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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Excited State Properties of Heteroleptic Cu(I) 4H‑Imidazolate
Complexes
Martin Schulz,*,†,‡ Christian Reichardt,†,‡ Carolin Müller,† Kilian R. A. Schneider,‡ Jonas Holste,†
and Benjamin Dietzek*,†,‡,§
†
Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany
Leibniz Institute of Photonic Technology (IPHT), Department Functional Interfaces, Albert-Einstein-Straße 9, 07745 Jena,
Germany
§
Center für Energy and Environmental Chemistry (CEEC), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena,
Germany
‡
S Supporting Information
*
ABSTRACT: The excited state properties of three heteroleptic
copper(I) xantphos 4H-imidazolate complexes are investigated by
means of femtosecond and nanosecond time-resolved transient
absorption spectroscopy in dichloromethane solution. The subpicosecond spectral changes observed after excitation into the MLCT
absorption band are interpreted as intersystem crossing from the
singlet to the triplet manifold. This interpretation is corroborated by
DFT and TD-DFT results, indicating a comparable molecular
geometry in the ground state (and hence the nonrelaxed singlet
state) and the excited triplet state. Population of the triplet state is
followed by planarization of the N-aryl rings of the 4H-imidazolate
ligand on a 10 ps time scale. The planarization strongly depends on
the substitution pattern of the N-aryls and correlates with the
reduced moment of inertia for the planarization motion. The triplet state subsequently decays to the ground state in about 100
ns. These results demonstrate that the excited state processes of copper(I) complexes depend on the specific ligand(s) and their
substitution pattern. Thus, the work presented points to a possibility to design copper(I) complexes with specific photophysical
properties.
■
INTRODUCTION
Understanding the excited-state processes of chromophores
and their dependence on the substitution pattern is a
prerequisite for custom-tailoring the photophysical properties
of chromophores. The excited state properties of homoleptic
copper(I) diimine complexes are generally quite well understood,1−14 while the excited state properties of heteroleptic
copper(I) phosphine diimine complexes15−22 have drawn much
less attention (vide inf ra for a brief summary). So far, for both
homoleptic and heteroleptic copper(I) complexes, comparable
results have been obtained in terms of the nature of the excited
state processes and the associated rate constants. However, it
should be noted that for most of the investigated homoleptic
and heteroleptic complexes, the diimine is a phenanthrolinetype ligand. Hence, it remains to be validated how generalizable
the model that has been put forward in understanding the
excited-state chemistry of the heteroleptic copper(I) complexes
carrying a phenanthroline derived ligand is.
We have recently reported on neutral, heteroleptic copper(I)
phosphine 4H-imidazolate complexes with strong electronic
absorption spanning almost the entire visible range (Figure
1).23 The 4H-imidazolate ligands are comparable to benchmark
© XXXX American Chemical Society
phenanthroline ligands such as neocuproine in terms of steric
demand in the vicinity of the copper center.24 However, the
4H-imidazolate ligand backbone is less rigid than phenanthroline and allows for rotational motion of the N-aryl rings, which
has been shown to dominate the ultrafast photophysics in
Ru(II) 4H-imidazolate complexes.25−28 Herein, we report on
the dependence of the excited state processes on the N-aryl
substituents of three Cu(I)-4H-imidazolate complexes by
means of transient absorption spectroscopy with femtosecond
and nanosecond temporal resolution.
