Photophysical processes in rhenium(I) diiminetricarbonyl arylisocyanides featuring three interacting triplet excited states

Article abstract of DOI:10.1021/acs.inorgchem.9b01155

We present a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal-organic chromophores of the generic molecular formula [Re(phen)-(CO)₃(CNAr)]⁺ [CNAr = 2,6-diisopropylphenyl isocyanide (1), 4-phenyl-2,6-diisopropylphenyl isocyanide (2), 4-phenylethynyl-2,6-diisopropylphenyl isocyanide (3), and 4-biphenyl-2,6-diisopropylphenyl isocyanide (4)]. This particular series features varied degrees of π-conjugation length in the CNAr moiety, resulting in significant modulation in the resultant photophysical properties. All molecules possess long-lived [8- 700 μs at room temperature (RT)], strongly blue-green photoluminescent and highly energetic excited states λ_max,em = 500-518 nm; Φ = 14-64%). Each of these chromophores has been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to suprananosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in manipulating the excited-state decay in three of these molecules (2-4), wherein the triplet metal-to-ligand charge-transfer (³MLCT) state along with two distinct triplet ligand-centered (³LC) excited-state configurations (phen and CNAr) conspire to produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1, the phenanthroline ³LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT character. With more extended π conjugation in the CNAr subunit (2-4), the initially generated ³LC(phen)/³MLCT excited state ultimately migrates to the CNAr ³LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature unique examples of inorganic excimer formation, as evidenced by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.

Full text of DOI:10.1021/acs.inorgchem.9b01155

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Photophysical Processes in Rhenium(I) Diiminetricarbonyl
Arylisocyanides Featuring Three Interacting Triplet Excited States
Joseph M. Favale, Jr., Evgeny O. Danilov, James E. Yarnell, and Felix N. Castellano*
Department of Chemistry, North Carolina State University (NCSU), Raleigh, North Carolina 27695-8204, United States
S
* Supporting Information
ABSTRACT: We present a series of four transition-metal
complexes based on the rhenium(I) tricarbonyl 1,10-
phenanthroline (phen) template, with a lone ancillary
arylisocyanide (CNAr) ligand to yield metal−organic
chromophores of the generic molecular formula [Re(phen)-
(CO)3(CNAr)]+ [CNAr = 2,6-diisopropylphenyl isocyanide
(1), 4-phenyl-2,6-diisopropylphenyl isocyanide (2), 4-phenyl-
ethynyl-2,6-diisopropylphenyl isocyanide (3), and 4-biphenyl-
2,6-diisopropylphenyl isocyanide (4)]. This particular series
features varied degrees of π-conjugation length in the CNAr
moiety, resulting in significant modulation in the resultant
photophysical properties. All molecules possess long-lived [8−
700 μs at room temperature (RT)], strongly blue-green
photoluminescent and highly energetic excited states (λmax,em = 500−518 nm; Φ = 14−64%). Each of these chromophores has
been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to supra-
nanosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as
a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy
with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental
evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density
functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in
manipulating the excited-state decay in three of these molecules (2−4), wherein the triplet metal-to-ligand charge-transfer
(3MLCT) state along with two distinct triplet ligand-centered (3LC) excited-state configurations (phen and CNAr) conspire to
produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting
shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1, the
3
phenanthroline LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT
3
character. With more extended π conjugation in the CNAr subunit (2−4), the initially generated LC(phen)/3MLCT excited
state ultimately migrates to the CNAr 3LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature
unique examples of inorganic excimer formation, as evidenced by dynamic self-quenching in the corresponding PL intensity
decays accompanied by the observation of a short-lived low-energy emission feature.
metal-to-ligand charge transfer (MLCT) in nature. This
manifests in broad, featureless emission spectra that are highly
dependent on environmental factors such as the solvent
polarity. It is also possible for low-energy ligand-centered (LC)
excited states to be present in these molecules lying in close
energetic proximity to the MLCT states. Any possible thermal
equilibrium or activation between these closely lying states
may lead to beneficial consequences, including dramatic
extension of the original MLCT excited-state lifetime. The
vast majority of previous studies have focused on tuning of the
MLCT states through manipulation of the lowest unoccupied
molecular orbital (LUMO) energetics directly resulting from
structural modifications in the appended diimine ligand. It is
INTRODUCTION
■
For approximately 4 decades, fac-rhenium(I) diiminetricar-
bonyl complexes have drawn considerable attention and
experimental investigation.1−15 These chromophores have
potential applications in a variety of photonics-relevant
technologies including solar energy conversion, photochemical
transformations, photocatalytic carbon dioxide and proton
reduction, photoluminescent molecular devices, and photo-
luminescence (PL) sensing of numerous analytes.16−29 These
attributes result from their low-energy visible-light absorption
properties with corresponding long excited-state lifetimes
suitable for sensitizing myriad intramolecular and bimolecular
electron- and energy-transfer reactions.
Prior investigations have concluded that the vast majority of
chromophores in the particular rhenium(I) tricarbonyl family
possess lowest-lying excited states, which are predominantly
Received: April 19, 2019
© XXXX American Chemical Society
A
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Scheme 1. General Synthetic Pathway and Molecular Structures of the fac-Rhenium(I) Complexes 1−4
important to note that through temperature-dependent
investigations, the proximity and ordering of multiple excited
states resident in these chromophores can be readily
elucidated.2,6−8,11,12,14,30
ligand and the photophysics are dominated by pure
3MLCT.47−49 because of their utilization primarily as an
ancillary ligand, the photophysical properties of lower-energy
CNAr moieties have rarely been explored in rhenium(I).
