Article
Title: Non symmetrical sterically challenged phenanthroline ligands and
their homoleptic copper(I) complexes with improved excited state
properties
Authors: Lea Gimeno, Errol Blart, Jean-Noël Rebilly, Marina Coupeau,
Magali Allain, Thierry Roisnel, Alexis Quarré de Verneuil,
Christophe Gourlaouen, Chantal Daniel, and Yann Pellegrin
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001209
Link to VoR: https://doi.org/10.1002/chem.202001209
01/2020

10.1002/chem.202001209
Chemistry - A European Journal
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Non symmetrical sterically challenged phenanthroline ligands
and their homoleptic copper(I) complexes with improved excited
state properties
Lea Gimeno,[a] Errol Blart,[a] Jean-Noël Rebilly,[b],* Marina Coupeau,[a] Magali Allain,[c] Thierry Roisnel,[d]
Alexis Quarré de Verneuil,[a] Christophe Gourlaouen,[e],* Chantal Daniel,[e] and Yann Pellegrin[a],*
Abstract: A strategy is presented to improve the excited state
excitation (equation 1), the metal formally shifts from Cu(I)
(favored tetrahedral geometry) to Cu(II) (favored square-planar
geometry).
+
reactivity of homoleptic copper-bis(diimine) complexes CuL2 by
increasing the steric bulk around Cu(I) while preserving their stability.
Substituting the phenanthroline on position 2 by a phenyl group allows
the implementation of stabilizing intramolecular π stacking within the
copper complex, while tethering a branched alkyl chain on position 9
provides enough steric bulk to rise the excited state energy E00. Two
novel complexes are thus studied and compared to symmetrical
models. The impact of breaking the symmetry of phenanthroline
ligands on the photophysical properties of the complexes is analyzed
and rationalized thanks to a combined theoretical and experimental
study. The importance of fine-tuning the steric bulk of the N-N chelate
in order to stabilize the coordination sphere is demonstrated.
Importantly, the excited state reactivity of the newly developed
complexes is improved as demonstrated in the frame of a reductive
quenching step, evidencing the relevance of our strategy.
[CuIL2]+ + hv → [CuIIL•-L]+ (equation 1)
The coordination sphere relaxes in the [CuIIL•-L]+ excited state by
flattening which opens several channels for efficient and counter-
productive extinction of the luminescence.[3] Consequently, the
excited state energy levels dynamically shift to lower energy on a
typical 10-100 ps time scale, favoring non radiative de-excitation
of the excited states (gap law). Besides, flattening of the transient
copper(II) ion opens a fifth binding site for a ligand (e.g. a solvent
molecule) leading again to the very fast so-called “exciplex
quenching” of the excited state.[4] Encumbering groups limit the
extent of the photo-induced flattening, thus promoting the
luminescence
of
copper(I)
complexes.
Consequently
considerable efforts have been made to improve the
luminescence of copper(I) complexes by increasing the steric bulk
around Cu(I).[5] The latter can be brought by either aliphatic (type
Introduction
I
complexes, figure 1) or aromatic (type-II complexes)
substituents. Impressive luminescence quantum yields have been
obtained with type-I complexes by Castellano and co-workers
with [Cu(dipp)2]+, [Cu(tmdsbp)2]+,[5b, c, 6] or Burstyn and co-workers
with [Cu(dtbp)2]+[5a, 7] (where dipp stands for 2,9-diisopropyl-1,10-
phenanthroline, tmdsbp stands for 2,9-di-secbutyl-3,4,7,8-
tetramethyl-1,10-phenanthroline and dtbp for 2,9-ditert-butyl-
1,10-phenanthroline). Remarkably, [Cu(dtbp)2]+ featured even
better photophysical parameters than [Ru(bpy)3]2+ (luminescence
lifetime of 3.26 µs vs. ca. 1 µs respectively) but the stability of the
coordination scaffold was sacrificed. Indeed, the copper(I)
complex is so sterically burdened that it quantitatively
decomposes in presence of a weak ligand like acetonitrile.[5a]
Type-II [Cu(dpp)2]+ reported by Sauvage and co-workers (and
parent complexes) also proved very luminescent[8] even at room
temperature in presence of strongly coordinating species (e.g.
water or acetonitrile): the complex is rigidified and shielded by the
phenyl groups, π-stacked to the vicinal phen ligand.[9] But the
latter interactions substantially distort the coordination cage
entailing a decrease of the singlet state energy E00.[10] This in turn
impact negatively the reactivity of the excited state, by virtue of
the Rehm and Weller equation.
Copper(I) complexes of the general formula [Cu(NN)2]+ where NN
is a diimine ligand bearing bulky substituents in α of the nitrogen
atoms are appealing luminophores in the frame of photosensitive
coordination compounds.[1] Indeed, they exhibit photo-physical
properties which are similar to those of benchmark [Ru(bpy)3]2+.[1a,
1c, 2]. In particular they are luminescent upon excitation in the Metal
to Ligand Charge Transfer (MLCT), provided that encumbering
groups are appended in α of the chelating nitrogen atoms. During
[a]
Lea Gimeno, Marina Coupeau, Alexis Quarre de Verneuil, Dr. Errol
Blart, Dr. Yann Pellegrin
Université de Nantes, CNRS
CEISAM UMR6230
F-44000 Nantes, France
E-mail: yann.pellegrin@univ-nantes.fr
Dr. Jean-Noël Rebilly
ICMMO, UMR 8182
Université Paris Sud, Université Paris Saclay
Orsay 91405 cedex, France
Magali Allain
Laboratoire MOLTECH-Anjou, UMR CNRS 6200
Univ Angers, SFR MATRIX
2 Bd Lavoisier, 49045 Angers Cedex, France
Thierry Roisnel
[b]
[c]
[d]
Université de Rennes CNRS
Institut des Sciences Chimiques de Rennes
UMR6226
F-35000 Rennes, France
[e]
Dr. Christophe Gourlaouen, Pr. Chantal Daniel
Laboratoire de Chimie Quantique Institut de Chimie UMR 7177
CNRS-Université de Strasbourg, 4, Rue Blaise Pascal CS 90032, F-
67081 Strasbourg Cedex, France
Supporting information for this article is given via a link at the end of
the document.
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energy of the excited state can be increased and how it impacts
the reactivity of the excited state.
Results and Discussion
The synthesis of ligands L1 and L2 was performed in two steps,
by addition of the corresponding organolithium reagents on
anhydrous phenanthroline and re-aromatization with MnO2
(scheme 1). Other synthetic pathways have been envisioned (for
instance, an improved Povarov approach has been successfully
developed to synthesize ligand L1)[11] but the organolithium route
proved to be more efficient in our hands. The synthesis of the
three corresponding copper(I) complexes proceeded smoothly,
by mixing two equivalents of ligand with one equivalent of
tetrakis(acetonitrile)copper hexafluorophosphate in degassed
dichloromethane.
Figure 1. Illustration of the strategy developed in this contribution
Summarily, very bulky aliphatic groups like tert-butyl in α of the
nitrogen atoms endow the corresponding copper(I) complex with
long lived, intense luminescence and high E00 while aromatic
groups confer improved stability to the coordination cage,
maintain significant luminescence lifetime and quantum yield, but
lead to a smaller E00.
Scheme 1. Synthesis of ligands L1, L2 and of complexes C1 and C2.
The bright red powders were characterized by 1H-NMR and mass
spectrometry, all the analyses confirm the proposed structures.
Monocrystals of ligand L2 and complex C2 were obtained by slow
In this contribution, we explore
a strategy to implement
diffusion of hexane in
a dichloromethane solution of the
intramolecular π-stacking while maintaining the tetrahedral
geometry to secure high E00. Our approach is based on non-
symmetrical phen ligands substituted by one phenyl group on one
side and one bulky branched alkyl chain (isopropyl: ligand L1; or
tert-butyl: ligand L2) on the other.
corresponding molecule, and their structures could be resolved
by X-ray diffraction. Relevant parameters are reported in table
ST1 (supporting information). The structure of L2 (figure S1,
supporting information) reveals as expected a very sterically
strained environment around chelating nitrogen atoms. The
phenyl group tethered to C12 is slightly twisted with respect to the
phenanthroline plane (twist angle = ca. 25.3°), due to repulsion
between protons H11 and H14 and possibly crystal packing forces.
The structure of complex C2 is given in figure 3 (ORTEP view
available in figure S2). Basically, the CuN4 coordination sphere is
a flattened tetrahedron; the τ4 parameter for C2 is almost exactly
the average between [Cu(dpp)2]+ and [Cu(dtbp)2]+ (figure S6).
However C2’s structure is closer to a type-I complex than a type-
II, despite the presence of the aryl group in α of the nitrogen atom.