In the following, the excited state processes reported for
homoleptic copper(I) diimine complexes7−9,12,29−33 as well as
heteroleptic copper(I) phosphine diimine complexes16,17 will
briefly be described (the description refers to complexes with a
substituted 1,10-phenanthroline as the most commonly
employed diimine ligand). In homoleptic copper(I) diimine
complexes, excitation of the MLCT absorption band into the
singlet manifold leads to the formation of a transient copper(II)
center, which is followed by a pseudo-Jahn−Teller flattening
Received: July 10, 2017
A
DOI: 10.1021/acs.inorgchem.7b01680
Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Investigated copper(I) 4H-imidazolate complexes and the ground state absorption spectra in CH2Cl2 (r.t., ca. 2 × 10−5 M). The 4Himidazolate chromophore of the ligand is highlighted in violet.
with respect to the 4H-imidazolate plane, which takes place on
a 10 ps time scale. This behavior will be derived and discussed
in the following sections.
distortion from the (pseudo) tetrahedral toward a (distorted)
square planar coordination geometry, usually on a subpicosecond time scale.9,29 Due to spin−orbit coupling of the copper
ion, intersystem crossing (ISC) to the triplet manifold is
possible and usually occurs on a 10 ps time scale.9,29,32 The
system then decays radiatively to the singlet ground state on
time scales up to the microsecond range.24 The formation of a
square planar coordination environment allows interactions
with donor molecules (e.g., solvent) via the apical coordination
site, which results in nonradiative decay of the formed
exciplex.9,24 Thus, hampering the flattening distortion by
introduction of bulky substituents resulted in greatly enhanced
emission time constants.7,34−36
A similar excited state behavior was reported for heteroleptic
[Cu(I)(xantphos)(phenanthroline)]+ complexes.16,17 In dependence on the substitution pattern of the phenanthroline
ligands, time constants for the flattening distortion and
intersystem crossing of 0.7−1.4 ps and 6.8−7.4 ps have been
observed, respectively.17 For the complexes reported in ref 17,
the rate of the flattening distortion depends on the molecular
weight of the phenanthroline ligands; larger distortion time
constants were observed for heavier ligands. The bulkiness,
however, seemed to play a minor role for the distortion time
scale. The intersystem crossing time constant, on the other
hand, was found to depend on the electronic properties of the
phenanthroline ligands showing shorter ISC time constants for
ligands with electron withdrawing substituents.16,17
In contrast, the copper(I) xantphos 4H-imidazolate complexes reported in this contribution undergo a fast subpicosecond ISC, followed by the planarization of the N-aryl rings
■
RESULTS AND DISCUSSION
Synthesis. The synthesis of the copper(I)4H-imidazolate
complexes CuN1P2 and CuN2P2 was previously described.23
CuN3P2 is described for the first time and was synthesized
according to the published procedure.23 Briefly, stepwise mixing
of [Cu(acetonitrile)4]PF6 with the xantphos ligand P2 and the
respective 4H-imidazole in the presence of an anion exchange
resin afforded deeply colored solutions. The pure product was
obtained by recrystallization. CuN3P2 was characterized by
one- and two-dimensional 1H, 13C, and 31P NMR spectroscopy;
mass spectrometry; and elemental analysis (see Supporting
Information for further details).
Ground State Absorption Properties. All complexes
feature an intense absorption band in the UV as well as a broad
absorption band that spans almost the entire visible spectrum
(Figure 1). In the UV, a strong narrow band between 250 and
350 nm is observed with a lower-energy shoulder at about 360
nm. In the visible range, a weaker but broader absorption band
ranges from about 400 to 600 nm. The unusually broad and
low-energy absorption is a feature of the polymethine character
of the monoanionic 4H-imidazolate ligands, which are best
described as (aza)-oxonoles (the neutral 4H-imidazole is best
described as merocyanine).37−40 The chromophore properties
of the 4H-imidazolate are retained upon complexation of the
copper(I) xantphos fragment. However, in the copper(I)
B
DOI: 10.1021/acs.inorgchem.7b01680
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390, 430, and 620 nm as well as positive differential absorption
features above 700 nm. In CuN1P2 and CuN2P2, the 430 nm
band is observed as a shoulder to the 390 nm band, while in
CuN3P2 the situation is reversed and the 390 nm band appears
as a shoulder to the 430 nm band. For all complexes, the
ground state bleach is observed at around the 550 nm, i.e., in
the long wavelength range of the respective steady state
absorption spectra.