The current work focuses on varying the π-conjugation
length in the ancillary CNAr ligand fused to the rhenium(I)
tricarbonyl 1,10-phenanthroline (phen) template to yield
metal−organic chromophores of the generic molecular formula
[Re(phen)(CO)3(CNAr)]+ (1−4 in Scheme 1) featuring long-
lived and highly energetic excited-state properties. All of these
molecules have been photophysically investigated using static
and dynamic spectroscopic techniques, the latter probed from
ultrafast to suprananosecond time scales using transient
absorption (TA) and PL. Electronic structure calculations
utilizing density functional theory (DFT) and time-dependent
DFT (TD-DFT) methods have been performed on this series
of chromophores to support the experimental spectroscopic
assignments. The combined experimental and computational
evidence illustrates that the CNAr subunit in 2−4 was actively
engaged in manipulating the excited-state decay in these
To date, a variety of ancillary ligands have been incorporated
in these rhenium(I) diiminetricarbonyl frameworks to system-
atically modulate excited-state decay properties, leveraging
deterministic energy gap law behavior.31,32 These ancillary
substitutions include the incorporation of halides,1,3,4 pyridine
and piperidine,2,6,7,11,12 and, of particular significance to the
present work, isocyanides.8,13,33−36 Isocyanide ligands, featur-
ing both alkyl and aryl substituents, serve as strong σ donors
and good π acceptors, similar to that imposed by carbon
monoxide (CO).37−39 Therefore, isocyanides impart a strong
ligand field to the rhenium(I) center while offering the
opportunity for deliberately installing organic subunits with
meaningful variation in their singlet and triplet excited-state
energies. Extensive work into perturbation of the photophysical
properties of rhenium(I) isocyano complexes has been
performed, via substituent modification on simple arylisocya-
nide (CNAr) ligands and the number of isocyano versus
carbonyl ligands on the complex.33 These studies have found
that the photophysical behavior of such complexes is
dependent on the dπ(Re)−ππ*(diimine) MLCT, with the
electronic influence of the isocyanide playing an ancilliary role
in the energetics of this excited state. It is noted that further
functionalization of the isocyanide provides a route to a more
drastic modification of the properties of these rhenium(I)
complexes. Such molecules are potentially valuable for driving
catalytic transformations initiated through triplet−triplet
energy transfer, as has been recently discovered.40,41 Beyond
rhenium(I) complexes, isocyanide ligands have been employed
as ancillary ligands in other photophysically active systems.
Most of these are derived from iridium(III) cyclometalate
archetypes, and all feature bright phosphorescence across the
3
3
molecules, wherein the MLCT and two distinct LC excited-
state configurations (phen and CNAr) conspire to produce the
resultant photophysical properties.
RESULTS AND DISCUSSION
■
Syntheses. The rhenium(I) complexes 1−4 were synthe-
sized using the general procedure outlined in Scheme 1. Each
of the isocyanide ligands was prepared following the general
synthetic procedure outlined in Figures S1−S3. Each of the
target compounds was purified through multiple precipitations
by dissolving the isolated product in methanol, followed by
treatment with aqueous NH4PF6. The final rhenium(I)
complexes were structurally characterized using H and 13C
1
NMR spectroscopy, high-resolution time-of-flight electrospray
ionization mass spectrometry (ESI-MS), and Fourier transform
infrared (FTIR) spectroscopy. The purified molecules were
determined to be both thermally and photochemically stable in
nonhalogenated solvents, with a minor degree of photo-
degradation observed in halogenated solvents upon extended
3
visible spectrum from a predominantly LC state, depending
on the cyclometalate chosen.42−46 Further utilization of
isocyanides has come in the form of homoleptic complexes
of group VI metals, where the isocyanide is the chromophoric
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Figure 1. Frontier molecular orbitals for 1−4. Calculations were performed at the DFT//M06/Def2-SVP/SDD level of theory.
Figure 2. Electronic UV−vis absorption (red) and normalized static PL (blue) spectra of 1−4 in THF at RT. The PL spectra were recorded with
samples having an o.d. = 0.1 at 355 nm excitation in deoxygenated THF.
light exposure. The new molecules are soluble in a variety of
polar organic solvents, and tetrahydrofuran (THF) was
selected as the solvent medium for all spectroscopic measure-
ments in this contribution.
demonstrating that modification of the ancillary isocyanide
ligand does not significantly impact the electronic structure of
the phen ligand. The highest occupied molecular orbital
(HOMO) for these complexes is largely localized on the CNAr
ligand, but there is a significant contribution from the Re d
orbitals that decreases upon going from 1 to 4. The increase in
the electronic localization on the isocyanide ligand also
correlates with the change in the energy where the HOMO
energy increases from −7.274 eV (in 1) to −6.676 eV (in 4).
HOMO−1 consists largely of Re d orbital contributions with
some contribution localized on the phen ligand (1 and 2) or
the CNAr ligand (3 and 4).
Static Absorption and PL Measurements. The UV−vis
electronic absorption and PL spectra of 1−4 measured in THF
are presented in Figure 2, with additional information collected
in Table 1. In all of the complexes in this study, the red edges
of the absorption bands greater than 350 nm have been
Electronic Structure Calculations. Prior to the dis-
cussion of the photophysical properties of complexes 1−4, a
brief DFT-based analysis is presented. The DFT and TD-DFT
calculations were performed at the M06/Def2-SVP/SDD level
of theory using the polarizable continuum model (PCM) to
simulate the THF solvent environment. The ground-state
structures for 1−4 were optimized, and the frontier molecular
orbitals are presented in Figure 1 and the resulting energies
collected in Table S1. In all four molecules, LUMO and
LUMO+1 are the in-phase and out-of-phase π*-orbital
combinations, respectively, localized on the phen ligand. The
energies of LUMO (−3.080 to −3.089 eV) and LUMO+1
(−2.865 to −2.872 eV) are also consistent across the series,
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between 300 and 350 nm when moving across the series is
attributed to the presence of additional aromatic rings on the
isocyanide ligands because extension of the π system lowers
the energy while raising the oscillator strength of the LC π−π*
transitions localized on the isocyanide ligand. Experimentally,
this results in the red shift and broadening of the main
absorption band in the electronic spectra.
Table 1. Static UV−Vis Absorption and PL Data for 1−4 at
a
RT and 77 K
RT PL
λmax, nm
77 K Em
λmax, nm
complex
Abs λmax, nm (ε, M−1 cm−1
)
1
2
273 (45700), 305 (16400), 365 (3000) 505
278 (57900), 315 (30200), 339 (15800), 507
367 (3600)
456
457
The static PL spectra for 1 and 2 measured in THF are
broad and featureless and are also highly quenched by
dissolved oxygen, suggesting significant triplet excited-state
character in the PL emission; all of these observations are
consistent with some degree of charge-transfer character. We
note that simple visual inspection of the RT PL spectra in these
molecules can be misleading because multiple closely lying
excited states each contribute PL intensity proportional to their
respective radiative rate constant.8 Therefore, unambiguous
identification of the lowest excited state requires more
sophisticated spectroscopic examination as described below.