Figure 2. Structures of the complexes investigated therein.
The resulting homoleptic copper complexes, C1 and C2 (figure 2)
are a “fusion” of [Cu(dpp)2]+ with [Cu(dipp)2]+ and [Cu(dtbp)2]+,
respectively, and would benefit from the stabilizing intramolecular
phenyl-phenanthroline π-stacking interactions while the steric
bulk provided by the alkyl chains (isopropyl or tert-butyl)
contribute reaching a high E00. We describe the synthesis and full
optical and electrochemical characterization of C1 and C2. The
structural and optical properties of the two complexes have been
analyzed by means of density functional theory (DFT). Our goals
are 1) to find out if intramolecular π-stacking can take place within
the complex despite the presence of bulky ramified chain on the
same ligand; 2) to assess the stability of the corresponding
coordination scaffold vs. the steric strain; 3) to observe if the
Figure 3. Two views for the structure of complex C2.
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Rather unfavorable angles between phenyl groups C and E and
phenanthroline B and A respectively (aromatic cycles and
relevant atom nomenclature is given in figure S3) are measured
line with the favorable π-stacking interactions between the phenyl
and the adjacent phenanthroline.[15]
+
(more than 30° vs. ca. 10° in the case of Cu(dpp)2 )[9a] and the
distance between the aromatic cores is larger (more than 4 Å)
meaning that the extent of intramolecular π stacking is likely
smaller than for [Cu(dpp)2]+ in the crystal phase. A relevant way
to quantitatively estimate how distorted from the ideal D2d
symmetry copper complexes are, consists in calculating θx, θy and
θz angles, respectively the rocking, waggling and flattening angles
(see figures S4-6 in supporting information for an illustration of
those angles).[12] For a perfectly tetrahedral geometry, θx = θy = θz
= 90°; this is roughly the case for [Cu(dtbp)2]+ (table ST1, figure
S6). For [Cu(dpp)2]+, the intramolecular stacking interactions lead
to a strongly distorted structure (θx = 104.6, θy = 69.8, θz = 100.2).
On the other hand, C2’s structure is not perfectly tetrahedral since
θz is significantly different from 90° revealing the coordination
sphere is substantially flattened in the ground state, but to a lesser
extent than [Cu(dpp)2]+, as exemplified by θx and θy values which
are close to 90°. All in all, the θx,y,z parameters of C2 are closer to
those of plain [Cu(dmp)2]+.[12] This means that the steric bulk
imposed by the tert-butyl group limits the development of
intramolecular π stacking within the complex in the crystal phase.
Nevertheless, let us pinpoint that the phenyl group is more twisted
with respect to the phenanthroline plane onto which it is tethered
(C on A, E on B) passing from the free ligand to the complex. This
could be a proof that some interaction between C and B (and E
and A) is taking place. CH-π interactions are possible in this frame
(Figure S7). Accordingly, crystal packing could account for this
observation too. Importantly, no significant intermolecular
interaction is observed in the structure. In solution, it is interesting
to notice that the chemical shifts assigned to hydrogen atoms on
the phenyl groups tethered on phenanthroline are step wisely
shielded when passing from [Cu(dpp)2]+ to C1 then C2 indicating
that the orientation of the bulky aromatic groups is different (table
ST2), resulting from a mixture of steric constraints, π-stacking and
possible CH-π interactions.
Figure 4. Electronic absorption spectra for complexes C1 (black trace), C2
(blue trace), [Cu(dipp)2]+ (green trace) and [Cu(dpp)2]+ (red trace) recorded in
dichloromethane at room temperature. Average concentrations 0.05 mM.
All other [Cu(NN)2]+ complexes are located in between those two
extremes, featuring shoulders of variable intensity at the stem of
the MLCT. In our case, one can learn from normalized absorption
spectra (figure S8) that the structure of C1 is substantially more
flattened in the ground state than C2 or benchmark [Cu(dipp)2]+.
This proves that the phenyl substituent in ligand L1 is capable of
intramolecular π-stacking with the vicinal phenanthroline. On the
other hand, the tert-butyl group in L2 imposes a stronger steric
bulk which restrains the flattening in C2 and limits the possibility
of π-π interactions. This corroborates the observations made on
the solid state where the 30° angle and the distance between
phenyl C and phenanthroline B (figure S3) are more in line with
CH-π interactions (figure 3).
Electrochemistry
The electrochemical behavior of complexes C1 and C2 was
explored by cyclic voltammetry in dichloromethane and
dimethylformamide (DMF) solutions, in presence of
tetrabutylammonium
hexafluorophosphate
as
supporting
UV-Vis absorption
electrolyte. The voltammograms are given in figures S9 and S10;
and related data are reported in table 1. One mono-electronic,
pseudo-reversible oxidation wave was monitored and assigned to
the CuII/CuI couple for both complexes (E1 = 0.49 and 0.55 V vs.
ferrocenium/ferrocene redox couple (Fc+/Fc) for C1 and C2
respectively). Generally speaking, C1, C2 and [Cu(dpp)2]+ are
oxidized at similar potentials, significantly less positive than
UV-Vis spectra were recorded for C1 and C2 in dichloromethane
solutions (figure 4) and experimental data are gathered in table 1.
Both complexes exhibit the same absorption features, namely
intense ligand centered π-π* transitions below 370 nm and a
broad band assigned to the MLCT spanning between 400 and 500
nm and responsible for the bright orange color of all complexes.
A shoulder is monitored above 500 nm for all complexes but is
more prominent for C1 than C2. For symmetry reasons, the
S0→S1 transition is forbidden in the case of tetrahedral copper(I)
complexes and the MLCT peaking at ca. 460 nm is thus a S0→Sn
(n > 1) transition. When the geometry moves away from
tetrahedral, the S0→S1 transition becomes partially allowed.
The intensity of the S0 to S1 transition thus allows to estimate the
degree of flattening in a copper(I) complex in its ground state.[5h,
13] [Cu(dpp)2]+ shows a rather significant S0 to S1 transition in the
shape of a broad shoulder,[14] whereas no such spectral feature
can be detected for [Cu(dtbp)2]+ (virtually perfectly tetrahedral).
This indicates a significant symmetry descent from D2d to D2, in
+
E1(Cu(dipp)2 ). Indeed, bulky substituents enforce a D2d symmetry
which stabilizes the copper(I) ion and induce a more anodic
5h, 16]
oxidation potential.[5d,
Phenyl substituents generate
intramolecular π-stacking and entail a descent to D2 symmetry
and thus a shift of the oxidation potential to less positive values
vs. [Cu(dipp)2]+.[5a] C2 is undeniably more sterically burdened than
C1 because of the tert-butyl group, which translates into E1(C2) >
E1(C1).
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+
Table 1. UV-Vis and electrochemical data for complexes C1, C2, [Cu(dpp)2]+
and [Cu(dipp)2]+ in dichloromethane. All data recorded at room temperature.
Electrochemical potential values reported vs. the Fc+/Fc couple taken as internal
standard.
ʎem(Cu(dpp)2 ) and E00 is larger. These results are comparable
with those obtained by Miller et al. where dpp-like ligands were
progressively encumbered with substituents, gradually limiting the
π-stacking interaction within the corresponding complexes.[5h]
C2’s emission wavelength is further blue-shifted due to the even
greater steric strain brought by tert-butyl (compared to isopropyl),
with similar ʎem and E00. Conclusively, increasing the steric bulk
with one ramified alkyl chain on one side of phenanthroline while
keeping a phenyl group on the other allowed to increase E00
compared to [Cu(dpp)2]+ and maintain significant π-stacking
interaction at least for the case of C1.
Complexes
λmax/nm
εʎmax/L.mol-1cm-1
E1 (ΔE)/V (mV)[a]
E2 (ΔE)/V (mV)[b]
C1
443, 540
450
3990, 1000
3850
0.49 (120)
0.55 (110)
0.53 (80)
-2.06 (130)
-2.08 (90)
-2.07 (120)
-2.10 (75)
C2
[Cu(dpp)2]+
[Cu(dipp)2]+
439, 550
453
3800, 1800
7260
0.65 (120)
[a] Degassed dichloromethane solutions, sweep rate for electrochemical
measurements = 100 mV.s-1. [b] Degassed DMF solutions, sweep rate for
electrochemical measurements = 1 V.s-1. E1 = (Ep,a1 + Ep,c1)/2, E2 = (Ep,a
+
2
Ep,c2)/2; ΔE = Ep,a1 – Ep,c1 or Ep,a2 – Ep,c2, where Ep,a1, Ep,c1, Ep,a2 and Ep,c2 are
anodic and cathodic peak potentials for electrochemical processes 1 and 2,
respectively.