Upon increasing the delay time, the 390, 430, and 620 nm
bands decrease, while at 490 nm a new band is growing in, as
are features above 700 nm. The decrease of the 390 nm band as
well as the increase of the features above 700 nm continue
during the first 5 ps, while the decrease of the 430 and 620 nm
bands progresses within about the first 100 ps. The observed
decrease/increase of the optical density difference at 430 nm/
490 nm bands is most pronounced for CuN3P2. Finally, the
bands at 490 nm/620 nm increase/decrease over about the first
100 ps for all three complexes. On longer time scales (>1 ns),
the spectral features of CuN1P2 and CuN3P2 remain
approximately unchanged. The spectral features of CuN2P2
slowly decay, after having reached a maximum at 490 nm within
about 100 ps. About 50 ns after the pump pulse (see
Supporting Information, Figures S7−S9), the absorption
difference spectra of all three complexes are qualitatively the
same as those obtained at about 1.8 ns, and the observed
difference absorption decays monoexponentially to zero.
Time Constants and Assignment of Underlying
Molecular Processes. Global fitting of the data obtained
between 100 fs and 1.8 ns (fs-time-resolved experiment)
required two time constants and an infinite component. In
order to temporally resolve the infinite component, a
nanosecond time-resolved experiment was carried out and
revealed a monoexponential decay (the initial delay time was 50
ns). The spectral signature of the infinite component
(femtosecond-time-resolved experiment) is reflected in the
initial spectrum of the nanosecond-time-resolved experiment.
The three time constants obtained are summarized in Table 1.
The shortest time constant τ1, i.e., 0.6 ps (CuN1P2), 0.8 ps
(CuN2P2), and 0.8 ps (CuN3P2), is associated with the
decrease of the 390 nm band and the rise of the features above
700 nm. This is reflected in the similar decay associated spectra,
DAS(τ1), for the individual complexes. The second time
constant τ2 obtained differs significantly among the three
complexes and is quantified as 6 ps (CuN3P2), 9 ps
(CuN1P2), and 18 ps (CuN2P2). Spectrally and irrespective
of the complex, τ2 is associated with the continuous increase of
the 490 nm band and the decrease of the 430 and 620 nm band.
Subsequently, the system decays monoexponentially to the
ground state with τ3: 73 ns (CuN2P2), 107 ns (CuN3P2), and
135 ns (CuN1P2).
The DAS (Figures 3−5) associated with the first time
constant reflects marked spectral changes over the entire visible
spectrum up to 780 nm as detailed above. The photophysical
process connected with the observed spectral changes is
assigned to intersystem crossing (ISC) from the singlet to the
triplet state. Similarly pronounced spectral changes were
observed for [Cu(dmp)2]+12,31,41 and [Cu(dmp)(xantphos)]+17
and assigned with ISC, however, on a picosecond time scale.
Flattening distortion might also take place on such short time
scales; however, this process is generally associated with only
very minor spectral changes in the UV/vis range.9,12,17,29 Thus,
the spectral changes reflected in the DAS(τ1) are likely due to
significant contributions of ISC to the ultrafast subpicosecond
complexes, the visible absorption band appears broadened,
leading to the complexes’ dark violet color (see Figure 2).23
Figure 2. Normalized absorption spectra of the neutral ligand HN3,
deprotonated ligand N3− (deprotonated with 1,8-diazabicyclo[5.4.0]undec-7-ene, DBU), and the complex CuN3P2 in dichloromethane
(also see Supporting Information Figure S4 for the neutral ligand
HN1, deprotonated ligand N1−, and CuN1P2 and Figure S5 for the
neutral ligand HN2, deprotonated ligand N2−, and CuN2P2).