The structured PL spectra exhibited by 3 and 4 (Figure 2) at
room temperature (RT) in THF immediately suggest
interactions between multiple excited states in close energetic
proximity. Here, 3 exhibits the most obvious vibronic
components, whereas 4 features much more subtle broadening
and shouldering toward lower emission energies. All PL
spectral features in both 3 and 4 are strongly quenched by
dissolved oxygen in THF solutions. This oxygen quenching
was found to be asymmetric over the entire emission envelope,
with the higher-energy emission more strongly quenched than
the lower-energy region. Given these initial steady-state
spectroscopic observations, the main PL emission features of
3
4
278 (40400), 300 (35900), 322 (40900), 503, 535
492
495
341 (33000), 370 (4000)
(sh)
277 (40600), 298 (38800), 319 (40500), 518
340 (29400), 370 (3800)
a
All RT spectra measured in THF (absorption), deoxygenated THF
(PL), and 77 K PL data recorded in Me-THF frozen glasses. All
wavelengths measured to 2 nm.
assigned as charge transfer in character. A previous study by
Rillema and co-workers demonstrated that the lowest-energy
absorption band on a rhenium(I) complex bearing a 2,6-
dimethylphenyl isocyanide was a metal−ligand-to-ligand
charge-transfer singlet.13 The TD-DFT calculations performed
here confirm this assignment as the lowest-energy transition in
1−4 consisting of a rhenium/isocyanide (metal/ligand)-to-
phen ligand charge-transfer transition (Figures S10−S13).
However, the oscillator strength of this transition is low, so it
likely contributes very little to the experimentally observed red
shoulder in the corresponding absorption spectra (Figure 2).
When the higher-lying transitions S0 → S2 through S0 → S4 in
complexes 1 and 2 are examined, the transition type changes to
a Re(dπ) → phen(π*) MLCT transition with higher oscillator
strength, contributing to the observed shoulder in the
measured spectra.
3
3 and 4 are tentatively assigned to a mixture of LC and
3MLCT excited states. In addition, the optimized triplet-state
spin densities obtained from DFT support this as the
localization of the unpaired spins to be localized on the
rhenium, the isocyanide, and the phen ligand (Figures S16 and
The high-energy portion (λ < 300 nm) of the electronic
spectrum of 1 is dominated by phen-based π−π* transitions
with some π−π* contribution from the isocyanide ligand. The
increased oscillator strength of the spectral features observed
Figure 3. Normalized static PL spectra of 1−4 measured in 2-MeTHF frozen glasses at 77 K (λex= 355 nm). The vertical dashed lines are drawn as
a guide for the eye to illustrate the marked differences in the PL spectral profiles in 1 and 2 with respect to 3 and 4.
D
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S17). Additional static and dynamic experiments described
below will help to elucidate the true nature of these excited
manifolds and their respective interactions.
Table 2. Summary of the Photophysical Parameters of 1−4
Measured in THF at RT
a
b
b
complex
Φ
τem (μs) τTA (μs) kr (×103 s−1
)
knr (×103 s−1
)
The static PL spectra for 1−4 were recorded at 77 K in 2-
methyltetrahydrofuran (2-MeTHF) glasses following 355 nm
excitation (Figure 3). In 1 and 2, the 77 K PL emission spectra
are significantly blue-shifted relative to their corresponding RT
spectra recorded in THF and a high degree of vibronic
structure also becomes evident. The thermally induced Stokes
shifts [ΔEs = E00(77 K) − E00(298 K)] observed in 1 and 2 are
quite large and quantitatively identical in magnitude given the
1 nm bandpass of the PL experiments, ΔEs = 2128 and 2158
cm−1, respectively. By comparison, the ΔEs measured for the
1
2
3
4
0.64
0.48
0.14
0.15
8.1
14.2
d
8.9
15.5
72.0
31.0
0.20
0.22
40.5
33.6
1.2
c
710
663
c
d
1.3
a
Measured relative to a 9,10-diphenylanthracene standard quantum
counter, 10%.56 Values calculated from the PL quantum yield and
b
c
TA decay lifetime values, 5%. Excited-state lifetime extrapolated to
infinite dilution, 5%. Values could not be accurately determined
d
because of the lower quantum yields and longer lifetimes. Higher
concentrations were necessary for an accurate kinetic fit, which
ultimately manifested quenched excited-state best fits using
biexponential kinetics.
3
rhenium(I) diiminetricarbonyl MLCT model complex Re-
(phen)(CO)3Cl is 2381 cm−1 in the same solvent measured
under identical experimental conditions (Figure S18). In all of
these instances, the magnitude of the ΔEs values remained
consistent with that anticipated for strongly polar charge-
transfer excited states. However, the measured 77 K PL spectra
higher concentrations (>10−5 M), the excited-state decay was
best fit with a biexponential function in 3 and 4 because there
was now clear evidence of an additional emissive species being
present in these samples as visualized in static and dynamic PL
experiments (Figure 4). Note that no aggregation was
3
of 1 and 2 quantitatively align with the LC phosphorescence
measured for phen,2 indicative of 3MLCT/3LC state crossover
occurring between RT and 77 K. In essence, the lack of solvent
3
reorganization in the vitrified medium renders the MLCT
state slightly higher in energy with respect to the lower-lying
1,10-phenantholine-centered 3LC state. This behavior is
analogous to previous work on rhenium(I) diimine complexes
containing alkylisocyanide ancillary ligands, in addition to
being observed in various MLCT excited states featuring other
transition metals.2,50,51 Of particular relevance to the current
observations, the 77 K emission spectrum of [Ir(phen)2Cl2]Cl
is quantitatively similar to the PL spectrum recorded for 1,
which is also similar to the phosphorescence of free phen.52
In 3 and 4, the static PL emission spectra exhibit
substantially smaller thermally induced Stokes shifts (ΔEs =
444 and 897 cm−1, respectively) in comparison to 1 and 2,
concomitant with the observation of distinct vibronic
progressions becoming revealed at 77 K in these molecules
(Figure 3). Because the MLCT states are expected to blue shift
over 2000 cm−1 in these chromophores while the phen-based
3LC states remain at much higher energy, this lower-energy
phosphorescence emission measured in 3 and 4 must be
emanating from a lower-energy triplet excited state. Because
molecules 3 and 4 feature aryl-based isocyanide substituents
possessing extended π conjugation (phenylacetylene and
biphenyl, respectively), it immediately becomes apparent that
these 77 K PL spectra are most appropriately assigned as the
3LC states localized on the CNAr substituents in 3 and 4.