Both C1 and C2 are expected to be less flattened than [Cu(dpp)2]+,
but nevertheless, E1([Cu(dpp)2]+) > E1(C1). Rationalizing the
order of their close oxidation potentials is made particularly
difficult by the interplay of both steric and electronic effects.
No reduction wave was observed within the solvent electro-
activity window of dichloromethane. In DMF however, a pseudo-
reversible wave was monitored below -2 V vs. Fc+/Fc as internal
standard for all complexes and was assigned to the addition of an
extra electron on one phenanthroline ligand. All potential values
E2 are very close to each other despite the differences between
the electronic effects from branched alkyl chains (electron rich)
and the aromatic groups (electron poor).
Figure 5. Emission spectra for complexes C1 (black trace), C2 (blue trace) and
+
Cu(dpp)2 (red trace) recorded in dilute dichloromethane solutions (ca. 0.05
mM) at room temperature.
C1 and C2 exhibit similar emission quantum yields than model
[Cu(dpp)2]+, but unexpectedly short lifetimes, especially for C2
(table 2). As a matter of fact, the emission lifetimes for C2 and
Excited state properties
Complexes were studied by steady state fluorescence and time-
correlated single photon counting in degassed dichloromethane.
All complexes are luminescent at room temperature in
dichloromethane dilute solutions. Steady state emission spectra
are given in figure 5, while decay traces are given in figures S11
and S12. Emission maximum wavelengths λem, luminescence
quantum yields ɸlum, lifetimes τ and E00 values (energy difference
between singlet excited state and the ground state in ground
vibrational states, estimated by the tangent method)[5b] are
reported in table 2.
Concerning model complexes [Cu(dpp)2]+ and [Cu(dipp)2]+, bulky
isopropyl groups in the latter tend to favour D2d ground state
geometry and a stabilized environment for copper(I) affording
short emission wavelength and large E00. Conversely,
intramolecular π-stacking in [Cu(dpp)2]+ entail the flattening of the
coordination cage towards D2 symmetry, destabilization of the
copper(I) ion and thus longer emission wavelength and smaller
E00. All figures of merit for C1 and C2 are comprised between
those of the model complexes. ʎem (C1) is red shifted compared
to ʎem([Cu(dipp)2]+) precisely because of the intramolecular π-
stacking interaction entailing some significant distortion of the
coordination sphere away from D2d; conversely the steric bulk
from the branched isopropyl chain tends to oppose to this ground
state distortion, forcing C1 in a less distorted geometry than
[Cu(dpp)2]+, hence ʎem (C1) is blue shifted compared to
+
simple Cu(dmp)2 are comparable, although the latter is less
sterically strained. In order to deepen the study, we extracted the
radiative and non-radiative rate constants (respectively kr and knr)
from our experimental data (table 2). C2 exhibits the second best
kr (behind [Cu(dipp)2]+), which can be assigned to its blue shifted
absorption and emission bands compared to C1 or [Cu(dpp)2]+,
because kr is proportional to the cube of the radiative energy
gap.[5h, 17] The reason for the weak emission properties of C2 are
therefore not grounded in kr. knr is however larger for C1 and
especially C2. A similar behavior was observed with the copper(I)
complex [Cu(MeMap)2]+ (where MeMap is a non-symmetrical
phenanthroline ligand bearing one anisyl and one methyl group in
α of the chelating nitrogen atoms)[18] and seems related to the
non-symmetrical nature of the ligands.
As reported in previous works, a phen ligand substituted by an
aromatic and an aliphatic groups provides
a very bulky
environment on one side of the corresponding complex and at the
same time a less bulky environment on the other.[19] This can be
exacerbated in the excited state and interactions of the exposed
copper(I) center with solvent molecules or counter-anions would
thus be favored.
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Table 2. Excited state parameters for complexes C1, C2, Cu(dpp)2+ and Cu(dipp)2+. Potentials are given vs. Fc+/Fc.
complexes
ʎem/nm
698
678
732
679
ɸlum/%
0.10
0.08
0.10
0.40
τ/ns
216
92
243
365
E00/eV
2.09
2.11
2.00
2.11
Eox*/V
0.03
0.03
-0.07
0.01
kr / s-1
knr / s-1
kACN / s-1.M-1
1.09x109
-
6.88x108
9.16x108
kRQ / s-1.M-1
1.8x109
C1
C2
Cu(dpp)2
4.63x103
8.70x103
4.12x103
1.10x104
4.63x106
1.09x107
4.11x106
2.73x106
2.8x109
+
0.8x109
3.1x109
+
Cu(dipp)2
Such a process is particularly prominent with coordinating species
(exciplex quenching) but has been monitored with non-
coordinating molecules too.[20] Generally speaking, the solvent is
known to have an influence on the ground and excited state
properties of copper(I) complexes,[21] and this influence could be
stronger when non-symmetrical ligands are coordinated to Cu(I)
and lead to large knr. Tentatively, we recorded the luminescence
decays of C1, C2, [Cu(dpp)2]+ and [Cu(dipp)2]+ in dichloromethane
while progressively adding acetonitrile, well known to quench the
emission of the copper complexes by exciplex formation, in order
to appreciate how affected C1 and C2 are compared to classical
[Cu(dipp)2]+ and [Cu(dpp)2]+. Following a Stern-Volmer analysis
we could retrieve the quenching constants kACN (figure S13, table
2) of the emission by acetonitrile. The exciplex quenching is
undeniably more efficient for C1 than [Cu(dipp)2]+ or [Cu(dpp)2]+,
the latter being as previously observed the most resilient.[8a] C2
proved to be too unstable in presence of acetonitrile to allow
recording spectra in the same conditions, but preliminary analysis
of the data shows a very efficient emission quenching too. The
enhanced exciplex quenching for C1 and C2 likely contributes to
their particularly high knr.
the theoretical results in light of those two possible conformers,
we will name RX the structures similar to what is pictured in Figure
3 (and calculated as presented in Figure 6, left side) and conf the
second computed structure (calculated as presented in figure 6,
right side) for [Cu(dpp)2]+, C1 and C2 complexes.
[Cu(dpp)2]+
C2
Computational chemistry
RX, D2
Conf, S4
Structures
Figure 6. NCI analysis performed on the RX (top left) and conf (top right)
structures of the [Cu(dpp)2]+ complex and on the RX (bottom left) and conf
(bottom right) structures of C2. In green are the basins corresponding to Van
der Waals forces and in red area of steric congestion. All the structures have
been fully optimized in the electronic ground state with GAUSSIAN (see
computational details).
The presence of phenyl moieties in the C1 and C2 complexes
generates several [Cu(dpp)2]+-type conformers, the structures of
which have been fully optimized at the DFT level. The structure of
[Cu(dipp)2]+ complex belongs to the D2d symmetry group. For
[Cu(dpp)2]+, two conformers are possible, one of D2 symmetry and
one of S4 symmetry (Figure 6). The D2 structure is significantly
more stable than the S4 one (G = 2.3 kcal mol-1 computed at the
DFT level) in accordance with experimental observations.[9a] The
origin of the greater stability of the D2 structure can be attributed
to -stacking interactions as shown by the analysis of Non-
Covalent Interactions (NCI) (figure 6). Whereas in the D2 structure,
π-stacking is enhanced by the interaction between the phenyl
groups of one ligand and the phenanthroline moiety of the second
one, it is weakened in the S4 structure where the two moieties are
less parallel. Furthermore, to ensure a greater parallelism
between these moieties, the D2 structure is already flattened in
the ground state. C1 and C2 also exhibit conformers due to
different orientations of the phenyl rings. In C1, the two computed
structures have the same energy (G = 0.0 kcal mol-1). For C2,
the structure issued from the X-ray data (figure 3) is slightly less
stable than the conformer (G = -0.6 kcal mol-1 computed at the
DFT level, Figure 6). Such variations are the result of the
competition between the stabilizing -stacking interactions
generated by the phenyl moieties and the steric congestion
induced by the isopropyl and tert-butyl groups. In order to discuss
Absorption
The absorption spectra were computed on the DFT/B3LYP
optimized structures (Table 3) of [Cu(dipp)2]+, [Cu(dpp)2]+, C1 and
C2 complexes and of their conformers (Figure 7). As observed
experimentally, the visible domain is assigned to MLCT
transitions and the UV-Visible domain to transitions localized on
the ligands. The computed spectra of [Cu(dipp)2]+ is very similar
to the experimental one with an intense band at 485 nm. Two
singlet states are present at higher wavelength (505 and 522 nm)
with no intensity for symmetry reasons. A shoulder is present at
lower wavelength (433 nm).
The agreement between experimental (figure 3) and theoretical
results (figure 7) is less good for the three other complexes in their
RX structure. We retrieve the drop of the main S0→Sn band
intensity and the appearance of smaller bands. For [Cu(dpp)2]+,
the overall structure of the spectrum is well reproduced though
being red shifted as compared to the experimental one. The main
absorbing band is located at 454 nm. Four absorbing singlet
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states are located at 549, 582, 584 and 628 nm generating the
band between 570 to 650 nm.