A tentative assignment of the observed electronic transitions
can be made by comparison of the spectra of the complexes
with the absorption spectra of the neutral ligands as well as the
deprotonated ligands (see Figure 2). The absorption spectra of
the neutral ligands HN1 and HN2 exhibit two bands in the UV
region around 280 and 350 nm with nearly the same intensity
ratio, while HN3 features a weak absorption band at 290 nm as
well as a structured band at around 360 nm similar to HN1. In
the spectra of the copper complexes, the absorption band
between 250 and 300 nm is attributed to the xantphos-based
transitions, while the 4H-imidazolate based transitions appear
as shoulders around 310 nm (CuN2P2) as well as at 360 nm
(CuN1P2, CuN3P2) and are allocated to intraligand transitions
(πim → πim*).
For the complexes, the absorption bands between 400 and
650 nm are significantly red-shifted with respect to the free
ligands. CuN1P2 shows a maximum at 551 as well as shoulders
at 520 and 580 nm, and CuN2P2 has a maximum at 517 nm
with shoulders at about 480 and 550 nm. CuN3P2 exhibits a
double peak at 531 and 454 nm as well as a shoulder at about
550 nm. Earlier TD-DFT calculations on CuN2P2 indicated
the occurrence of MLCT transitions in the visible range.23
However, the similarity of the overall structure of the visible
absorption bands of the complexes with respect to those of the
deprotonated ligands suggests that also 4H-imidazolate-based
intraligand transitions are involved.
Excited State Absorption Properties. All complexes
reported in this contribution were investigated in dichloromethane solution at room temperature by transient absorption
spectroscopy upon excitation at 520 nm, i.e., in the MLCT
absorption band. The transient absorption of the samples was
probed with a white light continuum (generated in CaF2). The
observed spectra are given in Figures 3−5 and in general show
comparable transient absorption spectra as well as consistent
time dependent changes. The spectra immediately observed
after excitation show excited state absorption bands at about
C
DOI: 10.1021/acs.inorgchem.7b01680
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Article
Inorganic Chemistry
Figure 3. (A) Selected femtosecond-time-resolved transient absorption spectra of CuN1P2 in dichloromethane at r.t. and the inverted ground state
absorption spectrum shaded in gray. (B) Decay associated spectra (DAS). (C) Kinetic traces recorded at various probe wavelengths.
reorganization processes, namely, the planarization of the Naryl rings of the 4H-imidazolate ligand toward the imidazole
plane (cf. Figure 6). Rotational motion of the N-aryl rings was
already described for ruthenium(II) 4H-imidazolate complexes47 and can be understood as stabilization of the excited
triplet state by the enlargement of the conjugated system. For
CuN3P2, which bears a small para-methyl substituent, the
observed time constant is the shortest, followed by CuN1P2
with the larger para-COOEt substituent and CuN2P2 with a
meta-CF3 substituent. The dependence of the time constants
on the substitution pattern can be rationalized when
considering the impact of the mass and position of the
substituents on the moment of inertia of the rotational motion,
i.e., planarization of the N-aryl rings. In CuN3P2, rotational
motion is comparatively fast, due to a small moment of inertia,
since the small methyl group lies along the rotation axis. The
COOEt group in CuN1P2 imposes a larger moment of inertia,
leading to a slowdown of the rotational motion, with respect to
CuN3P2, and consequently a larger planarization time constant
is observed. The meta-CF3 group sits off the rotational axis and
thus imposes the largest impact on the rotational motion
leading to the largest planarization time constant among the
studied complexes. Reduced moments of inertia were deduced
from optimized (DFT) ground state structures of all three
complexes using the method of Pitzer48,49 as implemented in
photoinduced kinetics. This is somehow unexpected, since for
most Cu(I) phenanthroline-type complexes ISC time constants
were reported to occur on a 10 ps time scale, as a consequence
of weak spin−orbit coupling, induced by the faster flattening
process.32,42 The attribution of the spectral changes associated
with the subpicosecond time constants to ISC can be
rationalized when considering strong spin−orbit coupling.