These particular molecules now emerge as quite interesting
because they likely feature RT electronic interactions occurring
between three distinct triplet excited states, namely, the lowest
Figure 4. Time-gated PL emission spectra recorded for 3 (bottom)
and 4 (top) measured in THF using nanosecond laser pulses (λex
=
355 nm; 7 ns fwhm) and ICCD detection. Spectra at 250 ns delay
(red), relative to the excitation pulse, recorded with a 50 ns gate
width. Spectra at 50 μs delay (blue) recorded with a 1 μs gate width.
Normalized steady-state PL spectra (black) are shown for comparison
along with the static PL spectrum that subtracts the normalized time-
resolved PL spectrum recorded at 50 μs delay.
observed in the ground states of 3 and 4, as evidenced by
the corresponding electronic absorption spectra being
consistent with the Beer−Lambert law over all chromophore
concentrations investigated. The static PL emission spectra
measured in concentrated samples of 3 and 4 contained a low-
energy shoulder; see the solid black lines in Figure 4. In
addition, time-resolved PL experiments revealed that both
molecules possessed shorter lifetimes than those recorded in
the corresponding diluted samples. Therefore, time-gated PL
emission spectra were recorded for concentrated samples of 3
and 4 at both short (250 ns) and long (50 μs) delay times in
order differentiate the high- and low-energy features observed
in the static PL spectra (Figure 4). The time-gated spectrum
recorded for 3 at the 250 ns delay time (solid red line) was
very broad in nature, spanning most of the visible region, and
featured rather intense emission toward the red region of the
3
MLCT state and two low-lying LC states resident on two
distinct chromophoric ligands (phen and CNAr) installed on
the same rhenium(I) center. To the best of our knowledge, 3
and 4 represent the first examples of rhenium(I) tricarbonyl
chromophores, with three distinct triplet excited states being
thermally accessible simultaneously. These interactions will be
further elucidated below through temperature-dependent
excited-state dynamics.
Concentration-Dependent Self-Quenching. Another
interesting phenomenon consistently observed in 3 and 4
was a concentration dependence of their excited-state lifetimes
in THF. The combined photophysical data measured for 1−4
under optically dilute conditions are presented in Table 2. At
E
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Figure 5. Ultrafast TA difference spectra of 1−4 recorded in THF. The ultrafast TA difference spectrum of the model chromophore
Re(phen)(CO)3Cl (red dashed line) at 2.12 ps delay is included with 1 for a direct comparison. All difference spectra were obtained using 352 nm
excitation (0.4−0.8 mJ pulse−1; 90 fs fwhm) with sample o.d.’s of ∼0.45.
spectrum. The 50 μs delay spectrum in 3 (solid blue line)
almost quantitatively reproduced the static PL spectrum at this
concentration, which possessed most of the spectral character-
istics of the diluted chromophore, as seen in Figure 2. Similar
results were obtained in analogous experiments, where 4 was
examined under identical experimental conditions.
significant amplification of the PL intensity on the red side of
the static spectral envelope was clearly observed (Figure S19).
The excited-state decay kinetics was measured as a function
of the chromophore concentration in order to use the Stern−
Volmer equation to quantify the rate of self-quenching (kq) in
3 and 4 as well as give an estimate of the excited-state lifetime
at infinite dilution (τ0). Figure S20 presents these Stern−
Volmer plots, where the kq values were found to be 8.8 × 108
and 5.80 × 108 M−1 s−1 for 3 and 4, respectively. Because the
extent of orbital overlap in an excimer is related to the relative
red shift observed between monomer and excimer emission, 3
is clearly red-shifted with respect to 4. To rationalize this
result, we postulate that the isocyanide ligand in 3 has the
potential for greater π overlap between distinct molecules due
to the more planar nature of the ethynyl linker used in this
instance. In 3, the red shift was approximately 2766 cm−1,
while that in 4 was found to be ∼1511 cm−1. Overall, the π-
conjugated CNAr subunits appear to facilitate intermolecular π
stacking during excited-state decay, and it is this interaction
that is most likely responsible for excimer formation in 3 and 4.
Moving forward, this suggests a general design strategy in
metal−organic chromophores wherein the efficient generation
of a long-lived triplet intraligand excited state can lead to
entirely new classes of excimer-based PL projecting from the
red into the near-IR.
In both instances, the PL decay at earlier times was
dominated by emission emanating from a characteristically
distinct species decaying in parallel with the longer-lived bluer
species corresponding to the photophysical response from the
diluted molecule. Because the shorter-lived PL emission in 3
and 4 was red-shifted, in addition to being broad and
featureless, we propose that an excimer-type interaction was
responsible for this behavior. The excited-state lifetimes in 3
and 4 are so long-lived (710 and 663 μs, respectively) that this
readily enables bimolecular reactions with ground-state
molecules, ultimately leading to low-energy excimer formation
in a portion of the decaying excited-state population. Although
rare, a handful of examples of inorganic triplet excimers have
been reported, almost exclusively based on square-planar
platinum(II) complexes.53−55 However, in the current
3
molecules, it is the LC excited states resident on the CNAr
ligands that are deemed responsible for producing the
proposed excimers in 3 and 4. In an attempt to generate the
true emission spectral profile of the proposed excimers, the
normalized time-gated spectrum at 50 μs was subtracted from
the corresponding static spectrum in both 3 and 4 (Figure 4,
dashed green lines). It is clear that these difference spectra
produce profiles that are substantially red-shifted and distinct
of those measured in the diluted molecules, indeed
qualitatively consistent with excimer formation. This excimer
interaction was also manifested in the static PL emission
spectra measured in concentrated solutions of 3 and 4, where
Excited-State Dynamics. In order to comprehend the
photophysical processes leading to formation of the lowest
excited-state configurations in 1−4, ultrafast TA experiments
were performed in THF solutions. The TA difference spectra
are shown in Figure 5, with the time constants fit to the most
prominent signals summarized in Table 3, with kinetic data
given in Figures S21 and S22. What becomes immediately
apparent is the difference in dynamics between 1 and 2−4. In
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3
nanoseconds time scale, which is the lowest-energy MLCT
excited state. By comparison, the prompt TA signatures
measured in 1 on femtosecond-to-nanosecond time scales are
clearly inconsistent with the expected salient features of the
phen radical anion. Instead, there is a prompt formation of a
broad transient absorbance (across most of the visible region)
with a discernible peak at 400 nm, which decays away only
slightly over the entire length of the delay line (5.5 ns).