I),[22] the present complexes are highly flexible and present
several minima that can contribute to the absorption spectrum.
The limitation of the static approach, which does not reflect this
flexibility, explains the difference between the experimental and
theoretical results. The influence of the dynamics on the
absorption spectrum of C2, which are known to strongly influence
the latter, are likely responsible for the differences between the
experimental and calculated spectra. Importantly, the calculations
match with the experimentally observed trend that the extinction
coefficients of the S0→Sn transitions decrease when phenyl
Table 3. Geometric parameters of the ground state of [Cu(dipp)2]+, [Cu(dpp)2]+,
C1 and C2 complexes and of their conformers. Distances are in Angstroms and
angles in degrees. For each complex are given the relative stabilities (E)
between the RX and conf structures from ADF calculations.
State
[Cu(dipp)2]+
[Cu(dpp)2]+
C1
C2
RX
conf
RX
conf
RX
conf
Cu-N1A
Cu-N2A
Cu-N1B
Cu-N2B
z
2.061
2.061
2.061
2.061
90.0
-
2.063
2.063
2.063
2.063
-118.7
0.0
2.095
2.146
2.088
2.031
2.037
groups are tethered in
phenanthroline.[23]
α
of the nitrogen atoms of
2.095
2.095
2.095
90.0
2.024
2.146
2.024
107.2
1.0
2.058
2.088
2.058
67.9
2.180
2.031
2.180
106.2
0.7
2.162
2.037
2.162
73.4
Emission spectra
Nuclear evolution, mainly the z angle (as defined in figure S4)
and the Cu-N distances, and electronic re-localization underlie
excited state relaxation. Delocalization of the excited electron
over the two phenanthroline ligands corresponds to D2 point
group for [Cu(dipp)2]+ and [Cu(dpp)2]+ complexes and to C2 point
group for C1 and C2 complexes. Localization on one
phenanthroline only leads to C2 point group for [Cu(dipp)2]+ and
[Cu(dpp)2]+ complexes and to C1 point group for C1 and C2
complexes (see figure S17 and table ST3).
E
3.8
0.0
0.0
[Cu(dipp)2]+
For [Cu(dipp)2]+, two symmetry point groups were considered, D2
allowing the flattening of the complex and C2 allowing the
electronic localization on one of the ligand (figure S17 and table
ST3). We retrieve similar results as for the [Cu(phenX2)2]+
complexes series.[22] The lowest energy structures belong to the
C2 symmetry with singlet and triplet states of MLCT character
(Figure 8, right). However, the lowest singlet and triplet states of
D2 symmetry are very close in energy (Table 4) and correspond
to transition states on the C2 potential energy profile joining the
two minima. Whereas the triplet emission wavelength (C2 and D2)
are underestimated as compared to the experimental values, the
emission wavelength of the lowest singlet state of D2 symmetry
(first singlet of B1 symmetry) agrees with the experiment
(calculated 673 nm in table 4 vs. measured 679 nm in table 2).
Figure 7. Theoretical absorption spectra in the visible domain for complexes C1
(black trace), C2 (blue trace), [Cu(dipp)2]+ (green trace) and [Cu(dpp)2]+ (red
trace) on the RX structures
The theoretical spectrum of C1 mostly reproduces the
characteristics of the experimental one; the main band is located
at 466 nm with again smaller intensity transitions between 500-
600 nm and corresponding to the lowest singlet excited states
calculated at 569, 532, 504 and 500 nm. The absorption spectrum
of C2 is characterized by three peaks at 445, 510 and 570 nm.
The most intense band at 510 nm is generated by two singlet
states calculated at 506 and 512 nm. The presence of accessible
conformers may explain the discrepancies between theoretical
and experimental absorption spectra.
The spectra computed on the conf structures present significant
shift of the bands. For the [Cu(dpp)2]+ complex, the conf structure
leads to tetrahedral arrangement around the copper cation and
the spectrum is very similar to that of [Cu(dipp)2]+ complex,
illustrating again the structural impact on the optical properties
(figure S14). Moreover, these results prove as well that the conf
structure of [Cu(dpp)2]+ does not participate in its absorption
spectrum. For C1 and C2 complexes, the differences between the
RX (Figure 7) and conf spectra (Figures S15 and S16) are less
drastic though significant, revealing that both structures are
substantially participating in the overall optical properties. As for
our previous work on the [Cu(phenX2)2]+ complexes (where
phenX2 stands for 2,9-diX-1,10-phenanthroline and X = Cl, Br or
Similarly to the [Cu(phenX2)2]+ complexes and as observed
22, 24]
experimentally in usual homoleptic[5b,
and heteroleptic
copper(I) complexes[25]
a
thermally activated delayed
fluorescence (TADF) is proposed. Indeed, SOC and ΔEST are
favorable to an efficient re-population of the singlet potential
energy surface (PES) after relaxation in the lowest triplet PES.
Molecular oscillatory motion between the two minima of C2
symmetry characterizes the dynamics of the system in both PES.
This results in a structure that is, in average, of D2 symmetry,
which can be considered as the emitting one. This is further
supported by the respective oscillator strength of the singlet states
in the C2 (S1A) and D2 (S1B1) symmetry. There is one order of
magnitude in favor of the D2 structure (Figure 8).
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Table 4. Computed emission wavelength (in nm), Singlet-Triplet energy splitting
(EST in eV) at the geometry of the triplet state and Spin-Orbit Coupling (SOC
in cm-1) at the same geometry for [Cu(dipp)2]+ complex in the D2 and C2 point
groups, energetic barrier (in cm-1) between the D2 geometry and the C2 minima,
flattening of structure (z in degrees) and oscillator strength of the singlet state.
singlet states of C2 symmetry is in very good agreement with the
experiment regardless of the considered conformer RX or conf.
As for [Cu(dpp)2]+, C1 and C2 present a flattening in the ground
state (Table 3) due to intramolecular π-stacking interactions. The
extra flattening generated by the excitation is small but larger for
the RX structures (around 7°) than for the conf structures (around
3°).
Symmetry
State
D2
C2
S1B1
T1A
S1A
T1A
em
EST
SOC
barrier
Cu-NA
Cu-NB
z
673
861
760
914
0.332
18.2
0.336
56.8
0
Table 5. Emission wavelength (in nm), Singlet-Triplet energy splitting (EST in
eV) at the geometry of the triplet state and Spin-Orbit Coupling (SOC in cm-1) at
the same geometry for [Cu(dpp)2]+ complex in the D2 and C2 point groups,
energetic barrier (in cm-1) between the D2 geometry and the C2 minima,
flattening of structure (z in degrees) and oscillator strength of the singlet state.
736
2.039
2.039
-110.0
7.9 10-2
502
0
2.017
2.017
-108.9
1.992
2.116
110.1
1.7 10-2
1.992
2.063
110.9
Symmetry
State
D2
C2
S1B1
T1A
S1A
T1A
em
EST
SOC
Barrier
Cu-NA
Cu-NB
z
698
905
754
949
fosc
0.331
15.3
0.353
49.5
252
2.041
2.041
-120.7
6.3 10-2
445
0
0
2.007
2.007
-121.3
1.995
2.106
-121.0
2.8 10-2
1.988
2.055
-120.9
D2
C2
fosc
Figure 8. Electronic density difference between ground and excited states of
T1a states of D2 symmetry (left) and of C2 symmetry (right) of the [Cu(dipp)2]+
complex. In red and green are the electron depleted and enriched zones
respectively.
Table 6. Emission wavelength (in nm), Singlet-Triplet energy splitting (EST in
eV) at the geometry of the triplet state and Spin-Orbit Coupling (SOC in cm-1) at
the same geometry for C1 complex in the C2 and C1 point groups, energetic
barrier (in cm-1) between all geometries and the RX C1 minima, flattening of
structure (z in degrees) and oscillator strength of the singlet state.
[Cu(dpp)2]+
The structures of the excited states generated by the conf
structure of [Cu(dpp)2]+ complex are much less stable than those
from the RX conformer and will not be detailed further (See table
ST4). The RX structures are already flattened in the ground state
(z=-119.7°) and almost no extra flattening is observed in the
excited state geometries which justifies its particularly good
photophysical behavior.[26] Similarly to [Cu(dipp)2]+ complex, the
triplet emission wavelengths are incompatible with the
experimental data (Table 5, Table 2) while the singlet emission
wavelength (D2 symmetry) reproduces rather well the experiment
leading to a nearly identical photophysical mechanism.