The availability of large spin−orbit couplings was reported
for copper(I) bis(2,9-dimethyl-1,10-phenanthroline) complexes
([Cu(dmp)2]+),32,43 and hence the general possibility of fast,
subpicosecond intersystem crossing was proposed.32,43−45
Furthermore, the spin−orbit coupling is reduced upon
structural rearrangement of the complex in the excited state,
i.e., upon flattening of the initial tetrahedral geometry,7,32 which
we assume takes place in parallel with ISC. Zou and co-workers
theoretically (DFT, TD-DFT) found for copper(I)(phenanthroline)(diphosphine)-type complexes that the dihedral angle (DHA) between the P−Cu−P and N−Cu−N planes
decreases in the relaxed S1 state but is very similar in the relaxed
T1 with respect to S0.46 Similarity between the singlet state
structure and the triplet state structure usually leads to
increased ISC rates.46
The spectral changes associated with τ2 are represented by
the decay of the 430 and 620 nm bands and the rise of the 490
nm band. These observations are interpreted as structural
D
DOI: 10.1021/acs.inorgchem.7b01680
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Article
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Figure 4. (A) Selected femtosecond-time-resolved transient absorption spectra of CuN2P2 in dichloromethane at r.t. and the inverted ground state
absorption spectrum shaded in gray. (B) Decay associated spectra (DAS). (C) Kinetic traces recorded at various probe wavelengths.
the GPOP program.50 The exocyclic 4H-imidazolate nitrogen
and the ipso-carbon of the N-aryl ring were set as pivot atoms
for the rotation along the N−C bond. A correlation between
the calculated moments of inertia and τ2 is given in Figure 6.
Computational Results. We have carried out DFT and
TD-DFT calculations with CuN2P2, in order to investigate the
lowest energy singlet and lowest energy triplet state with
respect to the ground state. All calculations were made with the
PBE0 functional, which has been found suitable for copper(I)
complexes (see Supporting Information for further details).20 A
solvent sphere (dichloromethane) was taken into account. The
obtained geometrical parameters for the calculated ground state
are in good agreement with X-ray data, i.e., bond lengths, bite
angles, as well as the DHA of the ground state (Table S1). The
energies of the S1 as well as T1 state cannot directly be
compared due to the different methodologies (DFT vs TDDFT). However, the relative differences with respect to the
ground state are 1.3 eV (S1) and 1.4 eV (T1), respectively.
Within the error of the calculation (ca. 0.2 eV) these energy
differences are equal (we note that the planarization of the Naryl rings (vide inf ra) was not taken into account for the
calculation of T1). The major contributions of the low energy
singlet transitions (Table 2) involve charge transfer transitions
from the HOMO, HOMO−1, HOMO−2, and HOMO−3 to
the 4H-imidazolate based LUMO (Figure S12). The four
highest energy occupied orbitals are mainly of copper parentage
with contributions from xantphos and the 4H-imidazolate
(Table S2). The transitions are therefore characterized as
MLCT transitions; however, some ILCT character is noted as
well.
The DFT calculations revealed that the dihedral angle
between the P−Cu−P and N−Cu−N planes (see Figure 7) in
the lowest energy triplet state (81.61°) is essentially the same as
in the singlet ground state (81.84°) but heavily distorted in the
relaxed S1 state (64.61°). The calculated DHA in the ground
state is in agreement with X-ray data CuN2P2 (82.46°).23 The
observation of a decreased DHA in the S1 state but essentially
no change in the T1 state, with respect to S0, is in agreement
with the calculated change of the DHAs in the S0, S1, and T1
states of [Cu(dmp)(PPh3)2]+, [Cu(dbp)(PPh3)2]+, and [Cu(dbp)(DPEPhos)]+, respectively (dmp = 2,9-dimethyl-(1,10phenanthroline); dbp = 2,9-di-n-butyl-(1,10-phenanthroline)).46 Contrarily, decreased DHAs in the T1 state with
respect to S0 were theoretically found for copper(I) bisphenanthroline complexes7,10,32,44 as well as [Cu(I)(dmp)(xantphos)]+17 and [Cu(I)(dmp)(DPEphos)]+.20 For the
latter, also the S1 geometry was calculated, revealing a similarly
decreased DHA as in the T1 state with respect to S0.20,46 The
computational results obtained by us as well as in the cited
literature support the idea that fast intersystem crossing is
E
DOI: 10.1021/acs.inorgchem.7b01680
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Inorganic Chemistry
Figure 5. (A) Selected femtosecond-time-resolved transient absorption spectra of CuN3P2 in dichloromethane at r.t. and the inverted ground state
absorption spectrum shaded in gray. (B) Decay associated spectra (DAS). (C) Kinetic traces recorded at various probe wavelengths.