Because the molecular photophysics from the lowest excited
state in 1 and related molecules based on iridium(III) and
platinum(II) is considered to be configurationally mixed
Table 3. Summary of the Time Constants from Ultrafast TA
Kinetic Modeling
a
complex
λfit (nm)
τ1 (fs)
τ2 (ps)
τ3 (ps)
1
2
3
4
400, 417, 552, 649
398, 536
523, 537, 580
572, 603
200 ( 50)
190 ( 50)
130 ( 60)
220 ( 20)
25 ( 10)
5 ( 1)
3.0 ( 0.2)
3.4 ( 0.3)
50 ( 8)
13.5 ( 0.4)
26 ( 1)
a
Fits and kinetic data from which these time constants were obtained
can be found in Figures S20 and S21.
3
3
1, there is the prompt (∼200 fs) formation of a broad excited-
state absorbance feature traversing most of the visible
spectrum. This is followed by very subtle shifts in the signal
maximum around 400−450 nm with a corresponding
formation time constant of 25 ps (Figure S22). These
because of energetically proximate MLCT and LC (phen)
manifolds,50,59−63 we assign the TAs observed in 1 to the
3phen moiety because the phen radical anion does not
contribute significant oscillator strength. In the available
published examples of TA studies executed on configuration-
ally mixed excited states, the dominating absorbing species is
3
dynamics differ significantly from those found in the MLCT
model compound Re(phen)(CO)3Cl, whose difference spec-
trum is presented in Figure 5 at 2.12 ps delay. In our
experiments (Figure S23) and previous literature results,57,58
100 fs pulsed-laser excitation of Re(phen)(CO)3Cl results in
the nearly prompt formation of its 3MLCT excited-state
absorption with a maximum near 475 nm, consistent with that
shown in Figure 5. This was followed by the growth of a
shoulder at 575 nm taking place on the order of tens of
picoseconds. The prompt signal has been assigned to rapid
dynamics associated with the formation of the phen radical
anion along with intersystem crossing to the 3MLCT manifold.
The longer-time-scale (picosecond) dynamics was assigned to
3
unequivocally the LC state in all instances. Similar results
have also been echoed in metal−organic molecules featuring
triplet excited-state equilibria.58,64−70
For 2−4, it is clear that the ultimate triplet excited state
formed is significantly different with respect to that measured
in 1, as evidenced by the continued growth in the TA signals
on the order of tens of picoseconds. These molecules also
possess a similar prompt (130−220 fs) formation of the broad
visible absorption signal observed in 1 at both short and long
delay times. Combined together, we assign this initial
absorption feature in 2−4 as resulting from intersystem
crossing from the singlet manifold to the configurationally
mixed triplet state (3MLCT/3LC) analogous to that assigned
3
vibrational relaxation occurring within the MLCT manifold
that ultimately produces a transient signal that persists into the
Figure 6. Normalized nanosecond sensitized triplet absorption difference spectra (red spectra) of the free isocyanide ligands corresponding to
those in molecules 2−4 compared to the normalized absorption difference spectra for 2−4 measured promptly in nanosecond TA experiments
(blue spectra). Excitation was performed at 355 nm (∼1.0 mJ pulse−1; 7 ns fwhm), and spectra were recorded with 5 μs delay relative to excitation
and 50 ns gate width. The triplet sensitizers were benzophenone for 2 and thioxanthen-9-one for 3 and 4.
G
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Article
Figure 7. Qualitative energy-level diagrams summarizing the photophysical behavior of 1−4.
in 1. This is followed by vibrational relaxation within this
configurationally mixed triplet excited state that is similar to 1,
now with a slightly faster time constant ranging between 3 and
5 ps. However, after the prompt formation and vibrational
cooling of this initially populated state, there is a significantly
slower process (ranging from 13 to 50 ps) believed to be triplet
migration to an excited state with a significantly higher
extinction coefficient, which is also responsible for the
substantial absorption growth observed for 2−4 in the visible
region over this time range.
Figures S24−S27. The PL and TA decay kinetics measured for
1−4 using conventional time-resolved PL and laser flash
photolysis experiments were all adequately modeled with
single-exponential decays and featured quantitative agreement
between both experiments in all instances (Table 2).
Consistent with the 3MLCT/3LC (phen; in 1) and 3LC
(isocyanide; in 2−4) excited-state assignments suggested
above, the long-lived excited-state lifetimes recorded here,
8.9, 15.5, 710, and 663 μs, respectively, are completely
congruent with those assignments. Figure 7 presents qualitative
Jablonski diagrams of 1−4, summarizing all of the photo-
physical processes and their corresponding assignments
resulting from the current investigation.
Temperature-Dependent Time-Resolved PL and TA
Experiments. In order to investigate the relative energies of
the closely lying triplet excited states featured in this series of
molecules, the excited-state decay rates of each chromophore
were measured as a function of the temperature. To best fit the
series dynamic temperature data, a three-state Boltzmann
distribution model was invoked; Figure 8 and eqs 1−3 describe
Because these data are quite distinguishing, we assign the
3
long-lived excited-state absorptions in 2−4 to the pure LC
excited states corresponding to the respective isocyanide ligand
residents in the structure. Further evidence for these
assignments emerges from the sensitized triplet spectra of
the free isocyanide ligands resulting from sensitization of 2
with benzophenone and sensitization of 3 and 4 with
thioxanthen-9-one (Figure 6). In order to best mimic the
Lewis acidic character of the rhenium(I) center, trifluoroacetic
acid (TFA) was added to the respective sensitization
experiments to singly protonate the relevant isocyanide ligand.