For [Cu(dipp)2]+ and [Cu(dpp)2]+ complexes only two geometrical
distortions are observed upon excitation. We observe a variation
of the z angle due to the flattening of the ligand and variation of
the Cu-N distances. Those distances are reduced for the ligand
accepting the excited electron and lengthened for the inert ligand.
No rocking nor wagging is observed.
Symmetry
State
C2(RX)
C1(RX)
C2(conf)
C1(conf)
S1B
T1A
894
S1
T1
S1B
T1A
892
S1
T1
em

684
757
939
679
764
933
EST

0.354
17.7
0.355
48.8
0.334
13.6
0.363
52.5
0
SOC
barrier
Cu-N1A
Cu-N2A
Cu-N1B
Cu-N2B
z
401
354
0
35
485
596
8
2.057
2.036
2.057
2.036
114.8
2.022
2.010
2.022
2.010
114.2
2.009
1.983
2.160
2.072
113.8
1.997
1.986
2.083
2.043
114.4
2.046
2.040
2.046
2.040
65.0
2.005
2.023
2.005
2.023
65.1
1.974
2.006
2.160
2.072
67.2
1.977
2.007
2.061
2.059
64.4
6.9
2.2
6.9
1.8
fosc
10-2
10-2
10-2
10-2
C1 and C2
The asymmetry in the substituents of the phenanthroline ligand is
reflected in the different Cu-N distances. For C1, the Cu-N1
distances are longer than the Cu-N2 distances. This is reversed
for C2. This is easily explained; in C2 the longer Cu-N2 distance
is due to the steric repulsion generated by the tert-butyl moiety
which is larger than isopropyl. The evolution of these distances in
C1 and C2 in the excited state depends if the molecule is excited
from the RX or from the conf structure. The most noticeable point
is the evolution of the Cu-N2B distances in the conf conformer of
Complexes C1 and C2 in their excited states are characterized by
several conformers close in energy with similar emission
wavelengths (table 6 and table 7). The singlet and triplet emission
wavelengths generated by the structures of C1 symmetry disagree
with the experimental data. The same conclusion can be drawn
for the emission from the structures of C2 symmetry in their triplet
state. However, the emission wavelength computed for the S1B
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C2. It rises up to 2.4 Å in the C1 point group symmetry (Table 7)
evidence that dynamically the excited state is delocalized over
two ligands. However, none of the discussed above
characteristics are responsible for the emission intensity drop of
C1, C2 and [Cu(dpp)2]+ compared to [Cu(dipp)2]+. One last
movement has to be considered. Upon excitation, the initially D2d
[Cu(dipp)2]+ complex flattens generating two degenerated
conformers (see figure S18). We found a transition state
connecting these two conformers on the lowest triplet PES with a
relatively low associated energy barrier (computed G = 3.5
kcal.mol-1). At this geometry, in which z is almost 90°, the
coupling between S0 and S1 is very small (it is strictly nil in the
ground state D2d structure). The emission of [Cu(dipp)2]+ is
relatively intense because it arises from a flattened conformation.
The observed stabilizing interactions between the ligand for
[Cu(dpp)2]+, C1 and C2 (which hold while shifting from one energy
well to the other) may lower this barrier so that dynamically, an
orthogonal structure significantly contributes to the emission
properties. As a consequence, the reverse intersystem crossing
channel would be less favorable, decreasing the TADF efficiency
and leading to the less intense emission for these complexes
compared to that of [Cu(dipp)2]+ complex (for which such structure
is too high in energy).
potentially opening the way for partial ligand decoordination.
Table 7. Emission wavelength (in nm), Singlet-Triplet energy splitting (EST in
eV) at the geometry of the triplet state and Spin-Orbit Coupling (SOC in cm-1) at
the same geometry for C2 complex in the C2 and C1 point groups, energetic
barrier (in cm-1) between all geometries and the RX C1 minima, flattening of
structure (z in degrees) and oscillator strength of the singlet state.
Symmetry
State
C2(RX)
C1(RX)
T1
C2(conf)
C1(conf)
S1B
T1A
830
S1
S1B
T1A
781
S1
T1
em

662
815
925
650
793
910
EST

0.339
35.5
0.270
63.4
0.287
32.9
0.250
56.9
0
SOC
barrier
Cu-N1A
Cu-N2A
Cu-N1B
Cu-N2B
z
1460
2.096
2.045
2.096
2.045
111.0
891
421
239
933
823
0
2.057
2.027
2.057
2.027
111.5
1.995
1.994
2.212
2.130
98.8
2.002
2.003
2.176
2.060
113.4
2.006
2.149
2.006
2.149
68.3
1.982
2.138
1.982
2.138
71.6
1.980
1.993
2.057
2.421
74.8
1.971
1.995
2.026
2.390
72.0
Overall, complexes C1 and C2 stand out in the series of
complexes by the fact that many different but equally probable
structures are in dynamic equilibrium, and this opens many non-
radiative pathways which entail a significant increase of knr and
thus weaker emission quantum yields.
5.2
1.5
5.6 10-
6.9
fosc
10-2
10-3
10-3
2
The presence of a flexible non symmetrical ligand generates
several minima on the lowest excited singlet and ground and
lowest excited singlet and triplet states PES. This conformational
flexibility has to be taken into account to reproduce the
experimental result, as the different conformers will be present in
the solution due to their close stability. This affects the calculation
of the absorption and emission spectra and may be the source of
the large band observed for the emission. The excitation can
localize on one of the two ligands. However, the energy of the
symmetric structure, which is the transition state between the two
minima, is so low that in reality the complexes freely oscillate in
between those two minima. As a consequence, in average the
structure is symmetric and explains the very good agreement
between the computed and experimental emission wavelengths
for the symmetric structures for all complexes. The value
computed for the triplet-singlet splitting (EST), between 0.3-0.4
eV and the spin-orbit coupling (SOC) values are compatible with
the TADF mechanism but from a structure that is not an energy
minimum. From this point of view the rather large ΔEST is
compensated not only by SOC but also by vibronic coupling
effects.[27] The presence of phenyl rings on the phenanthroline
ligand induces a flattening of the structure in the ground state,
which is almost identical in the excited states: no further flattening
is observed. This invalidates the hypothesis that the non-
symmetrical nature of the ligands is responsible for extra
flattening of the coordination sphere upon excitation.
Stability studies
Given the lability of the copper(I)-phenanthroline coordination
sphere, the stability of the complexes is strongly impacted by the
steric bulk around the metal ion. For example, [Cu(dtbp)2]+ is
quantitatively destroyed in presence of one equivalent of
acetonitrile[5a] thanks to a strong steric relief. We thus began by
recording the 1H NMR spectrum of C1 and C2 in deuterated
acetonitrile. C1’s spectrum featured the expected signals of the
homoleptic complex, and the spectra remained unchanged for
several weeks (figure S19). On the other hand, the 1H NMR
spectrum of C2 revealed the latter is in equilibrium with other
species (figure S20). Indeed, the UV-Vis spectrum of C2 in
dichloromethane is strongly affected by the addition of aliquots of
CH3CN (figure S21), the MLCT significantly collapsing upon
increasing the acetonitrile’s concentration while some weak
absorbance over the visible range confirms the formation of a
[Cu(L2)(CH3CN)]+ species following equation 2:
[Cu(L2)2]+ + 2CH3CN → [Cu(L2)(CH3CN)2]+ + L2 (equation 2)[19,
28]
This is not unexpected since the tert-butyl group leads to longer
Cu-N2A and Cu-N2B distances and limits the extent of stabilizing
intramolecular π-stacking.
Interestingly, C1’s spectrum was slightly affected by acetonitrile
too revealing some instability for dilute solutions, while
[Cu(dipp)2]+ and [Cu(dpp)2]+ spectra remained roughly idle in the
same conditions (figures S22 and S23). This prompted us to
inquire further about the behaviour of our complexes in presence
of nucleophiles.
The computed energies of the excited state (see table ST5)
correlate well with those measured by the tangent method
whether the conf or RX structures are considered for C1 and C2.
This confirms that increasing the steric burden around the copper
ion allows indeed to rise the energy gap between S1 and the
ground state.
The theoretical results have shown that the decrease in
absorption for [Cu(dpp)2]+, C1 and C2 is due to the initial flattening
of the complexes induced by the phenyl moieties. We also put in
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Stopped flow analysis
with the behavior hypothesized in the literature: the non-
symmetrical nature of the ligands provides a protected side by
stacking of the phenyl group which simultaneously opens a site
on the other side of the complexes, explaining their enhanced
reactivity towards cyanide (and acetonitrile).