Table 1. Time Constants τ Determined by a Global Fit of the
Ultrafast Transient Absorption Data As Well As Reduced
Moments of Inertia Ired for the Rotation of the N-Aryl
Moieties (vide inf ra)
complex
τ1/ps
τ2/ps
τ3/ns
Ired/amuÅ2
CuN1P2
CuN2P2
CuN3P2
0.6
0.8
0.8
9
18
6
135
73
107
359
920
87
is in contrast to earlier reports on homo- and heteroleptic
copper(I)−phenanthroline complexes. The fast ISC is rationalized on the basis of similar dihedral angles in the ground state
(and hence the excited singlet state prior to flattening)
presumably resulting in strong spin−orbit coupling. In this
respect, the flattening distortion represents a quenching
channel for the ISC. The spectral changes associated with
time constants between 5 and 18 ps are interpreted as
planarization of the N-aryl rings of the 4H-imidazolate ligand
on about a 10 ps time scale. The planarization time constants
vary markedly with the substitution pattern of the N-aryl rings.
A positive correlation of the reduced moment of inertia of the
rotational motion with the second time constant is obtained
and reflects the impact of the substitution on the planarization
of the N-aryl rings. All described processes, namely flattening,
ISC, and planarization, can be expected to depend on the
substitution pattern of N-aryl rings. In order to exploit the
fascinating features of copper(I) 4H-imidazolate complexes,
extended studies are necessary that include the investigation of
the emission properties and calculation of spin−orbit couplings.
Nonetheless, the reported observations demonstrate that the
understanding of copper(I) excited state processes and hence
the design of complexes with distinct properties is just at the
beginning.
possible on the basis of similar geometries in the undistorted,
i.e., not flattened, excited singlet and triplet states, as was earlier
proposed.46,51
■
CONCLUSION
The excited state properties of three neutral, heteroleptic
copper(I)4H-imidazolate complexes were investigated by
means of femtosecond- and nanosecond-time-resolved
pump−probe absorption spectroscopy as well as DFT
calculations (cf. Figure 8). Excitation of the complexes at 520
nm (MLCT band) in dichloromethane furnished distinct
excited state absorption patterns. The spectral changes
associated with the fast subpicosecond processes are interpreted
as ISC from the excited singlet to the triplet manifold, taking
place on the same time scale as the flattening distortion, which
F
DOI: 10.1021/acs.inorgchem.7b01680
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Figure 6. Correlation of the reduced moment of inertia for the rotation of the N-aryl rings with the second time constant τ2 (left). DFT optimized
structure of CuN2P2. The dashed line marks the axis for the rotation of the N-aryl ring. For the sake of clarity, the xantphos ligand is given in light
gray, and hydrogen atoms are omitted.