Although the positions of the resultant triplet absorbances in
the free ligands are somewhat shifted with respect to the TAs
observed in the analogous rhenium(I) chromophores, their
bandshapes are qualitatively similar and therefore consistent
3
with formation of the LC excited state on the respective
isocyanide moiety. Overall, the combined ultrafast TA data and
the nanosecond triplet sensitization experiments suggest triplet
3
energy migration from the initially populated MLCT/3LC
3
state to the LC state of the respective isocyanide ligand
occurring with time constants of 50, 13.5, and 26 ps in 2−4,
respectively. An extensive literature search revealed no related
reports of excited-state dynamics occurring between configura-
3
3
tionally mixed MLCT/3LC excited states, with a second LC
excited state located on an ancillary ligand. Clearly, there is
ample space for additional investigations in this general area,
and many combinations of metal centers and ligands can be
conceived to glean further insight into such intramolecular
triplet-energy migration processes.
Figure 8. Generalized energy-level diagram depicting the parameters
of three-state Boltzmann model used to model temperature-
dependent excited-state decay in molecules 1−4.
this three-state model.8,71 The three excited states involved
here are defined as two emissive triplet states of charge-transfer
and/or LC character and the third of a thermally deactivating
metal-centered ligand-field state. This three-state model has
been successfully applied to similar rhenium(I) diimine
complexes.8 Consistent with these previous studies, a simple
two-state model using one triplet excited state and one d−d
state does not adequately fit our data without giving physically
unreasonable results.
In 1−4, the persistent transient signals from the ultrafast TA
experiments measured at long delay times quantitatively match
the corresponding nanosecond TA difference spectra, indicat-
ing that no significant dynamics are taking place between the
5.5 ns delay time in the ultrafast experiment and 15 ns, the
earliest prompt signal resolvable in the nanosecond TA
apparatus. The nanosecond TA difference spectra of 1−4 as
well as their associated transient decay kinetics are provided in
H
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Article
kd1 = A1e−ΔE /kT
2
Complexes 3 and 4 feature an entirely different series of
molecular excited states that are in play. The first obvious
difference is that a much larger energy gap between the lowest
triplet and ligand-field states now exists, ∼5600−6000 cm−1.
Additionally, the decay rate from that lowest triplet excited
state is significantly slower in comparison to that observed in 1
and 2. If it is assumed that the differences in the ligand-field
state energy between all four molecules are negligible, we can
readily conclude that the lower triplet excited states in 3 and 4
are no longer 3LC (phen)/3MLCT-based but rather 3LC
(CNAr)-localized. This is consistent with the combined PL,
TA, and electronic structure calculations (Figures S16 and
S17), therefore rendering the upper excited state in 3 and 4
using the model presented in Figure 8 as an admixture of
(1)
(2)
kd2 = A2e−ΔE −ΔE /kT
2
1
1 + e−ΔE /RT
1
1
kobs
τ =
=
k1 + k2e−ΔE /kT + (A1 + A2)e−ΔE /kT
1
2
(3)
A few assumptions accompany this particular model. The
first is that the two lower excited states are photoluminescent.
The second is that the upper d−d ligand-field states deactivate
so efficiently that, once this state is populated, there are no
viable radiative decay pathways. Third, the role of the self-
quenching phenomenon noted for complexes 3 and 4 is
negated by conducting experiments at low concentrations
where self-quenching was not observed. Finally, it is also
assumed that ΔE2 is sufficiently greater than ΔE1 such that the
rates of thermal activation to the ligand-field state can be
combined into a single preexponential factor (A1 + A2). The
most adequate fits taking into account all of the time-resolved
temperature-dependent PL (1 and 2) or absorbance data (3
and 4) for all four complexes are presented in Figure S28, and
the values for each relevant fitting parameter are summarized
in Table 4.
3
nearly isoenergetic LC(phen)/3MLCT states.
CONCLUSIONS
■
This study investigated a series of rhenium(I)-based
diiminetricarbonyl arylisocyanide chromophores (1−4) featur-
ing interactions between three closely lying triplet excited
states. The combined experimental evidence illustrated that the
CNAr ligand was actively engaged in manipulating excited-
state decay in three of these molecules (2−4), wherein the
3MLCT state along with two LC excited-state configurations
3
Table 4. Summary of Fitting Values from the Three-State
Boltzmann Distribution Model
(phen and CNAr) conspired to produce the resultant
photophysical properties. As the π conjugation within the
CNAr ligand was extended, the initially generated 3LC-
(phen)/3MLCT excited state in 1 ultimately migrated to the
CNAr 3LC state on the order of tens of picoseconds in 2−4. It
should be noted that previous literature has described
a
b
b
c
c
complex
ΔE1
ΔE2
k1
k2
1
2
3
4
545
496
775
806
4042
4042
5622
5957
9.3
3.1
0.55
0.12
53.0
55.0
22.5
25.1
3
dπ(Re)−π*(CNAr) MLCT transitions existing in a number
of complexes not featuring a diimine moiety.35,36 The existence
of this transition and any role it may play in the photophysics
of the complexes in this current study was considered;
however, no spectroscopic or computational evidence was
found to support this consideration. Chromophores 3 and 4
with RT lifetimes near 700 μs represent unique examples of
excimer formation from inorganic molecules, albeit from
interactions between the 3LC state and a corresponding
ground-state molecule, as evidenced by dynamic self-
quenching in the corresponding PL intensity decays,
accompanied by the observation of a short-lived low-energy
orange-to-red emission feature in both instances. In the quest
for generating new classes of molecular-based charge-transfer
excited states intended for driving high-energy chemical
transformations, all excited configurations must be considered
to achieve the proper chromophore design. While 3LC excited
states on diimine and ancillary ligands are energetically out-of-
reach in most visible-absorbing MLCT chromophores, they are
quite relevant and can dominate excited-state decay in near-
UV-absorbing MLCT molecules, as was observed in 2−4 in
the present study.
a
The preexponential factor (A1 + A2) was fixed to 4 × 1012 for 1 and
b
28 and floated between 4 × 1014 and 8 × 1014 for 3 and 4. Units of
c
cm−1 Value × 104 s−1
From the extracted parameters, a number of conclusions can
be drawn. In 1 and 2, the energy gap between the lowest triplet
manifold and the d−d state is approximately 4000 cm−1, in
good agreement with similar molecules modeled in an identical
fashion.8 According to the relative rates of decay from the two
emissive excited states, the lower state is consistent with being
3LC in character, whereas the higher-lying excited state is more
3
in line with the CT character. Additionally, the ordering of
triplet states is consistent with the TD-DFT modeling of the
transitions, where S0 → T1 is LC on the phen ligand and S0 →
T2 is the MLCT transition (Figure S14). From a combination
of the kinetic and DFT modeling, 77 K PL emission spectra,
and conclusions drawn from the TA dynamics, it can be
3
concluded that the lower triplet excited state in 1 is LC
(phen) and the upper excited state is likely 3MLCT. Definitive
3
assignments of the lowest LC state in 2 are more difficult
because this molecule likely features a mixed 3LC state
resulting from the nearly isoenergetic combination of both the
phen and the isocyanide triplets. TD-DFT modeling reveals
that the energy difference between S0 → T1 (phen) and S0 →
T2 (isocyanide) is approximately 0.03 eV (Figure S15).