Monitoring the real time decomplexation of copper(I) complexes
by cyanide by stopped flow is a valuable tool to study the
accessibility of copper(I) towards nucleophilic attacks, and how
shielded is the latter by the groups tethered in α of the nitrogen
atoms of phenanthroline. The process is based on the chemical
equation:
kD values reflect the fact that acetonitrile can behave as a
competitor ligand, leading to partial dissociation of the complexes.
kD increases in the order [Cu(dipp)2]+ < [Cu(dpp)2]+ << C1: this
means that C1 dissociates faster than [Cu(dpp)2]+ and
[Cu(dipp)2]+ in acetonitrile, which is in line with the conclusions
from the cyanide induced decomplexation. The fact that
[Cu(dpp)2]+ dissociates faster than [Cu(dipp)2]+ is unexpected; we
propose that the electron-withdrawing character of phenyl
depletes the σ-donating power of dpp compared to dipp (where
electron rich isopropyl groups have the opposite effect) and this
could translate into the (slightly) faster dissociation of [Cu(dpp)2]+.
As a conclusion, the observation that complexes bearing non
symmetrical phenanthroline are more prone to nucleophilic attack
than classical complexes with symmetrically substituted ligands
can be relevantly compared to their enhanced exciplex quenching.
This fragility in the ground state is likely exacerbated in the excited
state.
Cu(NN)2+ + 4 CN- →Cu(CN)43- + 2NN (equation 3)
The huge complexation constant of Cu(CN)43- (1028 vs. 1010-12 for
Cu(NN)2+ complexes) is the main driving force for this quantitative
demetallation reaction.[29] Complexes bearing non-symmetrical
phenanthroline ligands have previously shown enhanced kinetics
towards cyanide assisted demetallation compared to symmetrical
systems.[19] Willing to assess if the behaviour of C1 and C2 in
those conditions would follow the same trend, the decay of the
MLCT with time in presence of excess CN- in a 1:1 (v/v)
dichloromethane:acetonitrile mixture was monitored by stopped
flow. [Cu(dipp)2]+ and [Cu(dpp)2]+ were studied in the same
conditions for comparison purpose.
C2 proved to be too unstable in the solvent mixture and was
therefore not further examined. In all other cases, decays are
monoexponential with measured decay constants kobs (figure
S24). In our experimental conditions (large excess of
tetrabutylammonium cyanide) the degradation kinetics can be
considered as pseudo first order in complex. Plots of kobs vs. [CN-]
yield straight lines (figure S25). The intersection with vertical axis
affords the constant kD insensitive to [CN-] and assigned to the
natural dissociation kinetics of the complexes in the analysis
medium.[18-19] The slopes of the straight lines yields the constant
kCN assigned to cyanide assisted demetallation. Significant values
of kCN indicate that cyanide is involved in the rate determining step
(figure S26). Constants are given in table 8 and experimental
details can be found in the SI (table ST6).
Reactivity
Using the Rehm and Weller equation, it is possible to estimate the
reductive and oxidative power of photosensitizers promoted in
their excited state, namely E(Cn+/Cn*) and E(Cn*/Cn-) (or Eox*)
for C1 and C2. Considering the case of Eox*, the Rehm and Weller
equation gives:
Eox* = E(Cn/Cn-) + E00 (equation 4)
Where E(Cn/Cn-) is the reduction potential of complexes Cn (E2
in table 1) and E00 is the energy of the excited state. Both C1 and
C2 exhibit more positive Eox* than [Cu(dpp)2]+, and comparable to
[Cu(dipp)2]+. Thus, the driving force for photo-induced electron
transfers is more favorable for C1 and C2 than for the model
complex [Cu(dpp)2]+. In order to test this assertion, we decided to
try and implement reductive quenching with C1 and C2 with
decamethylferrocene (dmFc) as electron donor (figure 9). The
latter is an excellent donor (E(dmFc+/dmFc) = -0.57 V vs. Fc+/Fc)
which has been successfully used by McMillin et al. in what are,
to the best of our knowledge, the only articles ever reporting
reductive quenching with homoleptic [Cu(NN)2]+ complexes
(namely [Cu(dipp)2]+ and [Cu(dpp)2]+).[10, 30] Indeed, the latter are
particularly poor photo-oxidants.[1a] Yet, successfully performing
reductive quenching with plain homoleptic copper(I) complexes is
a valuable endeavor leading to the photo-generation of very
reductive species [Cu(NN)2]0 (more precisely [CuI(NN)(NN•-)]0)
paving the way towards low-cost, visible light-activated photo-
reduction processes of difficult substrates or catalysts. We
however highlight that the reversibility of the dmFc+/dmFc couple
prevents accumulation of Cn- upon photoexcitation (figure 9, step
3).
Table 8. Dissociation rate constants of copper(I) bis(diimine) complexes C1,
[Cu(dpp)2]+ and [Cu(dipp)2]+ in acetonitrile, with [Bu4NPF6] = 0.05 M; T = 25.0 ±
0.2 °C.
Complexes
kD (s-1)
kCN(M-1s-1)
+
Cu(dpp)2
0.976 ± 0.07
11.50 ± 0.08
+
Cu(dipp)2
0.30 ± 0.04
14 ± 5
229 ± 4
C1
8526 ± 551
In line with previously published results,[19, 29] [Cu(dpp)2]+ is the
more resilient species vs. cyanide demetallation thanks to the
intramolecular π-stacking interactions between the phenyl and
the vicinal phenanthroline: the copper(I) ion is well protected in
this rigid cavity, as incidentally evidenced by its good
photophysical properties in strongly coordinating medium.
[Cu(dipp)2]+ is deprived of stabilizing interactions and therefore
more prone to react with cyanide, as indicated by a ca. 20 times
larger kCN compared to [Cu(dpp)2]+. As previously observed for
complexes bearing non-symmetrically substituted phenanthroline
ligands, C1 features a larger kCN than [Cu(dpp)2]+ by almost two
orders of magnitude, revealing that the non-symmetrical nature of
L1 promotes cyanide nucleophilic attack on the copper ion.[19]
Less intuitively, kCN is also much larger for C1 than for [Cu(dipp)2]+
despite an intermediate steric hindrance. However, this is in line
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Conclusions
The use of non-symmetrical phenanthroline ligands bearing one
phenyl and one ramified alkyl chain to respectively stabilize and
sterically strain corresponding copper(I) complexes is reported.
Tethering an isopropyl or a tert-butyl chain and a phenyl group in
α of the nitrogen atoms allowed to implement intramolecular π
stacking interactions and rise the complex excited state energy at
the same time by ca. 100 meV compared to symmetrical
[Cu(dpp)2]+. Calculations reveal that the stacking interactions lead
to a more rigid coordination sphere for C1 and C2, the extent of
computed photo-induced flattening being smaller than for
[Cu(dipp)2]+ where no stacking takes place. The stacking was
nevertheless not sufficient to stabilize the coordination sphere
when using tert-butyl instead of isopropyl substituents, because
of the strong steric strain imposed by the tertiary ramified alkyl
chain.
The unsymmetrical nature of the phenanthroline ligand promotes
nucleophilic attacks on the copper centre in C1 and C2 in their
ground states. We extend this statement to the excited state,
exciplex quenching being more efficient for non-symmetrical
complexes, and thus rationalize their rather weak photophysical
parameters (emission quantum yield and lifetime). Simulation of
the emission wavelengths was achieved with TD-DFT
calculations. Importantly, the fitting required to take into account
that the electron in the MLCT excited state oscillates between the
two ligands, leading to an overall symmetrical potential well. The
increased energy of the excited state for C1 and C2 lead to better
photo-oxidizing power, which materialized in improved reductive
quenching kinetics compared to [Cu(dpp)2]+.
Figure 9. The reductive quenching cycle in the case of Cn (n = 1 or 2) as
photosensitizer and dmFc as electron donor.
Using the same experimental protocol than McMillin et al.,[30] we
monitored the impact of increasing quantities of dmFc in
dichloromethane solutions of C1, C2, [Cu(dpp)2]+ and [Cu(dipp)2]+
on their luminescence lifetime and extracted bimolecular
quenching rates kQ using Stern-Volmer formalism (figure S27).