Table 2. Low Energy Transitions Calculated on the TD-DFT/PBE0 Level of Theory
transition
energy/eV
wavelength/nm
oscillator strength
assignment
S1
S2
S3
S4
2.49
2.54
2.66
2.75
498
489
467
450
0.1888
0.2299
0.0329
0.0073
MLCT
MLCT
MLCT
MLCT
(29%),
(68%),
(20%),
(70%),
HOMO→LUMO (59%)
HOMO→LUMO (24%)
H-2→LUMO (63%), HOMO→LUMO (14%)
H-2→LUMO (21%)
repetition rate), was used to produce the 520 nm pump pulse with a
TOPAS-C. A supercontinuum probe pulse generated in a CaF2 plate
served as a broad-band probe. The polarizations of pump and probe
were oriented at the magic angle. Probe and reference intensities were
detected on a double stripe diode array and converted into differential
absorption (DA) signals using a commercially available detection
system (Pascher Instruments AB). The time resolution of the
experiment was evaluated by the width of the coherent artifact,53,54
allowing an estimation of the cross correlation value between pump
and probe to be on the order of 80 fs. The femtosecond time-resolved
measurements were performed in quartz cells with a 1 mm optical path
length with an approximate optical density of 0.4 at 520 nm. The
integrity of the samples was ensured via absorption spectroscopy prior
to and after each measurement. The DA signals recorded as a function
of the delay time and the probe wavelength were chirp corrected and
subsequently subjected to a global fitting routine using a sum of
exponential functions for data analysis.55
The nanosecond time-resolved experimental setup was previously
described.56 The pump pulse was delivered by a Continuum Surelite
Laser, with repetition rate of 10 Hz and modified with a Continuum
optical parametric oscillator to obtain the pump wavelengths of 520
nm. Probe light was delivered by a 75 W xenon arc lamp and dispersed
by a ruled grating on to the sample using right angle geometry with
respect to the pump light. The probe light was detected by a
Hamamatsu R928 photomultiplier, and the signal was processed by a
commercially available detection system (Pascher Instruments AB).
The measurements were performed in a flow-through quartz cell (1
cm optical path length) at an approximate optical density of 0.4 at 520
nm. The integrity of the samples was ensured via absorption
spectroscopy prior to and after each measurement. The sample was
probed between 380 and 800 nm with 10 nm steps. The DA signals
were recorded as a function of the delay time, and the probe
wavelength was subjected to a global fitting routine using a sum of
exponential functions for data analysis.57
Figure 7. P−Cu−P and N−Cu−N planes of DFT-optimized CuN2P2
in the ground state (GS), relaxed singlet excited state (S1), and triplet
state (T1). The dihedral angles are in parentheses.
Figure 8. Overview of the investigated excited state processes (dark
gray arrows).
■
major contributions
H-1→LUMO
H-1→LUMO
H-3→LUMO
H-3→LUMO
EXPERIMENTAL SECTION
The setup for femtosecond-time-resolved transient absorption spectroscopy has been described previously.52 An 800 nm pulse, produced
by an amplified Ti:sapphire oscillator (Libra, Coherent Inc., 1 kHz
G
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■
Nuclear Interactions on the Excited-State Properties and Structural
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ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01680.
Synthesis and characterization details for CuN3P2,
comparison of the UV−vis absorption spectra of the
neutral and deprotonated ligands as well as the
complexes, transient absorption data obtained between
50 and 1000 ns, and computational details (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: martin.schulz.1@uni-jena.de.
*E-mail: benjamin.dietzek@uni-jena.de.
ORCID
Martin Schulz: 0000-0003-4989-5207
Benjamin Dietzek: 0000-0002-2842-3537
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
B.D. thanks the financial support by the FCI. C.R. is grateful for
financial support from the Jena Graduate Academy for a Ph.D.
scholarship. K.R.A.S. thanks the Carl-Zeiss-Foundation for a
Ph.D. scholarship. The authors thank Prof. Rainer Beckert for
putting the 4H-imidazoles at our disposal. Julia Preiss is
gratefully acknowledged for helpful discussions. Finally, we
would like to thank the COST Action CM1202.
■
■
DEDICATION
Dedicated to Prof. Rainer Beckert on the occasion of his
retirement
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