Because these two states are relatively isoenergetic, they
could not be distinguished with the use of an additional
parameter in the Boltzmann fit, so their relative ordering
simply cannot be determined using temperature-dependent PL
decay data. However, the RT TA data suggest that the lowest-
EXPERIMENTAL SECTION
General Procedures. All reagents and solvents used in the
■
synthetic procedures were obtained from commercial sources and
used as received, unless otherwise noted. H and 13C NMR spectra
1
were recorded on a Varian Innova 400 MHz spectrometer with
1
working frequencies of 400 and 100 MHz for H and 13C NMR,
respectively. All NMR spectra were plotted and processed using the
MestReNova software, version 10.0.2. High-resolution ESI-MS data
were measured at the Michigan State University Mass Spectrometry
and Metabolomics Core (Waters Xevo G2-XS QTOF). Electronic
3
energy state is most likely LC (CNAr) in the case of 2.
I
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Inorganic Chemistry
Article
absorption spectroscopic measurements were recorded on a Shimadzu
UV-3600 spectrophotometer in 1 cm quartz cuvettes with
spectrophotometric-grade THF as the solvent. Steady-state emission
measurements were made on an FS-980 fluorimeter (Edinburgh
Instruments) fitted with a 450 W xenon arc lamp and a PMT detector
in sealable 1 cm quartz cuvettes designed for air-free handling.
Ultrafast TA Spectroscopy. Time-resolved TA measurements
were performed at the NCSU Imaging and Kinetic Spectroscopy
Laboratory in the Department of Chemistry. Subpicosecond
absorption transients were detected using a Helios TA spectrometer
from Ultrafast Systems. A portion of the output from a 1 kHz
Ti:sapphire Coherent Libra regenerative amplifier (4 mJ; 100 fs fwhm
at 800 nm) was split into the pump and probe beams. The probe
beam was delayed in a 6 ns optical delay stage, while the pump beam
was directed into an optical parametric amplifier (Coherent OPerA
Solo) to generate tunable excitation. These measurements were
performed according to previously published methods.58,72
Nanosecond TA Spectroscopy, Time-Resolved PL, and
Time-Gated PL Spectroscopic Measurements. Nanosecond TA
measurements and time-gated PL spectra were collected with a LP920
laser flash photolysis system (Edinburgh Instruments) using, for
excitation, a Minilite 355 Nd:YAG (Continuum). TA difference and
PL spectra were collected using an iStar ICCD camera (Andor
Technology), controlled by the LP900 software (Edinburgh Instru-
ments). Emission decay kinetics were measured using Minilite 355
Nd:YAG (Continuum) excitation and collected using an apparatus
described previously.64 All samples were prepared air-free in a
glovebox and sealed in sealable 1 cm quartz cuvettes.
Triplet Sensitization of Free Ligands. In order to obtain free-
ligand triplet TA spectra, a sensitizer was added (to achieve an o.d. at
excitation of ∼0.2) to saturated solutions of the free ligands is THF.
All preparation was done air-free in a glovebox. For the phen and
isocyanide ligands of 2, benzophenone was used as the sensitizer and
excited at 355 nm. For the isocyanide ligands of 3 and 4, thioxanthone
was used as the sensitizer and excited at 410 nm using the Vibrant 355
Nd:YAG/OPO (OPOTEK). For the spectra of all isocyanide ligands,
TFA (∼10%) was added to mimic the Lewis acid effect of
complexation.
DFT Calculations. The calculations utilized in this study were
performed using the Gaussian 09 software package (revision D.01)73
and the computation resources of the NCSU High Performance
Computing Center. Ground- and triplet-state geometry optimizations
were performed using the M06 functional,74 along with the def2-SVP
basis set of Aldrich’s group, as implemented in Gaussian 09.75 The
Stuttgart−Dresden effective core potentials76 were used to replace the
core electrons in rhenium for all calculations. The PCM was used to
simulate the THF solvent environment for all calculations.77
Frequency calculations were performed on all optimized structures,
and no imaginary frequencies were obtained. The TD-DFT
calculations were performed using the same conditions as those
described for the geometry optimizations.78−80 The energy and
oscillator strengths were computed for each of the 50 lowest singlet
excitations and 10 lowest triplet excitations. The natural transition
orbitals of the low-lying singlet and triplet transitions were generated
using GaussView 5.0.81
dissolved in minimal methanol and recrystallized by the addition of a
saturated solution of NH4PF6 in deionized water. The pure
recrystallized solid was filtered, washed with excess deionized water
and pentane, and then dried under vacuum.
[Re(phen)(CO)3(DippCN)]PF6 (1). Re(phen)(CO)3Cl (340 mg,
0.699 mmol), AgCF3SO3 (185 mg, 0.720 mmol), and 2,6-
diisopropylphenyl isocyanide (132 mg, 0.705 mmol) were used.
Yield: 443 mg, 81%. 1H NMR (400 MHz, CD2Cl2): δ 9.46 (dd, 2H, J
= 5.2 and 1.4 Hz), 8.87 (dd, 2H, J = 8.3 and 1.4 Hz), 8.28 (s, 2H),
8.10 (dd, 2H, J = 8.3 and 5.2 Hz), 7.29 (t, 1H, J = 7.9 Hz), 7.03 (d,
2H, J = 7.9 Hz), 2.43 (sep, 2H, J = 6.9 Hz), 0.84 (d, 12H, J = 6.9 Hz).