Importantly, the quenching of the excited state of all copper
complexes may occur via two mechanisms: energy transfer (“EnT”
from MLCT excited state to low lying iron(II) based dd states,
kinetic constant kEnT) and electron transfer (from dmFc to excited
copper complexes, namely reductive quenching, kinetic constant
kRQ). Thus, kQ = kRQ + kEnT. As previously demonstrated, kEnT can
be considered similar for plain ferrocene and dmFc since the
energies of the final quencher d-d state are similar.[30-31] Electron
transfer processes between ferrocene and copper complexes
being thermodynamically uphill, one can roughly estimate kEnT (kQ
= kEnT when the quencher is plain ferrocene Fc) by monitoring
copper complexes luminescence quenching by plain ferrocene
(figure S28, table ST7). As previously observed, quenching by
energy transfer is one order of magnitude less efficient than
quenching by photo-induced electron transfer, making the latter
the dominant phenomenon in presence of dmFc. Thus, we could
estimate kRQ and the results are reported in table 2: C1 and C2’s
emissions are more efficiently quenched by dmFc than
[Cu(dpp)2]+, meaning that C1 and C2 are more efficient photo-
Experimental Section
General: chemicals were purchased from Sigma-Aldrich or Fisher
Scientific and used as received. Thin-layer chromatography (TLC) was
performed on aluminium sheets precoated with Merck 5735 Kieselgel
60F254. Column chromatography was carried out with Merck 5735
Kieselgel 60F (0.040-0.063 mm mesh). 1H spectra were recorded on an
AVANCE 300 UltraShield BRUKER. Chemical shifts for 1H NMR spectra
are referenced relative to residual protium in the deuterated solvent (CDCl3
δ = 7.26 ppm). NMR spectra were recorded at room temperature, chemical
shifts are written in ppm and coupling constants in Hz. Mass spectrometry
was performed with a JEOL JMS-700 B/E spectrometer. Electrochemical
measurements were made under an argon atmosphere in CH2Cl2 with 0.1
M Bu4NPF6. Cyclic voltammetry experiments were performed by using an
+
oxidants than Cu(dpp)2 , kRQ being twice and three times larger
for C1 and C2 respectively than kRQ([Cu(dpp)2]+). Interestingly
kRQ(C1) is smaller than kRQ([Cu(dipp)2]+) or kRQ(C2) although
those three complexes have comparable photo-oxidative powers
from the sole thermodynamic point of view (see Eox* table 2). This
is certainly due to kinetic factors. Indeed, the latter are believed
to be particularly prominent for reductive quenching,[8c, 30] because
the complex shifts from a flattened excited state toward a
tetrahedral reduced copper(I) complex (figure 9, step 2) whereas
it shifts from a flattened excited state to a flattened oxidized
copper(II) complex in the frame of oxidative quenching. The
reorganization energy is thus supposed to be a particularly
important factor to take into account here; given our results, C1
and C2 should thus display higher reorganization energies.
However, computed distortion energies do not reflect the
observed trend (table ST5) as [Cu(dipp)2]+ exhibits larger
reorganization energies than C1 and C2 for the computed states
(due to the flattening movement contribution which is more
prominent that in the three other complexes). Work is ongoing to
rationalize this observation.
Autolab PGSTAT 302N potentiostat/galvanostat.
A standard three-
electrode electrochemical cell was used. Potentials were referenced vs.
reversible ferrocenium/ferrocene couple. The working electrode was a
glassy carbon disk and the auxiliary electrode was a Pt wire. UV-visible
absorption spectra were recorded on an Analytik Jena spectrophotometer,
using 1 cm path length cells. Emission spectra were recorded on a
Fluoromax-3 Horiba spectrofluorimeter (1 cm quartz cells). E00 values
were measured by the tangent method: a tangent to the emission spectrum
was drawn on the blue side of the latter and the intersection with the
, = .
baseline gives wavelength λ00 used to calculate E00 1240/λ00
Luminescence decays were recorded with a DELTAFLEX time correlated
single photon counting system (HORIBA) on degassed dichloromethane
solutions. X-ray single-crystal diffraction data for L1 were collected at 150K
on a Rigaku Oxford Diffraction SuperNova diffractometer equipped with
Atlas CCD detector and micro-focus Cu-Kα radiation (λ = 1.54184 Å). The
structure was solved by direct methods and refined on F2 by full matrix
least-squares techniques using SHELX programs (G. M. Sheldrick 2013-
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10.1002/chem.202001209
Chemistry - A European Journal
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2016, SHELXS 2013/1 and SHELXL 2016/4). All non-H atoms were
refined anisotropically and multiscan empirical absorption was corrected
using CrysAlisPro program (CrysAlisPro, Agilent Technologies,
V1.171.38.46, 2015). The H atoms were included in the calculation without
refinement. CCDC-1972177 contains the supplementary crystallographic
data for this paper. Crystallographic data: C22H20N2, M = 312.40, colorless
prism, 0.326 x 0.298 x 0.254 mm3, orthorhombic, space group P212121,
a = 9.7972(1) Å, b = 11.6086(2) Å, c = 15.1268(2) Å, V = 1720.40(4) Å3, Z
= 4, ρcalc = 1.206 g/cm3, μ = 0.543 mm-1, F(000) = 664, θmin = 4.802 °,
θmax = 76.289°, 6751 reflections collected, 3339 unique (Rint = 0.0252),
parameters / restraints = 221 / 0, R1 = 0.0419 and wR2 = 0.1117 using
3278 reflections with I>2σ(I), R1 = 0.0445 and wR2 = 0.1155 using all data,
absolute structure parameter = 0.2(3), GOF = 1.041, -0.250 < Δρ < 0.387
e.Å-3. X-ray single-crystal diffraction data for C2 were collected at 150K
on a D8 VENTURE Bruker AXS diffractometer equipped with a (CMOS)
PHOTON 100 detector, Mo-Kα radiation (λ = 0.71073 Å, multilayer
monochromator). Crystallographic data: (C44H40CuN4•F6P•2(C7H8)); M =
1017.57. monoclinic P 21/n (I.T.#14), a = 14.9886(15), b = 19.446(2), c =
17.4437(15) Å, β = 105.329(3) °, V = 4903.3(8) Å3. Z = 4, d = 1.378 g.cm-
3, μ = 0.545 mm-1. The structure was solved by dual-space algorithm
using the SHELXT program,[32] and then refined with full-matrix least-
squares methods based on F2 (SHELXL).[33] All non-hydrogen atoms were
refined with anisotropic atomic displacement parameters. H atoms were
finally included in their calculated positions and treated as riding on their
parent atom with constrained thermal parameters. A final refinement on F2
with 11112 unique intensities and 639 parameters converged at ωRF2 =
0.1231 (RF = 0.0500) for 8360 observed reflections with I > 2σ(I).
2-(tert-butyl)-9-phenyl-1,10-phenanthroline (L2) was synthesized in a
similar manner to that of (1) using the following reagents: (1) (100 mg, 0.42
mmol), phenyllithium (2.0 M, 0.32 mL) and manganese dioxide (912 mg,
10.5 mmol). Yield: 117 mg (89%). 1H NMR spectrum (300MHz, CDCl3) δ=
8.48 (m, 2H), 8.30 (d, 3JH,H = 8.7Hz, 1H), 8.18 (d, 3JH,H = 8.7Hz, 1H), 8.15
3
(d, JH,H = 8.4Hz, 1H), 7.77 (m, 3H), 7.57 (m, 2H), 7.47 (m, 1H), 1.64 (s,
9H). 13C NMR spectrum (75MHz, CDCl3) δ= 156.34 (s), 139.43 (s), 136.84
(s), 136.05 (s), 129.39 (s), 128.82 (s), 127.42 (s), 126.99 (s), 126.08 (s),
125.34 (s), 120.08 (s), 119.24 (s), 38.76 (s), 30.36 (s). HRMS (ESI+) for
C22H20N2 [M+H]+, m/z 313.1714 found, 313.1705 calc.
Anal. Calcd for C22H20N2 . 0.05 CH2Cl2 . 0.05 C5H12 : C, 83.63; H, 6.51; N,
8.75. Found: C, 83.31; H, 6.19; N, 8.53.
[Cu(L1)2]PF6 (C1) Under argon atmosphere, [Cu(ACN)4]PF6 (58 mg,
0.16mmol) was dissolved in 4mL of degassed dichloromethane. This
solution was transferred into a vial that contains L1 (92 mg, 0.31mmol).
The red resulting solution was stirred overnight at room temperature. The
mixture was precipitate in hexanes to obtain a red powder (100%, 248mg)
1H NMR spectrum (300MHz, CDCl3) δ= 8.59 (d, 3JH,H= 8.4Hz, 2H), 8.52 (d,
3JH,H= 8.4Hz, 2H), 8.04 (s, 4H), 7.94 (d, 3JH,H= 8.4Hz, 2H), 7.72 (d, 3JH,H
8.4Hz, 2H), 7.24 (m, 4H), 6.69 (t, 3JH,H= 7.5Hz, 2H), 6.37 (d, 3JH,H= 7.8Hz,
4H), 3.21 (t, 3JH,H = 6.9Hz, 2H), 1.13 (d, 3JH,H= 7.2Hz, 6H), 0.92 (d, 3JH,H
=
=
6.9Hz, 6H). 13C NMR spectrum (125MHz, CDCl3) δ= 166.42 (s), 157.67
(s), 143.77 (s), 142.85 (s), 139.03 (s), 137.85 (s), 137.81 (s), 128.63 (s),
128.38 (s), 128.26 (s), 127.36 (s), 127.10 (s), 126.81 (s), 126.19 (s),
124.83 (s), 121.89 (s), 39.57 (s), 29.70 (s), 22.67 (s), 21.91 (s).