13C NMR (100 MHz, CD2Cl2): δ 191.5 (2C), 188.3, 155.0 (2C),
147.5, 146.0, 140.1, 132.0, 131.6 (2C), 129.1, 127.6, 124.4, 131.2
(2C), 128.8, 128.6, 128.5, 127.9, 127.8, 126.9, 126.7, 126.1, 125.8,
125.7, 125.4, 30.6, 22.1. HR-MS ([M − PF6]+). Calcd: m/z 636.1425.
Found: m/z 636.1426. FTIR (ATR, cm−1): 2172 (m), 2035 (s), 1961
(sh), 1925 (s).
[Re(phen)(CO)3(PhDippCN)]PF6 (2). Re(phen)(CO)3Cl (340
mg, 0.699 mmol), AgCF3SO3 (185 mg, 0.720 mmol), and 4-phenyl-
2,6-diisopropylphenyl isocyanide (187 mg, 0.710 mmol) were used.
Yield: 444 mg, 74%. 1H NMR (400 MHz, CD2Cl2): δ 9.48 (dd, 2H, J
= 5.2 and 1.4 Hz), 8.88 (dd, 2H, J = 8.3 and 1.4 Hz), 8.29 (s, 2H),
8.11 (dd, 2H, J = 8.3 and 5.2 Hz), 7.47−7.36 (m, 5H), 7.23 (s, 2H),
2.49 (sep, 2H, J = 6.9 Hz), 0.89 (d, 12H, J = 6.9 Hz). 13C NMR (100
MHz, CD2Cl2): δ 190.9 (2C), 187.8, 154.4 (3C), 146.9, 145.9, 143.8,
140.2, 139.6, 131.5, 128.9, 128.5, 128.3, 127.1, 127.0, 122.6, 30.2,
21.6. HR-MS ([M − PF6]+). Calcd: m/z 712.1738. Found: m/z
712.1730. FTIR (ATR, cm−1): 2174 (m), 2034 (s), 1965 (sh), 1938
(s).
[Re(phen)(CO)3(PhEthDippCN)]PF6 (3). Re(phen)(CO)3Cl
(340 mg, 0.699 mmol), AgCF3SO3 (185 mg, 0.720 mmol), and 4-
phenylethynyl-2,6-diisopropylphenyl isocyanide (202 mg, 0.703
mmol) were used. Yield: 463 mg, 75%. 1H NMR (400 MHz,
CD2Cl2): δ 9.47 (dd, 2H, J = 5.2 and 1.4 Hz), 8.88 (dd, 2H, J = 8.3
and 1.4 Hz), 8.29 (s, 2H), 8.11 (dd, 2H, J = 8.3 and 5.2 Hz), 7.48 (m,
2H), 7.35 (m, 3H), 7.20 (s, 2H), 2.43 (sep, 2H, J = 6.9 Hz), 0.87 (d,
12H, J = 6.9 Hz). 13C NMR (100 MHz, CD2Cl2): δ 191.3 (2C),
188.2, 155.0 (2C), 147.5, 146.2, 140.8, 132.2, 132.0, 129.6, 129.1,
127.6, 127.5, 126.7, 122.8, 93.0, 88.6, 30.6, 22.0. HR-MS ([M −
PF6]+). Calcd: m/z 736.1738. Found: m/z 736.1728. FTIR (ATR,
cm−1): 2172 (m), 2032 (s), 1959 (sh), 1947 (s).
[Re(phen)(CO)3(DiPhDippCN)] PF6 (4). Re(phen)(CO)3Cl (340
mg, 0.699 mmol), AgCF3SO3 (185 mg, 0.720 mmol), and 4-biphenyl-
2,6-diisopropylphenyl isocyanide (240 mg, 0.707 mmol) were used.
Yield: 516 mg, 79%. 1H NMR (400 MHz, CD2Cl2): δ 9.48 (dd, 2H, J
= 5.2 and 1.4 Hz), 8.88 (dd, 2H, J = 8.3 and 1.4 Hz), 8.29 (s, 2H),
8.12 (dd, 2H, J = 8.3 and 5.2 Hz), 7.66 (m, 2H), 7.62 (m, 2H), 7.56
(m, 2H), 7.45 (m, 2H), 7.36 (m, 1H), 7.29 (s, 2H), 2.43 (sep, 2H, J =
6.9 Hz), 0.84 (d, 12H, J = 6.9 Hz). 13C NMR (100 MHz, CD2Cl2): δ
191.5 (2C), 188.3, 155.0 (2C), 147.5, 146.5, 143.9, 141.7, 140.8,
140.6, 138.9, 132.0, 129.4, 129.1, 128.3, 128.1, 127.6, 127.5, 123.0
(2C), 30.8, 22.2. HR-MS ([M − PF6]+). Calcd: m/z 788.2051.
Found: m/z 788.2136. FTIR (ATR, cm−1): 2169 (m), 2035 (s), 1965
(sh), 1932 (s).
General Synthetic Procedure for the Re-CNAr Molecules.
This general preparation was adapted from the previously reported
synthesis.13 In a two-necked 100 mL round-bottom flask with a reflux
condenser, solid Re(phen)(CO)3Cl and silver triflate (slight excess)
were purged with a heavy N2 gas flow. To the solids was added
absolute ethanol (25 mL), and the mixture was sealed under N2. The
mixture was protected from light and heated to 85 °C for 4 h. The
mixture was then filtered hot over Celite. The filter solution was
returned to reflux and the appropriate isocyanide ligand was dissolved
in a minimal amount of absolute ethanol and added to the reflux
reaction via a syringe. After another 4 h. the reaction mixture was
cooled and the solvent concentrated to ∼5 mL in vacuo. This
concentrated solution was added dropwise to 150 mL of stirring
diethyl ether, producing a solid slurry. These solids were filtered and
dried to yield the crude product as triflate salt. This solid was
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01155.
■
S
Additional static and time-resolved spectra, temperature-
dependent data, structural characterization data, and
DFT calculations for 1−4 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: fncastel@ncsu.edu. Phone: (919) 515-3021.
■
J
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Article
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ORCID
Felix N. Castellano: 0000-0001-7546-8618
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under
Award DE-SC0011979. J.E.Y. was supported by the Air Force
Institute of Technology.
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