HRMS (ESI): m/z calcd for C42H36CuN4+ : 659.2236 [M]+; found 659.2250.
UV/Vis(CH2Cl2): MLCT ʎmax(ε)= 443 nm (3987 L.cm-1.mol-1); fluorescence
(CH2Cl2): λex= 443nm; λem= 698nm.
Stopped Flow absorption spectrophotometry was performed on a BioLogic
SFM-4000 coupled to a J&M Tidas diode array spectrometer. Experiments
were at least triplicated for kinetic fits. Monoexponential fits were
performed using the Biokine software (BioLogic).
Anal. Calcd for C42H36CuF6N4P.0.9 CH2Cl2.0.15 C5H12: C, 58.78; H, 4.47;
N, 6.28. Found C, 58.56; H, 4.82; N, 6.66.
2-(tert-butyl)-1,10-phenanthroline (1) Under argon, the 1,10-
phenanthroline (1.00 g, 5.56 mmol) is dissolved into dry toluene (48 mL).
The solution is cooled to 0°C before addition dropwise of tert-butyllithium
(1.7 M, 4.9 mL). The resulting solution was allowed to warm to room
temperature and stir for 18 h. Water (10 mL) was then added followed by
a dichloromethane extraction. Organics phases are stirred with activated
manganese dioxide (12.00 g, 139.00 mmol) for 6h. The mixture was dried
over magnesium sulfate, filtrated and evaporated. The orange oil is
chromatographed on silica column (0 to 5% of methanol in
dichloromethane) to obtain 931 mg (71%) of the product. 1H NMR
spectrum (300MHz, CDCl3) δ= 9.23 (dd, JH,H=1.8Hz, JH,H=4.5Hz, 1H),
8.24 (dd, 4JH,H=1.8Hz, 3JH,H = 8.1Hz, 1H), 8.19 (d, 3JH,H = 8.4Hz, 1H), 7.78
(m, 3H), 7.62 (dd, 3JH,H= 4.5Hz, 3JH,H= 8.1Hz, 1H), 1.61 (s, 9H). 13C NMR
spectrum (75MHz, CDCl3) δ= 169.80 (s), 150.38 (s), 146.53 (s), 145.00 (s),
136.10 (s), 128.93 (s), 126.72 (s), 126.41 (s), 125.62 (s), 122.43 (s),
119.95 (s), 38.66 (s), 30.51 (s). HRMS (ESI+) for C16H16N2 [M+H]+, m/z
237.1399 found, 237.1392 calc.
[Cu(L2)2]PF6 (C2) was synthesized in a similar manner to that of (C1)
using the following reagents: [Cu(ACN)4]PF6 (28 mg, 0.075mmol) and L1
(47 mg, 0.15mmol). Yield: (100%, 125mg). 1H NMR spectrum (300MHz,
CDCl3) δ= 8.60 (d, 3JH,H= 8.4Hz, 2H), 8.47 (d, 3JH,H= 8.7Hz, 2H), 8.07 (AB,
JAB = 12.6Hz, 4H), 7.99 (d, JH,H= 8.7Hz, 2H), 7.87 (d, JH,H= 8.1Hz, 2H),
7.14 (m, 4H), 6.65 (m, 2H), 6.32 (m, 4H), 1.24 (s, 18H). 13C NMR spectrum
(75MHz, CDCl3) δ= 168.57 (s), 157.28 (s), 143.09 (s), 139.21 (s), 137.95
(s), 137.80 (s), 128.85 (s), 128.56 (s), 128.38 (s), 127.50 (s), 126.90 (s),
126.55 (s), 125.39 (s), 124.21 (s), 38.52 (s), 30.56 (s). HRMS (ESI): m/z
calcd for C44H40CuN4+ : 687.2549 [M]+; found 687.2541. UV/Vis (CH2Cl2):
MLCT ʎmax(ε)= 450 nm (3851 L.cm-1.mol-1); fluorescence (CH2Cl2): λex=
450nm; λem= 695nm.
3
3
4
3
Anal. Calcd for C44H40CuF6N4P. 0.45 CH2Cl2: C, 61.26; H, 4.73; N, 6.43.
Found: C, 61.27; H, 4.85; N, 6.50.
Computational details All calculations were performed using ADF
2019[34] package at the DFT level of theory with B3LYP functional.[35] The
all-electrons slater type TZP basis set described all atoms.[36] Scalar
relativistic ZORA Hamiltonian was employed.[37] Van der Waals forces
were treated through the introduction of Grimme’s corrections
(grimme3).[38] Solvent corrections for dichloromethane were introduced
through a PCM model.[39] Structures were fully optimized and the
absorption spectrum was computed by mean of TD-DFT[40] on these
structures with the inclusion of the Tamm-Dancoff approximation.[41] Spin-
Orbit Coupling was computed by a perturbative approach. The excited
states (singlet or triplet) structures were computed by the same approach.
All calculations were first performed retaining the symmetry point group of
the ground state. Then symmetry was broken to allow complete relaxation
of the structures. For the study of Non-Covalent interactions, the
complexes were reoptimized with GAUSSIAN 09 (D01)[42] package at DFT
level of theory (B3LYP functional)[43] using 6-31+G** basis set[44] for all
atoms (the f polarization were deleted). Again, dispersion corrections were
introduced through Grimme’s correction GD3 and solvent introduced
through a PCM model of dichloromethane. These calculations were
performed on the ground state and on the lowest triplet state of the
2-isopropyl-1,10-phenanthroline (2) was synthesized in
a similar
manner to that of (1) using the following reagents: 1,10-phenanthroline
(200 mg, 1.11 mmol), isopropyllithium (0.7 M, 2.4 mL) and manganese
dioxyde (2.40 g, 27.8 mmol). Yield: 174 mg (71%). 1H NMR spectrum
(400MHz, CDCl3) δ= 9.25 (dd, 4JH,H = 1.6Hz, 3JH,H = 4.4Hz, 1H), 8.25 (m,
3
2H), 7.78 (AB, JAB= 15.6Hz, 2H), 7.64 (m, 2H), 3.69 (sept, JH,H = 7.2Hz,
1H), 1.49 (d, 3JH,H = 7.2Hz, 6H). 13C NMR spectrum (100MHz, CDCl3) δ=
206.82 (s), 150.34 (s), 145.41 (s), 136.58 (s), 135.99 (s), 126.47 (s),
125.56 (s), 122.61 (s), 120.05 (s), 37.71 (s), 30.89 (s), 23.16 (s). HRMS
(ESI+) for C15H14N2 [M+H]+, m/z 223.1243 found, 223.1235 calc.
2-isopropyl-9-phenyl-1,10-phenanthroline (L1) was synthesized in a
similar manner to that of (1) using the following reagents: (2) (174 mg, 0.78
mmol), phenyllithium (1.9 M, 0.62 mL) and manganese dioxide (1.69 g,
19.50 mmol). Yield: 224 mg (96%). All characterization data are in
agreement with the ones published by Kavita et al.[11]
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molecules. The natures of the encountered stationary points were
characterized by a frequency analysis. The wavefunction of these
complexes was then analyzed using the NCIplot package.[45]
Acknowledgements
The authors wish to thank the ANR (PERCO program n°ANR-16-
CE07-0012-01) for financial support, the CEISAM (Nantes
University) analysis platforms. YP is grateful to Dr. Eric Levillain
from MOLTECH-Anjou for granting access to his glove box
electrochemical setup. The calculations were carried out in part at
the IDRIS computer centers through a grant of computer time
from GENCI (A0050810629) and in part in the Strasbourg HPC.
Keywords: Copper(I) homoleptic complexes • Photochemistry •
reductive quenching • DFT calculation
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Substituting phenanthroline by one
phenyl and one branched alkyl chain
in α of the nitrogen atoms yields
copper(I) complexes gathering the
advantages of both kinds of
Lea Gimeno, Errol Blart, Jean-Noel
Rebilly, Marina Coupeau, Magali Allain,
Thierry Roisnel, Alexis Quarré de
Verneuil, Christophe Gourlaouen,
Chantal Daniel and Yann Pellegrin*
substituents, provided that the steric
bulk is finely tuned. This strategy
allowed to implement a reductive
quenching step with improved
kinetics, while laying stress under the
paramount importance of controlling
the reorganization energy.
Page No. – Page No.
Non symmetrical sterically
challenged phenanthroline ligands
and their homoleptic copper(I)
complexes with improved excited
state properties
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This article is protected by copyright. All rights reserved.